Lack of Cdkl5 Disrupts the Organization of Excitatory and Inhibitory Synapses and Parvalbumin Interneurons in the Primary Visual Cortex

Abstract

Cyclin-dependent kinase-like 5 (CDKL5) mutations are found in severe neurodevelopmental disorders, including the Hanefeld variant of Rett syndrome (RTT; CDKL5 disorder). CDKL5 loss-of-function murine models recapitulate pathological signs of the human disease, such as visual attention deficits and reduced visual acuity. Here we investigated the cellular and synaptic substrates of visual defects by studying the organization of the primary visual cortex (V1) of Cdkl5−/y mice. We found a severe reduction of c-Fos expression in V1 of Cdkl5−/y mutants, suggesting circuit hypoactivity. Glutamatergic presynaptic structures were increased, but postsynaptic PSD-95 and Homer were significantly downregulated in CDKL5 mutants. Interneurons expressing parvalbumin, but not other types of interneuron, had a higher density in mutant V1, and were hyperconnected with pyramidal neurons. Finally, the developmental trajectory of pavalbumin-containing cells was also affected in Cdkl5−/y mice, as revealed by fainter appearance perineuronal nets at the closure of the critical period (CP). The present data reveal an overall disruption of V1 cellular and synaptic organization that may cause a shift in the excitation/inhibition balance likely to underlie the visual deficits characteristic of CDKL5 disorder. Moreover, ablation of CDKL5 is likely to tamper with the mechanisms underlying experience-dependent refinement of cortical circuits during the CP of development.

Deficiency of cyclin-dependent kinase-like 5 causes spontaneous epileptic seizures in neonatal mice

“…However, the protein level of PSD-95, a CDKL5-interacting protein in the postsynaptic density, appeared to be reduced in pups lacking CDKL5 (0.812 ± 0.049 of WT, p < 0.01; Fig. 7A, D), consistent with previous studies in mutant mice and in cultured neurons (Ricciardi et al, 2012;Zhu et al, 2013;Della Sala et al, 2016;Pizzo et al, 2016;Lupori et al, 2019). Thus, KCC2 phosphorylation at S940 appears to be selectively down-regulated in Cdkl5 null pups, especially at the age when they are suffering from spontaneous seizures.…”

“…On the other hand, given that KCC2 has not been identified as a direct substrate of CDKL5 (Baltussen et al, 2018;Munoz et al, 2018), the CDKL5-dependent regulation of pKCC2 might be mediated indirectly through the action of protein kinase C, which has been reported to interact with CDKL5 and phosphorylate KCC2 at S940 (Lee et al, 2007;Wang et al, 2012). Although we did not find any alteration in the expression of proteins related to excitation/inhibition (E/I) balance, such as pKCC2-T1007, NKCC1, GABAA receptor, GAD2, vGAT, vGluT2 and CB1R, in the cortex at P12, other players, such as NMDA receptors, AMPA receptors and parvalbumin, could still play a role in CDKL5-deficiency-induced E/I imbalance (Pizzo et al, 2016;Okuda et al, 2017;Tang et al, 2017;Yennawar et al, 2019).…”

Loss of Mecp2 Causes Atypical Synaptic and Molecular Plasticity of Parvalbumin-Expressing Interneurons Reflecting Rett Syndrome–Like Sensorimotor Defects

“…Presynaptic puncta that contacted immunolabeled cell bodies and those in close juxtaposition to immunolabeled dendrites were identified as presynaptic varicosities by visual examination in three orthogonal planes using Imaris. Puncta immunolabeled for VGLUT1 or VGAT were considered to be in close apposition to either PV- or CR-positive somata or dendrites when there were no black pixels between the pre- and postsynaptic structures, as described in Pizzo et al (2016). The number of immunolabeled presynaptic puncta was counted manually using Imaris, and they were included in the analysis only if present in at least two consecutive optical sections in the 3D z -axis confocal image stack.…”

Molecular and Synaptic Bases of CDKL5 Disorder

“…In the whole‐body knockout mice, there is a significant reduction in spine density and the number of mature spines in dentate gyrus granule cells, hippocampal CA1 pyramidal neurons, and cortical layer V pyramidal neurons (Della Sala et al ., ; Trazzi et al ., ). The expression levels of several postsynaptic markers, such as PSD‐95 and homer were reduced (Pizzo et al ., ). However, in conditional knockout mice in which CDKL5 was ablated specifically from forebrain excitatory neurons, a trend toward an increase in spine density and volume was observed (Tang et al ., ).…”