Brown Fat Promotes Muscle Growth During Regeneration.

Accumulation of adipose tissue around and within muscles is highly correlated with reduced strength, functional limitations, and poor rehabilitative outcomes. Given the intimate physical contact between these tissues, paracrine cross-talk is a likely mediator of this association. The recent discovery that muscle-associated adipose tissue exhibits features of beige fat has suggested that this cross-talk may be modifiable, as beige fat can be stimulated to assume features of brown fat. In this work, we describe a novel intermuscular fat transplant model in the mouse rotator cuff to investigate cross-talk between muscle and adipose tissue. Specifically, we examine the role of transplanted fat phenotype on muscle regeneration by transplanting pieces of classical brown (interscapular), beige (inguinal), or white (epididymal) adipose tissue in conjunction with cardiotoxin injection to the adjacent supraspinatus muscle. Transplantation of brown fat, but not beige or white, significantly increased muscle mass, fiber cross-sectional area and contractile force production compared with sham injury. This effect was not seen when cardiotoxin was delivered to a distant muscle, or when adjacent muscles were injected with saline indicating that the effect is localized and specifically targeting the regenerative process. Thus, we conclude that local signaling between fat and muscle varies by phenotype and that brown fat supports regeneration. Clinical significance: Our findings suggest that the phenotype of muscle-associated fat could be a novel therapeutic target to modulate fat-muscle signaling. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1817-1826, 2019.