however , liver , muscle , and adipose tissue are the most sensitive to the hormone , rendering it the most important anabolic hormone identified to date . in vertebrates
, this peptide belongs to a superfamily of structurally related proteins , the insulin - related family of substances , that includes insulin - like growth factors-1 ( igf-1 ) and -2 ( igf-2 ) and relaxin . until three decades ago ,
insulin was considered only as a peripheral hormone , unable to cross the blood - brain barrier ( bbb ) and to affect the central nervous system ( cns ) [ 35 ] .
however , this idea was challenged after the detection of immunoreactive insulin in dog cerebrospinal fluid ( csf ) .
further studies provided clear evidence that insulin occurs in brain , where it may reach high levels [ 7 , 8 ] , exerting long - term trophic effects on cns neurons .
although the in vivo brain insulin levels remain controversial , they appear to be 10- to 100-fold higher than in plasma and to change during brain development , with the highest values in late fetal and early neonatal rabbit brain ( about 8090 and 195  ng / g , respectively ) , that decrease in the adult brain ( about 32  ng / g ) .  
insulin present in adult cns is primarily derived from pancreatic -cells and is transported by csf into the brain [ 35 , 9 , 10 ] .
this insulin crosses bbb mostly via a carrier - mediated , saturable , regulatable , and temperature - sensitive active process [ 4 , 5 , 9 , 10 ] that is limited by the barrier system formed by the tight junctions between endothelial cells . thus , it is not surprising that an acute increase in peripheral insulin levels leads to higher csf insulin , whilst chronic peripheral hyperinsulinemia ( as occurs in insulin resistance ) downregulates insulin receptors ( ir ) at bbb , impairing insulin transport into the brain .
moreover , an increase in circulating insulin has been shown to rapidly affect brain activity ( independently from its systemic effects ) , as occurs in healthy individuals submitted to a hyperinsulinemic - euglycemic clamp . alternatively ,
peripheral insulin can access cns directly through the area postrema , a circumventricular region with a  leaky  bbb that allows free diffusion of plasma solubles directly into this area [ 13 , 14 ] .  

de novo insulin synthesis in brain has been proposed as an alternative source of insulin in cns .
this hypothesis has been supported by the detection of preproinsulin i and ii mrna in rat fetal brain and cultured neurons and also by insulin immunoreactivity in neuronal endoplasmic reticulum , golgi apparatus , cytoplasm , axon , dendrites , and synapses [ 1521 ] .
additionally , high levels of insulin in brain extracts , its presence in immature nerve cell bodies [ 7 , 18 ] , the rapid transport of peripherally injected insulin into the csf , and the fact that less than 1% of the hormone crosses the bbb in dogs and rodents   further support the idea that insulin can be synthesized in brain . however , the unequivocal evidence supporting this hypothesis is that insulin can be synthesized in cultured rat brain neurons and released upon k- and ca - induced membrane depolarization .
more specifically , its synthesis seems to occur in pyramidal neurons ( e.g. , from hippocampus , prefrontal cortex , enthorhinal cortex , and olfactory bulb ) , but not in glial cells .
thus , it is not surprising that insulin is highly enriched in brain cortex , olfactory bulb , hippocampus , hypothalamus , and amygdala . similarly to insulin , insulin - growth factor-1 ( igf-1 ) occurs also in rodent and human brain and can cross the bbb [ 11 , 27 , 28 ] .   taken this together
herewith , we will first analyse brain ir - mediated signaling pathways and the pivotal role of insulin in brain ( and peripheral ) glucose metabolism , synaptic transmission , memory / learning , and neuroprotection . then
, we will focus preferentially on the protective effect of insulin against diabetes and its long - term complications in cns , aging / longevity , and age - related neurodegenerative disorders ( especially alzheimer 's disease ( ad ) ) .
finally , we will briefly discuss the pros and cons of the potential therapeutic interest of insulin in diabetes- and age - related neurodegeneration .
in fact , from the evidences presented herein , insulin appears to be a naturally occurring hormone essential for normal cns function .
once in brain , insulin rapidly binds to ir , which are highly abundant ( but selectively distributed ) throughout cns [ 15 , 20 , 26 , 29 ] , especially in olfactory bulb , hypothalamus , cerebral cortex , cerebellum , hippocampus , and striatum [ 5 , 9 , 30 ] .
this differential distribution of insulin and ir in brain suggests that insulin from different sources ( peripheral or local ) may reach ir from distinct brain regions to initiate neuronal signal transduction .
concerning intracellular localization , ir are highly abundant in neurons ( with high protein expression in cell bodies and synapses ) and less abundant in glia [ 3236 ] .
similarly , igf-1 receptors ( igf-1r ) were detected throughout neurons and glia , particularly in hippocampus , amygdala , parahippocampal gyrus , cerebellum , cerebral cortex , and caudate nucleus , being less abundant in substantia nigra , red nucleus , white matter , and cerebral peduncles .
ir and igf-1r are tetrameric glycoproteins that belong to the receptor tyrosine ( tyr ) kinase superfamily , composed of two  ( 120135  kda ) and two  ( 95  kda ) subunits [ 2 , 3840 ] .
brain ir subunits differ from peripheral ones by the slightly lower molecular weight and by the absence of downregulation after exposure to high insulin levels [ 20 , 41 , 42 ] .
besides ir and igf-1r , the tyr kinase superfamily comprises also human insulin receptor - related protein , drosophila homolog of ir and mollusc ir - related protein , suggesting that their features have been highly conserved throughout evolution .
interestingly , two different types of ir have been found in mammalian brain : a 
peripheral-  like type ( with lower density in glia cells ) and a neuron - specific type ( highly expressed in neurons ) . due to structural and functional homology , insulin and igf-1 can bind to ( and activate ) both ir and igf-1r , with insulin binding to the ir with higher affinity ( < 1  nm ) than igf-1 ( 100500-fold lower affinity ) , whereas igf-1r preferentially binds igf-1 ( < 1  nm ) as compared to insulin ( 100500-fold lower affinity ) . once bound to  subunits of the neuronal ir or igf-1r , insulin ( or igf-1 )
promotes autophosphorylation of the  subunit at tyr residues 1158 , 1162 , and 1163 , triggering its intrinsic tyr kinase activity [ 5 , 39 , 40 , 4446 ] and phosphorylating insulin receptor substrate ( irs ) docking proteins ( irs1 - 4 ) at tyr residues .
then , src homology-2 ( sh2 ) domain - containing signaling molecules ( namely , the p85 regulatory subunit of phospatidylinositol 3-kinase ( pi3k ) ) are recruited and activate the catalytic subunit of pi3k , as well as the growth factor receptor - bound protein 2 ( grb-2 ) [ 5 , 40 , 44 , 46 ] .
as a consequence , two major signaling pathways can be activated : the pi3k / akt / glycogen synthase kinase-3 ( gsk-3 ) and the ras / raf-1/extracellular signal - regulated kinases ( erk1 and erk2 , erk1/2 ) [ 5 , 39 , 40 , 46 ] . after binding of the p85 sh2 domain to the phosphorylated tyr of active irs , pi3k becomes active and p110 inhibition is relieved , allowing translocation of active pi3k to plasma membrane and subsequent formation of pi-3 , 4 , 5-trisphosphate , and pi-3,4-bisphosphate .
then , these molecules bind to and recruit downstream signaling proteins containing pleckstrin homology domains . amongst them , the serine ( ser)/threonine ( thr ) kinase akt can be recruited to the plasma membrane [ 30 , 40 , 4648 ] and phosphorylated by protein kinase 3-phosphoinositide - dependent protein kinase-1 [ 44 , 46 , 49 ] .
once activated , akt detaches from plasma membrane and translocates into cytosol and nucleus , where it phosphorylates target proteins at ser and thr residues .
these target proteins include the proapoptotic bad ( inactivated when phosphorylated at ser136 ) , caspase-9 ( inhibited by phosphorylation at ser196 ) , and gsk-3 .
of relevance in this signaling pathway is the gsk-3 , a ser / thr protein kinase that becomes inactive upon akt - mediated phosphorylation at the n - terminal ser9 , initiating multiple physiologic effects [ 39 , 5053 ] .
gsk-3 is negatively regulated also by pkc and c - amp - dependent protein kinase ( pka ) ( which are also activated by insulin or igf-1 ) [ 54 , 55 ] . depending on the cellular context , gsk-3 can be targeted to different intracellular locations ( e.g. , cytosol , mitochondria , or nucleus ) to easily access its substrates : active gsk-3 ( ser9 dephosphorylated ) appears mostly in nuclei , mitochondria , and membrane lipid rafts ( detergent - resistant plasma membrane microdomains involved in signal transduction ) , while inactive gsk-3 is mostly cytosolic . besides ser9 dephosphorylation , gsk-3 can be also activated by phosphorylation at tyr216 , and , although this mechanism remains unclear , changes in intracellular ca or fyn ( a member of the src tyr kinase family ) appear to be involved [ 58 , 59 ] .
the inhibition of gsk3 , occurring after pi3k inhibition , has been described to prevent apoptosis , while overexpression of a constitutively active gsk-3 resulted in pc12 cell death [ 6062 ] .
interestingly , lin et al .   described that maintenance of phospho - gsk-3 levels , after transient activation of pi3k / akt signalling , prevented apoptosis induced by hydrogen peroxide ( h2o2 ) in neural stem cells .
moreover , we previously showed that insulin prevented neuronal ir / igf-1r inactivation upon oxidative stress and , by inhibiting gsk-3 ( via pi-3k / akt activation ) , insulin stimulated the synthesis of proteins involved in neuronal antioxidant defence , glucose metabolism , and antiapoptotic mechanisms .
although the mechanisms underlying gsk-3-induced apoptosis remain unclear , decreased creb - mediated expression of bcl-2 [ 65 , 66 ] , mitochondrial cytochrome c release and caspase activation ( either via the intrinsic pathway or via translocation of the apoptosis - inducing factor , aif ) [ 6770 ] , phosphorylation and downregulation of heat shock proteins ( e.g. , heat shock factor-1 ) may be involved .  
besides gsk-3 , igf-1-induced activation of akt also phosphorylates and inhibits foxo3 , which is retained in cytoplasm by the 14 - 3 - 3 protein . as a result , nuclear translocation of foxo3 is blocked , as well as its subsequent deleterious targeting of nuclear genes , thus promoting hippocampal and cerebellar granule neuronal survival [ 49 , 71 , 72 ] .
this foxo3 inhibition was also shown to impair bim transcription in cerebellar granular neurons , contributing to the antiapoptotic effect of igf-1 [ 72 , 73 ] .
akt may also phosphorylate nuclear factor-b ( nf-b ) either directly [ 74 , 75 ] or via ikk - induced inhibitory protein of b ( ib ) phosphorylation at ser32 ( with the concomitant disintegration of the p65-nf-b / ib complex , translocation of p65-nf-b into the nucleus , and its heterodimerization with other nf-b members for transcriptional regulation ) [ 76 , 77 ] .
importantly , nf-b activation has been shown to increase cu / zn -superoxide dismutase ( sod ) expression and mnsod levels after h2o2 and amyloid beta ( a ) treatment , protecting against oxidative stress and apoptosis in pc12 and nt2 cells , respectively [ 77 , 78 ] .
alternatively , pi-3k / akt activation may promote neuronal and glial antioxidant defences by stimulation of the nrf-2/antioxidant responsive element ( are ) .
another target for insulin - induced pi-3k / akt pathway is creb , which , upon phosphorylation at ser133 , enhances mitochondrial membrane potential ( m ) , intracellular atp levels , nad(p)h redox state , and hexokinase activity , that is , improves adult neuronal glucose metabolism and axonal outgrowth .
interestingly , this akt - mediated modulation of neuronal mitochondrial function and caspase activity has been correlated with changes in bcl-2 expression ( and its interaction with bad ) and/or with increased glucose transporter-1 ( glut1 ) expression ( and stimulation of glycolysis and protection against mitochondrial permeability transition pore opening and cytochrome c release ) [ 49 , 80 ] .
this may , at least partially , constitute an explanation for the apoptotic cell death that arises from the loss of neurotrophic support and subsequent depletion of pi3k / akt signaling .
after phosphorylation of ir , the adapter protein shc ( src homology-2 domain - containing ) binds to ir and grb2 is attracted to bind to shc via its sh2 domains , thus activating the erk1/2 signaling pathway .
sh3 domains are other grb2 domains that interact with guanine nucleotide exchange son of sevenless ( sos ) protein , stimulating the exchange of gdp for gtp at ras , which becomes active and then recruits the ser / thr kinase raf .
subsequent activation of mek ( or map2k , mitogen - activated protein kinase kinase ) leads to phosphorylation ( and activation ) of erk1/2 on thr and tyr residues [ 52 , 8183 ] , culminating in activation of several transcription factors that control gene expression ( e.g. , ets - like protein-1 ( elk-1 ) and c - myc ) .  
interestingly , erk1/2 activation was originally described to play an antiapoptotic role in neurons via phosphorylation of bad at ser112 [ 84 , 85 ] .
conversely , others showed the involvement of active erk1/2 in synaptic plasticity and cell death [ 8689 ] , especially after oxidative stress   and n - methyl d - aspartate ( nmda ) receptor - mediated excitotoxicity .
traditionally , insulin signaling through pi3k - dependent pathway was considered as functionally separated from erk- or mapk - dependent signals , with pi3k controlling metabolism , while the mitogenic role was related with erk1/2 .
however , others proposed that some cross - talk may occur between both signaling pathways , with raf being the possible link , at least in glutamate - induced oxidative stress .
this cross - talk may have an anti - apoptotic role through bad phosphorylation at ser136 .
.   showed that erk1/2 could be inhibited by igf-1 via pi3k - dependent raf phosphorylation at ser259 , thus protecting cerebellar granule neurons from cell death .
so this could constitute an alternative pathway ( to pi3k / akt - induced phosphorylation of bad , caspases and foxo3 ) to promote neuronal survival .  
besides pi3k / akt and erk1/2 signaling pathways , insulin has also been described to protect against neuronal and non - neuronal apoptosis via activation of mapk signaling [ 93 , 94 ] ( namely , p38 mapk ) and suppression of caspase-3 activity .
this was further confirmed by other studies showing that c - jun n - terminal kinase ( jnk ) downregulation promoted cell survival [ 96 , 97 ] .
activation of p38 mapk by insulin / igf-1 was also shown to stimulate glucose transport [ 98102 ] and antioxidant - related genes in nonneuronal cells submitted to oxidative stress .
surprisingly , little or no effect on mapk activation occurred upon insulin / igf-1 administration in embryonic dorsal root ganglion or adult sensory neurons [ 80 , 104 ] .
taken together , we can infer that the complexity of insulin / igf-1-mediated ir / igf-1r signaling pathways may play a crucial role in cns , including regulation of brain metabolism [ 105108 ] , neuronal growth and differentiation [ 16 , 105 , 109 ] , or neuromodulation [ 105 , 110112 ] .
interestingly , tissue ( and cellular ) dependence on insulin is such that its restriction under some damaging conditions results in cell atrophy and apoptotic death .
the most well - known insulin effect is the regulation of peripheral glucose transport and metabolism [ 2 , 113 , 114 ] .
however , the precise underlying mechanisms were unravelled only in the last decade and appear to involve a major integration of nutritional and hormonal peripheral signals , mediated by specialized groups of neurons from the arcuate nucleus of the hypothalamus , the glucosensing neurons .
these neurons respond to peripheral signals required for the regulation of ingestive behaviour and energy homeostasis .
besides insulin , such signals also include the anorexigenic peptides proopiomelanocortin ( pomc ) and cocaine- and amphetamine - regulated transcript ( cart ) , as well as the orexigenic neuropeptide y ( npy ) and agouti - related peptide ( agrp ) . under supraphysiological glucose levels , brain insulin signaling activation results in hyperpolarization of glucosensing neurons , activating katp channels and reducing neuronal firing ( probably due to the inactivation of pomc neurons ) , thus decreasing body weight [ 39 , 45 , 116 ] .
conversely , impairment of brain insulin signaling ( as occurs in peripheral insulin resistance ) activates jnk , phosphorylating irs-1 at ser / thr and promoting a feedback inhibition of ir , thus resulting in increased body weight .
this orexigenic effect is mediated by the activation of arcuate neurons containing npy , agrp , and gaba [ 39 , 45 , 117 ] .
additionally , besides controlling body weight , hypothalamic insulin also regulates hepatic glucose production , and , upon impairment of hypothalamic insulin signaling , the subsequent decrease in hepatic sensitivity to the circulating insulin stimulates hepatic glucose production [ 39 , 118 , 119 ] .
this suggests that insulin elicits a cns ( hypothalamus)-liver axis response to regulate hepatic glucose production .
although traditionally brain glucose metabolism was considered essentially insulin insensitive , some evidences led to the hypothesis that insulin may regulate glucose metabolism only in glia [ 33 , 121 , 122 ] .
however , more recent studies suggested that cerebral glucose metabolism may be controlled by neuronal insulin / ir signaling pathways [ 2 , 80 , 123 , 124 ] .
this hypothesis is supported by the overlapping distributions of insulin , ir , and glucose transporters ( gluts ) isoforms 1 and 4 in selective brain regions ( e.g. , hippocampus and choroid plexus ) .
furthermore , changes in circulating insulin levels were described to modulate cerebellar glut4 expression , and hyperinsulinemia led to regional changes in glucose utilization in rodent brain .
but the most striking evidence for insulin - mediated brain glucose metabolism came from bingham et al . , showing that fasting insulin levels stimulate global glucose metabolism maximally in human brain cortex , either directly ( as in peripheral tissues ) or indirectly ( via insulin - stimulated neuronal activation ) .
these authors also suggested that , if the recruitment of gluts to the plasma membrane and subsequent increment in glucose uptake was a direct effect of insulin , this might involve the partially insulin - sensitive glial glut1 , since the main neuronal glucose transporter ( glut3 ) is insulin insensitive .
an alternative pathway for insulin to provide energy for neurons involves inhibition of neuronal norepinephrine uptake , with subsequent activation of glial -adrenoreceptors and glucose extrusion from glial glycogen stores , namely , in astrocytes [ 2 , 120 ] . as a result , astrocytic glycogen can be converted to glucose , which is then transported to the extracellular fluid via insulin - stimulated glut1 , constituting an additional energy source for neurons [ 2 , 128 ] .
taken together , these evidences suggest that any deleterious interference in the cross - talk between insulin and neuronal glucose metabolism may impair atp synthesis and culminate in neuronal apoptosis . in this regard , wu et al
.   showed that insulin prevented serumdeprivation - induced apoptosis in r28 rat retinal neurons , while koo and vaziri   hypothesized that insulin could also prevent oxidation of glucose transporters or stimulate antioxidant defence mechanisms in type 1 diabetic streptozotocin ( stz ) rats , thus stimulating intracellular metabolism .
additionally , we reported that insulin - induced ir / igf-1r activation and subsequent pi3k / akt signaling prevented the decrease in hexokinase - ii expression , thus stimulating glycolysis upon oxidative stress in rat brain cultured neurons [ 64 , 132 ] .
this further supports the idea that brain is both an insulin- and glucose - sensitive tissue .
conversely , other studies failed to show acute effects of insulin on glucose transport into the brain [ 134 , 135 ] .
some authors hypothesized that brain insulin may play other roles apart from glucose metabolism , based on the heterogenous distribution of ir in brain , the poor correlation between ir location and neuronal energy utilization , the insulin - independent neuronal glucose uptake , the neuromodulatory role of insulin in invertebrates , its action on neuronal norepinephrine and serotonin uptake , and its relation to npy . among such roles
, brain insulin has been proposed to increase neurite outgrowth [ 136 , 137 ] and regeneration of small myelinated fibers [ 2 , 138 ] , maintain cortical , sympathetic and sensory neuronal survival during nervous system development [ 139 , 140 ] , stimulate neuronal protein synthesis , and improve synaptic activity and plasticity , memory formation , and storage , as well as neuroprotection [ 141143 ] . in this
regard , insulin administration has been shown to improve memory / learning in rats   and in healthy humans ( after intranasal administration ) , without changes in peripheral glycemia [ 12 , 144 ] .
systemic insulin infusion also improved verbal memory and attention . interestingly , these results appear to correlate with high ir expression in hypothalamus and limbic system ( hippocampus , pyriform cortex , and amygdala ) [ 12 , 15 , 115 ] and may be associated with the well - known effect of insulin on synaptic transmission ( e.g. , monoamines ) [ 15 , 146 , 147 ] . at this respect , about 30 years ago , insulin was described to promote epinephrine and norepinephrine release in adrenergic terminals , inhibit synaptic reuptake of norepinephrine , modify catecholamine kinetics , and stimulate neuronal serotonin uptake [ 42 , 149152 ] .
more recently , some authors reported that insulin modulated the expression of nmda receptors , increasing neuronal ca influx and reinforcing synaptic communication between neurons , and also modulated long - term potentiation , a molecular model of learning [ 126 , 154 ] .
this was further supported by insulin - mediated control of cell surface glutamate and gaba receptor density ( via the modulation of receptor targeting to the membrane and endocytic internalization ) , thus affecting synaptic plasticity .
additionally , we showed that insulin prevented the decrease in gaba and glutamate uptake and their increased extrasynaptosomal levels in rat synaptosomes after oxidative stress and/or type 2 diabetes [ 155 , 156 ] . apparently ,
such neuromodulatory role of insulin could arise from its direct effect on neurotransmitter transport or from decreased atp levels and subsequent reversal of the amino acid transporters [ 155 , 156 ] , thus protecting neurons against damaging effects of excitotoxicity or oxidative stress .   regarding this issue
, we and others showed that insulin or igf-1 attenuated both retinal and brain neuronal apoptosis induced by damaging conditions ( e.g. , oxidative stress ) [ 64 , 130 , 157162 ] .
apparently , such neuroprotection could arise from restoration of ir / igf-1r signaling - mediated gene transcription ( e.g. , increased hexokinase - ii and bcl-2 and decreased glutathione peroxidase and caspase-3 expression ) , thus improving neuronal glucose metabolism [ 1 , 25 , 132 , 163 , 164 ] and antioxidant defences   ( figure 1 ) . others proposed an alternative antiapoptotic mechanism involving igf-1-induced activation of igf-1r / pi-3k / akt signaling and subsequent prevention of caspase activation via phosphorylation of the survival transcription factor creb ( activated ) , gsk-3 ( proapoptotic ) , and transcription factor of the forkhead box 1 ( foxo1 , inactivated ) [ 104 , 165 ] .
surprisingly , schubert et al .   described that , in nirko mice ( knocked out for neuronal ir ) , insulin treatment was capable to circumvent neuronal apoptosis in an ir - dependent manner .
in contrast , ryu et al .   failed to show protection by igf-1 against excitotoxic or oxidative stress - induced necrosis , despite a decrement in neuronal apoptosis .
additionally , insulin induced free radical generation and lipid oxidation ( leading to neuronal necrosis ) , in a protein - kinase - c ( pkc- ) dependent process [ 160 , 168 ] . independently of the underlying signaling pathway , insulin has been increasingly shown to play a neuroprotective role against several damaging conditions , including oxidative stress , and to mitigate neuronal apoptotic death [ 1 , 157 ] .
since some of these injuries may constitute the underlying mechanisms of brain dysfunction associated with several pathologies ( e.g. , diabetes , aging and age - related diseases , as alzheimer 's disease ( ad ) ) , next , we will discuss the effect of insulin under such pathologies .
diabetes mellitus , one of the most common metabolic disorders , is a major disorder of insulin regulation .
diabetes has reached epidemic proportions in western countries , and , according to world health organization estimates , it will affect ~300 million people worldwide in 2025 , rendering diabetes an important public health concern in 21st century .
this is further aggravated by the increasing prevalence of diabetes with aging , risk factors associated with modern lifestyle in developed and under development countries ( e.g. , higher life expectancy , obesity , sedentarism , hypertension , hyperlipidemia , and genetic factors ) , and its severe long - term complications   ( e.g. , cardiovascular disease , renal failure , retinopathy , stroke , and peripheral and autonomic neuropathy ) [ 84 , 172 ] .
a consequence of diabetes , mostly of type 2 diabetes , is insulin resistance and chronic peripheral hyperinsulinemia , accompanied by downregulated insulin transport into the brain and its subsequent deprivation from insulin beneficial effects . indeed ,
diabetes - associated disruption between insulin activity and glucose metabolism results in decreased cerebral blood flow and oxidative glucose metabolism [ 46 , 174 , 175 ] ( which may also arise from the impaired blood - brain glucose transport ) .
this hypothesis is supported by the downregulation of neuronal insulin signaling pathways and brain glycolytic enzymes in uncontrolled diabetes [ 25 , 176 ] ( probably due to the inhibition of ir phosphorylation ) that may culminate in progressive impairment in learning , memory , and cognition
. however , seaquist et al .   failed to show any effect of short - term hyperglycemia in glucose transport .   in previous studies
, we described that , despite the apparent protection against oxidative stress in type 2 diabetic goto - kakizaki ( gk ) rat brain synaptosomes ( probably due to higher plasma and brain vitamin e levels ) , they had lower synaptosomal membrane potential and atp / adp levels .
these results were ( at least partially ) explained by the diabetes - related impairment of brain mitochondrial electron transfer chain , which might be exacerbated by aging and/or amyloid  peptide ( a ) , and were counteracted by the antioxidant coenzyme coq10   or insulin .
this reinforces the involvement of oxidative stress and/or metabolic / mitochondrial dysfunction in long - term damaging effects of diabetes [ 178181 ] .
although the underlying mechanisms of diabetes - induced mitochondrial dysfunction are not completely understood , it has been proposed that increased mitochondrial nitric oxide synthase ( nos ) activity ( and subsequent nitric oxide ( no ) production ) may inhibit mitochondrial complexes iii and iv and atp synthase , either by nitrosylation or protein thiol oxidation .
this may lead to impairment of atp production , decreased m , opening of mitochondrial permeability transition pore ( mptp ) , and cell death .  
another consequence of diabetes is oxidative stress , which may arise from several mechanisms related with chronic hyperglycemia : advanced glycation endproducts ( ages ) formation , glucose autoxidation , mitochondrial dysfunction , endoplasmic reticulum stress , and impaired antioxidant defences [ 183 , 184 ] , culminating in caspase - dependent neuronal death [ 171 , 183 ] . indeed ,
dorsal root ganglion neurons from diabetic rats displayed increased apoptotic markers ( e.g. , cleaved caspase-3 , positive tunel staining , and parp-1 activation ) .
interestingly , li et al .   observed that hippocampal ca1 and ca2 neuronal density from type 1 diabetic rats was lower than in type 2 diabetic animals , whilst specific markers of apoptosis ( e.g. , tunel staining , fas and bax expression , cytosolic aif , expression and activity of caspases-3 and -12 ) and nuclear labeling of 8-hydroxy-2-deoxyguanosine ( 8-ohdg ) were higher , suggesting that oxidative stress could be the link to apoptotic activity in type 1 diabetes .   besides brain glucose metabolism and oxidative stress
indeed , glutamate affinity for ampa , but not nmda receptors , was shown to be decreased in type 1 diabetic stz rats .
others reported an increase in taurine and gaba transport in diabetic rat retina and retinal pigment epithelium   and that hyperglycemia impaired glutamate release upon ischemia / reperfusion in both nondiabetic   and diabetic animals [ 187 , 188 ] .
in this regard , we also showed that gaba levels were decreased in stz - diabetic synaptosomes compared to control synaptosomes   and that insulin could modulate rat brain synaptosomal gaba and/or glutamate transport under oxidation and/or type 2 diabetes [ 155 , 156 ] .
as an antidiabetes therapy , insulin has been the best studied and more efficient pharmacological compound mainly used in the treatment of type 1 diabetes .
nevertheless , it has been also increasingly used in the treatment of type 2 diabetes noncontrolled by diet , exercise or , oral antidiabetic agents , and its use has been shown to decrease microvascular complications and mortality [ 190 , 191 ] . in this perspective , it has been increasingly suggested that all diabetic patients could be insulin treated , independently of disease progression . in terms of subcellular effects of insulin on diabetes , guyot et al .  
reported that the insulin - induced increase in extraneuronal glutamate in type 1 diabetic stz rats submitted to ischemia could arise from stimulation of astrocytic glucose uptake ( instead of neuronal glucose uptake ) with subsequent decrement in neuronal glucose metabolism and in the energy available for neurotransmitter uptake .
this effect on neurotransmission was even more pronounced in stz rats given a high dose of insulin , resulting in a prolonged release of aspartate , glutamate , taurine , and gaba .
noteworthy , the extracellular increase in gaba as well as in the number of gabaa receptors upon insulin treatment might protect against the cytotoxic effects of released excitatory amino acids .
importantly , the damaging effects of diabetes on neurotransmission may be potentiated by impairment of ca uptake to the endoplasmic reticulum and/or mitochondria , further exacerbating neuronal excitotoxicity .   from the abovementioned
, it seems that only diabetes - related hyperglycemia account for the long - term damage and that , by normalizing blood glucose levels , insulin might protect against those injuries .
nevertheless , it has been also increasingly demonstrated that recurrent hypoglycemic episodes ( the most common side - effect of insulin therapy ) may adversely affect diabetic cns [ 84 , 172 ] , resulting in motor incapacity , seizures , or cognitive damage , mainly in aged patients . at this respect
, it has been shown that recurrent hypoglycemia affected the endogenous levels of metabolites indirectly linked with brain glucose metabolism ( e.g. , glutamate , leading to excitotoxicity ) , accelerated lipolysis ( increasing the formation of highly oxidizable polyunsatured fatty acids ) , impaired protein synthesis , ion homeostasis , and mitochondrial function , culminating in neuronal dysfunction [ 120 , 193 ] . in a recent study , our group showed that , although insulin was able to protect type 1 diabetic stz rats ( either exposed or not to a ) against brain oxidative stress and mitochondrial dysfunction [ 180 , 194 ] , this was not the case in stz rat brain after an acute hypoglycemia episode .
this suggested that poor glycemic control might damage brain areas involved in learning and memory , thus limiting the analysis of insulin and hyperglycemia roles in diabetic brain . under this perspective , it is not surprising that brain requires robust neuroendocrine counterregulatory mechanisms to maintain blood glucose within a narrow , nondeleterious range .  
as described before , diabetes has been widely associated with slowly progressive end - organ damage in brain [ 27 , 195 ] , resulting in diabetic neuropathy and/or mild to moderately impaired cognitive function , both in type 1 and type 2 diabetic patients . however
, clinically relevant deficits mostly occur in elderly type 2 diabetic patients , probably due to a complex interaction between diabetes and the normal brain aging .
although the molecular mechanisms underlying such long - term effects of diabetes in cns remain unclear , it has been hypothesized the involvement of ages formation , aldose reductase activity , oxidative stress , activation of protein kinase c , and increased hexosamine pathway flux [ 91 , 171 , 193 , 196 , 197 ] . in a very recent study , soeda et al .   described an increase in insulin signaling upon neuronal mutation of sh2-containing inositol 5-phosphatase 2 ( ship2 , a negative regulator of phosphatidylinositol 3 , 4 , 5-trisphosphate - mediated signals ) and that ship2 levels were also increased in type 2 diabetes db / db mice brain .
conversely , when ship2 was overexpressed in mice , akt - mediated ir / igf-1r signaling was impaired , the neuroprotection by insulin / igf-1 was attenuated , and increased neuronal apoptosis and impaired memory were also reported .  
interestingly , insulin treatment has been described to prevent biochemical and pathological indices of peripheral sensory neuropathy in animal models of type 1 diabetes at doses that did not impact on hyperglycemia , suggesting that insulin therapy might not only ameliorate peripheral diabetic complications but might also improve brain function in diabetic patients [ 199201 ] .
furthermore , this points to the existence of a link between insulin , cognitive decline and dementia , and type 2 diabetes [ 27 , 195 ] .
aging can be defined as a time - dependent loss of fitness that begins after the organism attains its maximum reproductive competence , that is , an increased susceptibility to stress   that culminates in an increased incidence of chronic diseases and an exponential increase in the chance of dying . although the molecular mechanisms underlying aging remain controversial , harman proposed , in 1968 , that oxidized macromolecules accumulate with age , decreasing cell function and shortening lifespan  the free radical theory of aging .
if this was as simple as harman proposed , antioxidant therapy would prevent aging - related damage to tissues .
however , nutritional and genetic studies aiming to increase lifespan by boosting antioxidant defences were mostly unsuccessful .
indeed , supplementation with vitamins e or c was not able to reduce mortality in several clinical trials [ 206 , 207 ] .
moreover , studies involving overexpression of antioxidant enzymes ( e.g. , superoxide dismutase and/or catalase ) in mice failed to increase lifespan , despite the decrease in oxidized macromolecules .
conversely , overexpression of the peroxidase and redox - active thioredoxin 1   and mitochondrial - targeted catalase   were shown to prolong mouse lifespan .  
a parallel antiaging paradigm has been increasingly proposed , in which the effect of genetic changes or caloric restriction in insulin signaling pathway might slow the rate of living , thus decreasing metabolism and oxidative stress [ 211214 ] .
this hypothesis has been supported by studies showing that caloric or dietary restriction inhibited insulin signaling cascades that regulate glucose intake , prolonging rodent longevity . however
, this does not explain why the decrease in ros levels occurs , despite the increased metabolic rate observed under such conditions .  
more recently , it has been suggested that epigenetic factors modulated by aging ( e.g. , histone deacetylase family , namely , sirtuins , histone acetylases and dna methyltransferases ) impose a metabolic ( redox ) shift towards an increased reliance on glycolysis ( instead of mitochondrial metabolism ) , probably mediated by age- and sedentarism - related insulin resistance ( conditions that require lower metabolic demands ) .
accordingly , oxidized kelch - like ech - associated protein 1 ( keap-1 ) may block nf - e2-related factor ( nrf ) release to signal mitochondrial biogenesis , thus decreasing mitochondrial synthesis and forcing cells to rely mostly on glycolysis . as a consequence ,
aged tissue becomes increasingly unable to cope with energy demands or stress , initiating a catastrophic cycle of oxidized membrane receptors , signaling molecules , transcription factors and epigenetic transcriptional regulators , culminating in cell death and organ failure .
although the mechanisms remain unknown , it is also believed that , in this age - related oxidized environment , ir / igf-1r may be constitutively oxidized ( by free radicals and/or ages , which are also increased in aging ) .
this process may be due to the failure of oxidized protein tyrosine phosphatase-1b ( ptp1b ) to reactivate the receptor , thus limiting subsequent insulin signaling progression and impairing translocation of glucose transporters into the membrane . as a consequence , glucose levels may rise , and , although insulin production may also be stimulated , it may have no effect  the well - known insulin resistance condition that occurs in type 2 diabetes and aging [ 1 , 205 ] .
insulin resistance further potentiates the formation of ages , creating a damaging vicious cycle on aging brain .
age - associated changes in ir signaling may also arise from increased cholesterol levels and decreased membrane fluidity ( impairing lateral movements of lipid rafts ) or from modification of ir internalization , reexpression , or degradation by the proteasome .
noteworthy , aging has also been associated with a decrease in brain ir number and binding capacity , namely , in hippocampus , cortex , and choroid plexus [ 31 , 219 , 220 ] .
undissociated from aging is the longevity , which can be defined as the property of approaching the species - specific maximum lifespan , that is , the oldest observed age of death in the species . rather than environmental conditions
, longevity depends mostly on genetics . increasing evidence points towards the idea that decreased insulin signaling
pathway may promote longevity in several species , including yeast , worms , drosophila , mice , and man [ 212 , 221225 ] . a hypothesis further supported by increased adiponectin and peroxisome proliferator - activated receptor -2 ( ppar-2 ) gene ( two well - known insulin sensitizers ) in centenarians and long - lived men , whose type 2 diabetes incidence was also dramatically decreased .
additionally , single mutation of daf-2 ( an ir / igf-1r homolog ) resulted in an increase in lifespan , via the nuclear translocation of the foxo family transcription factor daf-16 . in this regard , evidences suggested that 14 - 3 - 3 molecules may complex with sirtuin 2 ( sir2 ) upon stress , activating nuclear foxo / daf-16 , while the inactive foxo / daf-16 remained in cytoplasm upon normal insulin signaling .
null mutations in drosophila genes that encode ir or irs homologues were shown to extend lifespan of female fruit flies .
similarly , overexpression of irs-1 in mice was shown to decrease lifespan , and overexpression of irs-1 , 2 , 3 , or 4 in adipose cells was shown to stimulate glut4 translocation independently from insulin .
accordingly , mice lacking irs-2 had extended lifespan . surprisingly , ir overexpression in mammary epithelial cells resulted in a tumorigenic phenotype , suggesting that its continuous activation upon age - related oxidative stress might not be beneficial .
recent studies suggest that reversible control of cgmp via phosphodiesterase regulation might modulate insulin production and , thus , could constitute a key regulatory messenger of lifespan extension . studies in centenarian people and caloric restriction in rodents and nonhuman primates
suggested that prerequisites for longevity include ( besides the above - mentioned increased insulin sensitivity and subsequent normal ir signal transduction ) decreased fasting glucose and oxidative stress [ 1 , 231 ] . in line with this , decreased mitochondrial function has been suggested to , in some circumstances , increase lifespan .
this idea is supported by studies in the clk-1 mutant worms , showing reduced respiratory , developmental and behavioural rates , but longer lifespan .
such phenomena may be related with the fact that clk-1 is essential for ubiquinone synthesis , an important component of the mitochondrial electron respiratory chain .
similar results have been described in drosophila mutants with lower expression of electron transfer chain components in adult neurons   and in a mouse model with decreased activity of cytochrome c oxidase complex .   in this
regard , type 2 diabetes appears to be , at least partially , a model for premature aging .
this idea has been further confirmed by a decrease of cellular replicative senescence in diabetic subjects .
thus , one of the main challenges for the next decades will be to unravel the complex interactions between aging and diabetes that result in insulin resistance , allowing the development of more efficient preventive and therapeutic strategies to overcome age - related diseases ( e.g. , alzheimer 's disease ( ad ) ) .
taken together , if ( 1 ) aging is frequently related with sedentarism and ( 2 ) the organism does not require extra demands for energy , then it can survive longer on the lower energy levels resulting from glycolysis .
this can occur by downregulating the mitochondrial electron transport chain components and the activities of several redox - sensitive transcription factors , enzymes , transporters , and signaling proteins ( e.g. , ir ) , as has been widely described .
ad is a complex and common neurodegenerative disease that afflicted 26.6 million people worldwide in 2006 , a number that can quadruplicate by 2050 .
clinically , this disease has an insidious onset , typically beginning with a subtle decline in memory that progresses to global deterioration in cognitive and adaptive function .
neuropathologically , is characterized by the presence of extracellular senile plaques ( sp ) , intracellular neurofibrillary tangles ( nft ) , and loss of basal forebrain cholinergic neurons that innervate the hippocampus and cortex .
while nfts are formed from paired helical filaments composed of neurofilaments and hyperphosphorylated tau protein , sp arises mostly after deposition of amyloid  ( a ; a 3943 amino acid peptide derived from the proteolytic cleavage of a larger amyloid  precursor protein ( app ) by the - and -secretases ) .
the most common resulting fragments are either 40 or 42 amino acids in length ( a140 and a142 ) .
although the initiating factor(s ) for ad remain unclear , some authors suggested that they might involve intracellular ( rather than extracellular ) accumulation of a , leading to neurite dystrophy and degeneration , postsynaptic protein loss , and eventual death of neocortical , hippocampal , and subcortical neurons [ 39 , 44 , 179 ] .
importantly , a accumulation is also a common process in normal aged human , but its massive deposition in ad   may result from either an overproduction of app , impaired app proteolytic processing , and/or mutations in the genes encoding for app or presenilins .
only 5% ( or less ) of all ad cases have an early onset , autosomal dominant familial origin , probably due to missense mutations in presenilin ( ps ) genes 1 or 2 in chromosomes 14 and 1 , or in the app gene in chromosome 21 .
additionally , allelic abnormalities of the apolipoprotein e ( apoe ) gene on chromosome 19 are related with early onset and increased severity of inherited and sporadic ad . on the contrary ,
most of ad cases ( 95% or more ) are sporadic in origin , aging being the main risk factor [ 11 , 232 ] .
although sporadic ad is faced as a  disease  of aging , this does not necessarily imply that aging per se is a disease , and we must bear in mind that the boundaries between late - onset ad and  normal  aging are not absolute .
this may be due to ( 1 ) increasing evidences from epidemiological , immunohistochemical , and molecular genetics studies suggesting a heterogenous etiology for ad   and ( 2 ) the fact that an unquestionable ad diagnosis is only possible after the patient 's death , through post mortem morphological and histological brain analysis .
indeed , a decline in neuropsychologic test performance , brain atrophy , neuronal loss , and plaque / tangle deposition also occur with aging in the absence of dementia .
therefore , it is reasonable to consider the causal molecular events for sporadic ad within the aging spectrum , rather than distinct disease phenomena .  
although the exact pathophysiologic mechanisms underlying signal transduction abnormalities and neurodegeneration in ad brain remain unknown , oxidative stress and metabolic dysfunction appear to be involved [ 68 , 242 , 243 ] .
indeed , a was shown to directly induce overproduction of reactive oxygen and nitrogen species and neurotoxicity .
this was further exacerbated by a-associated decreased plasma antioxidant defences ( e.g. , uric acid , glutathione , catalase , superoxide dismutase , glutathione peroxidise , and , reductase ) , as described by several authors [ 245248 ] . additionally , antioxidant supplementation ( e.g. , vitamin e plus vitamin c , and donezepil , an acetylcholinesterase inhibitor , plus vitamin e ) was shown to have beneficial effects in ad [ 249 , 250 ] . similarly , vitamin e , idebenone , uric acid , or glutathione prevented a neurotoxicity in human and rat cortical and hippocampal neurons and delayed disease progression in ad patients
moreover , an induction of apolipoprotein d expression ( a protein that appears to function as lipid antioxidant and to extend lifespan in drosophila ) has been suggested to constitute an age - related stress - resistance mechanism in ad brains .   taken together , these data unequivocally demonstrate the involvement of oxidative stress in ad and suggest a potential therapeutic role for antioxidant supplementation [ 245 , 252 ] .
however , others studies failed to show any protection induced by antioxidant treatment in ad [ 249 , 253 ] .
although indirectly , the involvement of oxidative stress in ad pathophysiology was further reinforced by the observation of ad neuropil enrichment in ages and redox active metal ions ( e.g. , zinc , iron ) , particularly in sp , and nft [ 179 , 251 ] ( as detected in brain cortex , hippocampus , and basal nucleus of meynert from ad patients ) .
to further increase the complexity on this subject , oxidative stress in ad may also arise from mitochondrial dysfunction , involving decreased activity of mitochondrial complex iv and decreased atp / adp , as revised by moreira et al . .
in fact , evidences widely implicate metabolic defects in ad , the lower brain metabolic rate being one of the best - documented abnormalities that occurs early in the pathology .
this may involve impaired glucose uptake and metabolism and also a slightly decreased cerebral metabolic rate of oxygen ( at the beginning of ad ) [ 256 , 257 ] , preceding any evidence for functional impairment by neuropsychological testing or brain atrophy neuroimaging .
thus , it is believed that impaired ad brain metabolism may be a cause , rather than a consequence , of neurodegeneration .
although the underlying causes of reduced metabolism in ad are not completely understood , it has been reported that atrophy of cerebral vasculature ( the major metabolic exchange surface of brain ) , a decrement in brain glucose transport activity , or even impaired insulin signaling might play a crucial role .
indeed , liu et al .   proposed that glut1 and glut3 could be downregulated in ad , thus impairing brain glucose uptake / metabolism .
additionally , several studies ( including ours ) showed that levels or activity of enzymes from intermediary metabolism ( e.g. , aconitase , glutamine synthetase , creatine kinase , pyruvate dehydrogenase , and -ketoglutarate dehydrogenase ) were decreased in ad brains and cells exposed to a [ 261264 ] .
more specifically , bubber et al .   observed that changes in tricarboxylic acid cycle enzymes ' activities ( mainly of the pyruvate dehydrogenase complex ) correlated with the clinical state , suggesting a coordinated mitochondrial alteration in ad .
one of the most common changes in electron transport chain underlying mitochondrial dysfunction in ad is decreased cytochrome oxidase activity , described in such distinct ad human tissues as platelets [ 266 , 267 ] and post mortem brain tissue [ 268 , 269 ] .
furthermore , studies with cybrid cells demonstrated that ad platelet deficits in cytochrome oxidase could be transferred to cells depleted of mitochondrial dna ( mtdna ) ( rho0 cells ) , suggesting that mtdna - associated mitochondrial dysfunction might play a role in ad neurodegeneration [ 266 , 270 , 271 ] .
valla and collaborators   also reported significant declines in complexes iii and iv in ad lymphocyte mitochondria and a significant decline in complex iv in mild cognitive impairment lymphocyte mitochondria , suggesting that mitochondrial abnormalities could be present at the earliest symptomatic stages of the disease . finally , enhanced mitochondria degradation was described in ad , leaving behind lysosomal detritus containing non - functional mitochondrial components [ 261 , 273 ] .  
noteworthy , among dementia - type disorders , ad has been increasingly associated with type 2 diabetes [ 27 , 274 , 275 ] .
indeed , two epidemiological studies ( the honolulu - asia aging study , and the rotterdam and mayo studies ) reported that type 2 diabetes increases the risk for ad , dependently on vascular dementia [ 175 , 276 , 277 ] .
furthermore , both pathologies share several common aspects , including aging - related processes , degeneration , high cholesterol levels , peripheral and cns insulin resistance , dysfunctional ir and ir - mediated signaling , and decreased glucose transport and metabolism , despite the higher nonmetabolized glucose levels in cerebral blood [ 2 , 5 , 25 , 27 , 175 , 256 , 259 , 275 ] .
this led to the hypothesis that type-2-diabetes - mediated recurrent hyperinsulinemia / hypoglycemia episodes culminate in long - term changes in brain vasculature , neurodegeneration , and cognitive impairment , facilitating ad onset [ 30 , 126 , 275 ] .
the existence of a correlation between the severity of neuritic plaques , nft and cerebral amyloid angiopathy , and the presence of type 2 diabetes and apolipoprotein 4 ( apo4 ) allele further supports this hypothesis [ 25 , 27 , 278 ] . on the other hand
, ad patients exhibited a higher risk for developing type 2 diabetes , and this led to the idea that brain ir signaling might be the missing link between brain neuronal loss and pancreatic -cell loss in both diseases [ 27 , 279 ] .
more interestingly , a recent proposal refers that ad can be an  insulin - resistant brain state  , or even a  type 3 diabetes 
this can be supported by the reports on age- and ad - related decrease in insulin mrna and protein levels [ 11 , 282 , 283 ] , ir or igf-1r expression [ 11 , 40 , 219 , 284 ] , irs-1 and irs-2 levels , markers of tyr kinase activity ( namely , active irs-1 , pi3k , and erk1/2 ) [ 40 , 175 ] , or a reduced association of shc with grb2 .
additionally , soluble a oligomers were shown to interfere with ir function , probably due to a loss of these receptors at dendrites and their increased expression in the cell soma , despite no changes in absolute hippocampal neuronal ir levels . accordingly , moloney et al .   suggested that damaged ir - associated neurotrophic and metabolic brain functions in ad neurons might arise after persistent and pathological hyperactivation of the akt - mtor - s6k signaling pathway , increasing irs-1 phosphorylation at ser312 or 616 , culminating in irs-1/2 degradation .
concerning increased fasting plasma insulin , decreased csf insulin levels , and/or decreased csf / plasma insulin ratio in ad patients [ 11 , 175 ] , they are suggestive of insulin clearance impairment , which may elevate plasma a levels [ 27 , 274 ] , due to the role of insulin in modulation of amyloid processing both in vivo and in vitro .
in fact , brain insulin / igf-1/akt - mediated phosphorylation / inactivation of gsk-3 inhibited a production [ 274 , 287 ] and its abnormal intracellular accumulation , probably by increasing its extracellular secretion and by accelerating its trafficking from golgi and trans - golgi network to the plasma membrane [ 39 , 40 ] .
however , under insulin resistance conditions , despite the chronic peripheral hyperinsulinemia , downregulation of brain insulin synthesis and/or transport decreased brain insulin levels   and its subsequent signaling cascades , culminating in increased a levels , as in tg2576 ad transgenic mice .
more recently , freude et al .   reported that irs-2 deficiency in this mouse model of ad decreased both app cleavage and a levels in brain .
alternatively , insulin / igf-1 prevented a accumulation by promoting its transport into cns via a-binding carrier proteins ( e.g. , transthyretin and albumin ) [ 27 , 200 , 238 , 290292 ] , or by insulin interfering with extracellular proteolytic a degradation via insulin - degrading enzyme ( ide ) .
this metalloprotease , that also catabolizes insulin and igf-1 , can be competitively inhibited by insulin resistance [ 30 , 46 , 126 , 200 , 218 , 238 , 278 ] , impairing a degradation , increasing its neurotoxicity and promoting ad [ 11 , 46 , 278 ] .
noteworthy , since brain ir does not desensitize , ide may also constitute a negative feedback loop to control insulin action [ 31 , 46 ] , that is , pi3k / akt activation by insulin may upregulate ide , which may stop subsequent signaling and promote a clearance in hippocampal neurons .  
insulin and igf-1 were also described to modulate both physiological and abnormal neuronal tau protein phosphorylation , in a process involving akt , gsk-3 , erk1/2 , and cdk-5 [ 12 , 25 , 27 , 30 , 39 , 40 , 44 ] .
similarly , sui et al .   observed that gsk-3 inhibition blocked specific phosphorylation of tau protein in pc12 cells .
moreover , in diabetic animals treated with insulin , a complete prevention of tau protein hyperphosphorylation was reported , probably resulting from reestablishment of brain insulin signaling .
this is further supported by the age - related tau protein hyperphosphorylation and cns accumulation in both transgenic nirko mice ( mice lacking brain / neuronal ir )   and irs-2-deficient mice ( in which brain ir is dysregulated ) .
interestingly , hyperinsulinemia in nirko mice [ 12 , 167 ] and insulin treatment in human nt2 neurons   were described to decrease tau protein phosphorylation .
conversely , several authors described that in human sh - sy5y neuroblastoma cells [ 59 , 296 ] and rat primary cortical neurons , insulin exposure increased hyperphosphorylated tau protein levels , which was not transported into axons , thus accumulating and aggregating into nfts in neuronal perikarya , and thereby promoting oxidative stress , apoptotic or necrotic death , and mitochondrial dysfunction associated with ad .
from all the above - mentioned evidences , we believe that a valuable therapeutic window for insulin against diabetes- and age - related neurodegenerative disorders ( e.g. , ad ) may exist . under these conditions ,
an inadequate trophic support to brain may occur due to lower insulin / igf-1 levels and/or damaged ir / igf-1r - mediated signaling , affecting gene transcription and culminating in neurodegeneration / death , cognitive dysfunction , and , ultimately , in long - term complications ( in case of diabetes ) and/or in ad ( and other dementia ) .
thus , those symptoms should be alleviated upon increased insulin levels , accomplished by exogenous insulin plus glucose administration ( to maintain euglycemia , avoiding the deleterious effects of hypoglycemia on memory and cognition ) [ 30 , 164 , 175 ] .
interestingly , intranasal insulin administration has been increasingly considered as a potential peripheral therapy , with the advantage of penetrating the cns within minutes ( without affecting plasma glucose or insulin levels ) .
this can occur via extracellular bulk flow transport along olfactory and trigeminal perivascular channels , but also through more traditional axonal transport pathways , culminating in memory improvement [ 39 , 144 , 298300 ] . in this regard ,
a very recent clinical trial from craft et al .   showed that treatment of ad or mildly cognitively impaired adults with intranasal insulin stabilized or improved cognition and cerebral glucose metabolism .
similarly , direct intracerebroventricular insulin administration was shown to improve memory performance ( without changes in blood insulin or glucose concentrations ) , but this approach poses some questions concerning its human applicability . although the use of stem cells to deliver insulin or igf-1 into brain has been considered as a hypothetical beneficial therapy ( by increasing neuronal survival and decreasing oxidative stress in the cns ) , safety and efficacy issues must first be improved . however , chronically high insulin levels in brain can be deleterious , due to ( 1 ) desensitization of pi3k pathway and inadequate responses to other trophic factors , ( 2 ) potentiation of nmda receptors and resultant excitotoxicity , and/or ( 3 ) competition with a for ide , increasing extracellular a accumulation ( meaning that excessive insulin must be removed to alleviate the competitive blockade of ide ) [ 44 , 46 ] .
a therapeutical alternative could be the use of antidiabetic insulin sensitizers ( e.g. , thiazolidinediones ) that , under hyperinsulinemia , would decrease insulin availability to the brain without affecting glycemia .
unfortunately , clinical trials showed that rosiglitazone , a thiazolidinedione , decreased cognitive performance in ad patients [ 302 , 303 ] .
other therapeutical approaches could be ( 1 ) the small molecule insulin mimetic demethylasterriquinone b1 ( daqb1 ) that does not competitively inhibit ide and modulates ir , ( 2 ) the delivery of insulin antibodies , or ( 3 ) insulin - inhibiting peptides into the brain . increased cns ide levels ( via gene therapy or ide infusion )
given that activated pi3k / akt inhibits gsk-3 and subsequent a production and tau protein hyperphosphorylation ( as well as increased a clearance via stimulation of transthyretin and ide ) , another strategy could be the improvement of insulin signaling [ 44 , 46 , 218 ] . in line with this
, the use of gsk-3 inhibitors could be attractive , but it might also impair several vital physiological targets of this kinase or have no impact on other critical components of the neurodegenerative cascade [ 30 , 218 ] .
in the last three decades , brain insulin signaling has faced a novel and increased interest in neuroscience research , either in its signaling pathways and/or as a promising therapy against diabetes and age - related neurodegenerative disorders ( e.g. , ad ) . from the first studies recognizing the abundance of insulin and ir in brain to its potential involvement in numerous neurodegenerative diseases associated with diabetes ( particularly type 2 diabetes ) and aging mediated less than 20 years .
indeed , in 1999 , halter described that elderly people have impaired insulin sensitivity , which might account for by the slight age - related increase in fasting glucose levels and the delay in return to normal glucose levels after an oral glucose tolerance test .
however , more than 10 years later , the elucidation on whether the origin of this insulin resistant state relies on aging per se or on external , lifestyle factors remains a matter of debate . in this regard , one of the main challenges for the next decades will be to unravel the complex interactions between aging and diabetes that underlie insulin resistance , allowing the development of more efficient preventive and therapeutic strategies to overcome age - related neurodegenerative diseases . concerning ad , a recent hypothesis points towards the idea that ad is the  brain - type diabetes .
 this is supported by decreased number or binding capacity of brain ir in both ad patients and mouse models , increased risk for type 2 diabetes in ad patients ( and vice versa ) , and the accumulation of hyperphosphorylated tau in the cns of irs-2-disrupted mice , a model of type 2 diabetes .
thus , restoring insulin levels and/or its receptor - mediated signaling cascades ( without affecting blood glucose levels ) constitute a potentially interesting therapeutic strategy against ad , due to the inhibition of a production ( and its increased clearance ) and tau protein hyperphosphorylation , two well - known hallmarks of the pathology .
more recent studies focused on the importance of insulin / ir signaling in increased longevity . however , the results remain highly controversial .   in summary , in cns , rather than just an  acquired  peripheral hormone , insulin appears to be a naturally occurring peptide of the outmost importance .
 insulin signaling in central nervous system ( cns ) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning , memory , and age - related neurodegenerative diseases . 
 thus , besides its well - known role in longevity , insulin may constitute a promising therapy against diabetes- and age - related neurodegenerative disorders . 
 more interestingly , insulin has been also faced as the potential missing link between diabetes and aging in cns , with alzheimer 's disease ( ad ) considered as the  brain - type diabetes .  in fact , brain insulin has been shown to regulate both peripheral and central glucose metabolism , neurotransmission , learning , and memory and to be neuroprotective . and 
 a future challenge will be to unravel the complex interactions between aging and diabetes , which , we believe , will allow the development of efficient preventive and therapeutic strategies to overcome age - related diseases and to prolong human  
 healthy  longevity . 
 herewith , we aim to integrate the metabolic , neuromodulatory , and neuroprotective roles of insulin in two age - related pathologies : diabetes and ad , both in terms of intracellular signaling and potential therapeutic approach . 
1. Introduction
2. Neuronal IR/IGF-1R-Mediated Signaling Pathways
3. Role of Insulin and IGF-1 in the Brain
4. Protection by Insulin against Diabetes and Its Long-Term Complications in CNS
5. Insulin in Aging and Longevity
6. Insulin in an Age-Related Neurodegenerative Disorder: Alzheimer's Disease
7. Is There a Therapeutic Window for Insulin against Diabetes- and Age-Related Neurodegeneration?
8. Conclusion