Multiple Isoforms of Nesprin1 Are Integral Components of Ciliary Rootlets

The intercentriolar fibers function as docking sites of centriolar satellites for cilia assembly

Two mother centrioles in an animal cell are linked by intercentriolar fibers that have CROCC/rootletin as their main building block. Here, we investigated the regulatory role of intercentriolar/rootlet fibers in cilia assembly. The cilia formation rates were significantly reduced in the CEP250/C-NAP1 and CROCC/rootletin knockout (KO) cells, irrespective of the departure of the young mother centrioles from the basal bodies. In addition, centriolar satellites were dispersed throughout the cytoplasm in the CEP250 and CROCC KO cells. We observed that PCM1 directly binds to CROCC. Their interaction is critical not only for the accumulation of centriolar satellites near the centrosomes/basal bodies but also for cilia formation. Finally, we observed that the centriolar satellite proteins are localized at the intercentriolar/rootlet fibers in the kidney epithelial cells. Based on these findings, we propose that the intercentriolar/rootlet fibers function as docking sites for centriolar satellites near the centrosomes/basal bodies and facilitate the cilia assembly process.

Zebrafish crocc2 mutants exhibit divergent craniofacial shape, misregulated variability, and aberrant cartilage morphogenesis

BACKGROUND

Phenotypic variation is of paramount importance in development, evolution, and human health; however, the molecular mechanisms that influence organ shape and shape variability are not well understood. During craniofacial development, the behavior of skeletal precursors is regulated by both biochemical and environmental inputs, and the primary cilia play critical roles in transducing both types of signals. Here, we examine a gene that encodes a key constituent of the ciliary rootlets, crocc2, and its role in cartilage morphogenesis in larval zebrafish.

RESULTS

Geometric morphometric analysis of crocc2 mutants revealed altered craniofacial shapes and expanded variation. At the cellular level, we observed altered chondrocyte shapes and planar cell polarity across multiple stages in crocc2 mutants. Notably, cellular defects were specific to areas that experience direct mechanical input. Cartilage cell number, apoptosis, and bone patterning were not affected in crocc2 mutants.

CONCLUSIONS

Whereas "regulatory" genes are widely implicated in patterning the craniofacial skeleton, genes that encode "structural" aspects of the cell are increasingly implicated in shaping the face. Our results add crocc2 to this list, and demonstrate that it affects craniofacial geometry and canalizes phenotypic variation. We propose that it does so via mechanosensing, possibly through the ciliary rootlet. If true, this would implicate a new organelle in skeletal development and evolution.

Emerging roles of centrosome cohesion

The centrosome, consisting of centrioles and the associated pericentriolar material, is the main microtubule-organizing centre (MTOC) in animal cells. During most of interphase, the two centrosomes of a cell are joined together by centrosome cohesion into one MTOC. The most dominant element of centrosome cohesion is the centrosome linker, an interdigitating, fibrous network formed by the protein C-Nap1 anchoring a number of coiled-coil proteins including rootletin to the proximal end of centrioles. Alternatively, centrosomes can be kept together by the action of the minus end directed kinesin motor protein KIFC3 that works on interdigitating microtubules organized by both centrosomes and probably by the actin network. Although cells connect the two interphase centrosomes by several mechanisms into one MTOC, the general importance of centrosome cohesion, particularly for an organism, is still largely unclear. In this article, we review the functions of the centrosome linker and discuss how centrosome cohesion defects can lead to diseases.

Regulation of rod photoreceptor function by farnesylated G-protein γ-subunits

Heterotrimeric G-protein transducin, Gt, is a key signal transducer and amplifier in retinal rod and cone photoreceptor cells. Despite similar subunit composition, close amino acid identity, and identical posttranslational farnesylation of their Gγ subunits, rods and cones rely on unique Gγ1 (Gngt1) and Gγc (Gngt2) isoforms, respectively. The only other farnesylated G-protein γ-subunit, Gγ11 (Gng11), is expressed in multiple tissues but not retina. To determine whether Gγ1 regulates uniquely rod phototransduction, we generated transgenic rods expressing Gγ1, Gγc, or Gγ11 in Gγ1-deficient mice and analyzed their properties. Immunohistochemistry and Western blotting demonstrated the robust expression of each transgenic Gγ in rod cells and restoration of Gαt1 expression, which is greatly reduced in Gγ1-deficient rods. Electroretinography showed restoration of visual function in all three transgenic Gγ1-deficient lines. Recordings from individual transgenic rods showed that photosensitivity impaired in Gγ1-deficient rods was also fully restored. In all dark-adapted transgenic lines, Gαt1 was targeted to the outer segments, reversing its diffuse localization found in Gγ1-deficient rods. Bright illumination triggered Gαt1 translocation from the rod outer to inner segments in all three transgenic strains. However, Gαt1 translocation in Gγ11 transgenic mice occurred at significantly dimmer background light. Consistent with this, transretinal ERG recordings revealed gradual response recovery in moderate background illumination in Gγ11 transgenic mice but not in Gγ1 controls. Thus, while farnesylated Gγ subunits are functionally active and largely interchangeable in supporting rod phototransduction, replacement of retina-specific Gγ isoforms by the ubiquitous Gγ11 affects the ability of rods to adapt to background light.

The structure and function of centriolar rootlets

To gain a holistic understanding of cellular function, we must understand not just the role of individual organelles, but also how multiple macromolecular assemblies function collectively. Centrioles produce fundamental cellular processes through their ability to organise cytoskeletal fibres. In addition to nucleating microtubules, centrioles form lesser-known polymers, termed rootlets. Rootlets were identified over a 100 years ago and have been documented morphologically since by electron microscopy in different eukaryotic organisms. Rootlet-knockout animals have been created in various systems, providing insight into their physiological functions. However, the precise structure and function of rootlets is still enigmatic. Here, I consider common themes of rootlet function and assembly across diverse cellular systems. I suggest that the capability of rootlets to form physical links from centrioles to other cellular structures is a general principle unifying their functions in diverse cells and serves as an example of how cellular function arises from collective organellar activity.

Summary: This Review discusses the structure and function of enigmatic cytoskeletal fibres termed centriolar rootlets, suggesting that they form physical links between subcellular structures to allow collective organelle function.

The Nesprin-1/-2 ortholog ANC-1 regulates organelle positioning in C. elegans independently from its KASH or actin-binding domains

KASH proteins in the outer nuclear membrane comprise the cytoplasmic half of linker of nucleoskeleton and cytoskeleton (LINC) complexes that connect nuclei to the cytoskeleton. Caenorhabditis elegans ANC-1, an ortholog of Nesprin-1/2, contains actin-binding and KASH domains at opposite ends of a long spectrin-like region. Deletion of either the KASH or calponin homology (CH) domains does not completely disrupt nuclear positioning, suggesting neither KASH nor CH domains are essential. Deletions in the spectrin-like region of ANC-1 led to significant defects, but only recapitulated the null phenotype in combination with mutations in the transmembrane (TM) span. In anc-1 mutants, the endoplasmic reticulum ER, mitochondria, and lipid droplets were unanchored, moving throughout the cytoplasm. The data presented here support a cytoplasmic integrity model where ANC-1 localizes to the ER membrane and extends into the cytoplasm to position nuclei, ER, mitochondria, and other organelles in place.

Centrosome and ciliary abnormalities in fetal akinesia deformation sequence human fibroblasts

Ciliopathies are clinical disorders of the primary cilium with widely recognised phenotypic and genetic heterogeneity. Here, we found impaired ciliogenesis in fibroblasts derived from individuals with fetal akinesia deformation sequence (FADS), a broad spectrum of neuromuscular disorders arising from compromised foetal movement. We show that cells derived from FADS individuals have shorter and less primary cilia (PC), in association with alterations in post-translational modifications in α-tubulin. Similarly, siRNA-mediated depletion of two known FADS proteins, the scaffold protein rapsyn and the nucleoporin NUP88, resulted in defective PC formation. Consistent with a role in ciliogenesis, rapsyn and NUP88 localised to centrosomes and PC. Furthermore, proximity-ligation assays confirm the respective vicinity of rapsyn and NUP88 to γ-tubulin. Proximity-ligation assays moreover show that rapsyn and NUP88 are adjacent to each other and that the rapsyn-NUP88 interface is perturbed in the examined FADS cells. We suggest that the perturbed rapsyn-NUP88 interface leads to defects in PC formation and that defective ciliogenesis contributes to the pleiotropic defects seen in FADS.

Multimerization of Zika Virus-NS5 Causes Ciliopathy and Forces Premature Neurogenesis

Zika virus (ZikV) is a flavivirus that infects neural tissues, causing congenital microcephaly. ZikV has evolved multiple mechanisms to restrict proliferation and enhance cell death, although the underlying cellular events involved remain unclear. Here we show that the ZikV-NS5 protein interacts with host proteins at the base of the primary cilia in neural progenitor cells, causing an atypical non-genetic ciliopathy and premature neuron delamination. Furthermore, in human microcephalic fetal brain tissue, ZikV-NS5 persists at the base of the motile cilia in ependymal cells, which also exhibit a severe ciliopathy. Although the enzymatic activity of ZikV-NS5 appears to be dispensable, the amino acids Y25, K28, and K29 that are involved in NS5 oligomerization are essential for localization and interaction with components of the cilium base, promoting ciliopathy and premature neurogenesis. These findings lay the foundation for therapies that target ZikV-NS5 multimerization and prevent the developmental malformations associated with congenital Zika syndrome.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

Motile ciliogenesis and the mitotic prism

Motile cilia of epithelial multiciliated cells transport vital fluids along organ lumens to promote essential respiratory, reproductive and brain functions. Progenitors of multiciliated cells undergo massive and coordinated organelle remodelling during their differentiation for subsequent motile ciliogenesis. Defects in multiciliated cell differentiation lead to severe cilia-related diseases by perturbing cilia-based flows. Recent work designated the machinery of mitosis as the orchestrator of the orderly progression of differentiation associated with multiple motile cilia formation. By examining the events leading to motile ciliogenesis with a methodological prism of mitosis, we contextualise and discuss the recent findings to broaden the spectrum of questions related to the differentiation of mammalian multiciliated cells.

Chain reaction: LINC complexes and nuclear positioning

Nuclear positioning plays an essential role in defining cell architecture and behaviour in both development and disease, and nuclear location frequently adjusts according to internal and external cues. For instance, during periods of migration in many cell types, the nucleus may be actively repositioned behind the microtubule-organising centre. Nuclear movement, for the most part, is dependent upon coupling of the cytoskeleton to the nuclear periphery. This is accomplished largely through SUN and KASH domain proteins, which together assemble to form LINC (linker of the nucleoskeleton and cytoskeleton) complexes spanning the nuclear envelope. SUN proteins of the inner nuclear membrane provide a connection to nuclear structures while acting as a tether for outer nuclear membrane KASH proteins. The latter contain binding sites for diverse cytoskeletal components. Recent publications highlight new aspects of LINC complex regulation revealing that the interplay between SUN and KASH partners can strongly influence how the nucleus functionally engages with different branches of the cytoskeleton.

Energide-cell body as smallest unit of eukaryotic life

Background

The evolutionary origin of the eukaryotic nucleus is obscure and controversial. Currently preferred are autogenic concepts; ideas of a symbiotic origin are mostly discarded and forgotten. Here we briefly discuss these issues and propose a new version of the symbiotic and archaeal origin of the eukaryotic nucleus.

Scope and Conclusions

The nucleus of eukaryotic cells forms via its perinuclear microtubules, the primary eukaryotic unit known also as the Energide-cell body. As for all other endosymbiotic organelles, new Energides are generated only from other Energides. While the Energide cannot be generated de novo, it can use its secretory apparatus to generate de novo the cell periphery apparatus. We suggest that Virchow's tenet Omnis cellula e cellula should be updated as Omnis Energide e Energide to reflect the status of the Energide as the primary unit of the eukaryotic cell, and life. In addition, the plasma membrane provides feedback to the Energide and renders it protection via the plasma membrane-derived endosomal network. New discoveries suggest archaeal origins of both the Energide and its host cell.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

The KASH-containing isoform of Nesprin1 giant associates with ciliary rootlets of ependymal cells

Biallelic nonsense mutations of SYNE1 underlie a variable array of cerebellar and non-cerebellar pathologies of unknown molecular etiology. SYNE1 encodes multiple isoforms of Nesprin1 that associate with the nuclear envelope, with large cerebellar synapses and with ciliary rootlets of photoreceptors. Using two novel mouse models, we determined the expression pattern of Nesprin1 isoforms in the cerebellum whose integrity and functions are invariably affected by SYNE1 mutations. We further show that a giant isoform of Nesprin1 associates with the ciliary rootlets of ependymal cells that line brain ventricles and establish that this giant ciliary isoform of Nesprin1 harbors a KASH domain. Whereas cerebellar phenotypes are not recapitulated in Nes1g^STOP/STOP mice, these mice display a significant increase of ventricular volume. Together, these data fuel novel hypotheses about the molecular pathogenesis of SYNE1 mutations and support that KASH proteins may localize beyond the nuclear envelope in vivo.

Emerging views of the nucleus as a cellular mechanosensor

The ability of cells to respond to mechanical forces is critical for numerous biological processes. Emerging evidence indicates that external mechanical forces trigger changes in nuclear envelope structure and composition, chromatin organization and gene expression. However, it remains unclear if these processes originate in the nucleus or are downstream of cytoplasmic signals. Here we discuss recent findings that support a direct role of the nucleus in cellular mechanosensing and highlight novel tools to study nuclear mechanotransduction.

STED nanoscopy of the centrosome linker reveals a CEP68-organized, periodic rootletin network anchored to a C-Nap1 ring at centrioles

The centrosome linker proteins C-Nap1, rootletin, and CEP68 connect the two centrosomes of a cell during interphase into one microtubule-organizing center. This coupling is important for cell migration, cilia formation, and timing of mitotic spindle formation. Very little is known about the structure of the centrosome linker. Here, we used stimulated emission depletion (STED) microscopy to show that each C-Nap1 ring at the proximal end of the two centrioles organizes a rootletin ring and, in addition, multiple rootletin/CEP68 fibers. Rootletin/CEP68 fibers originating from the two centrosomes form a web-like, interdigitating network, explaining the flexible nature of the centrosome linker. The rootletin/CEP68 filaments are repetitive and highly ordered. Staggered rootletin molecules (N-to-N and C-to-C) within the filaments are 75 nm apart. Rootletin binds CEP68 via its C-terminal spectrin repeat-containing region in 75-nm intervals. The N-to-C distance of two rootletin molecules is ∼35 to 40 nm, leading to an estimated minimal rootletin length of ∼110 nm. CEP68 is important in forming rootletin filaments that branch off centrioles and to modulate the thickness of rootletin fibers. Thus, the centrosome linker consists of a vast network of repeating rootletin units with C-Nap1 as ring organizer and CEP68 as filament modulator.

Cone myoid elongation involves unidirectional microtubule movement mediated by dynein-1

Using structured illumination microscopy and photoconvertible tubulin in zebrafish photoreceptors, it is shown that microtubules move together during myoid elongation, a dark adaptive process in cone photoreceptors. Additionally, cytoplasmic dynein-1, localized at the base of the elongating myoid, mediates this unidirectional movement of microtubules.

Teleosts and amphibians exhibit retinomotor movements, morphological changes in photoreceptors regulated by light and circadian rhythms. Cone myoid elongation occurs during dark adaptation, leading to the positioning of the cone outer segment closer to the retinal pigment epithelium. Although it has been shown that microtubules are essential for cone myoid elongation, the underlying mechanism has not been established. In this work, we generated a transgenic line of zebrafish expressing a photoconvertible form of α-tubulin (tdEOS-tubulin) specifically in cone photoreceptors. Using superresolution structured illumination microscopy in conjunction with both pharmacological and genetic manipulation, we show that cytoplasmic dynein-1, which localizes to the junction between the ellipsoid and myoid, functions to shuttle microtubules from the ellipsoid into the myoid during the course of myoid elongation. We propose a novel model by which stationary complexes of cytoplasmic dynein-1 are responsible for the shuttling of microtubules between the ellipsoid and myoid is the underlying force for the morphological change of myoid elongation.