Taurine Transporter SLC6A6 Expression Promotes Mesenchymal Stromal Cell Function

Mesenchymal stromal cell (MSC) differentiation is critical for the development, maintenance, and repair of bone tissue. This occurs within the bone marrow microenvironment, which consists of various stromal populations and structural components that support skeletal growth, repair and homeostasis. MSCs also promote self-renewal of hematopoietic stem cells and regulate their differentiation. Analysis of our own and publicly available single-cell RNA sequencing datasets of the murine non-immune bone and bone marrow indicates that Slc6a6 expression is enriched in MSCs. Slc6a6 encodes for an ion-dependent transporter of taurine (TauT). Although taurine supplements have been shown to mitigate the onset of bone defects in aged populations, the absence of TauT expression in osteolineage cells suggests that taurine's effect on bone may be secondary to its function in other populations, such as MSCs. Using young TauT genetic loss-of-function murine models we find that TauT loss impacts MSC populations in vivo and impairs MSC osteogenic differentiation in vitro. This correlates with decreased bone mineral density and bone strength in young TauT knockout mice. Importantly, shRNA-based knockdown of SLC6A6 expression in primary human MSCs reduces osteogenic differentiation, indicating a key role of taurine uptake in human MSC function. Consistent with a decline in MSC function with TauT loss, we find that TauT null MSCs are unable to support self-renewal and expansion of co-cultured hematopoietic stem/progenitor populations. Mechanistically, our RNA-sequencing analysis identifies downregulation of Wnt/beta-catenin signaling in MSCs in the absence of TauT. These cells also show reduced oxidative phosphorylation, and increased ROS levels, indicating that impaired Wnt signaling and elevated oxidative stress may contribute to the observed defects in osteogenic differentiation capacity. Collectively, our data identify taurine uptake as a key regulator of mesenchymal stromal cell maintenance and osteogenic fate determination.