Hyponatremia elicits gene expression changes driving osteoclast differentiation and functions

Area of Science:

• Bone biology
• Endocrinology
• Cellular and Molecular Medicine

Background:

• Chronic hyponatremia is linked to increased bone loss, osteoporosis, and fractures, particularly in aging individuals.
• Prior research in a rat model and osteoclast cells suggested hyponatremia causes osteoporosis by enhancing osteoclastic bone resorption.
• Evidence pointed to a direct stimulatory effect of low extracellular sodium ion concentration ([Na+]) on osteoclasts, rather than low osmolality.

Purpose of the Study:

• To investigate the molecular mechanisms underlying low [Na+] -induced changes in osteoclast gene expression.
• To identify specific signaling pathways and cellular processes affected by low extracellular sodium.
• To provide a basis for developing therapeutic strategies for bone fragility in patients with chronic hyponatremia.

Main Methods:

• RNA sequencing and gene ontology analysis of osteoclasts exposed to low [Na+].
• Confirmation of gene expression findings using mouse whole genome microarray and quantitative RT-PCR.
• Functional assays to assess the independent effects of RANKL and low [Na+] on osteoclast activity.

Main Results:

• Low [Na+] significantly altered gene expression in osteoclasts, supporting their growth and differentiation.
• Identified activation of the receptor activator of nuclear factor kappa-B ligand (RANKL) and PI3K/Akt pathways.
• Revealed low [Na+] upregulates lysosomal genes, mitochondrial energy production, MMP-9 expression, and enhances osteoclast motility.

Conclusions:

• Low extracellular sodium concentration directly stimulates osteoclast growth, differentiation, and function through specific transcriptomic changes.
• RANKL and low [Na+] independently contribute to enhanced osteoclast activity, exacerbating bone resorption.
• Understanding these molecular pathways is crucial for developing treatments for hyponatremia-induced osteoporosis and bone fragility.