Search

Recent studies have ascribed many non-pumping functions to theNa/K-ATPase and it has been demonstrated to interact with a variety of proteins viaits cytosolic N-terminus domain (NT), which plays an essential role for itsnon-pumping functions.One of the proteins that interact with NT is caveolin-1 that is implicated incellular cholesterol trafficking and homeostasis. Since we previously showed that theNa/K-ATPase regulated membrane trafficking of caveolin-1, in this study we havefurther revealed that the Na/K-ATPase is able to regulate cellular cholesteroldistribution via its interaction with cavelin-1. Graded knockdown of the Na/K-ATPaseleads to redistribution of cholesterol from membranes to the cytosol and this effect isindependent of its pumping function. Moreover, this regulation is confirmed in α1+/-mouse liver. Functionally, the knockdown-induced redistribution appears to affect thecholesterol sensing in the endoplasmic reticulum because it activates the sterolregulatory element binding protein pathway in vivo. Interestingly, our subsequentstudy has demonstrated that plasma membrane cholesterol also regulates the cellsurface Na/K-ATPase α1 subunit by modulating membrane trafficking of α1.Depletion of plasma membrane cholesterol leads to endocytosis of α1 andaccumulation of α1 in the late endosome/lysosomes. Mechanistically, thecholesterol-regulated α1 trafficking appears to be related to cholesterol-α1 interactionat the cholesterol recognition/interaction amino acid consensus site (CRAC) withinNT. Disruption of the cholesterol-α1 interaction by mutating the key amino acid in CRAC blunts the regulation of the Na/K-ATPase trafficking by cholesterol. Thus, ourstudies have revealed a reciprocal regulation between the plasma membraneNa/K-ATPase and cholesterol and the Na/K-ATPase may represent a potential plasmamembrane cholesterol sensor for the regulation of cellular cholesterol. Furthermore,since the Na/K-ATPase is a signaling molecule that controls kinase activities, thereciprocal regulation between the Na/K-ATPase and cholesterol may serve as animportant link between kinase cascades and lipid homeostasis.To study the physiological role of the receptor function of the Na/K-ATPase,we first revealed that Cav-1 KO mice developed salt-sensitive hypertension, whichimplicated the signaling Na/K-ATPase in blood pressure regulation. Furthermore, wegenerated NT-YFP-expressing transgenic mice and showed that renal NT-YFPexpression results in salt-sensitive hypertension. Mechanistically, development ofsalt-sensitive hypertension is related to renal membrane α1 and sodium hydrogenexchanger 3 trafficking and urine excretion of the endogenous Na/K-ATPase ligand,marinobufagenin, which may influence renal sodium reabsorption and excretion.Taken together, this study has demonstrated that the Na/K-ATPase α1 subunitplays important roles in cellular cholesterol homeostasis and is also regulated byplasma membrane cholesterol content. Moreover, in vivo expression of NT-YFPaffects α1 trafficking and leads to salt-sensitive hypertension, which warrants furtherexamination of the physiological role of the Na/K-ATPase α1 in the renal andcardiovascular systems.

Modularization is a commonly-used technique for modeling and control of large-scale systems. It can reduce the complexity of controllers, enhance the flexibility in the design of system models and controllers, and facilitate the redesign of controllers when changes are required to the system models. However, the modular design of systems and controllers can also create unexpected conflicts among controllers. This work is set in the context of the theory of supervisory control of discrete event systems. The three main contributions of this thesis address modeling, control with priorities, and incremental model evolution of modular discrete event systems. An important motivation for this thesis is the problem of feature interactions in telecommunication networks. When a new feature, such as call waiting or call forwarding, is added to the system, it may interact unexpectedly with the existing features and result in undesirable system behavior. Meanwhile, the introduction of new system components or features sometimes requires the complete redesign of the existing features. In this thesis, we model the logical behavior of telecommunication networks as discrete event systems, where features are implemented as modular supervisors that jointly control the telecommunication system. Techniques are proposed to detect the potential interactions between these supervisors. We then propose a new scheme for modular control of discrete event systems. This scheme provides a new approach to resolve conflicts among modular supervisors, such as blocking and loss of functionality, through the use of a priority mechanism. We develop algorithms that use this priority mechanism to synthesize nonblocking modular supervisors. In the case of incremental system evolution, we investigate the feasibility of reusing the existing modular supervisors. We propose systematic methods for the reuse of existing supervisors based on the automatic synthesis of input and output interfaces. Although this work was originally motivated by the problem of feature interactions, the results presented in this thesis can be readily applied to other discrete event system applications such as manufacturing systems.