Mechanisms of adhesion and growth control in stem cells

Kapur, Reuben

Abstract

In adults, bone marrow (BM) is the major site for the continuous production of mature blood cells. Mature blood cells arise from hematopoieic stem cells (HSC) within the bone marrow cavity. Although, the regulation of HSCs is an extremely popular area of investigation; how this regulation occurs still remains an enigma. It is thought that part of this process involves self-renewal, differentiation and apoptosis or senescence. Although the precise mechanism(s) by which these fates are determined is still unclear, several molecules have been implicated in these processes, including components of the BM microenvironment. The BM microenvironment consists of extracellular matrix proteins as well as cells such as osteoblasts and stromal cells which contribute to the stem/progenitor cell fate by anchoring these cells in the BM cavity and delivering them adhesive signals as well as signals in the form of soluble ligands, including cytokines and chemokines. These signals induce intracellular activation of both the positive regulators of stem cell growth, self-renewal and differentiation as well as negative signals. Some of these pathways, such as those initiated by stem cell factor (SCF), the ligand for KIT, stromal cell derived factor-1 (SDF-1), and thrombopoietin (TPO), induce the activation of phosphatidylinositol 3? kinase (PI3K) and the formation of phosphatidylinositol 3,4,5-trisphosphate (PIP3). How PIP3 levels are regulated in stem/progenitor cells is poorly understood. This is important, as disregulated PIP3 levels in stem/progenitor cells have been shown to result in myeloproliferative disease (MPD) as well as acute myeloid leukemia (AML). Our long-range goal is to understand the signaling mechanisms that control the growth and survival signals in hematopoietic stem and progenitor cells (SC/Ps), including in myeloid lineage derived macrophages and neutrophils. The objective of this application is to determine how components of class IA PI3Kinase contribute to PIP3 induced SC/P cell growth under steady state as well as under stress induced hematopoiesis. Additional objective are to determine the extent to which PIP3 generated by class IA PI3K in SC/Ps is negatively regulated by phosphatases such as SHIP and PTEN, and what are some of the signaling mechanisms by which PTEN?s activity is regulated in these cells. The central hypothesis of this application, which has been formulated on the basis of our preliminary data, is that PIP3 generated via class IA PI3K contributes to both steady state as well as stress induced hematopoiesis, PIP3 generated by class IA PI3K contributes significantly to the MPD and AML phenotypes associated with SHIP and PTEN deficiency and Rho associated kinase 1 (ROCK1) positively regulates growth and survival of SC/Ps in part by regulating the activation of PTEN. Our proposed studies will provide unique insights into the physiologic significance of the in vivo interactions between class IA PI3K, PTEN and ROCK1 in regulating growth and survival in SC/Ps.