Emerging evidence suggests that the specialized Mitochondria-associated Endoplasmic Reticulum Contacts (MERCs), biochemically harvested as Mitochondria-Associated ER Membranes, often referred to as MAMs1,2 play a role in several neurodegenerative diseases, including AD3,4. These MAMs are composed of cholesterol-rich lipid raft-like microdomains in the ER and the outer membrane of mitochondria tethered by a series of proteins that create structural and functional diversities among the MAMs5,6,7. The recently coined MAM hypothesis posits that the increase of MAMs leads to enhanced Aβ production and the pathogenic cascade of AD, including neurofibrillary tangle (NFT) formation, calcium dyshomeostasis, and neuroinflammation3,8. About 5%-20% of mitochondria make physical contact with the ER to form MAMs9. The gap width of MAMs is determined by the smooth and rough ER (sER and rER, respectively). The variable gap width between sER-mitochondria (10-50 nm) and rER-mitochondria (50-80 nm) suggests that the gap width of MAMs has a long spectrum that ranges between tight (~10 nm) to loose (~80 nm)10,11,12,13. MAM gap width determines MAM functions, such as calcium homeostasis and lipid transport1,14. A recent report has shown that the MAMs formed between tightly (~10 nm) connected ER and mitochondria, called full MAMs, are apoptotic. In contrast, MAMs formed between loosely connected (~25 nm) ER and mitochondria, termed defective or medium MAMs, are anti-apoptotic14,15,16. Stabilization of MAMs with a gap width of 6 nm ± 1 nm increased Aβ generation from a novel 3-dimensional (3D) neural culture model of AD. In contrast, the stabilization of MAMs with a gap width 24 nm ± 3 nm has no effect on Aβ generation17. This finding suggests for the first time that regulating the degree of MAM stabilization, but not destabilizing MAMs, is the key to regulating Aβ generation. An attempt to completely destabilize MAMs may have unwanted consequences because MAMs maintain several cellular events critical for cell survival12.
The modulation of MAMs is an emerging area of research with potential implications for various disorders, including cancer, metabolic disorders, and neurodegenerative diseases18. Despite the availability of many MAM modulators, no major attempt has so far been taken to test their abilities to destabilize MAMs and lower AD pathology, primarily because the structural diversities of MAMs make them a highly complex system to target for drug discovery. But, the newly developed structural systems pharmacology, which considers the specific properties of the drug targets and their environment18,19 should overcome the difficulties and develop highly potent drugs targeting MAMs or MAM-associated proteins in AD. However, the search for an effective modulator of MAM stabilization requires methods to quantify the degree of MAM stabilization precisely. Traditional techniques like electron microscopy (EM) or super-resolution microscopy have limitations in determining MAM stabilization. Overcoming these challenges would likely require the development of novel, more dynamic imaging techniques or biochemical assays that can provide quantitative measures of MAM stabilization in living cells. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) of primary neurons revealed that the ER tends to form a network around mitochondria likely to limit mitochondrial motility20,21. The disruption of mitochondrial transport systems, either retrograde, anterograde, or both, had a profound impact on synaptic and neuronal function22. Thus, the novel live-cell imaging and kymography-based analysis of axonal velocity of ER-bound mitochondria described here as a metric to quantitatively measure MAM stabilization will facilitate the identification of MAM modulator(s) that can switch the MAM stabilization threshold to one that maintains or possibly lowers as opposed to increases Aβ generation.