Regular ArticleSprouts from Cut Corticospinal Axons Persist in the Presence of Astrocytic Scarring in Long-Term Lesions of the Adult Rat Spinal Cord
Abstract
Small, circumscribed electrolytic lesions were made in the corticospinal tract at the upper cervical level of the adult rat spinal cord. At increasing survival times, immunohistochemistry of glial fibrillary acidic protein and electron microscopy showed that the predominantly longitudinal astrocytic processes underwent a progressive hypertrophy, which spread from the lesion, increasing in intensity from 1 week and reaching a maximum at between 9.5 and 13 weeks, by which time the lesion was completely surrounded by a dense astrocytic scar. A previous study with orthograde transport of axonal tracers showed that from 2 weeks after the lesion the main axonal stems of both cut and adjacent uncut corticospinal axons had large varicosities. The swollen ends of the cut axons, and also the adjacent uncut axons, emitted extensive arborizations of sprouts directed into the central, macrophage-filled area of the lesion. The present experiments indicated that the axon sprouts persisted apparently undiminished over the period (from 9.5 to 13 weeks) when the astrocytic scarring process was reaching its maximum. Surrounding the center of the lesion was an area in which the axons had become demyelinated. By 3 weeks a few axons were remyelinated with peripheral myelin formed by Schwann cells which had migrated into the lesions. By 4 months the scar region was densely colonized by Schwann cells, which now had remyelinated a wide swath of both cut and uncut axons. The cut axons were myelinated by Schwann cells as far as their large terminal expansions, which were sheathed, but not myelinated, by satellitic Schwann cells. Thus, at survivals long enough for the formation of a dense, astrocytic scar, cut corticospinal axons retain extensive terminal and collateral arborizations even in the macrophage-filled central lesion area and are myelinated or ensheathed by endogenous Schwann cells.
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Regulation of autophagy by inhibitory CSPG interactions with receptor PTPσ and its impact on plasticity and regeneration after spinal cord injury
2020, Experimental NeurologyChondroitin sulfate proteoglycans (CSPGs), extracellular matrix molecules that increase dramatically following a variety of CNS injuries or diseases, have long been known for their potent capacity to curtail cell migrations as well as axon regeneration and sprouting. The inhibition can be conferred through binding to their major cognate receptor, Protein Tyrosine Phosphatase Sigma (PTPσ). However, the precise mechanisms downstream of receptor binding that mediate growth inhibition have remained elusive. Recently, CSPGs/PTPσ interactions were found to regulate autophagic flux at the axon growth cone by dampening the autophagosome-lysosomal fusion step. Because of the intense interest in autophagic phenomena in the regulation of a wide variety of critical cellular functions, we summarize here what is currently known about dysregulation of autophagy following spinal cord injury, and highlight this critical new mechanism underlying axon regeneration failure. Furthermore, we review how CSPGs/PTPσ interactions influence plasticity through autophagic regulation and how PTPσ serves as a switch to execute either axon outgrowth or synaptogenesis. This has exciting implications for the role CSPGs play not only in axon regeneration failure after spinal cord injury, but also in neurodegenerative diseases where, again, inhibitory CSPGs are upregulated.
Inhibiting store-operated calcium entry attenuates white matter secondary degeneration following SCI
2020, Neurobiology of DiseaseAxonal degeneration plays a key role in the pathogenesis of numerous neurological disorders including spinal cord injury. After the irreversible destruction of the white matter elements during the primary (mechanical) injury, spared axons and their supporting glial cells begin to breakdown causing an expansion of the lesion site. Here we mechanistically link external sources of calcium entry through axoplasmic reticulum calcium store depletion that contributes to secondary axonal degeneration through a process called store-operated calcium entry. There is increasing evidence suggesting that store-operated calcium entry impairment is responsible for numerous disorders. Nevertheless, its role following spinal cord injury remains poorly understood. We hypothesize that store-operated calcium entry mediates secondary white matter degeneration after spinal cord injury. We used our previously published model of laser-induced spinal cord injury to focally transect mid cervical dorsal column axons from live 6–8-week-old heterozygous CNPaseGFP/+: Thy1YFP+ double transgenic murine spinal cord preparations (five treated, eight controls) and documented the dynamic changes in axons over time using two-photon excitation microscopy. We report that 1 hour delayed treatment with YM-58483, a potent inhibitor of store-operated calcium entry, significantly decreased intra-axonal calcium accumulation, axonal dieback both proximal and distal to the lesion site, reduced secondary axonal “bystander” damage acutely after injury, and promoted greater oligodendrocyte survival compared to controls. We also targeted store-operated calcium entry following a clinically relevant contusion spinal cord injury model in vivo. Adult, 6–8-week-old Advillin-Cre: Ai9 mice were subjected to a mild 30 kdyn contusion and imaged to observe secondary axonal degeneration in live animals. We found that delayed treatment with YM-58483 increased axonal survival and reduced axonal spheroid formation compared to controls (n = 5 mice per group). These findings suggest that blocking store-operated calcium entry acutely is neuroprotective and introduces a novel target to prevent pathological calcium entry following spinal cord injury using a clinically relevant model.
Cytoskeleton dynamics in axon regeneration
2018, Current Opinion in NeurobiologyRecent years have seen cytoskeleton dynamics emerging as a key player in axon regeneration. The cytoskeleton, in particular microtubules and actin, ensures the growth of neuronal processes and maintains the singular, highly polarized shape of neurons. Following injury, adult central axons are tipped by a dystrophic structure, the retraction bulb, which prevents their regeneration. Abnormal cytoskeleton dynamics are responsible for the formation of this growth-incompetent structure but pharmacologically modulating cytoskeleton dynamics of injured axons can transform this structure into a growth-competent growth cone. The cytoskeleton also drives the migration of scar-forming cells after an injury. Targeting its dynamics modifies the composition of the inhibitory environment formed by scar tissue and renders it more permissive for regenerating axons. Hence, cytoskeleton dynamics represent an appealing target to promote axon regeneration. As some of cytoskeleton-targeting drugs are used in the clinics for other purposes, they hold the promise to be used as a basis for a regenerative therapy after a spinal cord injury.
Research on the biology of adult neural stem cells (NSCs) and induced NSCs (iNSCs), as well as NSC-based therapies for diseases in central nervous system (CNS) has started to generate the expectation that these cells may be used for treatments in CNS injuries or disorders. Recent technological progresses in both NSCs themselves and their derivatives have brought us closer to therapeutic applications. Adult neurogenesis presents in particular regions in mammal brain, known as neurogenic niches such as the dental gyrus (DG) in hippocampus and the subventricular zone (SVZ), within which adult NSCs usually stay for long periods out of the cell cycle, in G0. The reactivation of quiescent adult NSCs needs orchestrated interactions between the extrinsic stimulis from niches and the intrinsic factors involving transcription factors (TFs), signaling pathway, epigenetics, and metabolism to start an intracellular regulatory program, which promotes the quiescent NSCs exit G0 and reenter cell cycle. Extrinsic and intrinsic mechanisms that regulate adult NSCs are interconnected and feedback on one another. Since endogenous neurogenesis only happens in restricted regions and steadily fails with disease advances, interest has evolved to apply the iNSCs converted from somatic cells to treat CNS disorders, as is also promising and preferable. To overcome the limitation of viral-based reprogramming of iNSCs, bioactive small molecules (SM) have been explored to enhance the efficiency of iNSC reprogramming or even replace TFs, making the iNSCs more amenable to clinical application. Despite intense research efforts to translate the studies of adult and induced NSCs from the bench to bedside, vital troubles remain at several steps in these processes. In this review, we examine the present status, advancement, pitfalls, and potential of the two types of NSC technologies, focusing on each aspects of reactivation of quiescent adult NSC and reprogramming of iNSC from somatic cells, as well as on progresses in cell-based regenerative strategies for neural repair and criteria for successful therapeutic applications.
Determinants of Axon Growth, Plasticity, and Regeneration in the Context of Spinal Cord Injury
2018, American Journal of PathologyThe mechanisms that underlie recovery after injury of the central nervous system have rarely been definitively established. Axon regrowth remains the major prerequisite for plasticity, regeneration, circuit formation, and eventually functional recovery. The attributed functional relevance of axon regrowth, however, will depend on several subsequent conditional neurobiological modifications, including myelination and synapse formation, but also pruning of aberrant connectivity. Despite the ability to revamp axon outgrowth by altering an increasing number of extracellular and intracellular targets, disentangling which axons are responsible for the recovery of function from those that are functionally silent, or even contributing to aberrant functions, represents a pertinent void in our understanding, challenging the intuitive translational link between anatomical and functional regeneration. Anatomic hallmarks of regeneration are not static and are largely activity dependent. Herein, we survey mechanisms leading to the formation of dystrophic growth cone at the injured axonal tip, the subsequent axonal dieback, and the molecular determinants of axon growth, plasticity, and regeneration in the context of spinal cord injury.
A view from the ending: Axonal dieback and regeneration following SCI
2017, Neuroscience LettersCitation Excerpt :No obvious organelle accumulation or endocytic vesicles are present. In the CST endings shown by Li and Raisman [64], granular cytoplasm and small membranous vesicles/Golgi are present and condensed in patches within the center of the endings. Many small vesicles are observed along the inner plasma membrane in the two endings shown.
Following spinal cord injury (SCI), most axons fail to regenerate and instead form large, swollen endings generically called ‘retraction bulbs.’ These endings form and persist after SCI even under experimental therapeutic conditions where significant CNS regeneration occurs. Although retraction bulbs can arise from either activation of degenerative processes or deficits in regenerative processes, they are typically grouped as a single type of axonal ending. To facilitate the targeting of axonal endings for SCI repair, this review focuses on dissecting the different types of axonal endings present following injury by examining them in the context of the temporal, degenerative and regenerative changes that occur following injury. The stages of axonal dieback (also known as axonal retraction) and the steps necessary for successful axonal regeneration are outlined. The types of axonal endings that can arise as an axon successfully or unsuccessfully mounts a regenerative response are examined, with an emphasis on retraction bulbs, growth cones, and collapsed growth cones. Retraction bulbs are subdivided into those that arise from a failure to form a growth cone (endbulbs) and those that stall in response to inhibitory gradients (dystrophic axonal endings). The current understanding of the mechanisms that lead to the development of different types of axonal endings, how different experimental therapeutic interventions may act on different types of axonal endings, the current gaps in understanding the sites of action of some pro-regenerative therapies, and some of the methodological challenges to studying different types of axonal endings are discussed.