Axon regeneration in peripheral nerves is enhanced by proteoglycan degradation

Proteomic analysis of potential keratan sulfate, chondroitin sulfate A, and hyaluronic acid molecular interactions

PURPOSE

Corneal stroma extracellular matrix (ECM) glycosaminoglycans (GAGs) include keratan sulfate (KS), chondroitin sulfate A (CSA), and hyaluronic acid (HA). Embryonic corneal keratocytes and sensory nerve fibers grow and differentiate according to chemical cues they receive from the ECM. This study asked which of the proteins that may regulate keratocytes or corneal nerve growth cone immigration interact with corneal GAGs.

METHODS

Biotinylated KS (bKS), CSA (bCSA), and HA (bHA) were prepared and used in microarray protocols to assess their interactions with 8268 proteins and a custom microarray of 85 extracellular epitopes of nerve growth-related proteins. Surface plasmon resonance (SPR) was performed with bKS and SLIT2, and their ka, kd, and KD were determined.

RESULTS

Highly sulfated KS interacted with 217 microarray proteins, including 75 kinases, several membrane or secreted proteins, many cytoskeletal proteins, and many nerve function proteins. CSA interacted with 24 proteins, including 10 kinases and 2 cell surface proteins. HA interacted with 6 proteins, including several ECM-related structural proteins. Of 85 ECM nerve-related epitopes, KS bound 40 proteins, including SLIT, 2 ROBOs, 9 EPHs, 8 Ephrins (EFNs), 8 semaphorins (SEMAs), and 2 nerve growth factor receptors. CSA bound nine proteins, including ROBO2, 2 EPHs, 1 EFN, two SEMAs, and netrin 4. HA bound no ECM nerve-related epitopes. SPR confirmed that KS binds SLIT2 strongly. The KS core protein mimecan/osteoglycin bound 15 proteins.

CONCLUSIONS

Corneal stromal GAGs bind, and thus could alter the availability or conformation of, many proteins that may influence keratocyte and nerve growth cone behavior in the cornea.

Introduction to special issue: Challenges and opportunities for regeneration in the peripheral nervous system

Regeneration in the peripheral nervous system offers unique opportunities and challenges to medicine. Compared to the central nervous system, peripheral axons can and do regenerate resulting in functional recovery, especially if the distance to target is short as in distal limb injuries. However, this regenerative capacity is often incomplete and functional recovery with proximal lesions is limited. Furthermore, regeneration of axons to the appropriate targets remains a challenge with inappropriate reinnervation being an impediment to full recovery. The reviews and selected original research papers in this Special Issue will address some of these challenges and highlight new opportunities for development of effective therapies for nerve regeneration.

Treadmill training enhances axon regeneration in injured mouse peripheral nerves without increased loss of topographic specificity

We investigated the extent of misdirection of regenerating axons when that regeneration was enhanced by using treadmill training. Retrograde fluorescent tracers were applied to the cut proximal stumps of the tibial and common fibular nerves 2 or 4 weeks after transection and surgical repair of the mouse sciatic nerve. The spatial locations of retrogradely labeled motoneurons were studied in untreated control mice and in mice receiving 2 weeks of treadmill training, according to either a continuous protocol (10 m/minute, 1 hour/day, 5 days/week) or an interval protocol (20 m/minute for 2 minutes, followed by a 5-minute rest, repeated four times, 5 days/week). More retrogradely labeled motoneurons were found in both treadmill-trained groups. The magnitude of this increase was as great as or greater than that found after using other enhancement strategies. In both treadmill-trained groups, the proportions of motoneurons labeled from tracer applied to the common fibular nerve that were found in spinal cord locations reserved for tibial motoneurons in intact mice were no greater than in untreated control mice and significantly less than those found after electrical stimulation or chondroitinase treatment. Treadmill training in the first 2 weeks following peripheral nerve injury produces a marked enhancement of motor axon regeneration without increasing the propensity of those axons to choose pathways leading to functionally inappropriate targets.

Influence of aging on chondroitin sulfate proteoglycan expression and neural stem/progenitor cells in rat brain and improving effects of a herbal medicine, yokukansan

There is evidence of structural and functional deterioration in the brain, including the prefrontal cortex (PFC) and hippocampus, during the normal aging process in animals and humans. Extracellular matrix-associated glycoproteins, such as chondroitin sulfate proteoglycans (CSPGs), are involved in not only maintaining the structures and functions of adult neurons, but also regulating the proliferation, migration, and neurite outgrowth of neural stem cells in the brain. On the other hand, a herbal medicine, yokukansan (YKS), is used in a variety of clinical situations for treating symptoms associated with age-related neurodegenerative disorders such as Alzheimer's disease, but its pharmacological properties have not been fully understood. The present study was designed to clarify the influence of aging and the improving effects of YKS on the expression of aggrecan, a major molecule of CSPGs, and on the proliferation and migration of neural stem/progenitor cells identified by bromodeoxyuridine (BrdU) incorporation in the PFC and hippocampus including the dentate gyrus. Aged rats (24 months old) showed a significant increase in aggrecan expression throughout the PFC and in the hippocampus particularly in the CA3 subfield, but not the dentate gyrus compared to young rats (5 months old), evaluated by the immunohistochemical method. YKS treatment decreased the age-related increase in aggrecan expression as well as normal expression in young rats. Aged rats also showed a decreased number of BrdU-labeled cells in the PFC and hippocampus, and these decreases were improved by YKS treatment, which also increased the numbers in young rats. These results suggest that aging influences the microenvironment for adult and immature neurons in the brain, which may affect the proliferation and migration of neural stem/progenitor cells, and YKS has pharmacological potency for these age-related events. These findings help to understand the physiology and pathology of the aged brain and provide an anti-aging strategy for the brain.

The potentiation of peripheral nerve sheaths in regeneration and repair

Traumatic injury to the nervous system often results in life changing loss of neurological function. Spontaneous neural regeneration occurs rarely and the outcome of therapeutic intervention is most often unacceptable. An intensive effort is underway to improve methods and technologies for nervous system repair. To date, the most success has been attained in the outcomes of peripheral nerve restoration. The importance of the peripheral nerve sheaths in successful nerve regeneration has been long recognized. In particular, Schwann cells and their basal laminae play a central role in axon development, maintenance, physiology, and response to injury. The endoneurial basal lamina is rich in components that promote axonal growth. It is now evident that the bioactivities of these components are counterbalanced by various factors that impede axonal growth. The growth-promoting potential of peripheral nerve is realized in the degenerative processes that occur distal to a lesion. This potentiation involves precise spatiotemporal alterations in the balance of antagonistic regulators of axonal growth. Experimental alteration of nerve sheath composition can also potentiate nerve and improve key features of nerve regeneration. For instance, enzymatic degradation of inhibitory chondroitin sulfate proteoglycan mimics endogenous processes that potentiate degenerated nerve and improves the outcome of direct nerve repair and grafting in animal models. This review provides a perspective of the essential role that peripheral nerve sheaths play in regulating axonal regeneration and focuses on discoveries leading to the inception and development of novel therapies for nerve repair.

Treadmill training promotes axon regeneration in injured peripheral nerves

Physical activity after spinal cord injury promotes improvements in motor function, but its effects following peripheral nerve injury are less clear. Although axons in peripheral nerves are known to regenerate better than those in the CNS, methods of accelerating regeneration are needed due to the slow overall rate of growth. Therefore we studied the effect of two weeks of treadmill locomotion on the growth of regenerating axons in peripheral nerves following injury. The common fibular nerves of thy-1-YFP-H mice, in which a subset of axons in peripheral nerves express yellow fluorescent protein (YFP), were cut and repaired with allografts from non-fluorescent littermates, and then harvested two weeks later. Mice were divided into groups of low-intensity continuous training (CT, 60 min), low-intensity interval training (IT; one group, 10 reps, 20 min total), and high-intensity IT (three groups, 2, 4, and 10 reps). One repetition consisted of 2 min of running and 5 min of rest. Sixty minutes of CT resulted in the highest exercise volume, whereas 2 reps of IT resulted in the lowest volume of exercise. The lengths of regenerating YFP(+) axons were measured in images of longitudinal optical sections of nerves. Axon profiles were significantly longer than control in all exercise groups except the low-intensity IT group. In the CT group and the high-intensity IT groups that trained with 4 or 10 repetitions axons were more than twice as long as unexercised controls. The number of intervals did not impact axon elongation. Axon sprouting was enhanced in IT groups but not the CT group. Thus exercise, even in very small quantities, increases axon elongation in injured peripheral nerves whereas continuous exercise resulting in higher volume (total steps) may have no net impact on axon sprouting.

Adult bone marrow-derived mononuclear cells expressing chondroitinase AC transplanted into CNS injury sites promote local brain chondroitin sulphate degradation

Injury to the CNS of vertebrates leads to the formation of a glial scar and production of inhibitory molecules, including chondroitin sulphate proteoglycans. Various studies suggest that the sugar component of the proteoglycan is responsible for the inhibitory role of these compounds in axonal regeneration. By degrading chondroitin sulphate chains with specific enzymes, denominated chondroitinases, the inhibitory capacity of these proteoglycans is decreased. Chondroitinase administration involves frequent injections of the enzyme at the lesion site which constitutes a rather invasive method. We have produced a vector containing the gene for Flavobacterium heparinum chondroitinase AC for expression in adult bone marrow-derived cells which were then transplanted into an injury site in the CNS. The expression and secretion of active chondroitinase AC was observed in vitro using transfected Chinese hamster ovarian and gliosarcoma cells and in vivo by immunohistochemistry analysis which showed degraded chondroitin sulphate coinciding with the location of transfected bone marrow-derived cells. Immunolabelling of the axonal growth-associated protein GAP-43 was observed in vivo and coincided with the location of degraded chondroitin sulphate. We propose that bone marrow-derived mononuclear cells, transfected with our construct and transplanted into CNS, could be a potential tool for studying an alternative chondroitinase AC delivery method.

Chondrotinase ABC enhances axonal regeneration across nerve gaps

We evaluated the effects of chondroitinase ABC on axonal regeneration across peripheral nerve gaps. We compared axonal regeneration after 15-mm tibial nerve resection and repair with a silicone tube filled with type I collagen gel (negative control group), with a silicone tube filled with type I collagen gel containing chondroitinase ABC at three different concentrations (2.5 units/mL, 5 units/mL, 10 units/mL) (chondroitinase ABC groups), and with an autologous nerve segment (nerve autograft group). Electrophysiological and histological assessments were carried out 12 weeks after surgery. In the electrophysiological study, compound muscle action potentials (CMAPs) and nerve conduction velocities (NCVs) were recorded in all groups except the negative control group. Although both CMAPs and NCVs were highest in the nerve autograft group, there were no significant differences among the three chondroitinase ABC groups in either parameter. Histological findings were consistent with electrophysiological results. Based on these findings, we conclude that topical injection of chondroitinase ABC can significantly increase the critical length of nerve gap repair by tubulization or artificial nerve placement.

Chondroitinase ABC improves basic and skilled locomotion in spinal cord injured cats

Chondroitin sulfate proteoglycans (CSPGs) are upregulated in the central nervous system following injury. Chondroitin sulfate glycosaminoglycan (CS GAG) side chains substituted on this family of molecules contribute to the limited functional recovery following injury by restricting axonal growth and synaptic plasticity. In the current study, the effects of degrading CS GAGs with Chondroitinase ABC (Ch'ase ABC) in the injured spinal cords of adult cats were assessed. Three groups were evaluated for 5 months following T10 hemisections: lesion-only, lesion+control, and lesion+Ch'ase ABC. Intraspinal control and Ch'ase ABC treatments to the lesion site began immediately after injury and continued every other day, for a total of 15 treatments, using an injectable port system. Delivery and in vivo cleavage were verified anatomically in a subset of cats across the treatment period. Recovery of skilled locomotion (ladder, peg, and beam) was significantly accelerated, on average, by >3 weeks in Ch'ase ABC-treated cats compared to controls. Ch'ase ABC-treated cats also showed greater recovery of specific skilled locomotor features including intralimb movement patterns and significantly greater paw placement onto pegs. Although recovery of basic locomotion (bipedal treadmill and overground) was not accelerated, intralimb movement patterns were more normal in the Ch'ase ABC-treated cats. Qualitative assessment of serotonergic immunoreactivity also suggested that Ch'ase ABC treatment enhanced plasticity. Finally, analyses using fluorophore-assisted carbohydrate electrophoresis (FACE) indicate CS GAG content is similar in cat and human. These findings show, for the first time, that intraspinal cleavage of CS GAGs can enhance recovery of function following spinal cord injury in large animals with sophisticated motor behaviors and axonal growth requirements similar to those encountered in humans.

Effect of body temperature on chondroitinase ABC's ability to cleave chondroitin sulfate glycosaminoglycans

Chondroitinase ABC (Ch'ase ABC) is a bacterial lyase that degrades chondroitin sulfate (CS), dermatan sulfate, and hyaluronan glycosaminoglycans (GAGs). This enzyme has received significant attention as a potential therapy for promoting central nervous system and peripheral nervous system repair based on its degradation of CS GAGs. Determination of the stability of Ch'ase ABC activity at temperatures equivalent to normal (37 degrees C) and elevated (39 degrees C) body temperatures is important for optimizing its clinical usage. We report here data obtained from examining enzymatic activity at these temperatures across nine lots of commercially available protease-free Ch'ase ABC. CS GAG degrading activity was assayed by using 1) immunohistochemical detection of unsaturated disaccharide stubs generated by digestion of proteoglycans in tissue sections and 2) fluorophore-assisted carbohydrate electrophoresis (FACE) and/or high-performance liquid chromatography (HPLC) to separate and quantify unsaturated disaccharide digestion products. Our results indicate that there is a significant effect of lot and time on enzymatic thermostability. Average enzymatic activity is significantly decreased at 1 and 3 days at 39 degrees C and 37 degrees C, respectively. Furthermore, the average activity seen after 1 day was significantly different between the two temperatures. Addition of bovine serum albumin as a stabilizer significantly preserved enzymatic activity at 1 day, but not 3 days, at 39 degrees C. These results show that the CS GAG degrading activity of Ch'ase ABC is significantly decreased with incubation at body temperature over time and that all lots do not show equal thermostability. These findings are important for the design and interpretation of experimental and potential clinical studies involving Ch'ase ABC.

Mammalian-produced chondroitinase AC mitigates axon inhibition by chondroitin sulfate proteoglycans

Chondroitin sulfate proteoglycans (CSPGs) are up-regulated following spinal cord injury and are partly responsible for failed regeneration. Experimental paradigms in vivo that degrade chondroitin sulfate glycosaminoglycan chains with the bacterial enzyme, chondroitinase, greatly enhance the ability of axons to regenerate through the glial scar. Unfortunately, enthusiasm for this treatment paradigm is diminished by the lack of a minimally invasive and sustained delivery method. To address these deficits, we have engineered a Tet-On adenoviral vector encoding chondroitinase AC and have characterized its enzymatic function in vitro. U373 human astrocytoma cells were transduced with adenovirus and subsequently induced with doxycycline to secrete enzymatically active chondroitinase as detected by western blot and kinetic analyses. Enzymatic activity demonstrated biological relevance in studies where neurite outgrowth into and across CSPG-adsorbed regions pre-treated with conditioned media from chondroitinase secreting astrocytes was significantly increased compared with untreated controls (p < 0.0001). We also measured important parameters of enzyme activity including: pH, temperature, and enzyme stability that are fundamental to harnessing the true therapeutic potential of this approach. The use of resident cells for continuous secretion of CSPG-degrading enzymes at the site of the glial scar promises to be of greater clinical relevance than contemporary methods.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Electrical stimulation promotes peripheral axon regeneration by enhanced neuronal neurotrophin signaling

Electrical stimulation of cut peripheral nerves at the time of their surgical repair results in an enhancement of axon regeneration. Regeneration of axons through nerve allografts was used to evaluate whether this effect is due to an augmentation of cell autonomous neurotrophin signaling in the axons or signaling from neurotrophins produced in the surrounding environment. In the thy-1-YFP-H mouse, a single 1 h application of electrical stimulation at the time of surgical repair of the cut common fibular nerve results in a significant increase in the proportion of YFP+ dorsal root ganglion neurons, which were immunoreactive for BDNF or trkB, as well as an increase in the length of regenerating axons through allografts from wild type litter mates, both 1 and 2 weeks later. Axon growth through allografts from neurotrophin-4/5 knockout mice or grafts made acellular by repeated cycles of freezing and thawing is normally very poor, but electrical stimulation results in a growth of axons through these grafts, which is similar to that observed through grafts from wild type mice after electrical stimulation. When cut nerves in NT-4/5 knockout mice were electrically stimulated, no enhancement of axon regeneration was found. Electrical stimulation thus produces a potent enhancement of the regeneration of axons in cut peripheral nerves, which is independent of neurotrophin production by cells in their surrounding environment but is dependent on stimulation of trkB and its ligands in the regenerating axons themselves.

Proteoglycans as modulators of axon guidance cue function

Organizing a functional neuronal network requires the precise wiring of neuronal connections. In order to find their correct targets, growth cones navigate through the extracellular matrix guided by secreted and membrane-bound molecules of the slit, netrin, ephrin and semaphorin families. Although many of these axon guidance molecules are able to bind to heparan sulfate proteoglycans, the role of proteoglycans in regulating axon guidance cue function is only now beginning to be understood. Recent developmental studies in a wide range of model organisms have revealed a crucial role for heparan sulfate proteoglycans as modulators of key signaling pathways in axon guidance. In addition, emerging evidence indicates an essential role for chondroitin sulfate proteoglycans in modifying the guidance function of semaphorins. It is becoming increasingly clear that extracellular matrix molecules, rather than just constituting a structural scaffold, can critically influence axon guidance cue function in development, and may continue to do so in the injured adult nervous system.

Myelin-associated glycoprotein reduces axonal branching and enhances functional recovery after sciatic nerve transection in rats

The mature peripheral nervous system (PNS) generally shows better regeneration of injured axons as opposed to the central nervous system (CNS). However, complete functional recovery is rarely achieved even in the PNS although morphologically good axonal regeneration often occurs. This mainly results from aberrant reinnervation due to extensive branching of cut axons with consequent failure of synchronized movements of the muscles. Myelin-associated glycoprotein (MAG), a well-characterized molecule existing both in the CNS and PNS myelin, is considered to be a potent inhibitor of axonal regeneration especially in the CNS. In the present study, we investigated whether MAG has any effects not only on axonal elongation, but also on axonal branching. We show herein that MAG minimized branching of the peripheral axons both in vitro and in vivo via activation of RhoA. Furthermore, after sciatic nerve transection in rats, focal and temporary application of MAG to the lesion dramatically enhanced the functional recovery. Using double retrograde labeling and preoperative/postoperative labeling of spinal neurons, reduced hyperinnervation and improved accuracy of target reinnervation was confirmed, respectively. In conclusion, as MAG significantly improves the quality of axonal regeneration, it can be used as a new therapeutic approach for peripheral nerve repair with possible focal and temporary application.

Matrix metalloproteinases and proteoglycans in axonal regeneration

After an injury to the adult mammalian central nervous system (CNS), a variety of growth-inhibitory molecules are upregulated. A glial scar forms at the site of injury and is composed of numerous molecular substances, including chondroitin sulfate proteoglycans (CSPGs). These proteoglycans inhibit axonal growth in vitro and in vivo. Matrix metalloproteinases (MMPs) can degrade the core protein of some CSPGs as well as other growth-inhibitory molecules such as Nogo and tenascin-C. MMPs have been shown to facilitate axonal regeneration in the adult mammalian peripheral nervous system (PNS). This review will focus on the various roles of proteoglycans and MMPs within the injured nervous system. First, we will present a general background on the injured central nervous system and explore the roles that proteoglycans play in the injured PNS and CNS. Second, we will discuss the various functions of MMPs within the injured PNS and CNS. Special attention will be paid to the possibility of how MMPs might modify the growth-inhibitory extracellular environment of the injured adult mammalian spinal cord and facilitate axonal regeneration in the CNS.

Mechanisms of Disease: what factors limit the success of peripheral nerve regeneration in humans?

Functional recovery after repair of peripheral nerve injury in humans is often suboptimal. Over the past quarter of a century, there have been significant advances in human nerve repair, but most of the developments have been in the optimization of surgical techniques. Despite extensive research, there are no current therapies directed at the molecular mechanisms of nerve regeneration. Multiple interventions have been shown to improve nerve regeneration in small animal models, but have not yet translated into clinical therapies for human nerve injuries. In many rodent models, regeneration occurs over relatively short distances, so the duration of denervation is short. By contrast, in humans, nerves often have to regrow over long distances, and the distal portion of the nerve progressively loses its ability to support regeneration during this process. This can be largely attributed to atrophy of Schwann cells and loss of a Schwann cell basal lamina tube, which results in an extracellular environment that is inhibitory to nerve regeneration. To develop successful molecular therapies for nerve regeneration, we need to generate animal models that can be used to address the following issues: improving the intrinsic ability of neurons to regenerate to increase the speed of axonal outgrowth; preventing loss of basal lamina and chronic denervation changes in the denervated Schwann cells; and overcoming inhibitory cues in the extracellular matrix.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Heparan sulfate proteoglycans and the emergence of neuronal connectivity

With the identification of the molecular determinants of neuronal connectivity, our understanding of the extracellular information that controls axon guidance and synapse formation has evolved from single factors towards the complexity that neurons face in a living organism. As we move in this direction - ready to see the forest for the trees - attention is returning to one of the most ancient regulators of cell-cell interaction: the extracellular matrix. Among many matrix components that influence neuronal connectivity, recent studies of the heparan sulfate proteoglycans suggest that these ancient molecules function as versatile extracellular scaffolds that both sculpt the landscape of extracellular cues and modulate the way that neurons perceive the world around them.

Nerve injury induces the expression of EXT2, a glycosyltransferase required for heparan sulfate synthesis

Heparan sulfate proteoglycans, which bear long chains of heparan sulfate glycosaminoglycan, play significant roles during embryogenesis, including the formation of the CNS. However, their involvement in nerve regeneration has not yet been clarified. Here, we found that the mRNA expression of EXT2, one of the crucial enzymes for heparan sulfate-glycosaminoglycan synthesis, was markedly up-regulated in injured hypoglossal motor neurons after axotomy. In addition, immunohistochemical staining with an antibody specific for heparan sulfate-glycosaminoglycan chains demonstrated increased expression of heparan sulfate-glycosaminoglycan chains in the injured nucleus. Furthermore, the mRNA expressions of glypican-1 and syndecan-1, which are both well-known heparan sulfate proteoglycans, were prominently up-regulated in injured motor neurons. These results suggest that the biosynthesis of heparan sulfate chains promoted by EXT2 is activated in injured motor neurons, and that glypican-1 and syndecan-1 are potent candidates for heparan sulfate proteoglycans involved in peripheral nerve regeneration.

Neurotrophin-4/5 is required for the early growth of regenerating axons in peripheral nerves

The requirement of the trkB ligand, neurotrophin-4/5 (NT-4/5), for the growth of regenerating axons in the peripheral nervous system (PNS) is not well established. We studied regenerating axon growth in transected peripheral nerves of thy-1-YFP-H mice that had been repaired using allografts obtained from brain-derived neurotrophic factor (BDNF) or NT-4/5 knockout mice. Lengths of profiles of YFP+ axons measured in these grafts were compared with those measured in grafts obtained from wild-type donors. When compared with axon profiles measured in grafts from wild-type donors, axon profile lengths measured in grafts from homozygous (NT-4/5(-/-)) or heterozygous (NT-4/5(+/-)) mice were significantly shorter. In contrast, the lengths of axon profiles measured in grafts from BDNF(+/-) mice were not significantly different from those measured in grafts from wild-type mice. A reduced amount of BDNF, but not NT-4/5, is sufficient to promote the elongation of regenerating axons in the PNS. When grafts from wild-type or NT-4/5(-/-) mice were treated acutely at the time of surgical repair either with exogenous BDNF or NT-4/5, the lengths of axon profiles measured in the grafts were significantly longer than those measured in grafts from untreated wild-type mice. These findings are consistent with a requirement for NT-4/5 from within the pathway used by regenerating axons for the successful growth of those axons in peripheral nerves.