Identification of specific mRNAs affected by treatments producing long-term facilitation in Aplysia

Immediate and persistent transcriptional correlates of long-term sensitization training at different CNS loci in Aplysia californica

Repeated noxious stimulation produces long-term sensitization of defensive withdrawal reflexes in Aplysia californica, a form of long-term memory that requires changes in both transcription and translation. Previous work has identified 10 transcripts which are rapidly up-regulated after long-term sensitization training in the pleural ganglia. Here we use quantitative PCR to begin examining how these transcriptional changes are expressed in different CNS loci related to defensive withdrawal reflexes at 1 and 24 hours after long-term sensitization training. Specifically, we sample from a) the sensory wedge of the pleural ganglia, which exclusively contains the VC nociceptor cell bodies that help mediate input to defensive withdrawal circuits, b) the remaining pleural ganglia, which contain withdrawal interneurons, and c) the pedal ganglia, which contain many motor neurons. Results from the VC cluster show different temporal patterns of regulation: 1) rapid but transient up-regulation of Aplysia homologs of C/EBP, C/EBPγ, and CREB1, 2) delayed but sustained up-regulation of BiP, Tolloid/BMP-1, and sensorin, 3) rapid and sustained up-regulation of Egr, GlyT2, VPS36, and an uncharacterized protein (LOC101862095), and 4) an unexpected lack of regulation of Aplysia homologs of calmodulin (CaM) and reductase-related protein (RRP). Changes in the remaining pleural ganglia mirror those found in the VC cluster at 1 hour but with an attenuated level of regulation. Because these samples had almost no expression of the VC-specific transcript sensorin, our data suggests that sensitization training likely induces transcriptional changes in either defensive withdrawal interneurons or neurons unrelated to defensive withdrawal. In the pedal ganglia, we observed only a rapid but transient increase in Egr expression, indicating that long-term sensitization training is likely to induce transcriptional changes in motor neurons but raising the possibility of different transcriptional endpoints in this cell type.

Characterization of the rapid transcriptional response to long-term sensitization training in Aplysia californica

We used a custom-designed microarray and quantitative PCR to characterize the rapid transcriptional response to long-term sensitization training in the marine mollusk Aplysia californica. Aplysia were exposed to repeated noxious shocks to one side of the body, a procedure known to induce a long-lasting, transcription-dependent increase in reflex responsiveness that is restricted to the side of training. One hour after training, pleural ganglia from the trained and untrained sides of the body were harvested; these ganglia contain the sensory nociceptors which help mediate the expression of long-term sensitization memory. Microarray analysis from 8 biological replicates suggests that long-term sensitization training rapidly regulates at least 81 transcripts. We used qPCR to test a subset of these transcripts and found that 83% were confirmed in the same samples, and 86% of these were again confirmed in an independent sample. Thus, our new microarray design shows strong convergent and predictive validity for analyzing the transcriptional correlates of memory in Aplysia. Fully validated transcripts include some previously identified as regulated in this paradigm (ApC/EBP and ApEgr) but also include novel findings. Specifically, we show that long-term sensitization training rapidly up-regulates the expression of transcripts which may encode Aplysia homologs of a C/EBPγ transcription factor, a glycine transporter (GlyT2), and a vacuolar-protein-sorting-associated protein (VPS36).

Poly-(ADP-ribose) polymerase-1 is necessary for long-term facilitation in Aplysia

Activity-dependent long-term synaptic plasticity requires gene expression and protein synthesis. Identifying essential genes and studying their transcriptional and translational regulation are key steps to understanding how synaptic changes become long lasting. Recently, the enzyme poly-(ADP-ribose) polymerase 1 (PARP-1) was shown to be necessary for long-term memory (LTM) in Aplysia. Since PARP-1 decondenses chromatin, we hypothesize that this enzyme regulates the expression of specific genes essential for long-term synaptic plasticity that underlies LTM. We cloned Aplysia PARP-1 (ApPARP-1) and determined that its expression in sensory neurons is necessary for serotonin (5-HT)-mediated long-term facilitation (LTF) of sensorimotor neuron synapses. PARP enzymatic activity is also required, since transient application of PARP inhibitors blocked LTF. Differential display and RNA analysis of ganglia dissected from intact animals exposed to 5-HT identified the ribosomal RNA genes as PARP-dependent effector genes. The increase in the expression of rRNAs is long lasting and dynamic. Pulse-labeling RNA studies showed a PARP-dependent increase in rRNAs but not in the total RNA 24 h after 5-HT treatment. Moreover, the expression of both the AprpL27a (Aplysia ribosomal protein L27a) and the ApE2N (Aplysia ubiquitin-conjugating enzyme E2N) mRNAs also increased after 5-HT. Thus, our results suggest that 5-HT, in part by regulating PARP-1 activity, alters the expression of transcripts required for the synthesis of new ribosomes necessary for LTF.

Molecular memory traces

To understand the essence of memory, one must examine the working of the brain on many levels. It is important to find the appropriate level to study the particular aspect of memory under investigation. In this review, I will focus on insights gained from examining memory at the molecular level. I will illustrate these insights with specific examples from examining the molecular and cellular mechanisms underlying long-term facilitation in the marine mollusk Aplysia and long-term potentiation, studied mainly in rodents. In particular, I will discuss how molecular memory traces are formed and focus in detail on what role increasing the level of proteins through protein synthesis and gene expression plays in memory formation. I will point out three important constraints from molecular work that should impact on cognitive modeling of the nervous system: (i) the induction of plasticity depends on the 'state' of the synapse; (ii) there are multiple independent molecular traces formed after experience with different half-lives; and (iii) the requirement for the conjunction of synaptic activation and new protein synthesis implies that new conjunctions are required to induce long-term memory formation.