The challenges of long-distance axon regeneration in the injured CNS

Inhibition of CK2 Diminishes Fibrotic Scar Formation and Improves Outcomes After Ischemic Stroke via Reducing BRD4 Phosphorylation

Fibrotic scars play important roles in tissue reconstruction and functional recovery in the late stage of nervous system injury. However, the mechanisms underlying fibrotic scar formation and regulation remain unclear. Casein kinase II (CK2) is a protein kinase that regulates a variety of cellular functions through the phosphorylation of proteins, including bromodomain-containing protein 4 (BRD4). CK2 and BRD4 participate in fibrosis formation in a variety of tissues. However, whether CK2 affects fibrotic scar formation remains unclear, as do the mechanisms of signal regulation after cerebral ischemic injury. In this study, we assessed whether CK2 could modulate fibrotic scar formation after cerebral ischemic injury through BRD4. Primary meningeal fibroblasts were isolated from neonatal rats and treated with transforming growth factor-β1 (TGF-β1), SB431542 (a TGF-β1 receptor kinase inhibitor) or TBB (a highly potent CK2 inhibitor). Adult SD rats were intraperitoneally injected with TBB to inhibit CK2 after MCAO/R. We found that CK2 expression was increased in vitro in the TGF-β1-induced fibrosis model and in vivo in the MCAO/R injury model. The TGF-β1 receptor kinase inhibitor SB431542 decreased CK2 expression in fibroblasts. The CK2 inhibitor TBB reduced the increases in proliferation, migration and activation of fibroblasts caused by TGF-β1 in vitro, and it inhibited fibrotic scar formation, ameliorated histopathological damage, protected Nissl bodies, decreased infarct volume and alleviated neurological deficits after MCAO/R injury in vivo. Furthermore, CK2 inhibition decreased BRD4 phosphorylation both in vitro and in vivo. The findings of the present study suggested that CK2 may control BRD4 phosphorylation to regulate fibrotic scar formation, to affecting outcomes after ischemic stroke.

[Inhibition of Sonic Hedgehog signaling inhibits fibrous scar formation and adversely affects functional outcome after ischemic brain injury in rats]

OBJECTIVE

To investigate the effects of inhibiting Sonic Hedgehog (Shh) signaling on fibrous scar formation and functional outcome after ischemic brain injury.

METHODS

Adult SD rats were randomized into sham-operated group, middle cerebral artery occlusion (MCAO) and reperfusion (I/R) group, I/R with intraventricular empty adenoviral vector (rAd-NC) injection group, and I/R with adenovirus-mediated Shh knockdown (rAd-ShShh) group. After the treatments, the neurological deficits of the rats were assessed, and the protein and mRNA expressions of fibronectin (Fn), α-SMA, and Shh in the ischemic hemisphere were detected with immunofluorescence assay and qPCR; TUNEL staining was used for detecting neural cell apoptosis. In the cell experiment, primary meningeal fibroblasts isolated from neonatal SD rats were pretreated for 24 h with TGF-β1 or TGF-β1 plus cyclopamine (CYC) before oxygen-glucose deprivation for 150 min followed by reoxygenation for 72 h (OGD/R). CCK-8 assay and scratch test were performed to examine the changes in cell proliferation and migration, and immunofluorescence assay, qPCR and Western blotting were used for detecting cell transformation and the expressions of Shh, α-SMA, and Fn.

RESULTS

Cerebral I/R injury significantly increased the protein and mRNA expressions of Shh, α-SMA, and Fn in the ischemic hemisphere of the rats, but their expression levels were significantly lowered by intraventricular injection of rAd-Shshh (P < 0.05), which obviously increased cell apoptosis in the ischemic hemisphere (P < 0.05) and improved modified mNSS and modified Bederson scores of the rats (P < 0.05). In the cell experiment, pretreatment with TGF-β1 and TGF-β1+CYC both increased the viability of the primary meningeal fibroblasts after OGD/R. TGF-β1 significantly enhanced the migration ability and induced obvious transformation of the exposed cells (P < 0.05), but these effects were significantly attenuated by co-treatment with CYC (P < 0.05). The expressions of Shh, α-SMA and Fn in the TGF-β1 group were all significantly higher in TGF-β1-treated cells (P < 0.05) and were obviously lowered by co-treatment with CYC (P < 0.05).

CONCLUSION

Inhibition of Shh signaling may inhibit fibrous scar formation and functional recovery in rats after ischemic brain injury.

Advancing Peripheral Nerve Graft Transplantation for Incomplete Spinal Cord Injury Repair

Peripheral nerves have a propensity for axon growth and regeneration that the central nervous system lacks (CNS). However, CNS axons can also grow long distances if introduced to a graft harvested from a peripheral nerve (PNGs), which is the rationale for using PNGs as repair strategy for injuries of the spinal cord. From a clinical perspective, PNGs provide interesting possibilities with potential to repair the injured spinal cord. First, there are numerous options to harvest autologous grafts associated with low risk for the patient. Second, a PNG allow axons to grow considerable distances and can, by the surgical procedure, be navigated to specific target sites in the CNS. Furthermore, a PNG provides all necessary biological substrates for myelination of elongating axons. A PNG can thus be suited to bridge axons long distances across an injury site and restore long tracts in incomplete SCI. Experimentally, locomotor functions have been improved transplanting a PNG after incomplete injury. However, we still know little with regard to the formation of new circuitries and functional outcome in association to when, where, and how grafts are inserted into the injured spinal cord, especially for sensory functions. In this perspective, we discuss the advantages of PNG from a clinical and surgical perspective, the need for adding/repairing long tracts, how PNGs are best applied for incomplete injuries, and the unexplored areas we believe are in need of answers.

The Drama of Wallerian Degeneration: The Cast, Crew, and Script

Significant advances have been made in recent years in identifying the genetic components of Wallerian degeneration, the process that brings the progressive destruction and removal of injured axons. It has now been accepted that Wallerian degeneration is an active and dynamic cellular process that is well regulated at molecular and cellular levels. In this review, we describe our current understanding of Wallerian degeneration, focusing on the molecular players and mechanisms that mediate the injury response, activate the degenerative program, transduce the death signal, execute the destruction order, and finally, clear away the debris. By highlighting the starring roles and sketching out the molecular script of Wallerian degeneration, we hope to provide a useful framework to understand Wallerian and Wallerian-like degeneration and to lay a foundation for developing new therapeutic strategies to treat axon degeneration in neural injury as well as in neurodegenerative disease. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Growth-Promoting Treatment Screening for Corticospinal Neurons in Mouse and Man

Neurons of the central nervous system (CNS) that project long axons into the spinal cord have a poor axon regenerative capacity compared to neurons of the peripheral nervous system. The corticospinal tract (CST) is particularly notorious for its poor regeneration. Because of this, traumatic spinal cord injury (SCI) is a devastating condition that remains as yet uncured. Based on our recent observations that direct neuronal interleukin-4 (IL-4) signaling leads to repair of axonal swellings and beneficial effects in neuroinflammation, we hypothesized that IL-4 acts directly on the CST. Here, we developed a tissue culture model for CST regeneration and found that IL-4 promoted new growth cone formation after axon transection. Most importantly, IL-4 directly increased the regenerative capacity of both murine and human CST axons, which corroborates its regenerative effects in CNS damage. Overall, these findings serve as proof-of-concept that our CST regeneration model is suitable for fast screening of new treatments for SCI.

Toward Functional Restoration of the Central Nervous System: A Review of Translational Neuroscience Principles

Injury to the central nervous system (CNS) can leave patients with devastating neurological deficits that may permanently impair independence and diminish quality of life. Recent insights into how the CNS responds to injury and reacts to critically timed interventions are being translated into clinical applications that have the capacity to drastically improve outcomes for patients suffering from permanent neurological deficits due to spinal cord injury, stroke, or other CNS disorders. The translation of such knowledge into practical and impactful treatments involves the strategic collaboration between neurosurgeons, clinicians, therapists, scientists, and industry. Therefore, a common understanding of key neuroscientific principles is crucial. Conceptually, current approaches to CNS revitalization can be divided by scale into macroscopic (systems-circuitry) and microscopic (cellular-molecular). Here we review both emerging and well-established tenets that are being utilized to enhance CNS recovery on both levels, and we explore the role of neurosurgeons in developing therapies moving forward. Key principles include plasticity-driven functional recovery, cellular signaling mechanisms in axonal sprouting, critical timing for recovery after injury, and mechanisms of action underlying cellular replacement strategies. We then discuss integrative approaches aimed at synergizing interventions across scales, and we make recommendations for the basis of future clinical trial design. Ultimately, we argue that strategic modulation of microscopic cellular behavior within a macroscopic framework of functional circuitry re-establishment should provide the foundation for most neural restoration strategies, and the early involvement of neurosurgeons in the process will be crucial to successful clinical translation.

Catecholaminergic axons in the neocortex of adult mice regrow following brain injury

Serotonin axons in the adult rodent brain can regrow and recover their function following several forms of injury including controlled cortical impact (CCI), a neocortical stab wound, or systemic amphetamine toxicity. To assess whether this capacity for regrowth is unique to serotonergic fibers, we used CCI and stab injury models to assess whether fibers from other neuromodulatory systems can also regrow following injury. Using tyrosine-hydoxylase (TH) immunohistochemistry we measured the density of catecholaminergic axons before and at various time points after injury. One week after CCI injury we observed a pronounced loss, across cortical layers, of TH+ axons posterior to the site of injury. One month after CCI injury the same was true of TH+ axons both anterior and posterior to the site of injury. This loss was followed by significant recovery of TH+ fiber density across cortical layers, both anterior and posterior to the site of injury, measured three months after injury. TH+ axon loss and recovery over weeks to months was also observed throughout cortical layers using the stab injury model. Double label immunohistochemistry revealed that nearly all TH+ axons in neocortical layer 1/2 are also dopamine-beta-hyroxylase+ (DBH+; presumed norepinephrine), while TH+ axons in layer 5 are a mixture of DBH+ and dopamine transporter+ types. This suggests that noradrenergic axons can regrow following CCI or stab injury in the adult mouse neocortex and leaves open the question of whether dopaminergic axons can do the same.

The Extracellular Environment of the CNS: Influence on Plasticity, Sprouting, and Axonal Regeneration after Spinal Cord Injury

The extracellular environment of the central nervous system (CNS) becomes highly structured and organized as the nervous system matures. The extracellular space of the CNS along with its subdomains plays a crucial role in the function and stability of the CNS. In this review, we have focused on two components of the neuronal extracellular environment, which are important in regulating CNS plasticity including the extracellular matrix (ECM) and myelin. The ECM consists of chondroitin sulfate proteoglycans (CSPGs) and tenascins, which are organized into unique structures called perineuronal nets (PNNs). PNNs associate with the neuronal cell body and proximal dendrites of predominantly parvalbumin-positive interneurons, forming a robust lattice-like structure. These developmentally regulated structures are maintained in the adult CNS and enhance synaptic stability. After injury, however, CSPGs and tenascins contribute to the structure of the inhibitory glial scar, which actively prevents axonal regeneration. Myelin sheaths and mature adult oligodendrocytes, despite their important role in signal conduction in mature CNS axons, contribute to the inhibitory environment existing after injury. As such, unlike the peripheral nervous system, the CNS is unable to revert to a “developmental state” to aid neuronal repair. Modulation of these external factors, however, has been shown to promote growth, regeneration, and functional plasticity after injury. This review will highlight some of the factors that contribute to or prevent plasticity, sprouting, and axonal regeneration after spinal cord injury.

Human dental stem cell derived transgene-free iPSCs generate functional neurons via embryoid body-mediated and direct induction methods

Induced pluripotent stem cells (iPSCs) give rise to neural stem/progenitor cells, serving as a good source for neural regeneration. Here, we established transgene-free (TF) iPSCs from dental stem cells (DSCs) and determined their capacity to differentiate into functional neurons in vitro. Generated TF iPSCs from stem cells of apical papilla and dental pulp stem cells underwent two methods-embryoid body-mediated and direct induction, to guide TF-DSC iPSCs along with H9 or H9 Syn-GFP (human embryonic stem cells) into functional neurons in vitro. Using the embryoid body-mediated method, early stage neural markers PAX6, SOX1, and nestin were detected by immunocytofluorescence or reverse transcription-real time polymerase chain reaction (RT-qPCR). At late stage of neural induction measured at Weeks 7 and 9, the expression levels of neuron-specific markers Nav1.6, Kv1.4, Kv4.2, synapsin, SNAP25, PSD95, GAD67, GAP43, and NSE varied between stem cells of apical papilla iPSCs and H9. For direct induction method, iPSCs were directly induced into neural stem/progenitor cells and guided to become neuron-like cells. The direct method, while simpler, showed cell detachment and death during the differentiation process. At early stage, PAX6, SOX1 and nestin were detected. At late stage of differentiation, all five genes tested, nestin, βIII-tubulin, neurofilament medium chain, GFAP, and Nav, were positive in many cells in cultures. Both differentiation methods led to neuron-like cells in cultures exhibiting sodium and potassium currents, action potential, or spontaneous excitatory postsynaptic potential. Thus, TF-DSC iPSCs are capable of undergoing guided neurogenic differentiation into functional neurons in vitro, thereby may serve as a cell source for neural regeneration.

Injectable hydroxyphenyl derivative of hyaluronic acid hydrogel modified with RGD as scaffold for spinal cord injury repair

Hydrogel scaffolds which bridge the lesion, together with stem cell therapy represent a promising approach for spinal cord injury (SCI) repair. In this study, a hydroxyphenyl derivative of hyaluronic acid (HA-PH) was modified with the integrin-binding peptide arginine-glycine-aspartic acid (RGD), and enzymatically crosslinked to obtain a soft injectable hydrogel. Moreover, addition of fibrinogen was used to enhance proliferation of human Wharton's jelly-derived mesenchymal stem cells (hWJ-MSCs) on HA-PH-RGD hydrogel. The neuroregenerative potential of HA-PH-RGD hydrogel was evaluated in vivo in acute and subacute models of SCI. Both HA-PH-RGD hydrogel injection and implantation into the acute spinal cord hemisection cavity resulted in the same axonal and blood vessel density in the lesion area after 2 and 8 weeks. HA-PH-RGD hydrogel alone or combined with fibrinogen (HA-PH-RGD/F) and seeded with hWJ-MSCs was then injected into subacute SCI and evaluated after 8 weeks using behavioural, histological and gene expression analysis. A subacute injection of both HA-PH-RGD and HA-PH-RGD/F hydrogels similarly promoted axonal ingrowth into the lesion and this effect was further enhanced when the HA-PH-RGD/F was combined with hWJ-MSCs. On the other hand, no effect was found on locomotor recovery or the blood vessel ingrowth and density of glial scar around the lesion. In conclusion, we have developed and characterized injectable HA-PH-RGD based hydrogel, which represents a suitable material for further combinatorial therapies in neural tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1129-1140, 2018.

Retinoic acid synthesis by NG2 expressing cells promotes a permissive environment for axonal outgrowth

Stimulation of retinoic acid (RA) mediated signalling pathways following neural injury leads to regeneration in the adult nervous system and numerous studies have shown that the specific activation of the retinoic acid receptor β (RARβ) is required for this process. Here we identify a novel mechanism by which neuronal RARβ activation results in the endogenous synthesis of RA which is released in association with exosomes and acts as a positive cue to axonal/neurite outgrowth. Using an established rodent model of RARβ induced axonal regeneration, we show that neuronal RARβ activation upregulates the enzymes involved in RA synthesis in a cell specific manner; alcohol dehydrogenase7 (ADH7) in neurons and aldehyde dehydrogenase 2 (Raldh2) in NG2 expressing cells (NG2 + cells). These release RA in association with exosomes providing a permissive substrate to neurite outgrowth. Conversely, deletion of Raldh2 in the NG2 + cells in our in vivo regeneration model is sufficient to compromise axonal outgrowth. This hitherto unidentified RA paracrine signalling is required for axonal/neurite outgrowth and is initiated by the activation of neuronal RARβ signalling.

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Raldh2, the enzyme for retinoic acid synthesis, is upregulated in NG2 + cells during axonal regeneration.

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Deletion of Raldh2 in NG2 + cells prevents regeneration.

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RA signalling modulates axonal pathfinding.

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Fine-tuned regulation of RA distribution via exosome transport

let-7 miRNA controls CED-7 homotypic adhesion and EFF-1-mediated axonal self-fusion to restore touch sensation following injury

Neuronal injury often leads to devastating consequences such as loss of senses or locomotion. Restoration of function after injury relies on whether the injured axons can find their target cells. Although fusion between injured proximal axon and distal fragment has been observed in many organisms, its functional significance is not clear. Here, using Caenorhabditis elegans mechanosensory neurons, we address this question. Using two femtosecond lasers simultaneously, we could scan and sever posterior lateral microtubule neurons [posterior lateral microtubules (PLMs)] on both sides of the worm. We showed that axotomy of both PLMs leads to a dramatic loss of posterior touch sensation. During the regenerative phase, only axons that fuse to their distal counterparts contribute to functional recovery. Loss of let-7 miRNA promotes functional restoration in both larval and adult stages. In the L4 stage, loss of let-7 increases fusion events by increasing the mRNA level of one of the cell-recognition molecules, CED-7. The ability to establish cytoplasmic continuity between the proximal and distal ends declines with age. Loss of let-7 overcomes this barrier by promoting axonal transport and enrichment of the EFF-1 fusogen at the growing tip of cut processes. Our data reveal the functional property of a regenerating neuron.

In Vivo Imaging of CNS Injury and Disease

In vivo optical imaging has emerged as a powerful tool with which to study cellular responses to injury and disease in the mammalian CNS. Important new insights have emerged regarding axonal degeneration and regeneration, glial responses and neuroinflammation, changes in the neurovascular unit, and, more recently, neural transplantations. Accompanying a 2017 SfN Mini-Symposium, here, we discuss selected recent advances in understanding the neuronal, glial, and other cellular responses to CNS injury and disease with in vivo imaging of the rodent brain or spinal cord. We anticipate that in vivo optical imaging will continue to be at the forefront of breakthrough discoveries of fundamental mechanisms and therapies for CNS injury and disease.

Rewiring Neuronal Circuits: A New Method for Fast Neurite Extension and Functional Neuronal Connection

Brain and spinal cord injury may lead to permanent disability and death because it is still not possible to regenerate neurons over long distances and accurately reconnect them with an appropriate target. Here a procedure is described to rapidly initiate, elongate, and precisely connect new functional neuronal circuits over long distances. The extension rates achieved reach over 1.2 mm/h, 30-60 times faster than the in vivo rates of the fastest growing axons from the peripheral nervous system (0.02 to 0.04 mm/h)²⁸ and 10 times faster than previously reported for the same neuronal type at an earlier stage of development⁴. First, isolated populations of rat hippocampal neurons are grown for 2-3 weeks in microfluidic devices to precisely position the cells, enabling easy micromanipulation and experimental reproducibility. Next, beads coated with poly-D-lysine (PDL) are placed on neurites to form adhesive contacts and pipette micromanipulation is used to move the resulting bead-neurite complex. As the bead is moved, it pulls out a new neurite that can be extended over hundreds of micrometers and functionally connected to a target cell in less than 1 h. This process enables experimental reproducibility and ease of manipulation while bypassing slower chemical strategies to induce neurite growth. Preliminary measurements presented here demonstrate a neuronal growth rate far exceeding physiological ones. Combining these innovations allows for the precise establishment of neuronal networks in culture with an unprecedented degree of control. It is a novel method that opens the door to a plethora of information and insights into signal transmission and communication within the neuronal network as well as being a playground in which to explore the limits of neuronal growth. The potential applications and experiments are widespread with direct implications for therapies that aim to reconnect neuronal circuits after trauma or in neurodegenerative diseases.

Dorsal Root Ganglion Injection and Dorsal Root Crush Injury as a Model for Sensory Axon Regeneration

Achieving axon regeneration after nervous system injury is a challenging task. As different parts of the central nervous system (CNS) differ from each other anatomically, it is important to identify an appropriate model to use for the study of axon regeneration. By using a suitable model, we can formulate a specific treatment based on the severity of injury, the neuronal cell type of interest, and the desired spinal tract for assessing regeneration. Within the sensory pathway, DRG neurons are responsible for relaying sensory information from the periphery to the CNS. We present here a protocol that uses a DRG injection with a viral vector and a concurrent dorsal root crush injury in the lower cervical spinal cord of an adult rat as a model to study sensory axon regeneration. As demonstrated using a control virus, AAV5-GFP, we show the effectiveness of a direct DRG injection in transducing DRG neurons and tracing sensory axons into the spinal cord. We also show the effectiveness of the dorsal root crush injury in denervating the forepaw as an injury model for evaluating axon regeneration. Despite the requirement for specialized training to perform this invasive surgical procedure, the protocol is flexible, and potential users can modify many parts to accommodate their experimental requirements. Importantly, it can serve as a foundation for those in search of a suitable animal model for their studies. We believe that this article will help new users to learn the procedure in a very efficient and effective manner.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

The soft mechanical signature of glial scars in the central nervous system

Injury to the central nervous system (CNS) alters the molecular and cellular composition of neural tissue and leads to glial scarring, which inhibits the regrowth of damaged axons. Mammalian glial scars supposedly form a chemical and mechanical barrier to neuronal regeneration. While tremendous effort has been devoted to identifying molecular characteristics of the scar, very little is known about its mechanical properties. Here we characterize spatiotemporal changes of the elastic stiffness of the injured rat neocortex and spinal cord at 1.5 and three weeks post-injury using atomic force microscopy. In contrast to scars in other mammalian tissues, CNS tissue significantly softens after injury. Expression levels of glial intermediate filaments (GFAP, vimentin) and extracellular matrix components (laminin, collagen IV) correlate with tissue softening. As tissue stiffness is a regulator of neuronal growth, our results may help to understand why mammalian neurons do not regenerate after injury.

The Function of FGFR1 Signalling in the Spinal Cord: Therapeutic Approaches Using FGFR1 Ligands after Spinal Cord Injury

Extensive research is ongoing that concentrates on finding therapies to enhance CNS regeneration after spinal cord injury (SCI) and to cure paralysis. This review sheds light on the role of the FGFR pathway in the injured spinal cord and discusses various therapies that use FGFR activating ligands to promote regeneration after SCI. We discuss studies that use peripheral nerve grafts or Schwann cell grafts in combination with FGF1 or FGF2 supplementation. Most of these studies show evidence that these therapies successfully enhance axon regeneration into the graft. Further they provide evidence for partial recovery of sensory function shown by electrophysiology and motor activity evidenced by behavioural data. We also present one study that indicates that combination with additional, synergistic factors might further drive the system towards functional regeneration. In essence, this review summarises the potential of nerve and cell grafts combined with FGF1/2 supplementation to improve outcome even after severe spinal cord injury.

Restoring axonal localization and transport of transmembrane receptors to promote repair within the injured CNS: a critical step in CNS regeneration

Each neuronal subtype is distinct in how it develops, responds to environmental cues, and whether it is capable of mounting a regenerative response following injury. Although the adult central nervous system (CNS) does not regenerate, several experimental interventions have been trialled with successful albeit limited instances of axonal repair. We highlight here some of these approaches including extracellular matrix (ECM) modification, cellular grafting, gene therapy-induced replacement of proteins, as well as application of biomaterials. We also review the recent report demonstrating the failure of axonal localization and transport of growth-promoting receptors within certain classes of mature neurons. More specifically, we discuss an inability of integrin receptors to localize within the axonal compartment of mature motor neurons such as in the corticospinal and rubrospinal tracts, whereas in immature neurons of those pathways and in mature sensory tracts such as in the optic nerve and dorsal column pathways these receptors readily localize within axons. Furthermore we assert that this failure of axonal localization contributes to the intrinsic inability of axonal regeneration. We conclude by highlighting the necessity for both combined therapies as well as a targeted approach specific to both age and neuronal subtype will be required to induce substantial CNS repair.

Differential regenerative ability of sensory and motor neurons

After injury, the adult mammalian central nervous system (CNS) lacks long-distance axon regeneration. This review discusses the similarities and differences of sensory and motor neurons, seeking to understand how to achieve functional sensory and motor regeneration. As these two types of neurons respond differently to axotomy, growth environment and treatment, the future challenge will be on how to achieve full recovery in a way that allows regeneration of both types of fibres simultaneously.

Regrowth of Serotonin Axons in the Adult Mouse Brain Following Injury

It is widely believed that damaged axons in the adult mammalian brain have little capacity to regrow, thereby impeding functional recovery after injury. Studies using fixed tissue have suggested that serotonin neurons might be a notable exception, but remain inconclusive. We have employed in vivo two-photon microscopy to produce time-lapse images of serotonin axons in the neocortex of the adult mouse. Serotonin axons undergo massive retrograde degeneration following amphetamine treatment and subsequent slow recovery of axonal density, which is dominated by new growth with little contribution from local sprouting. A stab injury that transects serotonin axons running in the neocortex is followed by local regression of cut serotonin axons and followed by regrowth from cut ends into and across the stab rift zone. Regrowing serotonin axons do not follow the pathways left by degenerated axons. The regrown axons release serotonin and their regrowth is correlated with recovery in behavioral tests.

Restoration of Visual Function by Enhancing Conduction in Regenerated Axons

Although a number of repair strategies have been shown to promote axon outgrowth following neuronal injury in the mammalian CNS, it remains unclear whether regenerated axons establish functional synapses and support behavior. Here, in both juvenile and adult mice, we show that either PTEN and SOCS3 co-deletion, or co-overexpression of osteopontin (OPN)/insulin-like growth factor 1 (IGF1)/ciliary neurotrophic factor (CNTF), induces regrowth of retinal axons and formation of functional synapses in the superior colliculus (SC) but not significant recovery of visual function. Further analyses suggest that regenerated axons fail to conduct action potentials from the eye to the SC due to lack of myelination. Consistent with this idea, administration of voltage-gated potassium channel blockers restores conduction and results in increased visual acuity. Thus, enhancing both regeneration and conduction effectively improves function after retinal axon injury.

Rapid Mechanically Controlled Rewiring of Neuronal Circuits

CNS injury may lead to permanent functional deficits because it is still not possible to regenerate axons over long distances and accurately reconnect them with an appropriate target. Using rat neurons, microtools, and nanotools, we show that new, functional neurites can be created and precisely positioned to directly (re)wire neuronal networks. We show that an adhesive contact made onto an axon or dendrite can be pulled to initiate a new neurite that can be mechanically guided to form new synapses at up to 0.8 mm distance in <1 h. Our findings challenge current understanding of the limits of neuronal growth and have direct implications for the development of new therapies and surgical techniques to achieve functional regeneration. Significance statement: Brain and spinal cord injury may lead to permanent disability and death because it is still not possible to regenerate neurons over long distances and accurately reconnect them with an appropriate target. Using microtools and nanotools we have developed a new method to rapidly initiate, elongate, and precisely connect new functional neuronal circuits over long distances. The extension rates achieved are ≥60 times faster than previously reported. Our findings have direct implications for the development of new therapies and surgical techniques to achieve functional regeneration after trauma and in neurodegenerative diseases. It also opens the door for the direct wiring of robust brain-machine interfaces as well as for investigations of fundamental aspects of neuronal signal processing and neuronal function.

Glial progenitor cell migration promotes CNS axon growth on functionalized electroconducting microfibers

Electroactive systems that promote directional axonal growth and migration of glial progenitor cells (GPC) are needed for the treatment of neurological injuries. We report the functionalization of electroconducting microfibers with multiple biomolecules that synergistically stimulate the proliferation and migration of GPC, which in turn induce axonal elongation from embryonic cerebral cortex neurons. PEDOT doped with poly[(4-styrenesulfonic acid)-co-(maleic acid)] was synthesized on carbon microfibers and used for covalent attachment of molecules to the electroactive surface. The molecular complexes that promoted GPC proliferation and migration, followed by axonal extension, were composed of polylysine, heparin, basic fibroblast growth factor (bFGF), and matricellular proteins; the combination of bFGF with vitronectin or fibronectin being indispensable for sustained glial and axonal growth. The rate of glial-induced axonal elongation was about threefold that of axons growing directly on microfibers functionalized with polylysine alone. Electrical stimuli applied through the microfibers released bFGF and fibronectin from the polymer surface, consequently reducing GPC proliferation and promoting their differentiation into astrocytes, without causing cell detachment or toxicity. These results suggest that functionalized electroactive microfibers may provide a multifunctional tool for controlling neuron-glia interactions and enhancing neural repair.

STATEMENT OF SIGNIFICANCE

We report a multiple surface functionalization strategy for electroconducting microfibers (MFs), in order to promote proliferation and guided migration of glial precursor cells (GPC) and consequently create a permissive substrate for elongation of central nervous system (CNS) axons. GPC divided and migrated extensively on the functionalized MFs, leading to fast elongation of embryonic cerebral cortex axons. The application of electric pulses thorough the MFs controlled glial cell division and differentiation. The functionalized MFs provide an advanced tool for neural tissue engineering and for controlling neuron-glial interactions. CNS axonal growth associated to migratory glial precursors, together with the possibility of directing glial differentiation by electrical stimuli applied through the MFs, open a new research avenue to explore for CNS repair.

Treatment with analgesics after mouse sciatic nerve injury does not alter expression of wound healing-associated genes

Animal models of sciatic nerve injury are commonly used to study neuropathic pain as well as axon regeneration. Administration of post-surgical analgesics is an important consideration for animal welfare, but the actions of the analgesic must not interfere with the scientific goals of the experiment. In this study, we show that treatment with either buprenorphine or acetaminophen following a bilateral sciatic nerve crush surgery does not alter the expression in dorsal root ganglion (DRG) sensory neurons of a panel of genes associated with wound healing. These findings indicate that the post-operative use of buprenorphine or acetaminophen at doses commonly suggested by Institutional Animal Care and Use Committees does not change the intrinsic gene expression response of DRG neurons to a sciatic nerve crush injury, for many wound healing-associated genes. Therefore, administration of post-operative analgesics may not confound the results of transcriptomic studies employing this injury model.

Axon glycoprotein routing in nerve polarity, function, and repair

Nervous system function relies on the capacity of neurons to organize specialized domains for impulse reception or transmission. Such a polarized architecture relies on highly discriminatory and efficient mechanisms for the transport and targeting of required molecules to their functional positions. Glycans play a central role in polarized traffic based on their extraordinary capacity to encrypt bio-information. Glycan-based interactions exquisitely regulate cargo selection, trafficking, and targeting to the axon membrane. This generates segregated functional domains, where basal nerve processes such as axon growth, synaptic activity, or myelination take place. Deciphering the details of the glycan structures and carbohydrate-binding molecules that underlie these mechanisms improves our knowledge of nerve physiology and defines novel specific approaches for neurological treatments.

Shared Brain Connectivity Issues, Symptoms, and Comorbidities in Autism Spectrum Disorder, Attention Deficit/Hyperactivity Disorder, and Tourette Syndrome

The prevalence of neurodevelopmental disorders, including autism spectrum disorder (ASD), attention deficit/hyperactivity disorder (ADHD), and Tourette syndrome (TS), has increased over the past two decades. Currently, about one in six children in the United States is diagnosed as having a neurodevelopmental disorder. Evidence suggests that ASD, ADHD, and TS have similar neuropathology, which includes long-range underconnectivity and short-range overconnectivity. They also share similar symptomatology with considerable overlap in their core and associated symptoms and a frequent overlap in their comorbid conditions. Consequently, it is apparent that ASD, ADHD, and TS diagnoses belong to a broader spectrum of neurodevelopmental illness. Biologically, long-range underconnectivity and short-range overconnectivity are plausibly related to neuronal insult (e.g., neurotoxicity, neuroinflammation, excitotoxicity, sustained microglial activation, proinflammatory cytokines, toxic exposure, and oxidative stress). Therefore, these disorders may a share a similar etiology. The main purpose of this review is to critically examine the evidence that ASD, ADHD, and TS belong to a broader spectrum of neurodevelopmental illness, an abnormal connectivity spectrum disorder, which results from neural long-range underconnectivity and short-range overconnectivity. The review also discusses the possible reasons for these neuropathological connectivity findings. In addition, this review examines the role and issue of axonal injury and regeneration in order to better understand the neuropathophysiological interplay between short- and long-range axons in connectivity issues.

An ex vivo laser-induced spinal cord injury model to assess mechanisms of axonal degeneration in real-time

Injured CNS axons fail to regenerate and often retract away from the injury site. Axons spared from the initial injury may later undergo secondary axonal degeneration. Lack of growth cone formation, regeneration, and loss of additional myelinated axonal projections within the spinal cord greatly limits neurological recovery following injury. To assess how central myelinated axons of the spinal cord respond to injury, we developed an ex vivo living spinal cord model utilizing transgenic mice that express yellow fluorescent protein in axons and a focal and highly reproducible laser-induced spinal cord injury to document the fate of axons and myelin (lipophilic fluorescent dye Nile Red) over time using two-photon excitation time-lapse microscopy. Dynamic processes such as acute axonal injury, axonal retraction, and myelin degeneration are best studied in real-time. However, the non-focal nature of contusion-based injuries and movement artifacts encountered during in vivo spinal cord imaging make differentiating primary and secondary axonal injury responses using high resolution microscopy challenging. The ex vivo spinal cord model described here mimics several aspects of clinically relevant contusion/compression-induced axonal pathologies including axonal swelling, spheroid formation, axonal transection, and peri-axonal swelling providing a useful model to study these dynamic processes in real-time. Major advantages of this model are excellent spatiotemporal resolution that allows differentiation between the primary insult that directly injures axons and secondary injury mechanisms; controlled infusion of reagents directly to the perfusate bathing the cord; precise alterations of the environmental milieu (e.g., calcium, sodium ions, known contributors to axonal injury, but near impossible to manipulate in vivo); and murine models also offer an advantage as they provide an opportunity to visualize and manipulate genetically identified cell populations and subcellular structures. Here, we describe how to isolate and image the living spinal cord from mice to capture dynamics of acute axonal injury.

Kinesin KIF4A transports integrin β1 in developing axons of cortical neurons

CNS axons have poor regenerative ability compared to PNS axons, and mature axons regenerate less well than immature embryonic axons. The loss of regenerative ability with maturity is accompanied by the setting up of a selective transport filter in axons, restricting the types of molecule that are present. We confirm that integrins (represented by subunits β1 and α5) are present in early cortical axons in vitro but are excluded from mature axons. Ribosomal protein and L1 show selective axonal transport through association with kinesin kif4A; we have therefore examined the hypothesis that integrin transport might also be in association with kif4A. Kif4A is present in all processes of immature cortical neurons cultured at E18, then downregulated by 14days in vitro, coinciding with the exclusion of integrin from axons. Kif4a co-localises with β1 integrin in vesicles in neurons and non-neuronal cells, and the two molecules co-immunoprecipitate. Knockdown of KIF4A expression with shRNA reduced the level of integrin β1 in axons of developing neurons and reduced neurite elongation on laminin, an integrin-dependent substrate. Overexpression of kif4A triggered apoptosis in neuronal and non-neuronal cells. In mature neurons expression of kif4A-GFP at a modest level did not kill the cells, and the kif4A was detectable in their axons. However this was not accompanied by an increase in integrin β1 axonal transport, suggesting that kif4A is not the only integrin transporter, and that integrin exclusion from axons is controlled by factors other than the kif4A level.

AKAP12 mediates barrier functions of fibrotic scars during CNS repair

The repair process after CNS injury shows a well-organized cascade of three distinct stages: inflammation, new tissue formation, and remodeling. In the new tissue formation stage, various cells migrate and form the fibrotic scar surrounding the lesion site. The fibrotic scar is known as an obstacle for axonal regeneration in the remodeling stage. However, the role of the fibrotic scar in the new tissue formation stage remains largely unknown. We found that the number of A-kinase anchoring protein 12 (AKAP12)-positive cells in the fibrotic scar was increased over time, and the cells formed a structure which traps various immune cells. Furthermore, the AKAP12-positive cells strongly express junction proteins which enable the structure to function as a physical barrier. In in vivo validation, AKAP12 knock-out (KO) mice showed leakage from a lesion, resulting from an impaired structure with the loss of the junction complex. Consistently, focal brain injury in the AKAP12 KO mice led to extended inflammation and more severe tissue damage compared to the wild type (WT) mice. Accordingly, our results suggest that AKAP12-positive cells in the fibrotic scar may restrict excessive inflammation, demonstrating certain mechanisms that could underlie the beneficial actions of the fibrotic scar in the new tissue formation stage during the CNS repair process.

Signaling regulations of neuronal regenerative ability

Different from physiological axon growth during development, a major limiting factor for successful axon regeneration is the poor intrinsic regenerative capacity in mature neurons in the adult mammalian central nervous system (CNS). Recent studies identified several molecular pathways, including PTEN/mTOR, Jak/STAT, DLK/JNK, providing important probes in investigating the mechanisms by which the regenerative ability is regulated. This review will summarize these recent findings and speculate their implications.

Neu3 sialidase-mediated ganglioside conversion is necessary for axon regeneration and is blocked in CNS axons

PNS axons have a high intrinsic regenerative ability, whereas most CNS axons show little regenerative response. We show that activation of Neu3 sialidase, also known as Neuraminidase-3, causing conversion of GD1a and GT1b to GM1 ganglioside, is an essential step in regeneration occurring in PNS (sensory) but not CNS (retinal) axons in adult rat. In PNS axons, axotomy activates Neu3 sialidase, increasing the ratio of GM1/GD1a and GM1/GT1b gangliosides immediately after injury in vitro and in vivo. No change in the GM1/GD1a ratio after axotomy was observed in retinal axons (in vitro and in vivo), despite the presence of Neu3 sialidase. Externally applied sialidase converted GD1a ganglioside to GM1 and rescued axon regeneration in CNS axons and in PNS axons after Neu3 sialidase blockade. Neu3 sialidase activation in DRGs is initiated by an influx of extracellular calcium, activating P38MAPK and then Neu3 sialidase. Ganglioside conversion by Neu3 sialidase further activates the ERK pathway. In CNS axons, P38MAPK and Neu3 sialidase were not activated by axotomy.

DRG axon elongation and growth cone collapse rate induced by Sema3A are differently dependent on NGF concentration

Regeneration of embryonic and adult dorsal root ganglion (DRG) sensory axons is highly impeded when they encounter neuronal growth cone-collapsing factor semaphorin3A (Sema3A). On the other hand, increasing evidence shows that DRG axon's regeneration can be stimulated by nerve growth factor (NGF). In this study, we aimed to evaluate whether increased NGF concentrations can counterweight Sema3A-induced inhibitory responses in 15-day-old mouse embryo (E15) DRG axons. The DRG explants were grown in Neurobasal-based medium with different NGF concentrations ranging from 0 to 100 ng/mL and then treated with Sema3A at constant 10 ng/mL concentration. To evaluate interplay between NGF and Sema3A number of DRG axons, axon outgrowth distance and collapse rate were measured. We found that the increased NGF concentrations abolish Sema3A-induced inhibitory effect on axon outgrowth, while they have no effect on Sema3A-induced collapse rate.

Syntenin-a promotes spinal cord regeneration following injury in adult zebrafish

In contrast to mammals, adult zebrafish recover locomotor function after spinal cord injury, in part due to the capacity of the central nervous system to repair severed connections. To identify molecular cues that underlie regeneration, we conducted mRNA expression profiling and found that syntenin-a expression is upregulated in the adult zebrafish spinal cord caudal to the lesion site after injury. Syntenin is a scaffolding protein involved in mammalian cell adhesion and movement, axonal outgrowth, establishment of cell polarity, and protein trafficking. It could thus be expected to be involved in supporting regeneration in fish. Syntenin-a mRNA and protein are expressed in neurons, glia and newly generated neural cells, and upregulated caudal to the lesion site on days 6 and 11 following spinal cord injury. Treatment of spinal cord-injured fish with two different antisense morpholinos to knock down syntenin-a expression resulted in significant inhibition of locomotor recovery at 5 and 6 weeks after injury, when compared to control morpholino-treated fish. Knock-down of syntenin-a reduced regrowth of descending axons from brainstem neurons into the spinal cord caudal to the lesion site. These observations indicate that syntenin-a is involved in regeneration after traumatic insult to the central nervous system of adult zebrafish, potentially leading to novel insights into the cellular and molecular mechanisms that require activation in the regeneration-deficient mammalian central nervous system.