Separable mechanisms drive local and global polarity establishment in the Caenorhabditiselegans intestinal epithelium

Apico-basolateral polarization is essential for epithelial cells to function as selective barriers and transporters, and to provide mechanical resilience to organs. Epithelial polarity is established locally, within individual cells to establish distinct apical, junctional and basolateral domains, and globally, within a tissue where cells coordinately orient their apico-basolateral axes. Using live imaging of endogenously tagged proteins and tissue-specific protein depletion in the Caenorhabditiselegans embryonic intestine, we found that local and global polarity establishment are temporally and genetically separable. Local polarity is initiated prior to global polarity and is robust to perturbation. PAR-3 is required for global polarization across the intestine but local polarity can arise in its absence, as small groups of cells eventually established polarized domains in PAR-3-depleted intestines in a HMR-1 (E-cadherin)-dependent manner. Despite the role of PAR-3 in localizing PKC-3 to the apical surface, we additionally found that PAR-3 and PKC-3/aPKC have distinct roles in the establishment and maintenance of local and global polarity. Taken together, our results indicate that different mechanisms are required for local and global polarity establishment in vivo.

Claudin-7b and Claudin-h are required for controlling cilia morphogenesis in the zebrafish kidney

Claudins are a family of proteins which are the most important components of the tight junctions. The location of Claudins on the renal tubule epithelial determines its paracellular transport characteristics, but whether Claudins have other functions in kidneys remains still unclear. Here, we showed that the transcripts encoding two Claudin family proteins, claudin-7b (cldn-7b) and claudin-h (cldn-h), were expressed in the transporting cells in the zebrafish pronephros. By knocking down of cldn-7b and cldn-h in zebrafish, we showed that these claudins morphants exhibited cystic kidneys accompanied with body curvature. Further analysis showed that down regulation of cldn-7b or cldn-h led to multiple defects in apico-basolateral polarity, cilia morphology and ciliary function in kidney. Moreover, the ciliary defect was confirmed by depletion of Cldn-7b or Cldn-h using CRISPR/Cas9 system. We also showed that both cldn-7b and cldn-h were genetically interacted with a well-known ciliary gene, arl13b. Deletion of arl13b led to curly cilia in the pronephros that phenocopied with cldn-7b and cldn-h morphants. Taken together, our data suggested that the tight junction protein, Cldn-7b and Cldn-h, regulate kidney development and function by affecting cilia morphology.

Regulation of the apico-basolateral trafficking polarity of the homologous copper-ATPases ATP7A and ATP7B

The homologous P-type copper-ATPases (Cu-ATPases) ATP7A and ATP7B are the key regulators of copper homeostasis in mammalian cells. In polarized epithelia, upon copper treatment, ATP7A and ATP7B traffic from the trans-Golgi network (TGN) to basolateral and apical membranes, respectively. We characterized the sorting pathways of Cu-ATPases between TGN and the plasma membrane and identified the machinery involved. ATP7A and ATP7B reside on distinct domains of TGN in limiting copper conditions, and in high copper, ATP7A traffics to basolateral membrane, whereas ATP7B traverses common recycling, apical sorting and apical recycling endosomes en route to apical membrane. Mass spectrometry identified regulatory partners of ATP7A and ATP7B that include the adaptor protein-1 complex. Upon knocking out pan-AP-1, sorting of both Cu-ATPases is disrupted. ATP7A loses its trafficking polarity and localizes on both apical and basolateral surfaces in high copper. By contrast, ATP7B loses TGN retention but retained its trafficking polarity to the apical domain, which became copper independent. Using isoform-specific knockouts, we found that the AP-1A complex provides directionality and TGN retention for both Cu-ATPases, whereas the AP-1B complex governs copper-independent trafficking of ATP7B solely. Trafficking phenotypes of Wilson disease-causing ATP7B mutants that disrupts putative ATP7B-AP1 interaction further substantiates the role of AP-1 in apical sorting of ATP7B.