Adult-Onset Deletion of Cdkl5 in Forebrain Glutamatergic Neurons Impairs Synaptic Integrity and Behavior in Mice

Cyclin-dependent kinase-like 5 (CDKL5) deficiency disorder (CDD) is a severe X-linked neurodevelopmental condition characterized by early-onset, intractable epilepsy, motor and cognitive impairment, and autistic-like features. Although constitutive Cdkl5 knock-out (KO) models have established the importance of CDKL5 during early brain develop-ment, its role in the mature brain remains poorly defined. Here, we used an inducible, conditional KO model in which Cdkl5 is selectively deleted from forebrain glutamatergic neurons in adult mice to investigate the postdevelopmental functions of CDKL5. Tamoxi-fen-induced Cdkl5 deletion in adult Cdkl5flox/Y(Cre+) males led to prominent behavioral impairments, including deficits in motor coordination, reduced sociability, and impaired hippocampus-dependent spatial memory, while behavioral features such as hyperactivity and stereotypic jumping, typically present in germline KOs, were absent. Sensory func-tions, including olfaction and pain perception, were also preserved. At the cellular level, the loss of Cdkl5 resulted in a marked reduction of excitatory synapse density in the cortex and hippocampus, accompanied by increased numbers of immature dendritic spines and decreased mature spines. Neuronal loss in the hippocampal CA1 region and selective mi-croglial activation in the cortex were also observed. These alterations closely resemble those seen in constitutive KO models, underscoring the ongoing requirement for CDKL5 expression in excitatory neurons for maintaining synaptic integrity and neuronal homeo-stasis in the adult brain. This study underscores the importance of temporally controlled models for investigating the mechanisms underlying CDD pathophysiology in the adult brain.