Dissertation

The function of neuroligin-2 in neuronal development and neuropsychiatric disorders

Sun, Chicheng

2013

Overview

A delicate balance between excitation and inhibition is crucial for brain functions, and the disturbance of this balance is an emerging hypothesis underlying many neuropsychiatric disorders. Neuroligins are a family of cell adhesion molecules found at the postsynaptic sites of both excitatory glutamatergic and inhibitory GABAergic synapses. The trans-synaptic interactions between neuroligins and their presynaptic receptor neurexins regulate synapse formation and function. Accumulating genetic studies have implicated neuroligin mutations in neuropsychiatric disorders, such as autism spectrum disorders. However, the functional relevance of neuroligins and the molecular mechanisms of their contribution to disease onset are largely unknown. First, I investigated the functional defects of novel neuroligin-2 mutations linked to schizophrenia. Among neuroligins, neuroligin-2 is selectively localized at GABAergic synapses and is critical for regulating inhibitory synaptic transmission. Since GABAergic deficits have been implicated in schizophrenia by both human postmortem and genetic studies, we hypothesized that neuroligin-2 is a potential risk gene for schizophrenia. In a cohort of 584 schizophrenia patients, we identified novel neuroligin-2 missense point mutations. Among them, I identified R215H as a loss-of-function mutation applying a heterologous GABAergic synapse induction assay. The R215H mutant was defective in mediating cell adhesion and in promoting GABAergic synapse formation. Mechanistically, the R215H mutant showed significantly reduced cell surface expression possibly due to incomplete glycosylation. My work suggests that defect in GABAergic synapse formation may be a potential risk factor for schizophrenia. In the second part of my thesis, I reported a novel function of neuroligin-2 in regulating GABA functional switch from excitation to inhibition through KCC2. KCC2 is a neuron specific potassium-chloride co-transporter that exports chloride. The developmental up-regulation of KCC2 mediates GABA functional switch by decreasing intracellular chloride concentration. Surprisingly, KCC2 expression was significantly reduced after knockdown of neuroligin-2 by shRNA-mediated RNA interference. As functional consequences of decreased KCC2, knockdown of neuroligin-2 abolished GABA functional switch in developing neurons and reversed GABA action to excitatory in mature neurons. Overexpression of shRNA proof neuroligin-2, but not neuroligin-1, rescued both decreased KCC2 expression and delayed GABA functional switch induced by shRNAs. Using gramicidin-perforated patch clamp recordings, I further demonstrated that neuroligin-2 expression level directly regulates GABA equilibrium potential. It has been reported that knockdown of neuroligin-2 decreased the number of both GABAergic and glutamatergic synapses, but the mechanism was unknown. I showed that KCC2 overexpression rescued glutamatergic synapse loss induced by knockdown of neuroligin- 2, suggesting that neuroligin-2 regulates glutamatergic synapses through KCC2. In summary, my findings uncovered a new function of neuroligin-2 in regulating GABA functional switch and glutamatergic synapse formation. Therefore, in addition to its conventional role of cell adhesion at GABAergic synapses, neuroligin-2 may serve as a master regulator in balancing excitation and inhibition in the brain. Dysfunctions of neuroligin-2 may cause excitation/inhibition imbalance and contribute to the etiology of neuropsychiatric disorders, as indicated by the case of neuroligin-2 R215H mutant in schizophrenia.

• Publisher: ProQuest Dissertations & Theses
• Subject: Cellular biology / Developmental biology / Neurosciences
• ISBN: 9781321858181, 1321858183