The scaffolding protein KIBRA (also called WWC1) is involved in the regulation of important intracellular transport processes and the establishment of cell polarity. Furthermore, KIBRA/WWC1 is an upstream regulator of the Hippo signaling pathway that controls cell proliferation and organ size in animals. KIBRA/WWC1 represents only one member of the WWC protein family that also includes the highly similar proteins WWC2 and WWC3. Although the function of KIBRA/WWC1 was studied intensively in cells and animal models, the importance of WWC2 and WWC3 was not yet elucidated. Here, we describe evolutionary, molecular, and functional aspects of the WWC family. We show that the WWC genes arose in the ancestor of bilateral animals (clades such as insects and vertebrates) from a single founder gene most similar to the present KIBRA/WWC1-like sequence of Drosophila. This situation was still maintained until the common ancestor of lancelet and vertebrates. In fish, a progenitor-like sequence of mammalian KIBRA/WWC1 and WWC2 is expressed together with WWC3. Finally, in all tetrapods, the three family members, KIBRA/WWC1, WWC2, and WWC3, are found, except for a large genomic deletion including WWC3 in Mus musculus. At the molecular level, the highly conserved WWC proteins share a similar primary structure, the ability to form homo- and heterodimers and the interaction with a common set of binding proteins. Furthermore, all WWC proteins negatively regulate cell proliferation and organ growth due to a suppression of the transcriptional activity of YAP, the major effector of the Hippo pathway.

The WWC1 gene has been genetically associated with human episodic memory performance, and its product KIBRA has been shown to interact with the atypical protein kinase PKMζ. Although recently challenged, PKMζ remains a candidate postsynaptic regulator of memory maintenance. Here we show that PKMζ is subject to rapid proteasomal degradation and that KIBRA is both necessary and sufficient to counteract this process, thus stabilizing the kinase and maintaining its function for a prolonged time. We define the binding sequence on KIBRA, a short amino acid motif near the C-terminus. Both hippocampal knock-down of KIBRA in rats and KIBRA knock-out in mice result in decreased learning and memory performance in spatial memory tasks supporting the notion that KIBRA is a player in episodic memory. Interestingly, decreased memory performance is accompanied by decreased PKMζ protein levels. We speculate that the stabilization of synaptic PKMζ protein levels by KIBRA may be one mechanism by which KIBRA acts in memory maintenance.

The Hippo pathway fulfills a crucial function in controlling the balance between proliferation, differentiation and apoptosis in cells. Recent studies showed that G protein-coupled receptors (GPCRs) serve as upstream regulators of Hippo signaling, that either activate or inactivate the Hippo pathway via the large tumor suppressor kinase (LATS) and its substrate, the co-transcription factor Yes-associated protein (YAP). In this study, we focused on the Angiotensin II type 1 receptor (AT1R), which belongs to the GPCR family and has an essential role in the control of blood pressure and water homeostasis. We found that Angiotensin II (Ang II) inactivates the pathway by decreasing the activity of LATS kinase; therefore, leading to an enhanced nuclear shuttling of unphosphorylated YAP in HEK293T cells. This shuttling of YAP is actin-dependent as disruption of the actin cytoskeleton inhibited dephosphorylation of LATS and YAP. Interestingly, in contrast to HEK293T cells, podocytes, which are a crucial component of the glomerular filtration barrier, display a predominant nuclear YAP localization in vivo and in vitro. Moreover, stimulation with Ang II did not alter Hippo pathway activity in podocytes, which show a deactivated pathway. Reactivation of the LATS kinase activity in podocytes resulted in an increased cytoplasmic YAP localization accompanied by a strong induction of apoptosis. Thus, our work indicates that the control of LATS activation and subsequent YAP localization is important for podocyte homeostasis and survival.

In mammals, the KIBRA locus has been associated with memory performance and cognition by genome-wide single nucleotide polymorphism screening. Genetic studies in Drosophila and human cells have identified KIBRA as a novel regulator of the Hippo signaling pathway, which plays a critical role in human tumorigenesis. Recent studies also indicated that KIBRA is involved in other physiological processes including cell polarity, membrane/vesicular trafficking, mitosis and cell migration. At the biochemical level, KIBRA protein is highly phosphorylated by various kinases in epithelial cells. Here, we discuss the updates concerning the function and regulation of KIBRA in the brain and beyond.

The endothelial glycocalyx (eGC), a carbohydrate-rich layer lining the luminal surface of the endothelium, provides a first vasoprotective barrier against vascular leakage in sepsis. We hypothesized that angiopoietin-2 (Angpt-2), antagonist of the endothelium-stabilizing receptor Tie2, induces a rapid loss of the eGC in human sepsis. Using intravital microscopy, we measured the perfused boundary region (PBR), an inverse parameter of eGC dimensions in sublingual microvessels, in patients with sepsis and age-matched nonseptic subjects. Median PBR values were significantly higher in patients compared with controls and correlated with serum Angpt-2 levels. To transfer and further explore these findings in a cell culture system, we exposed endothelial cells (ECs) to serum (5%) from a subgroup of septic patients and nonseptic controls. Confocal and atomic force microscopy revealed that sepsis serum, but not control serum, induced thinning of the eGC on human ECs in vitro, which correlated with paired PBR values obtained in vivo (r = 0.96, p < 0.01). Inhibition of Angpt-2 or Tie2 activation completely abolished eGC damage. Mechanistically, sepsis-induced eGC breakdown required the loss of its main constituent heparan sulfate; a result of heparan sulfate-specific enzyme heparanase, which was suppressed by Tie2 activation. Finally, Tie2 activation, but not Angpt-2 inhibition, initiated after septic or enzymatic damage provoked rapid refurbishment of the eGC. Our data indicate that eGC breakdown in human sepsis is mediated via Tie2 deactivation by Angpt-2. Activation of Tie2 seems to accelerate recovery of the eGC and might hold promise as a therapeutic target in human sepsis.