N-wasp and the arp2/3 complex are critical regulators of actin in the development of dendritic spines and synapses

The interaction between KIF21A and KANK1 regulates dendritic morphology and synapse plasticity in neurons

JOURNAL/nrgr/04.03/01300535-202501000-00029/figure1/v/2024-05-14T021156Z/r/image-tiff Morphological alterations in dendritic spines have been linked to changes in functional communication between neurons that affect learning and memory. Kinesin-4 KIF21A helps organize the microtubule-actin network at the cell cortex by interacting with KANK1; however, whether KIF21A modulates dendritic structure and function in neurons remains unknown. In this study, we found that KIF21A was distributed in a subset of dendritic spines, and that these KIF21A-positive spines were larger and more structurally plastic than KIF21A-negative spines. Furthermore, the interaction between KIF21A and KANK1 was found to be critical for dendritic spine morphogenesis and synaptic plasticity. Knockdown of either KIF21A or KANK1 inhibited dendritic spine morphogenesis and dendritic branching, and these deficits were fully rescued by coexpressing full-length KIF21A or KANK1, but not by proteins with mutations disrupting direct binding between KIF21A and KANK1 or binding between KANK1 and talin1. Knocking down KIF21A in the hippocampus of rats inhibited the amplitudes of long-term potentiation induced by high-frequency stimulation and negatively impacted the animals' cognitive abilities. Taken together, our findings demonstrate the function of KIF21A in modulating spine morphology and provide insight into its role in synaptic function.

Dendritic Spines and Pain Memory

Neuropathic pain is a debilitating form of pain arising from injury or disease of the nervous system that affects millions of people worldwide. Despite its prevalence, the underlying mechanisms of neuropathic pain are still not fully understood. Dendritic spines are small protrusions on the surface of neurons that play an important role in synaptic transmission. Recent studies have shown that dendritic spines reorganize in the superficial and deeper laminae of the spinal cord dorsal horn with the development of neuropathic pain in multiple models of disease or injury. Given the importance of dendritic spines in synaptic transmission, it is possible that studying dendritic spines could lead to new therapeutic approaches for managing intractable pain. In this review article, we highlight the emergent role of dendritic spines in neuropathic pain, as well as discuss the potential for studying dendritic spines for the development of new therapeutics.

Actin-nucleation promoting factor N-WASP influences alpha-synuclein condensates and pathology

Abnormal intraneuronal accumulation of soluble and insoluble α-synuclein (α-Syn) is one of the main pathological hallmarks of synucleinopathies, such as Parkinson's disease (PD). It has been well documented that the reversible liquid-liquid phase separation of α-Syn can modulate synaptic vesicle condensates at the presynaptic terminals. However, α-Syn can also form liquid-like droplets that may convert into amyloid-enriched hydrogels or fibrillar polymorphs under stressful conditions. To advance our understanding on the mechanisms underlying α-Syn phase transition, we employed a series of unbiased proteomic analyses and found that actin and actin regulators are part of the α-Syn interactome. We focused on Neural Wiskott-Aldrich syndrome protein (N-WASP) because of its association with a rare early-onset familial form of PD. In cultured cells, we demonstrate that N-WASP undergoes phase separation and can be recruited to synapsin 1 liquid-like droplets, whereas it is excluded from α-Syn/synapsin 1 condensates. Consistently, we provide evidence that wsp-1/WASL loss of function alters the number and dynamics of α-Syn inclusions in the nematode Caenorhabditis elegans. Together, our findings indicate that N-WASP expression may create permissive conditions that promote α-Syn condensates and their potentially deleterious conversion into toxic species.

Alteration of synaptic protein composition during developmental synapse maturation

The postsynaptic density (PSD) is a collection of specialized proteins assembled beneath the postsynaptic membrane of dendritic spines. The PSD proteome comprises ~1000 proteins, including neurotransmitter receptors, scaffolding proteins and signalling enzymes. Many of these proteins have essential roles in synaptic function and plasticity. During brain development, changes are observed in synapse density and in the stability and shape of spines, reflecting the underlying molecular maturation of synapses. Synaptic protein composition changes in terms of protein abundance and the assembly of protein complexes, supercomplexes and the physical organization of the PSD. Here, we summarize the developmental alterations of postsynaptic protein composition during synapse maturation. We describe major PSD proteins involved in postsynaptic signalling that regulates synaptic plasticity and discuss the effect of altered expression of these proteins during development. We consider the abnormality of synaptic profiles and synaptic protein composition in the brain in neurodevelopmental disorders such as autism spectrum disorders. We also explain differences in synapse development between rodents and primates in terms of synaptic profiles and protein composition. Finally, we introduce recent findings related to synaptic diversity and nanoarchitecture and discuss their impact on future research. Synaptic protein composition can be considered a major determinant and marker of synapse maturation in normality and disease.

Whole-exome sequencing reveals genetic variants that may play a role in neurocytomas

OBJECTIVES

Neurocytomas (NCs) are rare intracranial tumors that can often be surgically resected. However, disease course is unpredictable in many patients and medical therapies are lacking. We have used whole exome sequencing to explore the molecular etiology for neurocytoma and assist in target identification to develop novel therapeutic interventions.

METHODS

We used whole exome sequencing (WES) to compare the molecular landscape of 21 primary & recurrent NCs to five normal cerebellar control samples. WES data was analyzed using the Qiagen Clinical Insight program, variants of interest (VOI) were interrogated using ConSurf, ScoreCons, & Ingenuity Pathway Analysis Software to predict their potential functional effects, and Copy number variations (CNVs) in the genes of interest were analyzed by Genewiz (Azenta Life Sciences).

RESULTS

Of 40 VOI involving thirty-six genes, 7 were pathogenic, 17 likely-pathogenic, and 16 of uncertain-significance. Of seven pathogenic NC associated variants, Glucosylceramidase beta 1 [GBA1 c.703T > C (p.S235P)] was mutated in 5/21 (24%), Coagulation factor VIII [F8 c.3637dupA (p.I1213fs*28)] in 4/21 (19%), Phenylalanine hydroxylase [PAH c.975C > A (p.Y325*)] in 3/21 (14%), and Fanconi anemia complementation group C [FANCC c.1162G > T (p.G388*)], Chromodomain helicase DNA binding protein 7 [CHD7 c.2839C > T (p.R947*)], Myosin VIIA [MYO7A c.940G > T (p.E314*)] and Dynein axonemal heavy chain 11 [DNAH11 c.3544C > T (p.R1182*)] in 2/21 (9.5%) NCs respectively. CNVs were noted in 85% of these latter 7 genes. Interestingly, a Carboxy-terminal domain RNA polymerase II polypeptide A small phosphatase 2 [CTDSP2 c.472G > A (p.E158K)] of uncertain significance was also found in > 70% of NC cases.

INTERPRETATION

The variants of interest we identified in the NCs regulate a variety of neurological processes including cilia motility, cell metabolism, immune responses, and DNA damage repair and provide novel insights into the molecular pathogenesis of these extremely rare tumors.

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.

Wiskostatin and other carbazole scaffolds as off target inhibitors of dynamin I GTPase activity and endocytosis

Wiskostatin (1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol) (1) is a carbazole-based compound reported as a specific and relatively potent inhibitor of the N-WASP actin remodelling complex (S-isomer EC₅₀ = 4.35 μM; R-isomer EC₅₀ = 3.44 μM). An NMR solution structure showed that wiskostatin interacts with a cleft in the regulatory GTPase binding domain of N-WASP. However, numerous studies have reported wiskostatin's actions on membrane transport and cytokinesis that are independent of the N-WASP-Arp2/3 complex pathway, but offer limited alternative explanation. The large GTPase, dynamin has established functional roles in these pathways. This study reveals that wiskostatin and its analogues, as well as other carbazole-based compounds, are inhibitors of helical dynamin GTPase activity and endocytosis. We characterise the effects of wiskostatin on in vitro dynamin GTPase activity, in-cell endocytosis, and determine the importance of wiskostatin functional groups on these activities through design and synthesis of libraries of wiskostatin analogues. We also examine whether other carbazole-based scaffolds frequently used in research or the clinic also modulate dynamin and endocytosis. Understanding off-targets for compounds used as research tools is important to be able to confidently interpret their action on biological systems, particularly when the target and off-targets affect overlapping mechanisms (e.g. cytokinesis and endocytosis). Herein we demonstrate that wiskostatin is a dynamin inhibitor (IC₅₀ 20.7 ± 1.2 μM) and a potent inhibitor of clathrin mediated endocytosis (IC₅₀ = 6.9 ± 0.3 μM). Synthesis of wiskostatin analogues gave rise to 1-(9H-carbazol-9-yl)-3-((4-methylbenzyl)amino)propan-2-ol (35) and 1-(9H-carbazol-9-yl)-3-((4-chlorobenzyl)amino)propan-2-ol (43) as potent dynamin inhibitors (IC₅₀ = 1.0 ± 0.2 μM), and (S)-1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol (8a) and (R)-1-(3,6-dibromo-9H-carbazol-9-yl)-3-(dimethylamino)propan-2-ol (8b) that are amongst the most potent inhibitors of clathrin mediated endocytosis yet reported (IC₅₀ = 2.3 ± 3.3 and 2.1 ± 1.7 μM, respectively).

N-WASP-Arp2/3 signaling controls multiple steps of dendrite maturation in Purkinje cells in vivo

During neural development, the actin filament network must be precisely regulated to form elaborate neurite structures. N-WASP tightly controls actin polymerization dynamics by activating an actin nucleator Arp2/3. However, the importance of N-WASP-Arp2/3 signaling in the assembly of neurite architecture in vivo has not been clarified. Here, we demonstrate that N-WASP-Arp2/3 signaling plays a crucial role in the maturation of cerebellar Purkinje cell (PC) dendrites in vivo in mice. N-WASP was expressed and activated in developing PCs. Inhibition of Arp2/3 and N-WASP from the beginning of dendrite formation severely disrupted the establishment of a single stem dendrite, which is a characteristic basic structure of PC dendrites. Inhibition of Arp2/3 after stem dendrite formation resulted in hypoplasia of the PC dendritic tree. Cdc42, an upstream activator of N-WASP, is required for N-WASP-Arp2/3 signaling-mediated PC dendrite maturation. In addition, overactivation of N-WASP is also detrimental to dendrite formation in PCs. These findings reveal that proper activation of N-WASP-Arp2/3 signaling is crucial for multiple steps of PC dendrite maturation in vivo.

The Long-Term Pannexin 1 Ablation Produces Structural and Functional Modifications in Hippocampal Neurons

Enhanced activity and overexpression of Pannexin 1 (Panx1) channels contribute to neuronal pathologies such as epilepsy and Alzheimer's disease (AD). The Panx1 channel ablation alters the hippocampus's glutamatergic neurotransmission, synaptic plasticity, and memory flexibility. Nevertheless, Panx1-knockout (Panx1-KO) mice still retain the ability to learn, suggesting that compensatory mechanisms stabilize their neuronal activity. Here, we show that the absence of Panx1 in the adult brain promotes a series of structural and functional modifications in the Panx1-KO hippocampal synapses, preserving spontaneous activity. Compared to the wild-type (WT) condition, the adult hippocampal neurons of Panx1-KO mice exhibit enhanced excitability, a more complex dendritic branching, enhanced spine maturation, and an increased proportion of multiple synaptic contacts. These modifications seem to rely on the actin-cytoskeleton dynamics as an increase in the actin polymerization and an imbalance between the Rac1 and the RhoA GTPase activities were observed in Panx1-KO brain tissues. Our findings highlight a novel interaction between Panx1 channels, actin, and Rho GTPases, which appear to be relevant for synapse stability.

TIAM-1 differentially regulates dendritic and axonal microtubule organization in patterning neuronal development through its multiple domains

Axon and dendrite development require the cooperation of actin and microtubule cytoskeletons. Microtubules form a well-organized network to direct polarized trafficking and support neuronal processes formation with distinct actin structures. However, it is largely unknown how cytoskeleton regulators differentially regulate microtubule organization in axon and dendrite development. Here, we characterize the role of actin regulators in axon and dendrite development and show that the RacGEF TIAM-1 regulates dendritic patterns through its N-terminal domains and suppresses axon growth through its C-terminal domains. TIAM-1 maintains plus-end-out microtubule orientation in posterior dendrites and prevents the accumulation of microtubules in the axon. In somatodendritic regions, TIAM-1 interacts with UNC-119 and stabilizes the organization between actin filaments and microtubules. UNC-119 is required for TIAM-1 to control axon growth, and its expression levels determine axon length. Taken together, TIAM-1 regulates neuronal microtubule organization and patterns axon and dendrite development respectively through its different domains.

A condensate dynamic instability orchestrates actomyosin cortex activation

A key event at the onset of development is the activation of a contractile actomyosin cortex during the oocyte-to-embryo transition^1-3. Here we report on the discovery that, in Caenorhabditis elegans oocytes, actomyosin cortex activation is supported by the emergence of thousands of short-lived protein condensates rich in F-actin, N-WASP and the ARP2/3 complex^4-8 that form an active micro-emulsion. A phase portrait analysis of the dynamics of individual cortical condensates reveals that condensates initially grow and then transition to disassembly before dissolving completely. We find that, in contrast to condensate growth through diffusion⁹, the growth dynamics of cortical condensates are chemically driven. Notably, the associated chemical reactions obey mass action kinetics that govern both composition and size. We suggest that the resultant condensate dynamic instability¹⁰ suppresses coarsening of the active micro-emulsion¹¹, ensures reaction kinetics that are independent of condensate size and prevents runaway F-actin nucleation during the formation of the first cortical actin meshwork.

Serum exosomal coronin 1A and dynamin 2 as neural tube defect biomarkers

Abstract

No highly specific and sensitive biomarkers have been identified for early diagnosis of neural tube defects (NTDs). In this study, we used proteomics to identify novel proteins specific for NTDs. Our findings revealed three proteins showing differential expression during fetal development. In a rat model of NTDs, we used western blotting to quantify proteins in maternal serum exosomes on gestational days E18, E16, E14, and E12, in serum on E18 and E12, in neural tubes on E18 and E12, and in fetal neural exosomes on E18. The expression of coronin 1A and dynamin 2 was exosome-specific and associated with spina bifida aperta embryogenesis. Furthermore, coronin 1A and dynamin 2 were significantly downregulated in maternal serum exosomes (E12–E18), neural tubes, and fetal neural exosomes. Although downregulation was also observed in serum, the difference was not significant. Differentially expressed proteins were further analyzed in the serum exosomes of pregnant women during gestational weeks 12–40 using enzyme-linked immunosorbent assays. The findings revealed that coronin 1A and dynamin 2 showed potential diagnostic efficacy during gestational weeks 12–40, particularly during early gestation (12–18 weeks). Therefore, these two targets are used as candidate NTD screening and diagnostic biomarkers during early gestation.

Key messages

We used proteomics to identify novel proteins specific for NTDs.

CORO1A and DNM2 showed exosome-specific expression and were associated with SBA.

CORO1A and DNM2 were downregulated in maternal serum exosomes and FNEs.

CORO1A and DNM2 showed good diagnostic efficacy for NTDs during early gestation.

These two targets may have applications as NTD screening and diagnostic biomarkers.

Supplementary information

The online version contains supplementary material available at 10.1007/s00109-022-02236-w.

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.

Regulation of Cdc42 signaling by the dopamine D2 receptor in a mouse model of Parkinson's disease

Substantial spine loss in striatal medium spiny neurons (MSNs) and abnormal behaviors are common features of Parkinson's disease (PD). The caudate putamen (CPu) mainly contains MSNs expressing dopamine D1 receptor (dMSNs) and dopamine D2 receptor (iMSNs) exerting critical effects on motor and cognition behavior. However, the molecular mechanisms contributing to spine loss and abnormal behaviors in dMSNs and iMSNs under parkinsonian state remain unknown. In the present study, we revealed that Cell division control protein 42 (Cdc42) signaling was significantly decreased in the caudate putamen (CPu) in parkinsonian mice. In addition, overexpression of constitutively active Cdc42 in the CPu reversed spine abnormalities and improved the behavior deficits in parkinsonian mice. Utilizing conditional dopamine D1 receptor (D1R) or D2 receptor (D2R) knockout mice, we found that such a decrease under parkinsonian state was further reduced by conditional knockout of the D2R but not D1R. Moreover, the thin spine loss in iMSNs and deficits in motor coordination and cognition induced by conditional knockout of D2R were reversed by overexpression of constitutively active Cdc42 in the CPu. Additionally, conditional knockout of Cdc42 from D2R‐positive neurons in the CPu was sufficient to induce spine and behavior deficits similar to those observed in parkinsonian mice. Overall, our results indicate that impaired Cdc42 signaling regulated by D2R plays an important role in spine loss and behavioral deficits in PD.

Vav3-Deficient Astrocytes Enhance the Dendritic Development of Hippocampal Neurons in an Indirect Co-culture System

Vav proteins belong to the class of guanine nucleotide exchange factors (GEFs) that catalyze the exchange of guanosine diphosphate (GDP) by guanosine triphosphate (GTP) on their target proteins. Here, especially the members of the small GTPase family, Ras homolog family member A (RhoA), Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division control protein 42 homolog (Cdc42) can be brought into an activated state by the catalytic activity of Vav-GEFs. In the central nervous system (CNS) of rodents Vav3 shows the strongest expression pattern in comparison to Vav2 and Vav1, which is restricted to the hematopoietic system. Several studies revealed an important role of Vav3 for the elongation and branching of neurites. However, little is known about the function of Vav3 for other cell types of the CNS, like astrocytes. Therefore, the following study analyzed the effects of a Vav3 knockout on several astrocytic parameters as well as the influence of Vav3-deficient astrocytes on the dendritic development of cultured neurons. For this purpose, an indirect co-culture system of native hippocampal neurons and Vav3-deficient cortical astrocytes was used. Interestingly, neurons cultured in an indirect contact with Vav3-deficient astrocytes showed a significant increase in the dendritic complexity and length after 12 and 17 days in vitro (DIV). Furthermore, Vav3-deficient astrocytes showed an enhanced regeneration in the scratch wound heal assay as well as an altered profile of released cytokines with a complete lack of CXCL11, reduced levels of IL-6 and an increased release of CCL5. Based on these observations, we suppose that Vav3 plays an important role for the development of dendrites by regulating the expression and the release of neurotrophic factors and cytokines in astrocytes.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Biological Implications of a Stroke Therapy Based in Neuroglobin Hyaluronate Nanoparticles. Neuroprotective Role and Molecular Bases

Exogenous neuroprotective protein neuroglobin (Ngb) cannot cross the blood-brain barrier. To overcome this difficulty, we synthesized hyaluronate nanoparticles (NPs), able to deliver Ngb into the brain in an animal model of stroke (MCAO). These NPs effectively reached neurons, and were microscopically identified after 24 h of reperfusion. Compared to MCAO non-treated animals, those treated with Ngb-NPs showed survival rates up to 50% higher, and better neurological scores. Tissue damage improved with the treatment, but no changes in the infarct volume or in the oxidative/nitrosative values were detected. A proteomics approach (p-value < 0.02; fold change = 0.05) in the infarcted areas showed a total of 219 proteins that significantly changed their expression after stroke and treatment with Ngb-NPs. Of special interest, are proteins such as FBXO7 and NTRK2, which were downexpressed in stroke, but overexpressed after treatment with Ngb-NPs; and ATX2L, which was overexpressed only under the effect of Ngb. Interestingly, the proteins affected by the treatment with Ngb were involved in mitochondrial function and cell death, endocytosis, protein metabolism, cytoskeletal remodeling, or synaptic function, and in regenerative processes, such as dendritogenesis, neuritogenesis, or sinaptogenesis. Consequently, our pharmaceutical preparation may open new therapeutic scopes for stroke and possibly for other neurodegenerative pathologies.

Poststroke dendritic arbor regrowth requires the actin nucleator Cobl

Ischemic stroke is a major cause of death and long-term disability. We demonstrate that middle cerebral artery occlusion (MCAO) in mice leads to a strong decline in dendritic arborization of penumbral neurons. These defects were subsequently repaired by an ipsilateral recovery process requiring the actin nucleator Cobl. Ischemic stroke and excitotoxicity, caused by calpain-mediated proteolysis, significantly reduced Cobl levels. In an apparently unique manner among excitotoxicity-affected proteins, this Cobl decline was rapidly restored by increased mRNA expression and Cobl then played a pivotal role in poststroke dendritic arbor repair in peri-infarct areas. In Cobl knockout (KO) mice, the dendritic repair window determined to span day 2 to 4 poststroke in wild-type (WT) strikingly passed without any dendritic regrowth. Instead, Cobl KO penumbral neurons of the primary motor cortex continued to show the dendritic impairments caused by stroke. Our results thereby highlight a powerful poststroke recovery process and identified causal molecular mechanisms critical during poststroke repair.

Ischemic stroke is a major cause of death and long-term disability. This study reveals that, in mice, stroke-induced damage to dendritic arborization in the area around an infarct is rapidly repaired via dendritic regrowth; this plasticity requires the actin nucleator Cobl.

Impact of Pharmacological and Non-Pharmacological Modulators on Dendritic Spines Structure and Functions in Brain

Dendritic spines are small, thin, hair-like protrusions found on the dendritic processes of neurons. They serve as independent compartments providing large amplitudes of Ca²⁺ signals to achieve synaptic plasticity, provide sites for newer synapses, facilitate learning and memory. One of the common and severe complication of neurodegenerative disease is cognitive impairment, which is said to be closely associated with spine pathologies viz., decreased in spine density, spine length, spine volume, spine size etc. Many treatments targeting neurological diseases have shown to improve the spine structure and distribution. However, concise data on the various modulators of dendritic spines are imperative and a need of the hour. Hence, in this review we made an attempt to consolidate the effects of various pharmacological (cholinergic, glutamatergic, GABAergic, serotonergic, adrenergic, and dopaminergic agents) and non-pharmacological modulators (dietary interventions, enriched environment, yoga and meditation) on dendritic spines structure and functions. These data suggest that both the pharmacological and non-pharmacological modulators produced significant improvement in dendritic spine structure and functions and in turn reversing the pathologies underlying neurodegeneration. Intriguingly, the non-pharmacological approaches have shown to improve intellectual performances both in preclinical and clinical platforms, but still more technology-based evidence needs to be studied. Thus, we conclude that a combination of pharmacological and non-pharmacological intervention may restore cognitive performance synergistically via improving dendritic spine number and functions in various neurological disorders.

Rho Signaling in Synaptic Plasticity, Memory, and Brain Disorders

Memory impairments are associated with many brain disorders such as autism, Alzheimer's disease, and depression. Forming memories involves modifications of synaptic transmission and spine morphology. The Rho family small GTPases are key regulators of synaptic plasticity by affecting various downstream molecules to remodel the actin cytoskeleton. In this paper, we will review recent studies on the roles of Rho proteins in the regulation of hippocampal long-term potentiation (LTP) and long-term depression (LTD), the most extensively studied forms of synaptic plasticity widely regarded as cellular mechanisms for learning and memory. We will also discuss the involvement of Rho signaling in spine morphology, the structural basis of synaptic plasticity and memory formation. Finally, we will review the association between brain disorders and abnormalities of Rho function. It is expected that studying Rho signaling at the synapse will contribute to the understanding of how memory is formed and disrupted in diseases.

Dendritic Spines: Mediators of Cognitive Resilience in Aging and Alzheimer's Disease

Cognitive resilience is often defined as the ability to remain cognitively normal in the face of insults to the brain. These insults can include disease pathology, such as plaques and tangles associated with Alzheimer's disease, stroke, traumatic brain injury, or other lesions. Factors such as physical or mental activity and genetics may contribute to cognitive resilience, but the neurobiological underpinnings remain ill-defined. Emerging evidence suggests that dendritic spine structural plasticity is one plausible mechanism. In this review, we highlight the basic structure and function of dendritic spines and discuss how spine density and morphology change in aging and Alzheimer's disease. We note evidence that spine plasticity mediates resilience to stress, and we tackle dendritic spines in the context of cognitive resilience to Alzheimer's disease. Finally, we examine how lifestyle and genetic factors may influence dendritic spine plasticity to promote cognitive resilience before discussing evidence for actin regulatory kinases as therapeutic targets for Alzheimer's disease.

Cortactin Contributes to Activity-Dependent Modulation of Spine Actin Dynamics and Spatial Memory Formation

Postsynaptic structures on excitatory neurons, dendritic spines, are actin-rich. It is well known that actin-binding proteins regulate actin dynamics and by this means orchestrate structural plasticity during the development of the brain, as well as synaptic plasticity mediating learning and memory processes. The actin-binding protein cortactin is localized to pre- and postsynaptic structures and translocates in a stimulus-dependent manner between spines and the dendritic compartment, thereby indicating a crucial role for synaptic plasticity and neuronal function. While it is known that cortactin directly binds F-actin, the Arp2/3 complex important for actin nucleation and branching as well as other factors involved in synaptic plasticity processes, its precise role in modulating actin remodeling in neurons needs to be deciphered. In this study, we characterized the general neuronal function of cortactin in knockout mice. Interestingly, we found that the loss of cortactin leads to deficits in hippocampus-dependent spatial memory formation. This impairment is correlated with a prominent dysregulation of functional and structural plasticity. Additional evidence shows impaired long-term potentiation in cortactin knockout mice together with a complete absence of structural spine plasticity. These phenotypes might at least in part be explained by alterations in the activity-dependent modulation of synaptic actin in cortactin-deficient neurons.

Proteomic Profiling of the Substantia Nigra to Identify Determinants of Lewy Body Pathology and Dopaminergic Neuronal Loss

Proteinaceous aggregates containing α-synuclein protein called Lewy bodies in the substantia nigra is a hallmark of Parkinson's disease. The molecular mechanisms of Lewy body formation and associated neuronal loss remain largely unknown. To gain insights into proteins and pathways associated with Lewy body pathology, we performed quantitative profiling of the proteome. We analyzed substantia nigra tissue from 51 subjects arranged into three groups: cases with Lewy body pathology, Lewy body-negative controls with matching neuronal loss, and controls with no neuronal loss. Using a label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach, we characterized the proteome both in terms of protein abundances and peptide modifications. Statistical testing for differential abundance of the most abundant 2963 proteins, followed by pathway enrichment and Bayesian learning of the causal network structure, was performed to identify likely drivers of Lewy body formation and dopaminergic neuronal loss. The identified pathways include (1) Arp2/3 complex-mediated actin nucleation; (2) synaptic function; (3) poly(A) RNA binding; (4) basement membrane and endothelium; and (5) hydrogen peroxide metabolic process. According to the data, the endothelial/basement membrane pathway is tightly connected with both pathologies and likely to be one of the drivers of neuronal loss. The poly(A) RNA-binding proteins, including the ones relevant to other neurodegenerative disorders (e.g., TDP-43 and FUS), have a strong inverse correlation with Lewy bodies and may reflect an alternative mechanism of nigral neurodegeneration.

Novel CTRP8-RXFP1-JAK3-STAT3 axis promotes Cdc42-dependent actin remodeling for enhanced filopodia formation and motility in human glioblastoma cells

C1q tumor necrosis factor‐related peptide 8 (CTRP8) is the least studied member of the C1Q‐TNF‐related peptide family. We identified CTRP8 as a ligand of the G protein‐coupled receptor relaxin family peptide receptor 1 (RXFP1) in glioblastoma multiforme (GBM). The CTRP8‐RXFP1 ligand–receptor system protects human GBM cells against the DNA‐alkylating damage‐inducing temozolomide (TMZ), the drug of choice for the treatment of patients with GBM. The DNA protective role of CTRP8 was dependent on a functional RXFP1‐STAT3 signaling cascade and targeted the monofunctional glycosylase N‐methylpurine DNA glycosylase (MPG) for more efficient base excision repair of TMZ‐induced DNA‐damaged sites. CTRP8 also improved the survival of GBM cells by upregulating anti‐apoptotic BCl‐2 and BCL‐XL. Here, we have identified Janus‐activated kinase 3 (JAK3) as a novel member of a novel CTRP8‐RXFP1‐JAK3‐STAT3 signaling cascade that caused an increase in cellular protein content and activity of the small Rho GTPase Cdc42. This is associated with significant F‐actin remodeling and increased GBM motility. Cdc42 was critically important for the upregulation of the actin nucleation complex N‐Wiskott–Aldrich syndrome protein/Arp3/4 and actin elongation factor profilin‐1. The activation of the RXFP1‐JAK3‐STAT3‐Cdc42 axis by both RXFP1 agonists, CTRP8 and relaxin‐2, caused extensive filopodia formation. This coincided with enhanced activity of ezrin, a key factor in tethering F‐actin to the plasma membrane, and inhibition of the actin filament severing activity of cofilin. The F‐actin remodeling and pro‐migratory activities promoted by the novel RXFP1‐JAK3‐STAT3‐Cdc42 axis were blocked by JAK3 inhibitor tofacitinib and STAT3 inhibitor STAT3 inhibitor VI. This provides a new rationale for the design of JAK3 and STAT3 inhibitors with better brain permeability for clinical treatment of the pervasive brain invasiveness of GBM.

Conditional RAC1 knockout in motor neurons restores H-reflex rate-dependent depression after spinal cord injury

A major complication with spinal cord injury (SCI) is the development of spasticity, a clinical symptom of hyperexcitability within the spinal H-reflex pathway. We have previously demonstrated a common structural motif of dendritic spine dysgenesis associated with hyperexcitability disorders after injury or disease insults to the CNS. Here, we used an adeno-associated viral (AAV)-mediated Cre-Lox system to knockout Rac1 protein expression in motor neurons after SCI. Three weeks after AAV9-Cre delivery into the soleus/gastrocnemius of Rac1-“floxed” adult mice to retrogradely infect spinal alpha-motor neurons, we observed significant restoration of RDD and reduced H-reflex excitability in SCI animals. Additionally, viral-mediated Rac1 knockdown reduced presence of dendritic spine dysgenesis on motor neurons. In control SCI animals without Rac1 knockout, we continued to observe abnormal dendritic spine morphology associated with hyperexcitability disorder, including an increase in mature, mushroom dendritic spines, and an increase in overall spine length and spine head size. Taken together, our results demonstrate that viral-mediated disruption of Rac1 expression in ventral horn motor neurons can mitigate dendritic spine morphological correlates of neuronal hyperexcitability, and reverse hyperreflexia associated with spasticity after SCI. Finally, our findings provide evidence of a putative mechanistic relationship between motor neuron dendritic spine dysgenesis and SCI-induced spasticity.

PIKfyve activity is required for lysosomal trafficking of tau aggregates and tau seeding

Tauopathies, such as Alzheimer's disease (AD), are neurodegenerative disorders characterized by the deposition of hyperphosphorylated tau aggregates. Proteopathic tau seeds spread through the brain in a temporospatial pattern, indicative of transsynaptic propagation. It is hypothesized that reducing the uptake of tau seeds and subsequent induction of tau aggregation could be a potential approach for abrogating disease progression in AD. Here, we studied to what extent different endosomal routes play a role in the neuronal uptake of preformed tau seeds. Using pharmacological and genetic tools, we identified dynamin-1, actin, and Rac1 as key players. Furthermore, inhibition of PIKfyve, a protein downstream of Rac1, reduced both the trafficking of tau seeds into lysosomes and the induction of tau aggregation. Our work shows that tau aggregates are internalized by a specific endocytic mechanism and that their fate once internalized can be pharmacologically modulated to reduce tau seeding in neurons.

RHO to the DOCK for GDP disembarking: Structural insights into the DOCK GTPase nucleotide exchange factors

The human dedicator of cytokinesis (DOCK) family consists of 11 structurally conserved proteins that serve as atypical RHO guanine nucleotide exchange factors (RHO GEFs). These regulatory proteins act as mediators in numerous cellular cascades that promote cytoskeletal remodeling, playing roles in various crucial processes such as differentiation, migration, polarization, and axon growth in neurons. At the molecular level, DOCK DHR2 domains facilitate nucleotide dissociation from small GTPases, a process that is otherwise too slow for rapid spatiotemporal control of cellular signaling. Here, we provide an overview of the biological and structural characteristics for the various DOCK proteins and describe how they differ from other RHO GEFs and between DOCK subfamilies. The expression of the family varies depending on cell or tissue type, and they are consequently implicated in a broad range of disease phenotypes, particularly in the brain. A growing body of available structural information reveals the mechanism by which the catalytic DHR2 domain elicits nucleotide dissociation and also indicates strategies for the discovery and design of high-affinity small-molecule inhibitors. Such compounds could serve as chemical probes to interrogate the cellular function and provide starting points for drug discovery of this important class of enzymes.

Astrocyte-derived small extracellular vesicles promote synapse formation via fibulin-2-mediated TGF-β signaling

Neuronal synapse formation is critical for brain development and depends on secreted factors from astrocytes. Here, we report that small extracellular vesicles (EVs) secreted from primary astrocytes, but not from neurons or C6 glioma cells, greatly enhance spine and synapse formation by primary cortical neurons. A comparative proteomics analysis of small EVs from astrocytes, neurons, and C6 glioma cells identified fibulin-2 as a promising EV cargo to regulate synaptogenesis. Treatment of cortical neurons with recombinant fibulin-2 increased the formation of spines and synapses, similar to the effect of small EVs. In addition, treatment of neurons with fibulin-2 or astrocyte-derived small EVs led to increased phosphorylation of Smad2, an indicator of TGF-β signaling. Finally, the effects of fibulin-2 and astrocyte-derived small EVs on synapse formation were reversed by inhibiting transforming growth factor β (TGF-β) signaling. These data suggest a model in which astrocyte EVs promote synapse formation via fibulin-2-mediated activation of TGF-β signaling.

Kinase Signaling in Dendritic Development and Disease

Dendrites undergo extensive growth and remodeling during their lifetime. Specification of neurites into dendrites is followed by their arborization, maturation, and functional integration into synaptic networks. Each of these distinct developmental processes is spatially and temporally controlled in an exquisite fashion. Protein kinases through their highly specific substrate phosphorylation regulate dendritic growth and plasticity. Perturbation of kinase function results in aberrant dendritic growth and synaptic function. Not surprisingly, kinase dysfunction is strongly associated with neurodevelopmental and psychiatric disorders. Herein, we review, (a) key kinase pathways that regulate dendrite structure, function and plasticity, (b) how aberrant kinase signaling contributes to dendritic dysfunction in neurological disorders and (c) emergent technologies that can be applied to dissect the role of protein kinases in dendritic structure and function.

Myosin 18Aα targets the guanine nucleotide exchange factor β-Pix to the dendritic spines of cerebellar Purkinje neurons and promotes spine maturation

Myosin 18Aα is a myosin 2-like protein containing unique N- and C-terminal protein interaction domains that co-assembles with myosin 2. One protein known to bind to myosin 18Aα is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42 that has been shown to promote dendritic spine maturation by activating the assembly of actin and myosin filaments in spines. Here, we show that myosin 18A⍺ concentrates in the spines of cerebellar Purkinje neurons via co-assembly with myosin 2 and through an actin binding site in its N-terminal extension. miRNA-mediated knockdown of myosin 18A⍺ results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18A⍺. Importantly, β-Pix co-localizes with myosin 18A⍺ in spines, and its spine localization is lost upon myosin 18A⍺ knockdown or when its myosin 18A⍺ binding site is deleted. Finally, we show that the spines of myosin 18A⍺ knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data argue that mixed filaments of myosin 2 and myosin 18A⍺ form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix's GEF activity.

Antipsychotic Drugs Reverse MK801-Inhibited Cell Migration and F-actin Condensation by Modulating the Rho Signaling Pathway in B35 Cells

Background and Aim. MK801-induced psychotic symptoms and also the Ras homolog family member A (RhoA) expression and cell division control protein 42 (cdc42) mRNA modulation in the rat brain have been investigated. Antipsychotic drugs (APDs) have been reported to induce Rho GDP-dissociation inhibitor (RhoGDI) pathway regulation related to cytoskeleton reorganization in neuronal cells. It will be necessary to clarify the effects of APDs on MK801-induced RhoGDI signaling regulation in neuronal cells. Methods. B35 neuronal cells were treated with MK801 for 7 days then treated with MK801 in combination with haloperidol or clozapine for a further 7 days. Cell migration, F-actin condensation, and RhoGDI signaling regulation were examined to investigate the regulatory effects of MK801, haloperidol, and clozapine in B35 neuronal cells. Results. MK801 reduced B35 cell migration, whereas both haloperidol and clozapine reversed the reduction in cell migration induced by MK801. Haloperidol and clozapine restored F-actin condensation after it was diminished by MK801 in B35 cell nuclei. MK801 increased the RhoGDI1 and RhoA expression, which was diminished by the addition of haloperidol and clozapine. MK801 reduced the CDC42 expression, which was restored by haloperidol and clozapine. MK801 reduced the Rho-associated coiled-coil containing protein kinase 1 (ROCK1), profilin1 (PFN1), and neuronal Wiskott–Aldrich Syndrome protein (N-WASP) expression, which was further reduced by haloperidol and clozapine. MK801 also increased the phosphorylated myosin light chain 2 (p-MLC2), postsynaptic density protein 95 (PSD-95), and c-jun expression, which was decreased by haloperidol and clozapine. p21 (RAC1-) activated kinase 1 (PAK1) expression was not affected by MK801.

Hyperactive ACC-MDT Pathway Suppresses Prepulse Inhibition in Mice

Altered prepulse inhibition (PPI) is an endophenotype associated with multiple brain disorders, including schizophrenia. Circuit mechanisms that regulate PPI have been suggested, but none has been demonstrated through direct manipulations. IRSp53 is an abundant excitatory postsynaptic scaffold implicated in schizophrenia, autism spectrum disorders, and attention-deficit/hyperactivity disorder. We found that mice lacking IRSp53 in cortical excitatory neurons display decreased PPI. IRSp53-mutant layer 6 cortical neurons in the anterior cingulate cortex (ACC) displayed decreased excitatory synaptic input but markedly increased neuronal excitability, which was associated with excessive excitatory synaptic input in downstream mediodorsal thalamic (MDT) neurons. Importantly, chemogenetic inhibition of mutant neurons projecting to MDT normalized the decreased PPI and increased excitatory synaptic input onto MDT neurons. In addition, chemogenetic activation of MDT-projecting layer 6 neurons in the ACC decreased PPI in wild-type mice. These results suggest that the hyperactive ACC-MDT pathway suppresses PPI in wild-type and IRSp53-mutant mice.

Molecular changes in glaucomatous trabecular meshwork. Correlations with retinal ganglion cell death and novel strategies for neuroprotection

Glaucoma is a chronic neurodegenerative disease characterized by retinal ganglion cell loss. Although significant advances in ophthalmologic knowledge and practice have been made, some glaucoma mechanisms are not yet understood, therefore, up to now there is no effective treatment able to ensure healing. Indeed, either pharmacological or surgical approaches to this disease aim in lowering intraocular pressure, which is considered the only modifiable risk factor. However, it is well known that several factors and metabolites are equally (if not more) involved in glaucoma. Oxidative stress, for instance, plays a pivotal role in both glaucoma onset and progression because it is responsible for the trabecular meshwork cell damage and, consequently, for intraocular pressure increase as well as for glaucomatous damage cascade. This review at first shows accurately the molecular-derived dysfunctions in antioxidant system and in mitochondria homeostasis which due to both oxidative stress and aging, lead to a chronic inflammation state, the trabecular meshwork damage as well as the glaucoma neurodegeneration. Therefore, the main molecular events triggered by oxidative stress up to the proapoptotic signals that promote the ganglion cell death have been highlighted. The second part of this review, instead, describes some of neuroprotective agents such as polyphenols or polyunsaturated fatty acids as possible therapeutic source against the propagation of glaucomatous damage.

Glial ATP and Large Pore Channels Modulate Synaptic Strength in Response to Chronic Inactivity

A key feature of neurotransmission is its ability to adapt to changes in neuronal environment, which is essential for many brain functions. Homeostatic synaptic plasticity (HSP) emerges as a compensatory mechanism used by neurons to adjust their excitability in response to changes in synaptic activity. Recently, glial cells emerged as modulators for neurotransmission by releasing gliotransmitters into the synaptic cleft through pathways that include P2X₇ receptors (P2X₇R), connexons, and pannexons. However, the role of gliotransmission in the activity-dependent adjustment of presynaptic strength is still an open question. Here, we investigated whether glial cells participate in HSP upon chronic inactivity and the role of adenosine triphosphate (ATP), connexin43 hemichannels (Cx43HCs), and pannexin1 (Panx1) channels in this process. We used immunocytochemistry against vesicular glutamate transporter 1 (vGlut1) to estimate changes in synaptic strength in hippocampal dissociated cultures. Pharmacological manipulations indicate that glial-derived ATP and P2X₇R are required for HSP. In addition, inhibition of Cx43 and Panx1 channels reveals a pivotal role for these channels in the compensatory adjustment of synaptic strength, emerging as new pathways for ATP release upon inactivity. The involvement of Panx1 channels was confirmed by using Panx1-deficient animals. Lacking Panx1 in neurons is sufficient to prevent the P2X₇R-dependent upregulation of presynaptic strength; however, the P2X₇R-dependent compensatory adjustment of synapse density requires both neuronal and glial Panx1. Together, our data supports an essential role for glial ATP signaling and Cx43HCs and Panx1 channels in the homeostatic adjustment of synaptic strength in hippocampal cultures upon chronic inactivity.

The Role of Rac GTPase in Dendritic Spine Morphogenesis and Memory

The ability to form memories in the brain is needed for daily functions, and its impairment is associated with human mental disorders. Evidence indicates that long-term memory (LTM)-related processes such as its consolidation, extinction and forgetting involve changes of synaptic efficacy produced by alterations in neural transmission and morphology. Modulation of the morphology and number of dendritic spines has been proposed to contribute to changes in neuronal transmission mediating such LTM-related processes. Rac GTPase activity is regulated by synaptic activation and it can affect spine morphology by controlling actin-regulatory proteins. Recent evidence shows that changes in Rac GTPase activity affect memory consolidation, extinction, erasure and forgetting and can affect spine morphology in brain areas that mediate these behaviors. Altered Rac GTPase activity is associated with abnormal spine morphology and brain disorders. By affecting Rac GTPase activity we can further understand the roles of spine morphogenesis in memory. Moreover, manipulation of Rac GTPase activity may serve as a therapeutic tool for the treatment of memory-related brain diseases.

Dendritic Spines in Alzheimer's Disease: How the Actin Cytoskeleton Contributes to Synaptic Failure

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by Aβ-driven synaptic dysfunction in the early phases of pathogenesis. In the synaptic context, the actin cytoskeleton is a crucial element to maintain the dendritic spine architecture and to orchestrate the spine’s morphology remodeling driven by synaptic activity. Indeed, spine shape and synaptic strength are strictly correlated and precisely governed during plasticity phenomena in order to convert short-term alterations of synaptic strength into long-lasting changes that are embedded in stable structural modification. These functional and structural modifications are considered the biological basis of learning and memory processes. In this review we discussed the existing evidence regarding the role of the spine actin cytoskeleton in AD synaptic failure. We revised the physiological function of the actin cytoskeleton in the spine shaping and the contribution of actin dynamics in the endocytosis mechanism. The internalization process is implicated in different aspects of AD since it controls both glutamate receptor membrane levels and amyloid generation. The detailed understanding of the mechanisms controlling the actin cytoskeleton in a unique biological context as the dendritic spine could pave the way to the development of innovative synapse-tailored therapeutic interventions and to the identification of novel biomarkers to monitor synaptic loss in AD.

Rapid Estrogen and Progesterone Signaling to Dendritic Spine Formation via Cortactin/Wave1-Arp2/3 Complex

BACKGROUND

Synaptic plasticity is the neuronal capacity to modify the function and structure of dendritic spines (DS) in response to neuromodulators. Sex steroids, particularly 17β-estradiol (E2) and progesterone (P4), are key regulators in the control of DS formation through multiprotein complexes including WAVE1 protein, and are thus fundamental for the development of learning and memory.

OBJECTIVES

The aim of this work was to evaluate the molecular switch Cdk5 kinase/protein phosphatase 2A (PP2A) in the control of WAVE1 protein (phosphorylation/dephosphorylation) and the regulation of WAVE1 and cortactin to the Arp2/3 complex, in response to rapid treatments with E2 and P4 in cortical neuronal cells.

RESULTS

Rapid treatment with E2 and P4 modified neuronal morphology and significantly increased the number of DS. This effect was reduced by the use of a Cdk5 inhibitor (Roscovitine). In contrast, inhibition of PP2A with PP2A dominant negative construct significantly increased DS formation, evidencing the participation of kinase/phosphatase in the regulation of WAVE1 in DS formation induced by E2 and P4. Cortactin regulates DS formation via Src and PAK1 kinase induced by E2 and P4. Both cortactin and WAVE1 signal to Arp2/3 complex to synergistically promote actin nucleation.

CONCLUSION

These results suggest that E2 and P4 dynamically regulate neuron morphology through nongenomic signaling via cortactin/WAVE1-Arp2/3 complex. The control of these proteins is tightly orchestrated by phosphorylation, where kinases and phosphatases are essential for actin nucleation and, finally, DS formation. This work provides a deeper understanding of the biological actions of sex steroids in the regulation of DS turnover and neuronal plasticity processes.

Distinct molecular mechanisms control levels of synaptic F-actin

Polymerization of filamentous (F)-actin at the neuronal synapse plays an important role in neuronal function. However, the regulatory mechanisms controlling the levels of synaptic actin remain incompletely understood. Here, I used established pharmacological blockers to acutely disrupt the function of actin polymerization machinery, then quantified their effect on synaptic F-actin levels. Synaptic F-actin was modestly decreased by inhibition of Arp2/3-dependent actin branching. Blockade of formin-dependent actin elongation resulted in an Arp2/3-dependent increase in synaptic actin that could be mimicked by limited actin depolymerization. Limited actin depolymerization was also sufficient to reverse a decrease in synaptic F-actin caused by prolonged blockade of synaptic NMDA-type glutamate receptors. These results suggest that interplay between different actin polymerization pathways may regulate synaptic actin dynamics.

Dopamine D(1) and D(2) Receptors Differentially Regulate Rac1 and Cdc42 Signaling in the Nucleus Accumbens to Modulate Behavioral and Structural Plasticity After Repeated Methamphetamine Treatment

BACKGROUND

Methamphetamine (METH) is a highly addictive psychostimulant that strongly activates dopamine receptor signaling in the nucleus accumbens (NAc). However, how dopamine D₁ and D₂ receptors (D₁Rs and D₂Rs, respectively) as well as downstream signaling pathways, such as those involving Rac1 and Cdc42, modulate METH-induced behavioral and structural plasticity is largely unknown.

METHODS

Using NAc conditional D₁R and D₂R deletion mice, Rac1 and Cdc42 mutant viruses, and a series of behavioral and morphological methods, we assessed the effects of D₁Rs and D₂Rs on Rac1 and Cdc42 in modulating METH-induced behavioral and structural plasticity in the NAc.

RESULTS

D₁Rs and D₂Rs in the NAc consistently regulated METH-induced conditioned place preference, locomotor activation, and dendritic and spine remodeling of medium spiny neurons but differentially regulated METH withdrawal-induced spatial learning and memory impairment and anxiety. Interestingly, Rac1 and Cdc42 signaling were oppositely modulated by METH, and suppression of Rac1 signaling and activation of Cdc42 signaling were crucial to METH-induced conditioned place preference and structural plasticity but not to locomotor activation. D₁Rs activated Rac1 and Cdc42 signaling, while D₂Rs inhibited Rac1 signaling but activated Cdc42 signaling to mediate METH-induced conditioned place preference and structural plasticity but not locomotor activation. In addition, NAc D₁R deletion aggravated METH withdrawal-induced spatial learning and memory impairment by suppressing Rac1 signaling but not Cdc42 signaling, while NAc D₂R deletion aggravated METH withdrawal-induced anxiety without affecting Rac1 or Cdc42 signaling.

CONCLUSIONS

D₁Rs and D₂Rs differentially regulate Rac1 and Cdc42 signaling to modulate METH-induced behavioral plasticity and the structural remodeling of medium spiny neurons in the NAc.

Long-term memory is maintained by continuous activity of Arp2/3 in lateral amygdala

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such changes induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. It has been shown that the actin nucleating protein Arp2/3 is essential for supporting neuronal morphology and synaptic transmission. We therefore hypothesized that continuous Arp2/3 activity is needed to maintain long-term memory over time. To test this hypothesis we microinjected into lateral amygdala (LA) of rats CK-666, a specific inhibitor of Arp2/3, two days after fear conditioning and tested the effect on long-term fear memory maintenance a day afterward. We found that injection of CK-666 two days after training abolished fear conditioning memory. Fear conditioning could be formed when a control compound CK-689 was applied two days after training. Microinjection of CK-666 a day before fear conditioning training had no effect on fear conditioning learning and long-term memory formation. We revealed that Arp2/3 is also needed to maintain long-term conditioned taste aversion (CTA) memory in LA. Microinjection of CK-666 two days after CTA training impaired long-term memory tested a day afterwards. We conclude that continuous activity of Arp2/3 in LA is essential for the maintenance of long-term memory.

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.

Fine-mapping scan of bipolar disorder susceptibility loci in Latino pedigrees

We previously identified bipolar disorder (BD) susceptibility loci on 8q24, 14q32, and 2q12-14 in a genome-wide nonparametric linkage screen in a Latino cohort. We now perform a fine mapping analysis using a dense map of additional SNPs to identify BD susceptibility genes within these regions. One thousand nine hundred and thirty-eight individuals with Latino ancestry (880 individuals with BD Type I or Schizoaffective, Bipolar Type) from 416 Latino pedigrees from the United States, Mexico, Costa Rica, and Guatemala were genotyped with 3,074 SNPs to provide dense coverage of the 8q24 (11.5 cM), 14q32 (7.5 cM), and 2q12-14 (6.5 cM) chromosomal loci. Single-marker association tests in the presence of linkage were performed using the LAMP software. The top linkage peak (rs7834818; LOD = 5.08, p = 3.30E - 5) and associated single marker (rs2280915, p = 2.70E - 12) were located within FBXO32 on 8q24. On chromosome 2, the top linkage peak (rs6750326; LOD = 5.06, p = 3.50E - 5) and associated single marker (rs11887088, p = 2.90E - 6) were located in intragenic regions near ACTR3 and DPP10. None of the additional markers in the region around chromosome 14q32 met significance levels for linkage or association. We identified six SNPs on 2q12-q14 and one SNP in FBXO32 on 8q24 that were significantly associated with BD in this Latino cohort.

Visual Defects and Ageing

Many diseases are related to age, among these neurodegeneration is particularly important. Alzheimer's disease Parkinson's and Glaucoma have many common pathogenic events including oxidative damage, Mitochondrial dysfunction, endothelial alterations and changes in the visual field. These are well known in the case of glaucoma, less in the case of neurodegeneration of the brain. Many other molecular aspects are common, such as the role of endoplasmic reticulum autophagy and neuronal apoptosis while others have been neglected due to lack of space such as inflammatory cytokine or miRNA. Moreover, the loss of specific neuronal populations, the induction of similar mechanisms of cell injury and the deposition of protein aggregates in specific anatomical areas are very similar events between these diseases. Intracellular and/or extracellular accumulation of protein aggregates is a key feature of many neurodegenerative disorders. The existence of abnormal protein aggregates has been documented in the RGCs of glaucomatous patients such as the anomalous Tau protein or the β-amyloid accumulations. Intra-cell catabolic processes also appear to be common in both glaucoma and neurodegeneration. They also help us to understand how the basis between these diseases is common and how the visual aspects can be a serious problem for those who are affected.

Structure and Function of an Actin-Based Filter in the Proximal Axon

The essential organization of microtubules within neurons has been described; however, less is known about how neuronal actin is arranged and the functional implications of its arrangement. Here, we describe, in live cells, an actin-based structure in the proximal axon that selectively prevents some proteins from entering the axon while allowing the passage of others. Concentrated patches of actin in proximal axons are present shortly after axonal specification in rat and zebrafish neurons imaged live, and they mark positions where anterogradely traveling vesicles carrying dendritic proteins halt and reverse. Patches colocalize with the ARP2/3 complex, and when ARP2/3-mediated nucleation is blocked, a dendritic protein mislocalizes to the axon. Patches are highly dynamic, with few persisting longer than 30 min. In neurons in culture and in vivo, actin appears to form a contiguous, semipermeable barrier, despite its apparently sparse distribution, preventing axonal localization of constitutively active myosin Va but not myosin VI.

The Role of Actin Cytoskeleton in Dendritic Spines in the Maintenance of Long-Term Memory

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such alterations induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. This is especially intriguing as the half-life of most of the proteins that underlie such changes is usually in the range of hours to days and these proteins may change their location over time. In this review we describe studies that indicate the involvement of dendritic spines in memory formation and its maintenance. These studies show that learning leads to changes in the number and morphology of spines. Disruption in spines morphology or manipulations that lead to alteration in their number after consolidation are associated with impairment in memory maintenance. We further ask how changes in dendritic spines morphology, induced by learning and reputed to encode memory, are maintained to preserve long-term memory. We propose a mechanism, based on studies described in the review, whereby the actin cytoskeleton and its regulatory proteins involved in the initial alteration in spine morphology induced by learning are also essential for spine structural stabilization that maintains long-term memory. In this model glutamate receptors and other synaptic receptors activation during learning leads to the creation of new actin cytoskeletal scaffold leading to changes in spines morphology and memory formation. This new actin cytoskeletal scaffold is preserved beyond actin and its regulatory proteins turnover and dynamics by active stabilization of the level and activity of actin regulatory proteins within these memory spines.

A Schizophrenia-Linked KALRN Coding Variant Alters Neuron Morphology, Protein Function, and Transcript Stability

BACKGROUND

Large-scale genetic studies have revealed that rare sequence variants, including single nucleotide variants (SNVs), in glutamatergic synaptic genes are enriched in schizophrenia patients. However, the majority are too rare to show any association with disease and have not been examined functionally. One such SNV, KALRN-P2255T, displays a penetrance that greatly exceeds that of previously identified schizophrenia-associated SNVs. Therefore, we sought to characterize its effects on the function of kalirin (Kal)-9, a dual Ras-related C3 botulinum toxin substrate 1 and Ras homologue gene family, member A (RhoA) guanine nucleotide exchange factor, upregulated in human schizophrenia brain tissue.

METHODS

Kal9 was overexpressed in primary rat cortical neurons or human embryonic kidney 293 (HEK293) cells. The effects of the P2255T variant on dendritic branching, dendritic spine morphology, protein and messenger RNA stability, and catalytic activity were examined.

RESULTS

Kal9-P2255T leads to diminished basal dendritic branching and dendritic spine size, compared with wild-type Kal9. The P2255T SNV directly affected Kal9 protein function, causing increased RhoA activation in HEK293 cells, but had no effect on Ras-related C3 botulinum toxin substrate 1 activation. Consistent with human postmortem findings, we found that Kal9-P2255T protein levels were higher than those of wild-type Kal9 in neurons. Increased messenger RNA stability was detected in HEK293 cells, indicating that this was the cause of the higher protein levels. When analyzed together, increased intrinsic RhoA guanine nucleotide exchange factor catalytic activity combined with increased messenger RNA expression led to net enhancement of RhoA activation, known to negatively impact neuronal morphology.

CONCLUSIONS

Taken together, our data reveal a novel mechanism for disease-associated SNVs and provide a platform for modeling morphological changes in mental disorders.

Actin polymerization is reduced in the anterior cingulate cortex of elderly patients with schizophrenia

Recent reports suggest abnormalities in the regulation of actin cytoskeletal dynamics in schizophrenia, despite consistent evidence for normal actin expression. We hypothesized that this may be explained by changes in the polymerization state of actin, rather than in total actin expression. To test this, we prepared filamentous actin (F-actin, polymeric) and globular actin (G-actin, monomeric) fractions from postmortem anterior cingulate cortex from 16 patients with schizophrenia and 14 comparison subjects. Additionally, binding of fluorescently-labeled phalloidin, a selectively F-actin-binding peptide, was measured in unfractionated samples from the same subjects. Western blot analysis of fractions revealed decreased F-actin, increased G-actin, and decreased ratios of F-actin/total actin and F-actin/G-actin in schizophrenia. Decreased phalloidin binding to F-actin in parallel experiments in the same subjects independently supports these findings. These results suggest a novel aspect of schizophrenia pathophysiology and are consistent with previous evidence of reduced dendritic spine density and altered synaptic plasticity in schizophrenia, both of which have been linked to cytoskeletal abnormalities.