Yingjie Shen

Traf fi c lights for axon growth: proteoglycans and their neuronal receptors

Yingjie Shen

Department of Neuroscience and Center for Brain and Spinal Cord Repair, Wexner Medical Center, The Ohio State University, 460 w 12thAve, Columbus, OH 43210, USA

Axon growth is a central event in the development and post-injury plasticity of the nervous system. Growing axons encounter a wide variety of environmental instructions. Much like traf fi c lights in controlling the migrating axons, chondroitin sulfate proteoglycans (CSPGs) and heparan sulfate proteoglycans (HSPGs) often lead to “stop” and “go” growth responses in the axons, respectively. Recently, the LAR family and NgR family molecules were identified as neuronal receptors for CSPGs and HSPGs. These discoveries provided molecular tools for further study of mechanisms underlying axon growth regulation. More importantly, the identi fi cation of these proteoglycan receptors offered potential therapeutic targets for promoting post-injury axon regeneration.

axonal regeneration; chondroitin sulfate and heparan sulfate proteoglycans

Shen Y. Traffic lights for axon growth: proteoglycans and their neuronal receptors. Neural Regen Res. 2014;9(4):356-361.

Chondroitin sulfate proteoglycans (CSPGs) and heparan sulfate proteoglycans (HSPGs)

Proteoglycans are proteins glycosylated with sulfated glycosaminoglycan (GAG) side chains. The diversity of proteoglycans arises from their core protein sequences, the degree of glycosylation, and the length and composition of the GAGs. Based on the types of associated GAGs, proteoglycans are classified into four families: chondroitin sulfate proteoglycans, heparan sulfate proteoglycans, dermatan sulfate proteoglycans, and keratan sulfate proteoglycans. CSPGs and HSPGs carry chondroitin sulfate (CS) and heparan sulfate (HS) GAGs, respectively. The CS and HS share similar polysaccharide structures, but differ in their sulfation patterns along the sugar chains. Many of these proteoglycans are secreted into the intercellular space, yet others are either transmembrane proteins or tethered to the cell membrane through a glycosylphosphatidylinositol (GPI) anchor. CSPGs and HSPGs are produced by many cell types and found in all vertebrate tissues and organs.

Expression patterns and functional signi fi cance in the nervous system

In the nervous system, CSPGs and HSPGs are synthesized by neurons and glia, and their spatiotemporal expression is dynamically regulated throughout the course of development (Brittis et al., 1992; Pindzola et al., 1993; Oohira et al., 1994; Stipp et al., 1994; Ivins et al., 1997; Milev et al., 1998; Hsueh and Sheng, 1999; Bovolenta and Fernaud-Espinosa, 2000). For example, Glypican-2, a nervous system speci fi c HSPG, is expressed only transiently when axons are actively growing, but not after cell migration has completed and axons have reached their targets (Stipp et al., 1994; Ivins et al., 1997). Many CSPGs are highly expressed in the developing central nervous system (CNS). They appear around the critical period in the perineuronal nets and in boundary regions where axon growth is normally restricted (Brittis et al., 1992; Wang and Fawcett, 2012). As the CNS matures, the expression of CSPGs gradually declines (Oohira et al., 1994; Milev et al., 1998). In rat embryonic retina, regression of CSPG level closely correlates with the onset of ganglion cell differentiation (Brittis et al., 1992).

Although attenuated in the mature nervous system, the expression of CSPGs and HSPGs may resurrect upon injuries (Garcia de Yebenes et al., 1999; Asher et al., 2000; Iseki et al., 2002; Jones et al., 2003; Bloechlinger et al., 2004; Properzi et al., 2008; Yi et al., 2012). Spinal cord and brain lesions in adult rodents induce waves of post-injury CSPG expression near the injury epicenters (Asher et al., 2000; Jones et al., 2003; Yi et al., 2012). CSPGs are also upregulated in the perineuronal nets of deafferented neurons distal to spinal cord lesion sites (Massey et al., 2006; Alilain et al., 2011). Glypican-1, an HSPG, is upregulated in adult rat dorsal root ganglion (DRG) neurons after sciatic nerve injury, and the elevated Glypican-1 expression persists until the injured axons reinnervated their peripheral targets (Bloechlinger et al., 2004).

The dynamic expression patterns of these proteoglycans during development and injuries are consistent with their functional roles in these processes, as HSPGs and CSPGs are found to be crucial players in regulating embryonic axon path fi nding and post-injury axonal plasticity (Brittis et al., 1992; Pindzola et al., 1993; Bandtlow and Zimmermann, 2000; Bovolenta and Fernaud-Espinosa, 2000; Wilson and Snow, 2000; Alilain et al., 2011; Maeda et al., 2011). Interestingly, HSPGs and CSPGs often have opposite effects onaxonal behavior. In the developing nervous system, CSPGs function as repulsive guidance molecules, whereas HSPGs often present attractive signals to axons (Brittis et al., 1992; Kantor et al., 2004; Wang et al., 2012). Likewise, CSPGs inhibit, while HSPGs promote, neurite outgrowth in cultured neurons (Snow et al., 1990; Snow et al., 1991; Coles et al., 2011). After injuries in the CNS, CSPGs deposited at the lesion sites form a potent inhibitory barrier that prevents the regeneration of severed axons (McKeon et al., 1991; Davies et al., 1997; Busch and Silver, 2007; Bartus et al., 2012; Garcia-Alias and Fawcett, 2012). On the contrary, treatment with Glypican, an HSPG that promotes axon growth, improves anatomical regeneration and functional recovery after cerebral ischemia (Hill et al., 2012).

Although the functional significance of these proteoglycans in axon growth has been documented for decades, the underlying mechanisms remained poorly understood, mainly because no molecular signaling was identi fi ed. Fortunately, a number of neuronal receptors for CSPGs and HSPGs have recently been discovered, allowing further research to decode the signaling mechanisms underlying proteoglycan regulation of axon growth.

Neuronal receptors of CSPGs and HSPGs

The LAR family RPTPs

Three transmembrane receptor protein tyrosine phosphatases (RPTPs), PTPsigma, PTPdelta, and leukocyte antigen-related (LAR), form a family that is often referred to as the LAR family RPTPs. These RPTPs have long been implicated in synaptogenesis and axon path fi nding during development (Garrity et al., 1999; Ledig et al., 1999; Johnson and Holt, 2000; Chagnon et al., 2004; Dunah et al., 2005; Fox and Zinn, 2005; Johnson et al., 2006; Kwon et al., 2010; Horn et al., 2012; Hendriks et al., 2013; Takahashi and Craig, 2013), and are now increasingly recognized as key regulators of post-injury axonal plasticity (Xie et al., 2001; McLean et al., 2002; Thompson, 2003; Van der Zee et al., 2003; Sapieha et al., 2005; Shen et al., 2009; Fry et al., 2010; Fisher et al., 2011; Takahashi et al., 2012; Gardner and Habecker, 2013).

In the nervous system, the LAR family RPTPs are expressed by both neurons and glia. In neurons, they are localized in soma, axons, and growth cones (Zhang et al., 1998; Mueller et al., 2000; McLean et al., 2002; Thompson, 2003; Fisher et al., 2011; Takahashi et al., 2012). The LAR family RPTPs are highly expressed in the developing nervous system of the vertebrates, with certain splicing isoforms of PTPsigma and PTPdelta exclusively confined to the CNS (Yan et al., 1993; Pulido et al., 1995). As the animal matures, expression levels of these RPTPs signi fi cantly decline in most areas of the CNS, except some brain regions such as the hippocampus, where PTPsigma and PTPdelta expression remains substantial (Yan et al., 1993; Wang et al., 1995; Schaapveld et al., 1998).

In the adult nervous system, injuries may also play a role in regulating the expression of the LAR family RPTPs. There have been a number of reports on injury-induced changes of LAR and PTPsigma expression; however, the results were not always consistent between studies, possibly due to the different analytical methods used (Haworth et al., 1998; Xie et al., 2001; McLean et al., 2002; Thompson, 2003; Sapieha et al., 2005; Fry et al., 2010). Nonetheless, a growing number of studies have shown consistent results on the functional roles of PTPsigma and LAR in nervous system injuries. Despite opposite findings on post-injury LAR expression, different research groups showed similar results that genetic depletion of LAR hampers axon regeneration after sciatic nerve injuries (Xie et al., 2001; Van der Zee et al., 2003). On the other hand, many research groups, using various injuries models of both central and peripheral nervous systems, showed unanimously that PTPsigma de fi ciency improves axon regeneration (McLean et al., 2002; Thompson, 2003; Sapieha et al., 2005; Shen et al., 2009; Fry et al., 2010; Gardner and Habecker, 2013).

Receptors of CSPGs and HSPGs

For many years, despite growing implications of the LAR RPTPs in axon path fi nding and regeneration, little was know about the underlying molecular mechanisms. Although these RPTPs are transmembrane proteins with structures that resemble cell surface receptors, no functional ligands had been identi fi ed until recently.

The initial evidence of the LAR family RPTPs serving as proteoglycan receptors emerged from the finding that in chick embryos, PTPsigma, a retinal axon protein, binds with high af fi nity to HSPGs in retinal basal lamina (Aricescu et al., 2002). Heparinase treatment, which digests the HS epitopes from the retinal basal lamina, effectively removes PTPsigma binding. Mutations in the fi rst N-terminal Ig-like domain of PTPsigma eliminate its binding to the HSPG ligands. These data suggest that the receptor-ligand binding is mediated by the N-terminal domain of PTPsigma and the GAG moieties of HSPGs. In addition, PTPsigma was recently shown to be a neuronal receptor for CSPGs (Shen et al., 2009). Through the same N-terminal domain, PTPsigma binds to CSPGs secreted by reactive astrocytes in spinal cord lesion sites. The same binding site mutations that abrogate PTPsigma-HSPG binding also eliminate PTPsigma-CSPG binding. Chondroitinase treatment, which degrades CS GAGs, abolishes PTPsigma binding to CSPGs. Therefore, PTPsigma serves as a common GAG receptor for both CSPGs and HSPGs. Furthermore, PTPsigma has similar af fi nity towards HSPGs and CSPGs. The fact that both CS and HS GAGs interact with PTPsigma through the same binding site and with comparable af fi nities suggests a competition between these ligands (Shen et al., 2009; Coles et al., 2011).

In neurons, PTPsigma mediates intracellular signaling from both classes of proteoglycans. The biological effects of HSPG and CSPG signaling through this same receptor, however, can be counteracting. In cultured neurons, PTPsigma-CSPG interaction inhibits neurite extension, and genetic depletion of PTPsigma desensitizes neurons to CSPG-inhibition. In injured spinal cord, deficiency of PTPsigma enhances axon extension into CSPG-enriched glial scar (Shen et al., 2009; Fry et al., 2010). On the other hand, HSPGs, such as Glypican-2, induce robust neurite outgrowth incultured neurons, and this effect is abolished in PTPsigma de fi cient neurons (Coles et al., 2011). In the presence of both CS and HS proteoglycans, the outcome of neurite outgrowth depends on the relative abundance of these competing PTPsigma ligands (Coles et al., 2011). PTPsigma therefore plays a role as a bifunctional receptor in the regulation of neurite extension by mediating “stop” and “go” signals from CSPGs and HSPGs, respectively.

Within the LAR family, the first N-terminal Ig-like domain, through which PTPsigma binds to the GAGs, is highly conserved (Coles et al., 2011). Thus it is conceivable that the other two members of the family, LAR and PTPdelta, may also function as receptors of CSPGs and HSPGs. In fact, all three LAR RPTPs, through their first N-terminal domains, bind to heparin, a structural analog of HS (Coles et al., 2011). Drosophila LAR, a remote homologue of the vertebrate LAR family RPTPs, functions as an axonal receptor of Syndecan, an HSPG, and mediates its positive regulatory signals in motor axon guidance and synaptic development (Fox and Zinn, 2005; Johnson et al., 2006). In zebra fi sh, LAR isoforms steer somatosensory innervation into the skin, presumably through a receptor-ligand interaction with HSPGs. Deleting the first N-terminal domain of LAR or removing the HS GAGs from the skin tissue disrupts innervation (Wang et al., 2012). Although yet to be veri fi ed, mammalian LAR may as well be an HSPG receptor, given that the N-terminal ligand-binding domain is highly conserved across species (Coles et al., 2011).

Moreover, LAR was recently found to be an additional receptor of CSPGs. Like PTPsigma, LAR binds to CSPGs through CS GAGs. De fi ciency of LAR eliminates CSPG signaling and mitigates CSPGs-inhibition of neurite outgrowth in cultured neuron, suggesting a functional role of LARCSPG interaction in neurite extension (Fisher et al., 2011). It is therefore reasonable to expect that LAR deficiency would promote post-injury axon regeneration in a lesion environment rich of CSPGs, in a similar fashion as PTPsigma deficiency. However, after sciatic nerve injuries, axon regeneration is hampered by LAR de fi ciency, but enhanced by PTPsigma de fi ciency (Xie et al., 2001; McLean et al., 2002; Van der Zee et al., 2003). It is not clear what results in the discrepancy between the in vitro and in vivo effects of LAR de fi ciency.

Opposite axon growth signals by CSPGs and HSPGs

As discussed above, it seems that HSPGs and CSPGs signal opposite instructions to the axons, through the same neuronal receptor PTPsigma, much like turning on and off a molecular switch. What are the underlying mechanisms of such bimodal signaling? To decode this paradox, a recent study showed that CSPGs and HSPGs induce differential molecular con fi guration of the LAR family RPTPs (Coles et al., 2011). When bound to these RPTPs, HS GAGs and their analogs cluster the receptors into oligomers, whereas CS GAGs maintain them in monomers. In the presence of both HS and CS, the degree of receptor clustering appears to depend on the relative abundance of these competitive ligands. As suggested by previous studies, oligomerization of these receptor tyrosine phosphatases may lead to the inhibition of their enzyme activity (Bilwes et al., 1996; Wallace et al., 1998). If so, the binding of CS and HS to the RPTPs would switch their phosphatase activity “on” and “off”, respectively. In a migrating growth cone, suppressed tyrosine phosphatase activity can be translated into a local increase of phosphotyrosine residues on many signaling molecules, thereby activating pathways leading to axon growth. In fact, Glypican-2, which possesses HS chains that cluster PTPsigma, stimulates robust neurite outgrowth in cultured neurons (Coles et al., 2011). Conversely, while CSPGs are notorious inhibitors of neurite extension, their CS GAGs, which dissociate the receptors in monomers, enhance LAR phosphatase activity in cultured cells (Fisher et al., 2011). Treatment with peptides that inhibit LAR phosphatase activity improves axon regeneration into CSPG-enriched glial scar after spinal cord injury, suggesting that CSPG-inhibition of axon growth is mediated by LAR enzyme activity (Fisher et al., 2011). In a similar vein, a very recent study showed that a PTPsigma inhibitory peptide signi fi cantly enhances axonal plasticity and promotes robust functional recovery following severe spinal cord injury (B. T. Lang, 2012).

Because of the counteracting functions of CSPGs and HSPGs in regulating axon growth, it is critical to maintain a homeostatic balance of these proteoglycans in the perineuronal environment. Pathological conditions of the nervous system, such as traumatic injuries, disrupt such balance and often lead to a CSPG-dominant lesion environment surrounding the injured neurons. As a consequence, this may deviate neuronal signaling through the LAR family RPTPs, which could result in downstream cellular events such as intense adhesion and low motility of the dystrophic axon tips, and eventual long-term entrapment of injured axons at the glial scar barriers or within the perineuronal nets (Busch et al., 2010; B. T. Lang, 2012).

The Nogo receptors (NgRs)

The Nogo receptor (NgR) family consists of three GPI-anchored receptors: NgR1, the bona fide “Nogo receptor”, and its two homologues, NgR2 and NgR3. The NgRs are predominantly expressed by neurons throughout development and remain highly expressed in the adult nervous system (Lauren et al., 2003). Subcellular localization analysis showed that NgR1 and NgR2 are expressed on axons, and in particular on growth cones (Wang et al., 2002b; Venkatesh et al., 2005). Earlier studies have identi fi ed NgR1 as a receptor of the myelin-associated inhibitors (MAIs), including Nogo-A, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp). NgR1 was shown to mediate MAI-inhibition of axon regeneration in injured adult CNS (Kim et al., 2004; McGee et al., 2005; Cafferty and Strittmatter, 2006), although results were sometimes inconsistent between different research groups (Zheng et al., 2005). While denoted as Nogo receptors, neither NgR2 nor NgR3 binds to Nogo-A. NgR2 was instead found to be a receptor of MAG, and NgR3 does not interact with any of the MAIs (Venkatesh et al., 2005; Lauren et al., 2007).

Receptors of CSPGs (and HSPGs?)

A recent study showed that NgR1 and NgR3 are also functional receptors of CSPGs (Dickendesher et al., 2012). NgR1 and NgR3 each bind with high af fi nity to puri fi ed CS GAGs, and to CSPGs in developing brains and injured optic nerves. The binding of CSPGs and MAIs to NgR1 appears to be mediated by distinct domains, suggesting independent receptor engagement and possible synergistic signaling by these ligands through this common receptor. Interestingly, binding of CSPGs induces the formation of a receptor complex that comprises NgR1 and NgR3, along with p75, an NgR1 co-receptor in MAI signaling (Wang et al., 2002a). These fi ndings therefore revealed a molecular platform shared by the MAIs and CSPGs, which involves multiple receptors and serves as a signal converging point for these axon growth inhibitors.

Although loss of Ngr1 or Ngr3 alone is not sufficient to overcome CSPG-inhibition, combined loss of Ngr1 and Ngr3 renders resistance to CSPG-inhibition in cultured neurons and promotes axon regeneration in injured optic nerves. A further enhancement of axon regeneration was observed with triple depletions of Ngr1, Ngr3, and PTPsigma, suggesting a functional redundancy among these receptors (Dickendesher et al., 2012).

The study also showed a robust binding of NgR1 to the HS GAGs in developing brains. The interactions of CS and HS with Ngr1 are mediated by the same motif on Ngr1, suggesting a competition between these GAG ligands for this common receptor. The biological relevance of HS-Ngr1 interaction, however, has not been characterized, leaving an interesting topic for future studies as this would reveal whether the NgRs behave as bifunctional receptors to mediate opposite axon growth instructions from CSPGs and HSPGs.

Semaphorin 5A

In addition to the LAR family and NgR family receptors, Semaphorin 5A, an axon guidance molecule, binds through its thrombospondin repeats to the GAGs of both CSPGs and HSPGs and thereby presents itself to the developing axons as a bifunctional guidance molecule (Kantor et al., 2004). Whereas HSPGs are required for axon attraction by Semaphorin 5A, CSPGs convert Semaphorin 5A into an inhibitory guidance cue.

Given the abundance of proteoglycans in the intercellular space, it would not be surprising that future studies may identify more receptors of CSPGs and HSPGs, and perhaps also receptors of other proteoglycans such as the dermatan sulfate proteoglycans and keratan sulfate proteoglycans.

Discussion

The functional significance of HSPGs and CSPGs in axon growth is known for decades; however, the underlying molecular mechanisms remain poorly understood. These recent discoveries of CSPG and HSPG receptors provided us with molecule tools to further dissect the mechanisms of axon growth regulation. More importantly, these proteoglycan receptors are likely potential therapeutic targets for promoting post-injury axon regeneration. The studies aforementioned in this review also give a few take-home messages regarding the treatment strategies that target these axon growth regulators.

Functional redundancy among CSPG receptors

The fact that CSPGs interact with multiple receptors in the LAR and NgR family suggests a functional redundancy among these receptors. Removing an individual CSPG receptor, or even a combined loss of NgR1 and NgR3 only results in a partial relief of CSPG-inhibition (Shen et al., 2009; Fisher et al., 2011; Dickendesher et al., 2012). Hence, a combinational targeting strategy that simultaneously interferes with multiple CSPG receptors may be necessary to achieve an optimal result in promoting post-injury axon regeneration. Nonetheless, in some occasions the injured axons may possess a predominant CSPG receptor. For example, in cardiac sympathetic neurons, where PTPsigma appears to be the major CSPG receptor, targeting PTPsigma alone is suf fi cient for a full reinnervation of the scar after ischemia-reperfusion injury (Gardner and Habecker, 2013). Therefore, the strategy to combat CSPG inhibition should be tailored based on the local distribution of the CSPG receptors.

Targeting a bifunctional receptor

For a bifunctional receptor such as PTPsigma, blocking ligand access to its binding domain will eliminate the inhibitory signals of CSPGs; however, this will also deprive the growth promoting signals of HSPGs. Such an approach therefore may not provide the best outcome in promoting axon growth. Instead, supplying HSPGs to the CSPG-enriched lesion environment is a simple strategy that eliminates CSPG-inhibition and stimulated axon growth simultaneously. Alternatively, inhibiting PTPsigma phosphatase activity can be another strategy to switch downstream signaling in favor of axon growth. Compared with the genetic deletion of PTPsigma, a PTPsigma inhibitory peptide evidently has much stronger effects in promoting axonal plasticity, presumably because the blockade of the phosphatase activity assimilates not only the removal of CS but also the addition of HS (as discussed above) (B. T. Lang, 2012).

Locations of treatment

In injured spinal cords, CSPGs are heavily deposited in the lesion areas, forming a barrier that halts the severed axons. It is therefore rational to apply CSPG-combatting treatments to the area of dystrophic axon tips. In fact, lesion site injection of Chondroitinase ABC, an enzyme that digests CS GAGs, effectively enhances axonal plasticity and functional recovery after spinal cord injuries (Bradbury et al., 2002; Garcia-Alias et al., 2009; Alilain et al., 2011).

However, the lesion sites are not the only location where CSPGs contact the injured neurons, and often neglected is the somatodendritic region where the proteoglycan receptors are surrounded by perineuronal CSPGs. Upon spinal cord injuries, CSPGs are upregulated in the perineuronal nets of deafferented neurons distal to the lesion sites (Massey et al., 2006; Alilain et al., 2011). In a recent study, systemic application of a PTPsigma inhibitory peptide achieved robust functional restoration, which appears to mostly result from sprouting fibers that presumably have broken through the perineuronal nets (B. T. Lang, 2012). These fi ndings therefore suggest that the cell bodies of deafferented neurons distalfrom the lesion site are additional locations where CSPG-receptor interaction plays a role in curtailing plasticity. Therefore, an optimal treatment regime should take into account both the axons and cell bodies of the injured neurons so as to maximize the opportunity for functional recovery.

Acknowledgments:The author thanks Dr. Jerry Silver and Bradley T. Lang (Department of Neurosciences, Case Western Reserve University) for comments and discussion.

Con fl icts of interest:None declared.

Alilain WJ, Horn KP, Hu H, Dick TE, Silver J (2011) Functional regeneration of respiratory pathways after spinal cord injury. Nature 475:196-200.

Aricescu AR, McKinnell IW, Halfter W, Stoker AW (2002) Heparan sulfate proteoglycans are ligands for receptor protein tyrosine phosphatase sigma. Mol Cell Biol 22:1881-1892.

Asher RA, Morgenstern DA, Fidler PS, Adcock KH, Oohira A, Braistead JE, Levine JM, Margolis RU, Rogers JH, Fawcett JW (2000) Neurocan is upregulated in injured brain and in cytokine-treated astrocytes. J Neurosci 20:2427-2438.

B. T. Lang JMC, M. A. Depaul, A. R. Filous, T. A. Evans, Y. L. Weng, A. Y. Huang, S. Li, J. Silver. (2012) Peptide inhibitors of LAR family phosphatases release CSPG mediated entrapment of axons and promote robust behavioral recovery following contusive spinal cord injury.

Bandtlow CE, Zimmermann DR (2000) Proteoglycans in the developing brain: new conceptual insights for old proteins. Physiol Rev 80:1267-1290.

Bartus K, James ND, Bosch KD, Bradbury EJ (2012) Chondroitin sulphate proteoglycans: key modulators of spinal cord and brain plasticity. Exp Neurol 235:5-17.

Bilwes AM, den Hertog J, Hunter T, Noel JP (1996) Structural basis for inhibition of receptor protein-tyrosine phosphatase-alpha by dimerization. Nature 382:555-559.

Bloechlinger S, Karchewski LA, Woolf CJ (2004) Dynamic changes in glypican-1 expression in dorsal root ganglion neurons after peripheral and central axonal injury. Eur J Neurosci 19:1119-1132.

Bovolenta P, Fernaud-Espinosa I (2000) Nervous system proteoglycans as modulators of neurite outgrowth. Prog Neurobiol 61:113-132.

Bradbury EJ, Moon LD, Popat RJ, King VR, Bennett GS, Patel PN, Fawcett JW, McMahon SB (2002) Chondroitinase ABC promotes functional recovery after spinal cord injury. Nature 416:636-640.

Brittis PA, Canning DR, Silver J (1992) Chondroitin sulfate as a regulator of neuronal patterning in the retina. Science 255:733-736.

Busch SA, Horn KP, Cuascut FX, Hawthorne AL, Bai L, Miller RH, Silver J (2010) Adult NG2+ cells are permissive to neurite outgrowth and stabilize sensory axons during macrophage-induced axonal dieback after spinal cord injury. J Neurosci 30:255-265.

Busch SA, Silver J (2007) The role of extracellular matrix in CNS regeneration. Curr Opin Neurobiol 17:120-127.

Cafferty WB, Strittmatter SM (2006) The Nogo-Nogo receptor pathway limits a spectrum of adult CNS axonal growth. J Neurosci 26:12242-12250.

Chagnon MJ, Uetani N, Tremblay ML (2004) Functional signi fi cance of the LAR receptor protein tyrosine phosphatase family in development and diseases. Biochem Cell Biol 82:664-675.

Coles CH, Shen Y, Tenney AP, Siebold C, Sutton GC, Lu W, Gallagher JT, Jones EY, Flanagan JG, Aricescu AR (2011) Proteoglycan-speci fi c molecular switch for RPTPsigma clustering and neuronal extension. Science 332:484-488.

Davies SJ, Fitch MT, Memberg SP, Hall AK, Raisman G, Silver J (1997) Regeneration of adult axons in white matter tracts of the central nervous system. Nature 390:680-683.

Dickendesher TL, Baldwin KT, Mironova YA, Koriyama Y, Raiker SJ, Askew KL, Wood A, Geoffroy CG, Zheng B, Liepmann CD, Katagiri Y, Benowitz LI, Geller HM, Giger RJ (2012) NgR1 and NgR3 are receptors for chondroitin sulfate proteoglycans. Nat Neurosci 15:703-712.

Dunah AW, Hueske E, Wyszynski M, Hoogenraad CC, Jaworski J, Pak DT, Simonetta A, Liu G, Sheng M (2005) LAR receptor protein tyrosine phosphatases in the development and maintenance of excitatory synapses. Nat Neurosci 8:458-467.

Fisher D, Xing B, Dill J, Li H, Hoang HH, Zhao Z, Yang XL, Bachoo R, Cannon S, Longo FM, Sheng M, Silver J, Li S (2011) Leukocyte common antigen-related phosphatase is a functional receptor for chondroitin sulfate proteoglycan axon growth inhibitors. J Neurosci 31:14051-14066.

Fox AN, Zinn K (2005) The heparan sulfate proteoglycan syndecan is an in vivo ligand for the Drosophila LAR receptor tyrosine phosphatase. Curr Biol 15:1701-1711.

Fry EJ, Chagnon MJ, Lopez-Vales R, Tremblay ML, David S (2010) Corticospinal tract regeneration after spinal cord injury in receptor protein tyrosine phosphatase sigma de fi cient mice. Glia 58:423-433.

Garcia de Yebenes E, Ho A, Damani T, Fillit H, Blum M (1999) Regulation of the heparan sulfate proteoglycan, perlecan, by injury and interleukin-1alpha. J Neurochem 73:812-820.

Garcia-Alias G, Barkhuysen S, Buckle M, Fawcett JW (2009) Chondroitinase ABC treatment opens a window of opportunity for task-speci fi c rehabilitation. Nat Neurosci 12:1145-1151.

Garcia-Alias G, Fawcett JW (2012) Training and anti-CSPG combination therapy for spinal cord injury. Exp Neurol 235:26-32.

Gardner RT, Habecker BA (2013) Infarct-derived chondroitin sulfate proteoglycans prevent sympathetic reinnervation after cardiac ischemia-reperfusion injury. J Neurosci 33:7175-7183.

Garrity PA, Lee CH, Salecker I, Robertson HC, Desai CJ, Zinn K, Zipursky SL (1999) Retinal axon target selection in Drosophila is regulated by a receptor protein tyrosine phosphatase. Neuron 22:707-717. Haworth K, Shu KK, Stokes A, Morris R, Stoker A (1998) The expression of receptor tyrosine phosphatases is responsive to sciatic nerve crush. Mol Cell Neurosci 12:93-104.

Hendriks WJ, Elson A, Harroch S, Pulido R, Stoker A, den Hertog J (2013) Protein tyrosine phosphatases in health and disease. FEBS J 280:708-730.

Hill JJ, Jin K, Mao XO, Xie L, Greenberg DA (2012) Intracerebral chondroitinase ABC and heparan sulfate proteoglycan glypican improve outcome from chronic stroke in rats. Proc Natl Acad Sci U S A 109: 9155-9160.

Horn KE, Xu B, Gobert D, Hamam BN, Thompson KM, Wu CL, Bouchard JF, Uetani N, Racine RJ, Tremblay ML, Ruthazer ES, Chapman CA, Kennedy TE (2012) Receptor protein tyrosine phosphatase sigma regulates synapse structure, function and plasticity. J Neurochem 122:147-161.

Hsueh YP, Sheng M (1999) Regulated expression and subcellular localization of syndecan heparan sulfate proteoglycans and the syndecan-binding protein CASK/LIN-2 during rat brain development. J Neurosci 19:7415-7425.

Iseki K, Hagino S, Mori T, Zhang Y, Yokoya S, Takaki H, Tase C, Murakawa M, Wanaka A (2002) Increased syndecan expression by pleiotrophin and FGF receptor-expressing astrocytes in injured brain tissue. Glia 39:1-9.

Ivins JK, Litwack ED, Kumbasar A, Stipp CS, Lander AD (1997) Cerebroglycan, a developmentally regulated cell-surface heparan sulfate proteoglycan, is expressed on developing axons and growth cones. Dev Biol 184:320-332.

Johnson KG, Holt CE (2000) Expression of CRYP-alpha, LAR, PTP-delta, and PTP-rho in the developing Xenopus visual system. Mech Dev 92:291-294.

Johnson KG, Tenney AP, Ghose A, Duckworth AM, Higashi ME, Par fi tt K, Marcu O, Heslip TR, Marsh JL, Schwarz TL, Flanagan JG, Van Vactor D (2006) The HSPGs Syndecan and Dallylike bind the receptor phosphatase LAR and exert distinct effects on synaptic development. Neuron 49:517-531.

Jones LL, Margolis RU, Tuszynski MH (2003) The chondroitin sulfate proteoglycans neurocan, brevican, phosphacan, and versican are differentially regulated following spinal cord injury. Exp Neurol 182:399-411.

Kantor DB, Chivatakarn O, Peer KL, Oster SF, Inatani M, Hansen MJ, Flanagan JG, Yamaguchi Y, Sretavan DW, Giger RJ, Kolodkin AL (2004) Semaphorin 5A is a bifunctional axon guidance cue regulated by heparan and chondroitin sulfate proteoglycans. Neuron 44:961-975.

Kim JE, Liu BP, Park JH, Strittmatter SM (2004) Nogo-66 receptor prevents raphespinal and rubrospinal axon regeneration and limits functional recovery from spinal cord injury. Neuron 44:439-451.

Kwon SK, Woo J, Kim SY, Kim H, Kim E (2010) Trans-synaptic adhesions between netrin-G ligand-3 (NGL-3) and receptor tyrosine phosphatases LAR, protein-tyrosine phosphatase delta (PTPdelta), and PTPsigma via speci fi c domains regulate excitatory synapse formation. J Biol Chem 285:13966-13978.

Lauren J, Airaksinen MS, Saarma M, Timmusk T (2003) Two novel mammalian Nogo receptor homologs differentially expressed in the central and peripheral nervous systems. Mol Cell Neurosci 24:581-594.

Lauren J, Hu F, Chin J, Liao J, Airaksinen MS, Strittmatter SM (2007) Characterization of myelin ligand complexes with neuronal Nogo-66 receptor family members. J Biol Chem 282:5715-5725.

Ledig MM, Haj F, Bixby JL, Stoker AW, Mueller BK (1999) The receptor tyrosine phosphatase CRYPalpha promotes intraretinal axon growth. J Cell Biol 147:375-388.

Maeda N, Ishii M, Nishimura K, Kamimura K (2011) Functions of chondroitin sulfate and heparan sulfate in the developing brain. Neurochem Res 36:1228-1240.

Massey JM, Hubscher CH, Wagoner MR, Decker JA, Amps J, Silver J, Onifer SM (2006) Chondroitinase ABC digestion of the perineuronal net promotes functional collateral sprouting in the cuneate nucleus after cervical spinal cord injury. J Neurosci 26:4406-4414.

McGee AW, Yang Y, Fischer QS, Daw NW, Strittmatter SM (2005) Experience-driven plasticity of visual cortex limited by myelin and Nogo receptor. Science 309:2222-2226.

McKeon RJ, Schreiber RC, Rudge JS, Silver J (1991) Reduction of neurite outgrowth in a model of glial scarring following CNS injury is correlated with the expression of inhibitory molecules on reactive astrocytes. J Neurosci 11:3398-3411.

McLean J, Batt J, Doering LC, Rotin D, Bain JR (2002) Enhanced rate of nerve regeneration and directional errors after sciatic nerve injury in receptor protein tyrosine phosphatase sigma knock-out mice. J Neurosci 22:5481-5491.

Milev P, Maurel P, Chiba A, Mevissen M, Popp S, Yamaguchi Y, Margolis RK, Margolis RU (1998) Differential regulation of expression of hyaluronan-binding proteoglycans in developing brain: aggrecan, versican, neurocan, and brevican. Biochem Biophys Res Commun 247:207-212.

Mueller BK, Ledig MM, Wahl S (2000) The receptor tyrosine phosphatase CRYPalpha affects growth cone morphology. J Neurobiol 44:204-218.

Oohira A, Matsui F, Watanabe E, Kushima Y, Maeda N (1994) Developmentally regulated expression of a brain speci fi c species of chondroitin sulfate proteoglycan, neurocan, identi fi ed with a monoclonal antibody IG2 in the rat cerebrum. Neuroscience 60:145-157.

Pindzola RR, Doller C, Silver J (1993) Putative inhibitory extracellular matrix molecules at the dorsal root entry zone of the spinal cord during development and after root and sciatic nerve lesions. Dev Biol 156:34-48.

Properzi F, Lin R, Kwok J, Naidu M, van Kuppevelt TH, Ten Dam GB, Camargo LM, Raha-Chowdhury R, Furukawa Y, Mikami T, Sugahara K, Fawcett JW (2008) Heparan sulphate proteoglycans in glia and in the normal and injured CNS: expression of sulphotransferases and changes in sulphation. Eur J Neurosci 27:593-604.

Pulido R, Serra-Pages C, Tang M, Streuli M (1995) The LAR/PTP delta/ PTPsigma subfamily of transmembrane protein-tyrosine-phosphatases: multiple human LAR, PTP delta, and PTPsigma isoforms are expressed in a tissue-speci fi c manner and associate with the LAR-interacting protein LIP.1. Proc Natl Acad Sci U S A 92:11686-11690.

Sapieha PS, Duplan L, Uetani N, Joly S, Tremblay ML, Kennedy TE, Di Polo A (2005) Receptor protein tyrosine phosphatase sigma inhibits axon regrowth in the adult injured CNS. Mol Cell Neurosci 28:625-635.

Schaapveld RQ, Schepens JT, Bachner D, Attema J, Wieringa B, Jap PH, Hendriks WJ (1998) Developmental expression of the cell adhesion molecule-like protein tyrosine phosphatases LAR, RPTPdelta and RPTPsigma in the mouse. Mech Dev 77:59-62.

Shen Y, Tenney AP, Busch SA, Horn KP, Cuascut FX, Liu K, He Z, Silver J, Flanagan JG (2009) PTPsigma is a receptor for chondroitin sulfate proteoglycan, an inhibitor of neural regeneration. Science 326:592-596.

Snow DM, Lemmon V, Carrino DA, Caplan AI, Silver J (1990) Sulfated proteoglycans in astroglial barriers inhibit neurite outgrowth in vitro. Exp Neurol 109:111-130.

Snow DM, Watanabe M, Letourneau PC, Silver J (1991) A chondroitin sulfate proteoglycan may in fl uence the direction of retinal ganglion cell outgrowth. Development 113:1473-1485.

Stipp CS, Litwack ED, Lander AD (1994) Cerebroglycan: an integral membrane heparan sulfate proteoglycan that is unique to the developing nervous system and expressed speci fi cally during neuronal differentiation. J Cell Biol 124:149-160.

Takahashi H, Craig AM (2013) Protein tyrosine phosphatases PTPdelta, PTPsigma, and LAR: presynaptic hubs for synapse organization. Trends Neurosci 36:522-534.

Takahashi H, Katayama K, Sohya K, Miyamoto H, Prasad T, Matsumoto Y, Ota M, Yasuda H, Tsumoto T, Aruga J, Craig AM (2012) Selective control of inhibitory synapse development by Slitrk3-PTPdelta trans-synaptic interaction. Nat Neurosci 15:389-398, S381-382.

Thompson K (2003) Receptor protein tyrosine phosphatase sigma inhibits axonal regeneration and the rate of axon extension. Mol Cell Neurosci 23:681-692.

Van der Zee CE, Man TY, Van Lieshout EM, Van der Heijden I, Van Bree M, Hendriks WJ (2003) Delayed peripheral nerve regeneration and central nervous system collateral sprouting in leucocyte common antigen-related protein tyrosine phosphatase-deficient mice. Eur J Neurosci 17:991-1005.

Venkatesh K, Chivatakarn O, Lee H, Joshi PS, Kantor DB, Newman BA, Mage R, Rader C, Giger RJ (2005) The Nogo-66 receptor homolog NgR2 is a sialic acid-dependent receptor selective for myelin-associated glycoprotein. J Neurosci 25:808-822.

Wallace MJ, Fladd C, Batt J, Rotin D (1998) The second catalytic domain of protein tyrosine phosphatase delta (PTP delta) binds to and inhibits the first catalytic domain of PTPsigma. Mol Cell Biol 18:2608-2616.

Wang D, Fawcett J (2012) The perineuronal net and the control of CNS plasticity. Cell Tissue Res 349:147-160.

Wang F, Wolfson SN, Gharib A, Sagasti A (2012) LAR receptor tyrosine phosphatases and HSPGs guide peripheral sensory axons to the skin. Curr Biol 22:373-382.

Wang H, Yan H, Canoll PD, Silvennoinen O, Schlessinger J, Musacchio JM (1995) Expression of receptor protein tyrosine phosphatase-sigma (RPTP-sigma) in the nervous system of the developing and adult rat. J Neurosci Res 41:297-310.

Wang KC, Kim JA, Sivasankaran R, Segal R, He Z (2002a) P75 interacts with the Nogo receptor as a co-receptor for Nogo, MAG and OMgp. Nature 420:74-78.

Wang X, Chun SJ, Treloar H, Vartanian T, Greer CA, Strittmatter SM (2002b) Localization of Nogo-A and Nogo-66 receptor proteins at sites of axon-myelin and synaptic contact. J Neurosci 22:5505-5515.

Wilson MT, Snow DM (2000) Chondroitin sulfate proteoglycan expression pattern in hippocampal development: potential regulation of axon tract formation. J Comp Neurol 424:532-546.

Xie Y, Yeo TT, Zhang C, Yang T, Tisi MA, Massa SM, Longo FM (2001) The leukocyte common antigen-related protein tyrosine phosphatase receptor regulates regenerative neurite outgrowth in vivo. J Neurosci 21:5130-5138.

Yan H, Grossman A, Wang H, D’Eustachio P, Mossie K, Musacchio JM, Silvennoinen O, Schlessinger J (1993) A novel receptor tyrosine phosphatase-sigma that is highly expressed in the nervous system. J Biol Chem 268:24880-24886.

Yi JH, Katagiri Y, Susarla B, Figge D, Symes AJ, Geller HM (2012) Alterations in sulfated chondroitin glycosaminoglycans following controlled cortical impact injury in mice. J Comp Neurol 520:3295-3313.

Zhang JS, Honkaniemi J, Yang T, Yeo TT, Longo FM (1998) LAR tyrosine phosphatase receptor: A developmental isoform is present in neurites and growth cones and its expression is regional- and cell-speci fi c. Mol Cell Neurosci 10:271-286.

Zheng B, Atwal J, Ho C, Case L, He XL, Garcia KC, Steward O, Tessier-Lavigne M (2005) Genetic deletion of the Nogo receptor does not reduce neurite inhibition in vitro or promote corticospinal tract regeneration in vivo. Proc Natl Acad Sci U S A 102:1205-1210.

Copyedited by Li CH, Song LP, Zhao M

10.4103/1673-5374.128236

Yingjie Shen, Ph.D., yingjie.shen@osumc. edu.

http://www.nrronline.org/

Accepted: 2014-01-08