Role of Aminophospholipids in the Formation of Lipid Rafts in Model Membranes

Chip-based sensing for release of unprocessed cell surface proteins in vitro and in serum and its (patho)physiological relevance

To study the possibility that certain components of eukaryotic plasma membranes are released under certain (patho)physiological conditions, a chip-based sensor was developed for the detection of cell surface proteins, which are anchored at the outer leaflet of eukaryotic plasma membranes by a covalently attached glycolipid, exclusively, and might be prone to spontaneous or regulated release on the basis of their amphiphilic character. For this, unprocessed, full-length glycosylphosphatidylinositol-anchored proteins (GPI-AP), together with associated phospholipids, were specifically captured and detected by a chip- and microfluidic channel-based sensor, leading to changes in phase and amplitude of surface acoustic waves (SAW) propagating over the chip surface. Unprocessed GPI-AP in complex with lipids were found to be released from rat adipocyte plasma membranes immobilized on the chip, which was dependent on the flow rate and composition of the buffer stream. The complexes were identified in the incubation medium of primary rat adipocytes, in correlation to the cell size, and in rat as well as human serum. With rats, the measured changes in SAW phase shift, reflecting specific mass/size or amount of the unprocessed GPI-AP in complex with lipids, and SAW amplitude, reflecting their viscoelasticity, enabled the differentiation between the lean and obese (high-fat diet) state, and the normal (Wistar) and hyperinsulinemic (Zucker fatty) as well as hyperinsulinemic hyperglycemic (Zucker diabetic fatty) state. Thus chip-based sensing for complexes of unprocessed GPI-AP and lipids reveals the inherently labile anchorage of GPI-AP at plasma membranes and their susceptibility for release in response to (intrinsic/extrinsic) cues of metabolic relevance and may, therefore, be useful for monitoring of (pre-)diabetic disease states.

Transverse distribution of plasma membrane bilayer cholesterol: Picking sides

The transverse asymmetry (sidedness) of phospholipids in plasma membrane bilayers is well characterized, distinctive, actively maintained and functionally important. In contrast, numerous studies using a variety of techniques have concluded that plasma membrane bilayer cholesterol is either mostly in the outer leaflet or the inner leaflet or is fairly evenly distributed. Sterols might simply partition according to their differing affinities for the asymmetrically disposed phospholipids, but some studies have proposed that it is actively transported to the outer leaflet. Other work suggests that the sterol is enriched in the inner leaflet, driven by either positive interactions with the phosphatidylethanolamine on that side or by its exclusion from the outer leaflet by the long chain sphingomyelin molecules therein. This uncertainty raises three questions: is plasma membrane cholesterol sidedness fixed in a given cell or cell type; is it generally the same among mammalian species; and does it serve specific physiological functions? This review grapples with these issues.