Endothelin-1 Mediates Brain Microvascular Dysfunction Leading to Long-Term Cognitive Impairment in a Model of Experimental Cerebral Malaria

Influence of Tau on Neurotoxicity and Cerebral Vasculature Impairment Associated with Alzheimer's Disease

Alzheimer's disease is a fatal chronic neurodegenerative condition marked by a gradual decline in cognitive abilities and impaired vascular function within the central nervous system. This affliction initiates its insidious progression with the accumulation of two aberrant protein entities including Aβ plaques and neurofibrillary tangles. These chronic elements target distinct brain regions, steadily erasing the functionality of the hippocampus and triggering the erosion of memory and neuronal integrity. Several assumptions are anticipated for AD as genetic alterations, the occurrence of Aβ plaques, altered processing of amyloid precursor protein, mitochondrial damage, and discrepancy of neurotropic factors. In addition to Aβ oligomers, the deposition of tau hyper-phosphorylates also plays an indispensable part in AD etiology. The brain comprises a complex network of capillaries that is crucial for maintaining proper function. Tau is expressed in cerebral blood vessels, where it helps to regulate blood flow and sustain the blood-brain barrier's integrity. In AD, tau pathology can disrupt cerebral blood supply and deteriorate the BBB, leading to neuronal neurodegeneration. Neuroinflammation, deficits in the microvasculature and endothelial functions, and Aβ deposition are characteristically detected in the initial phases of AD. These variations trigger neuronal malfunction and cognitive impairment. Intracellular tau accumulation in microglia and astrocytes triggers deleterious effects on the integrity of endothelium and cerebral blood supply resulting in further advancement of the ailment and cerebral instability. In this review, we will discuss the impact of tau on neurovascular impairment, mitochondrial dysfunction, oxidative stress, and the role of hyperphosphorylated tau in neuron excitotoxicity and inflammation.

Effects of anti-tau immunotherapy on reactive microgliosis, cerebral endotheliopathy, and cognitive function in an experimental model of cerebral malaria

Cerebral malaria (CM), a potentially fatal encephalopathy caused primarily by infection with Plasmodium falciparum, results in long-term adverse neuro-psychiatric sequelae. Neural cell injury contributes to the neurological deficits observed in CM. Abnormal regulation of tau, an axonal protein pathologically associated with the formation of neurofibrillary lesions in neurodegenerative diseases, has been linked to inflammation and cerebral microvascular compromise and has been reported in human and experimental CM (ECM). Immunotherapy with a monoclonal antibody to pathological tau (PHF-1 mAB) in experimental models of neurodegenerative diseases has been reported to mitigate cognitive decline. We investigated whether immunotherapy with PHF-1 mAB prevented cerebral endotheliopathy, neural cell injury, and neuroinflammation during ECM. Using C57BL/6 mice infected with either Plasmodium berghei ANKA (PbA), which causes ECM, Plasmodium berghei NK65 (PbN), which causes severe malaria, but not ECM, or uninfected mice (Un), we demonstrated that when compared to PbN infection or uninfected mice, PbA infection resulted in significant memory impairment at 6 days post-infection, in association with abnormal tau phosphorylation at Ser202 /Thr205 (pSer202 /Thr205 ) and Ser396-404 (pSer396-404 ) in mouse brains. ECM also resulted in significantly higher expression of inflammatory markers, in microvascular congestion, and glial cell activation. Treatment with PHF-1 mAB prevented PbA-induced cognitive impairment and was associated with significantly less vascular congestion, neuroinflammation, and neural cell activation in mice with ECM. These findings suggest that abnormal regulation of tau protein contributes to cerebral vasculopathy and is critical in the pathogenesis of neural cell injury during CM. Tau-targeted therapies may ameliorate the neural cell damage and subsequent neurocognitive impairment that occur during disease.

Interplay Between the Immune and Nervous Cognitive Systems in Homeostasis and in Malaria

The immune and nervous systems can be thought of as cognitive and plastic systems, since they are both involved in cognition/recognition processes and can be architecturally and functionally modified by experience, and such changes can influence each other's functioning. The immune system can affect nervous system function depending on the nature of the immune stimuli and the pro/anti-inflammatory responses they generate. Here we consider interactions between the immune and nervous systems in homeostasis and disease, including the beneficial and deleterious effects of immune stimuli on brain function and the impact of severe and non-severe malaria parasite infections on neurocognitive and behavioral parameters in human and experimental murine malaria. We also discuss the effect of immunization on the reversal of cognitive deficits associated with experimental non-severe malaria in a model susceptible to the development of the cerebral form of the illness. Finally, we consider the possibility of using human vaccines, largely exploited as immune-prophylactics for infectious diseases, as therapeutic tools to prevent or mitigate the expression of cognitive deficits in infectious and chronic degenerative diseases.

Cerebral Malaria Model Applying Human Brain Organoids

Neural injuries in cerebral malaria patients are a significant cause of morbidity and mortality. Nevertheless, a comprehensive research approach to study this issue is lacking, so herein we propose an in vitro system to study human cerebral malaria using cellular approaches. Our first goal was to establish a cellular system to identify the molecular alterations in human brain vasculature cells that resemble the blood-brain barrier (BBB) in cerebral malaria (CM). Through transcriptomic analysis, we characterized specific gene expression profiles in human brain microvascular endothelial cells (HBMEC) activated by the Plasmodium falciparum parasites. We also suggest potential new genes related to parasitic activation. Then, we studied its impact at brain level after Plasmodium falciparum endothelial activation to gain a deeper understanding of the physiological mechanisms underlying CM. For that, the impact of HBMEC-P. falciparum-activated secretomes was evaluated in human brain organoids. Our results support the reliability of in vitro cellular models developed to mimic CM in several aspects. These systems can be of extreme importance to investigate the factors (parasitological and host) influencing CM, contributing to a molecular understanding of pathogenesis, brain injury, and dysfunction.

Malaria Related Neurocognitive Deficits and Behavioral Alterations

Typical of tropical and subtropical regions, malaria is caused by protozoa of the genus Plasmodium and is, still today, despite all efforts and advances in controlling the disease, a major issue of public health. Its clinical course can present either as the classic episodes of fever, sweating, chills and headache or as nonspecific symptoms of acute febrile syndromes and may evolve to severe forms. Survivors of cerebral malaria, the most severe and lethal complication of the disease, might develop neurological, cognitive and behavioral sequelae. This overview discusses the neurocognitive deficits and behavioral alterations resulting from human naturally acquired infections and murine experimental models of malaria. We highlighted recent reports of cognitive and behavioral sequelae of non-severe malaria, the most prevalent clinical form of the disease worldwide. These sequelae have gained more attention in recent years and therapies for them are required and demand advances in the understanding of neuropathogenesis. Recent studies using experimental murine models point to immunomodulation as a potential approach to prevent or revert neurocognitive sequelae of malaria.

Hemozoin Promotes Lung Inflammation via Host Epithelial Activation

Respiratory distress (RD) is a complication of severe malaria associated with a particularly high risk for death in African children infected with the parasite Plasmodium falciparum. The pathophysiology underlying RD remains poorly understood, and the condition is managed supportively.

Respiratory distress in severe malaria is associated with high mortality, but its pathogenesis remains unclear. The malaria pigment hemozoin (HZ) is abundant in target organs of severe malaria, including the lungs, and is known to be a potent innate immune activator of phagocytes. We hypothesized that HZ might also stimulate lung epithelial activation and thereby potentiate lung inflammation. We show here that airway epithelium stimulated with HZ undergoes global transcriptional reprogramming and changes in cell surface protein expression that comprise an epithelial activation phenotype. Proinflammatory signaling is induced, and key cytoadherence molecules are upregulated, including several associated with severe malaria, such as CD36 and ICAM1. Epithelial and extracellular matrix remodeling pathways are transformed, including induction of key metalloproteases and modulation of epithelial junctions. The overall program induced by HZ serves to promote inflammation and neutrophil transmigration, and is recapitulated in a murine model of HZ-induced acute pneumonitis. Together, our data demonstrate a direct role for hemozoin in stimulating epithelial activation that could potentiate lung inflammation in malaria.

Interleukin-6: Important Mediator of Vasospasm Following Subarachnoid Hemorrhage

The correlation of neuroinflammation with the development of cerebral vasospasm following subarachnoid hemorrhage has been well documented in the literature; both clinical and preclinical. The exact mechanisms by which this process occurs, however, are poorly elucidated. Recent evidence indicates that interleukin-6 is not only an important prognostic biomarker for subarachnoid hemorrhage and subsequent vasospasm development but also an integral component in the progression of injury following initial insult. In this review, we briefly highlight other pathways under investigation and focus heavily on what has been discovered regarding the role of interleukin 6 and cerebral vasospasm following subarachnoid hemorrhage. A proposed mechanistic pathway is highlighted in written and graphical format. A discussion regarding the human correlative findings and initial pre-clinical mechanistic studies is addressed. Finally, in the future investigation section, innovative developments and a clear description of areas warranting further scientific inquiry are emphasized. This review will catalyze continued discovery in this area of emerging significance and aid in the quest for effective vasospasm treatment where limited clinical therapeutics currently exist.

Ideational Fluency in Patients with Rheumatoid Arthritis

BACKGROUND

Neuropsychiatric symptoms have been well documented in several systemic inflammatory conditions, for example, systemic lupus erythematosus (SLE). Increased prevalence of cognitive decline and psychiatric issues has been reported in patients with rheumatoid arthritis (RA). However, there is limited evidence of which exact cognitive domains are affected and to what degree.

AIM

To test the performance of cognition in the domain of ideational fluency (Thing Categories Test in particular) in patients with RA and compare the results with the general population and to the results with cognitive and depression screening scores in both groups.

METHODS

Patient Health Questionnaire 9 (PHQ-9), Generalized Anxiety Disorder 7 (GAD-7) assessment, Montreal Cognitive Assessment (MoCA), and Thing Categories Test (TCT) were used to evaluate patients with RA, as well as the control group.

RESULTS

Twenty patients with RA and 20 controls were tested, with 7 and 4 men, and 13 and 16 women in the study and control group, respectively. Average scores in TCT at three minutes were 7.50 (IQR6.0-10.0) and 6.0 (IQR3.0-8.0) for category "blue"; 17.50 (IQR15.0-19.0) and 16.0 (10.0-18.0) for category "round" in the control and study group, respectively. A statistically significant difference was established between the study and the control group in TCT for the category "blue" (p<0.025). The average score for GAD7 was 2.0 (IQR 0.0-5.75) and 3.0 (IQR0.50-6.00) in the control and study group, respectively. The average score for PHQ-9 was 2.0 (IQR0.25-4.75) and 4.0 (IQR2.00-5.50) in the control and study group, respectively. Finally, the average score for the MoCA scale was 27.0 (IQR25.25-28.00) and 26.0 (IQR23.50-28.00) in the control and study group, respectively.

CONCLUSION

Preliminary evidence suggests that RA at least partially affects the cognitive domain of ideational fluency. However, further research with larger experimental groups is needed to provide more conclusive evidence.

Pathophysiology and neurologic sequelae of cerebral malaria

Cerebral malaria (CM), results from Plasmodium falciparum infection, and has a high mortality rate. CM survivors can retain life-long post CM sequelae, including seizures and neurocognitive deficits profoundly affecting their quality of life. As the Plasmodium parasite does not enter the brain, but resides inside erythrocytes and are confined to the lumen of the brain's vasculature, the neuropathogenesis leading to these neurologic sequelae is unclear and under-investigated. Interestingly, postmortem CM pathology differs in brain regions, such as the appearance of haemorragic punctae in white versus gray matter. Various host and parasite factors contribute to the risk of CM, including exposure at a young age, parasite- and host-related genetics, parasite sequestration and the extent of host inflammatory responses. Thus far, several proposed adjunctive treatments have not been successful in the treatment of CM but are highly needed. The region-specific CM neuro-pathogenesis leading to neurologic sequelae is intriguing, but not sufficiently addressed in research. More attention to this may lead to the development of effective adjunctive treatments to address CM neurologic sequelae.

Retinopathy-Positive Cerebral Malaria Is Associated With Greater Inflammation, Blood-Brain Barrier Breakdown, and Neuronal Damage Than Retinopathy-Negative Cerebral Malaria

BACKGROUND

Our prior study findings suggest that Plasmodium falciparum is the cause of disease in both malaria retinopathy-positive (RP) and most retinopathy-negative (RN) cerebral malaria (CM), and that absence of retinopathy and decreased disease severity in RN CM may be due to shorter duration of illness, lower parasite biomass, and decreased var gene expression in RN compared to RP CM. In the present study, we assessed the pathophysiology of RP and RN CM.

METHODS

We compared markers of systemic and central nervous system inflammation, oxidative stress, neuronal injury, systemic endothelial activation, angiogenesis, and platelet activation in Ugandan children with RP (n = 167) or RN (n = 87) CM.

RESULTS

RP children had higher plasma C-reactive protein (P = .013), ferritin and erythropoietin (both P < .001) levels, an elevated cerebrospinal fluid (CSF):plasma albumin ratio (P < .001), and higher CSF tau protein levels (P = .049) than RN children. Levels of plasma and CSF proinflammatory and anti-inflammatory cytokines and oxidative stress markers did not differ between RP and RN children. RN children had higher plasma levels of endothelin 1 (P = .003), platelet-derived growth factor (P = .012), and platelet factor 4 (P = .034).

CONCLUSIONS

RP and RN CM may represent different phases of CM. RN CM may be driven by early vasospasm and platelet activation, whereas the more advanced RP CM is associated with greater inflammation, increased erythropoietic drive, blood-brain barrier breakdown, and neuronal injury, each of which may contribute to greater disease severity.

New Insights into Malaria Pathogenesis

Malaria remains a major public health threat in tropical and subtropical regions across the world. Even though less than 1% of malaria infections are fatal, this leads to about 430,000 deaths per year, predominantly in young children in sub-Saharan Africa. Therefore, it is imperative to understand why a subset of infected individuals develop severe syndromes and some of them die and what differentiates these cases from the majority that recovers. Here, we discuss progress made during the past decade in our understanding of malaria pathogenesis, focusing on the major human parasite Plasmodium falciparum.

Brain Vasculature and Cognition

There is a complex interaction between the brain and the cerebral vasculature to meet the metabolic demands of the brain for proper function. Preservation of cerebrovascular function and integrity has a central role in this sophisticated communication within the brain, and any derangements can have deleterious acute and chronic consequences. In almost all forms of cognitive impairment, from mild to Alzheimer disease, there are changes in cerebrovascular function and structure leading to decreased cerebral blood flow, which may initiate or worsen cognitive impairment. In this focused review, we discuss the contribution of 2 major vasoactive pathways to cerebrovascular dysfunction and cognitive impairment in an effort to identify early intervention strategies.

Effects of early administration of insulin-like growth factor-1 on cognitive function in septic encephalopathy

BACKGROUND

Both protective and therapeutic functions of insulin-like growth factor-1 (IGF-1) in brain injury have been reported, but its effects on cognitive sequelae after septic encephalopathy (SE) remain unclear.

MATERIALS AND METHODS

This study was divided into three parts, and a septic model was built by cecal ligation and puncture (CLP). First, survival analysis was performed, and IGF-1's effects on long-term cognition and depressive emotion were assessed. Second, the characteristics of IGF-1 function in cognition were evaluated. Finally, cytochrome C, caspase-9, tumor necrosis factor receptor (TNFR), and caspase-8 levels as well as cell apoptosis in the hippocampus were evaluated.

RESULTS

IGF-1 did not reduce mortality or alleviate depressive symptoms in septic rats, but improved the memory of noxious stimulation and spatial learning and memory. These effects were observed only when IGF-1 was administered within 0-6 hours after CLP. Moreover, cytochrome C and caspase-9 expression levels were increased at 6 hours after CLP in the hippocampus, while TNFR and caspase-8 amounts were not increased until 12 hours after CLP. Cell apoptosis increased at 12 hours after CLP, but was inhibited by IGF-1.

CONCLUSION

Cognitive impairment in rats recovering from SE is associated with cell apoptosis in the hippocampus. Supplementation of IGF-1 reduces cell apoptosis by preventing the over-expression of cytochrome C and TNFR, and results in improved cognitive function. However, improvement only occurs when IGF-1 is administered at the early stage (within 6 hours) of sepsis. As cytochrome C activation occurs earlier than that of TNFR in this study, cytochrome C may be the main factor inducing apoptosis in early SE.

CT perfusion imaging of cerebral microcirculatory changes following subarachnoid hemorrhage in rabbits: Specific role of endothelin-1 receptor antagonist

BACKGROUND

Cerebral vasospasm may lead to delayed ischemic neurological deficits following subarachnoid hemorrhage (SAH). Endothelin (ET-1) is an important factor participating in cerebral vasospasm underlying SAH. We used a specific endothelin receptor antagonist, BQ123 to assess the specific role of endothelin-1 receptor antagonist in cerebral vasospasm in a rabbit model of SAH by examining plasma ET-1 levels and the principal CT perfusion (CTP) parameters pertinent to the hemodynamic status of microcirculation following SAH.

METHODS

102 male New Zealand white rabbits were divided into control, SAH and SAH + BQ123 intervention group (BQ123 group). Rabbit SAH model was established by double hemorrhage injection of autologous blood into the cisterna magna; Aquilion ONE was used to collect cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) which were used to evaluate cerebral microcirculation hemodynamics; Elisa was used to assess plasma ET-1 levels. Data were collected on days 1, 4, 7 and 14 following SAH, respectively.

RESULTS

Compared with the control group, the CBF in the SAH group was significantly lower, while the MTT was significantly higher. The CBF decreased on the 4th day and reached the lowest on the 7th day. The MTT began to rise on the 4th day and peaked on the 7th day. While in the BQ123 intervention group, the CBF significantly increased while the MTT significantly decreased on the 1st and the 4th days, respectively. Compared with SAH group, plasma ET-1 levels in BQ123 group significantly increased on the earlier (1st and 4th days) but not later days (between the 7th and 14th days). In addition, the inflammatory infiltration of brain tissues in rabbits treated with BQ123 post-SAH was significantly reduced compared with SAH group.

CONCLUSION

CTP can quantify the therapeutic effect of BQ123 after SAH; Selective blockade of ET-1 endothelin receptor, BQ123 significantly improved microcirculatory perfusion along with a reduction in resultant vasogenic inflammatory responses. The effect of BQ123 on the cerebral microcirculation was lobe dependent.

Endothelin and diabetic complications: a brain-centric view

The global epidemic of diabetes is of significant concern. Diabetes associated vascular disease signifies the principal cause of morbidity and mortality in diabetic patients. It is also the most rapidly increasing risk factor for cognitive impairment, a silent disease that causes loss of creativity, productivity, and quality of life. Small vessel disease in the cerebral vasculature plays a major role in the pathogenesis of cognitive impairment in diabetes. Endothelin system, including endothelin-1 (ET-1) and the receptors (ET(A) and ET(B)), is a likely candidate that may be involved in many aspects of the diabetes cerebrovascular disease. In this review, we took a brain-centric approach and discussed the role of the ET system in cerebrovascular and cognitive dysfunction in diabetes.

Endothelins in inflammatory neurological diseases

Endothelins were discovered more than thirty years ago as potent vasoactive compounds. Beyond their well-documented cardiovascular properties, however, the contributions of the endothelin pathway have been demonstrated in several neuroinflammatory processes and the peptides have been reported as clinically relevant biomarkers in neurodegenerative diseases. Several studies report that endothelin-1 significantly contributes to the progression of neuroinflammatory processes, particularly during infections in the central nervous system (CNS), and is associated with a loss of endothelial integrity at the blood brain barrier level. Because of the paucity of clinical trials with endothelin-1 antagonists in several infectious and non-infectious neuroinflammatory diseases, it remains an open question whether the 21 amino acid peptide is a mediator/modulator rather than a biomarker of the progression of neurodegeneration. This review focuses on the potential roles of endothelins in the pathology of neuroinflammatory processes, including infectious diseases of viral, bacterial or parasitic origin in which the synthesis of endothelins or its pharmacology have been investigated from the cell to the bedside in several cases, as well as in non-infectious inflammatory processes such as neurodegenerative disorders like Alzheimers Disease or central nervous system vasculitis.

Cerebral Malaria Causes Enduring Behavioral and Molecular Changes in Mice Brain Without Causing Gross Histopathological Damage

Malaria, parasitic disease considered a major health public problem, is caused by Plasmodium protozoan genus and transmitted by the bite of infected female Anopheles mosquito genus. Cerebral malaria (CM) is the most severe presentation of malaria, caused by P. falciparum and responsible for high mortality and enduring development of cognitive deficits which may persist even after cure and cessation of therapy. In the present study we evaluated selected behavioral, neurochemical and neuropathologic parameters after rescue from experimental cerebral malaria caused by P. berghei ANKA in C57BL/6 mice. Behavioral tests showed impaired nest building activity as well as increased marble burying, indicating that natural behavior of mice remains altered even after cure of infection. Regarding the neurochemical data, we found decreased α2/α3 Na⁺,K⁺-ATPase activity and increased immunoreactivity of phosphorylated Na⁺,K⁺-ATPase at Ser⁹⁴³ in cerebral cortex after CM. In addition, [³H]-Flunitrazepam binding assays revealed a decrease of benzodiazepine/GABA_A receptor binding sites in infected animals. Moreover, in hippocampus, dot blot analysis revealed increased levels of protein carbonyls, suggesting occurrence of oxidative damage to proteins. Interestingly, no changes in the neuropathological markers Fluoro-Jade C, Timm staining or IBA-1 were detected. Altogether, present data indicate that behavioral and neurochemical alterations persist even after parasitemia clearance and CM recovery, which agrees with available clinical findings. Some of the molecular mechanisms reported in the present study may underlie the behavioral changes and increased seizure susceptibility that persist after recovery from CM and may help in the future development of therapeutic strategies for CM sequelae.

Endothelin-1 Treatment Induces an Experimental Cerebral Malaria-Like Syndrome in C57BL/6 Mice Infected with Plasmodium berghei NK65

Plasmodium berghei ANKA infection of C57BL/6 mice is a widely used model of experimental cerebral malaria (ECM). By contrast, the nonneurotropic P. berghei NK65 (PbN) causes severe malarial disease in C57BL/6 mice but does not cause ECM. Previous studies suggest that endothelin-1 (ET-1) contributes to the pathogenesis of ECM. In this study, we characterize the role of ET-1 on ECM vascular dysfunction. Mice infected with 10⁶ PbN-parasitized red blood cells were treated with either ET-1 or saline from 2 to 8 days postinfection (dpi). Plasmodium berghei ANKA-infected mice served as the positive control. ET-1-treated PbN-infected mice exhibited neurological signs, hypothermia, and behavioral alterations characteristic of ECM, dying 4 to 8 dpi. Parasitemia was not affected by ET-1 treatment. Saline-treated PbN-infected mice did not display ECM, surviving until 12 dpi. ET-1-treated PbN-infected mice displayed leukocyte adhesion to the vascular endothelia and petechial hemorrhages throughout the brain at 6 dpi. Intravital microscopic images demonstrated significant brain arteriolar vessel constriction, decreased functional capillary density, and increased blood-brain barrier permeability. These alterations were not present in either ET-1-treated uninfected or saline-treated PbN-infected mice. In summary, ET-1 treatment of PbN-infected mice induced an ECM-like syndrome, causing brain vasoconstriction, adherence of activated leukocytes in the cerebral microvasculature, and blood-brain barrier leakage, indicating that ET-1 is involved in the genesis of brain microvascular alterations that are the hallmark of ECM.