Two Signaling Pathways Regulate the Expression and Secretion of a Neuropeptide Required for Long-Term Facilitation inAplysia

Abstract

Activation of several signaling pathways contributes to long-term synaptic plasticity, but how brief stimuli produce coordinated activation of these pathways is not understood. In Aplysia, the long-term facilitation (LTF) of sensory neuron synapses by 5-hydroxytryptamine (serotonin; 5-HT) requires the activation of several kinases, including mitogen-activated protein kinase (MAPK). The 5-HT-enhanced secretion of the sensory neuron-specific neuropeptide sensorin mediates the activation of MAPK. We find that stimulus-induced activation of two signaling pathways, phosphoinositide 3-kinase (PI3K) and type II protein kinase A (PKA), regulate sensorin secretion and responses. Treatment with 5-HT produces a rapid increase in sensorin synthesis, especially at varicosities, which precedes the secretion of sensorin. PI3K inhibitor and rapamycin block LTF and the rapid synthesis of sensorin at varicosities even in the absence of sensory neuron cell bodies. Secretion of the newly synthesized sensorin from the varicosities and activation of the autocrine responses of sensorin to produce LTF require type II PKA interaction with AKAPs (A-kinase anchoring proteins). Thus, long-term synaptic plasticity is produced when multiple signaling pathways that are important for regulating distinct cellular functions are activated in a specific sequence and recruit the secretion of a neuropeptide to activate additional critical pathways.

Protein Kinase C Regulates Local Synthesis and Secretion of a Neuropeptide Required for Activity-Dependent Long-Term Synaptic Plasticity

“…In contrast, by 2 h after tetanus plus 5-HT (n ϭ 10) staining for sensorin decreased to control levels both in the axons (115 Ϯ 13%) and varicosities (92 Ϯ 8%). The decline after tetanus plus 5-HT parallel results observed after five applications of 5-HT that produce nonassociative LTF (Hu et al, 2006, their Fig. 1).…”

“…Tetanus plus 5-HT induces a rapid increase in sensorin expression and secretion to produce associative LTF Nonassociative LTF produced by five applications of 5-HT requires the rapid increase in sensorin synthesis and the release of the newly synthesized sensorin (Hu et al, 2004a(Hu et al, , 2006. We therefore tested whether a single pairing of tetanus plus 5-HT, known to produce associative LTF, evokes rapid changes in sensorin expression in the cell body, axon, and varicosities of sensory neurons.…”

“…5-HT can produce nonassociative LTF by activating several signaling pathways. LTF accompanied by structural plasticity requires new gene and protein expression (Bailey et al, 1992) mediated by the timely activation of phosphoinosotide 3-kinase (PI3K), protein kinase A (PKA), and mitogen-activated protein kinase (MAPK) in sensory neurons (Greenberg et al, 1987;Sweatt and Kandel, 1989;Nazif et al, 1991;Schacher et al, 1993;Martin et al, 1997;Michael et al, 1998;Muller and Carew, 1998;Purcell et al, 2003;Udo et al, 2005;Hu et al, 2006). The kinases phosphorylate both local cytoplasmic substrates (Schuster et al, 1985;Bailey et al, 1997;Martin et al, 1997;Michael et al, 1998;Angers et al, 2002;Liu et al, 2004) and transcription factors in the sensory neurons (Dash et al, 1990;Bacskai et al, 1993;Alberini et al, 1994;Martin et al, 1997;Bartsch et al, 1998;Yamamoto et al, 1999;Purcell et al, 2003).…”

“…Protein kinase C (PKC) activity is enhanced by 5-HT (Sacktor and Schwartz, 1990;Sossin and Schwartz, 1992) and participates in both short-and intermediate-term facilitation (ITF) (Sugita et al, 1992(Sugita et al, , 1997Wu et al, 1995;Sutton and Carew, 2000;Manseau et al, 2001;Sutton et al, 2004), but its role in nonassociative LTF is not clear (Sossin et al, 1994;Wu et al, 1995). Activation of MAPK in sensory neurons is mediated by 5-HT-induced increases in the synthesis, secretion, and autocrine action of sensorin, the sensory beginning at 30 min after tetanus plus 5-HT (Liu et al, 2004;Hu et al, 2006). Cocultures or sensory neurons plated alone were exposed for 5 min to 100 nM of active phorbol [phorbol dibutyrate (PDBu) dissolved in DMSO; Sigma] or control inactive phorbol (4␣-PDBu dissolved in DMSO; Sigma).…”

“…Immunocytochemistry was used to monitor the expression and distribution of sensorin throughout the sensory neurons and total p42/44 MAPK or phosphorylated p42/44 MAPK in the cell bodies of sensory neurons ( Hu et al, 2004aHu et al, , 2006. Cultures at various times after stimuli or after the application of control solutions were rinsed briefly in artificial seawater, fixed in 4% paraformaldehyde, and processed as described previously (Liu et al, 2003;Hu et al, 2004a).…”