Neuronal SEL1L-HRD1 ER-associated degradation is essential for motor function and survival in mice.

Hypomorphic variants in the SEL1L-HRD1 ER-associated degradation (ERAD) complex have been linked to severe neurological syndromes in children, including neurodevelopmental delay, intellectual disability, motor dysfunction, and early death. Despite this association, its physiological importance and underlying mechanisms in neurons remain poorly understood. Here, we show that neuronal SEL1L-HRD1 ERAD is essential for maintaining one-carbon metabolism, motor function, and overall viability. Neuron-specific deletion of Sel1L in mice (Sel1LSynCre) resulted in growth retardation, severe motor impairments, and early mortality by 9 weeks of age - mirroring core clinical features observed in affected patients - despite preserved neuronal numbers and only modest ER stress. Multiomics analyses, including single-nucleus RNA sequencing and metabolomics, revealed significant dysregulation of one-carbon metabolism in ERAD-deficient brains. This included activation of the serine, folate, and methionine pathways, accompanied by elevated levels of S-adenosylmethionine and related metabolites, likely resulting from induction of the integrated stress response. Together, these findings uncover a previously unappreciated role for neuronal SEL1L-HRD1 ERAD in coordinating ER protein quality control with metabolic adaptation, providing insight into the molecular basis of ERAD-related neurodevelopmental disease.