KIF1Bβ transports dendritically localized mRNPs in neurons and is recruited to synapses in an activity-dependent manner

Understanding how kinesin motor proteins regulate postsynaptic function in neuron

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

Matrix-screening reveals a vast potential for direct protein-protein interactions among RNA binding proteins

RNA-binding proteins (RBPs) are crucial factors of post-transcriptional gene regulation and their modes of action are intensely investigated. At the center of attention are RNA motifs that guide where RBPs bind. However, sequence motifs are often poor predictors of RBP-RNA interactions in vivo. It is hence believed that many RBPs recognize RNAs as complexes, to increase specificity and regulatory possibilities. To probe the potential for complex formation among RBPs, we assembled a library of 978 mammalian RBPs and used rec-Y2H matrix screening to detect direct interactions between RBPs, sampling > 600 K interactions. We discovered 1994 new interactions and demonstrate that interacting RBPs bind RNAs adjacently in vivo. We further find that the mRNA binding region and motif preferences of RBPs deviate, depending on their adjacently binding interaction partners. Finally, we reveal novel RBP interaction networks among major RNA processing steps and show that splicing impairing RBP mutations observed in cancer rewire spliceosomal interaction networks. The dataset we provide will be a valuable resource for understanding the combinatorial interactions of RBPs with RNAs and the resulting regulatory outcomes.

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

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

Cytoskeletal regulation guides neuronal trafficking to effectively supply the synapse

The development and proper function of the brain requires the formation of highly complex neuronal circuitry. These circuits are shaped from synaptic connections between neurons and must be maintained over a lifetime. The formation and continued maintenance of synapses requires accurate trafficking of presynaptic and postsynaptic components along the axon and dendrite, respectively, necessitating deliberate and specialized delivery strategies to replenish essential synaptic components. Maintenance of synaptic transmission also requires readily accessible energy stores, produced in part by localized mitochondria, that are tightly regulated with activity level. In this review, we focus on recent developments in our understanding of the cytoskeletal environment of axons and dendrites, examining how local regulation of cytoskeletal dynamics and organelle trafficking promotes synapse-specific delivery and plasticity. These new insights shed light on the complex and coordinated role that cytoskeletal elements play in establishing and maintaining neuronal circuitry.

Circular RNAs regulate its parental genes transcription in the AD mouse model using two methods of library construction

Circular RNA (circRNA) is an important class of noncoding RNA. Current, protocols to detected circRNAs have utilized nonpolyadenylated RNAs, ribosomal RNA-depleted RNA samples (rRNA- library), and rRNA-depleted RNA that has been treated with RNase R to digest linear RNA (rRNA- RNase R+ library). Accumulating evidence suggests the participation of circRNAs in Alzheimer's disease (AD)-associated pathophysiology, but the details remain largely unknown. Here, we elucidated the brain circRNAs profiles of AD-model and WT mice using two methods of library construction (rRNA- RNase R+ libraries and rRNA- libraries). We focused on the construction of libraries that best allow the identification of circRNAs from next-generation RNA sequencing data. We obtained a significantly higher abundance of circRNAs in the rRNA- RNase R+ libraries than in the rRNA- libraries. Additionally, the rRNA- RNase R+ libraries more clearly revealed differentially expressed circRNAs. We performed a correlation analysis between differentially expressed circRNAs and their parental genes and performed KEGG analysis of the parental genes to explore the role of circRNA in AD. Our results identified significantly dysregulated circRNAs and KEGG analysis revealed that the identified circRNAs are involved in regulating AD development from distinct origins, including cAMP signaling, MAPK signaling, insulin secretion, Axon guidance, Long-term potentiation, dopaminergic synapse, and ErBb signaling pathways. Following rigorous selection, we identified several important circRNAs and mRNAs and propose the circRNAs regulate parental gene transcription or affect variable splicing, which were discovered to be mainly involved in Aβ production and cognitive performance (TRPC6) and neuronal differentiation and development (NME7). These newly identified circRNAs should serve as a valuable resource for the future development of potential biomarkers and therapeutic targets for AD.

Spatial control of membrane traffic in neuronal dendrites

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

Specific depletion of the motor protein KIF5B leads to deficits in dendritic transport, synaptic plasticity and memory

The kinesin I family of motor proteins are crucial for axonal transport, but their roles in dendritic transport and postsynaptic function are not well-defined. Gene duplication and subsequent diversification give rise to three homologous kinesin I proteins (KIF5A, KIF5B and KIF5C) in vertebrates, but it is not clear whether and how they exhibit functional specificity. Here we show that knockdown of KIF5A or KIF5B differentially affects excitatory synapses and dendritic transport in hippocampal neurons. The functional specificities of the two kinesins are determined by their diverse carboxyl-termini, where arginine methylation occurs in KIF5B and regulates its function. KIF5B conditional knockout mice exhibit deficits in dendritic spine morphogenesis, synaptic plasticity and memory formation. Our findings provide insights into how expansion of the kinesin I family during evolution leads to diversification and specialization of motor proteins in regulating postsynaptic function.

Transporting molecules within a cell becomes a daunting task when the cell is a neuron, with fibers called axons and dendrites that can stretch as long as a meter. Neurons use many different molecules to send messages across the body and store memories in the brain. If the right molecules cannot be delivered along the length of nerve cells, connections to neighboring neurons may decay, which may impair learning and memory.

Motor proteins are responsible for transporting molecules within cells. Kinesins are a type of motor protein that typically transports materials from the body of a neuron to the cell’s periphery, including the dendrites, which is where a neuron receives messages from other nerve cells. Each cell has up to 45 different kinesin motors, but it is not known whether each one performs a distinct task or if they have overlapping roles.

Now, Zhao, Fok et al. have studied two similar kinesins, called KIF5A and KIF5B, in rodent neurons to determine their roles. First, it was shown that both proteins were found at dendritic spines, which are small outgrowths on dendrites where contact with other cells occurs. Next, KIF5A and KIF5B were depleted, one at a time, from neurons extracted from a brain region called the hippocampus. Removing KIF5B interfered with the formation of dendritic spines, but removing KIF5A did not have an effect. Dendritic spines are essential for learning and memory, so several behavioral tests were conducted on mice that had been genetically modified to express less KIF5B in the forebrain. These tests revealed that the mice performed poorly in tasks that tested their memory recall.

This work opens a new area of research studying the specific roles of different kinesin motor proteins in nerve cells. This could have important implications because certain kinesin motor proteins such as KIF5A are known to be defective in some inherited neurodegenerative diseases.

A novel strategy to visualize vesicle-bound kinesins reveals the diversity of kinesin-mediated transport

In mammals, 15 to 20 kinesins are thought to mediate vesicle transport. Little is known about the identity of vesicles moved by each kinesin or the functional significance of such diversity. To characterize the transport mediated by different kinesins, we developed a novel strategy to visualize vesicle-bound kinesins in living cells. We applied this method to cultured neurons and systematically determined the localization and transport parameters of vesicles labeled by different members of the Kinesin-1, -2, and -3 families. We observed vesicle labeling with nearly all kinesins. Only six kinesins bound vesicles that undergo long-range transport in neurons. Of these, three had an axonal bias (KIF5B, KIF5C and KIF13B), two were unbiased (KIF1A and KIF1Bβ), and one transported only in dendrites (KIF13A). Overall, the trafficking of vesicle-bound kinesins to axons or dendrites did not correspond to their motor domain preference, suggesting that on-vesicle regulation is crucial for kinesin targeting. Surprisingly, several kinesins were associated with populations of somatodendritic vesicles that underwent little long-range transport. This assay should be broadly applicable for investigating kinesin function in many cell types.

FIGNL1 associates with KIF1Bβ and BICD1 to restrict dynein transport velocity during axon navigation

Atkins et al. identify a new role for Fidgetin-like 1 in motor axon navigation via its regulation of bidirectional axonal transport. They show that Fidgetin-like 1 binds Kif1bβ and the opposed polarity-directed motor dynein/dynactin in a molecular complex and controls circuit wiring by reducing dynein velocity in developing motor axons.

Neuronal connectivity relies on molecular motor-based axonal transport of diverse cargoes. Yet the precise players and regulatory mechanisms orchestrating such trafficking events remain largely unknown. We here report the ATPase Fignl1 as a novel regulator of bidirectional transport during axon navigation. Using a yeast two-hybrid screen and coimmunoprecipitation assays, we showed that Fignl1 binds the kinesin Kif1bβ and the dynein/dynactin adaptor Bicaudal D-1 (Bicd1) in a molecular complex including the dynactin subunit dynactin 1. Fignl1 colocalized with Kif1bβ and showed bidirectional mobility in zebrafish axons. Notably, Kif1bβ and Fignl1 loss of function similarly altered zebrafish motor axon pathfinding and increased dynein-based transport velocity of Rab3 vesicles in these navigating axons, pinpointing Fignl1/Kif1bβ as a dynein speed limiter complex. Accordingly, disrupting dynein/dynactin activity or Bicd1/Fignl1 interaction induced motor axon pathfinding defects characteristic of Fignl1 gain or loss of function, respectively. Finally, pharmacological inhibition of dynein activity partially rescued the axon pathfinding defects of Fignl1-depleted larvae. Together, our results identify Fignl1 as a key dynein regulator required for motor circuit wiring.

KIF1Bβ mutations detected in hereditary neuropathy impair IGF1R transport and axon growth

Uncovering the mechanistic link between kinesin motors and neuropathy, Xu et al. identify functional KIF1Bβ mutations in human hereditary neuropathy to analyze them in mouse models. They propose that KIF1Bβ transports IGF1R and facilitates axonal outgrowth. Both of these effects are significantly affected by the clinical mutations.

KIF1Bβ is a kinesin-3 family anterograde motor protein essential for neuronal development, viability, and function. KIF1Bβ mutations have previously been reported in a limited number of pedigrees of Charcot-Marie-Tooth disease type 2A (CMT2A) neuropathy. However, the gene responsible for CMT2A is still controversial, and the mechanism of pathogenesis remains elusive. In this study, we show that the receptor tyrosine kinase IGF1R is a new direct binding partner of KIF1Bβ, and its binding and transport is specifically impaired by the Y1087C mutation of KIF1Bβ, which we detected in hereditary neuropathic patients. The axonal outgrowth and IGF-I signaling of Kif1b^−/− neurons were significantly impaired, consistent with decreased surface IGF1R expression. The complementary capacity of KIF1Bβ-Y1087C of these phenotypes was significantly impaired, but the binding capacity to synaptic vesicle precursors was not affected. These data have supported the relevance of KIF1Bβ in IGF1R transport, which may give new clue to the neuropathic pathogenesis.

Bioorthogonal Metabolic Labeling of Nascent RNA in Neurons Improves the Sensitivity of Transcriptome-Wide Profiling

Transcriptome-wide expression profiling of neurons has provided important insights into the underlying molecular mechanisms and gene expression patterns that transpire during learning and memory formation. However, there is a paucity of tools for profiling stimulus-induced RNA within specific neuronal cell populations. A bioorthogonal method to chemically label nascent (i.e., newly transcribed) RNA in a cell-type-specific and temporally controlled manner, which is also amenable to bioconjugation via click chemistry, was recently developed and optimized within conventional immortalized cell lines. However, its value within a more fragile and complicated cellular system such as neurons, as well as for transcriptome-wide expression profiling, has yet to be demonstrated. Here, we report the visualization and sequencing of activity-dependent nascent RNA derived from neurons using this labeling method. This work has important implications for improving transcriptome-wide expression profiling and visualization of nascent RNA in neurons, which has the potential to provide valuable insights into the mechanisms underlying neural plasticity, learning, and memory.

Cholinergic activity is essential for maintaining the anterograde transport of Choline Acetyltransferase in Drosophila

Cholinergic activity is essential for cognitive functions and neuronal homeostasis. Choline Acetyltransferase (ChAT), a soluble protein that synthesizes acetylcholine at the presynaptic compartment, is transported in bulk in the axons by the heterotrimeric Kinesin-2 motor. Axonal transport of soluble proteins is described as a constitutive process assisted by occasional, non-specific interactions with moving vesicles and motor proteins. Here, we report that an increase in the influx of Kinesin-2 motor and association between ChAT and the motor during a specific developmental period enhances the axonal entry, as well as the anterograde flow of the protein, in the sensory neurons of intact Drosophila nervous system. Loss of cholinergic activity due to  Hemicholinium and Bungarotoxin treatments, respectively, disrupts the interaction between ChAT and Kinesin-2 in the axon, and the episodic enhancement of axonal influx of the protein. Altogether, these observations highlight a phenomenon of synaptic activity-dependent, feedback regulation of a soluble protein transport in vivo, which could potentially define the quantum of its pre-synaptic influx.

Forniceal deep brain stimulation induces gene expression and splicing changes that promote neurogenesis and plasticity

Clinical trials are currently underway to assess the efficacy of forniceal deep brain stimulation (DBS) for improvement of memory in Alzheimer's patients, and forniceal DBS has been shown to improve learning and memory in a mouse model of Rett syndrome (RTT), an intellectual disability disorder caused by loss-of-function mutations in MECP2. The mechanism of DBS benefits has been elusive, however, so we assessed changes in gene expression, splice isoforms, DNA methylation, and proteome following acute forniceal DBS in wild-type mice and mice lacking Mecp2. We found that DBS upregulates genes involved in synaptic function, cell survival, and neurogenesis and normalized expression of ~25% of the genes altered in Mecp2-null mice. Moreover, DBS induced expression of 17-24% of the genes downregulated in other intellectual disability mouse models and in post-mortem human brain tissue from patients with Major Depressive Disorder, suggesting forniceal DBS could benefit individuals with a variety of neuropsychiatric disorders.

RNA sequencing of synaptic and cytoplasmic Upf1-bound transcripts supports contribution of nonsense-mediated decay to epileptogenesis

The nonsense mediated decay (NMD) pathway is a critical surveillance mechanism for identifying aberrant mRNA transcripts. It is unknown, however, whether the NMD system is affected by seizures in vivo and whether changes confer beneficial or maladaptive responses that influence long-term outcomes such the network alterations that produce spontaneous recurrent seizures. Here we explored the responses of the NMD pathway to prolonged seizures (status epilepticus) and investigated the effects of NMD inhibition on epilepsy in mice. Status epilepticus led to increased protein levels of Up-frameshift suppressor 1 homolog (Upf1) within the mouse hippocampus. Upf1 protein levels were also higher in resected hippocampus from patients with intractable temporal lobe epilepsy. Immunoprecipitation of Upf1-bound RNA from the cytoplasmic and synaptosomal compartments followed by RNA sequencing identified unique populations of NMD-associated transcripts and altered levels after status epilepticus, including known substrates such as Arc as well as novel targets including Inhba and Npas4. Finally, long-term video-EEG recordings determined that pharmacologic interference in the NMD pathway after status epilepticus reduced the later occurrence of spontaneous seizures in mice. These findings suggest compartment-specific recruitment and differential loading of transcripts by NMD pathway components may contribute to the process of epileptogenesis.

Molecular dynamics of FMRP and other RNA-binding proteins in MEG-01 differentiation: the role of mRNP complexes in non-neuronal development

Asymmetrically differentiating cells are formed with the aid of RNA-binding proteins (RBPs), which can bind, stabilize, regulate, and transport target mRNAs. The loss of RBPs in neurons may lead to severe neurodevelopmental diseases such as the Fragile X Syndrome with the absence of the Fragile X Mental Retardation Protein (FMRP). Because the latter is ubiquitous and shares many similarities with other RBPs involved in the development of peripheral cells, we suggest that FMRP would have a role in the differentiation of all tissues where it is expressed. A MEG-01 differentiation model was, therefore, established to study the global developmental functions of FMRP. PMA induction of MEG-01 cells causes important morphological changes driven by cytoskeletal dynamics. Cytoskeleton change and colocalization analyses were performed by confocal microscopy and sucrose gradient fractionation. Total cellular protein content and de novo synthesis were also analyzed. Microtubular transport mediates the displacement of FMRP and other RBP-containing mRNP complexes towards regions of the cell in development. De novo protein synthesis decreases significantly upon differentiation and total protein content composition is altered. Because those results are comparable with those obtained in neurons, the absence of FMRP would have significant consequences in cells everywhere in the body. The latter should be further investigated to give a better understanding of the systemic implications of imbalances of FMRP and other functionally similar RBPs.

Identification of expression patterns in the progression of disease stages by integration of transcriptomic data

Background

In the study of complex diseases using genome-wide expression data from clinical samples, a difficult case is the identification and mapping of the gene signatures associated to the stages that occur in the progression of a disease. The stages usually correspond to different subtypes or classes of the disease, and the difficulty to identify them often comes from patient heterogeneity and sample variability that can hide the biomedical relevant changes that characterize each stage, making standard differential analysis inadequate or inefficient.

Results

We propose a methodology to study diseases or disease stages ordered in a sequential manner (e.g. from early stages with good prognosis to more acute or serious stages associated to poor prognosis). The methodology is applied to diseases that have been studied obtaining genome-wide expression profiling of cohorts of patients at different stages. The approach allows searching for consistent expression patterns along the progression of the disease through two major steps: (i) identifying genes with increasing or decreasing trends in the progression of the disease; (ii) clustering the increasing/decreasing gene expression patterns using an unsupervised approach to reveal whether there are consistent patterns and find genes altered at specific disease stages. The first step is carried out using Gamma rank correlation to identify genes whose expression correlates with a categorical variable that represents the stages of the disease. The second step is done using a Self Organizing Map (SOM) to cluster the genes according to their progressive profiles and identify specific patterns. Both steps are done after normalization of the genomic data to allow the integration of multiple independent datasets. In order to validate the results and evaluate their consistency and biological relevance, the methodology is applied to datasets of three different diseases: myelodysplastic syndrome, colorectal cancer and Alzheimer’s disease. A software script written in R, named genediseasePatterns, is provided to allow the use and application of the methodology.

Conclusion

The method presented allows the analysis of the progression of complex and heterogeneous diseases that can be divided in pathological stages. It identifies gene groups whose expression patterns change along the advance of the disease, and it can be applied to different types of genomic data studying cohorts of patients in different states.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1290-4) contains supplementary material, which is available to authorized users.

Role of Dicer and the miRNA system in neuronal plasticity and brain function

MicroRNAs (miRNAs) are small regulatory non-coding RNAs that contribute to fine-tuning regulation of gene expression by mRNA destabilization and/or translational repression. Their abundance in the nervous system, their temporally and spatially regulated expression and their ability to respond in an activity-dependent manner make miRNAs ideal candidates for the regulation of complex processes in the brain, including neuronal plasticity, memory formation and neural development. The conditional ablation of the RNase III Dicer, which is essential for the maturation of most miRNAs, is a useful model to investigate the effect of the loss of the miRNA system, as a whole, in different tissues and cellular types. In this review, we first provide an overview of Dicer function and structure, and discuss outstanding questions concerning the role of miRNAs in the regulation of gene expression and neuronal function, to later focus on the insight derived from studies in which the genetic ablation of Dicer was used to determine the role of the miRNA system in the nervous system. In particular, we highlight the collective role of miRNAs fine-tuning plasticity-related gene expression and providing robustness to neuronal gene expression networks.

Activity-dependent FMRP requirements in development of the neural circuitry of learning and memory

The activity-dependent refinement of neural circuit connectivity during critical periods of brain development is essential for optimized behavioral performance. We hypothesize that this mechanism is defective in fragile X syndrome (FXS), the leading heritable cause of intellectual disability and autism spectrum disorders. Here, we use optogenetic tools in the Drosophila FXS disease model to test activity-dependent dendritogenesis in two extrinsic neurons of the mushroom body (MB) learning and memory brain center: (1) the input projection neuron (PN) innervating Kenyon cells (KCs) in the MB calyx microglomeruli and (2) the output MVP2 neuron innervated by KCs in the MB peduncle. Both input and output neuron classes exhibit distinctive activity-dependent critical period dendritic remodeling. MVP2 arbors expand in Drosophila mutants null for fragile X mental retardation 1 (dfmr1), as well as following channelrhodopsin-driven depolarization during critical period development, but are reduced by halorhodopsin-driven hyperpolarization. Optogenetic manipulation of PNs causes the opposite outcome--reduced dendritic arbors following channelrhodopsin depolarization and expanded arbors following halorhodopsin hyperpolarization during development. Importantly, activity-dependent dendritogenesis in both neuron classes absolutely requires dfmr1 during one developmental window. These results show that dfmr1 acts in a neuron type-specific activity-dependent manner for sculpting dendritic arbors during early-use, critical period development of learning and memory circuitry in the Drosophila brain.

Membrane-Coupled mRNA Trafficking in Fungi

Intracellular logistics are essential for delivery of newly synthesized material during polar growth of fungal hyphae. Proteins and lipids are actively transported throughout the cell by motor-dependent movement of small vesicles or larger units such as endosomes and the endoplasmic reticulum. A remarkably tight link is emerging between active membrane trafficking and mRNA transport, a process that determines the precise subcellular localization of translation products within the cell. Here, we report on recent insights into the mechanism and biological role of these intricate cotransport processes in fungal models such as Saccharomyces cerevisiae, Candida albicans, and Ustilago maydis. In the latter, we focus on the new finding of endosomal mRNA transport and its implications for protein targeting, complex assembly, and septin biology.

Transcriptional evidence for the role of chronic venlafaxine treatment in neurotrophic signaling and neuroplasticity including also Glutamatergic [corrected] - and insulin-mediated neuronal processes

OBJECTIVES

Venlafaxine (VLX), a serotonine-noradrenaline reuptake inhibitor, is one of the most commonly used antidepressant drugs in clinical practice for the treatment of major depressive disorder (MDD). Despite being more potent than its predecessors, similarly to them, the therapeutical effect of VLX is visible only 3-4 weeks after the beginning of treatment. Furthermore, recent papers show that antidepressants, including also VLX, enhance the motor recovery after stroke even in non depressed persons. In the present, transcriptomic-based study we looked for changes in gene expressions after a long-term VLX administration.

METHODS

Osmotic minipumps were implanted subcutaneously into Dark Agouti rats providing a continuous (40 mg/kg/day) VLX delivery for three weeks. Frontal regions of the cerebral cortex were isolated and analyzed using Illumina bead arrays to detect genes showing significant chances in expression. Gene set enrichment analysis was performed to identify specific regulatory networks significantly affected by long term VLX treatment.

RESULTS

Chronic VLX administration may have an effect on neurotransmitter release via the regulation of genes involved in vesicular exocytosis and receptor endocytosis (such as Kif proteins, Myo5a, Sv2b, Syn2 or Synj2). Simultaneously, VLX activated the expression of genes involved in neurotrophic signaling (Ntrk2, Ntrk3), glutamatergic transmission (Gria3, Grin2b and Grin2a), neuroplasticity (Camk2g/b, Cd47), synaptogenesis (Epha5a, Gad2) and cognitive processes (Clstn2). Interestingly, VLX increased the expression of genes involved in mitochondrial antioxidant activity (Bcl2 and Prdx1). Additionally, VLX administration also modulated genes related to insulin signaling pathway (Negr1, Ppp3r1, Slc2a4 and Enpp1), a mechanism that has recently been linked to neuroprotection, learning and memory.

CONCLUSIONS

Our results strongly suggest that chronic VLX treatment improves functional reorganization and brain plasticity by influencing gene expression in regulatory networks of motor cortical areas. These results are consonant with the synaptic (network) hypothesis of depression and antidepressant-induced motor recovery after stroke.

Impaired activity-dependent neural circuit assembly and refinement in autism spectrum disorder genetic models

Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics, transgenic calcium reporters, highly-targeted transgenic drivers for individually-identified neurons, and a vastly improved connectome of the brain are now being combined to provide unparalleled opportunities to both manipulate and monitor A-D processes during critical period brain development in defined neural circuits. The field is now poised to exploit this new Drosophila transgenic toolbox for the systematic dissection of A-D mechanisms in normal versus ASD brain development, particularly utilizing the well-established Drosophila FXS disease model.

The Fragile X mental retardation protein regulates matrix metalloproteinase 9 mRNA at synapses

Activity-dependent protein synthesis at synapses is dysregulated in the Fragile X syndrome (FXS). This process contributes to dendritic spine dysmorphogenesis and synaptic dysfunction in FXS. Matrix Metalloproteinase 9 (MMP-9) is an enzyme involved in activity-dependent reorganization of dendritic spine architecture and was shown to regulate spine morphology in a mouse model of FXS, the Fmr1 knock-out mice. Here we show that MMP-9 mRNA is part of the FMRP complex and colocalizes in dendrites. In the absence of FMRP MMP-9 mRNA translation is increased at synapses, suggesting that this mechanism contributes to the increased metalloproteinase level at synapses of Fmr1 knock-out mice. We propose that such a local effect can contribute to the aberrant dendritic spine morphology observed in the Fmr1 knock-out mice and in patients with FXS.

Endosomal transport of septin mRNA and protein indicates local translation on endosomes and is required for correct septin filamentation

Endosomes transport lipids and proteins over long distances by shuttling along microtubules. They also carry mRNAs on their surface, but the precise molecular function of this trafficking process is unknown. By live cell imaging of polarized fungal hyphae, we show microtubule-dependent transport of septin mRNA and encoded septin protein on the same shuttling endosomes. Consistent with the hypothesis that septin mRNA is translated on endosomes, the accumulation of septin protein on endosomes requires the recruitment of septin mRNA. Furthermore, ribosomal proteins co-localise with shuttling endosomes, but only if mRNA is present. Importantly, endosomal trafficking is essential for an efficient delivery of septin protein to filaments at growth poles, a process necessary to establish unipolar growth. Thus, we propose that local mRNA translation loads endosomes with septins for assembly and efficient delivery to septin filaments.

The FMRP regulon: from targets to disease convergence

The fragile X mental retardation protein (FMRP) is an RNA-binding protein that regulates mRNA metabolism. FMRP has been largely studied in the brain, where the absence of this protein leads to fragile X syndrome, the most frequent form of inherited intellectual disability. Since the identification of the FMRP gene in 1991, many studies have primarily focused on understanding the function/s of this protein. Hundreds of potential FMRP mRNA targets and several interacting proteins have been identified. Here, we report the identification of FMRP mRNA targets in the mammalian brain that support the key role of this protein during brain development and in regulating synaptic plasticity. We compared the genes from databases and genome-wide association studies with the brain FMRP transcriptome, and identified several FMRP mRNA targets associated with autism spectrum disorders, mood disorders and schizophrenia, showing a potential common pathway/s for these apparently different disorders.

MAPping out distribution routes for kinesin couriers

In the crowded environment of eukaryotic cells, diffusion is an inefficient distribution mechanism for cellular components. Long-distance active transport is required and is performed by molecular motors including kinesins. Furthermore, in highly polarised, compartmentalised and plastic cells such as neurons, regulatory mechanisms are required to ensure appropriate spatio-temporal delivery of neuronal components. The kinesin machinery has diversified into a large number of kinesin motor proteins as well as adaptor proteins that are associated with subsets of cargo. However, many mechanisms contribute to the correct delivery of these cargos to their target domains. One mechanism is through motor recognition of sub-domain-specific microtubule (MT) tracks, sign-posted by different tubulin isoforms, tubulin post-translational modifications, tubulin GTPase activity and MT-associated proteins (MAPs). With neurons as a model system, a critical review of these regulatory mechanisms is presented here, with a particular focus on the emerging contribution of compartmentalised MAPs. Overall, we conclude that - especially for axonal cargo - alterations to the MT track can influence transport, although in vivo, it is likely that multiple track-based effects act synergistically to ensure accurate cargo distribution.

New insights into mRNA trafficking in axons

In recent years, it has been demonstrated that mRNAs localize to axons of young and mature central and peripheral nervous system neurons in culture and in vivo. Increasing evidence is supporting a fundamental role for the local translation of these mRNAs in neuronal function by regulating axon growth, maintenance and regeneration after injury. Although most mRNAs found in axons are abundant transcripts and not restricted to the axonal compartment, they are sequestered into transport ribonucleoprotein particles and their axonal localization is likely the result of specific targeting rather than passive diffusion. It has been reported that long-distance mRNA transport requires microtubule-dependent motors, but the molecular mechanisms underlying the sorting and trafficking of mRNAs into axons have remained elusive. This review places particular emphasis on motor-dependent transport of mRNAs and presents a mathematical model that describes how microtubule-dependent motors can achieve targeted trafficking in axons. A future challenge will be to systematically explore how the numerous axonal mRNAs and RNA-binding proteins regulate different aspects of specific axonal mRNA trafficking during development and after regeneration.