Muscling in on neurorehabilitative strategies to counter respiratory motor dysfunction in cervical spinal cord injury

Severe trauma to the cervical spinal cord can have devastating consequences for breathing. The loss of excitatory drive to the phrenic motor pool gives rise to paralysis (in part or whole) of the diaphragm, the principal muscle of inspiration. The respiratory motor deficit and extent of the resultant respiratory-related consequences depend upon the site and completeness of the injury, although respiratory insufficiency is common in people with cervical spinal cord injury, underpinning much of the chronic burden of morbidity and serving as the major cause of mortality. There is considerable interest in neurorehabilitative strategies that strive to promote intrinsic compensatory mechanisms to rescue diaphragm contractile behaviour, through enhanced activity in spared latent neural pathways and/or via sprouting of phrenic axons. High unilateral cervical spinal cord injury results in long-term ipsilateral hemi-diaphragm paralysis. Recently, in a rodent model of C2 spinal hemi-section, rapid recovery of breathing was established following matrix remodelling by means of a single injection of chondroitinase ABC (ChABC) at the ipsilateral phrenic motor pool caudal to the site of injury (Warren et al. 2018). Intriguingly, treatment was more efficacious when applied late following injury compared to treatment in the acute phase. Impressive recovery was evoked 18 months after injury, revealed as the emergence of robust ipsilateral diaphragm electromyogram activity following treatment. This outcome is all the more remarkable in that chronic injury is usually considered as a significant barrier to treatment; moreover, therapies effective in the acute phase most often fail to show promise when applied late after injury. In this issue of The Journal of Physiology, Warren et al. (2021) build upon their prior work in a cohesive series of assessments, characterising diaphragm form and function following injury and treatment, centred on the importance of oxygen transport kinetics in the remodelling process. The comprehensive study illustrates injury-related deficits in muscle and the impressive capacity for recovery, albeit with a cautionary twist in the tale. Studies were performed in adult female rats following sham injury, or C2 spinal hemi-section with either immediate injection into the phrenic motor pool of saline or ChABC, or delayed injection of ChABC administered 4 weeks after injury. C2 spinal hemi-section resulted in a complete loss of function in the ipsilateral hemi-diaphragm that persisted for up to 6 weeks, with no spontaneous recovery, as reported in other models. Activity of the contralateral hemi-diaphragm was unchanged. Confirming previous observations (Warren et al. 2018), ChABC treatment applied within 1 week of injury resulted in meagre motor recovery compared to the robust synchronised recovery of ipsilateral diaphragm electromyogram activity at both 4 and 12 weeks following injury. There was evidence for remodelling in the ipsilateral hemi-diaphragm in response to chronic paralysis following injury. Surprisingly, given previous observations of diaphragm fibre atrophy following spinal cord injury, Warren et al. (2021) observed that diaphragm type 1 and 2a fibres were larger. Indeed, functional assessment of muscle work loops in ex vivo preparations revealed that ipsilateral hemi-diaphragm produced greater relative power at high cycle frequencies, but less power at cycle frequencies typical of resting breathing compared to contralateral hemi-diaphragm. As such, following C2 spinal hemi-section, there is discordance in the optimal cycle frequency between the injured and uninjured hemi-diaphragms. Increased cross-sectional area of type 1 and 2a fibres in the paralyzed hemi-diaphragm led to an effective decrease in capillary density (without a change in capillary number). Further measures of the spatial distribution of the microvasculature supply are described, collectively highlighting diminished oxygen transport capacity following injury. Warren et al. (2021) make use of an interesting modelling approach, incorporating an estimation of diaphragm mitochondrial content, based on citrate synthase activity, to reveal adequate oxygen supply, but decreased oxygen demand, in paralyzed hemi-diaphragms. Strikingly, the structural and functional properties of the ipsilateral hemi-diaphragm were largely restored following ChABC treatment. Fibre size and oxidative potential returned towards control values and peak power generation occurred at optimal cycle frequencies. Interestingly, the ipsilateral hemi-diaphragm demand for oxygen did not recover, revealing persistent metabolic remodelling resistant to treatment, with potential functional consequences. However, fatigue tolerance of ipsilateral hemi-diaphragm was essentially unchanged following C2 hemi-section or ChABC treatment. The study is remarkable on a number of fronts. The capacity for a single application of ChABC to elaborate considerable restoration of neuromuscular function is impressive. The increased efficacy in chronic injury clearly warrants further investigation to fully appreciate the mechanisms, which have considerable clinical relevance and challenge other findings in the field. The experimental approach of Warren et al. (2018, 2021) provides insight into the capacity to restore functional motor units that reverse aberrant structural changes in paralyzed muscle. It is interesting to consider and compare the relatively modest muscle remodelling and robust recovery following paralysis with the profound diaphragm atrophy that occurs during unloading in mechanically-ventilated animals and humans, which can significantly delay transition to spontaneous breathing. Warren et al. (2021) discuss the putative role of passive movement of the ipsilateral hemi-diaphragm, which may be adequate to protect the muscle from wasting that might otherwise have been expected and, indeed, is seen in other models. Net oxygen