Caspase signaling, a conserved inductive cue for metazoan cell differentiation

Transient caspase-mediated activation of caspase-activated DNase causes DNA damage required for phagocytic macrophage differentiation

Phagocytic macrophages are crucial for innate immunity and tissue homeostasis. Most tissue-resident macrophages develop from embryonic precursors that populate every organ before birth to lifelong self-renew. However, the mechanisms for versatile macrophage differentiation remain unknown. Here, we use in vivo genetic and cell biological analysis of the Drosophila larval hematopoietic organ, the lymph gland that produces macrophages. We show that the developmentally regulated transient activation of caspase-activated DNase (CAD)-mediated DNA strand breaks in intermediate progenitors is essential for macrophage differentiation. Insulin receptor-mediated PI3K/Akt signaling regulates the apoptosis signal-regulating kinase 1 (Ask1)/c-Jun kinase (JNK) axis to control sublethal levels of caspase activation, causing DNA strand breaks during macrophage development. Furthermore, caspase activity is also required for embryonic-origin macrophage development and efficient phagocytosis. Our study provides insights into developmental signaling and CAD-mediated DNA strand breaks associated with multifunctional and heterogeneous macrophage differentiation.

Caspase-mediated nuclear pore complex trimming in cell differentiation and endoplasmic reticulum stress

During apoptosis, caspases degrade 8 out of ~30 nucleoporins to irreversibly demolish the nuclear pore complex. However, for poorly understood reasons, caspases are also activated during cell differentiation. Here, we show that sublethal activation of caspases during myogenesis results in the transient proteolysis of four peripheral Nups and one transmembrane Nup. 'Trimmed' NPCs become nuclear export-defective, and we identified in an unbiased manner several classes of cytoplasmic, plasma membrane, and mitochondrial proteins that rapidly accumulate in the nucleus. NPC trimming by non-apoptotic caspases was also observed in neurogenesis and endoplasmic reticulum stress. Our results suggest that caspases can reversibly modulate nuclear transport activity, which allows them to function as agents of cell differentiation and adaptation at sublethal levels.

Non-apoptotic caspase activation ensures the homeostasis of ovarian somatic stem cells

Current evidence has associated caspase activation with the regulation of basic cellular functions without causing apoptosis. Malfunction of non-apoptotic caspase activities may contribute to specific neurological disorders, metabolic diseases, autoimmune conditions and cancers. However, our understanding of non-apoptotic caspase functions remains limited. Here, we show that non-apoptotic caspase activation prevents the intracellular accumulation of the Patched receptor in autophagosomes and the subsequent Patched-dependent induction of autophagy in Drosophila follicular stem cells. These events ultimately sustain Hedgehog signalling and the physiological properties of ovarian somatic stem cells and their progeny under moderate thermal stress. Importantly, our key findings are partially conserved in ovarian somatic cells of human origin. These observations attribute to caspases a pro-survival role under certain cellular conditions.

Mechanisms of Myofibre Death in Muscular Dystrophies: The Emergence of the Regulated Forms of Necrosis in Myology

Myofibre necrosis is a central pathogenic process in muscular dystrophies (MD). As post-lesional regeneration cannot fully compensate for chronic myofibre loss, interstitial tissue accumulates and impairs muscle function. Muscle regeneration has been extensively studied over the last decades, however, the pathway(s) controlling muscle necrosis remains largely unknown. The recent discovery of several regulated cell death (RCD) pathways with necrotic morphology challenged the dogma of necrosis as an uncontrolled process, opening interesting perspectives for many degenerative disorders. In this review, we focus on how cell death affects myofibres in MDs, integrating the latest research in the cell death field, with specific emphasis on Duchenne muscular dystrophy, the best-known and most common hereditary MD. The role of regulated forms of necrosis in myology is still in its infancy but there is increasing evidence that necroptosis, a genetically programmed form of necrosis, is involved in muscle degenerating disorders. The existence of apoptosis in myofibre demise will be questioned, while other forms of non-apoptotic RCDs may also have a role in myonecrosis, illustrating the complexity and possibly the heterogeneity of the cell death pathways in muscle degenerating conditions.

Non-apoptotic enteroblast-specific role of the initiator caspase Dronc for development and homeostasis of the Drosophila intestine

The initiator caspase Dronc is the only CARD-domain containing caspase in Drosophila and is essential for apoptosis. Here, we report that homozygous dronc mutant adult animals are short-lived due to the presence of a poorly developed, defective and leaky intestine. Interestingly, this mutant phenotype can be significantly rescued by enteroblast-specific expression of dronc⁺ in dronc mutant animals, suggesting that proper Dronc function specifically in enteroblasts, one of four cell types in the intestine, is critical for normal development of the intestine. Furthermore, enteroblast-specific knockdown of dronc in adult intestines triggers hyperplasia and differentiation defects. These enteroblast-specific functions of Dronc do not require the apoptotic pathway and thus occur in a non-apoptotic manner. In summary, we demonstrate that an apoptotic initiator caspase has a very critical non-apoptotic function for normal development and for the control of the cell lineage in the adult midgut and therefore for proper physiology and homeostasis.

Effect of IL‑7 on Th17 cell responses in a mouse model of neutrophilic asthma

Neutrophilic asthma (NA) is characterized by neutrophil-mediated inflammation and the presence of Th17 cells. However, the mechanisms underlying Th17 cell responses in NA remain unknown. The aim of the present study was to examine the effects of interleukin (IL)-7 on Th17 cell responses in NA. A NA mouse model was sensitized by airway delivery of ovalbumin (OVA) and lipopolysaccharide and challenged with 1% OVA aerosol from day 21 for 3 consecutive days. Airway resistance was then measured to assess airway hyper-responsiveness (AHR). Cells from bronchoalveolar lavage fluid (BALF) underwent Diff-Quick and hematoxylin and eosin staining for classification. The levels of IL-17 in the BALF were determined by ELISA. The effects of IL-7 administration and STAT5 inhibition on Th17 cells were also characterized in vitro using splenic CD4⁺ T cells. Ki-67, Bcl-2 and activated caspase-3 expression in differentiated Th17 cells were analyzed by flow cytometry. The mouse model of NA was characterized by increased AHR, elevated levels of IL-17, high neutrophil counts in BALF, accumulated inflammatory cells in the lung and Th17 cell responses. IL-7 promoted the expression of Ki-67 and Bcl-2 while reducing caspase-3 expression. STAT5 inhibitor treatment decreased the levels of Ki-67 and Bcl-2, and resulted in increased expression of caspase-3. These results suggested that the IL-7/JAK/STAT5 signaling pathway may be involved in Th17 cell responses in NA.

A requirement of Polo-like kinase 1 in murine embryonic myogenesis and adult muscle regeneration

Muscle development and regeneration require delicate cell cycle regulation of embryonic myoblasts and adult muscle satellite cells (MuSCs). Through analysis of the Polo-like kinase (Plk) family cell-cycle regulators in mice, we show that Plk1's expression closely mirrors myoblast dynamics during embryonic and postnatal myogenesis. Cell-specific deletion of Plk1 in embryonic myoblasts leads to depletion of myoblasts, developmental failure and prenatal lethality. Postnatal deletion of Plk1 in MuSCs does not perturb their quiescence but depletes activated MuSCs as they enter the cell cycle, leading to regenerative failure. The Plk1-null MuSCs are arrested at the M-phase, accumulate DNA damage, and apoptose. Mechanistically, Plk1 deletion upregulates p53, and inhibition of p53 promotes survival of the Plk1-null myoblasts. Pharmacological inhibition of Plk1 similarly inhibits proliferation but promotes differentiation of myoblasts in vitro, and blocks muscle regeneration in vivo. These results reveal for the first time an indispensable role of Plk1 in developmental and regenerative myogenesis.

Stochastic cellular automata model of neurosphere growth: Roles of proliferative potential, contact inhibition, cell death, and phagocytosis

Neural stem and progenitor cells isolated from the central nervous system form, under specific culture conditions, clonal cell clusters known as neurospheres. The neurosphere assay has proven to be a powerful in vitro system to study the behavior of such cells and the development of their progeny. However, the theory of neurosphere growth has remained poorly understood. To overcome this limitation, we have, in the present paper, developed a cellular automata model, with which we examined the effects of proliferative potential, contact inhibition, cell death, and clearance of dead cells on growth rate, final size, and composition of neurospheres. Simulations based on this model indicated that the proliferative potential of the founder cell and its progenitors has a major influence on neurosphere size. On the other hand, contact inhibition of proliferation limits the final size, and reduces the growth rate, of neurospheres. The effect of this inhibition is particularly dramatic when a stem cell becomes encapsulated by differentiated or other non-proliferating cells, thereby suppressing any further mitotic division - despite the existing proliferative potential of the stem cell. Conversely, clearance of dead cells through phagocytosis is predicted to accelerate growth by reducing contact inhibition. A surprising prediction derived from our model is that cell death, while resulting in a decrease in growth rate and final size of neurospheres, increases the degree of differentiation of neurosphere cells. It is likely that the cellular automata model developed as part of the present investigation is applicable to the study of tissue growth in a wide range of systems.

Low-level laser and adipose-derived stem cells altered remodelling genes expression and improved collagen reorganization during tendon repair

Objectives

The cellular therapy using adipose‐derived mesenchymal stem cells (ASCs) aims to improve tendon healing, considering that repaired tendons often result in a less resistant tissue. Our objective was to evaluate the effects of the ASCs combination with a low‐level laser (LLL), an effective photobiostimulation for the healing processes.

Materials and methods

Rats calcaneal tendons were divided into five groups: normal (NT), transected (T), transected and ASCs (SC) or LLL (L), or with ASCs and LLL (SCL).

Results

All treated groups presented higher expression of Dcn and greater organization of collagen fibres. In comparison with T, LLL also up‐regulated Gdf5 gene expression, ASCs up‐regulated the expression of Tnmd, and the association of LLL and ASCs down‐regulated the expression of Scx. No differences were observed for the expression of Il1b, Timp2, Tgfb1, Lox, Mmp2, Mmp8 and Mmp9, neither in the quantification of hydroxyproline, TNF‐α, PCNA and in the protein level of Tnmd. A higher amount of IL‐10 was detected in SC, L and SCL compared to T, and higher amount of collagen I and III was observed in SC compared to SCL.

Conclusions

Transplanted ASCs migrated to the transected region, and all treatments altered the remodelling genes expression. The LLL was the most effective in the collagen reorganization, followed by its combination with ASCs. Further investigations are needed to elucidate the molecular mechanisms involved in the LLL and ASCs combination during initial phases of tendon repair.

Novel initiator caspase reporters uncover previously unknown features of caspase-activating cells

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.

Characterization of the caspase family in zebrafish

First discovered for their role in mediating programmed cell death and inflammatory responses, caspases have now emerged as crucial regulators of other cellular and physiological processes including cell proliferation, differentiation, migration, and survival. In the developing nervous system, for instance, the non-apoptotic functions of caspases have been shown to play critical roles in the formation of neuronal circuits by regulating axon outgrowth, guidance and pruning. How caspase activity is spatially and temporally maintained at sub-lethal levels within cells remains however poorly understood, especially in vivo. Thanks to its transparency and accessibility, the zebrafish offers the unique ability to directly visualize caspase activation in vivo. Yet, detailed information about the caspase family in zebrafish is lacking. Here, we report the identification and characterization of 19 different caspase genes in zebrafish, and show that caspases have diverse expression profiles from cleavage to larval stages, suggesting highly specialized and/or redundant functions during embryonic development.

Learning on the Fly: The Interplay between Caspases and Cancer

The ease of genetic manipulation, as well as the evolutionary conservation of gene function, has placed Drosophila melanogaster as one of the leading model organisms used to understand the implication of many proteins with disease development, including caspases and their relation to cancer. The family of proteases referred to as caspases have been studied over the years as the major regulators of apoptosis: the most common cellular mechanism involved in eliminating unwanted or defective cells, such as cancerous cells. Indeed, the evasion of the apoptotic programme resulting from caspase downregulation is considered one of the hallmarks of cancer. Recent investigations have also shown an instrumental role for caspases in non-lethal biological processes, such as cell proliferation, cell differentiation, intercellular communication, and cell migration. Importantly, malfunction of these essential biological tasks can deeply impact the initiation and progression of cancer. Here, we provide an extensive review of the literature surrounding caspase biology and its interplay with many aspects of cancer, emphasising some of the key findings obtained from Drosophila studies. We also briefly describe the therapeutic potential of caspase modulation in relation to cancer, highlighting shortcomings and hopeful promises.

Structural control of caspase-generated glutamyl-tRNA synthetase by appended noncatalytic WHEP domains

Aminoacyl-tRNA synthetases are ubiquitous, evolutionarily conserved enzymes catalyzing the conjugation of amino acids onto cognate tRNAs. During eukaryotic evolution, tRNA synthetases have been the targets of persistent structural modifications. These modifications can be additive, as in the evolutionary acquisition of noncatalytic domains, or subtractive, as in the generation of truncated variants through regulated mechanisms such as proteolytic processing, alternative splicing, or coding region polyadenylation. A unique variant is the human glutamyl-prolyl-tRNA synthetase (EPRS) consisting of two fused synthetases joined by a linker containing three copies of the WHEP domain (termed by its presence in tryptophanyl-, histidyl-, and glutamyl-prolyl-tRNA synthetases). Here, we identify site-selective proteolysis as a mechanism that severs the linkage between the EPRS synthetases in vitro and in vivo Caspase action targeted Asp-929 in the third WHEP domain, thereby separating the two synthetases. Using a neoepitope antibody directed against the newly exposed C terminus, we demonstrate EPRS cleavage at Asp-929 in vitro and in vivo Biochemical and biophysical characterizations of the N-terminally generated EPRS proteoform containing the glutamyl-tRNA synthetase and most of the linker, including two WHEP domains, combined with structural analysis by small-angle neutron scattering, revealed a role for the WHEP domains in modulating conformations of the catalytic core and GSH-S-transferase-C-terminal-like (GST-C) domain. WHEP-driven conformational rearrangement altered GST-C domain interactions and conferred distinct oligomeric states in solution. Collectively, our results reveal long-range conformational changes imposed by the WHEP domains and illustrate how noncatalytic domains can modulate the global structure of tRNA synthetases in complex eukaryotic systems.