Xin-Kang Tong

Specific Subtypes of Cortical GABA Interneurons Contribute to the Neurovascular Coupling Response to Basal Forebrain Stimulation

Neurovascular coupling, or the tight coupling between neuronal activity and regional cerebral blood flow (CBF), seems largely driven by the local processing of incoming afferent signals within the activated area. To test if cortical γ-aminobutyric acid (GABA) interneurons—the local integrators of cortical activity—are involved in this coupling, we stimulated the basalocortical pathway in vivo, monitored cortical CBF, and identified the activated interneurons (c-Fos-immunopositive) and the neuromediators involved in this response. Basal forebrain (BF) stimulation induced ipsilateral increases in CBF and selective activation of layers II to VI somatostatin- and/or neuropeptide Y-containing, as well as layer I GABA interneurons. Nitric oxide synthase interneurons displayed weak bilateral activation, whereas vasoactive intestinal polypeptide- or acetylcholine (ACh)-containing GABA interneurons were not activated. Selective cholinergic deafferentation indicated that ACh released from stimulated BF afferents triggered the CBF response, but the latter was mediated, in part, by the local release of GABA from cholinoceptive cortical interneurons, and through GABA-A receptor-mediated transmission. These data show that activation of specific subsets of GABA interneurons and their GABA-A-mediated effects on neuronal, vascular, and/or astroglial targets are necessary for the full expression of the hemodynamic response to BF stimulation. Further, these findings highlight the importance of understanding the cellular networks and circuitry that underlie hemodynamic signals, as only specific subsets of neurons may be activated by a given stimulus, depending on the afferent inputs they receive and integrate.

Simvastatin improves cerebrovascular function and counters soluble amyloid-beta, inflammation and oxidative stress in aged APP mice.

Cerebrovascular dysfunctions appear to contribute to Alzheimers disease (AD) pathogenesis and the associated cognitive decline. Recently, it has been suggested that statins could be beneficial to AD patients independently from their cholesterol-lowering effects. Using 10 month-old amyloid precursor protein transgenic mice (APP mice), we sought to reverse cerebrovascular, neuronal and memory impairments with simvastatin (20 mg/kg/day, 8 weeks). Simvastatin improved reactivity of cerebral arteries, rescued the blood flow response to neuronal activation, attenuated oxidative stress and inflammation, and reduced cortical soluble amyloid-beta (Abeta) levels and the number of Abeta plaque-related dystrophic neurites. However, at such an advanced stage of the pathology, it failed to reduce Abeta plaque load and normalize cholinergic and memory deficits. These findings demonstrate that low-dose simvastatin treatment in aged APP mice largely salvages cerebrovascular function and has benefits on several AD landmarks, which could explain some of the positive effects of statins reported in AD patients.

GABA neurons provide a rich input to microvessels but not nitric oxide neurons in the rat cerebral cortex: a means for direct regulation of local cerebral blood flow.

Basal forebrain neurons project to microvessels and the somata of nitric oxide (NO) synthase‐containing neurons in the cerebral cortex, and their stimulation results in increases in cortical perfusion. γ‐Aminobutyric acid (GABA) is the second major neurotransmitter synthesized by these neurons and it has also been reported to modify cerebromicrovascular tone. We thus investigated by light and electron microscopy the association of GABA neurons (labeled for glutamic acid decarboxylase [GAD]) with cortical microvessels and/or NO neurons (identified by nicotinamide adenine dinucleotide [NADPH‐D] histochemistry) within the frontoparietal and perirhinal cerebral cortex in the rat. On thick and semithin sections, a high density of GAD puncta was observed, several surrounded intracortical blood vessels and neuronal perikarya. In contrast, NADPH‐D cell somata and proximal dendrites were only occasionally contacted by GAD nerve terminals. Perivascular and perisomatic GAD appositions were identified at the ultrastructural level as large (0.44–0.50 μm2) neuronal varicosities located in the immediate vicinity of, or being directly apposed to, vessels or unstained neuronal cell bodies. In both cortical areas, perivascular GAD terminals were located at about 1 μm from the vessels and were seen to frequently establish junctional contacts (synaptic frequency of 25–40% in single thin sections) with adjacent neuronal but not vascular elements. Ibotenic or quisqualic acid lesion of the substantia innominata did not significantly affect the density of cortical and perivascular GAD terminals, suggesting that they mostly originated locally in the cortex. These results suggest that GABA terminals can interact directly with the microvascular bed and that the somata and proximal dendrites of NO neurons are not a major target for cortical GABA neurotransmission. However, based on the colocalization of GABA and NADPH‐D in a subset of cortical neurons, we suggest that these interneurons could be implicated in the cortical vascular response elicited by stimulation of basal forebrain neurons. J. Comp. Neurol. 421:161–171, 2000.

Oxidative stress and cerebrovascular dysfunction in mouse models of Alzheimer's disease

Several factors have been implicated in Alzheimers disease (AD) but there is no definite conclusion as to the main pathogenic agents. Mutations in the amyloid precursor protein (APP) that lead to increased production of amyloid β peptide (Aβ) are associated with the early‐onset, familial forms of AD. However, in addition to ageing, the most common risk factors for the sporadic, prevalent form of AD are hypertension, hypercholesterolaemia, ischaemic stroke, the ApoE4 allele and diabetes, all characterized by a vascular pathology. In AD, the vascular pathology includes accumulation of Aβ in the vessel wall, vascular fibrosis, and other ultrastructural changes in constituent endothelial and smooth muscle cells. Moreover, the ensuing chronic cerebral hypoperfusion has been proposed as a determinant factor in the accompanying cognitive deficits. In transgenic mice that overexpress mutated forms of the human APP (APP mice), the increased production of Aβ results in vascular oxidative stress and loss of vasodilatory function. The culprit molecule, superoxide, triggers the synthesis of other reactive oxygen species and the sequestration of nitric oxide (NO), thus impairing resting cerebrovascular tone and NO‐dependent dilatations. The Aβ‐induced cerebrovascular dysfunction can be completely abrogated in aged APP mice with antioxidant therapy. In contrast, in mice that overproduce an active form of the cytokine transforming growth factor‐β1 and recapitulate the vascular structural changes seen in AD, antioxidants have no beneficial effect on the accompanying cerebrovascular deficits. This review discusses the beneficial role and limitations of antioxidant therapy in AD cerebrovascular pathology.

Angiotensin II type 1 receptor blocker losartan prevents and rescues cerebrovascular, neuropathological and cognitive deficits in an Alzheimer's disease model.

Angiotensin II (AngII) receptor blockers that bind selectively AngII type 1 (AT1) receptors may protect from Alzheimers disease (AD). We studied the ability of the AT1 receptor antagonist losartan to cure or prevent AD hallmarks in aged (~18months at endpoint, 3months treatment) or adult (~12months at endpoint, 10months treatment) human amyloid precursor protein (APP) transgenic mice. We tested learning and memory with the Morris water maze, and evaluated neurometabolic and neurovascular coupling using [(18)F]fluoro-2-deoxy-D-glucose-PET and laser Doppler flowmetry responses to whisker stimulation. Cerebrovascular reactivity was assessed with on-line videomicroscopy. We measured protein levels of oxidative stress enzymes (superoxide dismutases SOD1, SOD2 and NADPH oxidase subunit p67phox), and quantified soluble and deposited amyloid-β (Aβ) peptide, glial fibrillary acidic protein (GFAP), AngII receptors AT1 and AT2, angiotensin IV receptor AT4, and cortical cholinergic innervation. In aged APP mice, losartan did not improve learning but it consolidated memory acquisition and recall, and rescued neurovascular and neurometabolic coupling and cerebrovascular dilatory capacity. Losartan normalized cerebrovascular p67phox and SOD2 protein levels and up-regulated those of SOD1. Losartan attenuated astrogliosis, normalized AT1 and AT4 receptor levels, but failed to rescue the cholinergic deficit and the Aβ pathology. Given preventively, losartan protected cognitive function, cerebrovascular reactivity, and AT4 receptor levels. Like in aged APP mice, these benefits occurred without a decrease in soluble Aβ species or plaque load. We conclude that losartan exerts potent preventive and restorative effects on AD hallmarks, possibly by mitigating AT1-initiated oxidative stress and normalizing memory-related AT4 receptors.

Cognitive and cerebrovascular improvements following kinin B1 receptor blockade in Alzheimer’s disease mice

BackgroundRecent evidence suggests that the inducible kinin B1 receptor (B1R) contributes to pathogenic neuroinflammation induced by amyloid-beta (Aβ) peptide. The present study aims at identifying the cellular distribution and potentially detrimental role of B1R on cognitive and cerebrovascular functions in a mouse model of Alzheimer’s disease (AD).MethodsTransgenic mice overexpressing a mutated form of the human amyloid precursor protein (APPSwe,Ind, line J20) were treated with a selective and brain penetrant B1R antagonist (SSR240612, 10 mg/kg/day for 5 or 10 weeks) or vehicle. The impact of B1R blockade was measured on i) spatial learning and memory performance in the Morris water maze, ii) cerebral blood flow (CBF) responses to sensory stimulation using laser Doppler flowmetry, and iii) reactivity of isolated cerebral arteries using online videomicroscopy. Aβ burden was quantified by ELISA and immunostaining, while other AD landmarks were measured by western blot and immunohistochemistry.ResultsB1R protein levels were increased in APP mouse hippocampus and, prominently, in reactive astrocytes surrounding Aβ plaques. In APP mice, B1R antagonism with SSR240612 improved spatial learning, memory and normalized protein levels of the memory-related early gene Egr-1 in the dentate gyrus of the hippocampus. B1R antagonism restored sensory-evoked CBF responses, endothelium-dependent dilations, and normalized cerebrovascular protein levels of endothelial nitric oxide synthase and B2R. In addition, SSR240612 reduced (approximately 50%) microglial, but not astroglial, activation, brain levels of soluble Aβ1-42, diffuse and dense-core Aβ plaques, and it increased protein levels of the Aβ brain efflux transporter lipoprotein receptor-related protein-1 in cerebral microvessels.ConclusionThese findings show a selective upregulation of astroglial B1R in the APP mouse brain, and the capacity of the B1R antagonist to abrogate amyloidosis, cerebrovascular and memory deficits. Collectively, these findings provide convincing evidence for a role of B1R in AD pathogenesis.

Transgenic Mice Overexpressing APP and Transforming Growth Factor-β1 Feature Cognitive and Vascular Hallmarks of Alzheimer's Disease

High brain levels of amyloid-β (Aβ) and transforming growth factor-β1 (TGF-β1) have been implicated in the cognitive and cerebrovascular alterations of Alzheimers disease (AD). We sought to investigate the impact of combined increases in Aβ and TGF-β1 on cerebrovascular, neuronal, and mnemonic function using transgenic mice overproducing these peptides (A/T mice). In particular, we measured cerebrovascular reactivity, evoked cerebral blood flow and glucose uptake during brain activation, cholinergic status, and spatial memory, along with cerebrovascular fibrosis, amyloidosis, and astrogliosis, and their evolution with age. An assessment of perfusion and metabolic responses was considered timely, given ongoing efforts for their validation as AD biomarkers. Relative to wild-type littermates, A/T mice displayed an early progressive decline in cerebrovascular dilatory ability, preserved contractility, and reduction in constitutive nitric oxide synthesis that establishes resting vessel tone. Altered levels of vasodilator-synthesizing enzymes and fibrotic proteins, resistance to antioxidant treatment, and unchanged levels of the antioxidant enzyme, superoxide dismutase-2, accompanied these impairments. A/T mice featured deficient neurovascular and neurometabolic coupling to whisker stimulation, cholinergic denervation, cerebral and cerebrovascular Aβ deposition, astrocyte activation, and impaired Morris water maze performance, which gained severity with age. The combined Aβ- and TGF-β1-driven pathology recapitulates salient cerebrovascular, neuronal, and cognitive AD landmarks and yields a versatile model toward highly anticipated diagnostic and therapeutic tools for patients featuring Aβ and TGF-β1 increments.

Intact memory in TGF-β1 transgenic mice featuring chronic cerebrovascular deficit: recovery with pioglitazone

The roles of chronic brain hypoperfusion and transforming growth factor-beta 1 (TGF-β1) in Alzheimers disease (AD) are unresolved. We investigated the interplay between TGF-β1, cerebrovascular function, and cognition using transgenic TGF mice featuring astrocytic TGF-β1 overexpression. We further assessed the impact of short, late therapy in elderly animals with the antioxidant N-acetyl–cysteine (NAC) or the peroxisome proliferator-activated receptor-γ agonist pioglitazone. The latter was also administered to pups as a prophylactic 1-year treatment. Elderly TGF mice featured cerebrovascular dysfunction that was not remedied with NAC. In contrast, pioglitazone prevented or reversed this deficit, and rescued the impaired neurovascular coupling response to whisker stimulation, although it failed to normalize the vascular structure. In aged TGF mice, neuronal and cognitive indices—the stimulus-evoked neurometabolic response, cortical cholinergic innervation, and spatial memory in the Morris water maze—were intact. Our findings show that impaired brain hemodynamics and cerebrovascular function are not accompanied by memory impairment in this model. Conceivably in AD, they constitute aggravating factors against a background of aging and underlying pathology. Our data further highlight the ability of pioglitazone to protect the cerebrovasculature marked by TGF-β1 increase, aging, fibrosis, and antioxidant resistance, thus of high relevance for AD patients.

Brain endothelial TAK1 and NEMO safeguard the neurovascular unit

Ridder et al. show that deletion of NEMO, a component of NF-kB signaling, in brain endothelial cells results in increased cerebral vascular permeability and endothelial cell death, and recapitulates the neurological symptoms observed in the genetic disease incontinentia pigmenti.

Simvastatin Restored Vascular Reactivity, Endothelial Function and Reduced String Vessel Pathology in a Mouse Model of Cerebrovascular Disease:

Cerebrovascular dysfunction seen in Alzheimers disease (AD) and vascular dementia (VaD) is multifaceted and not limited to the amyloid-β (Aβ) pathology. It encompasses structural alterations in the vessel wall, degenerating capillaries (string vessels), vascular fibrosis and calcification, features recapitulated in transgenic mice that overexpress transforming growth factor-β1 (TGF mice). We recently found that simvastatin rescued Aβ-mediated cerebrovascular and cognitive deficits in a transgenic mouse model of AD. However, whether simvastatin can counteract Aβ-independent deficits remains unknown. Here, we evaluated the effects of simvastatin in aged TGF mice on cerebrovascular reactivity and structure, and on cognitive performance. Simvastatin restored baseline levels of nitric oxide (NO), NO-, and KATP channel-mediated dilations and endothelin-1-induced contractions. Simvastatin significantly reduced vasculopathy with arteriogenic remodeling and string vessel pathology in TGF mice. In contrast, simvastatin did not lessen gliosis, and the cerebrovascular levels of pro-fibrotic proteins and calcification markers remained elevated after treatment. The TGF mice displayed subtle cognitive decline that was not affected by simvastatin. Our results show potent benefits of simvastatin on endothelial- and smooth muscle cell-mediated vasomotor responses, endothelial NO synthesis and in preserving capillary integrity. We conclude that simvastatin could be indicated in the treatment of cerebrovascular dysfunction associated with VaD and AD.