Example research essay topic: Plasma Membrane Alzheimer Disease - 950 words

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Lipid Rafts A lipid raft is a micro domain within the membranes of cells that is cholesterol-rich. It has been believed since 1972 that, in cell membranes, membrane proteins and phospholipids are constantly and widely distributed as following a fluid mosaic model. Then in 1988, a scientist named Kai Simmons, who worked at the European Molecular Biology Laboratory located in Germany, suggested the notion that there exist micro domains, which are enriched with a lot of different kinds of lipids, such as cholesterol, glycolipids, and sphingolipids, all present within cell membranes (Simons). The special physical and functional properties ascribed to lipid rafts in biological membranes reflect their distinctive organization and composition, properties that are hypothesized to rest in part on the differential partitioning of various membrane components between liquid-ordered and liquid-disordered lipid environments. Lipid rafts are formed spontaneously in the plasma membrane of various cell types. They are recognized as playing an important part in a variety of different physiological processes such as protein trafficking, activation of ion channels, antigen recognition, and importantly cellular signaling and receptor activation (Simons).

Many functions have been attributed to rafts, from cholesterol transport, endocytosis and signal transduction (Kalman). The function of cholesterol transport is almost certainly the case. It has also been suggested that the primary function of caveolae, one type of lipid raft, was constitutive endocytic trafficking, however recent data displays that it is not truly the case, and instead caveolae are very stable regions of membranes that are not involved in the process of endocytosis. The lipid rafts also play an integral part with the cytoskeleton. Many actin binding proteins are known to bind to polyphosphoinositides and to be regulated by them, by a series of protein domains such as PH or Pleckstrin Homology, PX and ENTH. It is consequently scarcely surprising that some ABPs are suggested to link the actin cytoskeleton and PIP 2 -enriched rafts.

One of these is gels olin, a Ca 2 +, pH and polyphosphoinositide regulated actin capping and severing protein, which partitions into rafts that are isolated biochemically from brain. GEMs are also suggested to link to the actin cytoskeleton by way of ABPs, particularly ERM proteins through EBP 50, a protein that binds members of the ERM proteins through the ERM C-terminus. Although the precise mechanism or mechanisms by which lipid rafts put forth their influence are not completely known or understood, it is believed that their formation results in lateral phase separation, which in turn promotes compartment ation of raft-associated proteins and their spatial segregation from other proteins in the membrane (Silvius). Lipid rafts can also influence the percolation threshold of other mobile molecules, by creating large obstacles, which thereby affect the rate and extent of their interaction. Clustering of such a small number of receptors occurs on the nanometer scale, below the resolution limit of a lens-based optical microscope.

While cholesterol, the main lipid component of lipid rafts, is known to alter the membranes mechanical and dynamical properties, very little is known about the electrical properties of cholesterol-rich nano-domains (Gross). Cholesterol poses a large permanent dipole moment, which modifies the membranes dipole potential, a parameter that plays an important role in fine regulation of diverse physiological processes such as modulation of activation and inactivation potentials of voltage-gated ion channels and alteration of receptor-ligand binding interaction. Finally, lipid rafts can play an important part in certain problems, or can act as a signal of certain problems. Signals that promote proliferation and migration of smooth muscle cells have been implicated in pathologic growth of hollow organs.

Members of the platelet-derived growth factor family, potent mitogen's and motility factors for SMC, have been shown to signal through cholesterol-enriched lipid rafts. these observations suggest a role for lipid rafts in regulation of platelet-derived growth factor-stimulated changes in the cytoskeleton. The decreased CYP 46 A 1 activity in the brain of Alzheimer's disease patients raises membrane cholesterol levels, and as a consequence the amyloid precursor protein is shifted and deposited in the cholesterol rich lipid rafts leading to beta-amyloid peptide specific metabolism (Kalman). Lipid rafts even play a kep part when it comes to HIV. Adhesion molecules on cells bind or adhere to various particles found in the blood by way of their lipid rafts. They help transport material across the cell membrane and into the cell.

It has been shown that all major adhesion molecules interact with HIV. When these molecules are present on the cell surface, the quantities of HIV binding to a cell increases from a few hundred to thousands. These molecules not only increase Hiv's ability to bind to the cell, but they also increase its ability to infect the cell and help transport the virus into it. Moreover, when HIV is bound to one of these molecules, its extremely difficult for the immune system to target and neutralize or eliminate the molecule. In addition to how adhesion molecules' lipid rafts help HIV to bind and infect cells, they also play a key role in permitting infected cells release new HIV. HIV has to get into the cell in order to use it to reproduce, but it also has to get back out, and it also does that by way of the lipid rafts.

Reference Gross, E. and Singer, K. Optical Mapping of Iso-potential Nano-Domains in the Plasma Membrane of Neuronal Cells Using Electro chromic Dyes and Near-Field Scanning Optical Microscopy. Cleveland, OH: Case Western University, 2004. Kalman, J. Cholesterol and Alzheimer's Disease.

Orv Hotel. September, 2005. Silvius, J. R. Partitioning of Membrane Molecules Between Raft and Non-Raft Domains.

Quebec: Department of Biochemistry, McGill University, 2005. Simons, K. & Ikonen, E. Functional Rafts in Cell Membranes. Nature v. 387, 1997, pg 569 - 572.

Research essay sample on Plasma Membrane Alzheimer Disease