Contributions of de novo lipid synthesis to hematopoietic stem cell function - PROJECT SUMMARY/ABSTRACT Hematopoietic stem cells (HSCs) can self-renew and generate all mature blood cells, allowing them to be used for transplantation to treat hematological disorders. However, as many as 5-27% of patients experience poor graft function, or incomplete hematopoietic recovery after transplant, which has only 6% 2-year overall survival. This can be a direct result of defective HSC self-renewal and multilineage differentiation potential. However, the specific pathways that regulate HSC function remain poorly understood. HSCs are mostly quiescent with low metabolic and mitochondrial activity. They are activated into cell cycle and their mitochondrial activity increases following transplant or 5-fluorouracil (5-FU)-induced myeloablation. Prior work in our lab has shown that after replicative stress, HSCs have decreased regenerative ability, dysfunctional mitochondrial dynamics, and decreased expression of metabolic genes such as stearoyl-Coa desaturase 1 (Scd1). SCD1 is an enzyme that catalyzes the formation of monounsaturated fatty acids (MUFAs) in the de novo lipid synthesis pathway. These products are used to form triglycerides and phospholipids that alter cell membrane structure, signaling, and mitochondrial homeostasis. Specifically, lipids are necessary for proper mitochondrial membrane architecture, dynamics, and oxidative metabolism. Our preliminary data demonstrate that pharmacological inhibition of SCD1 in HSCs in vitro significantly decreases their regenerative ability. SCD1 inhibition also leads to changes in mitochondrial network structure. However, the contribution of MUFA synthesis to HSC function in vivo and why HSCs depend on this pathway is unknown. Our central hypothesis is that MUFA synthesis is important for maintaining HSC regenerative function by supporting mitochondrial function, and loss of this pathway after replicative stress causes HSC functional decline. We will address this hypothesis by determining the role of MUFA synthesis in HSC function in vivo (Aim 1) using an inducible SCD1 knockout mouse model. We will then assess HSC regenerative function by serial competitive transplantation and recovery after 5-FU-induced myeloablation. We will also determine the mechanism by which MUFA synthesis maintains HSC function (Aim 2) by assessing mitochondrial function and performing lipidomic and metabolomic analysis in SCD1-deficient conditions. Using oleic acid, a product of the SCD1-catalyzed reaction, to restore MUFAs, we will determine whether this improves HSC regenerative and mitochondrial function after replicative stress. These studies will lead to a global understanding of the importance of MUFA synthesis to HSC function and reveal pathways that can be targeted to improve HSC regenerative functions and clinical outcomes of HSC-based therapies.
