Role of extracellular factors in axon regeneration in the CNS: implications for therapy

Co-transplantation of novel Nano-SDF scaffold with human neural stem cells attenuates inflammatory responses and apoptosis in traumatic brain injury

Traumatic brain injury (TBI) causes long-term disability and mortality worldwide. The prime pathological players in TBI are neuroinflammation and apoptosis. These pathological changes lead to a limited capacity of regeneration after TBI. To alleviate inflammatory responses and apoptosis triggered by TBI, developing bioactive scaffolds conjoined with stem cells is a decisive approach in neural tissue engineering. The aim of this study was to fabricate a novel nano-scaffold made of RADA-16 with a bioactive motif of stromal cell-derived factor-1 α (SDF-1α) and evaluate its effects with stem cell transplantation on inflammatory pathways, reactive gliosis, and apoptosis after TBI. Co-transplantation of Nano-SDF and human neural stem cells (hNSCs) derived from fetus brain in adult rats subjected to TBI led to the improvement of motor activitycompared with the control group. The treated animals with hNSCs + Nano-SDF had a significantly lower expression of toll-like receptor 4 and nuclear factor-kappa B at the injury site than the control animals. A significant reduction in the number of reactive astrocytes was also observed in rats that received hNSCs + Nano-SDF compared with the vehicle and Nano-SDF groups. Furthermore, the TUNEL assay indicated a significant reduction in TUNEL positive cells in the hNSCs + Nano-SDF group compared with the TBI, vehicle, and Nano-SDF groups. These data demonstrated co-transplantation of hNSCs with Nano-SDF can reduce inflammatory responses and cell death after TBI via creating a more supportive microenvironment. Further research is required to establish the therapeutic efficacy of Nano-SDF with stem cells for TBI.

ARF6 and Rab11 as intrinsic regulators of axon regeneration

Adult central nervous system (CNS) axons do not regenerate after injury because of extrinsic inhibitory factors, and a low intrinsic capacity for axon growth. Developing CNS neurons have a better regenerative ability, but lose this with maturity. This mini-review summarises recent findings which suggest one reason for regenerative failure is the selective distribution of growth machinery away from axons as CNS neurons mature. These studies demonstrate roles for the small GTPases ARF6 and Rab11 as intrinsic regulators of polarised transport and axon regeneration. ARF6 activation prevents the axonal transport of integrins in Rab11 endosomes in mature CNS axons. Decreasing ARF6 activation permits axonal transport, and increases regenerative ability. The findings suggest new targets for promoting axon regeneration after CNS injury.

Astrocytic YAP Promotes the Formation of Glia Scars and Neural Regeneration after Spinal Cord Injury

Yes-associated protein (YAP) transcriptional coactivator is negatively regulated by the Hippo pathway and functions in controlling the size of multiple organs, such as liver during development. However, it is not clear whether YAP signaling participates in the process of the formation of glia scars after spinal cord injury (SCI). In this study, we found that YAP was upregulated and activated in astrocytes of C57BL/6 male mice after SCI in a Hippo pathway-dependent manner. Conditional knockout (KO) of yap in astrocytes significantly inhibited astrocytic proliferation, impaired the formation of glial scars, inhibited the axonal regeneration, and impaired the behavioral recovery of C57BL/6 male mice after SCI. Mechanistically, the bFGF was upregulated after SCI and induced the activation of YAP through RhoA pathways, thereby promoting the formation of glial scars. Additionally, YAP promoted bFGF-induced proliferation by negatively controlling nuclear distribution of p27^Kip1 mediated by CRM1. Finally, bFGF or XMU-MP-1 (an inhibitor of Hippo kinase MST1/2 to activate YAP) injection indeed activated YAP signaling and promoted the formation of glial scars and the functional recovery of mice after SCI. These findings suggest that YAP promotes the formation of glial scars and neural regeneration of mice after SCI, and that the bFGF-RhoA-YAP-p27^Kip1 pathway positively regulates astrocytic proliferation after SCI.SIGNIFICANCE STATEMENT Glial scars play critical roles in neuronal regeneration of CNS injury diseases, such as spinal cord injury (SCI). Here, we provide evidence for the function of Yes-associated protein (YAP) in the formation of glial scars after SCI through regulation of astrocyte proliferation. As a downstream of bFGF (which is upregulated after SCI), YAP promotes the proliferation of astrocytes through negatively controlling nuclear distribution of p27^Kip1 mediated by CRM1. Activation of YAP by bFGF or XMU-MP-1 injection promotes the formation of glial scar and the functional recovery of mice after SCI. These results suggest that the bFGF-RhoA-YAP-p27^Kip1 axis for the formation of glial scars may be a potential therapeutic strategy for SCI patients.

PTEN modulates neurites outgrowth and neuron apoptosis involving the PI3K/Akt/mTOR signaling pathway

The present study aimed to explore the role of the PTEN/Akt/mTOR signaling pathway in the neurite outgrowth and apoptosis of cortical neurons. Cortical neurons were seeded on or adjacent to chondroitin sulfate proteoglycans. The length, number and crossing behavior of the neurites were calculated. Immunohistochemical staining and TUNEL data were analyzed. Neurites treated with PTEN inhibitor exhibited significant enhancements in elongation, initiation and crossing abilities when they encountered chondroitin sulfate proteoglycans in vitro. These effects disappeared when the PTEN/Akt/mTOR signaling pathway was blocked. Neurons exhibited significant enhancements in survival ability following PTEN inhibition. The present study demonstrated that PTEN inhibition can promote axonal elongation and initiation in cerebral cortical neurons, as well as the ability to cross the chondroitin sulfate proteoglycan border. In addition, PTEN inhibition is useful for protecting the neuron from apoptosis. The PTEN/Akt/mTOR signaling pathway is an important signaling pathway.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Integrins promote axonal regeneration after injury of the nervous system

Integrins are cell surface receptors that form the link between extracellular matrix molecules of the cell environment and internal cell signalling and the cytoskeleton. They are involved in several processes, e.g. adhesion and migration during development and repair. This review focuses on the role of integrins in axonal regeneration. Integrins participate in spontaneous axonal regeneration in the peripheral nervous system through binding to various ligands that either inhibit or enhance their activation and signalling. Integrin biology is more complex in the central nervous system. Integrins receptors are transported into growing axons during development, but selective polarised transport of integrins limits the regenerative response in adult neurons. Manipulation of integrins and related molecules to control their activation state and localisation within axons is a promising route towards stimulating effective regeneration in the central nervous system.

Cell-type specific expression of constitutively-active Rheb promotes regeneration of bulbospinal respiratory axons following cervical SCI

Damage to respiratory neural circuitry and consequent loss of diaphragm function is a major cause of morbidity and mortality in individuals suffering from traumatic cervical spinal cord injury (SCI). Repair of CNS axons after SCI remains a therapeutic challenge, despite current efforts. SCI disrupts inspiratory signals originating in the rostral ventral respiratory group (rVRG) of the medulla from their phrenic motor neuron (PhMN) targets, resulting in loss of diaphragm function. Using a rat model of cervical hemisection SCI, we aimed to restore rVRG-PhMN-diaphragm circuitry by stimulating regeneration of injured rVRG axons via targeted induction of Rheb (ras homolog enriched in brain), a signaling molecule that regulates neuronal-intrinsic axon growth potential. Following C2 hemisection, we performed intra-rVRG injection of an adeno-associated virus serotype-2 (AAV2) vector that drives expression of a constitutively-active form of Rheb (cRheb). rVRG neuron-specific cRheb expression robustly increased mTOR pathway activity within the transduced rVRG neuron population ipsilateral to the hemisection, as assessed by levels of phosphorylated ribosomal S6 kinase. By co-injecting our novel AAV2-mCherry/WGA anterograde/trans-synaptic axonal tracer into rVRG, we found that cRheb expression promoted regeneration of injured rVRG axons into the lesion site, while we observed no rVRG axon regrowth with AAV2-GFP control. AAV2-cRheb also significantly reduced rVRG axonal dieback within the intact spinal cord rostral to the lesion. However, cRheb expression did not promote any recovery of ipsilateral hemi-diaphragm function, as assessed by inspiratory electromyography (EMG) burst amplitudes. This lack of functional recovery was likely because regrowing rVRG fibers did not extend back into the caudal spinal cord to synaptically reinnervate PhMNs that we retrogradely-labeled with cholera toxin B from the ipsilateral hemi-diaphragm. Our findings demonstrate that enhancing neuronal-intrinsic axon growth capacity can promote regeneration of injured bulbospinal respiratory axons after SCI, but this strategy may need to be combined with other manipulations to achieve reconnection of damaged neural circuitry and ultimately recovery of diaphragm function.

ADAMTS-4 in oligodendrocytes contributes to myelination with an impact on motor function

Myelination is a late developmental process regulated by a set of inhibitory and stimulatory factors, including extracellular matrix components. Accordingly, chondroitin sulfate proteoglycans (CSPGs) act as negative regulators of myelination processes. A disintegrin and metalloproteinase with thrombospondin motifs type 4 (ADAMTS-4) is an extracellular protease capable of degrading CSPGs. Although exogenous ADAMTS-4 has been proven to be beneficial in several models of central nervous system (CNS) injuries, the physiological functions of endogenous ADAMTS-4 remain poorly understood. We first used Adamts4/LacZ reporter mice to reveal that ADAMTS-4 is strongly expressed in the CNS, especially in the white matter, with a cellular profile restricted to mature oligodendrocytes. Interestingly, we evidenced an abnormal myelination in Adamts4^-/- mice, characterized by a higher diameter of myelinated axons with a shifting g-ratio. Accordingly, lack of ADAMTS-4 is accompanied by motor deficits and disturbed nervous electrical activity. In conclusion, we demonstrate that ADAMTS-4 is a new marker of mature oligodendrocytes contributing to the myelination processes and thus to the control of motor capacities.

Mdivi-1 Inhibits Astrocyte Activation and Astroglial Scar Formation and Enhances Axonal Regeneration after Spinal Cord Injury in Rats

After spinal cord injury (SCI), astrocytes become hypertrophic, and proliferative, forming a dense network of astroglial processes at the site of the lesion. This constitutes a physical and biochemical barrier to axonal regeneration. Mitochondrial fission regulates cell cycle progression; inhibiting the cell cycle of astrocytes can reduce expression levels of axon growth-inhibitory molecules as well as astroglial scar formation after SCI. We therefore investigated how an inhibitor of mitochondrial fission, Mdivi-1, would affect astrocyte proliferation, astroglial scar formation, and axonal regeneration following SCI in rats. Western blot and immunofluorescent double-labeling showed that Mdivi-1 markedly reduced the expression of the astrocyte marker glial fibrillary acidic protein (GFAP), and a cell proliferation marker, proliferating cell nuclear antigen, in astrocytes 3 days after SCI. Moreover, Mdivi-1 decreased the expression of GFAP and neurocan, a chondroitin sulfate proteoglycan. Notably, immunofluorescent labeling and Nissl staining showed that Mdivi-1 elevated the production of growth-associated protein-43 and increased neuronal survival at 4 weeks after SCI. Finally, hematoxylin-eosin staining, and behavioral evaluation of motor function indicated that Mdivi-1 also reduced cavity formation and improved motor function 4 weeks after SCI. Our results confirm that Mdivi-1 promotes motor function after SCI, and indicate that inhibiting mitochondrial fission using Mdivi-1 can inhibit astrocyte activation and astroglial scar formation and contribute to axonal regeneration after SCI in rats.

Neuroinflammation in animal models of traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of mortality and morbidity worldwide. Neuroinflammation is prominent in the short and long-term consequences of neuronal injuries that occur after TBI. Neuroinflammation involves the activation of glia, including microglia and astrocytes, to release inflammatory mediators within the brain, and the subsequent recruitment of peripheral immune cells. Various animal models of TBI have been developed that have proved valuable to elucidate the pathophysiology of the disorder and to assess the safety and efficacy of novel therapies prior to clinical trials. These models provide an excellent platform to delineate key injury mechanisms that associate with types of injury (concussion, contusion, and penetration injuries) that occur clinically for the investigation of mild, moderate, and severe forms of TBI. Additionally, TBI modeling in genetically engineered mice, in particular, has aided the identification of key molecules and pathways for putative injury mechanisms, as targets for development of novel therapies for human TBI. This Review details the evidence showing that neuroinflammation, characterized by the activation of microglia and astrocytes and elevated production of inflammatory mediators, is a critical process occurring in various TBI animal models, provides a broad overview of commonly used animal models of TBI, and overviews representative techniques to quantify markers of the brain inflammatory process. A better understanding of neuroinflammation could open therapeutic avenues for abrogation of secondary cell death and behavioral symptoms that may mediate the progression of TBI.

O-GlcNAc glycosylation of p27(kip1) promotes astrocyte migration and functional recovery after spinal cord contusion

Glial scar formation derived from astrocyte proliferation and migration influences the functional recovery after spinal cord injury. Cyclin-dependent kinase inhibitor p27(kip1), whose activity is closely related to its phosphorylation state, reportedly regulates astrocyte proliferation and migration. In this study, we reported that p27(Kip1) undergoes O-GlcNAc modification at Ser 2, Ser 110 and Thr 197. Inhibiting O-GlcNAcylation on Ser 2 by gene mutation (S2A) attenuated the phosphorylation of Ser 10, and vice versa. Interestingly, compared with wild type p27(Kip1), S2A p27(Kip1) displayed a decreased interaction with CRM1 and reduced nuclear export following serum starvation and release. In addition, the interaction between stathmin and S2A p27(Kip1) was also decreased. Cytoskeletal proteins microtubules appeared high density in astrocytes transfected with S2A p27(Kip1) especially at the leading edge of the scratch wound. Accordingly, scratch-wound assay revealed that the motility of astrocytes transfected with S2A p27(Kip1) was faster than that of control. Finally, we injected lentiviral vectors immediately after spinal cord contusion, and found the lesion volume of the rat injected with S2A p27(Kip1) was smaller than that of rat injected with wild type p27(Kip1). Besides, the BBB and CBS behavioral tests showed greater functional recovery in S2A p27(Kip1) treated rats. Taken together, our findings revealed a novel function of O-GlcNAc modification of p27(Kip1) in mediating astrocytes migration and functional recovery after spinal cord contusion.

Regulation of oligodendrocyte precursor maintenance by chondroitin sulphate glycosaminoglycans

Chondroitin sulfate proteoglycans (CSPGs) have been proven to inhibit morphological maturation of oligodendrocytes as well as their myelination capabilities. Yet, it remained unclear, whether CSPGs and/or their respective chondroitin sulfate glycosaminoglycan (CS-GAG) side chains also regulate the oligodendrocyte lineage progression. Here, we initially show that CS-GAGs detected by the monoclonal antibody 473HD are expressed by primary rat NG2-positive oligodendrocyte precursor cells (OPCs) and O4-positive immature oligodendrocytes. CS-GAGs become down-regulated with ongoing oligodendrocyte differentiation. Enzymatic removal of the CS-GAG chains by the bacterial enzyme Chondroitinase ABC (ChABC) promoted spontaneous differentiation of proliferating rat OPCs toward O4-positive immature oligodendrocytes. Upon forced differentiation, the enzymatic removal of the CS-GAGs accelerated oligodendrocyte differentiation toward both MBP-positive and membrane forming oligodendrocytes. These processes were attenuated on enriched CSPG fractions, mainly consisting of Phosphacan/RPTPβ/ζ and to less extent of Brevican and NG2. To qualify CS-GAGs as universal regulators of oligodendrocyte biology, we finally tested the effect of CS-GAG removal on OPCs from different sources such as mouse cortical oligospheres, mouse spinal cord neurospheres, and most importantly human-induced pluripotent stem cell-derived radial glia-like neural precursor cells. For all culture systems used, we observed a similar inhibitory effect of CS-GAGs on oligodendrocyte differentiation. In conclusion, this study clearly suggests an important fundamental principle for complex CS-GAGs to regulate the oligodendrocyte lineage progression. Moreover, the use of ChABC in order to promote oligodendrocyte differentiation toward myelin gene expressing cells might be an applicable therapeutic option to enhance white matter repair.

TCTP Expression After Rat Spinal Cord Injury: Implications for Astrocyte Proliferation and Migration

Translationally controlled tumor protein (TCTP) is a ubiquitous and highly conserved protein which plays a role in cell proliferation and growth, apoptosis, and cell cycle regulation. However, its expression and function in spinal cord injury (SCI) are still unknown. Here, we demonstrated that expression of TCTP was dynamic changed after acute spinal cord injury. Our results showed that TCTP gradually increased, reached a peak at 3 day, and then declined to basal levels at 14 days after spinal cord injury. Upregulation of TCTP was accompanied with an increase in the levels of proliferation proteins such as PCNA. Immunofluorescent labeling also showed that TCTP located in astrocytes and traumatic SCI induced TCTP colocalizated with PCNA. These results indicated that TCTP might play an important role in astrocyte proliferation. To further probe the role of TCTP, TCTP-specific siRNA-transfected astrocytes showed significant decrease of primary astrocyte proliferation. Surprisingly, TCTP knockdown also reduced primary astrocyte migration, as the reorganization of microtubules and F-actin was disturbed after siRNA transfection. All above indicated that TCTP might play a crucial role in astrocyte proliferation and migration. Collectively, our data suggested that TCTP might play important roles in CNS pathophysiology after SCI.

The Neural Stem Cell Microenvironment: Focusing on Axon Guidance Molecules and Myelin-Associated Factors

Neural stem cells (NSCs) could produce various cell phenotypes in the subventricular zone (SVZ) and dentate gyrus of the hippocampus in the central nervous system (CNS), where neurogenesis has been determined to occur. The extracellular microenvironment also influences the behaviors of NSCs during development and at CNS injury sites. Our previous study indicates that myelin, a component of the CNS, could regulate the differentiation of NSCs in vitro. Recent reports have implicated three myelin-derived inhibitors, NogoA, myelin-associated glycoprotein (MAG), and oligodendrocyte-myelin glycoprotein (OMgp), as well as several axon guidance molecules as regulators of NSC survival, proliferation, migration, and differentiation. However, the molecular mechanisms underlying the behavior of NSCs are not fully understood. In this study, we summarize the current literature on the effects of different extrinsic factors on NSCs and discuss possible mechanisms, as well as future possible clinical applications.

shRNA against PTEN promotes neurite outgrowth of cortical neurons and functional recovery in spinal cord contusion rats

AIM

To explore neurite growth/regeneration and spinal cord injury repair after PTEN silencing via lentivirus-mediated RNAi.

MATERIALS & METHODS

Cortical neurons were seeded on or adjacent to chondroitin sulfate proteoglycans. The length, number and crossing behavior of neurites were calculated. Lentivirus was locally injected into spinal cord contusion rats. The functional recovery and immunohistochemical staining were analyzed.

RESULTS

Neurites with PTEN silencing exhibited significant enhancements in elongation, initiation and crossing ability when they encountered chondroitin sulfate proteoglycans in vitro. In vivo PTEN silencing improved functional recovery significantly, and promoted axon and synapse formation, but not scar formation.

CONCLUSIONS

PTEN silencing may be promising for spinal cord injury repair.

Comparative assessment of iridium oxide and platinum alloy wires using an in vitro glial scar assay

The long-term effect of chronically implanted electrodes is the formation of a glial scar. Therefore, it is imperative to assess the biocompatibility of materials before employing them in neural electrode fabrication. Platinum alloy and iridium oxide have been identified as good candidates as neural electrode biomaterials due to their mechanical and electrical properties, however, effect of glial scar formation for these two materials is lacking. In this study, we applied a glial scarring assay to observe the cellular reactivity to platinum alloy and iridium oxide wires in order to assess the biocompatibility based on previously defined characteristics. Through real-time PCR, immunostaining and imaging techniques, we will advance the understanding of the biocompatibility of these materials. Results of this study demonstrate iridium oxide wires exhibited a more significant reactive response as compared to platinum alloy wires. Cells cultured with platinum alloy wires had less GFAP gene expression, lower average GFAP intensity, and smaller glial scar thickness. Collectively, these results indicated that platinum alloy wires were more biocompatible than the iridium oxide wires.

Traumatic white matter injury and glial activation: from basic science to clinics

An improved understanding and characterization of glial activation and its relationship with white matter injury will likely serve as a novel treatment target to curb post injury inflammation and promote axonal remyelination after brain trauma. Traumatic brain injury (TBI) is a significant public healthcare burden and a leading cause of death and disability in the United States. Particularly, traumatic white matter (WM) injury or traumatic axonal injury has been reported as being associated with patients' poor outcomes. However, there is very limited data reporting the importance of glial activation after TBI and its interaction with WM injury. This article presents a systematic review of traumatic WM injury and the associated glial activation, from basic science to clinical diagnosis and prognosis, from advanced neuroimaging perspective. It concludes that there is a disconnection between WM injury research and the essential role of glia which serve to restore a healthy environment for axonal regeneration following WM injury. Particularly, there is a significant lack of non-invasive means to characterize the complex pathophysiology of WM injury and glial activation in both animal models and in humans. An improved understanding and characterization of the relationship between glia and WM injury will likely serve as a novel treatment target to curb post injury inflammation and promote axonal remyelination.

Long-term viral brain-derived neurotrophic factor delivery promotes spasticity in rats with a cervical spinal cord hemisection

We have recently reported that rats with complete thoracic spinal cord injury (SCI) that received a combinatorial treatment, including viral brain-derived neurotrophic factor (BDNF) delivery in the spinal cord, not only showed enhanced axonal regeneration, but also deterioration of hind-limb motor function. By demonstrating that BDNF over-expression can trigger spasticity-like symptoms in a rat model of sacral SCI, we proposed a causal relationship between the observed spasticity-like symptoms (i.e., resistance to passive range of motion) and the over-expression of BDNF. The current study was originally designed to evaluate a comparable combined treatment for cervical SCI in the rat to improve motor recovery. Once again we found similar signs of spasticity involving clenching of the paws and wrist flexion. This finding changed the focus of the study and, we then explored whether this spasticity-like symptom is directly related to the over-expression of BDNF by administering a BDNF antagonist. Using electromyographic measurements we showed that this treatment gradually diminished the resistance to overcome forelimb flexion in an acute experiment. Thus, we conclude that neuro-excitatory effects of chronic BDNF delivery together with diminished descending control after SCI can result in adverse effects.

Therapeutic potential of amniotic fluid-derived cells for treating the injured nervous system

There is a need for improved therapy for acquired brain injury, which has proven resistant to treatment by numerous drugs in clinical trials and continues to represent one of the leading causes of disability worldwide. Research into cell-based therapies for the treatment of brain injury is growing rapidly, but the ideal cell source has yet to be determined. Subpopulations of cells found in amniotic fluid, which is readily obtained during routine amniocentesis, can be easily expanded in culture, have multipotent differentiation capacity, are non-tumourigenic, and avoid the ethical complications associated with embryonic stem cells, making them a promising cell source for therapeutic purposes. Beneficial effects of amniotic fluid cell transplantation have been reported in various models of nervous system injury. However, evidence that amniotic fluid cells can differentiate into mature, functional neurons in vivo and incorporate into the existing circuitry to replace lost or damaged neurons is lacking. The mechanisms by which amniotic fluid cells improve outcomes after experimental nervous system injury remain unclear. However, studies reporting the expression and release of neurotrophic, angiogenic, and immunomodulatory factors by amniotic fluid cells suggest they may provide neuroprotection and (or) stimulate endogenous repair and remodelling processes in the injured nervous system. In this paper, we address recent research related to the neuronal differentiation of amniotic fluid-derived cells, the therapeutic efficacy of these cells in animal models of nervous system injury, and the possible mechanisms mediating the positive outcomes achieved by amniotic fluid cell transplantation.

The glial scar in spinal cord injury and repair

Glial scarring following severe tissue damage and inflammation after spinal cord injury (SCI) is due to an extreme, uncontrolled form of reactive astrogliosis that typically occurs around the injury site. The scarring process includes the misalignment of activated astrocytes and the deposition of inhibitory chondroitin sulfate proteoglycans. Here, we first discuss recent developments in the molecular and cellular features of glial scar formation, with special focus on the potential cellular origin of scar-forming cells and the molecular mechanisms underlying glial scar formation after SCI. Second, we discuss the role of glial scar formation in the regulation of axonal regeneration and the cascades of neuro-inflammation. Last, we summarize the physical and pharmacological approaches targeting the modulation of glial scarring to better understand the role of glial scar formation in the repair of SCI.

Intermittent hypoxia training after C2 hemisection modifies the expression of PTEN and mTOR

In this study, we examined modulations in phosphatase and tensin homolog (PTEN) and mammalian target of rapamycin (mTOR) protein expression after a lateral C2 hemisection and subsequent intermittent hypoxia (IH) exposure and training, which initiates respiratory motor plasticity and recovery. PTEN and mTOR are significant molecules within a signaling pathway that directly influences dendritic sprouting, axonal plasticity, and regeneration. Expression levels of PTEN, mTOR and downstream effectors within this pathway were investigated, and it was found that following injury and IH exposure the expression of these molecules was significantly altered. This study directly demonstrates the implementation and feasibility of a non-invasive strategy to modulate the expression levels of intrinsic signaling molecules known to influence plasticity and regeneration in the CNS.

Nanopatterning effects on astrocyte reactivity

An array of design strategies have been targeted toward minimizing failure of implanted microelectrodes by minimizing the chronic glial scar around the microelectrode under chronic conditions. Current approaches toward inhibiting the initiation of glial scarring range from altering the geometry, roughness, size, shape, and materials of the device. Studies have shown materials which mimic the nanotopography of the natural environment in vivo will consequently result in an improved biocompatible response. Nanofabrication of electrode arrays is being pursued in the field of neuronal electrophysiology to increase sampling capabilities. Literature shows a gap in research of nanotopography influence in the reduction of astrogliosis. The aim of this study was to determine optimal feature sizes for neural electrode fabrication, which was defined as eliciting a nonreactive astrocytic response. Nanopatterned surfaces were fabricated with nanoimprint lithography on poly(methyl methacrylate) surfaces. The rate of protein adsorption, quantity of protein adsorption, cell alignment, morphology, adhesion, proliferation, viability, and gene expression was compared between nanopatterned surfaces of different dimensions and non-nanopatterned control surfaces. Results of this study revealed that 3600 nanopatterned surfaces elicited less of a response when compared with the other patterned and non-nanopatterned surfaces. The surface instigated cell alignment along the nanopattern, less protein adsorption, less cell adhesion, proliferation and viability, inhibition of glial fibrillary acidic protein, and mitogen-activated protein kinase kinase 1 compared with all other substrates tested.

Intrinsic mechanisms regulating axon regeneration: an integrin perspective

Adult central nervous system (CNS) axons fail to regenerate after injury because of inhibitory factors in the surrounding environment and a low intrinsic regenerative capacity. Axons in the adult peripheral nervous system have a higher regenerative capacity, due in part to the presence of certain integrins-receptors for the extracellular matrix. Integrins are critical for axon growth during the development of the nervous system but are absent from some adult CNS axons. Here, we discuss the intrinsic mechanisms that regulate axon regeneration and examine the role of integrins. As correct localization is paramount to integrin function, we further discuss the mechanisms that regulate integrin traffic toward the axonal growth cone.

Rodent retinal ganglion cell cultures

Central neurons lose the ability for axonal regrowth during development and typically do not regenerate their axons following axotomy once they become mature unless given a growth-permissive environment, for example, a peripheral nerve graft. Retinal ganglion cells (RGCs) of the optic nerve represent a highly useful cell model for the study of neurotrophic factor responsiveness, although the presence of nonneuronal cells in the retina makes it difficult to interpret the direct effects of tested factors on RGCs. Cultures of purified RGCs thus represent an excellent tool for the study of optic nerve cell trophic responsiveness, in terms of both survival and axonal regeneration.

Characterization of astrocyte reactivity and gene expression on biomaterials for neural electrodes

Neural electrode devices hold great promise to help people with the restoration of lost functions. However, research is lacking in the biomaterial design of a stable, long-term device. Glial scarring is initiated when a device is inserted into brain tissue and an inflammatory response ensues. Astrocytes become hypertrophic, hyperplastic, and upregulate glial-fibrillary acidic protein. This study was designed to investigate the astrocyte proliferation, viability, morphology, and gene expression to assess the reactive state of the cells on different material surfaces. Although platinum and silicon have been extensively characterized both in vivo and in vitro for their biocompatibility with neuronal cells, this study used the novel usage of PMMA and SU-8 in neural electrodes by comparative analysis of materials' biocompatibility. This study has shown evidence of noncytotoxicity of SU-8. We have also confirmed the biocompatibility of PMMA with astrocytes. Moreover, we have established sound guidelines of which neural implant materials should meet to be depicted biocompatible.

Human umbilical cord blood cells restore brain damage induced changes in rat somatosensory cortex

Intraperitoneal transplantation of human umbilical cord blood (hUCB) cells has been shown to reduce sensorimotor deficits after hypoxic ischemic brain injury in neonatal rats. However, the neuronal correlate of the functional recovery and how such a treatment enforces plastic remodelling at the level of neural processing remains elusive. Here we show by in-vivo recordings that hUCB cells have the capability of ameliorating the injury-related impairment of neural processing in primary somatosensory cortex. Intact cortical processing depends on a delicate balance of inhibitory and excitatory transmission, which is disturbed after injury. We found that the dimensions of cortical maps and receptive fields, which are significantly altered after injury, were largely restored. Additionally, the lesion induced hyperexcitability was no longer observed in hUCB treated animals as indicated by a paired-pulse behaviour resembling that observed in control animals. The beneficial effects on cortical processing were reflected in an almost complete recovery of sensorimotor behaviour. Our results demonstrate that hUCB cells reinstall the way central neurons process information by normalizing inhibitory and excitatory processes. We propose that the intermediate level of cortical processing will become relevant as a new stage to investigate efficacy and mechanisms of cell therapy in the treatment of brain injury.

Diffuse traumatic axonal injury in the mouse induces atrophy, c-Jun activation, and axonal outgrowth in the axotomized neuronal population

Traumatic axonal injury (TAI) is a consistent component of traumatic brain injury (TBI) and is associated with much of its morbidity. Little is known regarding the long-term retrograde neuronal consequences of TAI and/or the potential that TAI could lead to anterograde axonal reorganization and repair. To investigate the repertoire of anterograde and retrograde responses triggered by TIA, Thy1-YFP-H mice were subjected to mild central fluid percussion injury and killed at various times between 15 min and 28 d post-injury. Based upon confocal assessment of the endogenous neuronal fluorescence, such injury was found to result in diffuse TAI throughout layer V of the neocortex within yellow fluorescent protein (YFP)-positive axons. When these fluorescent approaches were coupled with various quantitative and immunohistochemical approaches, we found that this TAI did not result in neuronal death over the 28 d period assessed. Rather, it elicited neuronal atrophy. Within these same axotomized neuronal populations, TAI was also found to induce an early and sustained activation of the transcription factors c-Jun and ATF-3 (activating transcription factor 3), known regulators of axon regeneration. Parallel ultrastructural studies confirmed that these reactive changes are consistent with atrophy in the absence of neuronal death. Concurrent with those events ongoing in the neuronal cell bodies, their downstream axonal segments revealed, as early as 1 d post-injury, morphological changes consistent with reactive sprouting that was accompanied by significant axonal elongation over time. Collectively, these TAI-linked events are consistent with sustained neuronal recovery, an activation of a regenerative genetic program, and subsequent axonal reorganization suggestive of some form of regenerative response.

Regulatory mechanisms that mediate tenascin C-dependent inhibition of oligodendrocyte precursor differentiation

Here, we present mechanisms for the inhibition of oligodendendrocyte precursor cell (OPC) differentiation, a biological function of neural extracellular matrix (ECM). The differentiation of oligodendrocytes is orchestrated by a complex set of stimuli. In the present study, we investigated the signaling pathway elicited by the ECM glycoprotein tenascin C (Tnc). Tnc substrates inhibit myelin basic protein (MBP) expression of cultured rat oligodendrocytes, and, conversely, we found that the emergence of MBP expression is accelerated in forebrains of Tnc-deficient mice. Mechanistically, Tnc interfered with phosphorylation of Akt, which in turn reduced MBP expression. At the cell surface, Tnc associates with lipid rafts in oligodendrocyte membranes, together with the cell adhesion molecule contactin (Cntn1) and the Src family kinase (SFK) Fyn. Depletion of Cntn1 in OPCs by small interfering RNAs (siRNAs) abolished the Tnc-dependent inhibition of oligodendrocyte differentiation, while Tnc exposure impeded the activation of the tyrosine kinase Fyn by Cntn1. Concomitant with oligodendrocyte differentiation, Tnc antagonized the expression of the signaling adaptor and RNA-binding molecule Sam68. siRNA-mediated knockdown or overexpression of Sam68 delayed or accelerated oligodendrocyte differentiation, respectively. Inhibition of oligodendrocyte differentiation with the SFK inhibitor PP2 could be rescued by Sam68 overexpression, which may indicate a regulatory role for Sam68 downstream of Fyn. Our study therefore uncovers the first signaling pathways that underlie Tnc-induced, ECM-dependent maintenance of the immature state of OPCs.

Trophic responsiveness of purified postnatal and adult rat retinal ganglion cells

Central neurons lose the ability for axonal re-growth during development and typically do not regenerate their axons following axotomy once they become mature unless given a growth-permissive environment i.e. peripheral nerve graft. In the present study, the growth responsiveness of purified retinal ganglion cells (RGCs) at different ages to neurotrophic factors and Schwann cell (SC)-secreted factors were examined directly. The purity of adult RGCs was 97% as assessed by retrograde labelling with 4,6-diamidino-2-phenylindole. The stability of cultures were demonstrated by long-term survival (30 days) in medium contained brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and forskolin (F) (BCF). RGCs from postnatal (P) (P0, P4, P8, P21) and adult (P90) rats showed decreasing levels of survival and neuritogenesis when grown in BCF. In contrast, the opposite was observed in SC-conditioned medium (CM)-treated P0-P8 RGCs which were increasingly responsive. SCCM induced maximal neurite outgrowth in P8 RGCs via the activation of extracellular regulated kinase 1/2 (Erk1/2). Inhibition of mitogen-activated protein kinase-Erk1/2 signaling using an Erk1/2-specific inhibitor (UO126) abolished SCCM-induced Erk1/2 phosphorylation and neuritogenesis completely. Although both SCCM and BCF failed to sustain the same levels of growth in P21 or P90 cultures as observed in P8 cultures, SCCM promoted higher survival and neuritogenesis than BCF-treated adult RGCs. This study is the first report of adult rat RGC purification and demonstrates that mature RGCs need multiple factors for survival and neurite outgrowth.

ProBDNF inhibits infiltration of ED1+ macrophages after spinal cord injury

The central nervous system (CNS) does not regenerate partly due to the slow clearance of debris from the degenerated myelin sheath by Wallerian degeneration. The mechanism underlying the inefficiency in myelin clearance is not clear. Here we showed that endogenous proBDNF may inhibit the infiltration of ED1+ inflammatory cells after spinal cord injury. After injury, proBDNF and its receptors sortilin and p75NTR are expressed in the spinal cord as determined by Western blots and immunocytochemistry. ProBDNF and mature BDNF were released from macrophages in vitro. Macrophages in vivo (ED1+) and isolated in vitro (CD11b+) express moderate levels of proBDNF, sortilin and p75NTR. ProBDNF suppressed the migration of isolated macrophages in vitro and the antibody to proBDNF enhanced the migration. Suppression of proBDNF in vivo by administering the antiserum to the prodomain of BDNF after spinal cord injury (SCI) increased the infiltration of macrophages and increased number of neurons in the injured cord. BBB tests showed that the treatment of the antibody to proBDNF improved the functional recovery after spinal cord injury. Our data suggest that proBDNF is a suppressing factor for macrophage migration and infiltration and may play a detrimental role after SCI.

Bridging the Divide between Neuroprosthetic Design, Tissue Engineering and Neurobiology

Neuroprosthetic devices have made a major impact in the treatment of a variety of disorders such as paralysis and stroke. However, a major impediment in the advancement of this technology is the challenge of maintaining device performance during chronic implantation (months to years) due to complex intrinsic host responses such as gliosis or glial scarring. The objective of this review is to bring together research communities in neurobiology, tissue engineering, and neuroprosthetics to address the major obstacles encountered in the translation of neuroprosthetics technology into long-term clinical use. This article draws connections between specific challenges faced by current neuroprosthetics technology and recent advances in the areas of nerve tissue engineering and neurobiology. Within the context of the device-nervous system interface and central nervous system implants, areas of synergistic opportunity are discussed, including platforms to present cells with multiple cues, controlled delivery of bioactive factors, three-dimensional constructs and in vitro models of gliosis and brain injury, nerve regeneration strategies, and neural stem/progenitor cell biology. Finally, recent insights gained from the fields of developmental neurobiology and cancer biology are discussed as examples of exciting new biological knowledge that may provide fresh inspiration toward novel technologies to address the complexities associated with long-term neuroprosthetic device performance.

Induction of apoptotic cell death preferentially in reactive astrocytes by concanavalin A

When central nervous system is injured, reactive astrocytes form glial scar which prevents neuronal regeneration. We examined the effects of concanavalin A (ConA) in primary astrocytes, and found preferential apoptotic effect of ConA in migrating reactive astrocytes. Thus, ConA may be applicable for enhancing neuronal regeneration by preventing glial scar.

Novel multi-modal strategies to promote brain and spinal cord injury recovery

Stroke is the leading cause of disability in the United States, and yet no definitive interventions can drive the nervous system beyond its measurable but often limited spontaneous recovery. Treatment to limit injury progression and enhance repair after stroke or other types of central nervous system injury is complicated by the heterogeneous nature of cell death and wound healing mechanisms and the multiple barriers to functional recovery. The heterogeneity of injury and repair mechanisms requires interventions that are broad and multi-modal, but also intrinsically safe. We describe a process to identify such interventions by screening multiple individual targets in the historically separate realms of neuroprotection, repair, and regeneration against a library of FDA-approved compounds with known safety. We have identified nearly 10 compounds that are able to activate simultaneously protective and reparative genes. These compounds have a theoretical therapeutic window that spans from evolving injury (minutes to hours) to stable injury (days to months to years). It is our hypothesis that these compounds will be most efficacious when paired with training. The notion is that drugs will alter the propensity of the nervous system toward recovery, whereas specific training will engage the needed instructive cues to achieve this goal. Indeed, robotic training can provide a level of motor learning that seems to enhance the salutary effects of training. In a system that depends heavily on cells that do not easily replenish themselves, cellular therapies could also ultimately be an important part of the cocktail. We conclude that combinations of interventions will be needed to surmount the multiple barriers to recovery in stroke and other types of brain and spinal cord injury recovery.

Actin in axons: stable scaffolds and dynamic filaments

Actin filaments are thin polymers of the 42 kD protein actin. In mature axons a network of subaxolemmal actin filaments provide stability for membrane integrity and a substrate for short distance transport of cargos. In developing neurons dynamic regulation of actin polymerization and organization mediates axonal morphogenesis and axonal pathfinding to synaptic targets. Other changes in axonal shape, collateral branching, branch retraction, and axonal regeneration, also depend on actin filament dynamics. Actin filament organization is regulated by a diversity of actin-binding proteins (ABP). ABP are the focus of complex extrinsic and intrinsic signaling pathways, and many neurological pathologies and dysfunctions arise from defective regulation of ABP function.

Retrograde injury signaling in lesioned axons

The cell body of a lesioned neuron must receive accurate and timely information on the site and extent of axonal damage, in order to mount an appropriate response. Specific mechanisms must therefore exist to transmit such information along the length of the axon from the lesion site to the cell body. Three distinct types of signals have been postulated to underlie this process, starting with injury-induced discharge of axon potentials, and continuing with two distinct types of retrogradely transported macromolecular signals. The latter includes, on the one hand, an interruption of the normal supply of retrogradely transported trophic factors from the target, and, on the other hand, activated proteins originating from the injury site. This chapter reviews the progress on understanding the different mechanistic aspects of the axonal response to injury, and how the information is conveyed from the injury site to the cell body to initiate regeneration.