Volumetric muscle loss is a devastating muscle injury that beyond the capability of spontaneous muscle regeneration. Treating VML remains a clinical challenge to the effected patients. Current treatments for VML focus on fulfilling the wound with acellular biomaterial with or without combination with a cellular component that promotes muscle regeneration [24]. Cells currently used for VML treatments are mainly myoblasts or other myogenic stem/progenitor cells [25, 26]. In this study, we demonstrated that a non-myogenic progenitor—FAP, when induced to BAT differentiation, can be used in treating VML. BAT-differentiated FAPs, when transplanted into the wound with an acellular material of matrigel, significantly improved muscle regeneration and limb function after VML.
Different than myogenic progenitors that directly differentiate into myocytes and fuse into new myotubes, FAPs are likely to promote muscle regeneration by generating a pro-myogenic environment to facilitate intrinsic myogenic progenitor migration and differentiation. Using inducible FAP depletion mice, we demonstrated a beneficial role of FAPs in muscle regeneration after VML. Supportive role of FAP in muscle regeneration has been reported previously in other injury models. FAPs have been shown to support muscle stem cells (MuSCs) differentiation in vitro [11, 27]. In a recent study, Wosczyna et al. reported that depletion of FAPs resulted in loss of expansion of muscle stem cells (MuSCs) and CD45 + hematopoietic cells after barium chloride-induced injury and impaired skeletal muscle regeneration in mice [27]. In our current study, we used the same approach of PDGFRα-CREERT/DTA mice to deplete FAPs with Tamoxifen after VML. We found significantly reduced muscle regeneration and hindlimb function in Tamoxifen-induced FAP depletion mice after VML compared to the control. This data provided direct evidence in supporting the concluding that FAPs have a beneficial role in muscle regeneration after VML.
In our PDGFRα-CREERT/DTA mice, we observed about 50% FAP depletion in after 2 weeks of Tamoxifen induction. This is similar to 50%-75% FAP depletion rate of FAPs reported by Wosczyna et al. in their study [28]. Different than satellite cell depletion mouse model, which usually reaches over 90% satellite cell depletion rate [29–31], the same Cre/DTA mouse model only achieves 50%-75% depletion rate for FAPs. Nevertheless, our data suggested that 50% depletion of FAP is sufficient to alter muscle homeostatic conditions and impairs regeneration after VML. It is possible that the surviving FAPs are severely damaged by Tamoxifen-induced DTA toxin thus lost their role in promoting muscle regeneration. Future work is needed to future define the fate of surviving FAPs in this model.
FAPs are proven to be able to differentiate to mature adipocytes (white fat cells) and fibroblasts in vitro and in vivo. Recently, Gorski reported that FAPs can differentiate into brown/beige-like adipocytes (BAT) that express uncoupling protein 1 (UCP-1) [32]. In our currently study, we demonstrated that FAPs differentiate into UCP-1(+) BAT-like cells in muscle after VML. The function of UCP-1(+) BAT-like FAPs in muscle regeneration is not fully understood. Meyer et al. has demonstrated that healthy rotator cuff muscles are encapsulated by epimuscular adipose tissue that appear to adopt a beige phenotype when the RC tendon is torn [33]. The same group further showed that transplantation of brown fat into supraspinatus muscle improved muscle regeneration after cardiotoxin injury, suggesting a positive role for BAT in RC muscle regeneration following injury [34]. In our previous work, we also demonstrated that transplantation of BAT-FAPs promotes muscle regeneration and shoulder function after massive rotator cuff tears and repair [13, 14]. In this study, we demonstrated that both mouse and human BAT-like FAP transplantation improved muscle regeneration and hindlimb function after VML in our mouse model.
The underline of mechanism of BAT-FAP in promoting muscle regeneration remains unknown at this time. In addition to thermogenesis and energy metabolism, BAT has been shown to possess a non-thermogenic metabolite function through with an endocrine/paracrine manor. Insulin-like growth factor 1(IGF1) and follistatin, two important promyogenic growth factors are among BAT derived cytokines (also known as batokines) [35–38]. Previous work has suggested that follistatin is the soluble mediator of functional interactions of FAP and myogenic satellite cells during muscle regeneration [39]. However, we did not see change of IGF1 and follistatin gene expression change in FAPs following VML. It is possible FAP derived cytokines other than IGF1 and follistatin may play an important role of FAP-mediated muscle regeneration after VML. A recent study showed that WNT1 Inducible Signaling Pathway Protein 1 (WISP1) is an important FAP-derived matricellular signal indirectly affects the myogenic differentiation of MuSCs [40]. Future work is needed to define beige FAP-derived promyogenic factors that promote muscle regeneration after VML.
β3 adrenergic receptors (B3AR) are selectively expressed in adipose tissue and selective β3 adrenergic receptor agonists can effectively stimulate BAT activity in human without significant cardiovascular system side effects [41, 42]. In this study, we demonstrated that Amibegron treatment significantly improved muscle regeneration and hindlimb function after VML. Mirabegron, a β3 adrenergic receptor agonist, with a similar structure to Amibegron, has been approved by the FDA for treating over-reactive bladders [43, 44]. Though it may mildly increase heart rate and blood pressure in patients, Mirabegron could be considered as a potential novel treatment for VML in the future.
There are some limitations in this study. First, we did not measure the direct contractility of TA muscle after VML in this study. However, this may not be a critical flaw of this study since we measure the hindlimb gait as a functional measurement. Our results showed that both Amibegron injection and BAT-differentiated FAP transplantation significantly improve limb function after VML. Second, only one time point of 6 weeks was used to evaluate the functional role of gait and muscle histology in the cell transplantation in order to reduce the number of animals used in this study. We believe 6 weeks after VML is a reasonable time point for gait analysis and muscle histology based on results from FAP reporter mice and FAP depletion mice in this study. However, a single time point excluded the possibility of studying the dynamic change BAT-FAP after transplantation in VML. Last but not least, it should be also noted that only a single donor of human FAPs was recruited in this study. Though it is sufficient to prove the concept that BAT-differentiated human FAPs could be improve muscle regeneration after VML, more donors will be recruited in future human FAP studies.