Estela Area-Gomez

Upregulated function of mitochondria‐associated ER membranes in Alzheimer disease

Alzheimer disease (AD) is associated with aberrant processing of the amyloid precursor protein (APP) by γ‐secretase, via an unknown mechanism. We recently showed that presenilin‐1 and ‐2, the catalytic components of γ‐secretase, and γ‐secretase activity itself, are highly enriched in a subcompartment of the endoplasmic reticulum (ER) that is physically and biochemically connected to mitochondria, called mitochondria‐associated ER membranes (MAMs). We now show that MAM function and ER–mitochondrial communication—as measured by cholesteryl ester and phospholipid synthesis, respectively—are increased significantly in presenilin‐mutant cells and in fibroblasts from patients with both the familial and sporadic forms of AD. We also show that MAM is an intracellular detergent‐resistant lipid raft (LR)‐like domain, consistent with the known presence of presenilins and γ‐secretase activity in rafts. These findings may help explain not only the aberrant APP processing but also a number of other biochemical features of AD, including altered lipid metabolism and calcium homeostasis. We propose that upregulated MAM function at the ER–mitochondrial interface, and increased cross‐talk between these two organelles, may play a hitherto unrecognized role in the pathogenesis of AD.

Presenilins Are Enriched in Endoplasmic Reticulum Membranes Associated with Mitochondria

Presenilin-1 (PS1) and -2 (PS2), which when mutated cause familial Alzheimer disease, have been localized to numerous compartments of the cell, including the endoplasmic reticulum, Golgi, nuclear envelope, endosomes, lysosomes, the plasma membrane, and mitochondria. Using three complementary approaches, subcellular fractionation, gamma-secretase activity assays, and immunocytochemistry, we show that presenilins are highly enriched in a subcompartment of the endoplasmic reticulum that is associated with mitochondria and that forms a physical bridge between the two organelles, called endoplasmic reticulum-mitochondria-associated membranes. A localization of PS1 and PS2 in mitochondria-associated membranes may help reconcile the disparate hypotheses regarding the pathogenesis of Alzheimer disease and may explain many seemingly unrelated features of this devastating neurodegenerative disorder.

α-Synuclein is localized to mitochondria-associated ER membranes.

Familial Parkinson disease is associated with mutations in α-synuclein (α-syn), a presynaptic protein that has been localized not only to the cytosol, but also to mitochondria. We report here that wild-type α-syn from cell lines, and brain tissue from humans and mice, is present not in mitochondria but rather in mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a structurally and functionally distinct subdomain of the ER. Remarkably, we found that pathogenic point mutations in human α-syn result in its reduced association with MAM, coincident with a lower degree of apposition of ER with mitochondria, a decrease in MAM function, and an increase in mitochondrial fragmentation compared with wild-type. Although overexpression of wild-type α-syn in mutant α-syn-expressing cells reverted the fragmentation phenotype, neither overexpression of the mitochondrial fusion/MAM-tethering protein MFN2 nor inhibition/ablation of the mitochondrial fission protein DRP1 was able to do so, implying that α-syn operates downstream of the mitochondrial fusion/fission machinery. These novel results indicate that wild-type α-syn localizes to the MAM and modulates mitochondrial morphology, and that these behaviors are impaired by pathogenic mutations in α-syn. We believe that our results have far-reaching implications for both our understanding of α-syn biology and the treatment of synucleinopathies.

Is Alzheimer's disease a disorder of mitochondria-associated membranes?

The subcellular localization of presenilin-1 (PS1) and presenilin-2 (PS2), two proteins that, when mutated, cause familial Alzheimers disease (AD), is controversial. We have discovered that mitochondria-associated membranes (MAM) - a specialized subcompartment of the endoplasmic reticulum (ER) involved in lipid metabolism and calcium homeostasis that physically connects ER to mitochondria - is the predominant subcellular location for PS1 and PS2, and for gamma-secretase activity. We hypothesize that presenilins play a role in maintaining MAM function, and that not only altered amyloid-beta levels and hyperphosphorylated tau, but also many other features of AD (e.g., altered phospholipid and cholesterol metabolism, aberrant calcium homeostasis, and abnormal mitochondrial dynamics) result from compromised MAM function. The localization of presenilins and gamma-secretase in MAM may help reconcile disparate ideas regarding the pathogenesis of AD, under a unifying hypothesis that could explain many features of both sporadic and familial AD, thereby taking AD research in a new and fruitful direction.

ApoE4 upregulates the activity of mitochondria‐associated ER membranes

In addition to the appearance of senile plaques and neurofibrillary tangles, Alzheimers disease (AD) is characterized by aberrant lipid metabolism and early mitochondrial dysfunction. We recently showed that there was increased functionality of mitochondria‐associated endoplasmic reticulum (ER) membranes (MAM), a subdomain of the ER involved in lipid and cholesterol homeostasis, in presenilin‐deficient cells and in fibroblasts from familial and sporadic AD patients. Individuals carrying the ε4 allele of apolipoprotein E (ApoE4) are at increased risk for developing AD compared to those carrying ApoE3. While the reason for this increased risk is unknown, we hypothesized that it might be associated with elevated MAM function. Using an astrocyte‐conditioned media (ACM) model, we now show that ER–mitochondrial communication and MAM function—as measured by the synthesis of phospholipids and of cholesteryl esters, respectively—are increased significantly in cells treated with ApoE4‐containing ACM as compared to those treated with ApoE3‐containing ACM. Notably, this effect was seen with lipoprotein‐enriched preparations, but not with lipid‐free ApoE protein. These data are consistent with a role of upregulated MAM function in the pathogenesis of AD and may help explain, in part, the contribution of ApoE4 as a risk factor in the disease.

On the Pathogenesis of Alzheimer's Disease: The MAM Hypothesis

The pathogenesis of Alzheimers disease (AD) is currently unclear and is the subject of much debate. The most widely accepted hypothesis designed to explain AD pathogenesis is the amyloid cascade, which invokes the accumulation of extracellular plaques and intracellular tangles as playing a fundamental role in the course and progression of the disease. However, besides plaques and tangles, other biochemical and morphological features are also present in AD, often manifesting early in the course of the disease before the accumulation of plaques and tangles. These include altered calcium, cholesterol, and phospholipid metabolism; altered mitochondrial dynamics; and reduced bioenergetic function. Notably, these other features of AD are associated with functions localized to a subdomain of the endoplasmic reticulum (ER), known as mitochondria‐associated ER membranes (MAMs). The MAM region of the ER is a lipid raft‐like domain closely apposed to mitochondria in such a way that the 2 organelles are able to communicate with each other, both physically and biochemically, thereby facilitating the functions of this region. We have found that MAM‐localized functions are increased significantly in cellular and animal models of AD and in cells from patients with AD in a manner consistent with the biochemical findings noted above. Based on these and other observations, we propose that increased ER‐mitochondrial apposition and perturbed MAM function lie at the heart of AD pathogenesis.—Area‐Gomez, E., Schon, E. A. On the pathogenesis of Alzheimers disease: the MAM hypothesis. FASEB J. 31,864–867 (2017). www.fasebj.org

Novel subcellular localization for α-synuclein: possible functional consequences

α-synuclein (α-syn) is one of the genes that when mutated or overexpressed causes Parkinson’s Disease (PD). Initially, it was described as a synaptic terminal protein and later was found to be localized at mitochondria. Mitochondria-associated membranes (MAM) have emerged as a central endoplasmic reticulum (ER) subcellular compartments where key functions of the cell occur. These domains, enriched in cholesterol and anionic phospholipids, are where calcium homeostasis, lipid transfer, and cholesterol metabolism are regulated. Some proteins, related to mitochondrial dynamics and function, are also localized to this area. Several neurodegenerative diseases have shown alterations in MAM functions and resident proteins, including Charcot Marie-Tooth and Alzheimer’s disease (AD). We have recently reported that MAM function is downregulated in cell and mouse models of PD expressing pathogenic mutations of α-syn. This review focuses on the possible role of α-syn in these cellular domains and the early pathogenic features of PD that could be explained by α-syn-MAM disturbances.

Assessing the Function of Mitochondria-Associated ER Membranes

Mitochondria-associated membranes or MAMs are specific regions within the endoplasmic reticulum in close apposition to mitochondria. These contacts between both organelles are involved in the regulation of several cellular functions such as the import of phosphatidylserine into mitochondria from the ER for decarboxylation to phosphatidylethanolamine, cholesterol esterification, calcium signaling, mitochondrial shape and motility, autophagy, and apoptosis. Recently, MAM alterations have been described to underlie some neurodegenerative diseases, including Alzheimers disease. In this chapter, we describe and discuss some of the methods to isolate and assay this interesting cellular region.

Mitochondrial Genetics and Disease

Mitochondrial disease resulting in reduced bioenergetic output can be due to mutations in either nuclear DNA–encoded or mitochondrial DNA–encoded gene products. We summarize some of the underlying principles of mitochondrial genetics that impact the diagnosis and pathogenesis of mitochondrial disorders. In addition, we present a brief overview of a new frontier in the field, namely, mitochondrial “dynamics,” which controls organellar fusion, fission, trafficking, and positioning, and exerts mitochondrial “quality control” by maintaining organellar integrity and viability. Analysis of mutations in gene products associated with this latter area has opened up new vistas in the study of disorders associated with compromised energy production.

Increased localization of APP-C99 in mitochondria-associated ER membranes causes mitochondrial dysfunction in Alzheimer disease

In the amyloidogenic pathway associated with Alzheimer disease (AD), the amyloid precursor protein (APP) is cleaved by β‐secretase to generate a 99‐aa C‐terminal fragment (C99) that is then cleaved by γ‐secretase to generate the β‐amyloid (Aβ) found in senile plaques. In previous reports, we and others have shown that γ‐secretase activity is enriched in mitochondria‐associated endoplasmic reticulum (ER) membranes (MAM) and that ER–mitochondrial connectivity and MAM function are upregulated in AD. We now show that C99, in addition to its localization in endosomes, can also be found in MAM, where it is normally processed rapidly by γ‐secretase. In cell models of AD, however, the concentration of unprocessed C99 increases in MAM regions, resulting in elevated sphingolipid turnover and an altered lipid composition of both MAM and mitochondrial membranes. In turn, this change in mitochondrial membrane composition interferes with the proper assembly and activity of mitochondrial respiratory supercomplexes, thereby likely contributing to the bioenergetic defects characteristic of AD.