Impaired MSC Maturation May Lead To Fragile Bone Formation

Mitochondrial dysfunction may impair MSC maturation and inhibit cell proliferation, contributing to fragile bone formation in SRS patients.

Patients with Snyder-Robinson Syndrome (SRS), an X-linked recessive condition, exhibit deficient Spermidine Synthase (SMS) gene expression as well as neurodevelopmental defects and osteoporosis, often leading to extremely fragile bones. Although several studies have explored the possible mechanisms by which the disease causes impaired bone formation, the actual cause is yet unknown. One study proposes a correlation between impaired bone formation and dysfunctional multipotent stromal cells or mesenchymal stem cells (MSCs), cells that normally differentiate into mesenchymal lineage cells, such as osteocytes, adipocytes, and chondrocytes. Therefore, the failure of these MSCs to differentiate into osteocytes may explain the defects in proper bone formation. However, a correlation between deficient SMS gene expression and MSC dysfunction in SRS patients has yet to be investigated.

Bone fragility, the rarity of the disease, and donor-to-donor variation of SMS expression make working with MSCs from SRS patients problematic. Therefore, to determine the underlying mechanism for impaired bone formation, researchers at the University of California, Davis, modeled the disease by silencing SMS in human bone marrow MSCs derived from healthy male donors and performed a comparative analysis of the transcriptome and metabolome between normal and deficient SMS expressing cells.

To silence the translation of the SMS transcript in MSCs, the researchers transduced the cells with a lentiviral vector constructed to express either a short hairpin RNA (shRNA) that is complementary to the SMS transcript or an shRNA (shControl) that is not complementary to any transcript in the human genome. Expression of the shRNA down-regulated the SMS mRNA and protein levels in MSCs and changed the morphology of the cells, displaying a smaller, less adherent cell type with a reduced proliferative rate as compared to the control.

Gene expression analysis was performed on specific osteogenic markers to understand the role SMS plays in MSC development. Although the expression of markers involved in initial MSC differentiation in the osteocyte lineage did not change, the expression of specific markers involved in the final steps of MSC maturation decreased substantially in the SMS silenced MSCs, implying that SMS silenced MSCs have impaired maturation during osteogenesis. In addition, gene expression analysis of the transcriptome was used to determine which genes, other than osteogenic markers, are affected by SMS silencing. The analysis identified the differential expression of 1084 genes, with many relating to cell cycle progression and metabolism of polyamines and glucose.

To explore the metabolism of the cells more thoroughly, the researchers ran a comparative analysis of the metabolome of SMS silenced MSCs to that of control MSCs. Forty-five metabolites were identified that had a change in levels as compared to the control. These metabolites were found to be associated with lipids, mitochondrial lipids, and glucose metabolism. The alterations in glucose metabolism implied a possible defect in mitochondrial function. Further analysis into mitochondrial function revealed a reduced glucose consumption, but increased lactate secretion, indicating an impaired citric acid cycle.

The researchers show that mitochondrial dysfunction due to impaired gene expression of SMS may inhibit cell proliferation and impair the maturation of MSCs in SRS patients. However, they suggest that mitochondrial dysfunction in MSCs is not the underlying factor contributing to fragile bone formation and that further investigation as to other factors that may contribute to these observations remains a key challenge for understanding osteogenic defects in SRS patients.

The authors isolated human bone marrow MSCs from whole bone marrow procured by StemExpress.
Ramsay AL et al. (2019) Modeling Snyder-Robinson Syndrome in Multipotent Stromal Cells Reveals Impaired Mitochondrial Function as a Potential Cause for Deficient Osteogenesis. Sci Rep 9, 15395. Online publication –