ZO-1 and ZONAB Interact to Regulate Proximal Tubular Cell Differentiation

Renal tubular segments are specialized structures with selective functions defined by the specificity of highly differentiated epithelial cells. All polarized epithelial cells in tubular structures have distinct apical and basal plasma membrane domains as well as lateral surfaces that connect sister cells by specialized cellular junctions. The basal membrane of epithelial cells adhere to the extracellular matrix primarily by integrins and syndecans, whereas the lateral surfaces of the epithelial cells interact with each other through lateral cellular junctions. Among these junctions, the apical tight junction complex controls paracellular transport, the adherens junctions localized below the tight junctions allow cells to adhere to each other, and gap junctions often found below the adherens junctions allow intercellular exchange of ions and small molecules.^1,2

The tight junctions are composed of members of the occludin, claudin, and junctional adhesion molecule families. They are transmembrane proteins that connect two adjacent cells through interactions with their extracellular domains and link to respective actin cytoskeleton by their cytoplasmic domains. The major functions of the tight junctions are to form a barrier that separates the apical and basolateral aspects of epithelial cells. The degree of “tightness” of this barrier is determined by the expression of specific members of tight junction proteins during differentiation.³ As tubules elongate, epithelial cells are thought to switch from a proliferative phenotype, where many of the molecules forming cellular junctions are not expressed, to a highly differentiated cell type with well-defined adherens and tight junction complexes. How the formation of these complexes is signaled is one of the key unanswered questions in epithelial cell biology.

A key component of tight junctions is the adaptor protein zona occludens 1 (ZO-1). This scaffolding molecule interacts with multiple other proteins, including actin, claudins, occludins, α-catenin, signaling proteins, and the Y-box transcription factor ZONAB. The interaction with ZONAB is particularly interesting because this transcription factor is a member of a family of DNA-binding proteins that regulate the expression of genes involved in proliferation, including cyclins, PCNA, and Erb receptors. ZONAB shuttles between tight junctions and the nucleus, and its transcriptional activity is inversely correlated with ZO-1 expression.³ When polarized renal MDCK cells become confluent, they express more ZO-1, resulting in increased ZONAB localization to the tight junctions and decreased levels of ZONAB in the cytoplasm and nucleus.⁴ This finding suggests that ZONAB interactions with ZO-1 are critical components of a tight junction–associated signal transduction pathway regulating the transition of epithelial cells from a proliferative to a differentiated phenotype.

The article by Lima et al.⁵ in this issue of JASN shows in vivo and in vitro that ZONAB and ZO-1 are important in determining when proximal renal tubular cells stop proliferating and start differentiating into specialized cells. An inverse relationship between ZONAB expression and apical proximal epithelial differentiation is observed during kidney development. In proximal tubular cells in vitro, decreased ZONAB expression occurs concomitantly with increased expression of apical endocytic receptors (megalin) and maturation of primary cilium. Interestingly, ZONAB overexpression prevents polarization and differentiation of these cells while promoting a proliferative phenotype. Thus, these novel findings show the levels of ZO-1 and the localization of ZONAB, a transcription factor that normally interacts with ZO-1 and multiple other nuclear and cytoplasmic proteins, are key modulators defining when epithelial cells along proximal tubules switch from a proliferative (nuclear/cytoplasmic ZONAB) to differentiated (ZO-1–bound ZONAB) state.

These observations add to a growing literature showing that cellular junctions actively modulate epithelial phenotype. The mechanism of cellular function regulated by tight junctions mirrors those attributed to the E-cadherin–mediated adherens junctions in the process of epithelial-to-mesenchymal transition. In addition to mediating the physical association of epithelial cells, E-cadherin regulates cell proliferation and migration by its interactions with β-catenin. When E-cadherin binds β-catenin, the nuclear translocation and the transcription activity of β-catenin are prevented.⁶ In contrast, decreased E-cadherin expression, as observed in the course of tumor metastasis or epithelial-to-mesenchymal transition, leads to increased translocation of β-catenin to the nucleus followed by increased expression of genes involved in cell survival, proliferation, or migration.^7–9 This pathway plays a key role in development, organ fibrosis, and tumorogenesis in multiple tissues, including the kidney.^10–13

Tight junctions and adherens junctions not only mediate similar signaling pathways, such as the ZO-1/ZONAB and the E-cadherin/β-catenin–dependent pathways, but also link to each other by interactions with common scaffolding proteins such as α-catenin and the actin cytoskeleton.³ Thus, these different cell–cell junctional complexes might act as sensors of specific physical stimuli leading to cooperation with each other in the regulation of fundamental epithelial cell functions such as proliferation, polarity, and differentiation. Understanding how these pathways are regulated and intersect with each other may very well help us to define how cell–cell contact of tubular epithelial cells modulates their ability to switch from proliferative to highly differentiated phenotypes.

Disclosures

None.