Stefano Tarantini

Age-related autoregulatory dysfunction and cerebromicrovascular injury in mice with angiotensin II-induced hypertension

Hypertension in the elderly substantially contributes to cerebromicrovascular damage and promotes the development of vascular cognitive impairment. Despite the importance of the myogenic mechanism in cerebromicrovascular protection, it is not well understood how aging affects the functional adaptation of cerebral arteries to high blood pressure. Hypertension was induced in young (3 months) and aged (24 months) C57/BL6 mice by chronic infusion of angiotensin II (AngII). In young hypertensive mice, the range of cerebral blood flow autoregulation was extended to higher pressure values, and the pressure-induced tone of middle cerebral artery (MCA) was increased. In aged hypertensive mice, autoregulation was markedly disrupted, and MCAs did not show adaptive increases in myogenic tone. In young mice, the mechanism of adaptation to hypertension involved upregulation of the 20-HETE (20-hydroxy-5,8,11,14-eicosatetraenoic acid)/transient receptor potential cation channel, subfamily C (TRPC6) pathway and this mechanism was impaired in aged hypertensive mice. Downstream consequences of cerebrovascular autoregulatory dysfunction in aged AngII-induced hypertensive mice included exacerbated disruption of the blood–brain barrier and neuroinflammation (microglia activation and upregulation of proinflammatory cytokines and chemokines), which were associated with impaired hippocampal dependent cognitive function. Collectively, aging impairs autoregulatory protection in the brain of mice with AngII-induced hypertension, potentially exacerbating cerebromicrovascular injury and neuroinflammation.

Resveratrol treatment rescues neurovascular coupling in aged mice: role of improved cerebromicrovascular endothelial function and downregulation of NADPH oxidase

Moment-to-moment adjustment of cerebral blood flow (CBF) to neuronal activity via neurovascular coupling is essential for the maintenance of normal neuronal function. Increased oxidative stress that occurs with aging was shown to impair neurovascular coupling, which likely contributes to a significant age-related decline in higher cortical function, increasing the risk for vascular cognitive impairment. Resveratrol is a polyphenolic compound that exerts significant antiaging protective effects in large vessels, but its effects on the cerebromicrovasculature remain poorly defined. The present study was undertaken to investigate the capacity of resveratrol to improve neurovascular coupling in aging. In aged (24-mo-old) C57BL/6 mice N(ω)-nitro-l-arginine methyl ester-sensitive, nitric oxide-mediated CBF responses to whisker stimulation and to the endothelium-dependent dilator acethylcholine (ACh) were impaired compared with those in young (3-mo-old) mice. Treatment of aged mice with resveratrol rescued neurovascular coupling and ACh-induced responses, which was associated with downregulation of cortical expression of NADPH oxidase and decreased levels of biomarkers of oxidative/nitrative stress (3-nitrotyrosine, 8-isoprostanes). Resveratrol also attenuated age-related increases in reactive oxygen species (ROS) production in cultured cerebromicrovascular endothelial cells (DCF fluorescence, flow cytometry). In conclusion, treatment with resveratrol rescues cortical neurovascular coupling responses to increased neuronal activity in aged mice, likely by restoring cerebromicrovascular endothelial function via downregulation of NADPH oxidase-derived ROS production. Beneficial cerebromicrovascular effects of resveratrol may contribute to its protective effects on cognitive function in aging.

Functional vascular contributions to cognitive impairment and dementia: mechanisms and consequences of cerebral autoregulatory dysfunction, endothelial impairment, and neurovascular uncoupling in aging

Increasing evidence from epidemiological, clinical and experimental studies indicate that age-related cerebromicrovascular dysfunction and microcirculatory damage play critical roles in the pathogenesis of many types of dementia in the elderly, including Alzheimers disease. Understanding and targeting the age-related pathophysiological mechanisms that underlie vascular contributions to cognitive impairment and dementia (VCID) are expected to have a major role in preserving brain health in older individuals. Maintenance of cerebral perfusion, protecting the microcirculation from high pressure-induced damage and moment-to-moment adjustment of regional oxygen and nutrient supply to changes in demand are prerequisites for the prevention of cerebral ischemia and neuronal dysfunction. This overview discusses age-related alterations in three main regulatory paradigms involved in the regulation of cerebral blood flow (CBF): cerebral autoregulation/myogenic constriction, endothelium-dependent vasomotor function, and neurovascular coupling responses responsible for functional hyperemia. The pathophysiological consequences of cerebral microvascular dysregulation in aging are explored, including blood-brain barrier disruption, neuroinflammation, exacerbation of neurodegeneration, development of cerebral microhemorrhages, microvascular rarefaction, and ischemic neuronal dysfunction and damage. Due to the widespread attention that VCID has captured in recent years, the evidence for the causal role of cerebral microvascular dysregulation in cognitive decline is critically examined.

Aging Exacerbates Obesity-induced Cerebromicrovascular Rarefaction, Neurovascular Uncoupling, and Cognitive Decline in Mice

Epidemiological studies show that obesity has deleterious effects on the brain and cognitive function in the elderly population. However, the specific mechanisms through which aging and obesity interact to promote cognitive decline remain unclear. To test the hypothesis that aging exacerbates obesity-induced cerebromicrovascular impairment, we compared young (7 months) and aged (24 months) high-fat diet-fed obese C57BL/6 mice. We found that aging exacerbates the obesity-induced decline in microvascular density both in the hippocampus and in the cortex. The extent of hippocampal microvascular rarefaction and the extent of impairment of hippocampal-dependent cognitive function positively correlate. Aging exacerbates obesity-induced loss of pericyte coverage on cerebral microvessels and alters hippocampal angiogenic gene expression signature, which likely contributes to microvascular rarefaction. Aging also exacerbates obesity-induced oxidative stress and induction of NADPH oxidase and impairs cerebral blood flow responses to whisker stimulation. Collectively, obesity exerts deleterious cerebrovascular effects in aged mice, promoting cerebromicrovascular rarefaction and neurovascular uncoupling. The morphological and functional impairment of the cerebral microvasculature in association with increased blood-brain barrier disruption and neuroinflammation (Tucsek Z, Toth P, Sosnowsk D, et al. Obesity in aging exacerbates blood-brain barrier disruption, neuroinflammation and oxidative stress in the mouse hippocampus: effects on expression of genes involved in beta-amyloid generation and Alzheimers disease. J Gerontol Biol Med Sci. 2013. In press, PMID: 24269929) likely contribute to obesity-induced cognitive decline in aging.

Aging exacerbates hypertension-induced cerebral microhemorrhages in mice: role of resveratrol treatment in vasoprotection

Recent studies demonstrate that aging exacerbates hypertension‐induced cognitive decline, but the specific age‐related mechanisms remain elusive. Cerebral microhemorrhages (CMHs) are associated with rupture of small intracerebral vessels and are thought to progressively impair neuronal function. To determine whether aging exacerbates hypertension‐induced CMHs young (3 months) and aged (24 months) mice were treated with angiotensin II plus L‐NAME. We found that the same level of hypertension leads to significantly earlier onset and increased incidence of CMHs in aged mice than in young mice, as shown by neurological examination, gait analysis, and histological assessment of CMHs in serial brain sections. Hypertension‐induced cerebrovascular oxidative stress and redox‐sensitive activation of matrix metalloproteinases (MMPs) were increased in aging. Treatment of aged mice with resveratrol significantly attenuated hypertension‐induced oxidative stress, inhibited vascular MMP activation, significantly delayed the onset, and reduced the incidence of CMHs. Collectively, aging promotes CMHs in mice likely by exacerbating hypertension‐induced oxidative stress and MMP activation. Therapeutic strategies that reduce microvascular oxidative stress and MMP activation may be useful for the prevention of CMHs, protecting neurocognitive function in high‐risk elderly patients.

Caloric restriction confers persistent anti-oxidative, pro-angiogenic, and anti-inflammatory effects and promotes anti-aging miRNA expression profile in cerebromicrovascular endothelial cells of aged rats

In rodents, moderate caloric restriction (CR) without malnutrition exerts significant cerebrovascular protective effects, improving cortical microvascular density and endothelium-dependent vasodilation, but the underlying cellular mechanisms remain elusive. To elucidate the persisting effects of CR on cerebromicrovascular endothelial cells (CMVECs), primary CMVECs were isolated from young (3 mo old) and aged (24 mo old) ad libitum-fed and aged CR F344xBN rats. We found an age-related increase in cellular and mitochondrial oxidative stress, which is prevented by CR. Expression and transcriptional activity of Nrf2 are both significantly reduced in aged CMVECs, whereas CR prevents age-related Nrf2 dysfunction. Expression of miR-144 was upregulated in aged CMVECs, and overexpression of miR-144 significantly decreased expression of Nrf2 in cells derived from both young animals and aged CR rats. Overexpression of a miR-144 antagomir in aged CMVECs significantly decreases expression of miR-144 and upregulates Nrf2. We found that CR prevents age-related impairment of angiogenic processes, including cell proliferation, adhesion to collagen, and formation of capillary-like structures and inhibits apoptosis in CMVECs. CR also exerts significant anti-inflammatory effects, preventing age-related increases in the transcriptional activity of NF-κB and age-associated pro-inflammatory shift in the endothelial secretome. Characterization of CR-induced changes in miRNA expression suggests that they likely affect several critical functions in endothelial cell homeostasis. The predicted regulatory effects of CR-related differentially expressed miRNAs in aged CMVECs are consistent with the anti-aging endothelial effects of CR observed in vivo. Collectively, we find that CR confers persisting anti-oxidative, pro-angiogenic, and anti-inflammatory cellular effects, preserving a youthful phenotype in rat cerebromicrovascular endothelial cells, suggesting that through these effects CR may improve cerebrovascular function and prevent vascular cognitive impairment.

IGF‐1 deficiency impairs neurovascular coupling in mice: implications for cerebromicrovascular aging

Aging is associated with marked deficiency in circulating IGF‐1, which has been shown to contribute to age‐related cognitive decline. Impairment of moment‐to‐moment adjustment of cerebral blood flow (CBF) via neurovascular coupling is thought to play a critical role in the genesis of age‐related cognitive impairment. To establish the link between IGF‐1 deficiency and cerebromicrovascular impairment, neurovascular coupling mechanisms were studied in a novel mouse model of IGF‐1 deficiency (Igf1f/f‐TBG‐Cre‐AAV8) and accelerated vascular aging. We found that IGF‐1‐deficient mice exhibit neurovascular uncoupling and show a deficit in hippocampal‐dependent spatial memory test, mimicking the aging phenotype. IGF‐1 deficiency significantly impaired cerebromicrovascular endothelial function decreasing NO mediation of neurovascular coupling. IGF‐1 deficiency also impaired glutamate‐mediated CBF responses, likely due to dysregulation of astrocytic expression of metabotropic glutamate receptors and impairing mediation of CBF responses by eicosanoid gliotransmitters. Collectively, we demonstrate that IGF‐1 deficiency promotes cerebromicrovascular dysfunction and neurovascular uncoupling mimicking the aging phenotype, which are likely to contribute to cognitive impairment.

Impaired neurovascular coupling in aging and Alzheimer's disease: Contribution of astrocyte dysfunction and endothelial impairment to cognitive decline

The importance of (micro)vascular contributions to cognitive impairment and dementia (VCID) in aging cannot be overemphasized, and the pathogenesis and prevention of age-related cerebromicrovascular pathologies are a subject of intensive research. In particular, aging impairs the increase in cerebral blood flow triggered by neural activation (termed neurovascular coupling or functional hyperemia), a critical mechanism that matches oxygen and nutrient delivery with the increased demands in active brain regions. From epidemiological, clinical and experimental studies the picture emerges of a complex functional impairment of cerebral microvessels and astrocytes, which likely contribute to neurovascular dysfunction and cognitive decline in aging and in age-related neurodegenerative diseases. This overview discusses age-related alterations in neurovascular coupling responses responsible for impaired functional hyperemia. The mechanisms and consequences of astrocyte dysfunction (including potential alteration of astrocytic endfeet calcium signaling, dysregulation of eicosanoid gliotransmitters and astrocyte energetics) and functional impairment of the microvascular endothelium are explored. Age-related mechanisms (cellular oxidative stress, senescence, circulating IGF-1 deficiency) impairing the function of cells of the neurovascular unit are discussed and the evidence for the causal role of neurovascular uncoupling in cognitive decline is critically examined.

Treatment with the cytochrome P450 ω-hydroxylase inhibitor HET0016 attenuates cerebrovascular inflammation, oxidative stress and improves vasomotor function in spontaneously hypertensive rats.

Hypertension increases cerebrovascular oxidative stress and inflammation and impairs vasomotor function. These pathological alterations lead to dysregulation of cerebral blood flow and exacerbate atherogenesis, increasing the morbidity of ischaemic cerebrovascular diseases and promoting vascular cognitive impairment. We aimed to test the hypothesis that increased production of the arachidonic acid metabolite 20‐hydroxy‐5,8,11,14‐eicosatetraenoic acid (20‐HETE) contributes to hypertension‐induced cerebrovascular alterations.

IGF-1 deficiency impairs cerebral myogenic autoregulation in hypertensive mice

Aging impairs autoregulatory protection in the brain, exacerbating hypertension-induced cerebromicrovascular injury, neuroinflammation, and development of vascular cognitive impairment. Despite the importance of the age-related decline in circulating insulin-like growth factor-1 (IGF-1) levels in cerebrovascular aging, the effects of IGF-1 deficiency on functional adaptation of cerebral arteries to high blood pressure remain elusive. To determine whether IGF-1 deficiency impairs autoregulatory protection, hypertension was induced in control and IGF-1-deficient mice (Igf1 f/f +TBG-iCre-AAV8) by chronic infusion of angiotensin-II. In hypertensive control mice, cerebral blood flow (CBF) autoregulation was extended to higher pressure values and the pressure-induced tone of middle cerebral arteries (MCAs) was increased. In hypertensive IGF-1-deficient mice, autoregulation was markedly disrupted, and MCAs did not show adaptive increases in myogenic tone. In control mice, the mechanism of adaptation to hypertension involved upregulation of TRPC channels in MCAs and this mechanism was impaired in hypertensive IGF-1-deficient mice. Likely downstream consequences of cerebrovascular autoregulatory dysfunction in hypertensive IGF-1-deficient mice included exacerbated disruption of the blood-brain barrier and neuroinflammation (microglia activation and upregulation of proinflammatory cytokines and chemokines), which were associated with impaired hippocampal cognitive function. Collectively, IGF-1 deficiency impairs autoregulatory protection in the brain of hypertensive mice, potentially exacerbating cerebromicrovascular injury and neuroinflammation mimicking the aging phenotype.