Biochemical Properties and Functions

Lipid rafts have been defined as islands of highly ordered saturated lipids and cholesterol that are laterally mobile in the plane of a more disordered fluid bilayer of largely unsaturated lipids [4-6]. Because of their ability to segregate functional proteins, lipid rafts have been proposed to play a central role in many cellular processes, including intracellular signaling and protein and lipid sorting [4,5]. In particular, Simons and Ikonen in 1997 postulated that, in polarized epithelial cells, rafts can act as sorting platforms for inclusion of proteins into apical post-trans-Golgi network (TGN) sorting vesicles. Later studies subsequently provided evidence for the role of lipid rafts in signaling, as in the case of IgE receptor (FcRI) and T-cell antigen receptor (TCR) [7,8].

Association with detergent-resistant membranes (DRM) is a useful criterion to estimate whether a protein associates with lipid rafts [6]. After solubilization of membranes or cells with Triton X-100 at 4 °C, raft-associated proteins and lipids remain insoluble and can then be floated to low density by sucrose density gradient centrifugation. If cholesterol is extracted by using methyl-b-cyclodextrin (mbCD) or is complexed by saponin, the raft proteins usually (but not always) become detergent-soluble [9].

Constitutive raft residents include glycophosphatidylinositol (GPI)-anchored proteins (e. g., the prion protein), double acylated proteins (e. g., tyrosine kinases of the Src family), palmitate-anchored proteins and transmembrane proteins (e.g., b-secretase; BACE) [5].

One source of confusion in the field of rafts has been the inter-relationship between rafts and caveolae. Indeed, for a long time these two terms have been used interchangeably. However, this issue has now been clarified by the analysis of caveolin knockout mice [10,11].

Caveolae appear as "smooth" uncoated-pits of 50- to 100-nm flask-shaped invaginations of the plasma membrane, originally identified by electron microscopy in a wide variety of tissues and cell types [12,13]. They represent a morphologically identifiable subset of lipid rafts identified by the coat protein, caveolin. Whilst the biochemical composition of lipid rafts and caveolae is thought to overlap, these microdomains are not equivalent [14].

Caveolar invagination is possibly driven by the polymerization of caveolins, of which there are three types: caveolin-1, -2, and -3. Caveolin-1 appears to have a central role in the formation of caveolae, because it was shown that cells without caveolin-1 lacked morphological caveolae, and that reintroduction of the protein was sufficient to generate caveolae [15,16].

Caveolae usually remain attached to the cell surface, but their internalization can be stimulated under certain conditions; for example, by Simian virus-40 (SV40) [17] or by treatment with the phosphatase inhibitor okadaic acid [18,19].

Both caveolae and rafts mediate the internalization of sphingolipids and sphingo-lipid-binding toxins, GPI-anchored proteins [20], and the autocrine motility factor (AMF).

Internalizations via caveolae or via lipid rafts are fundamentally similar processes, defined by their clathrin independence and sensitivity to cholesterol depletion. However, the cholesterol-dependent invagination of rafts occurs independently of caveolin-1 and of dynamin 2, a GTPase, localized at the neck of the caveolae [21] which regulates their internalization. Interestingly, in some cases caveolin-1 acts as a negative regulator of the budding of caveolar invaginations but caveolae become competent for endocytosis after specific signalling stimuli [22].

Caveolae/raft dysfunction has been implicated recently in the pathogenesis of different human diseases. Several groups of pathogens, including bacteria, prions, viruses, and parasites appear to hijack lipid rafts during internalization [23-25]. In this chapter, we illustrate the proposed role of lipid rafts in the trafficking and processing of PrPc and APP and in the pathogenesis of their related diseases.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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