Expression of Drosophila lamin C is developmentally regulated: analogies with vertebrate A-type lamins

Drosophila anterior midgut internalization via collective epithelial-mesenchymal transition at the embryo surface and enclosure by surrounding tissues

Internal organ development requires cell internalization, which can occur individually or collectively. The best characterized mode of collective internalization is epithelial invagination. Alternate modes involving collective mesenchymal behaviours at the embryo surface have been documented, but their prevalence is unclear. The Drosophila embryo has been a major model for the study of epithelial invaginations. However, internalization of the Drosophila anterior midgut primordium is incompletely understood. Here, we report that an epithelial-mesenchymal transition (EMT) occurs across the internalizing primordium when it is still at the embryo surface. At the earliest internalization stage, the primordium displays less junctional DE-cadherin than surrounding tissues but still exhibits coordinated epithelial structure as it invaginates with the ventral furrow. This initial invagination is transient, and its loss correlates with the activation of an associated mitotic domain. Activation of a subsequent mitotic domain across the broader primordium results in cell divisions with mixed orientations that deposit some cells within the embryo. However, cell division is non-essential for primordium internalization. Post-mitotically, the surface primordium displays hallmarks of EMT: loss of adherens junctions, loss of epithelial cell polarity, and gain of cell protrusions. Primordium cells extend over each other as they internalize asynchronously as individuals or small groups, and the primordium becomes enclosed by the reorganizations of surrounding epithelial tissues. We propose that collective EMT at the embryo surface promotes anterior midgut internalization through both inwardly-directed divisions and movements of its cells, and that the latter process is facilitated by surrounding tissue remodeling.

Mechanoresponsive regulation of myogenesis by the force-sensing transcriptional regulator Tono

Muscle morphogenesis is a multi-step program, starting with myoblast fusion, followed by myotube-tendon attachment and sarcomere assembly, with subsequent sarcomere maturation, mitochondrial amplification, and specialization. The correct chronological order of these steps requires precise control of the transcriptional regulators and their effectors. How this regulation is achieved during muscle development is not well understood. In a genome-wide RNAi screen in Drosophila, we identified the BTB-zinc-finger protein Tono (CG32121) as a muscle-specific transcriptional regulator. tono mutant flight muscles display severe deficits in mitochondria and sarcomere maturation, resulting in uncontrolled contractile forces causing muscle rupture and degeneration during development. Tono protein is expressed during sarcomere maturation and localizes in distinct condensates in flight muscle nuclei. Interestingly, internal pressure exerted by the maturing sarcomeres deforms the muscle nuclei into elongated shapes and changes the Tono condensates, suggesting that Tono senses the mechanical status of the muscle cells. Indeed, external mechanical pressure on the muscles triggers rapid liquid-liquid phase separation of Tono utilizing its BTB domain. Thus, we propose that Tono senses high mechanical pressure to adapt muscle transcription, specifically at the sarcomere maturation stages. Consistently, tono mutant muscles display specific defects in a transcriptional switch that represses early muscle differentiation genes and boosts late ones. We hypothesize that a similar mechano-responsive regulation mechanism may control the activity of related BTB-zinc-finger proteins that, if mutated, can result in uncontrolled force production in human muscle.

Ninein domains required for its localization, association with partners dynein and ensconsin, and microtubule organization

Ninein (Nin) is a microtubule (MT) anchor at the subdistal appendages of mother centrioles and the pericentriolar material (PCM) of centrosomes that also functions to organize MTs at noncentrosomal MT-organizing centers (ncMTOCs). In humans, the NIN gene is mutated in Seckel syndrome, an inherited developmental disorder. Here, we dissect the protein domains involved in Nin's localization and interactions with dynein and ensconsin (ens/MAP7) and show that the association with ens cooperatively regulates MT assembly in Drosophila fat body cells. We define domains of Nin responsible for its localization to the ncMTOC on the fat body cell nuclear surface, localization within the nucleus, and association with Dynein light intermediate chain (Dlic) and ens, respectively. We show that Nin's association with ens synergistically regulates MT assembly. Together, these findings reveal novel features of Nin function and its regulation of a ncMTOC.

Confinement promotes nematic alignment of spindle-shaped cells during Drosophila embryogenesis

Tissue morphogenesis is often controlled by actomyosin networks pulling on adherens junctions (AJs), but junctional myosin levels vary. At an extreme, the Drosophila embryo amnioserosa forms a horseshoe-shaped strip of aligned, spindle-shaped cells lacking junctional myosin. What are the bases of amnioserosal cell interactions and alignment? Compared with surrounding tissue, we find that amnioserosal AJ continuity has lesser dependence on α-catenin, the mediator of AJ-actomyosin association, and greater dependence on Bazooka/Par-3, a junction-associated scaffold protein. Microtubule bundles also run along amnioserosal AJs and support their long-range curvilinearity. Amnioserosal confinement is apparent from partial overlap of its spindle-shaped cells, its outward bulging from surrounding tissue and from compressive stress detected within the amnioserosa. Genetic manipulations that alter amnioserosal confinement by surrounding tissue also result in amnioserosal cells losing alignment and gaining topological defects characteristic of nematically ordered systems. With Bazooka depletion, confinement by surrounding tissue appears to be relatively normal and amnioserosal cells align despite their AJ fragmentation. Overall, the fully elongated amnioserosa appears to form through tissue-autonomous generation of spindle-shaped cells that nematically align in response to confinement by surrounding tissue.

Multigenerational Effect of Heat Stress on the Drosophila melanogaster Sperm Proteome

The effect of the parental environment on offspring through non-DNA sequence-based mechanisms, such as DNA methylation, chromatin modifications, noncoding RNAs, and proteins, could only be established after the conception of "epigenetics". These effects are now broadly referred to as multigenerational epigenetic effects. Despite accumulating evidence of male gamete-mediated multigenerational epigenetic inheritance, little is known about the factors that underlie heat stress-induced multigenerational epigenetic inheritance via the male germline in Drosophila. In this study, we address this gap by utilizing an established heat stress paradigm in Drosophila and investigating its multigenerational effect on the sperm proteome. Our findings indicate that multigenerational heat stress during the early embryonic stage significantly influences proteins in the sperm associated with translation, chromatin organization, microtubule-based processes, and the generation of metabolites and energy. Assessment of life-history traits revealed that reproductive fitness and stress tolerance remained unaffected by multigenerational heat stress. Our study offers initial insights into the chromatin-based epigenetic mechanisms as a plausible means of transmitting heat stress memory through the male germline in Drosophila. Furthermore, it sheds light on the repercussions of early embryonic heat stress on male reproductive potential. The data sets from this study are available at the ProteomeXchange Consortium under the identifier PXD037488.

Lamins as structural nuclear elements through evolution

Lamins are nuclear intermediate filament proteins with important, well-established roles in humans and other vertebrates. Lamins interact with DNA and numerous proteins at the nuclear envelope to determine the mechanical properties of the nucleus, coordinate chromatin organization, and modulate gene expression. Many of these functions are conserved in the lamin homologs found in basal metazoan organisms, including Drosophila and Caenorhabditis elegans. Lamin homologs have also been recently identified in non-metazoans, like the amoeba Dictyostelium discoideum, yet how these proteins compare functionally to the metazoan isoforms is only beginning to emerge. A better understanding of these distantly related lamins is not only valuable for a more complete picture of eukaryotic evolution, but may also provide new insights into the function of vertebrate lamins.

Effects of mutant lamins on nucleo-cytoskeletal coupling in Drosophila models of LMNA muscular dystrophy

The nuclei of multinucleated skeletal muscles experience substantial external force during development and muscle contraction. Protection from such forces is partly provided by lamins, intermediate filaments that form a scaffold lining the inner nuclear membrane. Lamins play a myriad of roles, including maintenance of nuclear shape and stability, mediation of nuclear mechanoresponses, and nucleo-cytoskeletal coupling. Herein, we investigate how disease-causing mutant lamins alter myonuclear properties in response to mechanical force. This was accomplished via a novel application of a micropipette harpooning assay applied to larval body wall muscles of Drosophila models of lamin-associated muscular dystrophy. The assay enables the measurement of both nuclear deformability and intracellular force transmission between the cytoskeleton and nuclear interior in intact muscle fibers. Our studies revealed that specific mutant lamins increase nuclear deformability while other mutant lamins cause nucleo-cytoskeletal coupling defects, which were associated with loss of microtubular nuclear caging. We found that microtubule caging of the nucleus depended on Msp300, a KASH domain protein that is a component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Taken together, these findings identified residues in lamins required for connecting the nucleus to the cytoskeleton and suggest that not all muscle disease-causing mutant lamins produce similar defects in subcellular mechanics.

γ-secretase promotes Drosophila postsynaptic development through the cleavage of a Wnt receptor

Developing synapses mature through the recruitment of specific proteins that stabilize presynaptic and postsynaptic structure and function. Wnt ligands signaling via Frizzled (Fz) receptors play many crucial roles in neuronal and synaptic development, but whether and how Wnt and Fz influence synaptic maturation is incompletely understood. Here, we show that Fz2 receptor cleavage via the γ-secretase complex is required for postsynaptic development and maturation. In the absence of γ-secretase, Drosophila neuromuscular synapses fail to recruit postsynaptic scaffolding and cytoskeletal proteins, leading to behavioral deficits. Introducing presenilin mutations linked to familial early-onset Alzheimer's disease into flies leads to synaptic maturation phenotypes that are identical to those seen in null alleles. This conserved role for γ-secretase in synaptic maturation and postsynaptic development highlights the importance of Fz2 cleavage and suggests that receptor processing by proteins linked to neurodegeneration may be a shared mechanism with aspects of synaptic development.

extra macrochaetae , encoding Drosophila Id, controls apical cell shape in the hindgut epithelium

Inhibitor of DNA-binding (Id) transcription factor regulates the balance of cell differentiation and proliferation and is involved in organ morphogenesis in various species. Previously, we revealed that extra macrochaetae ( emc ), encoding the only Id protein in Drosophila , controls chirality of cell shape in hindgut epithelium. Here, to further understand functions of emc in cell-shape regulations, we analyzed apical cell shape in the hindgut epithelium of emc mutant embryos. We found that emc mutants showed expansion of their apical surface, but no abnormalities in cell differentiation and proliferation. Therefore, our results demonstrate that Id can control cell morphology without affecting specification and propagation of cells.

OUP accepted manuscript

Eukaryotic chromosomes are spatially segregated into topologically associating domains (TADs). Some TADs are attached to the nuclear lamina (NL) through lamina-associated domains (LADs). Here, we identified LADs and TADs at two stages of Drosophila spermatogenesis – in bam^Δ86 mutant testes which is the commonly used model of spermatogonia (SpG) and in larval testes mainly filled with spermatocytes (SpCs). We found that initiation of SpC-specific transcription correlates with promoters’ detachment from the NL and with local spatial insulation of adjacent regions. However, this insulation does not result in the partitioning of inactive TADs into sub-TADs. We also revealed an increased contact frequency between SpC-specific genes in SpCs implying their de novo gathering into transcription factories. In addition, we uncovered the specific X chromosome organization in the male germline. In SpG and SpCs, a single X chromosome is stronger associated with the NL than autosomes. Nevertheless, active chromatin regions in the X chromosome interact with each other more frequently than in autosomes. Moreover, despite the absence of dosage compensation complex in the male germline, randomly inserted SpG-specific reporter is expressed higher in the X chromosome than in autosomes, thus evidencing that non-canonical dosage compensation operates in SpG.

A single-molecule localization microscopy method for tissues reveals nonrandom nuclear pore distribution in Drosophila

Single-molecule localization microscopy (SMLM) can provide nanoscale resolution in thin samples but has rarely been applied to tissues because of high background from out-of-focus emitters and optical aberrations. Here, we describe a line scanning microscope that provides optical sectioning for SMLM in tissues. Imaging endogenously-tagged nucleoporins and F-actin on this system using DNA- and peptide-point accumulation for imaging in nanoscale topography (PAINT) routinely gives 30 nm resolution or better at depths greater than 20 µm. This revealed that the nuclear pores are nonrandomly distributed in most Drosophila tissues, in contrast to what is seen in cultured cells. Lamin Dm0 shows a complementary localization to the nuclear pores, suggesting that it corrals the pores. Furthermore, ectopic expression of the tissue-specific Lamin C causes the nuclear pores to distribute more randomly, whereas lamin C mutants enhance nuclear pore clustering, particularly in muscle nuclei. Given that nucleoporins interact with specific chromatin domains, nuclear pore clustering could regulate local chromatin organization and contribute to the disease phenotypes caused by human lamin A/C laminopathies.

Reorganization of the nuclear architecture in the Drosophila melanogaster Lamin B mutant lacking the CaaX box

Lamins interact with the nuclear membrane and chromatin but the precise players and mechanisms of these interactions are unknown. Here, we tested whether the removal of the CaaX motif from Lamin B disrupts its attachment to the nuclear membrane and affects chromatin distribution. We used Drosophila melanogaster Lam^A25 homozygous mutants that lack the CaaX box. We found that the mutant Lamin B was not confined to the nuclear periphery but was distributed throughout the nuclear interior, colocalizing with chromosomes in salivary gland and proventriculus. The peripheral position of Lamin C, nuclear pore complex (NPC), heterochromatin protein 1a (HP1a), H3K9me2- and H3K27me3-associated chromatin remained intact. The fluorescence intensity of the DAPI-stained peripheral chromatin significantly decreased and that of the central chromatin significantly increased in the proventriculus nuclei of the mutantflies compared to wild-type. However, the mutation had little effect on chromatin radial distribution inside highly polytenized salivary gland nuclei.

Mesoscale phase separation of chromatin in the nucleus

Intact-organism imaging of Drosophila larvae reveals and quantifies chromatin-aqueous phase separation. The chromatin can be organized near the lamina layer of the nuclear envelope, conventionally fill the nucleus, be organized centrally, or as a wetting droplet. These transitions are controlled by changes in nuclear volume and the interaction of chromatin with the lamina (part of the nuclear envelope) at the nuclear periphery. Using a simple polymeric model that includes the key features of chromatin self-attraction and its binding to the lamina, we demonstrate theoretically that it is the competition of these two effects that determines the mode of chromatin distribution. The qualitative trends as well as the composition profiles obtained in our simulations compare well with the observed intact-organism imaging and quantification. Since the simulations contain only a small number of physical variables we can identify the generic mechanisms underlying the changes in the observed phase separations.

Diverse cellular morphologies during lumen maturation in Anopheles gambiae larval salivary glands

Mosquitoes are the greatest animal threat to human health, causing hundreds of millions of infections and around 1 million deaths each year. All mosquito-borne pathogens must traverse the salivary glands (SGs) to be transmitted to the next host, making this organ an ideal target for interventions. The adult SG develops from precursor cells located in the larval SG duct bud. Characterization of the larval SG has been limited. We sought to better understand larval SG architecture, secretion and gene expression. We developed an optimized method for larval SG staining and surveyed hundreds of larval stage 4 (L4) SGs using fluorescence confocal microscopy. Remarkable variation in SG cell and chromatin organization differed among individuals and across the L4 stage. Lumen formation occurred during L4 stage through secretion likely involving a coincident cellular apical lipid enrichment and extracellular vesicle-like structures. Meta-analysis of microarray data showed that larval SG gene expression is divergent from adult SGs, more similar to larval gastric cecae, but different from other larval gut compartments. This work highlights the variable cell architecture of larval Anopheles gambiae SGs and provides candidate targets for genetic strategies aiming to disrupt SGs and transmission of mosquito-borne pathogens.

Drosophila ELYS regulates Dorsal dynamics during development

Embryonic large molecule derived from yolk sac (ELYS) is a constituent protein of nuclear pores. It initiates assembly of nuclear pore complexes into functional nuclear pores toward the end of mitosis. Using cellular, molecular, and genetic tools, including fluorescence and Electron microscopy, quantitative PCR, and RNAi-mediated depletion, we report here that the ELYS ortholog (dElys) plays critical roles during Drosophila development. dElys localized to the nuclear rim in interphase cells, but during mitosis it was absent from kinetochores and enveloped chromatin. We observed that RNAi-mediated dElys depletion leads to aberrant development and, at the cellular level, to defects in the nuclear pore and nuclear lamina assembly. Further genetic analyses indicated that dElys depletion re-activates the Dorsal (NF-κB) pathway during late larval stages. Re-activated Dorsal caused untimely expression of the Dorsal target genes in the post-embryonic stages. We also demonstrate that activated Dorsal triggers apoptosis during later developmental stages by up-regulating the pro-apoptotic genes reaper and hid The apoptosis induced by Reaper and Hid was probably the underlying cause for developmental abnormalities observed upon dElys depletion. Moreover, we noted that dElys has conserved structural features, but contains a noncanonical AT-hook-like motif through which it strongly binds to DNA. Together, our results uncover a novel epistatic interaction that regulates Dorsal dynamics by dElys during development.

Invertebrate models of lamin diseases

Lamins are evolutionarily conserved nuclear intermediate filament proteins. They provide structural support for the nucleus and help regulate many other nuclear activities. Mutations in human lamin genes, and especially in the LMNA gene, cause numerous diseases, termed laminopathies, including muscle, cardiac, metabolic, neuronal and early aging diseases. Most laminopathies arise from autosomal dominant missense mutations. Many of the mutant residues are conserved in the lamin genes of the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Our current understanding of the mechanisms leading to these diseases is mostly based on patients cell lines and animal models including C. elegans and D. melanogaster. The simpler lamin system and the powerful genetic tools offered by these invertebrate organisms greatly contributed to such studies. Here we provide an overview of the studies of laminopathies in Drosophila and C. elegans models.

Branch-restricted localization of phosphatase Prl-1 specifies axonal synaptogenesis domains

Central nervous system (CNS) circuit development requires subcellular control of synapse formation and patterning of synapse abundance. We identified the Drosophila membrane-anchored phosphatase of regenerating liver (Prl-1) as an axon-intrinsic factor that promotes synapse formation in a spatially restricted fashion. The loss of Prl-1 in mechanosensory neurons reduced the number of CNS presynapses localized on a single axon collateral and organized as a terminal arbor. Flies lacking all Prl-1 protein had locomotor defects. The overexpression of Prl-1 induced ectopic synapses. In mechanosensory neurons, Prl-1 modulates the insulin receptor (InR) signaling pathway within a single contralateral axon compartment, thereby affecting the number of synapses. The axon branch–specific localization and function of Prl-1 depend on untranslated regions of the prl-1 messenger RNA (mRNA). Therefore, compartmentalized restriction of Prl-1 serves as a specificity factor for the subcellular control of axonal synaptogenesis.

Cellular and Animal Models of Striated Muscle Laminopathies

The lamin A/C (LMNA) gene codes for nuclear intermediate filaments constitutive of the nuclear lamina. LMNA has 12 exons and alternative splicing of exon 10 results in two major isoforms—lamins A and C. Mutations found throughout the LMNA gene cause a group of diseases collectively known as laminopathies, of which the type, diversity, penetrance and severity of phenotypes can vary from one individual to the other, even between individuals carrying the same mutation. The majority of the laminopathies affect cardiac and/or skeletal muscles. The underlying molecular mechanisms contributing to such tissue-specific phenotypes caused by mutations in a ubiquitously expressed gene are not yet well elucidated. This review will explore the different phenotypes observed in established models of striated muscle laminopathies and their respective contributions to advancing our understanding of cardiac and skeletal muscle-related laminopathies. Potential future directions for developing effective treatments for patients with lamin A/C mutation-associated cardiac and/or skeletal muscle conditions will be discussed.

Nuclear lamina integrity is required for proper spatial organization of chromatin in Drosophila

How the nuclear lamina (NL) impacts on global chromatin architecture is poorly understood. Here, we show that NL disruption in Drosophila S2 cells leads to chromatin compaction and repositioning from the nuclear envelope. This increases the chromatin density in a fraction of topologically-associating domains (TADs) enriched in active chromatin and enhances interactions between active and inactive chromatin. Importantly, upon NL disruption the NL-associated TADs become more acetylated at histone H3 and less compact, while background transcription is derepressed. Two-colour FISH confirms that a TAD becomes less compact following its release from the NL. Finally, polymer simulations show that chromatin binding to the NL can per se compact attached TADs. Collectively, our findings demonstrate a dual function of the NL in shaping the 3D genome. Attachment of TADs to the NL makes them more condensed but decreases the overall chromatin density in the nucleus by stretching interphase chromosomes.

The role of the nuclear lamina (NL) in chromatin architecture is still poorly understood. Here, the authors provide evidence that disruption of the NL in Drosophila cells leads to overall chromatin compaction and repositioning from the nuclear envelope, whereas lamina-associated regions become less compacted and transcription within them is increased.

Midgut-derived neuropeptide F controls germline stem cell proliferation in a mating-dependent manner

Stem cell maintenance is established by neighboring niche cells that promote stem cell self-renewal. However, it is poorly understood how stem cell activity is regulated by systemic, tissue-extrinsic signals in response to environmental cues and changes in physiological status. Here, we show that neuropeptide F (NPF) signaling plays an important role in the pathway regulating mating-induced germline stem cell (GSC) proliferation in the fruit fly Drosophila melanogaster. NPF expressed in enteroendocrine cells (EECs) of the midgut is released in response to the seminal-fluid protein sex peptide (SP) upon mating. This midgut-derived NPF controls mating-induced GSC proliferation via ovarian NPF receptor (NPFR) activity, which modulates bone morphogenetic protein (BMP) signaling levels in GSCs. Our study provides a molecular mechanism that describes how a gut-derived systemic factor couples stem cell behavior to physiological status, such as mating, through interorgan communication.

Communication between different organs is essential to respond quickly to environmental cues or changes in the physiological status of an organism. Recent studies have shown the existence of humoral factors or hormones, which are transported by the circulatory system to different organs and achieve coordination between them. Here, we have analyzed the communication mechanism between organs that regulates proliferation of germline stem cells (GSCs) in the ovary of the fruit fly Drosophila melanogaster. We show that a peptide hormone called neuropeptide F (NPF) is a key player in this process. This peptide is produced in both the brain and the midgut, and, remarkably, we find that only NPF released from the midgut is crucial for controlling post-mating GSC proliferation. Our data suggest that mating stimulates the release of NPF from the endocrine cells of the midgut stimulated by the presence of a seminal peptide. Midgut-derived NPF is then transduced through an NPF-specific G-protein–coupled receptor expressed in the ovary, and this triggers GSC proliferation. Our study identifies an essential interaction between the digestive system and the ovary that regulates the size of stem cell populations in flies depending on mating.

Ma2/d promotes myonuclear positioning and association with the sarcoplasmic reticulum

The cytoplasm of striated myofibers contains a large number of membrane organelles, including sarcoplasmic reticulum (SR), T-tubules and the nuclear membrane. These organelles maintain a characteristic juxtaposition that appears to be essential for efficient inter-membranous exchange of RNA, proteins and ions. We found that the membrane-associated Muscle-specific α2/δ (Ma2/d) subunit of the Ca²⁺ channel complex localizes to the SR and T-tubules, and accumulates at the myonuclear surfaces. Furthermore, Ma2/d mutant larval muscles exhibit nuclear positioning defects, disruption of the nuclear-SR juxtapositioning, as well as impaired larval locomotion. Ma2/d localization at the nuclear membrane depends on the proper function of the nesprin ortholog Msp300 and the BAR domain protein Amphiphysin (Amph). Importantly, live imaging of muscle contraction in intact Drosophila larvae indicated altered distribution of Sarco/Endoplamic Reticulum Ca²⁺-ATPase (SERCA) around the myonuclei of Ma2/d mutant larvae. Co-immunoprecipitation analysis supports association between Ma2/d and Amph, and indirectly with Msp300. We therefore suggest that Ma2/d, in association with Msp300 and Amph, mediates interactions between the SR and the nuclear membrane.

Laminopathies: what can humans learn from fruit flies

Lamin proteins are type V intermediate filament proteins (IFs) located inside the cell nucleus. They are evolutionarily conserved and have similar domain organization and properties to cytoplasmic IFs. Lamins provide a skeletal network for chromatin, the nuclear envelope, nuclear pore complexes and the entire nucleus. They are also responsible for proper connections between the karyoskeleton and structural elements in the cytoplasm: actin and the microtubule and cytoplasmic IF networks. Lamins affect transcription and splicing either directly or indirectly. Translocation of active genes into the close proximity of nuclear lamina is thought to result in their transcriptional silencing. Mutations in genes coding for lamins and interacting proteins in humans result in various genetic disorders, called laminopathies. Human genes coding for A-type lamin (LMNA) are the most frequently mutated. The resulting phenotypes include muscle, cardiac, neuronal, lipodystrophic and metabolic pathologies, early aging phenotypes, and combined complex phenotypes. The Drosophila melanogaster genome codes for lamin B-type (lamin Dm), lamin A-type (lamin C), and for LEM-domain proteins, BAF, LINC-complex proteins and all typical nuclear proteins. The fruit fly system is simpler than the vertebrate one since in flies there is only single lamin B-type and single lamin A-type protein, as opposed to the complex system of B- and A-type lamins in Danio, Xenopus and Mus musculus. This offers a unique opportunity to study laminopathies. Applying genetic tools based on Gal4 and in vitro nuclear assembly system to the fruit fly model may successfully advance knowledge of laminopathies. Here, we review studies of the laminopathies in the fly model system.

Nuclear Lamin is required for Winged Eye-mediated transdetermination of Drosophila imaginal disc

Drosophila imaginal discs often change their cell fate under stress conditions, and this phenomenon, called transdetermination (TD), has long been a useful model for studying cell fate plasticity during regeneration. We previously identified a chromatin-associated protein, Winged Eye (Wge), which induces eye-to-wing TD upon its over-expression in eye imaginal discs. However, the molecular mechanism of Wge-mediated TD remains obscure. Here, we analyzed Wge-interacting proteins and found that several heterochromatin-related proteins, including a nuclear lamina protein, Lamin (Lam), were associated with Wge protein in cultured cells. Knockdown experiments revealed that Lam is indeed required for Wge-mediated eye-to-wing TD. Moreover, Wge over-expression altered the spatial organization of genomic DNA inside the cell nuclei. Accordingly, we suggest that Wge interacts with Lam to link some genomic regions with the nuclear periphery and regulates chromatin dynamics in imaginal disc TD.

Ultrastructural analysis of mitotic Drosophila S2 cells identifies distinctive microtubule and intracellular membrane behaviors

BACKGROUND

S2 cells are one of the most widely used Drosophila melanogaster cell lines. A series of studies has shown that they are particularly suitable for RNAi-based screens aimed at the dissection of cellular pathways, including those controlling cell shape and motility, cell metabolism, and host-pathogen interactions. In addition, RNAi in S2 cells has been successfully used to identify many new mitotic genes that are conserved in the higher eukaryotes, and for the analysis of several aspects of the mitotic process. However, no detailed and complete description of S2 cell mitosis at the ultrastructural level has been done. Here, we provide a detailed characterization of all phases of S2 cell mitosis visualized by transmission electron microscopy (TEM).

RESULTS

We analyzed by TEM a random sample of 144 cells undergoing mitosis, focusing on intracellular membrane and microtubule (MT) behaviors. This unbiased approach provided a comprehensive ultrastructural view of the dividing cells, and allowed us to discover that S2 cells exhibit a previously uncharacterized behavior of intracellular membranes, involving the formation of a quadruple nuclear membrane in early prometaphase and its disassembly during late prometaphase. After nuclear envelope disassembly, the mitotic apparatus becomes encased by a discontinuous network of endoplasmic reticulum membranes, which associate with mitochondria, presumably to prevent their diffusion into the spindle area. We also observed a peculiar metaphase spindle organization. We found that kinetochores with attached k-fibers are almost invariably associated with lateral MT bundles that can be either interpolar bundles or k-fibers connected to a different kinetochore. This spindle organization is likely to favor chromosome alignment at metaphase and subsequent segregation during anaphase.

CONCLUSIONS

We discovered several previously unknown features of membrane and MT organization during S2 cell mitosis. The genetic determinants of these mitotic features can now be investigated, for instance by using an RNAi-based approach, which is particularly easy and efficient in S2 cells.

Tissue and cellular rigidity and mechanosensitive signaling activation in Alexander disease

Glial cells have increasingly been implicated as active participants in the pathogenesis of neurological diseases, but critical pathways and mechanisms controlling glial function and secondary non-cell autonomous neuronal injury remain incompletely defined. Here we use models of Alexander disease, a severe brain disorder caused by gain-of-function mutations in GFAP, to demonstrate that misregulation of GFAP leads to activation of a mechanosensitive signaling cascade characterized by activation of the Hippo pathway and consequent increased expression of A-type lamin. Importantly, we use genetics to verify a functional role for dysregulated mechanotransduction signaling in promoting behavioral abnormalities and non-cell autonomous neurodegeneration. Further, we take cell biological and biophysical approaches to suggest that brain tissue stiffness is increased in Alexander disease. Our findings implicate altered mechanotransduction signaling as a key pathological cascade driving neuronal dysfunction and neurodegeneration in Alexander disease, and possibly also in other brain disorders characterized by gliosis.

Alexander disease is a rare neurodegeneration caused by mutations in a glial gene GFAP. Here, Wang and colleagues show in animal models of Alexander disease that GFAP mutant brain and cells have greater tissue and cellular stiffness and greater activation of mechanosensitive signaling cascade.

Ari-1 Regulates Myonuclear Organization Together with Parkin and Is Associated with Aortic Aneurysms

Nuclei are actively positioned and anchored to the cytoskeleton via the LINC (Linker of Nucleoskeleton and Cytoskeleton) complex. We identified mutations in the Parkin-like E3 ubiquitin ligase Ariadne-1 (Ari-1) that affect the localization and distribution of LINC complex members in Drosophila. ari-1 mutants exhibit nuclear clustering and morphology defects in larval muscles. We show that Ari-1 mono-ubiquitinates the core LINC complex member Koi. Surprisingly, we discovered functional redundancy between Parkin and Ari-1: increasing Parkin expression rescues ari-1 mutant phenotypes and vice versa. We further show that rare variants in the human homolog of ari-1 (ARIH1) are associated with thoracic aortic aneurysms and dissections, conditions resulting from smooth muscle cell (SMC) dysfunction. Human ARIH1 rescues fly ari-1 mutant phenotypes, whereas human variants found in patients fail to do so. In addition, SMCs obtained from patients display aberrant nuclear morphology. Hence, ARIH1 is critical in anchoring myonuclei to the cytoskeleton.

Dpp dependent Hematopoietic stem cells give rise to Hh dependent blood progenitors in larval lymph gland of Drosophila

Drosophila hematopoiesis bears striking resemblance with that of vertebrates, both in the context of distinct phases and the signaling molecules. Even though, there has been no evidence of Hematopoietic stem cells (HSCs) in Drosophila, the larval lymph gland with its Hedgehog dependent progenitors served as an invertebrate model of progenitor biology. Employing lineage-tracing analyses, we have now identified Notch expressing HSCs in the first instar larval lymph gland. Our studies clearly establish the hierarchical relationship between Notch expressing HSCs and the previously described Domeless expressing progenitors. These HSCs require Decapentapelagic (Dpp) signal from the hematopoietic niche for their maintenance in an identical manner to vertebrate aorta-gonadal-mesonephros (AGM) HSCs. Thus, this study not only extends the conservation across these divergent taxa, but also provides a new model that can be exploited to gain better insight into the AGM related Hematopoietic stem cells (HSCs).

Implications and Assessment of the Elastic Behavior of Lamins in Laminopathies

Lamins are mechanosensitive and elastic components of the nuclear lamina that respond to external mechanical cues by altering gene regulation in a feedback mechanism. Numerous mutations in A-type lamins cause a plethora of diverse diseases collectively termed as laminopathies, the majority of which are characterized by irregularly shaped, fragile, and plastic nuclei. These nuclei are challenged to normal mechanotransduction and lead to disease phenotypes. Here, we review our current understanding of the nucleocytoskeleton coupling in mechanotransduction mediated by lamins. We also present an up-to-date understanding of the methods used to determine laminar elasticity both at the bulk and single molecule level.

B-type nuclear lamin and the nuclear pore complex Nup107-160 influences maintenance of the spindle envelope required for cytokinesis in Drosophila male meiosis

In higher eukaryotes, nuclear envelope (NE) disassembly allows chromatin to condense and spindle microtubules to access kinetochores. The nuclear lamina, which strengthens the NE, is composed of a polymer meshwork made of A- and B-type lamins. We found that the B-type lamin (Lam) is not fully disassembled and continues to localize along the spindle envelope structure during Drosophila male meiosis I, while the A-type lamin (LamC) is completely dispersed throughout the cytoplasm. Among the nuclear pore complex proteins, Nup107 co-localized with Lam during this meiotic division. Surprisingly, Lam depletion resulted in a higher frequency of cytokinesis failure in male meiosis. We also observed the similar meiotic phenotype in Nup107-depleted cells. Abnormal localization of Lam was found in the Nup-depleted cells at premeiotic and meiotic stages. The central spindle microtubules became abnormal and recruitment of a contractile ring component to the cleavage sites was disrupted in Lam-depleted cells and Nup107-depleted cells. Therefore, we speculate that both proteins are required for a reinforcement of the spindle envelope, which supports the formation of central spindle microtubules essential for cytokinesis in Drosophila male meiosis.

Mating-Induced Increase in Germline Stem Cells via the Neuroendocrine System in Female Drosophila

Mating and gametogenesis are two essential components of animal reproduction. Gametogenesis must be modulated by the need for gametes, yet little is known of how mating, a process that utilizes gametes, may modulate the process of gametogenesis. Here, we report that mating stimulates female germline stem cell (GSC) proliferation in Drosophila melanogaster. Mating-induced increase in GSC number is not simply owing to the indirect effect of emission of stored eggs, but rather is stimulated by a male-derived Sex Peptide (SP) and its receptor SPR, the components of a canonical neuronal pathway that induces a post-mating behavioral switch in females. We show that ecdysteroid, the major insect steroid hormone, regulates mating-induced GSC proliferation independently of insulin signaling. Ovarian ecdysteroid level increases after mating and transmits its signal directly through the ecdysone receptor expressed in the ovarian niche to increase the number of GSCs. Impairment of ovarian ecdysteroid biosynthesis disrupts mating-induced increase in GSCs as well as egg production. Importantly, feeding of ecdysteroid rescues the decrease in GSC number caused by impairment of neuronal SP signaling. Our study illustrates how female GSC activity is coordinately regulated by the neuroendocrine system to sustain reproductive success in response to mating.

Confocal Analysis of Nuclear Lamina Behavior during Male Meiosis and Spermatogenesis in Drosophila melanogaster

Lamin family proteins are structural components of a filamentous framework, the nuclear lamina (NL), underlying the inner membrane of nuclear envelope. The NL not only plays a role in nucleus mechanical support and nuclear shaping, but is also involved in many cellular processes including DNA replication, gene expression and chromatin positioning. Spermatogenesis is a very complex differentiation process in which each stage is characterized by nuclear architecture dramatic changes, from the early mitotic stage to the sperm differentiation final stage. Nevertheless, very few data are present in the literature on the NL behavior during this process. Here we show the first and complete description of NL behavior during meiosis and spermatogenesis in Drosophila melanogaster. By confocal imaging, we characterized the NL modifications from mitotic stages, through meiotic divisions to sperm differentiation with an anti-laminDm0 antibody against the major component of the Drosophila NL. We observed that continuous changes in the NL structure occurred in parallel with chromatin reorganization throughout the whole process and that meiotic divisions occurred in a closed context. Finally, we analyzed NL in solofuso meiotic mutant, where chromatin segregation is severely affected, and found the strict correlation between the presence of chromatin and that of NL.

Non-farnesylated B-type lamin can tether chromatin inside the nucleus and its chromatin interaction requires the Ig-fold region

Lamins are thought to direct heterochromatin to the nuclear lamina (NL); however, this function of lamin has not been clearly demonstrated in vivo. To address this, we analyzed polytene chromosome morphology when artificial lamin variants were expressed in Drosophila endoreplicating cells. We found that the CaaX-motif-deleted B-type lamin Dm₀, but not A-type lamin C, was able to form a nuclear envelope-independent layer that was closely associated with chromatin. Other nuclear envelope proteins were not detected in this "ectopic lamina," and the associated chromatin showed a repressive histone modification maker but not a permissive histone modification marker nor RNA polymerase II proteins. Furthermore, deletion of the C-terminal lamin-Ig-fold domain prevents chromatin association with this ectopic lamina. Thus, non-farnesylated B-type lamin Dm₀ can form an ectopic lamina and induce changes to chromatin structure and status inside the interphase nucleus.

Lamin Dysfunction Mediates Neurodegeneration in Tauopathies

The filamentous meshwork formed by the lamin nucleoskeleton provides a scaffold for the anchoring of highly condensed heterochromatic DNA to the nuclear envelope, thereby establishing the three-dimensional architecture of the genome [1]. Insight into the importance of lamins to cellular viability can be gleaned from laminopathies, severe disorders caused by mutations in genes encoding lamins. A cellular consequence of lamin dysfunction in laminopathies is relaxation of heterochromatic DNA [1]. Similarly, we have recently reported the widespread relaxation of heterochromatin in tauopathies [1]: age-related progressive neurodegenerative disorders, including Alzheimer's disease, that are pathologically characterized by aggregates of phosphorylated tau protein in the brain [2, 3]. Here we demonstrate that acquired lamin misregulation though aberrant cytoskeletal-nucleoskeletal coupling promotes relaxation of heterochromatin and neuronal death in an in vivo model of neurodegenerative tauopathy. Genetic manipulation of lamin function significantly modifies neurodegeneration in vivo, demonstrating that lamin pathology plays a causal role in tau-mediated neurotoxicity. We show that lamin dysfunction is conserved in human tauopathy, as super-resolution microscopy reveals a significantly disrupted nuclear lamina in postmortem tissue from human Alzheimer's disease brain. Our study provides strong evidence that tauopathies are neurodegenerative laminopathies and identifies a new pathway mediating neuronal death in currently untreatable human neurodegenerative disorders, including Alzheimer's disease.

"Salivary gland cellular architecture in the Asian malaria vector mosquito Anopheles stephensi"

Background

Anopheles mosquitoes are vectors for malaria, a disease with continued grave outcomes for human health. Transmission of malaria from mosquitoes to humans occurs by parasite passage through the salivary glands (SGs). Previous studies of mosquito SG architecture have been limited in scope and detail.

Methods

We developed a simple, optimized protocol for fluorescence staining using dyes and/or antibodies to interrogate cellular architecture in Anopheles stephensi adult SGs. We used common biological dyes, antibodies to well-conserved structural and organellar markers, and antibodies against Anopheles salivary proteins to visualize many individual SGs at high resolution by confocal microscopy.

Results

These analyses confirmed morphological features previously described using electron microscopy and uncovered a high degree of individual variation in SG structure. Our studies provide evidence for two alternative models for the origin of the salivary duct, the structure facilitating parasite transport out of SGs. We compare SG cellular architecture in An. stephensi and Drosophila melanogaster, a fellow Dipteran whose adult SGs are nearly completely unstudied, and find many conserved features despite divergence in overall form and function. Anopheles salivary proteins previously observed at the basement membrane were localized either in SG cells, secretory cavities, or the SG lumen. Our studies also revealed a population of cells with characteristics consistent with regenerative cells, similar to muscle satellite cells or midgut regenerative cells.

Conclusions

This work serves as a foundation for linking Anopheles stephensi SG cellular architecture to function and as a basis for generating and evaluating tools aimed at preventing malaria transmission at the level of mosquito SGs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1229-z) contains supplementary material, which is available to authorized users.

Nesprin provides elastic properties to muscle nuclei by cooperating with spectraplakin and EB1

The myonuclear scaffold in Drosophila larval muscles exhibits both elastic features, contributed by the stretching capacity of MSP300/nesprin, and rigidity, provided by a perinuclear network of microtubules stabilized by Shot/spectraplakin and EB1.

Muscle nuclei are exposed to variable cytoplasmic strain produced by muscle contraction and relaxation, but their morphology remains stable. Still, the mechanism responsible for maintaining myonuclear architecture, and its importance, is currently elusive. Herein, we uncovered a unique myonuclear scaffold in Drosophila melanogaster larval muscles, exhibiting both elastic features contributed by the stretching capacity of MSP300 (nesprin) and rigidity provided by a perinuclear network of microtubules stabilized by Shot (spectraplakin) and EB1. Together, they form a flexible perinuclear shield that protects myonuclei from intrinsic or extrinsic forces. The loss of this scaffold resulted in significantly aberrant nuclear morphology and subsequently reduced levels of essential nuclear factors such as lamin A/C, lamin B, and HP1. Overall, we propose a novel mechanism for maintaining myonuclear morphology and reveal its critical link to correct levels of nuclear factors in differentiated muscle fibers. These findings may shed light on the underlying mechanism of various muscular dystrophies.

Myopathic lamin mutations cause reductive stress and activate the nrf2/keap-1 pathway

Mutations in the human LMNA gene cause muscular dystrophy by mechanisms that are incompletely understood. The LMNA gene encodes A-type lamins, intermediate filaments that form a network underlying the inner nuclear membrane, providing structural support for the nucleus and organizing the genome. To better understand the pathogenesis caused by mutant lamins, we performed a structural and functional analysis on LMNA missense mutations identified in muscular dystrophy patients. These mutations perturb the tertiary structure of the conserved A-type lamin Ig-fold domain. To identify the effects of these structural perturbations on lamin function, we modeled these mutations in Drosophila Lamin C and expressed the mutant lamins in muscle. We found that the structural perturbations had minimal dominant effects on nuclear stiffness, suggesting that the muscle pathology was not accompanied by major structural disruption of the peripheral nuclear lamina. However, subtle alterations in the lamina network and subnuclear reorganization of lamins remain possible. Affected muscles had cytoplasmic aggregation of lamins and additional nuclear envelope proteins. Transcription profiling revealed upregulation of many Nrf2 target genes. Nrf2 is normally sequestered in the cytoplasm by Keap-1. Under oxidative stress Nrf2 dissociates from Keap-1, translocates into the nucleus, and activates gene expression. Unexpectedly, biochemical analyses revealed high levels of reducing agents, indicative of reductive stress. The accumulation of cytoplasmic lamin aggregates correlated with elevated levels of the autophagy adaptor p62/SQSTM1, which also binds Keap-1, abrogating Nrf2 cytoplasmic sequestration, allowing Nrf2 nuclear translocation and target gene activation. Elevated p62/SQSTM1 and nuclear enrichment of Nrf2 were identified in muscle biopsies from the corresponding muscular dystrophy patients, validating the disease relevance of our Drosophila model. Thus, novel connections were made between mutant lamins and the Nrf2 signaling pathway, suggesting new avenues of therapeutic intervention that include regulation of protein folding and metabolism, as well as maintenance of redox homoeostasis.

Identification of tissue-specific regulatory region in the zebrafish lamin A promoter

Lamins are major structural proteins present in the nuclei of metazoan cells and contribute significantly to nuclear organization and function. The expression of different types of lamins is developmentally regulated and lamin A is detectable in most differentiated tissues. Although the proximal promoter of the mammalian lamin A gene has been characterized, the tissue-specific regulatory elements of the gene have not been identified. In this study, we have cloned and functionally characterized a 2.99 kb segment upstream of exon 1 in the zebrafish lamin A gene. This fragment was able to drive GFP expression in several tissues of the developing embryo at 14-72 h post fertilization in stable transgenic lines. Deletion fragments of the 2.99 kb promoter were analyzed by microinjection into zebrafish embryos in transient assays as well as by luciferase reporter assays in cultured cells. A minimal promoter segment of 1.24 kb conferred tissue-specific expression of GFP in the zebrafish embryo as well as in a myoblast cell line. An 86 bp fragment of this 1.24 kb segment was able to activate a heterologous promoter in myoblasts. Mutational analysis revealed the importance of muscle-specific regulatory motifs in the promoter. Our results have important implications for understanding the tissue-specific regulation and functions of the lamin A gene.

Detection of Cell Death and Phagocytosis in the Drosophila Ovary

Billions of cells die and are cleared throughout the development and homeostasis of an organism. Either improper death or clearance can lead to serious illnesses. In the adult Drosophila ovary, germline cells can die by programmed cell death (PCD) at three distinct stages; here we focus on cell death that occurs in mid- and late oogenesis. In mid-oogenesis, the germline of egg chambers can undergo apoptosis in response to nutrient deprivation. In late oogenesis, the nurse cells are eliminated through a developmentally regulated, non-apoptotic cell death. In this chapter, we describe several methods to detect cell death and phagocytosis in the Drosophila ovary. DAPI stains the chromatin of all cells and can be used to detect morphological changes in cells that die by different mechanisms. TUNEL labels fragmented DNA, which can occur in both apoptotic and non-apoptotic death. LysoTracker, an acidophilic dye, marks acidic vesicles and some dying cells; therefore, it can be used to study both death and phagocytosis. We also describe several antibodies that can be used to investigate cell death and/or phagocytosis: active caspase Dcp-1, membrane markers, and lamins. Many of these antibodies can be used in combination with GFP fusion transgenes for further analysis; we show Rab5-GFP and Rab7-GFP, which can be used to study phagocytosis in further detail.

Differential centrifugation-based biochemical fractionation of the Drosophila adult CNS

Drosophila is widely used as a genetic model in questions of development, cellular function and disease. Genetic screens in flies have proven to be incredibly powerful in identifying crucial components for synapse formation and function, particularly in the case of the presynaptic release machinery. Although modern biochemical methods can identify individual proteins and lipids (and their binding partners), they have typically been excluded from use in Drosophila for technical reasons. To bridge this essential gap between genetics and biochemistry, we developed a fractionation method to isolate various parts of the synaptic machinery from Drosophila, thus allowing it to be studied in unprecedented biochemical detail. This is only possible because our protocol has unique advantages in terms of enriching and preserving endogenous protein complexes. The procedure involves decapitation of adult flies, homogenization and differential centrifugation of fly heads, which allow subsequent purification of presynaptic (and to a limited degree postsynaptic) components. It is designed to require only a rudimentary knowledge of biochemical fractionation, and it takes ∼3.5 h. The yield is typically 4 mg of synaptic membrane protein per gram of Drosophila heads.

B-type lamins in health and disease

For over two decades, B-type lamins were thought to have roles in fundamental processes including correct assembly of nuclear envelopes, DNA replication, transcription and cell survival. Recent studies have questioned these roles and have instead emphasised the role of these proteins in tissue building and tissue integrity, particularly in tissues devoid of A-type lamins. Other studies have suggested that the expression of B-type lamins in somatic cells influences the rate of entry into states of cellular senescence. In humans duplication of the LMNB1 gene (encoding lamin B1) causes an adult onset neurodegenerative disorder, termed autosomal dominant leukodystrophy, whilst very recently, LMNB1 has been implicated as a susceptibility gene in neural tube defects. This is consistent with studies in mice that reveal a critical role for B-type lamins in neuronal migration and brain development. In this review, I will consider how different model systems have contributed to our understanding of the functions of B-type lamins and which of those functions are critical for human health and disease.

The Drosophila ubiquitin-specific protease Puffyeye regulates dMyc-mediated growth

The essential and highly conserved role of Myc in organismal growth and development is dependent on the control of Myc protein abundance. It is now well established that Myc levels are in part regulated by ubiquitin-dependent proteasomal degradation. Using a genetic screen for modifiers of Drosophila Myc (dMyc)-induced growth, we identified and characterized a ubiquitin-specific protease (USP), Puffyeye (Puf), as a novel regulator of dMyc levels and function in vivo. We show that puf genetically and physically interacts with dMyc and the ubiquitin ligase archipelago (ago) to modulate a dMyc-dependent cell growth phenotype, and that varying Puf levels in both the eye and wing phenocopies the effects of altered dMyc abundance. Puf containing point mutations within its USP enzymatic domain failed to alter dMyc levels and displayed no detectable phenotype, indicating the importance of deubiquitylating activity for Puf function. We find that dMyc induces Ago, indicating that dMyc triggers a negative-feedback pathway that is modulated by Puf. In addition to its effects on dMyc, Puf regulates both Ago and its cell cycle substrate Cyclin E. Therefore, Puf influences cell growth by controlling the stability of key regulatory proteins.

Protein trafficking abnormalities in Drosophila tissues with impaired activity of the ZIP7 zinc transporter Catsup

Developmental patterning requires the precise interplay of numerous intercellular signaling pathways to ensure that cells are properly specified during tissue formation and organogenesis. The spatiotemporal function of the Notch signaling pathway is strongly influenced by the biosynthesis and intracellular trafficking of signaling components. Receptors and ligands must be trafficked to the cell surface where they interact, and their subsequent endocytic internalization and endosomal trafficking is crucial for both signal propagation and its down-modulation. In a forward genetic screen for mutations that alter intracellular Notch receptor trafficking in Drosophila epithelial tissues, we recovered mutations that disrupt the Catsup gene, which encodes the Drosophila ortholog of the mammalian ZIP7 zinc transporter. Loss of Catsup function causes Notch to accumulate abnormally in the endoplasmic reticulum (ER) and Golgi compartments, resulting in impaired Notch signaling. In addition, Catsup mutant cells exhibit elevated ER stress, suggesting that impaired zinc homeostasis causes increased levels of misfolded proteins within the secretory compartment.

Diet controls Drosophila follicle stem cell proliferation via Hedgehog sequestration and release

Dietary cholesterol levels control follicle stem cell proliferation in the Drosophila ovary via regulation of Hedgehog protein localization.

A healthy diet improves adult stem cell function and delays diseases such as cancer, heart disease, and neurodegeneration. Defining molecular mechanisms by which nutrients dictate stem cell behavior is a key step toward understanding the role of diet in tissue homeostasis. In this paper, we elucidate the mechanism by which dietary cholesterol controls epithelial follicle stem cell (FSC) proliferation in the fly ovary. In nutrient-restricted flies, the transmembrane protein Boi sequesters Hedgehog (Hh) ligand at the surface of Hh-producing cells within the ovary, limiting FSC proliferation. Upon feeding, dietary cholesterol stimulates S6 kinase–mediated phosphorylation of the Boi cytoplasmic domain, triggering Hh release and FSC proliferation. This mechanism enables a rapid, tissue-specific response to nutritional changes, tailoring stem cell divisions and egg production to environmental conditions sufficient for progeny survival. If conserved in other systems, this mechanism will likely have important implications for studies on molecular control of stem cell function, in which the benefits of low calorie and low cholesterol diets are beginning to emerge.

Evolution of the lamin protein family: what introns can tell

Lamins are the major components of the nuclear lamina, a filamentous layer found at the interphase between chromatin and the inner nuclear membrane. The lamina supports the nuclear envelope and provides anchorage sites for chromatin. Lamins and their associated proteins are required for most nuclear activities, mitosis, and for linking the nucleoskeleton to the network of cytoskeletal filaments. Mutations in lamins and their associated proteins give rise to a wide range of diseases, collectively called laminopathies. This review focuses on the evolution of the lamin protein family. Evolution from basal metazoans to man will be described on the basis of protein sequence comparisons and analyses of their gene structure. Lamins are the founding members of the family of intermediate filament proteins. How genes encoding cytoplasmic IF proteins could have arisen from the archetypal lamin gene progenitor, can be inferred from a comparison of the respective gene structures. The lamin/IF protein family seems to be restricted to the metazoans. In general, invertebrate genomes harbor only a single lamin gene encoding a B-type lamin. The archetypal lamin gene structure found in basal metazoans is conserved up to the vertebrate lineage. The completely different structure of lamin genes in Caenorhabditis and Drosophila are exceptions rather than the rule within their systematic groups. However, variation in the length of the coiled-coil forming central domain might be more common than previously anticipated. The increase in the number of lamin genes in vertebrates can be explained by two rounds of genome duplication. The origin of lamin A by exon shuffling might explain the processing of prelamin A to the mature non-isoprenylated form of lamin A. By alternative splicing the number of vertebrate lamin proteins has increased even further. Lamin C, an alternative splice form of the LMNA gene, is restricted to mammals. Amphibians and mammals express germline-specific lamins that differ in their protein structure from that of somatic lamins. Evidence is provided that there exist lamin-like proteins outside the metazoan lineage.

Loss of Drosophila A-type lamin C initially causes tendon abnormality including disintegration of cytoskeleton and nuclear lamina in muscular defects

Lamins are the major components of nuclear envelope architecture, being required for both the structural and informational roles of the nuclei. Mutations of lamins cause a spectrum of diseases in humans, including muscular dystrophy. We report here that the loss of the A-type lamin gene, lamin C in Drosophila resulted in pupal metamorphic lethality caused by tendon defects, matching the characteristics of human A-type lamin revealed by Emery-Dreifuss muscular dystrophy (EDMD). In tendon cells lacking lamin C activity, overall cell morphology was affected and organization of the spectraplakin family cytoskeletal protein Shortstop which is prominently expressed in tendon cells gradually disintegrated, notably around the nucleus and in a manner correlating well with the degradation of musculature. Furthermore, lamin C null mutants were efficiently rescued by restoring lamin C expression to shortstop-expressing cells, which include tendon cells but exclude skeletal muscle cells. Thus the critical function of A-type lamin C proteins in Drosophila musculature is to maintain proper function and morphology of tendon cells.

The molecular control of meiotic chromosomal behavior: events in early meiotic prophase in Drosophila oocytes

We review the critical events in early meiotic prophase in Drosophila melanogaster oocytes. We focus on four aspects of this process: the formation of the synaptonemal complex (SC) and its role in maintaining homologous chromosome pairings, the critical roles of the meiosis-specific process of centromere clustering in the formation of a full-length SC, the mechanisms by which preprogrammed double-strand breaks initiate meiotic recombination, and the checkpoints that govern the progression and coordination of these processes. Central to this discussion are the roles that somatic pairing events play in establishing the necessary conditions for proper SC formation, the roles of centromere pairing in synapsis initiation, and the mechanisms by which oocytes detect failures in SC formation and/or recombination. Finally, we correlate what is known in Drosophila oocytes with our understanding of these processes in other systems.

Type-I prenyl protease function is required in the male germline of Drosophila melanogaster

Many proteins require the addition of a hydrophobic prenyl anchor (prenylation) for proper trafficking and localization in the cell. Prenyl proteases play critical roles in modifying proteins for membrane anchorage. The type I prenyl protease has a defined function in yeast (Ste24p/Afc1p) where it modifies a mating pheromone, and in humans (Zmpste24) where it has been implicated in a disease of premature aging. Despite these apparently very different biological processes, the type I prenyl protease gene is highly conserved, encoded by a single gene in a wide range of animal and plant groups. A notable exception is Drosophila melanogaster, where the gene encoding the type I prenyl protease has undergone an unprecedented series of duplications in the genome, resulting in five distinct paralogs, three of which are organized in a tandem array, and demonstrate high conservation, particularly in the vicinity of the active site of the enzyme. We have undertaken targeted deletion to remove the three tandem paralogs from the genome. The result is a male fertility defect, manifesting late in spermatogenesis. Our results also show that the ancestral type I prenyl protease gene in Drosophila is under strong purifying selection, while the more recent replicates are evolving rapidly. Our rescue data support a role for the rapidly evolving tandem paralogs in the male germline. We propose that potential targets for the male-specific type I prenyl proteases include proteins involved in the very dramatic cytoskeletal remodeling events required for spermatid maturation.

Nuclear envelope budding enables large ribonucleoprotein particle export during synaptic Wnt signaling

Localized protein synthesis requires assembly and transport of translationally silenced ribonucleoprotein particles (RNPs), some of which are exceptionally large. Where in the cell such large RNP granules first assemble was heretofore unknown. We previously reported that during synapse development, a fragment of the Wnt-1 receptor, DFrizzled2, enters postsynaptic nuclei where it forms prominent foci. Here we show that these foci constitute large RNP granules harboring synaptic protein transcripts. These granules exit the nucleus by budding through the inner and the outer nuclear membranes in a nuclear egress mechanism akin to that of herpes viruses. This budding involves phosphorylation of A-type lamin, a protein linked to muscular dystrophies. Thus nuclear envelope budding is an endogenous nuclear export pathway for large RNP granules.

The function of lamins in the context of tissue building and maintenance

Lamins are the major structural components of the nuclear lamina found in metazoan organisms. Extensive studies using tissue culture cells have shown that lamins are involved in a wide range of basic cell functions. This has led to the prevailing idea that a given animal cell needs at least one lamin protein for its basic proliferation and survival. However, recent studies have shown that lamins are dispensable for the proliferation and survival of mouse embryonic stem cells (ESC). In contrast to a lack of essential functions in ESCs, certain differentiated cells lacking B-type lamins exhibit increased cell cycle exit rates and enhanced senescence. In this Extra View, we discuss how studies using animal models and cell cultures have begun to reveal cell-type specific functions of lamins in tissue building and homeostasis.