Identification of 3' UTR motifs required for mRNA localization to myelin sheaths in vivo.

Myelin is a specialized membrane produced by oligodendrocytes that insulates and supports axons. Oligodendrocytes extend numerous cellular processes, as projections of the plasma membrane, and simultaneously wrap multiple layers of myelin membrane around target axons. Notably, myelin sheaths originating from the same oligodendrocyte are variable in size, suggesting local mechanisms regulate myelin sheath growth. Purified myelin contains ribosomes and hundreds of mRNAs, supporting a model that mRNA localization and local protein synthesis regulate sheath growth and maturation. However, the mechanisms by which mRNAs are selectively enriched in myelin sheaths are unclear. To investigate how mRNAs are targeted to myelin sheaths, we tested the hypothesis that transcripts are selected for myelin enrichment through consensus sequences in the 3' untranslated region (3' UTR). Using methods to visualize mRNA in living zebrafish larvae, we identified candidate 3' UTRs that were sufficient to localize mRNA to sheaths and enriched near growth zones of nascent membrane. We bioinformatically identified motifs common in 3' UTRs from 3 myelin-enriched transcripts and determined that these motifs are required and sufficient in a context-dependent manner for mRNA transport to myelin sheaths. Finally, we show that 1 motif is highly enriched in the myelin transcriptome, suggesting that this sequence is a global regulator of mRNA localization during developmental myelination.

Prdm8 regulates pMN progenitor specification for motor neuron and oligodendrocyte fates by modulating the Shh signaling response.

Spinal cord pMN progenitors sequentially produce motor neurons and oligodendrocyte precursor cells (OPCs). Some OPCs differentiate rapidly as myelinating oligodendrocytes, whereas others remain into adulthood. How pMN progenitors switch from producing motor neurons to OPCs with distinct fates is poorly understood. pMN progenitors express prdm8, which encodes a transcriptional repressor, during motor neuron and OPC formation. To determine whether prdm8 controls pMN cell fate specification, we used zebrafish as a model system to investigate prdm8 function. Our analysis revealed that prdm8 mutant embryos have fewer motor neurons resulting from a premature switch from motor neuron to OPC production. Additionally, prdm8 mutant larvae have excess oligodendrocytes and a concomitant deficit of OPCs. Notably, pMN cells of mutant embryos have elevated Shh signaling, coincident with the motor neuron to OPC switch. Inhibition of Shh signaling restored the number of motor neurons to normal but did not rescue the proportion of oligodendrocytes. These data suggest that Prdm8 regulates the motor neuron-OPC switch by controlling the level of Shh activity in pMN progenitors, and also regulates the allocation of oligodendrocyte lineage cell fates.This article has an associated 'The people behind the papers' interview.

Motor neurons and oligodendrocytes arise from distinct cell lineages by progenitor recruitment.

During spinal cord development, ventral neural progenitor cells that express the transcription factors Olig1 and Olig2, called pMN progenitors, produce motor neurons and then oligodendrocytes. Whether motor neurons and oligodendrocytes arise from common or distinct progenitors in vivo is not known. Using zebrafish, we found that motor neurons and oligodendrocytes are produced sequentially by distinct progenitors that have distinct origins. When olig2(+) cells were tracked during the peak period of motor neuron formation, most differentiated as motor neurons without further cell division. Using time-lapse imaging, we found that, as motor neurons differentiated, more dorsally positioned neuroepithelial progenitors descended to the pMN domain and initiated olig2 expression. Inhibition of Hedgehog signaling during motor neuron differentiation blocked the ventral movement of progenitors, the progressive initiation of olig2 expression, and oligodendrocyte formation. We therefore propose that the motor neuron-to-oligodendrocyte switch results from Hedgehog-mediated recruitment of glial-fated progenitors to the pMN domain subsequent to neurogenesis.

The Akt-mTOR Pathway Drives Myelin Sheath Growth by Regulating Cap-Dependent Translation.

In the vertebrate CNS, oligodendrocytes produce myelin, a specialized membrane, to insulate and support axons. Individual oligodendrocytes wrap multiple axons with myelin sheaths of variable lengths and thicknesses. Myelin grows at the distal ends of oligodendrocyte processes, and multiple lines of work have provided evidence that mRNAs and RNA binding proteins localize to myelin, together supporting a model where local translation controls myelin sheath growth. What signal transduction mechanisms could control this? One strong candidate is the Akt-mTOR pathway, a major cellular signaling hub that coordinates transcription, translation, metabolism, and cytoskeletal organization. Here, using zebrafish as a model system, we found that Akt-mTOR signaling promotes myelin sheath growth and stability during development. Through cell-specific manipulations to oligodendrocytes, we show that the Akt-mTOR pathway drives cap-dependent translation to promote myelination and that restoration of cap-dependent translation is sufficient to rescue myelin deficits in mTOR loss-of-function animals. Moreover, an mTOR-dependent translational regulator was phosphorylated and colocalized with mRNA encoding a canonically myelin-translated protein in vivo, and bioinformatic investigation revealed numerous putative translational targets in the myelin transcriptome. Together, these data raise the possibility that Akt-mTOR signaling in nascent myelin sheaths promotes sheath growth via translation of myelin-resident mRNAs during development.SIGNIFICANCE STATEMENT In the brain and spinal cord, oligodendrocytes extend processes that tightly wrap axons with myelin, a protein- and lipid-rich membrane that increases electrical impulses and provides trophic support. Myelin membrane grows dramatically following initial axon wrapping in a process that demands protein and lipid synthesis. How protein and lipid synthesis is coordinated with the need for myelin to be generated in certain locations remains unknown. Our study reveals that the Akt-mTOR signaling pathway promotes myelin sheath growth by regulating protein translation. Because we found translational regulators of the Akt-mTOR pathway in myelin, our data raise the possibility that Akt-mTOR activity regulates translation in myelin sheaths to deliver myelin on demand to the places it is needed.

Temporal single-cell transcriptomes of zebrafish spinal cord pMN progenitors reveal distinct neuronal and glial progenitor populations.

Ventral spinal cord progenitor cells, which express the basic helix loop helix transcription factor Olig2, sequentially produce motor neurons and oligodendrocyte precursor cells (OPCs). Following specification some OPCs differentiate as myelinating oligodendrocytes while others persist as OPCs. Though a considerable amount of work has described the molecular profiles that define motor neurons, OPCs, and oligodendrocytes, less is known about the progenitors that produce them. To identify the developmental origins and transcriptional profiles of motor neurons and OPCs, we performed single-cell RNA sequencing on isolated pMN cells from embryonic zebrafish trunk tissue at stages that encompassed motor neurogenesis, OPC specification, and initiation of oligodendrocyte differentiation. Downstream analyses revealed two distinct pMN progenitor populations: one that appears to produce neurons and one that appears to produce OPCs. This latter population, called Pre-OPCs, is marked by expression of GS Homeobox 2 (gsx2), a gene that encodes a homeobox transcription factor. Using fluorescent in situ hybridizations, we identified gsx2-expressing Pre-OPCs in the spinal cord prior to expression of canonical OPC marker genes. Our data therefore reveal heterogeneous gene expression profiles among pMN progenitors, supporting prior fate mapping evidence.

Mutations in KIF7 implicated in idiopathic scoliosis in humans and axial curvatures in zebrafish.

Idiopathic scoliosis (IS) is a spinal disorder affecting up to 3% of otherwise healthy children. IS has a strong familial genetic component and is believed to be genetically complex due to significant variability in phenotype and heritability. Previous studies identified putative loci and variants possibly contributing to IS susceptibility, including within extracellular matrix, cilia, and actin networks, but the genetic architecture and underlying mechanisms remain unresolved. Here, we used whole-exome sequencing from three affected individuals in a multigenerational family with IS and identified 19 uncommon variants (minor allele frequency < 0.05). Genotyping of additional family members identified a candidate heterozygous variant (H1115Q, G>C, rs142032413) within the ciliary gene KIF7, a regulator within the hedgehog (Hh) signaling pathway. Resequencing of the second cohort of unrelated IS individuals and controls identified several severe mutations in KIF7 in affected individuals only. Subsequently, we generated a mutant zebrafish model of kif7 using CRISPR-Cas9. kif7co63/co63 zebrafish displayed severe scoliosis, presenting in juveniles and progressing through adulthood. We observed no deformities in the brain, Reissner fiber, or central canal cilia in kif7co63/co63 embryos, although alterations were seen in Hh pathway gene expression. This study suggests defects in KIF7-dependent Hh signaling, which may drive pathogenesis in a subset of individuals with IS.

Fmrp regulates oligodendrocyte lineage cell specification and differentiation.

Neurodevelopment requires the precise integration of a wide variety of neuronal and glial cell types. During early embryonic development, motor neurons and then oligodendrocyte precursor cells (OPCs) are specified from neural progenitors residing in the periventricular pMN progenitor domain of the spinal cord. Following gliogenesis, OPCs can differentiate as oligodendrocytes (OLs)-the myelinating glial cells of the central nervous system-or remain as OPCs. To generate unique cell types capable of highly divergent functions, these specification and differentiation events require specialized gene expression programs. RNA binding proteins (RBPs) regulate mRNA localization and translation in the developing nervous system and are linked to many neurodevelopmental disorders. One example is Fragile X syndrome (FXS), caused by the loss of the RBP fragile X mental retardation protein (FMRP). Importantly, infants with FXS have reduced white matter and we previously showed that zebrafish Fmrp is autonomously required in OLs to promote myelin sheath growth. We now find that Fmrp regulates cell specification in pMN progenitor cells such that fmr1 mutant zebrafish generate fewer motor neurons and excess OPCs. Fmrp subsequently promotes differentiation of OPCs, leading to fewer differentiating OLs in the developing spinal cord of fmr1 larvae. Although the early patterning of spinal progenitor domains appears largely normal in fmr1 mutants during early embryogenesis, Shh signaling is greatly diminished. Taken together, these results suggest cell stage-specific requirements for Fmrp in the specification and differentiation of oligodendrocyte lineage cells.

The RNA binding protein fragile X mental retardation protein promotes myelin sheath growth.

During development, oligodendrocytes in the central nervous system extend a multitude of processes that wrap axons with myelin. The highly polarized oligodendrocytes generate myelin sheaths on many different axons, which are far removed from the cell body. Neurons use RNA binding proteins to transport, stabilize, and locally translate mRNA in distal domains of neurons. Local synthesis of synaptic proteins during neurodevelopment facilitates the rapid structural and functional changes underlying neural plasticity and avoids extensive protein transport. We hypothesize that RNA binding proteins also regulate local mRNA regulation in oligodendrocytes to promote myelin sheath growth. Fragile X mental retardation protein (FMRP), an RNA binding protein that plays essential roles in the growth and maturation of neurons, is also expressed in oligodendrocytes. To determine whether oligodendrocytes require FMRP for myelin sheath development, we examined fmr1-/- mutant zebrafish and drove FMR1 expression specifically in oligodendrocytes. We found oligodendrocytes in fmr1-/- mutants developed myelin sheaths of diminished length, a phenotype that can be autonomously rescued in oligodendrocytes with FMR1 expression. Myelin basic protein (Mbp), an essential myelin protein, was reduced in myelin tracts of fmr1-/- mutants, but loss of FMRP function did not impact the localization of mbpa transcript in myelin. Finally, expression of FMR1-I304N, a missense allele that abrogates FMRP association with ribosomes, failed to rescue fmr1-/- mutant sheath growth and induced short myelin sheaths in oligodendrocytes of wild-type larvae. Taken together, these data suggest that FMRP promotes sheath growth through local regulation of translation.

Sequential specification of oligodendrocyte lineage cells by distinct levels of Hedgehog and Notch signaling.

During development of the central nervous system oligodendrocyte precursor cells (OPCs) give rise to both myelinating oligodendrocytes and NG2 glia, which are the most proliferative cells in the adult mammalian brain. NG2 glia retain characteristics of OPCs, and some NG2 glia produce oligodendrocytes, but many others persist throughout adulthood. Why some OPCs differentiate as oligodendrocytes during development whereas others persist as OPCs and acquire characteristics of NG2 glia is not known. Using zebrafish spinal cord as a model, we found that OPCs that differentiate rapidly as oligodendrocytes and others that remain as OPCs arise in sequential waves from distinct neural progenitors. Additionally, oligodendrocyte and persistent OPC fates are specified during a defined critical period by small differences in Shh signaling and Notch activity, which modulates Shh signaling response. Thus, our data indicate that OPCs fated to produce oligodendrocytes or remain as OPCs during development are specified as distinct cell types, raising the possibility that the myelinating potential of OPCs is set by graded Shh signaling activity.

Mutations in THAP11 cause an inborn error of cobalamin metabolism and developmental abnormalities.

CblX (MIM309541) is an X-linked recessive disorder characterized by defects in cobalamin (vitamin B12) metabolism and other developmental defects. Mutations in HCFC1, a transcriptional co-regulator which interacts with multiple transcription factors, have been associated with cblX. HCFC1 regulates cobalamin metabolism via the regulation of MMACHC expression through its interaction with THAP11, a THAP domain-containing transcription factor. The HCFC1/THAP11 complex potentially regulates genes involved in diverse cellular functions including cell cycle, proliferation, and transcription. Thus, it is likely that mutation of THAP11 also results in biochemical and other phenotypes similar to those observed in patients with cblX. We report a patient who presented with clinical and biochemical phenotypic features that overlap cblX, but who does not have any mutations in either MMACHC or HCFC1. We sequenced THAP11 by Sanger sequencing and discovered a potentially pathogenic, homozygous variant, c.240C > G (p.Phe80Leu). Functional analysis in the developing zebrafish embryo demonstrated that both THAP11 and HCFC1 regulate the proliferation and differentiation of neural precursors, suggesting important roles in normal brain development. The loss of THAP11 in zebrafish embryos results in craniofacial abnormalities including the complete loss of Meckel's cartilage, the ceratohyal, and all of the ceratobranchial cartilages. These data are consistent with our previous work that demonstrated a role for HCFC1 in vertebrate craniofacial development. High throughput RNA-sequencing analysis reveals several overlapping gene targets of HCFC1 and THAP11. Thus, both HCFC1 and THAP11 play important roles in the regulation of cobalamin metabolism as well as other pathways involved in early vertebrate development.

miR-219 regulates neural progenitors by dampening apical Par protein-dependent Hedgehog signaling.

The transition of dividing neuroepithelial progenitors to differentiated neurons and glia is essential for the formation of a functional nervous system. Sonic hedgehog (Shh) is a mitogen for spinal cord progenitors, but how cells become insensitive to the proliferative effects of Shh is not well understood. Because Shh reception occurs at primary cilia, which are positioned within the apical membrane of neuroepithelial progenitors, we hypothesized that loss of apical characteristics reduces the Shh signaling response, causing cell cycle exit and differentiation. We tested this hypothesis using genetic and pharmacological manipulation, gene expression analysis and time-lapse imaging of zebrafish embryos. Blocking the function of miR-219, a microRNA that downregulates apical Par polarity proteins and promotes progenitor differentiation, elevated Shh signaling. Inhibition of Shh signaling reversed the effects of miR-219 depletion and forced expression of Shh phenocopied miR-219 deficiency. Time-lapse imaging revealed that knockdown of miR-219 function accelerates the growth of primary cilia, revealing a possible mechanistic link between miR-219-mediated regulation of apical Par proteins and Shh signaling. Thus, miR-219 appears to decrease progenitor cell sensitivity to Shh signaling, thereby driving these cells towards differentiation.

Cholesterol Biosynthesis Supports Myelin Gene Expression and Axon Ensheathment through Modulation of P13K/Akt/mTor Signaling.

UNLABELLED: Myelin, which ensheaths and insulates axons, is a specialized membrane highly enriched with cholesterol. During myelin formation, cholesterol influences membrane fluidity, associates with myelin proteins such as myelin proteolipid protein, and assembles lipid-rich microdomains within membranes. Surprisingly, cholesterol also is required by oligodendrocytes, glial cells that make myelin, to express myelin genes and wrap axons. How cholesterol mediates these distinct features of oligodendrocyte development is not known. One possibility is that cholesterol promotes myelination by facilitating signal transduction within the cell, because lipid-rich microdomains function as assembly points for signaling molecules. Signaling cascades that localize to cholesterol-rich regions of the plasma membrane include the PI3K/Akt pathway, which acts upstream of mechanistic target of rapamycin (mTOR), a major driver of myelination. Through manipulation of cholesterol levels and PI3K/Akt/mTOR signaling in zebrafish, we discovered that mTOR kinase activity in oligodendrocytes requires cholesterol. Drawing on a combination of pharmacological and rescue experiments, we provide evidence that mTOR kinase activity is required for cholesterol-mediated myelin gene expression. On the other hand, cholesterol-dependent axon ensheathment is mediated by Akt signaling, independent of mTOR kinase activity. Our data reveal that cholesterol-dependent myelin gene expression and axon ensheathment are facilitated by distinct signaling cascades downstream of Akt. Because mTOR promotes cholesterol synthesis, our data raise the possibility that cholesterol synthesis and mTOR signaling engage in positive feedback to promote the formation of myelin membrane. SIGNIFICANCE STATEMENT: The speed of electrical impulse movement through axons is increased by myelin, a specialized, cholesterol-rich glial cell membrane that tightly wraps axons. During development, myelin membrane grows dramatically, suggesting a significant demand on mechanisms that produce and assemble myelin components, while it spirally wraps axons. Our studies indicate that cholesterol is necessary for both myelin growth and axon wrapping. Specifically, we found that cholesterol facilitates signaling mediated by the PI3K/Akt/mTOR pathway, a powerful driver of myelination. Because mTOR promotes the expression of genes necessary for cholesterol synthesis, cholesterol formation and PI3K/Akt/mTOR signaling might function as a feedforward mechanism to produce the large amounts of myelin membrane necessary for axon ensheathment.

Notch3 establishes brain vascular integrity by regulating pericyte number.

Brain pericytes are important regulators of brain vascular integrity, permeability and blood flow. Deficiencies of brain pericytes are associated with neonatal intracranial hemorrhage in human fetuses, as well as stroke and neurodegeneration in adults. Despite the important functions of brain pericytes, the mechanisms underlying their development are not well understood and little is known about how pericyte density is regulated across the brain. The Notch signaling pathway has been implicated in pericyte development, but its exact roles remain ill defined. Here, we report an investigation of the Notch3 receptor using zebrafish as a model system. We show that zebrafish brain pericytes express notch3 and that notch3 mutant zebrafish have a deficit of brain pericytes and impaired blood-brain barrier function. Conditional loss- and gain-of-function experiments provide evidence that Notch3 signaling positively regulates brain pericyte proliferation. These findings establish a new role for Notch signaling in brain vascular development whereby Notch3 signaling promotes expansion of the brain pericyte population.

Fbxw7 Limits Myelination by Inhibiting mTOR Signaling.

An important characteristic of vertebrate CNS development is the formation of specific amounts of insulating myelin membrane on axons. CNS myelin is produced by oligodendrocytes, glial cells that extend multiple membrane processes to wrap multiple axons. Recent data have shown that signaling mediated by the mechanistic target of rapamycin (mTOR) serine/threonine kinase promotes myelination, but factors that regulate mTOR activity for myelination remain poorly defined. Through a forward genetic screen in zebrafish, we discovered that mutation of fbxw7, which encodes the substrate recognition subunit of a SCF ubiquitin ligase that targets proteins for degradation, causes hypermyelination. Among known Fbxw7 targets is mTOR. Here, we provide evidence that mTOR signaling activity is elevated in oligodendrocyte lineage cells of fbxw7 mutant zebrafish larvae. Both genetic and pharmacological inhibition of mTOR function suppressed the excess myelin gene expression resulting from loss of Fbxw7 function, indicating that mTOR is a functionally relevant target of Fbxw7 in oligodendrocytes. fbxw7 mutant larvae wrapped axons with more myelin membrane than wild-type larvae and oligodendrocyte-specific expression of dominant-negative Fbxw7 produced longer myelin sheaths. Our data indicate that Fbxw7 limits the myelin-promoting activity of mTOR, thereby serving as an important brake on developmental myelination. SIGNIFICANCE STATEMENT: Myelin, a specialized, proteolipid-rich membrane that ensheaths and insulates nerve fibers, facilitates the rapid conduction of electrical impulses over long distances. Abnormalities in myelin formation or maintenance result in intellectual and motor disabilities, raising a need for therapeutic strategies designed to promote myelination. The mTOR kinase is a powerful driver of myelination, but the mechanisms that regulate mTOR function in myelination are not well understood. Our studies reveal that Fbxw7, a subunit of a ubiquitin ligase that targets other proteins for degradation, acts as a brake on myelination by limiting mTOR function. These findings suggest that Fbxw7 helps tune the amount of myelin produced during development and raise the possibility that Fbxw7 could be a target of myelin-promoting therapies.

CNS myelination requires cytoplasmic dynein function.

BACKGROUND: Cytoplasmic dynein provides the main motor force for minus-end-directed transport of cargo on microtubules. Within the vertebrate central nervous system (CNS), proliferation, neuronal migration, and retrograde axon transport are among the cellular functions known to require dynein. Accordingly, mutations of DYNC1H1, which encodes the heavy chain subunit of cytoplasmic dynein, have been linked to developmental brain malformations and axonal pathologies. Oligodendrocytes, the myelinating glial cell type of the CNS, migrate from their origins to their target axons and subsequently extend multiple long processes that ensheath axons with specialized insulating membrane. These processes are filled with microtubules, which facilitate molecular transport of myelin components. However, whether oligodendrocytes require cytoplasmic dynein to ensheath axons with myelin is not known. RESULTS: We identified a mutation of zebrafish dync1h1 in a forward genetic screen that caused a deficit of oligodendrocytes. Using in vivo imaging and gene expression analyses, we additionally found evidence that dync1h1 promotes axon ensheathment and myelin gene expression. CONCLUSIONS: In addition to its well known roles in axon transport and neuronal migration, cytoplasmic dynein contributes to neural development by promoting myelination.

Mutation of 3-hydroxy-3-methylglutaryl CoA synthase I reveals requirements for isoprenoid and cholesterol synthesis in oligodendrocyte migration arrest, axon wrapping, and myelin gene expression.

Myelin membrane, which ensheaths axons, has an unusually high amount of cholesterol. Cholesterol influences membrane fluidity and assembles lipid-rich microdomains within membranes, and some studies have shown that cholesterol is important for myelination. How cholesterol influences the development and differentiation of oligodendrocytes, glial cells that make myelin, is not known nor is clear whether isoprenoids, which also are products of the cholesterol biosynthetic pathway, contribute to myelination. Through a forward genetic screen in zebrafish we discovered that mutation of hmgcs1, which encodes an enzyme necessary for isoprenoid and cholesterol synthesis, causes oligodendrocyte progenitor cells (OPCs) to migrate past their target axons and to fail to express myelin genes. Drawing on a combination of pharmacological inhibitor and rescue experiments, we provide evidence that isoprenoids and protein prenylation, but not cholesterol, are required in OPCs to halt their migration at target axons. On the other hand, cholesterol, but not isoprenoids, is necessary both for axon ensheathment and myelin gene expression. Our data reveal that different products of the cholesterol biosynthetic pathway have distinct roles in oligodendrocyte development and that they together help to coordinate directed migration, axon wrapping, and gene expression.

Hcfc1b, a zebrafish ortholog of HCFC1, regulates craniofacial development by modulating mmachc expression.

Mutations in HCFC1 (MIM300019), have been recently associated with cblX (MIM309541), an X-linked, recessive disorder characterized by multiple congenital anomalies including craniofacial abnormalities. HCFC1 is a transcriptional co-regulator that modulates the expression of numerous downstream target genes including MMACHC, but it is not clear how these HCFC1 targets play a role in the clinical manifestations of cblX. To begin to elucidate the mechanism by which HCFC1 modulates disease phenotypes, we have carried out loss of function analyses in the developing zebrafish. Of the two HCFC1 orthologs in zebrafish, hcfc1a and hcfc1b, the loss of hcfc1b specifically results in defects in craniofacial development. Subsequent analysis revealed that hcfc1b regulates cranial neural crest cell differentiation and proliferation within the posterior pharyngeal arches. Further, the hcfc1b-mediated craniofacial abnormalities were rescued by expression of human MMACHC, a downstream target of HCFC1 that is aberrantly expressed in cblX. Furthermore, we tested distinct human HCFC1 mutations for their role in craniofacial development and demonstrated variable effects on MMACHC expression in humans and craniofacial development in zebrafish. Notably, several individuals with mutations in either HCFC1 or MMACHC have been reported to have mild to moderate facial dysmorphia. Thus, our data demonstrates that HCFC1 plays a role in craniofacial development, which is in part mediated through the regulation of MMACHC expression.

A zebrafish model of lethal congenital contracture syndrome 1 reveals Gle1 function in spinal neural precursor survival and motor axon arborization.

In humans, GLE1 is mutated in lethal congenital contracture syndrome 1 (LCCS1) leading to prenatal death of all affected fetuses. Although the molecular roles of Gle1 in nuclear mRNA export and translation have been documented, no animal models for this disease have been reported. To elucidate the function of Gle1 in vertebrate development, we used the zebrafish (Danio rerio) model system. gle1 mRNA is maternally deposited and widely expressed. Altering Gle1 using an insertional mutant or antisense morpholinos results in multiple defects, including immobility, small eyes, diminished pharyngeal arches, curved body axis, edema, underdeveloped intestine and cell death in the central nervous system. These phenotypes parallel those observed in LCCS1 human fetuses. Gle1 depletion also results in reduction of motoneurons and aberrant arborization of motor axons. Unexpectedly, the motoneuron deficiency results from apoptosis of neural precursors, not of differentiated motoneurons. Mosaic analyses further indicate that Gle1 activity is required extrinsically in the environment for normal motor axon arborization. Importantly, the zebrafish phenotypes caused by Gle1 deficiency are only rescued by expressing wild-type human GLE1 and not by the disease-linked Fin(Major) mutant form of GLE1. Together, our studies provide the first functional characterization of Gle1 in vertebrate development and reveal its essential role in actively dividing cells. We propose that defective GLE1 function in human LCCS1 results in both neurogenic and non-neurogenic defects linked to the apoptosis of proliferative organ precursors.

Pard3 regulates contact between neural crest cells and the timing of Schwann cell differentiation but is not essential for neural crest migration or myelination.

BACKGROUND: Schwann cells, which arise from the neural crest, are the myelinating glia of the peripheral nervous system. During development neural crest and their Schwann cell derivatives engage in a sequence of events that comprise delamination from the neuroepithelium, directed migration, axon ensheathment, and myelin membrane synthesis. At each step neural crest and Schwann cells are polarized, suggesting important roles for molecules that create cellular asymmetries. In this work we investigated the possibility that one polarity protein, Pard3, contributes to the polarized features of neural crest and Schwann cells that are associated with directed migration and myelination. RESULTS: We analyzed mutant zebrafish embryos deficient for maternal and zygotic pard3 function. Time-lapse imaging revealed that neural crest delamination was normal but that migrating cells were disorganized with substantial amounts of overlapping membrane. Nevertheless, neural crest cells migrated to appropriate peripheral targets. Schwann cells wrapped motor axons and, although myelin gene expression was delayed, myelination proceeded to completion. CONCLUSIONS: Pard3 mediates contact inhibition between neural crest cells and promotes timely myelin gene expression but is not essential for neural crest migration or myelination.

Brain-derived neurotrophic factor (BDNF) induces polarized signaling of small GTPase (Rac1) protein at the onset of Schwann cell myelination through partitioning-defective 3 (Par3) protein.

Brain-derived neurotrophic factor (BDNF) was shown to play a role in Schwann cell myelination by recruiting Par3 to the axon-glial interface, but the underlying mechanism has remained unclear. Here we report that Par3 regulates Rac1 activation by BDNF but not by NRG1-Type III in Schwann cells, although both ligands activate Rac1 in vivo. During development, active Rac1 signaling is localized to the axon-glial interface in Schwann cells by a Par3-dependent polarization mechanism. Knockdown of p75 and Par3 individually inhibits Rac1 activation, whereas constitutive activation of Rac1 disturbs the polarized activation of Rac1 in vivo. Polarized Rac1 activation is necessary for myelination as Par3 knockdown attenuates myelination in mouse sciatic nerves as well as in zebrafish. Specifically, Par3 knockdown in zebrafish disrupts proper alignment between the axon and Schwann cells without perturbing Schwann cell migration, suggesting that localized Rac1 activation at the axon-glial interface helps identify the initial wrapping sites. We therefore conclude that polarization of Rac1 activation is critical for myelination.