Effects of photodegradation products of bilirubin on myelinating cerebellum cultures

The Effects of Bilirubin and Lumirubin on the Differentiation of Human Pluripotent Cell-Derived Neural Stem Cells

The ‘gold standard’ treatment of severe neonatal jaundice is phototherapy with blue–green light, which produces more polar photo-oxidation products that are easily excreted via the bile or urine. The aim of this study was to compare the effects of bilirubin (BR) and its major photo-oxidation product lumirubin (LR) on the proliferation, differentiation, morphology, and specific gene and protein expressions of self-renewing human pluripotent stem cell-derived neural stem cells (NSC). Neither BR nor LR in biologically relevant concentrations (12.5 and 25 µmol/L) affected cell proliferation or the cell cycle phases of NSC. Although none of these pigments affected terminal differentiation to neurons and astrocytes, when compared to LR, BR exerted a dose-dependent cytotoxicity on self-renewing NSC. In contrast, LR had a substantial effect on the morphology of the NSC, inducing them to form highly polar rosette-like structures associated with the redistribution of specific cellular proteins (β-catenin/N-cadherin) responsible for membrane polarity. This observation was accompanied by lower expressions of NSC-specific proteins (such as SOX1, NR2F2, or PAX6) together with the upregulation of phospho-ERK. Collectively, the data indicated that both BR and LR affect early human neurodevelopment in vitro, which may have clinical relevance in phototherapy-treated hyperbilirubinemic neonates.

In memoriam: Henry Szczȩsny Schutta, MD (1928-2020)

Polish-American neurologist and neurologic historian Henry Szczȩsny Schutta, MB BS, MD (1928-2020), was born in the Free City of Gdańsk. After surviving the trauma and devastation wrought on his family and his native country during World War II, Schutta met the love of his life in war-torn Bonn, Germany. Schutta completed premedical studies in Bonn, then medical school in Sydney, Australia, and neurology residency at the National Hospital, Queen Square, London. He then joined the faculty at the University of Pennsylvania (1962-1973), where he became a recognized authority on electron microscopy of the nervous system and muscle, particularly concerning bilirubin encephalopathy, brain swelling, and papilledema. He was subsequently appointed as the inaugural chair of neurology at Downstate Medical Center in Brooklyn (1973-1980) and then chair of neurology at the University of Wisconsin (1980-1995). His academic career spanned four countries on three continents, and included substantive contributions to basic research, clinical care, administration, and teaching. He was most proud of his contributions to education and mentoring. He pushed his trainees to excel, and modeled for them the traits of caring, compassion, dedication, diligence, reasonableness, and fidelity-leavened with wit, charm, and a penchant for humorous historical anecdotes. Many of Schutta's witticisms on rounds ("Schutta-isms") have been retold by subsequent generations of trainees. This biography provides first-person accounts of Schutta's struggles and successes based on his own autobiography, a formal oral history for the American Academy of Neurology Oral History Project, and supporting accounts of his family, trainees, and colleagues.

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.

Neuro-inflammatory effects of photodegradative products of bilirubin

Phototherapy was introduced in the early 1950’s, and is the primary treatment of severe neonatal jaundice or Crigler-Najjar syndrome. Nevertheless, the potential biological effects of the products generated from the photodegradation of bilirubin during phototherapy remain unknown. This is very relevant in light of recent clinical observations demonstrating that the use of aggressive phototherapy can increase morbidity or even mortality, in extremely low birthweight (ELBW) infants. The aim of our study was to investigate the effects of bilirubin, lumirubin (LR, its major photo-oxidative product), and BOX A and B (its monopyrrolic oxidative products) on the central nervous system (CNS) using in vitro and ex vivo experimental models. The effects of bilirubin photoproducts on cell viability and expression of selected genes were tested in human fibroblasts, three human CNS cell lines (neuroblastoma SH-SY5Y, microglial HMC3, and glioblastoma U-87 cell lines), and organotypic rat hippocampal slices. Neither bilirubin nor its photo-oxidative products affected cell viability in any of our models. In contrast, LR in biologically-relevant concentrations (25 μM) significantly increased gene expression of several pro-inflammatory genes as well as production of TNF-α in organotypic rat hippocampal slices. These findings might underlie the adverse outcomes observed in ELBW infants undergoing aggressive phototherapy.

Biology of Bilirubin Photoisomers

Phototherapy is the main treatment for neonatal hyperbilirubinemia. In acute treatment of extreme hyperbilirubinemia, intensive phototherapy may have a role in 'detoxifying' the bilirubin molecule to more polar photoisomers, which should be less prone to crossing the blood-brain barrier, providing a 'brain-sparing' effect. This article reviews the biology of bilirubin isomers. Although there is evidence supporting the lower toxicity of bilirubin photoisomers, there are studies showing the opposite. There are methodologic weaknesses in most studies and better-designed experiments are needed. In an infant acutely threatened by bilirubin-induced brain damage, intensified phototherapy should be used expediently and aggressively.

The Biological Effects of Bilirubin Photoisomers

Although phototherapy was introduced as early as 1950’s, the potential biological effects of bilirubin photoisomers (PI) generated during phototherapy remain unclear. The aim of our study was to isolate bilirubin PI in their pure forms and to assess their biological effects in vitro. The three major bilirubin PI (ZE- and EZ-bilirubin and Z-lumirubin) were prepared by photo-irradiation of unconjugated bilirubin. The individual photoproducts were chromatographically separated (TLC, HPLC), and their identities verified by mass spectrometry. The role of Z-lumirubin (the principle bilirubin PI) on the dissociation of bilirubin from albumin was tested by several methods: peroxidase, fluorescence quenching, and circular dichroism. The biological effects of major bilirubin PI (cell viability, expression of selected genes, cell cycle progression) were tested on the SH-SY5Y human neuroblastoma cell line. Lumirubin was found to have a binding site on human serum albumin, in the subdomain IB (or at a close distance to it); and thus, different from that of bilirubin. Its binding constant to albumin was much lower when compared with bilirubin, and lumirubin did not affect the level of unbound bilirubin (Bf). Compared to unconjugated bilirubin, bilirubin PI did not have any effect on either SH-SY5Y cell viability, the expression of genes involved in bilirubin metabolism or cell cycle progression, nor in modulation of the cell cycle phase. The principle bilirubin PI do not interfere with bilirubin albumin binding, and do not exert any toxic effect on human neuroblastoma cells.

The role of phototherapy in the crash-cart approach to extreme neonatal jaundice

Extreme neonatal jaundice occurs infrequently but carries a high risk of permanent sequelae (kernicterus) when it does. Rapid therapeutic intervention has the potential to reduce this risk in some infants. Several case reports of infants with acute intermediate to advanced bilirubin encephalopathy shows that reversal may be possible. Phototherapy can be instituted at the flip of a switch, whereas other therapeutic measures necessarily involve delays. Therefore, high-intensity phototherapy must be regarded as an emergency measure in infants presenting with extreme jaundice and even more so in the presence of neurological symptoms. The principal and well-described effect of phototherapy involves conversion of bilirubin IXα (z, z) to more polar isomers, which are excreted in bile and urine. When care is taken to maximize the spectral power of phototherapy lights, and whenever possible with measures added to reduce the enterohepatic circulation of bilirubin, very rapid reductions in total serum bilirubin levels are possible. A hypothesis has been advanced that conversion of bilirubin to more polar photoisomers, which can reach relative concentrations of 20%-25% of total serum bilirubin within 1-2 hours, might have a direct neuroprotective effect. This theory posits that because polar molecules generally require a transporter to cross the blood-brain barrier, bilirubin photoisomers should be less prone to enter the brain. Although this theory has some support in in vitro toxicity studies, the evidence is controversial. Until further experimental support can be gained, photoconversion of bilirubin does not constitute a viable argument against instituting further measures against bilirubin neurotoxicity, such as intravenous immune globulin (when indicated) and exchange transfusion. Conversely, neither is the state of evidence an argument against immediate and effective phototherapy in the medical emergency of extreme neonatal jaundice.

Phototherapy for neonatal jaundice--therapeutic effects on more than one level?

Phototherapy for jaundice is a common treatment in neonatal medicine and is used to prevent the neurotoxic effects of bilirubin. Studies have assessed the optimal wavelength of phototherapy light, the importance of irradiance and spectral power, and the types of light source, including the use of single versus multiple light sources. Outcome measures have been duration of need for phototherapy or rate of reduction of serum bilirubin over a given time. An apparent resurgence of kernicterus in recent years has forced us to focus on the emergency management of severely jaundiced infants. Several studies have shown that very rapid reductions of total serum bilirubin levels are possible. The speed with which photoisomers are formed appears to be important both from this perspective and theoretically may also be neuroprotective because of the more polar nature of the photoisomers. This work reviews the evidence concerning the speed of photoisomer formation, as well as the evidence regarding the relative neurotoxicity of bilirubin isomers.

Reassessment of the unbound concentrations of unconjugated bilirubin in relation to neurotoxicity in vitro

Most studies of the cellular toxicity of unconjugated bilirubin (UCB) have been performed at concentrations of unbound UCB (BF) that exceed those in the plasma of neonates with bilirubin encephalopathy. We assessed whether UCB could be toxic to neurons and astrocytes at clinically relevant BF values (<or=1.0 microM), a range in which spontaneous precipitation of UCB would be unlikely to occur, even though BF exceeded the aqueous saturation limit of 70 nM. A meta-analysis yielded twelve published studies that had determined the in vitro effects of UCB on the function of cultured neurons or astrocytes at calculable BF values <or= 1.0 microM. BF values were recalculated from the stated UCB, albumin, and chloride concentrations by applying affinity constants derived from ultrafiltration of comparable solutions containing 14C-UCB and delipidated human serum albumin. At BF slightly above aqueous solubility, UCB impaired mitochondrial function and viability of astrocytes. Exposure of neuroblastoma and embryonic neuronal cell lines to BF above 250 nM impaired cellular proliferation and mitochondrial function and increased apoptosis. Purified UCB inhibited the uptake of glutamate into astrocytes at BF as low as 309 nM and induced apoptosis in brain neurons at BF as low as 85 nM. UCB can impair various cellular functions of astrocytes and neurons exposed to BF near or modestly above its aqueous solubility limit, at which UCB exists as soluble oligomers and metastable microaggregates. The results render doubtful the long-held concept that precipitation of UCB in or on cells is required to produce neurotoxicity.

Mechanisms of bilirubin toxicity: clinical implications

The basic mechanism of kernicterus and bilirubin encephalopathy has not been unequivocally determined. Much knowledge has been gained about phenomena that contribute to bilirubin neurotoxicity, and this knowledge has implications for clinical practice. Conditions that impact on blood-brain barrier function, increase brain blood flow, or impact on bilirubin metabolism, including its transport in serum, should be avoided, if possible. Such conditions include drugs and drug stabilizers that compete with bilirubin binding to albumin, or that inhibit P-glycoprotein in the blood-brain barrier, prematurity/immaturity, and clinically significant illness in the infant that involves hemolysis, respiratory and metabolic acidosis, infection, asphyxia, hypoxia and (perhaps) hyperoxia, and hyperosmolality. If these conditions are not avoidable then there should be a more aggressive approach to the treatment of hyperbilirubinemia. The limits of tolerance for hyperbilirubinemia varies among neonates and there are no tools to determine with certainty when a particular infant is approaching the danger zone. Neurological symptoms in a jaundiced infant require extreme vigilance, and, in most cases, immediate intervention.

Bilirubin adsorption in vitro by foetal and adult human erythrocytes

Adsorption of bilirubin in vitro by human foetal erythrocytes from umbilical cords was significantly greater than the binding by adult erythrocytes. A difference in binding capacity was detected at 0.6 mmol/l bilirubin concentration (bilirubin/albumin ratio 2:1) immediately after mixing with the bilirubin solution. After 30 min incubation a further increase in bound bilirubin was found to be due only to binding in foetal cells. The results suggest that bilirubin from the medium was incorporated into at least two compartments in foetal erythrocytes. A possible role of human foetal erythrocytes in bilirubin distribution to tissues is stressed.