Regulation of human mesenchymal stem cell functions by an autocrine loop involving NAD+ release and P2Y11-mediated signaling.

In several cell types, a regulated efflux of NAD(+) across Connexin 43 hemichannels (Cx43 HC) can occur, and extracellular NAD(+) (NAD(+)(e)) affects cell-specific functions. We studied the capability of bone marrow-derived human mesenchymal stem cells (MSC) to release intracellular NAD(+) through Cx43 HC. NAD(+) efflux, quantified by a sensitive enzymatic cycling assay, was significantly upregulated by low extracellular Ca(2+) (5-6-fold), by shear stress (13-fold), and by inflammatory conditions (3.1- and 2.5-fold in cells incubated with lipopolysaccharide (LPS) or at 39°C, respectively), as compared with untreated cells, whereas it was downregulated in Cx43-siRNA-transfected MSC (by 53\%) and by cell-to-cell contact (by 45\%). Further, we show that NAD(+)(e) activates the purinergic receptor P2Y(11) and a cyclic adenosin monophosphate (cAMP)/cyclic ADP-ribose/[Ca(2+)](i) signaling cascade, involving the opening, unique to MSC, of L-type Ca(2+) channels. Extracellular NAD(+) enhanced nuclear translocation of cAMP/Ca(2+)-dependent transcription factors. Moreover, NAD(+), either extracellularly added or autocrinally released, resulted in stimulation of MSC functions, including proliferation, migration, release of prostaglandin E(2) and cytokines, and downregulation of T lymphocyte proliferation compared with controls. No detectable modifications of MSC markers and of adipocyte or osteocyte differentiation were induced by NAD(+)(e). Controls included Cx43-siRNA transfected and/or NAD(+)-glycohydrolase-treated MSC (autocrine effects), and NAD(+)-untreated or P2Y(11)-siRNA-transfected MSC (exogenous NAD(+)). These findings suggest a potential beneficial role of NAD(+)(e) in modulating MSC functions relevant to MSC-based cell therapies.