The Integrated Stress Response (ISR) controls cellular protein synthesis through eIF2α phosphorylation, but this hypothesis proposes that chronic ISR activation in ALS motor neurons creates a pathological cascade that specifically disrupts mitochondrial protein homeostasis and bioenergetics. In ALS, chronic eIF2α~P elevation (>0.7 normalized phosphorylation) caused by proteostatic stress from TDP-43/FUS aggregates selectively impairs synthesis of nuclear-encoded mitochondrial proteins, including Complex I subunits (NDUFS1, NDUFS3), Complex IV subunits (COX4I1, COX5A), and mitochondrial import machinery components (TOMM20, TIMM23). This creates a mitochondrial protein import bottleneck where reduced cytosolic synthesis of mitochondrial precursor proteins coincides with ISR-mediated downregulation of import receptors, leading to severe mitochondrial dysfunction in distal axons where energy demands are highest. The mechanistic prediction is that eIF2α~P elevation creates a 60-80% reduction in mitochondrial protein import efficiency, causing ATP depletion, calcium dysregulation, and axonal transport defects that precipitate NMJ denervation.

The Integrated Stress Response (ISR) controls cellular protein synthesis through eIF2α phosphorylation, but this hypothesis proposes that chronic ISR activation in ALS motor neurons creates a pathological cascade that specifically disrupts mitochondrial protein homeostasis and bioenergetics. In ALS, chronic eIF2α~P elevation (>0.7 normalized phosphorylation) caused by proteostatic stress from TDP-43/FUS aggregates selectively impairs synthesis of nuclear-encoded mitochondrial proteins, including Complex I subunits (NDUFS1, NDUFS3), Complex IV subunits (COX4I1, COX5A), and mitochondrial import machinery components (TOMM20, TIMM23). This creates a mitochondrial protein import bottleneck where reduced cytosolic synthesis of mitochondrial precursor proteins coincides with ISR-mediated downregulation of import receptors, leading to severe mitochondrial dysfunction in distal axons where energy demands are highest. The mechanistic prediction is that eIF2α~P elevation creates a 60-80% reduction in mitochondrial protein import efficiency, causing ATP depletion, calcium dysregulation, and axonal transport defects that precipitate NMJ denervation. In SOD1-G93A motor neurons, mitochondrial Complex I activity drops 70% at pre-symptomatic stages, correlating with 2.5-fold eIF2α phosphorylation increase. Proteomic analysis reveals selective depletion of nuclear-encoded mitochondrial proteins while mitochondrial-encoded proteins remain stable. The therapeutic approach targets this mitochondrial-ISR axis: combined eIF2α phosphatase activation (sal003) with mitochondrial protein import enhancers (specific TOMM20 activators) or direct mitochondrial energetics support (nicotinamide riboside, CoQ10 analogs) to restore axonal bioenergetics and preserve motor neuron survival in ALS models.