Peripheral thickening of the sarcomeres and pointed end elongation of the thin filaments are both promoted by SALS and its formin interaction partners.

During striated muscle development the first periodically repeated units appear in the premyofibrils, consisting of immature sarcomeres that must undergo a substantial growth both in length and width, to reach their final size. Here we report that, beyond its well established role in sarcomere elongation, the Sarcomere length short (SALS) protein is involved in Z-disc formation and peripheral growth of the sarcomeres. Our protein localization data and loss-of-function studies in the Drosophila indirect flight muscle strongly suggest that radial growth of the sarcomeres is initiated at the Z-disc. As to thin filament elongation, we used a powerful nanoscopy approach to reveal that SALS is subject to a major conformational change during sarcomere development, which might be critical to stop pointed end elongation in the adult muscles. In addition, we demonstrate that the roles of SALS in sarcomere elongation and radial growth are both dependent on formin type of actin assembly factors. Unexpectedly, when SALS is present in excess amounts, it promotes the formation of actin aggregates highly resembling the ones described in nemaline myopathy patients. Collectively, these findings helped to shed light on the complex mechanisms of SALS during the coordinated elongation and thickening of the sarcomeres, and resulted in the discovery of a potential nemaline myopathy model, suitable for the identification of genetic and small molecule inhibitors.

FRL and DAAM are required for lateral adhesion of interommatidial cells and patterning of the retinal floor.

Optical insulation of the unit eyes (ommatidia) is an important prerequisite of precise sight with compound eyes. Separation of the ommatidia is ensured by pigment cells that organize into a hexagonal lattice in the Drosophila eye, forming thin walls between the facets. Cell adhesion, mediated by apically and latero-basally located junctional complexes, is crucial for stable attachment of these cells to each other and the basal lamina. Whereas former studies have focused on the formation and remodelling of the cellular connections at the apical region, here, we report a specific alteration of the lateral adhesion of the lattice cells, leaving the apical junctions largely unaffected. We found that DAAM and FRL, two formin-type cytoskeleton regulatory proteins, play redundant roles in lateral adhesion of the interommatidial cells and patterning of the retinal floor. We show that formin-dependent cortical actin assembly is crucial for latero-basal sealing of the ommatidial lattice. We expect that the investigation of these previously unreported eye phenotypes will pave the way toward a better understanding of the three-dimensional aspects of compound eye development.

The oxoglutarate dehydrogenase complex is involved in myofibril growth and Z-disc assembly in Drosophila.

Myofibrils are long intracellular cables specific to muscles, composed mainly of actin and myosin filaments. The actin and myosin filaments are organized into repeated units called sarcomeres, which form the myofibrils. Muscle contraction is achieved by the simultaneous shortening of sarcomeres, which requires all sarcomeres to be the same size. Muscles have a variety of ways to ensure sarcomere homogeneity. We have previously shown that the controlled oligomerization of Zasp proteins sets the diameter of the myofibril. Here, we looked for Zasp-binding proteins at the Z-disc to identify additional proteins coordinating myofibril growth and assembly. We found that the E1 subunit of the oxoglutarate dehydrogenase complex localizes to both the Z-disc and the mitochondria, and is recruited to the Z-disc by Zasp52. The three subunits of the oxoglutarate dehydrogenase complex are required for myofibril formation. Using super-resolution microscopy, we revealed the overall organization of the complex at the Z-disc. Metabolomics identified an amino acid imbalance affecting protein synthesis as a possible cause of myofibril defects, which is supported by OGDH-dependent localization of ribosomes at the Z-disc.

Machine learning framework to segment sarcomeric structures in SMLM data.

Object detection is an image analysis task with a wide range of applications, which is difficult to accomplish with traditional programming. Recent breakthroughs in machine learning have made significant progress in this area. However, these algorithms are generally compatible with traditional pixelated images and cannot be directly applied for pointillist datasets generated by single molecule localization microscopy (SMLM) methods. Here, we have improved the averaging method developed for the analysis of SMLM images of sarcomere structures based on a machine learning object detection algorithm. The ordered structure of sarcomeres allows us to determine the location of the proteins more accurately by superimposing SMLM images of identically assembled proteins. However, the area segmentation process required for averaging can be extremely time-consuming and tedious. In this work, we have automated this process. The developed algorithm not only finds the regions of interest, but also classifies the localizations and identifies the true positive ones. For training, we used simulations to generate large amounts of labelled data. After tuning the neural network's internal parameters, it could find the localizations associated with the structures we were looking for with high accuracy. We validated our results by comparing them with previous manual evaluations. It has also been proven that the simulations can generate data of sufficient quality for training. Our method is suitable for the identification of other types of structures in SMLM data.

Detection of Actin in Nuclear Protein Fraction Isolated from Adult Drosophila Ovary.

Much evidence supports the presence of cytoskeletal elements in the nucleus; however, the exact functions of these proteins in the nucleus are still uncertain. Of the cytoskeletal proteins, the activity and biological significance of nuclear actin has been the most extensively researched. It is now clear that actin performs essential tasks both in the cytoplasm and the nucleus, and that the dynamic balance between the large cytoplasmic and the significantly smaller nuclear actin pools is maintained by robust transport mechanisms. Therefore, the compartment-specific manipulation or investigation of actin has been an enormous challenge. Here, we present a protocol for the detection of actin in isolated nuclear protein fractions from Drosophila ovaries.

The Mechanisms of Thin Filament Assembly and Length Regulation in Muscles.

The actin containing tropomyosin and troponin decorated thin filaments form one of the crucial components of the contractile apparatus in muscles. The thin filaments are organized into densely packed lattices interdigitated with myosin-based thick filaments. The crossbridge interactions between these myofilaments drive muscle contraction, and the degree of myofilament overlap is a key factor of contractile force determination. As such, the optimal length of the thin filaments is critical for efficient activity, therefore, this parameter is precisely controlled according to the workload of a given muscle. Thin filament length is thought to be regulated by two major, but only partially understood mechanisms: it is set by (i) factors that mediate the assembly of filaments from monomers and catalyze their elongation, and (ii) by factors that specify their length and uniformity. Mutations affecting these factors can alter the length of thin filaments, and in human cases, many of them are linked to debilitating diseases such as nemaline myopathy and dilated cardiomyopathy.

Drosophila Models Rediscovered with Super-Resolution Microscopy.

With the advent of super-resolution microscopy, we gained a powerful toolbox to bridge the gap between the cellular- and molecular-level analysis of living organisms. Although nanoscopy is broadly applicable, classical model organisms, such as fruit flies, worms and mice, remained the leading subjects because combining the strength of sophisticated genetics, biochemistry and electrophysiology with the unparalleled resolution provided by super-resolution imaging appears as one of the most efficient approaches to understanding the basic cell biological questions and the molecular complexity of life. Here, we summarize the major nanoscopic techniques and illustrate how these approaches were used in Drosophila model systems to revisit a series of well-known cell biological phenomena. These investigations clearly demonstrate that instead of simply achieving an improvement in image quality, nanoscopy goes far beyond with its immense potential to discover novel structural and mechanistic aspects. With the examples of synaptic active zones, centrosomes and sarcomeres, we will explain the instrumental role of super-resolution imaging pioneered in Drosophila in understanding fundamental subcellular constituents.

Superresolution Microscopy of Drosophila Indirect Flight Muscle Sarcomeres.

Sarcomeres are extremely highly ordered macromolecular assemblies where proper structural organization is an absolute prerequisite to the functionality of these contractile units. Despite the wealth of information collected, the exact spatial arrangement of many of the H-zone and Z-disk proteins remained unknown. Recently, we developed a powerful nanoscopic approach to localize the sarcomeric protein components with a resolution well below the diffraction limit. The ease of sample preparation and the near crystalline structure of the Drosophila flight muscle sarcomeres make them ideally suitable for single molecule localization microscopy and structure averaging. Our approach allowed us to determine the position of dozens of H-zone and Z-disk proteins with a quasi-molecular, ~5-10 nm localization precision. The protocol described below provides an easy and reproducible method to prepare individual myofibrils for dSTORM imaging. In addition, it includes an in-depth description of a custom made and freely available software toolbox to process and quantitatively analyze the raw localization data.

Nanoscopy reveals the layered organization of the sarcomeric H-zone and I-band complexes.

Sarcomeres are extremely highly ordered macromolecular assemblies where structural organization is intimately linked to their functionality as contractile units. Although the structural basis of actin and Myosin interaction is revealed at a quasiatomic resolution, much less is known about the molecular organization of the I-band and H-zone. We report the development of a powerful nanoscopic approach, combined with a structure-averaging algorithm, that allowed us to determine the position of 27 sarcomeric proteins in Drosophila melanogaster flight muscles with a quasimolecular, ∼5- to 10-nm localization precision. With this protein localization atlas and template-based protein structure modeling, we have assembled refined I-band and H-zone models with unparalleled scope and resolution. In addition, we found that actin regulatory proteins of the H-zone are organized into two distinct layers, suggesting that the major place of thin filament assembly is an M-line-centered narrow domain where short actin oligomers can form and subsequently anneal to the pointed end.

Drosophila Atg9 regulates the actin cytoskeleton via interactions with profilin and Ena.

Autophagy ensures the turnover of cytoplasm and requires the coordinated action of Atg proteins, some of which also have moonlighting functions in higher eukaryotes. Here we show that the transmembrane protein Atg9 is required for female fertility, and its loss leads to defects in actin cytoskeleton organization in the ovary and enhances filopodia formation in neurons in Drosophila. Atg9 localizes to the plasma membrane anchor points of actin cables and is also important for the integrity of the cortical actin network. Of note, such phenotypes are not seen in other Atg mutants, suggesting that these are independent of autophagy defects. Mechanistically, we identify the known actin regulators profilin and Ena/VASP as novel binding partners of Atg9 based on microscopy, biochemical, and genetic interactions. Accordingly, the localization of both profilin and Ena depends on Atg9. Taken together, our data identify a new and unexpected role for Atg9 in actin cytoskeleton regulation.

mmSTORM: Multimodal localization based super-resolution microscopy.

Super-resolution localization microscopy provides a powerful tool to study biochemical mechanisms at single molecule level. Although the lateral position of the fluorescent dye molecules can be determined routinely with high precision, measurement of other modalities such as 3D and multicolor without the degradation of the original super-resolved image is still in the focus. In this paper a dual-objective multimodal single molecule localization microscopy (SMLM) technique has been developed, optimized and tested. The proposed optical arrangement can be implemented onto a conventional inverted microscope without serious system modification. The performance of the method was tested using fluorescence beads, F-actin filaments and sarcomere structures. It was shown that the proposed imaging method does not degrade the image quality of the original SMLM 2D image but could provide information on the axial position or emission spectra of the dye molecules.

Microtubule organization in presynaptic boutons relies on the formin DAAM.

Regulation of the cytoskeleton is fundamental to the development and function of synaptic terminals, such as neuromuscular junctions. Despite the identification of numerous proteins that regulate synaptic actin and microtubule dynamics, the mechanisms of cytoskeletal control during terminal arbor formation have remained largely elusive. Here, we show that DAAM, a member of the formin family of cytoskeleton organizing factors, is an important presynaptic regulator of neuromuscular junction development in Drosophila We demonstrate that the actin filament assembly activity of DAAM plays a negligible role in terminal formation; rather, DAAM is necessary for synaptic microtubule organization. Genetic interaction studies consistently link DAAM with the Wg/Ank2/Futsch module of microtubule regulation and bouton formation. Finally, we provide evidence that DAAM is tightly associated with the synaptic active zone scaffold, and electrophysiological data point to a role in the modulation of synaptic vesicle release. Based on these results, we propose that DAAM is an important cytoskeletal effector element of the Wg/Ank2 pathway involved in the determination of basic synaptic structures, and, additionally, that DAAM may couple the active zone scaffold to the presynaptic cytoskeleton.

The activities of the C-terminal regions of the formin protein disheveled-associated activator of morphogenesis (DAAM) in actin dynamics.

Disheveled-associated activator of morphogenesis (DAAM) is a diaphanous-related formin protein essential for the regulation of actin cytoskeleton dynamics in diverse biological processes. The conserved formin homology 1 and 2 (FH1-FH2) domains of DAAM catalyze actin nucleation and processively mediate filament elongation. These activities are indirectly regulated by the N- and C-terminal regions flanking the FH1-FH2 domains. Recently, the C-terminal diaphanous-autoregulatory domain (DAD) and the C terminus (CT) of formins have also been shown to regulate actin assembly by directly interacting with actin. Here, to better understand the biological activities of DAAM, we studied the role of DAD-CT regions of Drosophila DAAM in its interaction with actin with in vitro biochemical and in vivo genetic approaches. We found that the DAD-CT region binds actin in vitro and that its main actin-binding element is the CT region, which does not influence actin dynamics on its own. However, we also found that it can tune the nucleating activity and the filament end-interaction properties of DAAM in an FH2 domain-dependent manner. We also demonstrate that DAD-CT makes the FH2 domain more efficient in antagonizing with capping protein. Consistently, in vivo data suggested that the CT region contributes to DAAM-mediated filopodia formation and dynamics in primary neurons. In conclusion, our results demonstrate that the CT region of DAAM plays an important role in actin assembly regulation in a biological context.

The formin DAAM is required for coordination of the actin and microtubule cytoskeleton in axonal growth cones.

Directed axonal growth depends on correct coordination of the actin and microtubule cytoskeleton in the growth cone. However, despite the relatively large number of proteins implicated in actin-microtubule crosstalk, the mechanisms whereby actin polymerization is coupled to microtubule stabilization and advancement in the peripheral growth cone remained largely unclear. Here, we identified the formin Dishevelled-associated activator of morphogenesis (DAAM) as a novel factor playing a role in concerted regulation of actin and microtubule remodeling in Drosophilamelanogaster primary neurons. In vitro, DAAM binds to F-actin as well as to microtubules and has the ability to crosslink the two filament systems. Accordingly, DAAM associates with the neuronal cytoskeleton, and a significant fraction of DAAM accumulates at places where the actin filaments overlap with that of microtubules. Loss of DAAM affects growth cone and microtubule morphology, and several aspects of microtubule dynamics; and biochemical and cellular assays revealed a microtubule stabilization activity and binding to the microtubule tip protein EB1. Together, these data suggest that, besides operating as an actin assembly factor, DAAM is involved in linking actin remodeling in filopodia to microtubule stabilization during axonal growth.

The actin-binding ERM protein Moesin directly regulates spindle assembly and function during mitosis.

Ezrin-Radixin-Moesin proteins are highly conserved, actin-binding cytoskeletal proteins that play an essential role in microvilli formation, T-cell activation, and tumor metastasis by linking actin filaments to the plasma membrane. Recent studies demonstrated that the only Ezrin-Radixin-Moesin protein of Drosophila melanogaster, Moesin, is involved in mitotic spindle function through stabilizing cell shape and microtubules at the cell cortex. We previously observed that Moesin localizes to the mitotic spindle; hence, we tested for the biological significance of this surprising localization and investigated whether it plays a direct role in spindle function. To separate the cortical and spindle functions of Moesin during mitosis we combined cell biological and genetic methods. We used early Drosophila embryos, in which mitosis occurs in the absence of a cell cortex, and found in vivo evidence for the direct requirement of Moesin in mitotic spindle assembly and function. We also found that the accumulation of Moesin precedes the construction of the microtubule spindle, and the fusiform structure formed by Moesin persists even after the microtubules have disassembled.

DAAM is required for thin filament formation and Sarcomerogenesis during muscle development in Drosophila.

During muscle development, myosin and actin containing filaments assemble into the highly organized sarcomeric structure critical for muscle function. Although sarcomerogenesis clearly involves the de novo formation of actin filaments, this process remained poorly understood. Here we show that mouse and Drosophila members of the DAAM formin family are sarcomere-associated actin assembly factors enriched at the Z-disc and M-band. Analysis of dDAAM mutants revealed a pivotal role in myofibrillogenesis of larval somatic muscles, indirect flight muscles and the heart. We found that loss of dDAAM function results in multiple defects in sarcomere development including thin and thick filament disorganization, Z-disc and M-band formation, and a near complete absence of the myofibrillar lattice. Collectively, our data suggest that dDAAM is required for the initial assembly of thin filaments, and subsequently it promotes filament elongation by assembling short actin polymers that anneal to the pointed end of the growing filaments, and by antagonizing the capping protein Tropomodulin.

'Poking' microtubules bring about nuclear wriggling to position nuclei.

Nuclei wriggle in the cells of the follicle epithelium of the Drosophila pre-vitellogenic egg primordia. Although similar phenomena have been reported for a number of cultured cell types and some neurons in the zebrafish embryo, the mechanism and importance of the process have remained unexplained. Wriggling involves successive sudden and random minor turns of the nuclei, approximately three twists per minute with roughly 12° per twist, one of which lasts typically for 14 seconds. Wriggling is generated by the growing microtubules seeded throughout the cell cortex, which, while poking the nuclei, buckle and exert 5-40 piconewtons over ∼16 seconds. While wriggling, the nuclei drift ∼5 µm in a day in the immensely growing follicle cells along the apical-basal axis from the apical to the basal cell region. A >2-fold excess of the microtubules nucleated in the apical cell region, as compared with those seeded in the basal cell cortex, makes the nuclei drift along the apical-basal axis. Nuclear wriggling and positioning appear to be tightly related processes: they cease simultaneously when the nuclei become anchored by the actin cytoskeleton; moreover, colchicine or taxol treatment eliminates both nuclear wriggling and positioning. We propose that the wriggling nuclei reveal a thus far undescribed nuclear positioning mechanism.

The involvement of Importin-ß and peroxiredoxin-6005 in mitochondrial biogenesis.

Importin-ß is encoded by the Ketel gene in Drosophila. Upon running out of the maternal Importin-ß dowry larvae without the Ketel gene slow down and before dying possess symptoms characteristic for mitochondrial cytopathies. Death of the larvae is almost certainly the consequence of ceasing import of proteins, including some of the transcription factors, into the nuclei. We report here that the ensuing altered gene expression pattern leads to cessation of mitochondrial biogenesis. A transcriptome comparison between larvae with and without Ketel gene revealed altered expression level for 30 genes that are all nuclear. The seven downregulated genes have C/EBP transcription factor binding site in their promoter. RNAi silencing the function of peroxiredoxin-6005, one of the 23 upregulated genes, leads to excessive mitochondrial biogenesis, free radical production and death of the larvae. It appears that peroxiredoxin-6005 is engaged in mitochondrial biogenesis possibly as a component of redox-signaling.

The DRE motif is a key component in the expression regulation of the importin-beta encoding Ketel gene in Drosophila.

Importin-beta, encoded by the Ketel gene in Drosophila, is a key component of nuclear protein import, the formation of the spindle microtubules and the assembly of the nuclear envelope. The Drosophila embryos rely on the maternal importin-beta dowry at the beginning of their life. Expression of the zygotic Ketel gene commences during gastrulation in every cell and while the expression is maintained in the mitotically active diploid cells it ceases in the non-dividing larval cells in which nuclear protein import is assured by the long persisting importin-beta molecules. How is the expression of the Ketel gene regulated? In silico analysis revealed several conserved transcription factor binding sequences in the Ketel gene promoter. Reporter genes in which different segments of the promoter ensured transient expression of the luciferase gene in S2 cells identified the sequences required for normal Ketel gene expression level. Gel retardation and band shift assays revealed that the DREF and the CFDD transcription factors play key roles in the regulation of Ketel gene expression. Transgenic LacZ reporter genes revealed the sequences that ensure tissue-specific gene expression. Apparently, the regulation of Ketel gene expression depends largely on a DRE motif and action of the DREF, CFDD, CF2-II and BEAF transcription factors.