Hematopoietic stem cells in the bone marrow sustain the production of all blood cell lineages through differentiation into progenitors with progressively more restricted lineage potential. Hematopoietic stem cells are remarkably quiescent, are multipotent and have the capacity of self renew. They are the cells that engraft after bone marrow transplantation. Over time they can be become a repository for mutations that give rise to leukemias. Despite decades of research, a coherent understanding of the mechanisms involved in the regulation of their quiescence, self-renewal and differentiation is still lacking however. Furthermore,
Despite significant progress in our understanding of mechanisms involved in their self-renewal, differentiation and quiescence, renewal and even maintenance of HSCs in vitro remains challenging, however. Strategies to maintain or expand HSCs in vitro would have enormous implications for the current practice of allogeneic and autologous hematopoietic stem cell transplantation, as well as gene therapy and genome editing targeting HSCs. Furthermore, although major progress was made in the derivation of cells derived from primitive and definitive hematopoietic programs from pluripotent stem cells (PSCs), it has thus far not been possible to reliably generate repopulating HSCs from PSCs. One reason may be that any HSCs generated cannot be maintained in vitro using current approaches, and would therefore go undetected. Devising strategies to maintain or expand HSCs in vitro is therefore essential, and this requires deeper insight to their specific biology to understand the barriers to their in vitro maintenance. This the goal of our studies on HSCs.
Among inbred mouse strains there is extensive genetically determined variation in the function and kinetics of hematopoietic stem and progenitor cells. These traits vary continuously across genetically different individuals as they are determined by the contribution of multiple loci, called quantitative trait loci. We found, through quantitative trait mapping, that allelic variation in the Prdm16 gene plays a role in genetic variation in the hematopoietic system of inbred mouse strains. Using knockout approaches, we discovered that deletion of Prdm16 potently affected HSC maintenance.
We then discovered that one downstream effect of Prdm16 is regulation of mitochondrial fusion and that Prdm16 induces Mitofusin 2 (Mfn2). Deletion of Mfn2 leads to mitochondrial fragmentation and impaired maintenance of subset of HSCs with extensive lymphoid potential. The underlying mechanism is based on regulation of intracellular calcium and NFAT. Further studies focus on the predominantly ATP-independent roles of mitochondria in HSC function and calcium regulation. We discovered that, despite their reliance on glycolysis, HSCs display elevated mitochondrial mass and reduced mitophagy. Furthermore, HSCs can be maintained in a low-calcium environment. This is the basis of further mechanistic studies in vivo and in vitro into the mechanisms underlying HSCs maintenance and function by environmental calcium.
Representative publications:
Avagyan S, Aguilo F, Kamezaki K, Snoeck HW (2011). Quantitative trait mapping reveals a regulatory axis involving peroxisome proliferator-activated receptors, PRDM16, transforming growth factor-b2 and FLT3 in hematopoiesis. Blood, 118:6078-6086.
Aguilo F, Avagyan S, Labar A, Sevilla A, Lee DF, Kumar P, Lemischka IR, Zhou BY, Snoeck HW. (2011). Prdm16 is a physiological regulator of hematopoietic stem cells. Blood, 117:267-272.
Luchsinger LL, Justino de Almeida M, Corrigan DJ, Mumau M, Snoeck HW. Mitofusin 2 maintains hematopoietic stem cells with extensive lymphoid potential. Nature, 529(7587):528-531.
Justino M, Luchsinger LL, Williams L, Snoeck HW (2017). Elevated mitochondrial mass in hematopoietic stem cells. Cell Stem Cell 21:725-729.
Snoeck HW (2017). Mitochondrial regulation of hematopoietic stem cells. Curr Opin Cell Biol, 49:91-99.
Corrigan DJ, Luchsinger LL, Williams LJ, de Almeida MJ, Strikoudis A, Snoeck HW (2018). PRDM16 isoforms differentially regulate normal and leukemic hematopoiesis and inflammatory gene signature. J. Clin Invest., 128:3250-3264.
Luchsinger LL, Strikoudis A, Danzl NM, Bush EC, Finlayson MO, Satwani P, Sykes M, Snoeck, HW. (2019). Harnessing hematopoietic stem cell low intracellular calcium improves their maintenance in vitro. Cell Stem Cell, 25:225-240.
Snoeck, HW. Calcium regulation of stem cells. EMBO Rep e50028, 2020