The C. elegans linker cell (LC) directs migration and development of the male gonad, dying in the L4-adult transition to allow vas deferens-cloaca fusion.We previously showed that the LC dies independently of apoptosis genes, displaying non-apoptotic features including nuclear membrane invagination, open chromatin, and organelle swelling.These features are conserved in dying cells in normal vertebrate development and in polyQ-expansion diseases.
let-70 encodes an E2 ubiquitin-conjugating enzyme we identified in an RNAi screen for LC death genes.
let-70(RNAi) males are defective in LC migration and death and LC-specific RNAi using
rde-1 rescue blocks death but not migration. Thus,
let-70 functions in the LC to promote cell death and non-autonomously to control migration.
let-70 is transcriptionally induced in the LC as the cell begins to die. Similarly,we found that expression of a ubiquitin gene reporter,
ubq-1::gfp,also increases, as does expression of PQN-41, a polyQ-repeat protein required for LC death (Blum et al, 2012).These results suggest that LC death is controlled by a transcriptional program. Inactivation of the MAPKK
sek-1 or the histone methyltransferase
set-16 blocks
let-70 expression and LC death (Blum et al, 2012).
nhr-67 RNAi induces early
let-70::GFP expression in the LC; however,
nhr-67(RNAi) blocks LC death and double RNAi against
nhr-67 and
let-70 synergistically increases cell survival.Thus, proper timing of expression and/or optimal levels of
let-70 may be crucial for its cell death role.We screened for E3 ligases needed for LC death and found that RNAi against seven-in-absentia homolog
siah-1 or the RING-box
rbx-1 blocked LC death in 10% of animals.
siah-1;
rbx-1 double mutants show nearly 30% LC survival, suggesting that they act in parallel. Furthermore, 2-hybrid studies show that LET-70 and SIAH-1 interact. We suggest, therefore, that
let-70 degrades proteins normally required to inhibit LC death. Supporting this notion, RNAi against proteasome regulatory components blocks LC death, as does RNAi against other SCF components.Our studies suggest a role for protein degradation in the control of a novel, conserved form of cell death with potential relevance to human neurodegeneration.