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Diffusion limited space contributes to K+ siphoning by regulation of K+ and water homeostasis in astrocytes
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.ORCID iD: 0000-0001-5178-7593
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Diffusion Limited Space (DLS) is defined as a region where diffusion is limited by the geometry. Two examples of DLS in the brain are the neuronal synapse, and the narrow region between astrocyte endfeet and blood capillaries. In a series of geometrical models we show that DLS plays a role in regulation of water and K+ homeostasis in the brain by an indirect functional coupling of aquaporins (AQPs) and inward rectifying K+ (Kir) channels in a membrane microdomain.

1. Simulations in geometrical models of a synapse region show that following a step increase in synaptic [K+], both K+ and water are taken up by astrocytes via AQPs and Kir channels lining the synapse.  This uptake creates a transient depletion of water in the synapse region that, enhanced by the DLS, facilitates K+ uptake and an efficient clearance of excess K+ from the synapse.

2. Simulations in a geometrical model of astrocytes show that the DLS formed between astrocyte endfeet and blood capillaries, facilitate the siphoning of accumulated K+ into the extracellular space facing the blood capillaries. The DLS geometry creates an efficient coupling between AQPs and Kir channels.

3. Furthermore, the models show that a local coupling between water and K+ transport is important for the maintenance of membrane potential and the net K+ spatial buffering capacity in the astrocytes.

4. In the full geometrical model of K+ spatial buffering we show that the geometry of the extracellular space both in the synapse region and in the endfeet is an essential component for the cell volume regulation.

Our results suggest that for regulation of K+ and water homeostasis in astrocytes, not only the classical aspects of functional couplings between proteins, but also the geometry of the cell and the microdomains are important. Further, our results suggest a central role for AQPs in the astrocyte endfeet and identify their contribution to K+ siphoning.

URN: urn:nbn:se:kth:diva-11872OAI: diva2:287220
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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