Tight Junctions of the Neurovascular Unit

Defining Retinal Vasculitis

PURPOSE

To assess the validity of retinal vasculitis as the preferred diagnostic term for multiple conditions.

DESIGN

Perspective.

METHODS

This Perspective is based on expert opinion and review of literature focused on the current nosology and pathology of retinal vasculitis. Interpretation of the subset of intraocular inflammation named "retinal vasculitis" based on fundamental knowledge of the blood-retinal barrier, the neurovascular unit, and pathological and functional responses to a variety of stimuli is provided. Correlation with multimodal imaging and known mechanisms of immunologically mediated disease are discussed.

RESULTS

A search of Medline in early 2024 for the phrase "retinal vasculitis" resulted in 2041 citations encompassing immunologic, genetic, neoplastic, infectious, drug-related, and ischemia-related disorders. Classification schemes and angiographic grading systems are descriptive and do not address pathologic mechanisms adequately, in part due to lack of histologic confirmation. Although optical coherence tomography (OCT) angiography holds promise for better imaging of retinal vascular changes, it does not reveal the key feature of leakage and only partially improves understanding of pathophysiology. Diagnosing catastrophic retinal vascular occlusion after intravitreal injections as a retinal vasculitis is the most recent example of speculative application of the term to complex and rare disorders.

CONCLUSIONS

Retinal vasculitis is a diagnostic term that is over-used and imprecise. Revised nosology should limit the term to primary inflammation of the retinal vasculature itself that results in opening of the blood-retinal barrier with or without retinal vascular occlusions. Pending new histologic or mechanistic evidence, the provisional term of "retinal vascular inflammation" or "retinal vasculopathy" should be used for leakage or occlusion occurring in the context of intraocular inflammation.

Human in vitro blood barrier models: architectures and applications

Blood barriers serve as key points of transport for essential molecules as well as lines of defense to protect against toxins. In vitro modeling of these barriers is common practice in the study of their physiology and related diseases. This review describes a common method of using an adaptable, low cost, semipermeable, suspended membrane to experimentally model three blood barriers in the human body: the blood-brain barrier (BBB), the gut-blood barrier (GBB), and the air-blood barrier (ABB). The GBB and ABB both protect from the outside environment, while the BBB protects the central nervous system from potential neurotoxic agents in the blood. These barriers share several commonalities, including the formation of tight junctions, polarized cellular monolayers, and circulatory system contact. Cell architectures used to mimic barrier anatomy as well as applications to study function, dysfunction, and response provide an overview of the versatility enabled by these cultural systems.

Progressive Age-Associated Blood-Brain Barrier Leak/Dysfunction-Nexus of Neurodegenerative Disease Using MRI Markers to Identify Preclinical Disease and Potential New Targets for Future Treatments

This review article focuses on the upstream pertinent pathophysiology leading to neurodegenerative disease. Specifically, the nexus appears to be blood-brain barrier (BBB) leakiness resulting in a two-prong inflammatory disease spectrum damaging the microvasculature and corrupting protein synthesis and degradation with accumulating misfolded toxic proteins. The suboptimal results of removing misfolded proteins mean a new approach to disease in the preclinical state is required aimed at other targets. Validated noninvasive imaging and serologic biomarkers of early preclinical disease implemented in the high-risk patient cohort along with periodic surveillance once effective treatments are developed will be required. This review discusses the physiology and pathophysiology of the BBB, new MRI imaging techniques identifying the leak, and altered fluid dynamic effects in the preclinical state. The risk factors for disease development, preventative measures, and potential treatment targets are also discussed.

Mechanisms of blood-retinal barrier disruption related to intraocular inflammation and malignancy

Blood-retinal barrier (BRB) disruption is a common accompaniment of intermediate, posterior and panuveitis causing leakage into the retina and macular oedema resulting in vision loss. It is much less common in anterior uveitis or in patients with intraocular lymphoma who may have marked signs of intraocular inflammation. New drugs used for chemotherapy (cytarabine, immune checkpoint inhibitors, BRAF inhibitors, EGFR inhibitors, bispecific anti-EGFR inhibitors, MET receptor inhibitors and Bruton tyrosine kinase inhibitors) can also cause different types of uveitis and BRB disruption. As malignant disease itself can cause uveitis, particularly from breast, lung and gastrointestinal tract cancers, it can be clinically difficult to sort out the cause of BRB disruption. Immunosuppression due to malignant disease and/or chemotherapy can lead to infection which can also cause BRB disruption and intraocular infection. In this paper we address the pathophysiology of BRB disruption related to intraocular inflammation and malignancy, methods for estimating the extent and effect of the disruption and examine why some types of intraocular inflammation and malignancy cause BRB disruption and others do not. Understanding this may help sort and manage these patients, as well as devise future therapeutic approaches.

Regulation of Epithelial and Endothelial Barriers by Molecular Chaperones

The integrity and permeability of epithelial and endothelial barriers depend on the formation of tight junctions, adherens junctions, and a junction-associated cytoskeleton. The establishment of this junction-cytoskeletal module relies on the correct folding and oligomerization of its protein components. Molecular chaperones are known regulators of protein folding and complex formation in different cellular compartments. Mammalian cells possess an elaborate chaperone network consisting of several hundred chaperones and co-chaperones. Only a small part of this network has been linked, however, to the regulation of intercellular adhesions, and the systematic analysis of chaperone functions at epithelial and endothelial barriers is lacking. This review describes the functions and mechanisms of the chaperone-assisted regulation of intercellular junctions. The major focus of this review is on heat shock protein chaperones, their co-chaperones, and chaperonins since these molecules are the focus of the majority of the articles published on the chaperone-mediated control of tissue barriers. This review discusses the roles of chaperones in the regulation of the steady-state integrity of epithelial and vascular barriers as well as the disruption of these barriers by pathogenic factors and extracellular stressors. Since cytoskeletal coupling is essential for junctional integrity and remodeling, chaperone-assisted assembly of the actomyosin cytoskeleton is also discussed.

Recent advances towards overcoming the blood-brain barrier

The blood-brain barrier (BBB) is a protective element of the neurovascular unit (NVU) surrounded by astrocytes, pericytes, extracellular matrix, and the tight junctional complex, which play a fundamental role in brain homeostasis. Due to its impeccable structural architecture, the BBB is referred to as the brain's gatekeeper, a near-impenetrable barrier to therapeutics. This review summarises the significant strides that have been made in the last 5 years towards circumventing the BBB and developing efficient drug delivery systems. Challenges associated with several CNS disorders related to BBB failure and exploitation of this unique NVU component for targeted treatment of brain-related disorders are also discussed.

Engineering choroid plexus-on-a-chip with oscillatory flow for modeling brain metastasis

The human brain choroid plexus (ChP) is a highly organized secretory tissue with a complex vascular system and epithelial layers in the ventricles of the brain. The ChP is the body's principal source of cerebrospinal fluid (CSF); it also functions as a barrier to separate the blood from CSF, because the movement of CSF through the body is pulsatile in nature. Thus far, it has been challenging to recreate the specialized features and dynamics of the ChP in a physiologically relevant microenvironment. In this study, we recapitulated the ChP structure by developing a microfluidic chip in accordance with established design rules. Furthermore, we used image processing and analysis to mimic CSF flow dynamics within a rlcking system; we also used a hydrogel containing laminin to mimic brain extracellular matrix (ECM). Human ChP cells were cultured in the ChP-on-a-chip with in vivo-like CSF dynamic flow and an engineered ECM. The key ChP characteristics of capillaries, the epithelial layer, and secreted components were recreated in the adjusted microenvironment of our human ChP-on-a-chip. The drug screening capabilities of the device were observed through physiologically relevant drug responses from breast cancer cells that had spread in the ChP. ChP immune responses were also recapitulated in this device, as demonstrated by the motility and cytotoxic effects of macrophages, which are the most prevalent immune cells in the ChP. Our human ChP-on-a-chip will facilitate the elucidation of ChP pathophysiology and support the development of therapeutics to treat cancers that have metastasized into the ChP.

Reactive astrocytes associated with prion disease impair the blood brain barrier

BACKGROUND

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

RESULTS

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

CONCLUSIONS

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Decellularised spinal cord matrix manipulates glial niche into repairing phase via serglycin-mediated signalling pathway

Astrocytes are the most abundant and widespread glial cells in the central nervous system. The heterogeneity of astrocytes plays an essential role in spinal cord injury (SCI) repair. Decellularised spinal cord matrix (DSCM) is advantageous for repairing SCI, but little is known regarding the exact mechanisms and niche alterations. Here, we investigated the DSCM regulatory mechanism of glial niche in the neuro-glial-vascular unit using single-cell RNA sequencing. Our single cell sequencing, molecular and biochemical experiments validated that DSCM facilitated the differentiation of neural progenitor cells through increasing the number of immature astrocytes. Upregulation of mesenchyme-related genes, which maintained astrocyte immaturity, causing insensitivity to inflammatory stimuli. Subsequently, we identified serglycin (SRGN) as a functional component of DSCM, which involves inducing CD44-AKT signalling to trigger human spinal cord-derived primary astrocytes (hspASCs) proliferation and upregulation of genes related to epithelial-mesenchymal transition, thus impeding astrocyte maturation. Finally, we verified that SRGN-COLI and DSCM had similar functions in the human primary cell co-culture system to mimic the glia niche. In conclusion, our work revealed that DSCM reverted astrocyte maturation and altered the glia niche into the repairing phase through the SRGN-mediated signalling pathway.

Vascular Signalling

In all vertebrates, closed blood and open lymph circulatory systems are essential for the delivery of nutrients and oxygen to tissues, waste clearance, and immune function [...].

Tight junction disruption and the pathogenesis of the chronic complications of diabetes mellitus: A narrative review

The chronic complications of diabetes mellitus constitute a major public health problem. For example, diabetic eye diseases are the most important cause of blindness, and diabetic nephropathy is the most frequent cause of chronic kidney disease worldwide. The cellular and molecular mechanisms of these chronic complications are still poorly understood, preventing the development of effective treatment strategies. Tight junctions (TJs) are epithelial intercellular junctions located at the most apical region of cell-cell contacts, and their main function is to restrict the passage of molecules through the paracellular space. The TJs consist of over 40 proteins, and the most important are occludin, claudins and the zonula occludens. Accumulating evidence suggests that TJ disruption in different organs, such as the brain, nerves, retina and kidneys, plays a fundamental pathophysiological role in the development of chronic complications. Increased permeability of the blood-brain barrier and the blood-retinal barrier has been demonstrated in diabetic neuropathy, brain injury and diabetic retinopathy. The consequences of TJ disruption on kidney function or progression of kidney disease are currently unknown. In the present review, we highlighted the molecular events that lead to barrier dysfunction in diabetes. Further investigation of the mechanisms underlying TJ disruption is expected to provide new insights into therapeutic approaches to ameliorate the chronic complications of diabetes mellitus.

Reactive astrocytes associated with prion disease impair the blood brain barrier

Background

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

Results

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

Conclusions

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Neurovascular Unit-Derived Extracellular Vesicles: From Their Physiopathological Roles to Their Clinical Applications in Acute Brain Injuries

Extracellular vesicles (EVs) form a heterogeneous group of membrane-enclosed structures secreted by all cell types. EVs export encapsulated materials composed of proteins, lipids, and nucleic acids, making them a key mediator in cell–cell communication. In the context of the neurovascular unit (NVU), a tightly interacting multicellular brain complex, EVs play a role in intercellular communication and in maintaining NVU functionality. In addition, NVU-derived EVs can also impact peripheral tissues by crossing the blood–brain barrier (BBB) to reach the blood stream. As such, EVs have been shown to be involved in the physiopathology of numerous neurological diseases. The presence of NVU-released EVs in the systemic circulation offers an opportunity to discover new diagnostic and prognostic markers for those diseases. This review outlines the most recent studies reporting the role of NVU-derived EVs in physiological and pathological mechanisms of the NVU, focusing on neuroinflammation and neurodegenerative diseases. Then, the clinical application of EVs-containing molecules as biomarkers in acute brain injuries, such as stroke and traumatic brain injuries (TBI), is discussed.

Need for a Paradigm Shift in the Treatment of Ischemic Stroke: The Blood-Brain Barrier

Blood-brain barrier (BBB) integrity is essential to maintaining brain health. Aging-related alterations could lead to chronic progressive leakiness of the BBB, which is directly correlated with cerebrovascular diseases. Indeed, the BBB breakdown during acute ischemic stroke is critical. It remains unclear, however, whether BBB dysfunction is one of the first events that leads to brain disease or a down-stream consequence. This review will focus on the BBB dysfunction associated with cerebrovascular disease. An added difficulty is its association with the deleterious or reparative effect, which depends on the stroke phase. We will first outline the BBB structure and function. Then, we will focus on the spatiotemporal chronic, slow, and progressive BBB alteration related to ischemic stroke. Finally, we will propose a new perspective on preventive therapeutic strategies associated with brain aging based on targeting specific components of the BBB. Understanding BBB age-evolutions will be beneficial for new drug development and the identification of the best performance window times. This could have a direct impact on clinical translation and personalised medicine.

Recurrent de novo mutations in CLDN5 induce an anion-selective blood-brain barrier and alternating hemiplegia

Claudin-5 is the most enriched tight junction protein at the blood–brain barrier. Perturbations in its levels of expression have been observed across numerous neurological and neuropsychiatric conditions; however, pathogenic variants in the coding sequence of the gene have never been reported previously. Here, we report the identification of a novel de novo mutation (c.178G>A) in the CLDN5 gene in two unrelated cases of alternating hemiplegia with microcephaly. This mutation (G60R) lies within the first extracellular loop of claudin-5 and based on protein modelling and sequence alignment, we predicted it would modify claudin-5 to become an anion-selective junctional component as opposed to a purely barrier-forming protein.

Generation of stably transfected cell lines expressing wild-type or G60R claudin-5 showed that the tight junctions could still form in the presence of the G60R mutation but that the barrier against small molecules was clearly attenuated and displayed higher Cl⁻ ion permeability and lower Na⁺ permeability.

While this study strongly suggests that CLDN5 associated alternating hemiplegia is a channelopathy, it is also the first study to identify the conversion of the blood–brain barrier to an anion-selective channel mediated by a dominant acting variant in CLDN5.