Abstract
Polarized epithelial cells can organize into complex structures with a characteristic central lumen. Lumen formation requires that cells coordinately orient their polarity axis so that the basolateral domain is on the outside and apical domain inside epithelial structures. Here we show that the transmembrane aminopeptidase, CD13, is a key determinant of epithelial polarity orientation. CD13 localizes to the apical membrane and associates with an apical complex with Par6. CD13-deficient cells display inverted polarity in which apical proteins are retained on the outer cell periphery and fail to accumulate at an intercellular apical initiation site. Here we show that CD13 is required to couple apical protein cargo to Rab11-endosomes and for capture of endosomes at the apical initiation site. This role in polarity utilizes the short intracellular domain but is independent of CD13 peptidase activity.
Highlights
Polarized epithelial cells can organize into complex structures with a characteristic central lumen
We first established that endogenous CD13 and exogenous CD13-V5 were enriched at the luminal membrane of 3D Caco-2 cysts and they colocalized with Par[6] and other apical markers (Fig. 1a, b; Supplementary Figs. 1a–d)
Silencing of CD13 induced an inverted polarity phenotype in Caco-2 cysts, which was characterized by apical complexes and tight junctions positioned at the periphery of cell aggregates, while basolateral proteins were excluded from the outer edge and interactions with the matrix were impaired (Fig. 8a)
Summary
Polarized epithelial cells can organize into complex structures with a characteristic central lumen. An important property of many epithelial cells is establishing polarity along an apical-basal axis such that the apical membrane faces a lumen that is contiguous with the external environment. This creates a barrier that allows selective vectoral transport of macromolecules for absorption or secretion. The majority of established regulators of apical-basal polarity are multi-domain scaffold and adaptor proteins, that associate with transmembrane proteins that anchor them to the plasma membrane[4]. Epithelial cells cultured in 3D basement membrane extract rich in laminin is a well-established model of apical-basal polarity[8,9] In this model, apical and basolateral proteins are initially codistributed on the plasma membrane of single cells. The mechanisms that control the orientation of apical-basal polarity remain to be fully elucidated
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