Apoptosis is the most abundant form of programmed cell death (PCD) in metazoans, but some cells seem to utilize different cell death pathways independent of caspases, the executioners of the apoptotic program. To date, very little is known about the signaling and molecules underlying such alternative cell death pathways. In a previous work (Yacobi-Sharon et al., Dev Cell, 2013), we discovered and delineated a cell death pathway which is utilized to normally and constantly eliminate about one quarter of the emerging (premeiotic) spermatogonial cells in the adult Drosophila testis. This new cell death pathway, also termed Germ Cell Death (GCD), displays distinct morphological features reminiscent of both apoptosis and necrosis, requires lysosomal and mitochondrial proteases, and is independent of the main apoptotic effector caspases. Whereas this study uncovered some of the GCD executioners, nothing is known about the signaling pathway which triggers this cell death pathway.
Similar to the male germ cells in mammals, which require constant contact with their somatic supporting (Sertoli) cells for proper development, the Drosophila male germ cells are enveloped by a pair of somatic supporting (cyst) cells throughout maturation. Interestingly, we found that following GCD, the cyst cells activate caspases and undergo apoptosis, thus suggesting that GCD may be regulated extrinsically by signals originating in these somatic supporting. Indeed, by knocking down different signaling pathways in the cyst cells we identified the Hippo/Warts pathway as an important regulator of GCD. The Hippo/Warts signaling pathway is known to limit organ/cell growth and promote apoptosis by inhibiting the anti-apoptotic and pro-proliferative activity of the transcriptional coactivator Yorkie. Cyst cell-specific knockdown of either the main kinase in this pathway, Warts, or its coactivator, Mats, significantly attenuated GCD levels, whereas knockdown of Yorkie resulted in the enhancement of GCD levels. Further genetic and electron microscopy experiments support a model in which Hippo/Warts signaling in the supporting cells triggers the alternative cell death of the germ cells, which is followed by apoptosis of the supporting cells. These results demonstrate a role of the Hippo/Warts pathway in a non- autonomous regulation of a cell death program.