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|Title:||A CAMK/CREB-DEPENDENT PATHWAY CONTRIBUTES TO THE MOLECULAR MECHANISM FOR LONG-TERM HABITUATION IN CAENORHABDITIS ELEGANS|
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|Authors/Affiliations:||1 Tiffany Timbers*; 1 Catharine Rankin; |
1 University of British Columbia, Vancouver, BC, Canada
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|Content:||Objectives: Habituation, the simplest form of learning, is a decrement in response to an irrelevant stimulus presented repeatedly. Long-term memories (LTM) of habituation can be formed, and thus this simple learning and memory task can be used to study the underlying molecular pathways. Coupling basic learning and memory with a simple, reductionist model organism, Caenorhabditis elegans, we have begun to piece together a molecular model for the induction of LTM of habituation.|
Materials and Methods: C. elegans strains containing identified mutations in CRH-1 (homologous to the mammalian transcription factor CREB), and several kinases were tested for deficits in LTM. Animals were administered distributed training (4 training blocks of 20 mechanical stimuli given at a 60 s interstimulus interval, with a one hour rest between blocks) and tested 24 hours later for memory by applying 5 test mechanical stimuli. The responses to test stimuli of trained and control animals were compared.
Confocal imaging of the interneurons of wild-type and CRH-1 mutant worms expressing GLR-1 (non-NMDA glutamate receptor subunit homologue) tagged with GFP was performed 24 hours after LTM training. The size of GFP clusters was compared between trained and control animals of each genotype.
Results: The kinase CMK-1 (homologous to mammalian CaMK1), and the transcription factor CRH-1 are necessary for LTM of habituation in C. elegans. A recent study showed that CMK-1 can phosphorylate CRH-1 and activate CRH-1-dependent gene transcription in vivo (Kimura et al, 2002). We hypothesize that this also occurs during the induction of LTM. Interestingly, animals with a mutation in CKK-1 (homologous to mammalian CaMKK), a kinase demonstrated to be upstream of CMK-1 (Kimura et al, 2002), showed normal LTM. This evidence suggests a novel kinase may play the role of activating CMK-1 in the context of LTM of habituation in C. elegans.
Our lab demonstrated that consolidation of memory for habituation is associated with a decrease in the size of GLR-1 synaptic clusters 24 hours after LTM training (Rose et al., 2003). Here we report that crh-1 mutants expressing a GLR-1::GFP transgene do not show a decrease in the size of GLR-1 synaptic clusters 24 hours after LTM training. This suggests that the decrease observed in wild-type animals is dependent upon CRH-1.
Conclusion: We present data that suggest that the kinase CMK-1 phosphorylaytes CRH-1, causing activation and gene transcription. This gene transcription, through an unidentified pathway, decreases the expression of GLR-1, and this decrease of AMPA-type glutamate receptors subunits then contributes to the behavioral expression of LTM for habituation in C. elegans.
Kimura et al. (2002) EMBO reports 3, 962-966.
Rose et al. (2003) J. Neurosci. 23, 9595-9599.
This work was supported by operating grants from NSERC to CHR and by Graduate Fellowships from MSFHR and CIHR to TAT.
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