Assorted dysfunctions of endosomal alkali cation/proton exchanger SLC9A6 variants linked to Christianson syndrome

Abstract

Genetic screening has identified several variants of the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na + , K + )/H + exchanger 6 (NHE6) gene that cause Christianson syndrome, a debilitating X-linked developmental disorder associated with a range of neurological, somatic and behavioral symptoms. Many of these variants cause complete loss of NHE6 expression, but how subtler missense substitutions or nonsense mutations that partially truncate its C-terminal cytoplasmic regulatory domain impair NHE6 activity and endosomal function are poorly understood. Here, we describe the molecular and cellular consequences of six unique mutations located in the N-terminal cytoplasmic segment (A9S), the membrane ion translocation domain (L188P and G383D) and the C-terminal regulatory domain (E547*, R568Q, W570*) of human NHE6 that purportedly cause disease. Using a heterologous NHE6-deficient cell expression system, we show that the biochemical, catalytic, and cellular properties of the A9S and R568Q variants were largely indistinguishable from those of the wildtype transporter which obscured their disease significance. By contrast, the L188P, G383D, E547* and W570* mutants exhibited variable deficiencies in biosynthetic post-translational maturation, membrane sorting, pH homeostasis in recycling endosomes, and cargo trafficking, and also triggered apoptosis. These findings https://www.jbc.org/cgi/

A guide to plasma membrane solute carrier proteins

“…SLC9A6 (NHE6) is primarily an endosomal transporter, although it localizes to cell membranes after certain stimuli [290]. This Na + or K + /H + exchanger helps regulate intraluminal pH and dysfunction of this transporter has been associated with severe neurological disorders like Christianson syndrome, X-linked intellectual disability, and Angelman-like syndrome [302,303]. The transport activity of the two SLC9B proteins, SLC9B1 (NHA1) and SLC9B2 (NHA2), remains relatively unclear, although they are thought to drive Na + /Li + counter-transport, as well as (Na + or Li + )/H + transport [290,291,304].…”

NHE6-Depletion Corrects ApoE4-Mediated Synaptic Impairments and Reduces Amyloid Plaque Load

“…Individuals with Christianson Syndrome and mice lacking NHE6 present with motor deficits due to a dramatic progressive loss of cerebellar Purkinje cells (Ouyang et al, 2013). However, there is evidence that NHE6-functions other than its regulation of endosomal pH cause Christianson syndrome, including Purkinje cell loss (Ilie et al, 2020; Ilie et al, 2019). We have reproduced the Purkinje cell loss in our germline NHE6-KO mice ( Figure 2C ).…”

“…Therefore, Purkinje cell degeneration manifests itself postnatally and is not developmentally determined by the absence of NHE6. However, it is possible that loss of scaffolding functions and proper sorting, rather than dysregulation of endosomal pH, could be the main mechanism that causes Christianson syndrome, including Purkinje cell loss (Ilie et al, 2020;Ilie et al, 2019). If this could be substantiated by the development or discovery of NHE6 mutants that selectively ablate its ion exchange capacity without affecting its subcellular sorting or interaction with cytoplasmic or luminal binding partners, this would further raise the potential of NHE6 as a novel drug target for neurodegenerative diseases.…”

“…NHE6-deficiency in humans causes neurodevelopmental defects, which result in Christianson syndrome, an X-linked genetic disorder characterized by cognitive dysfunction, autism, ataxia, and epilepsy. However, some NHE6 mutant variants causing Christianson syndrome in humans do not significantly alter the ion exchange properties of NHE6 (Ilie et al, 2020;Ilie et al, 2019) suggesting that Christianson syndrome could be caused by loss of NHE6 scaffolding functions and not by loss of endosomal pH regulation. To investigate whether NHE6-depletion can reverse ApoE4 pathology in vivo, we generated a conditional NHE6 knockout mouse line (NHE6-cKO) to avoid complications caused by neurodevelopmental defects by temporally and spatially controlling NHE6-ablation.…”

“…NHE6-deficiency in humans causes neurodevelopmental defects, which result in Christianson syndrome, an X-linked genetic disorder characterized by cognitive dysfunction, autism, ataxia, and epilepsy. Importantly, recent evidence suggests that it is not increased endosomal acidification, but likely the loss of NHE6 scaffolding functions that causes Christianson syndrome (Ilie et al, 2020; Ilie et al, 2019). To investigate whether NHE6-depletion can reverse ApoE4 pathology in vivo , we generated a conditional NHE6 knockout mouse line (NHE6-cKO) to avoid complications caused by neurodevelopmental defects by temporally and spatially controlling NHE6-ablation.…”