X-box binding protein 1–mediated COL4A1s secretion regulates communication between vascular smooth muscle and stem/progenitor cells

Abstract

Vascular smooth muscle cells (VSMCs) contribute to the deposition of extracellular matrix proteins (ECMs), including Type IV collagen, in the vessel wall. ECMs coordinate communication among different cell types, but mechanisms underlying this communication remain unclear. Our previous studies have demonstrated that X-box binding protein 1 (XBP1) is activated and contributes to VSMC phenotypic transition in response to vascular injury. In this study, we investigated the participation of XBP1 in the communication between VSMCs and vascular progenitor cells (VPCs). Immunofluorescence and immunohistology staining revealed that Xbp1 gene was essential for type IV collagen alpha 1 (COL4A1) expression during mouse embryonic development and vessel wall ECM deposition and stem cell antigen 1-positive (Sca1 + )-VPC recruitment in response to vascular injury. The Western blot analysis elucidated an Xbp1 gene dose-dependent effect on COL4A1 expression and that the spliced XBP1 protein (XBP1s) increased protease-mediated COL4A1 degradation as revealed by Zymography. RT-PCR analysis revealed that XBP1s in VSMCs not only upregulated COL4A1/2 transcription but also induced the occurrence of a novel transcript variant, soluble type IV collagen alpha 1 ( COL4A1s) , in which the front part of exon 4 is joined with the rear part of exon 42. Chromatin-immunoprecipitation, DNA/protein pulldown and in vitro transcription demonstrated that XBP1s binds to exon 4 and exon 42, directing the transcription from exon 4 to exon 42. This leads to transcription complex bypassing the internal sequences, producing a shortened COL4A1s protein that increased Sca1 + -VPC migration. Taken together, these results suggest that activated VSMCs may recruit Sca1 + -VPCs via XBP1s-mediated COL4A1s secretion, leading to vascular injury repair or neointima formation.

A Novel Isoform of COL4A1 in the Regulation of Vascular Intercellular Communication

“…Both proposed mechanisms rely heavily on the action of the active form of XBP1s to facilitate the development of the COL4A1s isomer. As such, we demonstrate knockout of the XBP1 gene in the VSMC of mice as resulting in decreased COL4A1 in the vessel wall [14]. XBP1 therefore remains as an essential transcription factor for the COL4A1 gene transcription.…”

“…In the context of vascular injury, this results in the recruitment of Sca1 + -VPCs to the site of injury, whereby promoting the development of re-stenosis following cardiovascular interventions. In this manner, the authors stress an intercellular communication between SMCs and VPCs in the adventitia after vascular injury [14].…”

“…The work of Angbohang et al seeks to bridge this incomplete understanding, by exploring the role of a novel soluble isoform of type IV collagen A1(COL4A1s) secretion mediated by X-binding Box Protein 1 splicing isoform (XBP1s), in contributing to vascular injury repair and neointima formation (14). Here, the authors described such an XBP1smediated COL4A1s secretion as being capable of activating VSMCs to recruit stem cell antigen 1-positive (Sca1 + ) [14]. In the context of vascular injury, this results in the recruitment of Sca1 + -VPCs to the site of injury, whereby promoting the development of re-stenosis following cardiovascular interventions.…”

“…Of paramount also in the discovery, is the identification of the novel protein isomer COL4A1s, as being the mediating factor in facilitating the identified intercellular communication between VSMCs and VPCs. This COL4A1s isoform largely resembles the main structural features of the COL4A1 protein but has its internal helix domain shortened [14]. In terms of cellular function, the COL4A1s isoform may utilise its NC-domain to bind to integrins, but interaction with other cell surface components may be facilitated via the longer N-terminal [14].…”

“…This COL4A1s isoform largely resembles the main structural features of the COL4A1 protein but has its internal helix domain shortened [14]. In terms of cellular function, the COL4A1s isoform may utilise its NC-domain to bind to integrins, but interaction with other cell surface components may be facilitated via the longer N-terminal [14]. Once translated however, the novel isoform of COL4A1 protein functions as a paracrine cytokine, encouraging the migration of VPCs into the site of injury.…”