Characterizing the effects of muscle-specific GSK3α/ß reduction on murine muscle contractility and metabolism in female mice.

Dysregulation of skeletal muscle morphology and metabolism is associated with chronic diseases such as obesity and type 2 diabetes. The enzyme glycogen synthase kinase 3 (GSK3) is highly involved in skeletal muscle physiology and metabolism, acting as a negative regulator of muscle size, strength, adaptive thermogenesis, and glucose homeostasis. Correspondingly, we have shown that partial knockdown (∼40%) of GSK3 specifically in skeletal muscle increases lean mass, reduces fat mass, and activates muscle-based adaptive thermogenesis via sarco(endo)plasmic reticulum Ca2+ (SERCA) uncoupling in male mice. However, the effects of GSK3 knockdown in female mice have yet to be investigated. Here, we examined the effects of muscle-specific GSK3 knockdown on body composition, muscle size and strength, and whole body metabolism in female C57BL/6J mice. Our results show that GSK3 content is higher in the female soleus versus the male soleus; however, there were no differences in the extensor digitorum longus (EDL). Furthermore, muscle-specific GSK3 knockdown did not alter body composition in female mice, nor did it alter daily energy expenditure, glucose/insulin tolerance, mitochondrial respiration, or the expression of the SERCA uncouplers sarcolipin and neuronatin. We also did not find any differences in soleus muscle size, strength, or fatigue resistance. In the EDL, we found that an increase in absolute and specific force production, but there were no differences in fatigability. Therefore, our study highlights sex differences in the response to genetic reduction of gsk3, with most of the effects previously observed in male mice being absent in females.NEW & NOTEWORTHY Here we show that partial GSK3 knockdown has minimal effects on whole body metabolism and muscle contractility in female mice. This is partly inconsistent with previous results found in male mice, which reveal a potential influence of biological sex.

Low-dose lithium supplementation promotes adipose tissue browning and sarco(endo)plasmic reticulum Ca2+ ATPase uncoupling in muscle.

Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) uncoupling in skeletal muscle and mitochondrial uncoupling via uncoupling protein 1 (UCP1) in brown/beige adipose tissue are two mechanisms implicated in energy expenditure. Here, we investigated the effects of glycogen synthase kinase 3 (GSK3) inhibition via lithium chloride (LiCl) treatment on SERCA uncoupling in skeletal muscle and UCP1 expression in adipose. C2C12 and 3T3-L1 cells treated with LiCl had increased SERCA uncoupling and UCP1 protein levels, respectively, ultimately raising cellular respiration; however, this was only observed when LiCl treatment occurred throughout differentiation. In vivo, LiCl treatment (10 mg/kg/day) increased food intake in chow-fed diet and high-fat diet (HFD; 60% kcal)-fed male mice without increasing body mass-a result attributed to elevated daily energy expenditure. In soleus muscle, we determined that LiCl treatment promoted SERCA uncoupling via increased expression of SERCA uncouplers, sarcolipin and/or neuronatin, under chow-fed and HFD-fed conditions. We attribute these effects to the GSK3 inhibition observed with LiCl treatment as partial muscle-specific GSK3 knockdown produced similar effects. In adipose, LiCl treatment inhibited GSK3 in inguinal white adipose tissue (iWAT) but not in brown adipose tissue under chow-fed conditions, which led to an increase in UCP1 in iWAT and a beiging-like effect with a multilocular phenotype. We did not observe this beiging-like effect and increase in UCP1 in mice fed a HFD, as LiCl could not overcome the ensuing overactivation of GSK3. Nonetheless, our study establishes novel regulatory links between GSK3 and SERCA uncoupling in muscle and GSK3 and UCP1 and beiging in iWAT.

Toward countering muscle and bone loss with spaceflight: GSK3 as a potential target.

We examined the effects of ∼30 days of spaceflight on glycogen synthase kinase 3 (GSK3) content and inhibitory serine phosphorylation in murine muscle and bone samples from four separate missions (BION-M1, rodent research [RR]1, RR9, and RR18). Spaceflight reduced GSK3ß content across all missions, whereas its serine phosphorylation was elevated with RR18 and BION-M1. The reduction in GSK3ß was linked to the reduction in type IIA fibers commonly observed with spaceflight as these fibers are particularly enriched with GSK3. We then tested the effects of inhibiting GSK3 before this fiber type shift, and we demonstrate that muscle-specific Gsk3 knockdown increased muscle mass, preserved muscle strength, and promoted the oxidative fiber type with Earth-based hindlimb unloading. In bone, GSK3 activation was enhanced after spaceflight; and strikingly, muscle-specific Gsk3 deletion increased bone mineral density in response to hindlimb unloading. Thus, future studies should test the effects of GSK3 inhibition during spaceflight.