Enrichment of the exocytosis protein STX4 in skeletal muscle remediates peripheral insulin resistance and alters mitochondrial dynamics via Drp1

Abstract

Mitochondrial dysfunction is implicated in skeletal muscle insulin resistance. Syntaxin 4 (STX4) levels are reduced in human diabetic skeletal muscle, and global transgenic enrichment of STX4 expression improves insulin sensitivity in mice. Here, we show that transgenic skeletal muscle-specific STX4 enrichment (skmSTX4tg) in mice reverses established insulin resistance and improves mitochondrial function in the context of diabetogenic stress. Specifically, skmSTX4tg reversed insulin resistance caused by high-fat diet (HFD) without altering body weight or food consumption. Electron microscopy of wild-type mouse muscle revealed STX4 localisation at or proximal to the mitochondrial membrane. STX4 enrichment prevented HFD-induced mitochondrial fragmentation and dysfunction through a mechanism involving STX4-Drp1 interaction and elevated AMPK-mediated phosphorylation at Drp1 S637, which favors fusion. Our findings challenge the dogma that STX4 acts solely at the plasma membrane, revealing that STX4 localises at/proximal to and regulates the function of mitochondria in muscle. These results establish skeletal muscle STX4 enrichment as a candidate therapeutic strategy to reverse peripheral insulin resistance.

The Effect of Aerobic Exercise on the Oxidative Capacity of Skeletal Muscle Mitochondria in Mice with Impaired Glucose Tolerance

“…As one of the important mechanisms for inducing insulin resistance, skeletal muscle mitochondria play a key role in regulating insulin resistance [ 34 ]. Merz et al demonstrated that skeletal muscle-specific STX4 remediates HFD-induced insulin resistance via suppressing mitochondrial fission [ 35 ]. The reduced mitochondrial capacity in skeletal muscle is suggested to underlie IR development in obesity and type 2 diabetes [ 36 ].…”

Changes in Skeletal Muscle PAK1 Levels Regulate Tissue Crosstalk to Impact Whole Body Glucose Homeostasis

“…Transfected myoblasts expressed ~2-fold more PAK1 protein compared to endogenous PAK1 in the control pCMV-vector-transfected myoblasts (Figure 5B), and incubation of INS-1 832/13 b-cells with CM from the PAK1overexpressing myoblasts resulted in enhanced GSIS relative to that of the b-cells incubated with CM from vector-expressing myoblasts (Figure 5C, bar 4 vs. bar 2); b-cell insulin content was unchanged (Figure 5D). The observation that CM from muscle cells overexpressing PAK1 enhances GSIS is unique to PAK1 overexpression to date, as other factors like Syntaxin 4 overexpressed (STX4 OE) in L6 myoblast cells had no enhancing effect upon GSIS compared with vector-transfected control cells (STX4 OE=11 ng insulin/mg total protein at 1 mM glucose; 24 ng insulin/mg total protein at 20 mM glucose, n=4, p > 0.05 vs. vector control (41). These data suggest that PAK1 levels in the skeletal muscle may mediate the release of factors into the circulation that crosstalk with the pancreatic b-cells.…”

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

“…In the skeletal muscle of diabetic mice, STX4 mRNA levels are significantly reduced ( 93 ), suggesting that STX4 deficiency may be important for diabetes development, and that re-expression of STX4 may be a therapeutic strategy to restore insulin sensitivity and reverse T2D. In alignment with this point of view, a recent study showed that enrichment of STX4 (~2 fold) in skeletal muscle remediates peripheral insulin resistance in obese mice, even under conditions of persisting intake of high-fat diet ( 94 ). Collectively, these findings highlight the important role and therapeutic potential of STX4 in remediating insulin resistance in skeletal muscle.…”

“…High-fat diets are known to increase mitochondrial fission and mitophagy ( 184 ), a hallmark of high-fat diet-induced diabetes. STX4 enrichment reversed both skeletal muscle insulin resistance and mitochondrial fragmentation and dysfunction in high-fat diet-fed mice ( 94 ). The mechanism is currently believed to involve suppression of mitochondrial fission.…”

“…Specifically, phosphorylation of Drp1 at Ser616 promotes mitochondrial fission by facilitating Drp1 translocation from the cytosol to the mitochondria ( 186 ), whereas phosphorylation of Drp1 at Ser637 inhibits its translocation, thereby promoting mitochondrial fusion ( 187 ). In primary skeletal muscle and L6 skeletal muscle cells, STX4 binds to Drp1 and promotes its phosphorylation at Ser637, but not Ser616, through a mechanism involving AMPK ( 94 ). In addition, knockdown of STX4 reduces Drp1 phosphorylation at Ser637 and AMPK activation ( Figure 4 ).…”