The apoptotic regulator Nrz controls cytoskeletal dynamics via the regulation of Ca2+ trafficking in the zebrafish blastula

The tissue-specific nature of physiological zebrafish mitochondrial bioenergetics

Zebrafish are a powerful tool to study a myriad of experimental conditions, including mitochondrial bioenergetics. Considering that mitochondria are different in many aspects depending on the tissue evaluated, in the zebrafish model there is still a lack of this investigation. Especially for juvenile zebrafish. In the present study, we examined whether different tissues from zebrafish juveniles show mitochondrial density- and tissue-specificity comparing brain, liver, heart, and skeletal muscle (SM). The liver and brain complex IV showed the highest O2 consumption of all ETC in all tissues (10x when compared to other respiratory complexes). The liver showed a higher potential for ROS generation. In this way, the brain and liver showed more susceptibility to O2- generation when compared to other tissues. Regarding Ca2+ transport, the brain showed greater capacity for Ca2+ uptake and the liver presented low Ca2+ uptake capacity. The liver and brain were more susceptible to producing NO. The enzymes SOD and Catalase showed high activity in the brain, whereas GPx showed higher activity in the liver and CS in the SM. TEM reveals, as expected, a physiological diverse mitochondrial morphology. The essential differences between zebrafish tissues investigated probably reflect how the mitochondria play a diverse role in systemic homeostasis. This feature may not be limited to normal metabolic functions but also to stress conditions. In summary, mitochondrial bioenergetics in zebrafish juvenile permeabilized tissues showed a tissue-specificity and a useful tool to investigate conditions of redox system imbalance, mainly in the liver and brain.

Actin-mediated endocytosis in the E-YSL helps drive epiboly in zebrafish

In zebrafish, a punctate band of F-actin is reported to develop in the external yolk syncytial layer (E-YSL) during the latter part of epiboly in zebrafish embryos. Here, electron microscopy (EM) and fluorescence confocal microscopy were conducted to investigate dynamic changes in the E-YSL membrane during epiboly. Using scanning EM, we report that the surface of the E-YSL is highly convoluted, consisting of a complex interwoven network of branching membrane surface microvilli-like protrusions. The region of membrane surface protrusions was relatively wide at 30% epiboly but narrowed as epiboly progressed. This narrowing was coincident with the formation of the punctate actin band. We also demonstrated using immunogold transmission EM that actin clusters were localized at the membrane surface mainly within the protrusions as well as in deeper locations of the E-YSL. Furthermore, during the latter part of epiboly, the punctate actin band was coincident with a region of highly dynamic endocytosis. Treatment with cytochalasin B led to the disruption of the punctate actin band and the membrane surface protrusions, as well as the attenuation of endocytosis. Together, our data suggest that, in the E-YSL, the region encompassing the membrane surface protrusions and its associated punctate actin band are likely to be an integral part of the localized endocytosis, which is important for the progression of epiboly in zebrafish embryos.

CCN6 influences transcription and controls mitochondrial mass and muscle organization

Mutations in Cellular Communication Network Factor 6 (CCN6) are linked to the debilitating musculoskeletal disease Progressive Pseudo Rheumatoid Dysplasia (PPRD), which disrupts mobility. Yet, much remains unknown about CCN6 function at the molecular level. In this study, we revealed a new function of CCN6 in transcriptional regulation. We demonstrated that CCN6 localizes to chromatin and associates with RNA Polymerase II in human chondrocyte lines. Using zebrafish as a model organism we validated the nuclear presence of CCN6 and its association with RNA Polymerase II in different developmental stages from 10 hpf embryo to adult fish muscle. In concurrence with these findings, we confirmed the requirement of CCN6 in the transcription of several genes encoding mitochondrial electron transport complex proteins in the zebrafish, both in the embryonic stages and in the adult muscle. Reduction in the expression of these genes upon morpholino-mediated knockdown of CCN6 protein expression led to reduced mitochondrial mass, which correlated with defective myotome organization during zebrafish muscle development. Overall, this study suggests that the developmental musculoskeletal abnormalities linked with PPRD could be contributed at least partly by impaired expression of genes encoding mitochondrial electron transport complexes due to defects in CCN6 associated transcriptional regulation.

Keeping Cell Death Alive: An Introduction into the French Cell Death Research Network

Since the Nobel Prize award more than twenty years ago for discovering the core apoptotic pathway in C. elegans, apoptosis and various other forms of regulated cell death have been thoroughly characterized by researchers around the world. Although many aspects of regulated cell death still remain to be elucidated in specific cell subtypes and disease conditions, many predicted that research into cell death was inexorably reaching a plateau. However, this was not the case since the last decade saw a multitude of cell death modalities being described, while harnessing their therapeutic potential reached clinical use in certain cases. In line with keeping research into cell death alive, francophone researchers from several institutions in France and Belgium established the French Cell Death Research Network (FCDRN). The research conducted by FCDRN is at the leading edge of emerging topics such as non-apoptotic functions of apoptotic effectors, paracrine effects of cell death, novel canonical and non-canonical mechanisms to induce apoptosis in cell death-resistant cancer cells or regulated forms of necrosis and the associated immunogenic response. Collectively, these various lines of research all emerged from the study of apoptosis and in the next few years will increase the mechanistic knowledge into regulated cell death and how to harness it for therapy.

Mitochondria Lead the Way: Mitochondrial Dynamics and Function in Cellular Movements in Development and Disease

The dynamics, distribution and activity of subcellular organelles are integral to regulating cell shape changes during various physiological processes such as epithelial cell formation, cell migration and morphogenesis. Mitochondria are famously known as the powerhouse of the cell and play an important role in buffering calcium, releasing reactive oxygen species and key metabolites for various activities in a eukaryotic cell. Mitochondrial dynamics and morphology changes regulate these functions and their regulation is, in turn, crucial for various morphogenetic processes. In this review, we evaluate recent literature which highlights the role of mitochondrial morphology and activity during cell shape changes in epithelial cell formation, cell division, cell migration and tissue morphogenesis during organism development and in disease. In general, we find that mitochondrial shape is regulated for their distribution or translocation to the sites of active cell shape dynamics or morphogenesis. Often, key metabolites released locally and molecules buffered by mitochondria play crucial roles in regulating signaling pathways that motivate changes in cell shape, mitochondrial shape and mitochondrial activity. We conclude that mechanistic analysis of interactions between mitochondrial morphology, activity, signaling pathways and cell shape changes across the various cell and animal-based model systems holds the key to deciphering the common principles for this interaction.

Bcl-2 Family of Proteins in the Control of Mitochondrial Calcium Signalling: An Old Chap with New Roles

Bcl-2 family proteins are considered as one of the major regulators of apoptosis. Indeed, this family is known to control the mitochondrial outer membrane permeabilization (MOMP): a central step in the mitochondrial pathway of apoptosis. However, in recent years Bcl-2 family members began to emerge as a new class of intracellular calcium (Ca²⁺) regulators. At mitochondria-ER contacts (MERCs) these proteins are able to interact with major Ca²⁺ transporters, thus controlling mitochondrial Ca²⁺ homeostasis and downstream Ca²⁺ signalling pathways. Beyond the regulation of cell survival, this Bcl-2-dependent control over the mitochondrial Ca²⁺ dynamics has far-reaching consequences on the physiology of the cell. Here, we review how the Bcl-2 family of proteins mechanistically regulate mitochondrial Ca²⁺ homeostasis and how this regulation orchestrates cell death/survival decisions as well as the non-apoptotic process of cell migration.

Moonlighting Proteins

The single gene, single protein, single function hypothesis is increasingly becoming obsolete. Numerous studies have demonstrated that individual proteins can moonlight, meaning they can have multiple functions based on their cellular or developmental context. In this review, we discuss moonlighting proteins, highlighting the biological pathways where this phenomenon may be particularly relevant. In addition, we combine genetic, cell biological, and evolutionary perspectives so that we can better understand how, when, and why moonlighting proteins may take on multiple roles.

Ancient and conserved functional interplay between Bcl-2 family proteins in the mitochondrial pathway of apoptosis

In metazoans, Bcl-2 family proteins are major regulators of mitochondrially mediated apoptosis; however, their evolution remains poorly understood. Here, we describe the molecular characterization of the four members of the Bcl-2 family in the most primitive metazoan, Trichoplax adhaerens All four trBcl-2 homologs are multimotif Bcl-2 group, with trBcl-2L1 and trBcl-2L2 being highly divergent antiapoptotic Bcl-2 members, whereas trBcl-2L3 and trBcl-2L4 are homologs of proapoptotic Bax and Bak, respectively. trBax expression permeabilizes the mitochondrial outer membrane, while trBak operates as a BH3-only sensitizer repressing antiapoptotic activities of trBcl-2L1 and trBcl-2L2. The crystal structure of a trBcl-2L2:trBak BH3 complex reveals that trBcl-2L2 uses the canonical Bcl-2 ligand binding groove to sequester trBak BH3, indicating that the structural basis for apoptosis control is conserved from T. adhaerens to mammals. Finally, we demonstrate that both trBax and trBak BH3 peptides bind selectively to human Bcl-2 homologs to sensitize cancer cells to chemotherapy treatment.

Bcl-2-Protein Family as Modulators of IP3 Receptors and Other Organellar Ca2+ Channels

The pro- and antiapoptotic proteins belonging to the B-cell lymphoma-2 (Bcl-2) family exert a critical control over cell-death processes by enabling or counteracting mitochondrial outer membrane permeabilization. Beyond this mitochondrial function, several Bcl-2 family members have emerged as critical modulators of intracellular Ca²⁺ homeostasis and dynamics, showing proapoptotic and antiapoptotic functions. Bcl-2 family proteins specifically target several intracellular Ca²⁺-transport systems, including organellar Ca²⁺ channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs), Ca²⁺-release channels mediating Ca²⁺ flux from the endoplasmic reticulum, as well as voltage-dependent anion channels (VDACs), which mediate Ca²⁺ flux across the mitochondrial outer membrane into the mitochondria. Although the formation of protein complexes between Bcl-2 proteins and these channels has been extensively studied, a major advance during recent years has been elucidating the complex interaction of Bcl-2 proteins with IP₃Rs. Distinct interaction sites for different Bcl-2 family members were identified in the primary structure of IP₃Rs. The unique molecular profiles of these Bcl-2 proteins may account for their distinct functional outcomes when bound to IP₃Rs. Furthermore, Bcl-2 inhibitors used in cancer therapy may affect IP₃R function as part of their proapoptotic effect and/or as an adverse effect in healthy cells.

The apoptosis inhibitor Bcl-xL controls breast cancer cell migration through mitochondria-dependent reactive oxygen species production

The Bcl-xL apoptosis inhibitor plays a major role in vertebrate development. In addition to its effect on apoptosis, Bcl-xL is also involved in cell migration and mitochondrial metabolism. These effects may favour the onset and dissemination of metastasis. However, the underlying molecular mechanisms remain to be fully understood. Here we focus on the control of cell migration by Bcl-xL in the context of breast cancer cells. We show that Bcl-xL silencing led to migration defects in Hs578T and MDA-MB231 cells. These defects were rescued by re-expressing mitochondria-addressed, but not endoplasmic reticulum-addressed, Bcl-xL. The use of BH3 mimetics, such as ABT-737 and WEHI-539 confirmed that the effect of Bcl-xL on migration did not depend on interactions with BH3-containing death accelerators such as Bax or BH3-only proteins. In contrast, the use of a BH4 peptide that disrupts the Bcl-xL/VDAC1 complex supports that Bcl-xL by acting on VDAC1 permeability contributes to cell migration through the promotion of reactive oxygen species production by the electron transport chain. Collectively our data highlight the key role of Bcl-xL at the interface between cell metabolism, cell death, and cell migration, thus exposing the VDAC1/Bcl-xL interaction as a promising target for anti-tumour therapy in the context of metastatic breast cancer.

Mechanisms of zebrafish epiboly: A current view

Epiboly is a conserved gastrulation movement describing the thinning and spreading of a sheet or multi-layer of cells. The zebrafish embryo has emerged as a vital model system to address the cellular and molecular mechanisms that drive epiboly. In the zebrafish embryo, the blastoderm, consisting of a simple squamous epithelium (the enveloping layer) and an underlying mass of deep cells, as well as a yolk nuclear syncytium (the yolk syncytial layer) undergo epiboly to internalize the yolk cell during gastrulation. The major events during zebrafish epiboly are: expansion of the enveloping layer and the internal yolk syncytial layer, reduction and removal of the yolk membrane ahead of the advancing blastoderm margin and deep cell rearrangements between the enveloping layer and yolk syncytial layer to thin the blastoderm. Here, work addressing the cellular and molecular mechanisms as well as the sources of the mechanical forces that underlie these events is reviewed. The contribution of recent findings to the current model of epiboly as well as open questions and future prospects are also discussed.

NADPH-Oxidase-derived reactive oxygen species are required for cytoskeletal organization, proper localization of E-cadherin and cell motility during zebrafish epiboly

Cell movements are essential for morphogenesis during animal development. Epiboly is the first morphogenetic process in zebrafish in which cells move en masse to thin and spread the deep and enveloping cell layers of the blastoderm over the yolk cell. While epiboly has been shown to be controlled by complex molecular networks, the contribution of reactive oxygen species (ROS) to this process has not previously been studied. Here, we show that ROS are required for epiboly in zebrafish. Visualization of ROS in whole embryos revealed dynamic patterns during epiboly progression. Significantly, inhibition of NADPH oxidase activity leads to a decrease in ROS formation, delays epiboly, alters E-cadherin and cytoskeleton patterns and, by 24 h post-fertilization, decreases embryo survival, effects that are rescued by hydrogen peroxide treatment. Our findings suggest that a delicate ROS balance is required during early development and that disruption of that balance interferes with cell adhesion, leading to defective cell motility and epiboly progression.

Prmt7 regulates epiboly and gastrulation cell movements by facilitating syntenin

Epiboly spreads and thins the blastoderm over the yolk cell during zebrafish gastrulation. Despite of its fundamental function, little is known about the molecular mechanisms that control this coordinated cell movement. In this study, we investigated protein arginine methyltransferase 7 (Prmt7) morphants with an epibolic delay defect in zebrafish. The ratio of morphants with epiboly delay phenotypes increased as the dose of the injected morpholino (MO) increased. Here, syntenin transcripts are maternally deposited and ubiquitously expressed from the oocyte period to the early larva stage. Furthermore, we demonstrated that Prmt7 modulates epibolic movements of the enveloping layer by regulating F-actin organization. These defects can be partially rescued by re-expression of Prmt7 or syntenin protein. Analysis of the earliest cellular defects suggested a role of Prmt7 in the autonomous vegetal expansion of the yolk syncytial layer and the rearrangement of the actin cytoskeleton in extra-embryonic tissues. By a combination of knockdown studies and rescue experiments in zebrafish, we showed that epiboly relies on the molecular networking of Prmt7 by facilitating syntenin, which acts as a regulator for cytoskeleton. This study identifies the important function of the Prmt7 for the progression of zebrafish epiboly and establishes its key role in directional cell movements during early development.

A structural investigation of NRZ mediated apoptosis regulation in zebrafish

Bcl-2 family proteins play a crucial role in regulating apoptosis, a process critical for development, eliminating damaged or infected cells, host-pathogen interactions and in disease. Dysregulation of Bcl-2 proteins elicits an expansive cell survival mechanism promoting cell migration, invasion and metastasis. Through a network of intra-family protein-protein interactions Bcl-2 family members regulate the release of cell death factors from mitochondria. NRZ is a novel zebrafish pro-survival Bcl-2 orthologue resident on mitochondria and the endoplasmic reticulum (ER). However, the mechanism of NRZ apoptosis inhibition has not yet been clarified. Here we examined the interactions of NRZ with pro-apoptotic members of the Bcl-2 family using a combination of isothermal calorimetry and mutational analysis of NRZ. We show that NRZ binds almost all zebrafish pro-apoptotic proteins and displays a broad range of affinities. Furthermore, we define the structural basis for apoptosis inhibition of NRZ by solving the crystal structure of both apo-NRZ and a holo form bound to a peptide spanning the binding motif of the pro-apoptotic zBad, a BH3-only protein orthologous to mammalian Bad. The crystal structure of NRZ revealed that it adopts the conserved Bcl-2 like fold observed for other cellular pro-survival Bcl-2 proteins and employs the canonical ligand binding groove to bind Bad BH3 peptide. NRZ engagement of Bad BH3 involves the canonical ionic interaction between NRZ R86 and Bad D104 and an additional ionic interaction between NRZ D79 and Bad R100, and substitution of either NRZ R86 or D79 to Ala reduces the binding to Bad BH3 tenfold or more. Our findings provide a detailed mechanistic understanding for NRZ mediated anti-apoptotic activity in zebrafish by revealing binding to both Bad and Noxa, suggesting that NRZ is likely to occupy a unique mechanistic role in zebrafish apoptosis regulation by acting as a highly promiscuous pro-apoptotic Bcl-2 binder.

Subcellular Localization and Dynamics of the Bcl-2 Family of Proteins

Bcl-2 family proteins are recognized as major regulators of the mitochondrial pathway of apoptosis. They control the mitochondrial outer membrane permeabilization (MOMP) by directly localizing to this organelle. Further investigations demonstrated that Bcl-2 related proteins are also found in other intracellular compartments such as the endoplasmic reticulum, the Golgi apparatus, the nucleus and the peroxisomes. At the level of these organelles, Bcl-2 family proteins not only regulate MOMP in a remote fashion but also participate in major cellular processes including calcium homeostasis, cell cycle control and cell migration. With the advances of live cell imaging techniques and the generation of fluorescent recombinant proteins, it became clear that the distribution of Bcl-2 proteins inside the cell is a dynamic process which is profoundly affected by changes in the cellular microenvironment. Here, we describe the current knowledge related to the subcellular distribution of the Bcl-2 family of proteins and further emphasize on the emerging concept that this highly dynamic process is critical for cell fate determination.

Breast Cancer Targeting through Inhibition of the Endoplasmic Reticulum-Based Apoptosis Regulator Nrh/BCL2L10

Drug resistance and metastatic relapse remain a top challenge in breast cancer treatment. In this study, we present preclinical evidence for a strategy to eradicate advanced breast cancers by targeting the BCL-2 homolog Nrh/BCL2L10, which we discovered to be overexpressed in >45% of a large cohort of breast invasive carcinomas. Nrh expression in these tumors correlated with reduced metastasis-free survival, and we determined it to be an independent marker of poor prognosis. Nrh protein localized to the endoplasmic reticulum. Mechanistic investigations showed that Nrh made BH4 domain-dependent interactions with the ligand-binding domain of the inositol-1,4,5-triphosphate receptor (IP3R), a type 1/3 Ca2⁺ channel, allowing Nrh to negatively regulate ER-Ca2⁺ release and to mediate antiapoptosis. Notably, disrupting Nrh/IP3R complexes by BH4 mimetic peptides was sufficient to inhibit the growth of breast cancer cells in vitro and in vivo Taken together, our results highlighted Nrh as a novel prognostic marker and a candidate therapeutic target for late stage breast cancers that may be addicted to Nrh.Significance: These findings offer a comprehensive molecular model for the activity of Nrh/BCL2L10, a little studied antiapoptotic molecule, prognostic marker, and candidate drug target in breast cancer. Cancer Res; 78(6); 1404-17. ©2018 AACR.

The primary role of zebrafish nanog is in extra-embryonic tissue

The role of the zebrafish transcription factor Nanog has been controversial. It has been suggested that Nanog is primarily required for the proper formation of the extra-embryonic yolk syncytial layer (YSL) and only indirectly regulates gene expression in embryonic cells. In an alternative scenario, Nanog has been proposed to directly regulate transcription in embryonic cells during zygotic genome activation. To clarify the roles of Nanog, we performed a detailed analysis of zebrafish nanog mutants. Whereas zygotic nanog mutants survive to adulthood, maternal-zygotic (MZnanog) and maternal mutants exhibit developmental arrest at the blastula stage. In the absence of Nanog, YSL formation and epiboly are abnormal, embryonic tissue detaches from the yolk, and the expression of dozens of YSL and embryonic genes is reduced. Epiboly defects can be rescued by generating chimeric embryos of MZnanog embryonic tissue with wild-type vegetal tissue that includes the YSL and yolk cell. Notably, cells lacking Nanog readily respond to Nodal signals and when transplanted into wild-type hosts proliferate and contribute to embryonic tissues and adult organs from all germ layers. These results indicate that zebrafish Nanog is necessary for proper YSL development but is not directly required for embryonic cell differentiation.

Remodeling of Ca2+ signaling in cancer: Regulation of inositol 1,4,5-trisphosphate receptors through oncogenes and tumor suppressors

The calcium ion (Ca²⁺) is a ubiquitous intracellular signaling molecule that regulates diverse physiological and pathological processes, including cancer. Increasing evidence indicates that oncogenes and tumor suppressors regulate the Ca²⁺ transport systems. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are IP₃-activated Ca²⁺ release channels located on the endoplasmic reticulum (ER). They play pivotal roles in the regulation of cell death and survival by controlling Ca²⁺ transfer from the ER to mitochondria through mitochondria-associated ER membranes (MAMs). Optimal levels of Ca²⁺ mobilization to mitochondria are necessary for mitochondrial bioenergetics, whereas excessive Ca²⁺ flux into mitochondria causes loss of mitochondrial membrane integrity and apoptotic cell death. In addition to well-known functions on outer mitochondrial membranes, B-cell lymphoma 2 (Bcl-2) family proteins are localized on the ER and regulate IP₃Rs to control Ca²⁺ transfer into mitochondria. Another regulatory protein of IP₃R, IP₃R-binding protein released with IP₃ (IRBIT), cooperates with or counteracts the Bcl-2 family member depending on cellular states. Furthermore, several oncogenes and tumor suppressors, including Akt, K-Ras, phosphatase and tensin homolog (PTEN), promyelocytic leukemia protein (PML), BRCA1, and BRCA1 associated protein 1 (BAP1), are localized on the ER or at MAMs and negatively or positively regulate apoptotic cell death through interactions with IP₃Rs and regulation of Ca²⁺ dynamics. The remodeling of Ca²⁺ signaling by oncogenes and tumor suppressors that interact with IP₃Rs has fundamental roles in the pathology of cancers.

Ca2+ dynamics in zebrafish morphogenesis

Intracellular calcium ion (Ca²⁺) signaling is heavily involved in development, as illustrated by the use of a number of Ca²⁺ indicators. However, continuous Ca²⁺ patterns during morphogenesis have not yet been studied using fluorescence resonance energy transfer to track the Ca²⁺ sensor. In the present study, we monitored Ca²⁺ levels during zebrafish morphogenesis and differentiation with yellow cameleon, YC2.12. Our results show not only clear changes in Ca²⁺ levels but also continuous Ca²⁺ patterns at 24 hpf and later periods for the first time. Serial Ca²⁺dynamics during early pharyngula period (Prim-5-20; 24–33 hpf) was successfully observed with cameleon, which have not reported anywhere yet. In fact, high Ca²⁺ level occurred concurrently with hindbrain development in segmentation and pharyngula periods. Ca²⁺ patterns in the late gastrula through segmentation periods which were obtained with cameleon, were similar to those obtained previously with other Ca²⁺sensor. Our results suggested that the use of various Ca²⁺ sensors may lead to novel findings in studies of Ca²⁺ dynamics. We hope that these results will prove valuable for further research in Ca²⁺ signaling.

Mitochondrial Ca2+ uptake controls actin cytoskeleton dynamics during cell migration

Intracellular Ca²⁺ signaling regulates cell migration by acting on cytoskeleton architecture, cell directionality and focal adhesions dynamics. In migrating cells, cytosolic Ca²⁺ pool and Ca²⁺ pulses are described as key components of these effects. Whereas the role of the mitochondrial calcium homeostasis and the Mitochondria Cacium Uniporter (MCU) in cell migration were recently highlighted in vivo using the zebrafish model, their implication in actin cystokeleton dynamics and cell migration in mammals is not totally characterized. Here, we show that mcu silencing in two human cell lines compromises their migration capacities. This phenotype is characterized by actin cytoskeleton stiffness, a cell polarization loss and an impairment of the focal adhesion proteins dynamics. At the molecular level, these effects appear to be mediated by the reduction of the ER and cytosolic Ca²⁺ pools, which leads to a decrease in Rho-GTPases, RhoA and Rac1, and Ca²⁺-dependent Calpain activites, but seem to be independent of intracellular ATP levels. Together, this study highlights the fundamental and evolutionary conserved role of the mitochondrial Ca²⁺ homeostasis in cytoskeleton dynamics and cell migration.

Polarized cortical tension drives zebrafish epiboly movements

The principles underlying the biomechanics of morphogenesis are largely unknown. Epiboly is an essential embryonic event in which three tissues coordinate to direct the expansion of the blastoderm. How and where forces are generated during epiboly, and how these are globally coupled remains elusive. Here we developed a method, hydrodynamic regression (HR), to infer 3D pressure fields, mechanical power, and cortical surface tension profiles. HR is based on velocity measurements retrieved from 2D+T microscopy and their hydrodynamic modeling. We applied HR to identify biomechanically active structures and changes in cortex local tension during epiboly in zebrafish. Based on our results, we propose a novel physical description for epiboly, where tissue movements are directed by a polarized gradient of cortical tension. We found that this gradient relies on local contractile forces at the cortex, differences in elastic properties between cortex components and the passive transmission of forces within the yolk cell. All in all, our work identifies a novel way to physically regulate concerted cellular movements that might be instrumental for the mechanical control of many morphogenetic processes.

[From dualism to multiplicity: seeing BCL-2 family proteins and cell death with new eyes]

The concept of cell death has many links to the concept of death itself, defined as the opposite of life. Achievements obtained through research on apoptosis have apparently allowed us to transcend this Manichean view. Death is no longer outside, but rather inside living systems, as a constitutive force at work within the living matter. Whereas the death of cells can be positive and breed "creation" (e.g. during morphogenesis), its dysregulation can also cause or contribute to fatal diseases including cancer. It is tempting to apply this biological discourse to illuminate the relations between life and death, taken in general terms, but does this generalization actually hold? Is this discourse not essentially a metaphor? If cell death is considered as a vital aspect of various biological processes, then are we not faced with some vitalistic conception of death? Are there one or more meanings to the word "death"? Does the power to self-destruct act in opposition to other key features of living entities, or rather in juxtaposition to them? In this article, we first describe how the field of cell death has been developed on the basis of perceived and built dichotomies, mirroring the original opposition between life and death. We detail the limitations of the current paradigm of apoptosis regulation by BCL-2 family proteins, which nicely illustrate the problem of binary thinking in biology. Last, we try to show a way out of this dualistic matrix, by drawing on the notions of multiplicity, complexity, diversity, evolution and contingency.

Cytoskeleton dynamics in early zebrafish development: A matter of phosphorylation?

Early morphogenic movements are an important feature of embryonic development in vertebrates. During zebrafish gastrulation, epiboly progression is driven by the coordinated remodeling of the YSL microtubule network and F-actin cables. We recently described the implication of Nrz, an anti-apoptotic Bcl-2 homolog, in the control of the YSL cytoskeleton dynamics. Nrz knock-down induces premature actin-myosin ring formation leading to margin constriction, epiboly arrest and embryo lethality. At the molecular level, the Nrz protein controls the actin-myosin dynamics through IP3R-dependent calcium levels variation. Here, we discuss these novel findings and propose a model in which reversible phosphorylation of the Nrz/IP3R complex modulates the permeability of the IP3R calcium channel and thus may explain the Nrz-dependent control of IP3R opening required for proper epiboly completion.

Bcl-2 regulation of the inositol 1,4,5-trisphosphate receptor and calcium signaling in normal and malignant lymphocytes: potential new target for cancer treatment

The anti-apoptotic protein Bcl-2 is a versatile regulator of cell survival. Its interactions with its own pro-apoptotic family members are widely recognized for their role in promoting the survival of cancer cells. These interactions are thus being targeted for cancer treatment. Less widely recognized is the interaction of Bcl-2 with the inositol 1,4,5-trisphosphate receptor (InsP3R), an InsP3-gated Ca(2+) channel located on the endoplasmic reticulum. The nature of this interaction, the mechanism by which it controls Ca(2+) release from the ER, its role in T-cell development and survival, and the possibility of targeting it as a novel cancer treatment strategy are summarized in this review. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Dynamic regulation of the microtubule and actin cytoskeleton in zebrafish epiboly

Gastrulation is a key developmental stage with striking changes in morphology. Coordinated cell movements occur to bring cells to their correct positions in a timely manner. Cell movements and morphological changes are accomplished by precisely controlling dynamic changes in cytoskeletal proteins, microtubules, and actin filaments. Among those cellular movements, epiboly produces the first distinct morphological changes in teleosts. In this review, I describe epiboly and its mechanics, and the dynamic changes in microtubule networks and actin structures, mainly in zebrafish embryos. The factors regulating those cytoskeletal changes will also be discussed.

Mitochondria-mediated apoptosis in mammals

The mitochondria-mediated caspase activation pathway is a major apoptotic pathway characterized by mitochondrial outer membrane permeabilization (MOMP) and subsequent release of cytochrome c into the cytoplasm to activate caspases. MOMP is regulated by the Bcl-2 family of proteins. This pathway plays important roles not only in normal development, maintenance of tissue homeostasis and the regulation of immune system, but also in human diseases such as immune disorders, neurodegeneration and cancer. In the past decades the molecular basis of this pathway and the regulatory mechanism have been comprehensively studied, yet a great deal of new evidence indicates that cytochrome c release from mitochondria does not always lead to irreversible cell death, and that caspase activation can also have non-death functions. Thus, many unsolved questions and new challenges are still remaining. Furthermore, the dysfunction of this pathway involved in cancer development is obvious, and targeting the pathway as a therapeutic strategy has been extensively explored, but the efficacy of the targeted therapies is still under development. In this review we will discuss the mitochondria-mediated apoptosis pathway and its physiological roles and therapeutic implications.

Calcium signaling in extraembryonic domains during early teleost development

It is becoming recognized that the extraembryonic domains of developing vertebrates, that is, those that make no cellular contribution to the embryo proper, act as important signaling centers that induce and pattern the germ layers and help establish the key embryonic axes. In the embryos of teleost fish, in particular, significant progress has been made in understanding how signaling activity in extraembryonic domains, such as the enveloping layer, the yolk syncytial layer, and the yolk cell, might help regulate development via a combination of inductive interactions, cellular dynamics, and localized gene expression. Ca(2+) signaling in a variety of forms that include propagating waves and standing gradients is a feature found in all three teleostean extraembryonic domains. This leads us to propose that in addition to their other well-characterized signaling activities, extraembryonic domains are well suited (due to their relative stability and continuity) to act as Ca(2+) signaling centers and conduits.

Bcl-wav and the mitochondrial calcium uniporter drive gastrula morphogenesis in zebrafish

Bcl-2 proteins are acknowledged as key regulators of programmed cell death. However, increasing data suggest additional roles, including regulation of the cell cycle, metabolism and cytoskeletal dynamics. Here we report the discovery and characterization of a new Bcl-2-related multidomain apoptosis accelerator, Bcl-wav, found in fish and frogs. Genetic and molecular studies in zebrafish indicate that Bcl-wav and the recently identified mitochondrial calcium uniporter (MCU) contribute to the formation of the notochord axis by controlling blastomere convergence and extension movements during gastrulation. Furthermore, we found that Bcl-wav controls intracellular Ca(2+) trafficking by acting on the mitochondrial voltage-dependent anion channel, and possibly on MCU, with direct consequences on actin microfilament dynamics and blastomere migration guidance. Thus, from an evolutionary point of view, the original function of Bcl-2 proteins might have been to contribute in controlling the global positioning system of blastomeres during gastrulation, a critical step in metazoan development.

The Bcl-2 homolog Nrz inhibits binding of IP3 to its receptor to control calcium signaling during zebrafish epiboly

Members of the Bcl-2 protein family regulate mitochondrial membrane permeability and also localize to the endoplasmic reticulum where they control Ca(2+) homeostasis by interacting with inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs). In zebrafish, Bcl-2-like 10 (Nrz) is required for Ca(2+) signaling during epiboly and gastrulation. We characterized the mechanism by which Nrz controls IP3-mediated Ca(2+) release during this process. We showed that Nrz was phosphorylated during early epiboly, and that in embryos in which Nrz was knocked down, reconstitution with Nrz bearing mutations designed to prevent its phosphorylation disrupted cyclic Ca(2+) transients and the assembly of the actin-myosin ring and led to epiboly arrest. In cultured cells, wild-type Nrz, but not Nrz with phosphomimetic mutations, interacted with the IP3 binding domain of IP3R1, inhibited binding of IP3 to IP3R1, and prevented histamine-induced increases in cytosolic Ca(2+). Collectively, these data suggest that Nrz phosphorylation is necessary for the generation of IP3-mediated Ca(2+) transients and the formation of circumferential actin-myosin cables required for epiboly. Thus, in addition to their role in apoptosis, by tightly regulating Ca(2+) signaling, Bcl-2 family members participate in the cellular events associated with early vertebrate development, including cytoskeletal dynamics and cell movement.

Nrz but not zBcl-xL antagonizes Bcl-wav pro-apoptotic activity in zebrafish

We recently identified a new highly divergent Bcl-2 related protein, named Bcl-wav, with phylogenetic pattern restricted to aquatic anamniotes. In zebrafish gastrula, bclwav gene silencing resulted in calcium-dependent cytoskeleton remodeling leading to convergence and extension movements defaults and abnormal orientation of the larva notochord. Beyond its function in cell migration, Bcl-wav was found to act as a pro-apoptotic Bcl-2 member inducing Bax/Bak dependent cell death. Here we show that, in zebrafish, pro-apoptotic Bcl-wav activity is selectively counteracted by the anti-apoptotic Nrz protein but not by zBcl-xL. Indeed Nrz but not zBcl-xL was able to decrease Bcl-wav dependent embryo mortality. Furthermore Nrz was able to prevent apoptosis induced by Bcl-wav ectopic expression in the embryo’s head and tail. Finally co-immunoprecipitation experiments in HeLa cells showed that Bcl-wav directly interacts with Nrz. Overall these results expand our current knowledge about Bcl-2 family proteins interactome during early zebrafish development.

Microtubule-associated protein 9 (Map9/Asap) is required for the early steps of zebrafish development

Microtubules are structural components of the cell cytoskeleton and key factors for mitosis and ciliogenesis in eukaryotes. The regulation of MT dynamics requires non-motor MAPs. We previously showed that, in human cells in culture, MAP9 (also named ASAP) is involved in MT dynamics and is essential for mitotic spindle formation and mitosis progression. Indeed, misexpression of MAP9 leads to severe mitotic defects and cell death. Here, we investigated the in vivo role of map9 during zebrafish development. Map9 is expressed mainly as a maternal gene. Within cells, Map9 is associated with the MT network of the mitotic spindle and with centrosomes. Morpholino-mediated depletion of map9 leads to early development arrest before completion of epiboly. Map9 localizes to the MT array of the YSL. This MT network is destroyed in Map9-depleted embryos, and injection of anti-map9 morpholinos directly in the nascent YSL leads to arrest of epiboly/gastrulation. Finally, map9 knockdown deregulates the expression of genes involved in endodermal differentiation, dorso-ventral and left-right patterning, and other MT-based functions. At low morpholino doses, the surviving embryos show dramatic developmental defects, spindle and mitotic defects, and increased apoptosis. Our findings suggest that map9 is a crucial factor in early zebrafish development by regulating different MT-based processes.

C2orf62 and TTC17 are involved in actin organization and ciliogenesis in zebrafish and human

Vertebrate genomes contain around 20,000 protein-encoding genes, of which a large fraction is still not associated with specific functions. A major task in future genomics will thus be to assign physiological roles to all open reading frames revealed by genome sequencing. Here we show that C2orf62, a highly conserved protein with little homology to characterized proteins, is strongly expressed in testis in zebrafish and mammals, and in various types of ciliated cells during zebrafish development. By yeast two hybrid and GST pull-down, C2orf62 was shown to interact with TTC17, another uncharacterized protein. Depletion of either C2orf62 or TTC17 in human ciliated cells interferes with actin polymerization and reduces the number of primary cilia without changing their length. Zebrafish embryos injected with morpholinos against C2orf62 or TTC17, or with mRNA coding for the C2orf62 C-terminal part containing a RII dimerization/docking (R2D2) - like domain show morphological defects consistent with imperfect ciliogenesis. We provide here the first evidence for a C2orf62-TTC17 axis that would regulate actin polymerization and ciliogenesis.

Cell death proteins: an evolutionary role in cellular adaptation before the advent of apoptosis

Programmed cell death (PCD) or apoptosis is a broadly conserved phenomenon in metazoans, whereby activation of canonical signal pathways induces an ordered dismantling and death of a cell. Paradoxically, the constituent proteins and pathways of PCD (most notably the metacaspase/caspase protease mediated signal pathways) have been demonstrated to retain non-death functions across all phyla including yeast, nematodes, drosophila, and mammals. The ancient conservation of both death and non-death functions of PCD proteins raises an interesting evolutionary conundrum: was the primordial intent of these factors to induce cell death or to regulate other cellular adaptations? Here, we propose the hypothesis that apoptotic behavior of PCD proteins evolved or were co-opted from core non-death functions.

The selective BH4-domain biology of Bcl-2-family members: IP3Rs and beyond

Anti-apoptotic Bcl-2-family members not only neutralize pro-apoptotic proteins but also directly regulate intracellular Ca(2+) signaling from the endoplasmic reticulum (ER), critically controlling cellular health, survival, and death initiation. Furthermore, distinct Bcl-2-family members may selectively regulate inositol 1,4,5-trisphosphate receptor (IP3R): Bcl-2 likely acts as an endogenous inhibitor of the IP3R, preventing pro-apoptotic Ca(2+) transients, while Bcl-XL likely acts as an endogenous IP3R-sensitizing protein promoting pro-survival Ca(2+) oscillations. Furthermore, distinct functional domains in Bcl-2 and Bcl-XL may underlie the divergence in IP3R regulation. The Bcl-2 homology (BH) 4 domain, which targets the central modulatory domain of the IP3R, is likely to be Bcl-2's determining factor. In contrast, the hydrophobic cleft targets the C-terminal Ca(2+)-channel tail and might be more crucial for Bcl-XL's function. Furthermore, one amino acid critically different in the sequence of Bcl-2's and Bcl-XL's BH4 domains underpins their selective effect on Ca(2+) signaling and distinct biological properties of Bcl-2 versus Bcl-XL. This difference is evolutionary conserved across five classes of vertebrates and may represent a fundamental divergence in their biological function. Moreover, these insights open novel avenues to selectively suppress malignant Bcl-2 function in cancer cells by targeting its BH4 domain, while maintaining essential Bcl-XL functions in normal cells. Thus, IP3R-derived molecules that mimic the BH4 domain's binding site on the IP3R may function synergistically with BH3-mimetic molecules selectivity suppressing Bcl-2's proto-oncogenic activity. Finally, a more general role for the BH4 domain on IP3Rs, rather than solely anti-apoptotic, may not be excluded as part of a complex network of molecular interactions.

Multiple functions of BCL-2 family proteins

BCL-2 family proteins are the regulators of apoptosis, but also have other functions. This family of interacting partners includes inhibitors and inducers of cell death. Together they regulate and mediate the process by which mitochondria contribute to cell death known as the intrinsic apoptosis pathway. This pathway is required for normal embryonic development and for preventing cancer. However, before apoptosis is induced, BCL-2 proteins have critical roles in normal cell physiology related to neuronal activity, autophagy, calcium handling, mitochondrial dynamics and energetics, and other processes of normal healthy cells. The relative importance of these physiological functions compared to their apoptosis functions in overall organismal physiology is difficult to decipher. Apoptotic and noncanonical functions of these proteins may be intertwined to link cell growth to cell death. Disentanglement of these functions may require delineation of biochemical activities inherent to the characteristic three-dimensional shape shared by distantly related viral and cellular BCL-2 family members.

Non-apoptotic roles of Bcl-2 family: the calcium connection

The existence of the bcl-2 (B-cell lymphoma-2) gene was reported nearly 30 years ago. Yet, Bcl-2 family group of proteins still surprises us with their structural and functional diversity. Since the discovery of the Bcl-2 family of proteins as one of the main apoptosis judges, the precise mechanism of their action remains a hot topic of intensive scientific research and debates. Although extensive work has been performed on the role of mitochondria in apoptosis, more and more studies point out an implication of the endoplasmic reticulum in this process. Interestingly, Bcl-2 family proteins could be localized to both the mitochondria and the endoplasmic reticulum highlighting their crucial role in apoptosis control. In particular, in these organelles Bcl-2 proteins seem to be involved in calcium homeostasis regulation although the mechanisms underlying this function are still misunderstood. We now assume with high degree of certainty that the majority of Bcl-2 family members take part not only in apoptosis regulation but also in other processes important for the cell physiology briefly denominated as "non-apoptotic" functions. Drawing a complete and comprehensive image of Bcl-2 family requires the understanding of their implications in all cellular processes. Here, we review the current knowledge on the control of calcium homeostasis by the Bcl-2 family at the endoplasmic reticulum and at the mitochondria. Then we focus on the non-apoptotic functions of the Bcl-2 proteins in relation with the regulation of this versatile intracellular messenger. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Characterization of Ca(2+) signaling in the external yolk syncytial layer during the late blastula and early gastrula periods of zebrafish development

Preferential loading of the complementary bioluminescent (f-aequorin) and fluorescent (Calcium Green-1 dextran) Ca(2+) reporters into the yolk syncytial layer (YSL) of zebrafish embryos, revealed the generation of stochastic patterns of fast, short-range, and slow, long-range Ca(2+) waves that propagate exclusively through the external YSL (E-YSL). Starting abruptly just after doming (~4.5h post-fertilization: hpf), and ending at the shield stage (~6.0hpf) these distinct classes of waves propagated at mean velocities of ~50 and ~4μm/s, respectively. Although the number and pattern of these waves varied between embryos, their initiation site and arcs of propagation displayed a distinct dorsal bias, suggesting an association with the formation and maintenance of the nascent dorsal-ventral axis. Wave initiation coincided with a characteristic clustering of YSL nuclei (YSN), and their associated perinuclear ER, in the E-YSL. Furthermore, the inter-YSN distance (IND) appeared to be critical such that Ca(2+) wave propagation occurred only when this was <~8μm; an IND >~8μm was coincidental with wave termination at shield stage. Treatment with the IP3R antagonist, 2-APB, the Ca(2+) buffer, 5,5'-dibromo BAPTA, and the SERCA-pump inhibitor, thapsigargin, resulted in a significant disruption of the E-YSL Ca(2+) waves, whereas exposure to the RyR antagonists, ryanodine and dantrolene, had no significant effect. These findings led us to propose that the E-YSL Ca(2+) waves are generated mainly via Ca(2+) release from IP3Rs located in the perinuclear ER, and that the clustering of the YSN is an essential step in providing a CICR pathway required for wave propagation. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Diva/BclB regulates differentiation by inhibiting NDPKB/Nm23H2-mediated neuronal differentiation in PC-12 cells

Background

Diva (death inducer binding to vBcl-2 and Apaf-1)/BclB is a Bcl-2 family member, which is known for its function in apoptosis. Diva/BclB has been shown to interact with NDPKB/Nm23H2, which is involved in cellular differentiation. Thus far, there has been no direct evidence of Diva/BclB having a role in differentiation. In the present study, we investigated the expression of Diva/BclB and NDPKB/Nm23H2 during differentiation in PC-12 cell line.

Results

Our results show that after differentiation, Diva/BclB expression was decreased and reciprocally, NDPKB/Nm23H2 expression was increased and it translocated into the nucleus. Overexpression of NDPKB/Nm23H2 promoted PC-12 neuronal differentiation by increasing neurite outgrowth and arresting cell cycle progression. There was a concurrent downregulation of Diva/Boo when NDPKB/Nm23H2 was overexpressed, which mirrors the effect of NGF on PC-12 cell differentiation. Overexpression of Diva/BclB did not change the expression level of NDPKB/Nm23H2, but inhibited its nuclear localization. Cells that overexpressed Diva/BclB presented a decreased percentage of differentiated cells and average neurite length was shortened. This was due to an increase in the formation of Diva/BclB and NDPKB/Nm23H2 complexes as well as Diva/BclB and β-tubulin complexes. Concomitantly, there was a decrease in formation of NDPKB/Nm23H2 and β-tubulin complexes. Overexpression of Diva/BclB also resulted in a higher percentage of S-phase cells.

Conclusion

Our results showed a novel role for Diva/BclB in neuronal differentiation. Its downregulation during neuronal differentiation may be necessary to allow NDPKB/Nm23H2 and β-tubulin interaction that promotes NDPKB/Nm23H2 mediated differentiation.

Calcium signaling in developing embryos: focus on the regulation of cell shape changes and collective movements

During morphogenesis tissues significantly remodel by coordinated cell migrations and cell rearrangements. Central to this problem are cell shape changes that are driven by distinct cytoskeletal reorganization responsible for force generation. Calcium is a versatile and universal messenger that is implicated in the regulation of embryonic development. Although calcium transients accrue clearly and more intensely in tissues undergoing rearrangement/migration, it is far from clear what the role of these calcium signals is. Here we summarize the evidence implicating calcium participation in tissue movements, cell shape changes and the reorganization of contractile cytoskeletal elements in developing embryos. We also discuss a novel hypothesis that short-lived calcium spikes are required in cells and tissues undergoing migration and rearrangements as a fine tuning response mechanism to prevent local, abnormally high fluctuations in cytoskeletal activities.

The yolk cell of the zebrafish blastula harbors functional apoptosis machinery

We recently described the implication of the Bcl-2 related antiapoptotic Nrz protein during early zebrafish development. Nrz knock-down induces calcium-dependent cytoskeleton remodeling leading to margin constriction and premature embryo lethality. In the YSL, nrz knock-down embryos exhibit some typical features of apoptosis such as mitochondrial transmembrane potential loss and cytochrome c release. However, downstream caspase-3 activation has not been detected so far. Here, we report that the YSL contains fully functional apoptotic machinery that can activate caspase-3 following zBax ectopic expression. Furthermore, we present evidence that caspase-3 activation is actually detectable in nrz knock-down embryos when premature margin constriction is prevented.

There is more to life than death: a moonlighting function of a Bcl-2 member

Members of the Bcl-2 family proteins are best known for their roles in apoptosis regulation. In this issue of Developmental Cell, Popgeorgiev et al. (2011) have uncovered a new, nonapoptotic role for a Bcl-2 homolog during early embryogenesis in zebrafish.