PrP fragment 106-126 is toxic to cerebral endothelial cells expressing PrP(C)

Identification of age-specific gene regulators of La Crosse virus neuroinvasion and pathogenesis

One of the key events in viral encephalitis is the ability of virus to enter the central nervous system (CNS). Several encephalitic viruses, including La Crosse Virus (LACV), primarily induce encephalitis in children, but not adults. This phenomenon is also observed in LACV mouse models, where the virus gains access to the CNS of weanling animals through vascular leakage of brain microvessels, likely through brain capillary endothelial cells (BCECs). To examine age and region-specific regulatory factors of vascular leakage, we used genome-wide transcriptomics and targeted siRNA screening to identify genes whose suppression affected viral pathogenesis in BCECs. Further analysis of two of these gene products, Connexin43 (Cx43/Gja1) and EphrinA2 (Efna2), showed a substantial effect on LACV pathogenesis. Induction of Cx43 by 4-phenylbutyric acid (4-PBA) inhibited neurological disease in weanling mice, while Efna2 deficiency increased disease in adult mice. Thus, we show that Efna2 and Cx43 expressed by BCECs are key mediators of LACV-induced neuroinvasion and neurological disease.

Cytoskeleton Elements Contribute to Prion Peptide-Induced Endothelial Barrier Breakdown in a Blood–Brain Barrier In Vitro System

The mechanisms involved in the interaction of PrP 106-126, a peptide corresponding to the prion protein amyloidogenic region, with the blood–brain barrier (BBB) were studied. PrP 106-126 treatment that was previously shown to impair BBB function, reduced cAMP levels in cultured brain endothelial cells, increased nitric oxide (NO) levels, and changed the activation mode of the small GTPases Rac1 (inactivation) and RhoA (activation). The latter are well established regulators of endothelial barrier properties that act via cytoskeletal elements. Indeed, liquid chromatography-mass spectrometry (LC-MS)-based proteomic profiling study revealed extensive changes in expression of cytoskeleton-related proteins. These results shed light on the nature of the interaction between the prion peptide PrP 106-126 and the BBB and emphasize the importance of the cytoskeleton in endothelium response to prion- induced stress.

Age influences susceptibility of brain capillary endothelial cells to La Crosse virus infection and cell death

Background

A key factor in the development of viral encephalitis is a virus crossing the blood-brain barrier (BBB). We have previously shown that age-related susceptibility of mice to the La Crosse virus (LACV), the leading cause of pediatric arbovirus encephalitis in the USA, was associated with the ability of the virus to cross the BBB. LACV infection in weanling mice (aged around 3 weeks) results in vascular leakage in the olfactory bulb/tract (OB/OT) region of the brain, which is not observed in adult mice aged > 6–8 weeks. Thus, we studied age-specific differences in the response of brain capillary endothelial cells (BCECs) to LACV infection.

Methods

To examine mechanisms of LACV-induced BBB breakdown and infection of the CNS, we analyzed BCECs directly isolated from weanling and adult mice as well as established a model where these cells were infected in vitro and cultured for a short period to determine susceptibility to virus infection and cell death. Additionally, we utilized correlative light electron microscopy (CLEM) to examine whether changes in cell morphology and function were also observed in BCECs in vivo.

Results

BCECs from weanling, but not adult mice, had detectable infection after several days in culture when taken ex vivo from infected mice suggesting that these cells could be infected in vitro. Further analysis of BCECs from uninfected mice, infected in vitro, showed that weanling BCECs were more susceptible to virus infection than adult BCECs, with higher levels of infected cells, released virus as well as cytopathic effects (CPE) and cell death. Although direct LACV infection is not detected in the weanling BCECs, CLEM analysis of brain tissue from weanling mice indicated that LACV infection induced significant cerebrovascular damage which allowed virus-sized particles to enter the brain parenchyma.

Conclusions

These findings indicate that BCECs isolated from adult and weanling mice have differential viral load, infectivity, and susceptibility to LACV. These age-related differences in susceptibility may strongly influence LACV-induced BBB leakage and neurovascular damage allowing virus invasion of the CNS and the development of neurological disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02173-4.

Surface charge, glycocalyx, and blood-brain barrier function

The negative surface charge of brain microvessel endothelial cells is derived from the special composition of their membrane lipids and the thick endothelial surface glycocalyx. They are important elements of the unique defense systems of the blood-brain barrier. The tissue-specific properties, components, function and charge of the brain endothelial glycocalyx have only been studied in detail in the past 15 years. This review highlights the importance of the negative surface charge in the permeability of macromolecules and nanoparticles as well as in drug interactions. We discuss surface charge and glycoxalyx changes in pathologies related to the brain microvasculature and protective measures against glycocalyx shedding and damage. We present biophysical techniques, including a microfluidic chip device, to measure surface charge of living brain endothelial cells and imaging methods for visualization of surface charge and glycocalyx.

Strain variation in treatment and prevention of human prion diseases

Transmissible spongiform encephalopathies or prion diseases describe a number of different human disorders that differ in their clinical phenotypes, which are nonetheless united by their transmissible nature and common pathology. Clinical variation in the absence of a conventional infectious agent is believed to be encoded by different conformations of the misfolded prion protein. This misfolded protein is the target of methods designed to prevent disease transmission in a surgical setting and reduction of the misfolded seed or preventing its continued propagation have been the focus of therapeutic strategies. It is therefore possible that strain variation may influence the efficacy of prevention and treatment approaches. Historically, an understanding of prion disease transmission and pathogenesis has been focused on research tools developed using agriculturally relevant strains of prion disease. However, an increased understanding of the molecular biology of human prion disorders has highlighted differences not only between different forms of the disease affecting humans and animals but also within diseases such as Creutzfeldt-Jakob Disease (CJD), which is represented by several sporadic CJD specific conformations and an additional conformation associated with variant CJD. In this chapter we will discuss whether prion strain variation can affect the efficacy of methods used to decontaminate prions and whether strain variation in pre-clinical models of prion disease can be used to identify therapeutic strategies that have the best possible chance of success in the clinic.

The Role of Shed PrPc in the Neuropathogenesis of HIV Infection

HIV-1 enters the CNS soon after peripheral infection and causes chronic neuroinflammation and neuronal damage that leads to cognitive impairment in 40-70% of HIV-infected people. The nonpathogenic cellular isoform of the human prion protein (PrP^c) is an adhesion molecule constitutively expressed in the CNS. Previously, our laboratory showed that shed PrP^c (sPrP^c) is increased in the cerebrospinal fluid of HIV-infected people with cognitive deficits as compared with infected people with no impairment. In this article, we demonstrate that CCL2 and TNF-α, inflammatory mediators that are elevated in the CNS of HIV-infected people, increase shedding of PrP^c from human astrocytes by increasing the active form of the metalloprotease ADAM10. We show that the consequence of this shedding can be the production of inflammatory mediators, because treatment of astrocytes with rPrP^c increased secretion of CCL2, CXCL-12, and IL-8. Supernatants from rPrP^c-treated astrocytes containing factors produced in response to this treatment, but not rPrP^c by itself, cause increased chemotaxis of both uninfected and HIV-infected human monocytes, suggesting a role for sPrP^c in monocyte recruitment into the brain. Furthermore, we examined whether PrP^c participates in glutamate uptake and found that rPrP^c decreased uptake of this metabolite in astrocytes, which could lead to neurotoxicity and neuronal loss. Collectively, our data characterize mediators involved in PrP^c shedding and the effect of this sPrP^c on monocyte chemotaxis and glutamate uptake from astrocytes. We propose that shedding of PrP^c could be a potential target for therapeutics to limit the cognitive impairment characteristic of neuroAIDS.

Membrane Disruption Mechanism of a Prion Peptide (106-126) Investigated by Atomic Force Microscopy, Raman and Electron Paramagnetic Resonance Spectroscopy

A fragment of the human prion protein spanning residues 106-126 (PrP106-126) recapitulates many essential properties of the disease-causing protein such as amyloidogenicity and cytotoxicity. PrP106-126 has an amphipathic characteristic that resembles many antimicrobial peptides (AMPs). Therefore, the toxic effect of PrP106-126 could arise from a direct association of monomeric peptides with the membrane matrix. Several experimental approaches are employed to scrutinize the impacts of monomeric PrP106-126 on model lipid membranes. Porous defects in planar bilayers are observed by using solution atomic force microscopy. Adding cholesterol does not impede defect formation. A force spectroscopy experiment shows that PrP106-126 reduces Young's modulus of planar lipid bilayers. We use Raman microspectroscopy to study the effect of PrP106-126 on lipid atomic vibrational dynamics. For phosphatidylcholine lipids, PrP106-126 disorders the intrachain conformation, while the interchain interaction is not altered; for phosphatidylethanolamine lipids, PrP106-126 increases the interchain interaction, while the intrachain conformational order remains similar. We explain the observed differences by considering different modes of peptide insertion. Finally, electron paramagnetic resonance spectroscopy shows that PrP106-126 progressively decreases the orientational order of lipid acyl chains in magnetically aligned bicelles. Together, our experimental data support the proposition that monomeric PrP106-126 can disrupt lipid membranes by using similar mechanisms found in AMPs.

Low dose cranial irradiation-induced cerebrovascular damage is reversible in mice

Background

High-dose radiation-induced blood-brain barrier breakdown contributes to acute radiation toxicity syndrome and delayed brain injury, but there are few data on the effects of low dose cranial irradiation. Our goal was to measure blood-brain barrier changes after low (0.1 Gy), moderate (2 Gy) and high (10 Gy) dose irradiation under in vivo and in vitro conditions.

Methodology

Cranial irradiation was performed on 10-day-old and 10-week-old mice. Blood-brain barrier permeability for Evans blue, body weight and number of peripheral mononuclear and circulating endothelial progenitor cells were evaluated 1, 4 and 26 weeks postirradiation. Barrier properties of primary mouse brain endothelial cells co-cultured with glial cells were determined by measurement of resistance and permeability for marker molecules and staining for interendothelial junctions. Endothelial senescence was determined by senescence associated β-galactosidase staining.

Principle Findings

Extravasation of Evans blue increased in cerebrum and cerebellum in adult mice 1 week and in infant mice 4 weeks postirradiation at all treatment doses. Head irradiation with 10 Gy decreased body weight. The number of circulating endothelial progenitor cells in blood was decreased 1 day after irradiation with 0.1 and 2 Gy. Increase in the permeability of cultured brain endothelial monolayers for fluorescein and albumin was time- and radiation dose dependent and accompanied by changes in junctional immunostaining for claudin-5, ZO-1 and β-catenin. The number of cultured brain endothelial and glial cells decreased from third day of postirradiation and senescence in endothelial cells increased at 2 and 10 Gy.

Conclusion

Not only high but low and moderate doses of cranial irradiation increase permeability of cerebral vessels in mice, but this effect is reversible by 6 months. In-vitro experiments suggest that irradiation changes junctional morphology, decreases cell number and causes senescence in brain endothelial cells.

The mechanism of membrane disruption by cytotoxic amyloid oligomers formed by prion protein(106-126) is dependent on bilayer composition

The formation of fibrillar aggregates has long been associated with neurodegenerative disorders such as Alzheimer and Parkinson diseases. Although fibrils are still considered important to the pathology of these disorders, it is now widely understood that smaller amyloid oligomers are the toxic entities along the misfolding pathway. One characteristic shared by the majority of amyloid oligomers is the ability to disrupt membranes, a commonality proposed to be responsible for their toxicity, although the mechanisms linking this to cell death are poorly understood. Here, we describe the physical basis for the cytotoxicity of oligomers formed by the prion protein (PrP)-derived amyloid peptide PrP(106-126). We show that oligomers of this peptide kill several mammalian cells lines, as well as mouse cerebellar organotypic cultures, and we also show that they exhibit antimicrobial activity. Physical perturbation of model membranes mimicking bacterial or mammalian cells was investigated using atomic force microscopy, polarized total internal reflection fluorescence microscopy, and NMR spectroscopy. Disruption of anionic membranes proceeds through a carpet or detergent model as proposed for other antimicrobial peptides. By contrast, when added to zwitterionic membranes containing cholesterol-rich ordered domains, PrP(106-126) oligomers induce a loss of domain separation and decreased membrane disorder. Loss of raft-like domains may lead to activation of apoptotic pathways, resulting in cell death. This work sheds new light on the physical mechanisms of amyloid cytotoxicity and is the first to clearly show membrane type-specific modes of action for a cytotoxic peptide.

Immunization with either prion protein fragment 95-123 or the fragment-specific antibodies rescue memory loss and neurodegenerative phenotype of neurons in olfactory bulbectomized mice

Epidemiological studies demonstrated association between head injury (HI) and the subsequent development of Alzheimer's disease (AD). Certain hallmarks of AD, e.g. amyloid-β (Aβ) containing deposits, may be found in patients following traumatic BI (TBI). Recent studies uncover the cellular prion protein, PrP(C), as a receptor for soluble polymeric forms of Aβ (sAβ) which are an intermediate of such deposits. We aimed to test the hypothesis that targeting of PrP(C) can prevent Aβ related spatial memory deficits in olfactory bulbectomized (OBX) mice utilized here to resemble some clinical features of AD, such as increased level of Aβ, memory loss and deficit of the CNS cholin- and serotonin-ergic systems. We demonstrated that immunization with the a.a. 95-123 fragment of cellular prion (PrP-I) recovered cortical and hippocampus neurons from OBX induced degeneration, rescued spatial memory loss in Morris water maze test and significantly decrease the Aβ level in brain tissue of these animals. Affinity purified anti-PrP-I antibodies rescued pre-synaptic biomarker synaptophysin eliciting similar effect on memory of OBX mice, and protected hippocampal neurones from Aβ25-35-induced toxicity in vitro. Immunization OBX mice with a.a. 200-213 fragment of cellular prion (PrP-II) did not reach a significance in memory protection albeit having similar to PrP-I immunization impact on Aβ level in brain tissue. The observed positive effect of targeting the PrP-I by either active or passive immunization on memory of OBX mice revealed the involvement of the PrP(C) in AD-like pathology induced by olfactory bulbectomy. This OBX model may be a useful tool for mechanistic and preclinical therapeutic investigations into the association between PrP(C) and AD.

Hyaluronan oligosaccharides perturb lymphocyte slow rolling on brain vascular endothelial cells: implications for inflammatory demyelinating disease

Inflammatory demyelinating diseases like multiple sclerosis are characterized by mononuclear cell infiltration into the central nervous system. The glycosaminoglycan hyaluronan and its receptor, CD44, are implicated in the initiation and progression of a mouse model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). Digestion of hyaluronan tethered to brain vascular endothelial cells by a hyaluronidase blocks the slow rolling of lymphocytes along activated brain vascular endothelial cells and delays the onset of EAE. These effects could be due to the elimination of hyaluronan or the generation of hyaluronan digestion products that influence lymphocytes or endothelial cells. Here, we found that hyaluronan dodecasaccharides impaired activated lymphocyte slow rolling on brain vascular endothelial cells when applied to lymphocytes but not to the endothelial cells. The effects of hyaluronan dodecasaccharides on lymphocyte rolling were independent of CD44 and a receptor for degraded hyaluronan, Toll-like receptor-4. Subcutaneous injection of hyaluronan dodecasaccharides or tetrasaccharides delayed the onset of EAE in a manner similar to subcutaneous injection of hyaluronidase. Hyaluronan oligosaccharides can therefore act directly on lymphocytes to modulate the onset of inflammatory demyelinating disease.

Hyaluronan anchored to activated CD44 on central nervous system vascular endothelial cells promotes lymphocyte extravasation in experimental autoimmune encephalomyelitis

The extravasation of lymphocytes across central nervous system (CNS) vascular endothelium is a key step in inflammatory demyelinating diseases including multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). The glycosaminoglycan hyaluronan (HA) and its receptor, CD44, have been implicated in this process but their precise roles are unclear. We find that CD44(-/-) mice have a delayed onset of EAE compared with wild type animals. Using an in vitro lymphocyte rolling assay, we find that fewer slow rolling (<1 μm/s) wild type (WT) activated lymphocytes interact with CD44(-/-) brain vascular endothelial cells (ECs) than with WT ECs. We also find that CD44(-/-) ECs fail to anchor HA to their surfaces, and that slow rolling lymphocyte interactions with WT ECs are inhibited when the ECs are treated with a pegylated form of the PH20 hyaluronidase (PEG-PH20). Subcutaneous injection of PEG-PH20 delays the onset of EAE symptoms by ~1 day and transiently ameliorates symptoms for 2 days following disease onset. These improved symptoms correspond histologically to degradation of HA in the lumen of CNS blood vessels, decreased demyelination, and impaired CD4(+) T-cell extravasation. Collectively these data suggest that HA tethered to CD44 on CNS ECs is critical for the extravasation of activated T cells into the CNS providing new insight into the mechanisms promoting inflammatory demyelinating disease.

Interactions of the prion peptide (PrP 106-126) with brain capillary endothelial cells: coordinated cell killing and remodeling of intercellular junctions

We studied here the interactions of PrP 106-126, a peptide corresponding to the prion protein (PrP) amyloidogenic region, with a blood-brain barrier in vitro model consisting of confluent porcine brain endothelial cells (PBEC). PrP 106-126 interacted selectively with PBEC via their luminal side, and caused cumulative cell death, as shown by lactate dehydrogenase release, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, Caspase 3 induction and direct cell counting. In addition, PrP 106-126, but not its corresponding scrambled peptide, produced a 50% reduction of the trans-endothelial electrical resistance, while the PBEC maintained confluency. This process was accompanied by a 23% increase of PBEC average size and the selective disappearance from the cell borders of the junction proteins occludin, claudin-5 and VE-cadherin (but not ZO-1), as evaluated by immunostaining. These results fit with a mechanism by which PrP 106-126 initiates a coordinated cell killing process ultimately causing the remaining cells to undergo a coordinated remodeling of the intercellular junctions and an expansion of their cell territory.

PrPC, the cellular isoform of the human prion protein, is a novel biomarker of HIV-associated neurocognitive impairment and mediates neuroinflammation

Of the 33 million people infected with the human immunodeficiency virus (HIV) worldwide, 40-60% of individuals will eventually develop neurocognitive sequelae that can be attributed to the presence of HIV-1 in the central nervous system (CNS) and its associated neuroinflammation despite antiretroviral therapy. PrP(C) (protease resistant protein, cellular isoform) is the nonpathological cellular isoform of the human prion protein that participates in many physiological processes that are disrupted during HIV-1 infection. However, its role in HIV-1 CNS disease is unknown. We demonstrate that PrP(C) is significantly increased in both the CNS of HIV-1-infected individuals with neurocognitive impairment and in SIV-infected macaques with encephalitis. PrP(C) is released into the cerebrospinal fluid, and its levels correlate with CNS compromise, suggesting it is a biomarker of HIV-associated neurocognitive impairment. We show that the chemokine (c-c Motif) Ligand-2 (CCL2) increases PrP(C) release from CNS cells, while HIV-1 infection alters PrP(C) release from peripheral blood mononuclear cells. Soluble PrP(C) mediates neuroinflammation by inducing astrocyte production of both CCL2 and interleukin 6. This report presents the first evidence that PrP(C) dysregulation occurs in cognitively impaired HIV-1-infected individuals and that PrP(C) participates in the pathogenesis of HIV-1-associated CNS disease.

The deletion of amino acids 114-121 in the TM1 domain of mouse prion protein stabilizes its conformation but does not affect the overall structure

A mutant of mouse prion protein (PrPC) carrying a deletion of residues 114-121 (PrPDelta114-121) has previously been described to lack convertibility into the scrapie-associated isoform of PrP (PrPSc) and to exhibit a dominant-negative effect on the conversion of wild-type PrPC into PrPSc in living cells. Here we report the characterization of recombinantly expressed PrPDelta114-121 by Fourier-transformation infrared spectroscopy (FTIR) and circular dichroism (CD) spectroscopy. The analysis of spectra revealed an increased antiparallel beta-sheet content in the deletion mutant compared to wild-type PrPC. This additional short beta-sheet stabilized the fold of the mutant protein by DeltaDeltaG(0)'=3.4+/-0.3 kJ mol(-1) as shown by chemical unfolding experiments using guanidine hydrochloride. Secondary structure predictions suggest that the additional beta-sheet in PrPDelta114-121 is close to the antiparallel beta-sheet in PrPC. The high-affinity Cu2+-binding site outside the octarepeats, which is located close to the deletion and involves His110 as a ligand, was not affected, as detected by electron paramagnetic resonance (EPR) spectroscopy, suggesting that Cu2+ binding does not contribute to the protection of PrPDelta114-121 from conversion into PrPSc. We propose that the deletion of residues 114-121 stabilizes the mutant protein. This stabilization most likely does not obstruct the interaction of PrPDelta114-121 with PrPSc but represents an energy barrier that blocks the conversion of PrPDelta114-121 into PrPSc.

Cellular prion protein is increased in the plasma and peri-infarcted brain tissue after acute stroke

The physiologic properties of the normal cellular prion protein (PrP(C)) have not been established fully, although recent evidence showed its upregulation in cerebral ischaemia. Using patients, animal models, and in vitro studies we aimed to identify in detail the expression and localization of PrP(C) in ischemic stroke. Patients in acute phase of ischaemic stroke had increased plasma levels of circulating PrP(C) as compared to healthy age- and gender-matched controls (3.1 +/- 1.4 vs. 1.9 +/- 0.7 ng/ml, P = 0.002). Immunohistochemistry showed increased expression of PrP(C) in the soma of peri-infarcted neurones as well as in the endothelial cells (EC) of micro-vessels and inflammatory cells in peri-infarcted brain tissue from patients who survived for 2-34 days after an initial stroke. The same pattern was repeated 1-48 hr after MCAO. RT-PCR showed increased gene expression of PrP(C) by human foetal neurons (HFN) after 12 hr of oxygen glucose deprivation (OGD), which remained increased after 24 hr reperfusion. Western blotting confirmed that protein expression was similarly upregulated, and fluorescent labeling showed a notable increase in peri-nuclear and axonal PrP(C) staining intensity. Increased plasma PrP(C) seems to reflect endogenous expression in acute stroke-affected brain tissue. Increased cellular expression in peri-infarcted regions may influence hypoxia-induced cell damage, although the effects on EC survival and angiogenesis remain to be elucidated.

Pentosan polysulfate protects brain endothelial cells against bacterial lipopolysaccharide-induced damages

Peripheral inflammation can aggravate local brain inflammation and neuronal death. The blood-brain barrier (BBB) is a key player in the event. On a relevant in vitro model of primary rat brain endothelial cells co-cultured with primary rat astroglia cells lipopolysaccharide (LPS)-induced changes in several BBB functions have been investigated. LPS-treatment resulted in a dose- and time-dependent decrease in the integrity of endothelial monolayers: transendothelial electrical resistance dropped, while flux of permeability markers fluorescein and albumin significantly increased. Immunostaining for junctional proteins ZO-1, claudin-5 and beta-catenin was significantly weaker in LPS-treated endothelial cells than in control monolayers. LPS also reduced the intensity and changed the pattern of ZO-1 immunostaining in freshly isolated rat brain microvessels. The activity of P-glycoprotein, an important efflux pump at the BBB, was also inhibited by LPS. At the same time production of reactive oxygen species and nitric oxide was increased in brain endothelial cells treated with LPS. Pentosan polysulfate, a polyanionic polysaccharide could reduce the deleterious effects of LPS on BBB permeability, and P-glycoprotein activity. LPS-stimulated increase in the production of reactive oxygen species and nitric oxide was also decreased by pentosan treatment. The protective effect of pentosan for brain endothelium can be of therapeutical significance in bacterial infections affecting the BBB.

Junctional expression of the prion protein PrPC by brain endothelial cells: a role in trans-endothelial migration of human monocytes

The conversion of prion protein (PrPC) to its protease-resistant isoform is involved in the pathogenesis of prion diseases. Although PrPC is highly expressed in neurons and other cell types, its physiological function still remains elusive. Here, we describe how we evaluated its expression, subcellular localization and putative function in brain endothelial cells, which constitute the blood-brain barrier. We detected its expression in microvascular endothelium in mouse brain sections and at intercellular junctions of freshly isolated brain microvessels and cultured brain endothelial cells of mouse, rat and human origin. PrPC co-localized with the adhesion molecule platelet endothelial cell adhesion molecule-1 (PECAM-1); moreover, both PrPC and PECAM-1 were present in raft membrane microdomains. Using mixed cultures of wild-type and PrPC-deficient mouse brain endothelial cells, we observed that PrPC accumulation at cell-cell contacts was probably dependent on homophilic interactions between adjacent cells. Moreover, we report that anti-PrPC antibodies unexpectedly inhibited transmigration of U937 human monocytic cells as well as freshly isolated monocytes through human brain endothelial cells. Significant inhibition was observed with various anti-PrPC antibodies or blocking anti-PECAM-1 antibodies as control. Our results strongly support the conclusion that PrPC is expressed by brain endothelium as a junctional protein that is involved in the trans-endothelial migration of monocytes.

Neuroprotective effect of pentosan polysulphate on ischemia-related neuronal death of the hippocampus

Pentosan polysulphate (PPS) negatively charged sulphated glycosaminoglycan was studied in ischemia-related hippocampal neuronal death and compared with a low molecular weight of heparin, named dalteparin in rats. Transient global ischemia was produced by four vessel-occlusion, the occlusion of the bilateral common carotid arteries following the electrocautherization of the vertebral arteries. 3mg/kg of PPS or 300IU/kg of dalteparin was administered i.v. immediately after 7min-occlusion/reperfusion. Seven days after the operation, the animals were perfused with 4% paraformaldehyde, and paraffinized coronal brain sections measuring 6microm in thickness were stained with hematoxylin and eosin. Neuronal damage was then estimated as a ratio of the number of degenerated neurons to that of both the surviving and degenerated neurons in three distinct area of the CA1 subfield. The ratio of neuronal death increased with the length of the occlusion-time, at 5, 7 and 10min. Both PPS and dalteparin significantly inhibited the neuronal damage induced by 7min-occlusion. These results demonstrated that both PPS and dalteparin could thus protect brain neurons against ischemia/reperfusion-induced damage thus suggesting that they may be potentially useful therapeutic agents for acute ischemic stroke.

Puromycin-based purification of rat brain capillary endothelial cell cultures. Effect on the expression of blood-brain barrier-specific properties

One of the main difficulties with primary rat brain endothelial cell (RBEC) cultures is obtaining pure cultures. The variation in purity limits the achievement of in vitro models of the rat blood-brain barrier. As P-glycoprotein expression is known to be much higher in RBECs than in any contaminating cells, we have tested the effect of five P-glycoprotein substrates (vincristine, vinblastine, colchicine, puromycin and doxorubicin) on RBEC cultures, assuming that RBECs would resist the treatment with these toxic compounds whereas contaminating cells would not. Treatment with either 4 microg/mL puromycin for the first 2 days of culture or 3 microg/mL puromycin for the first 3 days showed the best results without causing toxicity to the cells. Transendothelial electrical resistance was significantly increased in cell monolayers treated with puromycin compared with untreated cell monolayers. When cocultured with astrocytes in the presence of cAMP, the puromycin-treated RBEC monolayer showed a highly reduced permeability to sodium fluorescein (down to 0.75 x 10(-6) cm/s) and a high electrical resistance (up to 500 Omega x cm(2)). In conclusion, this method of RBEC purification will allow the production of in vitro models of the rat blood-brain barrier for cellular and molecular biology studies as well as pharmacological investigations.

In vitro methods in the study of viral and prion permeability across the blood-brain barrier

(1) Infectious agents capable of entering the central nervous system (CNS) produce some of the most dreaded diseases known to man. The infectious agent within the CNS is often protected by the blood-brain barrier (BBB), shielded from endogenous and exogenous anti-infectious agents. (2) The use of in vitro methods offers many advantages to the study of how infectious agents interact with the BBB. Two such agents which negotiate the BBB early in the course of disease before damage to the BBB are the autoimmune deficiency syndrome virus, or human immunodeficiency virus 1, and scrapie prion. Our laboratories have used in vitro methods to study these agents. (3) Here, we review some of the results form our laboratories and those of others.

An Apolipoprotein E-Derived Peptide Mediates Uptake of Sterically Stabilized Liposomes into Brain Capillary Endothelial Cells

A promising strategy to solve the problems of insufficient membrane penetration of drugs and low target specificity is the localization of targeting and uptake-facilitating ligands on the surface of drug-carrier systems. This study investigated the role of a peptide derived from the LDL receptor (LDLr)-binding domain of apolipoprotein E (apoE) in initiating endocytosis in brain capillary endothelial cells. The highly cationic tandem dimer of apoE residues (141-150) was coupled covalently onto poly(ethylene glycol)-derivatized liposomes. Membrane binding and cellular uptake was monitored qualitatively by confocal-laser-scanning microscopy as well as quantitatively using a fluorescence assay. The peptide mediated an efficient, energy-dependent translocation of liposomes across the membrane of brain capillary endothelial cells. Liposomes without surface-located peptides displayed neither membrane accumulation nor cellular uptake. Low peptide affinity to LDLr and internalization of the complex into fibroblasts with up- and down-regulated receptor expression levels, as well as complex translocation into cells incubated with an antibody against the LDLr, pointed to a dominating role of an LDLr-independent transport route. Enzymatic digestion of heparan sulfate proteoglycan (HSPG) with heparinase I and addition of heparin and poly-l-lysin as competitors of HSPG and HSPG ligands, respectively, resulted in a significant loss in liposome internalization. The results suggested that HSPG played a major role in the apoE-peptide-mediated uptake of liposomes into endothelial cells of brain microvessels.

Ageing and exposure to oxidative stress in vivo differentially affect cellular levels of PrP in mouse cerebral microvessels and brain parenchyma

The biological function of cellular prion protein PrPc has not been established, despite in vitro studies suggesting antioxidant activity or link to signal transduction pathways. In this study, mice were exposed to hyperoxia to establish whether oxidative stress affected prion expression in vivo. C57Bl/6J mice aged 6, 18, and 24 months, maintained under normoxic conditions, exhibited age-related increases in PrPc in both cerebral microvessels and in microvessel-depleted brain homogenate. We demonstrate that PrPc is differentially affected by exposure to hyperoxia in vivo for 1 (24 h) or 2 (48 h) days, or for 1 day hyperoxia, followed by 1 day normoxia. Brain parenchymal cells from 6-month-old mice exposed to 1 day hyperoxia showed elevation of a glycosylated approximately 36 kDa form, whereas in 24-month-old mice cellular prion level was substantially reduced. Extending hyperoxia from 1 to 2 days resulted in significantly reduced PrPc level, regardless of age. Parenchymal PrPc is substantially elevated in 6-month-old mice, but declines in 18- and 24-month-old animals following 1 day hyperoxia. By contrast, PrPc content in cerebral microvessels from 6-month-old mice declined after a 2 day exposure to hyperoxia, while microvessels from 24-month-old brains showed elevated prion levels 24 h after hyperoxia. Moreover, unglycosylated 25-30 kDa PrPc, and a previously undescribed 50-64 kDa band containing at least some glycosylated protein, predominated in microvessels with lesser content of the glycosylated approximately 36 kDa form. Cellular content of these unglycosylated forms was correlated with age, while the response to hyperoxia was evident in both unglycosylated and glycosylated forms of the protein following 1 and 2 day exposures. The observed elevation of the 25-30 and 50-64 kDa bands of microvessel PrPc is not sustainable following 1 day hyperoxia, but returns to near normoxic levels within 24 h after hyperoxia. We also show in a knockout mouse for methionine sulfoxide reductase (MsrA), the enzyme responsible for reducing methionine sulfoxide back to methionine, and a regulator of cellular antioxidant defence, that following hyperoxia brain PrPc in the null mutant is elevated relative to PrPc content in the parent strain. Our results show up-regulated PrPc expression or reduced turnover in response to age-related, and hyperoxia-induced oxidative stress.

Do antiangiogenic protein fragments have amyloid properties?

Tumor growth requires proteolytic activity. As a consequence, protein breakdown products are present in the circulation of patients with cancer. Within the past decade a large number of proteolytic fragments have been identified that inhibit angiogenesis and tumor growth. The mechanism of action of these inhibitors is still poorly understood. We recently found that the effects of the angiogenesis inhibitor endostatin on endothelial cells is critically dependent on the presence of cross-beta structure, a structure also present in amyloidogenic polypeptides in plaques of patients with amyloidosis, such as Alzheimer disease. We also showed that cross-beta structure containing endostatin is a ligand for tissue-type plasminogen activator (tPA). We noted that many angiogenesis inhibitors stimulate tPA-mediated plasminogen activation. Because the presence of cross-beta structure is the common denominator in tPA-binding ligands, we hypothesize that these endogenous antiangiogenic proteolytic fragments share features with amyloidogenic polypeptides. We postulate that the cross-beta structural fold is present in these antiangiogenic polypeptide fragments and that this structure mediates the inhibitory effects. The hypothesis provides new insights in the potential mechanisms of these angiogenesis inhibitors and offers opportunities to improve their use.

Post-translational import of the prion protein into the endoplasmic reticulum interferes with cell viability: a critical role for the putative transmembrane domain

Aberrant folding of the mammalian prion protein (PrP) is linked to prion diseases in humans and animals. We show that during post-translational targeting of PrP to the endoplasmic reticulum (ER) the putative transmembrane domain induces misfolding of PrP in the cytosol and interferes with its import into the ER. Unglycosylated and misfolded PrP with an uncleaved N-terminal signal sequence associates with ER membranes, and, moreover, decreases cell viability. PrP expressed in the cytosol, lacking the N-terminal ER targeting sequence, also adopts a misfolded conformation; however, this has no adverse effect on cell growth. PrP processing, productive ER import, and cellular viability can be restored either by deleting the putative transmembrane domain or by using a N-terminal signal sequence specific for co-translational ER import. Our study reveals that the putative transmembrane domain features in the formation of misfolded PrP conformers and indicates that post-translational targeting of PrP to the ER can decrease cell viability.

The expression of prion protein by endothelial cells: a source of the plasma form of prion protein?

The neuronal prion protein (PrPC) is also expressed within peripheral tissues including human blood. The majority of blood PrPC is found within the plasma fraction. We hypothesized that the vascular endothelium could be a source of this PrPC. Reverse transcription polymerase chain reaction demonstrated that both human umbilical vein endothelial cells (HUVEC) and human microvascular endothelial cells (HMEC-1) expressed PrPC mRNA. Flow cytometry confirmed PrPC expression on HMEC-1s and HUVECs (120900 +/- 15058 and 58327 +/- 4577 molecules PrPC/cell respectively), with no upregulation following cellular activation. Confocal immunofluorescence microscopy confirmed that HMEC-1s and HUVECs were positive for PrPC on the plasma membrane. Time-resolved dissociation-enhanced fluoroimmunoassay (DELFIA) analysis of cell culture medium demonstrated a slow constitutive release of soluble PrPC not associated with activation. In contrast to von Willebrand factor antigen, PrPC plasma levels in vivo decrease following desmopressin therapy in patients with von Willebrand disease. Measurement of PrPC plasma levels in patients with varying blood counts demonstrated no association between cell count and PrPC concentration. However, there was a higher level of PrPC in plasma from patients with end-stage renal failure. In conclusion, endothelial cells of both macrovascular and microvascular origin expressed high levels of PrPC which can be constitutively released into the cell culture medium.

Cellular prion protein is expressed on endothelial cells and is released during apoptosis on membrane microparticles found in human plasma

BACKGROUND

Blood and plasma of animals experimentally infected with transmissible spongiform encephalopathies (TSEs) can transmit TSE infection by transfusion. A conformational isoform of prion protein (PrPsc) is believed to be the TSE-infectious agent that propagates by converting the cellular prion protein (PrPc) to additional molecules of PrPsc. In orally infected animals, PrPsc accumulates in intestinal endothelial cells. In blood, two thirds of PrPc resides in plasma, but its source is not known.

STUDY DESIGN AND METHODS

The expression of PrPc in cultured human umbilical vein endothelial cells (HUVECs) was studied using flow cytometry, immunoblotting, and RT-PCR. Flow cytometry was used to characterize endothelial membrane microparticles (MPs) in cell culture supernatants and in normal human plasma.

RESULTS

HUVECs and bovine aorta endothelial cells express PrPc. The number of surface PrPc molecules per cell in HUVECs was 58,000 +/- 2,800. The induction of apoptosis in HUVECs led to a marked release of membrane MPs (60,000-80,000 MPs/10(3) cells) that expressed PrPc and other endothelial antigens. The presence of endothelial cell-derived MPs expressing PrPc was demonstrated in platelet-free human plasma.

CONCLUSION

Endothelial cell apoptosis is associated with the release of PrPc-positive MPs. These MPs contribute to the PrPc pool in plasma and may have a role in disseminating TSE infectivity in blood.