Date of Award
5-9-2025
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
The aim of this dissertation is to investigate the crosstalk between adipose and muscle tissues in physiology and pathophysiology, particularly focusing on the therapeutic potential of muscle-associated adipose tissue in muscle regeneration. Adipose tissue, an endocrine organ, secretes cytokines (a.k.a. adipokines) that significantly and negatively impact muscle function in conditions like diabetes, obesity, and aging. Whether similar signals mediate communication between muscle and its adjacent adipose tissue through paracrine signaling is unknown, as is whether this communication is therapeutically modifiable. Understanding the interactions between adipose and muscle tissues could lead to novel therapeutic strategies for improving muscle regeneration and function. To define and manipulate paracrine adipose-muscle signaling, we developed an experimental transplanted adipose-muscle interface in the mouse rotator cuff for in vivo studies and collected human muscle and adipose samples from individuals with and without diabetes for in vitro experiments. Our previous research demonstrated that transplanted brown adipose tissue (BAT) enhances muscle regeneration, compared with white adipose tissue (WAT), through paracrine signaling. Given the potential pro-regenerative role of BAT-secreted cytokines, we hypothesized that stimulating muscle-associated adipose to adopt BAT-like characteristics could improve muscle regeneration. To explore this hypothesis, we utilized RNA sequencing to profile transcriptional changes in muscle-resident cell populations during regeneration in our BAT transplant model, aiming to identify potential “batokines” and their target cells involved in muscle regeneration. Concurrently, we profiled intramuscular adipose tissue (IMAT) and subcutaneous fat in human subjects to characterize depot-specific differences in secreted cytokines and their browning and pro-myogenic potentials. Adipose progenitor cells (APCs) were isolated and used to condition primary human muscle myoblasts in vitro to assess the impact on myogenesis. Last but not least, we developed a tunable polyethylene glycol hydrogel embedded with nanosilicates, aiming for sustained local release of a browning factor, mirabegron. We then tested whether sustained local mirabegron will locally brown adipose tissue in vivo in mice without systemic off-target effects. Our key findings reveal that transplanted BAT increases muscle mass in the early stages of regeneration primarily by modulating fibro-adipogenic progenitor (FAP) signaling, potentially through immune regulation. Although there were no long-term improvements in muscle mass or function in wildtype mice, in the absence of endogenous BAT, BAT transplantation improved muscle contractile force independent of changes in muscle mass, suggesting potential direct regulation of muscle contraction by “batokines”. RNA sequencing of human IMAT and subcutaneous fat revealed distinct transcriptional profiles, highlighting depot-specific differences and their respective browning potentials. IMAT showed reduced inflammatory pathways and an anti-myogenic profile for secreted cytokines compared to SQ. Stimulation of isolated human APC from IMAT fail to push them toward a brown/beige profile, and thus did not improve myogenesis of myoblasts in vitro. The implantation of our biodegradable gel proved to be viable up to 2 weeks in vivo with minor surrounding inflammation. The mirabegron-binding efficiency to the NS was measured to be 87% ± 5 and released mirabegron retained bioactivity. The mouse injected with mirabegron-loaded gel as proof-of-principle exhibits evidence of increased lipolysis, but not substantial adipose browning, on the mirabegron treated side compared with the contralateral side treated with unloaded gel. Importantly, the gel-treated mice did not exhibit increased heart rate associated with bolus mirabegron injection, suggesting that gel loading the drug avoids unwanted off-target effects. These data support the potential efficacy of the biomaterials approach, but also the need to tailor dosing and timing. Taken together these studies shed new light on the role of muscle-associated adipose in regulating muscle physiology. While exogenous brown fat has a moderate impact on muscle regeneration, human IMAT secreted factors do not improve in vitro myogenesis and human IMAT lacks the capacity for browning, shifting our therapeutic interest to adipose outside the muscle. We show that it is possible to induce browning locally through sustained release mirabegron, suggesting that this could target endogenous epi-muscular adipose.
Language
English (en)