The genomic structure of the murine alpha 4 integrin gene

A novel role of BMP4 in adult hematopoietic stem and progenitor cell homing via Smad independent regulation of integrin-α4 expression

Although it is well established that BMP4 plays an important role in the development of hematopoietic system, it is less well understood whether BMP4 affects adult hematopoiesis and how. Here, we describe a novel mechanism by which BMP4 regulates homing of murine as well as human hematopoietic stem/progenitor cells (HSPCs). BMP4 treatment of murine BM derived c-kitLin-Sca-1 (KLS) and CD150CD48-KLS cells for up to 5 days in vitro prevented the culture-induced loss of Integrin-α4 (ITGA4) expression as well as homing. The effect on ITGA4 expression in response to BMP4 is mediated via SMAD-independent phosphorylation of p38 MAPK, which activates microphthalmia-associated transcription factor (MITF), known to induce ITGA4 expression. Elevated ITGA4 expression significantly enhanced HSPC attachment to bone marrow stromal cells, homing and long-term engraftment of the BMP4 treated cells compared with the cells cultured without BMP4. BMP4 also induced expression of ITGA4 on human BM derived Lin-CD34 cells in culture, which was associated with improved homing potential. Thus, BMP4 prevents culture-induced loss of ITGA4 expression on HSPCs in a SMAD-independent manner, resulting in improved homing of cultured HSPCs and subsequent hematopoietic reconstitution.

KEY POINTS

Cytokine-induced loss of murine as well as human HSPC homing during ex vivo culture can be prevented by addition of BMP4. In HSPCs, BMP4 directly regulates Integrin-α4 expression through SMAD-independent p38 MAPK-mediated signaling.

Frequencies of alpha4 A3061G variants and identification of three new variants of the human integrin alpha4-subunit

Human integrin alpha4beta1 (VLA-4) is thought to play a key role in immune responses. Only two variants of alpha4-subunit (alpha4-tex and alpha4-mas) have been described until now, and the information regarding these variants is scanty. In this study, we measured the frequency of both variants in healthy blood donors (n=252). Surprisingly, the frequency of alpha4-mas (3061G, 0.31) was lower than that of alpha4-tex (3061A, 0.69). In addition, we sequenced and analyzed the mRNA of the entire alpha4-subunit in eight unselected healthy blood donors. These studies revealed three new variants, including a C to A transversion at position 269 in the promoter region of exon 1 (accession no. AJ504733); a G to A transversion at position 2273 in exon 16 (accession no. AJ510246, AJ510247), and a T to C exchange at position 3311 in exon 26 (accession no. AJ510248, AJ510249). None of these variants led to amino acid (AA) substitutions. Interestingly, homozygosity of the new variant 269A was not found.