Campfield & Byersdorfer

Hematopoiesis occurs when a limited number of specialized hematopoietic stem cells (HSCs) give rise to the diverse panoply of mature blood and immune cells of the body. This complex process is tightly regulated, and, when dysfunctional, hematopoietic disorders manifest early in childhood as bone marrow (BM) failure or later in childhood/adolescence as combined variable immunodeficiency (CVID). Characterization of underlying genetic defects has been essential in understanding immune homeostasis/development and genetic regulation of HSCs, and HSC self-renewal has long been a focus of significant investigation and scientific discovery. However, while the HSC field has developed pipelines for novel gene discovery, the mechanistic function of many HSC genes during homeostasis, engraftment, and immune reconstitution is not well understood.

Recently, the protein Follistatin-like 1 (FSTL-1) was identified as being highly expressed in lineage negative (Lin-)Sca1+cKit+ (LSK) HSCs from murine BM. FSTL-1 influences a myriad of biological functions including embryogenesis, inflammation and tissue repair, cardiac regeneration via cell-cycle regulation, enhancement of mesenchymal stem cell regenerative potential, and facilitation of stemness in breast cancer cells. In BM cells, shRNA-mediated suppression of Fstl1 decreased HSC engraftment through an unknown mechanism. In further support of FSTL-1’s role in hematopoiesis, others identified that the epigenetic gene regulator Znhit1 was critical for LSK and long-term (LT)-HSC self-renewal and that Znhit1-/- LSK cells decreased Fstl1 expression. Furthermore, Znhit1-/- stem cells had reduced expression of another regulator of HSC self- renewal, nuclear receptor subfamily 4 group A member 1 (Nr4a1). Coincidentally, we recently described a critical role for FSTL-1 in lung cell homeostasis, whereby reduced Fstl1 expression in murine lung decreased Nr4a1 expression in lung immune cells, identifying a novel FSTL-1-mediated regulation of Nr4a1 function in vivo. These combined observations suggest that FSTL-1 represents an essential component of HSC homeostasis likely via a novel Znhit-FSTL-1-Nr4a1 signaling pathway. Based on this prediction, we set out to better understand the role of FSTL-1 in HSC biology and function.

In concert with results from the published literature and our previous work, these findings strongly support the underlying hypothesis that Fstl1 is critical for post-transplant cellular engraftment of multiple lineages through its regulation of LT-HSC self-renewal. The proposed studies will help to define the impact of FSTL-1 deficiency on LT-HSC recovery, self-renewal capacity and impact of FSTL-1 specifically in LT-HSCs on transplant outcomes.