Plekha7 is a key adherens junction component involved in numerous functions in mammalian cells. Plekha7 is the most studied member of the PLEKHA protein family, which includes eight members with diverse functions. However, the evolutionary history of Plekha7 remains unexplored. Here, we outline the phylogeny and identify the origins of this gene and its paralogs. We show that Plekha7, together with Plekha4, Plekha5, and Plekha6, belong to a subfamily that we name PLEKHA4/5/6/7. This subfamily is distinct from the other Plekha proteins, which form two additional separate subfamilies, namely PLEKHA1/2 and PLEKHA3/8. Sequence, phylogenetic, exon-intron organization, and syntenic analyses reveal that the PLEKHA4/5/6/7 subfamily is represented by a single gene in invertebrates, which remained single in the last common ancestor of all chordates and underwent gene duplications distinctly in jawless and jawed vertebrates. In the latter species, a first round of gene duplications gave rise to the Plekha4/7 and Plekha5/6 pairs and a second round to the four extant members of the subfamily. These observations are consistent with the 1R/2R hypothesis of vertebrate genome evolution. Plekha7 and Plekha5 also exist in two copies in ray-finned fishes, due to the Teleostei-specific whole genome duplication. Similarities between the vertebrate Plekha4/5/6/7 members and non-chordate sequences are restricted to their N-terminal PH domains, whereas similarities across the remaining protein molecule are only sporadically found among few invertebrate species and are limited to the coiled-coil and extreme C-terminal ends. The vertebrate Plekha4/5/6/7 proteins contain extensive intrinsically disordered domains, which are topologically and structurally conserved in all chordates, but not in non-chordate invertebrates. In summary, our study sheds light on the origins and evolution of Plekha7 and the PLEKHA4/5/6/7 subfamily and unveils new critical information suitable for future functional studies of this still understudied group of proteins.

β-catenin has been widely studied in many animal and organ systems across evolution, and gain or loss of function has been linked to a number of human diseases. Yet fundamental knowledge regarding its protein expression and localization remains poorly described. Thus, we sought to define whether there was a temporal and cell-specific regulation of β-catenin activities that correlate with distinct cardiac morphological events. Our findings indicate that activated nuclear β-catenin is primarily evident early in gestation. As development proceeds, nuclear β-catenin is down-regulated and becomes restricted to the membrane in a subset of cardiac progenitor cells. After birth, little β-catenin is detected in the heart. The co-expression of β-catenin with its main transcriptional co-factor, Lef1, revealed that Lef1 and β-catenin expression domains do not extensively overlap in the cardiac valves. These data indicate mutually exclusive roles for Lef1 and β-catenin in most cardiac cell types during development. Additionally, these data indicate diverse functions for β-catenin within the nucleus and membrane depending on cell type and gestational timing. Cardiovascular studies should take into careful consideration both nuclear and membrane β-catenin functions and their potential contributions to cardiac development and disease.

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Recent studies have shown that genomic instability, oncogene expression, and high mutation rates in more than 50% of tumors can be attributed to increased transposon activity. Transposons, also called transposable elements (TEs), are parasitic mobile DNA elements that make up 45% of the human genome and can cause DNA damage and gene mutations. The reasons for increased TE activity in somatic tumors are currently unknown. TE RNA transcripts are targeted for degradation by PIWI ribonucleases, which are members of the Argonaute superfamily of proteins, through regulation of small RNAs called piRNAs (PIWI-interacting RNAs). Although the function of the PIWI-piRNA pathway has been extensively studied in the germline, there is little understanding about the presence or functionality of the pathway in somatic tissues. We have evidence of a novel interaction of the adherens junctions of well-differentiated epithelial cells with PIWIL2 (PIWI-like RNA-Mediated Gene Silencing 2), a key catalytic component of the PIWI-piRNA complex. Through immunofluorescence staining, confocal microscopy, and co-immunoprecipitation studies, we found co-localization and association of PIWIL2, as well as of other key members of the PIWI complex, such as PIWIL4 and TDRD1, with E-cadherin and p120 catenin at adherens junctions of well-differentiated breast and colon epithelial cells, whereas this association is lost in cancer cells and patient tissues. Adherens junction disruption results in mis-localization of PIWIL2, PIWIL4, and TDRD1 from the adherens junctions and their accumulation either to the cytoplasm or to the nucleus. E-cadherin depletion particularly results in increased expression of the LINE1 transposon, which is the most abundant transposon in human cells, and in increased levels of γ-H2AX, an indicator of DNA damage. Our working hypothesis is that cadherin junctions of well-differentiated epithelial cells recruit and regulate the PIWIL2 complex to suppress transposon activity. Since both loss of junctional integrity and increased transposon activity are universal events in cancer, this study has the potential to deepen our understanding of the role of the PIWI-transposon pathway in somatic tumorigenesis. Citation Format: Alyssa Risner, Joyce Nair-Menon, Vamsi Gangaraju, Antonis Kourtidis. Cadherin complexes recruit PIWIL2 to suppress pro-tumorigenic transformation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5818.