The development of functional tissues relies on the fine balance between cell death and cell division. Little is known about how tissue size and cell numbers are maintained in stress situations when additional proliferation is needed to compensate for cell loss. The c-Jun N-terminal kinases (JNK) signalling pathway is a stress pathway that has been proposed to trigger not only cell death but also cell survival and proliferation. To decipher the regulatory events that control these diverse responses, we have set out to characterise the spatial-temporal dynamics of cell fate (proliferation, cell death, cell cycle arrest and migration) upon JNK activation. We optogenetically activate signalling in a subset of cells by expressing either a constitutively active form of the JNK Kinase hemipterous (hep). Then, we reconstruct the cell-autonomous and non-autonomous response to stress signalling. The response is very fast: we observe cell death and cell cycle arrest within two hours of Hep activation. However, the response lasts for an extended time, with a population expressing low JNK levels being observed 18 hours after the light pulse. We reconstructed the pattern of gene expression in time and space, using both biased and unbiased approaches. First, we used the Hybridization Chain Reaction to visualise the expression of genes known to be regulated by JNK signalling, such as Wg/Wnt and Dpp/BMP. Finally, we performed bulk RNA sequencing to identify the genes that could potentially account for the varied effect of JNK signalling activation. We discovered that under temporary and mild stress levels, there is no significant upregulation of morphogens/growth factors but instead of their modulators: glypicans. In the absence of glypicans, the replacement of lost cells is impaired. We hope that our results will guide further studies of more complex tissues of direct biomedical relevance.
