Katanin p80 regulates human cortical development by limiting centriole and cilia number

Structural basis for disassembly of katanin heterododecamers

The reorganization of microtubules in mitosis, meiosis, and development requires the microtubule-severing activity of katanin. Katanin is a heterodimer composed of an ATPase associated with diverse cellular activities (AAA) subunit and a regulatory subunit. Microtubule severing requires ATP hydrolysis by katanin's conserved AAA ATPase domains. Whereas other AAA ATPases form stable hexamers, we show that katanin forms only a monomer or dimers of heterodimers in solution. Katanin oligomers consistent with hexamers of heterodimers or heterododecamers were only observed for an ATP hydrolysis-deficient mutant in the presence of ATP. X-ray structures of katanin's AAA ATPase in monomeric nucleotide-free and pseudo-oligomeric ADP-bound states revealed conformational changes in the AAA subdomains that explained the structural basis for the instability of the katanin heterododecamer. We propose that the rapid dissociation of katanin AAA oligomers may lead to an autoinhibited state that prevents inappropriate microtubule severing or that cyclical disassembly into heterodimers may critically contribute to the microtubule-severing mechanism.

Primary cilia proteins: ciliary and extraciliary sites and functions

Primary cilia are immotile organelles known for their roles in development and cell signaling. Defects in primary cilia result in a range of disorders named ciliopathies. Because this organelle can be found singularly on almost all cell types, its importance extends to most organ systems. As such, elucidating the importance of the primary cilium has attracted researchers from all biological disciplines. As the primary cilia field expands, caution is warranted in attributing biological defects solely to the function of this organelle, since many of these "ciliary" proteins are found at other sites in cells and likely have non-ciliary functions. Indeed, many, if not all, cilia proteins have locations and functions outside the primary cilium. Extraciliary functions are known to include cell cycle regulation, cytoskeletal regulation, and trafficking. Cilia proteins have been observed in the nucleus, at the Golgi apparatus, and even in immune synapses of T cells (interestingly, a non-ciliated cell). Given the abundance of extraciliary sites and functions, it can be difficult to definitively attribute an observed phenotype solely to defective cilia rather than to some defective extraciliary function or a combination of both. Thus, extraciliary sites and functions of cilia proteins need to be considered, as well as experimentally determined. Through such consideration, we will understand the true role of the primary cilium in disease as compared to other cellular processes' influences in mediating disease (or through a combination of both). Here, we review a compilation of known extraciliary sites and functions of "cilia" proteins as a means to demonstrate the potential non-ciliary roles for these proteins.

A missense mutation in Katnal1 underlies behavioural, neurological and ciliary anomalies

Microtubule severing enzymes implement a diverse range of tissue-specific molecular functions throughout development and into adulthood. Although microtubule severing is fundamental to many dynamic neural processes, little is known regarding the role of the family member Katanin p60 subunit A-like 1, KATNAL1, in central nervous system (CNS) function. Recent studies reporting that microdeletions incorporating the KATNAL1 locus in humans result in intellectual disability and microcephaly suggest that KATNAL1 may play a prominent role in the CNS; however, such associations lack the functional data required to highlight potential mechanisms which link the gene to disease symptoms. Here we identify and characterise a mouse line carrying a loss of function allele in Katnal1. We show that mutants express behavioural deficits including in circadian rhythms, sleep, anxiety and learning/memory. Furthermore, in the brains of Katnal1 mutant mice we reveal numerous morphological abnormalities and defects in neuronal migration and morphology. Furthermore we demonstrate defects in the motile cilia of the ventricular ependymal cells of mutants, suggesting a role for Katnal1 in the development of ciliary function. We believe the data we present here are the first to associate KATNAL1 with such phenotypes, demonstrating that the protein plays keys roles in a number of processes integral to the development of neuronal function and behaviour.

Of rings and spines: The multiple facets of Citron proteins in neural development

The Citron protein was originally identified for its capability to specifically bind the active form of RhoA small GTPase, leading to the simplistic hypothesis that it may work as a RhoA downstream effector in actin remodeling. More than two decades later, a much more complex picture has emerged. In particular, it has become clear that in animals, and especially in mammals, the functions of the Citron gene (CIT) are intimately linked to many aspects of central nervous system (CNS) development and function, although the gene is broadly expressed. More specifically, CIT encodes two main isoforms, Citron-kinase (CIT-K) and Citron-N (CIT-N), characterized by complementary expression pattern and different functions. Moreover, in many of their activities, CIT proteins act more as upstream regulators than as downstream effectors of RhoA. Finally it has been found that, besides working through actin, CIT proteins have many crucial functional interactions with the microtubule cytoskeleton and may directly affect genome stability. In this review, we will summarize these advances and illustrate their actual or potential relevance for CNS diseases, including microcephaly and psychiatric disorders.

The evolution of cortical development: the synapsid-diapsid divergence

The cerebral cortex covers the rostral part of the brain and, in higher mammals and particularly humans, plays a key role in cognition and consciousness. It is populated with neuronal cell bodies distributed in radially organized layers. Understanding the common and lineage-specific molecular mechanisms that orchestrate cortical development and evolution are key issues in neurobiology. During evolution, the cortex appeared in stem amniotes and evolved divergently in two main branches of the phylogenetic tree: the synapsids (which led to present day mammals) and the diapsids (reptiles and birds). Comparative studies in organisms that belong to those two branches have identified some common principles of cortical development and organization that are possibly inherited from stem amniotes and regulated by similar molecular mechanisms. These comparisons have also highlighted certain essential features of mammalian cortices that are absent or different in diapsids and that probably evolved after the synapsid-diapsid divergence. Chief among these is the size and multi-laminar organization of the mammalian cortex, and the propensity to increase its area by folding. Here, I review recent data on cortical neurogenesis, neuronal migration and cortical layer formation and folding in this evolutionary perspective, and highlight important unanswered questions for future investigation.

Katanin-like 2 (KATNAL2) functions in multiple aspects of haploid male germ cell development in the mouse

The katanin microtubule-severing proteins are essential regulators of microtubule dynamics in a diverse range of species. Here we have defined critical roles for the poorly characterised katanin protein KATNAL2 in multiple aspects of spermatogenesis: the initiation of sperm tail growth from the basal body, sperm head shaping via the manchette, acrosome attachment, and ultimately sperm release. We present data suggesting that depending on context, KATNAL2 can partner with the regulatory protein KATNB1 or act autonomously. Moreover, our data indicate KATNAL2 may regulate δ- and ε-tubulin rather than classical α-β-tubulin microtubule polymers, suggesting the katanin family has a greater diversity of function than previously realised. Together with our previous research, showing the essential requirement of katanin proteins KATNAL1 and KATNB1 during spermatogenesis, our data supports the concept that in higher order species the presence of multiple katanins has allowed for subspecialisation of function within complex cellular settings such as the seminiferous epithelium.

Male infertility affects one in twenty men of reproductive age in western countries. Despite this, the biochemical basis of common defects, including reduced sperm count and abnormal sperm structure and function, remains poorly defined. Microtubules are cellular “scaffolds” that serve critical roles in all cells, including developing male germ cells wherein they facilitate mitosis and meiosis (cell division), sperm head remodelling and sperm tail formation. The precise regulation of microtubule number, length and movement is thus, essential for male fertility. Within this manuscript, we have used spermatogenesis to define the function of the putative microtubule-severing protein katanin-like 2 (KATNAL2). We show that mice with compromised KATNAL2 function are male sterile as a consequence of defects in the structural remodelling of germ cells. Notably, we show the function of microtubule-based structures involved in sperm head shaping and tail formation are disrupted. Further, we show for the first time, that KATNAL2 can function both independently or in concert with the katanin regulatory protein KATNB1 and that it can target the poorly characterized tubulin subunits delta and epsilon. Our research has immediate relevance to the origins of human male infertility and provides novel insights into aspects of microtubule regulation relevant to numerous tissues and species.

Structural basis of katanin p60:p80 complex formation

Interactions between microtubule (MT) interacting and trafficking (MIT) domains and their binding proteins are important for the accurate progression of many cellular processes that require the AAA+ ATPase machinery. Therefore, knowledge on the structural basis of MIT domain interactions is crucial for understanding the molecular mechanisms underlying AAA+ ATPase function. Katanin is a MT-severing AAA+ ATPase that consists of p60 and p80 subunits. Although, the hexameric p60 subunit is active alone, its association with the p80 subunit greatly enhances both the MT-binding and -severing activities of katanin. However, the molecular mechanism of how the p80 subunit contributes to katanin function is currently unknown. Here, we structurally and functionally characterized the interaction between the two katanin subunits that is mediated by the p60-MIT domain and the p80 C-terminal domain (p80-CTD). We show that p60-MIT and p80-CTD form a tight heterodimeric complex, whose high-resolution structure we determined by X-ray crystallography. Based on the crystal structure, we identified two conserved charged residues that are important for p60-MIT:p80-CTD complex formation and katanin function. Moreover, p60-MIT was compared with other MIT domain structures and similarities are discussed.

WDR81 mutations cause extreme microcephaly and impair mitotic progression in human fibroblasts and Drosophila neural stem cells

Microlissencephaly is a rare brain malformation characterized by congenital microcephaly and lissencephaly. Microlissencephaly is suspected to result from abnormalities in the proliferation or survival of neural progenitors. Despite the recent identification of six genes involved in microlissencephaly, the pathophysiological basis of this condition remains poorly understood. We performed trio-based whole exome sequencing in seven subjects from five non-consanguineous families who presented with either microcephaly or microlissencephaly. This led to the identification of compound heterozygous mutations in WDR81, a gene previously associated with cerebellar ataxia, intellectual disability and quadrupedal locomotion. Patient phenotypes ranged from severe microcephaly with extremely reduced gyration with pontocerebellar hypoplasia to moderate microcephaly with cerebellar atrophy. In patient fibroblast cells, WDR81 mutations were associated with increased mitotic index and delayed prometaphase/metaphase transition. Similarly, in vivo, we showed that knockdown of the WDR81 orthologue in Drosophila led to increased mitotic index of neural stem cells with delayed mitotic progression. In summary, we highlight the broad phenotypic spectrum of WDR81-related brain malformations, which include microcephaly with moderate to extremely reduced gyration and cerebellar anomalies. Our results suggest that WDR81 might have a role in mitosis that is conserved between Drosophila and humans.

Mutations in the Katnb1 gene cause left-right asymmetry and heart defects

BACKGROUND

The microtubule-severing protein complex katanin is composed two subunits, the ATPase subunit, KATNA1, and the noncatalytic regulatory subunit, KATNB1. Recently, the Katnb1 gene has been linked to infertility, regulation of centriole and cilia formation in fish and mammals, as well as neocortical brain development. KATNB1 protein is expressed in germ cells in humans and mouse, mitotic/meiotic spindles and cilia, although the full expression pattern of the Katnb1 gene has not been described.

RESULTS

Using a knockin-knockout mouse model of Katnb1 dysfunction we demonstrate that Katnb1 is ubiquitously expressed during embryonic development, although a stronger expression is seen in the crown cells of the gastrulation organizer, the murine node. Furthermore, null and hypomorphic Katnb1 gene mutations show a novel correlation between Katnb1 dysregulation and the development of impaired left-right signaling, including cardiac malformations.

CONCLUSIONS

Katanin function is a critical regulator of heart development in mice. These findings are potentially relevant to human cardiac development. Developmental Dynamics 246:1027-1035, 2017. © 2017 Wiley Periodicals, Inc.

Katanin spiral and ring structures shed light on power stroke for microtubule severing

Microtubule-severing enzymes katanin, spastin and fidgetin are AAA ATPases important for the biogenesis and maintenance of complex microtubule arrays in axons, spindles and cilia. Because of a lack of known 3D structures for these enzymes, their mechanism of action has remained poorly understood. Here we report the X-ray crystal structure of the monomeric AAA katanin module from Caenorhabditis elegans and cryo-EM reconstructions of the hexamer in two conformations. The structures reveal an unexpected asymmetric arrangement of the AAA domains mediated by structural elements unique to microtubule-severing enzymes and critical for their function. The reconstructions show that katanin cycles between open spiral and closed ring conformations, depending on the ATP occupancy of a gating protomer that tenses or relaxes interprotomer interfaces. Cycling of the hexamer between these conformations would provide the power stroke for microtubule severing.

Regulation of cytokinesis during corticogenesis: focus on the midbody

Development of the cerebral cortices depends on tight regulation of cell divisions. In this system, stem and progenitor cells undergo symmetric and asymmetric divisions to ultimately produce neurons that establish the layers of the cortex. Cell division culminates with the formation of the midbody, a transient organelle that establishes the site of abscission between nascent daughter cells. During cytokinetic abscission, the final stage of cell division, one daughter cell will inherit the midbody remnant, which can then maintain or expel the remnant, but mechanisms and circumstances influencing this decision are unclear. This review describes the midbody and its constituent proteins, as well as the known consequences of their manipulation during cortical development. The potential functional relevance of midbody mechanisms is discussed.

SPEF2 functions in microtubule-mediated transport in elongating spermatids to ensure proper male germ cell differentiation

Sperm differentiation requires specific protein transport for correct sperm tail formation and head shaping. A transient microtubular structure, the manchette, appears around the differentiating spermatid head and serves as a platform for protein transport to the growing tail. Sperm flagellar 2 (SPEF2) is known to be essential for sperm tail development. In this study we investigated the function of SPEF2 during spermatogenesis using a male germ cell-specific Spef2 knockout mouse model. In addition to defects in sperm tail development, we observed a duplication of the basal body and failure in manchette migration resulting in an abnormal head shape. We identified cytoplasmic dynein 1 and GOLGA3 as novel interaction partners for SPEF2. SPEF2 and dynein 1 colocalize in the manchette and the inhibition of dynein 1 disrupts the localization of SPEF2 to the manchette. Furthermore, the transport of a known SPEF2-binding protein, IFT20, from the Golgi complex to the manchette was delayed in the absence of SPEF2. These data indicate a possible novel role of SPEF2 as a linker protein for dynein 1-mediated cargo transport along microtubules.

Microtubule minus-end regulation at spindle poles by an ASPM-katanin complex

ASPM (known as Asp in fly and ASPM-1 in worm) is a microcephaly-associated protein family that regulates spindle architecture, but the underlying mechanism is poorly understood. Here, we show that ASPM forms a complex with another protein linked to microcephaly, the microtubule-severing ATPase katanin. ASPM and katanin localize to spindle poles in a mutually dependent manner and regulate spindle flux. X-ray crystallography revealed that the heterodimer formed by the N- and C-terminal domains of the katanin subunits p60 and p80, respectively, binds conserved motifs in ASPM. Reconstitution experiments demonstrated that ASPM autonomously tracks growing microtubule minus ends and inhibits their growth, while katanin decorates and bends both ends of dynamic microtubules and potentiates the minus-end blocking activity of ASPM. ASPM also binds along microtubules, recruits katanin and promotes katanin-mediated severing of dynamic microtubules. We propose that the ASPM-katanin complex controls microtubule disassembly at spindle poles and that misregulation of this process can lead to microcephaly.

Katanin p80, NuMA and cytoplasmic dynein cooperate to control microtubule dynamics

Human mutations in KATNB1 (p80) cause severe congenital cortical malformations, which encompass the clinical features of both microcephaly and lissencephaly. Although p80 plays critical roles during brain development, the underlying mechanisms remain predominately unknown. Here, we demonstrate that p80 regulates microtubule (MT) remodeling in combination with NuMA (nuclear mitotic apparatus protein) and cytoplasmic dynein. We show that p80 shuttles between the nucleus and spindle pole in synchrony with the cell cycle. Interestingly, this striking feature is shared with NuMA. Importantly, p80 is essential for aster formation and maintenance in vitro. siRNA-mediated depletion of p80 and/or NuMA induced abnormal mitotic phenotypes in cultured mouse embryonic fibroblasts and aberrant neurogenesis and neuronal migration in the mouse embryonic brain. Importantly, these results were confirmed in p80-mutant harboring patient-derived induced pluripotent stem cells and brain organoids. Taken together, our findings provide valuable insights into the pathogenesis of severe microlissencephaly, in which p80 and NuMA delineate a common pathway for neurogenesis and neuronal migration via MT organization at the centrosome/spindle pole.

Microcephaly Proteins Wdr62 and Aspm Define a Mother Centriole Complex Regulating Centriole Biogenesis, Apical Complex, and Cell Fate

Mutations in several genes encoding centrosomal proteins dramatically decrease the size of the human brain. We show that Aspm (abnormal spindle-like, microcephaly-associated) and Wdr62 (WD repeat-containing protein 62) interact genetically to control brain size, with mice lacking Wdr62, Aspm, or both showing gene dose-related centriole duplication defects that parallel the severity of the microcephaly and increased ectopic basal progenitors, suggesting premature delamination from the ventricular zone. Wdr62 and Aspm localize to the proximal end of the mother centriole and interact physically, with Wdr62 required for Aspm localization, and both proteins, as well as microcephaly protein Cep63, required to localize CENPJ/CPAP/Sas-4, a final common target. Unexpectedly, Aspm and Wdr62 are required for normal apical complex localization and apical epithelial structure, providing a plausible unifying mechanism for the premature delamination and precocious differentiation of progenitors. Together, our results reveal links among centrioles, apical proteins, and cell fate, and illuminate how alterations in these interactions can dynamically regulate brain size.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.

Severe NDE1-mediated microcephaly results from neural progenitor cell cycle arrests at multiple specific stages

Microcephaly is a cortical malformation disorder characterized by an abnormally small brain. Recent studies have revealed severe cases of microcephaly resulting from human mutations in the NDE1 gene, which is involved in the regulation of cytoplasmic dynein. Here using in utero electroporation of NDE1 short hairpin RNA (shRNA) in embryonic rat brains, we observe cell cycle arrest of proliferating neural progenitors at three distinct stages: during apical interkinetic nuclear migration, at the G2-to-M transition and in regulation of primary cilia at the G1-to-S transition. RNAi against the NDE1 paralogue NDEL1 has no such effects. However, NDEL1 overexpression can functionally compensate for NDE1, except at the G2-to-M transition, revealing a unique NDE1 role. In contrast, NDE1 and NDEL1 RNAi have comparable effects on postmitotic neuronal migration. These results reveal that the severity of NDE1-associated microcephaly results not from defects in mitosis, but rather the inability of neural progenitors to ever reach this stage.

Human mutations in the NDE1 gene have been associated with cortical malformations and severe microcephaly. Here, the authors show in embryonic rat brains that NDE1-depleted neural progenitors arrest at three specific cell cycle stages before mitosis, resulting in a severe decrease in neurogenesis.

Biallelic Mutations in Citron Kinase Link Mitotic Cytokinesis to Human Primary Microcephaly

Cell division terminates with cytokinesis and cellular separation. Autosomal-recessive primary microcephaly (MCPH) is a neurodevelopmental disorder characterized by a reduction in brain and head size at birth in addition to non-progressive intellectual disability. MCPH is genetically heterogeneous, and 16 loci are known to be associated with loss-of-function mutations predominantly affecting centrosomal-associated proteins, but the multiple roles of centrosomes in cellular function has left questions about etiology. Here, we identified three families affected by homozygous missense mutations in CIT, encoding citron rho-interacting kinase (CIT), which has established roles in cytokinesis. All mutations caused substitution of conserved amino acid residues in the kinase domain and impaired kinase activity. Neural progenitors that were differentiated from induced pluripotent stem cells (iPSCs) derived from individuals with these mutations exhibited abnormal cytokinesis with delayed mitosis, multipolar spindles, and increased apoptosis, rescued by CRISPR/Cas9 genome editing. Our results highlight the importance of cytokinesis in the pathology of primary microcephaly.

Fidgetin-like 1 is a ciliogenesis-inhibitory centrosome protein

Fidgetin-like 1 (FIGL-1) is a homolog of fidgetin, an AAA protein that was identified as the protein encoded by the gene mutated in fidget mice. Because the phenotypes of fidget mice are reminiscent of the phenotypes of ciliopathy diseases, and because fidgetin has microtubule-severing activity, we hypothesize that these proteins participate in cilia-related processes. Indeed, overexpression of FIGL-1 interfered with ciliogenesis in cultured cells. In particular, overexpressed FIGL-1 strongly accumulated at the centrosome, and, when highly expressed, perturbed the localization of centrosomal proteins such as pericentrin, CP110, and centrin. Using a polyclonal antibody against human FIGL-1, we found that endogenous FIGL-1 localized preferentially around the mother centriole. Consistently, depletion of FIGL-1 by shRNA treatment enhanced ciliogenesis in HEK293T cells. By checking the integrity of the cytoplasmic microtubule network in FIGL-1-overexpressing cells, we found that FIGL-1 probably has microtubule-severing activity, as suggested by its sequence homology with other microtubule-severing proteins. Furthermore, we showed that overexpression of FIGL-1 in zebrafish embryo decreased the length of cilia and perturbed the heart laterality. Taken together, these results demonstrate that FIGL-1 is a new centrosomal protein and inhibits ciliogenesis. These results extend the already long list of centrosomal proteins and provide new insights into the regulation of ciliogenesis.

CPAP promotes timely cilium disassembly to maintain neural progenitor pool

A mutation in the centrosomal‐P4.1‐associated protein (CPAP) causes Seckel syndrome with microcephaly, which is suggested to arise from a decline in neural progenitor cells (NPCs) during development. However, mechanisms of NPCs maintenance remain unclear. Here, we report an unexpected role for the cilium in NPCs maintenance and identify CPAP as a negative regulator of ciliary length independent of its role in centrosome biogenesis. At the onset of cilium disassembly, CPAP provides a scaffold for the cilium disassembly complex (CDC), which includes Nde1, Aurora A, and OFD1, recruited to the ciliary base for timely cilium disassembly. In contrast, mutated CPAP fails to localize at the ciliary base associated with inefficient CDC recruitment, long cilia, retarded cilium disassembly, and delayed cell cycle re‐entry leading to premature differentiation of patient iPS‐derived NPCs. Aberrant CDC function also promotes premature differentiation of NPCs in Seckel iPS‐derived organoids. Thus, our results suggest a role for cilia in microcephaly and its involvement during neurogenesis and brain size control.

Cellular Mechanisms of Ciliary Length Control

Cilia and flagella are evolutionarily conserved, membrane-bound, microtubule-based organelles on the surface of most eukaryotic cells. They play important roles in coordinating a variety of signaling pathways during growth, development, cell mobility, and tissue homeostasis. Defects in ciliary structure or function are associated with multiple human disorders called ciliopathies. These diseases affect diverse tissues, including, but not limited to the eyes, kidneys, brain, and lungs. Many processes must be coordinated simultaneously in order to initiate ciliogenesis. These include cell cycle, vesicular trafficking, and axonemal extension. Centrioles play a central role in both cell cycle progression and ciliogenesis, making the transition between basal bodies and mitotic spindle organizers integral to both processes. The maturation of centrioles involves a functional shift from cell division toward cilium nucleation which takes place concurrently with its migration and fusion to the plasma membrane. Several proteinaceous structures of the distal appendages in mother centrioles are required for this docking process. Ciliary assembly and maintenance requires a precise balance between two indispensable processes; so called assembly and disassembly. The interplay between them determines the length of the resulting cilia. These processes require a highly conserved transport system to provide the necessary substances at the tips of the cilia and to recycle ciliary turnover products to the base using a based microtubule intraflagellar transport (IFT) system. In this review; we discuss the stages of ciliogenesis as well as mechanisms controlling the lengths of assembled cilia.

A novel family of katanin-like 2 protein isoforms (KATNAL2), interacting with nucleotide-binding proteins Nubp1 and Nubp2, are key regulators of different MT-based processes in mammalian cells

Katanins are microtubule (MT)-severing AAA proteins with high phylogenetic conservation throughout the eukaryotes. They have been functionally implicated in processes requiring MT remodeling, such as spindle assembly in mitosis and meiosis, assembly/disassembly of flagella and cilia and neuronal morphogenesis. Here, we uncover a novel family of katanin-like 2 proteins (KATNAL2) in mouse, consisting of five alternatively spliced isoforms encoded by the Katnal2 genomic locus. We further demonstrate that in vivo these isoforms are able to interact with themselves, with each other and moreover directly and independently with MRP/MinD-type P-loop NTPases Nubp1 and Nubp2, which are integral components of centrioles, negative regulators of ciliogenesis and implicated in centriole duplication in mammalian cells. We find KATNAL2 localized on interphase MTs, centrioles, mitotic spindle, midbody and the axoneme and basal body of sensory cilia in cultured murine cells. shRNAi of Katnal2 results in inefficient cytokinesis and severe phenotypes of enlarged cells and nuclei, increased numbers of centrioles and the manifestation of aberrant multipolar mitotic spindles, mitotic defects, chromosome bridges, multinuclearity, increased MT acetylation and an altered cell cycle pattern. Silencing or stable overexpression of KATNAL2 isoforms drastically reduces ciliogenesis. In conclusion, KATNAL2s are multitasking enzymes involved in the same cell type in critically important processes affecting cytokinesis, MT dynamics, and ciliogenesis and are also implicated in cell cycle progression.

KIAA0556 is a novel ciliary basal body component mutated in Joubert syndrome

BACKGROUND

Joubert syndrome (JBTS) and related disorders are defined by cerebellar malformation (molar tooth sign), together with neurological symptoms of variable expressivity. The ciliary basis of Joubert syndrome related disorders frequently extends the phenotype to tissues such as the eye, kidney, skeleton and craniofacial structures.

RESULTS

Using autozygome and exome analyses, we identified a null mutation in KIAA0556 in a multiplex consanguineous family with hallmark features of mild Joubert syndrome. Patient-derived fibroblasts displayed reduced ciliogenesis potential and abnormally elongated cilia. Investigation of disease pathophysiology revealed that Kiaa0556 (-/-) null mice possess a Joubert syndrome-associated brain-restricted phenotype. Functional studies in Caenorhabditis elegans nematodes and cultured human cells support a conserved ciliary role for KIAA0556 linked to microtubule regulation. First, nematode KIAA0556 is expressed almost exclusively in ciliated cells, and the worm and human KIAA0556 proteins are enriched at the ciliary base. Second, C. elegans KIAA0056 regulates ciliary A-tubule number and genetically interacts with an ARL13B (JBTS8) orthologue to control cilium integrity. Third, human KIAA0556 binds to microtubules in vitro and appears to stabilise microtubule networks when overexpressed. Finally, human KIAA0556 biochemically interacts with ciliary proteins and p60/p80 katanins. The latter form a microtubule-severing enzyme complex that regulates microtubule dynamics as well as ciliary functions.

CONCLUSIONS

We have identified KIAA0556 as a novel microtubule-associated ciliary base protein mutated in Joubert syndrome. Consistent with the mild patient phenotype, our nematode, mice and human cell data support the notion that KIAA0556 has a relatively subtle and variable cilia-related function, which we propose is related to microtubule regulation.

De novo TUBB2B mutation causes fetal akinesia deformation sequence with microlissencephaly: An unusual presentation of tubulinopathy

Tubulinopathies are increasingly emerging major causes underlying complex cerebral malformations, particularly in case of microlissencephaly often associated with hypoplastic or absent corticospinal tracts. Fetal akinesia deformation sequence (FADS) refers to a clinically and genetically heterogeneous group of disorders with congenital malformations related to impaired fetal movement. We report on an early foetal case with FADS and microlissencephaly due to TUBB2B mutation. Neuropathological examination disclosed virtually absent cortical lamination, foci of neuronal overmigration into the leptomeningeal spaces, corpus callosum agenesis, cerebellar and brainstem hypoplasia and extremely severe hypoplasia of the spinal cord with no anterior and posterior horns and almost no motoneurons. At the cellular level, the p.Cys239Phe TUBB2B mutant leads to tubulin heterodimerization impairment, decreased ability to incorporate into the cytoskeleton, microtubule dynamics alteration, with an accelerated rate of depolymerization. To our knowledge, this is the first case of microlissencephaly to be reported presenting with a so severe and early form of FADS, highlighting the importance of tubulin mutation screening in the context of FADS with microlissencephaly.

A syndrome of microcephaly, short stature, polysyndactyly, and dental anomalies caused by a homozygous KATNB1 mutation

Using whole-exome sequencing, we identified a homozygous acceptor splice-site mutation in intron 6 of the KATNB1 gene in a patient from a consanguineous Turkish family who presented with congenital microcephaly, lissencephaly, short stature, polysyndactyly, and dental abnormalities. cDNA analysis revealed complete loss of the natural acceptor splice-site resulting either in the usage of an alternative, exonic acceptor splice-site inducing a frame-shift and premature protein truncation or, to a minor extent, in complete skipping of exon 7. Both effects most likely lead to complete loss of KATNB1 function. Homozygous and compound heterozygous mutations in KATNB1 have very recently been described as a cause of microcephaly with brain malformations and seizures. We extend the KATNB1 associated phenotype by describing a syndrome characterized by primordial dwarfism, lissencephaly, polysyndactyly, and dental anomalies, which is caused by a homozygous truncating KATNB1 mutation.

RTTN Mutations Cause Primary Microcephaly and Primordial Dwarfism in Humans

Primary microcephaly is a developmental brain anomaly that results from defective proliferation of neuroprogenitors in the germinal periventricular zone. More than a dozen genes are known to be mutated in autosomal-recessive primary microcephaly in isolation or in association with a more generalized growth deficiency (microcephalic primordial dwarfism), but the genetic heterogeneity is probably more extensive. In a research protocol involving autozygome mapping and exome sequencing, we recruited a multiplex consanguineous family who is affected by severe microcephalic primordial dwarfism and tested negative on clinical exome sequencing. Two candidate autozygous intervals were identified, and the second round of exome sequencing revealed a single intronic variant therein (c.2885+8A>G [p.Ser963(∗)] in RTTN exon 23). RT-PCR confirmed that this change creates a cryptic splice donor and thus causes retention of the intervening 7 bp of the intron and leads to premature truncation. On the basis of this finding, we reanalyzed the exome file of a second consanguineous family affected by a similar phenotype and identified another homozygous change in RTTN as the likely causal mutation. Combined linkage analysis of the two families confirmed that RTTN maps to the only significant linkage peak. Finally, through international collaboration, a Canadian multiplex family affected by microcephalic primordial dwarfism and biallelic mutation of RTTN was identified. Our results expand the phenotype of RTTN-related disorders, hitherto limited to polymicrogyria, to include microcephalic primordial dwarfism with a complex brain phenotype involving simplified gyration.

Characterization of Ambra1 in asexual cycle of a non-vertebrate chordate, the colonial tunicate Botryllus schlosseri, and phylogenetic analysis of the protein group in Bilateria

Ambra1 is a positive regulator of autophagy, a lysosome-mediated degradative process involved both in physiological and pathological conditions. Nowadays, Ambra1 has been characterized only in mammals and zebrafish. Through bioinformatics searches and targeted cloning, we report the identification of the complete Ambra1 transcript in a non-vertebrate chordate, the tunicate Botryllus schlosseri. Tunicata is the sister group of Vertebrata and the only chordate group possessing species that reproduce also by blastogenesis (asexual reproduction). B. schlosseri Ambra1 deduced amino acid sequence is shorter than vertebrate homologues but still contains the typical WD40 domain. qPCR analyses revealed that the level of B. schlosseri Ambra1 transcription is temporally regulated along the colonial blastogenetic cycle. By means of similarity searches we identified Wdr5 and Katnb1 as proteins evolutionarily associated to Ambra1. Phylogenetic analyses on Bilateria indicate that: (i) Wdr5 is the most related to Ambra1, so that they may derive from an ancestral gene, (ii) Ambra1 forms a group of ancient genes evolved before the radiation of the taxon, (iii) these orthologous Ambra1 share the two conserved WD40/YVTN repeat-like-containing domains, and (iv) they are characterized by ancient duplications of WD40 repeats within the N-terminal domain.

Novel POC1A mutation in primordial dwarfism reveals new insights for centriole biogenesis

POC1A encodes a WD repeat protein localizing to centrioles and spindle poles and is associated with short stature, onychodysplasia, facial dysmorphism and hypotrichosis (SOFT) syndrome. These main features are related to the defect in cell proliferation of chondrocytes in growth plate. In the current study, we aimed at identifying the molecular basis of two patients with primordial dwarfism (PD) in a single family through utilization of whole-exome sequencing. A novel homozygous p.T120A missense mutation was detected in POC1A in both patients, a known causative gene of SOFT syndrome, and confirmed using Sanger sequencing. To test the pathogenicity of the detected mutation, primary fibroblast cultures obtained from the patients and a control individual were used. For evaluating the global gene expression profile of cells carrying p.T120A mutation in POC1A, we performed the gene expression array and compared their expression profiles to those of control fibroblast cells. The gene expression array analysis showed that 4800 transcript probes were significantly deregulated in cells with p.T120A mutation in comparison to the control. GO term association results showed that deregulated genes are mostly involved in the extracellular matrix and cytoskeleton. Furthermore, the p.T120A missense mutation in POC1A caused the formation of abnormal mitotic spindle structure, including supernumerary centrosomes, and changes in POC1A were accompanied by alterations in another centrosome-associated WD repeat protein p80-katanin. As a result, we identified a novel mutation in POC1A of patients with PD and showed that this mutation causes the formation of multiple numbers of centrioles and multipolar spindles with abnormal chromosome arrangement.

Mutations in KATNB1 cause complex cerebral malformations by disrupting asymmetrically dividing neural progenitors

Exome sequencing analysis of over 2,000 children with complex malformations of cortical development identified five independent (four homozygous and one compound heterozygous) deleterious mutations in KATNB1, encoding the regulatory subunit of the microtubule-severing enzyme Katanin. Mitotic spindle formation is defective in patient-derived fibroblasts, a consequence of disrupted interactions of mutant KATNB1 with KATNA1, the catalytic subunit of Katanin, and other microtubule-associated proteins. Loss of KATNB1 orthologs in zebrafish (katnb1) and flies (kat80) results in microcephaly, recapitulating the human phenotype. In the developing Drosophila optic lobe, kat80 loss specifically affects the asymmetrically dividing neuroblasts, which display supernumerary centrosomes and spindle abnormalities during mitosis, leading to cell cycle progression delays and reduced cell numbers. Furthermore, kat80 depletion results in dendritic arborization defects in sensory and motor neurons, affecting neural architecture. Taken together, we provide insight into the mechanisms by which KATNB1 mutations cause human cerebral cortical malformations, demonstrating its fundamental role during brain development.