Complexin-2 redistributes to the membrane of muscle cells in response to insulin and contributes to GLUT4 translocation

Abstract

Insulin stimulates glucose uptake in muscle cells by rapidly redistributing vesicles containing GLUT4 glucose transporters from intracellular compartments to the plasma membrane (PM). GLUT4 vesicle fusion requires the formation of SNARE complexes between vesicular VAMP and PM syntaxin4 and SNAP23. SNARE accessory proteins usually regulate vesicle fusion processes. Complexins aide in neuro-secretory vesicle-membrane fusion by stabilizing trans-SNARE complexes but their participation in GLUT4 vesicle fusion is unknown. We report that complexin-2 is expressed and homogeneously distributed in L6 rat skeletal muscle cells. Upon insulin stimulation, a cohort of complexin-2 redistributes to the PM. Complexin-2 knockdown markedly inhibited GLUT4 translocation without affecting proximal insulin signalling of Akt/PKB phosphorylation and actin fiber remodelling. Similarly, complexin-2 overexpression decreased maximal GLUT4 translocation suggesting that the concentration of complexin-2 is finely tuned to vesicle fusion. These findings reveal an insulin-dependent regulation of GLUT4 insertion into the PM involving complexin-2.

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

“…The complexin proteins were first identified as regulatory proteins involved in synaptic vesicle exocytosis in neurons ( 153 , 154 ). Recently, the expression of complexin-2 was also identified in skeletal muscle cells ( 62 ). Complexins contain an N-terminal accessory helix and a central helix, which interacts with the ternary SNARE complex in a calcium-dependent manner ( 155 , 156 ).…”

“…Complexins contain an N-terminal accessory helix and a central helix, which interacts with the ternary SNARE complex in a calcium-dependent manner ( 155 , 156 ). In skeletal muscle cells, complexin-2 translocates to the PM via the insulin-induced activation of the Rho-family GTPase Rac1 in the noncanonical insulin signaling pathway ( 62 ). Depletion of complexin-2 using siRNA inhibits insulin-stimulated GLUT4 accumulation at the L6 GLUT4myc myoblast surface ( 62 ), but overexpression of complexin-2 also impairs insulin-stimulated GLUT4 translocation ( 62 ).…”

“…In skeletal muscle cells, complexin-2 translocates to the PM via the insulin-induced activation of the Rho-family GTPase Rac1 in the noncanonical insulin signaling pathway ( 62 ). Depletion of complexin-2 using siRNA inhibits insulin-stimulated GLUT4 accumulation at the L6 GLUT4myc myoblast surface ( 62 ), but overexpression of complexin-2 also impairs insulin-stimulated GLUT4 translocation ( 62 ). These findings suggest that complexin-2, similar to Munc18c, is needed in a precise quantity in muscle cells for GSV exocytosis.…”

“…In contrast, DOC2B is a positive regulator of SNARE-dependent GSV fusion in skeletal muscle, coordinating the activation/opening of STX4 to promote SNARE complex assembly ( 20 ). In addition, complexin-2 regulates the stability of SNARE complexes in skeletal muscle cells ( 62 ). This review will discuss the most recent detailed mechanistic findings related to SNARE-mediated GLUT4 vesicle docking and fusion in the following sections.…”

A new degree of complexi(n)ty in the regulation of GLUT4 trafficking

“…For example, type 2 diabetes (T2D) is characterised by an impaired glucose uptake in insulinsensitive tissues such as in muscles with, as consequences, hyperglycaemia and glucotoxicity [1]. In this context, the recent study of Pavarotti et al [2] is important as it identified a new actor, complexin-2, regulating the insulin-mediated translocation of glucose transporter 4 (GLUT4) to the plasma membrane (PM) in skeletal muscle.…”