A brain microvasculature endothelial cell-specific viral vector with the potential to treat neurovascular and neurological diseases

In vivo selection in non-human primates identifies AAV capsids for on-target CSF delivery to spinal cord

Systemic administration of adeno-associated virus (AAV) vectors for spinal cord gene therapy has challenges including toxicity at high doses and pre-existing immunity that reduces efficacy. Intrathecal (IT) delivery of AAV vectors into cerebral spinal fluid can avoid many issues, although distribution of the vector throughout the spinal cord is limited, and vector entry to the periphery sometimes initiates hepatotoxicity. Here we performed biopanning in non-human primates (NHPs) with an IT injected AAV9 peptide display library. We identified top candidates by sequencing inserts of AAV DNA isolated from whole tissue, nuclei, or nuclei from transgene-expressing cells. These barcoded candidates were pooled with AAV9 and compared for biodistribution and transgene expression in spinal cord and liver of IT injected NHPs. Most candidates displayed increased retention in spinal cord compared with AAV9. Greater spread from the lumbar to the thoracic and cervical regions was observed for several capsids. Furthermore, several capsids displayed decreased biodistribution to the liver compared with AAV9, providing a high on-target/low off-target biodistribution. Finally, we tested top candidates in human spinal cord organoids and found them to outperform AAV9 in efficiency of transgene expression in neurons and astrocytes. These capsids have potential to serve as leading-edge delivery vehicles for spinal cord-directed gene therapies.

Blood-brain barrier disruption: a culprit of cognitive decline?

Cognitive decline covers a broad spectrum of disorders, not only resulting from brain diseases but also from systemic diseases, which seriously influence the quality of life and life expectancy of patients. As a highly selective anatomical and functional interface between the brain and systemic circulation, the blood-brain barrier (BBB) plays a pivotal role in maintaining brain homeostasis and normal function. The pathogenesis underlying cognitive decline may vary, nevertheless, accumulating evidences support the role of BBB disruption as the most prevalent contributing factor. This may mainly be attributed to inflammation, metabolic dysfunction, cell senescence, oxidative/nitrosative stress and excitotoxicity. However, direct evidence showing that BBB disruption causes cognitive decline is scarce, and interestingly, manipulation of the BBB opening alone may exert beneficial or detrimental neurological effects. A broad overview of the present literature shows a close relationship between BBB disruption and cognitive decline, the risk factors of BBB disruption, as well as the cellular and molecular mechanisms underlying BBB disruption. Additionally, we discussed the possible causes leading to cognitive decline by BBB disruption and potential therapeutic strategies to prevent BBB disruption or enhance BBB repair. This review aims to foster more investigations on early diagnosis, effective therapeutics, and rapid restoration against BBB disruption, which would yield better cognitive outcomes in patients with dysregulated BBB function, although their causative relationship has not yet been completely established.

Systematic multi-trait AAV capsid engineering for efficient gene delivery

Broadening gene therapy applications requires manufacturable vectors that efficiently transduce target cells in humans and preclinical models. Conventional selections of adeno-associated virus (AAV) capsid libraries are inefficient at searching the vast sequence space for the small fraction of vectors possessing multiple traits essential for clinical translation. Here, we present Fit4Function, a generalizable machine learning (ML) approach for systematically engineering multi-trait AAV capsids. By leveraging a capsid library that uniformly samples the manufacturable sequence space, reproducible screening data are generated to train accurate sequence-to-function models. Combining six models, we designed a multi-trait (liver-targeted, manufacturable) capsid library and validated 88% of library variants on all six predetermined criteria. Furthermore, the models, trained only on mouse in vivo and human in vitro Fit4Function data, accurately predicted AAV capsid variant biodistribution in macaque. Top candidates exhibited production yields comparable to AAV9, efficient murine liver transduction, up to 1000-fold greater human hepatocyte transduction, and increased enrichment relative to AAV9 in a screen for liver transduction in macaques. The Fit4Function strategy ultimately makes it possible to predict cross-species traits of peptide-modified AAV capsids and is a critical step toward assembling an ML atlas that predicts AAV capsid performance across dozens of traits.

Chemical approaches to probe and engineer AAV vectors

Adeno-associated virus (AAV) has emerged as the most promising vector for in vivo human gene therapy, with several therapeutic approvals in the last few years and countless more under development. Underlying this remarkable success are several attractive features that AAV offers, including lack of pathogenicity, low immunogenicity, long-term gene expression without genomic integration, the ability to infect both dividing and non-dividing cells, etc. However, the commonly used wild-type AAV capsids in therapeutic development present significant challenges, including inadequate tissue specificity and the need for large doses to attain therapeutic effectiveness, raising safety concerns. Additionally, significant preexisting adaptive immunity against most natural capsids, and the development of such anti-capsid immunity after the first treatment, represent major challenges. Strategies to engineer the AAV capsid are critically needed to address these challenges and unlock the full promise of AAV gene therapy. Chemical modification of the AAV capsid has recently emerged as a powerful new approach to engineer its properties. Unlike genetic strategies, which can be more disruptive to the delicate capsid assembly and packaging processes, "late-stage" chemical modification of the assembled capsid-whether at natural amino acid residues or site-specifically installed noncanonical amino acid residues-often enables a versatile approach to introducing new properties to the capsid. This review summarizes the significant recent progress in AAV capsid engineering strategies, with a particular focus on chemical modifications in advancing the next generation of AAV-based gene therapies.

Rational correction of pathogenic conformational defects in HTRA1

Loss-of-function mutations in the homotrimeric serine protease HTRA1 cause cerebral vasculopathy. Here, we establish independent approaches to achieve the functional correction of trimer assembly defects. Focusing on the prototypical R274Q mutation, we identify an HTRA1 variant that promotes trimer formation thus restoring enzymatic activity in vitro. Genetic experiments in Htra1R274Q mice further demonstrate that expression of this protein-based corrector in trans is sufficient to stabilize HtrA1-R274Q and restore the proteomic signature of the brain vasculature. An alternative approach employs supramolecular chemical ligands that shift the monomer-trimer equilibrium towards proteolytically active trimers. Moreover, we identify a peptidic ligand that activates HTRA1 monomers. Our findings open perspectives for tailored protein repair strategies.

Recombinant Adeno-Associated Virus Vectors for Gene Therapy of the Central Nervous System: Delivery Routes and Clinical Aspects

The Central Nervous System (CNS) is vulnerable to a range of diseases, including neurodegenerative and oncological conditions, which present significant treatment challenges. The blood-brain barrier (BBB) restricts molecule penetration, complicating the achievement of therapeutic concentrations in the CNS following systemic administration. Gene therapy using recombinant adeno-associated virus (rAAV) vectors emerges as a promising strategy for treating CNS diseases, demonstrated by the registration of six gene therapy products in the past six years and 87 ongoing clinical trials. This review explores the implementation of rAAV vectors in CNS disease treatment, emphasizing AAV biology and vector engineering. Various administration methods-such as intravenous, intrathecal, and intraparenchymal routes-and experimental approaches like intranasal and intramuscular administration are evaluated, discussing their advantages and limitations in different CNS contexts. Additionally, the review underscores the importance of optimizing therapeutic efficacy through the pharmacokinetics (PK) and pharmacodynamics (PD) of rAAV vectors. A comprehensive analysis of clinical trials reveals successes and challenges, including barriers to commercialization. This review provides insights into therapeutic strategies using rAAV vectors in neurological diseases and identifies areas requiring further research, particularly in optimizing rAAV PK/PD.

Microglia targeting by adeno-associated viral vectors

Microglia play a crucial role in maintaining homeostasis of the central nervous system and they are actively involved in shaping the brain's inflammatory response to stress. Among the multitude of involved molecules, purinergic receptors and enzymes are of special importance due to their ability to regulate microglia activation. By investigating the mechanisms underlying microglial responses and dysregulation, researchers can develop more precise interventions to modulate microglial behavior and alleviate neuroinflammatory processes. Studying gene function selectively in microglia, however, remains technically challenging. This review article provides an overview of adeno-associated virus (AAV)-based microglia targeting approaches, discussing potential prospects for refining these approaches to improve both specificity and effectiveness and encouraging future investigations aimed at connecting the potential of AAV-mediated microglial targeting for therapeutic benefit in neurological disorders.

Human Brain In Vitro Model for Pathogen Infection-Related Neurodegeneration Study

Recent advancements in stem cell biology and tissue engineering have revolutionized the field of neurodegeneration research by enabling the development of sophisticated in vitro human brain models. These models, including 2D monolayer cultures, 3D organoids, organ-on-chips, and bioengineered 3D tissue models, aim to recapitulate the cellular diversity, structural organization, and functional properties of the native human brain. This review highlights how these in vitro brain models have been used to investigate the effects of various pathogens, including viruses, bacteria, fungi, and parasites infection, particularly in the human brain cand their subsequent impacts on neurodegenerative diseases. Traditional studies have demonstrated the susceptibility of different 2D brain cell types to infection, elucidated the mechanisms underlying pathogen-induced neuroinflammation, and identified potential therapeutic targets. Therefore, current methodological improvement brought the technology of 3D models to overcome the challenges of 2D cells, such as the limited cellular diversity, incomplete microenvironment, and lack of morphological structures by highlighting the need for further technological advancements. This review underscored the significance of in vitro human brain cell from 2D monolayer to bioengineered 3D tissue model for elucidating the intricate dynamics for pathogen infection modeling. These in vitro human brain cell enabled researchers to unravel human specific mechanisms underlying various pathogen infections such as SARS-CoV-2 to alter blood-brain-barrier function and Toxoplasma gondii impacting neural cell morphology and its function. Ultimately, these in vitro human brain models hold promise as personalized platforms for development of drug compound, gene therapy, and vaccine. Overall, we discussed the recent progress in in vitro human brain models, their applications in studying pathogen infection-related neurodegeneration, and future directions.

Brain microvascular endothelial cell metabolism and its ties to barrier function

Brain microvascular endothelial cells, which lie at the interface between blood and brain, are critical to brain energetics. These cells must precisely balance metabolizing nutrients for their own demands with transporting nutrients into the brain to sustain parenchymal cells. It is essential to understand this integrated metabolism and transport so that we can develop better diagnostics and therapeutics for neurodegenerative diseases such as Alzheimer's disease, multiple sclerosis, and traumatic brain injury. In this chapter, we first describe brain microvascular endothelial cell metabolism and how these cells regulate both blood flow and nutrient transport. We then explain the impact of brain microvascular endothelial cell metabolism on the integrity of the blood-brain barrier, as well as how metabolites produced by the endothelial cells impact other brain cells. We detail some ways that cell metabolism is typically measured experimentally and modeled computationally. Finally, we describe changes in brain microvascular endothelial cell metabolism in aging and neurodegenerative diseases. At the end of the chapter, we highlight areas for future research in brain microvascular endothelial cell metabolism. The goal of this chapter is to underscore the importance of nutrient metabolism and transport at the brain endothelium for cerebral health and neurovascular disease treatment.

Perivascular neurons instruct 3D vascular lattice formation via neurovascular contact

The vasculature of the central nervous system is a 3D lattice composed of laminar vascular beds interconnected by penetrating vessels. The mechanisms controlling 3D lattice network formation remain largely unknown. Combining viral labeling, genetic marking, and single-cell profiling in the mouse retina, we discovered a perivascular neuronal subset, annotated as Fam19a4/Nts-positive retinal ganglion cells (Fam19a4/Nts-RGCs), directly contacting the vasculature with perisomatic endfeet. Developmental ablation of Fam19a4/Nts-RGCs led to disoriented growth of penetrating vessels near the ganglion cell layer (GCL), leading to a disorganized 3D vascular lattice. We identified enriched PIEZO2 expression in Fam19a4/Nts-RGCs. Piezo2 loss from all retinal neurons or Fam19a4/Nts-RGCs abolished the direct neurovascular contacts and phenocopied the Fam19a4/Nts-RGC ablation deficits. The defective vascular structure led to reduced capillary perfusion and sensitized the retina to ischemic insults. Furthermore, we uncovered a Piezo2-dependent perivascular granule cell subset for cerebellar vascular patterning, indicating neuronal Piezo2-dependent 3D vascular patterning in the brain.

New AAV9 engineered variants with enhanced neurotropism and reduced liver off-targeting in mice and marmosets

Although adeno-associated virus 9 (AAV9) has been highly exploited as delivery platform for gene-based therapies, its efficacy is hampered by low efficiency in crossing the adult blood-brain barrier (BBB) and pronounced targeting to the liver upon intravenous delivery. We generated a new galactose binding-deficient AAV9 peptide display library and selected two new AAV9 engineered capsids with enhanced targeting in mouse and marmoset brains after intravenous delivery. Interestingly, the loss of galactose binding greatly reduced undesired targeting to peripheral organs, particularly the liver, while not compromising transduction of the brain vasculature. However, the galactose binding was necessary to efficiently infect non-endothelial brain cells. Thus, the combinatorial actions of the galactose-binding domain and the incorporated displayed peptide are crucial to enhance BBB crossing along with brain cell transduction. This study describes two novel capsids with high brain endothelial infectivity and extremely low liver targeting based on manipulating the AAV9 galactose-binding domain.

MEK signaling represents a viable therapeutic vulnerability of KRAS-driven somatic brain arteriovenous malformations

Brain arteriovenous malformations (bAVMs) are direct connections between arteries and veins that remodel into a complex nidus susceptible to rupture and hemorrhage. Most sporadic bAVMs feature somatic activating mutations within KRAS, and endothelial-specific expression of the constitutively active variant KRASG12D models sporadic bAVM in mice. By leveraging 3D-based micro-CT imaging, we demonstrate that KRASG12D-driven bAVMs arise in stereotypical anatomical locations within the murine brain, which coincide with high endogenous Kras expression. We extend these analyses to show that a distinct variant, KRASG12C, also generates bAVMs in predictable locations. Analysis of 15,000 human patients revealed that, similar to murine models, bAVMs preferentially occur in distinct regions of the adult brain. Furthermore, bAVM location correlates with hemorrhagic frequency. Quantification of 3D imaging revealed that G12D and G12C alter vessel density, tortuosity, and diameter within the mouse brain. Notably, aged G12D mice feature increased lethality, as well as impaired cognition and motor function. Critically, we show that pharmacological blockade of the downstream kinase, MEK, after lesion formation ameliorates KRASG12D-driven changes in the murine cerebrovasculature and may also impede bAVM progression in human pediatric patients. Collectively, these data show that distinct KRAS variants drive bAVMs in similar patterns and suggest MEK inhibition represents a non-surgical alternative therapy for sporadic bAVM.

Utility of protein-protein binding surfaces composed of anti-parallel alpha-helices and beta-sheets selected by phage display

Over the past 3 decades, a diverse collection of small protein domains have been used as scaffolds to generate general purpose protein-binding reagents using a variety of protein display and enrichment technologies. To expand the repertoire of scaffolds and protein surfaces that might serve this purpose, we have explored the utility of (i) a pair of anti-parallel alpha-helices in a small highly disulfide-bonded 4-helix bundle, the CC4 domain from reversion-inducing Cysteine-rich Protein with Kazal Motifs and (ii) a concave beta-sheet surface and two adjacent loops in the human FN3 domain, the scaffold for the widely used monobody platform. Using M13 phage display and next generation sequencing, we observe that, in both systems, libraries of ∼30 million variants contain binding proteins with affinities in the low μM range for baits corresponding to the extracellular domains of multiple mammalian proteins. CC4- and FN3-based binding proteins were fused to the N- and/or C-termini of Fc domains and used for immunostaining of transfected cells. Additionally, FN3-based binding proteins were inserted into VP1 of AAV to direct AAV infection to cells expressing a defined surface receptor. Finally, FN3-based binding proteins were inserted into the Pvc13 tail fiber protein of an extracellular contractile injection system particle to direct protein cargo delivery to cells expressing a defined surface receptor. These experiments support the utility of CC4 helices B and C and of FN3 beta-strands C, D, and F together with adjacent loops CD and FG as surfaces for engineering general purpose protein-binding reagents.

Brain endothelial GSDMD activation mediates inflammatory BBB breakdown

The blood-brain barrier (BBB) protects the central nervous system from infections or harmful substances1; its impairment can lead to or exacerbate various diseases of the central nervous system2-4. However, the mechanisms of BBB disruption during infection and inflammatory conditions5,6 remain poorly defined. Here we find that activation of the pore-forming protein GSDMD by the cytosolic lipopolysaccharide (LPS) sensor caspase-11 (refs. 7-9), but not by TLR4-induced cytokines, mediates BBB breakdown in response to circulating LPS or during LPS-induced sepsis. Mice deficient in the LBP-CD14 LPS transfer and internalization pathway10-12 resist BBB disruption. Single-cell RNA-sequencing analysis reveals that brain endothelial cells (bECs), which express high levels of GSDMD, have a prominent response to circulating LPS. LPS acting on bECs primes Casp11 and Cd14 expression and induces GSDMD-mediated plasma membrane permeabilization and pyroptosis in vitro and in mice. Electron microscopy shows that this features ultrastructural changes in the disrupted BBB, including pyroptotic endothelia, abnormal appearance of tight junctions and vasculature detachment from the basement membrane. Comprehensive mouse genetic analyses, combined with a bEC-targeting adeno-associated virus system, establish that GSDMD activation in bECs underlies BBB disruption by LPS. Delivery of active GSDMD into bECs bypasses LPS stimulation and opens the BBB. In CASP4-humanized mice, Gram-negative Klebsiella pneumoniae infection disrupts the BBB; this is blocked by expression of a GSDMD-neutralizing nanobody in bECs. Our findings outline a mechanism for inflammatory BBB breakdown, and suggest potential therapies for diseases of the central nervous system associated with BBB impairment.

Endothelial lincRNA-p21 alleviates cerebral ischemia/reperfusion injury by maintaining blood-brain barrier integrity

Blood-brain barrier (BBB) disruption is increasingly recognized as an early contributor to the pathophysiology of cerebral ischemia/reperfusion (I/R) injury, and is also a key event in triggering secondary damage to the central nervous system. Recently, long non-coding RNA (lncRNA) have been found to be associated with ischemic stroke. However, the roles of lncRNA in BBB homeostasis remain largely unknown. Here, we report that long intergenic non-coding RNA-p21 (lincRNA-p21) was the most significantly down-regulated lncRNA in human brain microvascular endothelial cells (HBMECs) after oxygen and glucose deprivation/reoxygenation (OGD/R) treatment among candidate lncRNA, which were both sensitive to hypoxia and involved in atherosclerosis. Exogenous brain-endothelium-specific overexpression of lincRNA-p21 could alleviate BBB disruption, diminish infarction volume and attenuate motor function deficits in middle cerebral artery occlusion/reperfusion (MCAO/R) mice. Further results showed that lincRNA-p21 was critical to maintain BBB integrity by inhibiting the degradation of junction proteins under MCAO/R and OGD/R conditions. Specifically, lincRNA-p21 could inhibit autophagy-dependent degradation of occludin by activating PI3K/AKT/mTOR signaling pathway. Besides, lincRNA-p21 could inhibit VE-cadherin degradation by binding with miR-101-3p. Together, we identify that lincRNA-p21 is critical for BBB integrity maintenance, and endothelial lincRNA-p21 overexpression could alleviate cerebral I/R injury in mice, pointing to a potential strategy to treat cerebral I/R injury.

Gene therapy targeting the blood-brain barrier

Endothelial cells are the building blocks of vessels in the central nervous system (CNS) and form the blood-brain barrier (BBB). An intact BBB limits permeation of large hydrophilic molecules into the CNS. Thus, the healthy BBB is a major obstacle for the treatment of CNS disorders with antibodies, recombinant proteins or viral vectors. Several strategies have been devised to overcome the barrier. A key principle often consists in attaching the therapeutic compound to a ligand of receptors expressed on the BBB, for example, the transferrin receptor (TfR). The fusion molecule will bind to TfR on the luminal side of brain endothelial cells, pass the endothelial layer by transcytosis and be delivered to the brain parenchyma. However, attempts to endow therapeutic compounds with the ability to cross the BBB can be difficult to implement. An alternative and possibly more straight-forward approach is to produce therapeutic proteins in the endothelial cells that form the barrier. These cells are accessible from blood circulation and have a large interface with the brain parenchyma. They may be an ideal production site for therapeutic protein and afford direct supply to the CNS.

Proteomics of mouse brain endothelium uncovers dysregulation of vesicular transport pathways during aging

Age-related decline in brain endothelial cell (BEC) function contributes critically to neurological disease. Comprehensive atlases of the BEC transcriptome have become available, but results from proteomic profiling are lacking. To gain insights into endothelial pathways affected by aging, we developed a magnetic-activated cell sorting-based mouse BEC enrichment protocol compatible with proteomics and resolved the profiles of protein abundance changes during aging. Unsupervised cluster analysis revealed a segregation of age-related protein dynamics with biological functions, including a downregulation of vesicle-mediated transport. We found a dysregulation of key regulators of endocytosis and receptor recycling (most prominently Arf6), macropinocytosis and lysosomal degradation. In gene deletion and overexpression experiments, Arf6 affected endocytosis pathways in endothelial cells. Our approach uncovered changes not picked up by transcriptomic studies, such as accumulation of vesicle cargo and receptor ligands, including Apoe. Proteomic analysis of BECs from Apoe-deficient mice revealed a signature of accelerated aging. Our findings provide a resource for analysing BEC function during aging.

Monocytes Release Pro-Cathepsin D to Drive Blood-to-Brain Transcytosis in Diabetes

BACKGROUND

Microvascular complications are the major outcome of type 2 diabetes progression, and the underlying mechanism remains to be determined.

METHODS

High-throughput RNA sequencing was performed using human monocyte samples from controls and diabetes. The transgenic mice expressing human CTSD (cathepsin D) in the monocytes was constructed using CD68 promoter. In vivo 2-photon imaging, behavioral tests, immunofluorescence, transmission electron microscopy, Western blot analysis, vascular leakage assay, and single-cell RNA sequencing were performed to clarify the phenotype and elucidate the molecular mechanism.

RESULTS

Monocytes expressed high-level CTSD in patients with type 2 diabetes. The transgenic mice expressing human CTSD in the monocytes showed increased brain microvascular permeability resembling the diabetic microvascular phenotype, accompanied by cognitive deficit. Mechanistically, the monocytes release nonenzymatic pro-CTSD to upregulate caveolin expression in brain endothelium triggering caveolae-mediated transcytosis, without affecting the paracellular route of brain microvasculature. The circulating pro-CTSD activated the caveolae-mediated transcytosis in brain endothelial cells via its binding with low-density LRP1 (lipoprotein receptor-related protein 1). Importantly, genetic ablation of CTSD in the monocytes exhibited a protective effect against the diabetes-enhanced brain microvascular transcytosis and the diabetes-induced cognitive impairment.

CONCLUSIONS

These findings uncover the novel role of circulatory pro-CTSD from monocytes in the pathogenesis of cerebral microvascular lesions in diabetes. The circulatory pro-CTSD is a potential target for the intervention of microvascular complications in diabetes.

Natural Adeno-Associated Virus Serotypes and Engineered Adeno-Associated Virus Capsid Variants: Tropism Differences and Mechanistic Insights

Today, adeno-associated virus (AAV)-based vectors are arguably the most promising in vivo gene delivery vehicles for durable therapeutic gene expression. Advances in molecular engineering, high-throughput screening platforms, and computational techniques have resulted in a toolbox of capsid variants with enhanced performance over parental serotypes. Despite their considerable promise and emerging clinical success, there are still obstacles hindering their broader use, including limited transduction capabilities, tissue/cell type-specific tropism and penetration into tissues through anatomical barriers, off-target tissue biodistribution, intracellular degradation, immune recognition, and a lack of translatability from preclinical models to clinical settings. Here, we first describe the transduction mechanisms of natural AAV serotypes and explore the current understanding of the systemic and cellular hurdles to efficient transduction. We then outline progress in developing designer AAV capsid variants, highlighting the seminal discoveries of variants which can transduce the central nervous system upon systemic administration, and, to a lesser extent, discuss the targeting of the peripheral nervous system, eye, ear, lung, liver, heart, and skeletal muscle, emphasizing their tissue and cell specificity and translational promise. In particular, we dive deeper into the molecular mechanisms behind their enhanced properties, with a focus on their engagement with host cell receptors previously inaccessible to natural AAV serotypes. Finally, we summarize the main findings of our review and discuss future directions.

AAV-BR1 does not target endothelial cells in Sprague Dawley rats unlike in mice

Adeno-associated viruses (AAVs) are a popular tool in gene therapy approaches and have been engineered to specifically target different cells. There is interest in targeting endothelial cells (ECs) of the blood brain barrier and the AAV2 capsid variant BR1 has been found to transduce ECs with high selectivity in various mice models. However, this has not been tested in rat models. Here, we show that there is no EC transduction with systemic injection of the AAV-BR1-CAG-GFP virus in Sprague-Dawley rats (n=3), but instead transduction of brain parenchymal cells with neuronal morphology. These findings emphasize the importance of species-differences in use of AAVs.

Biomacromolecule-tagged nanoscale constructs for crossing the blood-brain barrier

Access to the brain is restricted by the low permeability of the blood-brain barrier (BBB), greatly hampering modern drug delivery efforts. A promising approach to overcome this boundary is to utilize biomacromolecules (peptides, nucleic acids, carbohydrates) as targeting ligands on nanoscale delivery vehicles to shuttle cargo across the BBB. In this mini-review, we highlight the most recent approaches for crossing the BBB using synthetic nanoscale constructs decorated with members of these general classes of biomacromolecules to safely and selectively deliver therapeutic materials to the brain.

Challenges and prospects in geriatric epilepsy treatment: the role of the blood-brain barrier in pharmacotherapy and drug delivery

The blood-brain barrier (BBB) is pivotal in maintaining neuronal physiology within the brain. This review delves into the alterations of the BBB specifically in the context of geriatric epilepsy. We examine how age-related changes in the BBB contribute to the pathogenesis of epilepsy in the elderly and present significant challenges in pharmacotherapy. Subsequently, we evaluate recent advancements in drug delivery methods targeting the BBB, as well as alternative approaches that could bypass the BBB's restrictive nature. We particularly highlight the use of neurotropic viruses and various synthetic nanoparticles that have been investigated for delivering a range of antiepileptic drugs. Additionally, the advantage and limitation of these diverse delivery methods are discussed. Finally, we analyze the potential efficacy of different drug delivery approaches in the treatment of geriatric epilepsy, aiming to provide insights into more effective management of this condition in the elderly population.

Meningeal interleukin-17-producing T cells mediate cognitive impairment in a mouse model of salt-sensitive hypertension

Hypertension (HTN), a disease afflicting over one billion individuals worldwide, is a leading cause of cognitive impairment, the mechanisms of which remain poorly understood. In the present study, in a mouse model of HTN, we find that the neurovascular and cognitive dysfunction depends on interleukin (IL)-17, a cytokine elevated in individuals with HTN. However, neither circulating IL-17 nor brain angiotensin signaling can account for the dysfunction. Rather, IL-17 produced by T cells in the dura mater is the mediator released in the cerebrospinal fluid and activating IL-17 receptors on border-associated macrophages (BAMs). Accordingly, depleting BAMs, deleting IL-17 receptor A in brain macrophages or suppressing meningeal T cells rescues cognitive function without attenuating blood pressure elevation, circulating IL-17 or brain angiotensin signaling. Our data unveil a critical role of meningeal T cells and macrophage IL-17 signaling in the neurovascular and cognitive dysfunction in a mouse model of HTN.

C-kit controls blood-brain barrier permeability by regulating caveolae-mediated transcytosis after chronic cerebral hypoperfusion

Blood-brain barrier (BBB) dysfunction plays a pivotal role in the pathology of chronic cerebral hypoperfusion (CCH)-related neurodegenerative diseases. Continuous endothelial cells (EC) that line the blood vessels of the brain are important components of the BBB to strictly control the flow of substances and maintain the homeostatic environment of the brain. However, the molecular mechanisms from the perspective of EC-induced BBB dysfunction after CCH are largely unknown. In this study, the BBB function was assessed using immunostaining and transmission electron microscopy. The EC dysfunction profile was screened by using EC enrichment followed by RNA sequencing. After identified the key EC dysfunction factor, C-kit, we used the C-kit inhibition drug (imatinib) and C-kit down-regulation method (AAV-BR1-C-kit shRNA) to verify the role of C-kit on BBB integrity and EC transcytosis after CCH. Furthermore, we also activated C-kit with stem cell factor (SCF) to observe the effects of C-kit on BBB following CCH. We explored that macromolecular proteins entered the brain mainly through EC transcytosis after CCH and caused neuronal loss. Additionally, we identified receptor tyrosine kinase C-kit as a key EC dysfunction molecule. Furthermore, the pharmacological inhibition of C-kit with imatinib counteracted BBB leakage by reducing caveolae-mediated transcytosis. Moreover, treatment with AAV-BR1-C-kit shRNA, which targets brain EC to inhibit C-kit expression, also ameliorated BBB leakage by reducing caveolae-mediated transcytosis. Furthermore, the SCF increased the permeability of the BBB by actively increasing caveolae-mediated transcytosis. This study provides evidence that C-kit is a key BBB permeability regulator through caveolae-mediated transcytosis in EC after CCH.

ANKS1A regulates LDL receptor-related protein 1 (LRP1)-mediated cerebrovascular clearance in brain endothelial cells

Brain endothelial LDL receptor-related protein 1 (LRP1) is involved in the clearance of Aβ peptides across the blood-brain barrier (BBB). Here we show that endothelial deficiency of ankyrin repeat and SAM domain containing 1 A (ANKS1A) reduces both the cell surface levels of LRP1 and the Aβ clearance across the BBB. Association of ANKS1A with the NPXY motifs of LRP1 facilitates the transport of LRP1 from the endoplasmic reticulum toward the cell surface. ANKS1A deficiency in an Alzheimer's disease mouse model results in exacerbated Aβ pathology followed by cognitive impairments. These deficits are reversible by gene therapy with brain endothelial-specific ANKS1A. In addition, human induced pluripotent stem cell-derived BBBs (iBBBs) were generated from endothelial cells lacking ANKS1A or carrying the rs6930932 variant. Those iBBBs exhibit both reduced cell surface LRP1 and impaired Aβ clearance. Thus, our findings demonstrate that ANKS1A regulates LRP1-mediated Aβ clearance across the BBB.

A20 regulates lymphocyte adhesion in murine neuroinflammation by restricting endothelial ICOSL expression in the CNS

A20 is a ubiquitin-modifying protein that negatively regulates NF-κB signaling. Mutations in A20/TNFAIP3 are associated with a variety of autoimmune diseases, including multiple sclerosis (MS). We found that deletion of A20 in central nervous system (CNS) endothelial cells (ECs) enhances experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. A20ΔCNS-EC mice showed increased numbers of CNS-infiltrating immune cells during neuroinflammation and in the steady state. While the integrity of the blood-brain barrier (BBB) was not impaired, we observed a strong activation of CNS-ECs in these mice, with dramatically increased levels of the adhesion molecules ICAM-1 and VCAM-1. We discovered ICOSL to be expressed by A20-deficient CNS-ECs, which we found to function as adhesion molecules. Silencing of ICOSL in CNS microvascular ECs partly reversed the phenotype of A20ΔCNS-EC mice without reaching statistical significance and delayed the onset of EAE symptoms in WT mice. In addition, blocking of ICOSL on primary mouse brain microvascular ECs impaired the adhesion of T cells in vitro. Taken together, we propose that CNS EC-ICOSL contributes to the firm adhesion of T cells to the BBB, promoting their entry into the CNS and eventually driving neuroinflammation.

Thinking outside the black box: are the brain endothelial cells the new main target in Alzheimer's disease?

The blood-brain barrier is the interface through which the brain interacts with the milieu and consists mainly of a sophisticated network of brain endothelial cells that forms blood vessels and selectively moves molecules inside and outside the brain through multiple mechanisms of transport. Although brain endothelial cell function is crucial for brain homeostasis, their role in neurodegenerative diseases has historically not been considered with the same importance as other brain cells such as microglia, astroglia, neurons, or even molecules such as amyloid beta, Tau, or alpha-synuclein. Alzheimer's disease is the most common neurodegenerative disease, and brain endothelial cell dysfunction has been reported by several groups. However, its impairment has barely been considered as a potential therapeutic target. Here we review the most recent advances in the relationship between Alzheimer's disease and brain endothelial cells commitment and analyze the possible mechanisms through which their alterations contribute to this neurodegenerative disease, highlighting their inflammatory phenotype and the possibility of an impaired secretory pattern of brain endothelial cells that could contribute to the progression of this ailment. Finally, we discuss why shall brain endothelial cells be appreciated as a therapeutic target instead of solely an obstacle for delivering treatments to the injured brain in Alzheimer's disease.

Adeno-associated virus vectors for retinal gene therapy in basic research and clinical studies

Retinal degenerative diseases, including glaucoma, age-related macular degeneration, diabetic retinopathy, and a broad range of inherited retinal diseases, are leading causes of irreversible vision loss and blindness. Gene therapy is a promising and fast-growing strategy to treat both monogenic and multifactorial retinal disorders. Vectors for gene delivery are crucial for efficient and specific transfer of therapeutic gene(s) into target cells. AAV vectors are ideal for retinal gene therapy due to their inherent advantages in safety, gene expression stability, and amenability for directional engineering. The eye is a highly compartmentalized organ composed of multiple disease-related cell types. To determine a suitable AAV vector for a specific cell type, the route of administration and choice of AAV variant must be considered together. Here, we provide a brief overview of AAV vectors for gene transfer into important ocular cell types, including retinal pigment epithelium cells, photoreceptors, retinal ganglion cells, Müller glial cells, ciliary epithelial cells, trabecular meshwork cells, vascular endothelial cells, and pericytes, via distinct injection methods. By listing suitable AAV vectors in basic research and (pre)clinical studies, we aim to highlight the progress and unmet needs of AAV vectors in retinal gene therapy.

TIMP2 ameliorates blood-brain barrier disruption in traumatic brain injury by inhibiting Src-dependent VE-cadherin internalization

Blood-brain barrier (BBB) disruption is a serious pathological consequence of traumatic brain injury (TBI), for which there are limited therapeutic strategies. Tissue inhibitor of metalloproteinase-2 (TIMP2), a molecule with dual functions of inhibiting MMP activity and displaying cytokine-like activity through receptor binding, has been reported to inhibit VEGF-induced vascular hyperpermeability. Here, we investigate the ability of TIMP2 to ameliorate BBB disruption in TBI and the underlying molecular mechanisms. Both TIMP2 and AlaTIMP2, a TIMP2 mutant without MMP-inhibiting activity, attenuated neurological deficits and BBB leakage in TBI mice; they also inhibited junctional protein degradation and translocation to reduce paracellular permeability in human brain microvascular endothelial cells (ECs) exposed to hypoxic plus inflammatory insult. Mechanistic studies revealed that TIMP2 interacted with α3β1 integrin on ECs, inhibiting Src activation-dependent VE-cadherin phosphorylation, VE-cadherin/catenin complex destabilization, and subsequent VE-cadherin internalization. Notably, localization of VE-cadherin on the membrane was critical for TIMP2-mediated EC barrier integrity. Furthermore, TIMP2-mediated increased membrane localization of VE-cadherin enhanced the level of active Rac1, thereby inhibiting stress fiber formation. All together, our studies have identified an MMP-independent mechanism by which TIMP2 regulates EC barrier integrity after TBI. TIMP2 may be a therapeutic agent for TBI and other neurological disorders involving BBB breakdown.

mTORC1 Inhibitor Rapamycin Inhibits Growth of Cerebral Cavernous Malformation in Adult Mice

BACKGROUND

Cerebral cavernous malformations (CCMs) are vascular malformations that frequently cause stroke. CCMs arise due to loss of function in one of the genes that encode the CCM complex, a negative regulator of MEKK3-KLF2/4 signaling in vascular endothelial cells. Gain-of-function mutations in PIK3CA (encoding the enzymatic subunit of the PI3K (phosphoinositide 3-kinase) pathway associated with cell growth) synergize with CCM gene loss-of-function to generate rapidly growing lesions.

METHODS

We recently developed a model of CCM formation that closely reproduces key events in human CCM formation through inducible CCM loss-of-function and PIK3CA gain-of-function in mature mice. In the present study, we use this model to test the ability of rapamycin, a clinically approved inhibitor of the PI3K effector mTORC1, to treat rapidly growing CCMs.

RESULTS

We show that both intraperitoneal and oral administration of rapamycin arrests CCM growth, reduces perilesional iron deposition, and improves vascular perfusion within CCMs.

CONCLUSIONS

Our findings further establish this adult CCM model as a valuable preclinical model and support clinical testing of rapamycin to treat rapidly growing human CCMs.

In vivo selection in non-human primates identifies superior AAV capsids for on-target CSF delivery to spinal cord

Systemic administration of adeno-associated virus (AAV) vectors for spinal cord gene therapy has challenges including toxicity at high doses and pre-existing immunity that reduces efficacy. Intrathecal delivery of AAV vectors into the cerebral spinal fluid (CSF) can avoid many of the issues of systemic delivery, although achieving broad distribution of the vector and transgene expression throughout the spinal cord is challenging and vector entry to the periphery occurs, sometimes initiating hepatotoxicity. Here we performed two rounds of in vivo biopanning in non-human primates (NHPs) with an AAV9 peptide display library injected intrathecally and performed insert sequencing on DNA isolated from either whole tissue (conventional selection), isolated nuclei, or nuclei from transgene-expressing cells. A subsequent barcoded pool of candidates and AAV9 was compared at the DNA (biodistribution) and RNA (expression) level in spinal cord and liver of intrathecally injected NHPs. Most of the candidates displayed enhanced biodistribution compared to AAV9 at all levels of spinal cord ranging from 2 to 265-fold. Nuclear isolation or expression-based selection yielded 4 of 7 candidate capsids with enhanced transgene expression in spinal cord (up to 2.4-fold), while no capsid obtained by conventional selection achieved that level. Furthermore, several capsids displayed lower biodistribution to the liver of up to 1,250-fold, compared to AAV9, providing a remarkable on target/off target biodistribution ratio. These capsids may have potential for gene therapy programs directed at the spinal cord and the selection method described here should be useful in clinically relevant large animal models.

Somatic variants of MAP3K3 are sufficient to cause cerebral and spinal cord cavernous malformations

Cerebral cavernous malformations (CCMs) and spinal cord cavernous malformations (SCCMs) are common vascular abnormalities of the CNS that can lead to seizure, haemorrhage and other neurological deficits. Approximately 85% of patients present with sporadic (versus congenital) CCMs. Somatic mutations in MAP3K3 and PIK3CA were recently reported in patients with sporadic CCM, yet it remains unknown whether MAP3K3 mutation is sufficient to induce CCMs. Here we analysed whole-exome sequencing data for patients with CCM and found that ∼40% of them have a single, specific MAP3K3 mutation [c.1323C>G (p.Ile441Met)] but not any other known mutations in CCM-related genes. We developed a mouse model of CCM with MAP3K3I441M uniquely expressed in the endothelium of the CNS. We detected pathological phenotypes similar to those found in patients with MAP3K3I441M. The combination of in vivo imaging and genetic labelling revealed that CCMs were initiated with endothelial expansion followed by disruption of the blood-brain barrier. Experiments with our MAP3K3I441M mouse model demonstrated that CCM can be alleviated by treatment with rapamycin, the mTOR inhibitor. CCM pathogenesis has usually been attributed to acquisition of two or three distinct genetic mutations involving the genes CCM1/2/3 and/or PIK3CA. However, our results demonstrate that a single genetic hit is sufficient to cause CCMs.

An Engineered Adeno-Associated Virus Capsid Mediates Efficient Transduction of Pericytes and Smooth Muscle Cells of the Brain Vasculature

Neurodegeneration and cerebrovascular disease share an underlying microvascular dysfunction that may be remedied by selective transgene delivery. To date, limited options exist in which cellular components of the brain vasculature can be effectively targeted by viral vector therapeutics. In this study, we characterize the first engineered adeno-associated virus (AAV) capsid mediating high transduction of cerebral vascular pericytes and smooth muscle cells (SMCs). We performed two rounds of in vivo selection with an AAV capsid scaffold displaying a heptamer peptide library to isolate capsids that traffic to the brain after intravenous delivery. One identified capsid, termed AAV-PR, demonstrated high transduction of the brain vasculature, in contrast to the parental capsid, AAV9, which transduces mainly neurons and astrocytes. Further analysis using tissue clearing, volumetric rendering, and colocalization revealed that AAV-PR enabled high transduction of cerebral pericytes located on small-caliber vessels and SMCs in the larger arterioles and penetrating pial arteries. Analysis of tissues in the periphery indicated that AAV-PR also transduced SMCs in large vessels associated with the systemic vasculature. AAV-PR was also able to transduce primary human brain pericytes with higher efficiency than AAV9. Compared with previously published AAV capsids tropisms, AAV-PR represents the first capsid to allow for effective transduction of brain pericytes and SMCs and offers the possibility of genetically modulating these cell types in the context of neurodegeneration and other neurological diseases.

Reduced endothelial caveolin-1 underlies deficits in brain insulin signalling in type 2 diabetes

Patients with type 2 diabetes exhibit severe impairments in insulin signalling in the brain and are five times more likely to develop Alzheimer's disease. However, what leads to these impairments is not fully understood. Here, we show reduced expression of endothelial cell caveolin-1 (Cav-1) in the db/db (Leprdb) mouse model of type 2 diabetes. This reduction correlated with alterations in insulin receptor expression and signalling in brain microvessels as well as brain parenchyma. These findings were recapitulated in the brains of endothelial cell-specific Cav-1 knock-out (Tie2Cre; Cav-1fl/fl) mice. Lack of Cav-1 in endothelial cells led to reduced response to insulin as well as reduced insulin uptake. Furthermore, we observed that Cav-1 was necessary for the stabilization of insulin receptors in lipid rafts. Interactome analysis revealed that insulin receptor interacts with Cav-1 and caveolae-associated proteins, insulin-degrading enzyme and the tight junction protein Zonula Occludence-1 in brain endothelial cells. Restoration of Cav-1 in Cav-1 knock-out brain endothelial cells rescued insulin receptor expression and localization. Overall, these results suggest that Cav-1 regulates insulin signalling and uptake by brain endothelial cells by modulating IR-α and IR-β localization and function in lipid rafts. Furthermore, depletion of endothelial cell-specific Cav-1 and the resulting impairment in insulin transport leads to alteration in insulin signalling in the brain parenchyma of type 2 diabetics.

Targeted transport of biotherapeutics at the blood-brain barrier

INTRODUCTION

The treatment of neurological diseases is significantly hampered by the lack of available therapeutics. A major restraint for the development of drugs is denoted by the presence of the blood-brain barrier (BBB), which precludes the transfer of biotherapeutics to the brain due to size restraints.

AREAS COVERED

Novel optimism for transfer of biotherapeutics to the brain has been generated via development of targeted therapeutics to nutrient transporters expressed by brain capillary endothelial cells (BCECs). Targeting approaches with antibodies acting as biological drug carriers allow for proteins and genetic material to enter the brain, and qualified therapy using targeted proteins for protein replacement has been observed in preclinical models and now emerging in the clinic. Viral vectors denote an alternative for protein delivery to the brain by uptake and transduction of BCECs, or by transport through the BBB leading to neuronal transduction.

EXPERT OPINION

The breaching of the BBB to large molecules has opened for treatment of diseases in the brain. A sturdier understanding of how biotherapeutics undergo transport through the BBB and how successful transport into the brain can be monitored is required to further improve the translation from successful preclinical studies to the clinic.

Targeting AAV vectors to the central nervous system by engineering capsid-receptor interactions that enable crossing of the blood-brain barrier

Viruses have evolved the ability to bind and enter cells through interactions with a wide variety of cell macromolecules. We engineered peptide-modified adeno-associated virus (AAV) capsids that transduce the brain through the introduction of de novo interactions with 2 proteins expressed on the mouse blood-brain barrier (BBB), LY6A or LY6C1. The in vivo tropisms of these capsids are predictable as they are dependent on the cell- and strain-specific expression of their target protein. This approach generated hundreds of capsids with dramatically enhanced central nervous system (CNS) tropisms within a single round of screening in vitro and secondary validation in vivo thereby reducing the use of animals in comparison to conventional multi-round in vivo selections. The reproducible and quantitative data derived via this method enabled both saturation mutagenesis and machine learning (ML)-guided exploration of the capsid sequence space. Notably, during our validation process, we determined that nearly all published AAV capsids that were selected for their ability to cross the BBB in mice leverage either the LY6A or LY6C1 protein, which are not present in primates. This work demonstrates that AAV capsids can be directly targeted to specific proteins to generate potent gene delivery vectors with known mechanisms of action and predictable tropisms.

Targeting cerebral diseases with enhanced delivery of therapeutic proteins across the blood-brain barrier

INTRODUCTION

Cerebral diseases have been threatening public physical and psychological health in the recent years. With the existence of the blood-brain barrier (BBB), it is particularly hard for therapeutic proteins like peptides, enzymes, antibodies, etc. to enter the central nervous system (CNS) and function in diagnosis and treatment in cerebral diseases. Fortunately, the past decade has witnessed some emerging strategies of delivering macromolecular therapeutic proteins across the BBB.

AREAS COVERED

Based on the structure, functions, and substances transport mechanisms, various enhanced delivery strategies of therapeutic proteins were reviewed, categorized by molecule-mediated delivery strategies, carrier-mediated delivery strategies, and other delivery strategies.

EXPERT OPINION

As for molecule-mediated delivery strategies, development of genetic engineering technology, optimization of protein expression and purification techniques, and mature of quality control systems all help to realize large-scale production of recombinant antibodies, making it possible to apply to the clinical practice. In terms of carrier-mediated delivery strategies and others, although nano-carriers/adeno-associated virus (AAV) are also promising candidates for delivering therapeutic proteins or genes across the BBB, some issues still remain to be further investigated, including safety concerns related to applied materials, large-scale production costs, quality control standards, combination therapies with auxiliary delivery strategies like focused ultrasound, etc.

Receptor-mediated transcytosis of macromolecules across the blood-brain barrier

INTRODUCTION

The blood-brain barrier (BBB) restricts brain access of virtually all macromolecules. Receptor-mediated transcytosis (RMT) is one strategy toward their brain delivery. In this strategy, targeting ligands conjugated to therapeutic payload or decorating particles containing the payload interact with targets on brain capillary endothelial cells (BCEC), triggering internalization, trafficking, and release from BCEC.

AREAS COVERED

RMT at the BBB has leveraged multiple formats of macromolecules and large particles. Interactions between those and BCEC have been studied primarily using antibodies, with findings applicable to the design of larger particles. BBB-penetrant constructs have also been identified in screening campaigns and directed evolution, and subsequently found to interact with RMT targets. In addition, BCEC targeted by constructs incorporating genomic payload can be made to produce therapeutic proteins.

EXPERT OPINION

While targeting may not be strictly necessary to reach a therapeutic effect for all macromolecules, it can improve a molecule's BBB transport, exposing it to the entire brain parenchyma and enhancing its effect. Constructs with better BCEC transcytosis may be designed rationally, leveraging knowledge about BCEC trafficking, and found in screening campaigns, where this knowledge can reduce the search space and improve iterative refinement. Identification of new targets may also help generate BBB-crossing constructs.

Functional gene delivery to and across brain vasculature of systemic AAVs with endothelial-specific tropism in rodents and broad tropism in primates

Delivering genes to and across the brain vasculature efficiently and specifically across species remains a critical challenge for addressing neurological diseases. We have evolved adeno-associated virus (AAV9) capsids into vectors that transduce brain endothelial cells specifically and efficiently following systemic administration in wild-type mice with diverse genetic backgrounds, and in rats. These AAVs also exhibit superior transduction of the CNS across non-human primates (marmosets and rhesus macaques), and in ex vivo human brain slices, although the endothelial tropism is not conserved across species. The capsid modifications translate from AAV9 to other serotypes such as AAV1 and AAV-DJ, enabling serotype switching for sequential AAV administration in mice. We demonstrate that the endothelial-specific mouse capsids can be used to genetically engineer the blood-brain barrier by transforming the mouse brain vasculature into a functional biofactory. We apply this approach to Hevin knockout mice, where AAV-X1-mediated ectopic expression of the synaptogenic protein Sparcl1/Hevin in brain endothelial cells rescued synaptic deficits.

The Neurovasculome: Key Roles in Brain Health and Cognitive Impairment: A Scientific Statement From the American Heart Association/American Stroke Association

BACKGROUND

Preservation of brain health has emerged as a leading public health priority for the aging world population. Advances in neurovascular biology have revealed an intricate relationship among brain cells, meninges, and the hematic and lymphatic vasculature (the neurovasculome) that is highly relevant to the maintenance of cognitive function. In this scientific statement, a multidisciplinary team of experts examines these advances, assesses their relevance to brain health and disease, identifies knowledge gaps, and provides future directions.

METHODS

Authors with relevant expertise were selected in accordance with the American Heart Association conflict-of-interest management policy. They were assigned topics pertaining to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

The neurovasculome, composed of extracranial, intracranial, and meningeal vessels, as well as lymphatics and associated cells, subserves critical homeostatic functions vital for brain health. These include delivering O2 and nutrients through blood flow and regulating immune trafficking, as well as clearing pathogenic proteins through perivascular spaces and dural lymphatics. Single-cell omics technologies have unveiled an unprecedented molecular heterogeneity in the cellular components of the neurovasculome and have identified novel reciprocal interactions with brain cells. The evidence suggests a previously unappreciated diversity of the pathogenic mechanisms by which disruption of the neurovasculome contributes to cognitive dysfunction in neurovascular and neurodegenerative diseases, providing new opportunities for the prevention, recognition, and treatment of these conditions.

CONCLUSIONS

These advances shed new light on the symbiotic relationship between the brain and its vessels and promise to provide new diagnostic and therapeutic approaches for brain disorders associated with cognitive dysfunction.

Antigen recognition detains CD8+ T cells at the blood-brain barrier and contributes to its breakdown

Blood-brain barrier (BBB) breakdown and immune cell infiltration into the central nervous system (CNS) are early hallmarks of multiple sclerosis (MS). High numbers of CD8+ T cells are found in MS lesions, and antigen (Ag) presentation at the BBB has been proposed to promote CD8+ T cell entry into the CNS. Here, we show that brain endothelial cells process and cross-present Ag, leading to effector CD8+ T cell differentiation. Under physiological flow in vitro, endothelial Ag presentation prevented CD8+ T cell crawling and diapedesis resulting in brain endothelial cell apoptosis and BBB breakdown. Brain endothelial Ag presentation in vivo was limited due to Ag uptake by CNS-resident macrophages but still reduced motility of Ag-specific CD8+ T cells within CNS microvessels. MHC class I-restricted Ag presentation at the BBB during neuroinflammation thus prohibits CD8+ T cell entry into the CNS and triggers CD8+ T cell-mediated focal BBB breakdown.

Improving retinal vascular endothelial cell tropism through rational rAAV capsid design

Vascular endothelial cells (VEC) are essential for retinal homeostasis and their dysfunction underlies pathogenesis in diabetic retinopathy (DR) and exudative age-related macular degeneration (AMD). Studies have shown that recombinant adeno-associated virus (rAAV) vectors are effective at delivering new genetic material to neural and glial cells within the retina, but targeting VECs remains challenging. To overcome this limitation, herein we developed rAAV capsid mutant vectors with improved tropism towards retinal VEC. rAAV2/2, 2/2[QuadYF-TV], and rAAV2/9 serotype vectors (n = 9, capsid mutants per serotype) expressing GFP were generated by inserting heptameric peptides (7AA) designed to increase endothelial targeting at positions 588 (2/2 and 2/2[QuadYF-TV] or 589 (2/9) of the virus protein (VP 1-3). The packaging and transduction efficiency of the vectors were assessed in HEK293T and bovine VECs using Fluorescence microscopy and flow cytometry, leading to the identification of one mutant, termed EC5, that showed improved endothelial tropism when inserted into all three capsid serotypes. Intra-ocular and intravenous administration of EC5 mutants in C57Bl/6j mice demonstrated moderately improved transduction of the retinal vasculature, particularly surrounding the optic nerve head, and evidence of sinusoidal endothelial cell transduction in the liver. Most notably, intravenous administration of the rAAV2/2[QuadYF-TV] EC5 mutant led to a dramatic and unexpected increase in cardiac muscle transduction.

Tailoring vascular phenotype through AAV therapy promotes anti-tumor immunity in glioma

Glioblastomas are aggressive brain tumors that are largely immunotherapy resistant. This is associated with immunosuppression and a dysfunctional tumor vasculature, which hinder T cell infiltration. LIGHT/TNFSF14 can induce high endothelial venules (HEVs) and tertiary lymphoid structures (TLS), suggesting that its therapeutic expression could promote T cell recruitment. Here, we use a brain endothelial cell-targeted adeno-associated viral (AAV) vector to express LIGHT in the glioma vasculature (AAV-LIGHT). We found that systemic AAV-LIGHT treatment induces tumor-associated HEVs and T cell-rich TLS, prolonging survival in αPD-1-resistant murine glioma. AAV-LIGHT treatment reduces T cell exhaustion and promotes TCF1⁺CD8⁺ stem-like T cells, which reside in TLS and intratumoral antigen-presenting niches. Tumor regression upon AAV-LIGHT therapy correlates with tumor-specific cytotoxic/memory T cell responses. Our work reveals that altering vascular phenotype through vessel-targeted expression of LIGHT promotes efficient anti-tumor T cell responses and prolongs survival in glioma. These findings have broader implications for treatment of other immunotherapy-resistant cancers.

Endothelial depletion of Atg7 triggers astrocyte-microvascular disassociation at blood-brain barrier

Microvascular basement membrane (BM) plays a pivotal role in the interactions of astrocyte with endothelium to maintain the blood-brain barrier (BBB) homeostasis; however, the significance and precise regulation of the endothelial cell-derived BM component in the BBB remain incompletely understood. Here, we report that conditional knockout of Atg7 in endothelial cells (Atg7-ECKO) leads to astrocyte-microvascular disassociation in the brain. Our results reveal astrocytic endfeet detachment from microvessels and BBB leakage in Atg7-ECKO mice. Furthermore, we find that the absence of endothelial Atg7 downregulates the expression of fibronectin, a major BM component of the BBB, causing significantly reduced coverage of astrocytes along cerebral microvessels. We reveal Atg7 triggers the expression of endothelial fibronectin via regulating PKA activity to affect the phosphorylation of cAMP-responsive element-binding protein. These results suggest that Atg7-regulated endothelial fibronectin production is required for astrocytes adhesion to microvascular wall for maintaining the BBB homeostasis. Thus, endothelial Atg7 plays an essential role in astrocyte-endothelium interactions to maintain the BBB integrity.

Endothelial hyperactivation of mutant MAP3K3 induces cerebral cavernous malformation enhanced by PIK3CA GOF mutation

Cerebral cavernous malformations (CCMs) refer to a common vascular abnormality that affects up to 0.5% of the population. A somatic gain-of-function mutation in MAP3K3 (p.I441M) was recently reported in sporadic CCMs, frequently accompanied by somatic activating PIK3CA mutations in diseased endothelium. However, the molecular mechanisms of these driver genes remain elusive. In this study, we performed whole-exome sequencing and droplet digital polymerase chain reaction to analyze CCM lesions and the matched blood from sporadic patients. 44 of 94 cases harbored mutations in KRIT1/CCM2 or MAP3K3, of which 75% were accompanied by PIK3CA mutations (P = 0.006). AAV-BR1-mediated brain endothelial-specific MAP3K3I441M overexpression induced CCM-like lesions throughout the brain and spinal cord in adolescent mice. Interestingly, over half of lesions disappeared at adulthood. Single-cell RNA sequencing found significant enrichment of the apoptosis pathway in a subset of brain endothelial cells in MAP3K3I441M mice compared to controls. We then demonstrated that MAP3K3I441M overexpression activated p38 signaling that is associated with the apoptosis of endothelial cells in vitro and in vivo. In contrast, the mice simultaneously overexpressing PIK3CA and MAP3K3 mutations had an increased number of CCM-like lesions and maintained these lesions for a longer time compared to those with only MAP3K3I441M. Further in vitro and in vivo experiments showed that activating PI3K signaling increased proliferation and alleviated apoptosis of endothelial cells. By using AAV-BR1, we found that MAP3K3I441M mutation can provoke CCM-like lesions in mice and the activation of PI3K signaling significantly enhances and maintains these lesions, providing a preclinical model for the further mechanistic and therapeutic study of CCMs.

Advances in AAV technology for delivering genetically encoded cargo to the nonhuman primate nervous system

Modern neuroscience approaches including optogenetics, calcium imaging, and other genetic manipulations have facilitated our ability to dissect specific circuits in rodent models to study their role in neurological disease. These approaches regularly use viral vectors to deliver genetic cargo (e.g., opsins) to specific tissues and genetically-engineered rodents to achieve cell-type specificity. However, the translatability of these rodent models, cross-species validation of identified targets, and translational efficacy of potential therapeutics in larger animal models like nonhuman primates remains difficult due to the lack of efficient primate viral vectors. A refined understanding of the nonhuman primate nervous system promises to deliver insights that can guide the development of treatments for neurological and neurodegenerative diseases. Here, we outline recent advances in the development of adeno-associated viral vectors for optimized use in nonhuman primates. These tools promise to help open new avenues for study in translational neuroscience and further our understanding of the primate brain.

Dissecting the neurovascular unit in physiology and Alzheimer's disease: Functions, imaging tools and genetic mouse models

The neurovascular unit (NVU) plays an essential role in regulating neurovascular coupling, which refers to the communication between neurons, glia, and vascular cells to control the supply of oxygen and nutrients in response to neural activity. Cellular elements of the NVU coordinate to establish an anatomical barrier to separate the central nervous system from the milieu of the periphery system, restricting the free movement of substances from the blood to the brain parenchyma and maintaining central nervous system homeostasis. In Alzheimer's disease, amyloid-β deposition impairs the normal functions of NVU cellular elements, thus accelerating the disease progression. Here, we aim to describe the current knowledge of the NVU cellular elements, including endothelial cells, pericytes, astrocytes, and microglia, in regulating the blood-brain barrier integrity and functions in physiology as well as alterations encountered in Alzheimer's disease. Furthermore, the NVU functions as a whole, therefore specific labeling and targeting NVU components in vivo enable us to understand the mechanism mediating cellular communication. We review approaches including commonly used fluorescent dyes, genetic mouse models, and adeno-associated virus vectors for imaging and targeting NVU cellular elements in vivo.

Overexpression of transmembrane TNFα in brain endothelial cells induces schizophrenia-relevant behaviors

Upregulation of genes and coexpression networks related to immune function and inflammation have been repeatedly reported in the brain of individuals with schizophrenia. However, a causal relationship between the abnormal immune/inflammation-related gene expression and schizophrenia has not been determined. We conducted co-expression networks using publicly available RNA-seq data from prefrontal cortex (PFC) and hippocampus (HP) of 64 individuals with schizophrenia and 64 unaffected controls from the SMRI tissue collections. We identified proinflammatory cytokine, transmembrane tumor necrosis factor-α (tmTNFα), as a potential regulator in the module of co-expressed genes that we find related to the immune/inflammation response in endothelial cells (ECs) and/or microglia of the brain of individuals with schizophrenia. The immune/inflammation-related modules associated with schizophrenia and the TNF signaling pathway that regulate the network were replicated in an independent cohort of brain samples from 68 individuals with schizophrenia and 135 unaffected controls. To investigate the association between the overexpression of tmTNFα in brain ECs and schizophrenia-like behaviors, we induced short-term overexpression of the uncleavable form of (uc)-tmTNFα in ECs of mouse brain for 7 weeks. We found schizophrenia-relevant behavioral deficits in these mice, including cognitive impairment, abnormal sensorimotor gating, and sensitization to methamphetamine (METH) induced locomotor activity and METH-induced neurotransmitter levels. These uc-tmTNFα effects were mediated by TNF receptor2 (TNFR2) and induced activation of TNFR2 signaling in astrocytes and neurons. A neuronal module including neurotransmitter signaling pathways was down-regulated in the brain of mice by the short-term overexpression of the gene, while an immune/inflammation-related module was up-regulated in the brain of mice after long-term expression of 22 weeks. Our results indicate that tmTNFα may play a direct role in regulating neurotransmitter signaling pathways that contribute to the clinical features of schizophrenia.

AAV Engineering for Improving Tropism to the Central Nervous System

Adeno-associated virus (AAV) is a non-pathogenic virus that mainly infects primates with the help of adenoviruses. AAV is being widely used as a delivery vector for in vivo gene therapy, as evidenced by five currently approved drugs and more than 255 clinical trials across the world. Due to its relatively low immunogenicity and toxicity, sustained efficacy, and broad tropism, AAV holds great promise for treating many indications, including central nervous system (CNS), ocular, muscular, and liver diseases. However, low delivery efficiency, especially for the CNS due to the blood-brain barrier (BBB), remains a significant challenge for more clinical application of AAV gene therapy. Thus, there is an urgent need for utilizing AAV engineering to discover next-generation capsids with improved properties, e.g., enhanced BBB penetrance, lower immunogenicity, and higher packaging efficiency. AAV engineering methods, including directed evolution, rational design, and in silico design, have been developed, resulting in the discovery of novel capsids (e.g., PhP.B, B10, PAL1A/B/C). In this review, we discuss key studies that identified engineered CNS capsids and/or established methodological improvements. Further, we also discussed important issues that need to be addressed, including cross-species translatability, cell specificity, and modular engineering to improve multiple properties simultaneously.