The clock regulator Bmal1 protects against muscular dystrophy

Highlights

• Genetic loss of Bmal1 in mdx mice aggravated muscle damage with wasting and impaired function.

• Bmal1 deficiency in mdx mice resulted in severe muscle injury in diaphragm with defective regenerative response.

• Bmal1 is required for satellite cell proliferation and regenerative myogenesis to protect against dystrophic- injury.

Abstract

The muscle-intrinsic clock machinery is required for the maintenance of muscle growth, remodeling and function. Our previous studies demonstrated that the essential transcription activator of the molecular clock feed-back loop, Brain and Muscle Arnt-Like 1(Bmal1), plays a critical role in myogenic progenitor behavior to promote and regenerative myogenesis. Using genetic approaches targeting Bmal1 in the DMD^mdx (mdx) dystrophic mouse model, here we report that the loss of Bmal1 function significantly accelerated dystrophic disease progression. In contrast to the mild dystrophic changes in mdx mice, the genetic loss-of-function of Bmal1 aggravated muscle damage in this dystrophic disease background, as indicated by persistently elevated creatine kinase levels, increased injury area and reduced muscle grip strength. Mechanistic studies revealed that markedly impaired myogenic progenitor proliferation and myogenic response underlie the defective new myofiber formation in the chronic dystrophic milieu. Taken together, our study identified the function of pro-myogenic clock gene Bmal1 in protecting against dystrophic damage, suggesting the potential for augmenting Bmal1 function to ameliorate dystrophic or degenerative muscle diseases.

Introduction

Repeated contraction-induced degeneration-regeneration cycles, a hallmark of dystrophic diseases such as Duchene muscular dystrophy (DMD), lead to progressive loss of muscle mass and function [1,2]. Effective means to augment regenerative repair to promote muscle growth and improve muscle mass could ameliorate dystrophic pathophysiology and potentially benefit patients with DMD [[3], [4], [5]]. Therefore, better understanding of molecular events that promote regenerative capacity could offer novel options to preserves functional muscle mass and ameliorate muscular dystrophy.

The circadian clock that drives 24-h rhythms in physiology comprise of a transcriptional-translational feedback loop that ultimately coordinates physiological processes with external timing cues [6]. In addition to the central clock residing in the suprachiasmatic nuclei entrained directly to light input, nearly all tissue and cell types in our body possess cell-autonomous clock rhythms that are normally driven by central clock. Skeletal muscle possesses intrinsic clock activity, which can be shifted by timing of exercise besides being driven by the central clock output. The clock transcription negative feedback loop underlies the ~24 h rhythms in behavior and physiology. Brain and Muscle Arnt-Like 1 (Bmal1), a muscle-enriched clock transcription activator, is essential for driving molecular clock transcription together with its heterodimer partner, Circadian Locomotor Output Cycles Kaput (CLOCK). Transcription activation of the repressors of the clock loop, including Periods (Per) and Cryptochromes (Cry), leads to subsequent inhibition of transcription to form the feedback loop. Various posttranscriptional and posttranslational mechanisms further contribute to establish the circadian oscillation cycles.

Skeletal muscle possesses cell-autonomous molecular clocks that are critical for temporal control of muscle function by entraining locomotor activity [[7], [8], [9]]. Recent studies indicated that the muscle-intrinsic clock is required for maintenance of muscle mass, growth and metabolic regulation [10,11]. This temporal element is involved in muscle mass regulation through sarcomeric structural organization and myogenic progenitor behaviors [[12], [13], [14], [15]]. We demonstrated previously that the essential clock activator Bmal1 promotes regenerative myogenesis and prevents sarcopenia [14,[16], [17], [18]]. More recently, our study revealed that Rev-erbα, a ligand-dependent nuclear receptor and transcriptional repressor of Bmal1 in the core clock loop [19,20], suppresses myogenic progenitor proliferation and differentiation [21]. In addition, Cry2 was found to be critical for circadian regulation of myogenic differentiation [22]. The circadian clock circuit may exert temporal control of myogenic progression, and thus represent a potential novel pathway for muscle-wasting disease intervention.

Utilizing the mdx mice as a DMD disease model, we generated double-null mutant of Bmal1 with the DMD disease background to directly test whether Bmal1 protects against dystrophic muscle injury. Our study revealed that the loss of Bmal1 significantly exacerbates dystrophic pathology in the mdx mice, demonstrating that Bmal1 maintains muscle regenerative capacity in muscular dystrophy.