Carnosine and Homocarnosine, the Forgotten, Enigmatic Peptides of the Brain

Adult pituitary progenitors/stem cells: from in vitro characterization to in vivo function

Stem cells/progenitors are being discovered in a growing number of adult tissues. They have been hypothesized for a long time to exist in the pituitary, especially because this gland is characterized by its plasticity as it constantly adapts its hormonal response to evolving needs, under the control of the hypothalamus. Recently, five labs have reported the presence of adult progenitors in the gland and shown their endocrine differentiation potential, using different in vitro assays, selection methods and markers to purify and characterize these similar cell populations. These will be discussed here, highlighting common points, and also differences. Thanks to these recent developments it is now possible to integrate progenitors into the physiology of the gland, and uncover their participation in normal but also pathological situations. Moreover, experimental situations inducing generation of new endocrine cells can now be re-visited in light of the involvement of progenitors, and also used to better understand their role. Some of these aspects will also be developed in this review.

The 3-ureidopropionase of Caenorhabditis elegans, an enzyme involved in pyrimidine degradation

Pyrimidines are important metabolites in all cells. Levels of cellular pyrimidines are controlled by multiple mechanisms, with one of these comprising the reductive degradation pathway. In the model invertebrate Caenorhabditis elegans, two of the three enzymes of reductive pyrimidine degradation have previously been characterized. The enzyme catalysing the final step of pyrimidine breakdown, 3-ureidopropionase (β-alanine synthase), had only been identified based on homology. We therefore cloned and functionally expressed the 3-ureidopropionase of C. elegans as hexahistidine fusion protein. The purified recombinant enzyme readily converted the two pyrimidine degradation products: 3-ureidopropionate and 2-methyl-3-ureidopropionate. The enzyme showed a broad pH optimum between pH 7.0 and 8.0. Activity was highest at approximately 40 °C, although the half-life of activity was only 65 s at that temperature. The enzyme showed clear Michaelis-Menten kinetics, with a K(m) of 147 ± 26 μM and a V(max) of 1.1 ± 0.1 U·mg protein(-1). The quaternary structure of the recombinant enzyme was shown to correspond to a dodecamer by 'blue native' gel electrophoresis and gel filtration. The organ specific and subcellular localization of the enzyme was determined using a translational fusion to green fluorescent protein and high expression was observed in striated muscle cells. With the characterization of the 3-ureidopropionase, the reductive pyrimidine degradation pathway in C. elegans has been functionally characterized.

Molecular identification of carnosine synthase as ATP-grasp domain-containing protein 1 (ATPGD1)

Carnosine (beta-alanyl-L-histidine) and homocarnosine (gamma-aminobutyryl-L-histidine) are abundant dipeptides in skeletal muscle and brain of most vertebrates and some invertebrates. The formation of both compounds is catalyzed by carnosine synthase, which is thought to convert ATP to AMP and inorganic pyrophosphate, and whose molecular identity is unknown. In the present work, we have purified carnosine synthase from chicken pectoral muscle about 1500-fold until only two major polypeptides of 100 and 90 kDa were present in the preparation. Mass spectrometry analysis of these polypeptides did not yield any meaningful candidate. Carnosine formation catalyzed by the purified enzyme was accompanied by a stoichiometric formation, not of AMP, but of ADP, suggesting that carnosine synthase belongs to the "ATP-grasp family" of ligases. A data base mining approach identified ATPGD1 as a likely candidate. As this protein was absent from chicken protein data bases, we reconstituted its sequence from a PCR-amplified cDNA and found it to fit with the 100-kDa polypeptide of the chicken carnosine synthase preparation. Mouse and human ATPGD1 were expressed in HEK293T cells, purified to homogeneity, and shown to catalyze the formation of carnosine, as confirmed by mass spectrometry, and of homocarnosine. Specificity studies carried out on all three enzymes were in agreement with published data. In particular, they acted with 15-25-fold higher catalytic efficiencies on beta-alanine than on gamma-aminobutyrate. The identification of the gene encoding carnosine synthase will help for a better understanding of the biological functions of carnosine and related dipeptides, which still remain largely unknown.

Relevance of allosteric conformations and homocarnosine concentration on carnosinase activity

Activity of carnosinase (CN1), the only dipeptidase with substrate specificity for carnosine or homocarnosine, varies greatly between individuals but increases clearly and significantly with age. Surprisingly, the lower CN1 activity in children is not reflected by differences in CN1 protein concentrations. CN1 is present in different allosteric conformations in children and adults since all sera obtained from children but not from adults were positive in ELISA and addition of DTT to the latter sera increased OD450 values. There was no quantitative difference in the amount of monomeric CN1 between children and adults. Further, CN1 activity was dose dependently inhibited by homocarnosine. Addition of 80 microM homocarnosine lowered V (max) for carnosine from 440 to 356 pmol/min/microg and increased K (m) from 175 to 210 microM. The estimated K (i) for homocarnosine was higher (240 microM). Homocarnosine inhibits carnosine degradation and high homocarnosine concentrations in cerebrospinal fluid (CSF) may explain the lower carnosine degradation in CSF compared to serum. Because CN1 is implicated in the susceptibility for diabetic nephropathy (DN), our findings may have clinical implications for the treatment of diabetic patients with a high risk to develop DN. Homocarnosine treatment can be expected to reduce CN1 activity toward carnosine, resulting in higher carnosine levels.

Carnosinase levels in aging brain: redox state induction and cellular stress response

Carnosinase is a dipeptidase found almost exclusively in brain and serum. Its natural substrate carnosine, present at high concentration in the brain, has been proposed as an antioxidant in vivo. We investigated the role of carnosinase in brain aging to establish a possible correlation with age-related changes in cellular stress response and redox status. In addition, a stable HeLa cell line expressing recombinant human serum carnosinase CN1 was established. The enzyme was purified from transfected cells, and specific antibodies were produced against it. Brain expression of CN1, Hsp72, heme oxygenase-1, and thioredoxin reductase increased with age, with a maximal induction in hippocampus and substantia nigra, followed by cerebellum, cortex, septum, and striatum. Hsps induction was associated with significant changes in total SH groups, GSH redox state, carbonyls, and HNE levels. A positive correlation between decrease in GSH and increase in Hsp72 expression was observed in all brain regions examined during aging. Increased carnosinase activity in the brain can lead to decreased carnosine levels and GSH/GSSG ratio. These results, consistent with the current notion that oxidative stress and cellular damage are characteristic hallmarks of the aging process, sustain the critical role of cellular stress-response mechanisms as possible targets for novel antiaging strategies.

Carnosine and its possible roles in nutrition and health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosine's possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosine's ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosine's possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

Glial cell regulation of neurotransmission and behavior in Drosophila

Mounting evidence demonstrates that glial cells might have important roles in regulating the physiology and behavior of adult animals. We summarize some of this evidence here, with an emphasis on the roles of glia of the differentiated nervous system in controlling neuronal excitability, behavior and plasticity. In the review we highlight studies in Drosophila and discuss results from the analysis of mammalian astrocytes that demonstrate roles for glia in the adult nervous system.

Cellular stress response: a novel target for chemoprevention and nutritional neuroprotection in aging, neurodegenerative disorders and longevity

The predominant molecular symptom of aging is the accumulation of altered gene products. Moreover, several conditions including protein, lipid or glucose oxidation disrupt redox homeostasis and lead to accumulation of unfolded or misfolded proteins in the aging brain. Alzheimer's and Parkinson's diseases or Friedreich ataxia are neurological diseases sharing, as a common denominator, production of abnormal proteins, mitochondrial dysfunction and oxidative stress, which contribute to the pathogenesis of these so called "protein conformational diseases". The central nervous system has evolved the conserved mechanism of unfolded protein response to cope with the accumulation of misfolded proteins. As one of the main intracellular redox systems involved in neuroprotection, the vitagene system is emerging as a neurohormetic potential target for novel cytoprotective interventions. Vitagenes encode for cytoprotective heat shock proteins (Hsp) Hsp70 and heme oxygenase-1, as well as thioredoxin reductase and sirtuins. Nutritional studies show that ageing in animals can be significantly influenced by dietary restriction. Thus, the impact of dietary factors on health and longevity is an increasingly appreciated area of research. Reducing energy intake by controlled caloric restriction or intermittent fasting increases lifespan and protects various tissues against disease. Genetics has revealed that ageing may be controlled by changes in intracellular NAD/NADH ratio regulating sirtuin, a group of proteins linked to aging, metabolism and stress tolerance in several organisms. Recent findings suggest that several phytochemicals exhibit biphasic dose responses on cells with low doses activating signaling pathways that result in increased expression of vitagenes encoding survival proteins, as in the case of the Keap1/Nrf2/ARE pathway activated by curcumin and NAD/NADH-sirtuin-1 activated by resveratrol. Consistently, the neuroprotective roles of dietary antioxidants including curcumin, acetyl-L-carnitine and carnosine have been demonstrated through the activation of these redox-sensitive intracellular pathways. Although the notion that stress proteins are neuroprotective is broadly accepted, still much work needs to be done in order to associate neuroprotection with specific pattern of stress responses. In this review the importance of vitagenes in the cellular stress response and the potential use of dietary antioxidants in the prevention and treatment of neurodegenerative disorders is discussed.

Carnosine-like immunoreactivity in neurons of the brain of an advanced teleost, the gray mullet (Chelon labrosus, Risso)

The distribution of the dipeptide carnosine (beta-alanyl-L-histidine) and the related dipeptides anserine and homocarnosine has been studied by biochemical methods and immunohistochemistry in the brain of mammals and other tetrapods. These studies have indicated the presence of these dipeptides mainly in glial cells and in some neurons (olfactory receptor neurons and certain putative migrating neurons and neuroblasts). Here, we used immunohistochemistry with a polyclonal anti-carnosine antibody and the streptavidin-avidin method to study for the first time the distribution of carnosine/carnosine-related dipeptides in the brain of a teleost fish (the gray mullet Chelon labrosus). In order to assess the neuronal nature of carnosine-immunoreactive cells, we also used double immunofluorescence methods with antibodies to carnosine and to the neuronal protein HuC/D. The results obtained show that carnosine and/or related dipeptides are present in neurons of various brain regions. The carnosine-like-immunoreactive neuronal populations of the optic tectum and cerebellum are described in detail. In the optic tectum, immunoreactivity to carnosine/carnosine-related dipeptides is present in neurons of the stratum album and the stratum griseum centrale. In the cerebellum, immunoreactivity to these dipeptides is localized in Purkinje cells, in putative Golgi and stellate cells, and in many nerve fibers. Carnosine-like immunoreactive cells in mullet brain seem to be specific types of neurons, in line with previous results in a urodele but at variance with previous results in mammals, reptiles and anurans.

Neuroprotective actions of a histidine analogue in models of ischemic stroke

Histidine is a naturally occurring amino acid with antioxidant properties, which is present in low amounts in tissues throughout the body. We recently synthesized and characterized histidine analogues related to the natural dipeptide carnosine, which selectively scavenge the toxic lipid peroxidation product 4-hydroxynonenal (HNE). We now report that the histidine analogue histidyl hydrazide is effective in reducing brain damage and improving functional outcome in a mouse model of focal ischemic stroke when administered intravenously at a dose of 20 mg/kg, either 30 min before or 60 min and 3 h after the onset of middle cerebral artery occlusion. The histidine analogue also protected cultured rat primary neurons against death induced by HNE, chemical hypoxia, glucose deprivation, and combined oxygen and glucose deprivation. The histidine analogue prevented neuronal apoptosis as indicated by decreased production of cleaved caspase-3 protein. These findings suggest a therapeutic potential for HNE-scavenging histidine analogues in the treatment of stroke and related neurodegenerative conditions.

Homology Modeling of Human Serum Carnosinase, a Potential Medicinal Target, and MD Simulations of Its Allosteric Activation by Citrate

Recent biochemical and clinical evidence implicates human serum carnosinase in a variety of pathological conditions, such as neurological disorders and diabetic nephropathy, suggesting that this enzyme is of potential interest as a novel medicinal target. The present study was undertaken with a view to model the serum carnosinase and its catalytic site and to unravel the molecular mechanism by which citrate ions increase the catalytic efficiency of serum carnosinase. A homology model of the enzyme was obtained on the basis of beta-alanine synthetase, and its active center was found to bind known substrates carnosine, homocarnosine, and anserine in a binding mode conducive to catalysis. Citrate ions were shown to bind at only three well-defined sites involving both ion pairs and hydrogen bonds. Molecular dynamics simulations evidenced that citrate binding had a remarkable conformational influence on the 3D structure of carnosinase, increasing the binding affinity (i.e., binding score) of carnosine to the catalytic site. This is one of the first reports documenting the molecular mechanism of an allosteric enzyme activator using MD simulations.