Role of SIRT3 in Microgravity Response: A New Player in Muscle Tissue Recovery

Simulated microgravity activates autophagy expression in the rat retina

OBJECTIVE

This study aims to investigate the expression and possible role of autophagy in the retina of rats under microgravity.

METHODS

Adult Sprague-Dawley (SD) rats were randomly allocated to either the tail suspension group (TS) or the control group (CTRL). To simulate microgravity-induced redistribution of cephalad fluid observed in space, the rats in the TS group underwent tail suspension for a duration of 4 weeks. Optical coherence tomography angiography (OCTA) was applied to assess the ocular blood flow and thickness of the retina. Hematoxylin and eosin (H&E) staining, along with transmission electron microscopy (TEM), were used to investigate morphological changes and autophagosomes in the retina. Endoplasmic reticulum autophagy (ER-phagy) related proteins (ATF4, CHOP, and GRP78) in the rat retina were detected using an immunofluorescence assay (IFA). The levels of autophagy-related proteins (Beclin1, P62, LC3B, ATF4, CHOP, and GRP78) were quantified by Western blot (WB). The expression of ATG5 and ATG7 genes was examined via real-time quantitative PCR (qPCR).

RESULTS

In fundus imaging signs, microgravity increases retinal thickness and the retinal vascular perfusion area. Moreover, microgravity also upregulates Beclin1, LC3B, ATF4, CHOP, and GRP78 while downregulating P62 in retina. It elevates the number of autophagosomes and activates autophagy and ER-phagy signaling pathways in retina.

CONCLUSION

Simulated microgravity can trigger the organism's intrinsic protective mechanisms, inducing the activation of autophagy (ER-phagy) in the retina, which may represent a self-defense mechanism against adverse conditions of microgravity-related stressors.

Hypocalcemia in combination with hyperphosphatemia impairs muscle cell differentiation in vitro

PURPOSE

Hypoparathyroidism is a rare endocrine disorder characterized by low or absent secretion of parathyroid hormone (PTH), which leads to decreased calcium and increased phosphorus levels in the serum. The diagnosis of hypoparathyroidism is based on the identification of the aforementioned biochemical abnormalities, which may be accompanied by clinical manifestations. Symptoms of hypoparathyroidism, primarily attributed to hypocalcemia, include muscle cramps or spasms, facial, leg, and foot pain, seizures, and tingling in the lips or fingers. The treatment of hypoparathyroidism depends on the severity of symptoms and the underlying pathology. Over the long term, calcium supplements, active vitamin D analogs, and thiazide diuretics may be needed. In fact, in patient cohorts in which optimal disease control still remains elusive, replacement therapy with recombinant parathyroid hormone analogs may be contemplated. Despite the predominantly neuromuscular symptoms of hypoparathyroidism, further effects of parathyroid hormone deficiency at the muscle cell level remain poorly understood. Thus, the aim of our study was to evaluate the effects of hypocalcemia in combination with hyperphosphatemia on muscle cells differentiation in vitro.

METHODS

C2C12 cells, an in vitro model of muscle cells, were differentiated for 2 or 6 days in the presence of hypocalcemia (CaCl2 0.9 mmol/l) and moderate (PO4 1.4 mmol/l) or severe (PO4 2.9 mmol/l) hyperphosphatemia, or combinations of both conditions. Cell differentiation and expression of genes linked to muscle differentiation were evaluated.

RESULTS

The combination of hypocalcemia with hyperphosphatemia induced a significant reduction (50%) in differentiation marker levels, such as MyoD (protein 1 for myoblast determination) and myogenin on the 1st day of differentiation, and MHC (myosin heavy chains) after 6 days of differentiation compared to control. Furthermore, this condition induced a statistically significant reduction of insulin-like growth factor-1 (IGF-1) mRNA expression and inhibition of IGF signaling and decrease in ERK phosphorylation compared to control cells.

CONCLUSIONS

Our results showed that a condition of hypocalcemia with hyperphosphatemia induced an alteration of muscle cell differentiation in vitro. In particular, we observed the reduction of myogenic differentiation markers, IGF-1 signaling pathway, and ERK phosphorylation in differentiated skeletal myoblasts. These data suggest that this altered extracellular condition might contribute to the mechanisms causing persistence of symptoms in patients affected by hypoparathyroidism.

Role of SIRT3 in bone homeostasis and its application in preventing and treating bone diseases

Bone homeostasis refers to the balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption and the maintenance of stable bone mass. SIRT3 is a class of mitochondrial protein deacetylase that influences various mitochondrial functions and is involved in the mechanisms underlying resistance to aging; regulation of bone marrow mesenchymal stem cells, osteoblasts, and osteoclasts; and development of osteoporosis, osteoarthritis, and other bone diseases. Moreover, exercise affects bones through SIRT3. Thus, studies on SIRT3 may provide insights for the treatment of bone diseases. Although SIRT3 can exert multiple effects on bone, the specific mechanism by which it regulates bone homeostasis remains unclear. By evaluating the relevant literature, this review discusses the structure and function of SIRT3, reveals the role and associated mechanisms of SIRT3 in regulating bone homeostasis and mediating bone health during exercise, and highlights the potential pharmacological value of SIRT3 in treating bone diseases.