CD95 ligand--death factor and costimulatory molecule?

Abstract

The CD95 ligand is involved as a death factor in the regulation of activation-induced cell death, establishment of immune privilege and tumor cell survival. In addition, CD95L may serve as a costimulatory molecule for T-cell activation. Alterations in expression or shedding of membrane and soluble CD95L are associated with numerous diseases, and underscore the pathophysiological relevance of the CD95/CD95L system. In most cases, the causal link between altered CD95L expression and pathophysiology is unknown. Given the potency of the molecule to regulate death and survival of many different cell types, the control of CD95L production, transport, storage, shedding and inactivation is of tremendous biological and clinical interest. This review summarizes the current knowledge, hypotheses and controversies about CD95L as a multifunctional ligand and receptor. It considers the different roles of membrane and soluble forms of CD95L and the complex networks of intracellular dynamics of protein trafficking, as well as the potential bidirectional signal transduction capacity of CD95L, with a focus on molecular interactions that have been worked out over the past years.

Figure 1

Structural features of a multifunctional death factor. Human CD95L is a type-II transmembrane molecule with 281 amino acids. The N-terminal cytoplasmic part (aa 1–81) contains a specific ‘double’ CKI substrate motif (CKI-S, aa 17–21). It also harbors an absolutely unique PRD (aa 37–70) which functions as a docking site for SH3 and WW domain proteins. The transmembrane region spans aa 81–102. Within the extracellular part, several metalloprotease substrate sites (MP-S) have been named to serve as cleavage sites for one or more metalloproteases (aa 109–115 (ELAELR), S126/127L and K129/130Q). These cleavage sites are located outside the region that is required for trimerization and self-assembly (SA). Thus, an sCD95L may also form trimers. The C-terminal THD (aa 183–281) contains several putative N-glycosylation sites (aa 184, 250, 260) and the very C-terminal receptor-binding site (RB) with a critical phenylalanin–residue at position 275.

Figure 2

Dual function of CD95L. (a) CD95L trimers function as death factor to trigger CD95-induced apoptosis via FADD and caspase activation. This is the key event during activation-induced cell death. It plays a major role in target cell cytolysis, in the establishment of immune privilege and during tumor counterattack. (b) Model for the costimulatory function of CD95L. By virtue of assembling with a number of cytosolic SH3 domain proteins required for TCR-dependent activation (double-circled), CD95L could alter the TCR/CD3 stimulus in different ways. In such a scenario, CD95L may serve as a costimulatory molecule to either directly interfere with signal 1 given by the TCR or by interfering with signal 2 given by classical costimulatory molecules such as CD28. In addition, CD95L could also create signals independent of other surface receptors.

Figure 3

CD95L interacting proteins – facts and hypotheses. Recent work by Griffiths et al.^,, showed that in cytotoxic cells, CD95L is targeted to secretory granules/lysosomes. Besides CD95L, these lysosomes contain other cytotoxic molecules such as granzymes and perforins, which in concert contribute to an effective target cell lysis. Targeting of CD95L to these lysosomes strictly depends on the PRD within the cytoplasmic tail of the molecule. In this context, members of the FCH/SH3 family (including FBP17, PACSINs, CD2BP1 and others) are good candidates for directing the localization and transport of CD95L (and/or secretory lysosomes in general). All proteins of this family are involved in actin cytoskeleton reorganization and protein/organelle trafficking. Other interactors for CD95L are the small adapter proteins Grb2 and Nck-1. Therefore, a working model for CD95L function would be that the PRD is not only required for lysosomal targeting, but also for transport and fusion of these lysosomes to the area where the immunological synapse is formed. This could be followed by the release of the various cytolytic factors and in the presence of a MMP (such as matrilysin, MMP-7) by CD95L cleavage. Whether the sCD95L induces or inhibits apoptosis then depends on the potential to form active multimers. The fact that CD95L has been found to act as a ‘negative’ or ‘positive’ costimulatory molecule for T cells underscores the complex biology of the molecule. Whether the same (FCH/SH3-family, Grb2 or Nck-1) or other (e.g. CKI, PI 3-kinases or Src kinases) CD95L interacting molecules are involved in this aspect, remains open.