Crustacean neuropeptides

Figure 1 Examples of insect and crustacean neuropeptides of similar structure with the same or different functions. Pandanus borealis is a shrimp. pGlu, pyroglutamic acid.

A suite of methods has been successfully applied over the past few decades in the research on arthropod neuropeptides the relevant methods are mentioned only briefly here with some examples relating to crustacean neuropeptides the reader is referred to [10] for a more detailed description of the general methods employed in the isolation and characterization of insect neuropeptides. 

To date, the complete sequence information of a crustacean genome has not been published consequently, there is a wide gap in the available data on receptors for crustacean neuropeptides. 

Booking D, Dircksen H, Keller R (2002) The crustacean neuropeptides of the CHH/MIH/GIH family structures and biological activities. In Wiese K (ed) The crustacean nervous system. Springer, Berlin, pp 84-97... 

Torfs P., Nieto J., Cerstiaens A., Boon D., Baggerman G., Poulos C., Waelkens E., Derua R., Calderon J. and De Loof A., et al. (2001). Pyrokinin neuropeptides in a crustacean - isolation and identification in the white shrimp Penaeus vannamei. Eur. J. Biochem. 268, 149-154. 

Chen R, Hui L, Sturm R, Li L (2009) Three dimensional mapping of neuropeptides and lipids in crustacean brain by mass spectral imaging. 1 Am Soc Mass Spectrom 20 1068-1077. doi 10.1016/j.jasms.2009.01.017... 

The similarity of the crustacean blanching substance (Factor A) found in crustacean sinus glands and insect corpora cardiaca has been confirmed by the structural identification of a family of neuropeptides common to arthropods. The first identified member of this family is RPCH (red pigment concentrating hormone), an octapeptide (pGlu-Leu-Asn-Phe-Ser-Pro-Gly-Trp-amide) isolated from eyestalks of the shrimp Pandalus borealis (28). Shortly thereafter, a related peptide--designated as adipokinetic hormone (AKH)--was purified from locust... 

Like the AKH/RPCH peptides, insect PDFs and crustacean PDHs are now an established family of neuropeptides common to arthropods. Whereas the fimctions of AKHs are varied and well defined in insects, the function of PDFs in insects remains unknown. The immunocytochemical distribution of PDF in the cephalic nervous tissues of orthopteran insects points to a neuromodulatory role, with potential functions in circadian rhythms this would be an exciting area for further research. 

In this article we will review the existing literature on a selection of neuropeptides to find out whether the phylogenetically related decapod crustaceans and insects synthesize peptides of the same or very similar primary structures and if so, whether the neuropeptides are employed for the same function and whether this function is exerted by an identical pathway for their mode of action. Although such comparisons may shed some light on evolutionary aspects, by no means is this account meant to treat neuropeptides in a phylogenetic framework. 

It is by no means surprising that structurally homologous neuropeptides have been identified in crustaceans and insects - they share, after all, a common ancestry. The functions of some of these neuropeptides have not been conserved in these arthropod groups and we discuss here a selection of neuropeptide families common to both. For each peptide family, we... 

The first member of the cHH peptide family to be functionally and structurally characterized from crustaceans was the crustacean hyperglycaemic hormone (cHH) of the shore crab, Carcinus maenas [77] cHH got its name from the first function ascribed to this neuropeptide, viz. when injected into the haemolymph of a crustacean, there is a consequential and significant rise in the concentration of glucose in the haemolymph that peaks around 2 hours after injection. This is a relatively straight-forward biological assay to perform cHH does, however, also have other functions which will be outlined below. 

Like JH III in insects, MF seems to play a role in enhancing crustacean reproductive maturity, as well as maintaining the juvenile morphology [123]. The synthesis of JH HI in insects is also regulated by neuropeptides, one group of regulators (the allatostatins) is, in fact, similar in structure to neuropeptides that have been isolated from crustaceans (see Section 3.3) but there is no structural resemblance between the allatostatins and the MOIHs and there is no report on the role of allatostatins in regulating the crustacean MOs. 

In insects and in crustaceans, the synthesis of both ecdysteroids and JH/MF are subject to control by neuropeptides. Although ecdysteroids are considered to be growth promoting hormones in both insects and crustaceans, these steroid hormones are differently regulated in the two taxa. In insects, the synthesis and release of ecdysteroids from the prothoracic gland is positively regulated by the prothoracicotropic hormone (PTTH) which is synthesized in neurosecretory cells of the brain and released from the CA [37]. PTTH was first completely identified in B. 

JH in insects maintains larval and nymphal characteristics during development and suppresses metamorphosis into the adult form similarly, MF in crustaceans retards metamorphosis and larval development [123]. Two types of neuropeptides control the production of JH in the CAs of insects in vitro (a) allatotropins, which stimulate the biosynthesis of JH and have only been found in insects to date, and (b) allatostatins (ASTs) which, inter alia, inhibit the biosynthesis of JH but which also have numerous other effects (mainly myoinhibitory) [137]. Neuropeptides that are structurally similar to the ASTs are also present in crustaceans and, therefore, we will review here what is known from the ASTs of insects and crustaceans. 

Orcokinin, NEDEIDRSGF °GFN, a myo-tropic 13-peptide from the abdominal nerve cords of the crayfish Orconectes limosus). It acts as a highly potent stimulator ofhindgut contractions. More members of the orcokinin neuropeptide family have been isolated from several crustacean species [J. Stangier et al.. Peptides 1992, 13, 859 D. Bungart et al.. Peptides 1995, 16, 199 L. Li et al., J. Comp. Neurol. 2002, 444, 227]. 

Webster SG (1998) Neuropeptides inhibiting growth and reproduction in crustaceans. In Coast GM, Webster SG (eds) Recent advances in arthropod endocrinology. Cambridge University Press, Cambridge, pp 33-52... 

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