13. A common pathway for apoptosis functions via caspases

27.12 The Fas receptor is a major trigger for apoptosis

Figure 27.39 The Fas receptor and ligand are both membrane proteins. A target cell bearing Fas receptor apoptoses when it interacts with a cell bearing the Fas ligand.

The Fas receptor (called Fas or FasR) and Fas ligand (FasL) are a pair of plasma membrane proteins whose interaction triggers one of the major pathways for apoptosis. Figure 27.39 shows that the cell bearing the Fas receptor apoptoses when it interacts with the cell carrying the Fas ligand (for review see Nagata, 1999).

Figure 27.40 Fas forms trimers that are activated when binding to FasL causes aggregation.

Activation of Fas resembles other receptors in involving an aggregation step. However, Figure 27.40 shows that there are some interesting differences from the growth receptor model. First, Fas forms a homomeric trimer. Second, it may be the case that the trimer assembles before the interaction with ligand. The effect of ligand may be to cause the trimers to cluster into large aggregates (Chan et al., 2000; Siegel et al., 2000). At all events, when FasL interacts with Fas, there is an aggregation event that enables Fas to activate the next stage in the pathway.

The names of the two proteins (Fas receptor and Fas ligand) reflect the way the system was discovered. An antibody directed against Fas protein kills cells that express Fas on their surface. The reason is that the antibody-Fas reaction activates Fas, which triggers a pathway for apoptosis. This defines Fas as a receptor that activates a cellular pathway.

Fas is a cell surface receptor related to the TNF (tumor necrosis factor) receptor. The FasL ligand is a transmembrane protein related to TNF. A family of receptors related to TNF includes two TNF receptors, Fas, and several receptors found on T lymphocytes. A corresponding family of ligands comprises a series of transmembrane proteins. This suggests that there are several pathways, each of which can be triggered by a cell-cell interaction, in which the "ligand" on one cell surface interacts with the receptor on the surface of the other cell.

Both Fas- and TNF-receptors can activate apoptosis. And in addition to its membrane-bound form, TNF also exists as cleaved, soluble protein, which functions as a soluble growth factor. In this form, largely produced by macrophages, TNF is a pleiotropic factor that signals many cellular responses, including cytotoxicity. Most of its responses are triggered by interaction with one of the TNF receptors, TNF-R1. FasL is cleaved to generate a soluble form, but the soluble form is much less active than the membrane-bound form, so the reaction probably is used to reduce the activity of the cell bearing the ligand.

An assay for the capacity of the ligand-receptor interaction to trigger apoptosis is to introduce the receptor into cultured cells that do not usually express it. On treatment with the ligand, the transfected cells die by apoptosis, but the parental cells do not. Using this assay, similar results are obtained with FasL/Fas receptor and with TNF/TNF-R1. Mutant versions of the receptor show that the apoptic response is triggered by an ~80 amino acid intracellular domain near the C-terminus. This region is loosely conserved (~28%) between Fas and TNF-R1, and is called the death domain (Tartaglia et al., 1993).

An assay for components of the apoptotic pathway in the cell is to see whether their over-expression causes apoptosis. This is done by transfecting the gene for the protein into the cell (which results in over-expression of the protein). This assay identifies several proteins that interact specifically with the Fas and/or TNF receptors. All of these proteins themselves have death domains, and it is possible that a homomeric interaction between two death domains provides the means by which the signal is passed from the receptor to the next component of the pathway.

The validity of this pathway in vivo was demonstrated by the discovery of the mouse mutation lpr. This is a recessive mutation in the gene for Fas. It causes proliferation of lymphocytes, resulting in a complex immune disorder affecting both B cells and T cells. Another mutation with similar effects is gld (generalized lymphoproliferative disease). This turns out to lie in the gene that codes the FasL ligand. The related properties of these two loci suggest that this apoptotic pathway is triggered by an interaction between the FasL ligand (gld product) and Fas (lpr product). The pathway is required for limiting the numbers of mature lymphocytes (Ito et al., 1991; Watanabe-Fukunaga et al., 1992; Suda et al., 1993).

This section updated 8-22-2000

Reviews

Nagata, S. (1999). Fas ligand-induced apoptosis. Ann. Rev. Genet. 33, 29-55.

Research

Chan, F. K. , Chun, H. J. , Zheng, L. , Siegel, R. M. , Bui, K. L. , Lenardo, M. J. , Chan, F. K. , Chun, H. J. , Zheng, L. , Siegel, R. M. , Bui, K. L. , and Lenardo, M. J. (2000). A domain in TNF receptors that mediates ligand-independent receptor assembly and signaling . Science 288, 2351-2354.

Ito, N. et al. (1991). The polypeptide encoded by the cDNA for human cell surface antigen Fas can mediate apoptosis. Cell 66, 233-243.

Siegel, R. M. , Frederiksen, J. K. , Zacharias, D. A. , Chan, F. K. , Johnson, M. , Lynch, D. , Tsien, R. Y. , Lenardo, M. J. , Siegel, R. M. , Frederiksen, J. K. , Zacharias, D. A. , Chan, F. K. , Jo (2000). Fas preassociation required for apoptosis signaling and dominant /dt>

UI  - 20336850 AU  - Siegel RM AU  - Frederiksen JK AU  - Zacharias DA AU  - Chan FK AU  - Johnson M AU  - Lynch D AU  - Tsien RY AU  - Lenardo MJ TI  - . Science 288, 2354-2357.

Suda, T. et al. (1993). Molecular cloning and expression of the Fas ligand, a novel member of the TNF family. Cell 75, 1169-1178.

Tartaglia, L. A. et al. (1993). A novel domain within the 55 kD TNF receptor signals cell death. Cell 74, 845-853.

Watanabe-Fukunaga, R. et al. (1992). Lymphoproliferation disorder in mice explained by defects in Fas antigen that mediates apoptosis. Nature 356, 314-317.