Chd1 Regulates Primitive State of Bone Marrow Mesenchymal Stromal Cells without Affecting Hematopoietic Support; An Insight on the Dissociation Between Primitive State and Niche Function

Mesenchymal stromal cells (MSCs) are characterized by heterogeneity in the proliferation/self-renewal potentials and hematopoietic supporting activity among subpopulations. Numerous studies have suggested that a primitive state of MSC subpopulation are correlated to its niche function to support hematopoietic stem cells (HSCs), but the mechanisms regulating primitive state of MSCs remains poorly understood.

In the present study, we examined the role of a chromatin remodeling enzyme, chd1 in the maintenance of open chromatin and undifferentiated state of MSCs. We analyzed for expression in MSCs, the expression level of chd1 progressively decreased during in-vitro subculture (from 7 to 18 passages) in a manner proportional to the passage numbers. Moreover, chd1 expression was down regulated in the MSCs during their differentiation into adipogenic or osteogenic lineages, compared to proliferative state, indicating the correlations between MSC proliferation potentials and expression level of chd1. Next, we transduced human bone marrow-derived MSCs with shRNAs against chd1 and found that chd1 knock down MSCs (chd1-KD) exhibit significant loss of colony forming activity (CFU-F), decrease of cell proliferation and loss of multi-lineage differentiation towards osteogenic or adipogenic lineages. Moreover, chd1-KD MSCs exhibited lower level expression of pluripotency-related genes, oct-4, sox-2 and nanog, with concomitant increase of H3K9me3 on the promoters and decreased chromatin accessibility in the oct-4 promoter, suggesting that chd1 regulate open chromatin and multi-lineage potential of MSCs.

However, KD of chd1 in MSCs did not affect the HSC-supporting activity of MSCs; human cord blood-derived CD34+ cells co-cultured on chd1-KD MSCs exhibited rather higher maintenance of primitive phenotype (CD34+90+) and higher repopulating activity in NOD/SCID-ɤC KO mice compared to those co-cultured on control group MSCs. Together, these results show that, while primitive state of MSCs are regulated by chromatin remodeling complex,chd1, the hematopoietic niche activity of MSCs is not directly influenced by the primitive state of MSCs, raising a questions on the prevailing notion that undifferentiated MSCs can better support hematopoietic function.