Melatonin promotes skeletal muscle mitochondria-associated endoplasmic reticulum membrane contacts and restores organellar membrane integrity and phospholipid composition in Zücker diabetic fatty rats of both sexes

Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are essential for cellular homeostasis and play crucial roles in lipid metabolism and calcium signaling. In skeletal muscle (SKM), MAMs contribute to metabolic flexibility, and their disruption is implicated in metabolic disorders such as diabesity (obesity and its associated type 2 diabetes). Melatonin, known for its antioxidant, circadian rhythm and metabolic regulatory effects, has been shown to preserve ER and mitochondrial function. This study explored whether melatonin could restore organellar membrane integrity, phospholipid composition in SKM, and MAM reassembly under diabesity conditions. Five-week-old male and female Zücker diabetic fatty (ZDF) and lean (ZL) rats were subdivided into untreated control group (C) and treated with oral melatonin group (M, 10 mg/kg/day in drinking water for 12 weeks). MAM integrity and ER‒mitochondria interactions in the vastus lateralis (VL) muscle were assessed via both in situ proximity ligation assay (PLA) and transmission electron microscopy. Mitochondrial membrane integrity was evaluated by measuring mitochondrial permeability transition pore (mPTP) opening and cytochrome C release. Mitochondrial and ER phospholipid composition was analyzed using ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC‒MS/MS). In this study, we observed for the first time that melatonin promoted MAM assembly and ER‒mitochondria contacts in the VL from both sex obese diabetic rats. It also improved mitochondrial membrane integrity and reduced cytosolic cytochrome C release. In mitochondria, melatonin restored the oxidized cardiolipin (CLox) to total cardiolipin (CL) ratio by decreasing CLox and increasing total CL and its species (CL (72:7), CL (76:12)). In the ER, melatonin restored the phosphatidylcholine (PC) to phosphatidylethanolamine (PE) ratio by reducing total PC, and specific PC species (PC (38:2) and PC (42:6)) while increasing total PE and specific PE species (PE (34:0), PE (36:2), PE (40:7), and PE (40:8)). These findings highlight the role of melatonin as a therapeutic agent for preserving SKM mitochondrial membrane integrity and its potential effect on interorganellar communication via MAM assembly during diabesity. Furthermore, melatonin contributes to the maintenance of proper phospholipid composition, promoting an ideal environment for proper MAM assembly.