Polarization along an apico-basolateral axis is a hallmark of epithelial cells and is essential for their selective barrier and transporter functions, as well as for their ability to provide mechanical resiliency to organs. Loss of polarity along this axis perturbs development and is associated with a wide number of diseases. We describe three steps involved in polarization: symmetry breaking, polarity establishment, and polarity maintenance. While the proteins involved in these processes are highly conserved among epithelial tissues and species, the execution of these steps varies widely and is context dependent. We review both theoretical principles underlying these steps and recent work demonstrating how apico-basolateral polarity is established in vivo in different tissues, highlighting how developmental and physiological contexts play major roles in the execution of the epithelial polarity program.

Apico-basolateral polarization is essential for epithelial cells to function as selective barriers and transporters, and to provide mechanical resiliency 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 C. elegans 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 is not required for establishment of local polarity as small groups of cells are able to establish polarized domains in PAR-3 depleted intestines in an HMR-1/E-cadherin dependent manner. Despite the role of PAR-3 in localizing PKC-3 to the apical surface, we additionally find that PAR-3 and PKC-3/aPKC have distinct roles in the establishment and maintenance of local and global polarity. Together, our results indicate that different mechanisms are required for local and global polarity establishment in vivo.

SummaryTissue-wide patterning is essential to multicellular development, requiring cells to individually generate polarity axes and coordinate them in space and time with neighbors. Using the C. elegans intestinal epithelium, we identified a patterning mechanism informed by stabilized cell/cell contact and executed via the scaffolding protein PAR-3 and the transmembrane protein E-cadherin/HMR-1. Intestinal cells break symmetry as PAR-3 and HMR-1 recruit apical determinants into micron-scale ‘local polarity complexes’ (LPCs) at homotypic contacts. LPCs undergo a HMR-1-based migration to a common tissue midline, thereby establishing tissue-wide polarity. Thus, symmetry breaking results from PAR-3-dependent intracellular polarization coupled to HMR-1-based tissue-level communication that occurs through a non-adhesive signaling role for HMR-1. Intestinal cells gain initial asymmetry from differential contact duration as homotypic contacts last longer than heterotypic contacts, thus providing stable platforms for LPC assembly and offering a logical and likely conserved framework for how internal epithelia with no obvious pre-existing asymmetries can polarize.

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The Planar cell Polarity Effector (PPE) genes inturned, fuzzy, and fritz are downstream components in the frizzled/starry night signaling pathway, and their function is instructed by upstream Planar Cell Polarity (PCP) core genes such as frizzled and dishevelled. PPE proteins accumulate asymmetrically in wing cells and function in a protein complex mediated by direct interactions between In and Frtz and In and Fy. How the PCP proteins instruct the accumulation of PPE protein is unknown. We found a likely direct interaction between Dishevelled and Fritz and Dishevelled and Fuzzy that could play a role in this. We previously found that mild overexpression of frtz rescued a weak in allele. To determine if this was due to extra Frtz stabilizing mutant In or due to Frtz being able to bypass the need for In we generate a precise deletion of the inturned gene (in PD ). We found that mild overexpression of Fritz partially rescued in PD , indicating that fritz has In independent activity in PCP. Previous studies of PPE proteins used fixed tissues, and did not provide any insights into the dynamic properties of PPE proteins. We used CRISPR/Cas9 genome editing technology to edit the fritz gene to add a green fluorescent protein tag. fritz mNeonGreen provides complete rescue activity and works well for in vivo imaging. Our data showed that Fritz is very dynamic in epidermal cells and preferentially distributed to discrete membrane subdomains ("puncta"). Surprisingly, we found it in stripes in developing bristles.