Apoptotic Fragmentation of Tricellulin

Apoptosis is a regulated process that removes cells from epithelial tissues while maintaining tissue integrity. Tricellular tight junctions (tTJs) are structures formed at three-cell junctions and are marked by the presence of tricellulin and angulins. These junctions are essential for maintaining epithelial barrier function. Prior research has shown that tTJs are dynamic and can be restructured during cellular changes like apoptosis. However, the specific role of tricellulin in this process remains unclear. This gap motivated the current study to examine how tricellulin is affected during apoptosis. No prior work had resolved whether tricellulin is cleaved by caspases or how this cleavage might influence tTJ integrity. Understanding this could clarify the molecular basis of epithelial homeostasis during cell turnover.

The aim of this study was to investigate how tricellulin behaves during apoptosis in epithelial cells. The researchers focused on whether tricellulin is degraded and how this degradation might affect tTJ structure. They tested the hypothesis that caspases are involved in tricellulin cleavage. The study also sought to identify specific cleavage sites in tricellulin and determine how these changes impact its function. By analyzing tricellulin in apoptotic cells, the authors aimed to uncover a mechanism by which tTJs disassemble during apoptosis. This could help explain how epithelial layers maintain integrity despite cell loss. The study's design was driven by the need to connect molecular events in apoptosis with broader epithelial homeostasis.

The researchers used MDCKII and RT-112 epithelial cell lines to induce apoptosis with staurosporine or camptothecin. They analyzed tricellulin levels using Western blotting and observed its localization through immunofluorescence microscopy. Caspase activity was inhibited using Z-VAD-FMK or Z-DEVD-FMK to assess the role of these enzymes in tricellulin degradation. Site-directed mutagenesis was performed to replace aspartate residues 487 and 441 with asparagine, preventing potential caspase cleavage. The interaction between tricellulin and LSR/angulin-1 was tested using deletion constructs. The study combined biochemical and genetic approaches to determine the cleavage mechanism and functional consequences of tricellulin fragmentation. These methods allowed the researchers to isolate the effects of caspase activity on tricellulin and its role in tTJ dynamics.

Apoptosis led to a time-dependent decrease in tricellulin levels, indicating its degradation. Western blotting showed the appearance of cleavage fragments as early as 2 hours post-treatment. Caspase inhibition prevented tricellulin fragmentation, confirming caspase involvement. Two cleavage sites were identified: aspartate residues 487 and 441 in the C-terminal coiled-coil domain. Mutation of these residues to asparagine blocked cleavage, supporting their role in caspase recognition. Deletion of the C-terminal region disrupted tricellulin’s binding to LSR/angulin-1, suggesting a loss of tTJ localization. These findings indicate that tricellulin is a direct substrate of caspases during apoptosis. The cleavage of tricellulin likely contributes to the disassembly of tTJs, facilitating cell extrusion from epithelial layers.

The authors propose that tricellulin is cleaved by caspases during apoptosis, which may facilitate tTJ disassembly. This cleavage occurs at specific aspartate residues in the C-terminal domain. The study suggests that caspase activity is necessary for tricellulin fragmentation. Mutation of cleavage sites or deletion of the C-terminal domain prevents tricellulin from interacting with LSR/angulin-1. This interaction is important for tricellulin’s localization to tTJs. The findings support the idea that tricellulin degradation is a key step in tTJ dissolution during apoptosis. The authors do not claim this is the only mechanism but suggest it contributes to epithelial homeostasis. These results align with the broader role of caspases in restructuring cell junctions during programmed cell death.