Mustn1 ablation in skeletal muscle results in functional alterations

Abstract

Mustn1, a gene expressed exclusively in the musculoskeletal system, was shown in previous in vitro studies to be a key regulator of myogenic differentiation and myofusion. Other studies also showed Mustn1 expression associated with skeletal muscle development and hypertrophy. However, its specific role in skeletal muscle function remains unclear. This study sought to investigate the effects of Mustn1 in a conditional knockout (KO) mouse model in Pax7 positive skeletal muscle satellite cells. Specifically, we investigated the potential effects of Mustn1 on myogenic gene expression, grip strength, alterations in gait, ex vivo investigations of isolated skeletal muscle isometric contractions, and potential changes in the composition of muscle fiber types. Results indicate that Mustn1 KO mice did not present any substantial phenotypic changes or significant variations in genes related to myogenic differentiation and fusion. However, an approximately 10% decrease in overall grip strength was observed in the 2‐month‐old KO mice in comparison to the control wild type (WT), but this decrease was not significant when normalized by weight. KO mice also generated approximately 8% higher vertical force than WT at 4 months in the hindlimb. Ex vivo experiments revealed decreases in about 20 to 50% in skeletal muscle contractions and about 10%–20% fatigue in soleus of both 2‐ and 4‐month‐old KO mice, respectively. Lastly, immunofluorescent analyses showed a persistent increase of Type IIb fibers up to 15‐fold in the KO mice while Type I fibers decreased about 20% and 30% at both 2 and 4 months, respectively. These findings suggest a potential adaptive or compensatory mechanism following Mustn1 loss, as well as hinting at an association between Mustn1 and muscle fiber typing. Collectively, Mustn1's complex roles in skeletal muscle physiology requires further research, particularly in terms of understanding the potential role of Mustn1 in muscle repair and regeneration, as well as with influence of exercise. Collectively, these will offer valuable insights into Mustn1's key biological functions and regulatory pathways.

Mustn1 is a smooth muscle cell-secreted microprotein that modulates skeletal muscle extracellular matrix composition

“…Considering some of the age-dependent observations in Mustn1 loss-of-function models, it is possible that there are compensatory mechanisms activated in the absence of Mustn1 expression, which deserve further investigation. Indeed, this idea is supported by recent studies showing changes in muscle fiber type composition in young Mustn1 KOs generated with a Pax7-Cre strategy [ 17 ]. In addition, aging studies using the Mustn1 models could reveal additional roles for this protein.…”

“…Although the Mustn1 gene is known to be expressed in skeletal muscle during embryonic development and regeneration [ 14 ], little is known about its physiological function. Previous studies have suggested a role in myoblast differentiation [ 15 , 16 ], muscle function [ 17 ], and whole-body glucose homeostasis [ 18 ]. Here, we show that Mustn1 is expressed and secreted from SMCs into the muscle extracellular space.…”

Mustn1 in Skeletal Muscle: A Novel Regulator?

“…There is a potential specific relationship between Mustn1 and Type I fibers that may explain their reduction observed at both 2 and 4 months in the Pax7-Cre-mediated Mustn1 KO mice [78]. Further, considering that Type II fibers develop from Type I fibers [2], the decrease in the regional area percentage of Type IIa fibers could be a downstream effect of the diminished number of Type I fibers.…”

“…Yet, the role of Mustn1 in the maturation or transition of these muscle fibers from Type I remains speculative. Interestingly, Mustn1 KO mice generated Type IIb fibers when their WT counterparts did not [78], hinting at a possible compensatory mechanism to offset lower Type I fiber counts. However, the question remains why a Type I fiber would evolve into a Type IIb instead of a Type IIa fiber, providing an interesting direction for future research.…”

“…Given that the soleus, a major plantar flexor muscle, is essential for activities such as walking, running, and climbing, the observed increase in Type IIb fibers may contribute to several notable changes. These include the elevated single limb vertical force during walking and greater absolute contractile forces in the KO mice during the ex vivo isometric contraction tests [78].…”