A Christianson syndrome-linked deletion mutation (Δ287ES288) in SLC9A6 impairs hippocampal neuronal plasticity

• Published in: Neurobiology of disease Vol. 130; p. 104490
• Main Authors: Gao, Andy Y.L., Ilie, Alina, Chang, Philip K.Y., Orlowski, John, McKinney, R. Anne
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-10-2019; Elsevier
• Subjects: AMPA receptors; Animals; Ataxia - genetics; Christianson syndrome; Dendritic Spines - metabolism; Dendritic Spines - pathology; Endosomes; Epilepsy - genetics; Genetic Diseases, X-Linked - genetics; Hippocampus - metabolism; Hippocampus - pathology; Intellectual Disability - genetics; Mice; Microcephaly - genetics; Mutation; Neuronal Plasticity - genetics; Ocular Motility Disorders - genetics; Organellar pH; Plasticity; Protein Transport - genetics; Receptors, AMPA - metabolism; SLC9A6/NHE6; Sodium-Hydrogen Exchangers - genetics; X-linked intellectual disability; APP; mEPSC; UBE3A; Organellar pH; FGF1; AP-1; EEA1; Aβ; LAMP1; FBS; leu; PSD; SHV; AD; Stx-13; AF-Tfn; EGTA; BACE1; Arc; mCh; NMDAR; MES; PCR; SEP; LRP1; gly-ChemLTP; EGFP; nACSF; ΔES; DMEM; ANOVA; sGluA1; Plasticity; HEK-293; WT; SLC9A6/NHE6; AP2; KO; BDNF; tdT; Christianson syndrome; X-linked intellectual disability; AMPAR; CS; DIV; (RS)-CPP; TrkB; PD; NHE6/SLC9A6; DNA; HEPES; HA; AMPA receptors; HBSS; Endosomes
• ISSN: 0969-9961; 1095-953X
• DOI: 10.1016/j.nbd.2019.104490

Christianson Syndrome is a rare but increasingly diagnosed X-linked intellectual disability disorder that arises from mutations in SLC9A6/NHE6, a pH-regulating transporter that localizes to early and recycling endosomes. We have recently reported that one of the originally identified disease-causing mutations in NHE6 (p.E287-S288del, or ΔES) resulted in a loss of its pH regulatory function. However, the impact of this mutation upon neuronal synapse formation and plasticity is unknown. Here, we investigate the consequences of the ΔES mutant upon mouse hippocampal pyramidal neurons by expressing a fluorescently-labeled ΔES NHE6 construct into primary hippocampal neurons. Neurons expressing the ΔES mutant showed significant reductions in mature dendritic spine density with a concurrent increase in immature filopodia. Furthermore, compared to wild-type (WT), ΔES-containing endosomes are redirected away from early and recycling endosomes toward lysosomes. In parallel, the ΔES mutant reduced the trafficking of glutamatergic AMPA receptors to excitatory synapses and increased their accumulation within lysosomes for potential degradation. Upon long-term potentiation (LTP), neurons expressing ΔES failed to undergo significant structural and functional changes as observed in controls and WT transfectants. Interestingly, synapse density and LTP-induced synaptic remodeling in ΔES-expressing neurons were partially restored by bafilomycin, a vesicular alkalinisation agent, or by leupeptin, an inhibitor of lysosomal proteolytic degradation. Overall, our results demonstrate that the ∆ES mutation attenuates synapse density and structural and functional plasticity in hippocampal neurons. These deficits may be partially due to the mistargeting of AMPA receptors and other cargos to lysosomes, thereby preventing their trafficking during synaptic remodeling. This mechanism may contribute to the cognitive learning deficits observed in patients with Christianson Syndrome and suggests a potential therapeutic strategy for treatment.
•A deletion mutation in NHE6 (ΔES) reduces dendritic spines in hippocampal neurons.•NHE6 ΔES is mislocalized and increases AMPA receptor trafficking to lysosomes.•The ΔES mutant impairs structural and functional remodeling after induction of LTP.•Lysosomal inhibition partially restores spine density and plasticity in ΔES-expressing cells.