HSDL2 Suppresses Epileptic Seizures Through Phosphorylation‐Dependent Modulation of the PSD95 ‐ NMDAR Signaling Axis

ABSTRACT Background Temporal lobe epilepsy (TLE) is characterized by synaptic dysfunction for which targeted therapies are lacking. Hydroxysteroid dehydrogenase‐like 2 (HSDL2) was previously identified as a potential regulator in TLE, but its precise functional and mechanistic role remained unexplored. Methods We compared HSDL2 protein expression in cortical tissues from patients with drug‐resistant TLE and a kainic acid (KA)‐induced mouse model via western blotting. Cellular localization was determined by immunofluorescence co‐staining with neuronal (NeuN and PSD95), astrocytic (GFAP), and microglial (IBA1) markers. Adeno‐associated virus (AAV) vectors were used to overexpress or knock down HSDL2 in the mouse hippocampus, followed by behavioral seizure assessments using pentylenetetrazol (PTZ) and chronic monitoring of spontaneous recurrent seizures (SRS). Underlying mechanisms were investigated through protein–protein interaction, patch‐clamp electrophysiology, and quantitative co‐immunoprecipitation. Results HSDL2 was significantly upregulated in both human TLE foci and the KA‐induced epileptic mouse brain. It was localized to both neurons and astrocytes. In vivo, HSDL2 overexpression prolonged the latency to PTZ‐induced seizures and reduced SRS frequency, whereas its knockdown exacerbated seizure severity and duration. Mechanistically, HSDL2 enhanced the membrane localization of postsynaptic density protein 95 (PSD95) and promoted its phosphorylation. This modification disrupted the physical interaction between PSD95 and the N‐methyl‐D‐aspartate receptor (NMDAR) NR2B and NR2A subunits, leading to a reduction in NMDAR‐mediated synaptic currents and neuronal hyperexcitability. Conclusions Our findings identify HSDL2 as a novel endogenous antiseizure protein that confers protection in epilepsy by modulating synaptic excitability. Specifically, HSDL2 regulates the PSD95‐NMDAR complex through post‐translational modification of PSD95, thereby inhibiting excessive NMDAR activity. Its therapeutic modulation may offer a strategy for drug development in TLE.