Cellular Compartments as Signaling Microdomains A General Overview

A large number of pharmacologically important signaling molecules localize in the plasma membrane, and as such, differential expression of such molecules in various cell types and tissues is an over riding principle that dominates drug development

P.A. Insel

University of California, San Diego Department of Pharmacology, 9500 Gilman Drive, BSB 3076,

La Jolla, CA 92093-0636, USA

[email protected]

E. Klussmann, J. Scott (eds.) Protein-Protein Interactions as New Drug Targets. Handbook of Experimental Pharmacology 186, © Springer-Verlag Berlin Heidelberg 2008

and usage. The plasma membrane is one of several cellular organelles (others include the nucleus, Golgi apparatus and mitochondria) that can be readily recognized using light and/or electron microscopy. Subcellular fractionation methods have provided the principal starting point for the application of biochemical and biophysical methods to assess the properties and functions of such organelles. Substantial evidence, however, supports the view that preparations derived from the use of fractionation methods are quite heterogeneous, with differences in protein and lipid composition, as well as in organelle-specific functional activities. Thus, there are generally no "gold standard" methods to isolate "pure" organelles, in particular, the plasma membrane and its sub-domains. Such plasma membrane regions show unique patterns of expression of molecular components and functional activities, as well as, in some cases, unique anatomic features; examples include the apical and basolateral membranes of epithelial cells, luminal and ablu-minal membranes of endothelial cells, and dendritic, axonal and cell body membranes of neurons.

Plasma membrane receptors are critical sites of action for a large number of currently used drugs and drugs in development. Such receptors include G-protein-coupled receptors (GPCR), the largest membrane receptor super-family in eukaryotic genomes, and post-GPCR components, e.g., heterotrimeric G-proteins and G-protein-regulated effector molecules, which are the three necessary and sufficient components that mediate signal transduction by GPCR. In native cell systems, the level of expression of the three key components is: GPCR, typically <10,000-1 cell for individual GPCR that link to Gs and Gi; G-proteins (~1,000,000-1 cell) and effectors (~30,000-1 cell in the case of adenylyl cyclase), thus yielding a stoichiometric ratio of ~1:100:3 for GPCR that act via Gs and Gi to regulate the activity of adenylyl cyclase (Ostrom et al. 2000). Limited data are available for the stoichiometric ratios of other classes of heterotrimeric G-proteins and the GPCRs and effectors with which they interact.

Knowledge of these stoichiometric relationships among the critical GPCR signaling components is not sufficient information to explain the speed, fidelity and extent of response (measured as second messenger generation) to GPCR agonists, including the selectivity of activation of distal effectors. Such observations have provided a rationale in support of the concept of compartmentation of GPCR and components distal to the receptors (Lucero and Robbins 2004; Ostrom and Insel 2004; Pike 2004). The identification of plasma membrane domains that organize and compartmentalize GPCR and post-GPCR signaling components has helped provide both anatomic and functional evidence for membrane microdomains involved in signal transduction by GPCR. Such microdomains include clathrin-coated pits, membrane/lipid rafts, teteraspanins and caveolae ("little caves"), a subset of membrane/lipid rafts. In this review, we focus on rafts and caveolae. Other articles provide overviews on clathrin-coated pits (Lefkowitz and Shenoy 2005; Luttrell and Lefkowitz 2002; von Zastrow 2003) and tetraspanins (Berditchevski and Odintsova 2007).

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