Concurrent Compression of Phospholipid Membranes by
Calcium and Cholesterol
Posted on 2019-08-23 - 14:53
Regulation
of cell metabolism, membrane fusion, association of
proteins with cellular membranes, and cellular signaling altogether
would not be possible without Ca2+ ions. The distribution
of calcium within the cell is uneven with the negatively charged inner
leaflet of the plasma membrane being one of the primary targets of
its accumulation. Therefore, we decided to map the influence of Ca2+ on the properties of lipid bilayers closely resembling natural
lipid membranes. We combined fluorescence spectroscopy (analysis of
time-resolved emission spectra of Laurdan probe and derived parameters:
integrated relaxation time related to local lipid mobility, and total
emission shift reflecting membrane polarity and hydration) with molecular
dynamics simulations to determine the effect of the increasing CaCl2 concentration on model lipid membranes containing POPC, POPS,
and cholesterol. On top of that, the impact of calcium on the plasma
membranes isolated from HEK293 cells was investigated using the steady-state
fluorescence of Laurdan. We found that calcium increases rigidity
of all the model lipid membranes used, elevates their thickness, increases
lipid packing and ordering, and impedes the local lipid mobility.
All these effects were to a great extent similar to those elicited
by cholesterol. However, the changes of the membrane properties induced
by calcium and cholesterol seem largely independent from each other.
At sufficiently high concentrations of calcium or cholesterol, the
steric effects hindered a further alteration of membrane organization,
i.e., the compressibility limit of membrane structures was reached.
We found no indication for mutual interaction between Ca2+ and cholesterol, nor competition of Ca2+ ions and hydroxyl
groups of cholesterol for binding to phospholipids. Fluorescence measurements
indicated that Ca2+ adsorption decreases mobility within
the carbonyl region of model bilayers more efficiently than monovalent
ions do (Ca2+ ≫ Li+ > Na+ >
K+ > Cs+). The effects of calcium ions were
to a great extent mitigated in the plasma membranes isolated from
HEK293 cells when compared to the model lipid membranes. Noticeably,
the plasma membranes showed remarkably higher resistance toward rigidification
induced by calcium ions even when compared with the model membranes
containing cholesterol.
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Melcrová, Adéla; Pokorna, Sarka; Vošahlíková, Miroslava; Sýkora, Jan; Svoboda, Petr; Hof, Martin; et al. (2019). Concurrent Compression of Phospholipid Membranes by
Calcium and Cholesterol. ACS Publications. Collection. https://doi.org/10.1021/acs.langmuir.9b00477
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AUTHORS (8)
AM
Adéla Melcrová
SP
Sarka Pokorna
MV
Miroslava Vošahlíková
JS
Jan Sýkora
PS
Petr Svoboda
MH
Martin Hof
LC
Lukasz Cwiklik
PJ
Piotr Jurkiewicz