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A surviving intact branch stabilizes remaining axon architecture after injury as revealed by in vivo imaging in the mouse spinal cord.

ABSTRACT: The complex morphology of axons presents a challenge in understanding axonal responses to injury and disease. By in vivo two-photon imaging of spinal dorsal column sensory axons, we systematically examined the effect of injury location relative to the main bifurcation point on axon degeneration and regeneration following highly localized laser injuries. Retrograde but not anterograde degeneration was strongly blocked at the bifurcation point at both the acute and subacute phases. Eliminating either the ascending or descending branch led to a poor regenerative response, while eliminating both led to a strong regenerative response. Thus, a surviving intact branch suppresses both retrograde degeneration and regeneration of the injured branch, thereby preserving the remaining axon architecture. Regenerating axons exhibited a dynamic pattern with alternating phases of regeneration and pruning over a chronic period. In vivo imaging continues to reveal new insights on axonal responses to injury in the mammalian spinal cord.

SUBMITTER: Lorenzana AO

PROVIDER: S-EPMC4441580 | biostudies-literature | 2015 May

REPOSITORIES: biostudies-literature

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A surviving intact branch stabilizes remaining axon architecture after injury as revealed by in vivo imaging in the mouse spinal cord.

Lorenzana Ariana O AO Lee Jae K JK Mui Matthew M Chang Amy A Zheng Binhai B

Neuron 20150430 4

The complex morphology of axons presents a challenge in understanding axonal responses to injury and disease. By in vivo two-photon imaging of spinal dorsal column sensory axons, we systematically examined the effect of injury location relative to the main bifurcation point on axon degeneration and regeneration following highly localized laser injuries. Retrograde but not anterograde degeneration was strongly blocked at the bifurcation point at both the acute and subacute phases. Eliminating eit ...[more]

PMID: 25937174

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Project description:Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

Project description:Transected axons fail to regrow across anatomically complete spinal cord injuries (SCI) in adults. Diverse molecules can partially facilitate or attenuate axon growth during development or after injury1-3, but efficient reversal of this regrowth failure remains elusive4. Here we show that three factors that are essential for axon growth during development but are attenuated or lacking in adults-(i) neuron intrinsic growth capacity2,5-9, (ii) growth-supportive substrate10,11 and (iii) chemoattraction12,13-are all individually required and, in combination, are sufficient to stimulate robust axon regrowth across anatomically complete SCI lesions in adult rodents. We reactivated the growth capacity of mature descending propriospinal neurons with osteopontin, insulin-like growth factor 1 and ciliary-derived neurotrophic factor before SCI14,15; induced growth-supportive substrates with fibroblast growth factor 2 and epidermal growth factor; and chemoattracted propriospinal axons with glial-derived neurotrophic factor16,17 delivered via spatially and temporally controlled release from biomaterial depots18,19, placed sequentially after SCI. We show in both mice and rats that providing these three mechanisms in combination, but not individually, stimulated robust propriospinal axon regrowth through astrocyte scar borders and across lesion cores of non-neural tissue that was over 100-fold greater than controls. Stimulated, supported and chemoattracted propriospinal axons regrew a full spinal segment beyond lesion centres, passed well into spared neural tissue, formed terminal-like contacts exhibiting synaptic markers and conveyed a significant return of electrophysiological conduction capacity across lesions. Thus, overcoming the failure of axon regrowth across anatomically complete SCI lesions after maturity required the combined sequential reinstatement of several developmentally essential mechanisms that facilitate axon growth. These findings identify a mechanism-based biological repair strategy for complete SCI lesions that could be suitable to use with rehabilitation models designed to augment the functional recovery of remodelling circuits.

Dataset Information

A surviving intact branch stabilizes remaining axon architecture after injury as revealed by in vivo imaging in the mouse spinal cord.

Publications

A surviving intact branch stabilizes remaining axon architecture after injury as revealed by in vivo imaging in the mouse spinal cord.

Similar Datasets

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