Neurotoxicity of Unconjugated Bilirubin in Neonatal Hypoxic-Ischemic Brain Injury in vitro

Unraveling the dual role of bilirubin in neurological Diseases: A Comprehensive exploration of its neuroprotective and neurotoxic effects

Neurodegenerative disorders are characterized by a progressive loss of neurons, causing substantial deficits in motor and cognitive functioning. Bilirubin is a yellow by-product of heme, existing in two primary isoforms namely unconjugated and conjugated, while initially produced unconjugated isomer is lipophilic and cytotoxic in nature. At physiological levels, bilirubin has an important role in brain function by acting as a powerful antioxidant, preventing brain tissues from oxidative damage by eliminating reactive oxygen species (ROS). Additionally, it contributes to immune regulation through microglial activation, cytokine release, complement system interception, fragment crystallization (Fc) receptor modulation, and major histocompatibility complex (MHC II) expression modification, which lower the risk of inflammatory and autoimmune reactions in the central nervous system (CNS). As per the literature, serum bilirubin concentrations are associated with CNS diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), ischemic stroke, hemorrhagic stroke, traumatic brain injury (TBI), multiple sclerosis (MS), epilepsy, schizophrenia and kernicterus spectrum disorder (KSD), which causes neuronal damage, especially in regions like the basal ganglia and cerebellum, which causes movement abnormalities and cognitive deficits. The aim of this article is to explore the dual role of bilirubin as neuroprotective and neurotoxic, essential for establishing effective therapeutic outcomes for neurodegenerative diseases by looking at its cellular mechanisms and discussing how bilirubin's antioxidant properties can shield neurons and, in some situations, may induce oxidative stress and apoptosis.

Neuropathological implication of high blood bilirubin in patients and model rats with depression

PURPOSE

Elevated bilirubin levels have been associated with major depressive disorder (MDD); however, the exact impact of bilirubin on MDD and the underlying molecular mechanisms remain unclear. Here, we explored the influence of bilirubin on MDD and sought to identify the mechanisms via which bilirubin induces depressive-like behavior.

PATIENTS AND METHODS

Forty patients who were diagnosed with MDD and received treatment with selective serotonin reuptake inhibitors (SSRIs) were included, with 43 healthy volunteers serving as controls. Clinical symptoms were evaluated using Hamilton depression rating scale-24 (HAMD-24) and the Hamilton anxiety rating scale. Serum concentrations of total bilirubin (TBIL) and indirect bilirubin (IBIL) were measured at baseline and after treatment using an automated biochemical analyzer. The connection between clinical symptoms and TBIL or IBIL was examined using Pearson correlation. Chronic restraint stress (CRS) was employed to generate a rat model of depression. TBIL, IBIL in rat serum were measured by ELISA. Reactive oxygen species (ROS) contents in rat hippocampal tissues were quantified by flow cytometry. The levels of microglial markers and the extent of neuronal damage in the rat hippocampus were assessed by immunofluorescence and transmission electron microscopy, respectively.

RESULTS

Serum TBIL and IBIL levels were higher in patients with MDD than in the healthy controls. After treatment with SSRIs, the serum levels of TBIL and IBIL in MDD patients were significantly reduced. The levels of TBIL and IBIL were associated with HAMD-24 in MDD patients. Compared with the controls, the serum levels of TBIL, IBIL and the hippocampal ROS contents were elevated in CRS-exposed rats. Fluoxetine lowered inflammatory factor levels, mitigated oxidative stress.

CONCLUSION

Our findings indicate a possible correlation between elevated serum bilirubin and depressive symptoms. Increases in ROS levels, along with neuronal damage, may represent pathological mechanisms underlying MDD.

Unconjugated bilirubin is correlated with the severeness and neurodevelopmental outcomes in neonatal hypoxic-ischemic encephalopathy

Unconjugated bilirubin (UB) levels during the first week after birth are related to outcomes in neonatal hypoxic-ischemic encephalopathy (HIE). Clinical Sarnat staging of HIE, brain magnetic resonance imaging (MRI), hearing outcomes, and neurodevelopmental outcomes ≥ 1 year were used to correlate UB in 82 HIE patients. The initial UB level was significantly correlated with lactic acid levels. The peak UB was higher (p < 0.001) in stage I (10.13 ± 4.03 mg/dL, n = 34) than in stages II and III (6.11 ± 2.88 mg/dL, n = 48). Among the 48 patients receiving hypothermia treatment, a higher peak UB was significantly (p < 0.001) correlated with unremarkable brain MRI scans and unremarkable neurodevelopmental outcomes at age ≥ 1 year. The peak UB were higher (P = 0.015) in patients free of seizures until 1 year of age (6.63 ± 2.91 mg/dL) than in patients with seizures (4.17 ± 1.77 mg/dL). Regarding hearing outcomes, there were no significant differences between patients with and without hearing loss. The UB level in the first week after birth is an important biomarker for clinical staging, MRI findings, seizures after discharge before 1 year of age, and neurodevelopmental outcomes at ≥ 1 year of age.

Non-Enzymatically Colorimetric Bilirubin Sensing Based on the Catalytic Structure Disruption of Gold Nanocages

As an essential indicator of liver function, bilirubin is of great significance for clinical diagnosis. A non-enzymatic sensor has been established for sensitive bilirubin detection based on the bilirubin oxidation catalyzed by unlabeled gold nanocages (GNCs). GNCs with dual-localized surface plasmon resonance (LSPR) peaks were prepared by a one-pot method. One peak around 500 nm was ascribed to gold nanoparticles (AuNPs), and the other located in the near-infrared region was the typical peak of GNCs. The catalytic oxidation of bilirubin by GNCs was accompanied by the disruption of cage structure, releasing free AuNPs from the nanocage. This transformation changed the dual peak intensities in opposite trend, and made it possible to realize the colorimetric sensing of bilirubin in a ratiometric mode. The absorbance ratios showed good linearity to bilirubin concentrations in the range of 0.20~3.60 μmol/L with a detection limit of 39.35 nM (3σ, n = 3). The sensor exhibited excellent selectivity for bilirubin over other coexisting substances. Bilirubin in real human serum samples was detected with recoveries ranging from 94.5 to 102.6%. The method for bilirubin assay is simple, sensitive and without complex biolabeling.

Pharmaceutical strategies for preventing toxicity and promoting antioxidant and anti-inflammatory actions of bilirubin

Bilirubin (BR) is the final product of haem catabolism. Disruptions along BR metabolic/transport pathways resulting from inherited disorders can increase plasma BR concentration (hyperbilirubinaemia). Unconjugated hyperbilirubinemia may induce BR accumulation in brain, potentially causing irreversible neurological damage, a condition known as BR encephalopathy or kernicterus, to which newborns are especially vulnerable. Numerous pharmaceutical strategies, mostly based on hemoperfusion, have been proposed over the last decades to identify new valid, low-risk alternatives for BR removal from plasma. On the other hand, accumulating evidence indicates that BR produces health benefits due to its potent antioxidant, anti-inflammatory and immunomodulatory action with a significant potential for the treatment of a multitude of diseases. The present manuscript reviews both such aspects of BR pharmacology, gathering literature data on applied pharmaceutical strategies adopted to: (i) reduce the plasma BR concentration for preventing neurotoxicity; (ii) produce a therapeutic effect based on BR efficacy in the treatment of many disorders.