Expression of synaptopodin, an actin-associated protein, in the rat hippocampus after limbic epilepsy

Synaptopodin, a 100 kD protein, associated with the actin cytoskeleton of the postsynaptic density and dendritic spines, is thought to play a role in modulating actin-based shape and motility of dendritic spines during formation or elimination of synaptic contacts. Temporal lobe epilepsy in humans and in rats shows neuronal damage, aberrant sprouting of hippocampal mossy fibers and subsequent synaptic remodeling processes. Using kainic acid (KA) induced epilepsy in rats, the postictal hippocampal expression of synaptopodin was analyzed by in situ hybridization (ISH) and immunohistochemistry. Sprouting of mossy fibers was visualized by a modified Timm's staining. ISH showed elevated levels of Synaptopodin mRNA in perikarya of CA3 principal neurons, dentate granule cells and in surviving hilar neurons these levels persisted up to 8 weeks after seizure induction. Synaptopodin immunoreactivity in the dendritic layers of CA3, in the hilus and in the inner molecular layer of the dentate gyrus (DG) was initially reduced. Eight weeks after KA treatment Synaptopodin protein expression returned to control levels in dendritic layers of CA3 and in the entire molecular layer of the DG. The recovery of protein expression was accompanied by simultaneous supra- and infragranular mossy fiber sprouting. Postictal upregulation of Synaptopodin mRNA levels in target cell populations of limbic epilepsy-elicited damage and subsequent Synaptopodin protein expression largely co-localized with remodeling processes as demonstrated by mossy fiber sprouting. It may thus represent a novel postsynaptic molecular correlate of hippocampal neuroplasticity.