Quantitative description of the interactions among kinase cascades underlying long-term plasticity of Aplysia sensory neurons.

Kinases play critical roles in synaptic and neuronal changes involved in the formation of memory. However, significant gaps exist in the understanding of how interactions among kinase pathways contribute to the mechanistically distinct temporal domains of memory ranging from short-term memory to long-term memory (LTM). Activation of protein kinase A (PKA) and mitogen-activated protein kinase (MAPK)-ribosomal S6 kinase (RSK) pathways are critical for long-term enhancement of neuronal excitability (LTEE) and long-term synaptic facilitation (LTF), essential processes in memory formation. This study provides new insights into how these pathways contribute to the temporal domains of memory, using empirical and computational approaches. Empirical studies of Aplysia sensory neurons identified a positive feedforward loop in which the PKA and ERK pathways converge to regulate RSK, and a negative feedback loop in which p38 MAPK inhibits the activation of ERK and RSK. A computational model incorporated these findings to simulate the dynamics of kinase activity produced by different stimulus protocols and predict the critical roles of kinase interactions in the dynamics of these pathways. These findings may provide insights into the mechanisms underlying aberrant synaptic plasticity observed in genetic disorders such as RASopathies and Coffin-Lowry syndrome.

The 5-HT- and FMRFa-activated signaling pathways interact at the level of the Erk MAPK cascade: potential inhibitory constraints on memory formation.

The sensorimotor synapse of Aplysia exhibits long-term facilitation (LTF) and long-term depression (LTD) elicited by the neuromodulator serotonin (5-HT) and the peptide Phe-Met-Arg-Phe-NH(2), respectively. 5-HT-induced LTF engages extracellular-regulated kinase (Erk) and CREB1, whereas FMRFa-induced LTD engages p38 MAPK (mitogen-activated protein kinase) and CREB2. The interaction of the 5-HT and FMRFa pathways was recently investigated in Aplysia at the level of gene expression. However, little is known about crosstalk of these pathways at the level of the second messenger cascades. We investigated the potential interaction of the 5-HT and FMRFa pathways at the level of the Erk cascade. We found that FMRFa inhibited basal Erk activity through p38 MAPK. FMRFa also inhibited 5-HT-induced phosphorylation of Erk and nuclear accumulation of phospho-ERK, suggesting that FMRFa may place inhibitory constraints on memory formation through regulation of the Erk MAPK cascade.

cAMP-response elements in Aplysia creb1, creb2, and Ap-uch promoters: implications for feedback loops modulating long term memory.

The Aplysia genes encoding for cAMP-response element-binding protein 1 (CREB1), CREB2, and ubiquitin C-terminal hydrolase (Ap-uch) have been implicated in the formation of long term memory. However, nothing is known about the promoter regions of these genes or the transcription factors that regulate them. We cloned the promoter regions of creb1, creb2, and Ap-uch and identified a canonical cAMP-response element (CRE) in the promoter region of creb1. Variants of the canonical CRE were identified in all three promoters. TATA boxes and C/EBP-binding motifs are also present in the promoter regions of these genes. Promoter immunoprecipitation assays and chromatin immunoprecipitation assays indicated that CREB1 and CREB2 bind to the promoter regions of creb1 and creb2, suggesting that feedback loops modulate the formation of long term memory. In a positive feedback loop, phosphorylated CREB1 might induce its own gene via CREs. In support of this suggestion, treatment with serotonin enhanced binding of CREB1 to its promoter region and increased mRNA levels of creb1. Levels of Ap-uch mRNA also increased in response to serotonin; however, binding of CREB1 or CREB2 to the promoter region of Ap-uch was not detected. The finding that the promoter region of creb2 has a CRE raises the intriguing possibility that its expression is regulated by CREB1 and/or CREB2. CREB2 may repress its own gene, forming a negative feedback loop, and CREB2 up-regulation via CREB1 may limit the activity of the CREB1-mediated positive feedback loop.