Gamma motor neurons survive and exacerbate alpha motor neuron degeneration in ALS

Area of Science:

• Neuroscience
• Cellular Biology
• Genetics

Background:

• The selective vulnerability of motor neurons (MNs) in amyotrophic lateral sclerosis (ALS) remains poorly understood at the molecular and cellular levels.
• Distinct MN subtypes exhibit differential susceptibility in ALS pathogenesis.

Purpose of the Study:

• To investigate the cellular basis for selective motor neuron vulnerability in amyotrophic lateral sclerosis (ALS).
• To determine the role of synaptic inputs in motor neuron degeneration in ALS mouse models.

Main Methods:

• Comparative analysis of alpha (α-MNs) and gamma (γ-MNs) in genetically distinct familial ALS mouse models (SOD1, TDP-43, FUS).
• Investigation of synaptic contacts from primary afferent (I_A) fibers onto MNs.
• Assessment of the impact of eliminating I_A synapses on α-MN survival in SOD1 mutant mice.
• Evaluation of targeted γ-MN reduction effects on disease progression in SOD1^G93A mutants.

Main Results:

• Selective degeneration of α-MNs and complete sparing of γ-MNs observed across multiple ALS mouse models.
• γ-MNs, which innervate muscle spindles, lack synaptic contacts from I_A fibers, unlike vulnerable α-MNs.
• Eliminating I_A synapses protected α-MNs in SOD1 mutants, implicating excitatory input in degeneration.
• Reduced I_A activation via γ-MN reduction in SOD1^G93A mutants delayed symptom onset and prolonged lifespan.

Conclusions:

• γ-MN resistance is a conserved feature across various ALS mouse models.
• Synaptic excitation, particularly from I_A fibers, is a critical determinant of MN vulnerability in ALS.
• Targeting I_A input or γ-MNs may offer therapeutic strategies for ALS.