Research summary:

Using Drosophila as a model system our work has focussed primarily on the study of two major processes, regeneration and tumorigenesis, and in particular the role of the Jun N-terminal Kinase (JNK) pathway, a conserved pathway known to be associated with tumorigenesis and regeneration in metazoans.

The JNK signalling pathway is a principal inducer of apoptosis in Drosophila, an autocrine function aimed to eliminate damaged or aberrant cells. This pro-apoptotic function includes the elimination by cell competition of oncogenic cells that may appear in development. Recent work has shown that JNK also has a pro-proliferation function, mediated by paracrine signalling and associated with the activity of the JAK/STAT, Wingless (Wg) and Decapentaplegic (Dpp).

Regarding the role of JNK function in tumorigenesis, we have found that it is a major factor in the process. In previous publications (Ballesteros-Arias et al 2014) we proposed that JNK-mediated proliferative signals are responsible for the growth of tumours generated by oncogenic mutations. More recent work (Pinal et al 2018) has demonstrated the full tumorigenic potential of JNK: in absence of apoptosis a transient stress (e.g. ionising radiation) results in persistent JNK activity and of its target pathways JAK/Stat, Wg or Dpp. The end result is continuous paracrine proliferative signalling, which causes tumour overgrowths. Moreover, we have found that the persistent JNK activity after irradiation is due to a maintenance loop involving Reactive Oxygen Species (ROS) and the DUOX maturation factor Moladiev (Mol). It is the existence of this loop what makes the absence of apoptosis potentially tumorigenic. We are in the process of studying JNK gene targets responsible for the tumorigenic response; a recent RNAseq screen performed in the lab has revealed a number JNK targets, several of which are conserved and also associated with human cancer.

We are also investigating the role of JNK in the tumorigenic processes generated by mutations like scribble, erupted or polyhomeotic. These mutations alter respectively apico-basal polarity, endocytosis or identity of epithelial cells and are also conserved in humans. The mechanisms by which defects in such different functions cause tumorigenesis are still obscure.

We are also trying to elucidate the mechanism of JNK activation during cell competition, which is still unclear. Some of the upstream factors are known (flower, sas/ptpd10, spz/toll, azot) but their specific molecular roles are not clear. It is also possible that there may be different ways to activate JNK.

JNK signalling displays also a paracrine function in regeneration processes. We propose to study the regenerative potential of imaginal discs or specific regions of the discs in relation to the local activity of JNK. Regarding regeneration we have compared the regeneration potential of the trunk region of the disc (the thorax) with that of the appendage (the wing). A principal finding is that the wing region shows a strong regeneration capacity while the trunk area does not. Our results indicate that this is due to local differences in JNK function, which appears to activate different target genes in trunk and appendage. This finding may be of interest to solve a classical problem in the regeneration field: why some organs are more regenerative than others or why different animal groups exhibit distinct regeneration potential. Furthermore, we have provided the first direct demonstration that the proliferative response required for regeneration is mediated by JNK activity emanating from damaged cells. An additional point of interest is that the trunk and the appendage of the wing disc are developmentally isolated in spite of sharing lineage: they cannot be reprogrammed to generate each other. These results have been published recently (Martin et al 2017; Martin and Morata 2018).

Another goal in the laboratory is to study chromatin remodelling during tissue regeneration. Surviving cells of damaged tissues need to readapt their status for new tissue formation. They do so by remodelling their chromatin to activate or repress a new battery of genes. A recent report based on work with mouse embryos suggest that activation of retrotransposon LINE elements regulates global chromatin accessibility at the beginning of development (Jachowicz, et al., 2018) suggesting that retrotransposon activation is integral to the early developmental program. During regeneration, nuclear chromatin might acquire an “intermediate” state that resembles the young embryonic totipotent state and that is mediated by retrotransposons.

Previous results in the laboratory show that the Roo and F-Element retrotransposons of Drosophila suffer profound changes in their leveles in the first hours of embryonic development. We are in the process of measuring the levels of these retrotransposon during tissue regeneration as well as altering them to study their putative role in regeneration.