Putative long range mossy fiber sprouting and regional cytochrome c oxidase alteration in the hippocampus of patients with mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is known as a distributed network disorder, and regional neuroanatomical changes may significantly contribute to aberrant network formation. The major pathologies of MTLE include neuronal loss in the dentate hilus, CA3 and CA1 regions, and mossy fiber (MF) sprouting into the inner molecular layer (iML). The latter forms aberrant excitatory circuities that are considered to facilitate recurrent seizures. The subiculum is relatively preserved during hippocampal sclerosis but has been thought to play a critical role in the synchronization and propagation of epileptic activity especially to extrahippocampal brain regions. We recently identified a distinct expression of alpha-smooth muscle actin (αSMA) at the MF terminals in human hippocampus. This prompted us to explore MF sprouting in resected hippocampi (n = 20) from patients with MTLE relative to postmortem control (n = 20) using αSMA along with reference markers for pathological cross-validation. Immunolabeling of neuron-specific nuclear antigen and sortilin showed neuronal loss in the hilus/CA3 and CA1 and granule cell dispersion in all resected hippocampi relative to control. Immunolabeling of αSMA, zinc transporter 3 and β-secretase 1 displayed a widening of the iML with enhanced staining intensity and a fibrous band extending from CA2, CA1 towards subiculum in the resected hippocampi. Cytochrome c oxidase immunolabeling was increased in the iML and subiculum in the MTLE group. Taking together, the current findings suggest the possibility of long-range MF sprouting and regionally occurred hypermetabolic state in the hippocampal formation of patients with drug resistant MTLE.