Functions of the Zinc-Sensing Receptor GPR39 in Regulating Intestinal Health in Animals

Zinc Alleviates Diabetic Muscle Atrophy via Modulation of the SIRT1/FoxO1 Autophagy Pathway Through GPR39

BACKGROUND

Muscle atrophy is a severe complication of diabetes, with autophagy playing a critical role in its progression. Zinc has been shown to alleviate hyperglycaemia and several diabetes-related complications, but its direct role in mediating diabetic muscle atrophy remains unclear. This study explores the potential role of zinc in the pathogenesis of diabetic muscle atrophy.

METHODS

In vivo, C57BL/6J mice were induced with diabetes by streptozotocin (STZ) and treated with ZnSO₄ (25 mg/kg/day) for six weeks. Gastrocnemius muscles were collected for histological analysis, including transmission electron microscopy (TEM). Serum zinc levels were measured by ICP-MS. Protein expression was evaluated using immunofluorescence (IF), immunohistochemistry (IHC) and Western blotting (WB). Bioinformatics analysis was used to identify key genes associated with muscle atrophy. In vitro, a high-glucose-induced diabetic C2C12 cell model was established and received ZnSO₄, rapamycin, SRT1720, TC-G-1008, or GPR39-CRISPR Cas9 intervention. Autophagy was observed by TEM, and protein expression was assessed by IF and WB. Intracellular zinc concentrations were measured using fluorescence resonance energy transfer (FRET).

RESULTS

In vivo, muscle atrophy, autophagy activation, and upregulation of SIRT1 and FoxO1, along with downregulation of GPR39, were confirmed in the T1D group. ZnSO₄ protected against muscle atrophy and inhibited autophagy (T1D + ZnSO₄ vs. T1D, all p < 0.0001), as evidenced by increased grip strength (212.40 ± 11.08 vs. 163.90 ± 10.95 gf), gastrocnemius muscle index (10.67 ± 0.44 vs. 8.80 ± 0.72 mg/g), muscle fibre cross-sectional area (978.20 ± 144.00 vs. 580.20 ± 103.30 μm2), and serum zinc levels (0.2335 ± 0.0227 vs. 0.1561 ± 0.0123 mg/L). ZnSO₄ down-regulated the expression of Atrogin-1 and MuRF1, and decreased the formation of autophagosomes in the gastrocnemius muscle of T1D mice (all p < 0.0001). RNA-seq analysis indicated activation of the SIRT1/FoxO1 signalling pathway in diabetic mice. ZnSO₄ down-regulated LC3B, SIRT1 and FoxO1, while upregulating P62 and GPR39 (all p < 0.05). In vitro, muscle atrophy, autophagy activation, and down-regulation of GPR39 were confirmed in the diabetic cell model (all p < 0.05). Both ZnSO₄ and TC-G-1008 down-regulated Atrogin-1, LC3B, SIRT1, and FoxO1, and up-regulated P62 and GPR39, inhibiting autophagy and improving muscle atrophy (all p < 0.05). The beneficial anti-atrophic effects of ZnSO₄ are diminished following treatment with SRT1720 or RAPA. Upon GPR39 knockout, SIRT1, FoxO1, and Atrogin-1 were upregulated, while P62 was downregulated. Intracellular zinc concentrations in ZnSO₄-treated group remained unchanged (p > 0.05), indicating that zinc supplementation did not affect zinc ion entry but acted through the cell surface receptor GPR39.

CONCLUSION

ZnSO4 inhibits excessive autophagy in skeletal muscle and alleviates muscle atrophy in diabetic mice via the GPR39-SIRT1/FoxO1 axis. These findings suggest that zinc supplementation may offer a potential therapeutic strategy for managing diabetic muscle atrophy.

Unlocking the brain's zinc code: implications for cognitive function and disease

Zn2+ transport across neuronal membranes relies on two classes of transition metal transporters: the ZnT (SLC30) and ZIP (SLC39) families. These proteins function to decrease and increase cytosolic Zn2+ levels, respectively. Dysfunction of ZnT and ZIP transporters can alter intracellular Zn2+ levels resulting in deleterious effects. In neurons, imbalances in Zn2+ levels have been implicated as risk factors in conditions such as Alzheimer's disease and neurodegeneration, highlighting the pivotal role of Zn2+ homeostasis in neuropathologies. In addition, Zn2+ modulates the function of plasma membrane proteins, including ion channels and receptors. Changes in Zn2+ levels, on both sides of the plasma membrane, profoundly impact signaling pathways governing cell development, differentiation, and survival. This review is focused on recent developments of neuronal Zn2+ homeostasis, including the impact of Zn2+ dyshomeostasis in neurological disorders, therapeutic approaches, and the increasingly recognized role of Zn2+ as a neurotransmitter in the brain.

Zinc-sensing receptor activation induces endothelium-dependent hyperpolarization-mediated vasorelaxation of arterioles

BACKGROUND

The micronutrient zinc (Zn2+) is critical for cell function as intracellular signaling and endogenous ligand for Zn2+ sensing receptor (ZnR). Although cytosolic Zn2+ (cyt) signaling in the vascular system was studied previously, role of the ZnR has not been explored in vascular physiology.

METHODS

ZnR-mediated relaxation response of human submucosal arterioles and the mesenteric arterioles from wide-type (WT), ZnR-/- and TRPV4-/- mice were determined by a Mulvany-style wire myograph. The perfused vessel density (PVD) of mouse mesenteric arterioles was also measured in in vivo study. The expression of ZnR in arterioles and vascular endothelial cells (VEC) were examined by immunofluorescence staining, and its function was characterized in VEC by Ca2+ imaging and patch clamp study.

RESULTS

ZnR expression was detected on human submucosal arterioles, murine mesenteric arterioles and VEC but not in ZnR-/- mice. ZnR activation predominately induced endothelium-dependent hyperpolarization (EDH)-mediated vasorelaxation of arterioles in vitro and in vivo via Ca2+ signaling, which is totally different from endothelium-dependent vasorelaxation via Zn2+ (cyt) signaling reported previously. Furthermore, ZnR-induced vasorelaxation via EDH was significantly impaired in ZnR-/- and TRPV4-/- mice. Mechanistically, ZnR induced endothelium-dependent vasorelaxation predominately via PLC/IP3/IP3R and TRPV4/SOCE. The role of ZnR in regulating Ca2+ signaling and ion channels on VEC was verified by Ca2+ imaging and patch clamp techniques.

CONCLUSION

ZnR activation induces endothelium-dependent vasorelaxation of resistance vessels predominately via TRPV4/Ca2+/EDH pathway. We therefore not only provide new insights into physiological role of ZnR in vascular system but also may pave a potential pathway for developing Zn2+-based treatments for vascular disease.