Identification of direct connections between the dura and the brain

The arachnoid barrier delineates the border between the central nervous system and dura mater. Although the arachnoid barrier creates a partition, communication between the central nervous system and the dura mater is crucial for waste clearance and immune surveillance1,2. How the arachnoid barrier balances separation and communication is poorly understood. Here, using transcriptomic data, we developed transgenic mice to examine specific anatomical structures that function as routes across the arachnoid barrier. Bridging veins create discontinuities where they cross the arachnoid barrier, forming structures that we termed arachnoid cuff exit (ACE) points. The openings that ACE points create allow the exchange of fluids and molecules between the subarachnoid space and the dura, enabling the drainage of cerebrospinal fluid and limited entry of molecules from the dura to the subarachnoid space. In healthy human volunteers, magnetic resonance imaging tracers transit along bridging veins in a similar manner to access the subarachnoid space. Notably, in neuroinflammatory conditions such as experimental autoimmune encephalomyelitis, ACE points also enable cellular trafficking, representing a route for immune cells to directly enter the subarachnoid space from the dura mater. Collectively, our results indicate that ACE points are a critical part of the anatomy of neuroimmune communication in both mice and humans that link the central nervous system with the dura and its immunological diversity and waste clearance systems.

Age-related alterations in meningeal immunity drive impaired CNS lymphatic drainage

The meningeal lymphatic network enables the drainage of cerebrospinal fluid (CSF) and facilitates the removal of central nervous system (CNS) waste. During aging and in Alzheimer's disease, impaired meningeal lymphatic drainage promotes the buildup of toxic misfolded proteins in the CNS. Reversing this age-related dysfunction represents a promising strategy to augment CNS waste clearance; however, the mechanisms underlying this decline remain elusive. Here, we demonstrate that age-related alterations in meningeal immunity underlie this lymphatic impairment. Single-cell RNA sequencing of meningeal lymphatic endothelial cells from aged mice revealed their response to IFNγ, which was increased in the aged meninges due to T cell accumulation. Chronic elevation of meningeal IFNγ in young mice via AAV-mediated overexpression attenuated CSF drainage-comparable to the deficits observed in aged mice. Therapeutically, IFNγ neutralization alleviated age-related impairments in meningeal lymphatic function. These data suggest manipulation of meningeal immunity as a viable approach to normalize CSF drainage and alleviate the neurological deficits associated with impaired waste removal.

Parenchymal border macrophages regulate the flow dynamics of the cerebrospinal fluid

Macrophages are important players in the maintenance of tissue homeostasis1. Perivascular and leptomeningeal macrophages reside near the central nervous system (CNS) parenchyma2, and their role in CNS physiology has not been sufficiently well studied. Given their continuous interaction with the cerebrospinal fluid (CSF) and strategic positioning, we refer to these cells collectively as parenchymal border macrophages (PBMs). Here we demonstrate that PBMs regulate CSF flow dynamics. We identify a subpopulation of PBMs that express high levels of CD163 and LYVE1 (scavenger receptor proteins), closely associated with the brain arterial tree, and show that LYVE1+ PBMs regulate arterial motion that drives CSF flow. Pharmacological or genetic depletion of PBMs led to accumulation of extracellular matrix proteins, obstructing CSF access to perivascular spaces and impairing CNS perfusion and clearance. Ageing-associated alterations in PBMs and impairment of CSF dynamics were restored after intracisternal injection of macrophage colony-stimulating factor. Single-nucleus RNA sequencing data obtained from patients with Alzheimer's disease (AD) and from non-AD individuals point to changes in phagocytosis, endocytosis and interferon-γ signalling on PBMs, pathways that are corroborated in a mouse model of AD. Collectively, our results identify PBMs as new cellular regulators of CSF flow dynamics, which could be targeted pharmacologically to alleviate brain clearance deficits associated with ageing and AD.

Meprin β knockout reduces brain Aβ levels and rescues learning and memory impairments in the APP/lon mouse model for Alzheimer's disease

β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the major described β-secretase to generate Aβ peptides in Alzheimer's disease (AD). However, all therapeutic attempts to block BACE1 activity and to improve AD symptoms have so far failed. A potential candidate for alternative Aβ peptides generation is the metalloproteinase meprin β, which cleaves APP predominantly at alanine in p2 and in this study we can detect an increased meprin β expression in AD brain. Here, we report the generation of the transgenic APP/lon mouse model of AD lacking the functional Mep1b gene (APP/lon × Mep1b-/-). We examined levels of canonical and truncated Aβ species using urea-SDS-PAGE, ELISA and immunohistochemistry in brains of APP/lon mouse × Mep1b-/-. Additionally, we investigated the cognitive abilities of these mice during the Morris water maze task. Aβ1-40 and 1-42 levels are reduced in APP/lon mice when meprin β is absent. Immunohistochemical staining of mouse brain sections revealed that N-terminally truncated Aβ2-x peptide deposition is decreased in APP/lon × Mep1b-/- mice. Importantly, loss of meprin β improved cognitive abilities and rescued learning behavior impairments in APP/lon mice. These observations indicate an important role of meprin β within the amyloidogenic pathway and Aβ production in vivo.

Brain barriers virtual: an interim solution or future opportunity?

BACKGROUND

Scientific conferences are vital communication events for scientists in academia, industry, and government agencies. In the brain barriers research field, several international conferences exist that allow researchers to present data, share knowledge, and discuss novel ideas and concepts. These meetings are critical platforms for researchers to connect and exchange breakthrough findings on a regular basis. Due to the worldwide COVID-19 pandemic, all in-person meetings were canceled in 2020. In response, we launched the Brain Barriers Virtual 2020 (BBV2020) seminar series, the first stand-in virtual event for the brain barriers field, to offer scientists a virtual platform to present their work. Here we report the aggregate attendance information on two in-person meetings compared with BBV2020 and comment on the utility of the virtual platform.

METHODS

The BBV2020 seminar series was hosted on a Zoom webinar platform and was free of cost for participants. Using registration- and Zoom-based data from the BBV2020 virtual seminar series and survey data collected from BBV2020 participants, we analyzed attendance trends, global reach, participation based on career stage, and engagement of BBV2020. We compared these data with those from two previous in-person conferences, a BBB meeting held in 2018 and CVB 2019.

RESULTS

We found that BBV2020 seminar participation steadily decreased over the course of the series. In contrast, live participation was consistently above 100 attendees and recording views were above 200 views per seminar. We also found that participants valued BBV2020 as a supplement during the COVID-19 pandemic in 2020. Based on one post-BBV2020 survey, the majority of participants indicated that they would prefer in-person meetings but would welcome a virtual component to future in-person meetings. Compared to in-person meetings, BBV2020 enabled participation from a broad range of career stages and was attended by scientists in academic, industry, and government agencies from a wide range of countries worldwide.

CONCLUSIONS

Our findings suggest that a virtual event such as the BBV2020 seminar series provides easy access to science for researchers across all career stages around the globe. However, we recognize that limitations exist. Regardless, such a virtual event could be a valuable tool for the brain barriers community to reach and engage scientists worldwide to further grow the brain barriers research field in the future.

Brain endothelial LRP1 maintains blood-brain barrier integrity

The entry of blood-borne molecules into the brain is restricted by the blood–brain barrier (BBB). Various physical, transport and immune properties tightly regulate molecule movement between the blood and the brain to maintain brain homeostasis. A recent study utilizing a pan-endothelial, constitutive Tie2-Cre showed that paracellular passage of blood proteins into the brain is governed by endocytic and cell signaling protein low-density lipoprotein receptor–related protein 1 (LRP1). Taking advantage of conditional Slco1c1-CreER^T2 specific to CNS endothelial cells and choroid plexus epithelial cells we now supplement previous results and show that brain endothelial Lrp1 ablation results in protease-mediated tight junction degradation, P-glycoprotein (P-gp) reduction and a loss of BBB integrity.

Meprin β: A novel regulator of blood-brain barrier integrity

The metalloprotease meprin β (Mep1b) is capable of cleaving cell-adhesion molecules in different tissues (e.g. skin, kidney and intestine) and is dysregulated in several diseases associated with barrier breakdown (Alzheimer´s disease, kidney disruption, inflammatory bowel disease). In this study, we demonstrate that Mep1b is a novel regulator of tight junction (TJ) composition and blood-brain barrier (BBB) integrity in brain endothelium. In Mep1b-transfected mouse brain endothelial cells (bEnd.3), we observed a reduction of the TJ protein claudin-5, decreased transendothelial electrical resistance (TEER) and an elevated permeability to paracellular diffusion marker [¹⁴C]-inulin. Analysis of global Mep1b knock-out (Mep1b^-/-) mice showed increased TJ protein expression (claudin-5, occludin, ZO-1) in cerebral microvessels and increased TEER in cultivated primary mouse brain endothelial compared to wild-type (wt) mice. Furthermore, we investigated the IgG levels in cerebrospinal fluid (CSF) and the brain water content as additional permeability markers and detected lower IgG levels and reduced brain water content in Mep1b^-/- mice compared to wt mice. Showing opposing features in overexpression and knock-out, we conclude that Mep1b plays a role in regulating brain endothelial TJ-proteins and therefore affecting BBB tightness in vitro and in vivo.

Enzyme replacement therapy with recombinant pro-CTSD (cathepsin D) corrects defective proteolysis and autophagy in neuronal ceroid lipofuscinosis

CTSD (cathepsin D) is one of the major lysosomal proteases indispensable for the maintenance of cellular proteostasis by turning over substrates of endocytosis, phagocytosis and autophagy. Consequently, CTSD deficiency leads to a strong impairment of the lysosomal-autophagy machinery. In mice and humans CTSD dysfunction underlies the congenital variant (CLN10) of neuronal ceroid lipofuscinosis (NCL). NCLs are distinct lysosomal storage disorders (LSDs) sharing various hallmarks, namely accumulation of protein aggregates and ceroid lipofuscin leading to neurodegeneration and blindness. The most established and clinically approved approach to treat LSDs is enzyme replacement therapy (ERT) aiming to replace the defective hydrolase with an exogenously applied recombinant protein. Here we reveal that recombinant human pro-CTSD produced in a mammalian expression system can be efficiently taken up by a variety of cell models, is correctly targeted to lysosomes and processed to the active mature form of the protease. In proof-of-principle experiments we provide evidence that recombinant human CTSD (rhCTSD) can improve the biochemical phenotype of CTSD-deficient hippocampal slice cultures in vitro and retinal cells in vivo. Furthermore, we demonstrate that dosing of rhCTSD in the murine CLN10 model leads to a correction of lysosomal hypertrophy, storage accumulation and impaired autophagic flux in the viscera and central nervous system (CNS). We establish that direct delivery of the recombinant protease to the CNS is required for improvement of neuropathology and lifespan extension. Together these data support the continuation of the pre-clinical studies for the application of rhCTSD in the treatment of NCL.Abbreviations: AIF1/IBA1: allograft inflammatory factor 1; BBB: blood brain barrier; CNS: central nervous system; CTSB: cathepsin B; CTSD: cathepsin D; CTSL: cathepsin L; ERT: enzyme replacement therapy; GFAP: glial fibrillary acidic protein; INL: inner nuclear layer; LAMP1: lysosomal-associated membrane protein 1; LAMP2: lysosomal-associated membrane protein 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; LDL: low-density lipoprotein; LRP1: low density lipoprotein receptor-related protein 1; LSD: lysosomal storage disorder; MEFs: mouse embryonic fibroblasts; M6P: mannose 6-phosphate; mCTSD: mature CTSD; NCL: neuronal ceroid lipofuscinosis; ONL: outer nuclear layer; PB: phosphate buffer; proCTSD: pro-cathepsin D; LRPAP1: low density lipoprotein receptor-related protein associated protein 1; rhCTSD: human recombinant CTSD; SAPC: saposin C; SAPD: saposin D; ATP5G1: ATP synthase, H+ transporting, mitochondrial F0 complex, subunit C1 (subunit 9); SQSTM1/p62: sequestosome 1; TPP1: tripeptidyl peptidase I.

LRP1 Has a Predominant Role in Production over Clearance of Aβ in a Mouse Model of Alzheimer's Disease

The low-density lipoprotein receptor-related protein-1 (LRP1) has a dual role in the metabolism of the amyloid precursor protein (APP). In cellular models, LRP1 enhances amyloid-β (Aβ) generation via APP internalization and thus its amyloidogenic processing. However, conditional knock-out studies in mice define LRP1 as an important mediator for the clearance of extracellular Aβ from brain via cellular degradation or transcytosis across the blood-brain barrier (BBB). In order to analyze the net effect of LRP1 on production and clearance of Aβ in vivo, we crossed mice with impaired LRP1 function with a mouse model of Alzheimer's disease (AD). Analysis of Aβ metabolism showed that, despite reduced Aβ clearance due to LRP1 inactivation in vivo, less Aβ was found in cerebrospinal fluid (CSF) and brain interstitial fluid (ISF). Further analysis of APP metabolism revealed that impairment of LRP1 in vivo shifted APP processing from the Aβ-generating amyloidogenic cleavage by beta-secretase to the non-amyloidogenic processing by alpha-secretase as shown by a decrease in extracellular Aβ and an increase of soluble APP-α (sAPP-α). This shift in APP processing resulted in overall lower Aβ levels and a reduction in plaque burden. Here, we present for the first time clear in vivo evidence that global impairment of LRP1's endocytosis function favors non-amyloidogenic processing of APP due to its reduced internalization and subsequently, reduced amyloidogenic processing. By inactivation of LRP1, the inhibitory effect on Aβ generation overrules the simultaneous impaired Aβ clearance, resulting in less extracellular Aβ and reduced plaque deposition in a mouse model of AD.

The metalloprotease ADAMTS4 generates N-truncated Aβ4-x species and marks oligodendrocytes as a source of amyloidogenic peptides in Alzheimer's disease

Brain accumulation and aggregation of amyloid-β (Aβ) peptides is a critical step in the pathogenesis of Alzheimer's disease (AD). Full-length Aβ peptides (mainly Aβ1-40 and Aβ1-42) are produced through sequential proteolytic cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. However, studies of autopsy brain samples from AD patients have demonstrated that a large fraction of insoluble Aβ peptides are truncated at the N-terminus, with Aβ4-x peptides being particularly abundant. Aβ4-x peptides are highly aggregation prone, but their origin and any proteases involved in their generation are unknown. We have identified a recognition site for the secreted metalloprotease ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin motifs 4) in the Aβ peptide sequence, which facilitates Aβ4-x peptide generation. Inducible overexpression of ADAMTS4 in HEK293 cells resulted in the secretion of Aβ4-40 but unchanged levels of Aβ1-x peptides. In the 5xFAD mouse model of amyloidosis, Aβ4-x peptides were present not only in amyloid plaque cores and vessel walls, but also in white matter structures co-localized with axonal APP. In the ADAMTS4^-/- knockout background, Aβ4-40 levels were reduced confirming a pivotal role of ADAMTS4 in vivo. Surprisingly, in the adult murine brain, ADAMTS4 was exclusively expressed in oligodendrocytes. Cultured oligodendrocytes secreted a variety of Aβ species, but Aβ4-40 peptides were absent in cultures derived from ADAMTS4^-/- mice indicating that the enzyme was essential for Aβ4-x production in this cell type. These findings establish an enzymatic mechanism for the generation of Aβ4-x peptides. They further identify oligodendrocytes as a source of these highly amyloidogenic Aβ peptides.

The concerted amyloid-beta clearance of LRP1 and ABCB1/P-gp across the blood-brain barrier is linked by PICALM

The accumulation of neurotoxic amyloid-beta (Aβ) in the brain is a characteristic hallmark of Alzheimer's disease (AD). The blood-brain barrier (BBB) provides a large surface area and has been shown to be an important mediator for removal of brain Aβ. Both, the ABC transporter P-glycoprotein (ABCB1/P-gp) and the receptor low-density lipoprotein receptor-related protein 1 (LRP1) have been implicated to play crucial roles in Aβ efflux from brain. Here, with immunoprecipitation experiments, co-immunostainings and dual inhibition of ABCB1/P-gp and LRP1, we show that both proteins are functionally linked, mediating a concerted transcytosis of Aβ through endothelial cells. Late-onset AD risk factor Phosphatidylinositol binding clathrin assembly protein (PICALM) is associated with both ABCB1/P-gp and LRP1 representing a functional link and guiding both proteins through the brain endothelium. Together, our results give more mechanistic insight on Aβ transport across the BBB and show that the functional interplay of different clearance proteins is needed for the rapid removal of Aβ from the brain.

The LepR-mediated leptin transport across brain barriers controls food reward

OBJECTIVE

Leptin is a key hormone in the control of appetite and body weight. Predominantly produced by white adipose tissue, it acts on the brain to inhibit homeostatic feeding and food reward. Leptin has free access to circumventricular organs, such as the median eminence, but entry into other brain centers is restricted by the blood-brain and blood-CSF barriers. So far, it is unknown for which of its central effects leptin has to penetrate brain barriers. In addition, the mechanisms mediating the transport across barriers are unclear although high expression in brain barriers suggests an important role of the leptin receptor (LepR).

METHODS

We selectively deleted LepR in brain endothelial and epithelial cells of mice (LepRbeKO). The expression of LepR in fenestrated vessels of the periphery and the median eminence as well as in tanycytes was not affected.

RESULTS

Perfusion studies showed that leptin uptake by the brain depended on LepR in brain barriers. When being fed with a rewarding high-fat diet LepRbeKO mice gained more body weight than controls. The aggravated obesity of LepRbeKO mice was due to hyperphagia and a higher sensitivity to food reward.

CONCLUSIONS

The LepR-mediated transport of leptin across brain barriers in endothelial cells lining microvessels and in epithelial cells of the choroid plexus controls food reward but is apparently not involved in homeostatic control of feeding.

Endothelial LRP1 - A Potential Target for the Treatment of Alzheimer's Disease : Theme: Drug Discovery, Development and Delivery in Alzheimer's Disease Guest Editor: Davide Brambilla

The accumulation of the neurotoxin beta-amyloid (Aβ) is a major hallmark in Alzheimer's disease (AD). Aβ homeostasis in the brain is governed by its production and various clearance mechanisms. Both pathways are influenced by the ubiquitously expressed low-density lipoprotein receptor-related protein 1 (LRP1). In cerebral blood vessels, LRP1 is an important mediator for the rapid removal of Aβ from brain via transport across the blood-brain barrier (BBB). Here, we summarize recent findings on LRP1 function and discuss the targeting of LRP1 as a modulator for AD pathology and drug delivery into the brain.

LRP1 Modulates APP Intraneuronal Transport and Processing in Its Monomeric and Dimeric State

The low-density lipoprotein receptor-related protein 1, LRP1, interacts with APP and affects its processing. This is assumed to be mostly caused by the impact of LRP1 on APP endocytosis. More recently, also an interaction of APP and LRP1 early in the secretory pathway was reported whereat retention of LRP1 in the ER leads to decreased APP cell surface levels and in turn, to reduced Aβ secretion. Here, we extended the biochemical and immunocytochemical analyses by showing via live cell imaging analyses in primary neurons that LRP1 and APP are transported only partly in common (one third) but to a higher degree in distinct fast axonal transport vesicles. Interestingly, co-expression of LRP1 and APP caused a change of APP transport velocities, indicating that LRP1 recruits APP to a specific type of fast axonal transport vesicles. In contrast lowered levels of LRP1 facilitated APP transport. We further show that monomeric and dimeric APP exhibit similar transport characteristics and that both are affected by LRP1 in a similar way, by slowing down APP anterograde transport and increasing its endocytosis rate. In line with this, a knockout of LRP1 in CHO cells and in primary neurons caused an increase of monomeric and dimeric APP surface localization and in turn accelerated shedding by meprin β and ADAM10. Notably, a choroid plexus specific LRP1 knockout caused a much higher secretion of sAPP dimers into the cerebrospinal fluid compared to sAPP monomers. Together, our data show that LRP1 functions as a sorting receptor for APP, regulating its cell surface localization and thereby its processing by ADAM10 and meprin β, with the latter exhibiting a preference for APP in its dimeric state.

Endothelial LRP1 transports amyloid-β(1-42) across the blood-brain barrier

According to the neurovascular hypothesis, impairment of low-density lipoprotein receptor-related protein-1 (LRP1) in brain capillaries of the blood-brain barrier (BBB) contributes to neurotoxic amyloid-β (Aβ) brain accumulation and drives Alzheimer's disease (AD) pathology. However, due to conflicting reports on the involvement of LRP1 in Aβ transport and the expression of LRP1 in brain endothelium, the role of LRP1 at the BBB is uncertain. As global Lrp1 deletion in mice is lethal, appropriate models to study the function of LRP1 are lacking. Moreover, the relevance of systemic Aβ clearance to AD pathology remains unclear, as no BBB-specific knockout models have been available. Here, we developed transgenic mouse strains that allow for tamoxifen-inducible deletion of Lrp1 specifically within brain endothelial cells (Slco1c1-CreER(T2) Lrp1(fl/fl) mice) and used these mice to accurately evaluate LRP1-mediated Aβ BBB clearance in vivo. Selective deletion of Lrp1 in the brain endothelium of C57BL/6 mice strongly reduced brain efflux of injected [125I] Aβ(1-42). Additionally, in the 5xFAD mouse model of AD, brain endothelial-specific Lrp1 deletion reduced plasma Aβ levels and elevated soluble brain Aβ, leading to aggravated spatial learning and memory deficits, thus emphasizing the importance of systemic Aβ elimination via the BBB. Together, our results suggest that receptor-mediated Aβ BBB clearance may be a potential target for treatment and prevention of Aβ brain accumulation in AD.

Low density lipoprotein receptor-related protein 1 mediated endocytosis of β1-integrin influences cell adhesion and cell migration

The low density lipoprotein receptor-related protein 1 (LRP1) has been shown to interact with β1-integrin and regulate its surface expression. LRP1 knock-out cells exhibit altered cytoskeleton organization and decreased cell migration. Here we demonstrate coupled endocytosis of LRP1 and β1-integrin and the involvement of the intracellular NPxY2 motif of LRP1 in this process. Mouse embryonic fibroblasts harboring a knock in replacement of the NPxY2 motif of LRP1 by a multiple alanine cassette (AAxA) showed elevated surface expression of β1-integrin and decreased β1-integrin internalization rates. As a consequence, cell spreading was altered and adhesion rates were increased in our cell model. Cells formed more focal adhesion complexes, whereby in vitro cell migration rates were decreased. Similar results could be observed in a corresponding mouse model, the C57Bl6 LRP1 NPxYxxL knock in mice, therefore, the biochemistry of cellular adhesion was altered in primary cortical neurons. In vivo cell migration experiments demonstrated a disturbance of neuroblast cell migration along the rostral migratory stream. In summary, our results indicate that LRP1 interacts with β1-integrin mediating integrin internalization and thus correlates with downstream signaling of β1-integrin such as focal adhesion dynamics. Consequently, the disturbance of this interaction resulted in a dysfunction in in vivo and in vitro cell adhesion and cell migration.

Expression of the ALS-causing variant hSOD1(G93A) leads to an impaired integrity and altered regulation of claudin-5 expression in an in vitro blood-spinal cord barrier model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive paralysis due to the loss of primary and secondary motor neurons. Mutations in the Cu/Zn-superoxide dismutase (SOD1) gene are associated with familial ALS and to date numerous hypotheses for ALS pathology exist including impairment of the blood-spinal cord barrier. In transgenic mice carrying mutated SOD1 genes, a disrupted blood-spinal cord barrier as well as decreased levels of tight junction (TJ) proteins ZO-1, occludin, and claudin-5 were detected. Here, we examined TJ protein levels and barrier function of primary blood-spinal cord barrier endothelial cells of presymptomatic hSOD1(G93A) mice and bEnd.3 cells stably expressing hSOD1(G93A). In both cellular systems, we observed reduced claudin-5 levels and a decreased transendothelial resistance (TER) as well as an increased apparent permeability. Analysis of the β-catenin/AKT/forkhead box protein O1 (FoxO1) pathway and the FoxO1-regulated activity of the claudin-5 promoter revealed a repression of the claudin-5 gene expression in hSOD1(G93A) cells, which was depended on the phosphorylation status of FoxO1. These results strongly indicate that mutated SOD1 affects the expression and localization of TJ proteins leading to impaired integrity and breakdown of the blood-spinal cord barrier.