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The mitochondrial permeability transition pore is a nonspecific transmembrane channel. Inhibition of mitochondrial permeability transition pore opening has been shown to alleviate mitochondrial swelling, calcium overload, and axonal degeneration. Cyclophilin D is an important component of the mitochondrial permeability transition pore. Whether cyclophilin D participates in mitochondrial impairment and axonal injury after intracerebral hemorrhage is not clear. In this study, we established mouse models of intracerebral hemorrhage in vivo by injection of autologous blood and oxyhemoglobin into the striatum in Thy1-YFP mice, in which pyramidal neurons and axons express yellow fluorescent protein. We also simulated intracerebral hemorrhage in vitro in PC12 cells using oxyhemoglobin. We found that axonal degeneration in the early stage of intracerebral hemorrhage depended on mitochondrial swelling induced by cyclophilin D activation and mitochondrial permeability transition pore opening. We further investigated the mechanism underlying the role of cyclophilin D in mouse models and PC12 cell models of intracerebral hemorrhage. We found that both cyclosporin A inhibition and short hairpin RNA interference of cyclophilin D reduced mitochondrial permeability transition pore opening and mitochondrial injury. In addition, inhibition of cyclophilin D and mitochondrial permeability transition pore opening protected corticospinal tract integrity and alleviated motor dysfunction caused by intracerebral hemorrhage. Our findings suggest that cyclophilin D is used as a key mediator of axonal degeneration after intracerebral hemorrhage; inhibition of cyclophilin D expression can protect mitochondrial structure and function and further alleviate corticospinal tract injury and motor dysfunction after intracerebral hemorrhage. Our findings provide a therapeutic target for preventing axonal degeneration of white matter injury and subsequent functional impairment in central nervous diseases.

Cervus elaphus sibericus (CES), commonly known as deer antler, has been used as a medicinal herb because of its various pharmacological activities, including its anti-infective, anti-arthritic, anti-allergic, and anti-oxidative properties. However, the precise mechanisms by which CES functions as a potent anti-oxidative agent remain unknown; particularly, the effects of CES on cortical neurons and its neurobiological mechanism have not been examined. We used primary cortical neurons from the embryonic rat cerebral cortex and hydrogen peroxide to induce oxidative stress and damage in neurons. After post-treatment of CES at three concentrations (10, 50, and 200 µg/mL), the influence of CES on the neurobiological mechanism was assessed by immunocytochemistry, flow cytometry, and real-time PCR. CES effectively prevented neuronal death caused by hydrogen peroxide-induced damage by regulating oxidative signaling. In addition, CES significantly induced the expression of brain-derived neurotrophic factor and neurotrophin nerve growth factor, as well as regeneration-associated genes. We also observed newly processing elongated axons after CES treatment under oxidative conditions. In addition, filopodia tips generally do not form a retraction bulb, called swollen endings. Thus, CES shows therapeutic potential for treating neurological diseases by stimulating neuron repair and regeneration.

Primary somatosensory axons stop regenerating as they re-enter the spinal cord, resulting in incurable sensory loss. What arrests them has remained unclear. We previously showed that axons stop by forming synaptic contacts with unknown non-neuronal cells. Here, we identified these cells in adult mice as oligodendrocyte precursor cells (OPCs). We also found that only a few axons stop regenerating by forming dystrophic endings, exclusively at the CNS:peripheral nervous system (PNS) borderline where OPCs are absent. Most axons stop in contact with a dense network of OPC processes. Live imaging, immuno-electron microscopy (immuno-EM), and OPC-dorsal root ganglia (DRG) co-culture additionally suggest that axons are rapidly immobilized by forming synapses with OPCs. Genetic OPC ablation enables many axons to continue regenerating deep into the spinal cord. We propose that sensory axons stop regenerating by encountering OPCs that induce presynaptic differentiation. Our findings identify OPCs as a major regenerative barrier that prevents intraspinal restoration of sensory circuits following spinal root injury. [Display omitted] • Axons occasionally stop by forming dystrophic endings at the CNS:PNS borderline • Most axons cross the borderline and then stop in association with OPCs at the DREZ • DRG axons form synapses with OPCs at the DREZ and in vitro • OPC ablation enables continued axon regeneration at and beyond the DREZ Kim et al. identify oligodendrocyte precursor cells (OPCs) as the postsynaptic cells arresting sensory axons regenerating into the spinal cord at the CNS:PNS interface, the DREZ. Synaptogenic arrest by OPCs may play a decisive role in inhibiting axon regeneration at the DREZ and elsewhere in the CNS. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: After traumatic spinal cord injury, functional deficits increase as axons die back from the center of the lesion and the glial scar forms. Axonal dieback occurs in two phases: an initial axon intrinsic stage that occurs over the first several hours and a secondary phase which takes place over the first few weeks after injury. Here, we examine the secondary phase, which is marked by infiltration of macrophages. Using powerful time-lapse multi-photon imaging, we captured images of interactions between Cx3cr1 +/GFP macrophages and microglia and Thy-1YFP axons in a mouse dorsal column crush spinal cord injury model. Over the first few weeks after injury, axonal retraction bulbs within the lesion are static except when axonal fragments are lost by a blebbing mechanism in response to physical contact followed by phagocytosis by mobile Cx3Cr1 +/GFP cells. Utilizing a radiation chimera model to distinguish marrow-derived cells from radio-resistant CNS-resident microglia, we determined that the vast majority of accumulated cells in the lesion are derived from the blood and only these are associated with axonal damage. Interestingly, CNS-resident Cx3Cr1 +/GFP microglia did not increasingly accumulate nor participate in neuronal destruction in the lesion during this time period. Additionally, we found that the blood-derived cells consisted mainly of singly labeled Ccr2 +/RFP macrophages, singly labeled Cx3Cr1 +/GFP macrophages and a small population of double-labeled cells. Since all axon destructive events were seen in contact with a Cx3Cr1 +/GFP cell, we infer that the CCR2 single positive subset is likely not robustly involved in axonal dieback. Finally, in our model, deletion of CCR2, a chemokine receptor, did not alter the position of axons after dieback. Understanding the in vivo cellular interactions involved in secondary axonal injury may lead to clinical treatment candidates involving modulation of destructive infiltrating blood monocytes. [Copyright &y& Elsevier]

Traumatic brain injury, a silent epidemic of modern societies, is a largely neglected area in drug development and no drug is currently available for the treatment of patients suffering from brain trauma. Despite this grim situation, much progress has been made over the last two decades in closely related medical indications, such as spinal cord injury, giving rise to a more optimistic approach to drug development in brain trauma. Fundamental insights have been gained with animal models of central nervous system (CNS) trauma and spinal cord injury. Neuroregenerative drug candidates have been identified and two of these have progressed to clinical development for spinal cord injury patients. If successful, these drug candidates may be used to treat brain trauma patients. Significant progress has also been made in understanding the fundamental molecular mechanism underlying irreversible axonal growth arrest in the injured CNS of higher mammals. From these studies, we have learned that the axonal retraction bulb, previously regarded as a marker for failure of regenerative growth, is not static but dynamic and, therefore, amenable to pharmacotherapeutic approaches. With the development of modified magnetic resonance imaging methods, fibre tracts can be visualised in the living human brain and such imaging methods will soon be used to evaluate the neuroregenerative potential of drug candidates. These significant advances are expected to fundamentally change the often hopeless situation of brain trauma patients and will be the first step towards overcoming the silent epidemic of brain injury. [ABSTRACT FROM AUTHOR]

Clearance of cellular debris is a critical feature of the developing nervous system, as evidenced by the severe neurological consequences of lysosomal storage diseases in children. An important developmental process, which generates considerable cellular debris, is synapse elimination, in which many axonal branches are pruned. The fate of these pruned branches is not known. Here, we investigate the role of lysosomal activity in neurons and glia in the removal of axon branches during early postnatal life. Using a probe for lysosomal activity, we observed robust staining associated with retreating motor axons. Lysosomal function was involved in axon removal because retreating axons were cleared more slowly in a mouse model of a lysosomal storage disease. In addition, we found lysosomal activity in the cerebellum at the time of, and at sites where, climbing fibers are eliminated. We propose that lysosomal activity is a central feature of synapse elimination. Moreover, staining for lysosomal activity may serve as a marker for regions of the developing nervous system undergoing axon pruning. [ABSTRACT FROM AUTHOR]

Axons in the CNS do not regrow after injury, whereas lesioned axons in the peripheral nervous system (PNS) regenerate. Lesioned CNS axons form characteristic swellings at their tips known as retraction bulbs, which are the non-growing counterparts of growth cones. Although much progress has been made in identifying intracellular and molecular mechanisms that regulate growth cone locomotion and axonal elongation, a comprehensive understanding of how retraction bulbs form and why they are unable to grow is still elusive. Here we report the analysis of the morphological and intracellular responses of injured axons in the CNS compared with those in the PNS. We show that retraction bulbs of injured CNS axons increase in size over time, whereas growth cones of injured PNS axons remain constant. Retraction bulbs contain a disorganized microtubule network, whereas growth cones possess the typical bundling of microtubules. Using in vivo imaging, we find that pharmacological disruption of microtubules in growth cones transforms them into retraction bulb-like structures whose growth is inhibited. Correspondingly, microtubule destabilization of sensory neurons in cell culture induces retraction bulb formation. Conversely, microtubule stabilization prevents the formation of retraction bulbs and decreases axonal degeneration in vivo. Finally, microtubule stabilization enhances the growth capacity of CNS neurons cultured on myelin. Thus, the stability and organization of microtubules define the fate of lesioned axonal stumps to become either advancing growth cones or nongrowing retraction bulbs. Our data pinpoint microtubules as a key regulatory target for axonal regeneration. [ABSTRACT FROM AUTHOR]

Disturbance in the neuronal network leads to instability in the microtubule (MT) railroad of axons, causing hindrance in the intra-axonal transport and making it difficult to re-establish the broken network. Peripheral nervous system (PNS) neurons can stabilize their MTs, leading to the formation of regeneration-promoting structures called "growth cones". However, central nervous system (CNS) neurons lack this intrinsic reparative capability and, instead, form growth-incompetent structures called "retraction bulbs", which have a disarrayed MT network. It is evident from various studies that although axonal regeneration depends on both cell-extrinsic and cell-intrinsic factors, any therapy that aims at axonal regeneration ultimately converges onto MTs. Understanding the neuronal MT dynamics will help develop effective therapeutic strategies in diseases where the MT network gets disrupted, such as spinal cord injury, traumatic brain injury, multiple sclerosis, and amyotrophic lateral sclerosis. It is also essential to know the factors that aid or inhibit MT stabilization. In this review, we have discussed the MT dynamics postaxotomy in the CNS and PNS, and factors that can directly influence MT stability in various diseases.

Simple Summary: Oxidative stress plays a crucial role in nerve injury-induced changes, including the cellular redox state, greater disease severity, and reduced function. Most studies have demonstrated that the modulation of oxidative damage to neurons leads to improved survival and functional recovery. Deer antlers are considered a potent natural source of antioxidants. Thus, we examined the neurotherapeutic effect of Cervus elaphus sibericus in the axon regeneration and growth cone reformation of cortical neurons. The effect of three doses of C. elaphus sibericus on cortical neurons was determined in a hydrogen peroxide-induced injury model using immunocytochemistry, flow-cytometry, and real-time PCR. We found that C. elaphus sibericus effectively inhibited neuronal death following hydrogen peroxide-induced injury by significantly inhibiting oxidative stress and consequently promoting neuron repair and axon regeneration in cortical neurons. Cervus elaphus sibericus (CES), commonly known as deer antler, has been used as a medicinal herb because of its various pharmacological activities, including its anti-infective, anti-arthritic, anti-allergic, and anti-oxidative properties. However, the precise mechanisms by which CES functions as a potent anti-oxidative agent remain unknown; particularly, the effects of CES on cortical neurons and its neurobiological mechanism have not been examined. We used primary cortical neurons from the embryonic rat cerebral cortex and hydrogen peroxide to induce oxidative stress and damage in neurons. After post-treatment of CES at three concentrations (10, 50, and 200 µg/mL), the influence of CES on the neurobiological mechanism was assessed by immunocytochemistry, flow cytometry, and real-time PCR. CES effectively prevented neuronal death caused by hydrogen peroxide-induced damage by regulating oxidative signaling. In addition, CES significantly induced the expression of brain-derived neurotrophic factor and neurotrophin nerve growth factor, as well as regeneration-associated genes. We also observed newly processing elongated axons after CES treatment under oxidative conditions. In addition, filopodia tips generally do not form a retraction bulb, called swollen endings. Thus, CES shows therapeutic potential for treating neurological diseases by stimulating neuron repair and regeneration. [ABSTRACT FROM AUTHOR]

In addition to altered gene expression, pathological cytoskeletal dynamics in the axon are another key intrinsic barrier for axon regeneration in the central nervous system (CNS). Here, we show that knocking out myosin IIA and IIB (myosin IIA/B) in retinal ganglion cells alone, either before or after optic nerve crush, induces significant optic nerve regeneration. Combined Lin28a overexpression and myosin IIA/B knockout lead to an additive promoting effect and long-distance axon regeneration. Immunostaining, RNA sequencing, and western blot analyses reveal that myosin II deletion does not affect known axon regeneration signaling pathways or the expression of regeneration-associated genes. Instead, it abolishes the retraction bulb formation and significantly enhances the axon extension efficiency. The study provides clear evidence that directly targeting neuronal cytoskeleton is sufficient to induce significant CNS axon regeneration and that combining altered gene expression in the soma and modified cytoskeletal dynamics in the axon is a promising approach for long-distance CNS axon regeneration. • Myosin II KO in RGCs induces significant optic nerve regeneration • Myosin II KO does not affect gene transcription in RGCs • Myosin II KO reduces retraction bulbs and leads to efficient axon extension • Myosin II KO and Lin28a expression in RGCs produce long-distance axon regeneration Although modulating the neuronal cytoskeleton has been deemed a promising approach to enhance mammalian axon regeneration, only a few studies have shown convincing results, especially in the central nervous system. Wang et al. demonstrate that the deletion of non-muscle myosin II sufficiently induces significant mammalian CNS axon regeneration in vivo. [ABSTRACT FROM AUTHOR]