Solving an Old Dogma: Is it an Arteriole or a Venule?

The Venular Side of Cerebral Amyloid Angiopathy: Proof of Concept of a Neglected Issue

Small vessel diseases (SVD) is an umbrella term including several entities affecting small arteries, arterioles, capillaries, and venules in the brain. One of the most relevant and prevalent SVDs is cerebral amyloid angiopathy (CAA), whose pathological hallmark is the deposition of amyloid fragments in the walls of small cortical and leptomeningeal vessels. CAA frequently coexists with Alzheimer's Disease (AD), and both are associated with cerebrovascular events, cognitive impairment, and dementia. CAA and AD share pathophysiological, histopathological and neuroimaging issues. The venular involvement in both diseases has been neglected, although both animal models and human histopathological studies found a deposition of amyloid beta in cortical venules. This review aimed to summarize the available information about venular involvement in CAA, starting from the biological level with the putative pathomechanisms of cerebral damage, passing through the definition of the peculiar angioarchitecture of the human cortex with the functional organization and consequences of cortical arteriolar and venular occlusion, and ending to the hypothesized links between cortical venular involvement and the main neuroimaging markers of the disease.

Immunolocalization of zinc transporters and metallothioneins reveals links to microvascular morphology and functions

Zinc homeostasis is vital to immune and other organ system functions, yet over a quarter of the world’s population is zinc deficient. Abnormal zinc transport or storage protein expression has been linked to diseases, such as cancer and chronic obstructive pulmonary disorder. Although recent studies indicate a role for zinc regulation in vascular functions and diseases, detailed knowledge of the mechanisms involved remains unknown. This study aimed to assess protein expression and localization of zinc transporters of the SLC39A/ZIP family (ZIPs) and metallothioneins (MTs) in human subcutaneous microvessels and to relate them to morphological features and expression of function-related molecules in the microvasculature. Microvessels in paraffin biopsies of subcutaneous adipose tissues from 14 patients undergoing hernia reconstruction surgery were analysed for 9 ZIPs and 3 MT proteins by MQCM (multifluorescence quantitative confocal microscopy). Zinc regulation proteins detected in human microvasculature included ZIP1, ZIP2, ZIP8, ZIP10, ZIP12, ZIP14 and MT1-3, which showed differential localization among endothelial and smooth muscle cells. ZIP1, ZIP2, ZIP12 and MT3 showed significantly (p < 0.05) increased immunoreactivities, in association with increased microvascular muscularization, and upregulated ET-1, α-SMA and the active form of p38 MAPK (Thr180/Tyr182 phosphorylated, p38 MAPK-P). These findings support roles of the zinc regulation system in microvascular physiology and diseases.

The α-dystrobrevins play a key role in maintaining the structure and function of the extracellular matrix-significance for protein elimination failure arteriopathies

The extracellular matrix (ECM) of the cerebral vasculature provides a pathway for the flow of interstitial fluid (ISF) and solutes out of the brain by intramural periarterial drainage (IPAD). Failure of IPAD leads to protein elimination failure arteriopathies such as cerebral amyloid angiopathy (CAA). The ECM consists of a complex network of glycoproteins and proteoglycans that form distinct basement membranes (BM) around different vascular cell types. Astrocyte endfeet that are localised against the walls of blood vessels are tethered to these BMs by dystrophin associated protein complex (DPC). Alpha-dystrobrevin (α-DB) is a key dystrophin associated protein within perivascular astrocyte endfeet; its deficiency leads to a reduction in other dystrophin associated proteins, loss of AQP4 and altered ECM. In human dementia cohorts there is a positive correlation between dystrobrevin gene expression and CAA. In the present study, we test the hypotheses that (a) the positive correlation between dystrobrevin gene expression and CAA is associated with elevated expression of α-DB at glial-vascular endfeet and (b) a deficiency in α-DB results in changes to the ECM and failure of IPAD. We used human post-mortem brain tissue with different severities of CAA and transgenic α-DB deficient mice. In human post-mortem tissue we observed a significant increase in vascular α-DB with CAA (CAA vrs. Old p < 0.005, CAA vrs. Young p < 0.005). In the mouse model of α-DB deficiency, there was early modifications to vascular ECM (collagen IV and BM thickening) that translated into reduced IPAD efficiency. Our findings highlight the important role of α-DB in maintaining structure and function of ECM, particularly as a pathway for the flow of ISF and solutes out of the brain by IPAD.

NOTCH3 variant position is associated with NOTCH3 aggregation load in CADASIL vasculature

Aims

CADASIL, the most prevalent hereditary cerebral small vessel disease, is caused by cysteine‐altering NOTCH3 variants (NOTCH3^cys) leading to vascular NOTCH3 protein aggregation. It has recently been shown that variants located in one of NOTCH3 protein epidermal growth‐factor like repeat (EGFr) domains 1–6, are associated with a more severe phenotype than variants located in one of the EGFr domains 7–34. The underlying mechanism for this genotype–phenotype correlation is unknown. The aim of this study was to analyse whether NOTCH3^cys variant position is associated with NOTCH3 protein aggregation load.

Methods

We quantified vascular NOTCH3 aggregation in skin biopsies (n = 25) and brain tissue (n = 7) of CADASIL patients with a NOTCH3^cys EGFr 1–6 variant or a EGFr 7–34 variant, using NOTCH3 immunohistochemistry (NOTCH3 score) and ultrastructural analysis of granular osmiophilic material (GOM count). Disease severity was assessed by neuroimaging (lacune count and white matter hyperintensity volume) and disability (modified Rankin scale).

Results

Patients with NOTCH3^cys EGFr 7–34 variants had lower NOTCH3 scores (P = 1.3·10⁻⁵) and lower GOM counts (P = 8.2·10⁻⁵) than patients with NOTCH3^cys EGFr 1–6 variants in skin vessels. A similar trend was observed in brain vasculature. In the EGFr 7–34 group, NOTCH3 aggregation levels were associated with lacune count (P = 0.03) and white matter hyperintensity volume (P = 0.02), but not with disability.

Conclusions

CADASIL patients with an EGFr 7–34 variant have significantly less vascular NOTCH3 aggregation than patients with an EGFr 1–6 variant. This may be one of the factors underlying the difference in disease severity between NOTCH3^cys EGFr 7–34 and EGFr 1–6 variants.

Dissecting the microvascular contributions to diffuse correlation spectroscopy measurements of cerebral hemodynamics using optical coherence tomography angiography

Significance: Diffuse correlation spectroscopy (DCS) is an emerging noninvasive, diffuse optical modality that purportedly enables direct measurements of microvasculature blood flow. Functional optical coherence tomography angiography (OCT-A) can resolve blood flow in vessels as fine as capillaries and thus has the capability to validate key attributes of the DCS signal. Aim: To characterize activity in cortical vasculature within the spatial volume that is probed by DCS and to identify populations of blood vessels that are most representative of the DCS signals. Approach: We performed simultaneous measurements of somatosensory-evoked cerebral blood flow in mice in vivo using both DCS and OCT-A. Results: We resolved sensory-evoked blood flow in the somatosensory cortex with both modalities. Vessels with diameters smaller than 10    μ m featured higher peak flow rates during the initial poststimulus positive increase in flow, whereas larger vessels exhibited considerably larger magnitude of the subsequent undershoot. The simultaneously recorded DCS waveforms correlated most highly with flow in the smallest vessels, yet featured a more prominent undershoot. Conclusions: Our direct, multiscale, multimodal cross-validation measurements of functional blood flow support the assertion that the DCS signal preferentially represents flow in microvasculature. The significantly greater undershoot in DCS, however, suggests a more spatially complex relationship to flow in cortical vasculature during functional activation.

Cerebral Vessels: An Overview of Anatomy, Physiology, and Role in the Drainage of Fluids and Solutes

The cerebral vasculature is made up of highly specialized structures that assure constant brain perfusion necessary to meet the very high demand for oxygen and glucose by neurons and glial cells. A dense, redundant network of arteries is spread over the entire pial surface from which penetrating arteries dive into the cortex to reach the neurovascular units. Besides providing blood to the brain parenchyma, cerebral arteries are key in the drainage of interstitial fluid (ISF) and solutes such as amyloid-beta. This occurs along the basement membranes surrounding vascular smooth muscle cells, toward leptomeningeal arteries and deep cervical lymph nodes. The dense microvasculature is made up of fine capillaries. Capillary walls contain pericytes that have contractile properties and are lined by a highly specialized blood-brain barrier that regulates the entry of solutes and ions and maintains the integrity of the composition of ISF. They are also important for the production of ISF. Capillaries drain into venules that course centrifugally toward the cortex to reach cortical veins and empty into dural venous sinuses. The walls of the venous sinuses are also home to meningeal lymphatic vessels that support the drainage of cerebrospinal fluid, although such pathways are still poorly understood. Damage to macro- and microvasculature will compromise cerebral perfusion, hamper the highly synchronized movement of neurofluids, and affect the drainage of waste products leading to neuronal and glial degeneration. This review will present vascular anatomy, their role in fluid dynamics, and a summary of how their dysfunction can lead to neurodegeneration.

Perivascular macrophages produce type I collagen around cerebral small vessels under prolonged hypertension in rats

Hypertension leads to structural remodeling of cerebral blood vessels, which has been implicated in the pathophysiology of cerebrovascular diseases. The remodeling and progression of arteriolosclerosis under hypertension involve fibrosis along with the production of type I collagen around cerebral arterioles. However, the source and regulatory mechanisms of this collagen production remain elusive. In this study, we examined if perivascular macrophages (PVMs) are involved in collagen production around cerebral small vessels in hypertensive SHRSP/Izm rats. Immunoreactivity for type I collagen around cerebral small vessels in 12-week-old hypertensive rats tended to higher than those in 4-week-old hypertensive and 12-week-old control rats. In ultrastructural analyses using transmission electron microscopy, the substantial deposition of collagen fibers could be observed in the intercellular spaces around PVMs near the arterioles of rats with prolonged hypertension. In situ hybridization analyses revealed that cells positive for mRNA of Col1a1, which comprises type I collagen, were observed near cerebral small vessels. The Col1a1-positive cells around cerebral small vessels were colocalized with immunoreactivity for CD206, a marker for PVMs, but not with those for glial fibrillary acidic protein or desmin, markers for other perivascular cells such as astrocytes and vascular smooth muscle cells. These results demonstrated that enhanced production of type I collagen is observed around cerebral small vessels in rats with prolonged hypertension and Col1a1 is expressed by PVMs, and support the concept that PVMs are involved in collagen production and vascular fibrosis under hypertensive conditions.

A new approach for examining the neurovascular structure with phalloidin and calcitonin gene-related peptide in the rat cranial dura mater

The neurovascular structures in the cranial dura mater have been studied with various histological techniques in the past years. In order to obtain a proper approach to reveal the detailed structures, different labeling methods for the cranial vessels and nerve fibers were tested in this study. Firstly, the labeling characteristics of phalloidin, alpha smooth muscle actin (α-SMA), and CD31 were compared in rat whole-mount cranial dura mater by using fluorescent immunohistochemistry or histochemistry. Secondly, according to their properties, phalloidin and α-SMA were selected to combine with calcitonin gene-related peptide (CGRP) to further demonstrate the cranial neurovascular structure. By these approaches, a three-dimensional map of blood vessels and nerve fibers within the whole-mount rat cranial dura mater was obtained. The results showed that phalloidin, α-SMA, and CD31 were preferably expressed in the wall of cranial vessels, corresponding to the arteriors, venules, and capillaries, respectively. Additionally, CGRP + nerve fibers were clearly demonstrated together with phalloidin + or α-SMA + vessels, forming a delicate neurovascular network in the cranial dura mater. The thick nerve bundles ran closely to the phalloidin + or α-SMA + vessels in parallel pattern, while the thin nerve fibers branched off from the bundles tending to surround the phalloidin + arterioles rather than α-SMA + venules. These findings suggest that phalloidin could be an appropriate biochemical maker to be effectively used together with CGRP for experiments examining the detailed spatial correlation of cranial blood vessels and nerve fibers in a three-dimensional view, which may provide clues for understanding the underlying mechanisms of cranial neurovascular disorders.

Potential Role of Venular Amyloid in Alzheimer's Disease Pathogenesis

Insurmountable evidence has demonstrated a strong association between Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA), along with various other cerebrovascular diseases. One form of CAA, which is the accumulation of amyloid-beta peptides (Aβ) along cerebral vessel walls, impairs perivascular drainage pathways and contributes to cerebrovascular dysfunction in AD. To date, CAA research has been primarily focused on arterial Aβ, while the accumulation of Aβ in veins and venules were to a lesser extent. In this review, we describe preclinical models and clinical studies supporting the presence of venular amyloid and potential downstream pathological mechanisms that affect the cerebrovasculature in AD. Venous collagenosis, impaired cerebrovascular pulsatility, and enlarged perivascular spaces are exacerbated by venular amyloid and increase Aβ deposition, potentially through impaired perivascular clearance. Gaining a comprehensive understanding of the mechanisms involved in venular Aβ deposition and associated pathologies will give insight to how CAA contributes to AD and its association with AD-related cerebrovascular disease. Lastly, we suggest that special consideration should be made to develop Aβ-targeted therapeutics that remove vascular amyloid and address cerebrovascular dysfunction in AD.