Compensatory plasticity in diaphragm and intercostal muscle utilization in a rat model of ALS

Abstract

In SOD1 transgenic rat model of ALS, breathing capacity is preserved until late in disease progression despite profound respiratory motor neuron (MN) cell death. To explore mechanisms preserving breathing capacity, we assessed inspiratory EMG activity in diaphragm and external intercostal T2 (EIC2) and T5 (EIC5) muscles in anesthetized SOD1 rats at disease end-stage (20% decrease in body mass). We hypothesized that despite significant phrenic motor neuron loss and decreased phrenic nerve activity, diaphragm electrical activity and trans-diaphragmatic pressure (Pdi) are maintained to sustain ventilation. We alternatively hypothesized that EIC activity is enhanced, compensating for impaired diaphragm function. Diaphragm, EIC2 and EIC5 muscle EMGs and Pdi were measured in urethane-anesthetized, spontaneously breathing female SOD1 rats versus wild-type littermates during normoxia (arterial PO ~90mmHg, PCO ~45mmHg), maximal chemoreceptor stimulation (MCS: 10.5% O/7% CO), spontaneous augmented breaths and sustained tracheal occlusion. Phrenic MNs were counted in C3-5; T2 and T5 ventrolateral MNs were counted. In end-stage SOD1 rats, 29% of phrenic MNs survived (vs. wild-type), yet integrated diaphragm EMG amplitude was normal. Nevertheless, maximal Pdi decreased ~30% vs. wild type (p<0.01) and increased esophageal to gastric pressure ratio (p<0.05), consistent with persistent diaphragm weakness. Despite major T2 and T5 MN death, integrated EIC2 (100% greater than wild type) and EIC5 (300%) EMG amplitudes were increased in mutant rats during normoxia (p<0.01), possibly compensating for decreased Pdi. Thus, despite significant phrenic MN loss, diaphragm EMG activity is maintained; in contrast, Pdi was not, suggesting diaphragm dysfunction. Presumably, increased EIC EMG activity compensated for persistent diaphragm weakness. These adjustments contribute to remarkable preservation of breathing ability despite major respiratory motor neuron death and diaphragm dysfunction.

Mechanisms of compensatory plasticity for respiratory motor neuron death

“…In SOD1 G93A rats, although only ~50% of phrenic nerve activity is maintained, diaphragm EMG activity is fully preserved (21,22) consistent with improved neuromuscular transmission from phrenic nerve to diaphragm muscle. Increased motor unit innervation ratio (i.e.…”

“…At disease end-stage, spontaneous phrenic nerve activity and evoked compound diaphragm action potentials are blunted (16,20). On the other hand, spontaneous diaphragm EMG activity during near-maximal reflex activation remains unchanged (21). Nevertheless, trans-diaphragmatic pressure and the esophageal-to-gastric pressure ratio are decreased, suggesting reduced diaphragm muscle contributions to breathing.…”

“…As one example, rat and mouse models overexpressing superoxide dismutase 1 (SOD1) with specific gene mutations known to elicit familial ALS in humans (15) exhibit progressive motor neuron death, limb and respiratory muscle paralysis, and eventual death (16)(17)(18)(19). In the SOD1 G93A rat model, respiratory motor neuron cell death at disease endstage mimics the human condition (~75% phrenic, ~60% intercostal motor neuron loss; (20,21). At disease end-stage, spontaneous phrenic nerve activity and evoked compound diaphragm action potentials are blunted (16,20).…”

“…In pre-symptomatic SOD1 G93A mice, force per muscle fiber decreased ~60% (26). Consequently, trans-diaphragmatic pressure (in vivo surrogate of diaphragm force) in rats was reduced by ~30%, suggesting secondary diaphragm dysfunction (21).…”

“…Accessory muscle recruitment is a compensatory response utilized during other pathological conditions involving diaphragm paralysis (107,(113)(114)(115)(116). In SOD1 G93A rats, transdiaphragmatic pressure is impaired at the late stages of disease leading to powerful recruitment of external intercostal muscles that are normally quiescent, likely compensating for diaphragm dysfunction (21). A class of glutamatergic neurons (V2a neurons) were implicated in the recruitment of accessory respiratory muscles in SOD1 G93A mice suggesting that degeneration of V2a neurons would induce decompensation and likely lead to respiratory failure (117).…”