TNF-mediated hilar interneuron loss and aberrant granule cell migration are associated with chronic cognitive deficits following TBI

Chronic morbidities, including cognitive impairment, are a common consequence of traumatic brain injury (TBI), with millions currently living with permanent TBI-related disabilities. Recent work has indicated that altered cellular architecture in the dentate gyrus (DG) may play a significant role in the development of chronic cognitive impairment and excitotoxicity. However, current understanding of the temporal progression of these pathological changes in the context of neuroinflammation and chronic cognitive outcomes is limited. This study characterized temporospatial changes in the hilar region of the DG, showing that the population of reelin- and somatostatin-expressing inhibitory interneurons was significantly reduced as early as 7 days post-injury (dpi), and that aberrant migration of excitatory granule cells occurs gradually in the weeks to months following injury. These findings coincided with upregulation of monocyte/macrophage-associated inflammatory mediators, including MIP-1β, MIG, MCP-1, and TNF-α at 7 days dpi, with differential cytokine regulation persisting 120 dpi. Injury was associated with the development of chronic spatial memory impairment and reduced risk-assessment behavior, with a transient reduction in spontaneous anxiety. TNFR1 and TNFR2 were differentially expressed in inhibitory neurons, further implicating TNF-signaling as a driver of hilar neuron loss. Furthermore, systemic administration of anti-TNF-α monoclonal antibody induced significant neuroprotection, attenuated pro-inflammatory mediators, and hilar interneuron loss. These findings suggest that TNF-TNFR signaling plays a crucial role in driving hilar interneuron loss and aberrant granule cell migration, which, in turn, may contribute to the development of excitotoxicity and chronic cognitive deficits.