The presence and specificity of crustacean cardioactive peptide (CCAP)-immunoreactivity in gastropod neurons

Identification and developmental expression of mRNAs encoding crustacean cardioactive peptide (CCAP) in decapod crustaceans

Full-length cDNAs encoding crustacean cardioactive peptide (CCAP) were isolated from several decapod (brachyuran and astacuran) crustaceans: the blue crab Callinectes sapidus, green shore crab Carcinus maenas, European lobster Homarus gamarus and calico crayfish Orconectes immunis. The cDNAs encode open reading frames of 143 (brachyurans) and 139-140 (astacurans) amino acids. Apart from the predicted signal peptides (30-32 amino acids), the conceptually translated precursor codes for a single copy of CCAP and four other peptides that are extremely similar in terms of amino acid sequence within these species, but which clearly show divergence into brachyuran and astacuran groups. Expression patterns of CCAP mRNA and peptide were determined during embryonic development in Carcinus using quantitative RT-PCR and immunohistochemistry with whole-mount confocal microscopy, and showed that significant mRNA expression (at 50% embryonic development) preceded detectable levels of CCAP in the developing central nervous system (CNS; at 70% development). Subsequent CCAP gene expression dramatically increased during the late stages of embryogenesis (80-100%), coincident with developing immunopositive structures. In adult crabs, CCAP gene expression was detected exclusively in the eyestalk, brain and in particular the thoracic ganglia, in accord with the predominance of CCAP-containing cells in this tissue. Measurement of expression patterns of CCAP mRNA in Carcinus and Callinectes thoracic ganglia throughout the moult cycle revealed only modest changes, indicating that previously observed increases in CCAP peptide levels during premoult were not transcriptionally coupled. Severe hypoxic conditions resulted in rapid downregulation of CCAP transcription in the eyestalk, but not the thoracic ganglia in Callinectes, and thermal challenge did not change CCAP mRNA levels. These results offer the first tantalising glimpses of involvement of CCAP in environmental adaptation to extreme, yet biologically relevant stressors, and perhaps suggest that the CCAP-containing neurones in the eyestalk might be involved in adaptation to environmental stressors.

Crustacean cardioactive peptide (CCAP)-related molluscan peptides (M-CCAPs) are potential extrinsic modulators of the buccal feeding network in the pond snail Lymnaea stagnalis

We combine electrophysiological and immunocytochemical analyses in the snail Lymnaea stagnalis of M-CCAP1 and M-CCAP2, two molluscan peptides with structure similar to crustacean cardioactive peptide CCAP, originally isolated from the snail Helix pomatia. Both M-CCAP peptides (M-CCAP1 and M-CCAP2, 1 microM) had an excitatory effect, depolarizing all the identified neurons of the buccal feeding network (including motoneurons: B1, B2, B4 and modulatory interneurons SO, OC: 62 neurons in 33 preparations). Additionally, in 67% of preparations, rhythmic activity (fictive feeding) was recorded with a mean rate of 7 cycles/min. No significant difference in the proportion of preparations showing fictive feeding or mean feeding rate was found between M-CCAP1 and M-CCAP2. The extrinsic feeding modulator, the serotonergic CGC neuron, responds by increase of the spontaneous activity after M-CCAP application (9 of 18 preparations). Crustacean CCAP (1 microM) evokes a slight membrane depolarization in 3 out of 8 preparations but never evokes fictive feeding. Immunostaining revealed no cell bodies in the buccal ganglia, but a dense network of CCAP immunopositive fibers arborizing in the buccal neuropil. Many of these fibers originate from a symmetrical pair of CCAP-immunoreactive cerebro-buccal interneurons, which are the most likely candidates for extrinsic modulatory interneurons in the buccal feeding network. Our data are the first results suggesting that M-CCAP-peptides exist as effective modulators in mollusc.

IDENTIFICATION, SEQUENCE AND EXPRESSION OF A CRUSTACEAN CARDIOACTIVE PEPTIDE (CCAP) GENE IN THE MOTHMANDUCA SEXTA

The crustacean cardioactive peptide (CCAP) gene was isolated from the tobacco hawkmoth Manduca sexta. The gene has an open reading frame of 125 amino acid residues containing a single, complete copy of CCAP. Analysis of the gene structure revealed three introns interrupting the coding region. A comparison of the M. sexta CCAP gene with the Drosophila melanogaster genome database reveals significant similarities in sequence and gene structure.

The spatial and temporal expression patterns of the CCAP gene in the M. sexta central nervous system were determined in all major post-embryonic stages using in situ hybridization techniques. The CCAP gene is expressed in a total of 116 neurons in the post-embryonic M. sextacentral nervous system. Nine pairs of cells are observed in the brain, 4.5 pairs in the subesophageal ganglion, three pairs in each thoracic ganglion(T1-T3), three pairs in the first abdominal ganglion (A1), five pairs each in the second to sixth abdominal ganglia (A2-A6) and 7.5 pairs in the terminal ganglion. The CCAP gene is expressed in every ganglion in each post-embryonic stage, except in the thoracic ganglia of first- and second-instar larvae. The number of cells expressing the CCAP gene varies during post-embryonic life,starting at 52 cells in the first instar and reaching a maximum of 116 shortly after pupation. One set of thoracic neurons expressing CCAP mRNA shows unusual variability in expression levels immediately prior to larval ecdysis. Using previously published CCAP immunocytochemical data, it was determined that 91 of 95 CCAP-immunopositive neurons in the M. sexta central nervous system also express the M. sexta CCAP gene, indicating that there is likely to be only a single CCAP gene in M. sexta.