Convergence of synapses, endosomes, and prions in the biology of neurodegenerative diseases

Nano-Infrared Imaging of Primary Neurons

Alzheimer’s disease (AD) accounts for about 70% of neurodegenerative diseases and is a cause of cognitive decline and death for one-third of seniors. AD is currently underdiagnosed, and it cannot be effectively prevented. Aggregation of amyloid-β (Aβ) proteins has been linked to the development of AD, and it has been established that, under pathological conditions, Aβ proteins undergo structural changes to form β-sheet structures that are considered neurotoxic. Numerous intensive in vitro studies have provided detailed information about amyloid polymorphs; however, little is known on how amyloid β-sheet-enriched aggregates can cause neurotoxicity in relevant settings. We used scattering-type scanning near-field optical microscopy (s-SNOM) to study amyloid structures at the nanoscale, in individual neurons. Specifically, we show that in well-validated systems, s-SNOM can detect amyloid β-sheet structures with nanometer spatial resolution in individual neurons. This is a proof-of-concept study to demonstrate that s-SNOM can be used to detect Aβ-sheet structures on cell surfaces at the nanoscale. Furthermore, this study is intended to raise neurobiologists’ awareness of the potential of s-SNOM as a tool for analyzing amyloid β-sheet structures at the nanoscale in neurons without the need for immunolabeling.

Synapses have autophagy under control

Regulation of autophagy in neurons remains unclear. In this issue, Kulkarni et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202002084) show with elegant live imaging that in dendrites, but not in axons, autophagosome motility and function is regulated by synaptic activity.

The Role of the SLC Transporters Protein in the Neurodegenerative Disorders

The solute carrier (SLC) superfamily is one of the major sub-groups of membrane proteins in mammalian cells. The solute carrier proteins include more than 400 different membrane-spanning solute carriers organized with 65 families in the human. In solute carrier family neurons, neurotransmitter is considered to be a pharmacological target of neuropsychiatric drugs because of their important role in the recovery of neurotransmitters such as GABA, glutamate, serotonin, dopamine and noradrenaline and regulation of their concentration in synaptic regions. Therefore, solute carrier transporters play vital and different roles in neurodegenerative disorders. In this article, the role of solute carrier transporters in neurodegenerative disorders such as Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, Parkinson’s diseases, depression, post-traumatic stress disorder, dementia, schizophrenia, and Epilepsy reviewed and discussed to see how defects or absences in SLC transporter cause neurodegenerative disorders. In this review, we try to summarize what is known about solute carriers with respect to brain distribution and expression. The review summarizes current knowledge on the roles of solute carrier transporters in neurodegenerative disorders.

Analysis of RNA Expression Profiles Identifies Dysregulated Vesicle Trafficking Pathways in Creutzfeldt-Jakob Disease

Functional genomics applied to the study of RNA expression profiles identified several abnormal molecular processes in experimental prion disease. However, only a few similar studies have been carried out to date in a naturally occurring human prion disease. To better characterize the transcriptional cascades associated with sporadic Creutzfeldt-Jakob disease (sCJD), the most common human prion disease, we investigated the global gene expression profile in samples from the frontal cortex of 10 patients with sCJD and 10 non-neurological controls by microarray analysis. The comparison identified 333 highly differentially expressed genes (hDEGs) in sCJD. Functional enrichment Gene Ontology analysis revealed that hDEGs were mainly associated with synaptic transmission, including GABA (q value = 0.049) and glutamate (q value = 0.005) signaling, and the immune/inflammatory response. Furthermore, the analysis of cellular components performed on hDEGs showed a compromised regulation of vesicle-mediated transport with mainly up-regulated genes related to the endosome (q value = 0.01), lysosome (q value = 0.04), and extracellular exosome (q value < 0.01). A targeted analysis of the retromer core component VPS35 (vacuolar protein sorting-associated protein 35) showed a down-regulation of gene expression (p value= 0.006) and reduced brain protein levels (p value= 0.002). Taken together, these results confirm and expand previous microarray expression profile data in sCJD. Most significantly, they also demonstrate the involvement of the endosomal-lysosomal system. Since the latter is a common pathogenic pathway linking together diseases, such as Alzheimer's and Parkinson's, it might be the focus of future studies aimed to identify new therapeutic targets in neurodegenerative diseases.

Heterogeneous Association of Alzheimer's Disease-Linked Amyloid-β and Amyloid-β Protein Precursor with Synapses

Alzheimer’s disease (AD) is increasingly viewed as a disease of synapses. Loss of synapses correlates better with cognitive decline than amyloid plaques and neurofibrillary tangles, the hallmark neuropathological lesions of AD. Soluble forms of amyloid-β (Aβ) have emerged as mediators of synapse dysfunction. Aβ binds to, accumulates, and aggregates in synapses. However, the anatomical and neurotransmitter specificity of Aβ and the amyloid-β protein precursor (AβPP) in AD remain poorly understood. In addition, the relative roles of Aβ and AβPP in the development of AD, at pre- versus post-synaptic compartments and axons versus dendrites, respectively, remain unclear. Here we use immunogold electron microscopy and confocal microscopy to provide evidence for heterogeneity in the localization of Aβ/AβPP. We demonstrate that Aβ binds to a subset of synapses in cultured neurons, with preferential binding to glutamatergic compared to GABAergic neurons. We also highlight the challenge of defining pre- versus post-synaptic localization of this binding by confocal microscopy. Further, endogenous Aβ₄₂ accumulates in both glutamatergic and GABAergic AβPP/PS1 transgenic primary neurons, but at varying levels. Moreover, upon knock-out of presenilin 1 or inhibition of γ-secretase AβPP C-terminal fragments accumulate both pre- and post-synaptically; however earlier pre-synaptically, consistent with a higher rate of AβPP processing in axons. A better understanding of the synaptic and anatomical selectivity of Aβ/AβPP in AD can be important for the development of more effective new therapies for this major disease of aging.

Disruption of amyloid precursor protein ubiquitination selectively increases amyloid β (Aβ) 40 levels via presenilin 2-mediated cleavage

Amyloid plaques, a neuropathological hallmark of Alzheimer's disease, are largely composed of amyloid β (Aβ) peptide, derived from cleavage of amyloid precursor protein (APP) by β- and γ-secretases. The endosome is increasingly recognized as an important crossroad for APP and these secretases, with major implications for APP processing and amyloidogenesis. Among various post-translational modifications affecting APP accumulation, ubiquitination of cytodomain lysines may represent a key signal controlling APP endosomal sorting. Here, we show that substitution of APP C-terminal lysines with arginine disrupts APP ubiquitination and that an increase in the number of substituted lysines tends to increase APP metabolism. An APP mutant lacking all C-terminal lysines underwent the most pronounced increase in processing, leading to accumulation of both secreted and intracellular Aβ40. Artificial APP ubiquitination with rapalog-mediated proximity inducers reduced Aβ40 generation. A lack of APP C-terminal lysines caused APP redistribution from endosomal intraluminal vesicles (ILVs) to the endosomal limiting membrane, with a subsequent decrease in APP C-terminal fragment (CTF) content in secreted exosomes, but had minimal effects on APP lysosomal degradation. Both the increases in secreted and intracellular Aβ40 were abolished by depletion of presenilin 2 (PSEN2), recently shown to be enriched on the endosomal limiting membrane compared with PSEN1. Our findings demonstrate that ubiquitin can act as a signal at five cytodomain-located lysines for endosomal sorting of APP. They further suggest that disruption of APP endosomal sorting reduces its sequestration in ILVs and results in PSEN2-mediated processing of a larger pool of APP-CTF on the endosomal membrane.

Genetic Variants of SNCA Are Associated with Susceptibility to Parkinson's Disease but Not Amyotrophic Lateral Sclerosis or Multiple System Atrophy in a Chinese Population

Background

The polymorphisms of α-synuclein (SNCA), rs3775444, rs3822086 and rs11931074 that are strongly associated with Parkinson’s disease (PD) in Caucasian populations, were examined in this study to elucidate the role of polymorphisms in different ethnic backgrounds. The possible associations of these three polymorphisms were also investigated in PD, amyotrophic lateral sclerosis (ALS), and multiple system atrophy (MSA) in a Chinese population based on the overlapping of clinical manifestations and pathological characteristics of these three neurodegenerative diseases.

Methods

A total of 1276 PD, 885 sporadic ALS (SALS), 364 MSA patients, and 846 healthy controls (HCs) were included. All subjects were genotyped for the three polymorphisms using Sequenom iPLEX Assay technology.

Results

Significant differences in the genotype distributions (p = 5.99E-06 and p = 4.98E-06, respectively) and the minor allele frequency (MAF) (p = 2.16E-06 and p = 2.15E-06, respectively) of SNCA rs3822086 (C) and rs11931074 (G) were observed between PD and HCs. However, no differences were found in the genotype distributions and MAF of SNCA rs3775444 (T) between PD and HCs. Haplotype that incorporated the three SNPs further strengthened the association with PD (best haplotype, p = 9.62E-005). No significant differences in the genotype distributions and MAF of the SNPs were found between SALS and HCs, MSA and HCs, and subgroups of PD and SALS. However, the MAF of SNCA rs3775444 (T) was significantly higher in MSA patients with frontal lobe dysfunction than MSA patients without dysfunction (p = 0.0002, OR 2.53, 95%CI: 1.55-4.15).

Conclusion

Our results suggest that the rs3822086 (C) allele and rs11931074 (G) allele in SNCA decrease the risk for PD, and SNCA rs11931074 may affect frontal lobe dysfunction of MSA in the Chinese population. However, these SNCA polymorphisms are not likely a common cause of SALS or MSA.

MLC1 protein: a likely link between leukodystrophies and brain channelopathies

Megalencephalic leukoencephalopathy with subcortical cysts (MLCs) disease is a rare inherited, autosomal recessive form of childhood-onset spongiform leukodystrophy characterized by macrocephaly, deterioration of motor functions, epileptic seizures and mental decline. Brain edema, subcortical fluid cysts, myelin and astrocyte vacuolation are the histopathological hallmarks of MLC. Mutations in either the MLC1 gene (>75% of patients) or the GlialCAM gene (<20% of patients) are responsible for the disease. Recently, the GlialCAM adhesion protein was found essential for the membrane expression and function of the chloride channel ClC-2 indicating MLC disease caused by mutation in GlialCAM as the first channelopathy among leukodystrophies. On the contrary, the function of MLC1 protein, which binds GlialCAM, its functional relationship with ClC-2 and the molecular mechanisms underlying MLC1 mutation-induced functional defects are not fully understood yet. The human MLC1 gene encodes a 377-amino acid membrane protein with eight predicted transmembrane domains which shows very low homology with voltage-dependent potassium (K⁺) channel subunits. The high expression of MLC1 in brain astrocytes contacting blood vessels and meninges and brain alterations observed in MLC patients have led to hypothesize a role for MLC1 in the regulation of ion and water homeostasis. Recent studies have shown that MLC1 establishes structural and/or functional interactions with several ion/water channels and transporters and ion channel accessory proteins, and that these interactions are affected by MLC1 mutations causing MLC. Here, we review data on MLC1 functional properties obtained in in vitro and in vivo models and discuss evidence linking the effects of MLC1 mutations to brain channelopathies.

Bridging the age spectrum of neurodegenerative storage diseases

For over a century, researchers have observed similar neurodegenerative hallmarks in brains of people affected by rare early-onset lysosomal storage diseases and late-onset neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Increasing evidence suggests these apparently disparate diseases share a common underlying feature, namely, a dysfunctional clearance of cellular cargo through the secretory-endosomal-autophagic-lysosomal-exocytic (SEALE) network. By providing examples of rare and common neurodegenerative diseases known to have pathologically altered cargo flux through the SEALE network, we explore the unifying hypothesis that impaired catabolism or exocytosis of SEALE cargo, places a burden of stress on neurons that initiates pathogenesis. We also describe how a growing understanding of genetic, epigenetic and age-related modifications of the SEALE network, has inspired a number of novel disease-modifying therapeutic approaches aimed at alleviating SEALE storage and providing therapeutic benefit to people affected by these devastating diseases across the age spectrum.

β-Amyloid peptides and amyloid plaques in Alzheimer's disease

Many lines of evidence support that β-amyloid (Aβ) peptides play an important role in Alzheimer's disease (AD), the most common cause of dementia. But despite much effort the molecular mechanisms of how Aβ contributes to AD remain unclear. While Aβ is generated from its precursor protein throughout life, the peptide is best known as the main component of amyloid plaques, the neuropathological hallmark of AD. Reduction in Aβ has been the major target of recent experimental therapies against AD. Unfortunately, human clinical trials targeting Aβ have not shown the hoped-for benefits. Thus, doubts have been growing about the role of Aβ as a therapeutic target. Here we review evidence supporting the involvement of Aβ in AD, highlight the importance of differentiating between various forms of Aβ, and suggest that a better understanding of Aβ's precise pathophysiological role in the disease is important for correctly targeting it for potential future therapy.

SNCA variants rs2736990 and rs356220 as risk factors for Parkinson's disease but not for amyotrophic lateral sclerosis and multiple system atrophy in a Chinese population

Previous studies found that polymorphisms rs2736990 and rs356220 in the alpha-synuclein (SNCA) gene increase the risk for Parkinson's disease (PD) in a Caucasian population. In consideration of the overlapping of clinical manifestations and pathologic characteristics among PD, amyotrophic lateral sclerosis (ALS), and multiple system atrophy (MSA), the possible associations of these 2 polymorphisms and 3 neurodegenerative diseases were studied in the Chinese population. A total of 1011 PD, 778 sporadic ALS (SALS), 264 MSA patients, and 721 healthy controls (HCs) were studied. All subjects were genotyped for the 2 polymorphisms using polymerase chain reaction and direct sequencing. Significant differences in the genotype frequencies (p = 0.0188 and 0.0064, respectively) and minor allele frequencies (MAFs) (p = 0.0065 and 0.0095, respectively) of rs2736990 and rs356220 were observed between the PD patients and HCs. Moreover, significant differences were found between the early-onset PD patients (<50 years) and matched controls but not in the late-onset PD patients (≥50 years). However, no differences were observed between subgroups with regard to clinical features, such as sex, onset symptoms (tremor or rigidity), cognition (normal or abnormal), and anxiety and depression (presence or absence). No significant differences were found in the genotype frequencies and MAFs of these 2 single-nucleotide polymorphisms between SALS patients and HCs and between MSA patients and HCs. No significant differences were found between subgroups with regard to the clinical presentation of SALS and MSA. Our results show that rs2736990 and rs356220 in SNCA decreased the risk for PD in a Chinese population. These candidate polymorphisms were unlikely to be the causes of SALS and MSA in this population.