The linker cell, a migratory leader cell of the male gonad, undergoes programmed cell death (PCD) at the late L4 larval stage, just prior to the adult molt. The molecular regulation of this cell death is unknown. We demonstrated that linker cell death does not require
ced-3 activity, and it may be entirely caspase-independent, as it is not blocked by expression of
p35, a broad-spectrum caspase inhibitor (n=20). Mutations in the known PCD core pathway genes or engulfment genes do not block linker cell death, suggesting that understanding this cell death may reveal new molecular pathways for PCD. We have begun to image linker cell death using electron microscopy. Preliminary studies of wild-type animals reveal that the linker cell is engulfed by a U cell descendant even though the linker cell looks healthy. However, we have observed that the linker cell still dies in animals defective in both engulfment pathways, suggesting that engulfment is not required for linker cell death. Previous studies reported that the linker cell can survive in a U cell ablated animal or in animals in which the linker cell failed to migrate to its final position, leading to a model of linker cell murder by a locally-acting extrinsic signal from U cell descendants (1). Our U cell ablations and analyses of linker cell migration mutants are not consistent with this model, as we observe linker cell death in both contexts. This suggests that linker cell death may have a major cell-autonomous component. Because the linker cell dies at the L4/Adult transition, we wondered if cell-autonomous developmental timing signals might regulate this cell death. Interestingly, the heterochronic gene
lin-29, a zinc finger transcription factor, is expressed in the linker cell (2). Remarkably, in
lin-29 (
n333) mutants, morphologically wild-type linker cells survive in adult worms, even 24 hours after the normal time of linker cell death. In adult wild-type animals 0/120 linker cells survived, but in
lin-29 animals, 98/197 linker cells survived with normal morphology. We are performing mosaic and expression studies to characterize the cellular focus and timing requirements for LIN-29 function in linker cell death. We are also examining additional heterochronic mutants. We propose that
lin-29 targets may encode the killer activities needed to eliminate the linker cell during development. To identify these we are initiating a visual screen to find new regulators of linker cell death. 1. Sulston, J.E. et al. (1980) Dev. Biol 78, 542-576 2. Euling, S. et al. (1999) Dev. Biol 206, 142-156