Muscle protein synthesis, mTORC1/MAPK/Hippo signaling, and capillary density are altered by blocking of myostatin and activins

Abstract

Loss of muscle mass and function occurs in various diseases. Myostatin blocking can attenuate muscle loss, but downstream signaling is not well known. Therefore, to elucidate associated signaling pathways, we used the soluble activin receptor IIb (sActRIIB-Fc) to block myostatin and activins in mice. Within 2 wk, the treatment rapidly increased muscle size as expected but decreased capillary density per area. sActRIIB-Fc increased muscle protein synthesis 1–2 days after the treatment correlating with enhanced mTORC1 signaling (phosphorylated rpS6 and S6K1, r = 0.8). Concurrently, increased REDD1 and eIF2Bε protein contents and phosphorylation of 4E-BP1 and AMPK was observed. In contrast, proangiogenic MAPK signaling and VEGF-A protein decreased. Hippo signaling has been characterized recently as a regulator of organ size and an important regulator of myogenesis in vitro. The phosphorylation of YAP (Yes-associated protein), a readout of activated Hippo signaling, increased after short- and longer-term myostatin and activin blocking and in exercised muscle. Moreover, dystrophic mdx mice had elevated phosphorylated and especially total YAP protein content. These results show that the blocking of myostatin and activins induce rapid skeletal muscle growth. This is associated with increased protein synthesis and mTORC1 signaling but decreased capillary density and proangiogenic signaling. It is also shown for the first time that Hippo signaling is activated in skeletal muscle after myostatin blocking and exercise and also in dystrophic muscle. This suggests that Hippo signaling may have a role in skeletal muscle in various circumstances.

Treatment with soluble activin type IIB-receptor improves bone mass and strength in a mouse model of duchenne muscular dystrophy

“…However, it was interesting to note that the differences between ActRIIB-Fc and ActRIIB-Fc-R groups were miniscule showing that physical activity did not have a noticeable further effect on trabecular bone in the murine femur in this model. ActRIIB-Fc-R mice ran significantly less especially in the beginning of the study as previously reported [26], which could at least in part explain the lack of additional effect of running on bone mass or strength in the ActRIIB-Fc-R group. The nonsignificant change between PBS and PBS running group in bone mineral density was also surprising suggesting that physical activity in a form of voluntary running does not greatly affect bone quality in long bones in young mdx mice.…”

Effects of muscular dystrophy, exercise and blocking activin receptor IIB ligands on the unfolded protein response and oxidative stress

“…unpublished data) when protein synthesis is acutely already increased [19]. This suggests that the level of disruption to ER homeostasis through increased amounts of newly synthesized proteins by blocking AcvR2B ligands may not be severe enough to trigger a UPR ER response.…”

“…Muscle homogenates mixed in Laemmli sample buffer were heated at 95°C to denature proteins and processed as described previously [19,25] …”

“…Blockade of AcvR2B ligands can be achieved, e.g. by using the soluble ligand binding domain of type IIb activin receptor fused to the Fc domain (sAcvR2B-Fc) to effectively increase muscle size [18][19][20][21]. Blocking these proteins using various strategies has been shown to attenuate dystrophic pathology of the mdx mouse in some [18,22], but not in all studies [20,23].…”

Co-Administration of Myostatin-Targeting siRNA and ActRIIB-Fc Fusion Protein Increases Masseter Muscle Mass and Fiber Size

“…It is also reported that the potent cell surface receptor ActRIIB binds and inhibits myostatin, leading to dramatic muscle growth (24)(25)(26). Based on this background, we hypothesized that the combination of these two agents would have a therapeutic advantage.…”