Spiral ligament dysfunction and endocochlear potential loss drive hearing impairment in Niemann–Pick C1 mice

Sensorineural hearing loss is increasingly recognized in Niemann–Pick disease type C (NPC), but the underlying cochlear lesion remains undefined. While prior work emphasized hair-cell (HC) involvement, whether auditory dysfunction instead arises from lateral-wall failure and endocochlear potential (EP) decline is unknown. Npc1 −/− mice and littermate controls underwent auditory function test and electrophysiological recordings at postnatal day (P) 35 and P63. Cochlear cytoarchitecture was evaluated using immunohistochemistry and transmission electron microscopy (TEM). To probe cell-type susceptibility, NPC1 was inhibited in Spiral ligament (SLi)-like fibrocytes, HC-like HEI-OC1 cells in vitro. Npc1 −/− mice showed elevated low-frequency auditory brainstem response (ABR) thresholds at P35, progressing to pan-frequency impairment and prolonged ABR wave IV–V latencies by P63. HCs, stereocilia bundles, and spiral ganglion cells were preserved. In contrast, EP was markedly reduced. Na⁺/K⁺-ATPase α1 and connexin-26 immunolabeling in the SLi decreased significantly without strial thinning, indicating impaired ion recycling and gap-junction coupling. Filipin staining and TEM revealed progressive free-cholesterol accumulation and vacuolar inclusions in SLi fibrocytes and supporting cells, with secondary involvement of HC regions. In vitro, NPC1 inhibition increased cholesterol in SLi-like fibrocytes but not in HEI-OC1 cells. NPC-related hearing loss arises primarily from SLi dysfunction and EP failure, with secondary HC compromise, rather than degeneration. These findings reposition NPC hearing loss as a disorder of cochlear homeostasis and identify EP preservation and correction of cholesterol trafficking as rational therapeutic targets. Early auditory monitoring may improve clinical outcomes. • NPC hearing loss stems from lateral-wall dysfunction, not primary hair-cell damage • Endocochlear potential declines early, driving progressive auditory deficits • SLi fibrocytes accumulate cholesterol, impairing ion recycling and gap-junctions • NPC1 inhibition in SLi fibroblasts, not hair cells, mimics cochlear pathology • Early auditory monitoring may guide interventions and improve clinical outcomes