UC San Diego

Maintaining the correct ion homeostasis across membranes is a major challenge in both nature and artificial systems. Archaea, have evolved to solve membrane permeability problems to survive in extreme environments by incorporating unique structural features found in their lipid. Specifically, inclusion of phytanyl side chains, ether glycerol linkages, tethering of lipids, cycloalkanes, and different polar lipid headgroups into their lipid membrane are believed to contribute to membrane stability.

We sought to gain a better understanding of the functional benefits attributed to these structural features to membrane stability to design a new class of synthetic Archaea inspired lipid membranes that can be used to overcome limitations (i.e. unstable in serum environment, high background leakage, and prone to hydrolysis) found in current lipid based technologies. Leakage experiments revealed liposomes made from GMGTPC (glycerol monoalkyl glycerol tetraether lipid with phosphatidylcholine headgroup) demonstrated a two order magnitude reduction in membrane leakage to small ions when compared with liposomes made from EggPC. Additionally, liposomes composed of GMGTPC-CH (cyclohexane integrated) lipid displayed an additional 40% decrease in membrane leakage to small ions when compared with liposomes made from GMGTPC lipids. Furthermore, leakage experiments revealed a higher degree of tolerance to headgroup modifications to membrane leakage for liposomes made from GMGT lipid analogs when compared with liposomes made from POPC.

After designing an optimal tetraether lipid scaffold that incorporates key Archaeal structural features for membrane leakage, we explored to integrate strategies employed by eukaryotes to improve membrane properties (i.e. addition of cholesterol). Liposomes made from the hybrid lipid, GcGTPC-CH, displayed a five-fold decrease in membrane leakage when compared with liposomes made from GMGTPC-CH, while maintaining functional membrane properties similar to membranes made from diacyl lipids.

Lastly, we engineered a thiol responsive hybrid lipid, GcGT(S-S)PC-CH, that demonstrated similar membrane stability in serum as GcGTPC-CH. Gratifyingly, doxorubicin loaded liposomes composed of GcGT(S-S)PC-CH liposomes displayed a 4 or 20-fold increase in toxicity to HeLa cells when compared with liposomes made from GcGTPC-CH or Doxil, respectively.

This work represents a first step towards development of stimuli-responsive tetraether lipids that may offer advantages in membrane properties to be used in cancer therapy.