BMP signaling is required for postnatal murine hematopoietic stem cell self-renewal.

Life-long production of blood from hematopoietic stem cells (HSCs) is a process of strict modulation. Intrinsic and extrinsic signals govern fate options like self-renewal - a cardinal feature of HSCs. Bone morphogenetic proteins (BMP) have an established role in embryonic hematopoiesis, but less is known about its functions in adulthood. Previously, SMAD-mediated BMP signaling has been proven dispensable for HSCs. However, the BMP Type II receptor (BMPR-II) is highly expressed in HSCs, leaving the possibility that BMPs function via alternative pathways. Here, we establish that BMP signaling is required for self-renewal of adult HSCs. Through conditional knockout we show that BMPR-II deficient HSCs have impaired self-renewal and regenerative capacity. BMPR-II deficient cells have reduced p38 activation, implying that non-SMAD pathways operate downstream of BMPs in HSCs. Indeed, a majority of primitive hematopoietic cells do not engage in SMAD-mediated responses downstream of BMPs in vivo. Furthermore, deficiency of BMPR-II results in increased expression of TJP1, a known regulator of self-renewal in other stem cells, and knockdown of TJP1 in primitive hematopoietic cells partly rescues the BMPR-II null phenotype. This suggests TJP1 may be a universal stem cell regulator. In conclusion, BMP signaling, in part mediated through TJP1, is required endogenously by adult HSCs to maintain self-renewal capacity and proper resilience of the hematopoietic system during regeneration.

The stem cell regulator PEDF is dispensable for maintenance and function of hematopoietic stem cells.

Pigment epithelium derived factor (PEDF), a ubiquitously expressed 50 kDa secreted glycoprotein, was recently discovered to regulate self-renewal of neural stem cells and have a supportive effect on human embryonic stem cell growth. Here, we analyzed expression of PEDF in the murine hematopoietic stem cell (HSC) compartments and found that PEDF is highly expressed in primary long-term HSCs. Therefore, we characterized the hematopoietic system in a knockout mouse model for PEDF and using this model we surprisingly found that PEDF is dispensable for HSC regulation. PEDF knockout mice exhibit normal hematopoiesis in steady state conditions and the absence of PEDF lead to normal regeneration capacity in a serial competitive transplantation setting. Additionally, PEDF-deficient cells exhibit unaltered lineage distribution upon serial transplantations. When human cord blood stem and progenitor cells were cultured in media supplemented with recombinant PEDF they did not show changes in growth potential. Taken together, we report that PEDF is not a critical regulatory factor for HSC function during regeneration in vivo or growth of human stem/progenitor cells in vitro.

Signaling via Smad2 and Smad3 is dispensable for adult murine hematopoietic stem cell function in vivo.

Transforming growth factor-ß (TGFß) is a member of a large family of polypeptide growth factors. TGFß signals mainly through the intracellular proteins Smad2 and Smad3, which are highly similar in amino acid sequence identity. A number of studies have shown that these proteins, dependent on context, have distinct roles in the TGFß signaling pathway. TGFß is one of the most potent inhibitors of hematopoietic stem and progenitor cell proliferation in vitro, but its role in hematopoiesis in vivo is still being determined. To circumvent possible redundancies at the receptor level and to address specifically the role of the Smad circuitry downstream of TGFß and activin in hematopoiesis, we studied the effect of genetically deleting both Smad2 and Smad3 in adult murine hematopoietic cells. Indeed, TGFß signaling is impaired in vitro in primitive bone marrow (BM) cells of Smad2 and Smad3 single knockout models. However, blood parameters appear normal under steady state and in the transplantation setting. Interestingly, upon deletion of both Smad2 and Smad3 in vivo, mice quickly develop a lethal inflammatory disease, suggesting that activin/TGFß signaling is crucial for immune cell homeostasis in the adult context. Furthermore, concurrent deletion of Smad2 and Smad3 in BM cells in immune-deficient nude mice did not result in any significant alterations of the hematopoietic system. Our findings suggest that Smad2 and Smad3 function to mediate crucial aspects of the immunoregulatory properties of TGFß, but are dispensable for any effect that TGFß has on primitive hematopoietic cells in vivo.