Role of an apCAM-associated Protein in Long-Term Facilitation in Aplysia

Bong-Kiun Kaang

National Research Laboratory of Neurobiology, Institute of Molecular Biology and Genetics, School of Biological Sciences, College of Natural Sciences, Seoul National University, San 56-1 Silim-dong Kwanak-gu, Seoul 151-742, Korea

e-mail: kaang@snu.ac.kr 

The synaptic growth accompanying long-term facilitation induced by 5-HT in Aplysia is associated with the endocytosis of apCAM from the plasma membrane of the sensory neuron. Although phosphorylation of the cytoplasmic tail of apCAM by MAP kinase is known to be important for apCAM function in long-term facilitation, the identity of the molecules that interact with the cytoplasmic tail of apCAM remains unknown. To search for the binding partners of apCAM, we used the cytoplasmic tail of apCAM as bait in a LexA yeast two-hybrid screen and obtained an apCAM-Associated Protein (CAMAP) that was expressed in the sensory neurons of Aplysia and to a lesser degree in the central ganglia, the ovotestis, and the stomach. In sensory neurons, CAMAP co-localized with apCAM at the cytoplasmic face of the plasma membrane of the cell body and axonal neurites. CAMAP translocated into the nucleus following exposures to 5-HT that produces long-term facilitation either in a cell-wide manner or locally at the synapse. Microinjection of the double-strand RNA of CAMAP into the presynaptic sensory neuron of sensory-motor cocultures completely blocked 5-HT-induced long-term facilitation in a sequence specific manner without affecting short-term facilitation. Using Aplysia one-hybrid and in situ hybridization, we found that the N-terminal fragments of CAMAP induce the transcription of ApC/EBP, which is known to be regulated for long-term facilitation. The N-terminal fragment of CAMAP can convert 5-HT-induced short-term facilitation to the long-term form, most likely via the induction of ApC/EBP. However, the C-terminal fragment of CAMAP functions as an autoinhibitory domain against the N-terminal fragment and this, in turn, blocks the induction of ApC/EBP mRNA required for long-term facilitation. The expression of a C-terminal fragment of CAMAP blocked long-term facilitation induced by five pulses of 5-HT. Taken together, these results suggest that CAMAP may serve as an essential retrograde signal that translocates into the nucleus by following exposures to 5-HT to help activate transcription necessary for long-term facilitation.