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

Novel Influences of Sex and \u3ci\u3eAPOE\u3c/i\u3e Genotype on Spinal Plasticity and Recovery of Function after Spinal Cord Injury

Spinal cord injuries can abolish both motor and sensory function throughout the body. Spontaneous recovery after injury is limited and can vary substantially between individuals. Despite an abundance of therapeutic approaches that have shown promise in preclinical models, there is currently a lack of effective treatment strategies that have been translated to restore function after SCI in the human population. We hypothesized that sex and genetic background of injured individuals could impact how they respond to treatment strategies, presenting a barrier to translating therapies that are not tailored to the individual. One gene of particular interest is APOE, which has been extensively studied in the brain due to its allele-specific influences on synaptic plasticity, metabolism, inflammation, and neurodegeneration. Despite its prominence as a therapeutic target in brain injury and disease, little is known about how it influences neural plasticity and repair processes in the spinal cord. Utilizing humanized mice, we examined how the ε3 and ε4 alleles of APOE influence the efficacy of therapeutic intermittent hypoxia (IH) in inducing spinally-mediated plasticity after cervical SCI. IH is sufficient to enhance plasticity and restore motor function after experimental SCI in genetically similar rodent populations, but its effect in human subjects is more variable (Golder, 2005; Hayes et al., 2014). Our results demonstrate that both sex and APOE genotype determine the extent of respiratory motor plasticity that is elicited by IH, highlighting the importance of considering these clinically relevant variables when translating therapeutic approaches for the SCI community. Significance Statement There is currently a critical need for therapeutics that restore motor and sensory function effectively after cervical spinal cord injury. Although many therapeutic approaches, including intermittent hypoxia, are being investigated for their potential to enhance spinal plasticity and improve motor outcomes after SCI, it is unknown whether the efficacy of these treatment strategies is influenced by individuals’ genetic background. Here we show that APOE genotype and sex both play a role in determining the propensity for motor plasticity in humanized mice after cervical SCI. These results indicate that sex and genetic background dictate how individuals respond to therapeutic approaches, thereby emphasizing the importance of developing personalized medicine for the diverse SCI population

A Latent Propriospinal Network Can Restore Diaphragm Function After High Cervical Spinal Cord Injury

Spinal cord injury (SCI) above cervical level 4 disrupts descending axons from the medulla that innervate phrenic motor neurons, causing permanent paralysis of the diaphragm. Using an ex vivo preparation in neonatal mice, we have identified an excitatory spinal network that can direct phrenic motor bursting in the absence of medullary input. After complete cervical SCI, blockade of fast inhibitory synaptic transmission caused spontaneous, bilaterally coordinated phrenic bursting. Here, spinal cord glutamatergic neurons were both sufficient and necessary for the induction of phrenic bursts. Direct stimulation of phrenic motor neurons was insufficient to evoke burst activity. Transection and pharmacological manipulations showed that this spinal network acts independently of medullary circuits that normally generate inspiration, suggesting a distinct non-respiratory function. We further show that this “latent” network can be harnessed to restore diaphragm function after high cervical SCI in adult mice and rats

Clearing Up the Phrenic Motor Neuron Survival Debate After Cervical Spinal Cord Injury

The diaphragm is the major muscle involved in breathing. Innervated by the phrenic nerve, it is controlled by phrenic motor neurons (PMNs), which receive descending inputs from the medulla. When these bulbospinal-pathways are damaged or severed in spinal cord injury (SCI), the external effects of injury are seen immediately, as the diaphragm becomes paralyzed and the individual loses the ability to breathe. However, the effect of injury on the internal circuitry, specifically PMN survival, is largely unknown. Contradictory evidence has surfaced, suggesting that there is large PMN death after injury, or conversely, that there is an absence of PMN death. However, histological techniques utilized in these studies have exposed the data to factors through which certainty cannot be guaranteed. These discrepancies are important to parse out because characterization of PMN survival is integral to studies of plasticity. The present study attempted to bridge this gap in knowledge and used XClarity clearing methods to accurately determine PMN survival after cervical SCI. XClarity transforms the tissue into a transparent medium. This allows for the whole spinal cord to be analyzed without tissue loss, as is common in other histological techniques. In this study, Sprague-Dawley rats were hemisected at the second level of the cervical spinal cord (c2Hx), which is a common experimental model of cervical SCI. Animals were divided into three groups: naïve, two weeks post-c2Hx, and five weeks post-c2Hx. Before perfusion, these animals were intrapleurally injected bilaterally with CTB-488, a retrograde tracer that labels PMNs. Depending on their group assignment, animals were perfused at five weeks post-injury, two weeks post-injury, or immediately after CTB-488 uptake. Cords were then processed with XClarity and PMN survival was characterized with Lightsheet microscopy. Analysis of PMNs is ongoing, however, preliminary data suggests that XClarity techniques are the preferable route to characterize PMN survival after injury