Mouse models of neurological disorders: a view from the blood-brain barrier

Acrylonitrile induction of rodent neoplasia: Potential mechanism of action and relevance to humans

Acrylonitrile, an industrial chemical, is a multisite carcinogen in rats and mice, producing tumors in four tissues with barrier function, that is, brain, forestomach, Zymbal’s gland, and Harderian gland. To assess mechanism(s) of action (MoA) for induction of neoplasia and to evaluate whether the findings in rodents are indicative of human hazard, data on the potential key effects produced by acrylonitrile in the four rodent target tissues of carcinogenicity were evaluated. A notable finding was depletion of glutathione in various organs, including two target tissues, the brain, and forestomach, suggesting that this effect could be a critical initiating event. An additional combination of oxidative DNA damage and cytotoxic effects of acrylonitrile and its metabolites, cyanide, and 2-cyanoethylene oxide, could initiate pro-inflammatory signaling and sustained cell and tissue injury, leading to compensatory cell proliferation and neoplastic development. The in vivo DNA-binding and genotoxicity of acrylonitrile has been studied in several target tissues with no compelling positive results. Thus, while some mutagenic effects were reported in acrylonitrile-exposed rodents, data to determine whether this mutagenicity stems from direct DNA reactivity of acrylonitrile are insufficient. Accordingly, the induction of tumors in rodents is consistent primarily with a non-genotoxic MoA, although a contribution from weak mutagenicity cannot be ruled out. Mechanistic data to support conclusions regarding human hazard from acrylonitrile exposure is weak. Comparison of metabolism of acrylonitrile between rodents and humans provide little support for human hazard. Three of the tissues affected in bioassays (forestomach, Zymbal’s gland, and Harderian gland) are present only in rodents, while the brain is anatomically different between rodents and humans, diminishing relevance of tumor induction in these tissues to human hazard. Extensive epidemiological data has not revealed causation of human cancer by acrylonitrile.

Leveraging the Dynamic Blood-Brain Barrier for Central Nervous System Nanoparticle-based Drug Delivery Applications

Neurological diseases and injuries have profound impact on a patient's lifespan and functional capabilities, but often lack effective intervention strategies to address the underlying neuropathology. The blood-brain barrier (BBB) is a major hurdle in the effective delivery of therapeutics to the brain. Recent discoveries in BBB maintenance reveal a dynamic system where time of day, disease progression, and even biological variables all strongly influence its permeability and flux of molecules. Nanoparticles can be used to improve the efficacy of therapeutics by increasing circulation time, bioavailability, selectivity, and controlling the rate of payload release. Considering these recent findings, the next generation of pharmacological paradigms are evolving to leverage nanotechnology to turn therapeutic intervention to meet the needs of a specific patient (i.e. personalized medicine).

A microfluidic model of human brain (μHuB) for assessment of blood brain barrier

Microfluidic cellular models, commonly referred to as "organs-on-chips," continue to advance the field of bioengineering via the development of accurate and higher throughput models, captivating the essence of living human organs. This class of models can mimic key in vivo features, including shear stresses and cellular architectures, in ways that cannot be realized by traditional two-dimensional in vitro models. Despite such progress, current organ-on-a-chip models are often overly complex, require highly specialized setups and equipment, and lack the ability to easily ascertain temporal and spatial differences in the transport kinetics of compounds translocating across cellular barriers. To address this challenge, we report the development of a three-dimensional human blood brain barrier (BBB) microfluidic model (μHuB) using human cerebral microvascular endothelial cells (hCMEC/D3) and primary human astrocytes within a commercially available microfluidic platform. Within μHuB, hCMEC/D3 monolayers withstood physiologically relevant shear stresses (2.73 dyn/cm2) over a period of 24 hr and formed a complete inner lumen, resembling in vivo blood capillaries. Monolayers within μHuB expressed phenotypical tight junction markers (Claudin-5 and ZO-1), which increased expression after the presence of hemodynamic-like shear stress. Negligible cell injury was observed when the monolayers were cultured statically, conditioned to shear stress, and subjected to nonfluorescent dextran (70 kDa) transport studies. μHuB experienced size-selective permeability of 10 and 70 kDa dextrans similar to other BBB models. However, with the ability to probe temporal and spatial evolution of solute distribution, μHuBs possess the ability to capture the true variability in permeability across a cellular monolayer over time and allow for evaluation of the full breadth of permeabilities that would otherwise be lost using traditional end-point sampling techniques. Overall, the μHuB platform provides a simplified, easy-to-use model to further investigate the complexities of the human BBB in real-time and can be readily adapted to incorporate additional cell types of the neurovascular unit and beyond.

Astrocyte-derived interleukin-15 exacerbates ischemic brain injury via propagation of cellular immunity

Astrocytes are believed to bridge interactions between infiltrating lymphocytes and neurons during brain ischemia, but the mechanisms for this action are poorly understood. Here we found that interleukin-15 (IL-15) is dramatically up-regulated in astrocytes of postmortem brain tissues from patients with ischemic stroke and in a mouse model of transient focal brain ischemia. We generated a glial fibrillary acidic protein (GFAP) promoter-controlled IL-15-expressing transgenic mouse (GFAP-IL-15^tg) line and found enlarged brain infarcts, exacerbated neurodeficits after the induction of brain ischemia. In addition, knockdown of IL-15 in astrocytes attenuated ischemic brain injury. Interestingly, the accumulation of CD8⁺ T and natural killer (NK) cells was augmented in these GFAP-IL-15^tg mice after brain ischemia. Of note, depletion of CD8⁺ T or NK cells attenuated ischemic brain injury in GFAP-IL-15^tg mice. Furthermore, knockdown of the IL-15 receptor α or blockade of cell-to-cell contact diminished the activation and effector function of CD8⁺ T and NK cells in GFAP-IL-15^tg mice, suggesting that astrocytic IL-15 is delivered in trans to target cells. Collectively, these findings indicate that astrocytic IL-15 could aggravate postischemic brain damage via propagation of CD8⁺ T and NK cell-mediated immunity.

TNAP and EHD1 are over-expressed in bovine brain capillary endothelial cells after the re-induction of blood-brain barrier properties

Although the physiological properties of the blood-brain barrier (BBB) are relatively well known, the phenotype of the component brain capillary endothelial cells (BCECs) has yet to be described in detail. Likewise, the molecular mechanisms that govern the establishment and maintenance of the BBB are largely unknown. Proteomics can be used to assess quantitative changes in protein levels and identify proteins involved in the molecular pathways responsible for cellular differentiation. Using the well-established in vitro BBB model developed in our laboratory, we performed a differential nano-LC MALDI-TOF/TOF-MS study of Triton X-100-soluble protein species from bovine BCECs displaying either limited BBB functions or BBB functions re-induced by glial cells. Due to the heterogeneity of the crude extract, we increased identification yields by applying a repeatable, reproducible fractionation process based on the proteins' relative hydrophobicity. We present proteomic and biochemical evidence to show that tissue non-specific alkaline phosphatase (TNAP) and Eps15 homology domain-containing protein 1(EDH1) are over-expressed by bovine BCECs after the re-induction of BBB properties. We discuss the impact of these findings on current knowledge of endothelial and BBB permeability.

Loss of vascular early response gene reduces edema formation after experimental stroke

Vascular Early Response Gene (Verge) is an immediate early gene (IEG) that is up-regulated in endothelial cells in response to a number of stressors, including ischemic stroke. Endothelial cell lines that stably express Verge show enhanced permeability. Increased Verge expression has also been associated with blood brain barrier breakdown. In this study we investigated the role of Verge in ischemic injury induced by middle cerebral artery occlusion (MCAO) in both Verge knockout (KO) and wild type (WT) mice. Verge KO mice had significantly less cerebral edema formation after MCAO compared to WT mice. However, stroke outcome (infarct size and neurological deficit scores) evaluated at either 24 or 72 hours after stroke showed no differences between the two genotypes. Verge deletion leads to decreased edema formation after ischemia; however acute stroke outcomes were unchanged.

In vivo effects of leptin on lymphocyte subpopulations in mice

Leptin, a hormone-cytokine mainly produced by the adipose tissue, has pleitropic effects on many biological system including metabolic, endocrine, and immune system. Although it is well known that leptin controls food intake on hypothalamic regions of brain, the role of leptin in hematopoietic and immune processes has been mainly investigated with in vitro and transgenic mouse studies. The aim of this study was to investigate the effects of peripheral leptin on lymphocyte subpopulation. Initially forty male Swiss albino mice were divided into five groups. Mice in group I (Control) were given serum physiologic (SP) and group L100, group L250, group L500, and group L1000 were given 100, 250, 500 and 1000 μg/kg/day recombinant mouse leptin, respectively. Leptin or SP was injected subcutaneously for the next 6 days. Daily food/water intake was recorded for each group. At the end of the study, whole blood samples (500 μl) were obtained via intracardiac punction in anesthetized mice. Leptin levels and lymphocyte subpopulations in blood samples were analyzed. We show that no in vivo dose-dependent effect of leptin is existed on lymphocyte subpopulations count in mice. Treatment of mice with high-dose leptin led to increase only CD4+ cells (P<0.05). In addition, high-dose leptin slightly increased CD3+ cells but this was not statistically confirmed (P=0.08). Notably, it was found that leptin caused insignificant changes on body weight and food intake in normal body weight mice. The data support that high-dose leptin has proliferative effect on CD4+ cells in vivo. However, more in vivo study needs to be examined to clarify how leptin affect lymphocyte subpopulations.

Aspects of CNS lupus: mouse models of anti-NMDA receptor antibody mediated reactivity

This chapter describes methods utilized in establishing a mouse model of neuropsychiatric lupus encompassing both cognitive and emotional dysfunction, and a model of the influence of maternal antibody on the developing brain. The antibody of interest binds the N-methyl-D: -aspartate receptor (NMDAR), a receptor for glutamate that is a major excitatory neurotransmitter in the brain involved in synaptic plasticity, in memory and learning, and in emotional responses.We introduce basic concepts of these models and provide protocols for the following: (1) the induction of anti-dsDNA, anti-NMDAR antibodies, (2) testing serum antibody titer by ELISA, (3) breaching blood brain barrier (BBB) integrity with LPS and epinephrine, (4) passive transfer of pathology by injecting human and mouse brain-reactive antibodies into adult mouse as well as injecting the antibody into gestating mice and transfer of antibody from dam to fetus, (5) blocking NMDAR-mediated pathogenicity in vivo, (6) evacuation of blood from the brain by cardiac perfusion to preserve the brain for histology, (7) evaluating injured/apoptotic neurons in brain histology, (8) preparing membrane-enriched brain -fractions for NMDAR analysis.

Dendritic degeneration, neurovascular defects, and inflammation precede neuronal loss in a mouse model for tau-mediated neurodegeneration

Adeno-associated virus (AAV)-mediated expression of wild-type or mutant P301L protein tau produces massive degeneration of pyramidal neurons without protein tau aggregation. We probed this novel model for genetic and structural factors and early parameters of pyramidal neurodegeneration. In yellow fluorescent protein-expressing transgenic mice, intracerebral injection of AAV-tauP301L revealed early damage to apical dendrites of CA1 pyramidal neurons, whereas their somata remained normal. Ultrastructurally, more and enlarged autophagic vacuoles were contained in degenerating dendrites and manifested as dark, discontinuous, vacuolated processes surrounded by activated astrocytes. Dendritic spines were lost in AAV-tauP301L-injected yellow fluorescent protein-expressing transgenic mice, and ultrastructurally, spines appeared dark and degenerating. In CX3CR1(EGFP/EGFP)-deficient mice, microglia were recruited early to neurons expressing human tau. The inflammatory response was accompanied by extravasation of plasma immunoglobulins. α2-Macroglobulin, but neither albumin nor transferrin, became lodged in the brain parenchyma. Large proteins, but not Evans blue, entered the brain of mice injected with AAV-tauP301L. Ultrastructurally, brain capillaries were constricted and surrounded by swollen astrocytes with extensions that contacted degenerating dendrites and axons. Together, these data corroborate the hypothesis that neuroinflammation participates essentially in tau-mediated neurodegeneration, and the model recapitulates early dendritic defects reminiscent of "dendritic amputation" in Alzheimer's disease.

Glia-induced reversible disruption of blood-brain barrier integrity and neuropathological response of the neurovascular unit

The blood-brain barrier (BBB) is the regulated interface that mediates selective transcellular transport of nutrients and essential components from the blood into the brain parenchyma. Many neurodegenerative diseases including stroke, multiple sclerosis, rheumatoid arthritis, and AIDS dementia exhibit loss of BBB integrity. Despite the increasing body of evidence for the involvement of glia in maintaining the BBB, few studies have addressed glial/endothelial/extracellular matrix interactions. A chemically induced astrocyte lesion provides a noninvasive model to study reversible BBB dysfunction in vivo. Blood-brain barrier integrity was assessed with fluorescent dextran tracers (3-70 kDa) and magnetic resonance imaging, in parallel with confocal and electron microscopy imaging of the neurovascular unit. These studies demonstrated modified tight-junction protein expression with loss of vascular integrity. We propose that adherens junction proteins and extracellular matrix remodeling provide a temporary size-selective barrier, whereas astrocyte and microglia activation direct tight-junction proteins to paracellular domains and restore BBB integrity. Morphological comparisons were made with the area postrema, a circumventricular organ with a naturally porous BBB. Further studies into cellular mechanisms of glial/endothelial/extracellular matrix interactions may identify novel glial-based therapeutic targets and innovate therapies for modulating diseases in which gliosis and raised levels of pro-inflammatory mediators are central components.

Protein kinase C activation modulates reversible increase in cortical blood-brain barrier permeability and tight junction protein expression during hypoxia and posthypoxic reoxygenation

Hypoxia (Hx) is a component of many disease states including stroke. Ischemic stroke occurs when there is a restriction of cerebral blood flow and oxygen to part of the brain. During the ischemic, and subsequent reperfusion phase of stroke, blood-brain barrier (BBB) integrity is lost with tight junction (TJ) protein disruption. However, the mechanisms of Hx and reoxygenation (HR)-induced loss of BBB integrity are not fully understood. We examined the role of protein kinase C (PKC) isozymes in modifying TJ protein expression in a rat model of global Hx. The Hx (6% O(2)) induced increased hippocampal and cortical vascular permeability to 4 and 10 kDa dextran fluorescein isothiocyanate (FITC) and endogenous rat-IgG. Cortical microvessels revealed morphologic changes in nPKC-θ distribution, increased nPKC-θ and aPKC-ζ protein expression, and activation by phosphorylation of nPKC-θ (Thr538) and aPKC-ζ (Thr410) residues after Hx treatment. Claudin-5, occludin, and ZO-1 showed disrupted organization at endothelial cell margins, whereas Western blot analysis showed increased TJ protein expression after Hx. The PKC inhibition with chelerythrine chloride (5 mg/kg intraperitoneally) attenuated Hx-induced hippocampal vascular permeability and claudin-5, PKC (θ and ζ) expression, and phosphorylation. This study supports the hypothesis that nPKC-θ and aPKC-ζ signaling mediates TJ protein disruption resulting in increased BBB permeability.

Evaluation of soluble junctional adhesion molecule-A as a biomarker of human brain endothelial barrier breakdown

BACKGROUND

An inducible release of soluble junctional adhesion molecule-A (sJAM-A) under pro-inflammatory conditions was described in cultured non-CNS endothelial cells (EC) and increased sJAM-A serum levels were found to indicate inflammation in non-CNS vascular beds. Here we studied the regulation of JAM-A expression in cultured brain EC and evaluated sJAM-A as a serum biomarker of blood-brain barrier (BBB) function.

METHODOLOGY/PRINCIPAL FINDINGS

As previously reported in non-CNS EC types, pro-inflammatory stimulation of primary or immortalized (hCMEC/D3) human brain microvascular EC (HBMEC) induced a redistribution of cell-bound JAM-A on the cell surface away from tight junctions, along with a dissociation from the cytoskeleton. This was paralleled by reduced immunocytochemical staining of occludin and zonula occludens-1 as well as by increased paracellular permeability for dextran 3000. Both a self-developed ELISA test and Western blot analysis detected a constitutive sJAM-A release by HBMEC into culture supernatants, which importantly was unaffected by pro-inflammatory or hypoxia/reoxygenation challenge. Accordingly, serum levels of sJAM-A were unaltered in 14 patients with clinically active multiple sclerosis compared to 45 stable patients and remained unchanged in 13 patients with acute ischemic non-small vessel stroke over time.

CONCLUSION

Soluble JAM-A was not suited as a biomarker of BBB breakdown in our hands. The unexpected non-inducibility of sJAM-A release at the human BBB might contribute to a particular resistance of brain EC to inflammatory stimuli, protecting the CNS compartment.