A Novel Juxtamembrane Domain in Tumor Necrosis Factor Receptor Superfamily Molecules Activates Rac1 and Controls Neurite Growth

Abstract

Members of the tumor necrosis factor receptor (TNFR) superfamily control cell fate determination, including cell death and differentiation. Fas (CD95) is the prototypical "death receptor" of the TNFR superfamily and signals apoptosis through well established pathways. In the adult nervous system, Fas induces apoptosis in the context of neuropathology such as stroke or amyotrophic lateral sclerosis. However, during nervous system development, Fas promotes neurite growth and branching. The molecular mechanisms underlying Fas-induced process formation and branching have remained unknown to date. Here, we define the molecular pathway linking Fas to process growth and branching in cell lines and in developing neurons. We describe a new cytoplasmic membrane proximal domain (MPD) that is essential for Fas-induced process growth and that is conserved in members of the TNFR superfamily. We show that the Fas MPD recruits ezrin, a molecule that links transmembrane proteins to the cytoskeleton, and activates the small GTPase Rac1. Deletion of the MPD, but not the death domain, abolished Rac1 activation and process growth. Furthermore, an ezrin-derived inhibitory peptide prevented Fas-induced neurite growth in primary neurons. Our results define a new domain, topologically and functionally distinct from the death domain, which regulates neuritogenesis via recruitment of ezrin and activation of Rac1.

How CD95 stimulates invasion

“…By contrast, CD95-induced migration and invasion does not appear to require an intact DD. 9,12,19,20 Nevertheless, in some tumor cell lines, invasion signaling by CD95 depends on caspase 8 11 and, by inference, on the DD to which it is recruited.…”

“…In these cells, it stimulates the branching and outgrowth of neurites rather than apoptosis. 20,50,51 The molecular mechanisms driving neurite outgrowth share many features with the mechanisms driving (tumor) cell migration. Both processes are orchestrated by the coordinated activation and inactivation of the Rho family GTPases RhoA, Rac and CDC42.…”

“…CD95L stimulates Rac activity and this is essential for CD95-induced neurite outgrowth. 20 Interestingly, CD95-induced Rac activation occurs independently of the death domain (DD). 20 Likewise, CD95-induced tumor cell invasion and neutrophil migration also occur independently of the DD.…”

“…20 Interestingly, CD95-induced Rac activation occurs independently of the death domain (DD). 20 Likewise, CD95-induced tumor cell invasion and neutrophil migration also occur independently of the DD. 9,12 This strongly suggests that the invasion-and apoptosis-promoting signaling pathways already diverge at the level of the receptor.…”

“…A conserved membrane-proximal domain (MPD), but not the death domain, is required for binding of CD95 to Ezrin and for Rac activation. 20 The MPD is located between the transmembrane domain and the death domain. Ezrin can promote activation of Rho GTPases, including Rac, by binding and sequestering RhoGDI.…”