Unit: Pharmacology of Neuro-Endocrine Regulations
Director: Rougeot Catherine
The major focus of our research is the molecular characterization and in vivo functional exploration of the physiological inhibitors of the activity of the membrane-anchored metallo-ectopeptidases, in particular NEP (Neutral EndoPeptidase) and amino-peptidase AP-N. Indeed, these ectoenzymes are involved in the neuroendocrine regulation of important physiological functions in mammals, notably pain perception, emotional stress and motivation. We have characterized in rat and in human the first physiological antagonist of the ecto-enkephalinase activities NEP and AP-N; we have named it "sialorphin".
In mammals, the zinc-containing peptidases are located at the surface of cells in nervous and systemic tissues where they play important functions as ectoenzymes, catalyzing the post-secretory processing or metabolism of a number of neuropeptides and regulatory peptides. These neuroendocrine signal peptides are involved in the receptor dependent control of central pain perception and depression, and peripheral inflammatory phenomena, arterial tone and mineral homeostasis. Because of their physiological importance and the critical role of ectoenzymes in modulating their functional potency it is essential to investigate and understand whether they can be protected by endogenous inhibitors. The data is also crucial from physiopathological and therapeutic points of view.
From the gene to an integrated physiological function: In an integrative post-genomic approach, we had established the molecular and functional bases providing the existence of an endogenous antagonist of metallo-ectopeptidase activity in mammals. Hence, we characterized the peptide-mediator sialorphin, which is the first physiological inhibitor of membrane NEP (Neutral EndoPeptidase; Neprilysin) or Enkephalinase activity identified to date in rodent; this endocrine signal messenger of the adaptative response to stress is a powerful inhibitor of painful perception in rat. For instance, analyses of behavioral pharmaco-chemistry have revealed that sialorphin displays potent analgesic activity in two behavioral rat models of peripheral injury-induced acute pain. The analgesia induced by sialorphin required the activation of Mu- and Delta- opioid receptors, consistent with the involvement of the endogenous enkephalinergic pathways that are mediated by Mu- and Delta- opioid receptors. Indeed, these receptors are involved in the transmission of endogenous opiate signals such as the enkephalins, which are inactivated by both metallo-ectopeptidases NEP and AP-N, and also of the alkaloid opiate, morphine. Otherwise, in a model of analysis of behavioral despair, we showed that sialorphin displays a dose-dependent anti-depressant activity in male rat; while, in a model of analysis of behavioral anxiety sialorphin did not display any activity. Together the results obtained in vitro ex vivo and in vivo led us to propose that in rat the physiological effects of sialorphin are mainly associated with its capacity to entirely protect the endogenous enkephalins from inactivation by the enkephalin-degrading ectoenzymes, NEP and AP-N, thereby to potentiate their physiological role, in particular the enkephalin-dependent anti-nociceptive and anti-depressive pathways.
Otherwise, because androgen-regulated sialorphin is rapidly secreted in response to environmental acute stress in sexually mature male rats, this led us to postulate that it might play a role linked to reproduction. To test this hypothesis, we investigated the effects induced by sialorphin on male sexual behavior pattern. The data obtained showed that sialorphin has the ability to modulate, at doses related to physiological circulating levels, the male rat mating pattern, i.e., exerting, in a dose-dependent manner, a dual facilitative/inhibitory effect on sexual performance, while stimulating at all doses the apparent sexual arousal or motivation. Thus we propose that the endogenous androgen-regulated sialorphin helps modulate the adaptative balance between excitatory and inhibitory mechanisms, serving appropriate male rat sexual response depending on the context.
The physiological properties of rat sialorphin demonstrate that this endogenous inhibitor carries a structural signature remarkably adapted in vivo in the rat, in terms of selectivity, bioavailability and efficiency, to the topological and functional characteristics of its target-ectopeptidases and, as a consequence it is particularly efficient from an integrative point of view as an antinociceptive and an antidepressant molecule. Using a combined post-genomic and biochemical approach, we have now characterized the endogenous functional homolog of rat sialorphin in humans, at the molecular and integrated functional level (on human cell and behavioral rat models). To our knowledge it is the first demonstration of the existence in humans of a physiological dual inhibitor of NEP and APN enkephalin-inactivationg ectopeptidases. This discovery is of crucial interest from a physiological and physio-pathological point of view when the extent of the functions mediated by the endogenous enkephalinergic pathways are considered, notably in pain, motivation and well-being behavioral homeostasis and in defective emotional behaviors such as socio-relational behavioral disorders.
From a physiological function to a candidate drug: Sialorphin is a natural bioactive molecule, discovered using a genomic approach and authenticated by post genomic and physio-pharmacological research. Sialorphin's regulation as well as its important role and functions at the integrated physiological level have now been characterized and confirmed. Furthermore, because of its in vivo properties, sialorphin could be the progenitor of a new class of therapeutic molecules, specifically, novel anti-pain and anti-depression drug-candidates.
Keywords: Pharmacology, Pain, Stress, Peptide hormone mediator, Opiate, Enkephalinases