Details

Autor(en) / Beteiligte

• Hack, Anniesha

Titel

A neurobiological model of sodium hydrogen exchanger NHE9

Ort / Verlag

ProQuest Dissertations Publishing

Erscheinungsjahr

2013

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Neurological disorders including autism spectrum disorders (ASD), X-linked mental retardation, attention deficit hyperactivity disorder and epilepsy have been linked to variants, deletions, and truncations in a recently recognized subgroup of endosomal Na+(K+)/H + exchangers, which include NHE6 and NHE9. These transporters are widely expressed in the brain and localize to recycling and late endosomes where they mediate the electrically silent counter transport of cations with protons. Although nothing is known about their function in the brain, studies in yeast have demonstrated that the orthologous transporter, Nhxl, regulates vacuolar pH to control vesicular trafficking and biogenesis of the lysosome. Therefore, we extrapolated our findings from yeast to mammals where we hypothesized that the endosomal NHEs control intravesicular pH to impact cellular trafficking pathways. In the studies of this dissertation, we used homology modeling, yeast phenotype screening, and a physiologically relevant astrocyte cell model to propose a neurobiological function for NHE9 and evaluate the effect of polymorphisms that have been identified in patients with ASD and epilepsy. Because both ion and pH gradients must be tightly regulated for many aspects of neuronal signaling and synaptic cleft homeostasis, including neurotransmitter uptake and clearance, we tested the role of NHE9 on the delivery and turnover of glutamate transporters at the astrocyte membrane. We showed that isoform-specific overexpression of NHE9 altered vesicular trafficking, with increased surface expression of ubiquitously expressed (transferrin) receptors and astrocyte specific (glutamate) transporters due to alkalinization of the endosome. These novel observations allowed a functional evaluation of autism-associated variants in NHE9. We have found that three variants, L236S, S438P, and V1761, exhibit a loss of function phenotype. We then extended our studies to a glioblastoma multiforme model (GBM) in order to understand the molecular mechanisms of NHE9 overexpression and the consequent gain of function phenotype. The observed increase in transferrin accumulation was due to an increase in the rate of influx whereas exocytosis was unaffected. Overall, these studies show that NHE9 may be causal to a subset of ASD (NHE9 loss of function) and GBM (NHE9 gain of function) cases, and can be a potential drug target in the treatment of patients.