Janus effect of glucocorticoids on differentiation of muscle fibro/adipogenic progenitors

Abstract

Muscle resident fibro-adipogenic progenitors (FAPs), support muscle regeneration by releasing cytokines that stimulate the differentiation of myogenic stem cells. However, in non-physiological contexts (myopathies, atrophy, aging) FAPs cause fibrotic and fat infiltrations that impair muscle function. We set out to perform a fluorescence microscopy-based screening to identify compounds that perturb the differentiation trajectories of these multipotent stem cells. From a primary screen of 1,120 FDA/EMA approved drugs, we identified 34 compounds as potential inhibitors of adipogenic differentiation of FAPs isolated from the murine model (mdx) of Duchenne muscular dystrophy (DMD). The hit list from this screen was surprisingly enriched with compounds from the glucocorticoid (GCs) chemical class, drugs that are known to promote adipogenesis in vitro and in vivo. To shed light on these data, three GCs identified in our screening efforts were characterized by different approaches. We found that like dexamethasone, budesonide inhibits adipogenesis induced by insulin in sub-confluent FAPs. However, both drugs have a pro-adipogenic impact when the adipogenic mix contains factors that increase the concentration of cAMP. Gene expression analysis demonstrated that treatment with glucocorticoids induces the transcription of Gilz/Tsc22d3, an inhibitor of the adipogenic master regulator PPARγ, only in anti-adipogenic conditions. Additionally, alongside their anti-adipogenic effect, GCs are shown to promote terminal differentiation of satellite cells. Both the anti-adipogenic and pro-myogenic effects are mediated by the glucocorticoid receptor and are not observed in the presence of receptor inhibitors. Steroid administration currently represents the standard treatment for DMD patients, the rationale being based on their anti-inflammatory effects. The findings presented here offer new insights on additional glucocorticoid effects on muscle stem cells that may affect muscle homeostasis and physiology.In muscular dystrophies the degeneration of muscle tissue is initially compensated by efficient regeneration that neutralize muscle loss 1 . However, over time, the regenerative potential in muscles of patients affected by myopathies is impaired and myofibers repair is curbed by the formation of fibrotic scars and fat infiltrations, ultimately leading to decreased muscle function 2 . Muscle fibro-adipogenic progenitors play an important role in these processes. FAPs are muscle mesenchymal stem cells residing in the muscle fibers interstitial space. FAPs express the SCA1, CD34 and PDGFRα (CD140a) antigens while they are negative for the hematopoietic and endothelial markers, CD45 and CD31, and for the satellite marker α7 integrin (ITGA7) 2-4 . FAPs contribute to muscle regeneration by secreting IGF-1 and IL-6 3 , by facilitating the clearance of necrotic debris 5 and by promoting the formation of extra-cellular matrix 6 . In addition to this pro-regenerative roles, FAPs are responsible for the formation of ectopic tissue infiltrations ...

Molecular Mechanisms of Glucocorticoid-Induced Insulin Resistance

“…In contrast, in mature adipocytes, DEX treatment decreases the expression of PPARG2 , SREBP1C , C/EBPα , and FABP4, indicating a decrease in anabolic pathways while increasing lipolysis (increased expression of lipases LIPE , ATGL , MGLL ) and decreasing glucose uptake [ 92 ] ( Figure 2 ). More recently, a study demonstrated that GC can either induce or inhibit the adipogenic differentiation program of muscle-resident fibro-adipogenic progenitors depending on the cellular concentration in cAMP [ 95 ]. Moreover, the inhibitory effects of GC-bound GR on adipogenesis were mediated through the up-regulation of a known GR target: GILZ .…”

“…Moreover, the inhibitory effects of GC-bound GR on adipogenesis were mediated through the up-regulation of a known GR target: GILZ . Indeed, under antiadipogenic conditions, GILZ is known to bind PPARG2 and inhibit its expression in mesenchymal stem cells (MSCs) [ 95 ]. Overall, GCs increase but are not necessary to the recruitment of progenitor cells towards adipogenesis, while decreasing insulin sensitivity in mature cells, mirroring what can be observed in vivo [ 70 , 92 ].…”

SCA-1 micro-heterogeneity in the fate decision of dystrophic fibro/adipogenic progenitors

“…Muscles of the mdx mouse develop ectopic adipose tissue only in old animals 20 . However, when cultured ex vivo in the absence of the anti-adipogenic signals of the muscle niche, freshly isolated FAPs from young mdx mice readily differentiate into adipocytes 42 . We asked whether the micro-heterogeneity in the expression of SCA-1 has any impact on the ex vivo adipogenic potential.…”

“…To rule out the possibility that the observed differences in the adipogenic differentiation of FAP cell states could be a consequence of a different kinetics in reaching cell confluence, we plotted the total number of nuclei per field over the total number of adipocytes per field from the experiment in Fig. 3a–d and we calculated the trend lines 42 . Even though the trend line of SCA1-High-FAPs clearly reveals a correlation between cell confluence and adipogenic differentiation, the trend of SCA1-Low-FAPs shows no increment of adipocytes at increasing nuclei number (Supplementary Fig.…”

Characterization of the Skeletal Muscle Secretome Reveals a Role for Extracellular Vesicles and IL1α/IL1β in Restricting Fibro/Adipogenic Progenitor Adipogenesis

“…However, in addition to their pro-regeneration role, in pathological conditions, FAPs are responsible for the development of ectopic intramuscular adipose (IMAT) or connective tissues (IMCT) [16,18,19,23]. Several molecular cues have been reported to be involved in the modulation of FAP differentiation in vitro [15,[24][25][26][27][28][29][30]. However, their relative importance in physiology and pathology remains to be established.…”