Mitochondrial Failure May Be the Hidden Common Thread Linking Alzheimer's, Parkinson's, ALS and More

Neurodegenerative diseases are among the fastest-growing public health burdens worldwide. Alzheimer's disease (AD) affects more than 55 million people, Parkinson's disease (PD) over 8.5 million, and multiple sclerosis (MS) about 2.8 million, yet disease-modifying treatments remain scarce. This narrative review argues that secondary mitochondrial dysfunction is a convergent hub across these conditions. It is a downstream consequence of protein aggregates or inflammation, distinct from primary mitochondrial DNA disorders such as MELAS and MERRF.

The authors explain why neurons are so vulnerable. They are post-mitotic, so damaged organelles can't be diluted by cell division. They have very high energy demands for action potentials, synaptic vesicle cycling and axonal transport. Their long axons separate synapses from the soma, where most quality-control machinery sits, and their antioxidant reserves are limited. Mitochondria supply ATP through oxidative phosphorylation. They also synthesize glutamate, GABA and amino acids, buffer calcium, manage reactive oxygen species (ROS), and gate apoptosis through cytochrome c release.

Across PD, AD, Huntington's disease (HD) and ALS, misfolded proteins (α-synuclein, amyloid-β, hyperphosphorylated tau, mutant huntingtin, TDP-43, FUS) directly engage respiratory-chain subunits or assembly factors. Mutant huntingtin also represses PGC-1α-driven transcription of mitochondrial genes. Accumulating mtDNA deletions further reduce respiratory subunits. Oligomeric Aβ and α-synuclein over-activate IP3 receptors at mitochondria-associated membranes (MAMs), driving excess ER-to-mitochondria calcium transfer. Loss of MICU1-MICU2 gatekeeping then allows unregulated uptake, opening the cyclophilin-D-dependent permeability pore and triggering apoptosis. The authors stress that this damage appears first in distal axons and spreads retrogradely, before the cell body dies.

The review compiles quantitative ROS differences. ROS rises about 2.5-3-fold in PD substantia nigra dopaminergic neurons (driven by complex I deficiency), 1.8-2.8-fold in AD hippocampal neurons, and roughly 2-fold in ALS spinal motor neurons. In MS, the ROS burst is largely extrinsic, from activated microglia via NOX enzymes. Antioxidant enzymes (SOD2, catalase, GPx) are induced early as compensation. In later disease, oxidative modification inactivates them, and post-mortem AD and PD brains show significantly reduced activity. The result is a feed-forward cycle of mtDNA, lipid and protein damage.

In PD, pathological oligomeric and fibrillar α-synuclein binds outer-membrane proteins such as VDAC and TOM20. This permeabilizes membranes, inhibits complex I and blocks PINK1/Parkin-dependent mitophagy. Physiological monomeric α-synuclein does not do this. The abstract goes on to cover disease-specific lesions in AD, HD, MS and ALS. It also covers emerging mechanisms (mitocytosis, mitochondrial aminoacyl-tRNA synthetase dysfunction, MAM disruption). The therapeutic strategies it lists are small-molecule regulators, mitophagy activators, biogenesis promoters, gene editing and mitochondrial transplantation.

The main caveats are translational. Candidate drugs often penetrate the blood-brain barrier poorly. The therapeutic window after symptom onset is narrow. Disease heterogeneity complicates precise targeting. This is a narrative rather than systematic review, so study selection and evidence weighting aren't formally standardized. Much of the supporting evidence is preclinical or post-mortem, and the supplied text ends partway through the PD section, so details of the later sections are not covered here.