No effective therapeutic agents currently exist to improve outcomes in severe autism. We recently identified a new cellular mechanism in autism involving endosomal Na+/H+ exchanger 6 (NHE6). Studies of NHE6 offer a valuable opportunity for rapid development of innovative treatment strategies in severe autism and related dis- orders. Human mutations in endosomal NHE6 constitute a newly recognized, monogenic, autism-related disorder. Also, down-regulation of NHE6 gene expression is evident in postmortem brains in approximately 30% of patients with idiopathic autism. NHE6 is an integral membrane protein that regulates proton efflux out of endosomes. We have reported that loss of NHE6 in neurons leads to over-acidification of the endosome lumen, and diminished neuronal arborization and synapse development. We also have exciting, new data demonstrating that intra-endosomal pH in the mutant is normalized in vitro by FDA-approved medicines known to alka- linize endosomes. The objective for this research proposal is to elucidate the mechanisms by which NHE6 mu- tations lead to defects in circuit development, and to test the ability of alkalinizing agents to normalize these mutant phenotypes in mouse in vitro and in vivo. Our data support the following central hypothesis: Loss of NHE6 function leads to over-acidification of early endosomes, which enhances degradation of cargo. We also hypothesize that over-acidification of early endosomes drives trafficking to the degradative pathway (i.e., to late endosomes and lysosomes), and decreases endosome recycling and endosome signaling. Our data indicate that TrkB is cargo in NHE6-associated endosomes. We find attenuated TrkB signaling in NHE6 mutant neu- rons. The BDNF/TrkB pathway is well-known to govern circuit development via endosomal signaling, and has broad significance in neuropsychiatry.
Our Aims i nclude: 1) Determine the functional domains of NHE6 protein through study of patient mutations; 2) Determine the role of NHE6 and intra-endosomal pH regulation in TrkB trafficking and endosomal signaling; and 3) Determine the reversibility of defects in neuronal arborization and synapse development in NHE6 mutant neurons using alkalinizing medicines. These studies on the role of intra-endosomal proton concentration are innovative because they represent: 1) a new level of cellular analysis in endosome trafficking and neuronal differentiation; and 2) a novel, druggable cellular mechanism in autism and related disorders. This research is significant because it will lead to 1) elucidation of fundamental and disease- relevant mechanisms in endosome biology and circuit development; and 2) development of specific avenues for mechanism-based treatments for severe autism. This project is also a part of an integrated translational ap- proach in our research group that is coordinated with ongoing studies in patient-derived induced pluripotent stem cells (iPSCs) and with clinical studies in patients with NHE6 mutations. Finally, this application is strongly in line with the NIH Interagency Autism Coordinating Committee (IACC) Strategic Plan that calls for the identification of molecular targets amenable to interventions.

Public Health Relevance

The proposed research is relevant to public health because autism and related disorders are common and associated with high costs to both families and society. The cellular mechanisms under investigation in this proposal govern formation of neuronal circuitry, a process thought to be defective in these disorders. This proposal is relevant to the part of the NIH mission that pertains to the translation of basic mechanisms of mental disorders into innovative treatments for these illnesses.

Agency
National Institute of Health (NIH)
Institute
National Institute of Mental Health (NIMH)
Type
Research Project (R01)
Project #
5R01MH102418-04
Application #
9433679
Study Section
Neurodifferentiation, Plasticity, and Regeneration Study Section (NDPR)
Program Officer
Panchision, David M
Project Start
2015-04-01
Project End
2019-02-28
Budget Start
2018-03-01
Budget End
2019-02-28
Support Year
4
Fiscal Year
2018
Total Cost
Indirect Cost
Name
Brown University
Department
Biochemistry
Type
Schools of Medicine
DUNS #
001785542
City
Providence
State
RI
Country
United States
Zip Code
02912
Ma, Li; Ouyang, Qing; Werthmann, Gordon C et al. (2017) Live-cell Microscopy and Fluorescence-based Measurement of Luminal pH in Intracellular Organelles. Front Cell Dev Biol 5:71
van Dyck, Laura I; Morrow, Eric M (2017) Genetic control of postnatal human brain growth. Curr Opin Neurol 30:114-124
Xu, Meiyu; Ouyang, Qing; Gong, Jingyi et al. (2017) Mixed Neurodevelopmental and Neurodegenerative Pathology in Nhe6-Null Mouse Model of Christianson Syndrome. eNeuro 4:
Young-Pearse, Tracy L; Morrow, Eric M (2016) Modeling developmental neuropsychiatric disorders with iPSC technology: challenges and opportunities. Curr Opin Neurobiol 36:66-73
Gamsiz, Ece D; Sciarra, Laura N; Maguire, Abbie M et al. (2015) Discovery of Rare Mutations in Autism: Elucidating Neurodevelopmental Mechanisms. Neurotherapeutics 12:553-71
Brennand, Kristen J; Marchetto, M Carol; Benvenisty, Nissim et al. (2015) Creating Patient-Specific Neural Cells for the In Vitro Study of Brain Disorders. Stem Cell Reports 5:933-945
Howe, Yamini J; O'Rourke, Julia A; Yatchmink, Yvette et al. (2015) Female Autism Phenotypes Investigated at Different Levels of Language and Developmental Abilities. J Autism Dev Disord 45:3537-49