The novel synaptogenic protein Farp1 links postsynaptic cytoskeletal dynamics and transsynaptic organization

Environmentally dependent and independent control of 3D cell shape

How cancer cells determine their shape in response to three-dimensional (3D) geometric and mechanical cues is unclear. We develop an approach to quantify the 3D cell shape of over 60,000 melanoma cells in collagen hydrogels using high-throughput stage-scanning oblique plane microscopy (ssOPM). We identify stereotypic and environmentally dependent changes in shape and protrusivity depending on whether a cell is proximal to a flat and rigid surface or is embedded in a soft environment. Environmental sensitivity metrics calculated for small molecules and gene knockdowns identify interactions between the environment and cellular factors that are important for morphogenesis. We show that the Rho guanine nucleotide exchange factor (RhoGEF) TIAM2 contributes to shape determination in environmentally independent ways but that non-muscle myosin II, microtubules, and the RhoGEF FARP1 regulate shape in ways dependent on the microenvironment. Thus, changes in cancer cell shape in response to 3D geometric and mechanical cues are modulated in both an environmentally dependent and independent fashion.

Cell Adhesion Molecule Signaling at the Synapse: Beyond the Scaffold

Synapses are specialized intercellular junctions connecting pre- and postsynaptic neurons into functional neural circuits. Synaptic cell adhesion molecules (CAMs) constitute key players in synapse development that engage in homo- or heterophilic interactions across the synaptic cleft. Decades of research have identified numerous synaptic CAMs, mapped their trans-synaptic interactions, and determined their role in orchestrating synaptic connectivity. However, surprisingly little is known about the molecular mechanisms that translate trans-synaptic adhesion into the assembly of pre- and postsynaptic compartments. Here, we provide an overview of the intracellular signaling pathways that are engaged by synaptic CAMs and highlight outstanding issues to be addressed in future work.

Identification of Novel Genomic Loci for Anxiety and Extensive Genetic Overlap with Psychiatric Disorders

Background

Anxiety disorders are prevalent and anxiety symptoms co-occur with many psychiatric disorders. We aimed to identify genomic risk loci associated with anxiety, characterize its genetic architecture, and genetic overlap with psychiatric disorders.

Methods

We used the GWAS of anxiety symptoms, schizophrenia, bipolar disorder, major depression, and attention deficit hyperactivity disorder (ADHD). We employed MiXeR and LAVA to characterize the genetic architecture and genetic overlap between the phenotypes. Conditional and conjunctional false discovery rate analyses were performed to boost the identification of genomic loci associated with anxiety and those shared with psychiatric disorders. Gene annotation and gene set analyses were conducted using OpenTargets and FUMA, respectively.

Results

Anxiety was polygenic with 12.9k estimated genetic risk variants and overlapped extensively with psychiatric disorders (4.1-11.4k variants). MiXeR and LAVA revealed predominantly positive genetic correlations between anxiety and psychiatric disorders. We identified 114 novel loci for anxiety by conditioning on the psychiatric disorders. We also identified loci shared between anxiety and major depression (n = 47), bipolar disorder (n = 33), schizophrenia (n = 71), and ADHD (n = 20). Genes annotated to anxiety loci exhibit enrichment for a broader range of biological pathways and differential tissue expression in more diverse tissues than those annotated to the shared loci.

Conclusions

Anxiety is a highly polygenic phenotype with extensive genetic overlap with psychiatric disorders. These genetic overlaps enabled the identification of novel loci for anxiety. The shared genetic architecture may underlie the extensive cross-disorder comorbidity of anxiety, and the identified genetic loci implicate molecular pathways that may lead to potential drug targets.

Phosphatases modified by LH signaling in ovarian follicles: testing their role in regulating the NPR2 guanylyl cyclase†

In response to luteinizing hormone (LH), multiple proteins in rat and mouse granulosa cells are rapidly dephosphorylated, but the responsible phosphatases remain to be identified. Because the phosphorylation state of phosphatases can regulate their interaction with substrates, we searched for phosphatases that might function in LH signaling by using quantitative mass spectrometry. We identified all proteins in rat ovarian follicles whose phosphorylation state changed detectably in response to a 30-min exposure to LH, and within this list, identified protein phosphatases or phosphatase regulatory subunits that showed changes in phosphorylation. Phosphatases in the phosphoprotein phosphatase (PPP) family were of particular interest because of their requirement for dephosphorylating the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase in the granulosa cells, which triggers oocyte meiotic resumption. Among the PPP family regulatory subunits, PPP1R12A and PPP2R5D showed the largest increases in phosphorylation, with 4-10 fold increases in signal intensity on several sites. Although follicles from mice in which these phosphorylations were prevented by serine-to-alanine mutations in either Ppp1r12a or Ppp2r5d showed normal LH-induced NPR2 dephosphorylation, these regulatory subunits and others could act redundantly to dephosphorylate NPR2. Our identification of phosphatases and other proteins whose phosphorylation state is rapidly modified by LH provides clues about multiple signaling pathways in ovarian follicles.

SynCAMs in Normal Vertebrate Neural Development and Neuropsychiatric Disorders: from the Perspective of the OCAs

Neuronal synaptic junctions connect neurons to enable neuronal signal transmission in the nervous system. The proper establishment of synaptic connections required many adhesion molecules. Malfunctions of these adhesion molecules can result in neural development disorders and neuropsychiatric disorders. How specific synapses are established by various adhesion molecules for proper neural circuitry is a fundamental question of neuroscience. SynCAMs, also named CADMs, Necl, etc., are among the many adhesion proteins found in synapses. Here, we review the current understanding of the physical properties of SynCAMs and their roles in axon pathfinding, myelination, synaptogenesis, and synaptic plasticity. In addition, we discuss the involvement of SynCAMs in neuropsychiatric disorders. Finally, we propose that SynCAM functions can be better viewed and understood from the perspective of orientational cell adhesions (OCAs). In particular, we discuss the possibilities of how SynCAMs can be regulated at the cell-type specific expression, transcription variants, posttranslational modification, and subcellular localization to modulate the diversity of SynCAMs as OCA molecules. Being major components of the synapses, SynCAMs continue to be an important research topic of neuroscience, and many outstanding questions are waiting to be answered.

Neocortical neuronal production and maturation defects in the TcMAC21 mouse model of Down syndrome

Down syndrome (DS) results from trisomy of human chromosome 21 (HSA21), and DS research has been conducted by the use of mouse models. We previously generated a humanized mouse model of DS, TcMAC21, which carries the long arm of HSA21. These mice exhibit learning and memory deficits, and may reproduce neurodevelopmental alterations observed in humans with DS. Here, we performed histologic studies of the TcMAC21 forebrain from embryonic to adult stages. The TcMAC21 neocortex showed reduced proliferation of neural progenitors and delayed neurogenesis. These abnormalities were associated with a smaller number of projection neurons and interneurons. Further, (phospho-)proteomic analysis of adult TcMAC21 cortex revealed alterations in the phosphorylation levels of a series of synaptic proteins. The TcMAC21 mouse model shows similar brain development abnormalities as DS, and will be a valuable model to investigate prenatal and postnatal causes of intellectual disability in humans with DS.

DjFARP Contributes to the Regeneration and Maintenance of the Brain through Activation of DjRac1 in Dugesia japonica

FERM, RhoGEF, and Pleckstrin domain protein (FARP) mediated RhoGTPase pathways are involved in diverse biological processes, such as neuronal development and tumorigenesis. However, little is known about their role in neural regeneration. We uncovered for the first time that FARP-Rac1 signaling plays an important role in neural regeneration in Dugesia japonica, a planarian that possesses unparalleled regenerative capacities. The planarian FARP homolog DjFARP was primarily expressed in both intact and regenerating brain and pharynx tissue. Functional studies suggested that downregulation of DjFARP with dsRNA in Dugesia japonica led to smaller brain sizes, defects in brain lateral branches, and loss of cholinergic, GABAergic, and dopaminergic neurons in both intact and regenerating animals. Moreover, the Rho GTPase DjRac1 was shown to play a similar role in neural regeneration and maintenance. Rac1 activation assay showed that DjFARP acts as a guanine nucleotide exchange factor (GEF) for DjRac1. Together, these findings indicate that the brain defects seen in DjFARP knockdown animals may be attributable to DjRac1 inactivation. In conclusion, our study demonstrated that DjFARP-DjRac1 signaling was required for the maintenance and proper regeneration of the brain in Dugesia japonica.

Social defeat stress induces genome-wide 5mC and 5hmC alterations in the mouse brain

Stress is adverse experience that require constant adaptation to reduce the emotional and physiological burden, or "allostatic load", of an individual. Despite their everyday occurrence, a subpopulation of individuals is more susceptible to stressors, while others remain resilient with unknown molecular signatures. In this study, we investigated the contribution of the DNA modifications, 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC), underlying the individual differences in stress susceptibility and resilience. Genome-wide 5mC and 5hmC profiles from 3- and 6-month adult male mice that underwent various durations of social defeat were generated. In 3-month animals, 5mC and 5hmC work in parallel and do not distinguish between stress-susceptible and resilient phenotypes, while in 6-month animals, 5mC and 5hmC show distinct enrichment patterns. Acute stress responses may epigenetically "prime" the animals to either increase or decrease their predisposition to depression susceptibility. In support of this, re-exposure studies reveal that the enduring effects of social defeat affect differential biological processes between susceptible and resilient animals. Finally, the stress-induced 5mC and 5hmC fluctuations across the acute-chronic-longitudinal time course demonstrate that the negative outcomes of chronic stress do not discriminate between susceptible and resilient animals. However, resilience is more associated with neuroprotective processes while susceptibility is linked to neurodegenerative processes. Furthermore, 5mC appears to be responsible for acute stress response, whereas 5hmC may function as a persistent and stable modification in response to stress. Our study broadens the scope of previous research offering a comprehensive analysis of the role of DNA modifications in stress-induced depression.

Rapid effects of valproic acid on the fetal brain transcriptome: Implications for brain development and autism

There is an increased incidence of autism among the children of women who take the anti-epileptic, mood stabilizing drug, valproic acid (VPA) during pregnancy; moreover, exposure to VPA in utero causes autistic-like symptoms in rodents and non-human primates. Analysis of RNAseq data obtained from fetal mouse brains 3 hr after VPA administration revealed that VPA significantly [p(FDR) ≤ 0.025] increased or decreased the expression of approximately 7,300 genes. No significant sex differences in VPA-induced gene expression were observed. Expression of genes associated with neurodevelopmental disorders such as autism as well as neurogenesis, axon growth and synaptogenesis, GABAergic, glutaminergic and dopaminergic synaptic transmission, perineuronal nets, and circadian rhythms was dysregulated by VPA. Moreover, expression of 400 autism risk genes was significantly altered by VPA as was expression of 247 genes that have been reported to play fundamental roles in the development of the nervous system, but are not linked to autism by GWAS. The goal of this study was to identify mouse genes that are: (a) significantly up- or down-regulated by VPA in the fetal brain and (b) known to be associated with autism and/or to play a role in embryonic neurodevelopmental processes, perturbation of which has the potential to alter brain connectivity in the postnatal and adult brain. The set of genes meeting these criteria provides potential targets for future hypothesis-driven approaches to elucidating the proximal underlying causes of defective brain connectivity in neurodevelopmental disorders such as autism.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

Different Modes of Mechanism of Gamma-Mangostin and Alpha-Mangostin to Inhibit Cell Migration of Triple-Negative Breast Cancer Cells Concerning CXCR4 Downregulation and ROS Generation

Background

Two mangostin compounds, gamma-mangostin and alpha-mangostin, show anticancer properties through the inhibition of cell proliferation and cell migration. Metastatic triple-negative breast cancer (TNBC) cells, including MDA-MB-231, highly express C-X-C chemokine receptor type 4 (CXCR4) to maintain reactive oxygen species (ROS) and cell migration.

Objectives

This study was performed to analyze and compare different modes of action of γ-mangostin and α-mangostin as antimigratory effects targeted on CXCR4 in MDA-MB-231 as a model of TNBC cell.

Methods

This study investigated the effect of γ-mangostin and α-mangostin using a series of assays, including Cell Counting Kit-8 (CCK-8) assay for cytotoxicity, wound healing assay for migration study, quantitative real-time polymerase chain reaction (qRT-PCR) for gene expression analysis, and flow cytometry for ROS measurement, along with in silico study to observe the binding between the compound and CXCR4.

Results

The findings revealed half maximal inhibitory concentration (IC50) values of 25 and 20 μM for γ-mangostin and α-mangostin in MDA-MB 231 cells, respectively. Moreover, a concentration of 10 μM was used for the migration assay. Both γ-mangostin and α-mangostin significantly suppressed cell migration within 24 hours. The present gene expression studies revealed the downregulation of key migration-associated genes, namely Farp, CXCR4, and LPHN2, upon γ-mangostin treatment but not α-mangostin. Additionally, both γ-mangostin and α-mangostin increased cellular ROS generation, highlighting the same effect of γ-mangostin and α-mangostin ROS elevation to inhibit cancer cell migration. Molecular docking simulations further suggested a potential interaction between γ-mangostin and α-mangostin with CXCR4 in high affinity.

Conclusions

These findings suggest that both γ-mangostin and α-mangostin inhibit breast cancer cell migration and induce cellular ROS levels in MDA-MB-231 cells; notably, γ-mangostin suppresses CXCR4 mRNA expression that might correlate to its activity to inhibit MDA-MB-231 cell migration.

CASK and FARP localize two classes of post-synaptic ACh receptors thereby promoting cholinergic transmission

Changes in neurotransmitter receptor abundance at post-synaptic elements play a pivotal role in regulating synaptic strength. For this reason, there is significant interest in identifying and characterizing the scaffolds required for receptor localization at different synapses. Here we analyze the role of two C. elegans post-synaptic scaffolding proteins (LIN-2/CASK and FRM-3/FARP) at cholinergic neuromuscular junctions. Constitutive knockouts or muscle specific inactivation of lin-2 and frm-3 dramatically reduced spontaneous and evoked post-synaptic currents. These synaptic defects resulted from the decreased abundance of two classes of post-synaptic ionotropic acetylcholine receptors (ACR-16/CHRNA7 and levamisole-activated AChRs). LIN-2's AChR scaffolding function is mediated by its SH3 and PDZ domains, which interact with AChRs and FRM-3/FARP, respectively. Thus, our findings show that post-synaptic LIN-2/FRM-3 complexes promote cholinergic synaptic transmission by recruiting AChRs to post-synaptic elements.

Genome-wide translation control analysis of developing human neurons

During neuronal differentiation, neuroprogenitor cells become polarized, change shape, extend axons, and form complex dendritic trees. While growing, axons are guided by molecular cues to their final destination, where they establish synaptic connections with other neuronal cells. Several layers of regulation are integrated to control neuronal development properly. Although control of mRNA translation plays an essential role in mammalian gene expression, how it contributes temporarily to the modulation of later stages of neuronal differentiation remains poorly understood. Here, we investigated how translation control affects pathways and processes essential for neuronal maturation, using H9-derived human neuro progenitor cells differentiated into neurons as a model. Through Ribosome Profiling (Riboseq) combined with RNA sequencing (RNAseq) analysis, we found that translation control regulates the expression of critical hub genes. Fundamental synaptic vesicle secretion genes belonging to SNARE complex, Rab family members, and vesicle acidification ATPases are strongly translationally regulated in developing neurons. Translational control also participates in neuronal metabolism modulation, particularly affecting genes involved in the TCA cycle and glutamate synthesis/catabolism. Importantly, we found translation regulation of several critical genes with fundamental roles regulating actin and microtubule cytoskeleton pathways, critical to neurite generation, spine formation, axon guidance, and circuit formation. Our results show that translational control dynamically integrates important signals in neurons, regulating several aspects of its development and biology.

Control of CRK-RAC1 activity by the miR-1/206/133 miRNA family is essential for neuromuscular junction function

Formation and maintenance of neuromuscular junctions (NMJs) are essential for skeletal muscle function, allowing voluntary movements and maintenance of the muscle tone, thereby preventing atrophy. Generation of NMJs depends on the interaction of motor neurons with skeletal muscle fibers, which initiates a cascade of regulatory events that is essential for patterning of acetylcholine receptor (AChR) clusters at specific sites of the sarcolemma. Here, we show that muscle-specific miRNAs of the miR-1/206/133 family are crucial regulators of a signaling cascade comprising DOK7-CRK-RAC1, which is critical for stabilization and anchoring of postsynaptic AChRs during NMJ development and maintenance. We describe that posttranscriptional repression of CRK by miR-1/206/133 is essential for balanced activation of RAC1. Failure to adjust RAC1 activity severely compromises NMJ function, causing respiratory failure in neonates and neuromuscular symptoms in adult mice. We conclude that miR-1/206/133 serve a specific function for NMJs but are dispensable for skeletal muscle development.

The miR-1/133/206 gene family codes for the most abundant microRNAs in striated muscles. Here, Klockner et al show that inactivation of all family members in skeletal muscle prevents formation of normal neuromuscular junctions due to increased expression of the adaptor protein CRK.

The synaptic scaffold protein MPP2 interacts with GABAA receptors at the periphery of the postsynaptic density of glutamatergic synapses

Recent advances in imaging technology have highlighted that scaffold proteins and receptors are arranged in subsynaptic nanodomains. The synaptic membrane-associated guanylate kinase (MAGUK) scaffold protein membrane protein palmitoylated 2 (MPP2) is a component of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor–associated protein complexes and also binds to the synaptic cell adhesion molecule SynCAM 1. Using superresolution imaging, we show that—like SynCAM 1—MPP2 is situated at the periphery of the postsynaptic density (PSD). In order to explore MPP2-associated protein complexes, we used a quantitative comparative proteomics approach and identified multiple γ-aminobutyric acid (GABA)_A receptor subunits among novel synaptic MPP2 interactors. In line with a scaffold function for MPP2 in the assembly and/or modulation of intact GABA_A receptors, manipulating MPP2 expression had effects on inhibitory synaptic transmission. We further show that GABA_A receptors are found together with MPP2 in a subset of dendritic spines and thus highlight MPP2 as a scaffold that serves as an adaptor molecule, linking peripheral synaptic elements critical for inhibitory regulation to central structures at the PSD of glutamatergic synapses.

This study shows that the MAGUK scaffold protein MPP2 is located at the periphery of postsynaptic densities in excitatory neurons, where it interacts with GABA-A receptors, thereby serving as a functional adaptor that links excitatory and inhibitory components of synaptic transmission at glutamatergic synapses.

The Effect of Sleep Deprivation and Subsequent Recovery Period on the Synaptic Proteome of Rat Cerebral Cortex

Sleep deprivation (SD) is commonplace in the modern way of life and has a substantial social, medical, and human cost. Sleep deprivation induces cognitive impairment such as loss of executive attention, working memory decline, poor emotion regulation, increased reaction times, and higher cognitive functions are particularly vulnerable to sleep loss. Furthermore, SD is associated with obesity, diabetes, cardiovascular diseases, cancer, and a vast majority of psychiatric and neurodegenerative disorders are accompanied by sleep disturbances. Despite the widespread scientific interest in the effect of sleep loss on synaptic function, there is a lack of investigation focusing on synaptic transmission on the proteome level. In the present study, we report the effects of SD and recovery period (RP) on the cortical synaptic proteome in rats. Synaptosomes were isolated after 8 h of SD performed by gentle handling and after 16 h of RP. The purity of synaptosome fraction was validated with western blot and electron microscopy, and the protein abundance alterations were analyzed by mass spectrometry. We observed that SD and RP have a wide impact on neurotransmitter-related proteins at both the presynaptic and postsynaptic membranes. The abundance of synaptic proteins has changed to a greater extent in consequence of SD than during RP: we identified 78 proteins with altered abundance after SD and 39 proteins after the course of RP. Levels of most of the altered proteins were upregulated during SD, while RP showed the opposite tendency, and three proteins (Gabbr1, Anks1b, and Decr1) showed abundance changes with opposite direction after SD and RP. The functional cluster analysis revealed that a majority of the altered proteins is related to signal transduction and regulation, synaptic transmission and synaptic assembly, protein and ion transport, and lipid and fatty acid metabolism, while the interaction network analysis revealed several connections between the significantly altered proteins and the molecular processes of synaptic plasticity or sleep. Our proteomic data implies suppression of SNARE-mediated synaptic vesicle exocytosis and impaired endocytic processes after sleep deprivation. Both SD and RP altered GABA neurotransmission and affected protein synthesis, several regulatory processes and signaling pathways, energy homeostatic processes, and metabolic pathways.

FARP1, ARHGEF39, and TIAM2 are essential receptor tyrosine kinase effectors for Rac1-dependent cell motility in human lung adenocarcinoma

Despite the undisputable role of the small GTPase Rac1 in the regulation of actin cytoskeleton reorganization, the Rac guanine-nucleotide exchange factors (Rac-GEFs) involved in Rac1-mediated motility and invasion in human lung adenocarcinoma cells remain largely unknown. Here, we identify FARP1, ARHGEF39, and TIAM2 as essential Rac-GEFs responsible for Rac1-mediated lung cancer cell migration upon EGFR and c-Met activation. Noteworthily, these Rac-GEFs operate in a non-redundant manner by controlling distinctive aspects of ruffle dynamics formation. Mechanistic analysis reveals a leading role of the AXL-Gab1-PI3K axis in conferring pro-motility traits downstream of EGFR. Along with the positive association between the overexpression of Rac-GEFs and poor lung adenocarcinoma patient survival, we show that FARP1 and ARHGEF39 are upregulated in EpCam⁺ cells sorted from primary human lung adenocarcinomas. Overall, our study reveals fundamental insights into the complex intricacies underlying Rac-GEF-mediated cancer cell motility signaling, hence underscoring promising targets for metastatic lung cancer therapy.

Activity and Function of the PRMT8 Protein Arginine Methyltransferase in Neurons

Among the nine mammalian protein arginine methyltransferases (PRMTs), PRMT8 is unusual because it has restricted expression in the nervous system and is the only membrane-bound PRMT. Emerging studies have demonstrated that this enzyme plays multifaceted roles in diverse processes in neurons. Here we will summarize the unique structural features of PRMT8 and describe how it participates in various neuronal functions such as dendritic growth, synapse maturation, and synaptic plasticity. Recent evidence suggesting the potential role of PRMT8 function in neurological diseases will also be discussed.

Integrative functional genomic analysis of intron retention in human and mouse brain with Alzheimer's disease

Intron retention (IR) has been implicated in the pathogenesis of complex diseases such as cancers; its association with Alzheimer's disease (AD) remains unexplored. We performed genome-wide analysis of IR through integrating genetic, transcriptomic, and proteomic data of AD subjects and mouse models from the Accelerating Medicines Partnership-Alzheimer's Disease project. We identified 4535 and 4086 IR events in 2173 human and 1736 mouse genes, respectively. Quantitation of IR enabled the identification of differentially expressed genes that conventional exon-level approaches did not reveal. There were significant correlations of intron expression within innate immune genes, like HMBOX1, with AD in humans. Peptides with a high probability of translation from intron-retained mRNAs were identified using mass spectrometry. Further, we established AD-specific intron expression Quantitative Trait Loci, and identified splicing-related genes that may regulate IR. Our analysis provides a novel resource for the search for new AD biomarkers and pathological mechanisms.

The Protein Arginine Methyltransferase PRMT8 and Substrate G3BP1 Control Rac1-PAK1 Signaling and Actin Cytoskeleton for Dendritic Spine Maturation

Excitatory synapses of neurons are located on dendritic spines. Spine maturation is essential for the stability of synapses and memory consolidation, and overproduction of the immature filopodia is associated with brain disorders. The structure and function of synapses can be modulated by protein post-translational modification (PTM). Arginine methylation is a major PTM that regulates chromatin structure, transcription, and splicing within the nucleus. Here we find that the protein arginine methyltransferase PRMT8 is present at neuronal synapses and its expression is upregulated in the hippocampus when dendritic spine maturation occurs. Depletion of PRMT8 leads to overabundance of filopodia and mis-localization of excitatory synapses. Mechanistically, PRMT8 promotes dendritic spine morphology through methylation of the dendritic RNA-binding protein G3BP1 and suppression of the Rac1-PAK1 signaling pathway to control synaptic actin dynamics. Our findings unravel arginine methylation as a crucial regulatory mechanism for actin cytoskeleton during synapse development.

Rho GTPase regulatory proteins in podocytes

The Rho family of small GTPases (Rho GTPases) are the master regulators of the actin cytoskeleton and consist of 22 members. Previous studies implicated dysregulation of Rho GTPases in podocytes in the pathogenesis of proteinuric glomerular diseases. Rho GTPases are primarily regulated by the three families of proteins; guanine nucleotide exchange factors (GEFs; 82 members), GTPase-activating proteins (GAPs; 69 members), and GDP dissociation inhibitors (GDIs; 3 members). Since the regulatory proteins far outnumber their substrate Rho GTPases and act in concert in a cell/context-dependent manner, the upstream regulatory mechanism directing Rho GTPases in podocytes is largely unknown. In this review, we summarize recent advances in the understanding of the role of Rho GTPase regulatory proteins in podocytes, including the known mutations of these proteins that cause proteinuria in humans. We also provide critical appraisal of the in vivo and in vitro studies and identify the knowledge gap in the field that will require further studies.

FARP-1 deletion is associated with lack of response to autism treatment by early start denver model in a multiplex family

Background

Children with autism spectrum disorder (ASD) display impressive clinical heterogeneity, also involving treatment response. Genetic variants can contribute to explain this large interindividual phenotypic variability.

Methods

Array‐CGH (a‐CGH) and whole genome sequencing (WGS) were performed on a multiplex family with two small children diagnosed with ASD at 17 and 18 months of age. Both brothers received the same naturalistic intervention for one year according to the Early Start Denver Model (ESDM), applied by the same therapists, yielding dramatically different treatment outcomes.

Results

The older sibling came out of the autism spectrum, while the younger sibling displayed very little, in any, improvement. This boy was subsequently treated applying a structured Early Intensive Behavioral Intervention paired with Augmentative Alternative Communication, which yielded a partial response within another year. The ESDM nonresponsive child carries a novel maternally inherited 65 Kb deletion at chr. 13q32.2 spanning FARP1. Farp1 is a synaptic scaffolding protein, which plays a significant role in neural plasticity.

Conclusion

These results represent a paradigmatic example of the heuristic potential of genetic markers in predicting treatment response and possibly in supporting the targeted prescription of specific early intervention approaches.

The netrin receptor UNC-40/DCC assembles a postsynaptic scaffold and sets the synaptic content of GABAA receptors

Increasing evidence indicates that guidance molecules used during development for cellular and axonal navigation also play roles in synapse maturation and homeostasis. In C. elegans the netrin receptor UNC-40/DCC controls the growth of dendritic-like muscle cell extensions towards motoneurons and is required to recruit type A GABA receptors (GABA_ARs) at inhibitory neuromuscular junctions. Here we show that activation of UNC-40 assembles an intracellular synaptic scaffold by physically interacting with FRM-3, a FERM protein orthologous to FARP1/2. FRM-3 then recruits LIN-2, the ortholog of CASK, that binds the synaptic adhesion molecule NLG-1/Neuroligin and physically connects GABA_ARs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA_ARs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA_ARs through the UNC-40-dependent intracellular scaffold. Since DCC is detected at GABA synapses in mammals, DCC might also tune inhibitory neurotransmission in the mammalian brain.

The netrin receptor UNC-40/DCC is required to recruit GABA_AR at neuromuscular junctions in C. elegans. Here, the authors show that UNC-40/DCC assembles an intracellular synaptic scaffold, regulating the content of GABA_AR and inhibitory neurotransmission.

Temporal Regulation of Dendritic Spines Through NrCAM-Semaphorin3F Receptor Signaling in Developing Cortical Pyramidal Neurons

Neuron-glial related cell adhesion molecule NrCAM is a newly identified negative regulator of spine density that genetically interacts with Semaphorin3F (Sema3F), and is implicated in autism spectrum disorders (ASD). To investigate a role for NrCAM in spine pruning during the critical adolescent period when networks are established, we generated novel conditional, inducible NrCAM mutant mice (Nex1Cre-ERT2: NrCAMflox/flox). We demonstrate that NrCAM functions cell autonomously during adolescence in pyramidal neurons to restrict spine density in the visual (V1) and medial frontal cortex (MFC). Guided by molecular modeling, we found that NrCAM promoted clustering of the Sema3F holoreceptor complex by interfacing with Neuropilin-2 (Npn2) and PDZ scaffold protein SAP102. NrCAM-induced receptor clustering stimulated the Rap-GAP activity of PlexinA3 (PlexA3) within the holoreceptor complex, which in turn, inhibited Rap1-GTPase and inactivated adhesive β1 integrins, essential for Sema3F-induced spine pruning. These results define a developmental function for NrCAM in Sema3F receptor signaling that limits dendritic spine density on cortical pyramidal neurons during adolescence.

MicroRNA Regulation of the Small Rho GTPase Regulators-Complexities and Opportunities in Targeting Cancer Metastasis

The small Rho GTPases regulate important cellular processes that affect cancer metastasis, such as cell survival and proliferation, actin dynamics, adhesion, migration, invasion and transcriptional activation. The Rho GTPases function as molecular switches cycling between an active GTP-bound and inactive guanosine diphosphate (GDP)-bound conformation. It is known that Rho GTPase activities are mainly regulated by guanine nucleotide exchange factors (RhoGEFs), GTPase-activating proteins (RhoGAPs), GDP dissociation inhibitors (RhoGDIs) and guanine nucleotide exchange modifiers (GEMs). These Rho GTPase regulators are often dysregulated in cancer; however, the underlying mechanisms are not well understood. MicroRNAs (miRNAs), a large family of small non-coding RNAs that negatively regulate protein-coding gene expression, have been shown to play important roles in cancer metastasis. Recent studies showed that miRNAs are capable of directly targeting RhoGAPs, RhoGEFs, and RhoGDIs, and regulate the activities of Rho GTPases. This not only provides new evidence for the critical role of miRNA dysregulation in cancer metastasis, it also reveals novel mechanisms for Rho GTPase regulation. This review summarizes recent exciting findings showing that miRNAs play important roles in regulating Rho GTPase regulators (RhoGEFs, RhoGAPs, RhoGDIs), thus affecting Rho GTPase activities and cancer metastasis. The potential opportunities and challenges for targeting miRNAs and Rho GTPase regulators in treating cancer metastasis are also discussed. A comprehensive list of the currently validated miRNA-targeting of small Rho GTPase regulators is presented as a reference resource.

FARP1 boosts CDC42 activity from integrin αvβ5 signaling and correlates with poor prognosis of advanced gastric cancer

Considering the poor prognosis of most advanced cancers, prevention of invasion and metastasis is essential for disease control. Ras homologous (Rho) guanine exchange factors (GEFs) and their signaling cascade could be potential therapeutic targets in advanced cancers. We conducted in silico analyses of The Cancer Genome Atlas expression data to identify candidate Rho-GEF genes showing aberrant expression in advanced gastric cancer and found FERM, Rho/ArhGEF, and pleckstrin domain protein 1 (FARP1) expression is related to poor prognosis. Analyses in 91 clinical advanced gastric cancers of the relationship of prognosis and pathological factors with immunohistochemical expression of FARP1 indicated that high expression of FARP1 is significantly associated with lymphatic invasion, lymph metastasis, and poor prognosis of the patients (P = 0.025). In gastric cancer cells, FARP1 knockdown decreased cell motility, whereas FARP1 overexpression promoted cell motility and filopodium formation via CDC42 activation. FARP1 interacted with integrin β5, and a potent integrin αvβ5 inhibitor (SB273005) prevented cell motility in only high FARP1-expressing gastric cancer cells. These results suggest that the integrin αvβ5-FARP1-CDC42 axis plays a crucial role in gastric cancer cell migration and invasion. Thus, regulatory cascade upstream of Rho can be a specific and promising target of advanced cancer treatment.

Molecular Architecture of Genetically-Tractable GABA Synapses in C. elegans

Inhibitory synapses represent a minority of the total chemical synapses in the mammalian brain, yet proper tuning of inhibition is fundamental to shape neuronal network properties. The neurotransmitter γ-aminobutyric acid (GABA) mediates rapid synaptic inhibition by the activation of the type A GABA receptor (GABA_AR), a pentameric chloride channel that governs major inhibitory neuronal transduction in the nervous system. Impaired GABA transmission leads to a variety of neuropsychiatric diseases, including schizophrenia, autism, epilepsy or anxiety. From an evolutionary perspective, GABA_AR shows remarkable conservations, and are found in all eukaryotic clades and even in bacteria and archaea. Specifically, bona fide GABA_ARs are found in the nematode Caenorhabditis elegans. Because of the anatomical simplicity of the nervous system and its amenability to genetic manipulations, C. elegans provide a powerful system to investigate the molecular and cellular biology of GABA synapses. In this mini review article, we will introduce the structure of the C. elegans GABAergic system and describe recent advances that have identified novel proteins controlling the localization of GABA_ARs at synapses. In particular, Ce-Punctin/MADD-4 is an evolutionarily-conserved extracellular matrix protein that behaves as an anterograde synaptic organizer to instruct the excitatory or inhibitory identity of postsynaptic domains.

Emerging Roles of Synapse Organizers in the Regulation of Critical Periods

Experience remodels cortical connectivity during developmental windows called critical periods. Experience-dependent regulation of synaptic strength during these periods establishes circuit functions that are stabilized as critical period plasticity wanes. These processes have been extensively studied in the developing visual cortex, where critical period opening and closure are orchestrated by the assembly, maturation, and strengthening of distinct synapse types. The synaptic specificity of these processes points towards the involvement of distinct molecular pathways. Attractive candidates are pre- and postsynaptic transmembrane proteins that form adhesive complexes across the synaptic cleft. These synapse-organizing proteins control synapse development and maintenance and modulate structural and functional properties of synapses. Recent evidence suggests that they have pivotal roles in the onset and closure of the critical period for vision. In this review, we describe roles of synapse-organizing adhesion molecules in the regulation of visual critical period plasticity and we discuss the potential they offer to restore circuit functions in amblyopia and other neurodevelopmental disorders.

Downregulating SynCAM and MPP6 expression is associated with ovarian cancer progression

Synaptic cell adhesion molecules (SynCAMs) are single transmembrane proteins that belong to the immunoglobulin superfamily of cell adhesion molecules. In the present study, a decrease in SynCAM levels in ovarian tumor tissues compared with normal tissues is reported; the downregulation was accompanied by the grade malignancy. The observations suggested that SynCAM may be essential for important novel functions in ovarian cancer. Further experiments showed that low SynCAM expression inhibited membrane palmitoylated protein 6 (MPP6) expression, a member of the palmitoylated membrane protein subfamily of peripheral membrane-associated guanylate kinases. In addition, low levels of MPP6 in ovarian tumor tissues correlated with shorter patient survival. A SynCAM-regulated pathway may provide molecular targets for the treatment of ovarian cancer and novel biomarkers to be used in clinical diagnosis.

Close Homolog of L1 Regulates Dendritic Spine Density in the Mouse Cerebral Cortex Through Semaphorin 3B

Dendritic spines in the developing mammalian neocortex are initially overproduced and then eliminated during adolescence to achieve appropriate levels of excitation in mature networks. We show here that the L1 family cell adhesion molecule Close Homolog of L1 (CHL1) and secreted repellent ligand Semaphorin 3B (Sema3B) function together to induce dendritic spine pruning in developing cortical pyramidal neurons. Loss of CHL1 in null mutant mice in both genders resulted in increased spine density and a greater proportion of immature spines on apical dendrites in the prefrontal and visual cortex. Electron microscopy showed that excitatory spine synapses with postsynaptic densities were increased in the CHL1-null cortex, and electrophysiological recording in prefrontal slices from mutant mice revealed deficiencies in excitatory synaptic transmission. Mechanistically, Sema3B protein induced elimination of spines on apical dendrites of cortical neurons cultured from wild-type but not CHL1-null embryos. Sema3B was secreted by the cortical neuron cultures, and its levels increased when cells were treated with the GABA antagonist gabazine. In vivo CHL1 was coexpressed with Sema3B in pyramidal neuron subpopulations and formed a complex with Sema3B receptor subunits Neuropilin-2 and PlexinA4. CHL1 and NrCAM, a closely related L1 adhesion molecule, localized primarily to distinct spines and promoted spine elimination to Sema3B or Sema3F, respectively. These results support a new concept in which selective spine elimination is achieved through different secreted semaphorins and L1 family adhesion molecules to sculpt functional neural circuits during postnatal maturation.SIGNIFICANCE STATEMENT Dendritic spines in the mammalian neocortex are initially overproduced and then pruned in adolescent life through unclear mechanisms to sculpt maturing cortical circuits. Here, we show that spine and excitatory synapse density of pyramidal neurons in the developing neocortex is regulated by the L1 adhesion molecule, Close Homolog of L1 (CHL1). CHL1 mediated spine pruning in response to the secreted repellent ligand Semaphorin 3B and associated with receptor subunits Neuropilin-2 and PlexinA4. CHL1 and related L1 adhesion molecule NrCAM localized to distinct spines, and promoted spine elimination to Semaphorin 3B and -3F, respectively. These results support a new concept in which selective elimination of individual spines and nascent synapses can be achieved through the action of distinct secreted semaphorins and L1 adhesion molecules.

ROCK2 inhibition triggers the collective invasion of colorectal adenocarcinomas

The metastatic progression of cancer is a multi-step process initiated by the local invasion of the peritumoral stroma. To identify the mechanisms underlying colorectal carcinoma (CRC) invasion, we collected live human primary cancer specimens at the time of surgery and monitored them ex vivo. This revealed that conventional adenocarcinomas undergo collective invasion while retaining their epithelial glandular architecture with an inward apical pole delineating a luminal cavity. To identify the underlying mechanisms, we used microscopy-based assays on 3D organotypic cultures of Caco-2 cysts as a model system. We performed two siRNA screens targeting Rho-GTPases effectors and guanine nucleotide exchange factors. These screens revealed that ROCK2 inhibition triggers the initial leader/follower polarization of the CRC cell cohorts and induces collective invasion. We further identified FARP2 as the Rac1 GEF necessary for CRC collective invasion. However, FARP2 activation is not sufficient to trigger leader cell formation and the concomitant inhibition of Myosin-II is required to induce invasion downstream of ROCK2 inhibition. Our results contrast with ROCK pro-invasive function in other cancers, stressing that the molecular mechanism of metastatic spread likely depends on tumour types and invasion mode.

Open Up to Make New Contacts: Caldendrin Senses Postsynaptic Calcium Influx to Dynamically Organize Dendritic Spines

In this issue of Neuron, Mikhaylova et al. (2018) report how the Ca²⁺ sensor caldendrin interacts in dendritic spines with cortactin to control actin remodeling. Combining molecular and functional approaches, this work gains insights into postsynaptic dynamics relevant for synaptic plasticity.

Secretion of Mast Cell Inflammatory Mediators Is Enhanced by CADM1-Dependent Adhesion to Sensory Neurons

Neuroimmune interactions are important in the pathophysiology of many chronic inflammatory diseases, particularly those associated with alterations in sensory processing and pain. Mast cells and sensory neuron nerve endings are found in areas of the body exposed to the external environment, both are specialized to sense potential damage by injury or pathogens and signal to the immune system and nervous system, respectively, to elicit protective responses. Cell adhesion molecule 1 (CADM1), also known as SynCAM1, has previously been identified as an adhesion molecule which may couple mast cells to sensory neurons however, whether this molecule exerts a functional as well as structural role in neuroimmune cross-talk is unknown. Here we show, using a newly developed in vitro co-culture system consisting of murine bone marrow derived mast cells (BMMC) and adult sensory neurons isolated from dorsal root ganglions (DRG), that CADM1 is expressed in mast cells and adult sensory neurons and mediates strong adhesion between the two cell types. Non-neuronal cells in the DRG cultures did not express CADM1, and mast cells did not adhere to them. The interaction of BMMCs with sensory neurons was found to induce mast cell degranulation and IL-6 secretion and to enhance responses to antigen stimulation and activation of FcεRI receptors. Secretion of TNFα in contrast was not affected, nor was secretion evoked by compound 48/80. Co-cultures of BMMCs with HEK 293 cells, which also express CADM1, while also leading to adhesion did not replicate the effects of sensory neurons on mast cells, indicative of a neuron-specific interaction. Application of a CADM1 blocking peptide or knockdown of CADM1 in BMMCs significantly decreased BMMC attachment to sensory neurites and abolished the enhanced secretory responses of mast cells. In conclusion, CADM1 is necessary and sufficient to drive mast cell-sensory neuron adhesion and promote the development of a microenvironment in which neurons enhance mast cell responsiveness to antigen, this interaction could explain why the incidence of painful neuroinflammatory disorders such as irritable bowel syndrome (IBS) are increased in atopic patients.

Transient localization of the Arp2/3 complex initiates neuronal dendrite branching in vivo

The formation of neuronal dendrite branches is fundamental for the wiring and function of the nervous system. Indeed, dendrite branching enhances the coverage of the neuron's receptive field and modulates the initial processing of incoming stimuli. Complex dendrite patterns are achieved in vivo through a dynamic process of de novo branch formation, branch extension and retraction. The first step towards branch formation is the generation of a dynamic filopodium-like branchlet. The mechanisms underlying the initiation of dendrite branchlets are therefore crucial to the shaping of dendrites. Through in vivo time-lapse imaging of the subcellular localization of actin during the process of branching of Drosophila larva sensory neurons, combined with genetic analysis and electron tomography, we have identified the Actin-related protein (Arp) 2/3 complex as the major actin nucleator involved in the initiation of dendrite branchlet formation, under the control of the activator WAVE and of the small GTPase Rac1. Transient recruitment of an Arp2/3 component marks the site of branchlet initiation in vivo These data position the activation of Arp2/3 as an early hub for the initiation of branchlet formation.

Biophysical mechanisms underlying the membrane trafficking of synaptic adhesion molecules

Adhesion proteins play crucial roles at synapses, not only by providing a physical trans-synaptic linkage between axonal and dendritic membranes, but also by connecting to functional elements including the pre-synaptic neurotransmitter release machinery and post-synaptic receptors. To mediate these functions, adhesion proteins must be organized on the neuronal surface in a precise and controlled manner. Recent studies have started to describe the mobility, nanoscale organization, and turnover rate of key synaptic adhesion molecules including cadherins, neurexins, neuroligins, SynCAMs, and LRRTMs, and show that some of these proteins are highly mobile in the plasma membrane while others are confined at sub-synaptic compartments, providing evidence for different regulatory pathways. In this review article, we provide a biophysical view of the diffusional trapping of adhesion molecules at synapses, involving both extracellular and intracellular protein interactions. We review the methodology underlying these measurements, including biomimetic systems with purified adhesion proteins, means to perturb protein expression or function, single molecule imaging in cultured neurons, and analytical models to interpret the data. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Structural analyses of FERM domain-mediated membrane localization of FARP1

FARP1 is a multi-domain protein that is involved in regulating neuronal development through interacting with cell surface proteins such as class A Plexins and SynCAM 1. The N-terminal FERM domain in FARP1 is known to both promote membrane localization and mediate these protein interactions, for which the underlying molecular mechanisms remain unclear. Here we determined the crystal structures of the FERM domain of FARP1 from zebrafish, and those of FARP2 (a close homolog of FARP1) from mouse and zebrafish. These FERM domains adopt the three-leaved clover fold that is typical of all FERM domains. Our structures reveal a positively charged surface patch that is highly conserved in the FERM domain of FARP1 and FARP2. In vitro lipid-binding experiments showed that the FARP1 FERM domain binds specifically to several types of phospholipid, which is dependent on the positively charged surface patch. We further determined through cell-based analyses that this surface patch on the FERM domain underlies the localization of FARP1 to the plasma membrane, and that FERM domain interactions recruit it to postsynaptic sites in neurons.

The tetrapartite synapse: a key concept in the pathophysiology of schizophrenia

Growing evidence points to synaptic pathology as a core component of the pathophysiology of schizophrenia (SZ). Significant reductions of dendritic spine density and altered expression of their structural and molecular components have been reported in several brain regions, suggesting a deficit of synaptic plasticity. Regulation of synaptic plasticity is a complex process, one that requires not only interactions between pre- and post-synaptic terminals, but also glial cells and the extracellular matrix (ECM). Together, these elements are referred to as the ‘tetrapartite synapse’, an emerging concept supported by accumulating evidence for a role of glial cells and the extracellular matrix in regulating structural and functional aspects of synaptic plasticity. In particular, chondroitin sulfate proteoglycans (CSPGs), one of the main components of the ECM, have been shown to be synthesized predominantly by glial cells, to form organized perisynaptic aggregates known as perineuronal nets (PNNs), and to modulate synaptic signaling and plasticity during postnatal development and adulthood. Notably, recent findings from our group and others have shown marked CSPG abnormalities in several brain regions of people with SZ. These abnormalities were found to affect specialized ECM structures, including PNNs, as well as glial cells expressing the corresponding CSPGs. The purpose of this review is to bring forth the hypothesis that synaptic pathology in SZ arises from a disruption of the interactions between elements of the tetrapartite synapse.

Building branched tissue structures: from single cell guidance to coordinated construction

Branched networks are ubiquitous throughout nature, particularly found in tissues that require large surface area within a restricted volume. Many tissues with a branched architecture, such as the vasculature, kidney, mammary gland, lung and nervous system, function to exchange fluids, gases and information throughout the body of an organism. The generation of branched tissues requires regulation of branch site specification, initiation and elongation. Branching events often require the coordination of many cells to build a tissue network for material exchange. Recent evidence has emerged suggesting that cell cooperativity scales with the number of cells actively contributing to branching events. Here, we compare mechanisms that regulate branching, focusing on how cell cohorts behave in a coordinated manner to build branched tissues.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

A CDC42-centered signaling unit is a dominant positive regulator of endothelial integrity

Endothelial barrier function is carefully controlled to protect tissues from edema and damage inflicted by extravasated leukocytes. RhoGTPases, in conjunction with myriad regulatory proteins, exert both positive and negative effects on the endothelial barrier integrity. Precise knowledge about the relevant mechanisms is currently fragmented and we therefore performed a comprehensive analysis of endothelial barrier regulation by RhoGTPases and their regulators. Combining RNAi with electrical impedance measurements we quantified the relevance of 270 Rho-associated genes for endothelial barrier function. Statistical analysis identified 10 targets of which six promoted- and four reduced endothelial barrier function upon downregulation. We analyzed in more detail two of these which were not previously identified as regulators of endothelial integrity. We found that the Rac1-GEF (Guanine nucleotide Exchange Factor) TIAM2 is a positive regulator and the Cdc42(Rac1)-GAP (GTPase-Activating Protein) SYDE1 is a negative regulator of the endothelial barrier function. Finally, we found that the GAP SYDE1 is part of a Cdc42-centered signaling unit, also comprising the Cdc42-GEF FARP1 and the Cdc42 effector PAK7 which controls the integrity of the endothelial barrier. In conclusion, using a siRNA-based screen, we identified new regulators of barrier function and found that Cdc42 is a dominant positive regulator of endothelial integrity.

A Graphical User Interface for Software-assisted Tracking of Protein Concentration in Dynamic Cellular Protrusions

Filopodia are dynamic, finger-like cellular protrusions associated with migration and cell-cell communication. In order to better understand the complex signaling mechanisms underlying filopodial initiation, elongation and subsequent stabilization or retraction, it is crucial to determine the spatio-temporal protein activity in these dynamic structures. To analyze protein function in filopodia, we recently developed a semi-automated tracking algorithm that adapts to filopodial shape-changes, thus allowing parallel analysis of protrusion dynamics and relative protein concentration along the whole filopodial length. Here, we present a detailed step-by-step protocol for optimized cell handling, image acquisition and software analysis. We further provide instructions for the use of optional features during image analysis and data representation, as well as troubleshooting guidelines for all critical steps along the way. Finally, we also include a comparison of the described image analysis software with other programs available for filopodia quantification. Together, the presented protocol provides a framework for accurate analysis of protein dynamics in filopodial protrusions using image analysis software.

Redundant Postsynaptic Functions of SynCAMs 1-3 during Synapse Formation

Investigating the roles of synaptogenic adhesion molecules during synapse formation has proven challenging, often due to compensatory functions between additional family members. The synaptic cell adhesion molecules 1–3 (SynCAM1–3) are expressed both pre- and postsynaptically, share highly homologous domains and are synaptogenic when ectopically presented to neurons; yet their endogenous functions during synaptogenesis are unclear. Here we report that SynCAM1–3 are functionally redundant and collectively necessary for synapse formation in cultured hippocampal neurons. Only triple knockdown (KD) of SynCAM1–3 using highly efficient, chained artificial microRNAs (amiRNAs) reduced synapse density and increased synapse area. Electrophysiological recordings of quantal release events supported an increase in synapse size caused by SynCAM1–3 depletion. Furthermore, a combinatorial, mosaic lentiviral approach comparing wild type (WT) and SynCAM1–3 KD neurons in the same culture demonstrate that SynCAM1–3 set synapse number and size through postsynaptic mechanisms. The results demonstrate that the redundancy between SynCAM1–3 has concealed their synaptogenic function at the postsynaptic terminal.

Genetic predisposition to high anxiety- and depression-like behavior coincides with diminished DNA methylation in the adult rat amygdala

Understanding biological mechanisms that shape vulnerability to emotional dysfunction is critical for elucidating the neurobiology of psychiatric illnesses like anxiety and depression. To elucidate molecular and epigenetic alterations in the brain that contribute to individual differences in emotionality, our laboratory utilized a rodent model of temperamental differences. Rats bred for low response to novelty (Low Responders, LRs) are inhibited in novel situations and display high anxiety, helplessness, and diminished sociability compared to High Novelty Responder (HR) rats. Our current transcriptome profiling experiment identified widespread gene expression differences in the amygdala of adult HR/LR rats; we hypothesize that HR/LR gene expression and downstream behavioral differences stem from distinct epigenetic (specifically DNA methylation) patterning in the HR/LR brain. Although we found similar levels of DNA methyltransferase proteins in the adult HR/LR amygdala, next-generation sequencing analysis of the methylome revealed 793 differentially methylated genomic sites between the groups. Most of the differentially methylated sites were hypermethylated in HR versus LR, so we next tested the hypothesis that enhancing DNA methylation in LRs would improve their anxiety/depression-like phenotype. We found that increasing DNA methylation in LRs (via increased dietary methyl donor content) improved their anxiety-like behavior and decreased their typically high levels of Forced Swim Test (FST) immobility; however, dietary methyl donor depletion exacerbated LRs' high FST immobility. These data are generally consistent with findings in depressed patients showing that treatment with DNA methylation-promoting agents improves depressive symptoms, and highlight epigenetic mechanisms that may contribute to individual differences in risk for emotional dysfunction.

SynCAMs - From axon guidance to neurodevelopmental disorders

Many cell adhesion molecules are located at synapses but only few of them can be considered synaptic cell adhesion molecules in the strict sense. Besides the Neurexins and Neuroligins, the LRRTMs (leucine rich repeat transmembrane proteins) and the SynCAMs/CADMs can induce synapse formation when expressed in non-neuronal cells and therefore are true synaptic cell adhesion molecules. SynCAMs (synaptic cell adhesion molecules) are a subfamily of the immunoglobulin superfamily of cell adhesion molecules. As suggested by their name, they were first identified as cell adhesion molecules at the synapse which were sufficient to trigger synapse formation. They also contribute to myelination by mediating axon-glia cell contacts. More recently, their role in earlier stages of neural circuit formation was demonstrated, as they also guide axons both in the peripheral and in the central nervous system. Mutations in SynCAM genes were found in patients diagnosed with autism spectrum disorders. The diverse functions of SynCAMs during development suggest that neurodevelopmental disorders are not only due to defects in synaptic plasticity. Rather, early steps of neural circuit formation are likely to contribute.