Abstract
Epithelia are an eminent tissue type and a common driver of tumorigenesis, requiring continual precision in cell division to maintain tissue structure and genome integrity. Mitotic defects often trigger apoptosis, impairing cell viability as a tradeoff for tumor suppression. Identifying conditions that lead to cell death and understanding the mechanisms behind this response are therefore of considerable importance. Here we investigated how epithelia of the Drosophila wing disc respond to loss of Short stop (Shot), a cytoskeletal crosslinking spectraplakin protein that we previously found to control mitotic spindle assembly and chromosome dynamics. In contrast to other known spindle-regulating genes, Shot knockdown induces apoptosis in the absence of Jun kinase (JNK) activation, but instead leads to elevated levels of active p38 kinase. Shot loss leads to double-strand break (DSB) DNA damage, and the apoptotic response is exacerbated by concomitant loss of p53. DSB accumulation is increased by suppression of the spindle assembly checkpoint, suggesting this effect results from chromosome damage during error-prone mitoses. Consistent with DSB induction, we found that the DNA damage and stress response genes, Growth arrest and DNA damage (GADD45) and Apoptosis signal-regulating kinase 1 (Ask1), are transcriptionally upregulated as part of the shot-induced apoptotic response. Finally, co-depletion of Shot and GADD45 induced significantly higher rates of chromosome segregation errors in cultured cells and suppressed shot-induced mitotic arrest. Our results demonstrate that epithelia are capable of mounting molecularly distinct responses to loss of different spindle-associated genes and underscore the importance of proper cytoskeletal organization in tissue homeostasis.
Highlights
The diverse and complex architectures that form animal tissues require spatial and temporal precision in the cell divisions that underlie their developmental growth patterns and homeostasis throughout adulthood
Whereas sas-4RNAi lead to an increase in phosphorylated Jun-related kinase (JNK), shotRNAi-mediated apoptosis occurred in the absence of such JNK activation (Fig. 1I–L)
To investigate this distinction further, we examined the level of activated p38, a related MAP kinase that contributes to apoptosis signaling as well as playing a key role in stress signaling[18]
Summary
The diverse and complex architectures that form animal tissues require spatial and temporal precision in the cell divisions that underlie their developmental growth patterns and homeostasis throughout adulthood. Defects in numerous genes controlling core aspects of epithelia cell function, including cell polarity, spindle assembly and function, as well as oriented cell division have been shown to trigger cell death[7] In many of these cases, including those involving spindle malfunction, activation of the Jun-related kinase (JNK) appears to be a common signaling effector in the apoptotic r esponse[8]. JNK appears to play a central role in epithelial responses to defects in cell polarity and/or division In contrast to these results, we recently demonstrated that loss of the spectraplakin gene, Short stop (Shot), induces apoptosis in Drosophila wing discs in the absence of JNK a ctivation[15]. In addition to triggering apoptosis in vivo, Shot loss leads to SACdependent mitotic delay and DNA segregation errors in cell c ulture[15] Many of these mitotic defects are shared with SAS-4 knockdown, yet their apoptotic responses have opposite JNK dependencies. Our results demonstrate the ability of epithelia to elicit different responses to loss of genes controlling similar mitotic spindle functions and highlight a JNK-independent mode of epithelial apoptosis following disruption of a cytoskeletal-organizing gene
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