Presenilins Are Enriched in Endoplasmic Reticulum Membranes Associated with Mitochondria

Abstract

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, ␥-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. Alzheimer disease (AD) is a late onset neurodegenerative disorder characterized by progressive neuronal loss, especially in the cortex and the hippocampus. 1 The two main histopathological hallmarks of AD are the accumulation of extracellular neuritic plaques, consisting predominantly of ␤-amyloid (A␤), and of neurofibrillary tangles, consisting mainly of hyperphosphorylated forms of the microtubule-associated protein tau. 1The vast majority of AD is sporadic, but mutations in amyloid precursor protein (APP), presenilin-1 (PS1), and presenilin-2 (PS2) have been identified in the rarer familial form, which is similar to sporadic AD but has an earlier age of onset.PS1 and PS2 are aspartyl proteases that cleave their substrates within transmembrane regions. The active forms of PS1 and PS2 are N-and C-terminal fragments, which are produced by cleavage of full-length presenilin in its "loop" domain.2 PS1 and PS2 are components of the ␥-secretase complex that processes a number of plasma-membrane proteins, including Notch, Jagged, E-cadherin, and, most relevant to AD, APP. The ␥-secretase complex also contains three other structural subunits: APH1, nicastrin (also called APH2), and presenilin enhancer protein 2.2 Following cleavage of APP by ␤-secretase, ␥-secretase cleaves the ϳ100-aa C-terminal "␤-stub" to release small amyloidogenic fragments, 40-and 42-aa in length (A␤40 and A␤42), that have been implicated in the pathogenesis of AD, as well as a ϳ60-aa APP intracellular domain.

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

“…In previous reports, we showed that γ‐secretase activity is localized in MAM (Area‐Gomez et al , 2009) and that alterations in γ‐secretase activity result in the upregulation of MAM function and in increased ER–mitochondria apposition (Area‐Gomez et al , 2012). We now show that the γ‐secretase substrate C99, in addition to its endosomal localization, is also present in MAM domains.…”

“…Consistent with the data of others (Das et al , 2016), FL‐APP and BACE1 co‐migrated partially with a marker for endosomes (Rab7), but not with lysosomal, ER‐intermediate, or MAM markers (Fig 2C). Similarly, the APP‐CTFs C83 and C99 co‐migrated with endosomal and lysosomal markers (Rab5, Rab7, and LAMP‐2) (Haass et al , 2012; Das et al , 2016), whereas PS1 co‐migrated with MAM markers, such as FACL4 (Area‐Gomez et al , 2009; Newman et al , 2014; Schreiner et al , 2015). We reasoned that the difficulty in seeing APP‐CTFs and PS1 together was probably due to the rapid cleavage of the CTFs by γ‐secretase once both are in the same compartment.…”

“…We and others have shown that presenilins and γ‐secretase activity localize to MAM (Area‐Gomez et al , 2009; Newman et al , 2014; Schreiner et al , 2015). Moreover, MAM functionality (Area‐Gomez et al , 2012) and ER–mitochondrial apposition (Area‐Gomez et al , 2012; Hedskog et al , 2013) are increased in AD.…”

“…We propose that, while the majority of C99 resides in endosomes, C99 can traffic to MAM regions in the ER, where it is cleaved rapidly by γ‐secretase to produce Aβ and AICD (Area‐Gomez et al , 2009; Schreiner et al , 2015). We note that although the mechanism by which C99 translocates to ER‐MAM is unknown, recent work has demonstrated the existence of ER‐endosome contacts (Rowland et al , 2014) where lipid and protein exchange may occur (Wilhelm et al , 2017).…”

ApoE4 upregulates the activity of mitochondria‐associated ER membranes

“…We reported previously that PS1, PS2, and γ‐secretase activities are highly enriched in the MAM 10 and that ER–mitochondrial communication and MAM function, including phospholipid and cholesteryl ester synthesis, are increased dramatically in presenilin‐mutant and presenilin‐deficient cells and in cells from AD patients 7. Both lines of evidence support the view that PS1 and PS2, in addition to generating Aβ, are negative regulators of MAM function 7 and that these alterations play a critical role in the pathogenesis of the disease (the “MAM hypothesis”) 11, 12.…”