An insight into the ligand-receptor interactions involved in the translocation of pathogens across blood-brain barrier

The Role of Bacteria in Central Nervous System Tumors: Opportunities and Challenges

Tumors of the central nervous system (CNS) are severe and refractory diseases with poor prognosis, especially for patients with malignant glioblastoma and brain metastases. Currently, numerous studies have explored the potential role of bacteria and intestinal flora in tumor development and treatment. Bacteria can penetrate the blood-brain barrier (BBB), targeting the hypoxic microenvironment at the core of tumors, thereby eliminating tumors and activating both the innate and adaptive immune responses, rendering them promising therapeutic agents for CNS tumors. In addition, engineered bacteria and derivatives, such as bacterial membrane proteins and bacterial spores, can also be used as good candidate carriers for targeted drug delivery. Moreover, the intestinal flora can regulate CNS tumor metabolism and influence the immune microenvironment through the "gut-brain axis". Therefore, bacterial anti-tumor therapy, engineered bacterial targeted drug delivery, and intervention of the intestinal flora provide therapeutic modalities for the treatment of CNS tumors. In this paper, we performed a comprehensive review of the mechanisms and therapeutic practices of bacterial therapy for CNS tumors and discussed potential future research directions in this field.

Blood-brain barrier biomarkers

The blood-brain barrier (BBB) is a dynamic interface that regulates the exchange of molecules and cells between the brain parenchyma and the peripheral blood. The BBB is mainly composed of endothelial cells, astrocytes and pericytes. The integrity of this structure is essential for maintaining brain and spinal cord homeostasis and protection from injury or disease. However, in various neurological disorders, such as traumatic brain injury, Alzheimer's disease, and multiple sclerosis, the BBB can become compromised thus allowing passage of molecules and cells in and out of the central nervous system parenchyma. These agents, however, can serve as biomarkers of BBB permeability and neuronal damage, and provide valuable information for diagnosis, prognosis and treatment. Herein, we provide an overview of the BBB and changes due to aging, and summarize current knowledge on biomarkers of BBB disruption and neurodegeneration, including permeability, cellular, molecular and imaging biomarkers. We also discuss the challenges and opportunities for developing a biomarker toolkit that can reliably assess the BBB in physiologic and pathophysiologic states.

Differential transcriptome response of blood brain barrier spheroids to neuroinvasive Neisseria and Borrelia

Background

The blood-brain barrier (BBB), a highly regulated interface between the blood and the brain, prevents blood-borne substances and pathogens from entering the CNS. Nevertheless, pathogens like Neisseria meningitidis and Borrelia bavariensis can breach the BBB and infect the brain parenchyma. The self-assembling BBB-spheroids can simulate the cross talk occurring between the cells of the barrier and neuroinvasive pathogens.

Methods

BBB spheroids were generated by co-culturing human brain microvascular endothelial cells (hBMECs), pericytes and astrocytes. The BBB attributes of spheroids were confirmed by mapping the localization of cells, observing permeability of angiopep2 and non-permeability of dextran. Fluorescent Neisseria, Borrelia or E. coli (non-neuroinvasive) were incubated with spheroids to observe the adherence, invasion and spheroid integrity. Transcriptome analysis with NGS was employed to investigate the response of BBB cells to infections.

Results

hBMECs were localized throughout the spheroids, whereas pericytes and astrocytes were concentrated around the core. Within 1 hr of exposure, Neisseria and Borrelia adhered to spheroids, and their microcolonization increased from 5 to 24 hrs. Integrity of spheroids was compromised by both Neisseria and Borrelia, but not by E. coli infection. Transcriptome analysis revealed a significant change in the expression of 781 genes (467 up and 314 down regulated) in spheroids infected with Neisseria, while Borrelia altered the expression of 621 genes (225 up and 396 down regulated). The differentially expressed genes could be clustered into various biological pathways like cell adhesion, extracellular matrix related, metallothionines, members of TGF beta, WNT signaling, and immune response. Among the differentially expressed genes, 455 (48%) genes were inversely expressed during Neisseria and Borrelia infection.

Conclusion

The self-assembling spheroids were used to perceive the BBB response to neuroinvasive pathogens - Neisseria and Borrelia. Compromised integrity of spheroids during Neisseria and Borrelia infection as opposed to its intactness and non-adherence of E. coli (non-neuroinvasive) denotes the pathogen dependent fate of BBB. Genes categorized into various biological functions indicated weakened barrier properties of BBB and heightened innate immune response. Inverse expression of 48% genes commonly identified during Neisseria and Borrelia infection exemplifies unique response of BBB to varying neuropathogens.

Neisseria meningitidis Opca Protein/MnO2 Hybrid Nanoparticles for Overcoming the Blood-Brain Barrier to Treat Glioblastoma

The major hurdle in glioblastoma therapy is the low efficacy of drugs crossing the blood-brain barrier (BBB). Neisseria meningitidis is known to specifically enrich in the central nervous system through the guidance of an outer membrane invasion protein named Opca. Here, by loading a chemotherapeutic drug methotrexate (MTX) in hollow manganese dioxide (MnO₂ ) nanoparticles with surface modification of the Opca protein of Neisseria meningitidis, a bionic nanotherapeutic system (MTX@MnO₂ -Opca) is demonstrated to effectively overcome the BBB. The presence of the Opca protein enables the drug to cross the BBB and penetrate into tumor tissues. After accumulating in glioblastoma, the nanotherapeutic system catalyzes the decomposition of excess H₂ O₂ in the tumor tissue and thereby generates O₂ , which alleviates tumor hypoxia and enhances the effect of chemotherapy in the treatment of glioblastoma. This bionic nanotherapeutic system may exhibit great potential in the treatment of glioblastoma.

Comprehensive Mapping of the Cell Response to Borrelia bavariensis in the Brain Microvascular Endothelial Cells in vitro Using RNA-Seq

Borrelia bavariensis can invade the central nervous system (CNS) by crossing the blood-brain barrier (BBB). It is predicted that B. bavariensis evokes numerous signaling cascades in the human brain microvascular endothelial cells (hBMECs) and exploits them to traverse across the BBB. The complete picture of signaling events in hBMECs induced by B. bavariensis remains uncovered. Using RNA sequencing, we mapped 11,398 genes and identified 295 differentially expressed genes (DEGs, 251 upregulated genes and 44 downregulated genes) in B. bavariensis challenged hBMECs. The results obtained from RNA-seq were validated with qPCR. Gene ontology analysis revealed the participation of DEGs in a number of biological processes like cell communication, organization of the extracellular matrix, vesicle-mediated transport, cell response triggered by pattern recognition receptors, antigen processing via MHC class I, cellular stress, metabolism, signal transduction, etc. The expression of several non-protein coding genes was also evoked. In this manuscript, we discuss in detail the correlation between several signaling cascades elicited and the translocation of BBB by B. bavariensis. The data revealed here may contribute to a better understanding of the mechanisms employed by B. bavariensis to cross the BBB.

Screening for Interacting Proteins with Peptide Biomarker of Blood-Brain Barrier Alteration under Inflammatory Conditions

Neurodegenerative diseases are characterized by increased permeability of the blood-brain barrier (BBB) due to alterations in cellular and structural components of the neurovascular unit, particularly in association with neuroinflammation. A previous screening study of peptide ligands to identify molecular alterations of the BBB in neuroinflammation by phage-display, revealed that phage clone 88 presented specific binding affinity to endothelial cells under inflammatory conditions in vivo and in vitro. Here, we aimed to identify the possible target receptor of the peptide ligand 88 expressed under inflammatory conditions. A cross-link test between phage-peptide-88 with IL-1β-stimulated human hCMEC cells, followed by mass spectrometry analysis, was used to identify the target of peptide-88. We modeled the epitope-receptor molecular interaction between peptide-88 and its target by using docking simulations. Three proteins were selected as potential target candidates and tested in enzyme-linked immunosorbent assays with peptide-88: fibronectin, laminin subunit α5 and laminin subunit β-1. Among them, only laminin subunit β-1 presented measurable interaction with peptide-88. Peptide-88 showed specific interaction with laminin subunit β-1, highlighting its importance as a potential biomarker of the laminin changes that may occur at the BBB endothelial cells under pathological inflammation conditions.

Transcriptomic analysis of human brain microvascular endothelial cells exposed to laminin binding protein (adhesion lipoprotein) and Streptococcus pneumoniae

Streptococcus pneumoniae invades the CNS and triggers a strong cellular response. To date, signaling events that occur in the human brain microvascular endothelial cells (hBMECs), in response to pneumococci or its surface adhesins are not mapped comprehensively. We evaluated the response of hBMECs to the adhesion lipoprotein (a laminin binding protein—Lbp) or live pneumococci. Lbp is a surface adhesin recently identified as a potential ligand, which binds to the hBMECs. Transcriptomic analysis was performed by RNA-seq of three independent biological replicates and validated with qRT-PCR using 11 genes. In total 350 differentially expressed genes (DEGs) were identified after infection with S. pneumoniae, whereas 443 DEGs when challenged with Lbp. Total 231 DEGs were common in both treatments. Integrative functional analysis revealed participation of DEGs in cytokine, chemokine, TNF signaling pathways and phagosome formation. Moreover, Lbp induced cell senescence and breakdown, and remodeling of ECM. This is the first report which maps complete picture of cell signaling events in the hBMECs triggered against S. pneumoniae and Lbp. The data obtained here could contribute in a better understanding of the invasion of pneumococci across BBB and underscores role of Lbp adhesin in evoking the gene expression in neurovascular unit.

Identification of the proteins of Borrelia garinii interacting with human brain microvascular endothelial cells

Lyme borreliosis is one of the major tick-borne diseases in Europe. Events of the translocation of Borrelia across the blood-brain barrier (BBB) involve multiple interactions between borrelial surface proteins and receptors on the brain microvascular endothelial cells (hBMECs). In this study, we aimed to identify proteins of Borrelia that plausibly interact with hBMECs. The surface proteome of live Borrelia (a neuroinvasive strain of B. garinii) was crosslinked with biotin prior to its incubation with hBMECs. The interacting proteins were recovered by affinity purification, followed by SWATH-MS. Twenty-four interacting candidates were grouped into outer membrane proteins (n = 12) and inner membrane proteins (n = 12) based on the subcellular location as per the predictions of LocateP. Other algorithms like TMHMM 2.0 and LipoP, ontology search and literature review were subsequently applied to each of the identified protein candidates to shortlist the most probable interactors. Six proteins namely, LysM domain protein, BESBP-5, Antigen S1, CRASP-1 (Bg071), Erp23 protein and Mlp family Lipoprotein were selected to produce their recombinant forms and experimentally validate their interaction with hBMECs. All the recombinant proteins interacted with hBMECs, in ELISA and immunocytochemistry. We present here a high-throughput approach of generating a dataset of plausible borrelial ligands followed by a systematic bioinformatic pipeline to categorize the proteins for experimental validation.

Mycobacteria employ two different mechanisms to cross the blood-brain barrier

Central nervous system (CNS) infection by Mycobacterium tuberculosis is one of the most devastating complications of tuberculosis, in particular in early childhood. In order to induce CNS infection, M. tuberculosis needs to cross specialised barriers protecting the brain. How M. tuberculosis crosses the blood-brain barrier (BBB) and enters the CNS is not well understood. Here, we use transparent zebrafish larvae and the closely related pathogen Mycobacterium marinum to answer this question. We show that in the early stages of development, mycobacteria rapidly infect brain tissue, either as free mycobacteria or within circulating macrophages. After the formation of a functionally intact BBB, the infiltration of brain tissue by infected macrophages is delayed, but not blocked, suggesting that crossing the BBB via phagocytic cells is one of the mechanisms used by mycobacteria to invade the CNS. Interestingly, depletion of phagocytic cells did not prevent M. marinum from infecting the brain tissue, indicating that free mycobacteria can independently cause brain infection. Detailed analysis showed that mycobacteria are able to cause vasculitis by extracellular outgrowth in the smaller blood vessels and by infecting endothelial cells. Importantly, we could show that this second mechanism is an active process that depends on an intact ESX-1 secretion system, which extends the role of ESX-1 secretion beyond the macrophage infection cycle.

Physical insights into the blood-brain barrier translocation mechanisms

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood-brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Deciphering the protein interaction in adhesion of Francisella tularensis subsp. holarctica to the endothelial cells

Extracellular form of Francisella is able to cross various cell barriers and invade multiple organs, such as skin, liver, lung and central nervous system. Transient adhesion of Francisella to endothelial cells may trigger the process of translocation. In this report, we showed that Francisella tularensis subsp. holarctica (Fth) is able to adhere to the endothelial cells, while ICAM-1 may serve as an adhesion molecule for Fth. Pull down and affinity ligand binding assays indicated that the PilE4 could be the probable ligand for ICAM-1. Further deciphering of this ligand:receptor interaction revealed that PilE4 interacts with Ig-like C2-type 1 domain of ICAM-1. To corroborate the role of PilE4 and ICAM-1 interaction in adhesion of extracellular form of Fth to endothelial cells, ICAM-1 was blocked with monoclonal anti-ICAM-1 antibody prior to the incubation with Fth and numbers of adherent bacteria were counted. Blocking of the ICAM-1 significantly reduced (500-fold, P < 0.05) number of adherent Fth compared to unblocked cells. PilE4:ICAM-1 interaction unfolded here may provide a new perspective on molecules involved in the adhesion of extracellular form of Francisella to endothelial cells and probably its translocation across endothelial barriers.

Pathogen-inspired drug delivery to the central nervous system

For as long as the human blood-brain barrier (BBB) has been evolving to exclude bloodborne agents from the central nervous system (CNS), pathogens have adopted a multitude of strategies to bypass it. Some pathogens, notably viruses and certain bacteria, enter the CNS in whole form, achieving direct physical passage through endothelial or neuronal cells to infect the brain. Other pathogens, including bacteria and multicellular eukaryotic organisms, secrete toxins that preferentially interact with specific cell types to exert a broad range of biological effects on peripheral and central neurons. In this review, we will discuss the directed mechanisms that viruses, bacteria, and the toxins secreted by higher order organisms use to enter the CNS. Our goal is to identify ligand-mediated strategies that could be used to improve the brain-specific delivery of engineered nanocarriers, including polymers, lipids, biologically sourced materials, and imaging agents.

Time course Haemophilus parasuis infection reveals pathological differences between virulent and non-virulent strains in the respiratory tract

Haemophilus parasuis is a common inhabitant of the upper respiratory tract of pigs and the etiological agent of Glässer's disease. However, the host-pathogen interaction remains to be well understood. In this work, 33 colostrum-deprived pigs were divided in 4 groups and each group was inoculated intranasally with a different H. parasuis strain (non-virulent strains SW114 and F9, and virulent strains Nagasaki and IT29755). Animals were necropsied at different times in order to determine the location of the bacteria in the respiratory tract of the host during infection. An immunohistochemistry method was developed to detect H. parasuis in nasal turbinates, trachea and lung. Also, the co-localization of H. parasuis with macrophages or neutrophils was examined by double immunohistochemistry and double immunofluorescence. Virulent strains showed a biofilm-like growth in nasal turbinates and trachea and were found easily in lung. Some virulent bacteria were detected in association with macrophages and neutrophils, but also inside pneumocyte-like cells. On the other hand, non-virulent strains were seldom detected in nasal turbinates and trachea, where they showed a microcolony pattern. Non-virulent strains were essentially not detected in lung. In conclusion, this work presents data showing differential localization of H. parasuis bacteria depending on their virulence. Interestingly, the intracellular location of virulent H. parasuis bacteria in non-phagocytic cells in lung could allow the persistence of the bacteria and constitute a virulence mechanism.

Current state-of-the-art molecular dynamics methods and applications

Molecular dynamics simulations are used to describe the patterns, strength, and properties of protein behavior, drug-receptor interactions, the solvation of molecules, the conformational changes that a protein or molecule may undergo under various conditions, and other events that require the systematic evaluation of molecular properties in dynamic molecular systems. Only few years ago proteins were considered to be rigid body structures with very limited conformational flexibility. However, it is now clear that proteins are highly dynamic structures, the internal organization of which is the key to their 3D spatial arrangement and hence biological function. The study of protein dynamics in the lab is a very complicated, expensive, and time-consuming process. Therefore, a lot of effort and hope lies with the computers and the in silico study of protein structure and molecular dynamics. Herein, an effort has been made to describe the ever-evolving field of molecular dynamics, the different algorithms, and force fields that are being used as well as to provide some insight on what the near future holds for this auspicious field of computational structural biology.

Transmigration route of Campylobacter jejuni across polarized intestinal epithelial cells: paracellular, transcellular or both?

Intact intercellular junctions and cellular matrix contacts are crucial structural components for the formation and maintenance of epithelial barrier functions in humans to control the commensal flora and protect against intruding microbes. Campylobacter jejuni is one of the most important zoonotic pathogens causing food-borne gastroenteritis and potentially more severe diseases such as reactive arthritis or Guillain–Barré syndrome. Crossing the intestinal epithelial barrier and host cell invasion by C. jejuni are considered to represent the primary reasons of gut tissue damage in humans and various animal model systems including monkeys, piglets, rabbits, hamsters and ferrets. C. jejuni is also able to invade underlying tissues such as the lamina propria, can enter the bloodstream, and possibly reach distinct organs such as spleen, liver or mesenteric lymph nodes. However, the molecular mechanisms as well as major bacterial and host cell factors involved in these activities are poorly understood. Various models exist by which the pathogen can trigger its own transmigration across polarized intestinal epithelial cells in vitro, the paracellular and/or transcellular mechanism. Recent studies suggest that bacterial factors such as flagellum, serine protease HtrA and lipooligosaccharide LOS may play an active role in bacterial transmigration. Here we review our knowledge on transmigration of C. jejuni as well as some other Campylobacter species, and discuss the pros and cons for the route(s) taken to travel across polarized epithelial cell monolayers. These studies provide fresh insights into the infection strategies employed by this important pathogen.

The crossroads of neuroinflammation in infectious diseases: endothelial cells and astrocytes

Homeostasis implies constant operational defence mechanisms, against both external and internal threats. Infectious agents are prominent among such threats. During infection, the host elicits the release of a vast array of molecules and numerous cell-cell interactions are triggered. These pleiomorphic mediators and cellular effects are of prime importance in the defence of the host, both in the systemic circulation and at sites of tissue injury, for example, the blood-brain barrier (BBB). Here, we focus on the interactions between the endothelium, astrocytes, and the molecules they release. Our review addresses these interactions during infectious neurological diseases of various origins, especially cerebral malaria (CM). Two novel elements of the interplay between endothelium and astrocytes, microparticles and the kynurenine pathway, will also be discussed.

Passage of parasites across the blood-brain barrier

The blood-brain barrier (BBB) is a structural and functional barrier that protects the central nervous system (CNS) from invasion by blood-borne pathogens including parasites. However, some intracellular and extracellular parasites can traverse the BBB during the course of infection and cause neurological disturbances and/or damage which are at times fatal. The means by which parasites cross the BBB and how the immune system controls the parasites within the brain are still unclear. In this review we present the current understanding of the processes utilized by two human neuropathogenic parasites, Trypanosoma brucei spp and Toxoplasma gondii, to go across the BBB and consequences of CNS invasion. We also describe briefly other parasites that can invade the brain and how they interact with or circumvent the BBB. The roles played by parasite-derived and host-derived molecules during parasitic and white blood cell invasion of the brain are discussed.