Alpha protocadherins and Pyk2 kinase regulate cortical neuron migration and cytoskeletal dynamics via Rac1 GTPase and WAVE complex in mice

CCCTC-binding factor N-terminal domain regulates clustered protocadherin gene expression by enhancing cohesin processivity

CTCF (CCCTC-binding factor) instructs 3D genome folding by anchoring or forestalling cohesin loop extrusion, but the exact mechanism remains obscure. Here, using clustered protocadherins (cPcdh) as model genes, we report that CTCF assists or facilitates cohesin loop extrusion by enhancing its processivity. Specifically, we show that, compared with the Pcdh α and γ gene clusters, the Pcdhβ cluster is greatly affected upon CTCFY226A/F228A mutation in the N-terminal domain. Given the long-range distance of the Pcdhβ cluster from the distal enhancer, this finding has interesting implications in CTCF regulation of cohesin processivity along the linear chromatin during DNA loop extrusion. In particular, the effect on cohesin processivity upon CTCFY226A/F228A mutation is conspicuously similar to that of WAPL overexpression, suggesting that, in contrast to the general view of blocking or forestalling cohesin, CTCF may actually enhance or facilitate cohesin loop extrusion during 3D genome folding. We conclude that CTCF enhances cohesin enrichments via the N-terminal YDF motif in clustered protocadherin genes in a genomic-distance biased manner.

Both GEF domains of the autism and developmental epileptic encephalopathy-associated Trio protein are required for proper tangential migration of GABAergic interneurons

Recessive and de novo mutations in the TRIO gene are associated with intellectual deficiency (ID), autism spectrum disorder (ASD) and developmental epileptic encephalopathies (DEE). TRIO is a dual guanine nucleotide exchange factor (GEF) that activates Rac1, Cdc42 and RhoA. Trio has been extensively studied in excitatory neurons, and has recently been found to regulate the switch from tangential to radial migration in GABAergic interneurons (INs) through GEFD1-Rac1-dependent SDF1α/CXCR4 signaling. Given the central role of Rho-GTPases during neuronal migration and the implication of IN pathologies in ASD and DEE, we investigated the relative roles of both Trio's GEF domains in regulating the dynamics of INs tangential migration. In Trio-/- mice, we observed reduced numbers of tangentially migrating INs, with intact progenitor proliferation. Further, we noted increased growth cone collapse in developing INs, suggesting altered cytoskeleton dynamics. To bypass the embryonic mortality of Trio-/- mice, we generated Dlx5/6Cre;Trioc/c conditional mutant mice (TriocKO), which develop spontaneous seizures and behavioral deficits reminiscent of ASD and ID. These phenotypes are associated with reduced cortical IN density and functional cortical inhibition. Mechanistically, this reduction of cortical IN numbers reflects a premature switch to radial migration, with an aberrant early entry in the cortical plate, as well as major deficits in cytoskeletal dynamics, including enhanced leading neurite branching and slower nucleokinesis reflecting reduced actin filament condensation and turnover as well as a loss of response to the motogenic effect of EphA4/ephrin A2 reverse signaling. Further, we show that both Trio GEFD1 and GEFD2 domains are required for proper IN migration, with a dominant role of the RhoA-activating GEFD2 domain. Altogether, our data show a critical role of the DEE/ASD-associated Trio gene in the establishment of cortical inhibition and the requirement of both GEF domains in regulating IN migration dynamics.

Circulating tsRNAs serve as potential biomarkers for predicting postoperative delirium in elderly patients receiving lower extremity orthopedic surgery

Background

Postoperative delirium (POD) is a serious neuropsychiatric complication in elderly surgical patients, yet its pathogenesis remains incompletely understood. Transfer RNA-derived small RNAs (tsRNAs) have emerged as crucial regulators in neurological disorders. We investigated whether specific tsRNAs could serve as predictive biomarkers for POD.

Methods

This study conducted a prospective case-control study of 158 elderly patients (≥60 years) undergoing orthopedic surgery. Plasma samples were collected preoperatively and on postoperative day 3.tsRNA expression profiles were analyzed using RNA sequencing and validated by RT-qPCR. Propensity score matching was performed to balance demographic and clinical variables. The predictive value of candidate tsRNAs was assessed using ROC analysis, and their potential functions were explored through bioinformatic analyses.

Results

Among 128 non-POD and 30 POD patients, two tsRNAs (Other-14: 31-tRNA-Gly-CCC-3 and Other-39: 73-tRNA-Arg-TCG-5) showed significantly elevated preoperative levels in POD patients (p<0.001).ROC analysis revealed strong predictive performance (AUC=0.868 and 0.956, respectively).These differences persisted in the propensity-matched cohort (29 pairs).Bioinformatic analyses indicated enrichment in pathways related to neurotransmission, inflammation, and metabolism.

Conclusion

This study identified novel tsRNA biomarkers that robustly predict POD risk and provide insights into its molecular pathogenesis. These findings may facilitate early risk stratification and preventive interventions.

Altered cytoskeleton dynamics in patient-derived iPSC-based model of PCDH19 clustering epilepsy

Protocadherin 19 (PCDH19) is an adhesion molecule involved in cell-cell interaction whose mutations cause a drug-resistant form of epilepsy, named PCDH19-Clustering Epilepsy (PCDH19-CE, MIM 300088). The mechanism by which altered PCDH19 function drive pathogenesis is not yet fully understood. Our previous work showed that PCDH19 dysfunction is associated with altered orientation of the mitotic spindle and accelerated neurogenesis, suggesting a contribution of altered cytoskeleton organization in PCDH19-CE pathogenesis in the control of cell division and differentiation. Here, we evaluate the consequences of altered PCDH19 function on microfilaments and microtubules organization, using a disease model obtained from patient-derived induced pluripotent stem cells. We show that iPSC-derived cortical neurons are characterized by altered cytoskeletal dynamics, suggesting that this protocadherin has a role in modulating stability of MFs and MTs. Consistently, the levels of acetylated-tubulin, which is related with stable MTs, are significantly increased in cortical neurons derived from the patient's iPSCs compared to control cells, supporting the idea that the altered dynamics of the MTs depends on their increased stability. Finally, performing live-imaging experiments using fluorescence recovery after photobleaching and by monitoring GFP-tagged end binding protein 3 (EB3) "comets," we observe an impairment of the plus-end polymerization speed in PCDH19-mutated cortical neurons, therefore confirming the impaired MT dynamics. In addition to altering the mitotic spindle formation, the present data unveil that PCDH19 dysfunction leads to altered cytoskeletal rearrangement, providing therapeutic targets and pharmacological options to treat this disorder.

CKLF1 in cardiovascular and cerebrovascular diseases

Chemokine like factor 1 (CKLF1) is a novel atypical chemokine, playing a crucial role in cardiovascular and cerebrovascular diseases (CCVDs) demonstrated by a growing body of works. In cardiovascular diseases including atherosclerosis and myocardial infarction, meanwhile in cerebrovascular diseases such as ischemic stroke and hemorrhagic stroke, the expression levels of CKLF1 change markedly, which triggers downstream signaling pathways by binding with its functional receptors, and then exerts multiple effects to participate in the occurrence and development of these CCVDs. The functional roles of CKLF1 are dynamic and CKLF1 may act as a double-edged sword. The CCVDs-promoting role is related to recruiting inflammatory cells, enhancing the proliferation of vascular smooth muscle cells and endothelial cells, while the CCVDs-suppressing role may correlate with migration of nerve cells and promotion of hematopoietic stem cell proliferation which contributes to disease recovery. Based on this, the paper intends to review expression shifts, potential roles, and molecular mechanisms of CKLF1 in CCVDs, and the current status of CKLF1 targeted therapeutic strategies is also included. We hope this review may provide a valuable reference for using CKLF1 as a diagnostic and prognostic biomarker for CCVDs or developing novel treatments.

Gamma-protocadherins regulate dendrite self-recognition and dynamics to drive self-avoidance

Neurons form cell-type-specific morphologies that are shaped by cell-surface molecules and their cellular events governing dendrite growth. One growth rule is dendrite self-avoidance, whereby dendrites distribute uniformly within a neuron's territory by avoiding sibling branches. In mammalian neurons, dendrite self-avoidance is regulated by a large family of cell-recognition molecules called the clustered protocadherins (cPcdhs). Genetic and molecular studies suggest that the cPcdhs mediate homophilic recognition and repulsion between self-dendrites. However, this model has not been tested through direct investigation of self-avoidance during development. Here, we performed live imaging and four-dimensional (4D) quantifications of dendrite morphogenesis to define the dynamics and cPcdh-dependent mechanisms of self-avoidance. We focused on the mouse retinal starburst amacrine cell (SAC), which requires the gamma-Pcdhs (Pcdhgs) and self/non-self-recognition to establish a stereotypic radial morphology while permitting dendritic interactions with neighboring SACs. Through morphogenesis, SACs extend dendritic protrusions that iteratively fill the growing arbor and contact and retract from nearby self-dendrites. Compared to non-self-contacting protrusions, self-contacting events have longer lifetimes, and a subset persists as loops. In the absence of the Pcdhgs, non-self-contacting dynamics are unaffected but self-contacting retractions are significantly diminished. Self-contacting bridges accumulate, leading to the bundling of dendritic processes and disruption to the arbor shape. By tracking dendrite self-avoidance in real time, our findings establish that the γ-Pcdhs mediate self-recognition and retraction between contacting sibling dendrites. Our results also illustrate how self-avoidance shapes stochastic and space-filling dendritic outgrowth for robust pattern formation in mammalian neurons.

Early-Life Exposure to 4-Hydroxy-4'-Isopropoxydiphenylsulfone Induces Behavioral Deficits Associated with Autism Spectrum Disorders in Mice

Exposure to bisphenol A (BPA) during gestation and lactation is considered to be a potential risk factor for autism spectrum disorder (ASD) in both humans and animals. As a novel alternative to BPA, 4-hydroxy-4'-isopropoxydiphenylsulfone (BPSIP) is frequently detected in breast milk and placental barrier systems, suggesting potential transmission from the mother to offspring and increased risk of exposure. Gestation and lactation are critical periods for central nervous system development, which are vulnerable to certain environmental pollutants. Herein, we investigated the behavioral impacts and neurobiological effects of early-life exposure to BPSIP (0.02, 0.1, and 0.5 mg/kg body weight/day) in mice offspring. Behavioral studies indicated that BPSIP exposure induced ASD-like behaviors, including elevated anxiety-related behavior and decreased spatial memory, in both male and female pups. A distinct pattern of reduced social novelty was observed only in female offspring, accompanied by significant alterations in antioxidant levels. Transcriptome analysis demonstrated that differentially expressed genes (DEGs) were mainly enriched in pathways related to behaviors and neurodevelopment, which were consistent with the observed phenotype. Besides, a decrease in the protein levels of complex IV (COX IV) across all tested populations suggests a profound impact on mitochondrial function, potentially leading to abnormal energy metabolism in individuals with autism. Additionally, changes in synaptic proteins, evidenced by alterations in synapsin 1 (SYN1) and postsynaptic density protein-95 (PSD95) levels in the cerebellum and hippocampus, support the notion of synaptic involvement. These findings suggest that BPSIP may induce sex-specific neurotoxic effects that involve oxidative stress, energy generation, and synaptic plasticity.

Complex Regulation of Protocadherin Epigenetics on Aging-Related Brain Health

Life expectancy continues to increase in the high-income world due to advances in medical care; however, quality of life declines with increasing age due to normal aging processes. Current research suggests that various aspects of aging are genetically modulated and thus may be slowed via genetic modification. Here, we show evidence for epigenetic modulation of the aging process in the brain from over 1800 individuals as part of the Framingham Heart Study. We investigated the methylation of genes in the protocadherin (PCDH) clusters, including the alpha (PCHDA), beta (PCDHB), and gamma (PCDHG) clusters. Reduced PCDHG, elevated PCDHA, and elevated PCDHB methylation levels were associated with substantial reductions in the rate of decline of regional white matter volume as well as certain cognitive skills, independent of overall accelerated or retarded aging as estimated by a DNA clock. These results are likely due to the different effects of the expression of genes in the alpha, beta, and gamma PCHD clusters and suggest that experience-based aging processes related to a decline in regional brain volume and select cognitive skills may be slowed via targeted epigenetic modifications.

PCDHA9 as a candidate gene for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease. To identify additional genetic factors, we analyzed exome sequences in a large cohort of Chinese ALS patients and found a homozygous variant (p.L700P) in PCDHA9 in three unrelated patients. We generated Pcdhα9 mutant mice harboring either orthologous point mutation or deletion mutation. These mice develop progressive spinal motor loss, muscle atrophy, and structural/functional abnormalities of the neuromuscular junction, leading to paralysis and early lethality. TDP-43 pathology is detected in the spinal motor neurons of aged mutant mice. Mechanistically, we demonstrate that Pcdha9 mutation causes aberrant activation of FAK and PYK2 in aging spinal cord, and dramatically reduced NKA-α1 expression in motor neurons. Our single nucleus multi-omics analysis reveals disturbed signaling involved in cell adhesion, ion transport, synapse organization, and neuronal survival in aged mutant mice. Together, our results present PCDHA9 as a potential ALS gene and provide insights into its pathogenesis.

Confluence and convergence of Dscam and Pcdh cell-recognition codes

The ability of neurites of the same neuron to avoid each other (self-avoidance) is a conserved feature in both invertebrates and vertebrates. The key to self-avoidance is the generation of a unique subset of cell-surface proteins in individual neurons engaging in isoform-specific homophilic interactions that drive neurite repulsion rather than adhesion. Among these cell-surface proteins are fly Dscam1 and vertebrate clustered protocadherins (cPcdhs), as well as the recently characterized shortened Dscam (sDscam) in the Chelicerata. Herein, we review recent advances in our understanding of how cPcdh, Dscam, and sDscam cell-surface recognition codes are expressed and translated into cellular functions essential for neural wiring.

Lactoferrin deficiency during lactation increases the risk of depressive-like behavior in adult mice

BACKGROUND

Lactoferrin is an active protein in breast milk that plays an important role in the growth and development of infants and is implicated as a neuroprotective agent. The incidence of depression is currently increasing, and it is unclear whether the lack of lactoferrin during lactation affects the incidence of depressive-like behavior in adulthood.

RESULTS

Lack of lactoferrin feeding during lactation affected the barrier and innate immune functions of the intestine, disrupted the intestinal microflora, and led to neuroimmune dysfunction and neurodevelopmental delay in the hippocampus. When exposed to external stimulation, adult lactoferrin feeding-deficient mice presented with worse depression-like symptoms; the mechanisms involved were activation of the LPS-TLR4 signalling pathway in the intestine and hippocampus, reduced BDNF-CREB signaling pathway in hippocampus, increased abundance of depression-related bacteria, and decreased abundance of beneficial bacteria.

CONCLUSIONS

Overall, our findings reveal that lactoferrin feeding deficient during lactation can increase the risk of depressive-like behavior in adults. The mechanism is related to the regulatory effect of lactoferrin on the development of the "microbial-intestinal-brain" axis.

The intrinsically disordered cytoplasmic tail of a dendrite branching receptor uses two distinct mechanisms to regulate the actin cytoskeleton

Dendrite morphogenesis is essential for neural circuit formation, yet the molecular mechanisms underlying complex dendrite branching remain elusive. Previous studies on the highly branched Caenorhabditis elegans PVD sensory neuron identified a membrane co-receptor complex that links extracellular signals to intracellular actin remodeling machinery, promoting high-order dendrite branching. In this complex, the claudin-like transmembrane protein HPO-30 recruits the WAVE regulatory complex (WRC) to dendrite branching sites, stimulating the Arp2/3 complex to polymerize actin. We report here our biochemical and structural analysis of this interaction, revealing that the intracellular domain (ICD) of HPO-30 is intrinsically disordered and employs two distinct mechanisms to regulate the actin cytoskeleton. First, HPO-30 ICD binding to the WRC requires dimerization and involves the entire ICD sequence, rather than a short linear peptide motif. This interaction enhances WRC activation by the GTPase Rac1. Second, HPO-30 ICD directly binds to the sides and barbed end of actin filaments. Binding to the barbed end requires ICD dimerization and inhibits both actin polymerization and depolymerization, resembling the actin capping protein CapZ. These dual functions provide an intriguing model of how membrane proteins can integrate distinct mechanisms to fine-tune local actin dynamics.

Visualization of trans-interactions of a protocadherin-α between processes originating from single neurons

Clustered protocadherin (Pcdh), a cell adhesion protein, is involved in the self-recognition and non-self-discrimination of neurons by conferring diversity on the cell surface. Although the roles of Pcdh in neurons have been elucidated, it has been challenging to visualize its adhesion activity in neurons, which is a molecular function of Pcdh. Here, we present fluorescent indicators, named IPADs, which visualize the interaction of protocadherin-α4 isoform (α4). IPADs successfully visualize not only homophilic α4 trans-interactions, but also combinatorial homophilic interactions between cells. The reversible nature of IPADs overcomes a drawback of the split-GFP technique and allows for monitoring the dissociation of α4 trans-interactions. Specially designed IPADs for self-recognition are able to monitor the formation and disruption of α4 trans-interactions between processes originating from the same neurons. We expect that IPADs will be useful tools for obtaining spatiotemporal information on Pcdh interactions in neuronal self-recognition and non-self-discrimination processes.

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 ob-tained from E12.5 fetal mouse brains 3 hours after VPA administration revealed that VPA significant-ly increased or decreased the expression of approximately 7,300 genes. No significant sex differ-ences in VPA-induced gene expression were observed. Expression of genes associated with neu-rodevelopmental disorders (NDDs) such as autism as well as neurogenesis, axon growth and syn-aptogenesis, GABAergic, glutaminergic and dopaminergic synaptic transmission, perineuronal nets, and circadian rhythms was dysregulated by VPA. Moreover, expression of 399 autism risk genes was significantly altered by VPA as was expression of 252 genes that have been reported to play fundamental roles in the development of the nervous system but are not otherwise linked to autism. 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 pro-vides potential targets for future hypothesis-driven approaches to elucidating the proximal underly-ing causes of defective brain connectivity in NDDs such as autism.

Estimation of Discriminative Multimodal Brain Network Connectivity Using Message-Passing-Based Nonlinear Network Fusion

Effective estimation of brain network connectivity enables better unraveling of the extraordinary complexity interactions of brain regions and helps in auxiliary diagnosis of psychiatric disorders. Considering different modalities can provide comprehensive characterizations of brain connectivity, we propose the message-passing-based nonlinear network fusion (MP-NNF) algorithm to estimate multimodal brain network connectivity. In the proposed method, the initial functional and structural networks were computed from fMRI and DTI separately. Then, we update every unimodal network iteratively, making it more similar to the others in every iteration, and finally converge to one unified network. The estimated brain connectivities integrate complementary information from multiple modalities while preserving their original structure, by adding the strong connectivities present in unimodal brain networks and eliminating the weak connectivities. The effectiveness of the method was evaluated by applying the learned brain connectivity for the classification of major depressive disorder (MDD). Specifically, 82.18% classification accuracy was achieved even with the simple feature selection and classification pipeline, which significantly outperforms the competing methods. Exploration of brain connectivity contributed to MDD identification suggests that the proposed method not only improves the classification performance but also was sensitive to critical disease-related neuroimaging biomarkers.

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

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

GABAergic neurons maturation is regulated by a delicate network

Gamma-aminobutyric acid-expressing (GABAergic) neurons are implicated in a variety of neuropsychiatric disorders, such as epilepsy, anxiety, autism, and other pathological processes, including cerebral ischemia injury and drug addiction. Therefore, GABAergic neuronal processes warrant further research. The development of GABAergic neurons is a tightly controlled process involving the activity of multiple transcription and growth factors. Here, we focus on the gene expression pathways and the molecular modulatory networks that are engaged during the development of GABAergic neurons with the goal of exploring regulatory mechanisms that influence GABAergic neuron fate (i.e., maturation). Overall, we hope to provide a basis for clarifying the pathogenesis of neurodegenerative disorders.

Activated PyK2 and Its Associated Molecules Transduce Cellular Signaling from the Cancerous Milieu for Cancer Metastasis

PyK2 is a member of the proline-rich tyrosine kinase and focal adhesion kinase families and is ubiquitously expressed. PyK2 is mainly activated by stimuli, such as activated Src kinases and intracellular acidic pH. The mechanism of PyK2 activation in cancer cells has been addressed extensively. The up-regulation of PyK2 through overexpression and enhanced phosphorylation is a key feature of tumorigenesis and cancer migration. In this review, we summarized the cancer milieu, including acidification and cancer-associated molecules, such as chemical reagents, interactive proteins, chemokine-related molecules, calcium channels/transporters, and oxidative molecules that affect the fate of PyK2. The inhibition of PyK2 leads to a beneficial strategy to attenuate cancer cell development, including metastasis. Thus, we highlighted the effect of PyK2 on various cancer cell types and the distribution of molecules that affect PyK2 activation. In particular, we underlined the relationship between PyK2 and cancer metastasis and its potential to treat cancer cells.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

Signaling Proteins Recruited to the Sperm Binding Site: Role of β-Catenin and Rho A

Sperm interaction with the oocyte plasma membrane triggers a localized response in the mouse oocyte that leads to remodeling of oocyte surface as well as the underlying cortical actin layer. The recent demonstration that PTK2B is recruited and activated at the sperm binding site raised the possibility that multiple signaling events may be activated during this stage of fertilization. The present study demonstrated that β-catenin and Rho A were recruited to the cortex underlying bound/fused sperm. To determine whether sperm-oocyte contact was sufficient to initiate β-catenin recruitment, Cd9-null, and PTK2b-null oocytes were tested for the ability to recruit β-catenin to sperm binding sites. Both Cd9 and Ptk2b ablation reduced β-catenin recruitment raising the possibility that PTK2B may act downstream of CD9 in the response to sperm binding/fusion. Further immunofluorescence study revealed that β-catenin co-localized with f-actin in the interstitial regions between actin layer fenestrae. Rho A, in contrast, was arranged underneath the actin layer in both the fenestra and the interstitial regions suggesting that they may play different roles in the oocyte.

Platelet-released extracellular vesicles: the effects of thrombin activation

Platelets exert fundamental roles in thrombosis, inflammation, and angiogenesis, contributing to different pathologies from cardiovascular diseases to cancer. We previously reported that platelets release extracellular vesicles (pEVs) which contribute to thrombus formation. However, pEV composition remains poorly defined. Indeed, pEV quality and type, rather than quantity, may be relevant in intravascular cross-talk with either circulating or vascular cells. We aimed to define the phenotypic characteristics of pEVs released spontaneously and those induced by thrombin activation to better understand their role in disease dissemination. pEVs obtained from washed platelets from healthy donor blood were characterized by flow cytometry. pEVs from thrombin-activated platelets (T-pEVs) showed higher levels of P-selectin and active form of glycoprotein IIb/IIIa than baseline non-activated platelets (B-pEVs). Following mass spectrometry-based differential proteomic analysis, significant changes in the abundance of proteins secreted in T-pEVs compared to B-pEVs were found. These differential proteins were involved in coagulation, adhesion, cytoskeleton, signal transduction, metabolism, and vesicle-mediated transport. Interestingly, release of proteins relevant for cell adhesion, intrinsic pathway coagulation, and platelet activation signalling was significantly modified by thrombin stimulation. A novel pEV-associated protein (protocadherin-α4) was found to be significantly reduced in T-pEVs showing a shift towards increased expression in the membranes of activated platelets. In summary, platelet activation induced by thrombin triggers the shedding of pEVs with a complex proteomic pattern rich in procoagulant and proadhesive proteins. Crosstalk with other vascular and blood cells in a paracrine regulatory mode could extend the prothrombotic signalling as well as promote proteostasic changes in other cellular types.

How clustered protocadherin binding specificity is tuned for neuronal self-/nonself-recognition

The stochastic expression of fewer than 60 clustered protocadherin (cPcdh) isoforms provides diverse identities to individual vertebrate neurons and a molecular basis for self-/nonself-discrimination. cPcdhs form chains mediated by alternating cis and trans interactions between apposed membranes, which has been suggested to signal self-recognition. Such a mechanism requires that cPcdh cis dimers form promiscuously to generate diverse recognition units, and that trans interactions have precise specificity so that isoform mismatches terminate chain growth. However, the extent to which cPcdh interactions fulfill these requirements has not been definitively demonstrated. Here, we report biophysical experiments showing that cPcdh cis interactions are promiscuous, but with preferences favoring formation of heterologous cis dimers. Trans homophilic interactions are remarkably precise, with no evidence for heterophilic interactions between different isoforms. A new C-type cPcdh crystal structure and mutagenesis data help to explain these observations. Overall, the interaction characteristics we report for cPcdhs help explain their function in neuronal self-/nonself-discrimination.

Fat3 acts through independent cytoskeletal effectors to coordinate asymmetric cell behaviors during polarized circuit assembly

The polarized flow of information through neural circuits depends on the orderly arrangement of neurons, their processes, and their synapses. This polarity emerges sequentially in development, starting with the directed migration of neuronal precursors, which subsequently elaborate neurites that form synapses in specific locations. In other organs, Fat cadherins sense the position and then polarize individual cells by inducing localized changes in the cytoskeleton that are coordinated across the tissue. Here, we show that the Fat-related protein Fat3 plays an analogous role during the assembly of polarized circuits in the murine retina. We find that the Fat3 intracellular domain (ICD) binds to cytoskeletal regulators and synaptic proteins, with discrete motifs required for amacrine cell migration and neurite retraction. Moreover, upon ICD deletion, extra neurites form but do not make ectopic synapses, suggesting that Fat3 independently regulates synapse localization. Thus, Fat3 serves as a molecular node to coordinate asymmetric cell behaviors across development.

STK25 and MST3 Have Overlapping Roles to Regulate Rho GTPases during Cortical Development

Precise control of neuronal migration is required for the laminar organization of the neocortex and critical for brain function. We previously reported that the acute disruption of the Stk25 gene (Stk25 conditional knock-out; cKO) during mouse embryogenesis causes anomalous neuronal migration in the neocortex, but paradoxically the Stk25 cKO did not have a cortical phenotype, suggesting some forms of compensation exist. In this study, we report that MST3, another member of the GCKIII subgroup of the Ste20-like kinase family, compensates for loss of Stk25 and vice versa with sex independent manner. MST3 overexpression rescued neuronal migration deficit and abnormal axonogenesis in Stk25 cKO brains. Mechanistically, STK25 leads to Rac1 activation and reduced RhoA levels in the developing brain, both of which are required to fully restore neuronal migration in the Stk25 cKO brain. Abnormal migration phenotypes are also rescued by overexpression of Bacurd1and Cul3, which target RhoA for degradation, and activate Rac1. This study reveals that MST3 upregulation is capable of rescuing acute Stk25 deficiency and resolves details of signaling downstream STK25 required for corticogenesis both common to and distinct from MST3 signaling.SIGNIFICANCE STATEMENT Proper neuronal migration during cortical development is required for normal neuronal function. Here, we show that STK25 and MST3 kinases regulate neuronal migration and polarization in a mutually compensatory manner. Furthermore, STK25 balances Rac1 activity and RhoA level through forming complexes with α-PIX and β-PIX, GTPase regulatory enzymes, and Cullin3-Bacurd1/Kctd13, a pair of RhoA ubiquitination molecules in a kinase activity-independent manner. Our findings demonstrate the importance of overlapping and unique roles of STK25 and MST3 to regulate Rho GTPase activities in cortical development.

The Non-receptor Tyrosine Kinase Pyk2 in Brain Function and Neurological and Psychiatric Diseases

Pyk2 is a non-receptor tyrosine kinase highly enriched in forebrain neurons. Pyk2 is closely related to focal adhesion kinase (FAK), which plays an important role in sensing cell contacts with extracellular matrix and other extracellular signals controlling adhesion and survival. Pyk2 shares some of FAK’s characteristics including recruitment of Src-family kinases after autophosphorylation, scaffolding by interacting with multiple partners, and activation of downstream signaling pathways. Pyk2, however, has the unique property to respond to increases in intracellular free Ca²⁺, which triggers its autophosphorylation following stimulation of various receptors including glutamate NMDA receptors. Pyk2 is dephosphorylated by the striatal-enriched phosphatase (STEP) that is highly expressed in the same neuronal populations. Pyk2 localization in neurons is dynamic, and altered following stimulation, with post-synaptic and nuclear enrichment. As a signaling protein Pyk2 is involved in multiple pathways resulting in sometimes opposing functions depending on experimental models. Thus Pyk2 has a dual role on neurites and dendritic spines. With Src family kinases Pyk2 participates in postsynaptic regulations including of NMDA receptors and is necessary for specific types of synaptic plasticity and spatial memory tasks. The diverse functions of Pyk2 are also illustrated by its role in pathology. Pyk2 is activated following epileptic seizures or ischemia-reperfusion and may contribute to the consequences of these insults whereas Pyk2 deficit may contribute to the hippocampal phenotype of Huntington’s disease. Pyk2 gene, PTK2B, is associated with the risk for late-onset Alzheimer’s disease. Studies of underlying mechanisms indicate a complex contribution with involvement in amyloid toxicity and tauopathy, combined with possible functional deficits in neurons and contribution in microglia. A role of Pyk2 has also been proposed in stress-induced depression and cocaine addiction. Pyk2 is also important for the mobility of astrocytes and glioblastoma cells. The implication of Pyk2 in various pathological conditions supports its potential interest for therapeutic interventions. This is possible through molecules inhibiting its activity or increasing it through inhibition of STEP or other means, depending on a precise evaluation of the balance between positive and negative consequences of Pyk2 actions.

Pyk2 suppresses contextual fear memory in an autophosphorylation-independent manner

Clustered protocadherins (Pcdhs) are a large family of cadherin-like cell adhesion proteins that are central for neurite self-avoidance and neuronal connectivity in the brain. Their downstream non-receptor tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2, also known as Ptk2b, Cakb, Raftk, Fak2, and Cadtk) is predominantly expressed in the hippocampus. We constructed Pyk2 null mouse lines and found that these mutant mice showed enhancement in contextual fear memory, without any change in auditory-cued and spatial-referenced learning and memory. In addition, by preparing Y402F mutant mice, we observed that Pyk2 suppressed contextual fear memory in an autophosphorylation-independent manner. Moreover, using high-throughput RNA sequencing, we found that immediate early genes, such as Npas4, cFos, Zif268/Egr1, Arc, and Nr4a1, were enhanced in Pyk2 null mice. We further showed that Pyk2 disruption affected pyramidal neuronal complexity and spine dynamics. Thus, we demonstrated that Pyk2 is a novel fear memory suppressor molecule and Pyk2 null mice provide a model for understanding fear-related disorders. These findings have interesting implications regarding dysregulation of the Pcdh‒Pyk2 axis in neuropsychiatric disorders.

Gamma-protocadherin localization at the synapse is associated with parameters of synaptic maturation

Clustered protocadherins (Pcdhs) are a family of ~60 cadherin-like proteins (divided into subclasses α, β, and γ) that regulate dendrite morphology and neural connectivity. Their expression is controlled through epigenetic regulation at a gene cluster encoding the molecules. During neural development, Pcdhs mediate dendrite self-avoidance in some neuronal types through an uncharacterized anti-adhesive mechanism. Pcdhs are also important for dendritic complexity in cortical neurons likely through a pro-adhesive mechanism. Pcdhs have also been postulated to participate in synaptogenesis and connectivity. Some synaptic defects were noted in knockout animals, including synaptic number and physiology, but the role of these molecules in synaptic development is not understood. The effect of Pcdh knockout on dendritic patterning may present a confound to studying synaptogenesis. We showed previously that Pcdh-γs are highly enriched in intracellular compartments in dendrites and spines with localization at only a few synaptic clefts. To gain insight into how Pcdh-γs might affect synapses, we compared synapses that harbored Pcdh-γs versus those that did not for parameters of synaptic maturation including pre- and postsynaptic size, postsynaptic perforations, and spine morphology by light microscopy in cultured hippocampal neurons and by serial section immuno-electron microscopy in hippocampal CA1. In mature neurons, synapses immunopositive for Pcdh-γs were larger in diameter with more frequent perforations. Analysis of spines in cultured neurons revealed that mushroom spines were more frequently immunopositive for Pcdh-γs at their tips than thin spines. These results suggest that Pcdh-γ function at the synapse may be related to promotion of synaptic maturation and stabilization.

Rac1 Signaling in Amygdala Astrocytes Regulates Fear Memory Acquisition and Retrieval

The importance of astrocytes in behavior control is increasingly appreciated, but little is known about the effects of their dynamic activity in regulating learning and memory. In the present study, we constructed AAVs of photoactivatable and photoinactivatable Ras-related C3 botulinum toxin substrate 1 (Rac1) under the mGFAP promoter, which enabled the manipulation of Rac1 activity in astrocytes by optical stimulation in free-moving mice. We found that both up-regulation and down-regulation of astrocytic Rac1 activity in the basolateral amygdala (BLA) attenuated memory acquisition in a fear conditioning mouse model. Meanwhile, neuronal activation in the BLA induced by memory acquisition was inhibited under both the up- and down-regulation of astrocytic Rac1 activity during training. In terms of the impact on fear memory retrieval, we found both up- and down-regulation of BLA astrocytic Rac1 activity impaired memory retrieval of fear conditioning and memory retrieval-induced neuronal activation. Notably, the effect of astrocytic Rac1 on memory retrieval was reversible. Our results demonstrate that the normal activity of astrocytic Rac1 is necessary for the activation of neurons and memory formation. Both activation and inactivation of astrocytic Rac1 activity in the BLA reduced the excitability of neurons, and thereby impaired fear memory acquisition and retrieval.

Sperm-oocyte signaling: the role of IZUMO1R and CD9 in PTK2B activation and actin remodeling at the sperm binding site†

Sperm-oocyte binding initiates an outside-in signaling event in the mouse oocyte that triggers recruitment and activation of the cytosolic protein kinase PTK2B in the cortex underlying the bound sperm. While not involved in gamete fusion, PTK2B activity promotes actin remodeling events important during sperm incorporation. However, the mechanism by which sperm-oocyte binding activates PTK2B is unknown, and the present study examined the possibility that sperm interaction with specific oocyte surface proteins plays an important role in PTK2B activation. Imaging studies revealed that as IZUMO1R and CD9 became concentrated at the sperm binding site, activated (phosphorylated) PTK2B accumulated in the cortex underlying the sperm head and in microvilli partially encircling the sperm head. In order to determine whether IZUMO1R and/or CD9 played a significant role in PTK2B recruitment and activation at the sperm binding site, the ability of oocytes null for Izumo1r or Cd9, to initiate an increase in PTK2B content and activation was tested. The results revealed that IZUMO1R played a minor role in PTK2B activation and had no effect on actin remodeling; however, CD9 played a very significant role in PTK2B activation and subsequent actin remodeling at the sperm binding site. These findings suggest the possibility that interaction of sperm surface proteins with CD9 or CD9-associated oocyte proteins triggers PTK2B activation at the sperm binding site.

Clustered Protocadherins Emerge as Novel Susceptibility Loci for Mental Disorders

The clustered protocadherins (cPcdhs) are a subfamily of type I single-pass transmembrane cell adhesion molecules predominantly expressed in the brain. Their stochastic and combinatorial expression patterns encode highly diverse neural identity codes which are central for neuronal self-avoidance and non-self discrimination in brain circuit formation. In this review, we first briefly outline mechanisms for generating a tremendous diversity of cPcdh cell-surface assemblies. We then summarize the biological functions of cPcdhs in a wide variety of neurodevelopmental processes, such as neuronal migration and survival, dendritic arborization and self-avoidance, axonal tiling and even spacing, and synaptogenesis. We focus on genetic, epigenetic, and 3D genomic dysregulations of cPcdhs that are associated with various neuropsychiatric and neurodevelopmental diseases. A deeper understanding of regulatory mechanisms and physiological functions of cPcdhs should provide significant insights into the pathogenesis of mental disorders and facilitate development of novel diagnostic and therapeutic strategies.

The γ-Protocadherins Regulate the Survival of GABAergic Interneurons during Developmental Cell Death

Inhibitory interneurons integrate into developing circuits in specific ratios and distributions. In the neocortex, inhibitory network formation occurs concurrently with the apoptotic elimination of a third of GABAergic interneurons. The cell surface molecules that select interneurons to survive or die are unknown. Here, we report that members of the clustered Protocadherins (cPCDHs) control GABAergic interneuron survival during developmentally-regulated cell death. Conditional deletion of the gene cluster encoding the γ-Protocadherins (Pcdhgs) from developing GABAergic neurons in mice of either sex causes a severe loss of inhibitory populations in multiple brain regions and results in neurologic deficits such as seizures. By focusing on the neocortex and the cerebellar cortex, we demonstrate that reductions of inhibitory interneurons result from elevated apoptosis during the critical postnatal period of programmed cell death (PCD). By contrast, cortical interneuron (cIN) populations are not affected by removal of Pcdhgs from pyramidal neurons or glial cells. Interneuron loss correlates with reduced AKT signaling in Pcdhg mutant interneurons, and is rescued by genetic blockade of the pro-apoptotic factor BAX. Together, these findings identify the PCDHGs as pro-survival transmembrane proteins that select inhibitory interneurons for survival and modulate the extent of PCD. We propose that the PCDHGs contribute to the formation of balanced inhibitory networks by controlling the size of GABAergic interneuron populations in the developing brain.SIGNIFICANCE STATEMENT A pivotal step for establishing appropriate excitatory-inhibitory ratios is adjustment of neuronal populations by cell death. In the mouse neocortex, a third of GABAergic interneurons are eliminated by BAX-dependent apoptosis during the first postnatal week. Interneuron cell death is modulated by neural activity and pro-survival pathways but the cell-surface molecules that select interneurons for survival or death are unknown. We demonstrate that members of the cadherin superfamily, the clustered γ-Protocadherins (PCDHGs), regulate the survival of inhibitory interneurons and the balance of cell death. Deletion of the Pcdhgs in mice causes inhibitory interneuron loss in the cortex and cerebellum, and leads to motor deficits and seizures. Our findings provide a molecular basis for controlling inhibitory interneuron population size during circuit formation.

Roles of Pyk2 in signal transduction after gonadotropin-releasing hormone receptor stimulation

The receptor for gonadotropin-releasing hormone (GnRH) is highly expressed in hypothalamic neurons. It has been reported that GnRH treatment of cultured GnRH neurons (GT1-7 cells) activated proline-rich tyrosine kinase 2 (Pyk2), and Pyk2 was involved in the activation of extracellular signal-regulated protein kinase 1 (ERK1) and ERK2 (ERK1/2). In the present study, we first examined the possibility that GnRH treatment might activate epidermal growth factor receptor (EGFR). We found that activation of EGFR after GnRH treatment for 5 min was much less than after EGF or heparin-binding EGF treatment. Next, we examined whether or not Pyk2 bound to growth factor receptor-binding protein 2 (Grb2). We overexpressed FLAG-fused Pyk2 in GT1-7 cells, and immunoprecipitated Pyk2 using an anti-FLAG antibody. The binding of Pyk2 to Grb2 was detected only after GnRH treatment. In contrast, a site-directed mutant of Pyk2 wherein tyrosine 881 was mutated to phenylalanine did not bind to Grb2. Studies with small interfering RNA and inhibitors indicated that the activation of Grb2/Ras/Raf/MEK was a major pathway to ERK1/2 activation after the short-term treatment of GT1-7 cells with GnRH.

Wiring the Brain by Clustered Protocadherin Neural Codes

There are more than a thousand trillion specific synaptic connections in the human brain and over a million new specific connections are formed every second during the early years of life. The assembly of these staggeringly complex neuronal circuits requires specific cell-surface molecular tags to endow each neuron with a unique identity code to discriminate self from non-self. The clustered protocadherin (Pcdh) genes, which encode a tremendous diversity of cell-surface assemblies, are candidates for neuronal identity tags. We describe the adaptive evolution, genomic structure, and regulation of expression of the clustered Pcdhs. We specifically focus on the emerging 3-D architectural and biophysical mechanisms that generate an enormous number of diverse cell-surface Pcdhs as neural codes in the brain.

Molecular Mechanisms of Cadherin Function During Cortical Migration

During development of the cerebral cortex, different types of neurons migrate from distinct origins to create the different cortical layers and settle within them. Along their way, migrating neurons use cell adhesion molecules on their surface to interact with other cells that will play critical roles to ensure that migration is successful. Radially migrating projection neurons interact primarily with radial glia and Cajal-Retzius cells, whereas interneurons originating in the subpallium follow a longer, tangential route and encounter additional cellular substrates before reaching the cortex. Cell-cell adhesion is therefore essential for the correct migration of cortical neurons. Several members of the cadherin superfamily of cell adhesion proteins, which mediate cellular interactions through calcium-dependent, mostly homophilic binding, have been shown to play important roles during neuronal migration of both projection neurons and interneurons. Although several classical cadherins and protocadherins are involved in this process, the most prominent is CDH2. This mini review will explore the cellular and molecular mechanisms underpinning cadherin function during cortical migration, including recent advances in our understanding of the control of adhesive strength through regulation of cadherin surface levels.

Many facades of CTCF unified by its coding for three-dimensional genome architecture

CCCTC-binding factor (CTCF) is a multifunctional zinc finger protein that is conserved in metazoan species. CTCF is consistently found to play an important role in many diverse biological processes. CTCF/cohesin-mediated active chromatin 'loop extrusion' architects three-dimensional (3D) genome folding. The 3D architectural role of CTCF underlies its multifarious functions, including developmental regulation of gene expression, protocadherin (Pcdh) promoter choice in the nervous system, immunoglobulin (Ig) and T-cell receptor (Tcr) V(D)J recombination in the immune system, homeobox (Hox) gene control during limb development, as well as many other aspects of biology. Here, we review the pleiotropic functions of CTCF from the perspective of its essential role in 3D genome architecture and topological promoter/enhancer selection. We envision the 3D genome as an enormous complex architecture, with tens of thousands of CTCF sites as connecting nodes and CTCF proteins as mysterious bonds that glue together genomic building parts with distinct articulation joints. In particular, we focus on the internal mechanisms by which CTCF controls higher order chromatin structures that manifest its many façades of physiological and pathological functions. We also discuss the dichotomic role of CTCF sites as intriguing 3D genome nodes for seemingly contradictory 'looping bridges' and 'topological insulators' to frame a beautiful magnificent house for a cell's nuclear home.

The role of clustered protocadherins in neurodevelopment and neuropsychiatric diseases

During development, individual neurons extend highly branched arbors that innervate the surrounding territory, enabling the formation of appropriate synaptic connections. The clustered protocadherins (cPCDH), a family of diverse cell-surface homophilic proteins, provide each neuron with a cell specific identity required for distinguishing between self versus non-self. While only 52 unique cPcdh isoforms are encoded in the human genome, a combination of stochastic promoter choice and the formation of a protein lattice through engagement of adjacent cPCDH protein cis/trans-tetramers confer the high degree of cellular specificity required for self-recognition. Studies of mice bearing deletions of individual cPcdh gene clustees have identified deficits in circuit formation and behavior. In humans, single nucleotide variants scattered across the cPCDH locus have been identified, which associate with multiple neurodevelopmental disorders, including autism and schizophrenia. To advance our understanding of cPCDH stochastic choice and maintenance, function across cell types, and contribution to neuropsychiatric disease pathogenesis, hiPSC-based models have been developed. Ultimately, integration of human genetic data, biochemical assays, and functional studies is needed to uncover the mechanism underlying neurite repulsion, which has been implicated in neurodevelopmental disorders.

Protocadherins at the Crossroad of Signaling Pathways

Protocadherins (Pcdhs) are cell adhesion molecules that belong to the cadherin superfamily, and are subdivided into clustered (cPcdhs) and non-clustered Pcdhs (ncPcdhs) in vertebrates. In this review, we summarize their discovery, expression mechanisms, and roles in neuronal development and cancer, thereby highlighting the context-dependent nature of their actions. We furthermore provide an extensive overview of current structural knowledge, and its implications concerning extracellular interactions between cPcdhs, ncPcdhs, and classical cadherins. Next, we survey the known molecular action mechanisms of Pcdhs, emphasizing the regulatory functions of proteolytic processing and domain shedding. In addition, we outline the importance of Pcdh intracellular domains in the regulation of downstream signaling cascades, and we describe putative Pcdh interactions with intracellular molecules including components of the WAVE complex, the Wnt pathway, and apoptotic cascades. Our overview combines molecular interaction data from different contexts, such as neural development and cancer. This comprehensive approach reveals potential common Pcdh signaling hubs, and points out future directions for research. Functional studies of such key factors within the context of neural development might yield innovative insights into the molecular etiology of Pcdh-related neurodevelopmental disorders.

Actin Alpha 2 (ACTA2) Downregulation Inhibits Neural Stem Cell Migration through Rho GTPase Activation

Although neural stem cells (NSCs) could migrate towards lesions after central nervous system (CNS) injury, the migration ability always is restricted due to the disturbed composition and density of the adhesion ligands and extracellular matrix (ECM) gradient after CNS injury. To date, various methods have been developed to enhance NSC migration and a number of factors, which are affecting NSC migration potential, have been identified. Here, primary NSCs were cultured and the expression of actin alpha 2 (ACTA2) in NSCs was determined using reverse transcription polymerase chain reaction (RT-PCR) and immunostaining. Next, the role of ACTA2 in regulating NSC migration and the potential mechanism was explored. Our results demonstrated that ACTA2 expressed in NSCs. Meanwhile, downregulated ACTA2 using siRNA inhibited NSC migration through hindering actin filament polymerization via increasing RhoA expression and decreasing Rac1 expression. The present study might enrich the basic knowledge of ACTA2 in NSC migration and open an avenue for enhancing NSC migration potential, subsequently providing an intervention target for functional recovery after CNS injury.

Molecular function predictions and diagnostic value analysis of plasma exosomal miRNAs in Hirschsprung's disease

Aim: To discover the potential roles of plasma exosomal miRNAs in Hirschsprung's disease (HSCR) and identify potential noninvasive biomarkers for early diagnosis of HSCR. Materials & methods: Plasma samples were collected from HSCR patients and matched controls. Exosomes were isolated before high-throughput Illumina sequencing was utilized to gain a profile of dysregulated exosomal miRNAs, followed with further verification in two separate cohorts. Bioinformatics analyses were also adopted to explore the molecular functions of dysregulated miRNAs in Hirschsprung's disease. Results & conclusion: 31 dysregulated miRNAs were identified with five considered as promising HSCR signatures. Gene enrichment analysis disclosed that the upregulated miRNAs were most likely to participate in 'extracellular matrix-receptor interaction' and contribute to HSCR through interfering in cell junctions.

Tandem CTCF sites function as insulators to balance spatial chromatin contacts and topological enhancer-promoter selection

BACKGROUND

CTCF is a key insulator-binding protein, and mammalian genomes contain numerous CTCF sites, many of which are organized in tandem.

RESULTS

Using CRISPR DNA-fragment editing, in conjunction with chromosome conformation capture, we find that CTCF sites, if located between enhancers and promoters in the protocadherin (Pcdh) and β-globin clusters, function as an enhancer-blocking insulator by forming distinct directional chromatin loops, regardless whether enhancers contain CTCF sites or not. Moreover, computational simulation in silico and genetic deletions in vivo as well as dCas9 blocking in vitro revealed balanced promoter usage in cell populations and stochastic monoallelic expression in single cells by large arrays of tandem CTCF sites in the Pcdh and immunoglobulin heavy chain (Igh) clusters. Furthermore, CTCF insulators promote, counter-intuitively, long-range chromatin interactions with distal directional CTCF sites, consistent with the cohesin "loop extrusion" model. Finally, gene expression levels are negatively correlated with CTCF insulators located between enhancers and promoters on a genome-wide scale. Thus, single CTCF insulators ensure proper enhancer insulation and promoter activation while tandem CTCF topological insulators determine balanced spatial contacts and promoter choice.

CONCLUSIONS

These findings have interesting implications on the role of topological chromatin insulators in 3D genome folding and developmental gene regulation.

Spatiotemporal Regulation of Rho GTPases in Neuronal Migration

Neuronal migration is essential for the orchestration of brain development and involves several contiguous steps: interkinetic nuclear movement (INM), multipolar–bipolar transition, locomotion, and translocation. Growing evidence suggests that Rho GTPases, including RhoA, Rac, Cdc42, and the atypical Rnd members, play critical roles in neuronal migration by regulating both actin and microtubule cytoskeletal components. This review focuses on the spatiotemporal-specific regulation of Rho GTPases as well as their regulators and effectors in distinct steps during the neuronal migration process. Their roles in bridging extracellular signals and cytoskeletal dynamics to provide optimal structural support to the migrating neurons will also be discussed.

Visualization of clustered protocadherin neuronal self-recognition complexes

Neurite self-recognition and avoidance are fundamental properties of all nervous systems¹. These processes facilitate dendritic arborization^2,3, prevent formation of autapses⁴ and allow free interaction among non-self neurons^1,2,4,5. Avoidance among self neurites is mediated by stochastic cell-surface expression of combinations of about 60 isoforms of α-, β- and γ-clustered protocadherin that provide mammalian neurons with single-cell identities^1,2,4-13. Avoidance is observed between neurons that express identical protocadherin repertoires^2,5, and single-isoform differences are sufficient to prevent self-recognition¹⁰. Protocadherins form isoform-promiscuous cis dimers and isoform-specific homophilic trans dimers^10,14-20. Although these interactions have previously been characterized in isolation^15,17-20, structures of full-length protocadherin ectodomains have not been determined, and how these two interfaces engage in self-recognition between neuronal surfaces remains unknown. Here we determine the molecular arrangement of full-length clustered protocadherin ectodomains in single-isoform self-recognition complexes, using X-ray crystallography and cryo-electron tomography. We determine the crystal structure of the clustered protocadherin γB4 ectodomain, which reveals a zipper-like lattice that is formed by alternating cis and trans interactions. Using cryo-electron tomography, we show that clustered protocadherin γB6 ectodomains tethered to liposomes spontaneously assemble into linear arrays at membrane contact sites, in a configuration that is consistent with the assembly observed in the crystal structure. These linear assemblies pack against each other as parallel arrays to form larger two-dimensional structures between membranes. Our results suggest that the formation of ordered linear assemblies by clustered protocadherins represents the initial self-recognition step in neuronal avoidance, and thus provide support for the isoform-mismatch chain-termination model of protocadherin-mediated self-recognition, which depends on these linear chains¹¹.