Sex-dependent effects of Fingolimod on neuronal activity and architecture in a mouse model for CDKL5 deficiency disorder

CDKL5 deficiency disorder (CDD) is a rare X-linked neurodevelopmental disorder characterised by early-onset seizures, intellectual disability, motor impairments and often autistic-like social deficits. Mutations in the CDKL5 gene, highly expressed in the brain, disrupt dendritic development, neuronal structure and synaptic plasticity and neuronal survival. Female heterozygotes show a mosaic pattern of dysfunction with variable severity, whereas males typically present more severe phenotypes. Fingolimod, a sphingosine-1-phosphate receptor modulator, has shown promise in the treatment of related neurodevelopmental diseases by enhancing neuroplasticity and BDNF signalling, yet its efficacy in CDD, particularly its sex-specific effects in rescuing neuronal structure and function remains unexplored. The first part of this study used wild type organotypic hippocampal slice cultures (OHCs) to assess sex-specific effects of Fingolimod on neuronal architecture. Fingolimod increased overall spine density in both male and female OHCs. However, male OHCs exhibited a BDNF-dependent elevation in spine dynamics, while female OHCs showed a BDNF-dependent shift toward an immature spine phenotype. Moreover, field electrophysiology in CDKL5 heterozygous (HET) female mice at 6-8, 10-12 and 16-18 weeks showed an age-dependent modulation of LTP with an impairment at 6-8 weeks, which returned to wild type levels at 10-12 weeks (suggesting compensation), and an elevated LTP by 16-18 weeks. Both male CDKL5 knockout (KO) mice and female CDKL5 HET mice were treated with Fingolimod and both hippocampal CA1 synaptic function and spine density were assessed. In CDKL5 KO males, Fingolimod produced a robust rescue of LTP in the right hemisphere and a slight non-significant increase in spine density. Meanwhile, in CDKL5 HET female mice it led to a partial, non-significant rescue in LTP in both hemispheres and partially reversed maladaptive spine density in both hemispheres. Concurrently, Fingolimod reduced basal synaptic transmission in both sexes, likely via modulation of glutamatergic signalling. The second part characterised hippocampal CA2 synaptic function and social behaviour in both male CDKL5 KO and female CDKL5 HET mice. Male CDKL5 KO mice exhibited stable CA2 LTP across developmental stages, increased anxiety and hyperactivity but unaltered sociability and social novelty, while female CDKL5 HET mice showed unaltered anxiety and social memory with a mild CA2 LTP impairment at 6-8 weeks and a gradual recovery to wild type levels by 16-18 weeks. Taken together these findings demonstrate that Fingolimod modulates hippocampal synaptic plasticity and dendritic architecture in a sex-dependent manner both in healthy and CDKL5 KO mice. The study also uncovers dynamic, age-dependent LTP trajectories in female heterozygous CDKL5 mice, highlighting developmental windows for therapeutic intervention, a novel insight with significant implications for tailoring treatments for CDD.