Mir-886-3p Contributes to the Regulation of the hematopoietic microenvironment by down-Regulating SDF-1α (CXCL12).

Abstract 561

Two functionally distinct stromal cell lines were isolated from a primary long term culture (LTC) established from aspirated human marrow. Designated HS5 and HS27a, the lines were immortalized and extensively characterized including expression profiles for both messenger (mRNA) and micro-RNA (miRNA, a recently described class of small non-coding RNAs that regulate gene expression by binding to target mRNAs to prevent their translation). HS5 was found to secrete growth factors that stimulate proliferation and differentiation of hematopoietic progenitors (G-CSF, IL-6, IL-1α and IL1β), whereas HS27a expresses activities associated with the stem cell niche (SDF-1αa, angiopoietin-1 etc). In keeping with this HS5 conditioned media stimulated proliferation and differentiation of isolated CD34+ cells whereas HS27a supported CD34+ cells in an undifferentiated state. When cultured together to better mimic in vivo cell-cell interactions, the gene expression of HS27a and HS5 combined differed from the expected sum of the two parts, exemplified by the 5-fold down regulation of SDF-1α.

Comparisons of miRNA expression profiles of HS5 and HS27a determined that mir-886-3p, (previously described by deep sequencing of small RNA libraries) was expressed > 40 fold in HS5 compared to HS27a, this was then confirmed by quantitative RT-PCR. Given the abundance of mir-886-3p and the possibility that it could be secreted by HS5 and taken up by cells in contact with HS5, we tested its effect on gene expression in HS27a. Transcript levels of genes associated with the stem cell niche (Jagged1, BMP4, Angiopoietin-1, SDF-1α, VCAM-1 and N-Cadherin) were determined by quantitative RT-PCR after direct transfection of mir-886-3p precursors into HS27a cells and compared to appropriate controls. Results show SDF-1α mRNA expression was down-regulated by as much as 8 fold 3 days after transfection. Levels of secreted SDF-1α in culture media, as determined by ELISA, were also decreased. Since SDF-1α is a chemokine known to be critical for the homing of hematopoietic stem and progenitor cells to their niche, the functional significance of the SDF-1α down-regulation by mir-886-3p was confirmed by decreased chemotaxis of T-lymphocytic cells (Jurkat) following miRNA transfection of stromal cells. To determine if mir-886-3p directly effects the SDF-1α transcript, the 1.5 kbp 3'untranslated region (UTR) of SDF-1α gene was cloned downstream of the luciferase gene, and co-transfected with mir-886-3p into HS27a cells. Results showed the luciferase activity was down-regulated greater than 50% in the presence of mir-886-3p, suggesting a direct effect on the SDF-1 α transcript.

Given the concern over the relevance of immortalized cell lines we investigated Mir-886-3p expression in primary marrow stromal cells at early passage sorted on the basis of +/- expression of CD146. (CD146 or MCAM has been reported to define a population that supports the hematopoietic stem/ precursor cell niche and is expressed by HS27a and not HS5 cells). Results indicated that the CD146+ stromal cells had significantly lower expression of mir-886-3p when compared to CD146- cells.

In summary, these data suggest a role for miRNA in modulating the expression of gene products that are associated with the hematopoietic stem cell niche.