Ontogeny of preproenkephalin mRNA expression in the rat retina

The Opioid Growth Factor in Growth Regulation and Immune Responses in Cancer

It has become apparent that endogenous opioids act not only as neurotransmitters and neuromodulators, but have multiple functions in the body. Activation of the opioid system by opiate drugs is associated with a risk of cancer development through direct stimulation of tumor cell proliferation and through immunosuppression. In contrast, the endogenous peptide opioid [Met5]-enkephalin, now commonly referred to as Opioid Growth Factor (OGF), negatively regulates cell proliferation in a wide number of cells during development, homeostasis, and neoplasia. This action is mediated through the opioid growth factor receptor, originally designated the zeta (ζ) opioid receptor. Further, contrary to the traditional notion of opiates as immunosuppressive, endogenous OGF has been shown to possess a number of positive immunomodulatory properties and may provide a beneficial effect in cancer by augmenting the activity of cells involved in both innate and acquired immunity. Taken together, the evidence supports consideration of opioid peptides such as OGF as new strategies for cancer therapy.

Ocular surface complications in diabetes: The interrelationship between insulin and enkephalin

Diabetes is a multi-faceted disorder with increasing prevalence and rising healthcare costs. The burden of diabetes is increased because of associated complications affecting nearly all organs including the eye. The underlying pathophysiology for the onset of these ocular surface disorders is not well known. Enkephalins are endogenous opioids that originate in the brain and have numerous actions in the human body. Opioid growth factor (OGF), chemically termed [Met5]-enkephalin, binds to a novel, nuclear-associated receptor and mediates cellular homeostasis. Serum OGF levels are elevated in diabetic individuals and rodent models of diabetes. Sustained blockade of the OGF receptor (OGFr) with opioid receptor antagonists, such as naltrexone (NTX), reverses many complications of diabetes in the animal model, including delayed cutaneous wound healing, dry eye, altered corneal surface sensitivity, and keratopathy. The increased enkephalin levels observed in diabetes suggest a relationship between endogenous opioid peptides and the pathophysiology of diabetes. It is common for diabetic patients to undergo insulin therapy to restore normal blood glucose levels. However, this restoration does not alter OGF serum levels nor ameliorate ocular surface complications in the animal model of diabetes. Moreover, sex differences in the prevalence of diabetes, response to insulin therapy, and abnormalities in the OGF-OGFr axis have been reported. This review highlights current knowledge on the dysregulation of the OGF-OGFr pathway and possible relationships of insulin and enkephalins to the development of ocular surface defects in diabetes. It proposes that this dysregulation is a fundamental mechanism for the pathobiology of diabetic complications.

Mammal retinal distribution of ENKergic amacrine cells and their neurochemical features: evidence from the PPE-GFP transgenic mice

The neuroactive peptide enkephalin (ENK) has been postulated to play important roles in modulating visual information. The retinal presence of ENKergic cells has been revealed with conventional morphological protocols targeting ENK molecule especially in avian, however, the detailed distribution of ENKergic cells and their specific neurochemical features in the mammal retina remain unclear because of the difficulties in visualizing ENKergic cells efficiently and reliably. To address this question, we took advantage of the preproenkephalin-green fluorescent protein (PPE-GFP) transgenic mice previously generated and identified in our group, and identified the neurochemical characteristics of retinal ENKergic cells. The majority of ENKergic cells occupied the proximal inner nuclear layer with a few displaced in the ganglion cell layer. Further double labeling revealed that most of these ENKergic amacrine cells used inhibitory glycine or gamma-aminobutyric acid as the primary neurotransmitter. However, some of them also utilized excitatory glutamate as the primary neurotransmitter. The present findings suggest that the retinal ENKergic cells fall into a subpopulation of amacrine cells and show predominantly inhibitory as well as less dominantly excitatory neurochemical features. Our findings offered comprehensive morphological evidence for the function of ENKergic amacrine cells of mammal species.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Expression of aminopeptidase B in the developing and adult rat retina

Aminopeptidase B (Ap-B), a ubiquitous enzyme, catalyses the amino-terminal cleavage of basic residues of peptide or protein substrates, indicating a role in precursor processing. The physiological function of Ap-B still remains an open question, even though its activity suggests that it could be involved in inflammatory processes and proliferation of tumor cells. This study was conducted to determine the expression of Ap-B in the developing and adult retina as a path to envisage physiological roles of Ap-B. RT-PCR and in situ hybridization were used to detect expression of Ap-B mRNA and activity tests, Western blotting and immunofluorescence microscopy were performed to identify and localize the enzyme in the rat retina. These biochemical and morphological methods show that Ap-B is expressed in the retina from embryo to adult. Expression level is restricted to specific layers (pigmented epithelium, outer and inner plexiform layers and ganglion cell layer) and is developmentally regulated. Moreover, a preliminary analysis indicates that Ap-B, the glucose transporter GLUT3 and choline acetyltransferase (ChAT) share a similar expression pattern in retina. Altogether, Ap-B appears predominantly expressed in neuronal cells lying in retinal layers containing neuritic extensions and synaptic junctions. Such expression is up-regulated during ontogenesis allowing to hypothesized that Ap-B participates in processes accompanying retinal neuronal cell differentiation.

Ontogeny, regional and cellular distribution of the novel metalloprotease neprilysin 2 in the rat: a comparison with neprilysin and endothelin-converting enzyme-1

The localisation of the gene transcripts of a recently discovered peptidase, neprilysin 2 (NEP2), was established by in situ hybridisation in rat tissues during development and adulthood. It was compared with those of neprilysin (NEP), a closely related enzyme in terms of sequence homology or substrate specificity, and of endothelin-converting enzyme 1 (ECE-1) which, like the other two, belongs to the M-13 sub-family of zinc-dependent metallopeptidases. The ontogeny of the three enzymes differed markedly, the expression of NEP2 being restricted to developing and differentiating fields of the CNS, whereas NEP and ECE-1 genes were broadly expressed early on in the CNS and periphery. In contrast to the wide expression of NEP and ECE-1 in peripheral adult tissues and in CNS, NEP2 was almost exclusively expressed in selected neuronal populations of the brain and spinal cord. The only exceptions were the intermediate and anterior lobes of the pituitary as well as the choroid plexuses, where NEP2 was also strongly expressed. These localisations as well as those in the hypothalamic nuclei, together with the previously established pattern of cleaved peptides, suggest the involvement of NEP2 in the metabolism of neurohormones of the hypothalamo-pituitary axis.Complementary distributions of NEP and NEP2 mRNAs were observed in a large number of brain areas with, for instance the former being highly expressed in the striatum in which NEP2 transcripts were almost undetectable. In contrast, NEP2 was highly expressed in numerous thalamic, hypothalamic and brainstem nuclei from which NEP was absent. Since both peptidases are able to cleave the same neuropeptides, this pattern may suggest a complementary role in their peptide inactivation functions in the CNS. Finally, ECE-1 mRNAs were generally observed in neuronal populations known to express the pre-proendothelin-1 gene, confirming the function of the metallopeptidase in endothelin-1 generation.

Transplacental transfer of the opioid growth factor, [Met(5)]-enkephalin, in rats

Placental transfer of the pentapeptide [Met5]-enkephalin, known to function as a growth regulating factor and neuromodulatory agent, was studied in pregnant Sprague-Dawley rats. Using separation by reversed phase high-performance liquid chromatography, and analysis by derivative spectroscopy, [Met5]-enkephalin was detected in 20-day-old fetal tissue including brain, heart, lung, and kidney. Fetal tissues from pregnant rats given an injection of 40 mg/kg [Met5]-enkephalin on gestation day 20 had markedly elevated levels of peptide within 1 h, indicating the transplacental transfer of this opioid. [Met5]-enkephalin levels were increased from control samples at 1, 2, 4, and 14 h post-injection of peptide, but not at 24 h. Evaluation of breakdown products of [Met5]-enkephalin, along with the related peptide [Leu5]-enkephalin, revealed that elution times differed substantially from [Met5]-enkephalin. These data indicate that [Met5]-enkephalin is present in fetal organs, crosses the placenta, does not appear to be restrictive in organ specificity, and is sustained in fetal tissues at detectable levels for at least 14 h. Given that [Met5]-enkephalin tonically inhibits DNA synthesis in the fetus, these results raise the question of whether an elevated level of this peptide (either maternally or from the fetus) may be detrimental to cellular ontogeny in the fetus, and perhaps have long-term implications for postnatal development.

The effects of morphine on cell proliferation

There is increasing evidence that endogenous opioid peptides ("enkephalins") and other neurotransmitters have widespread, receptor-mediated roles as growth regulators in non-neuronal cells and tissues. For example, it is now believed that enkephalins produced in placental trophoblast giant cells have multiple roles in supporting embryo growth, and in maternal adaptation to pregnancy. Since plant and synthetic narcotics (e.g., morphine) bind to the same receptors, the questions immediately arise: Do narcotics also have actions as growth regulators? If so, do these actions have physiological significance in addicts? Recent work on the first of these questions is covered in this review. While the greatest volume of research has been focused on the proliferative effects of narcotics for cells of the immune system, the roles of opioid peptides and narcotics on the growth of a variety of other cells has come under study recently.

Opioid growth factor and organ development in rat and human embryos

In addition to neurotransmission, the native opioid peptide, [Met5]enkephalin, is a tonically active inhibitory growth molecule that is termed opioid growth factor (OGF). OGF interacts with the zeta (zeta) opioid receptor to influence cell proliferation and tissue organization. We now identify OGF and the zeta receptor in embryonic derivatives including ectoderm, mesoderm, and endoderm of the rat on gestation day 20. Messenger RNA for preproenkephalin (PPE), the precursor of OGF, was detected in the developing cells, suggesting an autocrine production of this peptide. Acute exposure of the pregnant female to OGF resulted in a decrease in DNA synthesis in cells of organs representing all three germ layers, and did so in a receptor-mediated fashion. The influence of OGF was direct, as evidenced in organ culture studies. Blockade of endogenous opioid interaction using naltrexone (NTX) produced an increase in DNA synthesis, indicating the constitutive and functional nature of opioid activity on growth during prenatal life. Human fetal cells contained OGF and the zeta receptor. These data support the hypothesis that endogenous opioid modulation of organ development is a fundamental principle of mammalian embryogenesis, and that OGF has a profound influence on ontogeny. Irregularities in the role of opioids as growth regulators in relationship to the more than 500,000 newborns suffering from birth defects each year in the US needs to be examined.

The autocrine derivation of the opioid growth factor, [Met5]-enkephalin, in ocular surface epithelium

Endogenous opioid peptides serve as growth factors in developing, renewing, healing, and neoplastic cells and tissues. A native opioid peptide, [Met5]-enkephalin, termed opioid growth factor (OGF), has been discovered to regulate DNA synthesis in the epithelium of the ocular surface. OGF and its receptor zeta have been localized in both the basal and suprabasal cells of the epithelium. This study examined the hypothesis that OGF is an autocrine growth factor. Using probe for preproenkephalin (PPE) mRNA that encodes OGF, and in situ hybridization techniques, silver grains related to PPE mRNA were detected in both basal and suprabasal cells of the central and peripheral cornea, limbus, and conjunctiva. No distinct regional differences in the presence or location of message, as reflected by the density and distribution of PPE mRNA signal, were noted. These results demonstrate that a growth factor known to serve as a tonic, inhibitory, and receptor-mediated influence on the epithelium of the ocular surface is derived in an autocrine manner, thereby permitting local control of homeostatic cellular replication.

Opioid receptor and peptide mRNA expression in proliferative zones of fetal rat central nervous system

There is increasing evidence to suggest that opioid peptides may have widespread effects as regulators of growth. To evaluate the hypothesis that endogenous opioids control cellular proliferation during neural development, we have used in situ hybridization to examine opioid peptide and receptor mRNA expression in neuroepithelial zones of fetal rat brain and spinal cord. Our data show that proenkephalin mRNA is widely expressed in forebrain germinal zones and choroid plexus during the second half of gestation. In contrast, prodynorphin mRNA expression is restricted to the periventricular region of the ventral spinal cord. Little µ or delta receptor mRNA expression was detected in any regions of neuronal proliferation prior to birth. However, kappa receptor mRNA is widely expressed in hindbrain germinal zones during the 3rd week of gestation. Our present findings support the hypothesis that endogenous opioids may regulate proliferation of both neuronal and non-neuronal cells during central nervous system development. Given the segregated expression of proenkephalin mRNA in forebrain neuroepithelium and kappa receptor mRNA within hindbrain, different opioid mechanisms may regulate cell division in rostral and caudal brain regions.Key words: enkephalin, dynorphin, ontogeny, neurogenesis.

Opioid gene expression in the developing and adult rat heart

Opioid peptides are known to play a role in the function and growth of the mammalian heart. Although some information about gene expression of opioids in the heart is available, there is no data on the cellular location of opioid gene expression during development or in the adult. Using in situ hybridization and rat heart ranging from embryonic day 14 (E14) to adulthood, we have evaluated the distribution of gene expression for proenkephalin, proopiomelanocortin, and prodynorphin. With respect to preproenkephalin mRNA (PPE mRNA), message in the ventricle was abundant from E14 (the first time point examined) until shortly after birth, with a marked reduction noted on postnatal days 5, 10, and 21. Adults displayed considerable message, though less than in preparations of embryonic and neonatal heart. PPE mRNA was detected in epicardial, myocardial, and endocardial cells, as well as the walls of blood vessels, capillaries, and fibroblasts. Preproopiomelanocortin (POMC) mRNA was only found in adults, and was localized to the myocardium. Message for preprodynorphin could not be observed in the ventricles of developing or adult rats. These results are the first to define the temporal and spatial ontogeny of opioid gene expression with regard to the emergence of cardiac architecture. The data suggest that gene expression for proenkephalin is especially prevalent in embryonic and neonatal rats and may be related to the modulatory activity of the opioid growth factor, [Met5]-enkephalin, on cell proliferation and differentiation. The role of PPE and POMC mRNA in adult rat heart requires elucidation.