Bilirubin binding to myelin basic protein, histones and its inhibition in vitro of cerebellar protein synthesis

The Role of Bilirubin and the Other "Yellow Players" in Neurodegenerative Diseases

Bilirubin is a yellow endogenous derivate of the heme catabolism. Since the 1980s, it has been recognized as one of the most potent antioxidants in nature, able to counteract 10,000× higher intracellular concentrations of H2O2. In the recent years, not only bilirubin, but also its precursor biliverdin, and the enzymes involved in their productions (namely heme oxygenase and biliverdin reductase; altogether the "yellow players"-YPs) have been recognized playing a protective role in diseases characterized by a chronic prooxidant status. Based on that, there is an ongoing effort in inducing their activity as a therapeutic option. Nevertheless, the understanding of their specific contributions to pathological conditions of the central nervous system (CNS) and their role in these diseases are limited. In this review, we will focus on the most recent evidence linking the role of the YPs specifically to neurodegenerative and neurological conditions. Both the protective, as well as potentially worsening effects of the YP's activity will be discussed.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Hemoglobin as a source of iron overload in multiple sclerosis: does multiple sclerosis share risk factors with vascular disorders?

Although iron is known to be essential for the normal development and health of the central nervous system, abnormal iron deposits are found in and around multiple sclerosis (MS) lesions that themselves are closely associated with the cerebral vasculature. However, the origin of this excess iron is unknown, and it is not clear whether this is one of the primary causative events in the pathogenesis of MS, or simply another consequence of the long-lasting inflammatory conditions. Here, applying a systems biology approach, we propose an additional way for understanding the neurodegenerative component of the disease caused by chronic subclinical extravasation of hemoglobin, in combination with multiple other factors including, but not limited to, dysfunction of different cellular protective mechanisms against extracellular hemoglobin reactivity and oxidative stress. Moreover, such considerations could also shed light on and explain the higher susceptibility of MS patients to a wide range of cardiovascular disorders.

The evolving landscape of neurotoxicity by unconjugated bilirubin: role of glial cells and inflammation

Unconjugated hyperbilirubinemia is a common condition in the first week of postnatal life. Although generally harmless, some neonates may develop very high levels of unconjugated bilirubin (UCB), which may surpass the protective mechanisms of the brain in preventing UCB accumulation. In this case, both short-term and long-term neurodevelopmental disabilities, such as acute and chronic UCB encephalopathy, known as kernicterus, or more subtle alterations defined as bilirubin-induced neurological dysfunction (BIND) may be produced. There is a tremendous variability in babies' vulnerability toward UCB for reasons not yet explained, but preterm birth, sepsis, hypoxia, and hemolytic disease are comprised as risk factors. Therefore, UCB levels and neurological abnormalities are not strictly correlated. Even nowadays, the mechanisms of UCB neurotoxicity are still unclear, as are specific biomarkers, and little is known about lasting sequelae attributable to hyperbilirubinemia. On autopsy, UCB was shown to be within neurons, neuronal processes, and microglia, and to produce loss of neurons, demyelination, and gliosis. In isolated cell cultures, UCB was shown to impair neuronal arborization and to induce the release of pro-inflammatory cytokines from microglia and astrocytes. However, cell dependent sensitivity to UCB toxicity and the role of each nerve cell type remains not fully understood. This review provides a comprehensive insight into cell susceptibilities and molecular targets of UCB in neurons, astrocytes, and oligodendrocytes, and on phenotypic and functional responses of microglia to UCB. Interplay among glia elements and cross-talk with neurons, with a special emphasis in the UCB-induced immunostimulation, and the role of sepsis in BIND pathogenesis are highlighted. New and interesting data on the anti-inflammatory and antioxidant activities of different pharmacological agents are also presented, as novel and promising additional therapeutic approaches to BIND.

Bilirubin injury to neurons and glial cells: new players, novel targets, and newer insights

Encephalopathy by hyperbilirubinemia in infants has been described for decades, but neither the underlying cellular and molecular mechanisms nor the selective pattern of bilirubin deposition in the brain is well understood. The brain is composed of highly specialized and diverse populations of cells, represented by neurons and glia that comprise astrocytes, oligodendrocytes, and microglia. Although microscopic evaluation of icteric brain sections revealed bilirubin within neurons, neuronal processes, and microglia, cell dependent-sensitivity to bilirubin toxicity and the role of each nerve cell type are poorly understood. Even less considered are glial and neuronal pathologic alterations as integrated phenomena. The available knowledge on reactivity of glial cells to bilirubin and on the impairment to neuronal network dynamics that it causes, here summarized, suggests that a better comprehension of the interplay between neurons and glia is essential to understand bilirubin neurotoxicity and highlight potential molecular targets that may lead to disease-modifying therapeutic approaches.

Modulation of the effect of bilirubin on protein phosphorylation by lysine-containing peptides

Protein phosphorylation is an important mechanism for regulation of cell processes. Bilirubin inhibits phosphorylation of several peptides/proteins by a number of different kinases, and this may contribute to the toxic effects of bilirubin on cells, particularly neurons. Bilirubin binds to lysine residues on both albumin and ligandin. The ATP-binding subdomain II on protein kinases contains an invariant lysine, which might hypothetically be involved in mediation or modulation of bilirubin-inhibition of protein phosphorylation. We have studied the ability of lysine-containing peptides to modulate the effects of bilirubin, using phosphorylation of a phospholemman peptide catalyzed by protein kinase A as a model system. Addition of bilirubin (50 microM) decreased the activity of the catalytic subunit of protein kinase A by 75%. A synthetic lysine-containing decapeptide which mimicked part of subdomain II on the protein kinase family was partially able to prevent the bilirubin effects. Similar effects were not observed with two other decapeptides in which lysine had been replaced by arginine or alanine. Polylysine (100 microM) completely prevented the inhibitory effect of 50 microM bilirubin, whereas polyglutamate and polyarginine did not have this effect. Poly-D-lysine and poly-L-lysine appeared to be equivalent in their ability to prevent the bilirubin effect. These data support the notion that binding of bilirubin to lysine may play a role in the mediation and/or modulation of bilirubin neurotoxicity.

Bilirubin has widespread inhibitory effects on protein phosphorylation

The neurotoxic effects of bilirubin may involve modulation of neuronal protein phosphorylation systems. Using in vitro phosphorylation assays and a variety of protein substrates and purified protein kinases, we have studied the mechanism of bilirubin-induced inhibition of protein phosphorylation. Bilirubin was found to inhibit cAMP-dependent, cGMP-dependent, Ca(2+)- calmodulin-dependent, and Ca(2+)-phospholipid-dependent protein kinases, irrespective of substrate properties. Fifty percent inhibition occurred at bilirubin concentrations varying from 20 to 125 microM. Kinetic analysis, using the isolated catalytic subunit of cAMP-dependent kinase and a synthetic peptide substrate derived from the protein phospholemman, indicated that bilirubin (50 microM) decreased the apparent Vmax of the reaction, irrespective of whether ATP or peptide levels were varied, without significantly altering the apparent K(m) value. Thus our results indicate that bilirubin can inhibit catalytic domain(s) of protein kinases by apparent noncompetitive mechanism(s), presumably by interacting with noncatalytic domains on the enzyme. Given the key role of protein phosphorylation in cellular regulation, the widespread inhibitory effect of bilirubin on protein kinases may contribute to bilirubin neurotoxicity.

On the interaction of polypeptides with bile salts or bilirubin-IX alpha

Aqueous solutions formed by polypeptides, simple models of proteins, and bile salts (sodium cholate and deoxycholate, NaC and NaDC, respectively) or bilirubin-IX alpha (BR) have been studied by CD measurements. They could mimic more complicated biliary systems, thus supplying a possible interpretation of the behavior of some amino acid residues in the biliary proteins. The aggregation of NaDC and NaC in water can be monitored by CD measurements. Bile salts, in submicellar and micellar form, stabilize poly(L-Lys) (PLL) in alpha-helical conformation. The alpha-helix content increases with increasing bile salt concentration and ionic strength. NaDC seems to be a slightly better stabilizing agent of the alpha-helix conformation than NaC. Models characterized by hydrogen bonds between bile salts and PLL are proposed, also resorting to previous data available on the systems formed by NaDC and poly(L-Leu-L-Leu-L-Lys) (PLLL) or poly(L-Leu-L-Leu-L-Asp) (PLLA). Binding of BR to PLL, poly(D-Lys), poly(L-Glu), PLLL, and PLLA in water has been investigated by CD spectra in order to clarify the nature of the association complexes and the mechanism of the BR enantioselective complexation. Potential energy calculations provide binding models capable of explaining the enantioselective ability of the PLL and PLLL alpha-helices toward the left- and right-handed enantiomer of BR, respectively. BR is bound to -NH2 groups of PLL and PLLL lying on a right- and left-handed spiral, respectively. These results, together with those formerly obtained for some bile salts-BR systems, indicate that the selectivity originates from a binding that involves large regions of the BR molecule and gives rise, very probably, to moderate conformational changes from the "ridge tile" structure observed in the crystals. In some cases van der Waals forces can play a crucial role in the chiral recognition of bilirubin. Moreover, possible interaction models of BR with human serum albumin are proposed on the basis of a recent x-ray crystal structure of the protein.

Sucking behaviour and milk intake in jaundiced neonates

Twenty seven healthy newborn infants who developed hyperbilirubinaemia during the first week of life were investigated by daily test feeds between the second and sixth days of life. Nutritive sucking was assessed by measurement of rate, duration and pressure of sucking and by clinical observation. Milk consumption, duration and pressure of sucking all showed a significant increase during the period of study. Sucking rate did not vary with age. Significant correlation of sucking time and sucking pressure with milk intake were observed. Serum bilirubin levels showed no correlation with milk consumption on each day of the study or with the pooled data of feeding parameters when compared independently of post-natal age. Hyperbilirubinaemia was significantly correlated with the infants who appeared disinterested in feeds when assessed by clinical observations of apparent hunger and alertness at the onset of the test feed. However, milk intake was not impaired in these infants.

Neonatal bilirubin toxicity. A review of kernicterus and the implications of drug-induced bilirubin displacement

Kernicterus, the primary manifestation of neonatal bilirubin toxicity, remains an important complication of unconjugated hyperbilirubinaemia despite advances made with phototherapy and exchange transfusions. It results from the penetration of bilirubin into neuronal tissues of the CNS with subsequent damage to the mitochondrion. A number of factors may modify or potentiate bilirubin toxicity, including drugs administered to the infant. The importance of drug-bilirubin interactions in the pathogenesis of kernicterus was first realised quite inadvertently in the 1950s, and the potential risk for significant drug-bilirubin interactions has since become an important consideration in neonatal drug therapy. All drugs intended for use in newborn infants should be evaluated for their capacity to displace bilirubin. A number of techniques have been developed which have facilitated investigation of the mechanisms mediating the bilirubin-displacing effects of drugs and the pharmacokinetics of drug-bilirubin interactions. Further, the clinical risk for inducing kernicterus has been investigated for many of the drugs to which neonates may be exposed by direct administration, transplacentally, or through breast milk. This review summarises the available knowledge concerning the physicochemical properties and toxicities of bilirubin, reviews the methodologies used in evaluating drug-bilirubin interactions, and focuses on the mechanisms, pharmacokinetics and clinical significance of the bilirubin displacing effects of antibiotics, anticonvulsants, diuretics, and other important drug classes used in the treatment of neonates.

A heme binding site on myelin basic protein: characterization, location, and significance

Myelin basic protein (MBP), an extrinsic membrane protein from the myelin sheath, binds dicyanohemin. The binding generates absorption bands in the Soret region and quenches the fluorescence emitted by the sole tryptophan residue. The absorption titration curves in the Soret demonstrate that the binding is stoichiometric, one heme per protein, with a large value of the extinction coefficient (8 X 10(4) M-1 cm-1 at 420 nm). Fluorescence quenching titration curves indicate an identical stoichiometry and a low quenching efficiency of 20%. From the heme titration curve the association constant between dicyanohemin and MBP is estimated to be greater than or equal to 10 nM-1 in 50 mM 4-morpholinepropanesulfonic acid buffer, pH 7.0, at 20 degrees C. Digestion of MBP by Staphylococcus aureus V8 protease yields a peptide (38-118) whose heme binding properties are identical to those of MBP. In contrast, peptides obtained by digestion of MBP with cathepsin D do not exhibit any specific binding of dicyanohemin. The cleavage of the Phe-Phe (42-43) bond appears to be critical in this respect. A comparison of the sequence immediately preceding, including these residues with a probable heme binding site of a mitochondrial cytochrome b, reveals a high degree of homology. The possible significance of heme binding is discussed.

Serum creatine kinase isoenzymes behavior in hepatic failure with encephalopathy

Serum creatine kinase isoenzymes were determined in 24 patients with hepatic failure. Hepatic failure was due to severe acute and chronic liver disease. The 24 patients presented different degrees of coma. Nineteen cases (seven hepatitis and 12 cirrhosis) in coma grades III and IV showed the presence in serum of BB, brain and MB, myocardial isoenzymes. Follow up in six of these cases demonstrated that worsening or improvement was accompanied by an increase or decrease of this brain isoenzyme concentration. The leakage from their respective tissues of the brain and myocardial creatine kinase isoenzymes is probably due to the toxic and surface activity properties of serum free fatty acids, bile salts, and bilirrubin, which increase, among other factors, in these pathological entities.

Characteristic changes of cerebellar proteins associated with cerebellar hypoplasia in jaundiced Gunn rats and the prevention of these by phototherapy

In the cerebellar particulate fractions from Gunn rat homozygotes 3 protein bands with apparent mol. wts. of 250,000, 50,000 and 33,000 in SDS-polyacrylamide gel disc electrophoresis underwent major changes, and phototherapy of the newborns could effectively prevent changes.

Age-dependent changes in the specificity of tRNA methyltransferases in the cerebellum of the icteric and nonicteric Gunn rat

The activity of tRNA methyltransferases present in the cerebellum of 6- and 21-day-old nonicteric and icteric Gunn rats was compared using purified E. coli tRNAs as substrates. At 6 days the tRNA methyltransferases of the icteric animals were significantly more effective in methylating tRNAGlu2 and tRNAPhe than were those of their nonicteric counterparts. This relationship reversed itself at 21 days. The action of the tRNA methyltransferases from the 6-day-old icteric animals led to higher proportions of 1-methyladenine in tRNAGlu2 and tRNAPhe than were obtained using the corresponding enzymes of the nonicteric animals. The proportion of N2-methylguanine was also higher, yet only in tRNAfMet and not in tRNAPhe. The study reveals much more extensive fluctuations in the activity and in the substrate recognition specificity among the cerebellar tRNA methyltransferases of the icteric than among those of the nonicteric controls during the crucial 6--21 day period of cerebellar development.

Increased radiosensitivity of cerebral capillaries in neonatal Gunn rats as compared to Sprague-Dawley rats

The extent of petechial haemorrhages of the cerebral cortex examined between 14 hours and 4 days after X-irradiation to the head was compared in Sprague-Dawley and homozygous Gunn rats with congenital hyperbilirubinaemia. Animals 1 to 2 days old received single doses of either 250, 500 or 750 rad. By means of a special scoring scale the degree of the damage to the microvasculature was semi-quantitatively estimated. In both strains a significant difference in effect was obtained between 250 and 500 rad, but not between 500 and 750 rad. The shape of the dose-effect curve in Gunn rats was similar to that of Sprague-Dawley rats, but displaced upwards. In Gunn rats the effect of 250 rad was greater than that of 750 rad in Sprague-Dawley rats. Possible radiosensitizing mechanisms are discussed with reference to the literature and our results.

Interaction of rabbit hemopexin with bilirubin

The interaction of hemopexin with bilirubin was characterized by spectrophotometric, fluorimetric and circular dichroic techniques. Hemopexin rapidly forms an equimolar complex with libirubin that has an apparent dissociation constant Kd, of 7.5.10(-7) M. The association alters the absorption band of bilirubin near 150 nm, quenches the fluorescence of tryptophan residues of hemopexin, enhances the fluorescence of bilirubin, and induces strong ellipticity extrema in bilirubin of --60 . 10(3) deg . cm2 . dmol-1 at 465 nm and +70 . 10(3) deg . cm2 . dmol-1 at 415 nm. However, the conformation-sensitive ellipticity aband at 231 nm of hemopexin is not altered. In displacement experiments using circular dichroism, heme readily replaced bound bilirubin, indicating that bilirubin and heme are bound at the same site on hemopexin. Even at molar ratios of hemopexin to albumin of 3 to 1, human serum albumin removes bilirubin from hemopexin. Hemopexin is thus unlikely to have a role in the transport of bilirubin in serum.

Bilirubin and biliverdin binding to rat Y protein (ligandin)

The binding of bilirubin to poly(L-lysine) produces an optically active complex at pH 10.1. Circular dichroism spectra of these complexes are distinguishable from those generated by binding of bilirubin to the high affinity sites on albumin. Comparison of the circular dichroism spectra of bilirubin bound to the hepatic protein ligandin with those of bilirubin complexed with albumin or polylysine indicates that binding of bilirubin to ligandin occurs at two types of sites. These are distinguishable on the basis of their spectral properties, one resembling the high affinity site of bovine serum albumin and the other resembling polylysine. Complexes of biliverdin with albumin and ligandin bear similarities to the bilirubin-protein complexes. The native protein itself has an ordered structure which consists of 41% alpha-helix and is not altered by the binding of bilirubin.