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.

Alzheimer's disease (AD) is pathologically characterized by the deposition of the β-amyloid (Aβ) 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 β-secretase (BACE1). In vitro evidence implicates BIN1 in endosomal sorting of BACE1 and Aβ 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 Aβ in nontransgenic mice. Furthermore, we found that reduction of BIN1 levels in the 5XFAD mouse model of amyloidosis does not alter Aβ 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 Aβ levels. These results indicate that BIN1 function does not regulate Aβ generation in vivo.

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

BIN1 was identified as the second most prevalent risk factor for late-onset Alzheimer's disease (AD) in genome-wide association studies. A reduction in neuronal BIN1 isoform has been reported in patients with AD. Though little is known about BIN1 function in the brain, the loss of BIN1 expression has been proposed to influence BACE1 trafficking and Aβ production in vitro. Mice with germline deletion of a single Bin1 allele were analysed for BACE1 and APP distribution by immunofluorescence microscopy. Following crossing to the 5XFAD model of amyloidosis, the amyloid burden was assessed at 4 months of age using mAb-3D6 (anti-Aβ) and ThioflavinS staining. Endogenous Aβ and soluble and insoluble Aβ were measured using MesoScale Multi-plex assays. Deletion of a single Bin1 allele in mice, resulted in 50% reduction in BIN1 protein levels in the brain. However, the partial loss of BIN1 expression did not grossly affect BACE1 distribution in mouse brain, alter endogenous Aβ levels or cause significant cognitive or motor deficits in a range of behavioural paradigms. The loss of a single Bin1 allele in 5XFAD mice resulted in no significant difference in amyloid burden, as observed by immunofluorescence microscopy or ELISA, compared to littermate controls with two Bin1 alleles. The partial reduction in BIN1 expression did not alter behavioural deficits in 5XFAD mice associated with amyloid deposition in vivo. Despite prior reports of reduced neuronal BIN1 levels associated with AD and a role in BACE1 trafficking and Aβ generation in vitro, reduction of BIN1 protein within the brain of non-transgenic and 5XFAD mice resulted in no significant change in amyloid production or deposition. Our results indicate that the mechanism(s) through which risk is conferred by polymorphisms in the BIN1 gene is not through a role in amyloid generation or deposition.

The deposition of amyloid-β (Aβ) peptide in extracellular senile plaques is a hallmark of Alzheimer's disease (AD). Aβ is derived from the cleavage of amyloid precursor protein (APP) by the β- and γ-secretases. It has been proposed as the key trigger in the complex cascade of events which lead to AD. Genome-wide association studies have recently identified BIN1 as a major susceptibility locus for late-onset AD (LOAD) We previously reported a predominant expression of BIN1 in mature oligodendrocytes and the white matter tracts. Interestingly, recent in vitro studies described a role for BIN1 in APP processing by BACE1 and Ab production. However, the role of BIN1 in Ab generation in vivo remains to be reported. In this study, we explored BIN1 localization within the amyloid deposits in different model for Alzheimer disease. We explored the change of solubility properties of BIN1 using sequential detergent extraction. We investigated BIN1 localization within the deposits using confocal and STED microscopy. We confirmed our observations by immunoEM. Here, we report an increase in the levels of insoluble BIN1 in the brains of 5XFAD transgenic mice. We also observe a striking accrual of BIN1 within the amyloid deposits in multiple transgenic models of AD and in the human brain. This aberrant BIN1 localization was distinct from dystrophic neurite accumulation of APP and BACE1. Our immunoEM results suggest that the accumulation of insoluble BIN1 appears along the amyloid processes. We hypothesize that BIN1 insolubility and accumulation is a consequence of myelin destruction and the extracellular release of BIN1. Altogether, our results suggest a reorganization of myelin proteins surrounding the amyloid deposit and the change of BIN1 biophysical properties in relation to Ab accumulation in the brain. Our results bring new evidence of myelin protein reorganization associated with amyloid deposition. This work opens new avenues related to myelin pathology observed in AD.

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 Aβ 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 Aβ deposition in vivo that is distinct from its well-characterized function as an adaptor protein in endocytosis and membrane remodeling.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by pathological brain lesions and a decline in cognitive function. β-Amyloid peptides (Aβ), derived from proteolytic processing of amyloid precursor protein (APP), play a central role in AD pathogenesis. β-Site APP cleaving enzyme 1 (BACE1), the transmembrane aspartyl protease which initiates Aβ 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 Aβ levels were similar, synaptic activity-induced endogenous Aβ 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.

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.