the family of homer scaffolding proteins consists of three family members : homer1 , homer2 , and homer3 , with respective splice variants for each member .
these proteins are typically expressed in a concentrated fashion at post - synaptic densities , but have been ascribed with certain non - neuronal functions ( babu et al . , 2004 ; stiber et al . ,
the majority of proteins that belong to the homer family share two structurally conserved features in their secondary form : an n - terminal enabled / vasodilator - stimulated phosphoprotein homology 1 ( evh1 ) domain responsible for associating with proline - rich sequences contained within its target / ligand proteins , and a c - terminal coiled - coil domain containing multiple leucine zipper motifs that control homo / heteromerization of homer proteins ( xiao et al . , 1998 ;
one short homer protein splice variant , homer1a , does not contain this c - terminal region ( brakeman et al . , 1997 ) , allowing it to exist as a dominant - negative inhibitor to the scaffolding functions of the long forms .
importantly , homer proteins have been demonstrated to associate with many different proteins , including several which are critical to the transduction of peripheral somatosensory information .
group 1 metabotropic glutamate receptors ( mglur1/5 ) expressed in the spinal cord ( yashpal et al . , 2001 ) and amygdala ( neugebauer et al . , 2003 ; kolber et al .
2011b ) have been shown to be important to pain processing , and are tightly modulated by homer scaffolding proteins ( brakeman et al .
as shown in figure 1 , homer proteins link mglur1/5 to intracellular calcium stores through inositol-1,4,5-trisphosphate ( ip3 ) receptor types 1 and 3 ( tu et al . , 1998 ) , thereby regulating calcium release and neuronal excitability .
also , homer proteins are suspected to mediate the coupling of mglur and n - methyl - d - aspartate ( nmda ) receptors , prevalent in post - synaptic densities ( guo et al . , 2004 ) .
homer proteins associate with numerous other proteins as well , but scaffolding combinations outlined herein have recently been demonstrated to have significant effects on multiple pain models .
the homer scaffolding complex links mglur1/5 receptors to intracellular calcium stores via ip3 receptors , and also links to nmda receptors to dynamically regulate the transfer of somatosensory information . the shortened splice variant homer1a and its dominant - negative ability to associate with target proteins but not scaffold additional homer proteins , has been demonstrated to be an important mediator of peripheral pain . in 2006 , tappe et al .
reported that homer1a protein expression is significantly greater in activated sensory synapses ( tappe et al . , 2006 ) , in agreement with other groups that report neuronal activation - dependent increases in protein expression ( brakeman et al .
, 1997 ; kato et al . , 1997 ; bottai et al . , 2002 ; vazdarjanova et al . ,
for example , short hairpin rna ( shrna ) designed to knock - down homer1a protein expression significantly prolongs thermal hyperalgesia following viral administration of cfa - injected animals ( tappe et al . , 2006 ) .
indeed , through viral expression of homer1a , tappe and colleagues demonstrated that the short splice variant functions as an activity - dependent negative modulator of mglur scaffolding to intracellular calcium stores .
this inhibition can serve as a negative modulator of neuronal sensitization at first afferent pain synapses following peripheral inflammation . in short , homer1a exists in post - synaptic densities to reduce inflammatory hyperalgesia by preventing the longer homer proteins from associating with mglur1/5 and scaffolding other proteins to the receptor .
multiple groups have since demonstrated that homer proteins participate as critical modulators of synaptic plasticity as scaffolding proteins . in the amygdala ,
homer1a expression reduces arthritic pain hypersensitivity and negatively affects typical changes in interneuron plasticity following an inflammatory challenge ( tappe - theodor et al . ,
further , the induction of chronic compression of the l4/l5 dorsal root ganglia ( ccd ) induces rapid expression of homer 1a in the spinal dorsal horn , thereby reducing synaptic plasticity and hence , associated pain ( ma et al . , 2009 ) .
given that homer1a can interfere with intracellular calcium mobilization ( yuan et al . , 2003 )
, it could also protect against inflammatory pain and other mglur - activated signaling mechanisms that influence second - order neuron sensitivity , such as mapk activation ( ji et al . , 2002 ) .
therefore , homer - dependent scaffolding mechanisms significantly affect afferent pain transduction , serving as an important modulator that could be pharmacologically manipulated in the future to provide therapeutic benefit .
neuronal plasticity is predominantly studied as a function of neuronal activation between pre- and post - synaptic neurons in the central nervous system . as the rate of depolarization increases ,
membrane receptors undergo alterations in expression , post - translational modification , and/or subcellular localization that directly affect the likelihood of repeated receptor activation , such as for homer1a - dependent manipulations of mglur systems . in this scenario , neurons work to endogenously protect the receptor , the terminal , and the neuron itself from potential damage due to over - activation .
however , few studies have dissected the molecular changes that occur within the primary afferent terminal at the site of tissue innervation .
recent reports have identified that the scaffolding protein a - kinase anchoring protein 79/150 ( akap 79/150 ) is expressed in the periphery and dynamically recruits enzymes to modify transient receptor potential ( trp ) receptors , thereby affecting receptor response to stimuli .
identified in the early 1990s as a scaffolding protein for protein kinase a ( pka ) ( bregman et al . , 1991 ; carr et al . , 1992 ) , akap 79/150 functions to localize certain enzymes to target substrates on a sub - cellular level . in terms of its designations
, akap79 refers to the human isoform of the scaffolding proteins , expressed at 79 kda in molecular weight , while akap150 is the rodent isoform with additional amino acids that push its molecular weight to 150 kda ( referred to as akap150 from here forth ) .
the additional amino acids in akap150 comprise a series of multiple repeats that have no significant effect on scaffolding actions , and may have been evolutionarily contracted down to the human akap79 analog .
multiple research groups have since identified a myriad of substrates and enzymes that akap scaffolds together to establish organized and efficient signal transduction . in terms of enzymes ,
akap150 is anchored predominantly at the plasma membrane in multiple cell types , including neurons , and scaffolds pka , protein kinase c ( pkc ) , and calcineurin ( can , pp2b ) ( coghlan et al . ,
1995 ) along discrete sections of its secondary structure ( hoshi et al . , 2005 ) .
these enzymes are then able to efficiently act upon membrane targets that akap150 is reported to associate with , including the nmda receptor ( colledge et al . , 2000 ) , -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ( ampa ) receptor ( colledge et al . , 2000 ) , kv7.2/kcnq2 potassium channel ( hoshi et al . , 2003 ) , and l - type voltage - gated calcium channels ( gao et al . , 1997 ) .
recent work by several labs has also confirmed akap150 association with trp channels , including trpv1 ( jeske et al . , 2008 ; schnizler et al .
this scaffolding association affords sensory neurons with the plasticity required to dynamically respond to various stimuli known to cause pain . the first link between akap150 and trpv1
was established in experiments investigating pka catalytic subunit proximity to neuronal plasma membranes following administration of inflammatory mediators ( rathee et al . , 2002 ) .
trpv1 was found to act as the pka - sensitive heat transducer responsible for inflammatory hypersensitivity to a thermal stimulus , although the hypersensitivity was itself sensitive to st - ht31 , a peptide that blocks pka association with akap150 .
indeed , forskolin  dependent sensitization of trpv1 currents in dorsal root ganglia ( drg ) neurons was blocked following pre - incubation with st - ht31 , suggesting that akap150 exists as part of the trpv1-signaling module that mediates inflammatory sensitization of the channel .
although reports at that time were unable to demonstrate a physical interaction between gs subunit , akap150 , and trpv1 in any model system , the significance was established that scaffolding proteins dynamically affected receptor sensitivities to pro - algesic effectors .
akap scaffolds support multiple enzymes to modify substrate proteins in specific sub - cellular compartments . while some akap scaffolds bind enzymes including phosphodiesterase 4 d3 ( pde4d3 ) ( dodge et al . , 2001 ;
2001 ) , protein kinase n ( takahashi et al . , 1999 ) , and protein phosphatase 2a ( takahashi et al . , 1999 ) , akap150 primarily orients pka , pkc , and can with plasma membrane substrates , including trpv1 ( figure 2 ) . sirna knock - down and genetic ablation studies indicate that the loss of akap150 impairs pka- and pkc - phosphorylation and sensitization of trpv1 ( jeske et al . , 2008 , 2009 ) , indicating that akap150 expression is essential to certain inflammatory signaling pathways that utilize these two kinases , including prostaglandin ( schnizler et al . , 2008 ) and bradykinin ( zhang et al . , 2008 ) receptor - activated pathways .
however , de - phosphorylation and desensitization of trpv1 by can ( docherty et al . , 1996 ;
2010 ) , suggesting that can may be attracted to trpv1 by other means , including calmodulin ( numazaki et al . , 2003 ) .
although kinase scaffolding by akap150 has been repeatedly demonstrated to regulate trpv1 activation by multiple stimuli , later studies demonstrate cellular mechanisms that dictate akap150:trpv1 association in sensory neurons .
akap 79/150 ( akap150 ) is natively anchored to the plasma membrane via pip2 linkages that are hydrolyzed following phospholipase c ( plc ) activation , releasing akap150 to associate with substrate receptors , such as trpv1 .
akap150 association with trpv1 allows for pka- and pkc - mediated phosphorylation and sensitization of the receptor to peripheral stimuli .
later work in this field would not only demonstrate physical association between akap150 and trpv1 in multiple cell models , but also show that the association is dynamic and controlled by intracellular factors .
several research groups have provided indirect evidence of physical protein - protein interaction(s ) between akap150 and trpv1 including co - immunoprecipitation from transfected homologous cell culture models as well as primary sensory neuron cultures ( jeske et al . , 2008 ; schnizler et al . , 2008 ; zhang et al . , 2008 ) .
additional total internal reflective fluorescence - forster resonance energy transfer ( tirf - fret ) findings indicate strong association at the plasma membrane in co - transfected cells ( chaudhury et al . , 2011 ) .
importantly , tirf - fret studies also demonstrate that akap150 association with trpv1 is a calcium - sensitive process , revealing that calcium - bound calmodulin significantly reduces trpv1 interaction with the scaffolding protein ( chaudhury et al . , 2011 ) .
this dissociative interaction between calmodulin : trpv1 and akap150:trpv1 likely exists as an endogenous desensitization mechanism , allowing for calcium - sensitive , calmodulin - bound can to associate with and de - phosphorylate trpv1 , while blocking akap150-bound pka and pkc from phosphorylating and re - sensitizing trpv1 .
akap150 association with trpv1 is also negatively controlled by the anchoring of the scaffolding protein to phosphatidylinositol-4,5-bisphosphate ( pip2 ) in the plasma membrane .
working from the previously described anchorage of akap150 with certain phosphoinositides ( dell'acqua et al . , 1998 ) , recent findings indicate that phospholipase c ( plc ) activation releases akap150 from its plasma membrane moorings by hydrolyzing pip2 into ip3 and diacylglycerol ( dag ) .
released akap150 is then free to associate with membrane - associated proteins , such as trpv1 ( jeske et al . , 2011 ) , thereby providing an explanation for the diverging roles for pip2 in trpv1 regulation ( prescott and julius , 2003 ; lukacs et al .
although homer and akap scaffolds work similarly to dictate protein - protein interactions within both subcellular and temporal foci , there are several types of scaffolds that are devoid of protein structure , yet similarly organize and mediate signal transduction . organized structures within the plasma membrane such as caveolae and lipid rafts serve to regulate the activation efficiencies of certain receptors by scaffolding these receptors and/or effector molecules to dictate intracellular signal transduction .
therefore , protein - lipid interactions are as important as protein - protein interactions in influencing afferent plasticity . the fluid - mosaic model for biological plasma membrane organization proposed by singer and nicholson 40 years ago ( singer and nicolson , 1972 ) has been modified in recent years with the discovery and characterization of lipid raft microdomains ( simons and ikonen , 1997 ) .
lipid rafts consist of densely packed cholesterol and sphingolipid moieties that impart a reduced specific density compared to adjacent plasma membrane constituents . for certain receptor complexes ,
lipid rafts serve to concentrate extracellular - to - intracellular communication , thereby focusing signaling cascades . indeed , certain receptors and signaling molecules associate with lipid raft microdomains , providing for energy efficient signal transduction . in this vein ,
lipid rafts can be considered scaffolding structures for organizing receptor activation , as the activation of certain receptor complexes by somatosensory stimuli have been demonstrated to be dependent upon lipid raft association .
importantly , the dynamic association / dissociation of receptors and down - stream effectors with lipid rafts constitute a role in the dynamic synaptic plasticity of afferent pain transduction .
trp channel function in both trigeminal ganglia ( tg ) and drg neurons is reported to be sensitive to cholesterol depletion ( reducing lipid raft integrity ) . in cultured drg neurons ,
cholesterol depletion by methyl--cyclodextrin resulted in a significant reduction in capsaicin - induced currents ( liu et al . ,
further , similar treatments to reduce cholesterol in tg neurons resulted in decreased calcium uptake following capsaicin or resiniferatoxin application in culture ( szoke et al . ,
the cold sensing channel trpm8 also associates with lipid rafts ( morenilla - palao et al . , 2009 ) , and differentially reacts to cold and menthol stimuli depending on channel association with the cholesterol - laden microdomains in drg neurons .
taken together , data suggest that trp channel association with lipid rafts affects channel activities ( figure 3 ) .
interestingly , trp channels are composed of multiple plasma membrane - spanning domains , negating the need for lipid raft association to target the channels to the extracellular surface .
however , n - terminal prenylation of trpm8 was found to regulate most of the channel association with lipid rafts , further indicating that association with lipid rafts may be dynamic , and that other biochemical forces dictate channel localization to these cholesterol microdomains . lipid raft scaffolding at plasma membranes .
lipid rafts centralize signal transduction through certain receptors , including the -opioid receptor ( mor ) , trpm8 and trpv1 , via their intracellular association with microdomains concentrated in cholesterol and sphingolipids .
lipid rafts also anchor extracellular peptidases ep24.15/16 , to metabolize neuropeptides such as bradykinin ( bk ) , reducing free extracellular content available for receptor activation .
g - protein coupled receptor ( gpcr ) complexes also demonstrate associative properties with lipid raft moieties ( navratil et al . , 2003 ; chini and parenti , 2004 ; monastyrskaya et al . ,
2005 ) , as well as g - protein association ( ross , 1995 ) . given the role of -opioid receptor ( mor ) in effectively reducing somatic pain , reported data demonstrating mor association with lipid rafts proves to be highly significant .
work from the law research group illustrates that association of the receptor with lipid raft microdomains ( figure 3 ) is affected by ligand specificity , demonstrating that etorphine induces receptor translocation out of lipid rafts , while morphine has no effect ( zheng et al . , 2008 ) .
receptor localization was found to be dependent upon association with its gi2 signaling molecule , such that agonist activation of mor stimulates translocation to non - raft plasma membrane domains .
however , etorphine agonism , which strongly promotes mor association with -arrestin , stimulates receptor complex dissociation from lipid rafts , while morphine agonism , which weakly promotes receptor association with -arrestin ( whistler and von zastrow , 1998 ) , results in mor re - association with gi2 in lipid rafts .
therefore , in the case of mor , dynamic association with and internalization by -arrestin dictates gpcr activation and propagation of signal , in so much that dynamic association of mor with lipid rafts maintains receptor quiescence , while dissociation from lipid rafts allows for proper signal transduction downstream .
in addition to receptors intracellularly associated with lipid rafts , certain proteins are also extracellularly bound to raft microdomains , and constitute important regulators of neuropeptide functions .
metalloendopeptidases ec 3.4.24.15 and ec 3.4.24.16 ( ep24.15/16 ) are two closely related peptidases that co - localize with bradykinin type-2 receptors ( b2rs ) in lipid raft domains in tg neurons ( jeske et al . , 2006a ) .
among the many substrates that ep24.15/16 are capable of degrading , bradykinin ( bk ) exists as a high affinity substrate ( rioli et al .
therefore , the extracellular tethering of ep24.15/16 allows for the peptidases to degrade free bk before it can bind to and activate lipid - raft - associated b2r ( gomez et al . , 2011 ) .
given the role of bk as an inflammatory mediator that sensitizes numerous trp channels to normally - innocuous stimuli , lipid raft association of the receptor , as well as extracellular ep24.15/16 , provide for a dynamic micro - environment capable of significantly influencing afferent somatic activation .
dynamic changes in somatosensory perception occur in many subcellular locales , including the plasma membrane , through the reorganization and redistribution of proteins and scaffolding complexes .
post - translational changes target transducers of environmental stimuli that typically inform an organism that danger and/or injury is present .
these receptor transducers serve as substrates and points of control in multiple pathways , and scaffolding structures maintain dynamic , yet strict energy - efficient control over reactions that significantly affect receptor response . although scaffolding structures exist as large membrane - associated proteins , as well as microdomains within the membranes themselves .
both structures function to provide a framework to support and regulate dynamic changes to transducers of somatosensory information , prompting continued research into the roles of scaffolding proteins and structures in injury and disease pathologies .
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 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 .
 dynamic changes in somatosensory perception occur as a result of multiple signaling events . in many instances , over - activation of sensory receptors results in the desensitization and subsequent increased threshold for activation of receptors . in other cases , 
 receptor sensitization can occur following tissue injury and/or inflammation . in both cases , signaling mechanisms that control alterations in receptor activities can significantly affect organism response to sensory stimuli , including thermal , mechanical , and chemical . 
 due to the homeostatic nature of somatosensory recognition , dynamic changes in receptor response can negatively affect an individual 's way of life , as well as alert individuals to tissue damage . here , we will focus on scaffolding structures that regulate somatosensory neuronal excitability . 
Homer scaffolds
Akap 79/150
Lipid rafts
Conclusion
Conflict of interest statement