3146 – INVESTIGATING THE UBIQUITIN E3 LIGASE, FBXO21, IN REGULATION OF HEMATOPOIETIC STEM CELLS

Abstract

The lifetime process of hematopoiesis is highly regulated through molecular pathways directing hematopoietic cell fate decisions such as self-renewal and differentiation in the hematopoietic stem and progenitor cell (HSPC) population. The ubiquitin proteasome system (UPS) regulates many cellular processes through controlling protein levels by tagging them with polyubiquitin chains and promoting their degradation through the proteasome. The substrate recognition component of the UPS is the ubiquitin E3 ligase. Through investigating a specific family of ubiquitin E3 ligases, the Fbox family of proteins, which contains about 70 E3 ligases, we discovered that Fbxo21 was highly expressed in the HSPC population. Fbxo21 was more highly expressed in the HSPC population when compared to other Fbox genes. In addition, Fbxo21 expression was primarily in immature hematopoietic populations, whereas, more mature hematopoietic populations had limited expression. Western blot and qRT-PCR analysis confirmed high expression of FBXO21 in HSPC, and revealed low to no expression in mature myeloid populations. To determine the role of FBXO21 on HSPC maintenance, self-renewal, and differentiation, we generated shRNAs against Fbxo21. We found the silencing of Fbxo21 in HSPC leads to a loss of colonies and differentiation towards the mature myeloid lineage. These findings lead to the hypothesis that the ubiquitin E3 ligase FBXO21, and its substrates regulate hematopoietic stem cell maintenance and loss promotes myeloid differentiation. Deciphering the role of FBXO21 could expand the current known molecular mechanisms that regulate hematopoietic lineage specification and stem cell maintenance. The lifetime process of hematopoiesis is highly regulated through molecular pathways directing hematopoietic cell fate decisions such as self-renewal and differentiation in the hematopoietic stem and progenitor cell (HSPC) population. The ubiquitin proteasome system (UPS) regulates many cellular processes through controlling protein levels by tagging them with polyubiquitin chains and promoting their degradation through the proteasome. The substrate recognition component of the UPS is the ubiquitin E3 ligase. Through investigating a specific family of ubiquitin E3 ligases, the Fbox family of proteins, which contains about 70 E3 ligases, we discovered that Fbxo21 was highly expressed in the HSPC population. Fbxo21 was more highly expressed in the HSPC population when compared to other Fbox genes. In addition, Fbxo21 expression was primarily in immature hematopoietic populations, whereas, more mature hematopoietic populations had limited expression. Western blot and qRT-PCR analysis confirmed high expression of FBXO21 in HSPC, and revealed low to no expression in mature myeloid populations. To determine the role of FBXO21 on HSPC maintenance, self-renewal, and differentiation, we generated shRNAs against Fbxo21. We found the silencing of Fbxo21 in HSPC leads to a loss of colonies and differentiation towards the mature myeloid lineage. These findings lead to the hypothesis that the ubiquitin E3 ligase FBXO21, and its substrates regulate hematopoietic stem cell maintenance and loss promotes myeloid differentiation. Deciphering the role of FBXO21 could expand the current known molecular mechanisms that regulate hematopoietic lineage specification and stem cell maintenance.