Requirement for Plk2 in orchestrated ras and rap signaling, homeostatic structural plasticity, and memory

Expression and Function of Long Non-coding RNA in Endemic Cretinism

Endemic cretinism (EC) is one of the most severe iodine deficiency disorders, leading to typical symptoms such as neurodevelopmental impairments or mental deficits. In addition to environmental factors, the pathogenesis of its genetic contribution remains unclear. The study revealed the differential expression profiles of long non-coding RNA(lncRNA) and messenger RNA(mRNA) based on high-throughput RNA-seq. GO and KEGG analyses were used to annotate the function and pathway of differentially expressed (DE) mRNA and co-expressed mRNA. The protein-protein interaction(PPI) network was established. The expression levels of three lncRNAs and six mRNAs were validated by quantitative real-time PCR analysis (qRT-PCR) and subjected to correlation analysis. Compared to controls, a total of 864 lncRNAs and 393 mRNAs were differentially expressed. The PPI network had 149 nodes and 238 edges, and three key protein-coding genes were observed. Levels of LINC01220 and target mRNA IDO1 were statistically elevated in EC patients. Differentially expressed lncRNA may be a new potential player in EC. LINC01220 and IDO1 might interact with each other to participate in EC. The biological process of regulation of postsynaptic membrane potential and the Rap1 signaling pathway might exert a regulating role in the pathophysiological process of EC. Our findings could provide more theoretical and experimental evidence for investigating the pathophysiological mechanisms of EC.

Amyloid precursor protein combinatorial phosphorylation code regulates AMPA receptor removal during distinct forms of synaptic plasticity

Synaptic plasticity is essential for memory encoding and stabilization of neural network activity. Plasticity is impaired in neurodegenerative conditions including Alzheimer disease (AD). A central factor in AD is amyloid precursor protein (APP). Previous studies have suggested APP involvement in synaptic plasticity, but physiological roles of APP are not well understood. Here, we identified combinatorial phosphorylation sites within APP that regulate AMPA receptor trafficking during different forms of synaptic plasticity. Dual phosphorylation sites at threonine-668/serine-675 of APP promoted endocytosis of the GluA2 subunit of AMPA receptors during homeostatic synaptic plasticity. APP was also required for GluA2 internalization during NMDA receptor-dependent long-term depression, albeit via a distinct pair of phosphoresidues at serine-655/threonine-686. These data implicate APP as a central gate for AMPA receptor internalization during distinct forms of plasticity, unlocked by specific combinations of phosphoresidues, and suggest that APP may serve broad functions in learning and memory.

Multiomics of early epileptogenesis in mice reveals phosphorylation and dephosphorylation-directed growth and synaptic weakening

To investigate the phosphorylation-based signaling and protein changes occurring early in epileptogenesis, the hippocampi of mice treated with pilocarpine were examined by quantitative mass spectrometry at 4 and 24 h post-status epilepticus at vast depth. Hundreds of posttranscriptional regulatory proteins were the major early targets of increased phosphorylation. At 24 h, many protein level changes were detected and the phosphoproteome continued to be perturbed. The major targets of decreased phosphorylation at 4 and 24 h were a subset of postsynaptic density scaffold proteins, ion channels, and neurotransmitter receptors. Many proteins targeted by dephosphorylation at 4 h also had decreased protein abundance at 24 h, indicating a phosphatase-mediated weakening of synapses. Increased translation was indicated by protein changes at 24 h. These observations, and many additional indicators within this multiomic resource, suggest that early epileptogenesis is characterized by signaling that stimulates both growth and a homeostatic response that weakens excitability.

Interactions between Ras and Rap signaling pathways during neurodevelopment in health and disease

The Ras family of small GTPases coordinates tissue development by modulating cell proliferation, cell-cell adhesion, and cellular morphology. Perturbations of any of these key steps alter nervous system development and are associated with neurological disorders. While the underlying causes are not known, genetic mutations in Ras and Rap GTPase signaling pathways have been identified in numerous neurodevelopmental disorders, including autism spectrum, neurofibromatosis, intellectual disability, epilepsy, and schizophrenia. Despite diverse clinical presentations, intersections between these two signaling pathways may provide a better understanding of how deviations in neurodevelopment give rise to neurological disorders. In this review, we focus on presynaptic and postsynaptic functions of Ras and Rap GTPases. We highlight various roles of these small GTPases during synapse formation and plasticity. Based on genomic analyses, we discuss how disease-related mutations in Ras and Rap signaling proteins may underlie human disorders. Finally, we discuss how recent observations have identified molecular interactions between these pathways and how these findings may provide insights into the mechanisms that underlie neurodevelopmental disorders.

The guanine nucleotide exchange factor RapGEF2 is required for ERK-dependent immediate-early gene (Egr1) activation during fear memory formation

The MAP kinase ERK is important for neuronal plasticity underlying associative learning, yet specific molecular pathways for neuronal ERK activation are undetermined. RapGEF2 is a neuron-specific cAMP sensor that mediates ERK activation. We investigated whether it is required for cAMP-dependent ERK activation leading to other downstream neuronal signaling events occurring during associative learning, and if RapGEF2-dependent signaling impairments affect learned behavior. Camk2α-cre+/-::RapGEF2fl/fl mice with depletion of RapGEF2 in hippocampus and amygdala exhibit impairments in context- and cue-dependent fear conditioning linked to corresponding impairment in Egr1 induction in these two brain regions. Camk2α-cre+/-::RapGEF2fl/fl mice show decreased RapGEF2 expression in CA1 and dentate gyrus associated with abolition of pERK and Egr1, but not of c-Fos induction, following fear conditioning, impaired freezing to context after fear conditioning, and impaired cAMP-dependent long-term potentiation at perforant pathway and Schaffer collateral synapses in hippocampal slices ex vivo. RapGEF2 expression is largely eliminated in basolateral amygdala, also involved in fear memory, in Camk2α-cre+/-::RapGEF2fl/fl mice. Neither Egr1 nor c-fos induction in BLA after fear conditioning, nor cue-dependent fear learning, are affected by ablation of RapGEF2 in BLA. However, Egr1 induction (but not that of c-fos) in BLA is reduced after restraint stress-augmented fear conditioning, as is freezing to cue after restraint stress-augmented fear conditioning, in Camk2α-cre+/-::RapGEF2fl/fl mice. Cyclic AMP-dependent GEFs have been genetically associated as risk factors for schizophrenia, a disorder associated with cognitive deficits. Here we show a functional link between one of them, RapGEF2, and cognitive processes involved in associative learning in amygdala and hippocampus.

Plk2 promotes synaptic destabilization through disruption of N-cadherin adhesion complexes during homeostatic adaptation to hyperexcitation

Synaptogenesis in the brain is highly organized and orchestrated by synaptic cellular adhesion molecules (CAMs) such as N-cadherin and amyloid precursor protein (APP) that contribute to the stabilization and structure of synapses. Although N-cadherin plays an integral role in synapse formation and synaptic plasticity, its function in synapse dismantling is not as well understood. Synapse weakening and loss are prominent features of neurodegenerative diseases, and can also be observed during homeostatic compensation to neuronal hyperexcitation. Previously, we have shown that during homeostatic synaptic plasticity, APP is a target for cleavage triggered by phosphorylation by Polo-like kinase 2 (Plk2). Here, we found that Plk2 directly phosphorylates N-cadherin, and during neuronal hyperexcitation Plk2 promotes N-cadherin proteolytic processing, degradation, and disruption of complexes with APP. We further examined the molecular mechanisms underlying N-cadherin degradation. Loss of N-cadherin adhesive function destabilizes excitatory synapses and promotes their structural dismantling as a prerequisite to eventual synapse elimination. This pathway, which may normally help to homeostatically restrain excitability, could also shed light on the dysregulated synapse loss that occurs in cognitive disorders.

Activity-dependent post-translational regulation of palmitoylating and depalmitoylating enzymes in the hippocampus

Activity-induced changes in protein palmitoylation can regulate the plasticity of synaptic connections, critically impacting learning and memory. Palmitoylation is a reversible post-translational modification regulated by both palmitoyl-acyl transferases that mediate palmitoylation and palmitoyl thioesterases that depalmitoylate proteins. However, it is not clear how fluctuations in synaptic activity can mediate the dynamic palmitoylation of neuronal proteins. Using primary hippocampal cultures, we demonstrate that synaptic activity does not impact the transcription of palmitoylating and depalmitoylating enzymes, changes in thioesterase activity, or post-translational modification of the depalmitoylating enzymes of the ABHD17 family and APT2 (also known as LYPLA2). In contrast, synaptic activity does mediate post-translational modification of the palmitoylating enzymes ZDHHC2, ZDHHC5 and ZDHHC9 (but not ZDHHC8) to influence protein-protein interactions, enzyme stability and enzyme function. Post-translational modifications of the ZDHHC enzymes were also observed in the hippocampus following fear conditioning. Taken together, our findings demonstrate that signaling events activated by synaptic activity largely impact activity of the ZDHHC family of palmitoyl-acyl transferases with less influence on the activity of palmitoyl thioesterases.

Reelin and APP Cooperatively Modulate Dendritic Spine Formation In Vitro and In Vivo

Amyloid precursor protein (APP) plays an important role in the pathogenesis of Alzheimer's disease (AD), but the normal function of APP at synapses is poorly understood. We and others have found that APP interacts with Reelin and that each protein is individually important for dendritic spine formation, which is associated with learning and memory, in vitro. However, whether Reelin acts through APP to modulate dendritic spine formation or synaptic function remains unknown. In the present study, we found that Reelin treatment significantly increased dendritic spine density and PSD-95 puncta number in primary hippocampal neurons. An examination of the molecular mechanisms by which Reelin regulates dendritic spinogenesis revealed that Reelin enhanced hippocampal dendritic spine formation in a Ras/ERK/CREB signaling-dependent manner. Interestingly, Reelin did not increase dendritic spine number in primary hippocampal neurons when APP expression was reduced or in vivo in APP knockout (KO) mice. Taken together, our data are the first to demonstrate that Reelin acts cooperatively with APP to modulate dendritic spine formation and suggest that normal APP function is critical for Reelin-mediated dendritic spinogenesis at synapses.

Dynamic physiological α-synuclein S129 phosphorylation is driven by neuronal activity

In Parkinson's disease and other synucleinopathies, the elevation of α-synuclein phosphorylated at Serine129 (pS129) is a widely cited marker of pathology. However, the physiological role for pS129 has remained undefined. Here we use multiple approaches to show for the first time that pS129 functions as a physiological regulator of neuronal activity. Neuronal activity triggers a sustained increase of pS129 in cultured neurons (200% within 4 h). In accord, brain pS129 is elevated in environmentally enriched mice exhibiting enhanced long-term potentiation. Activity-dependent α-synuclein phosphorylation is S129-specific, reversible, confers no cytotoxicity, and accumulates at synapsin-containing presynaptic boutons. Mechanistically, our findings are consistent with a model in which neuronal stimulation enhances Plk2 kinase activity via a calcium/calcineurin pathway to counteract PP2A phosphatase activity for efficient phosphorylation of membrane-bound α-synuclein. Patch clamping of rat SNCA-/- neurons expressing exogenous wild-type or phospho-incompetent (S129A) α-synuclein suggests that pS129 fine-tunes the balance between excitatory and inhibitory neuronal currents. Consistently, our novel S129A knock-in (S129AKI) mice exhibit impaired hippocampal plasticity. The discovery of a key physiological function for pS129 has implications for understanding the role of α-synuclein in neurotransmission and adds nuance to the interpretation of pS129 as a synucleinopathy biomarker.

Inhibition of PLK2 activity affects APP and tau pathology and improves synaptic content in a sex-dependent manner in a 3xTg mouse model of Alzheimer's disease

Converging lines of evidence suggest that abnormal accumulation of the kinase Polo-like kinase 2 (PLK2) might play a role in the pathogenesis of Alzheimer's disease (AD), possibly through its role in regulating the amyloid β (Aβ) cascade. In the present study, we investigated the effect of inhibiting PLK2 kinase activity in in vitro and in vivo models of AD neuropathology. First, we confirmed that PLK2 overexpression modulated APP and Tau protein levels and phosphorylation in cell culture, in a kinase activity dependent manner. Furthermore, a transient treatment of triple transgenic mouse model of AD (3xTg-AD) with a potent and specific PLK2 pharmacological inhibitor (PLK2i #37) reduced some neuropathological aspects in a sex-dependent manner. In 3xTg-AD males, treatment with PLK2i #37 led to lower Tau burden, higher synaptic protein content, and prevented learning and memory deficits. In contrast, treated females showed an exacerbation of Tau pathology, associated with a reduction in amyloid plaque accumulation. Overall, our findings suggest that PLK2 inhibition alters key components of AD neuropathology in a sex-dependent manner and might display a therapeutic potential for the treatment for AD and related dementia.

Polo-Like Kinase 2: From Principle to Practice

Polo-like kinase (PLK) 2 is an evolutionarily conserved serine/threonine kinase that shares the n-terminal kinase catalytic domain and the C-terminal Polo Box Domain (PBD) with other members of the PLKs family. In the last two decades, mounting studies have focused on this and tried to clarify its role in many aspects. PLK2 is essential for mitotic centriole replication and meiotic chromatin pairing, synapsis, and crossing-over in the cell cycle; Loss of PLK2 function results in cell cycle disorders and developmental retardation. PLK2 is also involved in regulating cell differentiation and maintaining neural homeostasis. In the process of various stimuli-induced stress, including oxidative and endoplasmic reticulum, PLK2 may promote survival or apoptosis depending on the intensity of stimulation and the degree of cell damage. However, the role of PLK2 in immunity to viral infection has been studied far less than that of other family members. Because PLK2 is extensively and deeply involved in normal physiological functions and pathophysiological mechanisms of cells, its role in diseases is increasingly being paid attention to. The effect of PLK2 in inhibiting hematological tumors and fibrotic diseases, as well as participating in neurodegenerative diseases, has been gradually recognized. However, the research results in solid organ tumors show contradictory results. In addition, preliminary studies using PLK2 as a disease predictor and therapeutic target have yielded some exciting and promising results. More research will help people better understand PLK2 from principle to practice.

Rho-Rho-Kinase Regulates Ras-ERK Signaling Through SynGAP1 for Dendritic Spine Morphology

The structural plasticity of dendritic spines plays a critical role in NMDA-induced long-term potentiation (LTP) in the brain. The small GTPases RhoA and Ras are considered key regulators of spine morphology and enlargement. However, the regulatory interaction between RhoA and Ras underlying NMDA-induced spine enlargement is largely unknown. In this study, we found that Rho-kinase/ROCK, an effector of RhoA, phosphorylated SynGAP1 (a synaptic Ras-GTPase activating protein) at Ser842 and increased its interaction with 14-3-3ζ, thereby activating Ras-ERK signaling in a reconstitution system in HeLa cells. We also found that the stimulation of NMDA receptor by glycine treatment for LTP induction stimulated SynGAP1 phosphorylation, Ras-ERK activation, spine enlargement and SynGAP1 delocalization from the spines in striatal neurons, and these effects were prevented by Rho-kinase inhibition. Rho-kinase-mediated phosphorylation of SynGAP1 appeared to increase its dissociation from PSD95, a postsynaptic scaffolding protein located at postsynaptic density, by forming a complex with 14-3-3ζ. These results suggest that Rho-kinase phosphorylates SynGAP1 at Ser842, thereby activating the Ras-ERK pathway for NMDA-induced morphological changes in dendritic spines.

A review of Plks: Thinking outside the (polo) box

The polo-like kinase (Plk) family is comprised of five different members (Plk1-5), each with their own distinct functions. Plk family members participate in pivotal cell division processes as well as in non-mitotic roles. Importantly, Plk expression has been correlated with various disease states, including cancer. Multiples therapies, which primarily target Plk1, are currently being investigated alone or in combination with other agents for clinical use in different cancers. As the role of Plks in disease progression becomes more prominent, it is important to outline their functions as cell cycle regulators and more. This review summarizes the structure and both mitotic and non-mitotic functions of each of the five Plk family members, sequentially. Additionally, the proposed mechanisms for how Plks contribute to tumorigenesis and the therapeutics currently under investigation are outlined.

Chronic neuronal excitation leads to dual metaplasticity in the signaling for structural long-term potentiation

Synaptic plasticity is long-lasting changes in synaptic currents and structure. When neurons are exposed to signals that induce aberrant neuronal excitation, they increase the threshold for the induction of long-term potentiation (LTP), known as metaplasticity. However, the metaplastic regulation of structural LTP (sLTP) remains unclear. We investigate glutamate uncaging/photoactivatable (pa)CaMKII-dependent sLTP induction in hippocampal CA1 neurons after chronic neuronal excitation by GABA_A receptor antagonists. We find that the neuronal excitation decreases the glutamate uncaging-evoked Ca²⁺ influx mediated by GluN2B-containing NMDA receptors and suppresses sLTP induction. In addition, single-spine optogenetic stimulation using paCaMKII indicates the suppression of CaMKII signaling. While the inhibition of Ca²⁺ influx is protein synthesis independent, the paCaMKII-induced sLTP suppression depends on it. Our findings demonstrate that chronic neuronal excitation suppresses sLTP in two independent ways (i.e., dual inhibition of Ca²⁺ influx and CaMKII signaling). This dual inhibition mechanism may contribute to robust neuronal protection in excitable environments.

Targeting miR-126 in inv(16) acute myeloid leukemia inhibits leukemia development and leukemia stem cell maintenance

Acute myeloid leukemia (AML) harboring inv(16)(p13q22) expresses high levels of miR-126. Here we show that the CBFB-MYH11 (CM) fusion gene upregulates miR-126 expression through aberrant miR-126 transcription and perturbed miR-126 biogenesis via the HDAC8/RAN-XPO5-RCC1 axis. Aberrant miR-126 upregulation promotes survival of leukemia-initiating progenitors and is critical for initiating and maintaining CM-driven AML. We show that miR-126 enhances MYC activity through the SPRED1/PLK2-ERK-MYC axis. Notably, genetic deletion of miR-126 significantly reduces AML rate and extends survival in CM knock-in mice. Therapeutic depletion of miR-126 with an anti-miR-126 (miRisten) inhibits AML cell survival, reduces leukemia burden and leukemia stem cell (LSC) activity in inv(16) AML murine and xenograft models. The combination of miRisten with chemotherapy further enhances the anti-leukemia and anti-LSC activity. Overall, this study provides molecular insights for the mechanism and impact of miR-126 dysregulation in leukemogenesis and highlights the potential of miR-126 depletion as a therapeutic approach for inv(16) AML.

miR-126 is highly expressed in inv(16) Acute myeloid leukemia (AML) but its role is unclear. Here, the authors show that the aberrant expression of miR-126 in inv(16) AML is directly due to the CBFB-MYH11 fusion gene and that it can promote AML development and leukemia stem cell maintenance, highlighting miR-126 as a therapeutic target for inv(16) AML patients

Systemic mesalazine treatment prevents spontaneous skin fibrosis in PLK2-deficient mice

Skin fibrosis is a complex biological remodeling process occurring in disease like systemic sclerosis, morphea, or eosinophilic fasciitis. Since the knowledge about the underlying pathomechanisms is still incomplete, there is currently no therapy, which prevents or reverses skin fibrosis sufficiently. The present study investigates the role of polo-like kinase 2 (PLK2) and the pro-fibrotic cytokine osteopontin (OPN) in the pathogenesis of cutaneous fibrosis and demonstrates the antifibrotic effects of systemic mesalazine treatment in vivo. Isolated primary dermal fibroblasts of PLK2 wild-type (WT) and knockout (KO) mice were characterized invitro. Skin thickness and histoarchitecture were studied in paraffin-embedded skin sections. The effects of mesalazine treatment were examined in isolated fibroblasts and PLK2 KO mice, which were fed 100 µg/g mesalazine for 6 months via the drinking water. Compared to WT, PLK2 KO fibroblasts displayed higher spontaneous myofibroblast differentiation, reduced proliferation rates, and overexpression of the fibrotic cytokine OPN. Invitro, 72 h of treatment with 10 mmol/L mesalazine induced phenotype conversion in PLK2 KO fibroblasts and attenuated OPN expression by inhibiting ERK1/2. In vivo, dermal myofibroblast differentiation, collagen accumulation, and skin thickening were prevented by mesalazine in PLK2 KO. Plasma creatinine levels indicated good tolerability of systemic long-term mesalazine treatment. The current study reveals a spontaneous fibrotic skin phenotype and ERK1/2-dependent OPN overexpression in PLK2 KO mice. We provide experimental evidence for the antifibrotic effectiveness of systemic mesalazine treatment to prevent fibrosis of the skin, suggesting further investigation in experimental and clinical settings.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

Homeostatic synaptic scaling establishes the specificity of an associative memory

Correlation-based (Hebbian) forms of synaptic plasticity are crucial for the initial encoding of associative memories but likely insufficient to enable the stable storage of multiple specific memories within neural circuits. Theoretical studies have suggested that homeostatic synaptic normalization rules provide an essential countervailing force that can stabilize and expand memory storage capacity. Although such homeostatic mechanisms have been identified and studied for decades, experimental evidence that they play an important role in associative memory is lacking. Here, we show that synaptic scaling, a widely studied form of homeostatic synaptic plasticity that globally renormalizes synaptic strengths, is dispensable for initial associative memory formation but crucial for the establishment of memory specificity. We used conditioned taste aversion (CTA) learning, a form of associative learning that relies on Hebbian mechanisms within gustatory cortex (GC), to show that animals conditioned to avoid saccharin initially generalized this aversion to other novel tastants. Specificity of the aversion to saccharin emerged slowly over a time course of many hours and was associated with synaptic scaling down of excitatory synapses onto conditioning-active neuronal ensembles within gustatory cortex. Blocking synaptic scaling down in the gustatory cortex enhanced the persistence of synaptic strength increases induced by conditioning and prolonged the duration of memory generalization. Taken together, these findings demonstrate that synaptic scaling is crucial for sculpting the specificity of an associative memory and suggest that the relative strengths of Hebbian and homeostatic plasticity can modulate the balance between stable memory formation and memory generalization.

Genetic Deletion of Polo-Like Kinase 2 Induces a Pro-Fibrotic Pulmonary Phenotype

Pulmonary fibrosis is the chronic-progressive replacement of healthy lung tissue by extracellular matrix, leading to the destruction of the alveolar architecture and ultimately death. Due to limited pathophysiological knowledge, causal therapies are still missing and consequently the prognosis is poor. Thus, there is an urgent clinical need for models to derive effective therapies. Polo-like kinase 2 (PLK2) is an emerging regulator of fibroblast function and fibrosis. We found a significant downregulation of PLK2 in four different entities of human pulmonary fibrosis. Therefore, we characterized the pulmonary phenotype of PLK2 knockout (KO) mice. Isolated pulmonary PLK2 KO fibroblasts displayed a pronounced myofibroblast phenotype reflected by increased expression of αSMA, reduced proliferation rates and enhanced ERK1/2 and SMAD2/3 phosphorylation. In PLK2 KO, the expression of the fibrotic cytokines osteopontin and IL18 was elevated compared to controls. Histological analysis of PLK2 KO lungs revealed early stage remodeling in terms of alveolar wall thickening, increased alveolar collagen deposition and myofibroblast foci. Our results prompt further investigation of PLK2 function in pulmonary fibrosis and suggest that the PLK2 KO model displays a genetic predisposition towards pulmonary fibrosis, which could be leveraged in future research on this topic.

Multi-parametric analysis of 57 SYNGAP1 variants reveal impacts on GTPase signaling, localization, and protein stability

SYNGAP1 is a neuronal Ras and Rap GTPase-activating protein with important roles in regulating excitatory synaptic plasticity. While many SYNGAP1 missense and nonsense mutations have been associated with intellectual disability, epilepsy, schizophrenia, and autism spectrum disorder (ASD), whether and how they contribute to individual disease phenotypes is often unknown. Here, we characterize 57 variants in seven assays that examine multiple aspects of SYNGAP1 function. Specifically, we used multiplex phospho-flow cytometry to measure variant impact on protein stability, pERK, pGSK3β, pp38, pCREB, and high-content imaging to examine subcellular localization. We find variants ranging from complete loss-of-function (LoF) to wild-type (WT)-like in their regulation of pERK and pGSK3β, while all variants retain at least partial ability to dephosphorylate pCREB. Interestingly, our assays reveal that a larger proportion of variants located within the disordered domain of unknown function (DUF) comprising the C-terminal half of SYNGAP1 exhibited higher LoF, compared to variants within the better studied catalytic domain. Moreover, we find protein instability to be a major contributor to dysfunction for only two missense variants, both located within the catalytic domain. Using high-content imaging, we find variants located within the C2 domain known to mediate membrane lipid interactions exhibit significantly larger cytoplasmic speckles than WT SYNGAP1. Moreover, this subcellular phenotype shows both correlation with altered catalytic activity and unique deviation from signaling assay results, highlighting multiple independent molecular mechanisms underlying variant dysfunction. Our multidimensional dataset allows clustering of variants based on functional phenotypes and provides high-confidence, multi-functional measures for making pathogenicity predictions.

RAPGEF2 mediates oligomeric Aβ-induced synaptic loss and cognitive dysfunction in the 3xTg-AD mouse model of Alzheimer's disease

AIMS

Amyloid-β (Aβ) oligomers trigger synaptic degeneration that precedes plaque and tangle pathology. However, the signalling molecules that link Aβ oligomers to synaptic pathology remain unclear. Here, we addressed the potential role of RAPGEF2 as a novel signalling molecule in Aβ oligomer-induced synaptic and cognitive impairments in human-mutant amyloid precursor protein (APP) mouse models of Alzheimer's disease (AD).

METHODS

To investigate the role of RAPGEF2 in Aβ oligomer-induced synaptic and cognitive impairments, we utilised a combination of approaches including biochemistry, molecular cell biology, light and electron microscopy, behavioural tests with primary neuron cultures, multiple AD mouse models and post-mortem human AD brain tissue.

RESULTS

We found significantly elevated RAPGEF2 levels in the post-mortem human AD hippocampus. RAPGEF2 levels also increased in the transgenic AD mouse models, generating high levels of Aβ oligomers before exhibiting synaptic and cognitive impairment. RAPGEF2 upregulation activated the downstream effectors Rap2 and JNK. In cultured hippocampal neurons, oligomeric Aβ treatment increased the fluorescence intensity of RAPGEF2 and reduced the number of dendritic spines and the intensities of synaptic marker proteins, while silencing RAPGEF2 expression blocked Aβ oligomer-induced synapse loss. Additionally, the in vivo knockdown of RAPGEF2 expression in the AD hippocampus prevented cognitive deficits and the loss of excitatory synapses.

CONCLUSIONS

These findings demonstrate that the upregulation of RAPGEF2 levels mediates Aβ oligomer-induced synaptic and cognitive disturbances in the AD hippocampus. We propose that an early intervention regarding RAPGEF2 expression may have beneficial effects on early synaptic pathology and memory loss in AD.

β-catenin-mediated hair growth induction effect of 3,4,5-tri-O-caffeoylquinic acid

The hair follicle is a complex structure that goes through a cyclic period of growth (anagen), regression (catagen), and rest (telogen) under the regulation of several signaling pathways, including Wnt/ β-catenin, FGF, Shh, and Notch. The Wnt/β-catenin signaling is specifically involved in hair follicle morphogenesis, regeneration, and growth. β-catenin is expressed in the dermal papilla and promotes anagen induction and duration, as well as keratinocyte regulation and differentiation. In this study, we demonstrated the activation of β-catenin by a polyphenolic compound 3,4,5-tri-O-caffeoylquinic acid (TCQA) in mice model and in human dermal papilla cells to promote hair growth cycle. A complete regrowth of the shaved area of C3H mice was observed upon treatment with TCQA. Global gene expression analysis using microarray showed an upregulation in hair growth-associated genes. Moreover, the expression of β-catenin was remarkably upregulated in vivo and in vitro. These findings suggest that β-catenin activation by TCQA promoted the initiation of the anagen phase of the hair cycle.

Social Stress-Induced Postsynaptic Hyporesponsiveness in Glutamatergic Synapses Is Mediated by PSD-Zip70-Rap2 Pathway and Relates to Anxiety-Like Behaviors

PSD-Zip70 is a postsynaptic protein that regulates glutamatergic synapse formation and maturation by modulation of Rap2 activity. PSD-Zip70 knockout (PSD-Zip70KO) mice exhibit defective glutamatergic synaptic transmission in the prefrontal cortex (PFC) with aberrant Rap2 activation. As prefrontal dysfunction is implicated in the pathophysiology of stress-induced psychiatric diseases, we examined PSD-Zip70KO mice in a social defeat (SD) stress-induced mouse model of depression to investigate stress-induced alterations in synaptic function. Compared with wild-type (WT) mice, PSD-Zip70KO mice exhibited almost normal responses to SD stress in depression-related behaviors such as social activity, anhedonia, and depressive behavior. However, PSD-Zip70KO mice showed enhanced anxiety-like behavior irrespective of stress conditions. The density and size of dendritic spines of pyramidal neurons were reduced in the medial PFC (mPFC) in mice exposed to SD stress. Phosphorylation levels of the AMPA–type glutamate receptor (AMPA-R) GluA2 subunit at Ser880 were prominently elevated in mice exposed to SD stress, indicating internalization of surface-expressed AMPA-Rs and decreased postsynaptic responsiveness. Structural and functional impairments in postsynaptic responsiveness were associated with Rap2 GTPase activation in response to SD stress. Social stress-induced Rap2 activation was regulated by a PSD-Zip70-dependent pathway via interaction with SPAR/PDZ-GEF1. Notably, features such as Rap2 activation, dendritic spine shrinkage, and increased GluA2 phosphorylation were observed in the mPFC of PSD-Zip70KO mice even without SD stress. Together with our previous results, the present findings suggest that SD stress-induced postsynaptic hyporesponsiveness in glutamatergic synapses is mediated by PSD-Zip70-Rap2 signaling pathway and closely relates to anxiety-like behaviors.

Inhibition of Polo-like kinase 2 ameliorates pathogenesis in Alzheimer's disease model mice

Alzheimer disease (AD) is a neurodegenerative disorder characterized by pathological hallmarks of neurofibrillary tangles and amyloid plaques. The plaques are formed by aggregation and accumulation of amyloid β (Aβ), a cleavage fragment of amyloid precursor protein (APP). Enhanced neuronal activity and seizure events are frequently observed in AD, and elevated synaptic activity promotes Aβ production. However, the mechanisms that link synaptic hyperactivity to APP processing and AD pathogenesis are not well understood. We previously found that Polo-like kinase 2 (Plk2), a homeostatic repressor of neuronal overexcitation, promotes APP β-processing in vitro. Here, we report that Plk2 stimulates Aβ production in vivo, and that Plk2 levels are elevated in a spatiotemporally regulated manner in brains of AD mouse models and human AD patients. Genetic disruption of Plk2 kinase function reduces plaque deposits and activity-dependent Aβ production. Furthermore, pharmacological Plk2 inhibition hinders Aβ formation, synapse loss, and memory decline in an AD mouse model. Thus, Plk2 links synaptic overactivity to APP β-processing, Aβ production, and disease-relevant phenotypes in vivo, suggesting that Plk2 may be a potential target for AD therapeutics.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

Systems level expression correlation of Ras GTPase regulators

BACKGROUND

Proteins of the ubiquitously expressed core proteome are quantitatively correlated across multiple eukaryotic species. In addition, it was found that many protein paralogues exhibit expression anticorrelation, suggesting that the total level of protein with a given functionality must be kept constant.

METHODS

We performed Spearman's rank correlation analyses of gene expression levels for the RAS GTPase subfamily and their regulatory GEF and GAP proteins across tissues and across individuals for each tissue. A large set of published data for normal tissues from a wide range of species, human cancer tissues and human cell lines was analysed.

RESULTS

We show that although the multidomain regulatory proteins of Ras GTPases exhibit considerable tissue and individual gene expression variability, their total amounts are balanced in normal tissues. In a given tissue, the sum of activating (GEFs) and deactivating (GAPs) domains of Ras GTPases can vary considerably, but each person has balanced GEF and GAP levels. This balance is impaired in cell lines and in cancer tissues for some individuals.

CONCLUSIONS

Our results are relevant for critical considerations of knock out experiments, where functionally related homologs may compensate for the down regulation of a protein.

Phosphorylation of synaptic GTPase-activating protein (synGAP) by polo-like kinase (Plk2) alters the ratio of its GAP activity toward HRas, Rap1 and Rap2 GTPases

SynGAP is a Ras and Rap GTPase-activating protein (GAP) found in high concentration in the postsynaptic density (PSD) fraction from mammalian forebrain where it binds to PDZ domains of PSD-95. Phosphorylation of pure recombinant synGAP by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) shifts the balance of synGAP's GAP activity toward inactivation of Rap1; whereas phosphorylation by cyclin-dependent kinase 5 (CDK5) has the opposite effect, shifting the balance toward inactivation of HRas. These shifts in balance contribute to regulation of the numbers of surface AMPA receptors, which rise during synaptic potentiation (CaMKII) and fall during synaptic scaling (CDK5). Polo-like kinase 2 (Plk2/SNK), like CDK5, contributes to synaptic scaling. These two kinases act in concert to reduce the number of surface AMPA receptors following elevated neuronal activity by tagging spine-associated RapGAP protein (SPAR) for degradation, thus raising the level of activated Rap. Here we show that Plk2 also phosphorylates and regulates synGAP. Phosphorylation of synGAP by Plk2 stimulates its GAP activity toward HRas by 65%, and toward Rap1 by 16%. Simultaneous phosphorylation of synGAP by Plk2 and CDK5 at distinct sites produces an additive increase in GAP activity toward HRas (∼230%) and a smaller, non-additive increase in activity toward Rap1 (∼15%). Dual phosphorylation also produces an increase in GAP activity toward Rap2 (∼40-50%), an effect not produced by either kinase alone. As we previously observed for CDK5, addition of Ca²⁺/CaM causes a substrate-directed doubling of the rate and stoichiometry of phosphorylation of synGAP by Plk2, targeting residues also phosphorylated by CaMKII. In summary, phosphorylation by Plk2, like CDK5, shifts the ratio of GAP activity of synGAP to produce a greater decrease in active Ras than in active Rap, which would produce a shift toward a decrease in the number of surface AMPA receptors in neuronal dendrites.

Dysregulation and restoration of homeostatic network plasticity in fragile X syndrome mice

Chronic activity perturbations in neurons induce homeostatic plasticity through modulation of synaptic strength or other intrinsic properties to maintain the correct physiological range of excitability. Although similar plasticity can also occur at the population level, what molecular mechanisms are involved remain unclear. In the current study, we utilized a multielectrode array (MEA) recording system to evaluate homeostatic neural network activity of primary mouse cortical neuron cultures. We demonstrated that chronic elevation of neuronal activity through the inhibition of GABA(A) receptors elicits synchronization of neural network activity and homeostatic reduction of the amplitude of spontaneous neural network spikes. We subsequently showed that this phenomenon is mediated by the ubiquitination of tumor suppressor p53, which is triggered by murine double minute-2 (Mdm2). Using a mouse model of fragile X syndrome, in which fragile X mental retardation protein (FMRP) is absent (Fmr1 knockout), we found that Mdm2-p53 signaling, network synchronization, and the reduction of network spike amplitude upon chronic activity stimulation were all impaired. Pharmacologically inhibiting p53 with Pifithrin-α or genetically employing p53 heterozygous mice to enforce the inactivation of p53 in Fmr1 knockout cultures restored the synchronization of neural network activity after chronic activity stimulation and partially corrects the homeostatic reduction of neural network spike amplitude. Together, our findings reveal the roles of both Fmr1 and Mdm2-p53 signaling in the homeostatic regulation of neural network activity and provide insight into the deficits of excitability homeostasis seen when Fmr1 is compromised, such as occurs with fragile X syndrome.

Homeostatic plasticity in neural development

Throughout life, neural circuits change their connectivity, especially during development, when neurons frequently extend and retract dendrites and axons, and form and eliminate synapses. In spite of their changing connectivity, neural circuits maintain relatively constant activity levels. Neural circuits achieve functional stability by homeostatic plasticity, which equipoises intrinsic excitability and synaptic strength, balances network excitation and inhibition, and coordinates changes in circuit connectivity. Here, we review how diverse mechanisms of homeostatic plasticity stabilize activity in developing neural circuits.

Npas4 Is a Critical Regulator of Learning-Induced Plasticity at Mossy Fiber-CA3 Synapses during Contextual Memory Formation

Synaptic connections between hippocampal mossy fibers (MFs) and CA3 pyramidal neurons are essential for contextual memory encoding, but the molecular mechanisms regulating MF-CA3 synapses during memory formation and the exact nature of this regulation are poorly understood. Here we report that the activity-dependent transcription factor Npas4 selectively regulates the structure and strength of MF-CA3 synapses by restricting the number of their functional synaptic contacts without affecting the other synaptic inputs onto CA3 pyramidal neurons. Using an activity-dependent reporter, we identified CA3 pyramidal cells that were activated by contextual learning and found that MF inputs on these cells were selectively strengthened. Deletion of Npas4 prevented both contextual memory formation and this learning-induced synaptic modification. We further show that Npas4 regulates MF-CA3 synapses by controlling the expression of the polo-like kinase Plk2. Thus, Npas4 is a critical regulator of experience-dependent, structural, and functional plasticity at MF-CA3 synapses during contextual memory formation.

What goes up must come down: homeostatic synaptic plasticity strategies in neurological disease

Brain activity levels are tightly regulated to minimize imbalances in activity state. Deviations from the normal range of activity are deleterious and often associated with neurological disorders. To maintain optimal levels of activity, regulatory mechanisms termed homeostatic synaptic plasticity establish desired 'set points' for neural activity, monitor the network for deviations from the set point and initiate compensatory responses to return activity to the appropriate level that permits physiological function [1,2]. We speculate that impaired homeostatic control may contribute to the etiology of various neurological disorders including epilepsy and Alzheimer's disease, two disorders that exhibit hyperexcitability as a key feature during pathogenesis. Here, we will focus on recent progress in developing homeostatic regulation of neural activity as a therapeutic tool.

The Rap activator Gef26 regulates synaptic growth and neuronal survival via inhibition of BMP signaling

In Drosophila, precise regulation of BMP signaling is essential for normal synaptic growth at the larval neuromuscular junction (NMJ) and neuronal survival in the adult brain. However, the molecular mechanisms underlying fine-tuning of BMP signaling in neurons remain poorly understood. We show that loss of the Drosophila PDZ guanine nucleotide exchange factor Gef26 significantly increases synaptic growth at the NMJ and enhances BMP signaling in motor neurons. We further show that Gef26 functions upstream of Rap1 in motor neurons to restrain synaptic growth. Synaptic overgrowth in gef26 or rap1 mutants requires BMP signaling, indicating that Gef26 and Rap1 regulate synaptic growth via inhibition of BMP signaling. We also show that Gef26 is involved in the endocytic downregulation of surface expression of the BMP receptors thickveins (Tkv) and wishful thinking (Wit). Finally, we demonstrate that loss of Gef26 also induces progressive brain neurodegeneration through Rap1- and BMP signaling-dependent mechanisms. Taken together, these results suggest that the Gef26-Rap1 signaling pathway regulates both synaptic growth and neuronal survival by controlling BMP signaling.

Rap1 in endothelial biology

PURPOSE OF REVIEW

Ubiquitously-expressed small GTPase Rap1 is a key modulator of integrin- and cadherin-regulated processes. In endothelium, Rap1 promotes angiogenesis and endothelial barrier function, acting downstream from cAMP-activated Rap1GEF, Epac. Recent in-vivo studies in mouse models have provided more information about the physiological role of Rap1 in vessel development and after birth under normal and pathologic conditions. Important molecular details of dynamic regulation of endothelial barrier are uncovered.

RECENT FINDINGS

Rap1 is not essential for initial vessel formation but is critical for vessel stabilization, as double knockout of the two Rap1 isoforms leads to hemorrhage and embryonic lethality. After development, Rap1 is not required for endothelial barrier maintenance but is critical for nitric oxide production and endothelial function. Radil and Afadin mediate Rap1 effects on endothelial barrier function by regulating connection with Rho GTPases, actomyosin cytoskeleton, and cell-cell adhesion receptors.

SUMMARY

Rap1 is critically required for nitric oxide release and normal endothelial function in vivo. Mechanistic studies lead to a novel paradigm of Rap1 as a critical regulator of endothelial cell shear stress responses and endothelial homeostasis. Increased understanding of molecular mechanisms underlying endothelial barrier regulation may identify novel pharmacological targets for retinopathies and conditions with altered endothelial barrier function or when increased endothelial barrier is desired.

NCS-Rapgef2, the Protein Product of the Neuronal Rapgef2 Gene, Is a Specific Activator of D1 Dopamine Receptor-Dependent ERK Phosphorylation in Mouse Brain

The neuritogenic cAMP sensor (NCS), encoded by the Rapgef2 gene, links cAMP elevation to activation of extracellular signal-regulated kinase (ERK) in neurons and neuroendocrine cells. Transducing human embryonic kidney (HEK)293 cells, which do not express Rapgef2 protein or respond to cAMP with ERK phosphorylation, with a vector encoding a Rapgef2 cDNA reconstituted cAMP-dependent ERK activation. Mutation of a single residue in the cyclic nucleotide-binding domain (CNBD) conserved across cAMP-binding proteins abrogated cAMP-ERK coupling, while deletion of the CNBD altogether resulted in constitutive ERK activation. Two types of mRNA are transcribed from Rapgef2 in vivo. Rapgef2 protein expression was limited to tissues, i.e., neuronal and endocrine, expressing the second type of mRNA, initiated exclusively from an alternative first exon called here exon 1’, and an alternative 5’ protein sequence leader fused to a common remaining open reading frame, which is termed here NCS-Rapgef2. In the male mouse brain, NCS-Rapgef2 is prominently expressed in corticolimbic excitatory neurons, and striatal medium spiny neurons (MSNs). Rapgef2-dependent ERK activation by the dopamine D1 agonist SKF81297 occurred in neuroendocrine neuroscreen-1 (NS-1) cells expressing the human D1 receptor and was abolished by deletion of Rapgef2. Corticolimbic [e.g., dentate gyrus (DG), basolateral amygdala (BLA)] ERK phosphorylation induced by SKF81297 was significantly attenuated in CamK2α-Cre^+/−; Rapgef2^cko/cko male mice. ERK phosphorylation in nucleus accumbens (NAc) MSNs induced by treatment with SKF81297, or the psychostimulants cocaine or amphetamine, was abolished in male Rapgef2^cko/cko mice with NAc NCS-Rapgef2-depleting AAV-Synapsin-Cre injections. We conclude that D1-dependent ERK phosphorylation in mouse brain requires NCS-Rapgef2 expression.

E46K α-synuclein pathological mutation causes cell-autonomous toxicity without altering protein turnover or aggregation

α-Synuclein (aSyn) is the main driver of neurodegenerative diseases known as "synucleinopathies," but the mechanisms underlying this toxicity remain poorly understood. To investigate aSyn toxic mechanisms, we have developed a primary neuronal model in which a longitudinal survival analysis can be performed by following the overexpression of fluorescently tagged WT or pathologically mutant aSyn constructs. Most aSyn mutations linked to neurodegenerative disease hindered neuronal survival in this model; of these mutations, the E46K mutation proved to be the most toxic. While E46K induced robust PLK2-dependent aSyn phosphorylation at serine 129, inhibiting this phosphorylation did not alleviate aSyn toxicity, strongly suggesting that this pathological hallmark of synucleinopathies is an epiphenomenon. Optical pulse-chase experiments with Dendra2-tagged aSyn versions indicated that the E46K mutation does not alter aSyn protein turnover. Moreover, since the mutation did not promote overt aSyn aggregation, we conclude that E46K toxicity was driven by soluble species. Finally, we developed an assay to assess whether neurons expressing E46K aSyn affect the survival of neighboring control neurons. Although we identified a minor non-cell-autonomous component spatially restricted to proximal neurons, most E46K aSyn toxicity was cell autonomous. Thus, we have been able to recapitulate the toxicity of soluble aSyn species at a stage preceding aggregation, detecting non-cell-autonomous toxicity and evaluating how some of the main aSyn hallmarks are related to neuronal survival.

Linking Network Activity to Synaptic Plasticity during Sleep: Hypotheses and Recent Data

Research findings over the past two decades have supported a link between sleep states and synaptic plasticity. Numerous mechanistic hypotheses have been put forth to explain this relationship. For example, multiple studies have shown structural alterations to synapses (including changes in synaptic volume, spine density, and receptor composition) indicative of synaptic weakening after a period of sleep. Direct measures of neuronal activity and synaptic strength support the idea that a period of sleep can reduce synaptic strength. This has led to the synaptic homeostasis hypothesis (SHY), which asserts that during slow wave sleep, synapses are downscaled throughout the brain to counteract net strengthening of network synapses during waking experience (e.g., during learning). However, neither the cellular mechanisms mediating these synaptic changes, nor the sleep-dependent activity changes driving those cellular events are well-defined. Here we discuss potential cellular and network dynamic mechanisms which could underlie reductions in synaptic strength during sleep. We also discuss recent findings demonstrating circuit-specific synaptic strengthening (rather than weakening) during sleep. Based on these data, we explore the hypothetical role of sleep-associated network activity patterns in driving synaptic strengthening. We propose an alternative to SHY—namely that depending on experience during prior wake, a variety of plasticity mechanisms may operate in the brain during sleep. We conclude that either synaptic strengthening or synaptic weakening can occur across sleep, depending on changes to specific neural circuits (such as gene expression and protein translation) induced by experiences in wake. Clarifying the mechanisms underlying these different forms of sleep-dependent plasticity will significantly advance our understanding of how sleep benefits various cognitive functions.

Polo-like kinase 2 modulates α-synuclein protein levels by regulating its mRNA production

Variations in the α-synuclein-encoding SNCA gene represent the greatest genetic risk factor for Parkinson's disease (PD), and duplications/triplications of SNCA cause autosomal dominant familial PD. These facts closely link brain levels of α-synuclein with the risk of PD, and make lowering α-synuclein levels a therapeutic strategy for the treatment of PD and related synucleinopathies. In this paper, we corroborate previous findings on the ability of overexpressed Polo-like kinase 2 (PLK-2) to decrease cellular α-synuclein, but demonstrate that the process is independent of PLK-2 phosphorylating S129 in α-synuclein because a similar reduction is achieved with the non-phosphorable S129A mutant α-synuclein. Using a specific PLK-2 inhibitor (compound 37), we demonstrate that endogenous PLK-2 phosphorylates S129 only in some cells, but increases α-synuclein protein levels in all tested cell cultures and brain slices. PLK-2 is found to regulate the transcription of α-synuclein mRNA from both the endogenous mouse SNCA gene and transgenic vectors that only contain the open reading frame. Moreover, we are the first to show that regulation of α-synuclein by PLK-2 is of physiological importance since 10days' inhibition of endogenous PLK-2 in wt C57BL/6 mice increases endogenous α-synuclein protein levels. Our findings collectively demonstrate that PLK-2 regulates α-synuclein levels by a previously undescribed transcription-based mechanism. This mechanism is active in cells and brain tissue, opening up for alternative strategies for modulating α-synuclein levels and thereby for the possibility of modifying disease progression in synucleinopaties.

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

The SNK and SPAR signaling pathway changes in hippocampal neurons treated with amyloid-beta peptide in vitro

Amyloid-β peptide (Aβ) is believed to be a primary cause of Alzheimer's disease. Many studies have demonstrated that Aβ causes morphological and functional alterations of dendritic spines, leading to synaptic dysfunction, but the effect of Aβ on damage to synaptic functions is not fully understood. Spine-associated Rap guanosine triphosphatase-activating protein (SPAR) is an important regulator of activity-dependent remodeling of synapses and is critically involved in both mature dendritic spine formation and the maintenance of spine maturity. Serum-inducible kinase (SNK) is an activity-inducible member of the polo-like family of serine/threonine kinases. Coordinated regulation of Ras and Rap by SNK is critical for homeostatic plasticity and memory. A previous study in which rats were injected with Aβ_1-40 into the hippocampus showed that the SNK and SPAR signaling pathway may play a crucial role in Aβ-induced excitotoxic damage in the central nervous system by regulating synaptic stability. The present study was designed to investigate whether the SNK and SPAR signaling pathway was involved in Aβ-induced neurotoxicity in rat primary neurons. We measured mRNA and protein expression levels of SNK and SPAR in primary hippocampal neurons following Aβ treatment and used RNA interference to knockdown SNK to investigate the underlying mechanism. Expression of SNK and SPAR was altered by Aβ treatment, indicating that the SNK and SPAR signaling pathways may be involved in the damage to dendritic spines in hippocampal neurons induced by Aβ.

Polo-like kinase 2 phosphorylation of amyloid precursor protein regulates activity-dependent amyloidogenic processing

Alzheimer's disease (AD) is a neurodegenerative disorder with cognitive deficits. Amyloidogenic processing of amyloid precursor protein (APP) produces amyloid β (Aβ), the major component of hallmark AD plaques. Synaptic activity stimulates APP cleavage, whereas APP promotes excitatory synaptic transmission, suggesting APP participates in neuronal homeostasis. However, mechanisms linking synaptic activity to APP processing are unclear. Here we show that Polo-like kinase 2 (Plk2), an activity-inducible regulator of homeostatic plasticity, directly binds and phosphorylates threonine-668 and serine-675 of APP in vitro and associates with APP in vivo. Plk2 accelerates APP amyloidogenic cleavage by β-secretase at synapses and is required for neuronal overactivity-stimulated Aβ secretion. These findings implicate Plk2 as a novel mediator of activity-dependent APP amyloidogenic processing.

Clinical genomics expands the morbid genome of intellectual disability and offers a high diagnostic yield

Intellectual disability (ID) is a measurable phenotypic consequence of genetic and environmental factors. In this study, we prospectively assessed the diagnostic yield of genomic tools (molecular karyotyping, multi-gene panel and exome sequencing) in a cohort of 337 ID subjects as a first-tier test and compared it with a standard clinical evaluation performed in parallel. Standard clinical evaluation suggested a diagnosis in 16% of cases (54/337) but only 70% of these (38/54) were subsequently confirmed. On the other hand, the genomic approach revealed a likely diagnosis in 58% (n=196). These included copy number variants in 14% (n=54, 15% are novel), and point mutations revealed by multi-gene panel and exome sequencing in the remaining 43% (1% were found to have Fragile-X). The identified point mutations were mostly recessive (n=117, 81%), consistent with the high consanguinity of the study cohort, but also X-linked (n=8, 6%) and de novo dominant (n=19, 13%). When applied directly on all cases with negative molecular karyotyping, the diagnostic yield of exome sequencing was 60% (77/129). Exome sequencing also identified likely pathogenic variants in three novel candidate genes (DENND5A, NEMF and DNHD1) each of which harbored independent homozygous mutations in patients with overlapping phenotypes. In addition, exome sequencing revealed de novo and recessive variants in 32 genes (MAMDC2, TUBAL3, CPNE6, KLHL24, USP2, PIP5K1A, UBE4A, TP53TG5, ATOH1, C16ORF90, SLC39A14, TRERF1, RGL1, CDH11, SYDE2, HIRA, FEZF2, PROCA1, PIANP, PLK2, QRFPR, AP3B2, NUDT2, UFC1, BTN3A2, TADA1, ARFGEF3, FAM160B1, ZMYM5, SLC45A1, ARHGAP33 and CAPS2), which we highlight as potential candidates on the basis of several lines of evidence, and one of these genes (SLC39A14) was biallelically inactivated in a potentially treatable form of hypermanganesemia and neurodegeneration. Finally, likely causal variants in previously published candidate genes were identified (ASTN1, HELZ, THOC6, WDR45B, ADRA2B and CLIP1), thus supporting their involvement in ID pathogenesis. Our results expand the morbid genome of ID and support the adoption of genomics as a first-tier test for individuals with ID.

SHANK proteins limit integrin activation by directly interacting with Rap1 and R-Ras

SHANK3, a synaptic scaffold protein and actin regulator, is widely expressed outside of the central nervous system with predominantly unknown function. Solving the structure of the SHANK3 N-terminal region revealed that the SPN-domain is an unexpected Ras-association domain with high affinity for GTP-bound Ras and Rap G-proteins. The role of Rap1 in integrin activation is well established but the mechanisms to antagonize it remain largely unknown. Here, we show that SHANK1 and SHANK3 act as integrin activation inhibitors by sequestering active Rap1 and R-Ras via the SPN-domain and thus limiting their bioavailability at the plasma membrane. Consistently, SHANK3 silencing triggers increased plasma membrane Rap1 activity, cell spreading, migration and invasion. Autism-related mutations within the SHANK3 SPN-domain (R12C and L68P) disrupt G-protein interaction and fail to counteract integrin activation along the Rap1/RIAM/talin axis in cancer cells and neurons. Altogether, we establish SHANKs as critical regulators of G-protein signalling and integrin-dependent processes.

Long-term depression-associated signaling is required for an in vitro model of NMDA receptor-dependent synapse pruning

Activity-dependent pruning of synaptic contacts plays a critical role in shaping neuronal circuitry in response to the environment during postnatal brain development. Although there is compelling evidence that shrinkage of dendritic spines coincides with synaptic long-term depression (LTD), and that LTD is accompanied by synapse loss, whether NMDA receptor (NMDAR)-dependent LTD is a required step in the progression toward synapse pruning is still unknown. Using repeated applications of NMDA to induce LTD in dissociated rat neuronal cultures, we found that synapse density, as measured by colocalization of fluorescent markers for pre- and postsynaptic structures, was decreased irrespective of the presynaptic marker used, post-treatment recovery time, and the dendritic location of synapses. Consistent with previous studies, we found that synapse loss could occur without apparent net spine loss or cell death. Furthermore, synapse loss was unlikely to require direct contact with microglia, as the number of these cells was minimal in our culture preparations. Supporting a model by which NMDAR-LTD is required for synapse loss, the effect of NMDA on fluorescence colocalization was prevented by phosphatase and caspase inhibitors. In addition, gene transcription and protein translation also appeared to be required for loss of putative synapses. These data support the idea that NMDAR-dependent LTD is a required step in synapse pruning and contribute to our understanding of the basic mechanisms of this developmental process.

A model for regulation by SynGAP-α1 of binding of synaptic proteins to PDZ-domain 'Slots' in the postsynaptic density

SynGAP is a Ras/Rap GTPase-activating protein (GAP) that is a major constituent of postsynaptic densities (PSDs) from mammalian forebrain. Its α1 isoform binds to all three PDZ (PSD-95, Discs-large, ZO-1) domains of PSD-95, the principal PSD scaffold, and can occupy as many as 15% of these PDZ domains. We present evidence that synGAP-α1 regulates the composition of the PSD by restricting binding to the PDZ domains of PSD-95. We show that phosphorylation by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and Polo-like kinase-2 (PLK2) decreases its affinity for the PDZ domains by several fold, which would free PDZ domains for occupancy by other proteins. Finally, we show that three critical postsynaptic signaling proteins that bind to the PDZ domains of PSD-95 are present in higher concentration in PSDs isolated from mice with a heterozygous deletion of synGAP.

Control of Dendritic Spine Morphological and Functional Plasticity by Small GTPases

Structural plasticity of excitatory synapses is a vital component of neuronal development, synaptic plasticity, and behaviour. Abnormal development or regulation of excitatory synapses has also been strongly implicated in many neurodevelopmental, psychiatric, and neurodegenerative disorders. In the mammalian forebrain, the majority of excitatory synapses are located on dendritic spines, specialized dendritic protrusions that are enriched in actin. Research over recent years has begun to unravel the complexities involved in the regulation of dendritic spine structure. The small GTPase family of proteins have emerged as key regulators of structural plasticity, linking extracellular signals with the modulation of dendritic spines, which potentially underlies their ability to influence cognition. Here we review a number of studies that examine how small GTPases are activated and regulated in neurons and furthermore how they can impact actin dynamics, and thus dendritic spine morphology. Elucidating this signalling process is critical for furthering our understanding of the basic mechanisms by which information is encoded in neural circuits but may also provide insight into novel targets for the development of effective therapies to treat cognitive dysfunction seen in a range of neurological disorders.

PSD-Zip70 Deficiency Causes Prefrontal Hypofunction Associated with Glutamatergic Synapse Maturation Defects by Dysregulation of Rap2 Activity

Dysregulation of synapse formation and plasticity is closely related to the pathophysiology of psychiatric and neurodevelopmental disorders. The prefrontal cortex (PFC) is particularly important for executive functions such as working memory, cognition, and emotional control, which are impaired in the disorders. PSD-Zip70 (Lzts1/FEZ1) is a postsynaptic density (PSD) protein predominantly expressed in the frontal cortex, olfactory bulb, striatum, and hippocampus. Here we found that PSD-Zip70 knock-out (PSD-Zip70KO) mice exhibit working memory and cognitive defects, and enhanced anxiety-like behaviors. These abnormal behaviors are caused by impaired glutamatergic synapse transmission accompanied by tiny-headed immature dendritic spines in the PFC, due to aberrant Rap2 activation, which has roles in synapse formation and plasticity. PSD-Zip70 modulates the Rap2 activity by interacting with SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) in the postsynapse. Furthermore, suppression of the aberrant Rap2 activation in the PFC rescued the behavioral defects in PSD-Zip70KO mice. Our data demonstrate a critical role for PSD-Zip70 in Rap2-dependent spine synapse development in the PFC and underscore the importance of this regulation in PFC-dependent behaviors.

SIGNIFICANCE STATEMENT

PSD-Zip70 deficiency causes behavioral defects in working memory and cognition, and enhanced anxiety due to prefrontal hypofunction. This study revealed that PSD-Zip70 plays essential roles in glutamatergic synapse maturation via modulation of the Rap2 activity in the PFC. PSD-Zip70 interacts with both SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) and modulates the Rap2 activity in postsynaptic sites. Our results provide a novel Rap2-specific regulatory mechanism in synaptic maturation involving PSD-Zip70.

Hexa (ethylene glycol) derivative of benzothiazole aniline promotes dendritic spine formation through the RasGRF1-Ras dependent pathway

Our recent study demonstrated that an amyloid-β binding molecule, BTA-EG4, increases dendritic spine number via Ras-mediated signaling. To potentially optimize the potency of the BTA compounds, we synthesized and evaluated an amyloid-β binding analog of BTA-EG4 with increased solubility in aqueous solution, BTA-EG6. We initially examined the effects of BTA-EG6 on dendritic spine formation and found that BTA-EG6-treated primary hippocampal neurons had significantly increased dendritic spine number compared to control treatment. In addition, BTA-EG6 significantly increased the surface level of AMPA receptors. Upon investigation into the molecular mechanism by which BTA-EG6 promotes dendritic spine formation, we found that BTA-EG6 may exert its effects on spinogenesis via RasGRF1-ERK signaling, with potential involvement of other spinogenesis-related proteins such as Cdc42 and CDK5. Taken together, our data suggest that BTA-EG6 boosts spine and synapse number, which may have a beneficial effect of enhancing neuronal and synaptic function in the normal healthy brain.