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A role for AQP4 in renal K+ transport
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.ORCID iD: 0000-0003-3402-9672
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The principal cells of the collecting duct carry out two major tasks: concentration of urine and regulation of K+ homeostasis. Two water channels, AQP3 and AQP4, are expressed in the principal cell basolateral membrane. We propose that AQP4 participates in the regulation of K+ transport in the principal cells. K+ enters the cell via Na+, K+-ATPase-mediated transport in the basolateral membrane. The presence of K+ channels in this membrane permits some K+ recirculation, considered important for maintenance of membrane potential. Here we show that AQP4, but not AQP3, assembles with both Na+, K+-ATPase and an inwardly rectifying K+ channel Kir7.1. We hypothesize that AQP4, Na+, K+-ATPase and Kir7.1 form a K+-transporting microdomain and that AQP4 serves to maintain a favorable concentration gradient for K+ efflux into the diffusion-limited space within the deep infoldings in principal cell basal membrane. The hypothesis is tested in a mathematical model. The model predicts that the impact of AQP-mediated water transport on K+ transport is more significant if AQP water permeability is sensitive to fluctuations in extracellular K+ concentration ([K+]e). We measured water permeability of AQP4 expressed in a renal epithelial cell line and found that it is upregulated when [K+]e is increased to 8 mM, and downregulated when [K+]e is decreased to 1 mM. Studies in an oocyte system indicate that AQP4 does not possess a voltage or K+ sensor. Finally, we show that the expression of AQP4 in rat renal medulla is, in contrast to the expression of AQP2 and AQP3, resistant to changes in K+ intake. Our experimental data, together with the mathematical model, support the concept that AQP4 is involved in principal cell K+ transport processes.

URN: urn:nbn:se:kth:diva-11874OAI: diva2:287225
QC20100727Available from: 2010-01-18 Created: 2010-01-18 Last updated: 2010-07-27Bibliographically approved
In thesis
1. Modeling Biophysical Mechanisms underlying Cellular Homeostasis
Open this publication in new window or tab >>Modeling Biophysical Mechanisms underlying Cellular Homeostasis
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cellular homeostasis is the effort of all living cells to maintain their intracellular content when facing physiological change(s) in the extracellular environment. To date, cellular homeostasis is known to be regulated mainly by time-consuming active mechanisms and via multiple signaling pathways within the cells. The aim of this thesis is to show that time-efficient passive (physical) mechanisms also, under the control and regulation of bio-physical factors such as cell morphology and distribution and co-localization of transport proteins in the cell membrane, can regulate cellular homeostasis. This thesis has been developed in an interface between physics and biology and focuses on critical cases in which cells face physiologically unstable environments at their steady state and therefore may need a constituent effort to maintain their homeostasis. The main hypothesis here is that the cell geometry is oriented in such a way that cellular homeostasis is preserved in a given environment. For exploring these cases, comparative spatial models have been developed that combine transporting function of membrane proteins with simple versus complex geometries of cells. Models confirm the hypothesis and show that cell morphology, size of extracellular space and intercellular distances are important for a dynamic regulation of water and ion homeostasis at steady state. The main clue is the existence of diffusion limited space (DLS) in the bulk extracellular space (ECS). DLS can, despite being ECS, maintain its ionic content and water balance due a controlled function of transport proteins in the membrane facing part of DLS. This can significantly regulate cellular water and ion homeostasis and play an important role in cell physiology. In paper I, the role of DLS is explored in the kidney whereas paper II addresses the brain.

The response of cells to change in osmolarity is of critical importance for water homeostasis. Cells primarily respond to osmotic challenge by transport of water via their membranes. As water moves into or out of cells, the volumes of intra- and extracellular compartments consequently change. Water transport across the cell membrane is enhanced by a family of water channel proteins (aquaporins) which play important roles in regulation of both cell and the extracellular space dimensions. Paper III explores a role for aquaporins in renal K+ transport. Experimentally this role is suggested to be different from bulk water transport. In a geometrical model of a kidney principal cell with several DLS in the basolateral membrane, a biophysical role for DLS-aquaporins is suggested that also provides physiological relevance for this study. The biophysical function of water channels is then extensively explored in paper IV where the main focus has been the dynamics of the brain extracellular space following water transport. Both modeling and experimental data in this paper confirmed the importance of aquaporin-4 expressed in astrocytes for potassium kinetics in the brain extracellular space.

Finally, geometrically controlled transport mechanisms are studied on a molecular level, using silicon particles as a simplified model system for cell studies (paper V and VI). In paper V the role of electrostatic forces (around the nano-pores and in between the loaded material and the silicon surface) is studied with regard to transport processes.  In paper VI the roles of pore size and molecular weight of loaded material are studied. All together this thesis presents various modeling approaches that employ biophysical aspects of transport mechanisms combined with cell geometry to explain cell homeostasis and address cell physiology-based questions.   

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xii, 60 p.
Trita-FYS, ISSN 0280-316X ; 2010:01
National Category
Condensed Matter Physics
urn:nbn:se:kth:diva-11880 (URN)978-91-7415-546-4 (ISBN)
Public defence
2010-02-04, FA32, AlbaNova University Center, Roslagstullsbacken 21, KTH, Stockholm, 13:00 (English)
QC20100727Available from: 2010-01-21 Created: 2010-01-18 Last updated: 2010-07-27Bibliographically approved

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