Caspase activation is required for terminal erythroid differentiation

Effects of Boron Supplementation on Expression of Hsp70 in the Spleen of African Ostrich

Increased synthesis of heat shock protein 70 (Hsp70) occurs in prokaryotes and eukaryotes in response to physiological, environmental, and chemical exposures, thus allowing the cell survival from fatal conditions. Hsp70 cytoprotective properties may be clarified by its anti-apoptotic function. Boron has been reported to play an essential role in various organ developments and metabolisms. However, it is not known if boron is also able to modulate the Hsp70. In the present study, the actions of boron on ostrich spleen and expression level of Hsp70 were investigated. Thirty healthy ostrich chicks were randomly assigned to six groups: groups I, II, III, IV, V, and VI and fed the basal diet spiked with 0-, 40-, 80-, 160-, 320-, and 640-mg boric acid (BA)/L, respectively, in drinking water. The histomorphological examination in the spleen was done by hematoxylin and eosin (HE) staining. The expression level of Hsp70 was analyzed by immunohistochemistry (IHC) and western blotting, and mRNA expression of Hsp70 was investigated by quantitative real-time PCR (qPCR). In order to investigate apoptosis, TUNEL assay reaction in all treatment groups was analyzed. Our results showed that the histological structure of spleen up to 160 mg/L BA supplementation groups well developed. The Hsp70 expression level first induced at low-dose groups (up to group IV) and then inhibited dramatically in high-dose groups (V and VI) while comparing with the group I (0 mg BA). The TUNEL assay reaction revealed that the cell apoptosis amount was decreased in group IV, but in group V and especially in group VI, it was significantly increased (P < 0.01). Taken altogether, proper dietary boron treatment might stimulate ostrich chick spleen development by promoting the Hsp70 expression level and inhibiting apoptosis, while a high amount of boron supplementation would impair the ostrich spleen structure by inhibiting Hsp70 expression level and promoting cell apoptosis.

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.

Activation of Pro-apoptotic Caspases in Non-apoptotic Cells During Odontogenesis and Related Osteogenesis

Caspases are well known proteases in the context of inflammation and apoptosis. Recently, novel roles of pro-apoptotic caspases have been reported, including findings related to the development of hard tissues. To further investigate these emerging functions of pro-apoptotic caspases, the in vivo localisation of key pro-apoptotic caspases (-3,-6,-7,-8, and -9) was assessed, concentrating on the development of two neighbouring hard tissues, cells participating in odontogenesis (represented by the first mouse molar) and intramembranous osteogenesis (mandibular/alveolar bone). The expression of the different caspases within the developing tissues was correlated with the apoptotic status of the cells, to produce a picture of whether different caspases have potentially distinct, or overlapping non-apoptotic functions. The in vivo investigation was additionally supported by examination of caspases in an osteoblast-like cell line in vitro. Caspases-3,-7, and -9 were activated in apoptotic cells of the primary enamel knot of the first molar; however, caspase-7 and -8 activation was also associated with the non-apoptotic enamel epithelium at the same stage and later with differentiating/differentiated odontoblasts and ameloblasts. In the adjacent bone, active caspases-7 and -8 were present abundantly in the prenatal period, while the appearance of caspases-3,-6, and -9 was marginal. Perinatally, caspases-3 and -7 were evident in some osteoclasts and osteoblastic cells, and caspase-8 was abundant mostly in osteoclasts. In addition, postnatal activation of caspases-7 and -8 was retained in osteocytes. The results provide a comprehensive temporo-spatial pattern of pro-apoptotic caspase activation, and demonstrate both unique and overlapping activation in non-apoptotic cells during development of the molar tooth and mandibular/alveolar bone. The importance of caspases in osteogenic pathways is highlighted by caspase inhibition in osteoblast-like cells, which led to a significant decrease in osteocalcin expression, supporting a role in hard tissue cell differentiation.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

Heat stress-induced nuclear transport mediated by Hikeshi confers nuclear function of Hsp70s

The prime feature of eukaryotic cells is the separation of the intracellular space into two compartments, the nucleus and the cytoplasm. Active nuclear transport is crucial for the maintenance of this separation. In this report, we focus on a nuclear transport receptor named Hikeshi, which mediates the heat stress-induced nuclear import of 70-kDa heat shock proteins (Hsp70s), and discuss how the same protein can function differently depending on the cellular compartment in which it is localized. Hsp70 is a molecular chaperone that is predominantly localized in the cytoplasm under normal conditions but is known to accumulate in the nucleus under conditions of heat stress. Although the reported function of Hsp70 is mostly attributed to its molecular function in the cytoplasm, the functions of Hsp70 may extend beyond molecular chaperone activity in the nucleus.

Modifications to a common phosphorylation network provide individualized control in caspases

Caspase-3 activation and function have been well-defined during programmed cell death, but caspase activity, at low levels, is also required for developmental processes such as lymphoid proliferation and erythroid differentiation. Post-translational modification of caspase-3 is one method used by cells to fine-tune activity below the threshold required for apoptosis, but the allosteric mechanism that reduces activity is unknown. Phosphorylation of caspase-3 at a conserved allosteric site by p38-MAPK (mitogen-activated protein kinase) promotes survival in human neutrophils, and the modification of the loop is thought to be a key regulator in many developmental processes. We utilized phylogenetic, structural, and biophysical studies to define the interaction networks that facilitate the allosteric mechanism in caspase-3. We show that, within the modified loop, Ser¹⁵⁰ evolved with the apoptotic caspases, whereas Thr¹⁵² is a more recent evolutionary event in mammalian caspase-3. Substitutions at Ser¹⁵⁰ result in a pH-dependent decrease in dimer stability, and localized changes in the modified loop propagate to the active site of the same protomer through a connecting surface helix. Likewise, a cluster of hydrophobic amino acids connects the conserved loop to the active site of the second protomer. The presence of Thr¹⁵² in the conserved loop introduces a "kill switch" in mammalian caspase-3, whereas the more ancient Ser¹⁵⁰ reduces without abolishing enzyme activity. These data reveal how evolutionary changes in a conserved allosteric site result in a common pathway for lowering activity during development or a more recent cluster-specific switch to abolish activity.

Osteogenic Potential of Caspases Related to Endochondral Ossification

Caspases have functions particularly in apoptosis and inflammation. Increasing evidence indicates novel roles of these proteases in cell differentiation, including those involved in osteogenesis. This investigation provides a complex screening of osteogenic markers affected by pan caspase inhibition in micromass cultures derived from mouse forelimbs. PCR Array analysis showed significant alterations in expression of 49 osteogenic genes after 7 days of inhibition. The largest change was a decrease in CD36 expression, which was confirmed at organ level by caspase inhibition in cultured mouse ulnae followed by CD36 immunohistochemical analysis. So far, available data point to osteogenic potential of pro-apoptotic caspases. Therefore, the expression of pro-apoptotic caspases (-3, -6, -7, -8, -9) within the growth plate of mouse forelimbs at the stage where the individual zones are clearly apparent was studied. Caspase-9 was reported in the growth plate for the first time as well as caspase-6 and -7 in the resting zone, caspase-7 in the proliferation, and caspase-6 and -8 in the ossification zone. For all caspases, there was a gradient increase in activation toward the ossification zone. The distribution of staining varied significantly from that of apoptotic cells, and thus, the results further support non-apoptotic participation of caspases in osteogenesis.

Reversible permeabilization of the mitochondrial membrane promotes human cardiomyocyte differentiation from embryonic stem cells

Cell death and differentiation appear to share similar cellular features. In this study, we aimed to investigate whether differentiation and mitochondrial cell death use a common pathway. We assessed the hallmarks of apoptosis during cardiomyocyte differentiation of human embryonic stem cells and found remarkable changes in P53, reactive oxygen species, apoptotic protease-activating factor 1, poly[ADP-ribose]polymerase 1, cellular adenosine triphosphate, and mitochondrial complex I activity. Furthermore, we observed reversible mitochondrial membrane permeabilization during cardiomyocyte differentiation accompanied by reversible loss of mitochondrial membrane potential, and these changes coincided with the fluctuating patterns of cytosolic cytochrome c accumulation and subsequent caspase-9 and -3/7 activation. Moreover, the use of apoptosis inhibitors (BCL2-associated X protein [BAX] inhibitor and caspase-3/7 inhibitor) during differentiation impaired cardiomyocyte development, resulting in substantial downregulation of T, MESP1, NKX2.5, and α-MHC. Additionally, although the expression of specific differentiation markers (T, MESP1, NKX2.5, MEF2C, GATA4, and SOX17) was enhanced in doxorubicin-induced human embryonic stem cells, the stemness-specific markers (OCT4 and NANOG) showed significant downregulation. With increasing doxorubicin concentration (0.03-0.6 µM; IC50  = 0.5 µM), we observed a marked increase in the expression of mesoderm and endoderm markers. In summary, we suggest that reversible mitochondrial outer membrane permeabilization promotes cardiomyocyte differentiation through an attenuated mitochondria-mediated apoptosis-like pathway.

APG101 efficiently rescues erythropoiesis in lower risk myelodysplastic syndromes with severe impairment of hematopoiesis

CD95, a member of the death receptor family initiates a caspase-dependent apoptosis, when activated by its ligand CD95L, thought to negatively regulate erythrocyte production in the bone marrow. We have previously shown that CD95 is overexpressed in two thirds of patients with a lower risk myelodysplastic syndrome (MDS) and that resistance to erythropoiesis-stimulating agents (ESA) is linked to poor residual erythropoiesis. In the present study, we show that CD95 overexpression and previous transfusion are independent predictive factors of ESA resistance. To investigate an alternative therapeutic strategy of anemia in ESA-resistant patients, we have conducted a preclinical study of the effects of APG101, a fusion protein consisting of the extracellular domain of human CD95 and the Fc region of human IgG1 on MDS erythropoiesis in vitro. APG101 increases the number of burst-forming unit-erythroid (BFU-E) progenitors derived from CD34+ progenitors in liquid culture and improves overall proliferation rate of erythroid precursors by inhibiting apoptosis. APG101 rescues BFU-E growth in MDS patients presenting with attrition of erythroid progenitors at baseline, independently of CD95 or CD95L expression level. Our data show that overexpression of CD95 at diagnosis is a hallmark of ESA resistance and that severe impairment of erythropoiesis is predictive of erythroid response to APG101 in vitro. These data provide a rationale for further clinical investigation of APG101 in an attempt to treat anemia in lower risk MDS patients.

Revisiting the Road Map of Medullary Thymic Epithelial Cell Differentiation

The basic two-step terminal differentiation model of the medullary thymic epithelial cell (mTEC) lineage from immature MHC class II (MHCII)lo to mature MHCIIhi mTECs has recently been extended to include a third stage, namely the post-Aire MHCIIlo subset as identified by lineage-tracing models. However, a suitable surface marker distinguishing the phenotypically overlapping pre- from the post-Aire MHCIIlo stage has been lacking. In this study, we introduce the lectin Tetragonolobus purpureas agglutinin (TPA) as a novel cell surface marker that allows for such delineation. Based on our data, we derived the following sequence of mTEC differentiation: TPAloMHCIIlo → TPAloMHCIIhi → TPAhiMHCIIhi → TPAhiMHCIIlo Surprisingly, in the steady-state postnatal thymus TPAloMHCIIlo pre-Aire rather than terminally differentiated post-Aire TPAhiMHCIIlo mTECs were marked for apoptosis at an exceptionally high rate of ∼70%. Hence, only the minor cycling fraction of the MHCIIlo subset (<20%) potentially qualified as mTEC precursors. FoxN1 expression inversely correlated with the fraction of slow cycling and apoptotic cells within the four TPA subsets. TPA also further subdivided human mTECs, although with different subset distribution. Our revised road map emphazises close parallels of terminal mTEC development with that of skin, undergoing an alternative route of cell death, namely cornification rather than apoptosis. The high rate of apoptosis in pre-Aire MHCIIlo mTECs points to a "quality control" step during early mTEC differentiation.

Caspase signaling, a conserved inductive cue for metazoan cell differentiation

Caspase signaling pathways were originally discovered as conveyors of programmed cell death, yet a compendium of research over the past two decades have demonstrated that these same conduits have a plethora of physiologic functions. Arguably the most extensive non-death activity that has been attributed to this protease clade is the capacity to induce cell differentiation. Caspase control of differentiation is conserved across diverse metazoan organisms from flies to humans, suggesting an ancient origin for this form of cell fate control. Here we discuss the mechanisms by which caspase enzymes manage differentiation, the targeted substrates that may be common across cell lineages, and the countervailing signals that may be essential for these proteases to 'execute' this non-death cell fate.

Orchestration of late events in erythropoiesis by KLF1/EKLF

PURPOSE OF REVIEW

Transcriptional regulators provide the molecular and biochemical basis for the cell specific properties and characteristics that follow from their central role in establishing tissue-restricted expression. Precise and sequential control of terminal cell divisions, nuclear condensation, and enucleation are defining characteristics within erythropoietic differentiation. This review is focused on KLF1, a central global regulator of this process.

RECENT FINDINGS

Studies in the past year have brought a number of proteins that are targets of KLF1 regulation into focus with respect to their roles in terminal erythroid differentiation. Many of these are involved in fine control of the cell cycle at both early (E2F2, Cyclin A2) and later (p18, p27, p19) stages of differentiation, or are directly involved in enucleation (p18, p27). Dramatic biophysical changes controlled at the nuclear lamin by caspase 3 enable histone release and nuclear condensation, whereas dematin association with structural proteins alters the timing of enucleation. Conditional ablation of mDia2 has established its role in late stage cell cycle and enucleation.

SUMMARY

Transcription factors such as KLF1, along with epigenetic modifiers, play crucial roles in establishing the proper onset and progression of terminal differentiation events. Studies from the past year show a remarkable multifaceted convergence on cell cycle control, and establish that the orthochromatic erythroblast stage is a critical nodal point for many of the effects on enucleation. These studies are relevant to understanding the underlying causes of anemia and hematologic disease where defective enucleation predicts a poor clinical outcome.

Dyserythropoiesis of myelodysplastic syndromes

PURPOSE OF REVIEW

Myelodysplastic syndromes (MDS) are heterogeneous diseases of the hematopoietic stem cell in the elderly. Anemia is the main symptom that mostly correlates with dysplastic erythropoiesis in the bone marrow. We will review the recent advances in understanding the diverse mechanisms of dyserythropoiesis.

RECENT FINDINGS

Dyserythropoiesis defined as 10% dysplastic erythroid cells in the bone marrow is found in more than 80% of early MDS. Immature erythroblasts accumulate at the expense of mature erythroblasts due to differentiation arrest and apoptosis. In early MDS with dyserythropoiesis, caspase-dependent cleavage of the erythroid transcription factor GATA-1 occurring in basophilic erythroblasts accounts for impairment of final maturation. Depending on initiating genetic alteration, specific mechanisms contribute to erythroid defect. In MDS with 5q deletion, the haploinsufficiency of ribosomal protein gene, RPS14, opposes the transition of immature to mature erythroblasts by inducing a p53-dependent ribosome stress, cell cycle arrest and apoptosis. Recent work identifies the activation of a p53-S100A8/9 innate immune pathway that both intrinsically and extrinsically contributes to defective erythropoiesis. In MDS with ring sideroblasts, a paradigm of dyserythropoiesis, a unique mutation in SF3B1 splicing factor gene induces a multiplicity of alterations at RNA level that deeply modifies the patterns of gene expression.

SUMMARY

Insights in the pathophysiology of MDS with dyserythropoiesis may guide the choice of the appropriate therapy, for instance lenalidomide in MDS with del(5q). A better understanding of the mechanisms of dyserthropoiesis is required to treat anemia in non-del(5q) MDS, especially in case of resistance to first-line therapy by erythropoiesis-stimulating agents.

Epoetin β pegol (continuous erythropoietin receptor activator, CERA) is another choice for the treatment of anemia in myelodysplastic syndrome: a case report

Background

In most patients, anemia is present when myelodysplastic syndrome is diagnosed. Although darbepoetin α is the first-choice supportive therapy for low-risk myelodysplastic syndrome, half of all patients develop a loss of response to darbepoetin α within 12 months. However, few reports have described supportive therapy after the loss of response to darbepoetin α.

Case presentation

We herein present a case involving a 65-year-old Japanese woman with low-risk myelodysplastic syndrome whose erythropoiesis-stimulating agent treatment was switched from darbepoetin α to epoetin β pegol (continuous erythropoietin receptor activator) to treat transfusion-dependent anemia. The frequent transfusions required to treat the anemia resulted in transfusion-associated circulatory overload. The transfusion-dependent anemia was initially treated with darbepoetin α, which negated the requirement for transfusion. However, after 12 months of darbepoetin α therapy, the hemoglobin concentration sharply declined. We switched her therapy from darbepoetin α to continuous erythropoietin receptor activator to avoid transfusion. After initiation of continuous erythropoietin receptor activator therapy, the hemoglobin concentration gradually increased and transfusion was not required. At the time of writing, no progression of the anemia had occurred.

Conclusions

Although darbepoetin α is the first-choice supportive therapy for low-risk myelodysplastic syndrome, continuous erythropoietin receptor activator might be considered the second-choice therapy.

The severe phenotype of Diamond-Blackfan anemia is modulated by heat shock protein 70

Diamond-Blackfan anemia (DBA) is a rare congenital bone marrow failure syndrome that exhibits an erythroid-specific phenotype. In at least 70% of cases, DBA is related to a haploinsufficient germ line mutation in a ribosomal protein (RP) gene. Additional cases have been associated with mutations in GATA1. We have previously established that the RPL11^+/Mut phenotype is more severe than RPS19^+/Mut phenotype because of delayed erythroid differentiation and increased apoptosis of RPL11^+/Mut erythroid progenitors. The HSP70 protein is known to protect GATA1, the major erythroid transcription factor, from caspase-3 mediated cleavage during normal erythroid differentiation. Here, we show that HSP70 protein expression is dramatically decreased in RPL11^+/Mut erythroid cells while being preserved in RPS19^+/Mut cells. The decreased expression of HSP70 in RPL11^+/Mut cells is related to an enhanced proteasomal degradation of polyubiquitinylated HSP70. Restoration of HSP70 expression level in RPL11^+/Mut cells reduces p53 activation and rescues the erythroid defect in DBA. These results suggest that HSP70 plays a key role in determining the severity of the erythroid phenotype in RP-mutation-dependent DBA.

Mechanism of M-cell differentiation accelerated by proliferation of indigenous bacteria in rat Peyer's patches

The mechanism by which indigenous bacteria on the follicle-associated epithelium (FAE) of lymphatic follicles (LFs) accelerate the differentiation of microvillous columnar epithelial cells (MV) into M-cells was immunohistochemically investigated in rat Peyer’s patches. The results showed that the number of Toll-like receptor (TLR) -4⁺ M-cells was greater in the FAE with expansion of bacterial colonies (LFs with bacterial colonies on the FAE: b-LF) than the FAE without expansion of bacterial colonies (nb-LF). TLR-4 was also expressed in the striated borders of MV upstream next to M-cells in the FAE of the b-LF. TLR-4⁺ vesicles were frequently detected in the cytoplasms of MV with TLR-4⁺ striated borders upstream next to TLR-4⁺ M-cells in the FAE of b-LF. These findings suggest that TLR-4⁺ MV take up TLR-4 ligands and differentiate into M-cells in the b-LF. Neither the distribution of RANK nor that of RANKL was coincident with that of M-cells in the b-LF. Moreover, RANK, but not RANKL, was expressed in intestinal villi, whereas cleaved caspase-3 was immunonegative in the MV and M-cells of the FAE, unlike in villous epithelial cells. Therefore, RANK/RANKL signaling in the LF might contribute to the down-regulation of epithelial apoptosis to facilitate the differentiation of MV into M-cells in rat Peyer’s patches.

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-3-dependent cleavage of Akt modulates tau phosphorylation via GSK3β kinase: implications for Alzheimer's disease

The pathological hallmark of Alzheimer's disease (AD) is accumulation of misfolded amyloid-β peptides and hyperphosphorylated tau protein in the brain. Increasing evidence suggests that serine-aspartyl proteases-caspases are activated in the AD brain. Previous studies identified a caspase-3 cleavage site within the amyloid-β precursor protein, and a caspase-3 cleavage of tau as the mechanisms involved in the development of Aβ and tau neuropathology, respectively. However, the potential role that caspase-3 could have on tau metabolism remains unknown. In the current studies, we provide experimental evidence that caspase-3 directly and specifically regulates tau phosphorylation, and demonstrate that this effect is mediated by the GSK3β kinase pathway via a caspase-3-dependent cleavage of the protein kinase B (also known as Akt). In addition, we confirm these results in vivo by using a transgenic mouse model of AD. Collectively, our findings demonstrate a new role for caspase-3 in the neurobiology of tau, and suggest that therapeutic strategies aimed at inhibiting this protease-dependent cleavage of Akt may prove beneficial in preventing tau hyperphosphorylation and subsequent neuropathology in AD and related tauopathies.

Caspase-9 has a nonapoptotic function in Xenopus embryonic primitive blood formation

Caspases constitute a family of cysteine proteases centrally involved in programmed cell death, which is an integral part of normal embryonic and fetal development. However, it has become clear that specific caspases also have functions independent of cell death. In order to identify novel apoptotic and nonapoptotic developmental caspase functions, we designed and transgenically integrated novel fluorescent caspase reporter constructs in developing Xenopus embryos and tadpoles. This model organism has an external development, allowing direct and continuous monitoring. These studies uncovered a nonapoptotic role for the initiator caspase-9 in primitive blood formation. Functional experiments further corroborated that caspase-9, but possibly not the executioners caspase-3 and caspase-7, are required for primitive erythropoiesis in the early embryo. These data reveal a novel nonapoptotic function for the initiator caspase-9 and, for the first time, implicate nonapoptotic caspase activity in primitive blood formation.

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.

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.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Protein-based therapeutic for anemia caused by dyserythropoiesis

INTRODUCTION

Major advances have been recently made in understanding the molecular determinants of dyserythropoiesis, particularly due to recent works in β-thalassemia. The purpose of this review is devoted to underline the role of some proteins recently evidenced in the field, that may be new alternative therapeutic targets in the near future to alleviate different types of anemia. Areas covered: This review covers the contemporary aspects of some proteins involved in various types of dyserythropoiesis, including the transcriptional factor GATA-1 and its protective chaperone HSP70, but also cytokines of the transforming growth factor beta (TFG-β) family, TGF-β1 and GDF-11, and hormones as erythroferrone. It will be not exhaustive, but based on major recent published works from the literature in the past three years. Expert commentary: Sotatercept and lustatercept, two activin receptor II ligand traps that block GDF-11, are candidate drugs providing therapeutic hope in different types of ineffective erythropoiesis, including myelodysplastic syndromes (MDS) and β-thalassemia. Furthermore, a new concept emerges to consider erythroid lineage in the bone marrow as an endocrine gland.

Long and short (timeframe) of endoplasmic reticulum stress-induced cell death

A number of age-dependent degenerative diseases are caused by chronic endoplasmic reticulum (ER) stress in vital cells. In many cases, the afflicted cells suffer from ER stress since birth, but the death of irreplaceable cells occurs only late in life. Although our understanding of ER stress-induced cell death has advanced significantly, most of the known mechanisms involve pathways that signal within hours, and it remains unclear how these pathways regulate cell death that occurs only decades later. Here, I highlight the conceptual issues and suggest ways to make sense of the age-related effect of ER stress-induced cell death in degenerative diseases.

The RNA-binding profile of Acinus, a peripheral component of the exon junction complex, reveals its role in splicing regulation

Acinus (apoptotic chromatin condensation inducer in the nucleus) is an RNA-binding protein (RBP) originally identified for its role in apoptosis. It was later found to be an auxiliary component of the exon junction complex (EJC), which is deposited at exon junctions as a consequence of pre-mRNA splicing. To uncover the cellular functions of Acinus and investigate its role in splicing, we mapped its endogenous RNA targets using the cross-linking immunoprecipitation protocol (iCLIP). We observed that Acinus binds to pre-mRNAs, associating specifically to a subset of suboptimal introns, but also to spliced mRNAs. We also confirmed the presence of Acinus as a peripheral factor of the EJC. RNA-seq was used to investigate changes in gene expression and alternative splicing following siRNA-mediated depletion of Acinus in HeLa cells. This analysis revealed that Acinus is preferentially required for the inclusion of specific alternative cassette exons and also controls the faithful splicing of a subset of introns. Moreover, a large number of splicing changes can be related to Acinus binding, suggesting a direct role of Acinus in exon and intron definition. In particular, Acinus regulates the splicing of DFFA/ICAD transcript, a major regulator of DNA fragmentation. Globally, the genome-wide identification of RNA targets of Acinus revealed its role in splicing regulation as well as its involvement in other cellular pathways, including cell cycle progression. Altogether, this study uncovers new cellular functions of an RBP transiently associated with the EJC.

Chromatin condensation during terminal erythropoiesis

Mammalian terminal erythropoiesis involves gradual but dramatic chromatin condensation steps that are essential for cell differentiation. Chromatin and nuclear condensation is followed by a unique enucleation process, which is believed to liberate more spaces for hemoglobin enrichment and enable the generation of a physically flexible mature red blood cell. Although these processes have been known for decades, the mechanisms are still unclear. Our recent study reveals an unexpected nuclear opening formation during mouse terminal erythropoiesis that requires caspase-3 activity. Major histones, except H2AZ, are partially released from the opening, which is important for chromatin condensation. Block of the nuclear opening through caspase inhibitor or knockdown of caspase-3 inhibits chromatin condensation and enucleation. We also demonstrate that nuclear opening and histone release are cell cycle regulated. These studies reveal a novel mechanism for chromatin condensation in mammalia terminal erythropoiesis.

Chromatin Condensation and Enucleation in Red Blood Cells: An Open Question

Differentiating erythroid cells undergo dramatic changes in morphology, with reduction in cell size, chromatin and nuclear condensation, and enucleation. In this issue of Developmental Cell, Zhao et al. (2016) show that these events are associated with the formation of transient, recurring nuclear openings and selective histone release mediated by caspase-3.

Nuclear Condensation during Mouse Erythropoiesis Requires Caspase-3-Mediated Nuclear Opening

Mammalian erythropoiesis involves chromatin condensation that is initiated in the early stage of terminal differentiation. The mechanisms of chromatin condensation during erythropoiesis are unclear. Here, we show that the mouse erythroblast forms large, transient, and recurrent nuclear openings that coincide with the condensation process. The opening lacks nuclear lamina, nuclear pore complexes, and nuclear membrane, but it is distinct from nuclear envelope changes that occur during apoptosis and mitosis. A fraction of the major histones are released from the nuclear opening and degraded in the cytoplasm. We demonstrate that caspase-3 is required for the nuclear opening formation throughout terminal erythropoiesis. Loss of caspase-3 or ectopic expression of a caspase-3 non-cleavable lamin B mutant blocks nuclear opening formation, histone release, chromatin condensation, and terminal erythroid differentiation. We conclude that caspase-3-mediated nuclear opening formation accompanied by histone release from the opening is a critical step toward chromatin condensation during erythropoiesis in mice.

Comprehensive Proteomic Analysis of Human Erythropoiesis

Mass spectrometry-based proteomics now enables the absolute quantification of thousands of proteins in individual cell types. We used this technology to analyze the dynamic proteome changes occurring during human erythropoiesis. We quantified the absolute expression of 6,130 proteins during erythroid differentiation from late burst-forming units-erythroid (BFU-Es) to orthochromatic erythroblasts. A modest correlation between mRNA and protein expression was observed. We identified several proteins with unexpected expression patterns in erythroid cells, highlighting a breakpoint in the erythroid differentiation process at the basophilic stage. We also quantified the distribution of proteins between reticulocytes and pyrenocytes after enucleation. These analyses identified proteins that are actively sorted either with the reticulocyte or the pyrenocyte. Our study provides the absolute quantification of protein expression during a complex cellular differentiation process in humans, and it establishes a framework for future studies of disordered erythropoiesis.

Advances in understanding the mechanisms of erythropoiesis in homeostasis and disease

Anaemia or decreased blood haemoglobin is the most common blood disorder often characterized by reduced red blood cell (RBC) numbers. RBCs are produced from differentiation and commitment of haematopoietic stem cells to the erythroid lineage by a process called erythropoiesis. Coordination of erythropoietin receptor signalling with several erythroid transcription factors including GATA1 is essential for this process. A number of additional players that are critical for RBC production have been identified in recent years. Major technological advances, such as the development of RNA interference, genetically modified animals, including zebrafish, and imaging flow cytometry have led to these discoveries; the emergence of -omics approaches in combination with the optimization of ex vivo erythroid cultures have also produced a more comprehensive understanding of erythropoiesis. Here we summarize studies describing novel regulators of erythropoiesis that modulate erythroid cell production in the context of human erythroid disorders involving hypoxia, iron regulation, immune-related molecules, and the transcription factor FOXO3.

Erythropoietin-regulated oxidative stress negatively affects enucleation during terminal erythropoiesis

Differentiating erythroblasts are exposed to an oxidative environment. The dynamics of oxidative status during terminal erythropoiesis and how they affect cell differentiation in response to erythropoietin (Epo) are unclear. Here, we show that Epo induces reactive oxygen species (ROS) production in the early stages of terminal erythropoiesis. The levels of ROS correlate with CD71 surface expression and the uptake of iron and transferrin. ROS decreases in the late stages of terminal erythropoiesis, when the cells are preparing for enucleation. Consistently, treatment of erythroblasts with a low dose (5 mM) of N-acetyl-cysteine (NAC), a ROS scavenger, promotes enucleation. However, a high dose (20 mM) of NAC leads to significant cell death. Our study reveals an important function of Epo in regulating the dynamics of oxidative status and enucleation.

A Krebs Cycle Component Limits Caspase Activation Rate through Mitochondrial Surface Restriction of CRL Activation

How cells avoid excessive caspase activity and unwanted cell death during apoptotic caspase-mediated removal of large cellular structures is poorly understood. We investigate caspase-mediated extrusion of spermatid cytoplasmic contents in Drosophila during spermatid individualization. We show that a Krebs cycle component, the ATP-specific form of the succinyl-CoA synthetase β subunit (A-Sβ), binds to and activates the Cullin-3-based ubiquitin ligase (CRL3) complex required for caspase activation in spermatids. In vitro and in vivo evidence suggests that this interaction occurs on the mitochondrial surface, thereby limiting the source of CRL3 complex activation to the vicinity of this organelle and reducing the potential rate of caspase activation by at least 60%. Domain swapping between A-Sβ and the GTP-specific SCSβ (G-Sβ), which functions redundantly in the Krebs cycle, show that the metabolic and structural roles of A-Sβ in spermatids can be uncoupled, highlighting a moonlighting function of this Krebs cycle component in CRL activation.

Esterification of Ginsenoside Rh2 Enhanced Its Cellular Uptake and Antitumor Activity in Human HepG2 Cells

Our previous research had indicated that the octyl ester derivative of ginsenoside Rh2 (Rh2-O) might have a higher bioavailability than Rh2 in the Caco-2 cell line. The aim of this study was to investigate the cellular uptake and antitumor effects of Rh2-O in human HepG2 cells as well as its underlying mechanism compared with Rh2. Results showed that Rh2-O exhibited a higher cellular uptake (63.24%) than Rh2 (36.76%) when incubated with HepG2 cells for 24 h. Rh2-O possessed a dose- and time-dependent inhibitory effect against the proliferation of HepG2 cells. The IC50 value of Rh2-O for inhibition of HepG2 cell proliferation was 20.15 μM, which was roughly half the value of Rh2. Rh2-O induced apoptosis of HepG2 cells through a mitochondrial-mediated intrinsic pathway. In addition, the accumulation of ROS was detected in Rh2-O-treated HepG2 cells, which participated in the apoptosis of HepG2 cells. Conclusively, the findings above all suggested that Rh2-O as well as Rh2 inducing HepG2 cells apoptosis might involve similar mechanisms; however, Rh2-O had better antitumor activities than Rh2, probably due to its higher cellular uptake.

Detection of Apoptotic Versus Autophagic Cell Death by Flow Cytometry

Different modes of regulated cell death (RCD) can be initiated by distinct molecular machineries and their morphological manifestations can be difficult to discriminate. Moreover, cells responding to stress often activate an adaptive response centered around autophagy, and whether such a response is cytoprotective or cytotoxic cannot be predicted based on morphological parameters only. Molecular definitions are therefore important to understand various RCD subroutines from a mechanistic perspective. In vitro, various forms of RCD including apoptosis and autophagic cell death can be easily discriminated from each other with assays that involve chemical or pharmacological interventions targeting key components of either pathway. Here, we detail a straightforward method to discriminate apoptosis from autophagic cell death by flow cytometry, based on the broad-spectrum caspase inhibitor Z-VAD-fmk and the genetic inhibition of ATG5.

CD40 ligand induces RIP1-dependent, necroptosis-like cell death in low-grade serous but not serous borderline ovarian tumor cells

Ovarian high-grade serous carcinomas (HGSCs) and invasive low-grade serous carcinomas (LGSCs) are considered to be distinct entities. In particular, LGSCs are thought to arise from non-invasive serous borderline ovarian tumors (SBOTs) and show poor responsiveness to conventional chemotherapy. The pro-apoptotic effects of CD40 ligand (CD40L) have been demonstrated in HGSC, though the underlying mechanisms are not fully understood. Conversely, the therapeutic potential of the CD40L-CD40 system has yet to be evaluated in LGSC. We now show that CD40 protein is focally expressed on tumor cells in two of five primary LGSCs compared with no expression in eight primary SBOTs. Treatment with CD40L or agonistic CD40 antibody decreased the viability of LGSC-derived MPSC1 and VOA1312 cells, but not SBOT3.1 cells. Small interfering RNA (siRNA) targeting CD40 was used to show that it is required for these reductions in cell viability. CD40L treatment increased cleaved caspase-3 levels in MPSC1 cells though, surprisingly, neither pan-caspase inhibitor nor caspase-3 siRNA reversed or even attenuated CD40L-induced cell death. In addition, CD40-induced cell death was not affected by knockdown of the mitochondrial proteins apoptosis-inducing factor (AIF) and endonuclease G (EndoG). Interestingly, CD40L-induced cell death was blocked by necrostatin-1, an inhibitor of receptor-interacting protein 1 (RIP1), and attenuated by inhibitors of RIP3 (GSK'872) or MLKL (mixed lineage kinase domain-like; necrosulfonamide). Our results indicate that the upregulation of CD40 may be relatively common in LGSC and that CD40 activation induces RIP1-dependent, necroptosis-like cell death in LGSC cells.

Intracellular transduction of TAT-Hsp27 fusion protein enhancing cell survival and regeneration capacity of cardiac stem cells in acute myocardial infarction

Myocardial infarction (MI) results in the substantial loss of functional cardiomyocytes, which frequently leads to intractable heart disorders. Cardiac stem cells (CSCs) that retain the capacity to replace all cardiac cells might be a promising strategy for providing a source of new functional cardiomyocytes; however, the poor survival and engraftment of transplanted CSCs in the hostile environment of MI critically mitigate their therapeutic benefits. To capitalize their therapeutic potential, an ex vivo strategy in which CSCs were introduced to the recombinant heat shock protein 27 (Hsp27) through a TAT protein transduction domain for increasing the viability and engraftment in the infarcted myocardium was designed. A recombinant TAT fused Hsp27 (TAT-Hsp27) was able to enter CSCs in a dose-dependent manner. CSCs transduced with TAT-Hsp27 expressed not only endogenous Hsp27 but externally introduced Hsp27, resulting in substantial increase of their anti-oxidative and anti-apoptotic properties via suppressing reactive oxygen species production, the MAPKs signaling pathway, and caspase activation. TAT-Hsp27 enabled CSCs to be protected from apoptotic- and hypoxic-induced cell death during in vitro cardiomyogenic differentiation. In vivo studies demonstrated that CSCs transduced TAT-Hsp27 significantly increased the survival and engraftment in the acutely infarcted myocardium, which is closely related to caspase activity suppression. Finally, CSCs transduced TAT-Hsp27 improved cardiac function and attenuated cardiac remodeling in comparison with non-transduced CSCs. Overall, our approach, which is based on the ex vivo intracellular transduction of TAT-Hsp27 into CSCs before myocardial delivery, might be effective in treating MI.

Analysis of expression patterns of IGF-1, caspase-3 and HSP-70 in developing human tooth germs

AIMS

To analyze expression patterns of IGF-1, caspase-3 and HSP-70 in human incisor and canine tooth germs during the late bud, cap and bell stages of odontogenesis.

MATERIALS AND METHODS

Head areas or parts of jaw containing teeth from 10 human fetuses aged between 9th and 20th developmental weeks were immunohistochemically analyzed using IGF-1, active caspase-3 and HSP-70 markers. Semi-quantitative analysis of each marker's expression pattern was also performed.

RESULTS

During the analyzed period, IGF-1 and HSP-70 were mostly expressed in enamel organ. As development progressed, expression of IGF-1 and HSP-70 became more confined to differentiating tissues in the future cusp tip area, as well as in highly proliferating cervical loops. Few apoptotic bodies highly positive to active caspase-3 were observed in enamel organ and dental papilla from the cap stage onward. However, both enamel epithelia moderately expressed active caspase-3 throughout the investigated period.

CONCLUSIONS

Expression patterns of IGF-1, active caspase-3 and HSP-70 imply importance of these factors for early human tooth development. IGF-1 and HSP-70 have versatile functions in control of proliferation, differentiation and anti-apoptotic protection of epithelial parts of human enamel organ. Active caspase-3 is partially involved in formation and apoptotic removal of primary enamel knot, although present findings might reflect its ability to perform other non-death functions such as differentiation of hard dental tissues secreting cells and guidance of ingrowth of proliferating cervical loops.

The SAP motif and C-terminal RS- and RD/E-rich region influences the sub-nuclear localization of Acinus isoforms

Acinus has been reported to function in apoptosis, RNA processing and regulation of gene transcription including RA-dependent transcription. There are three different isoforms of Acinus termed Acinus-L, Acinus-S', and Acinus-S. The isoforms of Acinus differ in their N-terminus while the C-terminus is consistent in all isoforms. The sub-nuclear localization of Acinus-L and Acinus-S' was determined using fluorescence microscopy. Acinus-S' colocalizes with SC35 in nuclear speckles while Acinus-L localizes diffusely throughout the nucleoplasm. RA treatment has little effect on the sub-nuclear localization of Acinus-L and Acinus-S'. The domains/regions necessary for the distinct sub-nuclear localization of Acinus-L and Acinus-S' were identified. The speckled sub-nuclear localization of Acinus-S' is dependent on its C-terminal RS- and RD/E-rich region but is independent of the phosphorylation status of Ser-453 and Ser-604 within this region. The unique N-terminal SAP motif of Acinus-L is responsible for its diffuse localization in the nucleus. Moreover, the sub-nuclear localization of Acinus isoforms is affected by each other, which is determined by the combinatorial effect of the more potent SAP motif of Acinus-L and the C-terminal RS- and RD/E-rich region in all Acinus isoforms. The C-terminal RS- and RD/E-rich region of Acinus mediates the colocalization of Acinus isoforms as well as with its interacting protein RNPS1. In conclusion, the SAP motif is responsible for the difference in the nuclear localization between Acinus-L and Acinus-S'. This difference in the nuclear localization of Acinus-S' and Acinus-L may suggest that these two isoforms have different functional roles.

Apoptosis: role in myeloid cell development

Hematopoiesis is the process that generates blood cells in an organism from the pluripotent stem cells. Hematopoietic stem cells are characterized by their ability to undergo self-renewal and differentiation. The self-renewing ability ensures that these pluripotent cells are not depleted from the bone marrow niche. A proper balance between cell death and cell survival is necessary to maintain a homeostatic condition, hence, apoptosis, or programmed cell death, is an essential step in hematopoiesis. Recent studies, however, have introduced a new aspect to this process, citing the significance of the apoptosis mediator, caspase, in cell development and differentiation. Extensive research has been carried out to study the possible role of caspases and other apoptosis related factors in the developmental processes. This review focuses on the various apoptotic factors involved in the development and differentiation of myeloid lineage cells: erythrocytes, megakaryocytes, and macrophages.

Specific spatial distribution of caspase-3 in normal lenses

PURPOSE

To determine the distribution of active caspase-3 in rat eye lens epithelium.

METHODS

In total, 120 sagittal sections from forty rats were assessed for active caspase-3 labelling using immunohistochemistry. Lens epithelial cells were counted, and the fraction of active caspase-3 labelled cells and their relative positions were identified in each section.

RESULTS

Active caspase-3 is present in normal lens epithelium. The active caspase-3 expression was higher in the anterior pole of the lens. Probability of radial spatial distribution of labelling was fitted with a logistic model. The increase rate and the inflection point were estimated as CI (0.95) to 23 ± 3 cells and 114 ± 3 cells, respectively.

CONCLUSION

The gradually decreasing active caspase-3 labelling from the anterior pole to the periphery suggests that active caspase-3 may be involved in normal protein turnover caused by, for example, incident light.

Blood and immune cell engineering: Cytoskeletal contractility and nuclear rheology impact cell lineage and localization: Biophysical regulation of hematopoietic differentiation and trafficking

Clinical success with human hematopoietic stem cell (HSC) transplantation establishes a paradigm for regenerative therapies with other types of stem cells. However, it remains generally challenging to therapeutically treat tissues after engineering of stem cells in vitro. Recent studies suggest that stem and progenitor cells sense physical features of their niches. Here, we review biophysical contributions to lineage decisions, maturation, and trafficking of blood and immune cells. Polarized cellular contractility and nuclear rheology are separately shown to be functional markers of a hematopoietic hierarchy that predict the ability of a lineage to traffic in and out of the bone marrow niche. These biophysical determinants are regulated by a set of structural molecules, including cytoplasmic myosin-II and nuclear lamins, which themselves are modulated by a diverse range of transcriptional and post-translational mechanisms. Small molecules that target these mechanobiological circuits, along with novel bioengineering methods, could prove broadly useful in programming blood and immune cells for therapies ranging from blood transfusions to immune attack of tumors.

Review: Beta-thalassemia and molecular chaperones

Thalassemia is known as a diverse single gene disorder, which is prevalent worldwide. The molecular chaperones are set of proteins that help in two important processes while protein synthesis and degradation include folding or unfolding and assembly or disassembly, thereby helping in cell homeostasis. This review recaps current knowledge regarding the role of molecular chaperones in thalassemia, with a focus on beta thalassemia.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Programmed necrosis in the cross talk of cell death and inflammation

Cell proliferation and cell death are integral elements in maintaining homeostatic balance in metazoans. Disease pathologies ensue when these processes are disturbed. A plethora of evidence indicates that malfunction of cell death can lead to inflammation, autoimmunity, or immunodeficiency. Programmed necrosis or necroptosis is a form of nonapoptotic cell death driven by the receptor interacting protein kinase 3 (RIPK3) and its substrate, mixed lineage kinase domain-like (MLKL). RIPK3 partners with its upstream adaptors RIPK1, TRIF, or DAI to signal for necroptosis in response to death receptor or Toll-like receptor stimulation, pathogen infection, or sterile cell injury. Necroptosis promotes inflammation through leakage of cellular contents from damaged plasma membranes. Intriguingly, many of the signal adaptors of necroptosis have dual functions in innate immune signaling. This unique signature illustrates the cooperative nature of necroptosis and innate inflammatory signaling pathways in managing cell and organismal stresses from pathogen infection and sterile tissue injury.