Nervous system-derived chondroitin sulfate proteoglycans regulate growth cone morphology and inhibit neurite outgrowth: a light, epifluorescence, and electron microscopy study

Intervertebral disc degeneration and how it leads to low back pain

The purpose of this review was to evaluate data generated by animal models of intervertebral disc (IVD) degeneration published in the last decade and show how this has made invaluable contributions to the identification of molecular events occurring in and contributing to pain generation. IVD degeneration and associated spinal pain is a complex multifactorial process, its complexity poses difficulties in the selection of the most appropriate therapeutic target to focus on of many potential candidates in the formulation of strategies to alleviate pain perception and to effect disc repair and regeneration and the prevention of associated neuropathic and nociceptive pain. Nerve ingrowth and increased numbers of nociceptors and mechanoreceptors in the degenerate IVD are mechanically stimulated in the biomechanically incompetent abnormally loaded degenerate IVD leading to increased generation of low back pain. Maintenance of a healthy IVD is, thus, an important preventative measure that warrants further investigation to preclude the generation of low back pain. Recent studies with growth and differentiation factor 6 in IVD puncture and multi-level IVD degeneration models and a rat xenograft radiculopathy pain model have shown it has considerable potential in the prevention of further deterioration in degenerate IVDs, has regenerative properties that promote recovery of normal IVD architectural functional organization and inhibits the generation of inflammatory mediators that lead to disc degeneration and the generation of low back pain. Human clinical trials are warranted and eagerly anticipated with this compound to assess its efficacy in the treatment of IVD degeneration and the prevention of the generation of low back pain.

Development of Neurogenic Detrusor Overactivity after Thoracic Spinal Cord Injury Is Accompanied by Time-Dependent Changes in Lumbosacral Expression of Axonal Growth Regulators

Thoracic spinal cord injury (SCI) results in urinary dysfunction, which majorly affects the quality of life of SCI patients. Abnormal sprouting of lumbosacral bladder afferents plays a crucial role in this condition. Underlying mechanisms may include changes in expression of regulators of axonal growth, including chondroitin sulphate proteoglycans (CSPGs), myelin-associated inhibitors (MAIs) and repulsive guidance molecules, known to be upregulated at the injury site post SCI. Here, we confirmed lumbosacral upregulation of the growth-associated protein GAP43 in SCI animals with bladder dysfunction, indicating the occurrence of axonal sprouting. Neurocan and Phosphacan (CSPGs), as well as Nogo-A (MAI), at the same spinal segments were upregulated 7 days post injury (dpi) but returned to baseline values 28 dpi. In turn, qPCR analysis of the mRNA levels for receptors of those repulsive molecules in dorsal root ganglia (DRG) neurons showed a time-dependent decrease in receptor expression. In vitro assays with DRG neurons from SCI rats demonstrated that exposure to high levels of NGF downregulated the expression of some, but not all, receptors for those regulators of axonal growth. The present results, therefore, show significant molecular changes at the lumbosacral cord and DRGs after thoracic lesion, likely critically involved in neuroplastic events leading to urinary impairment.

Role of Chondroitin Sulfation Following Spinal Cord Injury

Traumatic spinal cord injury produces long-term neurological damage, and presents a significant public health problem with nearly 18,000 new cases per year in the U.S. The injury results in both acute and chronic changes in the spinal cord, ultimately resulting in the production of a glial scar, consisting of multiple cells including fibroblasts, macrophages, microglia, and reactive astrocytes. Within the scar, there is an accumulation of extracellular matrix (ECM) molecules—primarily tenascins and chondroitin sulfate proteoglycans (CSPGs)—which are considered to be inhibitory to axonal regeneration. In this review article, we discuss the role of CSPGs in the injury response, especially how sulfated glycosaminoglycan (GAG) chains act to inhibit plasticity and regeneration. This includes how sulfation of GAG chains influences their biological activity and interactions with potential receptors. Comprehending the role of CSPGs in the inhibitory properties of the glial scar provides critical knowledge in the much-needed production of new therapies.

Mechanical properties of the sciatic nerve following combined transplantation of analytically extracted acellular allogeneic nerve and adipose-derived mesenchymal stem cells

Purpose

To investigate the effects of Chemically Extracted Acellular Nerves (CEANs) when combined with Adipose-Derived mesenchymal Stem Cell (ADSC) transplantation on the repair of sciatic nerve defects in rabbits.

Methods

A total of 71 six-month-old Japanese rabbit were used in this study. Twenty rabbits served as sciatic nerve donors, while the other 51 rabbits were randomly divided into Autologous Nerve Transplantation Group (ANT, n=17), CEAN group (n=17) and CEAN-ADSCs group (n=17). In all these groups, the rabbit’s left sciatic nerves were injured before the experiment, and the uninjured sciatic nerves on their right side were used as the control (CON). Electrophysiological tests were carried out and sciatic nerves were prepared for histomorphology and stretch testing at 24 weeks post-transplant.

Results

There were significant differences between ANT and Con groups in amplitude (AMP): P=0.031; motor nerve conduction velocity (MNCV): P=0.029; Maximum stress: P=0.029; and Maximum strain P=0.027. There were also differences between the CEAN and CEAN+ADSCs groups in AMP: P=0.026, MNCV: P=0.024; Maximum stress: P=0.025 and Maximum strain: P=0.030. No significant differences in these parameters were observed when comparing the ANT and CEAN+SACN groups (MNCV: P=0.071) or the CEAN and ANT groups (Maximum stress: P=0.069; Maximum strain P=0.077).

Conclusion

Addition of ADSCs has a significant impact on the recovery of nerve function, morphology, and tensile mechanical properties following sciatic nerve injury.

Contributions of Chondroitin Sulfate, Keratan Sulfate and N-linked Oligosaccharides to Inhibition of Neurite Outgrowth by Aggrecan

The role of proteoglycans in the central nervous system (CNS) is a rapidly evolving field and has major implications in the field of CNS injury. Chondroitin sulfate proteoglycans (CSPGs) increase in abundance following damage to the spinal cord and inhibit neurite outgrowth. Major advances in understanding the interaction between outgrowing neurites and CSPGs has created a need for more robust and quantitative analyses to further our understanding of this interaction. We report the use of a high-throughput assay to determine the effect of various post-translational modifications of aggrecan upon neurite outgrowth from NS-1 cells (a PC12 cell line derivative). Aggrecan contains chondroitin sulfate, keratan sulfate, and N-linked oligosaccharides (N-glycans), each susceptible to removal through different enzymatic digestions. Using a sequential digestion approach, we found that chondroitin sulfate and N-glycans, but not keratan sulfate, contribute to inhibition of neurite outgrowth by substrate-bound aggrecan. For the first time, we have shown that N-linked oligosaccharides on aggrecan contribute to its inhibition of neuritogenesis.

Recent advances in the therapeutic uses of chondroitinase ABC

Many studies, using pre-clinical models of SCI, have demonstrated the efficacy of chondroitinase ABC as a treatment for spinal cord injury and this has been confirmed in laboratories worldwide and in several animal models. The aim of this review is report the current state of research in the field and to compare the relative efficacies of these new interventions to improve outcomes in both acute and chronic models of SCI. We also report new methods of chondroitinase delivery and the outcomes of two clinical trials using the enzyme to treat spinal cord injury in dogs and disc herniation in human patients. Recent studies have assessed the outcomes of combining chondroitinase with other strategies known to promote recovery following spinal cord injury and new approaches. Evidence is emerging that one of the most powerful combinations is that of chondroitinase with cell transplants. The particular benefits of each of the different cell types used for these transplant experiments are discussed. Combining chondroitinase with rehabilitation also improves outcomes. Gene therapy is an efficient method of enzyme delivery to the injured spinal cord and circumvents the issue of the enzyme's thermo-instability. Other methods of delivery, such as via nanoparticles or synthetic scaffolds, have shown promise; however, the outcomes from these experiments suggest that these methods of delivery require further optimization to achieve similar levels of efficacy to that obtained by a gene therapy approach. Pre-clinical models have also shown chondroitinase is efficacious in the treatment of other conditions, such as peripheral nerve injury, stroke, coronary reperfusion, Parkinson's disease and certain types of cancer. The wide range of conditions where the benefits of chondroitinase treatment have been demonstrated reflects the complex roles that chondroitin sulphate proteoglycans (its substrate) play in health and disease and warrants the enzyme's further development as a therapy.

The Biology of Regeneration Failure and Success After Spinal Cord Injury

Since no approved therapies to restore mobility and sensation following spinal cord injury (SCI) currently exist, a better understanding of the cellular and molecular mechanisms following SCI that compromise regeneration or neuroplasticity is needed to develop new strategies to promote axonal regrowth and restore function. Physical trauma to the spinal cord results in vascular disruption that, in turn, causes blood-spinal cord barrier rupture leading to hemorrhage and ischemia, followed by rampant local cell death. As subsequent edema and inflammation occur, neuronal and glial necrosis and apoptosis spread well beyond the initial site of impact, ultimately resolving into a cavity surrounded by glial/fibrotic scarring. The glial scar, which stabilizes the spread of secondary injury, also acts as a chronic, physical, and chemo-entrapping barrier that prevents axonal regeneration. Understanding the formative events in glial scarring helps guide strategies towards the development of potential therapies to enhance axon regeneration and functional recovery at both acute and chronic stages following SCI. This review will also discuss the perineuronal net and how chondroitin sulfate proteoglycans (CSPGs) deposited in both the glial scar and net impede axonal outgrowth at the level of the growth cone. We will end the review with a summary of current CSPG-targeting strategies that help to foster axonal regeneration, neuroplasticity/sprouting, and functional recovery following SCI.

Spatial distribution affects the role of CSPGs in nerve regeneration via the actin filament-mediated pathway

CSPGs are components of the extracellular matrix in the nervous system, where they serve as cues for axon guidance during development. After a peripheral nerve injury, CSPGs switch roles and become axon inhibitors and become diffusely distributed at the injury site. To investigate whether the spatial distribution of CSPGs affects their role, we combined in vitro DRG cultures with CSPG stripe or coverage assays to simulate the effect of a patterned substrate or dispersive distribution of CSPGs on growing neurites. We observed neurite steering at linear CSPG interfaces and neurite inhibition when diffused CSPGs covered the distal but not the proximal segment of the neurite. The repellent and inhibitory effects of CSPGs on neurite outgrowth were associated with the disappearance of focal actin filaments on growth cones. The application of an actin polymerization inducer, jasplakinolide, allowed neurites to break through the CSPG boundary and grow on CSPG-coated surfaces. The results of our study collectively reveal a novel mechanism that explains how the spatial distribution of CSPGs determines whether they act as a cue for axon guidance or as an axon-inhibiting factor. Increasing our understanding of this issue may promote the development of novel therapeutic strategies that regulate the spatial distributions of CSPGs to use them as an axon guidance cue.

Lipopolysaccharide-induced functional and structural injury of the mitochondria in the nigrostriatal pathway

Accumulating evidence suggests that chronic inflammation plays a role in the progressive dopaminergic neurodegeneration that occurs in Parkinson's disease. It has been hypothesized that inflammation mediates neuronal damage via exacerbation of a vicious cycle of oxidative stress and mitochondrial dysfunction. The bacterial endotoxin, lipopolysaccharide (LPS), induces microglial activation and inflammation driven dopaminergic neurodegeneration. In order to test the hypothesis that LPS-induced inflammatory response might damage mitochondrial structure and function leading to nigral dopaminergic neuron loss, we injected LPS or saline into the striatum of rats. Here, we found that intrastriatal LPS induced deficit in mitochondrial respiration, damage to mitochondrial cristae, mitochondrial oxidation and nitration. Finally, we found significant loss of dopaminergic neurons in the substantia nigra one week after LPS injection. This study indicates that LPS-induced dopaminergic neurodegeneration might be exerted by mitochondrial injury.

The role of chondroitinase as an adjuvant to peripheral nerve repair

Chondroitin sulfate proteoglycans (CSPGs) are potent inhibitors of neural regeneration in the peripheral nervous system. Following nerve injury, inhibitory CSPGs accumulate within the endoneurium and Schwann cell basal lamina of the distal nerve stump. The utilization of chondroitinase ABC (chABC) has led to a marked increase in the ability of injured axons to regenerate across gaps through the CSPG-laden extracellular matrix. Experimental models have repeatedly shown chABC to be capable of degrading the CSPGs that hinder neurite outgrowth. In this article, the characterization of CSPGs, their upregulation following peripheral nerve injury, and potential mechanisms behind their growth and inhibition are described. To date, the literature supports that the adjunct use of chABC may be beneficial to peripheral nerve repair in digesting inhibitory CSPGs. chABC has also shown some indication of synergism with other therapies, such as stem cell transplantation. Evidence supporting the use of chondroitinase as a treatment modality in nerve repair, either alone or in combination with other agents, is reviewed within. Finally, several shortcomings of chABC are addressed, notably its thermal stability and physiologic longevity - both hindering its widespread clinical adoption. Future studies are warranted in order to optimize the therapeutic benefits of the chondroitinase enzyme.

Isolation and culture of dissociated sensory neurons from chick embryos

Neurons are multifaceted cells that carry information essential for a variety of functions including sensation, motor movement, learning, and memory. Studying neurons in vivo can be challenging due to their complexity, their varied and dynamic environments, and technical limitations. For these reasons, studying neurons in vitro can prove beneficial to unravel the complex mysteries of neurons. The well-defined nature of cell culture models provides detailed control over environmental conditions and variables. Here we describe how to isolate, dissociate, and culture primary neurons from chick embryos. This technique is rapid, inexpensive, and generates robustly growing sensory neurons. The procedure consistently produces cultures that are highly enriched for neurons and has very few non-neuronal cells (less than 5%). Primary neurons do not adhere well to untreated glass or tissue culture plastic, therefore detailed procedures to create two distinct, well-defined laminin-containing substrata for neuronal plating are described. Cultured neurons are highly amenable to multiple cellular and molecular techniques, including co-immunoprecipitation, live cell imagining, RNAi, and immunocytochemistry. Procedures for double immunocytochemistry on these cultured neurons have been optimized and described here.

Proteoglycans: road signs for neurite outgrowth

Proteoglycans in the central nervous system play integral roles as "traffic signals" for the direction of neurite outgrowth. This attribute of proteoglycans is a major factor in regeneration of the injured central nervous system. In this review, the structures of proteoglycans and the evidence suggesting their involvement in the response following spinal cord injury are presented. The review further describes the methods routinely used to determine the effect proteoglycans have on neurite outgrowth. The effects of proteoglycans on neurite outgrowth are not completely understood as there is disagreement on what component of the molecule is interacting with growing neurites and this ambiguity is chronicled in an historical context. Finally, the most recent findings suggesting possible receptors, interactions, and sulfation patterns that may be important in eliciting the effect of proteoglycans on neurite outgrowth are discussed. A greater understanding of the proteoglycan-neurite interaction is necessary for successfully promoting regeneration in the injured central nervous system.

Spinal motor neurite outgrowth over glial scar inhibitors is enhanced by coculture with bone marrow stromal cells

BACKGROUND CONTEXT

Transplantation of bone marrow cells into spinal cord lesions promotes functional recovery in animal models, and recent clinical trials suggest possible recovery also in humans. The mechanisms responsible for these improvements are still unclear.

PURPOSE

To characterize spinal cord motor neurite interactions with human bone marrow stromal cells (MSCs) in an in vitro model of spinal cord injury (SCI).

STUDY DESIGN/SETTING

Previously, we have reported that human MSCs promote the growth of extending sensory neurites from dorsal root ganglia (DRG), in the presence of some of the molecules present in the glial scar, which are attributed with inhibiting axonal regeneration after SCI. We have adapted and optimized this system replacing the DRG with a spinal cord culture to produce a central nervous system (CNS) model, which is more relevant to the SCI situation.

METHODS

We have developed and characterized a novel spinal cord culture system. Human MSCs were cocultured with spinal motor neurites in substrate choice assays containing glial scar-associated inhibitors of nerve growth. In separate experiments, MSC-conditioned media were analyzed and added to spinal motor neurites in substrate choice assays.

RESULTS

As has been reported previously with DRG, substrate-bound neurocan and Nogo-A repelled spinal neuronal adhesion and neurite outgrowth, but these inhibitory effects were abrogated in MSC/spinal cord cocultures. However, unlike DRG, spinal neuronal bodies and neurites showed no inhibition to substrates of myelin-associated glycoprotein. In addition, the MSC secretome contained numerous neurotrophic factors that stimulated spinal neurite outgrowth, but these were not sufficient stimuli to promote spinal neurite extension over inhibitory concentrations of neurocan or Nogo-A.

CONCLUSIONS

These findings provide novel insight into how MSC transplantation may promote regeneration and functional recovery in animal models of SCI and in the clinic, especially in the chronic situation in which glial scars (and associated neural inhibitors) are well established. In addition, we have confirmed that this CNS model predominantly comprises motor neurons via immunocytochemical characterization. We hope that this model may be used in future research to test various other potential interventions for spinal injury or disease states.

The role of astrocytes in mediating exogenous cell-based restorative therapy for stroke

Astrocytes have not been a major therapeutic target for the treatment of stroke, with most research emphasis on the neuron. Given the essential role that astrocytes play in maintaining physiological function of the central nervous system and the very rapid and sensitive reaction astrocytes have in response to cerebral injury or ischemic insult, we propose to replace the neurocentric view for treatment with a more nuanced astrocytic centered approach. In addition, after decades of effort in attempting to develop neuroprotective therapies, which target reduction of the ischemic lesion, there are no effective clinical treatments for stroke, aside from thrombolysis with tissue plasminogen activator, which is used in a small minority of patients. A more promising therapeutic approach, which may affect nearly all stroke patients, may be in promoting endogenous restorative mechanisms, which enhance neurological recovery. A focus of efforts in stimulating recovery post stroke is the use of exogenously administered cells. The present review focuses on the role of the astrocyte in mediating the brain network, brain plasticity, and neurological recovery post stroke. As a model to describe the interaction of a restorative cell-based therapy with astrocytes, which drives recovery from stroke, we specifically highlight the subacute treatment of stroke with multipotent mesenchymal stromal cell therapy.

Comparison of sensory neuron growth cone and filopodial responses to structurally diverse aggrecan variants, in vitro

Following spinal cord injury, a regenerating neurite encounters a glial scar enriched in chondroitin sulfate proteoglycans (CSPGs), which presents a major barrier. There are two points at which a neurite makes contact with glial scar CSPGs: initially, filopodia surrounding the growth cone extend and make contact with CSPGs, then the peripheral domain of the entire growth cone makes CSPG contact. Aggrecan is a CSPG commonly used to model the effect CSPGs have on elongating or regenerating neurites. In this study, we investigated filopodia and growth cone responses to contact with structurally diverse aggrecan variants using the common stripe assay. Using time-lapse imaging with 15-s intervals, we measured growth cone area, growth cone width, growth cone length, filopodia number, total filopodia length, and the length of the longest filopodia following contact with aggrecan. Responses were measured after both filopodia and growth cone contact with five different preparations of aggrecan: two forms of aggrecan derived from bovine articular cartilage (purified and prepared using different techniques), recombinant aggrecan lacking chondroitin sulfate side chains (produced in CHO-745 cells) and two additional recombinant aggrecan preparations with varying lengths of chondroitin sulfate side chains (produced in CHO-K1 and COS-7 cells). Responses in filopodia and growth cone behavior differed between the structurally diverse aggrecan variants. Mutant CHO-745 aggrecan (lacking chondroitin sulfate chains) permitted extensive growth across the PG stripe. Filopodia contact with the CHO-745 aggrecan caused a significant increase in growth cone width and filopodia length (112.7% ± 4.9 and 150.9% ± 7.2 respectively, p<0.05), and subsequently upon growth cone contact, growth cone width remained elevated along with a reduction in filopodia number (121.9% ± 4.2; 72.39% ± 6.4, p<0.05). COS-7 derived aggrecan inhibited neurite outgrowth following growth cone contact. Filopodia contact produced an increase in growth cone area and width (126.5% ± 8.1; 150.3% ± 13.31, p<0.001), and while these parameters returned to baseline upon growth cone contact, a reduction in filopodia number and length was observed (73.94% ± 5.8, 75.3% ± 6.2, p<0.05). CHO-K1 derived aggrecan inhibited neurite outgrowth following filopodia contact, and caused an increase in growth cone area and length (157.6% ± 6.2; 117.0% ± 2.8, p<0.001). Interestingly, the two bovine articular cartilage aggrecan preparations differed in their effects on neurite outgrowth. The proprietary aggrecan (BA I, Sigma-Aldrich) inhibited neurites at the point of growth cone contact, while our chemically purified aggrecan (BA II) inhibited neurite outgrowth at the point of filopodia contact. BA I caused a reduction in growth cone width following filopodia contact (91.7% ± 2.5, p<0.05). Upon growth cone contact, there was a further reduction in growth cone width and area (66.4% ± 2.2; 75.6% ± 2.9; p<0.05), as well as reductions in filopodia number, total length, and max length (75.9% ± 5.7, p<0.05; 68.8% ± 6.0; 69.6% ± 3.5, p<0.001). Upon filopodia contact, BA II caused a significant increase in growth cone area, and reductions in filopodia number and total filopodia length (115.9% ± 5.4, p<0.05; 72.5% ± 2.7; 77.7% ± 3.2, p<0.001). In addition, filopodia contact with BA I caused a significant reduction in growth cone velocity (38.6 nm/s ± 1.3 before contact, 17.1 nm/s ± 3.6 after contact). These data showed that neuron morphology and behavior are differentially dependent upon aggrecan structure. Furthermore, the behavioral changes associated with the approaching growth cone may be predictive of inhibition or growth.

Corneal sulfated glycosaminoglycans and their effects on trigeminal nerve growth cone behavior in vitro: roles for ECM in cornea innervation

PURPOSE

Sensory trigeminal nerve growth cones innervate the cornea in a highly coordinated fashion. The purpose of this study was to determine if extracellular matrix glycosaminoglycans (ECM-GAGs), including keratan sulfate (KS), dermatan sulfate (DS), and chondroitin sulfate A (CSA) and C (CSC), polymerized in developing eyefronts, may provide guidance cues to nerves during cornea innervation.

METHODS

Immunostaining using antineuron-specific-β-tubulin and monoclonal antibodies for KS, DS, and CSA/C was performed on eyefronts from embryonic day (E) 9 to E14 and staining visualized by confocal microscopy. Effects of purified GAGs on trigeminal nerve growth cone behavior were tested using in vitro neuronal explant cultures.

RESULTS

At E9 to E10, nerves exiting the pericorneal nerve ring grew as tight fascicles, advancing straight toward the corneal stroma. In contrast, upon entering the stroma, nerves bifurcated repeatedly as they extended anteriorly toward the epithelium. KS was localized in the path of trigeminal nerves, whereas DS and CSA/C-rich areas were avoided by growth cones. When E10 trigeminal neurons were cultured on different substrates comprised of purified GAG molecules, their neurite growth cone behavior varied depending on GAG type, concentration, and mode of presentation (immobilized versus soluble). High concentrations of immobilized KS, DS, and CSA/C inhibited neurite growth to varying degrees. Neurites traversing lower, permissive concentrations of immobilized DS and CSA/C displayed increased fasciculation and decreased branching, whereas KS caused decreased fasciculation and increased branching. Enzymatic digestion of sulfated GAGs canceled their effects on trigeminal neurons.

CONCLUSIONS

Data herein suggest that GAGs may direct the movement of trigeminal nerve growth cones innervating the cornea.

Degenerate human nucleus pulposus cells promote neurite outgrowth in neural cells

Innervation of nociceptive nerve fibres into the normally aneural nucleus pulposus (NP) of the intervertebral disc (IVD) occurs during degeneration resulting in discogenic back pain. The neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), which are associated with stimulation of axonal outgrowth and nociception by neuronal cells, are both expressed by NP cells, with BDNF levels increasing with disease severity. However the mechanism of interaction between human NP cells and neural cells has yet to be fully elucidated. Therefore the aim of this study was to determine whether non-degenerate or degenerate human NP cells inhibit or stimulate neural outgrowth and whether any outgrowth is mediated by NGF or BDNF. Human NP cells from non-degenerate and degenerate IVD were cultured in alginate beads then co-cultured for 48 hours with human SH-SY5Y neuroblastoma cells. Co-culture of non-degenerate NP cells with neural cells resulted in both an inhibition of neurite outgrowth and reduction in percentage of neurite expressing cells. Conversely co-culture with degenerate NP cells resulted in an increase in both neurite length and percentage of neurite expressing cells. Addition of anti-NGF to the co-culture with degenerate cells resulted in a decrease in percentage of neurite expressing cells, while addition of anti-BDNF resulted in a decrease in both neurite length and percentage of neurite expressing cells. Our findings show that while non-degenerate NP cells are capable of inhibiting neurite outgrowth from human neural cells, degenerate NP cells stimulate outgrowth. Neurotrophin blocking studies demonstrated that both NGF and BDNF, secreted by degenerate NP cells, may play a role in this stimulation with BDNF potentially playing the predominant role. These findings suggest that NP cells are capable of regulating nerve ingrowth and that neoinnervation occurring during IVD degeneration may be stimulated by the NP cells themselves.

The roles of neuronal and glial precursors in overcoming chondroitin sulfate proteoglycan inhibition

The extension of axons through the major inhibitory component of the glial scar, chondroitin sulfate proteoglycans (CSPGs), remains a key obstacle for regeneration following spinal cord injury (SCI). We have previously shown that transplants composed of neuronal and glial restricted precursors (NRP and GRP respectively) promote regeneration and connectivity in the injured spinal cord (Bonner et al., 2010, 2011), however, little is known about the properties of these precursors at a cellular level. We now report that NRP-derived neurons, in contrast to dorsal root ganglion (DRG) neurons, have the ability to extend axons and cross over from a permissive substratum (laminin) onto inhibitory CSPG in vitro. Growth cones of neurons derived from NRP, compared to DRG, exhibit significantly lower levels of the CSPG receptors protein tyrosine phosphatase sigma (PTPσ) and leukocyte common antigen-related phosphatase (LAR). GRP-conditioned medium prepared from the same cell densities did not affect the response of primary sensory neurons to CSPG confirming that the ability of NRP-derived neurons to cross onto CSPG is determined intrinsically. However, GRP-conditioned medium collected from high density cultures increased the probability of DRG axons to cross from LN onto CSPG and increased the length of DRG axons extending on CSPG. Collectively, these results suggest that (1) neurons derived from NRPs are intrinsically insensitive to CSPGs due to low levels of receptor expression, and (2) high levels of factors secreted by GRP can reduce the inhibitory effects of CSPG and promote axonal growth. These observations provide mechanistic insights into the specific roles of NRPs and GRPs in promoting regeneration and repair following SCI.

Determination of the therapeutic time window for human umbilical cord blood mononuclear cell transplantation following experimental stroke in rats

Experimental treatment strategies using human umbilical cord blood mononuclear cells (hUCB MNCs) represent a promising option for alternative stroke therapies. An important point for clinical translation of such treatment approaches is knowledge on the therapeutic time window. Although expected to be wider than for thrombolysis, the exact time window for hUCB MNC therapy is not known. Our study aimed to determine the time window of intravenous hUCB MNC administration after middle cerebral artery occlusion (MCAO). Male spontaneously hypertensive rats underwent MCAO and were randomly assigned to hUCB MNC administration at 4, 24, 72, and 120 or 14 days. Influence of cell treatment was observed by magnetic resonance imaging on days 1, 8, and 29 following MCAO and by assessment of functional neurological recovery. On day 30, brains were screened for glial scar development and presence of hUCB MNCs. Further, influence of hUCB MNCs on necrosis and apoptosis in postischemic neural tissue was investigated in hippocampal slices cultures. Transplantation within a 72-h time window resulted in an early improvement of functional recovery, paralleled by a reduction of brain atrophy and diminished glial scarring. Cell transplantation 120 h post-MCAO only induced minor functional recovery without changes in the brain atrophy rate and glial reactivity. Later transplantation (14 days) did not show any benefit. No evidence for intracerebrally localized hUCB MNCs was found in any treatment group. In vitro hUCB MNCs were able to significantly reduce postischemic neural necrosis and apoptosis. Our results for the first time indicate a time window of therapeutic hUCB MNC application of at least 72 h. The time window is limited, but wider than compared to conventional pharmacological approaches. The data furthermore confirms that differentiation and integration of administered cells is not a prerequisite for poststroke functional improvement and lesion size reduction.

Alterations in chondroitin sulfate proteoglycan expression occur both at and far from the site of spinal contusion injury

Chondroitin sulfate proteoglycans (CSPGs) present an inhibitory barrier to axonal growth and plasticity after trauma to the central nervous system. These extracellular and membrane bound molecules are altered after spinal cord injuries, but the magnitude, time course, and patterns of expression following contusion injury have not been fully described. Western blots and immunohistochemistry were combined to assess the expression of four classically inhibitory CSPGs, aggrecan, neurocan, brevican and NG2, at the lesion site and in distal segments of cervical and thoracic spinal cord at 3, 7, 14 and 28 days following a severe mid-thoracic spinal contusion. Total neurocan and the full-length (250 kDa) isoform were strongly upregulated both at the lesion epicenter and in cervical and lumbar segments. In contrast, aggrecan and brevican were sharply reduced at the injury site and were unchanged in distal segments. Total NG2 protein was unchanged across the injury site, while NG2+ profiles were distributed throughout the lesion site by 14 days post-injury (dpi). Far from the lesion, NG2 expression was increased at lumbar, but not cervical spinal cord levels. To determine if the robust increase in neurocan at the distal spinal cord levels corresponded to regions of increased astrogliosis, neurocan and GFAP immunoreactivity were measured in gray and white matter regions of the spinal enlargements. GFAP antibodies revealed a transient increase in reactive astrocyte staining in cervical and lumbar cord, peaking at 14 dpi. In contrast, neurocan immunoreactivity was specifically elevated in the cervical dorsal columns and in the lumbar ventral horn and remained high through 28 dpi. The long lasting increase of neurocan in gray matter regions at distal levels of the spinal cord may contribute to the restriction of plasticity in the chronic phase after SCI. Thus, therapies targeted at altering this CSPG both at and far from the lesion site may represent a reasonable addition to combined strategies to improve recovery after SCI.

Student award winner in the undergraduate's degree category for the Society for Biomaterials 35th Annual Meeting, Orlando, Florida, April 13-16, 2011. Neurite growth in PEG gels: effect of mechanical stiffness and laminin concentration

Within a 3D environment, the chemical and mechanical properties of a scaffold can significantly influence nerve behavior. How these properties influence with nerve cells is important for optimizing neurite extension within a scaffold. The purpose of this study was to investigate the effect of low concentration poly(ethylene glycol) (PEG) with added laminin on 3D growth of dissociated dorsal root ganglia. Because of its high affinity for neurite adhesion and ability to promote extension, laminin was conjugated to the PEG chain, as well as mixed in the gel, at various concentrations to provide chemical cues. Gel stiffness, as determined by G*, significantly decreased with decreasing PEG concentration and with increasing laminin conjugate. Extension within the gels increased as the concentration of laminin increased with no difference between how laminin was presented (mixed or conjugated) to the cells. For example, in 3% PEG, extension increased from 92.29 ± 5.27 μm to 146.35 ± 13.12 μm as laminin conjugate concentration increased from plain to 100 μg/ml. Results indicated that the chemical properties of the scaffold influenced neurite growth more than the mechanical properties as laminin concentration had a greater impact on growth than the stiffness of the gel over the range studied. Neurite length as a function of scaffold stiffness and adhesion properties was also characterized and demonstrated a positive linear relationship between rate of neurite extension and laminin concentration. This study further demonstrates the importance of characterizing interactions between cell behavior and the chemical and mechanical environment.

Neurite outgrowth of neural progenitors in presence of inhibitory proteoglycans

Attempts to promote host regeneration after spinal cord injury (SCI) have often resulted in poor axon extension due to formation of a glial scar, which creates a dense physical barrier around the injury and contains molecules that inhibit regeneration and repair of adult injured axons. Previous studies have shown that, while transplants of multipotent neural stem cells (NSC) integrate poorly in the injury site, the use of neuronal-restricted precursor cells (NRP) together with glial-restricted precursor cells (GRP) allow differentiation and integration of neurons, possibly because NRP are able to overcome chondroitin sulfate proteoglycan (CSPG) inhibition. To investigate this possibility, we grew mixed cultures of NRP/GRP on CSPG at inhibitory concentrations, using embryonic hippocampal cultures as controls. We found that NRP/GRP grown on CSPG survive and differentiate into neurons with no significant changes in neurite length, relative to growth on control polylysine substrate, and in contrast to a significant inhibition of axon growth in hippocampal cultures grown on CSPG-coated substrate. There was, however, a significant decrease in neurite number and branching in both cultures, indicating that CSPG also has important effects on neuronal morphology. These data suggest that embryonic neurons supported by glial cells derived from NRP/GRP transplants are less sensitive to inhibitory effects of CSPG in the glial scar, and are thus an appropriate source for neuronal cell replacement and reconnection of damaged circuits after SCI.

Chondroitin sulfate-binding peptides block chondroitin 6-sulfate inhibition of cortical neurite growth

Chondroitin sulfate (CS) expression is increased in the glial scar following spinal cord injury demonstrating the importance understanding the role of CS in the central nervous system (CNS). There have been conflicting studies on the effects of the most abundant types of CS, chondroitin 4-sulfate (C4S) and chondroitin 6-sulfate (C6S), found in the CNS. In this study, the effects of C4S and C6S on rat embryonic day 18 cortical neurons were investigated. C4S had no effect on neuron behavior whereas C6S inhibited neurite outgrowth at higher concentrations (>10mug/ml). Two C6S-binding peptides (C6S-1 and C6S-2) were tested for their ability to block the inhibitory activity of C6S on neurite outgrowth. Neurons cultured with C6S and C6S-binding peptide at higher peptide concentrations had neurite lengths similar to neurons cultured without C6S. Therefore, the C6S-binding peptides were effective at blocking the inhibitory activity of C6S. The C6S-1 peptide had a higher binding affinity than the C6S-2 peptide and was consequently more effective at blocking C6S inhibition of neurite growth. To date, this is the first study to employ an alternative strategy from enzymatic digestion of CS chains to increase neurite outgrowth. These studies warrant further investigation of the use of C6S-binding peptides to increase nerve regeneration following spinal cord injury.

Muscle contractions guide rohon-beard peripheral sensory axons

Multiple molecular cues guide neuronal axons to their targets during development. Previous studies in vitro have shown that mechanical stimulation also can affect axon growth; however, whether mechanical force contributes to axon guidance in vivo is unknown. We investigated the role of muscle contractions in the guidance of zebrafish peripheral Rohon-Beard (RB) sensory axons in vivo. We analyzed several mutants that affect muscle contraction through different molecular pathways, including a new mutant allele of the titin a (pik) gene, mutants that affect the hedgehog signaling pathway, and a nicotinic acetylcholine receptor mutant. We found RB axon defects in these mutants, the severity of which appeared to correlate with the extent of muscle contraction loss. These axons extend between the muscle and skin and normally have ventral trajectories and repel each other on contact. RB peripheral axons in muscle mutants extend longitudinally instead of ventrally, and the axons fail to repel one another on contact. In addition, we showed that limiting muscle movements by embedding embryos in agarose caused similar defects in peripheral RB axon guidance. This work suggests that the mechanical forces generated by muscle contractions are necessary for proper sensory axon pathfinding in vivo.

Corticospinal neurons respond differentially to neurotrophins and myelin-associated glycoprotein in vitro

Elucidating the mechanisms that regulate the survival and outgrowth of corticospinal tract (CST) neurons and other CNS tracts will be a key component in developing novel approaches for the treatment of central nervous system (CNS) disorders, including stroke, spinal cord injury (SCI), and motor neuron disease (MND). However, the in vivo complexities of these diseases make a systematic evaluation of potential therapeutics that directly affect corticospinal regeneration or survival very challenging. Here, we use Thy1.2 transgenic mice expressing yellow fluorescent protein (YFP) in postnatal day 8 (P8) corticospinal neurons, as a source of CST neurons that have already established synapses in the spinal cord, to assess factors that influence neurite outgrowth and survival of axotomized CST neurons. After culture, YFP-positive corticospinal neurons represent an enriched neuronal population over other glia and interneurons, survive, and extend processes over time. YFP-positive CST neurons also continue to express the corticospinal markers CTIP2 and Otx1. CST neurons display different degrees of axon extension, dendritic branch length and elaboration, and neurite elongation in response to neurotrophin-3 and ciliary neurotrophic factor, and an inhibitory outgrowth response when cultured on myelin-associated glycoprotein. Some CST neurons are lost with extended culture, which provides a baseline from which we can also assess factors that enhance CST neuron survival. This assay thus allows us to assess independent aspects of CST axonal and dendritic outgrowth kinetics, which allows for the rapid and sensitive investigation of new therapies to address corticospinal neuron outgrowth in the context of CNS injury and neurodegenerative disorders.

A new in vitro model of the glial scar inhibits axon growth

Astrocytes respond to central nervous system (CNS) injury with reactive astrogliosis and participate in the formation of the glial scar, an inhibitory barrier for axonal regeneration. Little is known about the injury-induced mechanisms underlying astrocyte reactivity and subsequent development of an axon-inhibitory scar. We combined two key aspects of CNS injury, mechanical trauma and co-culture with meningeal cells, to produce an in vitro model of the scar from cultures of highly differentiated astrocytes. Our model displayed widespread morphological signs of astrocyte reactivity, increases in expression of glial fibrillary acidic protein (GFAP), and accumulation of GFAP in astrocytic processes. Expression levels of scar-associated markers, phosphacan, neurocan, and tenascins, were also increased. Importantly, neurite growth from various CNS neuronal populations was significantly reduced when neurons were seeded on the scar-like cultures, compared with growth on cultures of mature astrocytes. Quantification of neurite growth parameters on the scar model demonstrated significant reductions in neuronal adhesion and neurite lengths. Interestingly, neurite outgrowth of postnatal neurons was reduced to a greater extent than that of embryonic neurons, and outgrowth inhibition varied among neuronal populations. Scar-like reactive sites and neurite-inhibitory patches were found throughout these cultures, creating a patchwork of growth-inhibitory areas mimicking a CNS injury site. Thus, our model showed relevant aspects of scar formation and produced widespread inhibition of axonal regeneration; it should be useful both for examining mechanisms underlying scar formation and to assess various treatments for their potential to improve regeneration after CNS injury. (c) 2008 Wiley-Liss, Inc.

GM-CSF inhibits glial scar formation and shows long-term protective effect after spinal cord injury

OBJECT

This study investigated the effects of granulocyte macrophage-colony stimulating factor (GM-CSF) on the scar formation and repair of spinal cord tissues in rat spinal cord injury (SCI) model.

METHODS

Sprague-Dawley male rats (8 weeks old) were randomly divided into the sham-operated group, spinal cord injury group, and injury with GM-CSF treated group. A spinal cord injury was induced at T9/10 levels of rat spinal cord using a vascular clip. GM-CSF was administrated via intraperitoneal (IP) injection or on the dural surface using Gelfoam at the time of SCI. The morphological changes, tissue integrity, and scar formation were evaluated until 4 weeks after SCI using histological and immunohistochemical analyses.

RESULTS

The administration of GM-CSF either via IP injection or local treatment significantly reduced the cavity size and glial scar formation at 3-4 weeks after SCI. GM-CSF also reduced the expression of core proteins of chondroitin sulfate proteoglycans (CSPGs) such as neurocan and NG2 but not phosphacan. In particular, an intensive expression of glial fibriallary acidic protein (GFAP) and neurocan found around the cavity at 4 weeks was obviously suppressed by GM-CSF. Immunostaining for neurofilament (NF) and Luxol fast blue (LFB) showed that GM-CSF preserved well the axonal arrangement and myelin structure after SCI. The expression of GAP-43, a marker of regenerating axons, also apparently increased in the rostral grey matter by GM-CSF.

CONCLUSION

These results suggest that GM-CSF could enhance long-term recovery from SCI by suppressing the glial scar formation and enhancing the integrity of axonal structure.

The influence of sub-micron inhibitory clusters on growth cone substratum attachments and CD44 expression

Proteoglycan expression patterns in the central nervous system guide neuronal pathfinding during development, but also disrupt regeneration after injuries. To deepen our understanding of the molecular level effects of proteoglycan spatial arrangements on neuronal pathfinding, we designed micropatterning stamps for the precise placement of near single molecule chondroitin sulfate proteoglycan (CSPG) clusters into regularly spaced arrays. Actin ultrastructural analysis in dorsal root ganglion neurons grown on laminin-coated substrata patterned with aggrecan cluster arrays revealed filopodial and lamellapodial edge contact avoidance of individual clusters, while growth cone lamellapodia and central domains were able to span multiple clusters over a range of cluster densities. Total internal reflection fluorescence microscopy interrogation of growth cone substratum morphology further revealed persistence of integrin mediated substratum adhesion and local out-of-plane membrane bending over clusters on the height scale of aggrecan glycosaminoglycan side chains. Direct imaging of cell adhesion molecule CD44 expression in growth cones revealed an aggrecan dose dependent upregulation in CD44 molecules. Evidence of CD44 clustering coinciding with underlying aggrecan molecules implies CSPG-CD44 interactions. The results reveal the limited local repulsive effect of CSPGs on neuronal structures and provide evidence that CD44 upregulation in neurons is affected by local CSPG expression.

Decorin promotes robust axon growth on inhibitory CSPGs and myelin via a direct effect on neurons

Inhibitory chondroitin sulfate proteoglycans (CSPGs) and myelin-associated molecules are major impediments to axon regeneration within the adult central nervous system (CNS). Decorin infusion can however suppress the levels of multiple inhibitory CSPGs and promote axon growth across spinal cord injuries [Davies, J.E., Tang, X., Denning, J.W., Archibald, S.J., and Davies, S.J., 2004. Decorin suppresses neurocan, brevican, phosphacan and NG2 expression and promotes axon growth across adult rat spinal cord injuries. Eur. J. Neurosci. 19, 1226-1242]. A question remained as to whether decorin can also increase axon growth on inhibitory CSPGs and myelin via a direct effect on neurons. We have therefore conducted an in vitro analysis of neurite extension by decorin-treated adult dorsal root ganglion (DRG) neurons cultured on substrates of inhibitory CSPGs or myelin membranes mixed with laminin. Decorin treatment promoted 14.5 and 5-fold increases in average neurite length/neuron over untreated controls on CSPGs or myelin membranes respectively. In addition to suppressing inhibitory scar formation, our present data shows that decorin can directly boost the ability of neurons to extend axons within CSPG or myelin rich environments.

Aggrecan components differentially modulate nerve growth factor-responsive and neurotrophin-3-responsive dorsal root ganglion neurite growth

Aggrecan is one of the major chondroitin sulfate proteoglycans (CSPGs) expressed in the central nervous system. The signaling pathways activated downstream of cell interaction with aggrecan and with CSPGs in general and the importance of chondroitin sulfate-glycosaminoglycan side chains in their inhibition are unclear. Therefore, to analyze the effect of different components of aggrecan in inhibiting neurite growth, neurite outgrowth was quantified in an in vitro model in which chick dorsal root ganglion (DRG) explants were grown on substrates containing aggrecan bound to hyaluronan and link protein as a macromolecular aggregate, aggrecan monomers, hyaluronan, or ChABC-treated aggrecan. Aggrecan aggregate, aggrecan monomer, and hyaluronan inhibited neurite outgrowth from nerve growth factor (NGF)- and neurotrophin-3 (NT3)-responsive DRG neurons. Aggrecan inhibition was dependent on its chondroitin sulfate-glycosaminoglycans, as ChABC digestion alleviated neurite inhibition because of aggrecan. Growth cones displayed full or partial collapse on aggrecan aggregate, hyaluronan, and ChABC-treated aggrecan. Inhibition of Rho kinase (ROCK) with Y27632 increased neurite growth on some but not all of the aggrecan components tested. With NGF in the culture medium, Y27632 increased neurite outgrowth on aggrecan aggregate, monomers, and ChABC-treated aggrecan, but not on hyaluronan. The ROCK inhibitor also increased NT3-responsive outgrowth on aggrecan aggregate and hyaluronan, but not on ChABC-treated aggrecan. This study showed that the matrix proteoglycan aggrecan and its components have multiple effects on neurite outgrowth and that some of these effects involve the Rho/ROCK pathway.

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.

Multimodal signaling by the ADAMTSs (a disintegrin and metalloproteinase with thrombospondin motifs) promotes neurite extension

Aggregating proteoglycans (PG) bearing chondroitin sulfate (CS) side chains associate with hyaluronan and various secreted proteins to form a complex of extracellular matrix (ECM) that inhibits neural plasticity in the central nervous system (CNS). Chondroitinase treatment depletes PGs of their CS side chains and enhances neurite extension. Increasing evidence from in vivo models indicates that proteolytic cleavage of the PG core protein by members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of glutamyl-endopeptidases also promotes neural plasticity. The purpose of this study was to determine whether proteolytic action of the ADAMTSs influences neurite outgrowth in cultured neurons. Transfection of primary rat neurons with ADAMTS4 cDNA induced longer neurites, whether the neurons were grown on a monolayer of astrocytes that secrete inhibitory PGs or on laminin/poly-L-lysine substrate alone. Similar results were found when neurons were transfected with a construct encoding a proteolytically inactive, point mutant of ADAMTS4. Addition of recombinant ADAMTS4 or ADAMTS5 protein to immature neuronal cultures also enhanced neurite extension in a dose-dependent manner, an effect demonstrated to be dependent on the activation of MAP ERK1/2 kinase. These results suggest that ADAMTS4 enhances neurite outgrowth via a mechanism that does not require proteolysis but is dependent on activation of the MAP kinase cascade. Thus a model to illustrate multimodal ADAMTS activity would entail proteolysis of CS-bearing PGs to create a loosened matrix environment more favorable for neurite outgrowth, and enhanced neurite outgrowth directly stimulated by ADAMTS signaling at the cell surface.

The effects of proteoglycan surface patterning on neuronal pathfinding

Protein micropatterning techniques are increasingly applied in cell choice assays to investigate fundamental biological phenomena that contribute to the host response to implanted biomaterials, and to explore the effects of protein stability and biological activity on cell behavior for in vitro cell studies. In the area of neuronal regeneration the protein micropatterning and cell choice assays are used to improve our understanding of the mechanisms directing nervous system during development and regenerative failure in the central nervous system (CNS) wound healing environment. In these cell assays, protein micropatterns need to be characterized for protein stability, bioactivity, and spatial distribution and then correlated with observed mammalian cell behavior using appropriate model system for CNS development and repair. This review provides the background on protein micropatterning for cell choice assays and describes some novel patterns that were developed to interrogate neuronal adaptation to inhibitory signals encountered in CNS injuries.

Analysis of axonal regeneration in the central and peripheral nervous systems of the NG2-deficient mouse

Background

The chondroitin sulphate proteoglycan NG2 blocks neurite outgrowth in vitro and has been proposed as a major inhibitor of axonal regeneration in the CNS. Although a substantial body of evidence underpins this hypothesis, it is challenged by recent findings including strong expression of NG2 in regenerating peripheral nerve.

Results

We studied axonal regeneration in the PNS and CNS of genetically engineered mice that do not express NG2, and in sex and age matched wild-type controls. In the CNS, we used anterograde tracing with BDA to study corticospinal tract (CST) axons after spinal cord injury and transganglionic labelling with CT-HRP to trace ascending sensory dorsal column (DC) axons after DC lesions and a conditioning lesion of the sciatic nerve. Injury to these fibre tracts resulted in no difference between knockout and wild-type mice in the ability of CST axons or DC axons to enter or cross the lesion site. Similarly, after dorsal root injury (with conditioning lesion), most regenerating dorsal root axons failed to grow across the dorsal root entry zone in both transgenic and wild-type mice.

Following sciatic nerve injuries, functional recovery was assessed by analysis of the toe-spreading reflex and cutaneous sensitivity to Von Frey hairs. Anatomical correlates of regeneration were assessed by: retrograde labelling of regenerating dorsal root ganglion (DRG) cells with DiAsp; immunostaining with PGP 9.5 to visualise sensory reinnervation of plantar hindpaws; electron microscopic analysis of regenerating axons in tibial and digital nerves; and by silver-cholinesterase histochemical study of motor end plate reinnervation. We also examined functional and anatomical correlates of regeneration after injury of the facial nerve by assessing the time taken for whisker movements and corneal reflexes to recover and by retrograde labelling of regenerated axons with Fluorogold and DiAsp. None of the anatomical or functional analyses revealed significant differences between wild-type and knockout mice.

Conclusion

These findings show that NG2 is unlikely to be a major inhibitor of axonal regeneration after injury to the CNS, and, further, that NG2 is unlikely to be necessary for regeneration or functional recovery following peripheral nerve injury.

Bone marrow stromal cells stimulate neurite outgrowth over neural proteoglycans (CSPG), myelin associated glycoprotein and Nogo-A

In animal models, transplantation of bone marrow stromal cells (MSC) into the spinal cord following injury enhances axonal regeneration and promotes functional recovery. How these improvements come about is currently unclear. We have examined the interaction of MSC with neurons, using an established in vitro model of nerve growth, in the presence of substrate-bound extracellular molecules that are thought to inhibit axonal regeneration, i.e., neural proteoglycans (CSPG), myelin associated glycoprotein (MAG) and Nogo-A. Each of these molecules repelled neurite outgrowth from dorsal root ganglia (DRG) in a concentration-dependent manner. However, these nerve-inhibitory effects were much reduced in MSC/DRG co-cultures. Video microscopy demonstrated that MSC acted as "cellular bridges" and also "towed" neurites over the nerve-inhibitory substrates. Whereas conditioned medium from MSC cultures stimulated DRG neurite outgrowth over type I collagen, it did not promote outgrowth over CSPG, MAG or Nogo-A. These findings suggest that MSC transplantation may promote axonal regeneration both by stimulating nerve growth via secreted factors and also by reducing the nerve-inhibitory effects of the extracellular molecules present.

Dynamic role of kallikrein 6 in traumatic spinal cord injury

Kallikrein 6 (K6) is a member of the kallikrein gene family that comprises 15 structurally and functionally related serine proteases. In prior studies we showed that, while this trypsin-like enzyme is preferentially expressed in neurons and oligodendroglia of the adult central nervous system (CNS), it is up-regulated at sites of injury due to expression by infiltrating immune and resident CNS cells. Given this background we hypothesized that K6 is a key contributor to the pathophysiology of traumatic spinal cord injury (SCI), influencing neural repair and regeneration. Examination of K6 expression following contusion injury to the adult rat cord, and in cases of human traumatic SCI, indicated significant elevations at acute and chronic time points, not only at the injury site but also in cord segments above and below. Elevations in K6 were particularly prominent in macrophages, microglia and reactive astrocytes. To determine potential effects of elevated K6 on the regeneration environment, the ability of neurons to adhere to and extend processes on substrata which had been exposed to recombinant K6 was examined. Limited (1 h) or excess (24 h) K6-mediated proteolytic digestion of a growth-facilitatory substrate, laminin, significantly decreased neurite outgrowth. By contrast, similar hydrolysis of a growth-inhibitory substrate, aggrecan, significantly increased neurite extension and cell adherence. These data support the hypothesis that K6 enzymatic cascades mediate events secondary to spinal cord trauma, including dynamic modification of the capacity for axon outgrowth.

Membrane-bound CSPG mediates growth cone outgrowth and substrate specificity by Schwann cell contact with the DRG neuron cell body and not via growth cone contact

The central nervous system and peripheral nervous system (CNS/PNS) contain factors that inhibit axon regeneration, including myelin-associated glycoprotein (MAG), the Nogo protein, and chondroitin sulfate proteoglycan (CSPG). They also contain factors that promote axon regeneration, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Axon regeneration into and within the CNS fails because the balance of factor favors inhibiting regeneration, while in the PNS, the balance of factor favors promoting regeneration. The balance of influences in the CNS can be shifted toward promoting axon regeneration by eliminating the regeneration-inhibiting factors, overwhelming them with regeneration-promoting factors, or making axon growth cones non-receptive to regeneration-inhibiting factors. The present in vitro experiments, using adult rat dorsal root ganglion (DRG) neurons, were designed to determine whether the regeneration-inhibiting influences of Schwann cell CSPG are mediated via Schwann cell membrane contact with the DRG neuron cell body or their growth cones. The average longest neurite of neurons in cell body contact with Schwann cells was 7.4-fold shorter than those of neurons without Schwann cell-neuron cell body contact (naked neurons), and the neurites showed substrate specificity, growing only on the Schwann cell membranes and not extending onto the laminin substrate. The neurites of naked neurons showed no substrate specificity and extended over the laminin substrate, as well as onto and off the Schwann cells. After digesting the Schwann cell CSPG with the enzyme C-ABC, neurons in cell body contact with Schwann cells extended neurites the same length as those of naked neurons, and their neurites showed no substrate selectivity. Further, the neurites of naked neurons were not longer than those of naked neurons not exposed to C-ABC. These data indicate that the extent of neurite outgrowth from adult rat DRG neurons and substrate specificity of their growth cone is mediated via contact between the Schwann cell membrane-bound CSPG and the DRG neuron cell body and not with their growth cones. Further, there was no apparent influence of diffusible or substrate-bound CSPG on neurite outgrowth. These results show that eliminating the CSPG of Schwann cells in contact with the cell body of DRG neurons eliminates the sensitivity of their growth cones to the CSPG-induced outgrowth inhibition. This may in turn allow the axons of these neurons to regenerate through the dorsal roots and into the spinal cord.

Chondroitinase ABC combined with neural stem/progenitor cell transplantation enhances graft cell migration and outgrowth of growth-associated protein-43-positive fibers after rat spinal cord injury

We previously reported that the transplantation of neural stem/progenitor cells (NSPCs) can contribute to the repair of injured spinal cord in adult rats and monkeys. In some cases, however, most of the transplanted cells adhered to the cavity wall and failed to migrate and integrate into the host spinal cord. In this study we focused on chondroitin sulfate proteoglycan (CSPG), a known constituent of glial scars that is strongly expressed after spinal cord injury (SCI), as a putative inhibitor of NSPC migration in vivo. We hypothesized that the digestion of CSPG by chondroitinase ABC (C-ABC) might promote the migration of transplanted cells and neurite outgrowth after SCI. An in vitro study revealed that the migration of NSPC-derived cells was inhibited by CSPG and that this inhibitory effect was attenuated by C-ABC pre-treatment. Consistently, an in vivo study of C-ABC treatment combined with NSPC transplantation into injured spinal cord revealed that C-ABC pre-treatment promoted the migration of the transplanted cells, whereas CSPG-immunopositive scar tissue around the lesion cavity prevented their migration into the host spinal cord in the absence of C-ABC pre-treatment. Furthermore, this combined treatment significantly induced the outgrowth of a greater number of growth-associated protein-43-positive fibers at the lesion epicentre, compared with NSPC transplantation alone. These findings suggested that the application of C-ABC enhanced the benefits of NSPC transplantation for SCI by reducing the inhibitory effects of the glial scar, indicating that this combined treatment may be a promising strategy for the regeneration of injured spinal cord.

Outgrowth of neurites from NIE-115 neuroblastoma cells is prevented on repulsive substrates through the action of PAK

In the central nervous system (CNS), damaged axons are inhibited from regeneration by glial scars, where secreted chondroitin sulfate proteoglycan (CSPG) and tenascin repulse outgrowth of neurites, the forerunners of axons and dendrites. During differentiation, these molecules are thought to form boundaries for guiding neurons to their correct targets. In neuroblastoma NIE-115 cells, outgrowth of neurites on laminin could be induced by serum starvation or inhibition of RhoA by Clostridium botulinum C3 toxin. The outgrowing neurites avoided crossing onto the repulsive substrate CSPG or tenascin. This avoidance response was partially overcome on expression of membrane-targeted and kinase-inactive forms of PAK. In these cells, the endogenous PAK isoforms colocalized with actin in distinctive sites, alphaPAK in the cell center as small clusters and along the neurite shaft and betaPAK and gammaPAK in areas with membrane ruffles and filopodia, respectively. When isoform-specific N-terminal PAK sequences were introduced to interfere with PAK function, substantially more neurites crossed onto CSPG when cells contained a gammaPAK-derived peptide but not the corresponding alphaPAK- or betaPAK-derived peptide. Thus, while neurite outgrowth can be promoted by RhoA inhibition, overcoming the accompanying repulsive guidance response will require modulation of PAK activity. These results have therapeutic implications for CNS repair processes.

Chondroitinase ABCI improves locomotion and bladder function following contusion injury of the rat spinal cord

Chondroitin sulfate proteoglycans are synthesized and deposited in the spinal cord following injury. These proteoglycans may restrict regeneration and plasticity and contribute to the limited recovery seen after an injury. Chondroitinase, a bacterial enzyme that catalyzes the hydrolysis of the chondroitin chains on proteoglycans, has been shown to improve motor and sensory function following partial transection lesions of the spinal cord. To assess the effects of chondroitinase in a clinically relevant model of spinal cord injury, 128 female Long-Evans rats received either a severe, moderate, or mild contusion injury at the vertebral level T9/T10 with a forceps model and were treated for 2 weeks with chondroitinase ABCI at 0.06 Units per dose, penicillinase, or vehicle control via an intrathecal catheter placed near the injury. Motor behavior was measured by open-field testing of locomotion and bladder function monitored by measuring daily residual urine volumes. Animals treated with chondroitinase showed significant improvements in open-field locomotor activity as measured by the Basso, Beattie and Bresnahan scoring system after both severe and moderate SCI (p<0.05 and 0.01, respectively). No significant locomotor differences were observed in the mild injury group. In the moderate injury group, residual urine volumes were reduced with chondroitinase treatment by 2 weeks after injury (p<0.05) and in the severe injury group, by 6 weeks after injury (NS). These results demonstrate that chondroitinase is effective at promoting both somatic and autonomic motor recovery following a clinically relevant contusion spinal cord injury and is a candidate as a therapeutic for human spinal cord injury.

Cellular and molecular tunnels surrounding the forebrain commissures of human fetuses

Glial cells and extracellular matrix (ECM) molecules surround developing fiber tracts and are implicated in axonal pathfinding. These and other molecules are produced by these strategically located glial cells and have been shown to influence axonal growth across the midline in rodents. We searched for similar cellular and molecular structures surrounding the telencephalic commissures of fetal human brains. Paraffin-embedded brain sections were immunostained for glial fibrillary acidic protein (GFAP) and vimentin (VN) to identify glial cells; for microtubule-associated protein-2 (MAP-2) and neuronal nuclear protein (NeuN) to document neurons; for neurofilament (NF) to identify axons; and for chondroitin sulfate (CS), tenascin (TN), and fibronectin (FN) to show the ECM. As in rodents, three cellular clusters surrounding the corpus callosum were identified by their expression of GFAP and VN (but not MAP-2 or NeuN) from 13 to at least 18 weeks postovulation (wpo): the glial wedge, the glia of the indusium griseum, and the midline sling. CS and TN (but not FN) were expressed pericellularly in these cell groups. The anterior commissure was surrounded by a GFAP+/VN+ glial tunnel from 12 wpo, with TN expression seen between the GFAP+ cell bodies. The fimbria showed GFAP+/VN+ cells at its lateral and medial borders from 12 wpo, with pericellular expression of CS. The fornix showed GFAP+ cells somewhat later (16 wpo). Because these structures are similar to those described for rodents, we concluded that the axon guiding mechanisms postulated for commissural formation in nonhuman mammals may also be operant in the developing human brain.

Proteoglycans and injury of the central nervous system

Proteoglycan is a family of glycoproteins which carry covalently-linked glycosaminoglycan chains, such as chondroitin sulfate and heparan sulfate. Proteoglycans are believed to play important roles in morphogenesis and maintenance of various tissues including the central nervous system (CNS) through interactions with cell adhesion molecules and growth factors. In the CNS, a significant amount of evidence has been accumulated to show that proteoglycans function as modulators in various cellular events not only in the development, but also in the pathogenesis of neuronal diseases and lesions. When the CNS is injured, several chondroitin sulfate proteoglycans (CSPG) are up-regulated in glial scars formed around the lesion site. The glial scar also contains some molecules inhibitory to axonal growth, such as myelin-associated glycoprotein, Nogo, and Semaphorin. In vitro studies revealed that CSPG largely exert a repulsive effect on axonal regeneration, and a signal from CSPG modulates the actin cytoskeleton of outgrowing neurites through the Rho/ROCK pathway. These findings suggest that CSPG are responsible for unsuccessful axonal regeneration in glial scars. Various attempts to overcome the inhibitory effect of CSPG have been pursued in vivo. Digestion of chondroitin sulfate chains by chondroitinase ABC, suppression of CSPG core protein synthesis by decorin, suppression of glycosaminoglycan chain synthesis by a DNA enzyme, and inhibition of the Rho/ROCK pathway with specific inhibitors were all successful for increasing axonal regeneration. For a clinical application, the most effective combination of these treatments needs to be examined in the future.

Immunolocalization of laminin-alpha1-like antigens around synapses in mouse cerebellar perineuronal nets

The hypothesis that extracellular matrix components may be related to neuronal development in the mouse cerebellar cortex was verified with immunohistochemistry by using an antibody against laminin-alpha1, a major extracellular matrix protein in various tissues. A commercially available polyclonal antibody, raised against the carboxyl-terminal 20-amino acid peptide of laminin-alpha1 was used. Some positive immunoreaction products were localized around large GABAergic interneurons in granular layers and others were around neurons in deep cerebellar nuclei. At the electron microscope level, diaminobenzidine immunoreaction products were localized around presynaptic boutons and in intercellular matrices around interneurons. Such immunoreaction products could be detected at postnatal day 20, when most of cerebellar synapses are assumed to be established. It has been known that a special feature of extracellular matrix, termed perineuronal nets, exists around specific subpopulation of neurons. In the mouse cerebellum, the present findings suggest that laminin itself or laminin-like-antigens exists in the perineuronal nets in relation to inhibitory neuron synapses.