Analysis and suppression of mutant sel-12 in Caenorhabditis elegans
sel-12 and hop-1 are two C. elegans genes which are structurally and functionally homologous to the human presenilins, PS1 and PS2. Mutations in the human presenilins contribute to the majority of familial Alzheimer's disease cases. Work in C. elegans also revealed that presenilins are involved in Notch signaling as a reduction of sel-12 presenilin activity is able to suppress the phenotype associated with lin-12/Notch gain-of-function mutations. Several lines of evidence suggest that presenilins are an active protease that mediate the transmembrane cleavage event that releases the LIN-12/Notch intra-cellular domain from the membrane. This step is crucial for nuclear Notch signaling. As a consequence, loss of all presenilin activity, in a sel-12; hop-1 double mutant, causes phenotypes associated with the complete absence of all Notch signaling in C. elegans. Loss of sel-12 function in C. elegans leads to a highly penetrant egg-laying defect (Egl) which resembles the Egl defect present in lin-12 hypomorphic alleles. The inability of sel-12 mutant animals to lay eggs is shown here to be caused by two defects: (i) a  cell specification defect which results in a blockage of the vulva-uterine connection and (ii) a sex-muscle patterning defect leading to morphologically abnormal and misattached vulva-muscles. In order to find new molecules that are able to influence presenilin mediated signaling we screened for extragenic suppressors of the sel-12 Egl defect. In several screens four spr genes (suppressor of presenilin) were identified, which are able to suppress both sel-12 defects in the egg-laying system. The spr genes are not able to influence LIN-12/Notch signaling directly but require the activity of the second C. elegans presenilin hop-1 for sel-12 suppression. Molecular analysis of SPR-5 revealed that it belongs to a conserved family of FAD containing polyamine-oxidase like proteins. The human homologue is found to be an integral component of the CoREST transcriptional co-repressor complex. Further analysis of the other spr genes revealed that SPR-1 is the C. elegans homolog of the human CoREST protein, which is also a component of the CoREST complex. Based on the physical interaction of SPR-5 and SPR-1 presented here, it is likely that a CoREST like complex also exists in C. elegans. Mutations in this complex lead to a stage specific de-repression of hop-1 expression which is then able to substitute for sel-12 presenilin activity during development of the egg-laying system. The two remaining suppressors, spr-3 and spr-4, encode nuclear C2H2 zinc finger proteins that have been shown to up-regulate hop-1 transcription in a similar way and thus may be involved in nuclear targeting of the C elegans CoREST complex. sel-12 is also highly expressed in neurons, which suggests that it may also be required for neuronal function. Experiments presented here show that sel-12 is, for example, involved in the function and morphology of the AIY interneurons, which control the thermotaxis behaviour in C. elegans.
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