Bifunctional glycosphingolipid (GSL) probes to investigate GSL-interacting proteins in cell membranes

Glycosphingolipids (GSLs) are abundant glycolipids on cells and essential for cell recognition, adhesion, signal transduction, and so on. However, their lipid anchors are not long enough to cross the membrane bilayer. To transduce transmembrane signals, GSLs must interact with other membrane components, whereas such interactions are difficult to investigate. To overcome this difficulty, bifunctional derivatives of II3-β-N-acetyl-D-galactosamine-GA2 (GalNAc-GA2) and β-N-acetyl-D-glucosamine-ceramide (GlcNAc-Cer) were synthesized as probes to explore GSL-interacting membrane proteins in live cells. Both probes contain photoreactive diazirine in the lipid moiety, which can crosslink with proximal membrane proteins upon photoactivation, and clickable alkyne in the glycan to facilitate affinity tag addition for crosslinked protein pull-down and characterization. The synthesis is highlighted by the efficient assembly of simple glycolipid precursors followed by on-site lipid remodeling. These probes were employed to profile GSL-interacting membrane proteins in HEK293 cells. The GalNAc-GA2 probe revealed 312 distinct proteins, with GlcNAc-Cer probe-crosslinked proteins as controls, suggesting the potential influence of the glycan on GSL functions. Many of the proteins identified with the GalNAc-GA2 probe are associated with GSLs, and some have been validated as being specific to this probe. The versatile probe design and experimental protocols are anticipated to be widely applicable to GSL research.

Sensitive Method To Analyze Cell Surface GPI-Anchored Proteins Using DNA Hybridization Chain Reaction-Mediated Signal Amplification

GPI-anchored proteins (GPI-APs) are ubiquitous and essential but exist in low abundances on the cell surface, making their analysis and investigation especially challenging. To tackle the problem, a new method to detect and study GPI-APs based upon GPI metabolic engineering and DNA-facilitated fluorescence signal amplification was developed. In this context, cell surface GPI-APs were metabolically engineered using azido-inositol derivatives to introduce an azido group. This allowed GPI-AP coupling with alkyne-functionalized multifluorophore DNA assemblies generated by hybridization chain reaction (HCR). It was demonstrated that this approach could significantly improve the detection limit and sensitivity of GPI-APs, thereby enabling various biological studies, including the investigation of live cells. This new, enhanced GPI-AP detection method has been utilized to successfully explore GPI-AP engineering, analyze GPI-APs, and profile GPI-AP expression in different cells.

Investigation of Glycosylphosphatidylinositol (GPI)-Plasma Membrane Interaction in Live Cells and the Influence of GPI Glycan Structure on the Interaction

Glycosylphosphatidylinositols (GPIs) need to interact with other components in the cell membrane to transduce transmembrane signals. A bifunctional GPI probe was employed for photoaffinity-based proximity labelling and identification of GPI-interacting proteins in the cell membrane. This probe contained the entire core structure of GPIs and was functionalized with photoreactive diazirine and clickable alkyne to facilitate its crosslinking with proteins and attachment of an affinity tag. It was disclosed that this probe was more selective than our previously reported probe containing only a part structure of the GPI core for cell membrane incorporation and an improved probe for studying GPI-cell membrane interaction. Eighty-eight unique membrane proteins, many of which are related to GPIs/GPI-anchored proteins, were identified utilizing this probe. The proteomics dataset is a valuable resource for further analyses and data mining to find new GPI-related proteins and signalling pathways. A comparison of these results with those of our previous probe provided direct evidence for the profound impact of GPI glycan structure on its interaction with the cell membrane.

Recent research progress in glycosylphosphatidylinositol-anchored protein biosynthesis, chemical/chemoenzymatic synthesis, and interaction with the cell membrane

Glycosylphosphatidylinositol (GPI) attachment to the C-terminus of proteins is a prevalent posttranslational modification in eukaryotic species, and GPIs help anchor proteins to the cell surface. GPI-anchored proteins (GPI-APs) play a key role in various biological events. However, GPI-APs are difficult to access and investigate. To tackle the problem, chemical and chemoenzymatic methods have been explored for the preparation of GPI-APs, as well as GPI probes that facilitate the study of GPIs on live cells. Substantial progress has also been made regarding GPI-AP biosynthesis, which is helpful for developing new synthetic methods for GPI-APs. This article reviews the recent advancements in the study of GPI-AP biosynthesis, GPI-AP synthesis, and GPI interaction with the cell membrane utilizing synthetic probes.

A Biotinylated Glycosylphosphatidylinositol (GPI) as the Universal Platform To Access GPI-Anchored Protein Analogues

A glycosylphosphatidylinositol (GPI) derivative with biotin linked to its mannose III 6-O-position was prepared by a convergent strategy. This biotinylated GPI was demonstrated to bind avidinated proteins readily through biotin-avidin interaction and, therefore, can serve as a universal platform to access various biologically significant GPI-anchored protein analogues.

Diversity-Oriented Synthesis of Glycosylphosphatidylinositol Probes Based on an Orthogonally Protected Pentasaccharide

Two glycosylphosphatidylinositol (GPI) derivatives having an alkynyl group at different positions were derived from the same orthogonally protected pentasaccharide that in turn was assembled by a convergent [3+2] glycosylation strategy. The resultant alkynylated GPIs are useful biological probes and are suitable for further modification by click reaction to obtain other GPI probes. The pentasaccharide is a versatile platform for the synthesis of various uniquely functionalized GPI probes.

Profiling Glycosylphosphatidylinositol (GPI)-Interacting Proteins in the Cell Membrane Using a Bifunctional GPI Analogue as the Probe

Glycosylphosphatidylinositol (GPI) anchorage of cell surface proteins to the membrane is biologically important and ubiquitous in eukaryotes. However, GPIs do not contain long enough lipids to span the entire membrane bilayer. To transduce binding signals, GPIs must interact with other membrane components, but such interactions are difficult to define. Here, a new method was developed to explore GPI-interacting membrane proteins in live cell with a bifunctional analogue of the glucosaminylphosphatidylinositol motif conserved in all GPIs as a probe. This probe contained a diazirine functionality in the lipid and an alkynyl group on the glucosamine residue to respectively facilitate the cross-linkage of GPI-binding membrane proteins with the probe upon photoactivation and then the installation of biotin to the cross-linked proteins via a click reaction for affinity-based protein isolation and analysis. Profiling the proteins pulled down from the Hela cells revealed 94 unique and 18 overrepresented proteins compared to the control, and most of them are membrane proteins and many are GPI-related. The results have proved not only the concept of using the new bifunctional GPI probe to investigate GPI-binding membrane proteins but also the important role of inositol in the biological functions of GPI anchors and GPI-anchored proteins.

Chemical Synthesis of GPI Glycan-Peptide Conjugates by Traceless Staudinger Ligation

A new strategy has been developed for GPI glycan-peptide conjugate synthesis based upon a traceless Staudinger reaction between a peptide phosphinothioester and a GPI glycan azide. The strategy was first studied and optimized with simple peptides and GPI glycans, which offered excellent yields of the desired conjugates in both organic and aqueous solvents. It was then used to successfully synthesize an analogue of the human CD52 antigen containing the whole CD52 peptide sequence and the conserved trimannose motif of all GPI anchors.

Downregulation of TFPI in breast cancer cells induces tyrosine phosphorylation signaling and increases metastatic growth by stimulating cell motility

BACKGROUND

Increased hemostatic activity is common in many cancer types and often causes additional complications and even death. Circumstantial evidence suggests that tissue factor pathway inhibitor-1 (TFPI) plays a role in cancer development. We recently reported that downregulation of TFPI inhibited apoptosis in a breast cancer cell line. In this study, we investigated the effects of TFPI on self-sustained growth and motility of these cells, and of another invasive breast cancer cell type (MDA-MB-231).

METHODS

Stable cell lines with TFPI (both α and β) and only TFPIβ downregulated were created using RNA interference technology. We investigated the ability of the transduced cells to grow, when seeded at low densities, and to form colonies, along with metastatic characteristics such as adhesion, migration and invasion.

RESULTS

Downregulation of TFPI was associated with increased self-sustained cell growth. An increase in cell attachment and spreading was observed to collagen type I, together with elevated levels of integrin α2. Downregulation of TFPI also stimulated migration and invasion of cells, and elevated MMP activity was involved in the increased invasion observed. Surprisingly, equivalent results were observed when TFPIβ was downregulated, revealing a novel function of this isoform in cancer metastasis.

CONCLUSIONS

Our results suggest an anti-metastatic effect of TFPI and may provide a novel therapeutic approach in cancer.

Rafts in adult peripheral nerve myelin contain major structural myelin proteins and myelin and lymphocyte protein (MAL) and CD59 as specific markers

The myelin and lymphocyte protein (MAL) proteolipid is localized in central and peripheral compact myelin membranes, as well as in apical membranes of particular polarized cells. In this study, we addressed the question whether MAL and other peripheral myelin proteins are sorted and targeted to myelin membranes using mechanisms similar to those observed in polarized epithelial cells. To investigate the presence of raft-mediated sorting pathways in Schwann cells, we have isolated and analysed their composition in myelin membranes. Here, we show that rafts are present in adult human and rat peripheral compact myelin membranes and contain MAL, the GPI-anchored protein CD59, and substantial amounts of the PMP22 and P0. Colocalization studies show that CD59, and MAL have an almost identical expression pattern within compact myelin. Moreover, immuno-electron microscopy revealed that MAL, besides its localization in compact myelin, is also localized to Schmidt-Lanterman incisures. Taken together, our results demonstrate the presence of detergent-insoluble glycolipid-enriched complexes (DIGs) in different compartments of myelin membranes and indicate an important role for DIG-mediated transport mechanisms in the maintenance of the adult myelin sheath.

Membrane traffic in myelinating oligodendrocytes

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.

Assembly of myelin by association of proteolipid protein with cholesterol- and galactosylceramide-rich membrane domains

Myelin is a specialized membrane enriched in glycosphingolipids and cholesterol that contains a limited spectrum of proteins. We investigated the assembly of myelin components by oligodendrocytes and analyzed the role of lipid-protein interactions in this process. Proteolipid protein (PLP), the major myelin protein, was recovered from cultured oligodendrocytes from a low-density CHAPS-insoluble membrane fraction (CIMF) enriched in myelin lipids. PLP associated with the CIMF after leaving the endoplasmic reticulum but before exiting the Golgi apparatus, suggesting that myelin lipid and protein components assemble in the Golgi complex. The specific association of PLP with myelin lipids in CIMF was supported by the finding that it was efficiently cross-linked to photoactivable cholesterol, but not to phosphatidylcholine, which is underrepresented in both myelin and CIMF. Furthermore, depletion of cholesterol or inhibition of sphingolipid synthesis in oligodendrocytes abolished the association of PLP with CIMF. Thus, PLP may be recruited to myelin rafts, represented by CIMF, via lipid-protein interactions. In contrast to oligodendrocytes, after transfection in BHK cells, PLP is absent from isolated CIMF, suggesting that PLP requires specific lipids for raft association. In mice deficient in the enzyme ceramide galactosyl transferase, which cannot synthesize the main myelin glycosphingolipids, a large fraction of PLP no longer associates with rafts. Formation of a cholesterol- and galactosylceramide-rich membrane domain (myelin rafts) may be critical for the sorting of PLP and assembly of myelin in oligodendrocytes.