Proposal Summary/Abstract
Adult hematopoietic stem cells (HSCs) are a rare and unique population of stem cells that reside in the
bone marrow, where they undergo self-renewal and differentiation to maintain the blood system. To properly
maintain the balance between self-renewal and differentiation, HSCs receive signals from growth factors and
chemokines to activate the evolutionarily conserved phosphoinositide 3-kinase/Protein Kinase B (PI3K/AKT)
signaling pathway. Pathologic activation of this pathway is frequently observed in cancers, including leukemia,
making it a desirable target for cancer treatment. Several PI3K inhibitors are already used in the clinic and to
better inform therapeutic targeting, it is crucial to understand the roles of PI3K in adult HSCs.
Hematopoietic cells express three Class IA catalytic isoforms of PI3K (p110a, ß, d), all of which can
transduce growth factor and cytokines signals. Out of these isoforms, p110ß is unique, since in addition to
transducing growth factor signals through receptor tyrosine kinases (RTK), it can directly interact with G-protein
coupled receptor Gß¿ subunits to transduce chemokines, and also binds to RAC and to RAB5 GTPases. In
mouse embryonic fibroblasts, the p110ß-RAB5 interaction was shown to be important for the induction of
autophagic cellular recycling process, which is essential for the maintenance of HSC metabolism and self-
renewal. Individual Class 1A PI3K isoforms have unique functions in mature hematopoietic lineages, but they
are dispensable for HSCs function. To study the redundant roles of Class 1A PI3K in HSCs, we have
generated a triple knockout (TKO) mouse model with conditional deletion of p110a and p110ß in hematopoietic
cells, and germline deletion of p110d. Analysis of these TKO mice reveals upon the loss of all three Class1A
isoforms causes an increase in HSCs numbers, but decreased self-renewal and differentiation, with inefficient
repopulation of all mature blood lineages. This phenotype is different from the phenotypes of any PI3K single
isoform knockout mouse model, and even from p110a;d double knockout animals, suggesting that p110ß
isoform plays an important compensatory role in HSCs. Moreover, my data suggests that loss of Class I PI3K
causes a decrease in autophagy induction upon growth factor deprivation, though autophagy can still be
induced with the mTOR inhibitor rapamycin. Thus, I hypothesize that loss of Class IA PI3K compromises
autophagy induction, which causes altered HSC metabolism and impaired HSCs fitness.
The proposed studies will use our PI3K TKO mouse model to elucidate in Aim 1 the cellular mechanism
for defective self-renewal in HSCs. Aim 2 will establish the roles of autophagy in TKO HSC dysfunction. Lastly,
Aim 3 will determine which binding interactions of p110ß are the most important for its compensatory role in
HSC function. In summary, this research will delineate the cellular and molecular mechanisms by which Class
1A PI3K supports HSC self-renewal and differentiation.