Alternative exon usage creates novel transcript variants of tumor suppressor SHREW-1 gene with differential tissue expression profile

Abstract

Shrew-1, also called AJAP1, is a transmembrane protein associated with E-cadherin-mediated adherence junctions and a putative tumor suppressor. Apart from its interaction with β-catenin and involvement in E-cadherin internalization, little structure or function information exists. Here we explored shrew-1 expression during postnatal differentiation of mammary gland as a model system. Immunohistological analyses with antibodies against either the extracellular or the cytoplasmic domains of shrew-1 consistently revealed the expression of full-length shrew-1 in myoepithelial cells, but only part of it in luminal cells. While shrew-1 localization remained unaltered in myoepithelial cells, nuclear localization occurred in luminal cells during lactation. Based on these observations, we identified two unknown shrew-1 transcript variants encoding N-terminally truncated proteins. The smallest shrew-1 protein lacks the extracellular domain and is most likely the only variant present in luminal cells. RNA analyses of human tissues confirmed that the novel transcript variants of shrew-1 exist in vivo and exhibit a differential tissue expression profile. We conclude that our findings are essential for the understanding and interpretation of future functional and interactome analyses of shrew-1 variants.

Endogenous AJAP1 associates with the cytoskeleton and attenuates angiogenesis in endothelial cells

“…However, there is increasing evidence that the expression of AJAP1 is not restricted to the adherens junction, but depends on the developmental stage and tissue type (Klemmt et al, 2016). To assess the molecular function of AJAP1 underlying angiogenic sprouting, we investigated the cellular localization of endogenous AJAP1 in endothelial cells.…”

“…Initially, it had been shown that AJAP1 localizes at the plasma membrane in the adhesion complex of epithelial cells, where it binds to the E-cadherin-catenin complex ( Bharti et al, 2004 ). However, there is increasing evidence that the expression of AJAP1 is not restricted to the adherens junction, but depends on the developmental stage and tissue type ( Klemmt et al, 2016 ). To assess the molecular function of AJAP1 underlying angiogenic sprouting, we investigated the cellular localization of endogenous AJAP1 in endothelial cells.…”

“…This association was persistent and was not altered by cell confluence or duration of culture or in a three-dimensional cell context during spheroid sprouting. Recently, a first hint on this altered localization has been given by demonstrating a cytoplasmic localization of AJAP1 in epithelial cells of the murine mammary gland in vivo ( Klemmt et al, 2016 ).…”

“…AJAP1 overexpressing glioblastoma cell lines display a change in the cellular distribution of F-actin and β-tubulin ( Han et al, 2014 ). It has been shown recently that at least three protein isoforms of AJAP1 exist in the human context which show different expression patterns in organs and during developmental processes ( Klemmt et al, 2016 ). This also suggests that the different AJAP1 isoforms can localize differentially within cells.…”

“…Isoform 2, like isoform 1, comprises an extracellular and intracellular domain, whereas the first eleven amino acids are truncated. Isoform 3 consists of only 120 amino acids, and the extracellular domain is truncated ( Klemmt et al, 2016 ).…”