Overexpression of Synaptopodin increases the number of spine apparatuses and active synapses of dentate granule cells.

Synaptopodin (SP) is a plasticity-related actin-modulating protein found in a subpopulation of mature cortical spines. Its presence within spines is essential for the formation of the spine apparatus (SA) organelle, a local calcium store. SP has been functionally linked to enhanced synaptic strength and long-term spine stability, i.e., indicators of potentiated synapses involved in memory trace formation. As SP is transcriptionally upregulated under contextual learning conditions in dentate granule cells (GCs), we generated a gain-of-function mouse model (Cyan Fluorescent Protein-SP transgenic mouse; CSPtg) with a ~threefold overexpression of SP to study the cellular effects of an increased availability of SP in GCs. Like the wild-type (WT) protein, transgenic SP was sorted to spines and rescued the SA organelle in GCs of SP-deficient mice. In CSPtg mice, SP was found in twice as many spines compared to WT. At the ultrastructural level, a ~threefold increase in the number of SAs was observed. Other structural properties, including spine density, average spine head size, and average SP puncta size, were not significantly altered. Patch-clamp recordings of GCs in CSPtg mice revealed an ~ 3.5-fold increase in miniature excitatory postsynaptic current frequency, indicating that the increased expression of SP/SA in spines resulted in a larger number of active synapses. Thus, higher levels of SP in GCs may increase the number of synapses available for strengthening and plasticity. This could enhance the ability of GCs to be recruited into new ensembles under contextual learning conditions.