In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 10 ( 2012-03-06)
Abstract:
Thus, in summary, these findings show the existence of a mechanism of cell death in Drosophila other than developmental apoptosis and suggest its transcriptional control by NF-κB. These results also assign roles in addition to immunity to previously characterized genes of the immune signaling pathway. They also provide a basis for exploring NF-κB signaling as a therapeutic target in retinal degeneration. To understand further the role played by Relish in cell death, we asked if this prodeath signal could be attributed to its transcriptional activity (i.e., its effects on DNA in the nucleus) or to as-yet-unknown functions it has in the cell fluid (cytosol) outside the nucleus. To answer this question, we carried out gain-of-function studies by expressing certain parts of the Relish protein: ( i ) its transcriptionally active N-terminal region (domain) or ( ii ) the cytosol-localized C-terminal domain. Expression of the N-terminal domain in a variety of tissues proved detrimental ( Fig. P1 D – F ). However, overexpressed C-terminal domain or full-length Relish did not have any effects. These data suggest that Relish exerts a toxic effect via its transcriptional activity. We next examined the role played by genes that act downstream of Dredd during immune responses. The immune response pathway in Drosophila relies on the expression of antimicrobial peptides by enhancing the transcription of their genes. In this case, the transcription is activated via a transcription factor protein, NF-κB. During infection by Gram-negative bacteria, the immune response is mediated by the NF-κB homolog Relish. We observed that mutations in the gene relish completely prevented photoreceptor cell death Fig. P1 ( A – C ). We also observed that Relish was transcriptionally active specifically in light-treated norpA flies but not in wild-type (nonmutated) flies. Furthermore, mutations that block Relish activation rescue photoreceptor degeneration as well. These results all indicate its key role in retinal degeneration. To identify the players involved in photoreceptor cell death, we undertook a candidate gene approach. We reasoned that activation of effector caspases must rely on apical (initiator) caspases. Surprisingly, a mutation in the caspase-8 homolog, Dredd, rescued the light-induced cell death but had no adverse effects on retinal development, indicating that Dredd plays a crucial role in retinal degeneration but is dispensable during developmental apoptosis. Dredd is known to play a key role in the activation of the innate immune response against certain bacterial infections (Gram-negative bacteria). Interestingly, the upstream activators of Dredd in immune response were not involved in retinal degeneration in norpA flies. Thus, alternative and yet-uncharacterized mechanisms also can activate Dredd. We investigated the cell death signaling pathway activated in norpA flies. Previous analysis of norpA retinal degeneration suggested a marginal role for apoptotic players usually involved in developmental processes ( 5 ), indicating that the photoreceptor cell death might involve an alternative mechanism. Our data further clarify that the machinery in place for activating the major initiator caspase in this familiar apoptotic pathway, Dronc, is not engaged during retinal degeneration. The data presented here also suggest that effector caspases are involved, although only to a marginal extent. Caspases are enzymes that in general play a central role in apoptotic pathways. To model retinal degeneration, we used a Drosophila mutant, no receptor potential A ( norpA ). These flies lack a functional eye-specific protein, phospholipase C, which is the effector enzyme of the visual transduction pathway. Hence, these flies are defective in generating a response to light. The photoreceptors of these flies are viable when flies are reared in the dark but rapidly undergo cell death when the flies are exposed to light. Previous work on norpA mutants has revealed that exposure to light leads to massive endocytosis (internalization from the plasma membrane into the cell body) of rhodopsin, the light-sensing molecule ( 2 ). This internalized rhodopsin accumulates in the cell body and fails to undergo timely degradation ( 3 ). Preventing rhodopsin endocytosis or its accumulation rescues the photoreceptors from death ( 3 , 4 ). Retinal degeneration causes irreversible loss of vision in humans. In most cases, the cells affected are the neurons in the retina. The death of the retinal neurons and an inability to replace them causes permanent vision loss. Among the retinal neuronal cell types, photoreceptors seem to be especially susceptible to damage causing retinal degeneration ( 1 ) In most cases, the precise mechanism of photoreceptor death is not understood completely. We investigated the cell death (apoptosis) signaling pathways behind photoreceptor cell death and found that they involve players different from those of the known apoptotic pathways—specifically, the gene Relish , which is involved in immune responses.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1110666109
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2012
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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