Hematopoietic stem cell responsiveness to exogenous signals is limited by caspase-3

Loss of non-muscle myosin II Zipper leads to apoptosis-induced compensatory proliferation in Drosophila

Drosophila Non-muscle myosin II Zipper (Zip) belongs to a functionally divergent class of molecular motors that play a vital role in various cellular processes including cell adhesion, cell migration, cell protrusion, and maintenance of polarity via its cross-linking property with actin. To further determine its role in cell proliferation and apoptosis, we carried out Zip loss of function studies that led to compromised epithelial integrity in Drosophila wing imaginal discs as evident from the perturbed expression pattern of cell-cell junction proteins Cadherin, Actin, and Armadillo. Disruption of these adhesion proteins resulted in the cells undergoing apoptosis as evident from the increased level of effector caspase, cDcp-1. The induction of cell death due to the loss of function of Zip was accompanied by proliferation as apparent from increased PH3 staining. The control of apoptosis-induced compensatory proliferation lies under the caspase cascade. We carried out experiments that suggested that the apical caspase Dronc is responsible for the apoptosis-induced compensatory proliferation due to the loss of Zip function and not the effector caspase Drice/Dcp-1. Further, it was observed that Dronc leads to the subsequent activation of Jun N-terminal kinase pathway (JNK) pathway and Wingless (Wg) mitogen that diffuse to the neighboring cells and prompt them to undergo cell division. Taken together, our results suggest that loss of function of Zip leads to apoptosis-induced compensatory proliferation.

A near death experience: The secret stem cell life of caspase-3

Caspase-3 is known to play a pivotal role in mediating apoptosis, a key programmed cell death pathway. While extensive research has focused on understanding how caspase-3 is activated and functions during apoptosis, emerging evidence has revealed its significant non-apoptotic roles across various cell types, including stem cells. This review explores the critical involvement of caspase-3 in regulating stem cell properties, maintaining stem cell populations, and facilitating tissue regeneration. We also explore the potential pathological consequences of caspase-3 dysfunction in stem cells and cancer cells alongside the therapeutic opportunities of targeting caspase-3.

miRNA Expression Profile in Primary Limbal Epithelial Cells of Aniridia Patients

Purpose

This study evaluates the microRNA (miRNA) expression profile in primary limbal epithelial cells (pLECs) of patients with aniridia.

Methods

Primary human LECs were sampled and isolated from 10 patients with aniridia and 10 healthy donors. The miRNA profile was analyzed using miRNA microarrays. The biological roles of miRNA-validated target genes were delineated in silico by the enrichment analyses of the gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway. The expression of the most deregulated miRNAs was analyzed using quantitative real-time PCR (qRT-PCR).

Results

Microarray analysis revealed 10 differentially expressed miRNAs in pLECs of patients with aniridia relative to healthy controls (fold change = ≤ -2 or ≥ +2), nevertheless these were only differentially expressed using an unadjusted P value < 0.05. The qRT-PCR validation confirmed the significantly altered expression of miR-138-5p in pLECs of patients with aniridia (P = 0.005). In silico GO analysis of miR-138-5p target genes revealed the potential biological functions of miR-138-5p in regulating various cellular and molecular processes, including the positive regulation of cell motility, G1/S phase cell cycle transition, and cell migration, as well as the negative role in regulating epithelial cell differentiation. Pathway analysis highlighted the main involvement of the PI3K-Akt, Hippo, Wnt, Focal adhesion, cAMP, p53, IL-17, Jak-STAT, and MAPK-signaling pathways.

Conclusions

This study revealed miRNA expression profile in pLECs of patients with aniridia using miRNA microarray and identified miRNAs that had not been previously reported for aniridia LECs. Our study also provides functional and pathway information that can be used to predict possible mechanism of miRNA function in LECs, thereby bridging the gap in the pathogenesis of AAK studies.

Non-apoptotic caspase events and Atf3 expression underlie direct neuronal differentiation of adult neural stem cells

Neural stem cells (NSCs) generate neurons over a lifetime in adult vertebrate brains. In the adult zebrafish pallium, NSCs persist long term through balanced fate decisions. These decisions include direct neuronal conversions, i.e. delamination and neurogenesis without a division. To characterize this process, we reanalyze intravital imaging data of adult pallial NSCs, and observe shared delamination dynamics between NSCs and committed neuronal progenitors. Searching for mechanisms predicting direct NSC conversions, we build an NSC-specific genetic tracer of Caspase3/7 activation (Cas3*/Cas7*) in vivo. We show that non-apoptotic Cas3*/7* events occur in adult NSCs and are biased towards lineage termination under physiological conditions, with a predominant generation of single neurons. We further identify the transcription factor Atf3 as necessary for this bias. Finally, we show that the Cas3*/7* pathway is engaged by NSCs upon parenchymal lesion and correlates with NSCs more prone to lineage termination and neuron formation. These results provide evidence for non-apoptotic caspase events occurring in vertebrate adult NSCs and link these events with the NSC fate decision of direct conversion, which is important for long-term NSC population homeostasis.

Caspase-9 Is a Positive Regulator of Osteoblastic Cell Migration Identified by diaPASEF Proteomics

Caspase-9 is traditionally considered the initiator caspase of the intrinsic apoptotic pathway. In the past decade, however, other functions beyond initiation/execution of cell death have been described including cell type-dependent regulation of proliferation, differentiation/maturation, mitochondrial, and endosomal/lysosomal homeostasis. As previous studies revealed nonapoptotic functions of caspases in osteogenesis and bone homeostasis, this study was performed to identify proteins and pathways deregulated by knockout of caspase-9 in mouse MC3T3-E1 osteoblasts. Data-independent acquisition-parallel accumulation serial fragmentation (diaPASEF) proteomics was used to compare protein profiles of control and caspase-9 knockout cells. A total of 7669 protein groups were quantified, and 283 upregulated/141 downregulated protein groups were associated with the caspase-9 knockout phenotype. The deregulated proteins were mainly enriched for those associated with cell migration and motility and DNA replication/repair. Altered migration was confirmed in MC3T3-E1 cells with the genetic and pharmacological inhibition of caspase-9. ABHD2, an established regulator of cell migration, was identified as a possible substrate of caspase-9. We conclude that caspase-9 acts as a modulator of osteoblastic MC3T3-E1 cell migration and, therefore, may be involved in bone remodeling and fracture repair.

Exploring caspase functions in mouse models

Caspases are enzymes with protease activity. Despite being known for more than three decades, caspase investigation still yields surprising and fascinating information. Initially associated with cell death and inflammation, their functions have gradually been revealed to extend beyond, targeting pathways such as cell proliferation, migration, and differentiation. These processes are also associated with disease mechanisms, positioning caspases as potential targets for numerous pathologies including inflammatory, neurological, metabolic, or oncological conditions. While in vitro studies play a crucial role in elucidating molecular pathways, they lack the context of the body's complexity. Therefore, laboratory animals are an indispensable part of successfully understanding and applying caspase networks. This paper aims to summarize and discuss recent knowledge, understanding, and challenges in caspase knock-out mice.

Lethal and sublethal effects of programmed cell death pathways on hematopoietic stem cells

Programmed cell death is an evolutionally conserved cellular process in multicellular organisms that eliminates unnecessary or rogue cells during development, infection, and carcinogenesis. Hematopoietic stem cells (HSCs) are a rare, self-renewing, and multipotent cell population necessary for the establishment and regeneration of the hematopoietic system. Counterintuitively, key components necessary for programmed cell death induction are abundantly expressed in long-lived HSCs, which often survive myeloablative stress by engaging a prosurvival response that counteracts cell death-inducing stimuli. Although HSCs are well known for their apoptosis resistance, recent studies have revealed their unique vulnerability to certain types of programmed necrosis, such as necroptosis and ferroptosis. Moreover, emerging evidence has shown that programmed cell death pathways can be sublethally activated to cause nonlethal consequences such as innate immune response, organelle dysfunction, and mutagenesis. In this review, we summarized recent findings on how divergent cell death programs are molecularly regulated in HSCs. We then discussed potential side effects caused by sublethal activation of programmed cell death pathways on the functionality of surviving HSCs.

Caspase 8 deletion causes infection/inflammation-induced bone marrow failure and MDS-like disease in mice

Myelodysplastic syndromes (MDS) are a heterogeneous group of pre-leukemic hematopoietic disorders characterized by cytopenia in peripheral blood due to ineffective hematopoiesis and normo- or hypercellularity and morphologic dysplasia in bone marrow (BM). An inflammatory BM microenvironment and programmed cell death of hematopoietic stem/progenitor cells (HSPCs) are thought to be the major causes of ineffective hematopoiesis in MDS. Pyroptosis, apoptosis and necroptosis (collectively, PANoptosis) are observed in BM tissues of MDS patients, suggesting an important role of PANoptosis in MDS pathogenesis. Caspase 8 (Casp8) is a master regulator of PANoptosis, which is downregulated in HSPCs from most MDS patients and abnormally spliced in HSPCs from MDS patients with SRSF2 mutation. To study the role of PANoptosis in hematopoiesis, we generated inducible Casp8 knockout mice (Casp8-/-). Mx1-Cre-Casp8-/- mice died of BM failure within 10 days of polyI:C injections due to depletion of HSPCs. Rosa-ERT2Cre-Casp8-/- mice are healthy without significant changes in BM hematopoiesis within the first 1.5 months after Casp8 deletion. Such mice developed BM failure upon infection or low dose polyI:C/LPS injections due to the hypersensitivity of Casp8-/- HSPCs to infection or inflammation-induced necroptosis which can be prevented by Ripk3 deletion. However, impaired self-renewal capacity of Casp8-/- HSPCs cannot be rescued by Ripk3 deletion due to activation of Ripk1-Tbk1 signaling. Most importantly, mice transplanted with Casp8-/- BM cells developed MDS-like disease within 4 months of transplantation as demonstrated by anemia, thrombocytopenia and myelodysplasia. Our study suggests an essential role for a balance in Casp8, Ripk3-Mlkl and Ripk1-Tbk1 activities in the regulation of survival and self-renewal of HSPCs, the disruption of which induces inflammation and BM failure, resulting in MDS-like disease.

Inflammasomes as regulators of non-infectious disease

Inflammasomes are cytoplasmic organelles that stimulate inflammation upon cellular detection of infectious or non-infectious stress. While much foundational work has focused on the infection-associated aspects of inflammasome activities, recent studies have highlighted the role of inflammasomes in non-infectious cellular and organismal functions. Herein, we discuss the evolution of inflammasome components and highlight characteristics that permit inflammasome regulation of physiologic processes. We focus on emerging data that highlight the importance of inflammasome proteins in the regulation of reproduction, development, and malignancy. A framework is proposed to contextualize these findings.

The clinically relevant CHK1 inhibitor MK-8776 induces the degradation of the oncogenic protein PML-RARα and overcomes ATRA resistance in acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) is a hematological disease characterized by the expression of the oncogenic fusion protein PML-RARα. The current treatment approach for APL involves differentiation therapy using all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). However, the development of resistance to therapy, occurrence of differentiation syndrome, and relapses necessitate the exploration of new treatment options that induce differentiation of leukemic blasts with low toxicity. In this study, we investigated the cellular and molecular effects of MK-8776, a specific inhibitor of CHK1, in ATRA-resistant APL cells. Treatment of APL cells with MK-8776 resulted in a decrease in PML-RARα levels, increased expression of CD11b, and increased granulocytic activity consistent with differentiation. Interestingly, we showed that the MK-8776-induced differentiating effect resulted synergic with ATO. We found that the reduction of PML-RARα by MK-8776 was dependent on both proteasome and caspases. Specifically, both caspase-1 and caspase-3 were activated by CHK1 inhibition, with caspase-3 acting upstream of caspase-1. Activation of caspase-3 was necessary to activate caspase-1 and promote PML-RARα degradation. Transcriptomic analysis revealed significant modulation of pathways and upstream regulators involved in the inflammatory response and cell cycle control upon MK-8776 treatment. Overall, the ability of MK-8776 to induce PML-RARα degradation and stimulate differentiation of immature APL cancer cells into more mature forms recapitulates the concept of differentiation therapy. Considering the in vivo tolerability of MK-8776, it will be relevant to evaluate its potential clinical benefit in APL patients resistant to standard ATRA/ATO therapy, as well as in patients with other forms of acute leukemias.

Indoxyl sulfate impairs erythropoiesis at BFU-E stage in chronic kidney disease

Chronic kidney disease (CKD) is a global health condition characterized by a progressive deterioration of kidney function. It is associated with high serum levels of uremic toxins (UT), such as Indoxyl Sulfate (IS), which may participate in the genesis of several uremic complications. Anemia is one of the major complications in CKD patients that contribute to cardiovascular disease, increase morbi-mortality, and is associated with a deterioration of kidney failure in these patients. Our study aimed to characterize the impact of IS on CKD-related erythropoiesis. Using cellular and pre-clinical models, we studied cellular and molecular effects of IS on the growth and differentiation of erythroid cells. First, we examined the effect of clinically relevant concentrations of IS (up to 250 μM) in the UT7/EPO cell line. IS at 250 μM increased apoptosis of UT7/EPO cells at 48 h compared to the control condition. We confirmed this apoptotic effect of IS in erythropoiesis in human primary CD34⁺ cells during the later stages of erythropoiesis. Then, in IS-treated human primary CD34⁺ cells and in a (5/6 Nx) mice model, a blockage at the burst-forming unit-erythroid (BFU-E) stage of erythropoiesis was also observed. Finally, IS deregulates a number of erythropoietic related genes such as GATA-1, Erythropoietin-Receptor (EPO-R), and β-globin. Our findings suggest that IS could affect cell viability and differentiation of erythroid progenitors by altering erythropoiesis and contributing to the development of anemia in CKD.

High caspase 3 and vulnerability to dual BCL2 family inhibition define ETO2::GLIS2 pediatric leukemia

Pediatric acute myeloid leukemia expressing the ETO2::GLIS2 fusion oncogene is associated with dismal prognosis. Previous studies have shown that ETO2::GLIS2 can efficiently induce leukemia development associated with strong transcriptional changes but those amenable to pharmacological targeting remained to be identified. By studying an inducible ETO2::GLIS2 cellular model, we uncovered that de novo ETO2::GLIS2 expression in human cells led to increased CASP3 transcription, CASP3 activation, and cell death. Patient-derived ETO2::GLIS2+ leukemic cells expressed both high CASP3 and high BCL2. While BCL2 inhibition partly inhibited ETO2::GLIS2+ leukemic cell proliferation, BH3 profiling revealed that it also sensitized these cells to MCL1 inhibition indicating a functional redundancy between BCL2 and MCL1. We further show that combined inhibition of BCL2 and MCL1 is mandatory to abrogate disease progression using in vivo patient-derived xenograft models. These data reveal that a transcriptional consequence of ETO2::GLIS2 expression includes a positive regulation of the pro-apoptotic CASP3 and associates with a vulnerability to combined targeting of two BCL2 family members providing a novel therapeutic perspective for this aggressive pediatric AML subgroup.

Mitochondrial DNA in cell death and inflammation

Cytosolic DNA is recognized by the innate immune system as a potential threat. During apoptotic cell death, mitochondrial DNA (mtDNA) release activates the DNA sensor cyclic GMP-AMP synthase (cGAS) to promote a pro-inflammatory type I interferon response. Inflammation following mtDNA release during apoptotic cell death can be exploited to engage anti-tumor immunity and represents a potential avenue for cancer therapy. Additionally, various studies have described leakage of mtDNA, independent of cell death, with different underlying cues such as pathogenic infections, changes in mtDNA packaging, mtDNA stress or reduced mitochondrial clearance. The interferon response in these scenarios can be beneficial but also potentially disadvantageous, as suggested by a variety of disease phenotypes. In this review, we discuss mechanisms underlying mtDNA release governed by cell death pathways and summarize release mechanisms independent of cell death. We further highlight the similarities and differences in mtDNA release pathways, outlining gaps in our knowledge and questions for further research. Together, a deeper understanding of how and when mtDNA is released may enable the development of drugs to specifically target or inhibit mtDNA release in different disease settings.

Sub-lethal signals in the mitochondrial apoptosis apparatus: pernicious by-product or physiological event?

One of the tasks of mitochondria is the rule over life and death: when the outer membrane is permeabilized, the release of intermembrane space proteins causes cell death by apoptosis. For a long time, this mitochondrial outer membrane permeabilization (MOMP) has been accepted as the famous step from which no cell returns. Recent results have however shown that this quite plainly does not have to be the case. A cell can also undergo only a little MOMP, and it can efficiently repair damage it has incurred in the process. There is no doubt now that such low-scale permeabilization occurs. A major unclarified issue is the biological relevance. Is small-scale mitochondrial permeabilization an accident, a leakiness of the apoptosis apparatus, perhaps during restructuring of the mitochondrial network? Is it attempted suicide, where cell death by apoptosis is the real goal but the stimulus failed to reach the threshold? Or, more boldly, is there a true biological meaning behind the event of the release of low amounts of mitochondrial components? We will here explore this last possibility, which we believe is on one hand appealing, on the other hand plausible and supported by some evidence. Recent data are consistent with the view that sub-lethal signals in the mitochondrial apoptosis pathway can drive inflammation, the first step of an immune reaction. The apoptosis apparatus is almost notoriously easy to trigger. Sub-lethal signals may be even easier to set off. We suggest that the apoptosis apparatus is used in this way to sound the call when the first human cell is infected by a pathogen.

Paradoxical roles of caspase-3 in regulating cell survival, proliferation, and tumorigenesis

Caspase-3 is a widely expressed member of a conserved family of proteins, generally recognized for their activated proteolytic roles in the execution of apoptosis in cells responding to specific extrinsic or intrinsic inducers of this mode of cell death. However, accumulating evidence indicates that caspase-3 also plays key roles in regulating the growth and homeostatic maintenance of both normal and malignant cells and tissues in multicellular organisms. Given that yeast possess an ancestral caspase-like gene suggests that the caspase-3 protein may have acquired different functions later during evolution to better meet the needs of more complex multicellular organisms, but without necessarily losing all of the functions of its ancestral yeast precursor. This review provides an update on what has been learned about these interesting dichotomous roles of caspase-3, their evolution, and their potential relevance to malignant as well as normal cell biology.

Caspase-3/NLRP3 signaling in the mesenchymal stromal niche regulates myeloid-biased hematopoiesis

Background

The fate of hematopoietic stem cells (HSCs) is determined by a complex regulatory network that includes both intrinsic and extrinsic signals. In the past decades, many intrinsic key molecules of HSCs have been shown to control hematopoiesis homeostasis. Non-hematopoietic niche cells also contribute to the self-renewal, quiescence, and differentiation of HSCs. Mesenchymal stromal cells (MSCs) have been identified as important components of the niche. However, the regulatory role of MSCs in hematopoiesis has not been fully understood.

Methods

Caspase-3 and NLRP3 gene knockout mice were generated respectively, and hematopoietic development was evaluated in the peripheral circulation and bone marrow by flow cytometry, colony formation assay, and bone marrow transplantation. Bone-associated MSCs (BA-MSCs) were then isolated from gene knockout mice, and the effect of Caspase-3/NLRP3 deficient BA-MSCs on hematopoiesis regulation was explored in vivo and ex vivo.

Results

We report that Caspase-3 deficient mice exhibit increased myelopoiesis and an aberrant HSC pool. Ablation of Caspase-3 in BA-MSCs regulates myeloid lineage expansion by altering the expression of hematopoietic retention cytokines, including SCF and CXCL12. Interestingly, NLRP3 gene knockout mice share phenotypic similarities with Caspase-3 deficient mice. Additionally, we found that NLRP3 may play a role in myeloid development by affecting the cell cycle and apoptosis of hematopoietic progenitors.

Conclusions

Our data demonstrate that the Caspase-3/NLRP3 signaling functions as an important regulator in physiological hematopoiesis, which provides new insights regarding niche signals that influence hematopoiesis regulation in the bone marrow.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02640-y.

Caspase-3: Structure, function, and biotechnological aspects

Caspase-3, a cysteine-aspartic acid protease, has recently attracted much attention because of its incredible roles in tissue differentiation, regeneration, and neural development. This enzyme is a key zymogen in cell apoptosis and is not activated until it is cleaved by initiator caspases during apoptotic flux. Since caspase-3 has represented valuable capabilities in the field of medical research, biotechnological aspects of this enzyme, including the production of recombinant type, protein engineering, and designing delivery systems, have been considered as emerging therapeutic strategies in treating the apoptosis-related disorders. To date, several advances have been made in the therapeutic use of caspase-3 in the management of some diseases such as cancers, heart failure, and neurodegenerative disorders. In the current review, we intend to discuss the caspase-3's structure, functions, therapeutic applications, as well as its molecular cloning, protein engineering, and relevant delivery systems.

Apoptosis - Fueling the oncogenic fire

Apoptosis, the most extensively studied form of programmed cell death, is essential for organismal homeostasis. Apoptotic cell death has widely been reported as a tumor suppressor mechanism. However, recent studies have shown that apoptosis exerts noncanonical functions and may paradoxically promote tumor growth and metastasis. The hijacking of apoptosis by cancer cells may arise at different levels, either via the interaction of apoptotic cells with their local or distant microenvironment, or through the abnormal pro-oncogenic roles of the main apoptosis effectors, namely caspases and mitochondria, particularly upon failed apoptosis. In this review, we highlight some of the recently described mechanisms by which apoptosis and these effectors may promote cancer aggressiveness. We believe that a better understanding of the noncanonical roles of apoptosis may be crucial for developing more efficient cancer therapies.

An Early Myelosuppression in the Acute Mouse Sepsis Is Partly Outcome-Dependent

Adult hematopoietic stem and progenitor cells (HSPCs) respond to bacterial infections by expansion to myeloid cells. Sepsis impairs this process by suppressing differentiation of stem cells subsequently contributing to an ineffective immune response. Whether the magnitude of HSPCs impairment in sepsis is severity-dependent remains unknown. This study investigated dynamics of the HSPC immune-inflammatory response in the bone marrow, splenic, and blood compartments in moribund and surviving septic mice. The 12-week-old outbred CD-1 female mice (n=65) were subjected to a cecal ligation and puncture (CLP) sepsis, treated with antibiotics and fluid resuscitation, and stratified into predicted-to-die (P-DIE) and predicted-to-survive (P-SUR) cohorts for analysis. CLP strongly reduced the common myeloid and multipotent progenitors, short- and long-term hematopoietic stem cell (HSC) counts in the bone marrow; lineage⁻ckit⁺Sca-1⁺ and short-term HSC suppression was greater in P-DIE versus P-SUR mice. A profound depletion of the common myeloid progenitors occurred in the blood (by 75%) and spleen (by 77%) of P-DIE. In P-SUR, most common circulating HSPCs subpopulations recovered to baseline by 72 h post-CLP. Analysis of activated caspase-1/-3/-7 revealed an increased apoptotic (by 30%) but not pyroptotic signaling in the bone marrow HSCs of P-DIE mice. The bone marrow from P-DIE mice revealed spikes of IL-6 (by 5-fold), CXCL1/KC (15-fold), CCL3/MIP-1α (1.7-fold), and CCL2/MCP-1 (2.8-fold) versus P-SUR and control (TNF, IFN-γ, IL-1β, -5, -10 remained unaltered). Summarizing, our findings demonstrate that an early sepsis-induced impairment of myelopoiesis is strongly outcome-dependent but varies among compartments. It is suggestive that the HSCPC loss is at least partly due to an increased apoptosis but not pyroptosis.

Mutagenic Consequences of Sublethal Cell Death Signaling

Many human cancers exhibit defects in key DNA damage response elements that can render tumors insensitive to the cell death-promoting properties of DNA-damaging therapies. Using agents that directly induce apoptosis by targeting apoptotic components, rather than relying on DNA damage to indirectly stimulate apoptosis of cancer cells, may overcome classical blocks exploited by cancer cells to evade apoptotic cell death. However, there is increasing evidence that cells surviving sublethal exposure to classical apoptotic signaling may recover with newly acquired genomic changes which may have oncogenic potential, and so could theoretically spur the development of subsequent cancers in cured patients. Encouragingly, cells surviving sublethal necroptotic signaling did not acquire mutations, suggesting that necroptosis-inducing anti-cancer drugs may be less likely to trigger therapy-related cancers. We are yet to develop effective direct inducers of other cell death pathways, and as such, data regarding the consequences of cells surviving sublethal stimulation of those pathways are still emerging. This review details the currently known mutagenic consequences of cells surviving different cell death signaling pathways, with implications for potential oncogenic transformation. Understanding the mechanisms of mutagenesis associated (or not) with various cell death pathways will guide us in the development of future therapeutics to minimize therapy-related side effects associated with DNA damage.

Non-lethal message from the Holy Land: The first international conference on nonapoptotic roles of apoptotic proteins

Apoptosis is a major form of programmed cell death (PCD) that eliminates unnecessary and potentially dangerous cells in all metazoan organisms, thus ensuring tissue homeostasis and many developmental processes. Accordingly, defects in the activation of the apoptotic pathway often pave the way to disease. After several decades of intensive research, the molecular details controlling the apoptosis program have largely been unraveled, as well as the regulatory mechanisms of caspase activation during apoptosis. Nevertheless, an ever-growing list of studies is suggesting the essential role of caspases and other apoptotic proteins in ensuring nonlethal cellular functions during normal development, tissue repair, and regeneration. Moreover, if deregulated, these novel nonapoptotic functions can also instigate diseases. The difficulty of identifying and manipulating the caspase-dependent nonlethal cellular processes (CDPs), as well as the nonlethal functions of other cell death proteins (NLF-CDPs), meant that CDPs and NLF-CDPs have been only curiosities within the apoptotic field; however, the recent technical advancements and the latest biological findings are assigning an unanticipated biological significance to these nonapoptotic functions. Here, we summarize the various talks presented in the first international conference fully dedicated to discuss CDPs and NFL-CDPs and named 'The Batsheva de Rothschild Seminar on Non-Apoptotic Roles of Apoptotic Proteins'. The conference was organized between September 22, 2019, and 25, 2019, by Eli Arama (Weizmann Institute of Science), Luis Alberto Baena-Lopez (University of Oxford), and Howard O. Fearnhead (NUI Galway) at the Weizmann Institute of Science in Israel, and hosted a large international group of researchers.

GSDME maintains hematopoietic stem cells by balancing pyroptosis and apoptosis

GSDME contains a pore-forming domain at its N-terminal region to execute pyroptosis. Our previous study has reported that forced expression of Gsdme impairs the reconstitution capacity of hematopoietic stem cells (HSCs). While, how GSDME-mediated pyroptosis regulates HSCs remains unknown. Here, we show that hematopoietic stem and progenitor cells are capable to undergo pyroptosis in response to cisplatin treatment and GSDME is one of the genes mediating such process. Gsdme -/- mice revealed no difference in the steady state of blood system while Gsdme -/- HSCs exhibited compromised reconstitution capacity due to increased apoptosis. Briefly, this study reveals that GSDME modulates HSC function by coordinating pyroptosis and apoptosis.

Evaluation of Caspase-3 Activity During Apoptosis with Fluorescence Lifetime-Based Cytometry Measurements and Phasor Analyses

Caspase-3 is a well-described protease with many roles that impact the fate of a cell. During apoptosis, caspase-3 acts as an executioner caspase with important proteolytic functions that lead to the final stages of programmed cell death. Owing to this key role, caspase-3 is exploited intracellularly as a target of control of apoptosis for therapeutic outcomes. Yet the activation of caspase-3 during apoptosis is challenged by other roles and functions (e.g., paracrine signaling). This brief report presents a way to track caspase-3 levels using a flow cytometer that measures excited state fluorescence lifetimes and a signal processing approach that leads to a graphical phasor-based interpretation. An established Förster resonance energy transfer (FRET) bioprobe was used for this test; the connected donor and acceptor fluorophore is cleavable by caspase-3 during apoptosis induction. With the cell-by-cell decay kinetic data and phasor analyses we generate a caspase activation trajectory, which is used to interpret activation throughout apoptosis. When lifetime-based cytometry is combined with a FRET bioprobe and phasor analyses, enzyme activation can be simplified and quantified with phase and modulation data. We envision extrapolating this approach to high content screening, and reinforce the power of phasor approaches with cytometric data. Analyses such as these can be used to cluster cells by their phase and modulation "lifetime fingerprint" when the intracellular fluorescent probe is utilized as a sensor of enzyme activity. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC on behalf of International Society for Advancement of Cytometry.

Should we reconsider the apoptosis as a strategic player in tissue regeneration?

Apoptosis plays a central role in organs development and homeostasis. Impaired regulation of this process is often associated with the onset of several human diseases, such as developmental disorders and cancer. The last scientific investigations have discovered interesting connections between apoptosis, stem cells, tissue regeneration and cancer. The role of "programmed cell death" in stem cells and tissue engineering is extremely promising; in fact, it holds great potential for regenerative purposes. However, several questions still remain unsolved: do we really know all the main molecular actors able to switch ON/OFF the apoptosis? Is it possible to modulate these players, to obtain a predictable regeneration of tissues and organs? But primarily: should we reconsider the apoptosis as a strategic player in tissue regeneration? In this topical review, we have carefully examined the most recent discoveries about the role of apoptosis in stem cells and, specifically, in mesenchymal stem cells. The pivotal molecules involved in the activation and inhibition of the apoptotic pathways will be carefully described, with the aim to shed an overall light on the complex scenario of stem cell life and death, and on a novel strategy for tissue regeneration.

Efficient apoptosis requires feedback amplification of upstream apoptotic signals by effector caspase-3 or -7

Apoptosis is a complex multi-step process driven by caspase-dependent proteolytic cleavage cascades. Dysregulation of apoptosis promotes tumorigenesis and limits the efficacy of chemotherapy. To assess the complex interactions among caspases during apoptosis, we disrupted caspase-8, -9, -3, -7, or -6 and combinations thereof, using CRISPR-based genome editing in living human leukemia cells. While loss of apical initiator caspase-8 or -9 partially blocked extrinsic or intrinsic apoptosis, respectively, only combined loss of caspase-3 and -7 fully inhibited both apoptotic pathways, with no discernible effect of caspase-6 deficiency alone or in combination. Caspase-3/7 double knockout cells exhibited almost complete inhibition of caspase-8 or -9 activation. Furthermore, deletion of caspase-3 and -7 decreased mitochondrial depolarization and cytochrome c release upon apoptosis activation. Thus, activation of effector caspase-3 or -7 sets off explosive feedback amplification of upstream apoptotic events, which is a key feature of apoptotic signaling essential for efficient apoptotic cell death.

Placental cell death patterns exhibit differences throughout gestation in two strains of laboratory mice

Cell death is an essential physiological process required for the proper development and function of the human placenta. Although the mouse is a commonly used animal model for development studies, little is known about the extent and distribution of cell death in the mouse placenta throughout development and its physiological relevance. In the present study, we report the results of a systematic and quantitative assessment of cell death patterns in the placentae of two strains of laboratory mice commonly used for developmental studies-ICR and C57Bl/6. TUNEL staining revealed that ICR and C57Bl/6 placentae exhibited similar cell death patterns to those reported in human placentae during pregnancy, with comparatively infrequent death observed during early gestation, which increased and became more organized towards term. Interestingly, when comparing strain differences, increased cell death was observed in almost all regions of the inbred C57Bl/6 placentae compared to the outbred ICR strain. Finally, since Bcl-2 ovarian killer (Bok) has been reported to be a key player in human placental cell death, we examined its expression in murine placentae throughout gestation. Bok protein expression was observed in all placental regions and increased towards term in both strains. The results of this study indicate that although strain-specific differences in placental cell death exist, the overall rates and patterns of cell death during murine placentation parallel those previously described in humans. Thus, the murine placenta is a useful model to investigate molecular pathways involved in cell death signaling during human placentation.

Effects of 5-FU and anti-EGFR antibody in combination with ASA on the spherical culture system of HCT116 and HT29 colorectal cancer cell lines

The aim of this study was to examine the effects of 5‑fluorouracil (5‑FU), anti‑epidermal growth factor receptor (EGFR) antibody and aspirin (ASA) on the characteristics of two CRC cell lines, HCT116 and HT29, maintained in a spherical culture system. We observed that the morphology of both the HCT116 and HT29 cell‑derived spheres was significantly impaired and the size of the colonospheres was markedly reduced following treatment with the aforementioned three drugs. In contrast to adherent cultures, the spherical cultures were more resistant to the tested drugs, as was reflected by their capacity to re‑create the colonospheres when sustained in serum‑free medium. Flow cytometric analysis of the drug‑treated HCT116 cell‑derived spheres revealed changes in the fraction of cells expressing markers of cancer stem cells (CSCs), whereas the CSC phenotype of HT29 cell‑derived colonospheres was affected to a lesser extent. All reagents enhanced the percentage of non‑viable cells in the colonospheres despite the diminished fraction of active caspase‑3‑positive cells following treatment of the HT29 cell‑derived spheres with anti‑EGFR antibody. Increased autophagy, assessed by acridine orange staining, was noted following the incubation of the HT29‑colonospheres with ASA and 5‑FU in comparison to the control. Notably, the percentage of cyclooxygenase (COX)‑2‑positive cells was not affected by ASA, although its activity was markedly elevated in the colonospheres incubated with anti‑EGFR antibody. On the whole, the findings of this study indicate that all the tested drugs were involved in different cellular processes, which suggests that they should be considered for the combined therapeutic treatment of CRC, particularly for targeting the population of CSC‑like cells. Thus, cancer cell‑derived spheres may be used as a preferable model for in vitro anticancer drug testing.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

Proteinase 3 Limits the Number of Hematopoietic Stem and Progenitor Cells in Murine Bone Marrow

Hematopoietic stem and progenitor cells (HSPCs) undergo self-renewal and differentiation to guarantee a constant supply of short-lived blood cells. Both intrinsic and extrinsic factors determine HSPC fate, but the underlying mechanisms remain elusive. Here, we report that Proteinase 3 (PR3), a serine protease mainly confined to granulocytes, is also expressed in HSPCs. PR3 deficiency intrinsically suppressed cleavage and activation of caspase-3, leading to expansion of the bone marrow (BM) HSPC population due to decreased apoptosis. PR3-deficient HSPCs outcompete the long-term reconstitution potential of wild-type counterparts. Collectively, our results establish PR3 as a physiological regulator of HSPC numbers. PR3 inhibition is a potential therapeutic target to accelerate and increase the efficiency of BM reconstitution during transplantation.

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Proteinase 3 (PR3) is expressed in hematopoietic stem and progenitor cells (HSPCs)

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Deficiency of PR3 leads to expansion of HSPCs in murine bone marrow

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PR3 regulates spontaneous HSPC apoptosis by cleaving and activating caspase-3

Non-apoptotic Caspase regulation of stem cell properties

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.

Low-Dose Arsenic Trioxide Modulates the Differentiation of Mouse Embryonic Stem Cells

Arsenic (As) is a well-known environmental pollutant, while arsenic trioxide (ATO) has been proven to be an effective treatment for acute promyelocytic leukemia, however, the mechanism underlying its dual effects is not fully understood. Embryonic stem cells (ESCs) exhibit properties of stemness and serve as a popular model to investigate epigenetic modifiers including environmental pollutants. Herein, the effects of low-dose ATO on differentiation were evaluated in vitro using a mouse ESCs (mESCs) cell line, CGR8. Cells treated with 0.2-0.5 μM ATO for 3-4 days had slight inhibition of proliferation with elevation of apoptosis, but obvious alterations of differentiation by morphological checking and alkaline phosphatase (AP) staining. Moreover, ATO exposure significantly decreased the mRNA expression of the stemness maintenance genes including Oct4, Nanog, and Rex-1 ( P < 0.01), whereas obviously increased some tissue-specific differentiation marker genes such as Gata4, Gata-6, AFP, and IHH. These alterations were consistent with the differentiation phenotype induced by retinoic acid (RA) and the expression patterns of distinct pluripotency markers such as SSEA-1 and Oct4. Furthermore, low-dose ATO led to a quantitative increase in Caspase 3 (CASP3) activation and subsequent cleavage of Nanog around 27 kDa, which corresponded with the mouse Nanog cleaved by CASP3 in a tube cleavage assay. Taken together, we suggest that low-dose ATO exposure will induce differentiation, other than apoptosis, of ESCs, such effects might be tuned partially by ATO-induced CASP3 activation and Nanog cleavage coupling with other differentiation related genes involved. The present findings provide a preliminary action mechanism of arsenic on the cell fate determination.

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.

Apoptotic Caspases: Multiple or Mistaken Identities?

The mitochondrial caspase cascade was originally thought to be required for apoptotic death driven by Bak/Bax-mediated intrinsic apoptosis. It has also been ascribed several 'non-apoptotic' functions, including differentiation, proliferation, and cellular reprogramming. Recent work has demonstrated that, during apoptosis, the caspase cascade suppresses damage-associated molecular pattern (DAMP)-initiated production of cytokines such as type I interferon by the dying cell. The caspase cascade is not required for death to occur; instead, it shapes the immunogenic properties of the apoptotic cell. This raises questions about the role of apoptotic caspases in regulating DAMP signaling more generally, puts a new perspective on their non-apoptotic functions, and suggests that pharmacological caspase inhibitors might find new applications as antiviral or anticancer agents.

Murine hematopoietic stem cell reconstitution potential is maintained by osteopontin during aging

In adult mammals, hematopoietic stem cells (HSCs) reside in the bone marrow and are in part regulated by the bone marrow microenvironment, called the stem cell niche. We have previously identified the bone marrow morphogen osteopontin (OPN), which is abundantly present in the bone marrow extracellular matrix, as a negative regulator of the size of the HSC pool under physiological conditions. Here, we study the impact of OPN on HSC function during aging using an OPN-knockout mouse model. We show that during aging OPN deficiency is associated with an increase in lymphocytes and a decline in erythrocytes in peripheral blood. In a bone marrow transplantation setting, aged OPN-deficient stem cells show reduced reconstitution ability likely due to insufficient differentiation of HSCs into more mature cells. In serial bone marrow transplantation, aged OPN^−/− bone marrow cells fail to adequately reconstitute red blood cells and platelets, resulting in severe anemia and thrombocytopenia as well as premature deaths of recipient mice. Thus, OPN has different effects on HSCs in aged and young animals and is particularly important to maintain stem cell function in aging mice.

Embryonic Intra-Aortic Clusters Undergo Myeloid Differentiation Mediated by Mesonephros-Derived CSF1 in Mouse

The aorta-gonad-mesonephros (AGM) region contains intra-aortic clusters (IACs) thought to have acquired hematopoietic stem cell (HSC) potential in vertebrate embryos. To assess extrinsic regulation of IACs in the AGM region, we employed mouse embryos harboring a Sall1-GFP reporter gene, which allows identification of mesonephros cells based on GFP expression. Analysis of AGM region tissue sections confirmed mesonephros GFP expression. Mesonephric cells sorted at E10.5 expressed mRNA encoding Csf1, a hematopoietic cytokine, and corresponding protein, based on real-time PCR and immunocytochemistry, respectively. Further analysis indicated that some IACs express the CSF1 receptor, CSF1R. Expression of Cebpa and Irf8 mRNAs was higher in CSF1R-positive IACs, whereas that of Cebpε and Gfi1 mRNAs was lower relative to CSF1R-negative IACs, suggesting that CSF1/CSF1R signaling functions in IAC myeloid differentiation by modulating expression of these transcription factors. Colony formation assays using CSF1R-positive IACs revealed increased numbers of myeloid colonies in the presence of CSF1. Analysis using an intra-cellular signaling array indicated the greatest fold increase of Cleaved Caspase-3 in AGM cells in the presence of CSF1. Immunohistochemistry revealed that Cleaved Caspase-3 is primarily expressed in IACs in the AGM region, and incubation of IACs with CSF1 up-regulated Cleaved Caspase-3. Overall, our findings suggest that CSF1 secreted from mesonephros accelerates IAC myeloid differentiation in the AGM region, possibly via Caspase-3 cleavage.

Sub-lethal oxidative stress induces lysosome biogenesis via a lysosomal membrane permeabilization-cathepsin-caspase 3-transcription factor EB-dependent pathway

Here we provide evidence to link sub-lethal oxidative stress to lysosome biogenesis. Exposure of cells to sub-lethal concentrations of exogenously added hydrogen peroxide resulted in cytosol to nuclear translocation of the Transcription Factor EB (TFEB), the master controller of lysosome biogenesis and function. Nuclear translocation of TFEB was dependent upon the activation of a cathepsin-caspase 3 signaling pathway, downstream of lysosomal membrane permeabilization and accompanied by a significant increase in lysosome numbers as well as induction of TFEB-dependent lysosome-associated genes expression such as Ctsl, Lamp2 and its spliced variant Lamp2a, Neu1and Ctsb and Sqstm1 and Atg9b. The effects of sub-lethal oxidative stress on lysosomal gene expression and biogenesis were rescued upon gene silencing of caspase 3 and TFEB. Notably, caspase 3 activation was not associated with phenotypic hallmarks of apoptosis, evidenced by the absence of caspase 3 substrate cleavage, such as PARP, Lamin A/C or gelsolin. Taken together, these data demonstrate for the first time an unexpected and non-canonical role of a cathepsin-caspase 3 axis in the nuclear translocation of TFEB leading to lysosome biogenesis under conditions of sub-lethal oxidative stress.

Caspase-dependent non-apoptotic processes in development

Caspases are at the core of executing apoptosis by orchestrating cellular destruction with proteolytic cascades. Caspase-mediated proteolysis also controls diverse nonlethal cellular activities such as proliferation, differentiation, cell fate decision, and cytoskeletal reorganization. During the last decade or so, genetic studies of Drosophila have contributed to our understanding of the in vivo mechanism of the non-apoptotic cellular responses in developmental contexts. Furthermore, recent studies using C. elegans suggest that apoptotic signaling may play unexpected roles, which influence ageing and normal development at the organism level. In this review, we describe how the caspase activity is elaborately controlled during vital cellular processes at the level of subcellular localization, the duration and timing to avoid full apoptotic consequences, and also discuss the novel roles of non-apoptotic caspase signaling in adult homeostasis and physiology.

Caspase-3 controls AML1-ETO-driven leukemogenesis via autophagy modulation in a ULK1-dependent manner

AML1-ETO (AE), a fusion oncoprotein generated by t(8;21), can trigger acute myeloid leukemia (AML) in collaboration with mutations including c-Kit, ASXL1/2, FLT3, N-RAS, and K-RAS. Caspase-3, a key executor among its family, plays multiple roles in cellular processes, including hematopoietic development and leukemia progression. Caspase-3 was revealed to directly cleave AE in vitro, suggesting that AE may accumulate in a Caspase-3-compromised background and thereby accelerate leukemogenesis. Therefore, we developed a Caspase-3 knockout genetic mouse model of AML and found that loss of Caspase-3 actually delayed AML1-ETO9a (AE9a)-driven leukemogenesis, indicating that Caspase-3 may play distinct roles in the initiation and/or progression of AML. We report here that loss of Caspase-3 triggers a conserved, adaptive mechanism, namely autophagy (or macroautophagy), which acts to limit AE9a-driven leukemia. Furthermore, we identify ULK1 as a novel substrate of Caspase-3 and show that upregulation of ULK1 drives autophagy initiation in leukemia cells and that inhibition of ULK1 can rescue the phenotype induced by Caspase-3 deletion in vitro and in vivo. Collectively, these data highlight Caspase-3 as an important regulator of autophagy in AML and demonstrate that the balance and selectivity between its substrates can dictate the pace of disease.

Evolution of caspase-mediated cell death and differentiation: twins separated at birth

The phenotypic and biochemical similarities between caspase-mediated apoptosis and cellular differentiation are striking. They include such diverse phenomenon as mitochondrial membrane perturbations, cytoskeletal rearrangements and DNA fragmentation. The parallels between the two disparate processes suggest some common ancestry and highlight the paradoxical nature of the death-centric view of caspases. That is, what is the driving selective pressure that sustains death-inducing proteins throughout eukaryotic evolution? Plausibly, caspase function may be rooted in a primordial non-death function, such as cell differentiation, and was co-opted for its role in programmed cell death. This review will delve into the links between caspase-mediated apoptosis and cell differentiation and examine the distinguishing features of these events. More critically, we chronicle the evolutionary origins of caspases and propose that caspases may have held an ancient role in mediating the fidelity of cell division/differentiation through its effects on proteostasis and protein quality control.

T11TS repress gliomagenic apoptosis of bone marrow hematopoietic stem cells

Combating gliomagenic global immunosuppression is one of the emerging key for improving prognosis in malignant glioma. Apoptosis plays a pivotal role within the adult hematopoietic system particularly in regulating the cells of immune system. Gliomagenic regulation of apoptotic mediators within bone marrow milieu has not been elucidated. We previously demonstrated that administration of membrane glycopeptides T11 target structure (T11TS) not only rejuvenate bone marrow hematopoietic stem cells (BMHSCs) from glioma mediated hibernation by inhibiting gliomagenic overexpression of Ang-1/Tie-2 but also stimulate glioma mediated diminution of expression CD34, c-kit, and Sca-1 markers. In the present study, we investigated the impact of glioma on apoptotic signaling cascades of BMHSCs and consequences following T11TS therapy. Bone marrow smear and Annexin V staining confirm gliomagenic acceleration of apoptotic fate of BMHSCs whereas T11TS treatment in glioma-bearing rats disrupted apoptosis of BMHSCs. Flowcytometry, immunoblotting, and immunofluorescence imagining results revealed multi potent T11TS not only significantly downregulates gliomagenic overexpression of Fas, Fas L, Bid, and caspase-8, the pro-apoptotic extrinsic mediators but also strongly inhibits cytosolic release of cytochrome-c, Apf-1, and Bax to deactivate gliomagenic caspase-9, 3 the key intrinsic apoptotic mediators followed by up modulation of anti-apoptotic Bcl-2 in glioma associated HSCs. T11TS is also able to diminish the perforin-granzyme B mediated apoptotic verdict of BMHSCs during gliomagenesis. The anti-apoptotic action of T11TS on glioma associated BMHSCs provide a crucial insight into how T11TS exerts its immunomodulatory action against glioma mediated immune devastation.

Non-apoptotic functions of caspases in myeloid cell differentiation

Subtle caspase activation is associated with the differentiation of several myeloid lineages. A tightly orchestrated dance between caspase-3 activation and the chaperone HSP70 that migrates to the nucleus to protect the master regulator GATA-1 from cleavage transiently occurs in basophilic erythroblasts and may prepare nucleus and organelle expel that occurs at the terminal phase of erythroid differentiation. A spatially restricted activation of caspase-3 occurs in maturing megakaryocytes to promote proplatelet maturation and platelet shedding in the bloodstream. In a situation of acute platelet need, caspase-3 could be activated in response to IL-1α and promote megakaryocyte rupture. In peripheral blood monocytes, colony-stimulating factor-1 provokes the formation of a molecular platform in which caspase-8 is activated, which downregulates nuclear factor-kappa B (NF-κB) activity and activates downstream caspases whose target fragments such as those generated by nucleophosmin (NPM1) cleavage contribute to the generation of resting macrophages. Human monocytes secrete mature IL-1β in response to lipopolysaccharide through an alternative inflammasome activation that involves caspase-8, a pathway that does not lead to cell death. Finally, active caspase-3 is part of the proteases contained in secretory granules of mast cells. Many questions remain on how these proteases are activated in myeloid cell lineages, which target proteins are cleaved, whereas other are protected from proteolysis, the precise role of cleaved proteins in cell differentiation and functions, and the link between these non-apoptotic functions of caspases and the death of these diverse cell types. Better understanding of these functions may generate therapeutic strategies to control cytopenias or modulate myeloid cell functions in various pathological situations.

Knocking down of heat-shock protein 27 directs differentiation of functional glutamatergic neurons from placenta-derived multipotent cells

This study presents human placenta-derived multipotent cells (PDMCs) as a source from which functional glutamatergic neurons can be derived. We found that the small heat-shock protein 27 (HSP27) was downregulated during the neuronal differentiation process. The in vivo temporal and spatial profiles of HSP27 expression were determined and showed inverted distributions with neuronal proteins during mouse embryonic development. Overexpression of HSP27 in stem cells led to the arrest of neuronal differentiation; however, the knockdown of HSP27 yielded a substantially enhanced ability of PDMCs to differentiate into neurons. These neurons formed synaptic networks and showed positive staining for multiple neuronal markers. Additionally, cellular phenomena including the absence of apoptosis and rare proliferation in HSP27-silenced PDMCs, combined with molecular events such as cleaved caspase-3 and the loss of stemness with cleaved Nanog, indicated that HSP27 is located upstream of neuronal differentiation and constrains that process. Furthermore, the induced neurons showed increasing intracellular calcium concentrations upon glutamate treatment. These differentiated cells co-expressed the N-methyl-D-aspartate receptor, vesicular glutamate transporter, and synaptosomal-associated protein 25 but did not show expression of tyrosine hydroxylase, choline acetyltransferase or glutamate decarboxylase 67. Therefore, we concluded that HSP27-silenced PDMCs differentiated into neurons possessing the characteristics of functional glutamatergic neurons.

Normal hematopoiesis and lack of β-catenin activation in osteoblasts of patients and mice harboring Lrp5 gain-of-function mutations

Osteoblasts are emerging regulators of myeloid malignancies since genetic alterations in them, such as constitutive activation of β-catenin, instigate their appearance. The LDL receptor-related protein 5 (LRP5), initially proposed to be a co-receptor for Wnt proteins, in fact favors bone formation by suppressing gut-serotonin synthesis. This function of Lrp5 occurring in the gut is independent of β-catenin activation in osteoblasts. However, it is unknown whether Lrp5 can act directly in osteoblast to influence other functions that require β-catenin signaling, particularly, the deregulation of hematopoiesis and leukemogenic properties of β-catenin activation in osteoblasts, that lead to development of acute myeloid leukemia (AML). Using mice with gain-of-function (GOF) Lrp5 alleles (Lrp5(A214V)) that recapitulate the human high bone mass (HBM) phenotype, as well as patients with the T253I HBM Lrp5 mutation, we show here that Lrp5 GOF mutations in both humans and mice do not activate β-catenin signaling in osteoblasts. Consistent with a lack of β-catenin activation in their osteoblasts, Lrp5(A214V) mice have normal trilinear hematopoiesis. In contrast to leukemic mice with constitutive activation of β-catenin in osteoblasts (Ctnnb1(CAosb)), accumulation of early myeloid progenitors, a characteristic of AML, myeloid-blasts in blood, and segmented neutrophils or dysplastic megakaryocytes in the bone marrow, are not observed in Lrp5(A214V) mice. Likewise, peripheral blood count analysis in HBM patients showed normal hematopoiesis, normal percentage of myeloid cells, and lack of anemia. We conclude that Lrp5 GOF mutations do not activate β-catenin signaling in osteoblasts. As a result, myeloid lineage differentiation is normal in HBM patients and mice. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis, Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

New Views on the Misconstrued: Executioner Caspases and Their Diverse Non-apoptotic Roles

Initially characterized for their roles in apoptosis, executioner caspases have emerged as important regulators of an array of cellular activities. This is especially true in the nervous system, where sublethal caspase activity has been implicated in axonal pathfinding and branching, axonal degeneration, dendrite pruning, regeneration, long-term depression, and metaplasticity. Here we examine the roles of sublethal executioner caspase activity in nervous system development and maintenance, consider the mechanisms that locally activate and restrain these potential killers, and discuss how their activity be subverted in neurodegenerative disease.

Caspases uncouple p27(Kip1) from cell cycle regulated degradation and abolish its ability to stimulate cell migration and invasion

In addition to their role in programmed cell death, caspases exert non-lethal functions in diverse developmental processes including cell differentiation or tissue remodeling. Terminal cell cycle exit and differentiation can be promoted by increased level of the CDK inhibitor p27^Kip1. Activated caspases cause proteolytic processing of p27, and we identified a novel caspase cleavage site in human p27 that removes a C-terminal fragment of 22 amino acids from the CDK inhibitor, including a phosphodegron. Thereby, caspases protect the inhibitor from SCF-Skp2-mediated degradation in S, G2 and M phases of the cell cycle. As a consequence, p27 becomes stabilized and remains an efficient nuclear inhibitor of cell cycle progression. Besides controlling cyclin/CDK kinase activity, p27 also regulates cytoskeletal dynamics, cell motility and cell invasion. Following processing by caspases, p27 fails to bind to RhoA and to inhibit its activation, and thereby abolishes the ability of p27 to stimulate cell migration and invasion. We propose that the stabilization of the CDK inhibitor and elimination of RhoA-induced cytoskeletal remodeling upon caspase processing could contribute to cell cycle exit and cytoskeletal remodeling during non-lethal caspase controlled differentiation processes.