UBC Theses and Dissertations
Exploring the physiological function of the brain-enriched Na+/H+ exchanger NHE5 Diering, Graham Hugh
In general, electrical activity in the brain is enhanced or suppressed by increases or decreases in pH respectively. Moreover, many components of the synaptic machinery are sensitive to changes in pH within the physiological range. Therefore, given this sensitivity to pH, it is likely that pH-regulatory proteins at the synapse may play an essential role in synaptic transmission and synaptic plasticity. However, the identity of the molecules responsible for regulating pH at the synapse has never been determined. NHE5 is a unique member of the Na+/H+ exchanger (NHE) gene family that is highly enriched in brain, yet a physiological role for NHE5 has never been described. In this thesis work, I show for the first time that NHE5 protein is expressed in neurons. In these cells a portion of NHE5 is targeted to synapses; NHE5 abundance at synapses and on the neuronal cell-surface increases in response to synaptic activity. Further I found that dendritic spines, the location of the excitatory post-synapse, experience an activity and NHE5-dependent change in local pH. Blocking NHE5 activity by expression of a transport-deficient dominant-negative mutant or by knock-down resulted in spontaneous exuberant dendritic spine outgrowth, suggesting that NHE5 is a negative regulator of dendritic spine growth. Interestingly, this spine growth required an active NMDA receptor, a pH-sensitive protein, suggesting that the action of NHE5 is mediated through NMDA receptors. Thus, I propose that NHE5 is recruited to synapses during synaptic activity as part of a negative feed-back loop in order to regulate nearby pH-sensitive synapse components to control or stabilize dendritic spine growth. I have also expanded on the current understanding of the molecular regulation of NHE5 by the identification and characterization of an NHE5-SCAMP2 interaction. SCAMP2 binds to NHE5 in recycling endosomes and promotes its delivery to the cell surface, in a pathway that requires the small GTPase Arf6. Therefore, I have uncovered part of the physiological function of NHE5 and also identified the very first molecule involved specifically in regulation of pH at the synapse. Future work on the action of NHE5 at synapses and the regulation of NHE5 by protein binding partners and signaling pathways is discussed.
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