Structural characterization of osmoregulator peptides from the brain of the leech Theromyzon tessulatum: IPEPYVWD and IPEPYVWD-amide

Discovering new invertebrate neuropeptides using mass spectrometry

Neuropeptides are a complex set of messenger molecules controlling a wide array of regulatory functions and behaviors within an organism. These neuromodulators are cleaved from longer protein molecules and often experience numerous post-translational modifications to achieve their bioactive form. As a result of this complexity, sensitive and versatile analysis schemes are needed to characterize neuropeptides. Mass spectrometry (MS) through a variety of approaches has fueled the discovery of hundreds of neuropeptides in invertebrate species in the last decade. Particularly successful are direct tissue and single neuron analyses by matrix-assisted laser desorption/ionization (MALDI) MS, which has been used to elucidate approximately 440 neuropeptides, and examination of neuronal homogenates by electrospray ionization techniques (ESI), also leading to the characterization of over 450 peptides. Additional MS methods with great promise for the discovery of neuropeptides are MS imaging and large-scale peptidomics studies in combination with a sequenced genome.

Evidence for stanniocalcin and a related receptor in annelids

Stanniocalcin (STC) is a prime example of a hormone whose discovery in fish led to its subsequent discovery in mammals. STC is considered to be first and foremost a vertebrate polypeptide hormone with regulatory effects on ion transport, mitochondrial function and steroid hormone synthesis. The gene is widely expressed in both fishes and mammals, and the hormone can operate via both local and endocrine signaling pathways. In spite of the growing catalogue of vertebrate hormones and receptors with homologues in invertebrates, the notion that there might be an invertebrate STC homolog has received scant attention to date. In the present study, we have provided evidence for STC in annelid worms (freshwater leeches). Western blot analysis revealed the presence of two STC immunoreactive (STCir) proteins in leech tissue extracts of 100 and 193 kDa. These same extracts significantly lowered the rate of gill calcium transport upon injection into fish. Similarly, fish STC increased the rate of whole body calcium uptake when administered to leeches, and STC receptors of high affinity were identified on isolated leech plasma membranes. Two discrete populations of STC-positive cells were also identified in leeches using antibodies to fish STC and fish STC cRNA probes. One of the cell types was confined to the skin. The second cell type was confined to the coelomic cavity and identified as an adipose cell, which in leeches is a major repository of fat. Collectively, the data constitutes compelling evidence for the existence of STC-related proteins and receptors in annelids that share structural and functional similarities with those in vertebrates.

Angiotensin-converting enzyme inhibition studies by natural leech inhibitors by capillary electrophoresis and competition assay

A protocol to follow the processing of angiotensin I into angiotensin II by rabbit angiotensin-converting enzyme (ACE) and its inhibition by a novel natural antagonist, the leech osmoregulator factor (LORF) using capillary zonal electrophoresis is described. The experiment was carried out using the Beckman PACE system and steps were taken to determine (a) the migration profiles of angiotensin and its yielded peptides, (b) the minimal amount of angiotensin II detected, (c) the use of different electrolytes and (d) the concentration of inhibitor. We demonstrated that LORF (IPEPYVWD), a neuropeptide previously found in leech brain, is able to inhibit rabbit ACE with an IC(50) of 19.8 micro m. Interestingly, its cleavage product, IPEP exhibits an IC(50) of 11.5 micro m. A competition assay using p-benzoylglycylglycylglycine and insect ACE established that LORF and IPEP fragments are natural inhibitors for invertebrate ACE. Fifty-four percent of insect ACE activity is inhibited with 50 micro m IPEP and 35% inhibition with LORF (25 mm). Extending the peptide at both N- and C-terminus (GWEIPEPYVWDES) and the cleavage of IPEP in IP abolished the inhibitory activity of both peptides. Immunocytochemical data obtained with antisera raised against LORF and leech ACE showed a colocalization between the enzyme and its inhibitor in the same neurons. These results showed that capillary zonal electrophoresis is a useful technique for following enzymatic processes with small amounts of products and constitutes the first evidence of a natural ACE inhibitor in invertebrates.

Epithelial transport and osmoregulation in annelids

Epithelial transport related to osmoregulation has so far not been extensively investigated in annelids. Compared with the large body of information about ion transport across crustacean or insect epithelia, only a few studies have been done with isolated preparations of annelids, using the body wall of marine polychaetes or Hirudinea. Nephridial function and general body homeostasis have received more attention, and have probably been best investigated in Hirudinea. With recent advances in the molecular physiology of epithelial transport systems in vertebrates, the cloning of various transporters and ion channels, and the considerable number of osmoregulatory peptides that have now been found and analyzed from annelids, it should now be possible, and is timely, to conduct functional studies on individual selected epithelial preparations or isolated cells from annelids. Such studies may be important for establishing useful models with somewhat less complexity than mammalian systems. For example, annelids lack aldosterone, an important osmoregulatory hormone, which is a key factor in the regulation of sodium reabsorption in vertebrates. Therefore, not only would such studies contribute to annelid physiology, but they would be important in a broader sense for understanding osmoregulation and its evolution. They should also facilitate the discovery and investigation of new specific regulatory pathways.

The neuroendocrine system of annelids

In vertebrates the neuroendocrine system is based on chemical signaling between neural and endocrine structures. Final outcomes may be realized via chemical messengers traveling through circulatory conduits to their specific target sites. This process may rely, in part, on neurosecretion of the signaling molecules. The complexity of this system can be readily visualized when one considers the way in which interactions among classical neurotransmitters, cytokines, growth factors, and neuroendocrine hormones, in combination with autocrine and paracrine communication, can regulate cells and tissues. Apart from the neuroendocrine system there is also neuroimmune communication, consisting of reciprocal signaling between neuroendocrine and immune cells, which use the same molecules to coordinate their activity. Thus, our concept of the neuroendocrine system is constantly growing, despite its complexity, but it may be simply summarized as allowing bidirectional communication between neural and endocrine structures over distances greater than that achieved by synaptic communication. In the light of this, I demonstrate in this review that annelids, which are considered "simple" animals, also possess a neuroendocrine system.

PLGamide characterization and role in osmoregulation in leech brain

Neurons immunoreactive to an antiserum specifically directed against the Prolyl-Leucyl-Glycinamide peptide (PLGamide: Melanocyte Inhibiting Factor: MIF) were detected in the brain of the leech Theromyzon tessulatum. Radioimmunoassay titrations of the PLGamide-like material at different physiological stages of the life cycle indicated a maximal amount at stage 3B, which is correlated to phase of both maximal water uptake and coelomic vitellogenin accumulation. In vivo experiments demonstrate that this cerebral PLGamide-like material is an anti-diuretic factor that would act at stage 3B in order to permit a water uptake leading to water retention allowing coelomic yolk protein accumulation. In brains of the Gnatobdellid leech Hirudo medicinalis and the Pharyngobdellid leech Erpobdella octoculata, anti-PLGamide material was also detected with an amount not differing with the degree of sex maturation of the animals, confirming the link between osmoregulation and ovogenesis in rhynchobdellid leeches. Using a combination of biochemical techniques including high-pressure gel permeation chromatography followed by reversed-phase HPLC on brain extracts and Edman degradation, we demonstrated the presence of an authentic MIF-1 peptide in leech brain. Finally, since in vertebrates MIF-1 belongs to the non-classical opioid peptide family, we studied its binding displacement, in contrast to morphine, on mu-receptors and on nitric oxide (NO) release experiments in leech brain. PLGamide did not bind to mu-alkaloid opioid receptors and did not stimulate NO release.

Isolation and characterization of a leech neuropeptide in rat brains: coupling to nitric oxide release in leech, rat and human tissues

The osmoregulator peptide (leech osmoregulatory factor, LORF; IPEPYVWD) was first found in the leech central nervous system (CNS). Given the fact that certain peptides can be found in mammals and invertebrates, e.g., opioid, we examined rat brains to determine if LORF was present. This peptide was found and isolated by successive reversed-phase HPLC purification steps and characterized by electrospray mass spectrometry measurement. It was sequenced by Edman degradation and quantified in different tissues by ELISA. Our results demonstrate the presence of LORF in the hypothalamus, thalamus, and striatum (6 pmol/mg of protein extract) and in other brain areas at lower levels. This octapeptide is also present in the rat duodenum and liver (10 to 14 pmol/mg) and at lower levels in heart, lung, pancreas and caudal spinal cord (< 5 pmol/mg). The testes, adrenals and kidneys have the lowest levels of all the tissues examined (ca. 0.5 pmol/mg of protein). Furthermore, we also demonstrate that LORF is coupled to nitric oxide (NO) release in leech CNS, rat hypothalamus and human saphenous vein in a manner which is inhibited by a nitric oxide synthase inhibitor as well as an antibody directed toward LORF. The study demonstrates that LORF, and its function in relation to NO release, has been conserved over more than 400 million years of evolution.