A role for Sec8 in oligodendrocyte morphological differentiation

The exocyst complex is required for the trafficking and delivery of KCa3.1 to the basolateral membrane of polarized epithelia

Control of the movement of ions and water across epithelia is essential for homeostasis. Changing the number or activity of ion channels at the plasma membrane is a significant regulator of epithelial transport. In polarized epithelia, the intermediate-conductance calcium-activated potassium channel, KCa3.1 is delivered to the basolateral membrane where it generates and maintains the electrochemical gradients required for epithelial transport. The mechanisms that control the delivery of KCa3.1 to the basolateral membrane are still emerging. Herein, we investigated the role of the highly conserved tethering complex exocyst. In epithelia, exocyst is involved in the tethering of post-Golgi secretory vesicles with the basolateral membrane, which is required before membrane fusion. In our Fisher rat thyroid cell line that stably expresses KCa3.1, siRNA knockdown of either of the exocyst subunits Sec3, Sec6, or Sec8 significantly decreased KCa3.1-specific current. In addition, knockdown of exocyst complex subunits significantly reduced the basolateral membrane protein level of KCa3.1. Finally, co-immunoprecipitation experiments suggest associations between Sec6 and KCa3.1, but not between Sec8 and KCa3.1. Collectively, based on these data and our previous studies, we suggest that components of exocyst complex are crucially important in the tethering of KCa3.1 to the basolateral membrane. After which, Soluble N-ethylmaleimide-sensitive factor (SNF) Attachment Receptors (SNARE) proteins aid in the insertion of KCa3.1-containing vesicles into the basolateral membrane of polarized epithelia.NEW & NOTEWORTHY Our Ussing chamber and immunoblot experiments demonstrate that when subunits of the exocyst complex were transiently knocked down, this significantly reduced the basolateral population and functional expression of KCa3.1. These data suggest, combined with our protein association experiments, that the exocyst complex regulates the tethering of KCa3.1-containing vesicles to the basolateral membrane prior to the SNARE-dependent insertion of channels into the basolateral membrane of epithelial cells.

CASK loss of function differentially regulates neuronal maturation and synaptic function in human induced cortical excitatory neurons

Loss-of-function (LOF) mutations in CASK cause severe developmental phenotypes, including microcephaly with pontine and cerebellar hypoplasia, X-linked intellectual disability, and autism. Unraveling the pathological mechanisms of CASK-related disorders has been challenging owing to limited human cellular models to study the dynamic roles of this molecule during neuronal maturation and synapse development. Here, we investigate cell-autonomous functions of CASK in cortical excitatory induced neurons (iNs) generated from CASK knockout (KO) isogenic human embryonic stem cells (hESCs) using gene expression, morphometrics, and electrophysiology. While immature CASK KO iNs show robust neuronal outgrowth, mature CASK KO iNs display severe defects in synaptic transmission and synchronized network activity without compromising neuronal morphology and synapse numbers. In the developing human cortical excitatory neurons, CASK functions to promote both structural integrity and establishment of cortical excitatory neuronal networks. These results lay the foundation for future studies identifying suppressors of such phenotypes relevant to human patients.

Secretory Vesicles Targeted to Plasma Membrane During Pollen Germination and Tube Growth

Pollen germination and pollen tube growth are important biological events in the sexual reproduction of higher plants, during which a large number of vesicle trafficking and membrane fusion events occur. When secretory vesicles are transported via the F-actin network in proximity to the apex of the pollen tube, the secretory vesicles are tethered and fused to the plasma membrane by tethering factors and SNARE proteins, respectively. The coupling and uncoupling between the vesicle membrane and plasma membrane are also regulated by dynamic cytoskeleton, proteins, and signaling molecules, including small G proteins, calcium, and PIP2. In this review, we focus on the current knowledge regarding secretory vesicle delivery, tethering, and fusion during pollen germination and tube growth and summarize the progress in research on how regulators and signaling molecules participate in the above processes.

LRRK2 Modulates the Exocyst Complex Assembly by Interacting with Sec8

Mutations in LRRK2 play a critical role in both familial and sporadic Parkinson's disease (PD). Up to date, the role of LRRK2 in PD onset and progression remains largely unknown. However, experimental evidence highlights a critical role of LRRK2 in the control of vesicle trafficking, likely by Rab phosphorylation, that in turn may regulate different aspects of neuronal physiology. Here we show that LRRK2 interacts with Sec8, one of eight subunits of the exocyst complex. The exocyst complex is an evolutionarily conserved multisubunit protein complex mainly involved in tethering secretory vesicles to the plasma membrane and implicated in the regulation of multiple biological processes modulated by vesicle trafficking. Interestingly, Rabs and exocyst complex belong to the same protein network. Our experimental evidence indicates that LRRK2 kinase activity or the presence of the LRRK2 kinase domain regulate the assembly of exocyst subunits and that the over-expression of Sec8 significantly rescues the LRRK2 G2019S mutant pathological effect. Our findings strongly suggest an interesting molecular mechanism by which LRRK2 could modulate vesicle trafficking and may have important implications to decode the complex role that LRRK2 plays in neuronal physiology.

Glutamatergic Receptor Trafficking and Delivery: Role of the Exocyst Complex

Cells comprise several intracellular membrane compartments that allow them to function properly. One of these functions is cargo movement, typically proteins and membranes within cells. These cargoes ride microtubules through vesicles from Golgi and recycling endosomes to the plasma membrane in order to be delivered and exocytosed. In neurons, synaptic functions employ this cargo trafficking to maintain inter-neuronal communication optimally. One of the complexes that oversee vesicle trafficking and tethering is the exocyst. The exocyst is a protein complex containing eight subunits first identified in yeast and then characterized in multicellular organisms. This complex is related to several cellular processes, including cellular growth, division, migration, and morphogenesis, among others. It has been associated with glutamatergic receptor trafficking and tethering into the synapse, providing the molecular machinery to deliver receptor-containing vesicles into the plasma membrane in a constitutive manner. In this review, we discuss the evidence so far published regarding receptor trafficking and the exocyst complex in both basal and stimulated levels, comparing constitutive trafficking and long-term potentiation-related trafficking.

Assessing non-Mendelian inheritance in inherited axonopathies

PURPOSE

Inherited axonopathies (IA) are rare, clinically and genetically heterogeneous diseases that lead to length-dependent degeneration of the long axons in central (hereditary spastic paraplegia [HSP]) and peripheral (Charcot-Marie-Tooth type 2 [CMT2]) nervous systems. Mendelian high-penetrance alleles in over 100 different genes have been shown to cause IA; however, about 50% of IA cases do not receive a genetic diagnosis. A more comprehensive spectrum of causative genes and alleles is warranted, including causative and risk alleles, as well as oligogenic multilocus inheritance.

METHODS

Through international collaboration, IA exome studies are beginning to be sufficiently powered to perform a pilot rare variant burden analysis. After extensive quality control, our cohort contained 343 CMT cases, 515 HSP cases, and 935 non-neurological controls. We assessed the cumulative mutational burden across disease genes, explored the evidence for multilocus inheritance, and performed an exome-wide rare variant burden analysis.

RESULTS

We replicated the previously described mutational burden in a much larger cohort of CMT cases, and observed the same effect in HSP cases. We identified a preliminary risk allele for CMT in the EXOC4 gene (p value= 6.9 × 10-6, odds ratio [OR] = 2.1) and explored the possibility of multilocus inheritance in IA.

CONCLUSION

Our results support the continuing emergence of complex inheritance mechanisms in historically Mendelian disorders.

The formation of paranodal spirals at the ends of CNS myelin sheaths requires the planar polarity protein Vangl2

During axonal ensheathment, noncompact myelin channels formed at lateral edges of the myelinating process become arranged into tight paranodal spirals that resemble loops when cut in cross section. These adhere to the axon, concentrating voltage-dependent sodium channels at nodes of Ranvier and patterning the surrounding axon into distinct molecular domains. The signals responsible for forming and maintaining the complex structure of paranodal myelin are poorly understood. Here, we test the hypothesis that the planar cell polarity determinant Vangl2 organizes paranodal myelin. We show that Vangl2 is concentrated at paranodes and that, following conditional knockout of Vangl2 in oligodendrocytes, the paranodal spiral loosens, accompanied by disruption to the microtubule cytoskeleton and mislocalization of autotypic adhesion molecules between loops within the spiral. Adhesion of the spiral to the axon is unaffected. This results in disruptions to axonal patterning at nodes of Ranvier, paranodal axon diameter and conduction velocity. When taken together with our previous work showing that loss of the apico-basal polarity protein Scribble has the opposite phenotype-loss of axonal adhesion but no effect on loop-loop autotypic adhesion-our results identify a novel mechanism by which polarity proteins control the shape of nodes of Ranvier and regulate conduction in the CNS.

Peripheral axonal ensheathment is regulated by RalA GTPase and the exocyst complex

Axon ensheathment is fundamental for fast impulse conduction and the normal physiological functioning of the nervous system. Defects in axonal insulation lead to debilitating conditions, but, despite its importance, the molecular players responsible are poorly defined. Here, we identify RalA GTPase as a key player in axon ensheathment in Drosophila larval peripheral nerves. We demonstrate through genetic analysis that RalA action through the exocyst complex is required in wrapping glial cells to regulate their growth and development. We suggest that the RalA-exocyst pathway controls the targeting of secretory vesicles for membrane growth or for the secretion of a wrapping glia-derived factor that itself regulates growth. In summary, our findings provide a new molecular understanding of the process by which axons are ensheathed in vivo, a process that is crucial for normal neuronal function.

Oligodendrocytes express synaptic proteins that modulate myelin sheath formation

Vesicular release from neurons promotes myelin sheath growth on axons. Oligodendrocytes express proteins that allow dendrites to respond to vesicular release at synapses, suggesting that axon-myelin contacts use similar communication mechanisms as synapses to form myelin sheaths. To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion in zebrafish during developmental myelination and investigated expression and localization of PSD95, a dendritic post-synaptic protein, within oligodendrocytes. Synaptic vesicles accumulate and exocytose at ensheathment sites with variable patterning and most sheaths localize PSD95 with patterning similar to exocytosis site location. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes cause variable effects on sheath length and number. One candidate, Cadm1b, localizes to myelin sheaths where both PDZ binding and extracellular adhesion to axons mediate sheath growth. Our work raises the possibility that axon-glial communication contributes to myelin plasticity, providing new targets for mechanistic unraveling of developmental myelination.

X-linked intellectual disability gene CASK regulates postnatal brain growth in a non-cell autonomous manner

The phenotypic spectrum among girls with heterozygous mutations in the X-linked intellectual disability (XLID) gene CASK (calcium/calmodulin-dependent serine protein kinase) includes postnatal microcephaly, ponto-cerebellar hypoplasia, seizures, optic nerve hypoplasia, growth retardation and hypotonia. Although CASK knockout mice were previously reported to exhibit perinatal lethality and a 3-fold increased apoptotic rate in the brain, CASK deletion was not found to affect neuronal physiology and their electrical properties. The pathogenesis of CASK associated disorders and the potential function of CASK therefore remains unknown. Here, using Cre-LoxP mediated gene excision experiments; we demonstrate that deleting CASK specifically from mouse cerebellar neurons does not alter the cerebellar architecture or function. We demonstrate that the neuron-specific deletion of CASK in mice does not cause perinatal lethality but induces severe recurrent epileptic seizures and growth retardation before the onset of adulthood. Furthermore, we demonstrate that although neuron-specific haploinsufficiency of CASK is inconsequential, the CASK mutation associated human phenotypes are replicated with high fidelity in CASK heterozygous knockout female mice (CASK^(+/-)). These data suggest that CASK-related phenotypes are not purely neuronal in origin. Surprisingly, the observed microcephaly in CASK^(+/-) animals is not associated with a specific loss of CASK null brain cells indicating that CASK regulates postnatal brain growth in a non-cell autonomous manner. Using biochemical assay, we also demonstrate that CASK can interact with metabolic proteins. CASK knockdown in human cell lines cause reduced cellular respiration and CASK^(+/-) mice display abnormalities in muscle and brain oxidative metabolism, suggesting a novel function of CASK in metabolism. Our data implies that some phenotypic components of CASK heterozygous deletion mutation associated disorders represent systemic manifestation of metabolic stress and therefore amenable to therapeutic intervention.

The polarity protein Scribble regulates myelination and remyelination in the central nervous system

The development and regeneration of myelin by oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), requires profound changes in cell shape that lead to myelin sheath initiation and formation. Here, we demonstrate a requirement for the basal polarity complex protein Scribble in CNS myelination and remyelination. Scribble is expressed throughout oligodendroglial development and is up-regulated in mature oligodendrocytes where it is localised to both developing and mature CNS myelin sheaths. Knockdown of Scribble expression in cultured oligodendroglia results in disrupted morphology and myelination initiation. When Scribble expression is conditionally eliminated in the myelinating glia of transgenic mice, myelin initiation in CNS is disrupted, both during development and following focal demyelination, and longitudinal extension of the myelin sheath is disrupted. At later stages of myelination, Scribble acts to negatively regulate myelin thickness whilst suppressing the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAP) kinase pathway, and localises to non-compact myelin flanking the node of Ranvier where it is required for paranodal axo-glial adhesion. These findings demonstrate an essential role for the evolutionarily-conserved regulators of intracellular polarity in myelination and remyelination.

The polarity protein Scribble regulates the formation and properties of myelin sheaths in the central nervous system during development and after demyelinating injury.

The formation of myelin, a fatty, multilayered structure that surrounds certain neuronal axons in the nervous system, is essential for the proper communication of electrical signals by neurons, acting both as an insulator and to promote metabolic support to the axon. Loss of myelin can have severe functional consequences and trigger serious diseases, such as multiple sclerosis. Bidirectional communication between the oligodendrocytes, the myelinating cells of the central nervous system, and the axon is essential for the proper formation and function of myelin membranes; however, the signals that control myelination by oligodendrocytes in the central nervous system are poorly understood. In this paper, we use a combination of cell culture and animal studies to demonstrate that the protein Scribble, which is known to be a highly evolutionarily conserved regulator of cell polarity, plays a role in controlling whether oligodendrocytes myelinate axons. We show that Scribble regulates the length and thickness of myelin sheaths formed, as well as the tight adhesion of oligodendroglial membranes to the axonal surface, which is required for the organization of the axon into specialized domains at the nodes of Ranvier (gaps formed between the myelin sheaths generated by different cells). In addition, we show that Scribble plays a key role in the repair of myelin sheaths in a mouse model of demyelinating disease. The discovery of novel regulators of myelination in the central nervous system may allow for the identification of novel therapeutic targets for the promotion of myelin repair in patients suffering from demyelinating diseases.

Lysosomal storage disease in the brain: mutations of the β-mannosidase gene identified in autosomal dominant nystagmus

PURPOSE

Genetic etiology of congenital/infantile nystagmus remains largely unknown. This study aimed to identify genomic mutations in patients with infantile nystagmus and an associated disease network.

METHODS

Patients with inherited and sporadic infantile nystagmus were recruited for whole-exome and Sanger sequencing. β-Mannosidase activities were measured. Gene expression, protein-protein interaction, and nystagmus-associated lysosomal storage disease (LSD) genes were analyzed.

RESULTS

A novel heterozygous mutation (c.2013G>A; p.R638H) of MANBA, which encodes lysosomal β-mannosidase, was identified in patients with autosomal-dominant nystagmus. An additional mutation (c.2346T>A; p.L749H) in MANBA was found by screening patients with sporadic nystagmus. MANBA was expressed in the pretectal nucleus of the developing midbrain, known to be involved in oculomotor and optokinetic nystagmus. Functional validation of these mutations demonstrated a significant decrease of β-mannosidase activities in the patients as well as in mutant-transfected HEK293T cells. Further analysis revealed that nystagmus is present in at least 24 different LSDs involving the brain.

CONCLUSION

This is the first identification of MANBA mutations in patients with autosomal-dominant nystagmus, suggesting a new clinical entity. Because β-mannosidase activities are required for development of the oculomotor nervous system, our findings shed new light on the role of LSD-associated genes in the pathogenesis of infantile nystagmus.

Knockdown of Sec8 promotes cell-cycle arrest at G1/S phase by inducing p21 via control of FOXO proteins

p21(Cip1) protein inhibits the activity of cyclins at the G(1) checkpoint and influences transition of cells from the G(1) to the S phase of the cell cycle. Moreover, expression of members of the FOXO family (active form of forkhead transcription factors of the O class) in dividing cells promotes cell-cycle arrest at the G(1)/S boundary via regulation of p21(Cip1). Recently, the exocyst complex, including Sec8, has been implicated in various roles independent of its role in secretion, such as cell migration, invadopodia formation, cytokinesis, glucose uptake and neural development. Given the essential roles of the exocyst complex in cellular and developmental processes, disruption of its function may be involved in various diseases such as cancer, diabetes and neuronal disorders. However, the relationship between Sec8 and the cell cycle remains to be elucidated. In this study, knockdown of Sec8 inhibited cell growth and promoted cell-cycle arrest at the G(1)/S phase by control of p21 expression and retinoblastoma protein phosphorylation. Furthermore, Sec8 regulated FOXO family proteins via ubiquitin-proteasome degradation by regulating the expression of the murine double minute 2 (Mdm2) protein but not S-phase kinase-associated protein 2 (Skp2).

Pals1 is a major regulator of the epithelial-like polarization and the extension of the myelin sheath in peripheral nerves

Diameter, organization, and length of the myelin sheath are important determinants of the nerve conduction velocity, but the basic molecular mechanisms that control these parameters are only partially understood. Cell polarization is an essential feature of differentiated cells, and relies on a set of evolutionarily conserved cell polarity proteins. We investigated the molecular nature of myelin sheath polarization in connection with the functional role of the cell polarity protein pals1 (Protein Associated with Lin Seven 1) during peripheral nerve myelin sheath extension. We found that, in regard to epithelial polarity, the Schwann cell outer abaxonal domain represents a basolateral-like domain, while the inner adaxonal domain and Schmidt-Lanterman incisures form an apical-like domain. Silencing of pals1 in myelinating Schwann cells in vivo resulted in a severe reduction of myelin sheath thickness and length. Except for some infoldings, the structure of compact myelin was not fundamentally affected, but cells produced less myelin turns. In addition, pals1 is required for the normal polarized localization of the vesicular markers sec8 and syntaxin4, and for the distribution of E-cadherin and myelin proteins PMP22 and MAG at the plasma membrane. Our data show that the polarity protein pals1 plays an essential role in the radial and longitudinal extension of the myelin sheath, likely involving a functional role in membrane protein trafficking. We conclude that regulation of epithelial-like polarization is a critical determinant of myelin sheath structure and function.

The SNARE protein SNAP-29 interacts with the GTPase Rab3A: Implications for membrane trafficking in myelinating glia

During myelin formation, vast amounts of specialized membrane proteins and lipids are trafficked toward the growing sheath in cell surface-directed transport vesicles. Soluble N-ethylmaleimide-sensitive factor (NSF) attachment proteins (SNAPs) are important components of molecular complexes required for membrane fusion. We have analyzed the expression profile and molecular interactions of SNAP-29 in the nervous system. In addition to its known enrichment in neuronal synapses, SNAP-29 is abundant in oligodendrocytes during myelination and in noncompact myelin of the peripheral nervous system. By yeast two-hybrid screen and coimmunoprecipitation, we found that the GTPases Rab3A, Rab24, and septin 4 bind to the N-terminal domain of SNAP-29. The interaction with Rab24 or septin 4 was GTP independent. In contrast, interaction between SNAP-29 and Rab3A was GTP dependent, and colocalization was extensive both in synapses and in myelinating glia. In HEK293 cells, cytoplasmic SNAP-29 pools were redistributed upon coexpression with Rab3A, and surface-directed trafficking of myelin proteolipid protein was enhanced by overexpression of SNAP-29 and Rab3A. Interestingly, the abundance of SNAP-29 in sciatic nerves was increased during remyelination and in a rat model of Charcot-Marie-Tooth disease, two pathological situations with increased myelin membrane biogenesis. We suggest that Rab3A may regulate SNAP-29-mediated membrane fusion during myelination.

On the biogenesis of myelin membranes: sorting, trafficking and cell polarity

In the central nervous system, a multilayered membrane layer known as the myelin sheath enwraps axons, and is required for optimal saltatory signal conductance. The sheath develops from membrane processes that extend from the plasma membrane of oligodendrocytes and displays a unique lipid and protein composition. Myelin biogenesis is carefully regulated, and multiple transport pathways involving a variety of endosomal compartments are involved. Here we briefly summarize how the major myelin proteins proteolipid protein and myelin basic protein reach the sheath, and highlight potential mechanisms involved, including the role of myelin specific lipids and cell polarity related transport pathways.

Dlg1, Sec8, and Mtmr2 regulate membrane homeostasis in Schwann cell myelination

How membrane biosynthesis and homeostasis is achieved in myelinating glia is mostly unknown. We previously reported that loss of myotubularin-related protein 2 (MTMR2) provokes autosomal recessive demyelinating Charcot-Marie-Tooth type 4B1 neuropathy, characterized by excessive redundant myelin, also known as myelin outfoldings. We generated a Mtmr2-null mouse that models the human neuropathy. We also found that, in Schwann cells, Mtmr2 interacts with Discs large 1 (Dlg1), a scaffold involved in polarized trafficking and membrane addition, whose localization in Mtmr2-null nerves is altered. We here report that, in Schwann cells, Dlg1 also interacts with kinesin 13B (kif13B) and Sec8, which are involved in vesicle transport and membrane tethering in polarized cells, respectively. Taking advantage of the Mtmr2-null mouse as a model of impaired membrane formation, we provide here the first evidence for a machinery that titrates membrane formation during myelination. We established Schwann cell/DRG neuron cocultures from Mtmr2-null mice, in which myelin outfoldings were reproduced and almost completely rescued by Mtmr2 replacement. By exploiting this in vitro model, we propose a mechanism whereby kif13B kinesin transports Dlg1 to sites of membrane remodeling where it coordinates a homeostatic control of myelination. The interaction of Dlg1 with the Sec8 exocyst component promotes membrane addition, whereas with Mtmr2, negatively regulates membrane formation. Myelin outfoldings thus arise as a consequence of the loss of negative control on the amount of membrane, which is produced during myelination.

RalA and the exocyst complex influence neuronal polarity through PAR-3 and aPKC

Neuronal polarization requires localized cytoskeletal changes and polarized membrane traffic. Here, I report that the small GTPase RalA, previously shown to control neurite branching, also regulates neuronal polarity. RalA depletion, or ectopic expression of constitutively active RalA in cultured neurons inhibit axon formation. However, expression of a constitutively active RalA mutant that is unable to interact with the exocyst complex has no effect on neuronal polarization. Furthermore, depletion of the Sec6, Sec8 or Exo84 subunits of the exocyst complex also leads to unpolarized neurons. Early stages of neuronal polarization are accompanied by increasing levels of interaction of the exocyst complex with PAR-3 and atypical protein kinase C (aPKC), and by the RalA-dependent association of the exocyst complex with PAR-3. Thus, neuronal polarization involves a RalA-regulated association between mediators of vesicle trafficking (exocyst complex) and cell polarity (PAR-3).

Role for Rab3a in oligodendrocyte morphological differentiation

Rab3a, a small GTPase important for exocytosis, is uniquely up-regulated as oligodendrocytes enter terminal differentiation and initiate myelin biosynthesis. In this study, we analyze the role of this protein in oligodendrocyte morphological differentiation by using Rab3a overexpression and siRNAi-mediated Rab3a silencing. We found that Rab3a silencing delayed mature oligodendrocyte morphological differentiation but did not interfere with lineage progression of OL progenitors; this is consistent with the high levels of Rab3a expressed by mature oligodendrocytes compared with progenitor cells. Overexpression of GTP-bound, but not that of wild-type, Rab3a delayed OL morphological differentiation; this suggests that expression of a GTP-bound Rab3a mutant interferes with the normal function of endogenous Rab3a. We have also identified in oligodendrocytes two other exocytic small GTPases, Rab27B and RalA. Together, these findings indicate that Rab3a specifically stimulates morphological differentiation of mature oligodendrocytes and thus may be part of the necessary machinery for myelin membrane biogenesis.

Polarity proteins in glial cell functions

Glial cells, which include myelinating oligodendrocytes, Schwann cells and astrocytes, fulfil a large variety of functions that are critical for the development, functioning and regeneration of neurons. Some of these glial functions have been shown to require polarization of the intracellular machinery. Although the initial signals leading to glial cell polarization during development and in the adult are not completely elucidated, crucial molecules such as proteins of the extracellular matrix and their membrane receptors have been identified. A general picture of the intracellular signalling pathways controlling polarity in glial cells is also emerging and shows that highly conserved and ubiquitously expressed polarity proteins are involved.

Ral-regulated interaction between Sec5 and paxillin targets Exocyst to focal complexes during cell migration

Changes in cellular behavior that cause epithelial cells to lose adhesiveness, acquire a motile invasive phenotype and metastasize to secondary sites are complex and poorly understood. Molecules that normally function to integrate adhesive spatial information with cytoskeleton dynamics and membrane trafficking probably serve important functions in cellular transformation. One such complex is the Exocyst, which is essential for targeted delivery of membrane and secretory proteins to specific plasma membrane sites to maintain epithelial cell polarity. Upon loss of cadherin-mediated adhesion in Dunning R3327-5'A prostate tumor cells, Exocyst localization shifts from lateral membranes to tips of protrusive membrane extensions. Here, it colocalizes and co-purifies with focal complex proteins that regulate membrane trafficking and cytoskeleton dynamics. These sites are the preferred destination of post-Golgi transport vesicles ferrying biosynthetic cargo, such as alpha(5)-integrin, which mediates adhesion of cells to the substratum, a process essential to cell motility. Interference with Exocyst activity impairs integrin delivery to plasma membrane and inhibits tumor cell motility and matrix invasiveness. Localization of Exocyst and, by extension, targeting of Exocyst-dependent cargo, is dependent on Ral GTPases, which control association between Sec5 and paxillin. Overexpression of Ral-uncoupled Sec5 mutants inhibited Exocyst interaction with paxillin in 5'A cells, as did RNAi-mediated reduction of either RalA or RalB. Reduction of neither GTPase significantly altered steady-state levels of assembled Exocyst in these cells, but did change the observed localization of Exocyst proteins.

Distinct endocytic recycling of myelin proteins promotes oligodendroglial membrane remodeling

The central nervous system myelin sheath is a multilayered specialized membrane with compacted and non-compacted domains of defined protein composition. How oligodendrocytes regulate myelin membrane trafficking and establish membrane domains during myelination is largely unknown. Oligodendroglial cells respond to neuronal signals by adjusting the relative levels of endocytosis and exocytosis of the major myelin protein, proteolipid protein (PLP). We investigated whether endocytic trafficking is common to myelin proteins and analyzed the endocytic fates of proteins with distinct myelin subdomain localization. Interestingly, we found that PLP, myelin-associated glycoprotein (MAG) and myelin-oligodendrocyte glycoprotein (MOG), which localize to compact myelin, periaxonal loops and abaxonal loops, respectively, exhibit distinct endocytic fates. PLP was internalized via clathrin-independent endocytosis, whereas MAG was endocytosed by a clathrin-dependent pathway, although both proteins were targeted to the late-endosomal/lysosomal compartment. MOG was also endocytosed by a clathrin-dependent pathway, but in contrast to MAG, trafficked to the recycling endosome. Endocytic recycling resulted in the association of PLP, MAG and MOG with oligodendroglial membrane domains mimicking the biochemical characteristics of myelin domains. Our results suggest that endocytic sorting and recycling of myelin proteins may assist plasma membrane remodeling, which is necessary for the morphogenesis of myelin subdomains.

Wrapping it up: the cell biology of myelination

During nervous system development, oligodendroglia in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) synthesise large amounts of specific proteins and lipids to generate myelin, a specialised membrane that spirally ensheathes axons and facilitates fast conduction of the action potential. Myelination is initiated after glial processes have attached to the axon and polarisation of the plasma membrane has been triggered. Myelin assembly is a multi-step process that occurs in spatially distinct regions of the cell. We propose that assembly of myelin proteins and lipids starts during their transport through the biosynthetic pathway and continues at the plasma membrane aided by myelin-basic protein (MBP). These sequential processes create the special lipid and protein composition necessary for myelin to perform its insulating function during nerve conduction.

The molecular mechanisms of the mammalian exocyst complex in exocytosis

Exocytosis is a highly ordered vesicle trafficking pathway that targets proteins to the plasma membrane for membrane addition or secretion. Research over the years has discovered many proteins that participate at various stages in the mammalian exocytotic pathway. At the early stage of exocytosis, co-atomer proteins and their respective adaptors and GTPases have been shown to play a role in the sorting and incorporation of proteins into secretory vesicles. At the final stage of exocytosis, SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) and SNARE-associated proteins are believed to mediate the fusion of secretory vesicles at the plasma membrane. There are multiple events that may occur between the budding of secretory vesicles from the Golgi and the fusion of these vesicles at the plasma membrane. The most obvious and best-known event is the transport of secretory vesicles from Golgi to the vicinity of the plasma membrane via microtubules and their associated motors. At the vicinity of the plasma membrane, however, it is not clear how vesicles finally dock and fuse with the plasma membrane. Identification of proteins involved in these events should provide important insights into the mechanisms of this little known stage of the exocytotic pathway. Currently, a protein complex, known as the sec6/8 or the exocyst complex, has been implicated to play a role at this late stage of exocytosis.

Neuron-glia communication in the control of oligodendrocyte function and myelin biogenesis

During the development of the central nervous system the reciprocal communication between neurons and oligodendrocytes is essential for the generation of myelin, a multilamellar insulating membrane that ensheathes the axons. Neuron-derived signalling molecules regulate the proliferation, differentiation and survival of oligodendrocytes. Furthermore, neurons control the onset and timing of myelin membrane growth. In turn, signals from oligodendrocytes to neurons direct the assembly of specific subdomains in neurons at the node of Ranvier. Recent work has begun to shed light on the molecules and signaling systems used to coordinate the interaction of neurons and oligodendrocytes. For example, the neuronal signals seem to control the membrane trafficking machinery in oligodendrocytes that leads to myelination. These interconnections at multiple levels show how neurons and glia cooperate to build a complex network during development.

Myelin Biogenesis: Sorting out Protein Trafficking

Myelin biogenesis is a complex process involving coordinated exocytosis, endocytosis, mRNA transport and cytoskeletal dynamics. Recent studies indicate that soluble neuronal signals may control the surface expression of proteolipid protein, a process that involves reduced endocytosis and/or increased transport carrier recruitment from an intracellular pool.