Novel aspects of Na+,K+-ATPase
Abstract: Na,K-ATPase, an integral membrane protein expressed in each eukaryotic cell, serves as the major determinant of intracellular ion composition. In the current study we investigated novel aspects of Na,K-ATPase function and regulation. It is well established that Na,K-ATPase activity is regulated by reversible phosphorylation. New findings in this study are: 1) the level of intracellular Ca 2. concentration determines the functional effects of PKA and PKC-mediated Na,K-ATPase phosphorylation. At low [Ca 2+]i , activation of PKA or PKC resulted in inhibition of Na,K-ATPase activity, while at high (Ca 2+]i a stimulatory effect was observed. 2) Dynamic regulation of Na,K-ATPase activity plays a functional role in striatal neurons. Activation of D1 subtype of dopamine receptors resulted in inhibition of Na,K-ATPase activity. An additional unexpected and intriguing finding was colocalization of DI and D2 subtypes of dopamine receptors in virtually all striatal neurons. Activation of D2 receptors had no effect on Na,K-ATPase activity. D1 and D2 receptor activation had opposite effects on TTX-sensitive Na+-channels. A role for Na,K-ATPase in the regulation of cell adhesion was demonstrated. Partial inhibition of Na,K-ATPase activity significantly reduced cell attachment to fibronectin. Our results suggest that this effect is mediated by perturbation of normal Ca 2+ signaling and a reduction of focal adhesion kinase activity. These findings indicate the importance of Na,KATPase during development and differentiation. A role for Na,K-ATPase as a signal transducer involved in transcriptional regulation was demonstrated. Ouabain, the enclogenous steroid ligand of Na,K-ATPase, was found to act as a biological inducer of slow [Ca 2+]i oscillations. This signal activated the transcription factor NF-kappaB. The oscillations required Ca 2+ influx from extracellular space and release from intracellular stores. The IP3R, located in the ER membrane, was found to play a pivotal role in initiation and maintenance of Ca 2+ oscillations. By use of FRET and co-immunoprecipitation studies, we demonstrated an interaction between Na,K-ATPase and IP3R. This interaction was enhanced by ouabain. Our data indicate a novel mechanism for IP3R activation and a novel role for Na,K-ATPase as a modulator of I P3R-mediated Ca 2+ release. In conclusion, this thesis demonstrates that Na,K-ATPase has a multifunctional role. In addition to being the dynamic modulator of ion transport, Na,K-ATPase also serves as a receptor and signal transducer involved in the regulation of diverse cellular functions.
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