Targeting of tetraspanin proteins--potential benefits and strategies

Abstract

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

Figure 1. Tetraspanin structural features

a | Shown in this ‘unfolded’ tetraspanin are membrane-proximal palmitoylations (red), hydrophilic residues within transmembrane domains (grey balls), and extracellular loops (EC1 and EC2). EC2, also called large extracellular loop (LEL) is divided into a constant region, containing A, B and E helices, and a variable region, containing the signature tetraspanin CCG motif, two conserved disulphide bonds (red) and a third loop and disulphide bond (dashed) that appears in some tetraspanins. b | This more realistic scheme emphasizes the close packing of the four transmembrane domains, the proximity of EC1 and EC2, and the overall rod shaped structure of tetraspanins. Disulphide bonds are not shown. This scheme is based on structural results seen for uroplakin tetraspanins UPK1A and UPK1B, and modelled for other tetraspanins^,.

Figure 2. Strategies for targeting tetraspanins

a | Monoclonal antibodies (mAbs) might interfere with tetraspanins by blocking lateral interactions, although this has not yet been well documented. Also, they may sequester tetraspanins, leading to disruption of tetra spanin-enriched domains (TEMs), down-modulation or triggering of apoptosis. In addition, mAbs can be used to deliver a lethal hit to cells expressing particular tetraspanins. For example, the antibody might trigger apoptosis, complement-dependent cytotoxicity or antibody-dependent cellular cytotoxicity, or deliver a conjugated toxin or lethal radioisotope or nanoparticle. b | Recombinant soluble large extracelullar loops (sLELs) most probably insert into TEMs, to disrupt lateral interactions among tetraspanins, or between tetraspanins and their partner molecules. In a special case, hepatitis C virus (HCV) infection, the CD81 sLEL acts in trans by binding to the virus and preventing interaction with cellular CD81. c | RNAi strategies are well documented to demonstrate the functional importance of tetraspanins. d | Other approaches might be useful for tetraspanin targeting. For example, a small molecule could be designed to inhibit the interaction of a C-terminal tail with a PDZ-domain protein. A peptide specifically mimicking a tetraspanin transmembrane domain might be effective in disrupting lateral interactions. Interference with the appropriate protein acyl transferase in the Golgi would prevent tetraspanin palmitoylation, leading to an inability to assemble TEMs and resulting in impaired functions.