Expression and Branch-Specific Export of mRNA Are Regulated by Synapse Formation and Interaction with Specific Postsynaptic Targets

Abstract

Mechanosensory neurons (SNs) of Aplysia form synapses in culture with some targets (L7), but not others (L11), even when a SN is plated with both targets. We examined whether branch-specific net export of mRNA encoding synapse-specific molecules might contribute to branch-specific synapse formation. Single-cell RT-PCR was used to assay levels of mRNA encoding the SN-specific neuropeptide (sensorin A) and other transcripts in cell bodies and neuritic processes of SNs cultured alone or with synaptic targets. Some mRNAs are exported to neurites, but not others. Sensorin A mRNA is detected only in SN cell bodies and neurites, and expression levels correlate with the strength of the synaptic connections formed with L7 after 4 d in culture. After 4 d, more sensorin A transcripts are detected in SN neurites contacting L7 than in SN neurites contacting L11. The differential expression at 4 d is found even when a single SN contacts both targets simultaneously. By contrast, no significant difference in expression is detected in SN neurites contacting L7 versus L11 after 1 d of coculture. The results suggest that interaction and synapse formation with a specific target lead to a time-dependent change in the branch-specific accumulation of sensorin A mRNA in SNs. Because local protein synthesis at synaptic sites might contribute to synaptic function or plasticity, the results suggest that branch-specific targeting of mRNA encoding synapse-related molecules may contribute to the formation of specific synapses.

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

“…PKC and rapamycin regulate the local synthesis of sensorin after tetanus plus 5-HT A single pairing produced a rapid increase in sensorin translation from sensorin mRNA that is located in the cell body, axons, and the distal varicosities (Schacher et al, 1999;Hu et al, 2002;Lyles et al, 2006). Tetanus plus 5-HT, however, modestly increased sensorin in the cell body of the sensory neurons.…”

“…The rapid and significant increase in sensorin at distal varicosities by 0.5 h after stimulation suggested that local translation of sensorin mRNA, which accumulates at varicosities after being transported down sensory neuron axons (Schacher et al, 1999;Hu et al, 2002Hu et al, , 2006Lyles et al, 2006), contributes to local changes in sensorin levels. Local synthesis of sensorin is detected after removing the sensory neuron cell body; five applications of 5-HT produced a rapid rapamycin-sensitive and PI3K-dependent increase in sensorin synthesis in both the axons and varicosities (Hu et al, 2006).…”

Target-Dependent Release of a Presynaptic Neuropeptide Regulates the Formation and Maturation of Specific Synapses inAplysia

“…Sensory neurons were isolated from pleural ganglia dissected from adult animals (80 -100 gm); motor cell L7 and L11 were isolated from juvenile (1-3 gm) abdominal ganglia and maintained in culture medium (2 ml) from 1 to 7 d as described previously (Rayport and Schacher, 1986;Schacher et al, 1999). Culture medium was a 1:1 (by volume) mixture of Aplysia hemolymph and L15 medium with added salts to match ionic composition of seawater (Rayport and Schacher, 1986).…”

“…In culture, Aplysia neurons (sensory neurons or interneuron L10) form specific synapses when confronted with either individual targets or multiple targets simultaneously (Camardo et al, 1983;Hawver and Schacher, 1993;Schacher et al, 1999). Many important cellular changes in sensory neurons accompany contact and synapse formation with motor neuron L7 compared with contact with motor neuron L11, which fails to form synapses with sensory neurons: an increase in presynaptic axon growth and varicosity formation contacting the target (Glanzman et al, 1989;Schacher and Montarolo, 1991), the full development of transmitter release sites in varicosities contacting L7 (Glanzman et al, 1989;Hatada et al, 1999, Kim et al, 2003, and differences in the expression and distribution of specific mRNAs that encode proteins critical for synaptic function Hu et al, 2002Hu et al, , 2003.…”

“…The distribution of syntaxin mRNA in the cell body of sensory neurons is regulated by target interaction both during the early phase of synapse formation and when synapses are stabilized . Target interaction and synapse formation also regulate the expression and distribution of sensorin mRNA and protein in sensory neuron cell bodies and varicosities and transport of sensorin mRNA to distal sites (Santarelli et al, 1996;Schacher et al, 1999;Hu et al, 2002). Sensorin, the sensory neuron-specfic neuropeptide, produces hyperpolarizations in some unidentified postsynaptic targets that matches responses produced by electrical activity in sensory neurons (Brunet et al, 1991).…”