Cyclohexane Rings Reduce Membrane Permeability to Small Ions in Archaea-Inspired Tetraether Lipids

Article abstract of DOI:10.1002/anie.201510445

Extremophile archaeal organisms overcome problems of membrane permeability by producing lipids with structural elements that putatively improve membrane integrity compared to lipids from other life forms. Herein, we describe a series of lipids that mimic some key structural features of archaeal lipids, such as: 1) single tethering of lipid tails to create fully transmembrane tetraether lipids and 2) the incorporation of small rings into these tethered segments. We found that membranes formed from pure tetraether lipids leaked small ions at a rate that was about two orders of magnitude slower than common bilayer-forming lipids. Incorporation of cyclopentane rings into the tetraether lipids did not affect membrane leakage, whereas a cyclohexane ring reduced leakage by an additional 40 %. These results show that mimicking certain structural features of natural archaeal lipids results in improved membrane integrity, which may help overcome limitations of many current lipid-based technologies.

Full text of DOI:10.1002/anie.201510445

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International Edition: DOI: 10.1002/anie.201510445
German Edition: DOI: 10.1002/ange.201510445
Synthetic Lipids
Cyclohexane Rings Reduce Membrane Permeability to Small Ions in
Archaea-Inspired Tetraether Lipids
Takaoki Koyanagi+, Geoffray Leriche+, David Onofrei, Gregory P. Holland, Michael Mayer, and
Jerry Yang*
Abstract: Extremophile archaeal organisms overcome prob-
lems of membrane permeability by producing lipids with
structural elements that putatively improve membrane integrity
compared to lipids from other life forms. Herein, we describe
a series of lipids that mimic some key structural features of
archaeal lipids, such as: 1) single tethering of lipid tails to
create fully transmembrane tetraether lipids and 2) the incor-
poration of small rings into these tethered segments. We found
that membranes formed from pure tetraether lipids leaked
small ions at a rate that was about two orders of magnitude
slower than common bilayer-forming lipids. Incorporation of
cyclopentane rings into the tetraether lipids did not affect
membrane leakage, whereas a cyclohexane ring reduced
leakage by an additional 40%. These results show that
mimicking certain structural features of natural archaeal
lipids results in improved membrane integrity, which may
help overcome limitations of many current lipid-based
technologies.
in their lipid membrane and environmental growth temper-
ature.[1,7] This unique structural feature has been proposed to
decrease membrane leakage by increasing lipid packing.[8]
Additionally, Thaumarchaeota have cyclohexane rings incor-
porated into their lipids,[7] which have also been suggested to
affect membrane packing.[9]
Modification of membranes by addition of cholesterol,
PEG lipids, and lipids with high phase-transition temper-
atures is a common strategy to reduce membrane leakage in
laboratory settings.[10] However, the incorporation of addi-
tives onto membranes can be problematic as a result of, for
example, leaching,[11] potential long-term toxicity,[12–14] or
difficulty with liposome preparation.[15]
Several attempts have been made to address problems
with membrane leakage by using lipids extracted from
archaeal species or through chemical synthesis of archaea-
inspired lipids. For instance, polar lipid fraction E (PLFE)
extracted from Sulfobus acidocaldarius exhibit low perme-
ability, tight membrane packing, and high stability.[16,17]
However, harvesting reproducible and large quantities of
specific lipid compositions from cultured archaeal species can
be challenging.[18] While a few groups have also reported the
synthesis of singly tethered transmembrane spanning tet-
raether lipids,[19–23] the relationship between structure and
function of these lipids remains unclear because of the limited
data available on their membrane permeability properties. A
systematic study of the effect of specific structural features
(taking inspiration from archaeal lipids) on membrane
leakage could make it possible to design lipids with improved
integrity under a variety of environmental conditions.
Herein, we describe a series of synthetic lipids inspired by
those found in archaeal organisms, namely glycerol mono-
alkyl glycerol tetraether lipids with phosphocholine head
groups (GMGTPC), which exhibit excellent membrane
integrity without the necessity of additives (Scheme 1).
Collectively, the structural features of these new synthetic
lipids attempt to mimic lipids derived from Crenarchaeota or
Thaumarchaeota.[6] By incorporating the essence of some key
structural features (for example, the ether glycerol linkage,
tethering of lipids, and incorporation of rings) found in
natural archaeal lipids, we generated a set of synthetic lipids
that formed stable liposomal membranes at room temper-
ature with reduced leakage properties compared to commer-
cially available EggPC lipids.
I
n nature, ion pumps, molecular transporters, and alterations
of membrane composition are used to reduce membrane
leakage and maintain gradients.[1–4] Archaeal organisms
(halophiles, thermophiles, acidophiles, nitrifiers, and metha-
nogens), one of the three domains of life, have evolved
mechanically and chemically robust membrane compositions
that allow survival in extreme environments.[5] For instance,
Crenarchaeota, a kingdom of Archaea, have an optimal
survival temperature above 808C.[6] Interestingly, the mem-
branes of these hyperthermophiles are comprised of lipids
containing cyclopentane rings, with a positive correlation
found between the number of cyclopentane rings integrated
[*] T. Koyanagi,[+] Dr. G. Leriche,[+] Prof. J. Yang
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0358 (USA)
E-mail: jerryyang@ucsd.edu
D. Onofrei, Prof. G. P. Holland
Department of Chemistry and Biochemistry
San Diego State University, San Diego, CA 92182-1030 (USA)
Prof. M. Mayer
Department of Biomedical Engineering
University of Michigan, Ann Arbor, MI 48109 (USA)
[+] These authors contributed equally to this work.
Supporting Information (including additional details for the syn-
thesis and characterization of GMGTPC lipids, for the formation and
characterization of liposomes, and for kinetic analysis for the pH
equilibration studies) and ORCID(s) from the author(s) and for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510445.
We designed the series of synthetic lipids shown in
Scheme 1 to evaluate the effects on membrane permeability
of two important structural components found in many
archaeal lipids: 1) the effect of the tethering of alkyl tails to
create bolaform amphiphiles capable of spanning the length
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diameter of about 130 nm, as determined by dynamic light
scattering (DLS; Figure S8). We expect the lipids in these
liposomes to predominantly adopt a transmembrane config-
uration.[31,32] However, it is plausible that some fraction of the
lipids can adopt a hairpin configuration to help stabilize the
high curvature of the liposomes. The average size of these
liposomes was stable over at least 6 hours, and we did not
observe evidence of liposome aggregation or fusion when
subjected to a liposome fusion assay[33] (Figures S8 and S9).
The absence of a lipid phase transition or aggregation/fusion
of liposomes near room temperature suggests that the
GMGTPC lipids are suitable for evaluating the effects on
membrane leakage of the incorporation of small rings into
tetraether lipids.
To evaluate the relative permeability of membranes
formed from these different lipids, we developed a modified
pH equilibration assay that was previously reported by
Kakinuma and co-workers.[20] In this assay, we encapsulated
CF within the liposomes with an initial internal liposomal
pH value of 7.2. The liposomes were then incubated in
a buffered solution with an external pH value of 5.8, and the
change in fluorescence intensity of the CF unit was monitored
over time as the internal liposome pH equilibrated to pH 5.8.
We chose to use pH values of 5.8 and 7.2 as external and
internal liposomal pH values, respectively, because CF exhib-
its a linear correlation between its fluorescence intensity and
the environmental pH value within this pH range.[34] We also
chose to use CF as the fluorescent reporter of pH because we
did not detect any appreciable leakage of CF from the
liposomes formed from any of the lipids used in this study
over the time required to observe pH equilibration at room
temperature (Figure S10).
While we expect all of the membranes to be most
permeable to protons over any other ionic species under
these experimental conditions, it is possible the intraliposo-
mal buffer and other ions, such as OHÀ, Na+, or ClÀ, could
also contribute to the observed rate of pH equilibration.[35]
We therefore consider the measured initial rates of pH
equilibration from membranes composed of the different
lipids to represent an estimate of their overall permeability to
small ions rather than an estimate of their permeability to
a specific single ionic species.
To understand whether tethering of lipids affected mem-
brane leakage of small ions, we evaluated the detected initial
rates of pH equilibration of liposomes composed of EggPC
and GMGTPC (Figure 1A; Figure S11). Similar to previously
reported trends comparing the permeability of liposomes
composed of lipids derived from archaea versus bacteria,[17]
GMGTPC liposomes exhibited a decrease in the rate of
leakage of small ions of about two orders of magnitude when
compared to liposomes formed from EggPC. This result could
arise, in part, through a combination of the absence of the
ester linkage found in EggPC, the presence of the branched
alkane network provided by the phytanyl group in GMGTPC,
and the minimization of the small aqueous layer found in
between the two lipid leaflets of a bilayer-forming lipid
Scheme 1. Structures of synthesized tetraether lipids.
of the membranes, and 2) the effect of the incorporation of
cyclopentane or cyclohexane rings within the tethered lipid
chain. In all lipids synthesized, we included ether linkages
instead of ester groups between the lipid tails and the head
groups because ether functional groups are expected to be
more chemically stable than the ester groups commonly
found in eukaryotic and prokaryotic lipid membranes.
Tetraether archaeal lipids found in nature contain either
a single transmembrane tether or are macrocyclic (that is,
both transmembrane lipid tails are tethered).[24] Although one
example of a macrocyclic tetraether lipid has been prepared
by total synthesis,[25] the preparation required over 20
synthetic steps (without incorporation of rings).[26,27] Thus,
obtaining access to a series of synthetic macrocyclic tetraether
lipids containing rings was not practical. We therefore
synthesized lipids containing a single tether, which made it
possible to prepare a series of transmembrane spanning lipids
containing zero to three rings in sufficient quantities (150–
420 mg) to evaluate their leakage properties. Phosphocholine
head groups were incorporated into all lipids because these
zwitterionic groups are known to produce stable liposomes.[28]
We used a series of Wittig and SN2 reactions[29,30] to
generate a set of five GMGTPC tethered lipids (Scheme 1)
that differ from each other by the number and type of rings in
the hydrophobic core. The preparation of each of the lipids
required about 10 synthetic steps (see Figures S1–S6 in the
Supporting Information). Phytanyl groups were incorporated
as the untethered lipid chain in all GMGTPC lipids to avoid
occurrence of the phase transition temperatures of the lipids
near room temperature. Differential scanning calorimetry
(DSC) measurements support that all newly synthesized
lipids in liposome form do not undergo a phase transition over
a temperature range of 5–708C (Figure S7).
We prepared liposomes from pure GMGTPC lipids by
hydration of thin films of each lipid in buffer containing 5,6-
carboxyfluorescein (CF), followed by extrusion. This prepa-
ration afforded liposomes with an average hydrodynamic
(which, presumably, would be sparse in
a membrane
comprised of pure tethered-GMGTPC lipids).[16,36,37]
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Figure 2. Comparison of the detected initial rates of pH equilibration
of liposomes composed of lipids with zero rings (GMGTPC), one
cyclopentane ring (GMGTPC-CP1), or one cyclohexane ring (GMGTPC-
CH1). A) Natural log of the relative fluorescence intensity of CF (first
15% of pH-dependent fluorescence decrease of CF) plotted against
time and B) the detected initial rates of pH equilibration from
liposomes composed of GMGTPC, GMGTPC-CP1, or GMGTPC-CH1.
Statistical analyses were performed using a paired t-test. ** indicate
a p-value <0.01.
Figure 1. Observed rate of pH equilibration from liposomes formed
from EggPC or synthetic lipids. A) The change in CF fluorescence
intensity (monitoring at l(Ex/Em)=485/517 nm) from CF-encapsu-
lated GMGTPC or EggPC liposomes versus time. B) Comparison of the
observed initial rates of decreased CF fluorescence intensity from
CF-encapsulated liposomes composed of different lipids. C) Average
detected initial rates of pH equilibration in liposomes composed of
different lipids. Standard errors of the mean are provided based on
nine measurements each. Statistical analyses were performed using
a paired t-test. ** indicate a p-value <0.01.
1,3 stereochemistry as in the GMGTPC-CP1–3 lipids, avoid-
ing introduction of an additional variable when comparing
their leakage properties. Interestingly, Figure 2 shows that
liposomes composed of GMGTPC-CH1 lipids exhibited an
additional circa 40% decrease in the rate of small-ion
membrane leakage (kobsd = 0.8 Æ 0.1 hÀ1) compared to lip-
osomes comprised of lipids with no rings (GMGTPC) or with
one cyclopentane ring (GMGTPC-CP1). Presumably, the
difference in flexibility of the cyclohexane ring compared to
a cyclopentane ring affects lipid packing, which leads to
decreased membrane permeability to small ions in
GMGTPC-CH1 liposomes.[45] Additionally, intercalation of
cyclohexane or cycloheptane between the hydrophobic tails
of a bilayer lipid is believed to decrease proton permeabil-
ity.[46] These results reveal one potential functional benefit of
cyclohexane ring incorporation found in lipids derived from
Thaumarchaeota, which grow optimally in environments
between pH 5–8 and temperatures of 20–458C.[6]
We have thus presented a systematic study of the effects
on membrane leakage of some key structural features
inspired by natural archaeal lipids. As expected, the incorpo-
ration of phytanyl groups, the presence of tethering, and the
incorporation of ether glycerol backbones in tetraether lipids
substantially decreased the membrane permeability of small
ions when compared to commercially available EggPC lipids.
Surprisingly, we found that incorporation of cis-1,3 cyclo-
pentane rings had no effect on leakage of small ions in
GMGTPC lipids. In contrast, incorporation of a cis-1,3
cyclohexane ring significantly decreased small-ion membrane
leakage compared to all other GMGTPC lipids studied. Such
differences in small-ion permeability between lipids contain-
ing cyclopentane versus cyclohexane rings could reflect
differences in ring flexibility as it relates to lipid packing.
This work represents an important step towards establishing
A common feature found in many natural lipids derived
from Crenarchaeota is the presence of cyclopentane rings
within the tethered transmembrane core of tetraether
lipids.[38–40] To examine the effect of the integration of
cyclopentane rings in the lipid on small-ion membrane
leakage, we evaluated the rate of pH equilibration from
liposomes formed from GMGTPC-CP1–3 (Figure 1B and C),
which contained 1, 2, or
3 cis-1,3-cyclopentane rings
(Scheme 1). Interestingly, we found that the presence or
number of rings had no detectable effect at room temperature
on the rate of pH equilibration compared to GMGTPC
(which has zero rings within its tethered hydrophobic core).
These results are in contrast to reported computational
studies suggesting that cyclopentane rings[41] in tethered
lipids increased lipid packing.[7,42] Although the stereochem-
istry of the cyclopentane rings (cis versus trans) may account
for the discrepancy between our experimental results and the
reported computational studies, conformational analysis[43]
and X-ray studies[44] of polymers containing multiple 1,3-
cyclopentane rings support that the energy of interstrand
packing of these polymers is independent of the cis or
trans configuration of the rings. Nevertheless, the results
shown in Figure 1 demonstrate that any structural effects to
the lipid through introduction of cis cyclopentane rings is not
sufficient to significantly affect membrane leakage of small
ions under the conditions used in these studies.
To examine the effects on membrane leakage of a cyclo-
hexane ring incorporated into tethered lipids, we synthesized
GMGTPC-CH1 (Scheme 1) containing a cis-1,3-cyclohexane
group. In this case, the molecule maintained the same cis-
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Acknowledgements
Financial support from the Air Force Office of Scientific
Research (FA9550-12-1-0435 and FA9550-14-1-0014) is grate-
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Received: November 10, 2015
Published online: December 23, 2015
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