NXF2 is involved in cytoplasmic mRNA dynamics through interactions with motor proteins

The kinesin motor Kif9 regulates centriolar satellite positioning and mitotic progression

Centrosomes are the principal microtubule-organizing centers of the cell and play an essential role in mitotic spindle function. Centrosome biogenesis is achieved by strict control of protein acquisition and phosphorylation prior to mitosis. Defects in this process promote fragmentation of pericentriolar material culminating in multipolar spindles and chromosome missegregation. Centriolar satellites, membrane-less aggrupations of proteins involved in the trafficking of proteins toward and away from the centrosome, are thought to contribute to centrosome biogenesis. Here we show that the microtubule plus-end directed kinesin motor Kif9 localizes to centriolar satellites and regulates their pericentrosomal localization during interphase. Lack of Kif9 leads to aggregation of satellites closer to the centrosome and increased centrosomal protein degradation that disrupts centrosome maturation and results in chromosome congression and segregation defects during mitosis. Our data reveal roles for Kif9 and centriolar satellites in the regulation of cellular proteostasis and mitosis.

FMRP Long-Range Transport and Degradation Are Mediated by Dynlrb1 in Sensory Neurons

The fragile X messenger ribonucleoprotein 1 (FMRP) is a multifunctional RNA-binding protein implicated in human neurodevelopmental and neurodegenerative disorders. FMRP mediates the localization and activity-dependent translation of its associated mRNAs through the formation of phase-separated condensates that are trafficked by microtubule-based motors in axons. Axonal transport and localized mRNA translation are critical processes for long-term neuronal survival and are closely linked to the pathogenesis of neurological diseases. FMRP dynein-mediated axonal trafficking is still largely unexplored but likely to constitute a key process underlying FMRP spatiotemporal translational regulation. Here, we show that dynein light chain roadblock 1 (Dynlrb1), a subunit of the dynein complex, is a critical regulator of FMRP function. In sensory axons, FMRP associates with endolysosomal organelles, likely through annexin A11, and is retrogradely trafficked by the dynein complex in a Dynlrb1-dependent manner. Moreover, Dynlrb1 silencing induced FMRP granule accumulation and repressed the translation of microtubule-associated protein 1b, one of its primary mRNA targets. Our findings suggest that Dynlrb1 regulates FMRP function through the control of its transport and targeted degradation.

Understanding how kinesin motor proteins regulate postsynaptic function in neuron

The Kinesin superfamily proteins (KIFs) are major molecular motors that transport diverse set of cargoes along microtubules to both the axon and dendrite of a neuron. Much of our knowledge about kinesin function is obtained from studies on axonal transport. Emerging evidence reveals how specific kinesin motor proteins carry cargoes to dendrites, including proteins, mRNAs and organelles that are crucial for synapse development and plasticity. In this review, we will summarize the major kinesin motors and their associated cargoes that have been characterized to regulate postsynaptic function in neuron. We will also discuss how specific kinesins are selectively involved in the development of excitatory and inhibitory postsynaptic compartments, their regulation by post-translational modifications (PTM), as well as their roles beyond conventional transport carrier.

Mammalian Neuronal mRNA Transport Complexes: The Few Knowns and the Many Unknowns

Hundreds of messenger RNAs (mRNAs) are transported into neurites to provide templates for the assembly of local protein networks. These networks enable a neuron to configure different cellular domains for specialized functions. According to current evidence, mRNAs are mostly transported in rather small packages of one to three copies, rarely containing different transcripts. This opens up fascinating logistic problems: how are hundreds of different mRNA cargoes sorted into distinct packages and how are they coupled to and released from motor proteins to produce the observed mRNA distributions? Are all mRNAs transported by the same transport machinery, or are there different adaptors or motors for different transcripts or classes of mRNAs? A variety of often indirect evidence exists for the involvement of proteins in mRNA localization, but relatively little is known about the essential activities required for the actual transport process. Here, we summarize the different types of available evidence for interactions that connect mammalian mRNAs to motor proteins to highlight at which point further research is needed to uncover critical missing links. We further argue that a combination of discovery approaches reporting direct interactions, in vitro reconstitution, and fast perturbations in cells is an ideal future strategy to unravel essential interactions and specific functions of proteins in mRNA transport processes.

Spatial control of membrane traffic in neuronal dendrites

Neuronal dendrites are highly branched and specialized compartments with distinct structures and secretory organelles (e.g., spines, Golgi outposts), and a unique cytoskeletal organization that includes microtubules of mixed polarity. Dendritic membranes are enriched with proteins, which specialize in the formation and function of the post-synaptic membrane of the neuronal synapse. How these proteins partition preferentially in dendrites, and how they traffic in a manner that is spatiotemporally accurate and regulated by synaptic activity are long-standing questions of neuronal cell biology. Recent studies have shed new insights into the spatial control of dendritic membrane traffic, revealing new classes of proteins (e.g., septins) and cytoskeleton-based mechanisms with dendrite-specific functions. Here, we review these advances by revisiting the fundamental mechanisms that control membrane traffic at the levels of protein sorting and motor-driven transport on microtubules and actin filaments. Overall, dendrites possess unique mechanisms for the spatial control of membrane traffic, which might have specialized and co-evolved with their highly arborized morphology.

Rab23/Kif17 regulate oocyte meiotic progression by modulating tubulin acetylation and actin dynamics

Cytoskeletal dynamics are involved in multiple cellular processes during oocyte meiosis, including spindle organization, actin-based spindle migration, and polar body extrusion. Here, we report that the vesicle trafficking protein Rab23, a GTPase, drives the motor protein Kif17 and that this is important for spindle organization and actin dynamics during mouse oocyte meiosis. GTP-bound Rab23 accumulated at the spindle and promoted migration of Kif17 to the spindle poles. Depletion of Rab23 or Kif17 caused polar body extrusion failure. Further analysis showed that depletion of Rab23/Kif17 perturbed spindle formation and chromosome alignment, possibly by affecting tubulin acetylation. Kif17 regulated tubulin acetylation by associating with αTAT and Sirt2, and depletion of Kif17 altered expression of these proteins. Moreover, depletion of Kif17 decreased the level of cytoplasmic actin, which abrogated spindle migration to the cortex. The tail domain of Kif17 associated with constituents of the RhoA-ROCK-LIMK-cofilin pathway to modulate assembly of actin filaments. Taken together, our results demonstrate that the Rab23-Kif17-cargo complex regulates tubulin acetylation for spindle organization and drives actin-mediated spindle migration during meiosis.

Interactions between the Translation Machinery and Microtubules

Microtubules are components of eukaryotic cytoskeleton that are involved in the transport of various components from the nucleus to the cell periphery and back. They also act as a platform for assembly of complex molecular ensembles. Ribonucleoprotein (RNP) complexes, such as ribosomes and mRNPs, are transported over significant distances (e.g. to neuronal processes) along microtubules. The association of RNPs with microtubules and their transport along these structures are essential for compartmentalization of protein biosynthesis in cells. Microtubules greatly facilitate assembly of stress RNP granules formed by accumulation of translation machinery components during cell stress response. Microtubules are necessary for the cytoplasm-to-nucleus transport of proteins, including ribosomal proteins. At the same time, ribosomal proteins and RNA-binding proteins can influence cell mobility and cytoplasm organization by regulating microtubule dynamics. The molecular mechanisms underlying the association between the translation machinery components and microtubules have not been studied systematically; the results of such studies are mostly fragmentary. In this review, we attempt to fill this gap by summarizing and discussing the data on protein and RNA components of the translation machinery that directly interact with microtubules or microtubule motor proteins.

Intron retention in viruses and cellular genes: Detention, border controls and passports

Intron retention (IR), where one or more introns remain in the RNA after splicing, was long thought to be rare in mammalian cells, albeit common in plants and some viruses. Largely due to the development of better methods for RNA analysis, it has now been recognized that IR is much more common than previously thought and that this mechanism is likely to play an important role in mammalian gene regulation. To date, most publications and reviews about IR have described the resulting mRNAs as "dead end" products, with no direct consequence for the proteome. However, there are also many reports of mRNAs with retained introns giving rise to alternative protein isoforms. Although this was originally revealed in viral systems, there are now numerous examples of bona fide cellular proteins that are translated from mRNAs with retained introns. These new isoforms have sometimes been shown to have important regulatory functions. In this review, we highlight recent developments in this area and the research on viruses that led the way to the realization of the many ways in which mRNAs with retained introns can be regulated. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing RNA Export and Localization > Nuclear Export/Import RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

RNA-binding proteins of the NXF (nuclear export factor) family and their connection with the cytoskeleton

The mutual relationship between mRNA and the cytoskeleton can be seen from two points of view. On the one hand, the cytoskeleton is necessary for mRNA trafficking and anchoring to subcellular domains. On the other hand, cytoskeletal growth and rearrangement require the translation of mRNAs that are connected to the cytoskeleton. β-actin mRNA localization may influence dynamic changes in the actin cytoskeleton. In the cytoplasm, long-lived mRNAs exist in the form of RNP (ribonucleoprotein) complexes, where they interact with RNA-binding proteins, including NXF (Nuclear eXport Factor). Dm NXF1 is an evolutionarily conserved protein in Drosophila melanogaster that has orthologs in different animals. The universal function of nxf1 genes is the nuclear export of different mRNAs in various organisms. In this mini-review, we briefly discuss the evidence demonstrating that Dm NXF1 fulfils not only universal but also specialized cytoplasmic functions. This protein is detected not only in the nucleus but also in the cytoplasm. It is a component of neuronal granules. Dm NXF1 marks nuclear division spindles during early embryogenesis and the dense body on one side of the elongated spermatid nuclei. The characteristic features of sbr mutants (sbr¹⁰ and sbr⁵ ) are impairment of chromosome segregation and spindle formation anomalies during female meiosis. sbr¹² mutant sterile males with immobile spermatozoa exhibit disturbances in the axoneme, mitochondrial derivatives and cytokinesis. These data allow us to propose that the Dm NXF1 proteins transport certain mRNAs in neurites and interact with localized mRNAs that are necessary for dynamic changes of the cytoskeleton.

Functional significance of the sex chromosomes during spermatogenesis

Mammalian sex chromosomes arose from an ordinary pair of autosomes. Over hundreds of millions of years, they have evolved into highly divergent X and Y chromosomes and have become increasingly specialized for male reproduction. Both sex chromosomes have acquired and amplified testis-specific genes, suggestive of roles in spermatogenesis. To understand how the sex chromosome genes participate in the regulation of spermatogenesis, we review genes, including single-copy, multi-copy, and ampliconic genes, whose spermatogenic functions have been demonstrated in mouse genetic studies. Sex chromosomes are subject to chromosome-wide transcriptional silencing in meiotic and postmeiotic stages of spermatogenesis. We also discuss particular sex-linked genes that escape postmeiotic silencing and their evolutionary implications. The unique gene contents and genomic structures of the sex chromosomes reflect their strategies to express genes at various stages of spermatogenesis and reveal the driving forces that shape their evolution.Free Chinese abstract: A Chinese translation of this abstract is freely available at http://www.reproduction-online.org/content/149/6/R265/suppl/DC1.Free Japanese abstract: A Japanese translation of this abstract is freely available at http://www.reproduction-online.org/content/149/6/R265/suppl/DC2.

Septin 9 interacts with kinesin KIF17 and interferes with the mechanism of NMDA receptor cargo binding and transport

Kinesin motor interactions with cargo and their regulation are not well understood. Septin 9 (SEPT9) interacts directly with kinesin KIF17 and interferes with binding of the cargo adaptor/scaffold mLin-10/Mint1, which links the NMDA receptor subunit 2B (NR2B) to KIF17. SEPT9 down-regulates NR2B transport without affecting the motile properties of KIF17.

Intracellular transport involves the regulation of microtubule motor interactions with cargo, but the underlying mechanisms are not well understood. Septins are membrane- and microtubule-binding proteins that assemble into filamentous, scaffold-like structures. Septins are implicated in microtubule-dependent transport, but their roles are unknown. Here we describe a novel interaction between KIF17, a kinesin 2 family motor, and septin 9 (SEPT9). We show that SEPT9 associates directly with the C-terminal tail of KIF17 and interacts preferentially with the extended cargo-binding conformation of KIF17. In developing rat hippocampal neurons, SEPT9 partially colocalizes and comigrates with KIF17. We show that SEPT9 interacts with the KIF17 tail domain that associates with mLin-10/Mint1, a cargo adaptor/scaffold protein, which underlies the mechanism of KIF17 binding to the NMDA receptor subunit 2B (NR2B). Significantly, SEPT9 interferes with binding of the PDZ1 domain of mLin-10/Mint1 to KIF17 and thereby down-regulates NR2B transport into the dendrites of hippocampal neurons. Measurements of KIF17 motility in live neurons show that SEPT9 does not affect the microtubule-dependent motility of KIF17. These results provide the first evidence of an interaction between septins and a nonmitotic kinesin and suggest that SEPT9 modulates the interactions of KIF17 with membrane cargo.

KIF17 regulates RhoA-dependent actin remodeling at epithelial cell-cell adhesions

The kinesin KIF17 localizes at microtubule plus-ends where it contributes to regulation of microtubule stabilization and epithelial polarization. We now show that KIF17 localizes at cell-cell adhesions and that KIF17 depletion inhibits accumulation of actin at the apical pole of cells grown in 3D organotypic cultures and alters the distribution of actin and E-cadherin in cells cultured in 2D on solid supports. Overexpression of full-length KIF17 constructs or truncation mutants containing the N-terminal motor domain resulted in accumulation of newly incorporated GFP-actin into junctional actin foci, cleared E-cadherin from cytoplasmic vesicles and stabilized cell-cell adhesions to challenge with calcium depletion. Expression of these KIF17 constructs also increased cellular levels of active RhoA, whereas active RhoA was diminished in KIF17-depleted cells. Inhibition of RhoA or its effector ROCK, or expression of LIMK1 kinase-dead or activated cofilin(S3A) inhibited KIF17-induced junctional actin accumulation. Interestingly, KIF17 activity toward actin depends on the motor domain but is independent of microtubule binding. Together, these data show that KIF17 can modify RhoA-GTPase signaling to influence junctional actin and the stability of the apical junctional complex of epithelial cells.

Nxf7 deficiency impairs social exploration and spatio-cognitive abilities as well as hippocampal synaptic plasticity in mice

Nuclear RNA export factors (NXF) are conserved in all metazoans and are deemed essential for shuttling RNA across the nuclear envelope and other post-transcriptional processes (such as mRNA metabolism, storage and stability). Disruption of human NXF5 has been implicated in intellectual and psychosocial disabilities. In the present report, we use recently described Nxf7 knockout (KO) mice as an experimental model to analyze in detail the behavioral consequences of clinical NXF5 deficiency. We examined male Nxf7 KO mice using an extended cognitive and behavioral test battery, and recorded extracellular field potentials in the hippocampal CA1 region. We observed various cognitive and behavioral changes including alterations in social exploration, impaired spatial learning and spatio-cognitive abilities. We also defined a new experimental paradigm to discriminate search strategies in Morris water maze and showed significant differences between Nxf7 KO and control animals. Furthermore, while we observed no difference in a nose poke suppression in an conditioned emotional response (CER) protocol, Nxf7 KO mice were impaired in discriminating between differentially reinforced cues in an auditory fear conditioning protocol. This distinct neurocognitive phenotype was accompanied by impaired hippocampal Long-term potentiation (LTP), while long-term depression (LTD) was not affected by Nxf7 deficiency. Our data demonstrate that disruption of murine Nxf7 leads to behavioral phenotypes that may relate to the intellectual and social deficits in patients with NXF5 deficiency.

Evolutionarily diverged regulation of X-chromosomal genes as a primal event in mouse reproductive isolation

Improper gene regulation is implicated in reproductive isolation, but its genetic and molecular bases are unknown. We previously reported that a mouse inter-subspecific X chromosome substitution strain shows reproductive isolation characterized by male-specific sterility due to disruption of meiotic entry in spermatogenesis. Here, we conducted comprehensive transcriptional profiling of the testicular cells of this strain by microarray. The results clearly revealed gross misregulation of gene expression in the substituted donor X chromosome. Such misregulation occurred prior to detectable spermatogenetic impairment, suggesting that it is a primal event in reproductive isolation. The misregulation of X-linked genes showed asymmetry; more genes were disproportionally downregulated rather than upregulated. Furthermore, this misregulation subsequently resulted in perturbation of global transcriptional regulation of autosomal genes, probably by cascading deleterious effects. Remarkably, this transcriptional misregulation was substantially restored by introduction of chromosome 1 from the same donor strain as the X chromosome. This finding implies that one of regulatory genes acting in trans for X-linked target genes is located on chromosome 1. This study collectively suggests that regulatory incompatibility is a major cause of reproductive isolation in the X chromosome substitution strain.

Reproductive isolation characterized by male sterility and decreased viability is important for speciation, because it suppresses free genetic exchange between two diverged populations and accelerates the genetic divergence. One of the reproductive isolation phenomena, hybrid sterility (sterility in hybrid animals), is possibly caused by deleterious interactions between diverged genetic factors brought by two distinct populations. The polymorphism not only in protein-coding sequences but also in transcriptional regulatory sequences can cause the genetic incompatibility in hybrid animals. However, the precise genetic mechanisms of hybrid sterility are mostly unknown. Here, we report that the expression of X-linked genes derived from one mouse subspecies was largely misregulated in the genetic background of another subspecies. The misregulated expression of the X-linked genes subsequently affected the global expression of autosomal genes. The results collectively indicate that hybrid sterility between the two mouse subspecies is caused by misregulation of gene expression due to genetic incompatibility in the transcriptional regulatory circuitry. Such genetic incompatibility in transcriptional regulation likely underlies reproductive isolation in general.

Nuclear export factor 3 is involved in regulating the expression of TGF-β3 in an mRNA export activity-independent manner in mouse Sertoli cells

The NXF (nuclear export factor) family members are implicated in the transport of mRNA from the nucleus to the cytoplasm. Recently, some members of the NXF family have been reported to play divergent functional roles, such as post-transcriptional regulation, translational control, regulation of mRNA stability and trafficking. However, little is known about the roles of NXF3 in spermatogenesis. In the present study, we found that mouse NXF3, specifically expressed in principal cells in segment II of the caput epididymis, as well as Sertoli cells in the mouse testis, was required to mediate TGF-β (transforming growth factor β)-induced down-regulation of Tgfb3/TGF-β3 mRNA expression and protein secretion in Sertoli cells. In addition, NXF3 was also involved in TGF-β-induced transcriptional regulation of other genes associated with Sertoli cell maturation and the restructuring of the Sertoli cell BTB (blood-testis barrier), such as Gata1 (GATA-binding protein 1), Wt1 (Wilms's tumour homologue 1), Cldn11 (claudin11) and Cdkn1a (cyclin-dependent kinase inhibitor 1A or p21(Cip1)). The transcriptional regulation of NXF3 was mediated through physical interaction with STRAP (serine/threonine kinase receptor-associated protein), where NXF3 inhibited the complex formation among Smad7, STRAP and activated type I TGF-β receptor. Taken together, our data provide mechanistic insights into the roles of NXF3 in TGF-β-mediated expression of Tgfb3 and other genes. NXF3 may be implicated in Sertoli cell maturation and the extensive restructuring of the Sertoli cell BTB.

Generation and characterization of an Nxf7 knockout mouse to study NXF5 deficiency in a patient with intellectual disability

Members of the Nuclear eXport Factor (NXF) family are involved in the export of mRNA from the nucleus to the cytoplasm, or hypothesized to play a role in transport of cytoplasmic mRNA. We previously reported on the loss of NXF5 in a male patient with a syndromic form of intellectual disability. To study the functional role of NXF5 we identified the mouse counterpart. Based on synteny, mouse Nxf2 is the ortholog of human NXF5. However, we provide several lines of evidence that mouse Nxf7 is the actual functional equivalent of NXF5. Both Nxf7 and NXF5 are predominantly expressed in the brain, show cytoplasmic localization, and present as granules in neuronal dendrites suggesting a role in cytoplasmic mRNA metabolism in neurons. Nxf7 was primarily detected in the pyramidal cells of the hippocampus and in layer V of the cortex. Similar to human NXF2, mouse Nxf2 is highly expressed in testis and shows a nuclear localization. Interestingly, these findings point to a different evolutionary path for both NXF genes in human and mouse. We thus generated and validated Nxf7 knockout mice, which were fertile and did not present any gross anatomical or morphological abnormalities. Expression profiling in the hippocampus and the cortex did not reveal significant changes between wild-type and Nxf7 knockout mice. However, impaired spatial memory was observed in these KO mice when evaluated in the Morris water maze test. In conclusion, our findings provide strong evidence that mouse Nxf7 is the functional counterpart of human NXF5, which might play a critical role in mRNA metabolism in the brain.

Multiple cancer testis antigens function to support tumor cell mitotic fidelity

While the expression of genes that are normally involved in spermatogenesis is frequently detected in tumors, the extent to which these gene products are required for neoplastic behaviors is unclear. To begin to address their functional relevance to tumorigenesis, we identified a cohort of proteins which display synthetic lethality with paclitaxel in non-small-cell lung cancer and whose expression is biased toward testes and tumors. Remarkably, these testis proteins, FMR1NB, NXF2, MAGEA5, FSIP1, and STARD6, are required for accurate chromosome segregation in tumor cells. Their individual depletion enhances the generation of multipolar spindles, increases mitotic transit time, and induces micronucleation in response to an otherwise innocuous dose of paclitaxel. The underlying basis for abnormal mitosis is an alteration in microtubule function, as their depletion increases microtubule cytaster formation and disrupts microtubule stability. Given these observations, we hypothesize that reactivated testis proteins may represent unique tumor cell vulnerabilities which, if targeted, could enhance responsiveness to antimitotic therapy. Indeed, we demonstrate that combining paclitaxel with a small-molecule inhibitor of the gametogenic and tumor cell mitotic protein TACC3 leads to enhanced centrosomal abnormalities, activation of death programs, and loss of anchorage-independent growth.

The kinesin superfamily protein KIF17: one protein with many functions

Kinesins are ATP-dependent molecular motors that carry cargos along microtubules, generally in an anterograde direction. They are classified into 14 distinct families with varying structural and functional characteristics. KIF17 is a member of the kinesin-2 family that is plus end-directed. It is a homodimer with a pair of head motor domains that bind microtubules, a coiled-coil stalk, and a tail domain that binds cargos. In neurons, KIF17 transports N-methyl-D-aspartate receptor NR2B subunit, kainate receptor GluR5, and potassium Kv4.2 channels from cell bodies exclusively to dendrites. These cargos are necessary for synaptic transmission, learning, memory, and other functions. KIF17's interaction with NXF2 enables the transport of mRNA bidirectionally in dendrites. KIF17 or its homolog OSM-3 also mediates intraflagellar transport of cargos to the distal tips of flagella or cilia, thereby aiding in ciliogenesis. In many invertebrate and vertebrate sensory cells, KIF17 delivers cargos that contribute to chemosensory perception and signal transduction. In vertebrate photoreceptors, KIF17 is necessary for outer segment development and disc morphogenesis. In the testis, KIF17 (KIF17b) mediates microtubule-independent delivery of ACT from the nucleus to the cytoplasm and microtubule-dependent transport of Spatial-ε, both are presumably involved in spermatogenesis. KIF17 is also implicated in epithelial polarity and morphogenesis, placental transport and development, and the development of specific brain regions. The transcriptional regulation of KIF17 has recently been found to be mediated by nuclear respiratory factor 1 (NRF-1), which also regulates NR2B as well as energy metabolism in neurons. Dysfunctions of KIF17 are linked to a number of pathologies.

De novo duplication of Xq22.1→q24 with a disruption of the NXF gene cluster in a mentally retarded woman with short stature and premature ovarian failure

OBJECTIVE

To present molecular cytogenetic characterization of a de novo duplication of Xq22.1→q24 in a mentally retarded woman with short stature and premature ovarian failure.

MATERIALS AND METHODS

A 19-year-old woman presented with psychomotor retardation, developmental delay, mental retardation, short stature, low body weight, general muscle hypotonia, distal muscle hypotrophy of the lower extremities, elongated digits, scanty pubic and axillary hair, hypoplastic external female genitalia, and secondary amenorrhea but no clinical features of Pelizaeus-Merzbacher disease. Conventional cytogenetic analysis revealed a karyotype of 46,X,dup(X)(q22.1q24). Fluorescence in situ hybridization determined a direct duplication with a linear tandem orientation. Array comparative genomic hybridization demonstrated partial trisomy Xq [arr cgh Xq22.1q24 (101,490,234-119,070,188 bp)×3] with a 17.6-Mb duplication.

RESULTS

The duplicated region contained NXF2B, NXF4, NXF3, PLP1, and PGRMC1 genes. There was a disruption of the NXF gene cluster of Xcen-NXF5-NXF2-NXF2B-NXF4-NXF3-Xqter.

CONCLUSION

A duplication of Xq22.1→q24 with a disruption of the NXF gene cluster in female patients can be associated with clinical manifestations of mental retardation in addition to short stature and premature ovarian failure.

Spinal muscular atrophy: the role of SMN in axonal mRNA regulation

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by homozygous mutations or deletions in the survival of motor neuron (SMN1) gene, encoding the ubiquitously expressed SMN protein. SMN associates with different proteins (Gemins 2-8, Unrip) to form a multimeric complex involved in the assembly of small nuclear ribonucleoprotein complexes (snRNPs). Since this activity is essential for the survival of all cell types, it still remains unclear why motor neurons are selectively vulnerable to low levels of SMN protein. Aside from its housekeeping role in the assembly of snRNPs, additional functions of SMN have been proposed. The well-documented localization of SMN in axonal transport granules and its interaction with numerous mRNA-binding proteins not involved in splicing regulation suggest a role in axonal RNA metabolism. This review will focus on the neuropathological and experimental evidence supporting a role for SMN in regulating the assembly, localization, or stability of axonal messenger ribonucleoprotein complexes (mRNPs). Furthermore, how defects in this non-canonical SMN function may contribute to the motor neuron pathology observed in SMA will be discussed. This article is part of a Special Issue entitled RNA-Binding Proteins.

Unraveling regulation and new components of human P-bodies through a protein interaction framework and experimental validation

The cellular factors involved in mRNA degradation and translation repression can aggregate into cytoplasmic domains known as GW bodies or mRNA processing bodies (P-bodies). However, current understanding of P-bodies, especially the regulatory aspect, remains relatively fragmentary. To provide a framework for studying the mechanisms and regulation of P-body formation, maintenance, and disassembly, we compiled a list of P-body proteins found in various species and further grouped both reported and predicted human P-body proteins according to their functions. By analyzing protein-protein interactions of human P-body components, we found that many P-body proteins form complex interaction networks with each other and with other cellular proteins that are not recognized as P-body components. The observation suggests that these other cellular proteins may play important roles in regulating P-body dynamics and functions. We further used siRNA-mediated gene knockdown and immunofluorescence microscopy to demonstrate the validity of our in silico analyses. Our combined approach identifies new P-body components and suggests that protein ubiquitination and protein phosphorylation involving 14-3-3 proteins may play critical roles for post-translational modifications of P-body components in regulating P-body dynamics. Our analyses provide not only a global view of human P-body components and their physical interactions but also a wealth of hypotheses to help guide future research on the regulation and function of human P-bodies.

Cross-talk between distinct nuclear import pathways enables efficient nuclear import of E47 in conjunction with its partner transcription factors

Study of the nuclear import behavior of E47 in conjunction with its partner transcription factors shows that although the nuclear import of E47 is importin α dependent, it is capable of accumulating in the nucleus under importin α–blocked conditions by virtue of its interaction with its binding partners NeuroD1 and MyoD.

Nuclear import of karyophilic proteins is carried out by a variety of mechanisms. We previously showed that two basic helix-loop-helix proteins, NeuroD1 and E47, synergistically affect each other's nuclear import. In this study, we dissected the molecular pathways underlying nuclear import of the NeuroD1/E47 heterodimer. In vitro nuclear import assays indicated that importin α family members are the major nuclear import receptors for E47. However, inhibition of importin α resulted in cytoplasmic retention of E47 that could be rescued by its binding partner, NeuroD1, through heterodimerization. In addition, nuclear import of NeuroD1 was importin α independent but importin β1 dependent. In primary neurons, localization of endogenous E47 was not affected by importin α inhibition, suggesting that neuronal E47 could be imported into the nucleus as a heterodimer with NeuroD1 by using importin β1 alone. We also found that E47 had similar nuclear import characteristics in C2C12 cells, where E47 heterodimerized with MyoD, another helix-loop-helix protein, suggesting functional conservation within the same family of transcription factors. Collectively, our data reveal that E47 is imported into the nucleus via multiple pathways, depending on the molecular binding mode, establishing a previously uncharacterized cross-talk between two distinct nuclear import pathways.

Nxf3 is expressed in Sertoli cells, but is dispensable for spermatogenesis

In eukaryotes, mRNA is actively exported to the cytoplasm by a family of nuclear RNA export factors (NXF). Four Nxf genes have been identified in the mouse: Nxf1, Nxf2, Nxf3, and Nxf7. Inactivation of Nxf2, a germ cell-specific gene, causes defects in spermatogenesis. Here we report that Nxf3 is expressed exclusively in Sertoli cells of the postnatal testis, in a developmentally regulated manner. Expression of Nxf3 coincides with the cessation of Sertoli cell proliferation and the beginning of their differentiation. Continued expression of Nxf3 in mature Sertoli cells of the adult is spermatogenesis stage-independent. Nxf3 is not essential for spermatogenesis, however, suggesting functional redundancy among Nxf family members. With its unique expression pattern in the testis, the promoter of Nxf3 can be used to drive postnatal Sertoli cell-specific expression of other proteins such as Cre recombinase.

Autoinhibition of the kinesin-2 motor KIF17 via dual intramolecular mechanisms

Kinesin-2 motor KIF17 autoinhibition is visualized in vivo; in the absence of cargo, this homodimer’s C-terminal tail blocks microtubule binding, and a coiled-coil segment blocks motility.

Long-distance transport in cells is driven by kinesin and dynein motors that move along microtubule tracks. These motors must be tightly regulated to ensure the spatial and temporal fidelity of their transport events. Transport motors of the kinesin-1 and kinesin-3 families are regulated by autoinhibition, but little is known about the mechanisms that regulate kinesin-2 motors. We show that the homodimeric kinesin-2 motor KIF17 is kept in an inactive state in the absence of cargo. Autoinhibition is caused by a folded conformation that enables nonmotor regions to directly contact and inhibit the enzymatic activity of the motor domain. We define two molecular mechanisms that contribute to autoinhibition of KIF17. First, the C-terminal tail interferes with microtubule binding; and second, a coiled-coil segment blocks processive motility. The latter is a new mechanism for regulation of kinesin motors. This work supports the model that autoinhibition is a general mechanism for regulation of kinesin motors involved in intracellular trafficking events.

Regulation of male fertility by X-linked genes

Infertility is a worldwide reproductive health problem, affecting men and women about equally. Mouse genetic studies demonstrate that more than 200 genes specifically or predominantly regulate fertility. However, few genetic causes of infertility in humans have been identified. Here, we focus on the regulation of male fertility by X-linked, germ cell-specific genes. Previous genomic studies reveal that the mammalian X chromosome is enriched for genes expressed in early spermatogenesis. Recent genetic studies in mice show that X-linked, germ cell-specific genes, such as A-kinase anchor protein 4 (Akap4), nuclear RNA export factor 2 (Nxf2), TBP-associated factor 7l (Taf7l), and testis-expressed gene 11 (Tex11), indeed play important roles in the regulation of male fertility. Moreover, we find that the Taf7l Tex11 double-mutant males exhibit much more severe defects in meiosis than either single mutant, suggesting that these 2 X-linked genes regulate male meiosis synergistically. The X-linked, germ cell-specific genes are particularly attractive in the study of male infertility in humans. Because males are hemizygous for X-linked genes, loss-of-function mutations in the single-copy X-linked genes, unlike in autosomal genes, would not be masked by a normal allele. The genetic studies of X-linked, germ cell-specific genes in mice have laid a foundation for mutational analysis of their human orthologues in infertile men.

Maximum-entropy network analysis reveals a role for tumor necrosis factor in peripheral nerve development and function

Gene regulatory interactions that shape developmental processes can often can be inferred from microarray analysis of gene expression, but most computational methods used require extensive datasets that can be difficult to generate. Here, we show that maximum-entropy network analysis allows extraction of genetic interactions from limited microarray datasets. Maximum-entropy networks indicated that the inflammatory cytokine TNF-alpha plays a pivotal role in Schwann cell-axon interactions, and these data suggested that TNF mediates its effects by orchestrating cytoplasmic movement and axon guidance. In vivo and in vitro experiments confirmed these predictions, showing that Schwann cells in TNF(-/-) peripheral sensory bundles fail to envelop axons efficiently, and that recombinant TNF can partially correct these defects. These data demonstrate the power of maximum-entropy network-based methods for analysis of microarray data, and they indicate that TNF-alpha plays a direct role in Schwann cell-axon communication.

The RNA-binding motif protein 15B (RBM15B/OTT3) acts as cofactor of the nuclear export receptor NXF1

The human SPEN family proteins SHARP, RBM15/OTT1, and RBM15B/OTT3 share the structural domain architecture but show distinct functional properties. Here, we examined the function of OTT3 and compared it with its paralogues RBM15 and SHARP. We found that OTT3, like RBM15, has post-transcriptional regulatory activity, whereas SHARP does not, supporting a divergent role of RBM15 and OTT3. OTT3 shares with RBM15 the association with the splicing factor compartment and the nuclear envelope as well as the binding to mRNA export factors NXF1 and Aly/REF. Mutational analysis revealed direct interaction of OTT3 and RBM15 with NXF1 via their C-terminal regions. Biochemical and subcellular localization studies showed that OTT3 and RBM15 also interact with each other in vivo, further supporting a shared function. Genetic knockdown of RBM15 in mouse is embryonically lethal, indicating that OTT3 cannot compensate for the RBM15 loss, which supports the notion that these proteins, in addition to sharing similar activities, likely have distinct biological roles.

Male infertility and the involvement of the X chromosome

BACKGROUND

Male infertility is a worldwide problem, keeping many researchers puzzled. Besides environmental factors, much attention is paid to single gene defects. In this view, the sex chromosomes are particularly interesting since men only have a single copy of these chromosomes. The involvement of the Y chromosome in male infertility is obvious since the detection of Yq microdeletions. The role of the X chromosome, however, remains less understood.

METHODS

Articles were obtained by searching PubMed until December 2008. A first search attempted to identify genes located on the X chromosome potentially important for spermatogenesis. A second part of the study was focused on those genes for which the role has already been studied in infertile patients.

RESULTS

Multiple genes located on the X chromosome are expressed in testicular tissues. The function of many genes, especially the cancer-testis genes, has not been studied so far. There were striking differences between mouse and human genes. In the second part of the study, the results of mutation analyses of seven genes (AR, SOX3, USP26, NXF2, TAF7L, FATE and AKAP4) are described. Except for AR, no infertility causing mutations have, thus far, been described. It cannot be excluded that some of the observed changes should be considered as risk factors for impaired spermatogenesis.

CONCLUSIONS

It can be concluded that, so far, the mutation analysis of X-linked genes in humans, presumed to be crucial for spermatogenesis or sperm quality, has been disappointing. Other approaches to learn more about male infertility are necessary.

Inactivation of Nxf2 causes defects in male meiosis and age-dependent depletion of spermatogonia

In eukaryotes, mRNA is actively transported from nucleus to cytoplasm by a family of nuclear RNA export factors (NXF). While yeast harbors only one such factor (Mex67p), higher eukaryotes encode multiple NXFs. In mouse, four Nxf genes have been identified: Nxf1, Nxf2, Nxf3, and Nxf7. To date, the function of mouse Nxf genes has not been studied by targeted gene deletion in vivo. Here we report the generation of Nxf2 null mutant mice by homologous recombination in embryonic stem cells. Nxf2-deficient male mice exhibit fertility defects that differ between mouse strains. One third of Nxf2-deficient males on a mixed (C57BL/6x129) genetic background exhibit meiotic arrest and thus are sterile, whereas the remaining males are fertile. Disruption of Nxf2 in inbred (C57BL/6J) males impairs spermatogenesis, resulting in male subfertility, but causes no meiotic arrest. Testis weight and sperm output in C57BL/6J Nxf2(-/Y) mice are sharply reduced. Mutant epididymal sperm exhibit diminished motility. Importantly, proliferation of spermatogonia in Nxf2(-/Y) mice is significantly decreased. As a result, inactivation of Nxf2 causes depletion of germ cells in a substantial fraction of seminiferous tubules in aged mice. These studies demonstrate that Nxf2 plays a dual function in spermatogenesis: regulation of meiosis and maintenance of spermatogonial stem cells.

Treatment of chronic lymphocytic leukemia with a hypomethylating agent induces expression of NXF2, an immunogenic cancer testis antigen

PURPOSE

Critical to the success of active immunotherapy against cancer is the identification of immunologically recognized cancer-specific proteins with low tolerogenic potential. Cancer testis antigens (CTA), in particular, fulfill this requirement as a result of their aberrant expression restricted to cancer cells and lack of expression in normal tissues bypassing tolerogenic mechanisms against self. Although CTAs have been extensively studied in solid malignancies, little is known regarding their expression in chronic lymphocytic leukemia (CLL).

EXPERIMENTAL DESIGN

Using a two-pronged approach we evaluated the immunogenicity of 29 CTAs in 22 patients with CLL and correlated these results to reverse transcriptase PCR data from CLL cell lines and patient cells.

RESULTS

We identified IgG-specific antibodies for one antigen, NXF2, and confirmed this response by ELISA and Western blot. We found that treatment of CLL with 5-aza-2'-deoxycytidine can induce expression of NXF2 that lasted for several weeks after treatment. Treatment also increased levels of MHC and costimulatory molecules (CD80, CD86, and CD40) necessary for antigen presentation. In addition, we identified other promising antigens that may have potential immunotherapeutic application.

CONCLUSIONS

Our findings suggest that NXF2 could be further pursued as an immunotherapeutic target in CLL, and that treatment with demethylating agents could be exploited to specifically modulate CTA expression and effective antigen presentation in malignant B cells.

Synergistic nuclear import of NeuroD1 and its partner transcription factor, E47, via heterodimerization

The transition from undifferentiated pluripotent cells to terminally differentiated neurons is coordinated by a repertoire of transcription factors. NeuroD1 is a type II basic helix loop helix (bHLH) transcription factor that plays critical roles in neuronal differentiation and maintenance in the central nervous system. Its dimerization with E47, a type I bHLH transcription factor, leads to the transcriptional regulation of target genes. Mounting evidence suggests that regulating the localization of transcription factors contributes to the regulation of their activity during development as defects in their localization underlie a variety of developmental disorders. In this study, we attempted to understand the nuclear import mannerisms of NeuroD1 and E47. We found that the nuclear import of NeuroD1 and E47 is energy-dependent and involves the Ran-mediated pathway. Herein, we demonstrate that NeuroD1 and E47 can dimerize inside the cytoplasm before their nuclear import. Moreover, this dimerization promotes nuclear import as the nuclear accumulation of NeuroD1 was enhanced in the presence of E47 in an in vitro nuclear import assay, and NLS-deficient NeuroD1 was successfully imported into the nucleus upon E47 overexpression. NeuroD1 also had a similar effect on the nuclear accumulation of NLS-deficient E47. These findings suggest a novel role for dimerization that may promote, at least partially, the nuclear import of transcription factors allowing them to function efficiently in the nucleus.

Understanding the importance of mRNA transport in memory

RNA localization is an important mechanism to sort proteins to specific subcellular domains. In neurons, several mRNAs are localized in dendrites and their presence allows autonomous control of local translation in response to stimulation of specific synapses. Active constitutive and activity-induced mechanisms of mRNA transport have been described that represent critical steps in the establishment and maintenance of synaptic plasticity. In recent years, the molecular composition of different transporting units has been reported and the identification of proteins and mRNAs in these RNA granules contributes to our understanding of the key steps that regulate mRNA transport and translation. Although RNA granules are heterogeneous, several proteins are common to different RNA granule populations, suggesting that they play important roles in the formation of the granules and/or their regulation during transport and translation. About 1-4% of the neuron transcriptome is found in RNA granules and the characterization of bound mRNAs reveal that they encode proteins of the cytoskeleton, the translation machinery, vesicle trafficking, and/or proteins involved in synaptic plasticity. Non-coding RNAs and microRNAs are also found in dendrites and likely regulate RNA translation. These mechanisms of mRNA transport and local translation are critical for synaptic plasticity mediated by activity or experience and memory.

The signal sequence coding region promotes nuclear export of mRNA

In eukaryotic cells, most mRNAs are exported from the nucleus by the transcription export (TREX) complex, which is loaded onto mRNAs after their splicing and capping. We have studied in mammalian cells the nuclear export of mRNAs that code for secretory proteins, which are targeted to the endoplasmic reticulum membrane by hydrophobic signal sequences. The mRNAs were injected into the nucleus or synthesized from injected or transfected DNA, and their export was followed by fluorescent in situ hybridization. We made the surprising observation that the signal sequence coding region (SSCR) can serve as a nuclear export signal of an mRNA that lacks an intron or functional cap. Even the export of an intron-containing natural mRNA was enhanced by its SSCR. Like conventional export, the SSCR-dependent pathway required the factor TAP, but depletion of the TREX components had only moderate effects. The SSCR export signal appears to be characterized in vertebrates by a low content of adenines, as demonstrated by genome-wide sequence analysis and by the inhibitory effect of silent adenine mutations in SSCRs. The discovery of an SSCR-mediated pathway explains the previously noted amino acid bias in signal sequences and suggests a link between nuclear export and membrane targeting of mRNAs.

In eukaryotic cells, precursors of messenger RNAs (mRNAs) are synthesized and processed in the nucleus. During processing, noncoding introns are spliced out, and a cap and poly-adenosine sequence are added to the beginning and end of the transcript, respectively. The resulting mature mRNA is exported from the nucleus to the cytoplasm by crossing the nuclear pore. Both the introns and the cap help to recruit factors that are necessary for nuclear export of an mRNA. Here we provide evidence for a novel mRNA export pathway that is specific for transcripts coding for secretory proteins. These proteins contain signal sequences that target them for translocation across the endoplasmic reticulum membrane. We made the surprising observation that the signal sequence coding region (SSCR) can serve as a nuclear export signal of an mRNA that lacks an intron or functional cap. Even the export of an intron-containing natural mRNA was enhanced by its SSCR. The SSCR export signal appears to be characterized in vertebrates by a low content of adenines. Our discovery of an SSCR-mediated pathway explains the previously noted amino acid bias in signal sequences, and suggests a link between nuclear export and membrane targeting of mRNAs.

Signal sequences, which target newly synthesized secretory proteins to the endoplasmic reticulum, are encoded by adenine-depleted nucleotide sequences that promote the nuclear export of the mRNAs.

Is there a role for the nuclear export factor 2 gene in male infertility?

OBJECTIVE

To determine the presence of mutations in the NXF2 gene of patients with nonobstructive azoospermia.

DESIGN

Molecular analysis of male infertility.

SETTING

University genetic laboratory and reproductive clinic.

PATIENT(S)

Sixty-five patients with Sertoli cell-only syndrome (SCOS) and 20 control men.

INTERVENTION(S)

Polymerase chain reaction, sequencing analysis, RNA extraction, and reverse transcription polymerase chain reaction.

MAIN OUTCOME MEASURE(S)

Expression of NXF2 messenger RNA and analysis of the NXF2 gene for the presence of mutations and polymorphisms.

RESULT(S)

Messenger RNA derived from the NXF2 gene could be amplified from normal human testicular tissue. Sequencing analysis showed the presence of two polymorphisms in the NXF2 gene. A first alteration, c.1779C>T, was observed in one man who had complete SCOS. Although it is located near an intron-exon boundary, this change probably does not affect splicing. The second alteration, c.1857A>G, was detected in 22 patients with complete SCOS and in 13 patients with incomplete SCOS. Also, 15 of 20 men with normal spermatogenesis had this alteration. Neither of these alterations causes a change at the amino acid level.

CONCLUSION(S)

No mutations were detected in the NXF2 gene, from which we concluded that there is no need to screen for mutations in the NXF2 gene in a routine IVF program.

Nuclear RNA export factor 7 is localized in processing bodies and neuronal RNA granules through interactions with shuttling hnRNPs

The nuclear RNA export factor (NXF) family proteins have been implicated in various aspects of post-transcriptional gene expression. This study shows that mouse NXF7 exhibits heterologous localization, i.e. NXF7 associates with translating ribosomes, stress granules (SGs) and processing bodies (P-bodies), the latter two of which are believed to be cytoplasmic sites of storage, degradation and/or sorting of mRNAs. By yeast two-hybrid screening, a series of heterogeneous nuclear ribonucleoproteins (hnRNPs) were identified as possible binding partners for NXF7. Among them, hnRNP A3, which is believed to be involved in translational control and/or cytoplasmic localization of certain mRNAs, formed a stable complex with NXF7 in vitro. Although hnRNP A3 was not associated with translating ribosomes, it was co-localized with NXF7 in P-bodies. After exposing to oxidative stress, NXF7 trans-localized to SGs, whereas hnRNP A3 did not. In differentiated neuroblastoma Neuro2a cells, NXF7 was co-localized with hnRNP A3 in cell body and neurites. The amino terminal half of NXF7, which was required for stable complex formation with hnRNP A3, coincided with the region required for localization in both P-bodies and neuronal RNA granules. These findings suggest that NXF7 plays a role in sorting, transport and/or storage of mRNAs through interactions with hnRNP A3.