Sensory deficits in mice hypomorphic for a mammalian homologue of unc-53

Variants of NAV3, a neuronal morphogenesis protein, cause intellectual disability, developmental delay, and microcephaly

Microtubule associated proteins (MAPs) are widely expressed in the central nervous system, and have established roles in cell proliferation, myelination, neurite formation, axon specification, outgrowth, dendrite, and synapse formation. We report eleven individuals from seven families harboring predicted pathogenic biallelic, de novo, and heterozygous variants in the NAV3 gene, which encodes the microtubule positive tip protein neuron navigator 3 (NAV3). All affected individuals have intellectual disability (ID), microcephaly, skeletal deformities, ocular anomalies, and behavioral issues. In mouse brain, Nav3 is expressed throughout the nervous system, with more prominent signatures in postmitotic, excitatory, inhibiting, and sensory neurons. When overexpressed in HEK293T and COS7 cells, pathogenic variants impaired NAV3 ability to stabilize microtubules. Further, knocking-down nav3 in zebrafish led to severe morphological defects, microcephaly, impaired neuronal growth, and behavioral impairment, which were rescued with co-injection of WT NAV3 mRNA and not by transcripts encoding the pathogenic variants. Our findings establish the role of NAV3 in neurodevelopmental disorders, and reveal its involvement in neuronal morphogenesis, and neuromuscular responses.

Neuron navigators: A novel frontier with physiological and pathological implications

Neuron navigators are microtubule plus-end tracking proteins containing basic and serine rich regions which are encoded by neuron navigator genes (NAVs). Neuron navigator proteins are essential for neurite outgrowth, neuronal migration, and overall neurodevelopment along with some other functions as well. The navigator proteins are substantially expressed in the developing brain and have been reported to be differentially expressed in various tissues at different ages. Over the years, the research has found neuron navigators to be implicated in a spectrum of pathological conditions such as developmental anomalies, neurodegenerative disorders, neuropathic pain, anxiety, cancers, and certain inflammatory conditions. The existing knowledge about neuron navigators remains sparse owing to their differential functions, undiscovered modulators, and unknown molecular mechanisms. Investigating the possible role of neuron navigators in various physiological processes and pathological conditions pose as a novel field that requires extensive research and might provide novel mechanistic insights and understanding of these aspects.

Loss of Neuron Navigator 2 Impairs Brain and Cerebellar Development

Cerebellar hypoplasia and dysplasia encompass a group of clinically and genetically heterogeneous disorders frequently associated with neurodevelopmental impairment. The Neuron Navigator 2 (NAV2) gene (MIM: 607,026) encodes a member of the Neuron Navigator protein family, widely expressed within the central nervous system (CNS), and particularly abundant in the developing cerebellum. Evidence across different species supports a pivotal function of NAV2 in cytoskeletal dynamics and neurite outgrowth. Specifically, deficiency of Nav2 in mice leads to cerebellar hypoplasia with abnormal foliation due to impaired axonal outgrowth. However, little is known about the involvement of the NAV2 gene in human disease phenotypes. In this study, we identified a female affected with neurodevelopmental impairment and a complex brain and cardiac malformations in which clinical exome sequencing led to the identification of NAV2 biallelic truncating variants. Through protein expression analysis and cell migration assay in patient-derived fibroblasts, we provide evidence linking NAV2 deficiency to cellular migration deficits. In model organisms, the overall CNS histopathology of the Nav2 hypomorphic mouse revealed developmental anomalies including cerebellar hypoplasia and dysplasia, corpus callosum hypo-dysgenesis, and agenesis of the olfactory bulbs. Lastly, we show that the NAV2 ortholog in Drosophila, sickie (sick) is widely expressed in the fly brain, and sick mutants are mostly lethal with surviving escapers showing neurobehavioral phenotypes. In summary, our results unveil a novel human neurodevelopmental disorder due to genetic loss of NAV2, highlighting a critical conserved role of the NAV2 gene in brain and cerebellar development across species.

The Neuron Navigators: Structure, function, and evolutionary history

Neuron navigators (Navigators) are cytoskeletal-associated proteins important for neuron migration, neurite growth, and axon guidance, but they also function more widely in other tissues. Recent studies have revealed novel cellular functions of Navigators such as macropinocytosis, and have implicated Navigators in human disorders of axon growth. Navigators are present in most or all bilaterian animals: vertebrates have three Navigators (NAV1-3), Drosophila has one (Sickie), and Caenorhabditis elegans has one (Unc-53). Structurally, Navigators have conserved N- and C-terminal regions each containing specific domains. The N-terminal region contains a calponin homology (CH) domain and one or more SxIP motifs, thought to interact with the actin cytoskeleton and mediate localization to microtubule plus-end binding proteins, respectively. The C-terminal region contains two coiled-coil domains, followed by a AAA+ family nucleoside triphosphatase domain of unknown activity. The Navigators appear to have evolved by fusion of N- and C-terminal region homologs present in simpler organisms. Overall, Navigators participate in the cytoskeletal response to extracellular cues via microtubules and actin filaments, in conjunction with membrane trafficking. We propose that uptake of fluid-phase cues and nutrients and/or downregulation of cell surface receptors could represent general mechanisms that explain Navigator functions. Future studies developing new models, such as conditional knockout mice or human cerebral organoids may reveal new insights into Navigator function. Importantly, further biochemical studies are needed to define the activities of the Navigator AAA+ domain, and to study potential interactions among different Navigators and their binding partners.

Using coding and non-coding rare variants to target candidate genes in patients with severe tinnitus

Tinnitus is the phantom percept of an internal non-verbal set of noises and tones. It is reported by 15% of the population and it is usually associated with hearing and/or brain disorders. The role of structural variants (SVs) in coding and non-coding regions has not been investigated in patients with severe tinnitus. In this study, we performed whole-genome sequencing in 97 unrelated Swedish individuals with chronic tinnitus (TIGER cohort). Rare single nucleotide variants (SNV), large structural variants (LSV), and copy number variations (CNV) were retrieved to perform a gene enrichment analysis in TIGER and in a subgroup of patients with severe tinnitus (SEVTIN, n = 34), according to the tinnitus handicap inventory (THI) scores. An independent exome sequencing dataset of 147 Swedish tinnitus patients was used as a replication cohort (JAGUAR cohort) and population-specific datasets from Sweden (SweGen) and Non-Finish Europeans (NFE) from gnomAD were used as control groups. SEVTIN patients showed a higher prevalence of hyperacusis, hearing loss, and anxiety when they were compared to individuals in the TIGER cohort. We found an enrichment of rare missense variants in 6 and 8 high-constraint genes in SEVTIN and TIGER cohorts, respectively. Of note, an enrichment of missense variants was found in the CACNA1E gene in both SEVTIN and TIGER. We replicated the burden of missense variants in 9 high-constrained genes in the JAGUAR cohort, including the gene NAV2, when data were compared with NFE. Moreover, LSVs in constrained regions overlapping CACNA1E, NAV2, and TMEM132D genes were observed in TIGER and SEVTIN.

Chicory: Understanding the Effects and Effectors of This Functional Food

Industrial chicory has been the subject of numerous studies, most of which provide clinical observations on its health effects. Whether it is the roasted root, the flour obtained from the roots or the different classes of molecules that enter into the composition of this plant, understanding the molecular mechanisms of action on the human organism remains incomplete. In this study, we were interested in three molecules or classes of molecules present in chicory root: fructose, chlorogenic acids, and sesquiterpene lactones. We conducted experiments on the murine model and performed a nutrigenomic analysis, a metabolic hormone assay and a gut microbiota analysis, associated with in vitro observations for different responses. We have highlighted a large number of effects of all these classes of molecules that suggest a pro-apoptotic activity, an anti-inflammatory, antimicrobial, antioxidant, hypolipidemic and hypoglycemic effect and also an important role in appetite regulation. A significant prebiotic activity was also identified. Fructose seems to be the most involved in these activities, contributing to approximately 83% of recorded responses, but the other classes of tested molecules have shown a specific role for these different effects, with an estimated contribution of 23-24%.

The +TIP Navigator-1 is an actin-microtubule crosslinker that regulates axonal growth cone motility

Microtubule (MT) plus-end tracking proteins (+TIPs) are central players in the coordination between the MT and actin cytoskeletons in growth cones (GCs) during axon guidance. The +TIP Navigator-1 (NAV1) is expressed in the developing nervous system, yet its neuronal functions remain poorly elucidated. Here, we report that NAV1 controls the dynamics and motility of the axonal GCs of cortical neurons in an EB1-dependent manner and is required for axon turning toward a gradient of netrin-1. NAV1 accumulates in F-actin-rich domains of GCs and binds actin filaments in vitro. NAV1 can also bind MTs independently of EB1 in vitro and crosslinks nonpolymerizing MT plus ends to actin filaments in axonal GCs, preventing MT depolymerization in F-actin-rich areas. Together, our findings pinpoint NAV1 as a key player in the actin-MT crosstalk that promotes MT persistence at the GC periphery and regulates GC steering. Additionally, we present data assigning to NAV1 an important role in the radial migration of cortical projection neurons in vivo.

Expression pattern of Nav2 in the murine CNS with development

Neuron navigator 2 (NAV2, RAINB1, POMFIL2, HELAD1, unc53H2) is essential for nervous system development. In the present study the spatial distribution of Nav2 transcript in mouse CNS during embryonic, postnatal and adult life is examined. Because multiple NAV2 proteins are predicted based on alternate promoter usage and RNA splicing, in situ hybridization was performed using probes designed to the 5' and 3' ends of the Nav2 transcript, and PCR products using primer sets spanning the length of the mRNA were also examined by real time PCR (qPCR). These studies support full-length Nav2 transcript as the predominant form in the wild-type mouse CNS. The developing cortex, hippocampus, thalamus, olfactory bulb, and granule cells (GC) within the cerebellum show the highest expression, with a similar staining pattern using either the 5'Nav2 or 3'Nav2 probe. Nav2 is expressed in GC precursors migrating over the cerebellar primordium as well as in the postmitotic premigratory cells of the external granule cell layer (EGL). It is expressed in the cornu ammonis (CA) and dentate gyrus (DG) throughout hippocampal development. In situ hybridization was combined with immunohistochemistry for Ki67, CTIP2 and Nissl staining to follow Nav2 transcript location during cortical development, where it is observed in neuroepithelial cells exiting the germinal compartments, as well as later in the cortical plate (CP) and developing cortical layers. The highest levels of Nav2 in all brain regions studied are observed in late gestation and early postnatal life which coincides with times when neurons are migrating and differentiating. A hypomorphic mouse that lacks the full-length transcript but expresses shorter transcript shows little staining in the CNS with either probe set except at the base of the cerebellum, where a shorter Nav2 transcript is detected. Using dual fluorescent probe in situ hybridization studies, these cells are identified as oligodendrocytes and are detected using both Olig1 and the 3'Nav2 probe. The identification of full-length Nav2 as the primary transcript in numerous brain regions suggests NAV2 could play a role in CNS development beyond that of its well-established role in the cerebellum.

The Zn2+ and Ca2+ -binding S100B and S100A1 proteins: beyond the myths

The S100 genes encode a conserved group of 21 vertebrate-specific EF-hand calcium-binding proteins. Since their discovery in 1965, S100 proteins have remained enigmatic in terms of their cellular functions. In this review, we summarize the calcium- and zinc-binding properties of the dimeric S100B and S100A1 proteins and highlight data that shed new light on the extracellular and intracellular regulation and functions of S100B. We point out that S100B and S100A1 homodimers are not functionally interchangeable and that in a S100A1/S100B heterodimer, S100A1 acts as a negative regulator for the ability of S100B to bind Zn²⁺ . The Ca²⁺ and Zn²⁺ -dependent interactions of S100B with a wide array of proteins form the basis of its activities and have led to the derivation of some initial rules for S100B recognition of protein targets. However, recent findings have strongly suggested that these rules need to be revisited. Here, we describe a new consensus S100B binding motif present in intracellular and extracellular vertebrate-specific proteins and propose a new model for stable interactions of S100B dimers with full-length target proteins. A chaperone-associated function for intracellular S100B in adaptive cellular stress responses is also discussed. This review may help guide future studies on the functions of S100 proteins in general.

Urban colonization through multiple genetic lenses: The city-fox phenomenon revisited

Urbanization is driving environmental change on a global scale, creating novel environments for wildlife to colonize. Through a combination of stochastic and selective processes, urbanization is also driving evolutionary change. For instance, difficulty in traversing human-modified landscapes may isolate newly established populations from rural sources, while novel selective pressures, such as altered disease risk, toxicant exposure, and light pollution, may further diverge populations through local adaptation. Assessing the evolutionary consequences of urban colonization and the processes underlying them is a principle aim of urban evolutionary ecology. In the present study, we revisited the genetic effects of urbanization on red foxes (Vulpes vulpes) that colonized Zurich, Switzerland. Through use of genome-wide single nucleotide polymorphisms and microsatellite markers linked to the major histocompatibility complex (MHC), we expanded upon a previous neutral microsatellite study to assess population structure, characterize patterns of genetic diversity, and detect outliers associated with urbanization. Our results indicated the presence of one large evolutionary cluster, with substructure evident between geographic sampling areas. In urban foxes, we observed patterns of neutral and functional diversity consistent with founder events and reported increased differentiation between populations separated by natural and anthropogenic barriers. We additionally reported evidence of selection acting on MHC-linked markers and identified outlier loci with putative gene functions related to energy metabolism, behavior, and immunity. We concluded that demographic processes primarily drove patterns of diversity, with outlier tests providing preliminary evidence of possible urban adaptation. This study contributes to our overall understanding of urban colonization ecology and emphasizes the value of combining datasets when examining evolutionary change in an increasingly urban world.

Family-based association analysis of NAV2 gene with the risk and age at onset of Alzheimer's disease

The neuron navigator 2 (NAV2) gene is highly expressed in brain and involved in the nervous system development and may play a role in Alzheimer's disease (AD). We aimed to investigate the associations of 317 single-nucleotide polymorphisms (SNPs) in the NAV2 gene with the risk and age at onset (AAO) of AD using a family-based sample (1266 AD cases and 1279 healthy relatives). Association with the risk of AD was assessed using family-based association test -generalized estimating equations (FBAT- GEE) statistics while the association with AAO as a quantitative trait was evaluated using the FBAT-Wilcoxon statistic. Single marker analysis showed that 20 SNPs were significantly associated with the risk of AD (top SNP rs7112354 with p=8.46×10-4) and 11 SNPs were associated with AAO (top SNP rs1354269 with p=2.87×10-3). Interestingly, two SNPs rs17614100 and rs12364788 were associated with both the risk (p=1.7×10-2 and 2.71×10-2; respectively) and AAO (p=1.85×10-3 and 6.06×10-3; respectively). Haplotype analyses further supported the results of single marker analyses. In addition, functional analysis showed that NAV2 mRNA had significant expression across ten human brain regions examined and significantly correlated with APOE expression in four of ten regions. The present study is the first study providing evidence of several genetic variants within the NAV2 gene influencing the risk and AAO of AD.

Microtubule plus-end tracking proteins in neuronal development

Regulation of the microtubule cytoskeleton is of pivotal importance for neuronal development and function. One such regulatory mechanism centers on microtubule plus-end tracking proteins (+TIPs): structurally and functionally diverse regulatory factors, which can form complex macromolecular assemblies at the growing microtubule plus-ends. +TIPs modulate important properties of microtubules including their dynamics and their ability to control cell polarity, membrane transport and signaling. Several neurodevelopmental and neurodegenerative diseases are associated with mutations in +TIPs or with misregulation of these proteins. In this review, we focus on the role and regulation of +TIPs in neuronal development and associated disorders.

14-3-3ε and NAV2 interact to regulate neurite outgrowth and axon elongation

Neuron navigator 2 (NAV2) is required for all-trans retinoic acid (atRA) to induce neurite outgrowth in human neuroblastoma cells. Further, ectopic overexpression of full-length human NAV2 rescues an axonal elongation defect in the Caenorhabditis elegans unc-53 (NAV2 ortholog) mutant. Using a region of NAV2 that independently associates with the cytoskeleton as bait in a yeast-two-hybrid screen, 14-3-3ε was identified as a novel NAV2 interacting partner. Amino acids 761-960 of NAV2 are sufficient to confer a positive interaction with 14-3-3ε as evidenced by a two-hybrid screen and co-immunoprecipitation assay. Knockdown of 14-3-3ε leads to a decrease in atRA-mediated neurite outgrowth, similar to the elongation defects observed when NAV2 is depleted or mutated. Likewise, posterior lateral microtubule (PLM) defects in C. elegans fed unc-53 RNAi are similar to those fed ftt-2 (14-3-3 homolog) RNAi. The discovery of an interaction between NAV2 and 14-3-3ε could provide insight into the mechanism by which NAV2 participates in promoting cell migration and neuronal elongation.

Distinct cell guidance pathways controlled by the Rac and Rho GEF domains of UNC-73/TRIO in Caenorhabditis elegans

The cytoskeleton regulator UNC-53/NAV2 is required for both the anterior and posterior outgrowth of several neurons as well as that of the excretory cell while the kinesin-like motor VAB-8 is essential for most posteriorly directed migrations in Caenorhabditis elegans. Null mutations in either unc-53 or vab-8 result in reduced posterior excretory canal outgrowth, while double null mutants display an enhanced canal extension defect, suggesting the genes act in separate pathways to control this posteriorly directed outgrowth. Genetic analysis of putative interactors of UNC-53 or VAB-8, and cell-specific rescue experiments suggest that VAB-8, SAX-3/ROBO, SLT-1/Slit, and EVA-1 are functioning together in the outgrowth of the excretory canals, while UNC-53 appears to function in a parallel pathway with UNC-71/ADAM. The known VAB-8 interactor, the Rac/Rho GEF UNC-73/TRIO operates in both pathways, as isoform specific alleles exhibit enhancement of the phenotype in double-mutant combination with either unc-53 or vab-8. On the basis of these results, we propose a bipartite model for UNC-73/TRIO activity in excretory canal extension: a cell autonomous function that is mediated by the Rho-specific GEF domain of the UNC-73E isoform in conjunction with UNC-53 and UNC-71 and a cell nonautonomous function that is mediated by the Rac-specific GEF domain of the UNC-73B isoform, through partnering with VAB-8 and the receptors SAX-3 and EVA-1.

Nav2 hypomorphic mutant mice are ataxic and exhibit abnormalities in cerebellar development

Development of the cerebellum involves a coordinated program of neuronal process outgrowth and migration resulting in a foliated structure that plays a key role in motor function. Neuron navigator 2 (Nav2) is a cytoskeletal-interacting protein that functions in neurite outgrowth and axonal elongation. Herein we show that hypomorphic mutant mice lacking the full-length Nav2 transcript exhibit ataxia and defects in cerebellar development. At embryonic day (E)17.5, the mutant cerebellum is reduced in size and exhibits defects in vermal foliation. Reduction in cell proliferation at early times (E12.5 and E14.5) may contribute to this size reduction. The full-length Nav2 transcript is expressed in the premigratory zone of the external granule layer (EGL). Granule cells in the germinal zone of the EGL appear to proliferate normally, however, due to the reduction in cerebellar circumference there are fewer total BrdU-labeled granule cells in the mutants, and these fail to migrate normally toward the interior of the cerebellum. In Nav2 hypomorphs, fewer granule cells migrate out of cerebellar EGL explants and neurite outgrowth from both explants and isolated external granule cell cultures is reduced. This suggests that the formation of parallel axon fibers and neuronal migration is disrupted in Nav2 mutants. This work supports an essential role for full-length Nav2 in cerebellar development, including axonal elongation and migration of the EGL neurons.

Neuron navigator 3a regulates liver organogenesis during zebrafish embryogenesis

Endodermal organogenesis requires a precise orchestration of cell fate specification and cell movements, collectively coordinating organ size and shape. In Caenorhabditis elegans, uncoordinated-53 (unc-53) encodes a neural guidance molecule that directs axonal growth. One of the vertebrate homologs of unc-53 is neuron navigator 3 (Nav3). Here, we identified a novel vertebrate neuron navigator 3 isoform in zebrafish, nav3a, and we provide genetic evidence in loss- and gain-of-function experiments showing its functional role in endodermal organogenesis during zebrafish embryogenesis. In zebrafish embryos, nav3a expression was initiated at 22 hpf in the gut endoderm and at 40 hpf expanded to the newly formed liver bud. Endodermal nav3a expression was controlled by Wnt2bb signaling and was independent of FGF and BMP signaling. Morpholino-mediated knockdown of nav3a resulted in a significantly reduced liver size, and impaired development of pancreas and swim bladder. In vivo time-lapse imaging of liver development in nav3a morphants revealed a failure of hepatoblast movement out from the gut endoderm during the liver budding stage, with hepatoblasts being retained in the intestinal endoderm. In hepatocytes in vitro, nav3a acts as a positive modulator of actin assembly in lamellipodia and filipodia extensions, allowing cellular movement. Knockdown of nav3a in vitro impeded hepatocyte movement. Endodermal-specific overexpression of nav3a in vivo resulted in additional ectopic endodermal budding beyond the normal liver and pancreatic budding sites. We conclude that nav3a is required for directing endodermal organogenesis involving coordination of endodermal cell behavior.

Nav2 is necessary for cranial nerve development and blood pressure regulation

BACKGROUND

All-trans retinoic acid (atRA) is required for nervous system development, including the developing hindbrain region. Neuron navigator 2 (Nav2) was first identified as an atRA-responsive gene in human neuroblastoma cells (retinoic acid-induced in neuroblastoma 1, Rainb1), and is required for atRA-mediated neurite outgrowth. In this paper, we explore the importance of Nav2 in nervous system development and function in vivo.

RESULTS

Nav2 hypomorphic homozygous mutants show decreased survival starting at birth. Nav2 mutant embryos show an overall reduction in nerve fiber density, as well as specific defects in cranial nerves IX (glossopharyngeal) and X (vagus). Nav2 hypomorphic mutant adult mice also display a blunted baroreceptor response compared to wild-type controls.

CONCLUSIONS

Nav2 functions in mammalian nervous system development, and is required for normal cranial nerve development and blood pressure regulation in the adult.

Navigating the cell: UNC-53 and the navigators, a family of cytoskeletal regulators with multiple roles in cell migration, outgrowth and trafficking

Changes in cell shape are associated with a variety of processes including cell migration, axon outgrowth, cell division and vesicle trafficking. C. elegans UNC-53 and its vertebrate homologs, the Navigators, are required for the migration of cells and the outgrowth of neuronal processes. The identification of novel molecular interactions and live imaging studies have revealed that UNC-53/NAVs are signal transducers associated with actin filaments, microtubules and intermediate filaments. In addition to modulating cytoskeletal dynamics at the leading edge of migrating or outgrowing cells, both UNC-53 and the navigators are expressed in adult cells, conspicuously those with specialized roles in endocytosis or secretion. Collectively, these results suggest that UNC-53/NAVs may be a central regulator of cytoskeletal dynamics, responsible for integrating signaling cues to multiple components of the cytoskeleton to coordinate rearrangement during cell outgrowth or trafficking.

The cell migration molecule UNC-53/NAV2 is linked to the ARP2/3 complex by ABI-1

The shape changes that are required to position a cell to migrate or grow out in a particular direction involve a coordinated reorganization of the actin cytoskeleton. Although it is known that the ARP2/3 complex nucleates actin filament assembly, exactly how the information from guidance cues is integrated to elicit ARP2/3-mediated remodeling during outgrowth remains vague. Previous studies have shown that C. elegans UNC-53 and its vertebrate homolog NAV (Neuronal Navigators) are required for the migration of cells and neuronal processes. We have identified ABI-1 as a novel molecular partner of UNC-53/NAV2 and have found that a restricted calponin homology (CH) domain of UNC-53 is sufficient to bind ABI-1. ABI-1 and UNC-53 have an overlapping expression pattern, and display similar cell migration phenotypes in the excretory cell, and in mechanosensory and motoneurons. Migration defects were also observed after RNAi of proteins known to function with abi-1 in actin dynamics, including nck-1, wve-1 and arx-2. We propose that UNC-53/NAV2, through its CH domain, acts as a scaffold that links ABI-1 to the ARP2/3 complex to regulate actin cytoskeleton remodeling.

The atRA-responsive gene neuron navigator 2 functions in neurite outgrowth and axonal elongation

Neuron navigator 2 (Nav2) was first identified as an all-trans retinoic acid (atRA)-responsive gene in human neuroblastoma cells (retinoic acid-induced in neuroblastoma 1, RAINB1) that extend neurites after exposure to atRA. It is structurally related to the Caenorhabditis elegans unc-53 gene that is required for cell migration and axonal outgrowth. To gain insight into NAV2 function, the full-length human protein was expressed in C. elegans unc-53 mutants under the control of a mechanosensory neuron promoter. Transgene expression of NAV2 rescued the defects in unc-53 mutant mechanosensory neuron elongation, indicating that Nav2 is an ortholog of unc-53. Using a loss-of-function approach, we also show that Nav2 induction is essential for atRA to induce neurite outgrowth in SH-SY5Y cells. The NAV2 protein is located both in the cell body and along the length of the growing neurites of SH-SY5Y cells in a pattern that closely mimics that of neurofilament and microtubule proteins. Transfection of Nav2 deletion constructs in Cos-1 cells reveals a region of the protein (aa 837-1065) that directs localization with the microtubule cytoskeleton. Collectively, this work supports a role for NAV2 in neurite outgrowth and axonal elongation and suggests this protein may act by facilitating interactions between microtubules and other proteins such as neurofilaments that are key players in the formation and stability of growing neurites.

Microtubule plus-end tracking proteins in differentiated mammalian cells

Differentiated mammalian cells are often characterized by highly specialized and polarized structure. Its formation and maintenance depends on cytoskeletal components, among which microtubules play an important role. The shape and dynamic properties of microtubule networks are controlled by multiple microtubule-associated factors. These include molecular motors and non-motor proteins, some of which accumulate specifically at the growing microtubule plus-ends (the so-called microtubule plus-end tracking proteins). Plus-end tracking proteins can contribute to the regulation of microtubule dynamics, mediate the cross-talk between microtubule ends, the actin cytoskeleton and the cell cortex, and participate in transport and positioning of structural and regulatory factors and membrane organelles. Malfunction of these proteins results in various human diseases including some forms of cancer, neurodevelopmental disorders and mental retardation. In this article we discuss recent data on microtubule dynamics and activities of microtubule plus-end binding proteins important for the physiology and pathology of differentiated mammalian cells such as neurons, polarized epithelia, muscle and sperm cells.

Primary Cutaneous T-Cell Lymphomas Show a Deletion or Translocation AffectingNAV3, the HumanUNC-53Homologue

Multicolor fluorescent in situ hybridization (FISH) was used to identify acquired chromosomal aberrations in 12 patients with mycosis fungoides or Sézary syndrome, the most common forms of primary cutaneous T-cell lymphoma (CTCL). The most frequently affected chromosome was 12, which showed clonal deletions or translocations with a break point in 12q21 or 12q22 in five of seven consecutive Sézary syndrome patients and a clonal monosomy in the sixth patient. The break point of a balanced translocation t(12;18)(q21;q21.2), mapped in the minimal common region of two deletions, fine mapped to 12q2. By locus-specific FISH, the translocation disrupted one gene, NAV3 (POMFIL1), a human homologue of unc-53 in Caenorhabditis elegans. A missense mutation in the remaining NAV3 allele was found in one of six cases with a deletion or translocation. With locus-specific FISH, NAV3 deletions were found in the skin lesions of four of eight (50%) patients with early mycosis fungoides (stages IA-IIA) and in the skin or lymph node of 11 of 13 (85%) patients with advanced mycosis fungoides or Sézary syndrome. Preliminary functional studies with lentiviral small interfering RNA-based NAV3 silencing in Jurkat cells and in primary lymphocytes showed enhanced interleukin 2 expression (but not CD25 expression). Thus, NAV3 may contribute to the growth, differentiation, and apoptosis of CTCL cells as well as to the skewing from Th1-type to Th2-type phenotype during disease progression. NAV3, a novel putative haploinsufficient tumor suppressor gene, is disrupted in most cases of the commonest types of CTCL and may thus provide a new diagnostic tool.

Mouse neuron navigator 1, a novel microtubule-associated protein involved in neuronal migration

The development of the nervous system (NS) requires the coordinated migration of multiple waves of neurons and subsequent processes of neurite maturation, both involving selective guidance mechanisms. In Caenorhabditis elegans, unc-53 codes for a new multidomain protein involved in the directional migration of a subset of cells. We describe here the first functional characterization of the mouse homologue, mouse Neuron navigator 1 (mNAV1), whose expression is largely restricted to the NS during development. EGFP-mNAV1 associates with microtubules (MTs) plus ends present in the growth cone through a new microtubule-binding (MTB) domain. Moreover, its overexpression in transfected cells leads to MT bundling. The abolition of mNAV1 causes loss of directionality in the leading processes of pontine-migrating cells, providing evidence for a role of mNAV1 in mediating Netrin-1-induced directional migration.

LL5beta: a regulator of postsynaptic differentiation identified in a screen for synaptically enriched transcripts at the neuromuscular junction

In both neurons and muscle fibers, specific mRNAs are concentrated beneath and locally translated at synaptic sites. At the skeletal neuromuscular junction, all synaptic RNAs identified to date encode synaptic components. Using microarrays, we compared RNAs in synapse-rich and -free regions of muscles, thereby identifying transcripts that are enriched near synapses and that encode soluble membrane and nuclear proteins. One gene product, LL5β, binds to both phosphoinositides and a cytoskeletal protein, filamin, one form of which is concentrated at synaptic sites. LL5β is itself associated with the cytoplasmic face of the postsynaptic membrane; its highest levels border regions of highest acetylcholine receptor (AChR) density, which suggests a role in “corraling” AChRs. Consistent with this idea, perturbing LL5β expression in myotubes inhibits AChR aggregation. Thus, a strategy designed to identify novel synaptic components led to identification of a protein required for assembly of the postsynaptic apparatus.