The evolutionarily conserved family of proteins called caspases are the main factors mediating the orchestrated programme of cell suicide known as apoptosis. Since this protein family was associated with this essential biological function, the majority of scientific efforts were focused towards understanding their molecular activation and function during cell death. However, an emerging body of evidence has highlighted a repertoire of non-lethal roles within a large variety of cell types, including stem cells. Here we intend to provide a comprehensive overview of the key role of caspases as regulators of stem cell properties. Finally, we briefly discuss the possible pathological consequences of caspase malfunction in stem cells, and the therapeutic potential of caspase regulation applied to this context.

The caspase-mediated regulation of many cellular processes, including apoptosis, justifies the substantial interest in understanding all of the biological features of these enzymes. To complement functional assays, it is crucial to identify caspase-activating cells in live tissues. Our work describes novel initiator caspase reporters that, for the first time, provide direct information concerning the initial steps of the caspase activation cascade in Drosophila tissues. One of our caspase sensors capitalises on the rapid subcellular localisation change of a fluorescent marker to uncover novel cellular apoptotic events relating to the actin-mediated positioning of the nucleus before cell delamination. The other construct benefits from caspase-induced nuclear translocation of a QF transcription factor. This feature enables the genetic manipulation of caspase-activating cells and reveals the spatiotemporal patterns of initiator caspase activity. Collectively, our sensors offer experimental opportunities not available by using previous reporters and have proven useful to illuminate previously unknown aspects of caspase-dependent processes in apoptotic and non-apoptotic cellular scenarios.

Caspase malfunction in stem cells often precedes the appearance and progression of multiple types of cancer, including human colorectal cancer. However, the caspase‐dependent regulation of intestinal stem cell properties remains poorly understood. Here, we demonstrate that Dronc , the Drosophila ortholog of caspase‐ 9/2 in mammals, limits the number of intestinal progenitor cells and their entry into the enterocyte differentiation programme. Strikingly, these unexpected roles for Dronc are non‐apoptotic and have been uncovered under experimental conditions without epithelial replenishment. Supporting the non‐apoptotic nature of these functions, we show that they require the enzymatic activity of Dronc, but are largely independent of the apoptotic pathway. Alternatively, our genetic and functional data suggest that they are linked to the caspase‐mediated regulation of Notch signalling. Our findings provide novel insights into the non‐apoptotic, caspase‐dependent modulation of stem cell properties that could improve our understanding of the origin of intestinal malignancies.

Resistance to apoptosis due to caspase deregulation is considered one of the main hallmarks of cancer. However, the discovery of novel non-apoptotic caspase functions has revealed unknown intricacies about the interplay between these enzymes and tumor progression. To investigate this biological problem, we capitalized on a Drosophila tumor model with human relevance based on the simultaneous overactivation of the EGFR and the JAK/STAT signaling pathways. Our data indicate that widespread non-apoptotic activation of initiator caspases limits JNK signaling and facilitates cell fate commitment in these tumors, thus preventing the overgrowth and exacerbation of malignant features of transformed cells. Intriguingly, caspase activity also reduces the presence of macrophage-like cells with tumor-promoting properties in the tumor microenvironment. These findings assign tumor-suppressing activities to caspases independent of apoptosis, while providing molecular details to better understand the contribution of these enzymes to tumor progression.

Cellular stress plays a pivotal role in the onset of numerous human diseases. Consequently, the removal of dysfunctional cells, which undergo excessive stress-induced damage via various cell death pathways, including apoptosis, is essential for maintaining organ integrity and function. The evolutionarily conserved family of cysteine-aspartic-proteases, known as caspases, has been a key player in orchestrating apoptosis. However, recent research has unveiled the capability of these enzymes to govern fundamental cellular processes without triggering cell death. Remarkably, some of these non-lethal functions of caspases may contribute to restoring cellular equilibrium in stressed cells. This manuscript discusses how caspases can function as cellular stress managers and their potential impact on human health and disease. Additionally, it sheds light on the limitations of caspase-based therapies, given our still incomplete understanding of the biology of these enzymes, particularly in non-apoptotic contexts.