the central nervous system ( cns ) represents an important target for hiv infection during multiple stages of the disease : early , after invasion of the host , since the virus rapidly enters the cns , which then constantly acts as a viral reservoir ; lately , subverting its function and causing peripheral neuropathies and neurocognitive disorders ; and lastly , during the final stage of neuroaids , triggering opportunistic cns infections , cancers , and dementia ( tardieu and boutet , 2002 ) .
highly active antiretroviral therapy ( haart ) , a combination of drugs that inhibits enzymes essential for hiv replication , can reduce the viremia and the onset of opportunistic infections in most patients , and prolong the survival ( see table 1 ) .
although haart has reduced the incidence of clinical signs of neurological disease in hiv - infected individuals , autopsy studies suggest that there has been no corresponding decline in the incidence of inflammatory lesions in the cns .
haart currently in use in the clinical practice . among the limits of the current treatments
the most noticeable is the inability to eradicate hiv - infected cells , both , limiting the time frame in which haart initiated after exposure to hiv can prevent infection , and allowing replication - competent virus that persists in infected cells to emerge rapidly after the cessation of haart . according to those considerations
, there is evidence that haart is less effective in lowering virus replication in the cns than in the blood ( gisolf et al . , 2000 ) ; and haart resistant viruses are more often found in the cerebrospinal fluid ( csf ) than in biological samples ( cinque et al . ,
the principal challenges in drug development for cns  hiv infections are : ( 1 ) understanding the pathways for viral entry , replication , reservoirs , and glial activation in order to identify new targets for treatment , ( 2 ) developing selective therapeutics agents for those targets , or shaping effective strategies to deliver them within the cns , and finally but not to underestimate ( 3 ) collaborating with pharmaceutical companies to test promising drugs in clinical trials , and eventually exploit them in clinical practice .
hiv is known to productively replicate in macrophages and microglia of the cns , but it also infects , via a cd4-independent pathway , macroglial cells such as astrocytes and most - importantly brain microvascular endothelial cells , which represent the major component of the blood brain barrier ( bbb ; bissel and wiley , 2004 ) . although it is generally accepted that neurons do not become infected by hiv , recent reports have shown that neuronal progenitor brain cells can be infected by various viral strains ( i.e. , hiv-1 ) , eventually playing a role as viral reservoirs ( lawrence et al . , 2004 ) . moreover , one of the important features of cns infection is that its viral populations can diverge from those of plasma : indeed , genetic compartmentalization of csf hiv-1 infection has been nicely demonstrated in a series of studies using the heteroduplex tracking essay to compare blood and csf viral populations cross - sectionally and longitudinally ( ritola et al . , 2005 ) ; therefore even if during acute infection the two population are identical , during chronic infection and especially in cases of hiv encephalitis they genetically and functionally diverge , and may justify the failure of antiretroviral therapy ( strain et al . , 2005 ) .
identification of viral and cellular mechanisms in the cns that control virus replication after acute infection , maintain latency during the asymptomatic stage , and mediate resurgence of virus replication and development of hiv - associated encephalitis during late infection , is critical .
the dynamic signaling pathways that are activated by virus replication and active / infiltrating macrophages and lymphocytes and the cytokines , chemokines , and neurotoxic products they elaborate may interfere with the homeostatic signaling pathways that maintain normal quiescent brain function . to understand the correlates in terms of neuroprotection , neuroinflammation , and neurodegeneration processes
many preclinical models have been developed , such as : hiv transgenic rat ( i.e. , tg rat from reid et al . , 2001 ) , or mice ( i.e. , gp 120 tg mice from toggas et al . , 1994 ) , but also those obtained by injection of hiv proteins ( i.e. , gp 120 from sundar et al . , 1991 ; or tat and bansal et al . , 2000 ) , or hiv - infected macrophages into the brain of mice ( anderson et al . , 2003 )
the most studied signaling pathways include , among others , the gsk3 , cdk5 , erk , notch , p38 , and jnk cascades .
for instance , it is noteworthy that many mitogen - activated protein kinases are pivotal during acute , asymptomatic , and terminal infection : some of them seems to promote cell growth and differentiation in response to initial infection ( i.e. , erk ) , whereas others are associated to growth arrest , apoptosis , and oncogenic transformation during aids .
cns infection : in contrast to hiv - infected cd4 + t cells , infection in brain macrophages and microglial cell lines leads to an extended life span and elevated survival against apoptotic stresses ( chugh et al . , 2007 ) , and only during terminal infection ,
when there is resurgent virus replication , the balance favors neuronal death ( i.e. , via activation of jnk and p38 ) reflecting a failure to maintain homeostasis in the cns ( barber et al . , 2004 ) with the consequent motor and cognitive impairments .
many strategies are currently under evaluation to improve hiv treatment , but a relatively small number are primarily dedicated to the alleviation of neurological complaints or the control of neuroaids , and unfortunately more than 98% of drug candidates for cns disorders never make it to the clinic ( pardridge , 2003 ) .
herein are a couple of examples picturing how researches , initially conducted for neoplastic , neurodegenerative , or epileptic disorders , on specific pathways and delivery systems , might be adapted and applied to the field of cns  hiv infection ( see figure 1 ) .
neutralization of vif on microvascular endothelial cells ( ifn and ifn ) and subsequent inhibition of viral entry into the cns ; 3 .
inhibition of hiv replication in macrophages and viral spread into the cns ( nk-1-agonists ) ; 4 .
inhibition of cytoprotective effect exerted by tat proteins over microglial cells which act as cns viral reservoirs ( pi3k and akt inhibitors ) ; 5 .
protection of neural integrity and plasticity along with prevention of neurotoxicity and neuronal death ( glutamate receptors- , gsk3- , jnk- , and p38-inhibitors ) . discovered in 1898 ,
mitoguazone ( mgbm ) had been investigated but abandoned as an anti - cancer drug , recently it was found to cause selective loss of cd16 + hiv - infected cells , specifically by blocking osteopontin needed to transform a circulating macrophage into a cns / resident one .
mgbg although ineffective in reducing hiv viremia , does appear to shut down viral evolution by selectively killing recent migrants macrophages rather than resident cells , and to be safe over the long - term treatment in aids patients ( jeymohan et al . , 2009 ) .
the manipulation of ifn and ifn apobec pathways could prevent hiv invasion of cns because of the potent intrinsic immunity exerted by the expression of apobec3 g gene in microvascular endothelial cells ( argyris et al . , 2007 ) .
this gene code for proteins possessing cytidine deaminase activity and allow the bbb to actively neutralize the effects of viral infectivity factors ( vif ) , which are essentials for viral entry and spread into the cns ( jeong et al . , 2008 ) .
nk-1 , also known as substance p , was the first neuropeptide discovered in 1931 , it is widely distributed in the central and peripheral nervous system where it functions as a potent mediator of immunoregulation .
agonists to nk-1 receptor potently inhibit both in vitro and in vivo hiv replication in human macrophages , apparently down regulating ccr5 ( jeymohan et al . , 2009 ) .
initially recognized , and extensively studied as a promising target for anti - cancer therapies because of its role during tumorigenesis , the pi3k / akt cell survival pathway became a target for anti - hiv treatment after the observation that pi3k inhibitors ( such as wortmannin and ly294002 ) or akt inhibitors ( such as miltefosine ) , both able to cross the bbb , contrast the cytoprotective effect exerted by tat proteins over macrophage / microglial cells , and make them again susceptible to cell death following an apoptotic stimulus ( chugh et al .
most interestingly pi3k / akt inhibitors proved to be effective without harming uninfected cells and therefore experimental data as far available support their possible use for hiv - therapy and targeting of long - lived viral reservoirs .
nanoformulated haart essentially manipulates the immune system for therapeutic purposes by using synthetic or biological nanoengineered drug carriers .
those nanoformulated drugs have been proven to be efficient delivery vehicles for a wide range of therapeutic agents , and can be easily injected in the blood circulation or virtually into any site of the brain with minimal invasiveness .
major concerns to this therapeutic approach are the choice of drug administration ( systemic or local ) and consequently the residence in the systemic blood circulation , or in the brain tissue adjacent to topic injection , the drug loading efficiency , and the burst release effect .
furthermore , many other question marks are still to be answered , or even formulated .
concerning synthetic carriers , hyaluronic acid , a linear polysaccharide composed of alternating d - glucuronic acid and n - acetyl - d - glucosamine units , because of its immunoneutrality , has been proposed as a biocompatible and biodegradable biomaterial for tissue engineering , and in drug delivery systems into the cns : particularly appealing would seem the opportunity to produce drug - incorporated hyaluronic acid nanoparticles ( jeong et al . , 2008 ) . as well monocytes , because of their abilities to move drugs inside the cells have advantages as a depot for haart , but also they might improve compliance as drugs can be released from cells for periods up to weeks or longer .
nevertheless , further application of biological nanoengineered drug carriers will require optimization of nanoparticles uptake , deeper knowledge of their kinetics across the bbbs , and more sophisticated imaging systems to track migration of monocyte  macrophages to the brain and target the delivery of specific therapeutics ( jeymohan et al . , 2009 ) . beside compounds aimed at control the virus ,
others try to treat the effects of the underlying neuroinflammation and neurodegeneration mechanisms , and might provide a beneficial supplement to haart .
neurotoxicity is mainly mediated through glutamate production , which is triggered by nmda subtypes of glutamate receptors , and correlated to caspase activation and subsequent neuronal apoptosis ( erdmann et al . ,
2006 ) ; accordingly , pharmacologic inhibition of glutaminase directly correlate to neuronal survival ( tian et al . , 2008 ) .
those insights support the hypothesis that inhibition of glutamate production by blocking mitochondrial glutaminase activity may clinically prevent neurotoxicity during hiv-1 infection . taken together the gsk3 , cdk5 , jnk , and p38 signaling pathways are important regulators of neurotoxicity , being responsible for synaptic and dendritic damage to pyramidal neurons , loss of immunoreactive neurons , and demyelinization .
gsk3 inhibitors ( i.e. , lithium , valproic acid ) as well as cdk5 inhibitors ( i.e. , roscovitine ) seems to protect neuronal integrity and plasticity ( crews et al . , 2009 ) , while jnk- and p38-inhibitors ( i.e. , minocycline ) may prevent neuronal death ( lin et al .
, 2001 ; bozyczko - coyne et al . , 2002 ) each of those target holds promise for the development of treatment strategies to ameliorated the neuropathological effects exerted by hiv proteins ( tat , vif , etc . ) .
discovered in 1898 , mitoguazone ( mgbm ) had been investigated but abandoned as an anti - cancer drug , recently it was found to cause selective loss of cd16 + hiv - infected cells , specifically by blocking osteopontin needed to transform a circulating macrophage into a cns / resident one .
mgbg although ineffective in reducing hiv viremia , does appear to shut down viral evolution by selectively killing recent migrants macrophages rather than resident cells , and to be safe over the long - term treatment in aids patients ( jeymohan et al . , 2009 ) .
the manipulation of ifn and ifn apobec pathways could prevent hiv invasion of cns because of the potent intrinsic immunity exerted by the expression of apobec3 g gene in microvascular endothelial cells ( argyris et al . , 2007 ) .
this gene code for proteins possessing cytidine deaminase activity and allow the bbb to actively neutralize the effects of viral infectivity factors ( vif ) , which are essentials for viral entry and spread into the cns ( jeong et al .
nk-1 , also known as substance p , was the first neuropeptide discovered in 1931 , it is widely distributed in the central and peripheral nervous system where it functions as a potent mediator of immunoregulation .
agonists to nk-1 receptor potently inhibit both in vitro and in vivo hiv replication in human macrophages , apparently down regulating ccr5 ( jeymohan et al . , 2009 ) .
initially recognized , and extensively studied as a promising target for anti - cancer therapies because of its role during tumorigenesis , the pi3k / akt cell survival pathway became a target for anti - hiv treatment after the observation that pi3k inhibitors ( such as wortmannin and ly294002 ) or akt inhibitors ( such as miltefosine ) , both able to cross the bbb , contrast the cytoprotective effect exerted by tat proteins over macrophage / microglial cells , and make them again susceptible to cell death following an apoptotic stimulus ( chugh et al .
most interestingly pi3k / akt inhibitors proved to be effective without harming uninfected cells and therefore experimental data as far available support their possible use for hiv - therapy and targeting of long - lived viral reservoirs .
nanoformulated haart essentially manipulates the immune system for therapeutic purposes by using synthetic or biological nanoengineered drug carriers .
those nanoformulated drugs have been proven to be efficient delivery vehicles for a wide range of therapeutic agents , and can be easily injected in the blood circulation or virtually into any site of the brain with minimal invasiveness .
major concerns to this therapeutic approach are the choice of drug administration ( systemic or local ) and consequently the residence in the systemic blood circulation , or in the brain tissue adjacent to topic injection , the drug loading efficiency , and the burst release effect . furthermore , many other question marks are still to be answered , or even formulated . concerning synthetic carriers , hyaluronic acid , a linear polysaccharide composed of alternating d
- glucuronic acid and n - acetyl - d - glucosamine units , because of its immunoneutrality , has been proposed as a biocompatible and biodegradable biomaterial for tissue engineering , and in drug delivery systems into the cns : particularly appealing would seem the opportunity to produce drug - incorporated hyaluronic acid nanoparticles ( jeong et al . , 2008 ) .
as well monocytes , because of their abilities to move drugs inside the cells have advantages as a depot for haart , but also they might improve compliance as drugs can be released from cells for periods up to weeks or longer . nevertheless , further application of biological nanoengineered drug carriers will require optimization of nanoparticles uptake , deeper knowledge of their kinetics across the bbbs , and more sophisticated imaging systems to track migration of monocyte  macrophages to the brain and target the delivery of specific therapeutics ( jeymohan et al . , 2009 ) .
beside compounds aimed at control the virus , others try to treat the effects of the underlying neuroinflammation and neurodegeneration mechanisms , and might provide a beneficial supplement to haart .
neurotoxicity is mainly mediated through glutamate production , which is triggered by nmda subtypes of glutamate receptors , and correlated to caspase activation and subsequent neuronal apoptosis ( erdmann et al . , 2006 ) ; accordingly , pharmacologic inhibition of glutaminase directly correlate to neuronal survival ( tian et al . , 2008 ) .
those insights support the hypothesis that inhibition of glutamate production by blocking mitochondrial glutaminase activity may clinically prevent neurotoxicity during hiv-1 infection . taken together the gsk3 , cdk5 , jnk , and p38 signaling pathways are important regulators of neurotoxicity , being responsible for synaptic and dendritic damage to pyramidal neurons , loss of immunoreactive neurons , and demyelinization .
gsk3 inhibitors ( i.e. , lithium , valproic acid ) as well as cdk5 inhibitors ( i.e. , roscovitine ) seems to protect neuronal integrity and plasticity ( crews et al . , 2009 ) , while jnk- and p38-inhibitors ( i.e. , minocycline ) may prevent neuronal death ( lin et al . , 2001 ; bozyczko - coyne et al . , 2002 ) each of those target holds promise for the development of treatment strategies to ameliorated the neuropathological effects exerted by hiv proteins ( tat , vif , etc . )
combinations of drugs work better and longer than seen previously , patients benefit demonstrably from the magnitude and duration of viral suppression , and require more and more symptomatic treatment of neuroaids cognitive impairment .
however haart is very demanding for patients , non - adherence can lead to therapeutic failure , and in addition drug resistance is a continuing problem ; therefore , the need for improved drug and delivery systems , but also an interest for adaptive treatment strategies and mathematical modeling to manage structured treatment interruptions ( rosenberg et al . , 2007 ) , have been outlined by many researchers . to this
regard a thigh collaboration with industry is strictly required and introduces a new set of challenges such as : ( 1 ) circulation of promising ideas , often just developed with university grants , ( 2 ) protection of intellectual property and patent rights , ( 3 ) fund - raising opportunities for start - ups during preclinical researches on novel compounds , until ( 4 ) identification of potential blockbusters appealing to big pharma companies for introduction in the market and their clinical exploitation .
finally , current criteria for initiation of haart do not consider cns outcomes as a benchmark of efficacy , thus establishing cns - based criteria for initiation of protocols should be the ultimate goal not only for therapeutic but also for neuroprotection strategies ; as recently stated by jeymohan et al .
( 2009 ) on behalf of the neuroaids research participants these priorities should be seen as a template for future growth of the field and its long - term impact .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 the central nervous system ( cns ) represents an important target for hiv infection during multiple stages of the disease : early , after invasion of the host , acting as a viral reservoir ; lately , subverting its function and causing peripheral neuropathies and neurocognitive disorders ; and lastly , during the final stage of neuroaids , triggering opportunistic infections , cancers , and dementia . 
 highly active antiretroviral therapy , a combination of drugs that inhibits enzymes essential for hiv replication , can reduce the viremia and the onset of opportunistic infections in most patients , and prolong the survival . among the limits of the current treatments 
 the most noticeable is the inability to eradicate hiv - infected cells , both , limiting the time frame in which antiretroviral therapies initiated after exposure to hiv can prevent infection , and allowing replication - competent virus that persists in infected cells to emerge rapidly after the cessation of treatments . 
 many strategies are currently under evaluation to improve hiv treatment , unfortunately more than 98% of drug candidates for cns disorders never make it to the clinic ; here in we report how nanoformulated strategies might be adapted and applied to the field of cns  hiv infection . 
Backgroung
Current Understanding of CNSHIV Infection
Developing Strategies
Innovatively targeted pathways
Nanoformulated HAART
Neuroprotective strategy
Conclusion
Conflict of Interest Statement