Exocytosis of the silicified cell wall of diatoms involves extensive membrane disintegration.

Diatoms are unicellular algae characterized by silica cell walls. These silica elements are known to be formed intracellularly in membrane-bound silica deposition vesicles and exocytosed after completion. How diatoms maintain membrane homeostasis during the exocytosis of these large and rigid silica elements remains unknown. Here we study the membrane dynamics during cell wall formation and exocytosis in two model diatom species, using live-cell confocal microscopy, transmission electron microscopy and cryo-electron tomography. Our results show that during its formation, the mineral phase is in tight association with the silica deposition vesicle membranes, which form a precise mold of the delicate geometrical patterns. We find that during exocytosis, the distal silica deposition vesicle membrane and the plasma membrane gradually detach from the mineral and disintegrate in the extracellular space, without any noticeable endocytic retrieval or extracellular repurposing. We demonstrate that within the cell, the proximal silica deposition vesicle membrane becomes the new barrier between the cell and its environment, and assumes the role of a new plasma membrane. These results provide direct structural observations of diatom silica exocytosis, and point to an extraordinary mechanism in which membrane homeostasis is maintained by discarding, rather than recycling, significant membrane patches.

Efficient gene replacement by CRISPR/Cas-mediated homologous recombination in the model diatom Thalassiosira pseudonana.

CRISPR/Cas enables targeted genome editing in many different plant and algal species including the model diatom Thalassiosira pseudonana. However, efficient gene targeting by homologous recombination (HR) to date is only reported for photosynthetic organisms in their haploid life-cycle phase. Here, a CRISPR/Cas construct, assembled using Golden Gate cloning, enabled highly efficient HR in a diploid photosynthetic organism. Homologous recombination was induced in T. pseudonana using sequence-specific CRISPR/Cas, paired with a dsDNA donor matrix, generating substitution of the silacidin, nitrate reductase and urease genes by a resistance cassette (FCP:NAT). Up to c. 85% of NAT-resistant T. pseudonana colonies screened positive for HR by nested PCR. Precise integration of FCP:NAT at each locus was confirmed using an inverse PCR approach. The knockout of the nitrate reductase and urease genes impacted growth on nitrate and urea, respectively, while the knockout of the silacidin gene in T. pseudonana caused a significant increase in cell size, confirming the role of this gene for cell-size regulation in centric diatoms. Highly efficient gene targeting by HR makes T. pseudonana as genetically tractable as Nannochloropsis and Physcomitrella, hence rapidly advancing functional diatom biology, bionanotechnology and biotechnological applications targeted on harnessing the metabolic potential of diatoms.

Complex morphologies of biogenic crystals emerge from anisotropic growth of symmetry-related facets.

Directing crystal growth into complex morphologies is challenging, as crystals tend to adopt thermodynamically stable morphologies. However, many organisms form crystals with intricate morphologies, as exemplified by coccoliths, microscopic calcite crystal arrays produced by unicellular algae. The complex morphologies of the coccolith crystals were hypothesized to materialize from numerous crystallographic facets, stabilized by fine-tuned interactions between organic molecules and the growing crystals. Using electron tomography, we examined multiple stages of coccolith development in three dimensions. We found that the crystals express only one set of symmetry-related crystallographic facets, which grow differentially to yield highly anisotropic shapes. Morphological chirality arises from positioning the crystals along specific edges of these same facets. Our findings suggest that growth rate manipulations are sufficient to yield complex crystalline morphologies.

Structural evidence for extracellular silica formation by diatoms.

The silica cell wall of diatoms, a widespread group of unicellular microalgae, is an exquisite example for the ability of organisms to finely sculpt minerals under strict biological control. The prevailing paradigm for diatom silicification is that this is invariably an intracellular process, occurring inside specialized silica deposition vesicles that are responsible for silica precipitation and morphogenesis. Here, we study the formation of long silicified extensions that characterize many diatom species. We use cryo-electron tomography to image silica formation in situ, in 3D, and at a nanometer-scale resolution. Remarkably, our data suggest that, contradictory to the ruling paradigm, these intricate structures form outside the cytoplasm. In addition, the formation of these silica extensions is halted at low silicon concentrations that still support the formation of other cell wall elements, further alluding to a different silicification mechanism. The identification of this unconventional strategy expands the suite of mechanisms that diatoms use for silicification.

In situ electron microscopy characterization of intracellular ion pools in mineral forming microalgae.

The formation of coccoliths, intricate calcium carbonate scales that cover the cells of unicellular marine microalgae, is a highly regulated biological process. For decades, scientists have tried to elucidate the cellular, chemical, and structural mechanisms that control the precise mineralogy and shape of the inorganic crystals. Transmission electron microscopy was pivotal in characterizing some of the organelles that orchestrate this process. However, due to the difficulties in preserving soluble inorganic phases during sample preparation, only recently, new intracellular ion-pools were detected using state-of-the-art cryo X-ray and electron microscopy techniques. Here, we combine a completely non-aqueous sample preparation procedure and room temperature electron microscopy, to investigate the presence, cellular location, and composition, of mineral phases inside mineral forming microalga species. This methodology, which fully preserves the forming coccoliths and the recently identified Ca-P-rich bodies, allowed us to identify a new class of ion-rich compartments that have complex internal structure. In addition, we show that when carefully choosing heavy metal stains, elemental analysis of the mineral phases can give accurate chemical signatures of the inorganic phases. Applying this approach to mineral forming microalgae will bridge the gap between the low-preservation power for inorganic phases of conventional chemical-fixation based electron microscopy, and the low-yield of advanced cryo techniques.

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 ß subunit (A-Sß), 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-Sß and the GTP-specific SCSß (G-Sß), which functions redundantly in the Krebs cycle, show that the metabolic and structural roles of A-Sß in spermatids can be uncoupled, highlighting a moonlighting function of this Krebs cycle component in CRL activation.

The unconventional myosin CRINKLED and its mammalian orthologue MYO7A regulate caspases in their signalling roles.

Caspases provide vital links in non-apoptotic regulatory networks controlling inflammation, compensatory proliferation, morphology and cell migration. How caspases are activated under non-apoptotic conditions and process a selective set of substrates without killing the cell remain enigmatic. Here we find that the Drosophila unconventional myosin CRINKLED (CK) selectively interacts with the initiator caspase DRONC and regulates some of its non-apoptotic functions. Loss of CK in the arista, border cells or proneural clusters of the wing imaginal discs affects DRONC-dependent patterning. Our data indicate that CK acts as substrate adaptor, recruiting SHAGGY46/GSK3-ß to DRONC, thereby facilitating caspase-mediated cleavage and localized modulation of kinase activity. Similarly, the mammalian CK counterpart, MYO7A, binds to and impinges on CASPASE-8, revealing a new regulatory axis affecting receptor interacting protein kinase-1 (RIPK1)>CASPASE-8 signalling. Together, our results expose a conserved role for unconventional myosins in transducing caspase-dependent regulation of kinases, allowing them to take part in specific signalling events.

Sporoptosis: sowing the seeds of nuclear destruction.

What are the origins of programmed cell death (PCD)? In this issue of Developmental Cell, Eastwood et al. (2012) uncover an ancient developmental program of nuclear destruction in yeast, implying that some PCD mechanisms could have emerged from nonlethal processes before the divergence of fungi and metazoan.

Apoptosis is regulated by the VDAC1 N-terminal region and by VDAC oligomerization: release of cytochrome c, AIF and Smac/Diablo.

Mitochondria, central to basic life functions due to their generation of cellular energy, also serve as the venue for cellular decisions leading to apoptosis. A key protein in mitochondria-mediated apoptosis is the voltage-dependent anion channel (VDAC), which also mediates the exchange of metabolites and energy between the cytosol and the mitochondria. In this study, the functions played by the N-terminal region of VDAC1 and by VDAC1 oligomerization in the release of cytochrome c, Smac/Diablo and apoptosis-inducing factor (AIF) and subsequent apoptosis were addressed. We demonstrate that cells undergoing apoptosis induced by STS or cisplatin and expressing N-terminally truncated VDAC1 do not release cytochrome c, Smac/Diablo or AIF. Ruthenium red (RuR), AzRu, DIDS and hexokinase-I (HK-I), all known to interact with VDAC, inhibited the release of cytochrome c, Smac/Diablo and AIF, while RuR-mediated inhibition was not observed in cells expressing RuR-insensitive E72Q-VDAC1. These findings suggest that VDAC1 is involved in the release of not only cytochrome c but also of Smac/Diablo and AIF. We also demonstrate that apoptosis induction is associated with VDAC oligomerization, as revealed by chemical cross-linking and monitoring in living cells using Bioluminescence Resonance Energy Transfer. Apoptosis induction by STS, H2O2 or selenite augmented the formation of VDAC oligomers several fold. The results show VDAC1 to be a component of the apoptosis machinery and offer new insight into the functions of VDAC1 oligomerization in apoptosis and of the VDAC1 N-terminal domain in the release of apoptogenic proteins as well as into regulation of VDAC by anti-apoptotic proteins, such as HK and Bcl2.

VDAC1 cysteine residues: topology and function in channel activity and apoptosis.

The VDAC (voltage-dependent anion channel) is proposed to control metabolic cross-talk between mitochondria and the cytosol, as well as apoptotic cell death. It has been suggested that apoptosis is modulated by the oxidation state of VDAC. Since cysteine residues are the major target for oxidation/reduction, we verified whether one or both VDAC1 cysteine residues are involved in VDAC1-mediated transport or apoptosis activities. To assess the function of VDAC1 cysteine residues in channel activity and to probe cysteine topology with respect to facing the pore or the bilayer, we used thiol-modifying agents, namely membrane-permeable NEM (N-ethylmaleimide), bulky charged 5-FM (fluorescein-5-maleimide) and the cross-linking reagent BMOE [bis(maleimido)ethane]. Bilayer-reconstituted VDAC conductance was decreased by 5-FM, but not by NEM, whereas 5-FM had no effect on NEM-labelled VDAC conductance. BMOE caused the formation of dimeric VDAC1, suggesting that one of the two VDAC1 cysteine residues is exposed and available for cross-linking. The results thus suggest that one of the VDAC1 cysteine residues faces the VDAC pore, whereas the second is oriented towards the lipid bilayer. Mutated rat VDAC1 in which the two cysteine residues, Cys127 and Cys232, were replaced by alanine residues showed channel activity like native VDAC1 and, when expressed in cells, was localized to mitochondria. Human VDAC1-shRNA (small hairpin RNA)- or -siRNA (small interfering RNA)-treated cells, expressing low levels of endogenous human VDAC1 together with native or cysteine-less rat VDAC1, undergo apoptosis as induced by overexpression of these VDAC1 or upon treatment with reactive oxygen species-producing agents, H2O2, As2O3 or selenite, suggesting that the two cysteine residues are not required for apoptosis or VDAC1 oligomerization.