Alteration of scaffold: Possible role of MACF1 in Alzheimer's disease pathogenesis

Differential splicing of neuronal genes in a Trem2*R47H mouse model mimics alterations associated with Alzheimer's disease

BACKGROUND

Molecular characterization of late-onset Alzheimer's disease (LOAD), the leading cause of age-related dementia, has revealed transcripts, proteins, and pathway alterations associated with disease. Assessing these postmortem signatures of LOAD in experimental model systems can further elucidate their relevance to disease origins and progression. Model organisms engineered with human genetic factors further link these signatures to disease-associated variants, especially when studies are designed to leverage homology across species. Here we assess differential gene splicing patterns in aging mouse models carrying humanized APOE4 and/or the Trem2*R47H variant on a C57BL/6J background. We performed a differential expression of gene (DEG) and differential splicing analyses on whole brain transcriptomes at multiple ages. To better understand the difference between differentially expressed and differentially spliced genes, we evaluated enrichment of KEGG pathways and cell-type specific gene signatures of the adult brain from each alteration type. To determine LOAD relevance, we compared differential splicing results from mouse models with multiple human AD splicing studies.

RESULTS

We found that differentially expressed genes in Trem2*R47H mice were significantly enriched in multiple AD-related pathways, including immune response, osteoclast differentiation, and metabolism, whereas differentially spliced genes were enriched for neuronal related functions, including GABAergic synapse and glutamatergic synapse. These results were reinforced by the enrichment of microglial genes in DEGs and neuronal genes in differentially spliced genes in Trem2*R47H mice. We observed significant overlap between differentially spliced genes in Trem2*R47H mice and brains from human AD subjects. These effects were absent in APOE4 mice and suppressed in APOE4.Trem2*R47H double mutant mice relative to Trem2*R47H mice.

CONCLUSIONS

The cross-species observation that alternative splicing observed in LOAD are present in Trem2*R47H mouse models suggests a novel link between this candidate risk gene and molecular signatures of LOAD in neurons and demonstrates how deep molecular analysis of new genetic models links molecular disease outcomes to a human candidate gene.

Decreased Expression of Protein O-linked Mannose β 1,2-N-Acetylglucosaminyltransferase 1 Contributes to Alzheimer's Disease-like Pathologies

Alzheimer's disease (AD) is pathologically characterized by senile plaques and neurofibrillary tangles composed of β-amyloid peptide (Aβ) and tau hyperphosphorylation, respectively. Mannosylation, a particular type of post-translational modification, may be involved in the pathogenesis of AD. However, its underlying mechanism remains unclear. Protein O-linked mannose β 1,2-N-acetylglucosaminyltransferase 1 (POMGnT1) catalyzes the formation of the N-acetylglucosamine β-1,2-Man linkage of O-mannosylglycan, which can increase the protein post-translational mannosylation level. The defective POMGnT1 gene leads to the hypomannosylation of proteins, which may cause cognitive decline in aged people. This study aimed to investigate whether POMGnT1 participated in the pathogenesis of AD and explore its underlying role using AD mouse and cell models. In this study, the expression of POMGnT1 was measured in AD models [β-amyloid precursor protein (APP)/presenilin-1 (PS1) transgenic mice, an AD mouse model; N2a cells stably transfected with Swedish mutant APP (N2a/APP), an AD cell model]. The results revealed that the expression of POMGnT1 decreased in AD mouse and cell models. Additionally, POMGnT1-overexpressing N2a/APP cells were built by retroviral transfection. POMGnT1 overexpression may lower Aβ levels by reducing APP production and downregulating β-and γ-secretase activities. It also promoted clearance of Aβ by upregulating insulin-degrading enzymes and ameliorated tau hyperphosphorylation. Hence, it was concluded that POMGnT1 was involved in the pathogenic process of AD. The decreased expression of POMGnT1 contributes to AD-like pathologies.

Strength Through Unity: The Power of the Mega-Scaffold MACF1

The tight coordination of diverse cytoskeleton elements is required to support several dynamic cellular processes involved in development and tissue homeostasis. The spectraplakin-family of proteins are composed of multiple domains that provide versatility to connect different components of the cytoskeleton, including the actin microfilaments, microtubules and intermediates filaments. Spectraplakins act as orchestrators of precise cytoskeletal dynamic events. In this review, we focus on the prototypical spectraplakin MACF1, a protein scaffold of more than 700 kDa that coordinates the crosstalk between actin microfilaments and microtubules to support cell-cell connections, cell polarity, vesicular transport, proliferation, and cell migration. We will review over two decades of research aimed at understanding the molecular, physiological and pathological roles of MACF1, with a focus on its roles in developmental and cancer. A deeper understanding of MACF1 is currently limited by technical challenges associated to the study of such a large protein and we discuss ideas to advance the field.

Much More Than a Scaffold: Cytoskeletal Proteins in Neurological Disorders

Recent observations related to the structure of the cytoskeleton in neurons and novel cytoskeletal abnormalities involved in the pathophysiology of some neurological diseases are changing our view on the function of the cytoskeletal proteins in the nervous system. These efforts allow a better understanding of the molecular mechanisms underlying neurological diseases and allow us to see beyond our current knowledge for the development of new treatments. The neuronal cytoskeleton can be described as an organelle formed by the three-dimensional lattice of the three main families of filaments: actin filaments, microtubules, and neurofilaments. This organelle organizes well-defined structures within neurons (cell bodies and axons), which allow their proper development and function through life. Here, we will provide an overview of both the basic and novel concepts related to those cytoskeletal proteins, which are emerging as potential targets in the study of the pathophysiological mechanisms underlying neurological disorders.