Models of bilirubin neurological damage: lessons learned and new challenges

Association of exposure to organochlorine pesticides with the antioxidant bilirubin: Mediation analysis in the NHANES

Exposure to organochlorine pesticides (OCPs) has been linked to adverse health effects through oxidative stress. Bilirubin, an endogenous antioxidant, may play a key role in regulating oxidative stress. This study utilized NHANES data (2007-2016) to investigate the relationship between exposure to seven OCPs and serum total bilirubin (TB) levels in American adults, as well as potential mediating effects. Among the 6583 adults studied, HCB, β-HCCH, p, p'-DDE, p, p'-DDT, OXYCHLOR, and T-NONA were all significantly and positively correlated with TB levels among the seven OCPs analyzed (p < 0.05). HCB, β-HCCH, and OXYCHLOR showed inverted "J"-, a similar "S"-, and "U"-shaped nonlinear relationships with TB, respectively (p for nonlinear < 0.05). OXYCHLOR had the strongest cumulative effect, and platelets partially mediated the association between p, p'-DDE, p, p'-DDT, T-NONA, OXYCHLOR, and TB. Exposure to OPCs was significantly positively associated with antioxidant TB levels, and platelets played a partially negative mediating role in this association. This finding suggests that OCPs exposure may adversely affect human health by impacting the redox system.

The impact of brominated flame retardants exposure on serum total bilirubin: A cross-sectional analysis

BACKGROUND

Brominated flame retardants (BFRs) are harmful, bioaccumulative, and persistent environmental pollutants, posing significant health risks. Elevated bilirubin levels can cause neurotoxicity and damage to the heart, liver, kidneys, and other organs. This study utilizes National Health and Nutrition Examination Survey (NHANES) data to investigate the association between exposure to BFRs and total bilirubin (TB) levels in adult participants.

METHODS

Based on data from the NHANES 2007-2016, TB levels were divided into tertiles. Spearman's rank correlation was used to analyze the relationship between individual BFRs and TB levels. Weighted linear regression analysis, restricted cubic splines (RCS), and stratified analysis were conducted to assess the correlation between individual BFRs and TB levels. Weighted quantile sum (WQS) regression and quantile-based g-computation (QGC) analysis were used to comprehensively evaluate the impact of BFRs exposure on serum TB levels.

RESULTS

The study included 5831 participants. The results showed that PBB153, PBDE17, PBDE47, PBDE85, PBDE99, PBDE100, PBDE209, and PBDE183 were significantly correlated with TB levels (p <  0.05), with PBDE183 having the highest Spearman rank correlation coefficient of 0.292. After adjusting for confounding factors, most BFR remained significantly positively correlated with TB, while PBDE153 (β: - 0.031, 95%CI: - 0.317, 0.255, p =  0.829) and PBDE66 (β: 0.285, 95%CI: - 0.208, 0.777, p =  0.253) were not statistically significant. RCS analysis indicated that PBDE153 concentration had a significant U-shaped correlation with TB (p <  0.05), while PBDE17, PBDE99, PBDE154, and PBDE209 had an inverted "J"-shaped correlation (p <  0.05). PBB153, PBDE66, PBDE85, and PBDE183 also exhibited significant nonlinear S-shaped correlations with TB (p <  0.05). After stratification by age and gender, most individual BFR remained significantly positively correlated with TB levels (p <  0.05). WQS regression and QGC analysis indicated that mixed BFRs exposure was positively correlated with TB levels (β: 0.553, 95%CI: 0.384, 0.722, p <  0.001 and β: 1.060, 95%CI: 0.587, 1.532, p <  0.001), with PBDE183 contributing the most.

CONCLUSIONS

BFRs exposure is significantly positively correlated with TB levels, further suggesting the potential health impact of BFRs exposure on humans.

Iron Overload, Microbleeding and the Role of Bilirubin in Alzheimer's Disease Brain: Revisiting the Vascular Hypothesis

Alzheimer's disease (AD) and vascular dementia (VaD) are the two most prevalent forms of dementia, sharing overlapping clinical features yet distinct pathophysiological mechanisms. While AD is primarily driven by amyloid-beta (Aβ) plaques and tau neurofibrillary tangles, VaD results from cerebrovascular pathology, including ischemic lesions and chronic hypoperfusion. However, accumulating evidence suggests that vascular dysfunction is a crucial contributor to both conditions, bridging neurodegenerative and cerebrovascular pathologies. In this review, we explore the interplay between AD and VaD, focusing on shared pathways such as blood-brain barrier (BBB) breakdown, neuroinflammation, and microvascular damage. Notably, cerebral microbleeds have emerged as a common feature in both AD and VaD, further linking vascular pathology to neurodegeneration. Microbleeding contributes to BBB disruption, iron deposition, and exacerbated oxidative stress, creating a vicious cycle that accelerates cognitive decline. We highlight the role of iron dysregulation as a key driver in AD, exacerbating Aβ accumulation, tau hyperphosphorylation, and ferroptosis. Conversely, bilirubin emerges as a molecule with theranostic potential, acting as both a biomarker and a neuroprotective agent due to its antioxidant and anti-inflammatory properties. Despite its protective role, bilirubin's dysregulation under pathological conditions may contribute to oxidative damage and neurovascular dysfunction. In this context, the accumulation of iron from recurrent microbleeds may further disrupt bilirubin homeostasis, amplifying oxidative injury and inflammation. We propose a vascular hypothesis that integrates iron metabolism and bilirubin homeostasis, suggesting that their imbalance plays a central role in AD pathogenesis and worsening. Understanding the intricate molecular interplay between neurodegeneration and vascular dysfunction could provide novel insights into targeted interventions aimed at mitigating cognitive decline. Finally, we discuss the potential of bilirubin-based therapeutic strategies, including its role in counteracting oxidative stress and modulating neuroinflammatory pathways, offering promising avenues for future research and precision medicine in dementia.

Effect of moderate hyperbilirubinemia on an infant's brain: a quantitative susceptibility mapping and 1H-MRS study

Objective

The effects of moderate neonatal hyperbilirubinemia (NHB) remain unknown. The aim of this work was to investigate whether moderate NHB has an impact on an infant's brain and explore the relationship between brain magnetic susceptibility, brain metabolites, and biochemical tests in moderate NHB using quantitative susceptibility mapping (QSM) and magnetic resonance spectroscopy (MRS).

Materials and methods

In total, 28 term babies with moderate levels of blood bilirubin were enrolled in the NHB group, and 16 term infants were enrolled in the control group. All the patients underwent biochemical tests, 1H-MRS, and QSM examinations. Biochemical test results [e.g., direct bilirubin (DBiL)], metabolite ratios [e.g., glycerophosphocholine (GPC)], and susceptibility values were collected. The Mann-Whitney U-test was used to assess the differences between the NHB and control groups. Partial least square correlation (PLSC) analyses were performed to analyze the correlations between the biochemical results and the metabolite ratios and susceptibility values.

Results

The Mann-Whitney U-test showed that significant differences were observed in the biochemical results, susceptibility values of the left putamen, and absolute concentrations of GPC between the NHB group and the controls. No significant differences were found in the metabolite ratios between the two groups. The PLSC analysis demonstrated that the ratios of myo-inositol (Ins), N-acetylaspartate (NAA), and GPC relative to creatine and phosphocreatine had a robust correlation with DBiL in the NHB group. Furthermore, increasing susceptibility values of putamen, globus pallidus, caudate nucleus, and thalamus had a moderate correlation with decreasing DBiL and increasing TSH concentrations in the NHB group.

Conclusion

This study demonstrated that moderate hyperbilirubinemia could induce metabolic and susceptibility changes in an infant's brain (e.g., decreased susceptibility values and metabolite values) and these changes have a correlation with biochemical test results.

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.

Unbound bilirubin and risk of severe neurodevelopmental impairment in extremely low birthweight newborns

BACKGROUND

Unbound bilirubin (UB) was measured on day 5 ± 1 in 1101 ELBW newborns in the Aggressive vs Conservative Phototherapy randomized controlled trial. We accessed this dataset to quantify the UB-mediated risk of severe neurodevelopmental impairment (sNDI) in extremely low birthweight (ELBW) newborns.

METHODS

UB levels were standardized within laboratories as z-score percentiles. A dichotomous UB exposure variable was based on sharply increased risk of sNDI at the 85th percentile (UBz>85th). Targeted maximum likelihood estimation with ensemble machine learning (TMLE + ML) and logistic regression (LR) were used to estimate sNDI risk attributable to UBz>85th.

RESULTS

UB - total bilirubin correlation was poor: R2 0.24, 95% CI: 0.19, 0.30. The UB level corresponding to the UBz>85th cut-point in this cohort was estimated to be between 0.56 and 0.79 µg/dL. In 825 survivors with known sNDI status the crude risk difference (RD) associated with UBz>85th was 19.5%. With TMLE + ML, the RD was 14.4% (95% CI: 5.7%, 23.1%) corresponding to a number need to harm (NNH) of 7 (95% CI: 4, 18). LR results were similar: RD 12.4% (95% CI: 4.7%, 20.1%), NNH 8 (95% CI: 5, 21).

CONCLUSIONS

Exposure to UBz>85th was associated with a large increase in risk of sNDI in ELBW newborns.

IMPACT

Accessing the only large dataset with measures of unbound bilirubin (UB) and neurodevelopmental outcomes in extremely low birthweight (ELBW) newborns we found poor correlation between UB and total bilirubin (TB). The risk of severe neurodevelopmental impairment (sNDI) associated with UB rose non-linearly with an inflection point at UB z-score 85th percentile (UBz>85th), increasing the risk of sNDI by an estimated 14% (number need to harm: 7). The current incidence of exposure to harmful UB levels in very premature newborns is unlikely to be rare. TB levels provide a false sense of security to clinicians caring for very premature newborns.

Severe neonatal hyperbilirubinaemia in European and Indian subcontinent descendent newborns: a retrospective cohort study

Neonatal hyperbilirubinaemia is more common in Asian-descendent populations, but differences in disease severity are poorly reported. Our study aimed to compare neonatal hyperbilirubinaemia severity between European and Indian subcontinent descendent newborns. We conducted a single-centre retrospective cohort study including newborns admitted with unconjugated hyperbilirubinaemia (January 2016 to December 2021). Patients were followed during admission, comparing those with European ancestry (control group) and Indian subcontinent ancestry (India, Pakistan, Bangladesh and Nepal) (study group). The primary outcome was severe hyperbilirubinemia (TSB > 25 mg/dL, phototherapy > 6 h or need for exchange transfusion [ET]), and the secondary was TSB levels. Adjusted analysis for potential confounding factors was performed using binary logistic regression models. Of 110 newborns included, 27 (24.5%) had Indian subcontinent ancestry. Occurrence of TSB > 25 mg/dL was significantly higher in the study group (22.2% vs. 4.8%, p = 0.006), while no differences were noted in exposure to phototherapy > 6 h and ET therapy. Logistic regression models for confounding factors adjustment showed Indian subcontinent ancestry as an independent risk factor for TSB > 25 mg/dL (OR 7.49, CI 95% [1.23-45.50]). The study group revealed also higher absolute values of TSB both at admission (22.0 mg/dL vs. 19.6 mg/dL, p = 0.013) and at discharge (13.6 mg/dL vs. 11.4 mg/dL, p = 0.005). Conclusion: Our findings suggest that newborns with Indian subcontinent ancestry might show a higher risk for the development of severe hyperbilirubinemia compared to European ancestry newborns. Implementing earlier treatment thresholds in this subset of patients may help prevent severe hyperbilirubinemia.  What is Known: • Indian subcontinent descendent populations have high incidence of neonatal hyperbilirubinaemia but data regarding its severity are scarce. What is New: • This article shows that, compared to European descendent newborns, Indian subcontinent descendent newborns might be at higher risk for severe hyperbilirubinaemia.

Molecular events in brain bilirubin toxicity revisited

The mechanisms involved in bilirubin neurotoxicity are still far from being fully elucidated. Several different events concur to damage mainly the neurons among which inflammation and alteration of the redox state play a major role. An imbalance of cellular calcium homeostasis has been recently described to be associated with toxic concentrations of bilirubin, and this disequilibrium may in turn elicit an inflammatory reaction. The different and age-dependent sensitivity to bilirubin damage must also be considered in describing the dramatic clinical picture of bilirubin-induced neurological damage (BIND) formerly known as kernicterus spectrum disorder (KSD). This review aims to critically address what is known and what is not in the molecular events of bilirubin neurotoxicity to provide hints for a better diagnosis and more successful treatments. Part of these concepts have been presented at the 38th Annual Audrey K. Brown Kernicterus Symposium of Pediatric American Society, Washington DC, May 1, 2023.

Inflammatory liver diseases and susceptibility to sepsis

Patients with inflammatory liver diseases, particularly alcohol-associated liver disease and metabolic dysfunction-associated fatty liver disease (MAFLD), have higher incidence of infections and mortality rate due to sepsis. The current focus in the development of drugs for MAFLD is the resolution of non-alcoholic steatohepatitis and prevention of progression to cirrhosis. In patients with cirrhosis or alcoholic hepatitis, sepsis is a major cause of death. As the metabolic center and a key immune tissue, liver is the guardian, modifier, and target of sepsis. Septic patients with liver dysfunction have the highest mortality rate compared with other organ dysfunctions. In addition to maintaining metabolic homeostasis, the liver produces and secretes hepatokines and acute phase proteins (APPs) essential in tissue protection, immunomodulation, and coagulation. Inflammatory liver diseases cause profound metabolic disorder and impairment of energy metabolism, liver regeneration, and production/secretion of APPs and hepatokines. Herein, the author reviews the roles of (1) disorders in the metabolism of glucose, fatty acids, ketone bodies, and amino acids as well as the clearance of ammonia and lactate in the pathogenesis of inflammatory liver diseases and sepsis; (2) cytokines/chemokines in inflammatory liver diseases and sepsis; (3) APPs and hepatokines in the protection against tissue injury and infections; and (4) major nuclear receptors/signaling pathways underlying the metabolic disorders and tissue injuries as well as the major drug targets for inflammatory liver diseases and sepsis. Approaches that focus on the liver dysfunction and regeneration will not only treat inflammatory liver diseases but also prevent the development of severe infections and sepsis.

Bilirubin and Redox Stress in Age-Related Brain Diseases

Cellular redox status has a crucial role in brain physiology, as well as in pathologic conditions. Physiologic senescence, by dysregulating cellular redox homeostasis and decreasing antioxidant defenses, enhances the central nervous system's susceptibility to diseases. The reduction of free radical accumulation through lifestyle changes, and the supplementation of antioxidants as a prophylactic and therapeutic approach to increase brain health, are strongly suggested. Bilirubin is a powerful endogenous antioxidant, with more and more recognized roles as a biomarker of disease resistance, a predictor of all-cause mortality, and a molecule that may promote health in adults. The alteration of the expression and activity of the enzymes involved in bilirubin production, as well as an altered blood bilirubin level, are often reported in neurologic conditions and neurodegenerative diseases (together denoted NCDs) in aging. These changes may predict or contribute both positively and negatively to the diseases. Understanding the role of bilirubin in the onset and progression of NCDs will be functional to consider the benefits vs. the drawbacks and to hypothesize the best strategies for its manipulation for therapeutic purposes.

Inflammation, Dopaminergic Brain and Bilirubin

Dopamine is a well-known neurotransmitter due to its involvement in Parkinson's disease (PD). Dopamine is not only involved in PD but also controls multiple mental and physical activities, such as the pleasure of food, friends and loved ones, music, art, mood, cognition, motivation, fear, affective disorders, addiction, attention deficit disorder, depression, and schizophrenia. Dopaminergic neurons (DOPAn) are susceptible to stressors, and inflammation is a recognized risk for neuronal malfunctioning and cell death in major neurodegenerative diseases. Less is known for non-neurodegenerative conditions. Among the endogenous defenses, bilirubin, a heme metabolite, has been shown to possess important anti-inflammatory activity and, most importantly, to prevent DOPAn demise in an ex vivo model of PD by acting on the tumor necrosis factor-alpha (TNFα). This review summarizes the evidence linking DOPAn, inflammation (when possible, specifically TNFα), and bilirubin as an anti-inflammatory in order to understand what is known, the gaps that need filling, and the hypotheses of anti-inflammatory strategies to preserve dopamine homeostasis with bilirubin included.

Bilirubin-Induced Transcriptomic Imprinting in Neonatal Hyperbilirubinemia

Recent findings indicated aberrant epigenetic control of the central nervous system (CNS) development in hyperbilirubinemic Gunn rats as an additional cause of cerebellar hypoplasia, the landmark of bilirubin neurotoxicity in rodents. Because the symptoms in severely hyperbilirubinemic human neonates suggest other regions as privileged targets of bilirubin neurotoxicity, we expanded the study of the potential impact of bilirubin on the control of postnatal brain development to regions correlating with human symptoms. Histology, transcriptomic, gene correlation, and behavioral studies were performed. The histology revealed widespread perturbation 9 days after birth, restoring in adulthood. At the genetic level, regional differences were noticed. Bilirubin affected synaptogenesis, repair, differentiation, energy, extracellular matrix development, etc., with transient alterations in the hippocampus (memory, learning, and cognition) and inferior colliculi (auditory functions) but permanent changes in the parietal cortex. Behavioral tests confirmed the presence of a permanent motor disability. The data correlate well both with the clinic description of neonatal bilirubin-induced neurotoxicity, as well as with the neurologic syndromes reported in adults that suffered neonatal hyperbilirubinemia. The results pave the way for better deciphering the neurotoxic features of bilirubin and evaluating deeply the efficacy of new therapeutic approaches against the acute and long-lasting sequels of bilirubin neurotoxicity.