Twist‐1 Enhances Bone Marrow Mesenchymal Stromal Cell Support of Hematopoiesis by Modulating CXCL12 Expression

Twist‐1 encodes a basic helix‐loop‐helix transcription factor, known to contribute to mesodermal and skeletal tissue development. We have reported previously that Twist‐1 maintains multipotent human bone marrow‐derived mesenchymal stem/stromal cells (BMSC) in an immature state, enhances their life‐span, and influences cell fate determination. In this study, human BMSC engineered to express high levels of Twist‐1 were found to express elevated levels of the chemokine, CXCL12. Analysis of the CXCL12 proximal promoter using chromatin immunoprecipitation analysis identified several E‐box DNA sites bound by Twist‐1. Functional studies using a luciferase reporter construct showed that Twist‐1 increased CXCL12 promoter activity in a dose dependent manner. Notably, Twist‐1 over‐expressing BMSC exhibited an enhanced capacity to maintain human CD34 + hematopoietic stem cells (HSC) in long‐term culture‐initiating cell (LTC‐IC) assays. Moreover, the observed increase in HSC maintenance by Twist‐1 over‐expressing BMSC was blocked in the presence of the CXCL12 inhibitor, AMD3100. Supportive studies, using Twist‐1 deficient heterozygous mice demonstrated a significant decrease in the frequency of stromal progenitors and increased numbers of osteoblasts within the bone. These observations correlated to a decreased incidence in the number of clonogenic stromal progenitors (colony forming unit–fibroblasts) and lower levels of CXCL12 in Twist‐1 mutant mice. Furthermore, Twist‐1 deficient murine stromal feeder layers, exhibited a significant decrease in CXCL12 levels and lower numbers of hematopoietic colonies in LTC‐IC assays, compared with wild type controls. These findings demonstrate that Twist‐1, which maintains BMSC at an immature state, endows them with an increased capacity for supporting hematopoiesis via direct activation of CXCL12 gene expression. Stem Cells 2016;34:504–509