Abnormalities in the genioglossus muscle and its neuromuscular synapse in leptin-deficient male mice.

Abstract

Background: The genioglossus (GG) muscle, the largest upper airway dilator muscle, plays a crucial role in maintaining pharyngeal airway patency. It is innervated by hypoglossal motoneurons, and its tone is often reduced in patients with obstructive sleep apnea (OSA), leading to tongue collapse and airway obstruction during sleep. Although the mechanisms underlying this disorder are not fully understood, the neuromuscular junction (NMJ) of the GG muscle, essential for communication between motor neurons and skeletal muscle, has largely been overlooked.

Methods: In this study, we explored whether obesity impacts the NMJ of the GG muscle. Using the leptin-deficient obese mouse model, Lep^ob/ob, which exhibits pharyngeal collapsibility and hypoventilation, we analyzed the GG muscle and its NMJ in both male and female mice. We conducted morphological and histochemical studies of the GG muscle; quantitative fluorescence imaging to assess the density and dynamics of nicotinic acetylcholine receptors (nAChRs) at the NMJ; high-resolution confocal microscopy to evaluate structural changes in the pre- and postsynaptic apparatus; and transmission electron microscopy for ultrastructural analysis. Additionally, we examined the diaphragm (DIA) and sternomastoid (ST) muscles for comparative analysis.

Results: Our results show that the GG muscle and its NMJs exhibit significant alterations in Lep^ob/ob male mice, while the ST and DIA muscles remain unaffected. Lep^ob/ob males displayed altered GG muscle morphology, changes in synapse structure, and reduced postsynaptic AChR density compared to both controls and Lep^ob/ob females. Additionally, AChR turnover and the morphology of the presynaptic apparatus were impaired in Lep^ob/ob male mice. In contrast, Lep^ob/ob females exhibited NMJs similar to those of wild-type mice.

Conclusions: These findings suggest that the GG muscle is particularly susceptible to degeneration in obesity induced by leptin deficiency, with distinct alterations observed in both the muscle and the NMJ. This specificity underscores the complex impact of obesity on NMJ health and highlights the need for further investigation into muscle-specific responses to obesity-related stress. Additionally, the degeneration of the GG muscle appears to reflect a sex-specific impact of obesity on neuromuscular integrity and may contribute to the pathogenesis of OSA.