Lehrke, Michael J

Iron-sulfur (Fe-S) clusters are cofactors essential for the activity of numerous enzymes including DNA polymerases, helicases, and glycosylases. They are synthesized in the mitochondria as Fe-S intermediates and are exported to the cytoplasm for maturation by the mitochondrial transporter ABCB7. Here, we demonstrate that ABCB7 is required for bone marrow B cell development, proliferation, and class switch recombination, but is dispensable for peripheral B cell homeostasis in mice. Conditional deletion of ABCB7 using Mb1-cre resulted in a severe block in bone marrow B cell development at the pro-B cell stage. The loss of ABCB7 did not alter expression of transcription factors required for B cell specification or commitment. While increased intracellular iron was observed in ABCB7-deficient pro-B cells, this did not lead to increased cellular or mitochondrial reactive oxygen species, ferroptosis, or apoptosis. Interestingly, loss of ABCB7 led to replication-induced DNA damage in pro-B cells, independent of VDJ recombination, and these cells had evidence of slowed DNA replication. Stimulated ABCB7-deficient splenic B cells from CD23-cre mice also had a striking loss of proliferation and a defect in class switching. Thus, ABCB7 is essential for early B cell development, proliferation, and class switch recombination.

Hematopoietic cell homing after hematopoietic cell transplant (HCT) is governed by several pathways involving marrow niche cells that are evoked after pre-HCT conditioning. To understand the factors that play a role in homing, we performed expression analysis on the zebrafish marrow niche cells following conditioning. We determined that the non-collagenous protein extracellular matrix related protein dermatopontin (Dpt) was upregulated seven-fold in response to irradiation. Studies in mice revealed DPT induction both with radiation and lipopolysaccharide exposure. Interestingly, we found that co-incubation of zebrafish or murine hematopoietic cells with rDPT impedes hematopoietic stem and progenitor cell homing by 50% and 86%, respectively. Similarly, this translated into a 24% reduction in long term engraftment (versus control, p = 0.01). We found DPT to interact with VLA-4 and block hematopoietic - endothelial cell adhesion and transendothelial migration. Finally, a DPT knockout mouse displayed a 60% increase in homing of hematopoietic cells versus wildtype (p = 0.03) with slight improvement in long-term LSK-SLAM engraftment (2-fold, p = 0.04). These data show that the extracellular matrix (ECM)-related protein DPT increases with radiation and transiently impedes the transendothelial migration of hematopoietic cells to the marrow.

Abstract NKAP is a multifunctional nuclear protein that associates with the histone deacetylase HDAC3. Although both NKAP and HDAC3 are critical for hematopoietic stem cell (HSC) maintenance and survival, it was not known whether these two proteins work together. To assess the importance of their association in vivo, serial truncation and alanine scanning was performed on NKAP to identify the minimal binding site for HDAC3. Mutation of either Y352 or F347 to alanine abrogated the association of NKAP with HDAC3, but did not alter NKAP localization or expression. Using a linked conditional deletion/re-expression system in vivo, we demonstrated that re-expression of the Y352A NKAP mutant failed to restore HSC maintenance and survival in mice when endogenous NKAP expression was eliminated using Mx1-cre and poly-IC, whereas re-expression of wild type NKAP maintained the HSC pool. However, Y352A NKAP did restore proliferation in murine embryonic fibroblasts when endogenous NKAP expression was eliminated using ER-cre and tamoxifen. Therefore, Y352 in NKAP is critical for association with HDAC3 and for HSC maintenance and survival but is not important for proliferation of murine embryonic fibroblasts, demonstrating that NKAP functions in different complexes in different cell types.