Loss of TDP-43 oligomerization or RNA binding elicits distinct aggregation patterns.

Aggregation of the RNA-binding protein TAR DNA-binding protein 43 (TDP-43) is the key neuropathological feature of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). In physiological conditions, TDP-43 is predominantly nuclear, forms oligomers, and is contained in biomolecular condensates assembled by liquid-liquid phase separation (LLPS). In disease, TDP-43 forms cytoplasmic or intranuclear inclusions. How TDP-43 transitions from physiological to pathological states remains poorly understood. Using a variety of cellular systems to express structure-based TDP-43 variants, including human neurons and cell lines with near-physiological expression levels, we show that oligomerization and RNA binding govern TDP-43 stability, splicing functionality, LLPS, and subcellular localization. Importantly, our data reveal that TDP-43 oligomerization is modulated by RNA binding. By mimicking the impaired proteasomal activity observed in ALS/FTLD patients, we found that monomeric TDP-43 forms inclusions in the cytoplasm, whereas its RNA binding-deficient counterpart aggregated in the nucleus. These differentially localized aggregates emerged via distinct pathways: LLPS-driven aggregation in the nucleus and aggresome-dependent inclusion formation in the cytoplasm. Therefore, our work unravels the origins of heterogeneous pathological species reminiscent of those occurring in TDP-43 proteinopathy patients.

Comprehensive evaluation of human-derived anti-poly-GA antibodies in cellular and animal models of C9orf72 disease.

Hexanucleotide G4C2 repeat expansions in the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Dipeptide repeat proteins (DPRs) generated by translation of repeat-containing RNAs show toxic effects in vivo as well as in vitro and are key targets for therapeutic intervention. We generated human antibodies that bind DPRs with high affinity and specificity. Anti-GA antibodies engaged extra- and intra-cellular poly-GA and reduced aggregate formation in a poly-GA overexpressing human cell line. However, antibody treatment in human neuronal cultures synthesizing exogenous poly-GA resulted in the formation of large extracellular immune complexes and did not affect accumulation of intracellular poly-GA aggregates. Treatment with antibodies was also shown to directly alter the morphological and biochemical properties of poly-GA and to shift poly-GA/antibody complexes to more rapidly sedimenting ones. These alterations were not observed with poly-GP and have important implications for accurate measurement of poly-GA levels including the need to evaluate all centrifugation fractions and disrupt the interaction between treatment antibodies and poly-GA by denaturation. Targeting poly-GA and poly-GP in two mouse models expressing G4C2 repeats by systemic antibody delivery for up to 16 mo was well-tolerated and led to measurable brain penetration of antibodies. Long-term treatment with anti-GA antibodies produced improvement in an open-field movement test in aged C9orf72450 mice. However, chronic administration of anti-GA antibodies in AAV-(G4C2)149 mice was associated with increased levels of poly-GA detected by immunoassay and did not significantly reduce poly-GA aggregates or alleviate disease progression in this model.

FTLD-TDP assemblies seed neoaggregates with subtype-specific features via a prion-like cascade.

Morphologically distinct TDP-43 aggregates occur in clinically different FTLD-TDP subtypes, yet the mechanism of their emergence and contribution to clinical heterogeneity are poorly understood. Several lines of evidence suggest that pathological TDP-43 follows a prion-like cascade, but the molecular determinants of this process remain unknown. We use advanced microscopy techniques to compare the seeding properties of pathological FTLD-TDP-A and FTLD-TDP-C aggregates. Upon inoculation of patient-derived aggregates in cells, FTLD-TDP-A seeds amplify in a template-dependent fashion, triggering neoaggregation more efficiently than those extracted from FTLD-TDP-C patients, correlating with the respective disease progression rates. Neoaggregates are sequentially phosphorylated with N-to-C directionality and with subtype-specific timelines. The resulting FTLD-TDP-A neoaggregates are large and contain densely packed fibrils, reminiscent of the pure compacted fibrils present within cytoplasmic inclusions in postmortem brains. In contrast, FTLD-TDP-C dystrophic neurites show less dense fibrils mixed with cellular components, and their respective neoaggregates are small, amorphous protein accumulations. These cellular seeding models replicate aspects of the patient pathological diversity and will be a useful tool in the quest for subtype-specific therapeutics.

Reduction of the expression of the late-onset Alzheimer's disease (AD) risk-factor BIN1 does not affect amyloid pathology in an AD mouse model.

Alzheimer's disease (AD) is pathologically characterized by the deposition of the ß-amyloid (Aß) peptide in senile plaques in the brain, leading to neuronal dysfunction and eventual decline in cognitive function. Genome-wide association studies have identified the bridging integrator 1 (BIN1) gene within the second most significant susceptibility locus for late-onset AD. BIN1 is a member of the amphiphysin family of proteins and has reported roles in the generation of membrane curvature and endocytosis. Endocytic dysfunction is a pathological feature of AD, and endocytosis of the amyloid precursor protein is an important step in its subsequent cleavage by ß-secretase (BACE1). In vitro evidence implicates BIN1 in endosomal sorting of BACE1 and Aß generation in neurons, but a role for BIN1 in this process in vivo is yet to be described. Here, using biochemical and immunohistochemistry analyses we report that a 50% global reduction of BIN1 protein levels resulting from a single Bin1 allele deletion in mice does not change BACE1 levels or localization in vivo, nor does this reduction alter the production of endogenous murine Aß in nontransgenic mice. Furthermore, we found that reduction of BIN1 levels in the 5XFAD mouse model of amyloidosis does not alter Aß deposition nor behavioral deficits associated with cerebral amyloid burden. Finally, a conditional BIN1 knockout in excitatory neurons did not alter BACE1, APP, C-terminal fragments derived from BACE1 cleavage of APP, or endogenous Aß levels. These results indicate that BIN1 function does not regulate Aß generation in vivo.

Lack of BACE1 S-palmitoylation reduces amyloid burden and mitigates memory deficits in transgenic mouse models of Alzheimer's disease.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by pathological brain lesions and a decline in cognitive function. ß-Amyloid peptides (Aß), derived from proteolytic processing of amyloid precursor protein (APP), play a central role in AD pathogenesis. ß-Site APP cleaving enzyme 1 (BACE1), the transmembrane aspartyl protease which initiates Aß production, is axonally transported in neurons and accumulates in dystrophic neurites near cerebral amyloid deposits in AD. BACE1 is modified by S-palmitoylation at four juxtamembrane cysteine residues. S-palmitoylation is a dynamic posttranslational modification that is important for trafficking and function of several synaptic proteins. Here, we investigated the in vivo significance of BACE1 S-palmitoylation through the analysis of knock-in mice with cysteine-to-alanine substitution at the palmitoylated residues (4CA mice). BACE1 expression, as well as processing of APP and other neuronal substrates, was unaltered in 4CA mice despite the lack of BACE1 S-palmitoylation and reduced lipid raft association. Whereas steady-state Aß levels were similar, synaptic activity-induced endogenous Aß production was not observed in 4CA mice. Furthermore, we report a significant reduction of cerebral amyloid burden and BACE1 accumulation in dystrophic neurites in the absence of BACE1 S-palmitoylation in mouse models of AD amyloidosis. Studies in cultured neurons suggest that S-palmitoylation is required for dendritic spine localization and axonal targeting of BACE1. Finally, the lack of BACE1 S-palmitoylation mitigates cognitive deficits in 5XFAD mice. Using transgenic mouse models, these results demonstrate that intrinsic posttranslational S-palmitoylation of BACE1 has a significant impact on amyloid pathogenesis and the consequent cognitive decline.

VEGF modulates NMDA receptors activity in cerebellar granule cells through Src-family kinases before synapse formation.

NMDA type glutamate receptors (NMDARs) are best known for their role in synaptogenesis and synaptic plasticity. Much less is known about their developmental role before neurons form synapses. We report here that VEGF, which promotes migration of granule cells (GCs) during postnatal cerebellar development, enhances NMDAR-mediated currents and Ca(2+) influx in immature GCs before synapse formation. The VEGF receptor Flk1 forms a complex with the NMDAR subunits NR1 and NR2B. In response to VEGF, the number of Flk1/NR2B coclusters on the cell surface increases. Stimulation of Flk1 by VEGF activates Src-family kinases, which increases tyrosine phosphorylation of NR2B. Inhibition of Src-family kinases abolishes the VEGF-dependent NR2B phosphorylation and amplification of NMDAR-mediated currents and Ca(2+) influx in GCs. These findings identify VEGF as a modulator of NMDARs before synapse formation and highlight a link between an activity-independent neurovascular guidance cue (VEGF) and an activity-regulated neurotransmitter receptor (NMDAR).

Disrupted surface cross-talk between NMDA and Ephrin-B2 receptors in anti-NMDA encephalitis.

Autoimmune synaptic encephalitides are recently described human brain diseases leading to psychiatric and neurological syndromes through inappropriate brain-autoantibody interactions. The most frequent synaptic autoimmune encephalitis is associated with autoantibodies against extracellular domains of the glutamatergic N-methyl-d-aspartate receptor, with patients developing psychotic and neurological symptoms in an autoantibody titre-dependent manner. Although N-methyl-d-aspartate receptors are the primary target of these antibodies, the cellular and molecular pathway(s) that rapidly lead to N-methyl-d-aspartate receptor dysfunction remain poorly understood. In this report, we used a unique combination of high-resolution nanoparticle and bulk live imaging approaches to demonstrate that anti-N-methyl-d-aspartate receptor autoantibodies from patients with encephalitis strongly alter, in a time-dependent manner, the surface content and trafficking of GluN2-NMDA receptor subtypes. Autoantibodies laterally displaced surface GluN2A-NMDA receptors out of synapses and completely blocked synaptic plasticity. This loss of extrasynaptic and synaptic N-methyl-d-aspartate receptor is prevented both in vitro and in vivo, by the activation of EPHB2 receptors. Indeed, the anti-N-methyl-d-aspartate receptor autoantibodies weaken the interaction between the extracellular domains of the N-methyl-d-aspartate and Ephrin-B2 receptors. Together, we demonstrate that the anti-N-methyl-d-aspartate receptor autoantibodies from patients with encephalitis alter the dynamic retention of synaptic N-methyl-d-aspartate receptor through extracellular domain-dependent mechanism(s), shedding new light on the pathology of the neurological and psychiatric disorders observed in these patients and opening possible new therapeutic strategies.

Synaptic FUS accumulation triggers early misregulation of synaptic RNAs in a mouse model of ALS.

Mutations disrupting the nuclear localization of the RNA-binding protein FUS characterize a subset of amyotrophic lateral sclerosis patients (ALS-FUS). FUS regulates nuclear RNAs, but its role at the synapse is poorly understood. Using super-resolution imaging we determined that the localization of FUS within synapses occurs predominantly near the vesicle reserve pool of presynaptic sites. Using CLIP-seq on synaptoneurosomes, we identified synaptic FUS RNA targets, encoding proteins associated with synapse organization and plasticity. Significant increase of synaptic FUS during early disease in a mouse model of ALS was accompanied by alterations in density and size of GABAergic synapses. mRNAs abnormally accumulated at the synapses of 6-month-old ALS-FUS mice were enriched for FUS targets and correlated with those depicting increased short-term mRNA stability via binding primarily on multiple exonic sites. Our study indicates that synaptic FUS accumulation in early disease leads to synaptic impairment, potentially representing an initial trigger of neurodegeneration.

LAG3 is not expressed in human and murine neurons and does not modulate α-synucleinopathies.

While the initial pathology of Parkinson's disease and other α-synucleinopathies is often confined to circumscribed brain regions, it can spread and progressively affect adjacent and distant brain locales. This process may be controlled by cellular receptors of α-synuclein fibrils, one of which was proposed to be the LAG3 immune checkpoint molecule. Here, we analysed the expression pattern of LAG3 in human and mouse brains. Using a variety of methods and model systems, we found no evidence for LAG3 expression by neurons. While we confirmed that LAG3 interacts with α-synuclein fibrils, the specificity of this interaction appears limited. Moreover, overexpression of LAG3 in cultured human neural cells did not cause any worsening of α-synuclein pathology ex vivo. The overall survival of A53T α-synuclein transgenic mice was unaffected by LAG3 depletion, and the seeded induction of α-synuclein lesions in hippocampal slice cultures was unaffected by LAG3 knockout. These data suggest that the proposed role of LAG3 in the spreading of α-synucleinopathies is not universally valid.

Cytoplasmic FUS triggers early behavioral alterations linked to cortical neuronal hyperactivity and inhibitory synaptic defects.

Gene mutations causing cytoplasmic mislocalization of the RNA-binding protein FUS lead to severe forms of amyotrophic lateral sclerosis (ALS). Cytoplasmic accumulation of FUS is also observed in other diseases, with unknown consequences. Here, we show that cytoplasmic mislocalization of FUS drives behavioral abnormalities in knock-in mice, including locomotor hyperactivity and alterations in social interactions, in the absence of widespread neuronal loss. Mechanistically, we identified a progressive increase in neuronal activity in the frontal cortex of Fus knock-in mice in vivo, associated with altered synaptic gene expression. Synaptic ultrastructural and morphological defects were more pronounced in inhibitory than excitatory synapses and associated with increased synaptosomal levels of FUS and its RNA targets. Thus, cytoplasmic FUS triggers synaptic deficits, which is leading to increased neuronal activity in frontal cortex and causing related behavioral phenotypes. These results indicate that FUS mislocalization may trigger deleterious phenotypes beyond motor neuron impairment in ALS, likely relevant also for other neurodegenerative diseases characterized by FUS mislocalization.

Phase Separation and Neurodegenerative Diseases: A Disturbance in the Force.

Protein aggregation is the main hallmark of neurodegenerative diseases. Many proteins found in pathological inclusions are known to undergo liquid-liquid phase separation, a reversible process of molecular self-assembly. Emerging evidence supports the hypothesis that aberrant phase separation behavior may serve as a trigger of protein aggregation in neurodegeneration, and efforts to understand and control the underlying mechanisms are underway. Here, we review similarities and differences among four main proteins, α-synuclein, FUS, tau, and TDP-43, which are found aggregated in different diseases and were independently shown to phase separate. We discuss future directions in the field that will help shed light on the molecular mechanisms of aggregation and neurodegeneration.

Neuronal BIN1 Regulates Presynaptic Neurotransmitter Release and Memory Consolidation.

BIN1, a member of the BAR adaptor protein family, is a significant late-onset Alzheimer disease risk factor. Here, we investigate BIN1 function in the brain using conditional knockout (cKO) models. Loss of neuronal Bin1 expression results in the select impairment of spatial learning and memory. Examination of hippocampal CA1 excitatory synapses reveals a deficit in presynaptic release probability and slower depletion of neurotransmitters during repetitive stimulation, suggesting altered vesicle dynamics in Bin1 cKO mice. Super-resolution and immunoelectron microscopy localizes BIN1 to presynaptic sites in excitatory synapses. Bin1 cKO significantly reduces synapse density and alters presynaptic active zone protein cluster formation. Finally, 3D electron microscopy reconstruction analysis uncovers a significant increase in docked and reserve pools of synaptic vesicles at hippocampal synapses in Bin1 cKO mice. Our results demonstrate a non-redundant role for BIN1 in presynaptic regulation, thus providing significant insights into the fundamental function of BIN1 in synaptic physiology relevant to Alzheimer disease.

Aberrant accrual of BIN1 near Alzheimer's disease amyloid deposits in transgenic models.

Bridging integrator 1 (BIN1) is the most significant late-onset Alzheimer's disease (AD) susceptibility locus identified via genome-wide association studies. BIN1 is an adaptor protein that regulates membrane dynamics in the context of endocytosis and membrane remodeling. An increase in BIN1 expression and changes in the relative levels of alternatively spliced BIN1 isoforms have been reported in the brains of patients with AD. BIN1 can bind to Tau, and an increase in BIN1 expression correlates with Tau pathology. In contrast, the loss of BIN1 expression in cultured cells elevates Aß production and Tau propagation by insfluencing endocytosis and recycling. Here, we show that BIN1 accumulates adjacent to amyloid deposits in vivo. We found an increase in insoluble BIN1 and a striking accrual of BIN1 within and near amyloid deposits in the brains of multiple transgenic models of AD. The peri-deposit aberrant BIN1 localization was conspicuously different from the accumulation of APP and BACE1 within dystrophic neurites. Although BIN1 is highly expressed in mature oligodendrocytes, BIN1 association with amyloid deposits occurred in the absence of the accretion of other oligodendrocyte or myelin proteins. Finally, super-resolution microscopy and immunogold electron microscopy analyses highlight the presence of BIN1 in proximity to amyloid fibrils at the edges of amyloid deposits. These results reveal the aberrant accumulation of BIN1 is a feature associated with AD amyloid pathology. Our findings suggest a potential role for BIN1 in extracellular Aß deposition in vivo that is distinct from its well-characterized function as an adaptor protein in endocytosis and membrane remodeling.

BIN1 localization is distinct from Tau tangles in Alzheimer's disease.

BIN1 is the second most significant Alzheimer's disease (AD) risk factor gene identified through genome-wide association studies. BIN1 is an adaptor protein that can bind to several proteins including c-Myc, clathrin, adaptor protein-2 and dynamin. BIN1 is widely expressed in the brain and peripheral tissue as ubiquitous and tissue-specific alternatively spliced isoforms that regulate membrane dynamics and endocytosis in multiple cell types. The function of BIN1 in the brain and the mechanism(s) by which AD-associated BIN1 alleles increase the risk for the disease are not known. BIN1 has been shown to interact with Tau and two studies reported a positive correlation between BIN1 expression and neurofibrillary tangle pathology in AD. However, an inverse correlation between BIN1 expression and Tau propagation has also been reported. Moreover, there have been conflicting reports on whether BIN1 is present in tangles. A recent study characterized predominant BIN1 expression in mature oligodendrocytes in the gray matter and the white matter in rodent, and the human brain. Here, we have examined BIN1 localization in the brains of patients with AD using immunohistochemistry and immunofluorescence techniques to analyze BIN1 cellular expression in relation to cellular markers and pathological lesions in AD. We report that BIN1 immunoreactivity in human AD is not associated with neurofibrillary tangles or senile plaques. Moreover, our results show that BIN1 is not expressed by resting and activated microglia, astrocytes, or macrophages in human AD. In accordance with a recent report, low-level de novo BIN1 expression can be observed in a subset of neurons in the AD brain. Further investigations are warranted to understand the complex cellular mechanisms underlying the observed correlation between BIN1 expression and the severity of tangle pathology in AD.

Predominant expression of Alzheimer's disease-associated BIN1 in mature oligodendrocytes and localization to white matter tracts.

BACKGROUND: Genome-wide association studies have identified BIN1 within the second most significant susceptibility locus in late-onset Alzheimer's disease (AD). BIN1 undergoes complex alternative splicing to generate multiple isoforms with diverse functions in multiple cellular processes including endocytosis and membrane remodeling. An increase in BIN1 expression in AD and an interaction between BIN1 and Tau have been reported. However, disparate descriptions of BIN1 expression and localization in the brain previously reported in the literature and the lack of clarity on brain BIN1 isoforms present formidable challenges to our understanding of how genetic variants in BIN1 increase the risk for AD. METHODS: In this study, we analyzed BIN1 mRNA and protein levels in human brain samples from individuals with or without AD. In addition, we characterized the BIN1 expression and isoform diversity in human and rodent tissue by immunohistochemistry and immunoblotting using a panel of BIN1 antibodies. RESULTS: Here, we report on BIN1 isoform diversity in the human brain and document alterations in the levels of select BIN1 isoforms in individuals with AD. In addition, we report striking BIN1 localization to white matter tracts in rodent and the human brain, and document that the large majority of BIN1 is expressed in mature oligodendrocytes whereas neuronal BIN1 represents a minor fraction. This predominant non-neuronal BIN1 localization contrasts with the strict neuronal expression and presynaptic localization of the BIN1 paralog, Amphiphysin 1. We also observe upregulation of BIN1 at the onset of postnatal myelination in the brain and during differentiation of cultured oligodendrocytes. Finally, we document that the loss of BIN1 significantly correlates with the extent of demyelination in multiple sclerosis lesions. CONCLUSION: Our study provides new insights into the brain distribution and cellular expression of an important risk factor associated with late-onset AD. We propose that efforts to define how genetic variants in BIN1 elevate the risk for AD would behoove to consider BIN1 function in the context of its main expression in mature oligodendrocytes and the potential for a role of BIN1 in the membrane remodeling that accompanies the process of myelination.