Regeneration of acutely and chronically injured descending respiratory pathways within post-traumatic nerve grafts

Contrasting Experimental Rodent Aftercare With Human Clinical Treatment for Cervical Spinal Cord Injury: Bridging the Translational "Valley of Death"

More than half of all spinal cord injuries (SCIs) occur at the cervical level and often lead to life-threatening breathing motor dysfunction. The C2 hemisection (C2Hx) and high cervical contusion mouse and rat models of SCI are widely utilized both to understand the pathological effects of SCI and to develop potential therapies. Despite rigorous research effort, pre-clinical therapeutics studied in those animal models of SCI sometimes fail when evaluated in the clinical setting. Differences between standard-of-care treatment for acute SCI administered to clinical populations and experimental animal models of SCI could influence the heterogeneity of outcome between pre-clinical and clinical studies. In this review, we have summarized both the standard clinical interventions used to treat patients with cervical SCI and the various veterinary aftercare protocols used to care for rats and mice after experimentally induced C2Hx and high cervical contusion models of SCI. Through this analysis, we have identified areas of marked dissimilarity between clinical and veterinary protocols and suggest the modification of pre-clinical animal care particularly with respect to analgesia, anticoagulative measures, and stress ulcer prophylaxis. In our discussion, we intend to inspire consideration of potential changes to aftercare for animal subjects of experimental SCI that may help to bridge the translational "Valley of Death" and ultimately contribute more effectively to finding treatments capable of restoring independent breathing function to persons with cervical SCI.

Therapeutic Strategies Targeting Respiratory Recovery after Spinal Cord Injury: From Preclinical Development to Clinical Translation

High spinal cord injuries (SCIs) lead to permanent functional deficits, including respiratory dysfunction. Patients living with such conditions often rely on ventilatory assistance to survive, and even those that can be weaned continue to suffer life-threatening impairments. There is currently no treatment for SCI that is capable of providing complete recovery of diaphragm activity and respiratory function. The diaphragm is the main inspiratory muscle, and its activity is controlled by phrenic motoneurons (phMNs) located in the cervical (C3-C5) spinal cord. Preserving and/or restoring phMN activity following a high SCI is essential for achieving voluntary control of breathing. In this review, we will highlight (1) the current knowledge of inflammatory and spontaneous pro-regenerative processes occurring after SCI, (2) key therapeutics developed to date, and (3) how these can be harnessed to drive respiratory recovery following SCIs. These therapeutic approaches are typically first developed and tested in relevant preclinical models, with some of them having been translated into clinical studies. A better understanding of inflammatory and pro-regenerative processes, as well as how they can be therapeutically manipulated, will be the key to achieving optimal functional recovery following SCIs.

Respiratory plasticity following spinal cord injury: perspectives from mouse to man

The study of respiratory plasticity in animal models spans decades. At the bench, researchers use an array of techniques aimed at harnessing the power of plasticity within the central nervous system to restore respiration following spinal cord injury. This field of research is highly clinically relevant. People living with cervical spinal cord injury at or above the level of the phrenic motoneuron pool at spinal levels C3-C5 typically have significant impairments in breathing which may require assisted ventilation. Those who are ventilator dependent are at an increased risk of ventilator-associated co-morbidities and have a drastically reduced life expectancy. Pre-clinical research examining respiratory plasticity in animal models has laid the groundwork for clinical trials. Despite how widely researched this injury is in animal models, relatively few treatments have broken through the preclinical barrier. The three goals of this present review are to define plasticity as it pertains to respiratory function post-spinal cord injury, discuss plasticity models of spinal cord injury used in research, and explore the shift from preclinical to clinical research. By investigating current targets of respiratory plasticity research, we hope to illuminate preclinical work that can influence future clinical investigations and the advancement of treatments for spinal cord injury.

Sustained cell body reactivity and loss of NeuN in a subset of axotomized bulbospinal neurons after a chronic high cervical spinal cord injury

Following central nervous system lesion, the ability of injured axons to regrowth may depend on the level and duration of the injured cell body response (CBR). Therefore, to investigate whether axotomized brainstem neurons maintain a durable growth-competent state after spinal cord injury, we studied the effect of a chronic C2 hemisection in rats on the expression of various CBR markers involved in axon regeneration, such as c-Jun, ATF-3, HSP27, NO synthase (NOS), and also of the neural mature phenotype marker NeuN, in the bulbospinal respiratory neurons as compared to the gigantocellularis nucleus. Both at 7 and 30 days post-lesion (DPL), c-Jun and HSP27 were present in, respectively, ~60 and ~20% of the axotomized respiratory neurons, whereas the apoptotic factor caspase 3 was not detected in these cells. NOS appeared belatedly, and it was detected in ~20% of the axotomized respiratory neurons at 30DPL. At 30DPL, these different CBR markers were strongly colocalized in a sub-population of axotomized respiratory neurons and also in a sub-population of injured neurons within the gigantocellularis nucleus. Such CBR was also accompanied by a sustained alteration of the neural mature phenotype, as indicated by a loss of NeuN immunoreactivity selectively in HSP27⁺ bulbospinal neurons at 7DPL and 30DPL. Altogether, this study shows that a subset of axotomized medullary respiratory neurons remains in a growth-competent state after a chronic injury, suggesting that they may play a preferential role in long-lasting respiratory neuroplasticity processes.

Thermogelling chitosan lactate hydrogel improves functional recovery after a C2 spinal cord hemisection in rat

The present study was designed to provide an appropriate micro-environment for regenerating axotomized neurons and proliferating/migrating cells. Because of its intrinsic permissive properties, biocompatibility and biodegradability, we chose to evaluate the therapeutic effectiveness of a chitosan-based biopolymer. The biomaterial toxicity was measured through in vitro test based on fibroblast cell survival on thermogelling chitosan lactate hydrogel substrate and then polymer was implanted into a C2 hemisection of the rat spinal cord. Animals were randomized into three experimental groups (Control, Lesion and Lesion + Hydrogel) and functional tests (ladder walking and forelimb grip strength tests, respiratory assessment by whole-body plethysmography measurements) were used, once a week during 10 weeks, to evaluate post-traumatic recoveries. Then, electrophysiological examinations (reflexivity of the sub-lesional region, ventilatory adjustments to muscle fatigue known to elicit the muscle metaboreflex and phrenic nerve recordings during normoxia and temporary hypoxia) were performed. In vitro results indicated that the chitosan matrix is a non-toxic biomaterial that allowed fibroblast survival. Furthermore, implanted animals showed improvements of their ladder walking scores from the 4th week post-implantation. Finally, electrophysiological recordings indicated that animals receiving the chitosan matrix exhibited recovery of the H-reflex rate sensitive depression, the ventilatory response to repetitive muscle stimulation and an increase of the phrenic nerve activity to asphyxia compared to lesioned and nonimplanted animals. This study indicates that hydrogel based on chitosan constitute a promising therapeutic approach to repair damaged spinal cord or may be used as an adjuvant with other treatments to enhance functional recovery after a central nervous system damage. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2004-2019, 2017.

Neuroprotective and Neurorestorative Processes after Spinal Cord Injury: The Case of the Bulbospinal Respiratory Neurons

High cervical spinal cord injuries interrupt the bulbospinal respiratory pathways projecting to the cervical phrenic motoneurons resulting in important respiratory defects. In the case of a lateralized injury that maintains the respiratory drive on the opposite side, a partial recovery of the ipsilateral respiratory function occurs spontaneously over time, as observed in animal models. The rodent respiratory system is therefore a relevant model to investigate the neuroplastic and neuroprotective mechanisms that will trigger such phrenic motoneurons reactivation by supraspinal pathways. Since part of this recovery is dependent on the damaged side of the spinal cord, the present review highlights our current understanding of the anatomical neuroplasticity processes that are developed by the surviving damaged bulbospinal neurons, notably axonal sprouting and rerouting. Such anatomical neuroplasticity relies also on coordinated molecular mechanisms at the level of the axotomized bulbospinal neurons that will promote both neuroprotection and axon growth.

Harnessing the power of cell transplantation to target respiratory dysfunction following spinal cord injury

The therapeutic benefit of cell transplantation has been assessed in a host of central nervous system (CNS) diseases, including disorders of the spinal cord such as traumatic spinal cord injury (SCI). The promise of cell transplantation to preserve and/or restore normal function can be aimed at a variety of therapeutic mechanisms, including replacement of lost or damaged CNS cell types, promotion of axonal regeneration or sprouting, neuroprotection, immune response modulation, and delivery of gene products such as neurotrophic factors, amongst other possibilities. Despite significant work in the field of transplantation in models of SCI, limited attention has been directed at harnessing the therapeutic potential of cell grafting for preserving respiratory function after SCI, despite the critical role pulmonary compromise plays in patient outcome in this devastating disease. Here, we will review the limited number of studies that have demonstrated the therapeutic potential of intraspinal transplantation of a variety of cell types for addressing respiratory dysfunction in SCI.

Respiration following spinal cord injury: evidence for human neuroplasticity

Respiratory dysfunction is one of the most devastating consequences of cervical spinal cord injury (SCI) with impaired breathing being a leading cause of morbidity and mortality in this population. However, there is mounting experimental and clinical evidence for moderate spontaneous respiratory recovery, or "plasticity", after some spinal cord injuries. Pre-clinical models of respiratory dysfunction following SCI have demonstrated plasticity at neural and behavioral levels that result in progressive recovery of function. Temporal changes in respiration after human SCI have revealed some functional improvements suggesting plasticity paralleling that seen in experimental models-a concept that has been previously under-appreciated. While the extent of spontaneous recovery remains limited, it is possible that enhancing or facilitating neuroplastic mechanisms may have significant therapeutic potential. The next generation of treatment strategies for SCI and related respiratory dysfunction should aim to optimize these recovery processes of the injured spinal cord for lasting functional restoration.

Peripheral nerve grafts support regeneration after spinal cord injury

Traumatic insults to the spinal cord induce both immediate mechanical damage and subsequent tissue degeneration leading to a substantial physiological, biochemical, and functional reorganization of the spinal cord. Various spinal cord injury (SCI) models have shown the adaptive potential of the spinal cord and its limitations in the case of total or partial absence of supraspinal influence. Meaningful recovery of function after SCI will most likely result from a combination of therapeutic strategies, including neural tissue transplants, exogenous neurotrophic factors, elimination of inhibitory molecules, functional sensorimotor training, and/or electrical stimulation of paralyzed muscles or spinal circuits. Peripheral nerve grafts provide a growth-permissive substratum and local neurotrophic factors to enhance the regenerative effort of axotomized neurons when grafted into the site of injury. Regenerating axons can be directed via the peripheral nerve graft toward an appropriate target, but they fail to extend beyond the distal graft-host interface because of the deposition of growth inhibitors at the site of SCI. One method to facilitate the emergence of axons from a graft into the spinal cord is to digest the chondroitin sulfate proteoglycans that are associated with a glial scar. Importantly, regenerating axons that do exit the graft are capable of forming functional synaptic contacts. These results have been demonstrated in acute injury models in rats and cats and after a chronic injury in rats and have important implications for our continuing efforts to promote structural and functional repair after SCI.

Transplantation-mediated strategies to promote axonal regeneration following spinal cord injury

Devastating central nervous system injuries and diseases continue to occur in spite of the tremendous efforts of various prevention programs. The enormity and annual escalation of healthcare costs due to them require that therapeutic strategies be responsibly developed. The dysfunctions that occur after injury and disease are primarily due to neurotransmission damage. The last two decades of both experimental and clinical research have demonstrated that neural and non-neural tissue and cell transplantation is a viable option for ameliorating dysfunctions to markedly improve quality of life. Moreover, significant progress has been made with tissue and cell transplantation in studies of pathophysiology, plasticity, sprouting, regeneration, and functional recovery. This article will review information about the ability and potential, particularly for traumatic spinal cord injury, that neural and non-neural tissue and cell transplantation has to replace lost neurons and glia, to reconstruct damaged neural circuitry, and to restore neurotransmitters, hormones, neurotrophic factors, and neurotransmission. Donor tissues and cells to be discussed include peripheral nerve, fetal spinal cord and brain, central and peripheral nervous systems' glia, stem cells, those that have been genetically engineered, and non-neural ones. Combinatorial approaches and clinical research are also reviewed.

Shedding light on restoring respiratory function after spinal cord injury

Loss of respiratory function is one of the leading causes of death following spinal cord injury. Because of this, much work has been done in studying ways to restore respiratory function following spinal cord injury (SCI) – including pharmacological and regeneration strategies. With the emergence of new and powerful tools from molecular neuroscience, new therapeutically relevant alternatives to these approaches have become available, including expression of light sensitive proteins called channelrhodopsins. In this article we briefly review the history of various attempts to restore breathing after C2 hemisection, and focus on our recent work using the activation of light sensitive channels to restore respiratory function after experimental SCI. We also discuss how such light-induced activity can help shed light on the inner workings of the central nervous system respiratory circuitry that controls diaphragmatic function.

Descending bulbospinal pathways and recovery of respiratory motor function following spinal cord injury

The rodent respiratory system is a relevant model for study of the intrinsic post-lesion mechanisms of neuronal plasticity and resulting recovery after high cervical spinal cord injury. An unilateral cervical injury (hemisection, lateral section or contusion) interrupts unilaterally bulbospinal respiratory pathways to phrenic motor neurons innervating the diaphragm and leads to important respiratory defects on the injured side. However, the ipsilateral phrenic nerve exhibits a spontaneous and progressive recovery with post-lesion time. Shortly after a lateral injury, this partial recovery depends on the activation of contralateral pathways that cross the spinal midline caudal to the injury. Activation of these crossed phrenic pathways after the injury depends on the integrity of phrenic sensory afferents. These pathways are located principally in the lateral part of the spinal cord and involve 30% of the medullary respiratory neurons. By contrast, in chronic post-lesion conditions, the medial part of the spinal cord becomes sufficient to trigger substantial ipsilateral respiratory drive. Thus, after unilateral cervical spinal cord injury, respiratory reactivation is associated with a time-dependent anatomo-functional reorganization of the bulbospinal respiratory descending pathways, which represents an adaptative strategy for functional compensation.

Are rodents an appropriate pre-clinical model for treating spinal cord injury? Examples from the respiratory system

Because most studies of the effects of spinal cord injury (SCI) and resulting repair and treatments use rodent models, it is important to determine if these models are relevant to humans. In this review, we focus on alterations in respiratory function as a result of SCI. Several injury paradigms have been used in the rat to examine restoration of post-lesion respiratory function and potential benefits from repair strategies designed for humans. Unlike the corticospinal locomotor system, respiratory neural organization is well preserved between rodents and humans, and resembles the general organization of motor pathways in primates. These similarities justify the use of the rodent respiratory system as a model to analyze SCI and putative repair strategies.

Effect of spinal cord injury on the neural regulation of respiratory function

Injury at any level of the spinal cord can impair respiratory motor function. Indeed, complications associated with respiratory function are the number one cause of mortality in humans following spinal cord injury (SCI) at any level of the cord. This review is aimed at describing the effect of SCI on respiratory function while highlighting the recent advances made by basic science research regarding the neural regulation of respiratory function following injury. Models of SCI that include upper cervical hemisection and contusion injury have been utilized to examine the underlying neural mechanisms of respiratory control following injury. The approaches used to induce motor recovery in the respiratory system are similar to other studies that examine recovery of locomotor function after SCI. These include strategies to initiate regeneration of damaged axons, to reinnervate paralyzed muscles with peripheral nerve grafts, to use spared neural pathways to induce motor function, and finally, to initiate mechanisms of neural plasticity within the spinal cord to increase motoneuron firing. The ultimate goals of this research are to restore motor function to previously paralyzed respiratory muscles and improve ventilation in patients with SCI.

Specific and artifactual labeling in the rat spinal cord and medulla after injection of monosynaptic retrograde tracers into the diaphragm

The use of fluorescent dyes has been a major improvement for paths tracing studies. However, these tracers present different properties and have to be chosen carefully. The present study compares the ability of different tracers to specifically label phrenic motoneurons (PMNs) innervating the rat diaphragm. The administration of fluorogold (FG) from the transected phrenic nerve specifically labeled PMNs in the ipsilateral spinal cord. However, when FG was injected into one hemidiaphragm, in addition with ipsilateral PMNs, a less intense artifactual labeling was observed in the spinal cord (mainly in contralateral PMNs) and in the medulla oblongata (mainly in the area postrema and cranial motor nuclei). Similar results were observed using horseradish peroxidase, while no labeling was observed after injection of nuclear yellow or diamidino yellow into the diaphragm. By contrast, the dextran amine fluororuby (FR) and the carbocyanine DiAsp selectively and exclusively labeled ipsilateral PMNs 2 or 3 weeks after injection into the diaphragm, respectively. The lipophilic properties of DiAsp and the high molecular weight of FR may prevent their diffusion to adjacent tissues and into the blood stream which seems to account for the artifactual labeling observed with the other tracers. The higher homogeneity and quality of the labeling observed with FR compared to DiAsp make it the most appropriate tracer for the specific monosynaptic fluorescent labeling of PMNs after injection into the diaphragm.

Survival, integration, and axon growth support of glia transplanted into the chronically contused spinal cord

Due to an ever-growing population of individuals with chronic spinal cord injury, there is a need for experimental models to translate efficacious regenerative and reparative acute therapies to chronic injury application. The present study assessed the ability of fluid grafts of either Schwann cells (SCs) or olfactory ensheathing glia (OEG) to facilitate the growth of supraspinal and afferent axons and promote restitution of hind limb function after transplantation into a 2-month-old, moderate, thoracic (T8) contusion in the rat. The use of cultured glial cells, transduced with lentiviral vectors encoding enhanced green fluorescent protein (EGFP), permitted long-term tracking of the cells following spinal cord transplantation to examine their survival, migration, and axonal association. At 3 months following grafting of 2 million SCs or OEG in 6 microl of DMEM/F12 medium into the injury site, stereological quantification of the three-dimensional reconstructed spinal cords revealed that an average of 17.1 +/- 6.8% of the SCs and 2.3 +/- 1.4% of the OEG survived from the number transplanted. In the OEG grafted spinal cord, a limited number of glia were unable to prevent central cavitation and were found in patches around the cavity rim. The transplanted SCs, however, formed a substantive graft within the injury site capable of supporting the ingrowth of numerous, densely packed neurofilament-positive axons. The SC grafts were able to support growth of both ascending calcitonin gene-related peptide (CGRP)-positive and supraspinal serotonergic axons and, although no biotinylated dextran amine (BDA)-traced corticospinal axons were present within the center of the grafts, the SC transplants significantly increased corticospinal axon numbers immediately rostral to the injury-graft site compared with injury-only controls. Moreover, SC grafted animals demonstrated modest, though significant, improvements in open field locomotion and exhibited less foot position errors (base of support and foot rotation). Whereas these results demonstrate that SC grafts survive, support axon growth, and can improve functional outcome after chronic contusive spinal cord injury, further development of OEG grafting procedures in this model and putative combination strategies with SC grafts need to be further explored to produce substantial improvements in axon growth and function.

Phrenic rehabilitation and diaphragm recovery after cervical injury and transplantation of olfactory ensheathing cells

Functional respiratory recovery was evaluated by recording diaphragm and phrenic nerve activity several months after cervical cord hemisection followed by olfactory ensheathing cell (OEC) transplantation. The intact side was taken as a control in each rat. Sham-transplanted rats did not recover respiratory activity from the ipsilateral lesioned side. By contrast, ipsilateral phrenic and diaphragmatic activities recovered in transplanted rats amounted to 80.7% and 73% of their controls, respectively. After contralateral acute C1 section eliminating any contralateral influence from crossed compensatory pathways, the ipsilateral phrenic activity remained at 57.5% of the control, indicating that the phrenic recovery originated from the ipsilateral side. Supralesional stimulation in these rats elicited sublesional ipsilateral postsynaptic phrenic responses showing that transplantation helped ipsilateral fibers to again transmit nervous messages to the phrenic target, leading to substantial functional recovery. The origin of mechanisms involved in respiratory recovery (regeneration, resurrection, sprouting, sparing, demasking of latent pathways) is discussed.