Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Advances in understanding the roles of actin scaffolding and membrane trafficking in dendrite development

Dendritic morphology is typically highly branched, and the branching and synaptic abundance of dendrites can enhance the receptive range of neurons and the diversity of information received, thus providing the basis for information processing in the nervous system. Once dendritic development is aberrantly compromised or damaged, it may lead to abnormal connectivity of the neural network, affecting the function and stability of the nervous system and ultimately triggering a series of neurological disorders. Research on the regulation of dendritic developmental processes has flourished, and much progress is now being made in its regulatory mechanisms. Noteworthily, dendrites are characterized by an extremely complex dendritic arborization that cannot be attributed to individual protein functions alone, requiring a systematic analysis of the intrinsic and extrinsic signals and the coordinated roles among them. Actin cytoskeleton organization and membrane vesicle trafficking are required during dendrite development, with actin providing tracks for vesicles and vesicle trafficking in turn providing material for actin assembly. In this review, we focus on these two basic biological processes and discuss the molecular mechanisms and their synergistic effects underlying the morphogenesis of neuronal dendrites. We also offer insights and discuss strategies for the potential preventive and therapeutic treatment of neuropsychiatric disorders.

Protein translation rate determines neocortical neuron fate

The mammalian neocortex comprises an enormous diversity regarding cell types, morphology, and connectivity. In this work, we discover a post-transcriptional mechanism of gene expression regulation, protein translation, as a determinant of cortical neuron identity. We find specific upregulation of protein synthesis in the progenitors of later-born neurons and show that translation rates and concomitantly protein half-lives are inherent features of cortical neuron subtypes. In a small molecule screening, we identify Ire1α as a regulator of Satb2 expression and neuronal polarity. In the developing brain, Ire1α regulates global translation rates, coordinates ribosome traffic, and the expression of eIF4A1. Furthermore, we demonstrate that the Satb2 mRNA translation requires eIF4A1 helicase activity towards its 5'-untranslated region. Altogether, we show that cortical neuron diversity is generated by mechanisms operating beyond gene transcription, with Ire1α-safeguarded proteostasis serving as an essential regulator of brain development.

Early signs of neurodegenerative diseases: Possible mechanisms and targets for Golgi stress

The Golgi apparatus plays a crucial role in mediating the modification, transport, and sorting of intracellular proteins and lipids. The morphological changes occurring in the Golgi apparatus are exceptionally important for maintaining its function. When exposed to external pressure or environmental stimulation, the Golgi apparatus undergoes adaptive changes in both structure and function, which are known as Golgi stress. Although certain signal pathway responses or post-translational modifications have been observed following Golgi stress, further research is needed to comprehensively summarize and understand the related mechanisms. Currently, there is evidence linking Golgi stress to neurodegenerative diseases; however, the role of Golgi stress in the progression of neurodegenerative diseases such as Alzheimer's disease remains largely unexplored. This review focuses on the structural and functional alterations of the Golgi apparatus during stress, elucidating potential mechanisms underlying the involvement of Golgi stress in regulating immunity, autophagy, and metabolic processes. Additionally, it highlights the pivotal role of Golgi stress as an early signaling event implicated in the pathogenesis and progression of neurodegenerative diseases. Furthermore, this study summarizes prospective targets that can be therapeutically exploited to mitigate neurodegenerative diseases by targeting Golgi stress. These findings provide a theoretical foundation for identifying novel breakthroughs in preventing and treating neurodegenerative diseases.

Activity-dependent dendrite patterning in the postnatal barrel cortex

For neural circuit construction in the brain, coarse neuronal connections are assembled prenatally following genetic programs, being reorganized postnatally by activity-dependent mechanisms to implement area-specific computational functions. Activity-dependent dendrite patterning is a critical component of neural circuit reorganization, whereby individual neurons rearrange and optimize their presynaptic partners. In the rodent primary somatosensory cortex (barrel cortex), driven by thalamocortical inputs, layer 4 (L4) excitatory neurons extensively remodel their basal dendrites at neonatal stages to ensure specific responses of barrels to the corresponding individual whiskers. This feature of barrel cortex L4 neurons makes them an excellent model, significantly contributing to unveiling the activity-dependent nature of dendrite patterning and circuit reorganization. In this review, we summarize recent advances in our understanding of the activity-dependent mechanisms underlying dendrite patterning. Our focus lays on the mechanisms revealed by in vivo time-lapse imaging, and the role of activity-dependent Golgi apparatus polarity regulation in dendrite patterning. We also discuss the type of neuronal activity that could contribute to dendrite patterning and hence connectivity.

Polarized transport requires AP-1-mediated recruitment of KIF13A and KIF13B at the trans-Golgi

Neurons are polarized cells that require accurate membrane trafficking to maintain distinct protein complements at dendritic and axonal membranes. The Kinesin-3 family members KIF13A and KIF13B are thought to mediate dendrite-selective transport, but the mechanism by which they are recruited to polarized vesicles and the differences in the specific trafficking role of each KIF13 have not been defined. We performed live-cell imaging in cultured hippocampal neurons and found that KIF13A is a dedicated dendrite-selective kinesin. KIF13B confers two different transport modes, dendrite- and axon-selective transport. Both KIF13s are maintained at the trans-Golgi network by interactions with the heterotetrameric adaptor protein complex AP-1. Interference with KIF13 binding to AP-1 resulted in disruptions to both dendrite- and axon-selective trafficking. We propose that AP-1 is the molecular link between the sorting of polarized cargoes into vesicles and the recruitment of kinesins that confer polarized transport.

Split but merge: Golgi fragmentation in physiological and pathological conditions

The Golgi complex is a highly dynamic and tightly regulated cellular organelle with essential roles in the processing as well as the sorting of proteins and lipids. Its structure undergoes rapid disassembly and reassembly during normal physiological processes, including cell division, migration, polarization, differentiation, and cell death. Golgi dispersal or fragmentation also occurs in pathological conditions, such as neurodegenerative diseases, infectious diseases, congenital disorders of glycosylation diseases, and cancer. In this review, current knowledge about both structural organization and morphological alterations in the Golgi in physiological and pathological conditions is summarized together with the methodologies that help to reveal its structure.

The organization and function of the Golgi apparatus in dendrite development and neurological disorders

Dendrites are specialized neuronal compartments that sense, integrate and transfer information in the neural network. Their development is tightly controlled and abnormal dendrite morphogenesis is strongly linked to neurological disorders. While dendritic morphology ranges from relatively simple to extremely complex for a specified neuron, either requires a functional secretory pathway to continually replenish proteins and lipids to meet dendritic growth demands. The Golgi apparatus occupies the center of the secretory pathway and is regulating posttranslational modifications, sorting, transport, and signal transduction, as well as acting as a non-centrosomal microtubule organization center. The neuronal Golgi apparatus shares common features with Golgi in other eukaryotic cell types but also forms distinct structures known as Golgi outposts that specifically localize in dendrites. However, the organization and function of Golgi in dendrite development and its impact on neurological disorders is just emerging and so far lacks a systematic summary. We describe the organization of the Golgi apparatus in neurons, review the current understanding of Golgi function in dendritic morphogenesis, and discuss the current challenges and future directions.

Role of Nanoparticle-Conjugates and Nanotheranostics in Abrogating Oxidative Stress and Ameliorating Neuroinflammation

Oxidative stress is a deteriorating condition that arises due to an imbalance between the reactive oxygen species and the antioxidant system or defense of the body. The key reasons for the development of such conditions are malfunctioning of various cell organelles, such as mitochondria, endoplasmic reticulum, and Golgi complex, as well as physical and mental disturbances. The nervous system has a relatively high utilization of oxygen, thus making it particularly vulnerable to oxidative stress, which eventually leads to neuronal atrophy and death. This advances the development of neuroinflammation and neurodegeneration-associated disorders such as Alzheimer's disease, Parkinson's disease, epilepsy, dementia, and other memory disorders. It is imperative to treat such conditions as early as possible before they worsen and progress to irreversible damage. Oxidative damage can be negated by two mechanisms: improving the cellular defense system or providing exogenous antioxidants. Natural antioxidants can normally handle such oxidative stress, but they have limited efficacy. The valuable features of nanoparticles and/or nanomaterials, in combination with antioxidant features, offer innovative nanotheranostic tools as potential therapeutic modalities. Hence, this review aims to represent novel therapeutic approaches like utilizing nanoparticles with antioxidant properties and nanotheranostics as delivery systems for potential therapeutic applications in various neuroinflammation- and neurodegeneration-associated disease conditions.

Neural deficits in a mouse model of PACS1 syndrome are corrected with PACS1- or HDAC6-targeting therapy

PACS1 syndrome is a neurodevelopmental disorder (NDD) caused by a recurrent de novo missense mutation in PACS1 (p.Arg203Trp (PACS1R203W)). The mechanism by which PACS1R203W causes PACS1 syndrome is unknown, and no curative treatment is available. Here, we use patient cells and PACS1 syndrome mice to show that PACS1 (or PACS-1) is an HDAC6 effector and that the R203W substitution increases the PACS1/HDAC6 interaction, aberrantly potentiating deacetylase activity. Consequently, PACS1R203W reduces acetylation of α-tubulin and cortactin, causing the Golgi ribbon in hippocampal neurons and patient-derived neural progenitor cells (NPCs) to fragment and overpopulate dendrites, increasing their arborization. The dendrites, however, are beset with varicosities, diminished spine density, and fewer functional synapses, characteristic of NDDs. Treatment of PACS1 syndrome mice or patient NPCs with PACS1- or HDAC6-targeting antisense oligonucleotides, or HDAC6 inhibitors, restores neuronal structure and synaptic transmission in prefrontal cortex, suggesting that targeting PACS1R203W/HDAC6 may be an effective therapy for PACS1 syndrome.

Unraveling the Multifaceted Role of the Golgi Apparatus: Insights into Neuronal Plasticity, Development, Neurogenesis, Alzheimer's Disease, and SARS-CoV-2 Interactions

This article critically evaluates the multifunctional role of the Golgi apparatus within neurological paradigms. We succinctly highlight its influence on neuronal plasticity, development, and the vital trafficking and sorting mechanisms for proteins and lipids. The discourse further navigates to its regulatory prominence in neurogenesis and its implications in Alzheimer's Disease pathogenesis. The emerging nexus between the Golgi apparatus and SARS-CoV-2 underscores its potential in viral replication processes. This consolidation accentuates the Golgi apparatus's centrality in neurobiology and its intersections with both neurodegenerative and viral pathologies. In essence, understanding the Golgi's multifaceted functions harbors profound implications for future therapeutic innovations in neurological and viral afflictions.

Patronin/CAMSAP promotes reactivation and regeneration of Drosophila quiescent neural stem cells

The ability of stem cells to switch between quiescent and proliferative states is crucial for maintaining tissue homeostasis and regeneration. Drosophila quiescent neural stem cells (qNSCs) extend a primary protrusion that is enriched in acentrosomal microtubules and can be regenerated upon injury. Arf1 promotes microtubule growth, reactivation (exit from quiescence), and regeneration of qNSC protrusions upon injury. However, how Arf1 is regulated in qNSCs remains elusive. Here, we show that the microtubule minus-end binding protein Patronin/CAMSAP promotes acentrosomal microtubule growth and quiescent NSC reactivation. Patronin is important for the localization of Arf1 at Golgi and physically associates with Arf1, preferentially with its GDP-bound form. Patronin is also required for the regeneration of qNSC protrusion, likely via the regulation of microtubule growth. Finally, Patronin functions upstream of Arf1 and its effector Msps/XMAP215 to target the cell adhesion molecule E-cadherin to NSC-neuropil contact sites during NSC reactivation. Our findings reveal a novel link between Patronin/CAMSAP and Arf1 in the regulation of microtubule growth and NSC reactivation. A similar mechanism might apply to various microtubule-dependent systems in mammals.

Microtubule-associated septin complexes modulate kinesin and dynein motility with differential specificities

Long-range membrane traffic is guided by microtubule-associated proteins and posttranslational modifications, which collectively comprise a traffic code. The regulatory principles of this code and how it orchestrates the motility of kinesin and dynein motors are largely unknown. Septins are a large family of GTP-binding proteins, which assemble into complexes that associate with microtubules. Using single-molecule in vitro motility assays, we tested how the microtubule-associated SEPT2/6/7, SEPT2/6/7/9, and SEPT5/7/11 complexes affect the motilities of the constitutively active kinesins KIF5C and KIF1A and the dynein-dynactin-bicaudal D (DDB) motor complex. We found that microtubule-associated SEPT2/6/7 is a potent inhibitor of DDB and KIF5C, preventing mainly their association with microtubules. SEPT2/6/7 also inhibits KIF1A by obstructing stepping along microtubules. On SEPT2/6/7/9-coated microtubules, KIF1A inhibition is dampened by SEPT9, which alone enhances KIF1A, showing that individual septin subunits determine the regulatory properties of septin complexes. Strikingly, SEPT5/7/11 differs from SEPT2/6/7, in permitting the motility of KIF1A and immobilizing DDB to the microtubule lattice. In hippocampal neurons, filamentous SEPT5 colocalizes with somatodendritic microtubules that underlie Golgi membranes and lack SEPT6. Depletion of SEPT5 disrupts Golgi morphology and polarization of Golgi ribbons into the shaft of somato-proximal dendrites, which is consistent with the tethering of DDB to microtubules by SEPT5/7/11. Collectively, these results suggest that microtubule-associated complexes have differential specificities in the regulation of the motility and positioning of microtubule motors. We posit that septins are an integral part of the microtubule-based code that spatially controls membrane traffic.

Golgi polarity shift instructs dendritic refinement in the neonatal cortex by mediating NMDA receptor signaling

Dendritic refinement is a critical component of activity-dependent neuronal circuit maturation, through which individual neurons establish specific connectivity with their target axons. Here, we demonstrate that the developmental shift of Golgi polarity is a key process in dendritic refinement. During neonatal development, the Golgi apparatus in layer 4 spiny stellate (SS) neurons in the mouse barrel cortex lose their original apical positioning and acquire laterally polarized distributions. This lateral Golgi polarity, which is oriented toward the barrel center, peaks on postnatal days 5-7 (P5-P7) and disappears by P15, which aligns with the developmental time course of SS neuron dendritic refinement. Genetic ablation of N-methyl-D-aspartate (NMDA) receptors, key players in dendritic refinement, disturbs the lateral Golgi polarity. Golgi polarity manipulation disrupts the asymmetric dendritic projection pattern and the primary-whisker-specific response of SS neurons. Our results elucidate activity-dependent Golgi dynamics and their critical role in developmental neuronal circuit refinement.

Lipidation states orchestrate CLICK-III/CaMKIγ's stepwise association with Golgi and rafts-enriched membranes and specify its functional coupling to STEF-Rac1-dependent neurite extension

CLICK-III/CaMKIγ is a lipid-anchored neuronal isoform of multifunctional Ca2+/calmodulin-dependent protein kinases, which mediates BDNF-dependent dendritogenesis in cultured cortical neurons. We found that two distinct lipidation states of CaMKIγ, namely, prenylation and palmitoylation, controlled its association with detergent-resistant microdomains in the dendrites and were essential for its dendritogenic activity. However, the impact of each lipid modification on membrane targeting/trafficking and how it specifies functional coupling leading to polarized changes in neuronal morphology are not clear. Here, we show that prenylation induces membrane anchoring of CaMKIγ, permitting access to the Golgi apparatus, and a subsequent palmitoylation facilitates association with cholesterol-enriched lipid microdomains or lipid rafts, in particular at the Golgi. To specifically test the role of palmitoylated CaMKγ in neurite extension, we identified and took advantage of a cell system, PC12, which, unlike neurons, conveniently lacked CaMKIγ and was deficient in the activity-dependent release of a neuritogenic growth factor while possessing the ability to activate polarized rafts signaling for morphogenesis. This system allowed us to rigorously demonstrate that an activity-dependent, lipid rafts-restricted Rac activation leading to neuritogenesis could be functionally rescued by dually lipidated CaMKIγ expression, revealing that not only prenylation but also palmitoylation is essential for CaMKIγ to activate a compartmentalized STEF-Rac1 pathway. These results shed light on the significance of recruiting prenylated and palmitoylated CaMKIγ into the coalescing signalosomes at lipid rafts together with Rac1 and its specific GEF and STEF and forming a compartmentalized Ca2+ signaling pathway that underlies activity-dependent neuritogenesis and morphogenesis during axodendritic polarization critical for brain development and circuitogenesis.

The Golgi Apparatus: A Voyage through Time, Structure, Function and Implication in Neurodegenerative Disorders

This comprehensive review article dives deep into the Golgi apparatus, an essential organelle in cellular biology. Beginning with its discovery during the 19th century until today's recognition as an important contributor to cell function. We explore its unique organization and structure as well as its roles in protein processing, sorting, and lipid biogenesis, which play key roles in maintaining homeostasis in cellular biology. This article further explores Golgi biogenesis, exploring its intricate processes and dynamics that contribute to its formation and function. One key focus is its role in neurodegenerative diseases like Parkinson's, where changes to the structure or function of the Golgi apparatus may lead to their onset or progression, emphasizing its key importance in neuronal health. At the same time, we examine the intriguing relationship between Golgi stress and endoplasmic reticulum (ER) stress, providing insights into their interplay as two major cellular stress response pathways. Such interdependence provides a greater understanding of cellular reactions to protein misfolding and accumulation, hallmark features of many neurodegenerative diseases. In summary, this review offers an exhaustive examination of the Golgi apparatus, from its historical background to its role in health and disease. Additionally, this examination emphasizes the necessity of further research in this field in order to develop targeted therapeutic approaches for Golgi dysfunction-associated conditions. Furthermore, its exploration is an example of scientific progress while simultaneously offering hope for developing innovative treatments for neurodegenerative disorders.

Organelle mapping in dendrites of human iPSC-derived neurons reveals dynamic functional dendritic Golgi structures

Secretory pathways within dendrites of neurons have been proposed for local transport of newly synthesized proteins. However, little is known about the dynamics of the local secretory system and whether the organelles are transient or stable structures. Here, we quantify the spatial and dynamic behavior of dendritic Golgi and endosomes during differentiation of human neurons generated from induced pluripotent stem cells (iPSCs). In early neuronal development, before and during migration, the entire Golgi apparatus transiently translocates from the soma into dendrites. In mature neurons, dynamic Golgi elements, containing cis and trans cisternae, are transported from the soma along dendrites, in an actin-dependent process. Dendritic Golgi outposts are dynamic and display bidirectional movement. Similar structures were observed in cerebral organoids. Using the retention using selective hooks (RUSH) system, Golgi resident proteins are transported efficiently into Golgi outposts from the endoplasmic reticulum. This study reveals dynamic, functional Golgi structures in dendrites and a spatial map for investigating dendrite trafficking in human neurons.

A distinct Golgi-targeting mechanism of dGM130 in Drosophila neurons

GM130 is a matrix protein that is conserved in metazoans and involved in the architecture of the Golgi apparatus. In neurons, Golgi apparatus and dendritic Golgi outposts (GOs) have different compartmental organizations, and GM130 localization is present in both, indicating that GM130 has a unique Golgi-targeting mechanism. Here, we investigated the Golgi-targeting mechanism of the GM130 homologue, dGM130, using in vivo imaging of Drosophila dendritic arborization (da) neurons. The results showed that two independent Golgi-targeting domains (GTDs) with different Golgi localization characteristics in dGM130, together determined the precise localization of dGM130 in both the soma and dendrites. GTD1, covering the first coiled-coil region, preferentially targeted to somal Golgi rather than GOs; whereas GTD2, containing the second coiled-coil region and C-terminus, dynamically targeted to Golgi in both soma and dendrites. These findings suggest that there are two distinct mechanisms by which dGM130 targets to the Golgi apparatus and GOs, underlying the structural differences between them, and further provides new insights into the formation of neuronal polarity.

Palmitoylation regulates neuropilin-2 localization and function in cortical neurons and conveys specificity to semaphorin signaling via palmitoyl acyltransferases

Secreted semaphorin 3F (Sema3F) and semaphorin 3A (Sema3A) exhibit remarkably distinct effects on deep layer excitatory cortical pyramidal neurons; Sema3F mediates dendritic spine pruning, whereas Sema3A promotes the elaboration of basal dendrites. Sema3F and Sema3A signal through distinct holoreceptors that include neuropilin-2 (Nrp2)/plexinA3 (PlexA3) and neuropilin-1 (Nrp1)/PlexA4, respectively. We find that Nrp2 and Nrp1 are S-palmitoylated in cortical neurons and that palmitoylation of select Nrp2 cysteines is required for its proper subcellular localization, cell surface clustering, and also for Sema3F/Nrp2-dependent dendritic spine pruning in cortical neurons, both in vitro and in vivo. Moreover, we show that the palmitoyl acyltransferase ZDHHC15 is required for Nrp2 palmitoylation and Sema3F/Nrp2-dependent dendritic spine pruning, but it is dispensable for Nrp1 palmitoylation and Sema3A/Nrp1-dependent basal dendritic elaboration. Therefore, palmitoyl acyltransferase-substrate specificity is essential for establishing compartmentalized neuronal structure and functional responses to extrinsic guidance cues.

Golgi organization is a determinant of stem cell function in the small intestine

Cell-to-cell signalling between niche and stem cells regulates tissue regeneration. While the identity of many mediating factors is known, it is largely unknown whether stem cells optimize their receptiveness to niche signals according to the niche organization. Here, we show that Lgr5+ small intestinal stem cells (ISCs) regulate the morphology and orientation of their secretory apparatus to match the niche architecture, and to increase transport efficiency of niche signal receptors. Unlike the progenitor cells lacking lateral niche contacts, ISCs orient Golgi apparatus laterally towards Paneth cells of the epithelial niche, and divide Golgi into multiple stacks reflecting the number of Paneth cell contacts. Stem cells with a higher number of lateral Golgi transported Epidermal growth factor receptor (Egfr) with a higher efficiency than cells with one Golgi. The lateral Golgi orientation and enhanced Egfr transport required A-kinase anchor protein 9 (Akap9), and was necessary for normal regenerative capacity in vitro . Moreover, reduced Akap9 in aged ISCs renders ISCs insensitive to niche-dependent modulation of Golgi stack number and transport efficiency. Our results reveal stem cell-specific Golgi complex configuration that facilitates efficient niche signal reception and tissue regeneration, which is compromised in the aged epithelium.

Microglia reactivity entails microtubule remodeling from acentrosomal to centrosomal arrays

Microglia reactivity entails a large-scale remodeling of cellular geometry, but the behavior of the microtubule cytoskeleton during these changes remains unexplored. Here we show that activated microglia provide an example of microtubule reorganization from a non-centrosomal array of parallel and stable microtubules to a radial array of more dynamic microtubules. While in the homeostatic state, microglia nucleate microtubules at Golgi outposts, and activating signaling induces recruitment of nucleating material nearby the centrosome, a process inhibited by microtubule stabilization. Our results demonstrate that a hallmark of microglia reactivity is a striking remodeling of the microtubule cytoskeleton and suggest that while pericentrosomal microtubule nucleation may serve as a distinct marker of microglia activation, inhibition of microtubule dynamics may provide a different strategy to reduce microglia reactivity in inflammatory disease.

Live Imaging of Golgi Outposts in Drosophila Dendritic Arbors

Neurons are polarized cells whose polarity and morphology rely on the robust localization of cellular organelles and cargo to axons or dendrites. Developing neurons require an active secretory pathway, which includes the endoplasmic reticulum and Golgi apparatus, to supply membrane and proteins to growing dendrites and axons. In some neurons, a subset of the Golgi called Golgi "outposts" localize to dendrites and contribute to local secretory networks. The movement and positioning of Golgi outposts have been correlated with dendrite branch growth and stabilization as the dendritic arbor is established. Live imaging is essential to capture the dynamic nature of these organelles. Here we outline a protocol to image and quantify Golgi outposts in peripheral sensory neurons in live, intact Drosophila larvae.

Schizophrenia-associated Mitotic Arrest Deficient-1 (MAD1) regulates the polarity of migrating neurons in the developing neocortex

Although large-scale genome-wide association studies (GWAS) have identified an association between MAD1L1 (Mitotic Arrest Deficient-1 Like 1) and the pathology of schizophrenia, the molecular mechanisms underlying this association remain unclear. In the present study, we aimed to address these mechanisms by examining the role of MAD1 (the gene product of MAD1L1) in key neurodevelopmental processes in mice and human organoids. Our findings indicated that MAD1 is highly expressed during active cortical development and that MAD1 deficiency leads to impairments in neuronal migration and neurite outgrowth. We also observed that MAD1 is localized to the Golgi apparatus and regulates vesicular trafficking from the Golgi apparatus to the plasma membrane, which is required for the growth and polarity of migrating neurons. In this process, MAD1 physically interacts and collaborates with the kinesin-like protein KIFC3 (kinesin family member C3) to regulate the morphology of the Golgi apparatus and neuronal polarity, thereby ensuring proper neuronal migration and differentiation. Consequently, our findings indicate that MAD1 is an essential regulator of neuronal development and that alterations in MAD1 may underlie schizophrenia pathobiology.

The function of Golgi apparatus in LRRK2-associated Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disease associated with the intracellular organelles. Leucine-rich repeat kinase 2 (LRRK2) is a large multi-structural domain protein, and mutation in LRRK2 is associated with PD. LRRK2 regulates intracellular vesicle transport and function of organelles, including Golgi and lysosome. LRRK2 phosphorylates a group of Rab GTPases, including Rab29, Rab8, and Rab10. Rab29 acts in a common pathway with LRRK2. Rab29 has been shown to recruit LRRK2 to the Golgi complex (GC) to stimulate LRRK2 activity and alter the Golgi apparatus (GA). Interaction between LRRK2 and Vacuolar protein sorting protein 52 (VPS52), a subunit of the Golgi-associated retrograde protein (GARP) complex, mediates the function of intracellular soma trans-Golgi network (TGN) transport. VPS52 also interacts with Rab29. Knockdown of VPS52 leads to the loss of LRRK2/Rab29 transported to the TGN. Rab29, LRRK2, and VPS52 work together to regulate functions of the GA, which is associated with PD. We highlight recent advances in the roles of LRRK2, Rabs, VPS52, and other molecules, such as Cyclin-dependent kinase 5 (CDK5) and protein kinase C (PKC) in the GA, and discuss their possible association with the pathological mechanisms of PD.

The exocyst complex is required for developmental and regenerative neurite growth in vivo

The exocyst complex is an important regulator of intracellular trafficking and tethers secretory vesicles to the plasma membrane. Understanding of its role in neuron outgrowth remains incomplete, and previous studies have come to different conclusions about its importance for axon and dendrite growth, particularly in vivo. To investigate exocyst function in vivo we used Drosophila sensory neurons as a model system. To bypass early developmental requirements in other cell types, we used neuron-specific RNAi to target seven exocyst subunits. Initial neuronal development proceeded normally in these backgrounds, however, we considered this could be due to residual exocyst function. To probe neuronal growth capacity at later times after RNAi initiation, we used laser microsurgery to remove axons or dendrites and prompt regrowth. Exocyst subunit RNAi reduced axon regeneration, although new axons could be specified. In control neurons, a vesicle trafficking marker often concentrated in the new axon, but this pattern was disrupted in Sec6 RNAi neurons. Dendrite regeneration was also severely reduced by exocyst RNAi, even though the trafficking marker did not accumulate in a strongly polarized manner during normal dendrite regeneration. The requirement for the exocyst was not limited to injury contexts as exocyst subunit RNAi eliminated dendrite regrowth after developmental pruning. We conclude that the exocyst is required for injury-induced and developmental neurite outgrowth, but that residual protein function can easily mask this requirement.

Contributions of microtubule dynamics and transport to presynaptic and postsynaptic functions

Microtubules (MT) are elongated, tubular, cytoskeletal structures formed from polymerization of tubulin dimers. They undergo continuous cycles of polymerization and depolymerization, primarily at their plus ends, termed dynamic instability. Although this is an intrinsic property of MTs, there are a myriad of MT-associated proteins that function in regulating MT dynamic instability and other dynamic processes that shape the MT array. Additionally, MTs assemble into long, semi-rigid structures which act as substrates for long-range, motor-driven transport of many different types of cargoes throughout the cell. Both MT dynamics and motor-based transport play important roles in the function of every known type of cell. Within the last fifteen years many groups have shown that MT dynamics and transport play ever-increasing roles in the neuronal function of mature neurons. Not only are neurons highly polarized cells, but they also connect with one another through synapses to form complex networks. Here we will focus on exciting studies that have illuminated how MTs function both pre-synaptically in axonal boutons and post-synaptically in dendritic spines. It is becoming clear that MT dynamics and transport both serve important functions in synaptic plasticity. Thus, it is not surprising that disruption of MTs, either through hyperstabilization or destabilization, has profound consequences for learning and memory. Together, the studies described here suggest that MT dynamics and transport play key roles in synaptic function and when disrupted result in compromised learning and memory.

Primary neurons lacking the SNAREs vti1a and vti1b show altered neuronal development

BACKGROUND

Neurons are highly specialized cells with a complex morphology generated by various membrane trafficking steps. They contain Golgi outposts in dendrites, which are formed from somatic Golgi tubules. In trafficking membrane fusion is mediated by a specific combination of SNARE proteins. A functional SNARE complex contains four different helices, one from each SNARE subfamily (R-, Qa, Qb and Qc). Loss of the two Qb SNAREs vti1a and vti1b from the Golgi apparatus and endosomes leads to death at birth in mice with massive neurodegeneration in peripheral ganglia and defective axon tracts.

METHODS

Hippocampal and cortical neurons were isolated from Vti1a^-/- Vti1b^-/- double deficient, Vti1a^-/- Vti1b^+/-, Vti1a^+/- Vti1b^-/- and Vti1a^+/- Vti1b^+/- double heterozygous embryos. Neurite outgrowth was determined in cortical neurons and after stimulation with several neurotrophic factors or the Rho-associated protein kinase ROCK inhibitor Y27632, which induces exocytosis of enlargeosomes, in hippocampal neurons. Moreover, postsynaptic densities were isolated from embryonic Vti1a^-/- Vti1b^-/- and Vti1a^+/- Vti1b^+/- control forebrains and analyzed by western blotting.

RESULTS

Golgi outposts were present in Vti1a^-/- Vti1b^+/- and Vti1a^+/- Vti1b^-/- dendrites of hippocampal neurons but not detected in the absence of vti1a and vti1b. The length of neurites was significantly shorter in double deficient cortical neurons. These defects were not observed in Vti1a^-/- Vti1b^+/- and Vti1a^+/- Vti1b^-/- neurons. NGF, BDNF, NT-3, GDNF or Y27632 as stimulator of enlargeosome secretion did not increase the neurite length in double deficient hippocampal neurons. Vti1a^-/- Vti1b^-/- postsynaptic densities contained similar amounts of scaffold proteins, AMPA receptors and NMDA receptors compared to Vti1a^+/- Vti1b^+/-, but much more TrkB, which is the receptor for BDNF.

CONCLUSION

The absence of Golgi outposts did not affect the amount of AMPA and NMDA receptors in postsynaptic densities. Even though TrkB was enriched, BDNF was not able to stimulate neurite elongation in Vti1a^-/- Vti1b^-/- neurons. Vti1a or vti1b function as the missing Qb-SNARE together with VAMP-4 (R-SNARE), syntaxin 16 (Qa-SNARE) and syntaxin 6 (Qc-SNARE) in induced neurite outgrowth. Our data show the importance of vti1a or vti1b for two pathways of neurite elongation.

PP2A phosphatase regulates cell-type specific cytoskeletal organization to drive dendrite diversity

Uncovering molecular mechanisms regulating dendritic diversification is essential to understanding the formation and modulation of functional neural circuitry. Transcription factors play critical roles in promoting dendritic diversity and here, we identify PP2A phosphatase function as a downstream effector of Cut-mediated transcriptional regulation of dendrite development. Mutant analyses of the PP2A catalytic subunit (mts) or the scaffolding subunit (PP2A-29B) reveal cell-type specific regulatory effects with the PP2A complex required to promote dendritic growth and branching in Drosophila Class IV (CIV) multidendritic (md) neurons, whereas in Class I (CI) md neurons, PP2A functions in restricting dendritic arborization. Cytoskeletal analyses reveal requirements for Mts in regulating microtubule stability/polarity and F-actin organization/dynamics. In CIV neurons, mts knockdown leads to reductions in dendritic localization of organelles including mitochondria and satellite Golgi outposts, while CI neurons show increased Golgi outpost trafficking along the dendritic arbor. Further, mts mutant neurons exhibit defects in neuronal polarity/compartmentalization. Finally, genetic interaction analyses suggest β-tubulin subunit 85D is a common PP2A target in CI and CIV neurons, while FoxO is a putative target in CI neurons.

Dominant ARF3 variants disrupt Golgi integrity and cause a neurodevelopmental disorder recapitulated in zebrafish

Vesicle biogenesis, trafficking and signaling via Endoplasmic reticulum-Golgi network support essential developmental processes and their disruption lead to neurodevelopmental disorders and neurodegeneration. We report that de novo missense variants in ARF3, encoding a small GTPase regulating Golgi dynamics, cause a developmental disease in humans impairing nervous system and skeletal formation. Microcephaly-associated ARF3 variants affect residues within the guanine nucleotide binding pocket and variably perturb protein stability and GTP/GDP binding. Functional analysis demonstrates variably disruptive consequences of ARF3 variants on Golgi morphology, vesicles assembly and trafficking. Disease modeling in zebrafish validates further the dominant behavior of the mutants and their differential impact on brain and body plan formation, recapitulating the variable disease expression. In-depth in vivo analyses traces back impaired neural precursors' proliferation and planar cell polarity-dependent cell movements as the earliest detectable effects. Our findings document a key role of ARF3 in Golgi function and demonstrate its pleiotropic impact on development.

ER and Golgi trafficking in axons, dendrites, and glial processes

Both neurons and glia in mammalian brains are highly ramified. Neurons form complex neural networks using axons and dendrites. Axons are long with few branches and form pre-synaptic boutons that connect to target neurons and effector tissues. Dendrites are shorter, highly branched, and form post-synaptic boutons. Astrocyte processes contact synapses and blood vessels in order to regulate neuronal activity and blood flow, respectively. Oligodendrocyte processes extend toward axons to make myelin sheaths. Microglia processes dynamically survey their environments. Here, we describe the local secretory system (ER and Golgi) in neuronal and glial processes. We focus on Golgi outpost functions in acentrosomal microtubule nucleation, cargo trafficking, and protein glycosylation. Thus, satellite ER and Golgi are critical for local structure and function in neurons and glia.

Fringe-positive Golgi outposts unite temporal Furin 2 convertase activity and spatial Delta signal to promote dendritic branch retraction

Golgi outposts (GOPs) in dendrites are known for their role in promoting branch extension, but whether GOPs have other functions is unclear. We found that terminal branches of Drosophila class IV dendritic arborization (C4da) neurons actively grow during the early third-instar (E3) larval stage but retract in the late third (L3) stage. Interestingly, the Fringe (Fng) glycosyltransferase localizes increasingly at GOPs in distal dendritic regions through the E3 to the L3 stage. Expression of the endopeptidase Furin 2 (Fur2), which proteolyzes and inactivates Fng, decreases from E3 to L3 in C4da neurons, thereby increasing Fng-positive GOPs in dendrites. The epidermal Delta ligand and neuronal Notch receptor, the substrate for Fng-mediated O-glycosylation, also negatively regulate dendrite growth. Fng inhibits actin dynamics in dendrites, linking dendritic branch retraction to suppression of the C4da-mediated thermal nociception response in late larval stages. Thus, Fng-positive GOPs function in dendrite retraction, which would add another function to the repertoire of GOPs in dendrite arborization.

Regulators of the secretory pathway have distinct inputs into single-celled branching morphogenesis and seamless tube formation in the Drosophila trachea

Biological tubes serve as conduits through which gas, nutrients and other important fluids are delivered to tissues. Most biological tubes consist of multiple cells connected by epithelial junctions. Unlike these multicellular tubes, seamless tubes are unicellular and lack junctions. Seamless tubes are present in various organ systems, including the vertebrate vasculature, C.elegans excretory system, and Drosophila tracheal system. The Drosophila tracheal system is a network of air-filled tubes that delivers oxygen to all tissues. Specialized cells within the tracheal system, called terminal cells, branch extensively and form seamless tubes. Terminal tracheal tubes are polarized; the lumenal membrane has apical identity whereas the outer membrane exhibits basal characteristics. Although various aspects of membrane trafficking have been implicated in terminal cell morphogenesis, the precise secretory pathway requirements for basal and apical membrane growth have yet to be elucidated. In the present study, we demonstrate that anterograde trafficking, retrograde trafficking and Golgi-to-plasma membrane vesicle fusion are each required for the complex branched architecture of the terminal cell, but their inputs during seamless lumen formation are more varied. The COPII subunit, Sec31, and ER exit site protein, Sec16, are critical for subcellular tube architecture, whereas the SNARE proteins Syntaxin 5, Syntaxin 1 and Syntaxin 18 are more generally required for seamless tube growth and maintenance. These data suggest that distinct components of the secretory pathway have differential contributions to basal and apical membrane growth and maintenance during terminal cell morphogenesis.

Directed mechanisms for apical dendrite development during neuronal polarization

The development of the dendrite and the axon during neuronal polarization underlies the directed flow of information in the brain. Seminal studies on axon development have dominated the mechanistic analysis of neuronal polarization. These studies, many originating from examinations in cultured hippocampal and cortical neurons in vitro, have established a prevalent view that axon formation precedes and is necessary for neuronal polarization. There is also in vivo evidence supporting this view. Nevertheless, the establishment of bipolar polarity and the leading edge, and apical dendrite development in pyramidal neurons in vivo occur when axon formation is prevented. Furthermore, recent mounting evidence suggest that directed mechanisms might mediate bipolar polarity/leading process and subsequent apical dendrite development. In the presence of spatially directed extracellular cues in the developing brain, these events may operate independently of axon forming events. In this perspective we summarize evidence in support of these evolving views in neuronal polarization and highlight recent findings on dedicated mechanisms acting in apical dendrite development.

Microtubule polarity is instructive for many aspects of neuronal polarity

Many neurons in bilaterian animals are polarized with functionally distinct axons and dendrites. Microtubule polarity, microtubule stability, and the axon initial segment (AIS) have all been shown to influence polarized transport in neurons. Each of these cytoskeletal cues could act independently to control axon and dendrite identity, or there could be a hierarchy in which one acts upstream of the others. Here we test the hypothesis that microtubule polarity acts as a master regulator of neuronal polarity by using a Drosophila genetic background in which some dendrites have normal minus-end-out microtubule polarity and others have the axonal plus-end-out polarity. In these mosaic dendrite arbors, we found that ribosomes, which are more abundant in dendrites than axons, were reduced in plus-end-out dendrites, while an axonal cargo was increased. In addition, we determined that microtubule stability was different in plus-end-out and minus-end-out dendrites, with plus-end-out ones having more stable microtubules like axons. Similarly, we found that ectopic diffusion barriers, like those at the AIS, formed at the base of dendrites with plus-end-out regions. Thus, changes in microtubule polarity were sufficient to rearrange other cytoskeletal features associated with neuronal polarization. However, overall neuron shape was maintained with only subtle changes in branching in mosaic arbors. We conclude that microtubule polarity can act upstream of many aspects of intracellular neuronal polarization, but shape is relatively resilient to changes in microtubule polarity in vivo.

Neuronal mTOR Outposts: Implications for Translation, Signaling, and Plasticity

The kinase mTOR is a signaling hub for pathways that regulate cellular growth. In neurons, the subcellular localization of mTOR takes on increased significance. Here, we review findings on the localization of mTOR in axons and offer a perspective on how these may impact our understanding of nervous system development, function, and disease. We propose a model where mTOR accumulates in local foci we term mTOR outposts, which can be found in processes distant from a neuron’s cell body. In this model, pathways that funnel through mTOR are gated by local outposts to spatially select and amplify local signaling. The presence or absence of mTOR outposts in a segment of axon or dendrite may determine whether regional mTOR-dependent signals, such as nutrient and growth factor signaling, register toward neuron-wide responses. In this perspective, we present the emerging evidence for mTOR outposts in neurons, their putative roles as spatial gatekeepers of signaling inputs, and the implications of the mTOR outpost model for neuronal protein synthesis, signal transduction, and synaptic plasticity.

Doublecortin mutation leads to persistent defects in the Golgi apparatus and mitochondria in adult hippocampal pyramidal cells

Human doublecortin (DCX) mutations are associated with severe brain malformations leading to aberrant neuron positioning (heterotopia), intellectual disability and epilepsy. DCX is a microtubule-associated protein which plays a key role during neurodevelopment in neuronal migration and differentiation. Dcx knockout (KO) mice show disorganized hippocampal pyramidal neurons. The CA2/CA3 pyramidal cell layer is present as two abnormal layers and disorganized CA3 KO pyramidal neurons are also more excitable than wild-type (WT) cells. To further identify abnormalities, we characterized Dcx KO hippocampal neurons at subcellular, molecular and ultrastructural levels. Severe defects were observed in mitochondria, affecting number and distribution. Also, the Golgi apparatus was visibly abnormal, increased in volume and abnormally organized. Transcriptome analyses from laser microdissected hippocampal tissue at postnatal day 60 (P60) highlighted organelle abnormalities. Ultrastructural studies of CA3 cells performed in P60 (young adult) and > 9 months (mature) tissue showed that organelle defects are persistent throughout life. Locomotor activity and fear memory of young and mature adults were also abnormal: Dcx KO mice consistently performed less well than WT littermates, with defects becoming more severe with age. Thus, we show that disruption of a neurodevelopmentally-regulated gene can lead to permanent organelle anomalies contributing to abnormal adult behavior.

The Dendritic Ergic: Microtubule And Actin Cytoskeletons Participate In Stop-And-Go Movement Of Mobile Carriers Between Stable Structures

The ER-to-Golgi intermediate compartment (ERGIC) is a membranous organelle that mediates protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus. In neurons, clusters of these vesiculotubular structures are situated throughout the cell in proximity to the ER, passing cargo to the cis-Golgi cisternae, located mainly in the perinuclear region. Although ERGIC markers have been identified in neurons, the distribution and dynamics of neuronal ERGIC structures have not been characterized yet. Here, we show that long-distance ERGIC transport occurs via an intermittent mechanism in dendrites, with mobile elements moving between stationary structures. Slow and fast live-cell imaging have captured stable ERGIC structures remaining in place over long periods of time, as well as mobile ERGIC structures advancing very short distances along dendrites. These short distances have been consistent with the lengths between the stationary ERGIC structures. Kymography revealed ERGIC elements that moved intermittently, emerging from and fusing with stationary ERGIC structures. Interestingly, this movement apparently depends not only on the integrity of the microtubule cytoskeleton, as previously reported, but on the actin cytoskeleton as well. Our results indicate that the dendritic ERGIC has a dual nature, with both stationary and mobile structures. The neural ERGIC network transports proteins via a stop-and-go movement in which both the microtubule and the actin cytoskeletons participate. This article is protected by copyright. All rights reserved.

NgCAM and VAMP2 reveal that direct delivery and dendritic degradation maintain axonal polarity

Neurons are polarized cells of extreme scale and compartmentalization. To fulfill their role in electrochemical signaling, axons must maintain a specific complement of membrane proteins. Despite being the subject of considerable attention, the trafficking pathway of axonal membrane proteins is not well understood. Two pathways, direct delivery and transcytosis, have been proposed. Previous studies reached contradictory conclusions about which of these mediates delivery of axonal membrane proteins to their destination, in part because they evaluated long-term distribution changes and not vesicle transport. We developed a novel strategy to selectively label vesicles in different trafficking pathways and determined the trafficking of two canonical axonal membrane proteins, neuron-glia cell adhesion molecule and vesicle-associated membrane protein-2. Results from detailed quantitative analyses of transporting vesicles differed substantially from previous studies and found that axonal membrane proteins overwhelmingly undergo direct delivery. Transcytosis plays only a minor role in axonal delivery of these proteins. In addition, we identified a novel pathway by which wayward axonal proteins that reach the dendritic plasma membrane are targeted to lysosomes. These results redefine how axonal proteins achieve their polarized distribution, a crucial requirement for elucidating the underlying molecular mechanisms.

Protein kinase D promotes activity-dependent AMPA receptor endocytosis in hippocampal neurons

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors (AMPARs) mediate the majority of fast excitatory neurotransmission in the brain. The continuous trafficking of AMPARs into and out of synapses is a core feature of synaptic plasticity, which is considered as the cellular basis of learning and memory. The molecular mechanisms underlying the postsynaptic AMPAR trafficking, however, are still not fully understood. In this work, we demonstrate that the protein kinase D (PKD) family promotes basal and activity-induced AMPAR endocytosis in primary hippocampal neurons. Pharmacological inhibition of PKD increased synaptic levels of GluA1-containing AMPARs, slowed down their endocytic trafficking and increased neuronal network activity. By contrast, ectopic expression of constitutive active PKD decreased the synaptic level of AMPARs, while increasing their colocalization with early endosomes. Our results thus establish an important role for PKD in the regulation of postsynaptic AMPAR trafficking during synaptic plasticity.

CREB3L2 Modulates Nerve Growth Factor-Induced Cell Differentiation

Nerve growth factor (NGF) stimulates numerous cellular physiological processes, including growth, differentiation, and survival, and maintains the phenotype of several neuronal types. Most of these NGF-induced processes require adaptation of the secretory pathway since they involve extensive remodeling of membranes and protein redistribution along newly formed neuritic processes. CREB3 transcription factors have emerged as signaling hubs for the regulation of numerous genes involved in the secretory pathway and Golgi homeostasis, integrating stimuli from multiple sources to control secretion, posttranslational modifications and trafficking of proteins. Although recent studies have focused on their role in the central nervous system, little is known about their participation in cell differentiation. Therefore, we aimed to analyze the expression and signaling mechanism of CREB3 transcription factor family members, using the NGF-induced PC12 cell differentiation model. Results show that NGF treatment causes Golgi enlargement and a parallel increased expression of proteins and mRNAs encoding for proteins required for membrane transport (transport factors). Additionally, a significant increase in CREB3L2 protein and mRNA levels is detected in response to NGF. Both MAPK and cAMP signaling pathways are required for this response. Interestingly, CREB3L2 overexpression hampers the NGF-induced neurite outgrowth while its inhibition enhances the morphological changes driven by NGF. In agreement, CREB3L2 overexpressing cells display higher immunofluorescence intensity of Rab5 GTPase (a negative regulator of PC12 differentiation) than control cells. Also, Rab5 immunofluorescence levels decrease in CREB3L2-depleted cells. Taken together, our findings imply that CREB3L2 is an important downstream effector of NGF-activated pathways, leading to neuronal differentiation.

The Golgi-Associated PDZ Domain Protein Gopc/PIST Is Required for Synaptic Targeting of mGluR5

In neuronal cells, many membrane receptors interact via their intracellular, C-terminal tails with PSD-95/discs large/ZO-1 (PDZ) domain proteins. Some PDZ proteins act as scaffold proteins. In addition, there are a few PDZ proteins such as Gopc which bind to receptors during intracellular transport. Gopc is localized at the trans-Golgi network (TGN) and binds to a variety of receptors, many of which are eventually targeted to postsynaptic sites. We have analyzed the role of Gopc by knockdown in primary cultured neurons and by generating a conditional Gopc knockout (KO) mouse line. In neurons, targeting of neuroligin 1 (Nlgn1) and metabotropic glutamate receptor 5 (mGlu5) to the plasma membrane was impaired upon depletion of Gopc, whereas NMDA receptors were not affected. In the hippocampus and cortex of Gopc KO animals, expression levels of Gopc-associated receptors were not altered, while their subcellular localization was disturbed. The targeting of mGlu5 to the postsynaptic density was reduced, coinciding with alterations in mGluR-dependent synaptic plasticity and deficiencies in a contextual fear conditioning paradigm. Our data imply Gopc in the correct subcellular sorting of its associated mGlu5 receptor in vivo.

Maternal Ethanol Exposure Acutely Elevates Src Family Kinase Activity in the Fetal Cortex

Fetal alcohol syndrome (FAS) is characterized by disrupted fetal brain development and postnatal cognitive impairment. The targets of alcohol are diverse, and it is not clear whether there are common underlying molecular mechanisms producing these disruptions. Prior work established that acute ethanol exposure causes a transient increase in tyrosine phosphorylation of multiple proteins in cultured embryonic cortical cells. In this study, we show that a similar tyrosine phosphorylation transient occurs in the fetal brain after maternal dosing with ethanol. Using phospho-specific antibodies and immunohistochemistry, we mapped regions of highest tyrosine phosphorylation in the fetal cerebral cortex and found that areas of dendritic and axonal growth showed elevated tyrosine phosphorylation 10 min after maternal ethanol exposure. These were also areas of Src expression and Src family kinase (SFK) activation loop phosphorylation (pY416) expression. Importantly, maternal pretreatment with the SFK inhibitor dasatinib completely prevents both the pY416 increase and the tyrosine phosphorylation response. The phosphorylation response was observed in the perisomatic region and neurites of immature migrating and differentiating primary neurons. Importantly, the initial phosphotyrosine transient (~ 30 min) targets both Src and Dab1, two critical elements in Reelin signaling, a pathway required for normal cortical development. This initial phosphorylation response is followed by sustained reduction in Ser3 phosphorylation of n-cofilin, a critical actin severing protein and an identified downstream effector of Reelin signaling. This biochemical disruption is associated with sustained reduction of F-actin content and disrupted Golgi apparatus morphology in developing cortical neurons. The finding outlines a model in which the initial activation of SFKs by ethanol has the potential to disrupt multiple developmentally important signaling systems for several hours after maternal exposure.

Amyloid β production along the neuronal secretory pathway: Dangerous liaisons in the Golgi?

β-amyloid peptides (Aβ) are generated in intracellular compartments of neurons and secreted to form cytotoxic fibrils and plaques. Dysfunctional membrane trafficking contributes to aberrant Aβ production and Alzheimer's disease. Endosomes represent one of the major sites for Aβ production and recently the Golgi has re-emerged also as a major location for amyloid precursor protein (APP) processing and Aβ production. Based on recent findings, here we propose that APP processing in the Golgi is finely tuned by segregating newly-synthesised APP and the β-secretase BACE1 within the Golgi and into distinct trans-Golgi network transport pathways. We hypothesise that there are multiple mechanisms responsible for segregating APP and BACE1 during transit through the Golgi, and that perturbation in Golgi morphology associated with Alzheimer's disease, and or changes in cholesterol metabolism associated with Alzheimer's disease risk factors, may lead to a loss of partitioning and enhanced Aβ production.

zapERtrap: A light-regulated ER release system reveals unexpected neuronal trafficking pathways

Here we introduce zapalog-mediated endoplasmic reticulum trap (zapERtrap), which allows one to use light to precisely trigger forward trafficking of diverse integral membrane proteins from internal secretory organelles to the cell surface with single cell and subcellular spatial resolution. To demonstrate its utility, we use zapERtrap in neurons to dissect where synaptic proteins emerge at the cell surface when processed through central (cell body) or remote (dendrites) secretory pathways. We reveal rapid and direct long-range trafficking of centrally processed proteins deep into the dendritic arbor to synaptic sites. Select proteins were also trafficked to the plasma membrane of the axon initial segment, revealing a novel surface trafficking hotspot. Proteins locally processed through dendritic secretory networks were widely dispersed before surface insertion, challenging assumptions for precise trafficking at remote sites. These experiments provide new insights into compartmentalized secretory trafficking and showcase the tunability and spatiotemporal control of zapERtrap, which will have broad applications for regulating cell signaling and function.

Amyloid-β Impairs Dendritic Trafficking of Golgi-Like Organelles in the Early Phase Preceding Neurite Atrophy: Rescue by Mirtazapine

Neurite atrophy with loss of neuronal polarity is a pathological hallmark of Alzheimer's disease (AD) and other neurological disorders. While there is substantial agreement that disruption of intracellular vesicle trafficking is associated with axonal pathology in AD, comparatively less is known regarding its role in dendritic atrophy. This is a significant gap of knowledge because, unlike axons, dendrites are endowed with the complete endomembrane system comprising endoplasmic reticulum (ER), ER-Golgi intermediate compartment (ERGIC), Golgi apparatus, post-Golgi vesicles, and a recycling-degradative route. In this study, using live-imaging of pGOLT-expressing vesicles, indicative of Golgi outposts and satellites, we investigate how amyloid-β (Aβ) oligomers affect the trafficking of Golgi-like organelles in the different dendritic compartments of cultured rat hippocampal neurons. We found that short-term (4 h) treatment with Aβ led to a decrease in anterograde trafficking of Golgi vesicles in dendrites of both resting and stimulated (with 50 mM KCl) neurons. We also characterized the ability of mirtazapine, a noradrenergic and specific serotonergic tetracyclic antidepressant (NaSSA), to rescue Golgi dynamics in dendrites. Mirtazapine treatment (10 μM) increased the number and both anterograde and retrograde motility, reducing the percentage of static Golgi vesicles. Finally, mirtazapine reverted the neurite atrophy induced by 24 h treatment with Aβ oligomers, suggesting that this drug is able to counteract the effects of Aβ by improving the dendritic trafficking of Golgi-related vesicles.

Sunday Driver Mediates Multi-Compartment Golgi Outposts Defects Induced by Amyloid Precursor Protein

Golgi defects including Golgi fragmentation are pathological features of Alzheimer’s disease (AD). As a pathogenic factor in AD, amyloid precursor protein (APP) induces Golgi fragmentation in the soma. However, how APP regulates Golgi outposts (GOs) in dendrites remains unclear. Given that APP resides in and affects the movements of GOs, and in particular, reverses the distribution of multi-compartment GOs (mcGOs), we investigated the regulatory mechanism of mcGO movements in the Drosophila larvae. Knockdown experiments showed that the bidirectional mcGO movements were cooperatively controlled by the dynein heavy chain (Dhc) and kinesin heavy chain subunits. Notably, only Dhc mediated APP’s regulation of mcGO movements. Furthermore, by loss-of-function screening, the adaptor protein Sunday driver (Syd) was identified to mediate the APP-induced alteration of the direction of mcGO movements and dendritic defects. Collectively, by elucidating a model of bidirectional mcGO movements, we revealed the mechanism by which APP regulates the direction of mcGO movements. Our study therefore provides new insights into AD pathogenesis.

The neuronal ceroid lipofuscinosis-related protein CLN8 regulates endo-lysosomal dynamics and dendritic morphology

BACKGROUND INFORMATION

The endo-lysosomal system (ELS) comprises a set of membranous organelles responsible for transporting intracellular and extracellular components within cells. Defects in lysosomal proteins usually affect a large variety of processes and underlie many diseases, most of them with a strong neuronal impact. Mutations in the endoplasmic reticulum-resident CLN8 protein cause CLN8 disease. This condition is one of the 14 known neuronal ceroid lipofuscinoses (NCLs), a group of inherited diseases characterised by accumulation of lipofuscin-like pigments within lysosomes. Besides mediating the transport of soluble lysosomal proteins, recent research suggested a role for CLN8 in the transport of vesicles and lipids, and autophagy. However, the consequences of CLN8 deficiency on ELS structure and activity, as well as the potential impact on neuronal development, remain poorly characterised. Therefore, we performed CLN8 knockdown in neuronal and non-neuronal cell models to analyse structural, dynamic and functional changes in the ELS and to assess the impact of CLN8 deficiency on axodendritic development.

RESULTS

CLN8 knockdown increased the size of the Golgi apparatus, the number of mobile vesicles and the speed of endo-lysosomes. Using the fluorescent fusion protein mApple-LAMP1-pHluorin, we detected significant lysosomal alkalisation in CLN8-deficient cells. In turn, experiments in primary rat hippocampal neurons showed that CLN8 deficiency decreased the complexity and size of the somatodendritic compartment.

CONCLUSIONS

Our results suggest the participation of CLN8 in vesicular distribution, lysosomal pH and normal development of the dendritic tree. We speculate that the defects triggered by CLN8 deficiency on ELS structure and dynamics underlie morphological alterations in neurons, which ultimately lead to the characteristic neurodegeneration observed in this NCL.

SIGNIFICANCE

This is, to our knowledge, the first characterisation of the effects of CLN8 dysfunction on the structure and dynamics of the ELS. Moreover, our findings suggest a novel role for CLN8 in somatodendritic development, which may account at least in part for the neuropathological manifestations associated with CLN8 disease.

CAMSAPs organize an acentrosomal microtubule network from basal varicosities in radial glial cells

Neurons of the neocortex are generated by stem cells called radial glial cells. These polarized cells extend a short apical process toward the ventricular surface and a long basal fiber that acts as a scaffold for neuronal migration. How the microtubule cytoskeleton is organized in these cells to support long-range transport is unknown. Using subcellular live imaging within brain tissue, we show that microtubules in the apical process uniformly emanate for the pericentrosomal region, while microtubules in the basal fiber display a mixed polarity, reminiscent of the mammalian dendrite. We identify acentrosomal microtubule organizing centers localized in varicosities of the basal fiber. CAMSAP family members accumulate in these varicosities, where they control microtubule growth. Double knockdown of CAMSAP1 and 2 leads to a destabilization of the entire basal process. Finally, using live imaging of human fetal cortex, we reveal that this organization is conserved in basal radial glial cells, a related progenitor cell population associated with human brain size expansion.

Astrocytes close a motor circuit critical period

Critical periods-brief intervals during which neural circuits can be modified by activity-are necessary for proper neural circuit assembly. Extended critical periods are associated with neurodevelopmental disorders; however, the mechanisms that ensure timely critical period closure remain poorly understood^1,2. Here we define a critical period in a developing Drosophila motor circuit and identify astrocytes as essential for proper critical period termination. During the critical period, changes in activity regulate dendrite length, complexity and connectivity of motor neurons. Astrocytes invaded the neuropil just before critical period closure³, and astrocyte ablation prolonged the critical period. Finally, we used a genetic screen to identify astrocyte-motor neuron signalling pathways that close the critical period, including Neuroligin-Neurexin signalling. Reduced signalling destabilized dendritic microtubules, increased dendrite dynamicity and impaired locomotor behaviour, underscoring the importance of critical period closure. Previous work defined astroglia as regulators of plasticity at individual synapses⁴; we show here that astrocytes also regulate motor circuit critical period closure to ensure proper locomotor behaviour.

Molecular mechanisms that mediate dendrite morphogenesis

Neurons develop dendritic morphologies that bear cell type-specific features in dendritic field size and geometry, branch placement and density, and the types and distributions of synaptic contacts. Dendritic patterns influence the types and numbers of inputs a neuron receives, and the ways in which neural information is processed and transmitted in the circuitry. Even subtle alterations in dendritic structures can have profound consequences on neuronal function and are implicated in neurodevelopmental disorders. In this chapter, I review how growing dendrites acquire their exquisite patterns by drawing examples from diverse neuronal cell types in vertebrate and invertebrate model systems. Dendrite morphogenesis is shaped by intrinsic and extrinsic factors such as transcriptional regulators, guidance and adhesion molecules, neighboring cells and synaptic partners. I discuss molecular mechanisms that regulate dendrite morphogenesis with a focus on five aspects of dendrite patterning: (1) Dendritic cytoskeleton and cellular machineries that build the arbor; (2) Gene regulatory mechanisms; (3) Afferent cues that regulate dendritic arbor growth; (4) Space-filling strategies that optimize dendritic coverage; and (5) Molecular cues that specify dendrite wiring. Cell type-specific implementation of these patterning mechanisms produces the diversity of dendrite morphologies that wire the nervous system.

YIF1B mutations cause a post-natal neurodevelopmental syndrome associated with Golgi and primary cilium alterations

Human post-natal neurodevelopmental delay is often associated with cerebral alterations that can lead, by themselves or associated with peripheral deficits, to premature death. Here, we report the clinical features of 10 patients from six independent families with mutations in the autosomal YIF1B gene encoding a ubiquitous protein involved in anterograde traffic from the endoplasmic reticulum to the cell membrane, and in Golgi apparatus morphology. The patients displayed global developmental delay, motor delay, visual deficits with brain MRI evidence of ventricle enlargement, myelination alterations and cerebellar atrophy. A similar profile was observed in the Yif1b knockout (KO) mouse model developed to identify the cellular alterations involved in the clinical defects. In the CNS, mice lacking Yif1b displayed neuronal reduction, altered myelination of the motor cortex, cerebellar atrophy, enlargement of the ventricles, and subcellular alterations of endoplasmic reticulum and Golgi apparatus compartments. Remarkably, although YIF1B was not detected in primary cilia, biallelic YIF1B mutations caused primary cilia abnormalities in skin fibroblasts from both patients and Yif1b-KO mice, and in ciliary architectural components in the Yif1b-KO brain. Consequently, our findings identify YIF1B as an essential gene in early post-natal development in human, and provide a new genetic target that should be tested in patients developing a neurodevelopmental delay during the first year of life. Thus, our work is the first description of a functional deficit linking Golgipathies and ciliopathies, diseases so far associated exclusively to mutations in genes coding for proteins expressed within the primary cilium or related ultrastructures. We therefore propose that these pathologies should be considered as belonging to a larger class of neurodevelopmental diseases depending on proteins involved in the trafficking of proteins towards specific cell membrane compartments.