The F-BAR protein Rapostlin regulates dendritic spine formation in hippocampal neurons

F-BAR proteins CIP4 and FBP17 function in cortical neuron radial migration and process outgrowth

Neurite initiation from newly born neurons is a critical step in neuronal differentiation and migration. Neuronal migration in the developing cortex is accompanied by dynamic extension and retraction of neurites as neurons progress through bipolar and multipolar states. However, there is a relative lack of understanding regarding how the dynamic extension and retraction of neurites is regulated during neuronal migration. In recent work we have shown that CIP4, a member of the F-BAR family of membrane bending proteins, inhibits cortical neurite formation in culture, while family member FBP17 induces premature neurite outgrowth. These results beg the question of how CIP4 and FBP17 function in radial neuron migration and differentiation in vivo, including the timing and manner of neurite extension and retraction. Indeed, the regulation of neurite outgrowth is essential for the transitions between bipolar and multipolar states during radial migration. To examine the effects of modulating expression of CIP4 and FBP17 in vivo, we used in utero electroporation, in combination with our published Double UP technique, to compare knockdown or overexpression cells with control cells within the same mouse tissue of either sex. We show that either knockdown or overexpression of CIP4 and FBP17 results in the marked disruption of radial neuron migration by modulating neuronal morphology and neurite outgrowth, consistent with our findings in culture. Our results demonstrate that the F-BAR proteins CIP4 and FBP17 are essential for proper radial migration in the developing cortex and thus play a key role in cortical development.

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

Mechanical regulation of synapse formation and plasticity

Dendritic spines are small protrusions arising from dendrites and constitute the major compartment of excitatory post-synapses. They change in number, shape, and size throughout life; these changes are thought to be associated with formation and reorganization of neuronal networks underlying learning and memory. As spines in the brain are surrounded by the microenvironment including neighboring cells and the extracellular matrix, their protrusion requires generation of force to push against these structures. In turn, neighboring cells receive force from protruding spines. Recent studies have identified BAR-domain proteins as being involved in membrane deformation to initiate spine formation. In addition, forces for dendritic filopodium extension and activity-induced spine expansion are generated through cooperation between actin polymerization and clutch coupling. On the other hand, force from expanding spines affects neurotransmitter release from presynaptic terminals. Here, we review recent advances in our understanding of the physical aspects of synapse formation and plasticity, mainly focusing on spine dynamics.

A genetic correlation and bivariate genome-wide association study of grip strength and depression

Grip strength is an important biomarker reflecting muscle strength, and depression is a psychiatric disorder all over the world. Several studies found a significant inverse association between grip strength and depression, and there is also evidence for common physiological mechanisms between them. We used twin data from Qingdao, China to calculate genetic correlations, and we performed a bivariate GWAS to explore potential SNPs, genes, and pathways in common between grip strength and depression. 139 pairs of Dizygotic twins were used for bivariate GWAS. VEAGSE2 and PASCAL software were used for gene-based analysis and pathway enrichment analysis, respectively. And the resulting SNPs were subjected to eQTL analysis and pleiotropy analysis. The genetic correlation coefficient between grip strength and depression was -0.41 (-0.96, -0.15). In SNP-based analysis, 7 SNPs exceeded the genome-wide significance level (P<5×10-8) and a total of 336 SNPs reached the level of suggestive significance (P<1×10-5). Gene-based analysis and pathway-based analysis identified genes and pathways related to muscle strength and the nervous system. The results of eQTL analysis were mainly enriched in tissues such as the brain, thyroid, and skeletal muscle. Pleiotropy analysis shows that 9 of the 15 top SNPs were associated with both grip strength and depression. In conclusion, this bivariate GWAS identified potentially common pleiotropic SNPs, genes, and pathways in grip strength and depression.

Mechanical Principles Governing the Shapes of Dendritic Spines

Dendritic spines are small, bulbous protrusions along the dendrites of neurons and are sites of excitatory postsynaptic activity. The morphology of spines has been implicated in their function in synaptic plasticity and their shapes have been well-characterized, but the potential mechanics underlying their shape development and maintenance have not yet been fully understood. In this work, we explore the mechanical principles that could underlie specific shapes using a minimal biophysical model of membrane-actin interactions. Using this model, we first identify the possible force regimes that give rise to the classic spine shapes-stubby, filopodia, thin, and mushroom-shaped spines. We also use this model to investigate how the spine neck might be stabilized using periodic rings of actin or associated proteins. Finally, we use this model to predict that the cooperation between force generation and ring structures can regulate the energy landscape of spine shapes across a wide range of tensions. Thus, our study provides insights into how mechanical aspects of actin-mediated force generation and tension can play critical roles in spine shape maintenance.

Pathophysiological functions of Rnd proteins

Rnd proteins constitute a subfamily of Rho GTPases represented in mammals by Rnd1, Rnd2 and Rnd3. Despite their GTPase structure, their specific feature is the inability to hydrolyse GTP-bound nucleotide. This aspect makes them atypical among Rho GTPases. Rnds are regulated for their expression at the transcriptional or post-transcriptional levels and they are activated through post-translational modifications and interactions with other proteins. Rnd proteins are mainly involved in the regulation of the actin cytoskeleton and cell proliferation. Whereas Rnd3 is ubiquitously expressed, Rnd1 and 2 are tissue-specific. Increasing data has described their important role during development and diseases. Herein, we describe their involvement in physiological and pathological conditions with a focus on the neuronal and vascular systems, and summarize their implications in tumorigenesis.

Cyclin-Dependent Kinase-Like 5 (CDKL5): Possible Cellular Signalling Targets and Involvement in CDKL5 Deficiency Disorder

Cyclin-dependent kinase-like 5 (CDKL5, also known as STK9) is a serine/threonine protein kinase originally identified in 1998 during a transcriptional mapping project of the human X chromosome. Thereafter, a mutation in CDKL5 was reported in individuals with the atypical Rett syndrome, a neurodevelopmental disorder, suggesting that CDKL5 plays an important regulatory role in neuronal function. The disease associated with CDKL5 mutation has recently been recognised as CDKL5 deficiency disorder (CDD) and has been distinguished from the Rett syndrome owing to its symptomatic manifestation. Because CDKL5 mutations identified in patients with CDD cause enzymatic loss of function, CDKL5 catalytic activity is likely strongly associated with the disease. Consequently, the exploration of CDKL5 substrate characteristics and regulatory mechanisms of its catalytic activity are important for identifying therapeutic target molecules and developing new treatment. In this review, we summarise recent findings on the phosphorylation of CDKL5 substrates and the mechanisms of CDKL5 phosphorylation and dephosphorylation. We also discuss the relationship between changes in the phosphorylation signalling pathways and the Cdkl5 knockout mouse phenotype and consider future prospects for the treatment of mental and neurological disease associated with CDKL5 mutations.

SH3Ps-Evolution and Diversity of a Family of Proteins Engaged in Plant Cytokinesis

SH3P2 (At4g34660), an Arabidopsis thaliana SH3 and Bin/amphiphysin/Rvs (BAR) domain-containing protein, was reported to have a specific role in cell plate assembly, unlike its paralogs SH3P1 (At1g31440) and SH3P3 (At4g18060). SH3P family members were also predicted to interact with formins—evolutionarily conserved actin nucleators that participate in microtubule organization and in membrane–cytoskeleton interactions. To trace the origin of functional specialization of plant SH3Ps, we performed phylogenetic analysis of SH3P sequences from selected plant lineages. SH3Ps are present in charophytes, liverworts, mosses, lycophytes, gymnosperms, and angiosperms, but not in volvocal algae, suggesting association of these proteins with phragmoplast-, but not phycoplast-based cell division. Separation of three SH3P clades, represented by SH3P1, SH3P2, and SH3P3 of A. thaliana, appears to be a seed plant synapomorphy. In the yeast two hybrid system, Arabidopsis SH3P3, but not SH3P2, binds the FH1 and FH2 domains of the formin FH5 (At5g54650), known to participate in cytokinesis, while an opposite binding specificity was found for the dynamin homolog DRP1A (At5g42080), confirming earlier findings. This suggests that the cytokinetic role of SH3P2 is not due to its interaction with FH5. Possible determinants of interaction specificity of SH3P2 and SH3P3 were identified bioinformatically.

Opposing functions of F-BAR proteins in neuronal membrane protrusion, tubule formation, and neurite outgrowth

Neurite formation is a fundamental antecedent to axon and dendrite formation, but the mechanisms that underlie this important process are poorly characterized. Here, we demonstrate that two F-BAR proteins, CIP4 and FBP17, have opposing functions in early cortical neuron development.

The F-BAR family of proteins play important roles in many cellular processes by regulating both membrane and actin dynamics. The CIP4 family of F-BAR proteins is widely recognized to function in endocytosis by elongating endocytosing vesicles. However, in primary cortical neurons, CIP4 concentrates at the tips of extending lamellipodia and filopodia and inhibits neurite outgrowth. Here, we report that the highly homologous CIP4 family member, FBP17, induces tubular structures in primary cortical neurons and results in precocious neurite formation. Through domain swapping and deletion experiments, we demonstrate that a novel polybasic region between the F-BAR and HR1 domains is required for membrane bending. Moreover, the presence of a poly-PxxP region in longer splice isoforms of CIP4 and FBP17 largely reverses the localization and function of these proteins. Thus, CIP4 and FBP17 function as an antagonistic pair to fine-tune membrane protrusion, endocytosis, and neurite formation during early neuronal development.

High expression of FBP17 in invasive breast cancer cells promotes invadopodia formation

Metastatic spread of the cancer is usually the consequence of the activation of signaling pathways that generate cell motility and tissue invasion. Metastasis involves the reorganization of cytoskeleton and cell shape for the swift movement of the cells through extracellular matrix. Previously, we have described the invasive and metastatic role played by one of the members (Toca-1) of CIP4 subfamily of F-BAR proteins. In the present study, we address the role of another member (FBP17) of same family in the invasion breast cancer cells. Here, we report that the formin-binding protein 17 (FBP17) is highly expressed at both mRNA and protein levels in breast cancer cells. The study showed the association of FBP17 with cytoskeletal actin regulatory proteins like dynamin and cortactin. To determine its role in extracellular matrix (ECM) degradation, we achieved stable knockdown of FBP17 in MDA-MB-231 cells. FBP17 knockdown cells showed a defect and were found to be compromised in the degradation of ECM indicating the role of FBP17 in the invasion of breast cancer cells. Our results suggest that FBP17 is highly expressed in breast cancer cells and facilitates the invasion of breast cancer cells.

BAR Domain Proteins Regulate Rho GTPase Signaling

The Bin-Amphiphysin-Rvs (BAR) domain is a membrane lipid binding domain present in a wide variety of proteins, often proteins with a role in Rho-regulated signaling pathways. BAR domains do not only confer binding to lipid bilayers, they also possess a membrane sculpturing ability and thereby directly control the topology of biomembranes. BAR domain-containing proteins participate in a plethora of physiological processes but the common denominator is their capacity to link membrane dynamics to actin dynamics and thereby integrate processes such as endocytosis, exocytosis, vesicle trafficking, cell morphogenesis and cell migration. The Rho family of small GTPases constitutes an important bridging theme for many BAR domain-containing proteins. This review article will focus predominantly on the role of BAR proteins as regulators or effectors of Rho GTPases and it will only briefly discuss the structural and biophysical function of the BAR domains.

Upregulation of Rho7 in the temporal lobe tissue of humans with intractable epilepsy

Patients with intractable epilepsy (IE) exhibit an increased risk of premature death, psychosocial dysfunction and decreasing quality of life. The present study aimed to investigate the alteration in the expression of Rho7 in brain tissue from patients with IE, and to examine the association between Rho7 protein expression and IE. Temporal lobe samples were collected from the temporal lobes of 33 patients with IE patients and 10 age‑ and gender‑matched histologically healthy controls. Immunohistochemical staining was conducted to assess the number of Rho7‑positive cells. In addition, double‑label immunofluorescent staining was performed to examine the cellular localization of Rho7. The protein expression of Rho7 was examined using western blotting. Marked immunoreactivity for Rho7 was detected in the IE group, while faint and scattered immunoreactive staining was observed in the control group. The count of Rho7 positive cells in the IE patients was significantly increased compared with the control subjects (23.47±3.9% vs. 12.09±1.05%; P<0.01). Double‑label immunofluorescent staining indicated that Rho7 was primarily expressed in the cell membrane and cytoplasm, and colocalized with neuron‑specific enolase. Western blot analysis demonstrated that the expression of Rho7 in the IE group was significantly increased compared with the control group (0.41±0.031 vs. 0.25±0.025; P<0.01). The results of the present study demonstrated that upregulation of Rho7 immunoreactivity occurs in the brains of patients with IE, suggesting that Rho7 may be associated with the progression of IE or act as a potential treatment target.

Luciferase shRNA Presents off-Target Effects on Voltage-Gated Ion Channels in Mouse Hippocampal Pyramidal Neurons

RNA interference (RNAi) is a straightforward approach to study gene function from the in vitro cellular level to in vivo animal behavior. Although RNAi-mediated gene knockdown has become essentially routine in neuroscience over the past ten years, off-target effects of short hairpin RNAs (shRNAs) should be considered as the proper choice of control shRNA is critical in order to perform meaningful experiments. Luciferase shRNA (shLuc), targeting firefly luciferase, and scrambled shRNAs (shScrs) have been widely used as controls for vertebrate cell research. However, thorough validation of control shRNAs has not been made to date. Here, we performed thorough physiological and morphological studies against control shRNAs in mouse hippocampal CA1 pyramidal neurons. As expected, all control shRNAs exhibited normal basal synaptic transmission and dendritic morphology. However, to our surprise, shLuc exerted severe off-target effects on voltage-gated ion channel function, while the shScr had no detectable changes. These results indicate that thorough validation of shRNA is imperative and, in the absence of such validation, that shScr is the best available negative control for gene knockdown studies.

Different functional modes of BAR domain proteins in formation and plasticity of mammalian postsynapses

A plethora of cell biological processes involve modulations of cellular membranes. By using extended lipid-binding interfaces, some proteins have the power to shape membranes by attaching to them. Among such membrane shapers, the superfamily of Bin-Amphiphysin-Rvs (BAR) domain proteins has recently taken center stage. Extensive structural work on BAR domains has revealed a common curved fold that can serve as an extended membrane-binding interface to modulate membrane topologies and has allowed the grouping of the BAR domain superfamily into subfamilies with structurally slightly distinct BAR domain subtypes (N-BAR, BAR, F-BAR and I-BAR). Most BAR superfamily members are expressed in the mammalian nervous system. Neurons are elaborately shaped and highly compartmentalized cells. Therefore, analyses of synapse formation and of postsynaptic reorganization processes (synaptic plasticity) - a basis for learning and memory formation - has unveiled important physiological functions of BAR domain superfamily members. These recent advances, furthermore, have revealed that the functions of BAR domain proteins include different aspects. These functions are influenced by the often complex domain organization of BAR domain proteins. In this Commentary, we review these recent insights and propose to classify BAR domain protein functions into (1) membrane shaping, (2) physical integration, (3) action through signaling components, and (4) suppression of other BAR domain functions.

BAR domain proteins regulate Rho GTPase signaling

BAR proteins comprise a heterogeneous group of multi-domain proteins with diverse biological functions. The common denominator is the Bin-Amphiphysin-Rvs (BAR) domain that not only confers targeting to lipid bilayers, but also provides scaffolding to mold lipid membranes into concave or convex surfaces. This function of BAR proteins is an important determinant in the dynamic reconstruction of membrane vesicles, as well as of the plasma membrane. Several BAR proteins function as linkers between cytoskeletal regulation and membrane dynamics. These links are provided by direct interactions between BAR proteins and actin-nucleation-promoting factors of the Wiskott-Aldrich syndrome protein family and the Diaphanous-related formins. The Rho GTPases are key factors for orchestration of this intricate interplay. This review describes how BAR proteins regulate the activity of Rho GTPases, as well as how Rho GTPases regulate the function of BAR proteins. This mutual collaboration is a central factor in the regulation of vital cellular processes, such as cell migration, cytokinesis, intracellular transport, endocytosis, and exocytosis.

Function and regulation of Rnd proteins in cortical projection neuron migration

The mammalian cerebral cortex contains a high variety of neuronal subtypes that acquire precise spatial locations and form long or short-range connections to establish functional neuronal circuits. During embryonic development, cortical projection neurons are generated in the areas lining the lateral ventricles and they subsequently undergo radial migration to reach the position of their final maturation within the cortical plate. The control of the neuroblast migratory behavior and the coordination of the migration process with other neurogenic events such as cell cycle exit, differentiation and final maturation are crucial to normal brain development. Among the key regulators of cortical neuron migration, the small GTP binding proteins of the Rho family and the atypical Rnd members play important roles in integrating intracellular signaling pathways into changes in cytoskeletal dynamics and motility behavior. Here we review the role of Rnd proteins during cortical neuronal migration and we discuss both the upstream mechanisms that regulate Rnd protein activity and the downstream molecular pathways that mediate Rnd effects on cell cytoskeleton.

Transcription factor Sp4 regulates expression of nervous wreck 2 to control NMDAR1 levels and dendrite patterning

Glutamatergic signaling through N-methyl-d-aspartate receptors (NMDARs) is important for neuronal development and plasticity and is often dysregulated in psychiatric disorders. Mice mutant for the transcription factor Sp4 have reduced levels of NMDAR subunit 1 (NR1) protein, but not mRNA, and exhibit behavioral and memory deficits (Zhou et al., [2010] Human Molecular Genetics 19: 3797-3805). In developing cerebellar granule neurons (CGNs), Sp4 controls dendrite patterning (Ramos et al., [2007] Proc Natl Acad Sci USA 104: 9882-9887). Sp4 target genes that regulate dendrite pruning or NR1 levels are not known. Here we report that Sp4 activates transcription of Nervous Wreck 2 (Nwk2; also known as Fchsd1) and, further, that Nwk2, an F-BAR domain-containing protein, mediates Sp4-dependent regulation of dendrite patterning and cell surface expression of NR1. Knockdown of Nwk2 in CGNs increased primary dendrite number, phenocopying Sp4 knockdown, and exogenous expression of Nwk2 in Sp4-depleted neurons rescued dendrite number. We observed that acute Sp4 depletion reduced levels of surface, but not total, NR1, and this was rescued by Nwk2 expression. Furthermore, expression of Nr1 suppressed the increase in dendrite number in Sp4- or Nwk2- depleted neurons. We previously reported that Sp4 protein levels were reduced in cerebellum of subjects with bipolar disorder (BD) (Pinacho et al., [2011] Bipolar Disorders 13: 474-485). Here we report that Nwk2 mRNA and NR1 protein levels were also reduced in postmortem cerebellum of BD subjects. Our data suggest a role for Sp4-regulated Nwk2 in NMDAR trafficking and identify a Sp4-Nwk2-NMDAR1 pathway that regulates neuronal morphogenesis during development and may be disrupted in bipolar disorder.

Formins and membranes: anchoring cortical actin to the cell wall and beyond

Formins are evolutionarily conserved eukaryotic proteins participating in actin and microtubule organization. Land plants have three formin clades, with only two - Class I and II - present in angiosperms. Class I formins are often transmembrane proteins, residing at the plasmalemma and anchoring the cortical cytoskeleton across the membrane to the cell wall, while Class II formins possess a PTEN-related membrane-binding domain. Lower plant Class III and non-plant formins usually contain domains predicted to bind RHO GTPases that are membrane-associated. Thus, some kind of membrane anchorage appears to be a common formin feature. Direct interactions between various non-plant formins and integral or peripheral membrane proteins have indeed been reported, with varying mechanisms and biological implications. Besides of summarizing new data on Class I and Class II formin-membrane relationships, this review surveys such "non-classical" formin-membrane interactions and examines which, if any, of them may be evolutionarily conserved and operating also in plants. FYVE, SH3 and BAR domain-containing proteins emerge as possible candidates for such conserved membrane-associated formin partners.

Lysophosphatidic acid induces neurite branch formation through LPA3

Although neurite branching is crucial for neuronal network formation after birth, its underlying mechanisms remain unclear. Here, we demonstrate that lysophosphatidic acid (LPA) stimulates neurite branching through a novel signaling pathway. Treatment of neuronal cell lines with LPA resulted in neurite branch formation when LPA(3) receptor was introduced. The effects of LPA were blocked by inhibition of G(q) signaling. Furthermore, expression of inhibitory mutants of the small GTPase Rnd2/Rho7 or an Rnd2 effector rapostlin abolished LPA(3)-mediated neurite branching. The LPA(3) agonist 2(S)-OMPT or LPA also induced axonal branch formation in hippocampal neurons, which was blocked by G(q) and Rnd2 pathway inhibition or LPA(3) knockdown. These findings suggest that the novel signaling pathway involving LPA(3), G(q), and Rnd2 may play an important role in neuronal network formation.