A chemical approach to unraveling the biological function of the glycosylphosphatidylinositol anchor

Bioartificial pancreas microencapsulation and conformal coating of islet of Langerhans

Type 1 diabetes has been successfully treated by transplanting islets of Langerhans (islets), endocrine tissue releasing insulin. Serious issues, however, still remain. The administration of immunosuppressive drugs is required to prolong graft functioning; however, side effects of their long-term use on recipients are not fully understood, and cell transplantation therapy without the use of immunosuppressive drugs is desired. To resolve these issues, the encapsulation of isles with a semi-permeable membrane, or bioartificial pancreas, has been attempted. Many groups have reported that it functions very well in small animal models. Few of the bioartificial pancreases, however, were applied to human patients and their clinical outcome was not clear. In this review, we address obstacles and overview new techniques to overcome these issues, such as conformal coating and islet enclosure with cells.

Retention dynamics of amphiphilic polymers PEG-lipids and PVA-Alkyl on the cell surface

We tested two kinds of amphiphilic polymers for cell surface modification: a poly(ethylene glycol)-conjugated phospholipid (PEG-lipid) and a poly(vinyl alcohol) that carried alkyl side chains (PVA-alkyl). Both polymers were expected to anchor to the lipid bilayer of the cell membrane through hydrophobic interactions. We followed the kinetics of these fluorescently labeled amphiphilic polymers (fPEG-lipid, fPVA-alkyl) over time on living cells with confocal scanning laser microscopy and flow cytometry. We found that fPEG-lipids and fPVA-alkyl polymers were not cytotoxic, and they were released from the cell surface without triggering endocytosis. The gradual release from the cell surface was influenced by the hydrophobicity of the alkyl chains, which affected their stability. The amphiphilic polymers tended to aggregate on the cell surface; in particular, the aggregation of PVA-alkyl was clearly identified. Although most of PEG-lipids and PVA-alkyl polymers did not appear to in the cytoplasm, the cells were able to endocytose lipid molecules, as expected. These results suggested that the retention time of modified amphiphilic polymers on the cell surface should be a consideration when modifying cell surfaces to enhance cell transplantation.

Chemical tools for understanding protein lipidation in eukaryotes

Lipidation of proteins is an important mechanism to regulate protein trafficking and activity in cell and tissues. The targeting of proteins to membranes by lipidation plays key roles in many physiological processes and when not regulated properly can lead to cancer and neurological disorders. Dissecting the precise roles of protein lipidation in physiology and disease is a major challenge. Recent advances in chemical biology have now enabled the semisynthesis of lipidated proteins for fundamental biochemical and cellular studies. In addition, new chemical reporters of protein lipidation have improved the detection and enabled the proteomic analysis of lipidated proteins. The expanding efforts in chemical biology are therefore providing new tools to dissect the mechanisms and functions of protein lipidation as well as develop therapeutics targeted at protein lipidation pathways in disease.

Steric and not structure-specific factors dictate the endocytic mechanism of glycosylphosphatidylinositol-anchored proteins

Diverse glycosylphosphatidylinositol (GPI)-anchored proteins enter mammalian cells via the clathrin- and dynamin-independent, Arf1-regulated GPI-enriched early endosomal compartment/clathrin-independent carrier endocytic pathway. To characterize the determinants of GPI protein targeting to this pathway, we have used fluorescence microscopic analyses to compare the internalization of artificial lipid-anchored proteins, endogenous membrane proteins, and membrane lipid markers in Chinese hamster ovary cells. Soluble proteins, anchored to cell-inserted saturated or unsaturated phosphatidylethanolamine (PE)-polyethyleneglycols (PEGs), closely resemble the GPI-anchored folate receptor but differ markedly from the transferrin receptor, membrane lipid markers, and even protein-free PE-PEGs, both in their distribution in peripheral endocytic vesicles and in the manner in which their endocytic uptake responds to manipulations of cellular Arf1 or dynamin activity. These findings suggest that the distinctive endocytic targeting of GPI proteins requires neither biospecific recognition of their GPI anchors nor affinity for ordered-lipid microdomains but is determined by a more fundamental property, the steric bulk of the lipid-anchored protein.

Lipid modification of proteins through sortase-catalyzed transpeptidation

A general chemoenzymatic method for the site-specific attachment of lipids to protein substrates is described. Sortase A is used to append short lipid-modified oligoglycine peptides to the C terminus of protein substrates bearing a five amino acid sortase A recognition sequence (LPETG). We demonstrate the attachment of a range of hydrophobic modifications in excellent yield (60-90%), including a simple step for removing the sortase enzyme postreaction. Lipoproteins prepared using these procedures were subsequently shown to associate with mammalian cells in a lipid tail-dependent fashion and localized to the plasma membrane and endosomes.

The glycosylphosphatidylinositol anchor: a complex membrane-anchoring structure for proteins

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified protein in the outer leaflet of the cell membrane. The GPI anchor is a complex structure comprising a phosphoethanolamine linker, glycan core, and phospholipid tail. GPI-anchored proteins are structurally and functionally diverse and play vital roles in numerous biological processes. While several GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. This review discusses the structural diversity of the GPI anchor and its putative cellular functions, including involvement in lipid raft partitioning, signal transduction, targeting to the apical membrane, and prion disease pathogenesis. We specifically highlight studies in which chemically synthesized GPI anchors and analogues have been employed to study the roles of this unique posttranslational modification.

Noncovalent cell surface engineering: incorporation of bioactive synthetic glycopolymers into cellular membranes

The controlled addition of structurally defined components to live cell membranes can facilitate the molecular level analysis of cell surface phenomena. Here we demonstrate that cell surfaces can be engineered to display synthetic bioactive polymers at defined densities by exogenous membrane insertion. The polymers were designed to mimic native cell-surface mucin glycoproteins, which are defined by their dense glycosylation patterns and rod-like structures. End-functionalization with a hydrophobic anchor permitted incorporation into the membranes of live cultured cells. We probed the dynamic behavior of cell-bound glycopolymers bearing various hydrophobic anchors and glycan structures using fluorescence correlation spectroscopy (FCS). Their diffusion properties mirrored those of many natural membrane-associated biomolecules. Furthermore, the membrane-bound glycopolymers were internalized into early endosomes similarly to endogenous membrane components and were capable of specific interactions with protein receptors. This system provides a platform to study cell-surface phenomena with a degree of chemical control that cannot be achieved using conventional biological tools.