Molecular Mechanisms of Hematopoietic Stem Cell Development- The Role of Retinoic Acid Signaling Bhaskar Chanda For the Doctor of Philosophy Medical Biophysics University of Toronto 2013 Abstract During mouse embryonic development, the formation of blood or hematopoiesis occurs in multiple phases. The first phase or primitive hematopoiesis generates a restricted subset of blood cell lineages but is devoid of lymphoid and hematopoietic stem cell (HSC) potential. The next phase of hematopoiesis, also known as definitive hematopoiesis, is characterized by its ability to generate multilineage hematopoietic progenitors and HSCs from a specialized population of endothelial cells known as hemogenic endothelium (HE). Such endothelial to hematopoietic transitions (EHT) have been recently observed at a clonal level, however, molecular mechanisms that underlie EHT leading to the specification of HSCs have remained poorly understood. Here we show that retinoic acid (RA) signaling plays a pivotal role in embryonic hematopoiesis and HSC development. RA signaling inhibits primitive hematopoiesis, and promotes definitive hematopoiesis. This inductive effect of RA signaling extends to the specification of HSCs. Activation of the RA signaling pathway ex vivo in AGM-derived HE dramatically enhanced the repopulating potential, whereas its conditional inhibition in vivo abrogated HSC development. These repressive and inductive effects of RA signaling were mediated primarily via retinoic acid receptor (RAR)- α. We further analyzed the mechanistic basis of RA signaling with a combined use of cellular, molecular and biochemical assays, and show that β-catenin dependent Wnt signaling is the downstream mediator of RA signaling. Collectively, this thesis provides new insight into molecular mechanisms that control embryonic hematopoiesis and identify the RA pathway as a key regulator of definitive hematopoiesis and HSC specification.

Molecular Mechanisms of Hematopoietic Stem Cell Development- The Role of Retinoic Acid Signaling Bhaskar Chanda For the Doctor of Philosophy Medical Biophysics University of Toronto 2013 Abstract During mouse embryonic development, the formation of blood or hematopoiesis occurs in multiple phases. The first phase or primitive hematopoiesis generates a restricted subset of blood cell lineages but is devoid of lymphoid and hematopoietic stem cell (HSC) potential. The next phase of hematopoiesis, also known as definitive hematopoiesis, is characterized by its ability to generate multilineage hematopoietic progenitors and HSCs from a specialized population of endothelial cells known as hemogenic endothelium (HE). Such endothelial to hematopoietic transitions (EHT) have been recently observed at a clonal level, however, molecular mechanisms that underlie EHT leading to the specification of HSCs have remained poorly understood. Here we show that retinoic acid (RA) signaling plays a pivotal role in embryonic hematopoiesis and HSC development. RA signaling inhibits primitive hematopoiesis, and promotes definitive hematopoiesis. This inductive effect of RA signaling extends to the specification of HSCs. Activation of the RA signaling pathway ex vivo in AGM-derived HE dramatically enhanced the repopulating potential, whereas its conditional inhibition in vivo abrogated HSC development. These repressive and inductive effects of RA signaling were mediated primarily via retinoic acid receptor (RAR)- α. We further analyzed the mechanistic basis of RA signaling with a combined use of cellular, molecular and biochemical assays, and show that β-catenin dependent Wnt signaling is the downstream mediator of RA signaling. Collectively, this thesis provides new insight into molecular mechanisms that control embryonic hematopoiesis and identify the RA pathway as a key regulator of definitive hematopoiesis and HSC specification.