Non-lethal stress treatments (X-radiation or heat shock) administered to Drosophila imaginal discs induce massive apoptosis, which may eliminate more that 50% of the cells. Yet the discs are able to recover to form final structures of normal size and pattern. Thus, the surviving cells have to undergo additional proliferation to compensate for the cell loss. The finding that apoptotic cells ectopically express dpp and wg suggested that ectopic Dpp/Wg signalling might be responsible for compensatory proliferation. We have tested this hypothesis by analysing the response to irradiation-induced apoptosis of disc compartments that are mutant for dpp, for wg, or for both. We find that there is compensatory proliferation in these compartments, indicating that the ectopic Dpp/Wg signalling generated by apoptotic cells is not involved. However, we demonstrate that this ectopic Dpp/Wg signalling is responsible for the hyperplastic overgrowths that appear when apoptotic ('undead') cells are kept alive with the caspase inhibitor P35. We also show that the ectopic Dpp/Wg signalling and the overgrowths caused by undead cells are due to a non-apoptotic function of the JNK pathway. We propose that the compensatory growth is simply a homeostatic response of wing compartments, which resume growth after massive cellular loss until they reach the final correct size. The ectopic Dpp/Wg signalling associated with apoptosis is inconsequential in compartments with normal apoptotic cells, which die soon after the stress event. In compartments containing undead cells, the adventitious Dpp/Wg signalling results in hyperplastic overgrowths.

Programmed cell death (apoptosis) is a conserved process aimed to eliminate unwanted cells. The key molecules are a group of proteases called caspases that cleave vital proteins, which leads to the death of cells. In Drosophila, the apoptotic pathway is usually represented as a cascade of events in which an initial stimulus activates one or more of the proapoptotic genes (hid, rpr, grim), which in turn activate caspases. In stress-induced apoptosis, the dp53 (Drosophila p53) gene and the Jun N-terminal kinase (JNK) pathway function upstream in the activation of the proapoptotic genes. Here we demonstrate that dp53 and JNK also function downstream of proapoptotic genes and the initiator caspase Dronc (Drosophila NEDD2-like caspase) and that they establish a feedback loop that amplifies the initial apoptotic stimulus. This loop plays a critical role in the apoptotic response because in its absence there is a dramatic decrease in the amount of cell death after a pulse of the proapoptotic proteins Hid and Rpr. Thus, our results indicate that stress-induced apoptosis in Drosophila is dependant on an amplification loop mediated by dp53 and JNK. Furthermore, they also demonstrate a mechanism of mutual activation of proapoptotic genes.

The PTEN-induced putative kinase 1 (PINK1)/Parkin pathway can tag damaged mitochondria and trigger their degradation by mitophagy. Before the onset of mitophagy, the pathway blocks mitochondrial motility by causing Miro degradation. PINK1 activates Parkin by phosphorylating both Parkin and ubiquitin. PINK1, however, has other mitochondrial substrates, including Miro (also called RhoT1 and -2), although the significance of those substrates is less clear. We show that mimicking PINK1 phosphorylation of Miro on S156 promoted the interaction of Parkin with Miro, stimulated Miro ubiquitination and degradation, recruited Parkin to the mitochondria, and via Parkin arrested axonal transport of mitochondria. Although Miro S156E promoted Parkin recruitment it was insufficient to trigger mitophagy in the absence of broader PINK1 action. In contrast, mimicking phosphorylation of Miro on T298/T299 inhibited PINK1-induced Miro ubiquitination, Parkin recruitment, and Parkin-dependent mitochondrial arrest. The effects of the T298E/T299E phosphomimetic were dominant over S156E substitution. We propose that the status of Miro phosphorylation influences the decision to undergo Parkin-dependent mitochondrial arrest, which, in the context of PINK1 action on other substrates, can restrict mitochondrial dynamics before mitophagy.is the second most common neurodegenerative disorder, and is closely linked to mitochondrial dysfunction (1, 2). Two hereditary forms of recessive PD are caused by mutations in PINK1 (PTEN-induced putative kinase 1), a Ser/Thr mitochondrial kinase, and Parkin, a cytosolic E3 ubiquitin ligase (3, 4). The realization that these proteins are in a single pathway, with PINK1 acting upstream of Parkin to influence mitochondrial properties, was a critical step in uncovering the underlying pathological mechanisms of PD (5-7). This pathway can trigger the selective autophagy of damaged mitochondria, termed mitophagy (8, 9), but additional cellular functions have also been indicated for PINK1 and Parkin (10-15). Much, however, remains unclear about how the PINK1/Parkin pathway is regulated.In current models of PINK1/Parkin mitophagy (reviewed in ref. 1), healthy mitochondria import a PINK1 precursor constitutively to the inner membrane, where it is cleaved (16-18). The cleaved form then returns to the cytoplasm and is degraded by the N-end rule pathway (19). Mitochondrial depolarization, or protein misfolding in the matrix of energized mitochondria (20), prevent the import and degradation of PINK1, resulting in the accumulation of PINK1 on the outer mitochondrial membrane (OMM) (9,21,22). Once on the OMM, PINK1 kinase activity recruits Parkin from the cytosol (8, 9). Although Parkin adopts a self-inhibited conformation in solution (23-25), it becomes fully activated in a PINK1-dependent manner on the mitochondria (9, 21). Parkin ubiquitinates numerous proteins of the OMM (26, 27), and thereby recruits autophagy-related proteins to the damaged mitochondrion for autophagosome assembly (28-30).How PINK1 recruits and activa...