The Calcium Connection: Explaining Motor Neuron Vulnerability in ALS

ALS is a severe neuromuscular disease classically characterized by the progressive loss of motor neurons, leading to incremental muscle weakness and eventually death. Current treatment options for ALS have proven to have limited effect, merely delaying the progression of symptoms and prolonging patient survival. This motor neuron subtype-related differential vulnerability has been linked to neuron excitability, metabolism, and protein aggregation. Calcium dysregulation, which serves as an important second messenger in neural signaling pathways, has been implicated in each of these mechanisms and represents a potential target for therapeutic intervention. Armed with cutting-edge tools for visualizing and recording calcium transients in vivo, ALS researchers have delved deeper into the role of calcium dysregulation in disease in recent years. Vulnerable motor neuron populations display an excess of calcium-permeable ion channels together with reduced expression of calcium-binding proteins, generating a cellular environment primed for excitotoxic stress. Loss of inhibitory synaptic input further heightens susceptibility to calcium overload. Paradoxically, some evidence suggests that elevated neuronal activity can exert neuroprotective effects, highlighting the complexity of activity-dependent calcium signaling in ALS. Additionally, ALS-related toxic protein accumulation disrupts calcium homeostasis, contributing to endoplasmic reticulum stress and mitochondrial dysfunction. Emerging data indicate that calcium dysregulation impairs neuron-glia communication, amplifying neuroinflammation and accelerating disease progression. This review aims to synthesize current evidence on how calcium imbalance contributes to motor neuron vulnerability and degeneration in ALS. By exploring the cellular, synaptic, and network-level mechanisms of calcium dysregulation in ALS, the review examines its interplay with mitochondrial and ER stress and explores its impact on neuron-glia interactions with the aim of synthesizing key mechanistic insights into the disease pathogenesis and therapeutic targets.