Interaction of Calcium Ions with Lipid Membranes
Doktorsavhandling, 2019

Bilayer membranes enclose and shield the biological cell and its inner compartments, as well as the tubular networks that exist within and between the cells. Due to their fluidic nature, the membranes are incredibly dynamic and flexible, which allows them to bend, reshape and fuse in response to mechanical and chemical stimuli within their natural microenvironments. Variations in calcium ion concentration are of particular importance for chemical stimulation, as calcium ions play a major role in many cellular processes, including signaling, proliferation, cell division, migration, and exocytosis. Previous studies with lipid membranes showed that binding of calcium ions to the membrane results in dehydration of the lipid head groups, ordering of the hydrocarbon tails, and in the increase of membrane rigidity and tension. At the same time, direct membrane remodeling upon stimulation with calcium ions remains largely unknown. In this thesis, giant unilamellar vesicles (GUVs) and nanotubes were used as model systems of cellular membranes. These lipid membranes were subjected to variations in local calcium ion concentration close to the membrane surface using the microinjection technique and imaged with fluorescence microscopy.
           
The results of our studies on the membrane model systems provide evidence for formation of highly curved membrane structures such as membrane tubular protrusions in GUVs (Paper I, II), and lipid aggregates (bulges) in lipid nanotubes (Paper III), as consequences of controlled calcium ion exposure, which cannot be obtained under bulk conditions. It is also possible to move these highly curved membrane structures by repositioning the source of the calcium ion gradients along the GUV surfaces. Our findings demonstrate how elevated calcium ion concentration close to the GUV surface can trigger membrane remodeling, and how calcium gradients can be used to manipulate and guide lipid-based systems. It is further shown that calcium ion gradients can trigger directed movement and reorientation of phase-separated free-floating GUVs towards the calcium ion source (Paper IV), suggesting possible aspects of protocell migration due to changes in the chemical microenvironment. Lastly, in Paper V, we developed a technique to produce artificial intracellular vesicles (AIVs) inside a GUV. The formation of AIVs occurred upon localized microinjection of calcium solution using a glass micropipette, which was placed in direct contact with the outer surface of the GUV membrane. The AIVs can be used to study mechanisms of exo- and endocytosis, and due to the capability to entrap high concentrations of proteins within the AIVs, the findings bear potential for drug delivery applications.

phospholipid bilayer membrane

membrane dynamics

micromanipulation

calcium ions

cell migration.

membrane nanotube

membrane tubular protrusions

calcium ion concentration gradient

membrane bending

giant unilamellar vesicle

KA, Chemistry building
Opponent: Prof. Atul N. Parikh, University of California in Davis, USA

Författare

Baharan Ali Doosti

Chalmers, Kemi och kemiteknik, Kemi och biokemi, Fysikalisk kemi

Calcium ion gradients can remodel, control and manipulate lipid membranes
 
Calcium ions are important in numerous cellular events, ranging from participating in the embryonal development, to controlling our heartbeats and storing our memories, to cell death. Interestingly, calcium ions are also critical in many processes that require cell membranes to bend and remodel, such as cell migration, release and uptake of membrane bound material within the cell or from adjacent cells, and for fusing with other cells. To investigate the interplay between calcium ions and cellular membranes, we exposed calcium ions to lipid membranes mimicking the cell membranes. In our studies, the lipid membranes were giant unilamellar vesicles (GUVs) and lipid nanotubes. We used micromanipulation techniques, which allowed precise and controlled release of small volumes of calcium ions to the surfaces of the lipid membranes, permitting formation of small and confined calcium ion microenvironments. The morphological changes to the lipid membranes upon binding of the calcium ions were monitored using the optical microscopy technique.
 
Our studies demonstrate that local application of calcium ions can remodel the exposed site of the lipid membranes, causing the membrane to bend and generate tubular protrusions pointed away from the calcium ion source. The findings suggest that calcium ions may be important alone or in concert with other molecular species such as proteins in remodeling the shape of biological cells and affecting structural organization of cell membranes. We also demonstrate that by the repositioning of the calcium ion source, the remodeled membranes can be manipulated to be translated along the surface of the lipid membranes. This opens up new venues for application purposes, for contactless stimulation and manipulation of surfactant- or polymeric soft matter systems. We further demonstrate that calcium ions together with a direct mechanical stimulation of GUVs can generate micrometer-sized vesicles within the GUVs that mimic the intracellular compartments of biological cells. Creating such vesicles can help understanding of vesicle formation in the cell membrane, as well as in development of carriers for encapsulation of proteins in drug delivery applications. Finally, we found that free floating GUVs can reorient and move in the direction of the calcium ion source, thereby mimicking migratory behavior of the cell in the early life.

Styrkeområden

Nanovetenskap och nanoteknik

Ämneskategorier

Fysikalisk kemi

Biofysik

ISBN

978-91-7597-842-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4523

Utgivare

Chalmers tekniska högskola

KA, Chemistry building

Opponent: Prof. Atul N. Parikh, University of California in Davis, USA

Mer information

Senast uppdaterat

2019-01-04