Loss of sorting nexin 27 contributes to excitatory synaptic dysfunction by modulating glutamate receptor recycling in Down's syndrome

The schizophrenia risk gene C4 induces pathological synaptic loss by impairing AMPAR trafficking

Neuroimmune interactions play a significant role in regulating synaptic plasticity in both the healthy and diseased brain. The complement pathway, an extracellular proteolytic cascade, exemplifies these interactions. Its activation triggers microglia-dependent synaptic elimination via the complement receptor 3 (CR3). Current models of pathological complement activity in the brain propose that accelerated synaptic loss resulting from overexpression of C4 (C4-OE), a gene associated with schizophrenia, follows this pathway. Here, we report that C4-mediated cortical hypoconnectivity is CR3-independent. Instead, C4-OE triggers impaired GluR1 trafficking through an intracellular mechanism involving the endosomal protein SNX27, resulting in pathological synaptic loss. Moreover, C4 circuit alterations in the prefrontal cortex, a brain region associated with neuropsychiatric disorders, were rescued by increasing neuronal levels of SNX27, which we identify as an interacting partner of this neuroimmune protein. Our results link excessive complement activity to an intracellular endo-lysosomal trafficking pathway altering synaptic plasticity.

The crucial role of VPS35 and SHH in Parkinson's disease: Understanding the mechanisms behind the neurodegenerative disorder

Parkinson's disease (PD) is indeed a complex neurodegenerative disorder recognized by the progressive depletion of dopaminergic neurons in the brain, particularly in the substantia nigra region, leading to motor impairments and other symptoms. But at the molecular level, the study about PD still lacks. As the number of cases worldwide continues to increase, it is critical to focus on the cellular and molecular mechanisms of the disease's presentation and neurodegeneration to develop novel therapeutic approaches. At the molecular level, the complexity is more due to the involvement of vacuolar protein sorting 35 (VPS35) and sonic hedgehog (SHH) signaling in PD (directly or indirectly), leading to one of the most prominent hallmarks of the disease, which is an accumulation of α-synuclein. This elevated pathogenesis may result from impaired autophagy due to mutation in the case of VPS35 and impairment in SHH signaling at the molecular level. The traditional understanding of PD is marked by the disruption of dopaminergic neurons and dopaminergic signaling, which exacerbates symptoms of motor function deficits. However, the changes at the molecular level that are being disregarded also impact the overall health of the dopaminergic system. Gaining insight into these two unique mechanisms is essential to determine whether they give neuroprotection or have no effect on the health of neurons. Hence, here we tried to simplify the understanding of the role of VPS35 and SHH signaling to comprehend it in one direction.

Shaping down syndrome brain cognitive and molecular changes due to aging using adult animals from the Ts66Yah murine model

Down syndrome (DS) is the most common condition with intellectual disability and is caused by trisomy of Homo sapiens chromosome 21 (HSA21). The increased dosage of genes on HSA21 is associated with early neurodevelopmental changes and subsequently at adult age with the development of Alzheimer-like cognitive decline. However, the molecular mechanisms promoting brain pathology along aging are still missing. The novel Ts66Yah model represents an evolution of the Ts65Dn, used in characterizing the progression of brain degeneration, and it manifest phenotypes closer to human DS condition. In this study we performed a longitudinal analysis (3-9 months) of adult Ts66Yah mice. Our data support the behavioural alterations occurring in Ts66Yah mice at older age with improvement in the detection of spatial memory defects and also a new anxiety-related phenotype. The evaluation of hippocampal molecular pathways in Ts66Yah mice, as effect of age, demonstrate the aberrant regulation of redox balance, proteostasis, stress response, metabolic pathways, programmed cell death and synaptic plasticity. Intriguingly, the genotype-driven changes observed in those pathways occur early promoting altered brain development and the onset of a condition of premature aging. In turn, aging may account for the subsequent hippocampal deterioration that fall in characteristic neuropathological features. Besides, the analysis of sex influence in the alteration of hippocampal mechanisms demonstrate only a mild effect. Overall, data collected in Ts66Yah provide novel and consolidated insights, concerning trisomy-driven processes that contribute to brain pathology in conjunction with aging. This, in turn, aids in bridging the existing gap in comprehending the intricate nature of DS phenotypes.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Menin Deficiency Induces Autism-Like Behaviors by Regulating Foxg1 Transcription and Participates in Foxg1-Related Encephalopathy

FOXG1 syndrome is a developmental encephalopathy caused by FOXG1 (Forkhead box G1) mutations, resulting in high phenotypic variability. However, the upstream transcriptional regulation of Foxg1 expression remains unclear. This report demonstrates that both deficiency and overexpression of Men1 (protein: menin, a pathogenic gene of MEN1 syndrome known as multiple endocrine neoplasia type 1) lead to autism-like behaviors, such as social defects, increased repetitive behaviors, and cognitive impairments. Multifaceted transcriptome analyses revealed that Foxg1 signaling is predominantly altered in Men1 deficiency mice, through its regulation of the Alpha Thalassemia/Mental Retardation Syndrome X-Linked (Atrx) factor. Atrx recruits menin to bind to the transcriptional start region of Foxg1 and mediates the regulation of Foxg1 expression by H3K4me3 (Trimethylation of histone H3 lysine 4) modification. The deficits observed in menin deficient mice are rescued by the over-expression of Foxg1, leading to normalized spine growth and restoration of hippocampal synaptic plasticity. These findings suggest that menin may have a putative role in the maintenance of Foxg1 expression, highlighting menin signaling as a potential therapeutic target for Foxg1-related encephalopathy.

Metabolic reprogramming in astrocytes results in neuronal dysfunction in intellectual disability

Astrocyte aerobic glycolysis provides vital trophic support for central nervous system neurons. However, whether and how astrocytic metabolic dysregulation contributes to neuronal dysfunction in intellectual disability (ID) remain unclear. Here, we demonstrate a causal role for an ID-associated SNX27 mutation (R198W) in cognitive deficits involving reshaping astrocytic metabolism. We generated SNX27R196W (equivalent to human R198W) knock-in mice and found that they displayed deficits in synaptic function and learning behaviors. SNX27R196W resulted in attenuated astrocytic glucose uptake via GLUT1, leading to reduced lactate production and a switch from homeostatic to reactive astrocytes. Importantly, lactate supplementation or a ketogenic diet restored neuronal oxidative phosphorylation and reversed cognitive deficits in SNX27R196W mice. In summary, we illustrate a key role for astrocytic SNX27 in maintaining glucose supply and glycolysis and reveal that altered astrocytic metabolism disrupts the astrocyte-neuron interaction, which contributes to ID. Our work also suggests a feasible strategy for treating ID by restoring astrocytic metabolic function.

P-tau217 correlates with neurodegeneration in Alzheimer's disease, and targeting p-tau217 with immunotherapy ameliorates murine tauopathy

Neuronal loss is the central issue in Alzheimer's disease (AD), yet no treatment developed so far can halt AD-associated neurodegeneration. Here, we developed a monoclonal antibody (mAb2A7) against 217 site-phosphorylated human tau (p-tau217) and observed that p-tau217 levels positively correlated with brain atrophy and cognitive impairment in AD patients. Intranasal administration efficiently delivered mAb2A7 into male PS19 tauopathic mouse brain with target engagement and reduced tau pathology/aggregation with little effect on total soluble tau. Further, mAb2A7 treatment blocked apoptosis-associated neuronal loss and brain atrophy, reversed cognitive deficits, and improved motor function in male tauopathic mice. Proteomic analysis revealed that mAb2A7 treatment reversed alterations mainly in proteins associated with synaptic functions observed in murine tauopathy and AD brain. An antibody (13G4) targeting total tau also attenuated tau-associated pathology and neurodegeneration but impaired the motor function of male tauopathic mice. These results implicate p-tau217 as a potential therapeutic target for AD-associated neurodegeneration.

SNX27: A trans-species cognitive modulator with implications for anxiety and stress susceptibility

Sorting Nexin 27 (SNX27) is a brain-enriched endosome-associated cargo adaptor that shapes excitatory control, being relevant for cognitive and reward processing, and for several neurological conditions. Despite this, SNX27's role in the nervous system remains poorly explored. To further understand SNX27 function, we performed an extensive behavioral characterization comprising motor, cognitive and emotional dimensions of SNX27+/- mice. Furthermore, attending on the recently described association between SNX27 function and cellular stress signaling mechanisms in vitro, we explored SNX27-stress interplay using a Caenorhabditis elegans Δsnx-27 mutant and wild-type (WT) rodents after stress exposure. SNX27+/- mice, as C. elegans Δsnx-27 mutants, present cognitive impairments, highlighting a conserved role for SNX27 in cognitive modulation across species. Interestingly, SNX27 downmodulation leads to anxiety-like behavior in mice evaluated in the Elevated Plus Maze (EPM). This anxious phenotype is associated with increased dendritic complexity of the bed nucleus of the stria terminalis (BNST) neurons, and increased complexity of the basolateral amygdala (BLA) pyramidal neurons. These findings highlight the still unknown role of SNX27 in anxiety regulation. Moreover, we uncovered a direct link between SNX27 dysfunction and stress susceptibility in C. elegans and found that stress-exposed rodents display decreased SNX27 levels in stress-susceptible brain regions. Altogether, we provided new insights on SNX27's relevance in anxiety-related behaviors and neuronal structure in stress-associated brain regions.

SNX17 Mediates Dendritic Spine Maturation via p140Cap

Sorting nexin17 (SNX17) is a member of the sorting nexin family, which plays a crucial role in endosomal trafficking. Previous research has shown that SNX17 is involved in the recycling or degradation of various proteins associated with neurodevelopmental and neurological diseases in cell models. However, the significance of SNX17 in neurological function in the mouse brain has not been thoroughly investigated. In this study, we generated Snx17 knockout mice and observed that the homozygous deletion of Snx17 (Snx17-/-) resulted in lethality. On the other hand, heterozygous mutant mice (Snx17+/-) exhibited anxiety-like behavior with a reduced preference for social novelty. Furthermore, Snx17 haploinsufficiency led to impaired synaptic transmission and reduced maturation of dendritic spines. Through GST pulldown and interactome analysis, we identified the SRC kinase inhibitor, p140Cap, as a potential downstream target of SNX17. We also demonstrated that the interaction between p140Cap and SNX17 is crucial for dendritic spine maturation. Together, this study provides the first in vivo evidence highlighting the important role of SNX17 in maintaining neuronal function, as well as regulating social novelty and anxiety-like behaviors.

Transcriptome- and proteome-wide effects of a circular RNA encompassing four early exons of the spinal muscular atrophy genes

Spinal muscular atrophy (SMA) genes, SMN1 and SMN2, produce multiple circular RNAs (circRNAs), including C2A-2B-3-4 that encompasses early exons 2A, 2B, 3 and 4. Here we report the transcriptome- and proteome-wide effects of overexpression of C2A-2B-3-4 in inducible HEK293 cells. Our RNA-Seq analysis revealed altered expression of ~ 15% genes (4,172 genes) by C2A-2B-3-4. About half of the affected genes by C2A-2B-3-4 remained unaffected by L2A-2B-3-4, a linear transcript encompassing exons 2A, 2B, 3 and 4 of SMN1/SMN2. These fifindings underscore the unique role of the structural context of C2A-2B-3-4 in gene regulation. A surprisingly high number of upregulated genes by C2A-2B-3-4 were located on chromosomes 4 and 7, whereas many of the downregulated genes were located on chromosomes 10 and X. Supporting a cross-regulation of SMN1/SMN2 transcripts, C2A-2B-3-4 and L2A-2B-3-4 upregulated and downregulated SMN1/SMN2 mRNAs, respectively. Proteome analysis revealed 61 upregulated and 57 downregulated proteins by C2A-2B-3-4 with very limited overlap with those affected by L2A-2B-3-4. Independent validations confirmed the effect of C2A-2B-3-4 on expression of genes associated with chromatin remodeling, transcription, spliceosome function, ribosome biogenesis, lipid metabolism, cytoskeletal formation, cell proliferation and neuromuscular junction formation. Our findings reveal a broad role of C2A-2B-3-4, a universally expressed circRNA produced by SMN1/SMN2.

Increased burden of rare protein-truncating variants in constrained, brain-specific and synaptic genes in extremely impulsively violent males with antisocial personality disorder

The genetic correlates of extreme impulsive violence are poorly understood, and there have been few studies that have characterized a large group of affected individuals both clinically and genetically. We performed whole exome sequencing (WES) in 290 males with the life-course-persistent, extremely impulsively violent form of antisocial personality disorder (APD) and analyzed the spectrum of rare protein-truncating variants (rPTVs). Comparisons were made with 314 male controls and publicly available genotype data. Functional annotation tools were used for biological interpretation. Participants were significantly more likely to harbor rPTVs in genes that are intolerant to loss-of-function variants (odds ratio [OR] 2.06; p < 0.001), specifically expressed in brain (OR 2.80; p = 0.036) and enriched for those involved in neurotransmitter transport and synaptic processes. In 60 individuals (20%), we identified rPTVs that we classified as clinically relevant based on their clinical associations, biological function and gene expression patterns. Of these, 37 individuals harbored rPTVs in 23 genes that are associated with a monogenic neurological disorder, and 23 individuals harbored rPTVs in 20 genes reportedly intolerant to loss-of-function variants. The analysis presents evidence in support of a model where presence of either one or several private, functionally relevant mutations contribute significantly to individual risk of life-course-persistent APD and reveals multiple individuals who could be affected by clinically unrecognized neuropsychiatric Mendelian disease. Thus, Mendelian diseases and increased rPTV burden may represent important factors for the development of extremely impulsive violent life-course-persistent forms of APD irrespective of their clinical presentation.

Partial loss of Sorting Nexin 27 resembles age- and Down syndrome-associated T cell dysfunctions

BACKGROUND

Sorting Nexin 27 (SNX27)-retromer complex facilitates cargo recycling from endosomes to the plasma membrane. SNX27 downregulation in neurons, as the result of Trisomy 21 (T21), has been linked with cognitive deficits due to impairment of AMPA and NMDA receptor recycling. Studies in human T cell lines likewise demonstrated that SNX27 regulates the correct delivery of cargoes to the immune synapse limiting the activation of pro-inflammatory pathways. Nevertheless, the physiological consequences of partial SNX27 loss in T cell homeostasis are still unclear.

RESULTS

In this study, we have explored the consequences of T cell specific partial SNX27 downregulation in mice. T cells with partial SNX27 deficiency show a marked deficit in the CD4+ T cell pool, a hallmark of aging in mice and humans, and a well-characterized comorbidity of individuals with Down syndrome (DS). When analyzed ex vivo, CD4+ T cells with partial SNX27 deletion demonstrate enhanced proliferation but diminished IL-2 production. In contrast, the CD8+ population show enhanced expression of pro-inflammatory cytokines and lytic enzymes.

CONCLUSIONS

This mouse model supports the relevance of SNX27 in the organization of the immune synapse, previously described in cell lines, as well as in the control of T cell homeostasis. Individuals with DS experiment an acceleration of the aging process, which particularly affects the immune and central nervous systems. Thus, we hypothesize that reduced SNX27 expression in DS could contribute to the dysregulation of these systems and further research in SNX27 will shed light on the molecular factors underlying the phenotypes observed in people with DS and its contribution to aging.

Understanding the genetic mechanisms and cognitive impairments in Down syndrome: towards a holistic approach

The most common genetic cause of intellectual disability is Down syndrome (DS), trisomy 21. It commonly results from three copies of human chromosome 21 (HC21). There are no mutations or deletions involved in DS. Instead, the phenotype is caused by altered transcription of the genes on HC21. These transcriptional variations are responsible for a myriad of symptoms affecting every organ system. A very debilitating aspect of DS is intellectual disability (ID). Although tremendous advances have been made to try and understand the underlying mechanisms of ID, there is a lack of a unified, holistic view to defining the cause and managing the cognitive impairments. In this literature review, we discuss the mechanisms of neuronal over-inhibition, abnormal morphology, and other genetic factors in contributing to the development of ID in DS patients and to gain a holistic understanding of ID in DS patients. We also highlight potential therapeutic approaches to improve the quality of life of DS patients.

Retromer stabilization using a pharmacological chaperone protects in an α-synuclein based mouse model of Parkinson's

Background

In the present study we assessed the protective effects of a pharmacological approach to stabilize the retromer complex in a PD mouse model. Missense mutations in the VPS35 gene are a rare cause of familial PD. The VPS35 protein is a subunit of the retromer cargo recognition complex and has a variety of functions within neurons, many of which are potentially relevant for the pathophysiology of PD. Prior studies have revealed a role for the retromer complex in controlling accumulation and clearance of α-synuclein aggregates. We previously identified an aminoguanidine hydrazone, 1,3 phenyl bis guanylhydrazone (compound 2a), as a pharmacological stabilizer of the retromer complex that increases retromer subunit protein levels and function.

Methods

Here, we validate the efficacy of 2a in protecting against αSynuclein pathology and dopaminergic neuronal degeneration in a PD mouse model generated by unilateral injection of AAV-A53T-αSynuclein in the substantia nigra.

Results

Daily intraperitoneal administration of 2a at 10 mg/Kg for 100 days led to robust protection against behavioral deficits, dopaminergic neuronal loss and loss of striatal dopaminergic fibers and striatal monoamines. Treatment with 2a activated αSynuclein degradation pathways in the SN and led to significant reductions in aggregates and pathological αSynuclein.

Conclusion

These data suggest retromer stabilization as a promising therapeutic strategy for Parkinson's disease leading to neuroprotection of dopaminergic neurons and rescue in the accumulation of pathological and aggregates αSynuclein. We identified 2a compound as potential clinical drug candidate for future testing in Parkinson's disease patients.

Pathogenesis of miR-155 on nonmodifiable and modifiable risk factors in Alzheimer's disease

Alzheimer's disease (AD) is a common age-related neurodegenerative disease in the central nervous system and is the primary cause of dementia. It is clinically characterized by the memory impairment, aphasia, apraxia, agnosia, visuospatial and executive dysfunction, behavioral changes, and so on. Incidence of this disease was bound up with age, genetic factors, cardiovascular and cerebrovascular dysfunction, and other basic diseases, but the exact etiology has not been clarified. MicroRNAs (miRNAs) are small endogenous non-coding RNAs that were involved in the regulation of post-transcriptional gene expression. miRNAs have been extensively studied as noninvasive potential biomarkers for disease due to their relative stability in bodily fluids. In addition, they play a significant role in the physiological and pathological processes of various neurological disorders, including stroke, AD, and Parkinson's disease. MiR-155, as an important pro-inflammatory mediator of neuroinflammation, was reported to participate in the progression of β-amyloid peptide and tau via regulating immunity and inflammation. In this review, we put emphasis on the effects of miR-155 on AD and explore the underlying biological mechanisms which could provide a novel approach for diagnosis and treatment of AD.

Recruitment of the SNX17-Retriever recycling pathway regulates synaptic function and plasticity

Trafficking of cell-surface proteins from endosomes to the plasma membrane is a key mechanism to regulate synaptic function. In non-neuronal cells, proteins recycle to the plasma membrane either via the SNX27-Retromer-WASH pathway or via the recently discovered SNX17-Retriever-CCC-WASH pathway. While SNX27 is responsible for the recycling of key neuronal receptors, the roles of SNX17 in neurons are less understood. Here, using cultured hippocampal neurons, we demonstrate that the SNX17 pathway regulates synaptic function and plasticity. Disruption of this pathway results in a loss of excitatory synapses and prevents structural plasticity during chemical long-term potentiation (cLTP). cLTP drives SNX17 recruitment to synapses, where its roles are in part mediated by regulating the surface expression of β1-integrin. SNX17 recruitment relies on NMDAR activation, CaMKII signaling, and requires binding to the Retriever and PI(3)P. Together, these findings provide molecular insights into the regulation of SNX17 at synapses and define key roles for SNX17 in synaptic maintenance and in regulating enduring forms of synaptic plasticity.

Receptor Recycling by Retromer

The highly conserved retromer complex controls the fate of hundreds of receptors that pass through the endolysosomal system and is a central regulatory node for diverse metabolic programs. More than 20 years ago, retromer was discovered as an essential regulator of endosome-to-Golgi transport in yeast; since then, significant progress has been made to characterize how metazoan retromer components assemble to enable its engagement with endosomal membranes, where it sorts cargo receptors from endosomes to the trans-Golgi network or plasma membrane through recognition of sorting motifs in their cytoplasmic tails. In this review, we examine retromer regulation by exploring its assembled structure with an emphasis on how a range of adaptor proteins shape the process of receptor trafficking. Specifically, we focus on how retromer is recruited to endosomes, selects cargoes, and generates tubulovesicular carriers that deliver cargoes to target membranes. We also examine how cells adapt to distinct metabolic states by coordinating retromer expression and function. We contrast similarities and differences between retromer and its related complexes: retriever and commander/CCC, as well as their interplay in receptor trafficking. We elucidate how loss of retromer regulation is central to the pathology of various neurogenerative and metabolic diseases, as well as microbial infections, and highlight both opportunities and cautions for therapeutics that target retromer. Finally, with a focus on understanding the mechanisms that govern retromer regulation, we outline new directions for the field moving forward.

Altered Purinergic Signaling in Neurodevelopmental Disorders: Focus on P2 Receptors

With the umbrella term 'neurodevelopmental disorders' (NDDs) we refer to a plethora of congenital pathological conditions generally connected with cognitive, social behavior, and sensory/motor alterations. Among the possible causes, gestational and perinatal insults have been demonstrated to interfere with the physiological processes necessary for the proper development of fetal brain cytoarchitecture and functionality. In recent years, several genetic disorders caused by mutations in key enzymes involved in purine metabolism have been associated with autism-like behavioral outcomes. Further analysis revealed dysregulated purine and pyrimidine levels in the biofluids of subjects with other NDDs. Moreover, the pharmacological blockade of specific purinergic pathways reversed the cognitive and behavioral defects caused by maternal immune activation, a validated and now extensively used rodent model for NDDs. Furthermore, Fragile X and Rett syndrome transgenic animal models as well as models of premature birth, have been successfully utilized to investigate purinergic signaling as a potential pharmacological target for these diseases. In this review, we examine results on the role of the P2 receptor signaling in the etiopathogenesis of NDDs. On this basis, we discuss how this evidence could be exploited to develop more receptor-specific ligands for future therapeutic interventions and novel prognostic markers for the early detection of these conditions.

β2-microglobulin functions as an endogenous NMDAR antagonist to impair synaptic function

Down syndrome (DS) is a neurological disorder with multiple immune-related symptoms; however, crosstalk between the CNS and peripheral immune system remains unexplored. Using parabiosis and plasma infusion, we found that blood-borne factors drive synaptic deficits in DS. Proteomic analysis revealed elevation of β2-microglobulin (B2M), a major histocompatibility complex class I (MHC-I) component, in human DS plasma. Systemic administration of B2M in wild-type mice led to synaptic and memory defects similar to those observed in DS mice. Moreover, genetic ablation of B2m or systemic administration of an anti-B2M antibody counteracts synaptic impairments in DS mice. Mechanistically, we demonstrate that B2M antagonizes NMDA receptor (NMDAR) function through interactions with the GluN1-S2 loop; blocking B2M-NMDAR interactions using competitive peptides restores NMDAR-dependent synaptic function. Our findings identify B2M as an endogenous NMDAR antagonist and reveal a pathophysiological role for circulating B2M in NMDAR dysfunction in DS and related cognitive disorders.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Roles of the miR-155 in Neuroinflammation and Neurological Disorders: A Potent Biological and Therapeutic Target

Neuroinflammation plays a crucial role in the development and progression of neurological disorders. MicroRNA-155 (miR-155), a miR is known to play in inflammatory responses, is associated with susceptibility to inflammatory neurological disorders and neurodegeneration, including Alzheimer's disease, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis as well as epilepsy, stroke, and brain malignancies. MiR-155 damages the central nervous system (CNS) by enhancing the expression of pro-inflammatory cytokines, like IL-1β, IL-6, TNF-α, and IRF3. It also disturbs the blood-brain barrier by decreasing junctional complex molecules such as claudin-1, annexin-2, syntenin-1, and dedicator of cytokinesis 1 (DOCK-1), a hallmark of many neurological disorders. This review discusses the molecular pathways which involve miR-155 as a critical component in the progression of neurological disorders, representing miR-155 as a viable therapeutic target.

Sorting nexins: role in the regulation of blood pressure

Sorting nexins (SNXs) are a family of proteins that regulate cellular cargo sorting and trafficking, maintain intracellular protein homeostasis, and participate in intracellular signaling. SNXs are also important in the regulation of blood pressure via several mechanisms. Aberrant expression and dysfunction of SNXs participate in the dysregulation of blood pressure. Genetic studies show a correlation between SNX gene variants and the response to antihypertensive drugs. In this review, we summarize the progress in SNX-mediated regulation of blood pressure, discuss the potential role of SNXs in the pathophysiology and treatment of hypertension, and propose novel strategies for the medical therapy of hypertension.

Accumulation of pTau231 at the Postsynaptic Density in Early Alzheimer's Disease

BACKGROUND

Phosphorylated cytoplasmic tau inclusions correlate with and precede cognitive deficits in Alzheimer's disease (AD). However, pathological tau accumulation and relationships to synaptic changes remain unclear.

OBJECTIVE

To address this, we examined postmortem brain from 50 individuals with the full spectrum of AD (clinically and neuropathologically). Total tau, pTau231, and AMPA GluR1 were compared across two brain regions (entorhinal and middle frontal cortices), as well as clinically stratified groups (control, amnestic mild cognitive impairment, AD dementia), NIA-AA Alzheimer's Disease Neuropathologic Change designations (Not, Low, Intermediate, High), and Braak tangle stages (1-6). Significant co-existing pathology was excluded to isolate changes attributed to pathologic AD.

METHODS

Synaptosomal fractionation and staining were performed to measure changes in total Tau, pTau231, and AMPA GluR1. Total Tau and pTau231 were quantified in synaptosomal fractions using Quanterix Simoa HD-X.

RESULTS

Increasing pTau231 in frontal postsynaptic fractions correlated positively with increasing clinical and neuropathological AD severity. Frontal cortex is representative of early AD, as it does not become involved by tau tangles until late in AD. Entorhinal total tau was significantly higher in the amnestic mild cognitive impairment group when compared to AD, but only after accounting for AD associated synaptic changes. Alterations in AMPA GluR1 observed in the entorhinal cortex, but not middle frontal cortex, suggest that pTau231 mislocalization and aggregation in postsynaptic structures may impair glutamatergic signaling by promoting AMPA receptor dephosphorylation and internalization.

CONCLUSION

Results highlight the potential effectiveness of early pharmacological interventions targeting pTau231 accumulation at the postsynaptic density.

Endosomal Sorting Protein SNX27 and Its Emerging Roles in Human Cancers

SNX27 belongs to the sorting nexin (SNX) family of proteins that play a critical role in protein sorting and trafficking in the endocytosis pathway. This protein family is characterized by the presence of a Phox (PX) domain; however, SNX27 is unique in containing an additional PDZ domain. Recently, SNX27 has gained popularity as an important sorting protein that is associated with the retromer complex and mediates the recycling of internalized proteins from endosomes to the plasma membrane in a PDZ domain-dependent manner. Over 100 cell surface proteins have been identified as binding partners of the SNX27-retromer complex. However, the roles and underlying mechanisms governed by SNX27 in tumorigenesis remains to be poorly understood. Many of its known binding partners include several G-protein coupled receptors, such as β2-andrenergic receptor and parathyroid hormone receptor, are associated with multiple pathways implicated in oncogenic signaling and tumorigenesis. Additionally, SNX27 mediates the recycling of GLUT1 and the activation of mTORC1, both of which can regulate intracellular energy balance and promote cell survival and proliferation under conditions of nutrient deprivation. In this review, we summarize the structure and fundamental roles of SNX proteins, with a focus on SNX27, and provide the current evidence indicating towards the role of SNX27 in human cancers. We also discuss the gap in the field and future direction of SNX27 research. Insights into the emerging roles and mechanism of SNX27 in cancers will provide better development strategies to prevent and treat tumorigenesis.

Brain circuit pathology in Down syndrome: from neurons to neural networks

Down syndrome (DS), a genetic pathology caused by triplication of chromosome 21, is characterized by brain hypotrophy and impairment of cognition starting from infancy. While studies in mouse models of DS have elucidated the major neuroanatomical and neurochemical defects of DS, comparatively fewer investigations have focused on the electrophysiology of the DS brain. Electrical activity is at the basis of brain functioning. Therefore, knowledge of the way in which brain circuits operate in DS is fundamental to understand the causes of behavioral impairment and devise targeted interventions. This review summarizes the state of the art regarding the electrical properties of the DS brain, starting from individual neurons and culminating in signal processing in whole neuronal networks. The reported evidence derives from mouse models of DS and from brain tissues and neurons derived from individuals with DS. EEG data recorded in individuals with DS are also provided as a key tool to understand the impact of brain circuit alterations on global brain activity.

Downregulating PTBP1 Fails to Convert Astrocytes into Hippocampal Neurons and to Alleviate Symptoms in Alzheimer's Mouse Models

Conversion of astroglia into functional neurons has been considered a promising therapeutic strategy for neurodegenerative diseases. Recent studies reported that downregulation of the RNA binding protein, polypyrimidine tract-binding protein 1 (PTBP1), converts astrocytes into neurons in situ in multiple mouse brain regions, consequently improving pathologic phenotypes associated with Parkinson's disease, RGC loss, and aging. Here, we demonstrate that PTBP1 downregulation using an astrocyte-specific AAV-mediated shRNA system fails to convert hippocampal astrocytes into neurons in both male and female wild-type (WT) and β-amyloid (5×FAD) and tau (PS19) Alzheimer's disease (AD) mouse models and fails to reverse synaptic/cognitive deficits and AD-associated pathology in male mice. Similarly, PTBP1 downregulation cannot convert astrocytes into neurons in the striatum and substantia nigra in both male and female WT mice. Together, our study suggests that cell fate conversion strategy for neurodegenerative disease therapy through manipulating one single gene, such as PTBP1, warrants more rigorous scrutiny.SIGNIFICANCE STATEMENT Our results do not support some of the recent extraordinary and revolutionary claims that resident astrocytes can be directly and efficiently converted into neurons. Our study is critical for the field of neural regeneration and degeneration. In addition, our study is financially important because it may prevent other researchers/organizations from wasting a vast amount of time and resources on relevant investigations.

Enhanced GIRK2 channel signaling in Down syndrome: A feasible role in the development of abnormal nascent neural circuits

The most distinctive feature of Down syndrome (DS) is moderate to severe cognitive impairment. Genetic, molecular, and neuronal mechanisms of this complex DS phenotype are currently under intensive investigation. It is becoming increasingly clear that the abnormalities arise from a combination of initial changes caused by triplication of genes on human chromosome 21 (HSA21) and later compensatory adaptations affecting multiple brain systems. Consequently, relatively mild initial cognitive deficits become pronounced with age. This pattern of changes suggests that one approach to improving cognitive function in DS is to target the earliest critical changes, the prevention of which can change the ‘trajectory’ of the brain development and reduce the destructive effects of the secondary alterations. Here, we review the experimental data on the role of KCNJ6 in DS-specific brain abnormalities, focusing on a putative role of this gene in the development of abnormal neural circuits in the hippocampus of genetic mouse models of DS. It is suggested that the prevention of these early abnormalities with pharmacological or genetic means can ameliorate cognitive impairment in DS.

Tex264 Binding to SNX27 Regulates Itgα5 Receptor Membrane Recycling and Affects Cell Migration

Tex264 is an endoplasmic reticulum (ER) membrane protein that was recently demonstrated to act as an ER-phagy receptor under starvation conditions to mediate endoplasmic reticulum autophagy. However, how Tex264 functions in the central nervous system (CNS) and tumors is unclear. Here, we identified 89 proteins from the rat brain that may specifically interact with Tex264 and confirmed the interaction between sorting nexin 27 (SNX27) and Tex264 by coimmunoprecipitation and immunofluorescence. Our results indicated that Tex264 may promote recycling of membrane proteins from endosomes to the cell plasma membrane by recruiting SNX27 retromer vesicles. siRNA-mediated knockdown of TEX264 in HeLa cells did not affect cell proliferation but did significantly inhibit cell migration through a mechanism that may involve a reduction in SNX27-mediated Itgα5 receptor membrane recycling. Results of this study helped identify potential binding Tex264 partners and provide insights into Tex264 functions in the CNS and in tumors.

Retromer dysfunction in amyotrophic lateral sclerosis

Retromer is a heteropentameric complex that plays a specialized role in endosomal protein sorting and trafficking. Here, we report a reduction in the retromer proteins-vacuolar protein sorting 35 (VPS35), VPS26A, and VPS29-in patients with amyotrophic lateral sclerosis (ALS) and in the ALS model provided by transgenic (Tg) mice expressing the mutant superoxide dismutase-1 G93A. These changes are accompanied by a reduction of levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluA1, a proxy of retromer function, in spinal cords from Tg SOD1G93A mice. Correction of the retromer deficit by a viral vector expressing VPS35 exacerbates the paralytic phenotype in Tg SOD1G93A mice. Conversely, lowering Vps35 levels in Tg SOD1G93A mice ameliorates the disease phenotype. In light of these findings, we propose that mild alterations in retromer inversely modulate neurodegeneration propensity in ALS.

Microglial Tmem59 Deficiency Impairs Phagocytosis of Synapse and Leads to Autism-Like Behaviors in Mice

Synaptic abnormality is an important pathologic feature of autism spectrum disorders (ASDs) and responsible for various behavioral defects in these neurodevelopmental disorders. Microglia are the major immune cells in the brain and also play an important role in synapse refinement. Although dysregulated synaptic pruning by microglia during the brain development has been associated with ASDs, the underlying mechanism has yet to be fully elucidated. Herein, we observed that expression of Transmembrane protein 59 (TMEM59), a protein recently shown to regulate microglial function, was decreased in autistic patients. Furthermore, we found that both male and female mice with either complete or microglia-specific loss of Tmem59 developed ASD-like behaviors. Microglial TMEM59-deficient mice also exhibited enhanced excitatory synaptic transmission, increased dendritic spine density, and elevated levels of excitatory synaptic proteins in synaptosomes. TMEM59-deficient microglia had impaired capacity for synapse engulfment both in vivo and in vitro. Moreover, we demonstrated that TMEM59 interacted with the C1q receptor CD93 and TMEM59 deficiency promoted CD93 protein degradation in microglia. Downregulation of CD93 in microglia also impaired synapse engulfment. These findings identify a crucial role of TMEM59 in modulating microglial function on synapse refinement during brain development and suggest that TMEM59 deficiency may contribute to ASDs through disrupting phagocytosis of excitatory synapse and thus distorting the excitatory-inhibitory (E/I) neuronal activity balance.SIGNIFICANCE STATEMENT Microglia play an important role in synapse refinement. Dysregulated synaptic pruning by microglia during brain development has been associated with autism spectrum disorders (ASDs). However, the underlying mechanism has yet to be fully elucidated. Herein, we observe that the expression of Transmembrane protein 59 (TMEM59), an autophagy-related protein, is decreased in autistic patients. Moreover, we find ASD-like behaviors in mice with complete loss and with microglia-specific loss of Tmem59 Mechanistic studies reveal that TMEM59 deficiency in microglia impairs their synapse engulfment ability likely through destabilizing the C1q receptor CD93, thereby leading to enhanced excitatory neurotransmission and increased dendritic spine density. Our findings demonstrate a crucial role of microglial TMEM59 in early neuronal development and provide new insight into the etiology of ASDs.

Blocking Site-Specific Cleavage of Human Tau Delays Progression of Disease-Related Phenotypes in Genetically Matched Tau-Transgenic Mice Modeling Frontotemporal Dementia

Studies have recently demonstrated that a caspase-2-mediated cleavage of human tau (htau) at asparate-314 (D314) is responsible for cognitive deficits and neurodegeneration in mice modeling frontotemporal dementia (FTD). However, these animal studies may be confounded by flaws in their model systems, such as endogenous functional gene disruption and inequivalent transgene expression. To avoid these weaknesses, we examined the pathogenic role of this site-specific htau cleavage in FTD using genetically matched htau targeted-insertion mouse lines: rT2 and rT3. Both male and female mice were included in this study. rT2 mice contain a single copy of the FTD-linked htau proline-to-leucine mutation at amino acid 301 (htau P301L), inserted into a neutral site to avoid dysregulation of host gene expression. The similarly constructed rT3 mice harbor an additional D314-to-glutamate (D314E) mutation that blocks htau cleavage. We demonstrate that htau transgene expression occurs primarily in the forebrain at similar levels in rT2 and rT3 mice. Importantly, expression of the cleavage-resistant D314E mutant delays transgene-induced tau accumulation in the postsynaptic density, brain atrophy, hippocampal neurodegeneration, and spatial memory impairment, without altering age-related progression of pathologic tau conformation and phosphorylation. Our comprehensive investigation of age-dependent disease phenotypes associated with the htau P301L variant in precisely engineered FTD-modeling mice unveils a transiently protective effect of blocking htau cleavage at D314. Findings of this study advance our understanding of the contribution of this tau cleavage to the pathogenesis of FTD, and aid the development of effective dementia-targeting therapies.SIGNIFICANCE STATEMENT A site-specific and caspase-2-mediated cleavage of human tau plays a pathologic role in dementia. In this study, we investigate the contribution of this cleavage to the pathogenesis of frontotemporal dementia (FTD) using two genetically matched, tau-transgene targeted-insertion mouse lines that differ only by a cleavage-resistant mutation. The use of these mice avoids confounding effects associated with the random integration of tau transgenes to the mouse genome and allows us to comprehensively evaluate the impact of the tau cleavage on FTD phenotypes. Our data reveal that blocking this tau cleavage delays memory impairment and neurodegeneration of FTD-modeling mice. These findings improve our understanding of the pathogenic mechanisms underlying FTD and will facilitate the development of effective therapeutics.

Down Syndrome Fetal Fibroblasts Display Alterations of Endosomal Trafficking Possibly due to SYNJ1 Overexpression

Endosomal trafficking is essential for cellular homeostasis. At the crossroads of distinct intracellular pathways, the endolysosomal system is crucial to maintain critical functions and adapt to the environment. Alterations of endosomal compartments were observed in cells from adult individuals with Down syndrome (DS), suggesting that the dysfunction of the endosomal pathway may contribute to the pathogenesis of DS. However, the nature and the degree of impairment, as well as the timing of onset, remain elusive. Here, by applying imaging and biochemical approaches, we demonstrate that the structure and dynamics of early endosomes are altered in DS cells. Furthermore, we found that recycling trafficking is markedly compromised in these cells. Remarkably, our results in 18–20 week-old human fetal fibroblasts indicate that alterations in the endolysosomal pathway are already present early in development. In addition, we show that overexpression of the polyphosphoinositide phosphatase synaptojanin 1 (Synj1) recapitulates the alterations observed in DS cells, suggesting a role for this lipid phosphatase in the pathogenesis of DS, likely already early in disease development. Overall, these data strengthen the link between the endolysosomal pathway and DS, highlighting a dangerous liaison among Synj1, endosomal trafficking and DS.

Endosomal structure and APP biology are not altered in a preclinical mouse cellular model of Down syndrome

Individuals who have Down syndrome (trisomy 21) are at greatly increased risk of developing Alzheimer's disease, characterised by the accumulation in the brain of amyloid-β plaques. Amyloid-β is a product of the processing of the amyloid precursor protein, encoded by the APP gene on chromosome 21. In Down syndrome the first site of amyloid-β accumulation is within endosomes, and changes to endosome biology occur early in Alzheimer's disease. Here, we determine if primary mouse embryonic fibroblasts isolated from a mouse model of Down syndrome can be used to study endosome and APP cell biology. We report that in this cellular model, endosome number, size and APP processing are not altered, likely because APP is not dosage sensitive in the model, despite three copies of App.

MicroRNAs in the pathophysiology of Alzheimer's disease and Parkinson's disease: an overview

Neurodegenerative diseases are characterized by a progressive loss of neurons of the central nervous system (CNS) and serve as a major cause of morbidity, mortality and functional dependence especially among the elderly. Despite extensive research and development efforts, the success rate of clinical pipelines has been very limited. However, microRNAs (miRs) have been proved to be of crucial importance in regulating intracellular pathways for various pathologic conditions including those of a neurodegenerative nature. There is ample evidence of altered levels of various miRs in clinical samples of Alzheimer's disease and Parkinson's disease patients with potentially major clinical implications. In the current review, we aim to summarize the relevant literature on the role of miRs in the pathophysiology of Alzheimer's disease (AD) and Parkinson's disease (PD) as the two globally predominant neurodegenerative conditions.

GIRK2 Channels in Down Syndrome and Alzheimer's Disease

Cognitive impairment in Down syndrome (DS) results from the abnormal expression of hundreds of genes. However, the impact of KCNJ6, a gene located in the middle of the 'Down syndrome critical region' of chromosome 21, seems to stand out. KCNJ6 encodes GIRK2 (KIR3.2) subunits of G protein-gated inwardly rectifying potassium channels, which serve as effectors for GABAB, m2, 5HT1A, A1, and many other postsynaptic metabotropic receptors. GIRK2 subunits are heavily expressed in neocortex, cerebellum, and hippocampus. By controlling resting membrane potential and neuronal excitability, GIRK2 channels may thus affect both synaptic plasticity and stability of neural circuits in the brain regions important for learning and memory. Here, we discuss recent experimental data regarding the role of KCNJ6/GIRK2 in neuronal abnormalities and cognitive impairment in models of DS and Alzheimer's disease (AD). The results compellingly show that signaling through GIRK2 channels is abnormally enhanced in mouse genetic models of Down syndrome and that partial suppression of GIRK2 channels with pharmacological or genetic means can restore synaptic plasticity and improve impaired cognitive functions. On the other hand, signaling through GIRK2 channels is downregulated in AD models, such as models of early amyloidopathy. In these models, reduced GIRK2 channel signaling promotes neuronal hyperactivity, causing excitatory-inhibitory imbalance and neuronal death. Accordingly, activation of GABAB/GIRK2 signaling by GIRK channel activators or GABAB receptor agonists may reduce Aβ-induced hyperactivity and subsequent neuronal death, thereby exerting a neuroprotective effect in models of AD.

PDZ-Containing Proteins Targeted by the ACE2 Receptor

Angiotensin-converting enzyme 2 (ACE2) is a main receptor for SARS-CoV-2 entry to the host cell. Indeed, the first step in viral entry is the binding of the viral trimeric spike (S) protein to ACE2. Abundantly present in human epithelial cells of many organs, ACE2 is also expressed in the human brain. ACE2 is a type I membrane protein with an extracellular N-terminal peptidase domain and a C-terminal collectrin-like domain that ends with a single transmembrane helix and an intracellular 44-residue segment. This C-terminal segment contains a PDZ-binding motif (PBM) targeting protein-interacting domains called PSD-95/Dlg/ZO-1 (PDZ). Here, we identified the human PDZ specificity profile of the ACE2 PBM using the high-throughput holdup assay and measuring the binding intensities of the PBM of ACE2 against the full human PDZome. We discovered 14 human PDZ binders of ACE2 showing significant binding with dissociation constants' values ranging from 3 to 81 μM. NHERF, SHANK, and SNX27 proteins found in this study are involved in protein trafficking. The PDZ/PBM interactions with ACE2 could play a role in ACE2 internalization and recycling that could be of benefit for the virus entry. Interestingly, most of the ACE2 partners we identified are expressed in neuronal cells, such as SHANK and MAST families, and modifications of the interactions between ACE2 and these neuronal proteins may be involved in the neurological symptoms of COVID-19.

SNX14 deficiency-induced defective axonal mitochondrial transport in Purkinje cells underlies cerebellar ataxia and can be reversed by valproate

Loss-of-function mutations in sorting nexin 14 (SNX14) cause autosomal recessive spinocerebellar ataxia 20, which is a form of early-onset cerebellar ataxia that lacks molecular mechanisms and mouse models. We generated Snx14-deficient mouse models and observed severe motor deficits and cell-autonomous Purkinje cell degeneration. SNX14 deficiency disrupted microtubule organization and mitochondrial transport in axons by destabilizing the microtubule-severing enzyme spastin, which is implicated in dominant hereditary spastic paraplegia with cerebellar ataxia, and compromised axonal integrity and mitochondrial function. Axonal transport disruption and mitochondrial dysfunction further led to degeneration of high-energy-demanding Purkinje cells, which resulted in the pathogenesis of cerebellar ataxia. The antiepileptic drug valproate ameliorated motor deficits and cerebellar degeneration in Snx14-deficient mice via the restoration of mitochondrial transport and function in Purkinje cells. Our study revealed an unprecedented role for SNX14-dependent axonal transport in cerebellar ataxia, demonstrated the convergence of SNX14 and spastin in mitochondrial dysfunction, and suggested valproate as a potential therapeutic agent.

Nuclear speckle specific hnRNP D-like prevents age- and AD-related cognitive decline by modulating RNA splicing

BACKGROUND

Aberrant alternative splicing plays critical role in aging and age-related diseases. Heterogeneous nuclear ribonucleoproteins (hnRNPs) reportedly regulate RNA splicing process. Whether and how hnRNPs contribute to age-related neurodegenerative diseases, especially Alzheimer's disease (AD), remain elusive.

METHODS

Immunoblotting and immunostaining were performed to determine expression patterns and cellular/subcellular localization of the long isoform of hnRNP D-like (L-DL), which is a hnRNP family member, in mouse hippocampus. Downregulation of L-DL in WT mice was achieved by AAV-mediated shRNA delivery, followed by memory-related behavioural tests. L-DL interactome was analysed by affinity-precipitation and mass spectrometry. Alternative RNA splicing was measured by RNA-seq and analyzed by bioinformatics-based approaches. Downregulation and upregulation of L-DL in APP/PS1 mice were performed using AAV-mediated transduction.

RESULTS

We show that L-DL is specifically localized to nuclear speckles. L-DL levels are decreased in the hippocampus of aged mouse brains and downregulation of L-DL impairs cognition in mice. L-DL serves as a structural component to recruit other speckle proteins, and regulates cytoskeleton- and synapse-related gene expression by altering RNA splicing. Mechanistically, these splicing changes are modulated via L-DL-mediated interaction of SF3B3, a core component of U2 snRNP, and U2AF65, a U2 spliceosome protein that guides U2 snRNP's binding to RNA. In addition, L-DL levels are decreased in APP/PS1 mouse brains. While downregulation of L-DL deteriorates memory deficits and overexpression of L-DL improves cognitive function in AD mice, by regulating the alternative splicing and expression of synaptic gene CAMKV.

CONCLUSIONS

Our findings define a molecular mechanism by which hnRNP L-DL regulates alternative RNA splicing, and establish a direct role for L-DL in AD-related synaptic dysfunction and memory decline.

SNX27-FERM-SNX1 complex structure rationalizes divergent trafficking pathways by SNX17 and SNX27

The molecular events that determine the recycling versus degradation fates of internalized membrane proteins remain poorly understood. Two of the three members of the SNX-FERM family, SNX17 and SNX31, utilize their FERM domain to mediate endocytic trafficking of cargo proteins harboring the NPxY/NxxY motif. In contrast, SNX27 does not recycle NPxY/NxxY-containing cargo but instead recycles cargo containing PDZ-binding motifs via its PDZ domain. The underlying mechanism governing this divergence in FERM domain binding is poorly understood. Here, we report that the FERM domain of SNX27 is functionally distinct from SNX17 and interacts with a novel DLF motif localized within the N terminus of SNX1/2 instead of the NPxY/NxxY motif in cargo proteins. The SNX27-FERM-SNX1 complex structure reveals that the DLF motif of SNX1 binds to a hydrophobic cave surrounded by positively charged residues on the surface of SNX27. The interaction between SNX27 and SNX1/2 is critical for efficient SNX27 recruitment to endosomes and endocytic recycling of multiple cargoes. Finally, we show that the interaction between SNX27 and SNX1/2 is critical for brain development in zebrafish. Altogether, our study solves a long-standing puzzle in the field and suggests that SNX27 and SNX17 mediate endocytic recycling through fundamentally distinct mechanisms.

Increased AT1 receptor expression mediates vasoconstriction leading to hypertension in Snx1-/- mice

Angiotensin II type 1 receptor (AT1R) is a vital therapeutic target for hypertension. Sorting nexin 1 (SNX1) participates in the sorting and trafficking of the renal dopamine D5 receptor, while angiotensin and dopamine are counterregulatory factors in the regulation of blood pressure. The effect of SNX1 on AT1R is not known. We hypothesized that SNX1, through arterial AT1R sorting and trafficking, is involved in blood pressure regulation. CRISPR/Cas9 system-generated SNX1-/- mice showed dramatic elevations in blood pressure compared to their wild-type littermates. The angiotensin II-mediated contractile reactivity of the mesenteric arteries and AT1R expression in the aortas were also increased. Moreover, immunofluorescence and immunoprecipitation analyses revealed that SNX1 and AT1R were colocalized and interacted in the aortas of wild-type mice. In vitro studies revealed that AT1R protein levels and downstream calcium signaling were upregulated in A10 cells treated with SNX1 siRNA. This may have resulted from decreased AT1R protein degradation since the AT1R mRNA levels showed no changes. AT1R protein was less degraded when SNX1 was downregulated, as reflected by a cycloheximide chase assay. Furthermore, proteasomal rather than lysosomal inhibition increased AT1R protein content, and this effect was accompanied by decayed binding of ubiquitin and AT1R after SNX1 knockdown. Confocal microscopy revealed that AT1R colocalized with PSMD6, a proteasomal marker, and the colocalization was reduced after SNX1 knockdown. These findings suggest that SNX1 sorts AT1R for proteasomal degradation and that SNX1 impairment increases arterial AT1R expression, leading to increased vasoconstriction and blood pressure.

SNX27-driven membrane localisation of OTULIN antagonises linear ubiquitination and NF-κB signalling activation

BACKGROUND

Linear ubiquitination is a novel type of ubiquitination that plays important physiological roles in signalling pathways such as tumour necrosis factor (TNF) signalling. However, little is known about the regulatory mechanisms of linear ubiquitination, except the well-described enzymatic regulators E3 ligase linear ubiquitin chain assembly complex (LUBAC) and deubiquitinase OTULIN.

RESULTS

Previously, we identified SNX27, a member of the sorting nexin family protein, as a selective linear ubiquitin chain interactor in mass spectrometry-based ubiquitin interaction screening. Here, we demonstrated that the interaction between the linear ubiquitin chain and SNX27 is mediated by the OTULIN. Furthermore, we found that SNX27 inhibits LUBAC-mediated linear ubiquitin chain formation and TNFα-induced signalling activation. Mechanistic studies showed that, upon TNFα stimulation, OTULIN-SNX27 is localised to membrane-associated TNF receptor complex, where OTULIN deubiquitinates the linear polyubiquitin chain that formed by the LUBAC complex. Significantly, chemical inhibition of SNX27-retromer translocation by cholera toxin inhibits OTULIN membrane localization.

CONCLUSIONS

In conclusion, our study demonstrated that SNX27 inhibits TNFα induced NF-κB signalling activation via facilitating OTULIN to localize to TNF receptor complex.

Regulation of NMDA receptor trafficking and gating by activity-dependent CaMKIIα phosphorylation of the GluN2A subunit

NMDA receptor (NMDAR)-dependent Ca2+ influx underpins multiple forms of synaptic plasticity. Most synaptic NMDAR currents in the adult forebrain are mediated by GluN2A-containing receptors, which are rapidly inserted into synapses during long-term potentiation (LTP); however, the underlying molecular mechanisms remain poorly understood. In this study, we show that GluN2A is phosphorylated at Ser-1459 by Ca2+/calmodulin-dependent kinase IIα (CaMKIIα) in response to glycine stimulation that mimics LTP in primary neurons. Phosphorylation of Ser-1459 promotes GluN2A interaction with the sorting nexin 27 (SNX27)-retromer complex, thereby enhancing the endosomal recycling of NMDARs. Loss of SNX27 or CaMKIIα function blocks the glycine-induced increase in GluN2A-NMDARs on the neuronal membrane. Interestingly, mutations of Ser-1459, including the rare S1459G human epilepsy variant, prolong the decay times of NMDAR-mediated synaptic currents in heterosynapses by increasing the duration of channel opening. These findings not only identify a critical role of Ser-1459 phosphorylation in regulating the function of NMDARs, but they also explain how the S1459G variant dysregulates NMDAR function.

Restoring neuronal chloride homeostasis with anti-NKCC1 gene therapy rescues cognitive deficits in a mouse model of Down syndrome

A common feature of diverse brain disorders is the alteration of GABA-mediated inhibition because of aberrant, intracellular chloride homeostasis induced by changes in the expression and/or function of chloride transporters. Notably, pharmacological inhibition of the chloride importer NKCC1 is able to rescue brain-related core deficits in animal models of these pathologies and in some human clinical studies. Here, we show that reducing NKCC1 expression by RNA interference in the Ts65Dn mouse model of Down syndrome (DS) restores intracellular chloride concentration, efficacy of gamma-aminobutyric acid (GABA)-mediated inhibition, and neuronal network dynamics in vitro and ex vivo. Importantly, adeno-associated virus (AAV)-mediated, neuron-specific NKCC1 knockdown in vivo rescues cognitive deficits in diverse behavioral tasks in Ts65Dn animals. Our results highlight a mechanistic link between NKCC1 expression and behavioral abnormalities in DS mice and establish a molecular target for new therapeutic approaches, including gene therapy, to treat brain disorders characterized by neuronal chloride imbalance.

An Epilepsy-Associated GRIN2A Rare Variant Disrupts CaMKIIα Phosphorylation of GluN2A and NMDA Receptor Trafficking

Rare variants in GRIN genes, which encode NMDAR subunits, are strongly associated with neurodevelopmental disorders. Among these, GRIN2A, which encodes the GluN2A subunit of NMDARs, is widely accepted as an epilepsy-causative gene. Here, we functionally characterize the de novo GluN2A-S1459G mutation identified in an epilepsy patient. We show that S1459 is a CaMKIIα phosphorylation site, and that endogenous phosphorylation is regulated during development and in response to synaptic activity in a dark rearing model. GluN2A-S1459 phosphorylation results in preferential binding of NMDARs to SNX27 and a corresponding decrease in PSD-95 binding, which consequently regulates NMDAR trafficking. Furthermore, the epilepsy-associated GluN2A-S1459G variant displays defects in interactions with both SNX27 and PSD-95, resulting in trafficking deficits, reduced spine density, and decreased excitatory synaptic transmission. These data demonstrate a role for CaMKIIα phosphorylation of GluN2A in receptor targeting and implicate NMDAR trafficking defects as a link to epilepsy.

Sorting Out Sorting Nexins Functions in the Nervous System in Health and Disease

Endocytosis is a fundamental process that controls protein/lipid composition of the plasma membrane, thereby shaping cellular metabolism, sensing, adhesion, signaling, and nutrient uptake. Endocytosis is essential for the cell to adapt to its surrounding environment, and a tight regulation of the endocytic mechanisms is required to maintain cell function and survival. This is particularly significant in the central nervous system (CNS), where composition of neuronal cell surface is crucial for synaptic functioning. In fact, distinct pathologies of the CNS are tightly linked to abnormal endolysosomal function, and several genome wide association analysis (GWAS) and biochemical studies have identified intracellular trafficking regulators as genetic risk factors for such pathologies. The sorting nexins (SNXs) are a family of proteins involved in protein trafficking regulation and signaling. SNXs dysregulation occurs in patients with Alzheimer’s disease (AD), Down’s syndrome (DS), schizophrenia, ataxia and epilepsy, among others, establishing clear roles for this protein family in pathology. Interestingly, restoration of SNXs levels has been shown to trigger synaptic plasticity recovery in a DS mouse model. This review encompasses an historical and evolutionary overview of SNXs protein family, focusing on its organization, phyla conservation, and evolution throughout the development of the nervous system during speciation. We will also survey SNXs molecular interactions and highlight how defects on SNXs underlie distinct pathologies of the CNS. Ultimately, we discuss possible strategies of intervention, surveying how our knowledge about the fundamental processes regulated by SNXs can be applied to the identification of novel therapeutic avenues for SNXs-related disorders.

Neuroinflammation in HIV-Related Neuropathic Pain

In the management of human immunodeficiency virus (HIV) infection around the world, chronic complications are becoming a new problem along with the prolonged life expectancy. Chronic pain is widespread in HIV infected patients and even affects those with a low viral load undergoing long-term treatment with antiviral drugs, negatively influencing the adherence to disease management and quality of life. A large proportion of chronic pain is neuropathic pain, which defined as chronic pain caused by nervous system lesions or diseases, presenting a series of nervous system symptoms including both positive and negative signs. Injury caused by HIV protein, central and peripheral sensitization, and side effects of antiretroviral therapy lead to neuroinflammation, which is regarded as a maladaptive mechanism originally serving to promote regeneration and healing, constituting the main mechanism of HIV-related neuropathic pain. Gp120, as HIV envelope protein, has been found to be the major toxin that induces neuropathic pain. Particularly, the microglia, releasing numerous pro-inflammatory substances (such as TNFα, IL-1β, and IL-6), not only sensitize the neurons but also are the center part of the crosstalk bridging the astrocytes and oligodendrocytes together forming the central sensitization during HIV infection, which is not discussed detailly in recent reviews. In the meantime, some NRTIs and PIs exacerbate the neuroinflammation response. In this review, we highlight the importance of clarifying the mechanism of HIV-related neuropathic pain, and discuss about the limitation of the related studies as future research directions.

Toward Understanding the Molecular Role of SNX27/Retromer in Human Health and Disease

Aberrations in membrane trafficking pathways have profound effects in cellular dynamics of cellular sorting processes and can drive severe physiological outcomes. Sorting nexin 27 (SNX27) is a metazoan-specific sorting nexin protein from the PX-FERM domain family and is required for endosomal recycling of many important transmembrane receptors. Multiple studies have shown SNX27-mediated recycling requires association with retromer, one of the best-known regulators of endosomal trafficking. SNX27/retromer downregulation is strongly linked to Down's Syndrome (DS) via glutamate receptor dysfunction and to Alzheimer's Disease (AD) through increased intracellular production of amyloid peptides from amyloid precursor protein (APP) breakdown. SNX27 is further linked to addiction via its role in potassium channel trafficking, and its over-expression is linked to tumorigenesis, cancer progression, and metastasis. Thus, the correct sorting of multiple receptors by SNX27/retromer is vital for normal cellular function to prevent human diseases. The role of SNX27 in regulating cargo recycling from endosomes to the cell surface is firmly established, but how SNX27 assembles with retromer to generate tubulovesicular carriers remains elusive. Whether SNX27/retromer may be a putative therapeutic target to prevent neurodegenerative disease is now an emerging area of study. This review will provide an update on our molecular understanding of endosomal trafficking events mediated by the SNX27/retromer complex on endosomes.