Biological markers of vascular damage in Alzheimer’s disease patients

Alzheimer's disease (AD) represents the most common type of dementia, accounting for 50% of the total amount of cognitive impairment, while vascular dementia (VD) accounts for approximately 20% of the cases. AD has traditionally been considered a neurodegenerative condition in which vascular dysfunction plays a marginal role. On the other hand, VD is thought to be caused by a subacute or chronic reduction in cerebral blood flow (CBF) leading to neuronal dysfunction and death. However, it is not clear if these two major causes of dementia also share pathogenetic mechanisms. Many evidences point out a vascular pathogenetic involvement in its etiology. The recent finding that Abeta has also harmful effects on vessels indicates that vascular damage could be involved in the pathogenesis of AD, thus explaining how AD and VD are not always distinct entities but overlap by varying degrees. Abeta peptide, which plays a central role in AD, not only exerts harmful effects on the vessel walls increasing the risk of silent hemorrhagic/ischemic strokes, but it also facilitates the ultrastructural degeneration of the vessels, reducing vessels’ diameters, cerebral blood flow and energetic metabolism. Conversely, vascular damage which results in hypoxia/ischemia, inflammation, microglia activation and oxidative stress, can influence APP processing, modulating the expression of enzymes responsible for Abeta production. These mechanisms have been described in animal models, while few independent observations have been performed in humans. Classical neuropathological markers of AD are: (i) deposits of amyloid β (Abeta) in brain tissue (neuritic plaques), as well as within the wall of cerebral blood vessels; (ii) microglia activation; (iii) dystrophic neuronal processes in proximity and within Abeta plaques; (iv) progressive loss of synapses and neurons; and (v) severe structural damage of cerebral blood vessels. Nonetheless, many vascular risk factors have been also associated to AD, i.e. ApoE-e4 genotype, diabetes and hyperinsulinemia, high systolic blood pressure in midlife to late life and low diastolic blood pressure in late life, smoking, stroke, traumatic brain injury, elevated serum homocysteine (Hcy) levels, hypercholesterolemia and atherosclerosis. Furthermore, there are many evidences of peripheral haemostatic abnormalities, in particular platelets alterations, Von Willebrand Factor and Activated Factor VII, and increased level of thrombomodulin and E-selectin in AD, suggesting that an endothelial dysfunction may be involved in AD pathogenesis. Based on these evidences, a possible hypothesis is that Abeta induces endothelial injury, thus promoting ischemic damage, which may in turn affect APP processing and Abeta production. This reciprocal interaction may provide an explanation to the pathogenetic link between these two conditions. In this contest, elevated plasma levels of Homocysteine (Hcy), also know as Hyperhomocysteinemia (HHcy), is one of the strongest independent risk factors for vascular and cerebrovascular disorders and it has been associated to the risk of develop AD in elderly people. Recently, our group has published evidences of elevated plasma levels of Hcy in AD, correlated with folate deficiencies. Moreover, we have demonstrated that Post Methionine Load (PML) test is able to reveal twice as many HHcy AD subjects with respect to the fasting analysis, suggesting PML as useful test in detecting AD patients who may have the chance of an early folate treatment. Since vascular lesions often coexist with Abeta deposits in AD, and aberrant Abeta deposition in the intima may be pathologically important, it is possible that this phenomena is not only the consequence of the AD-related aberrant APP processing but may represent the early trigger of Amyloid deposition, in response to the primary endothelial damage. Moreover, after Abeta or hypoxia exposure, the endothelium undergoes changes which trigger the inflammatory response, as demonstrated in cerebral small vessel disease where there is histopathological evidence of endothelial cell activation. The increased vascular permeability is one of the features of endothelial cell activation, and it is thought that entry of serum proteins, such as coagulation and/or inflammatory mediators, into the vascular wall and perivascular neural parenchyma may sustain toxic effects. Indeed the blood-brain barrier (BBB) microvasculature, plays a crucial role in the regulation of cerebral blood flow (CBF) and may also play a pivotal role in AD pathogenesis by regulating the entry into brain parenchyma of a plethora of circulating molecules and xenobiotics, also triggering inflammation and oxidative stress. Cerebral endothelium could be of clinical relevance to investigate BBB permeability, indicating early endothelial perturbation as a consequence of hypoxia or Abeta deposition, events involved in inflammatory and oxidative cerebrovascular activity. Indeed, it has been previously demonstrated that proinflammatory cytokines alter the expression and processing of Abeta precursor protein, and fibrillar Abeta itself in turn promotes the production of proinflammatory cytokines by microglial and monocytic cell lines. Microglia is the major component of the intrinsic brain immune system and its pivotal role in cerebral inflammation-like responses could trigger and sustain neurodegenerative events. However, clinical observations on the potential role of inflammation in AD have yielded inconsistent results. Whereas several community-based studies have linked antiinflammatory interventions to a lowered risk of developing AD, a randomized, placebo-controlled clinical trial failed to demonstrate a beneficial effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on the progression of disease. It is noteworthy that, in the brain, perivascular macrophages and microglia that participate in intraparenchymal inflammation are derived from circulating macrophages. Previous studies have reported higher CSF levels of TNF-alpha than serum levels in AD patients, strengthening the hypothesis of a pivotal role of BBB and microglia activity in the pathogenetic mechanisms of AD. Activated microglia may also be involved in mechanisms of impaired glial glutamate uptake and reduced expression of glutamate transporters, or increased free radicals and nitric oxide synthesis in brain parenchyma. Central nervous system is particularly exposed to free radical injury, given its high metal content, which can catalyze the formation of oxygen free radicals, and the relatively low content of antioxidant defenses. Indeed, several studies show markers of oxidative damage (lipid peroxidation, protein oxidation, DNA oxidation and glycosidation markers) in brain areas affected by neurodegenerative disorders. Our group published several works demonstrating a link between oxidative stress and excitotoxicity in AD, and described peripheral markers of these mechanisms, that may be analyzed in patients as possible diagnostic and therapeutic tools. On the other hand, hypoxia and stroke could influence Abeta processing, as demonstrated by the hypoxia-inducible factor1 alpha (HIF-1alpha) regulation of BACE promoter or increased production of Abeta after stroke, which may increase caspase 3 cleavage of the GGA3 protein carrier resulting in decreased degradation of BACE. Studies of the effect of vascular risk factors on Abeta processing could help to elucidate whether vascular disease has only an additive effect on cognitive performance or it is also intrinsic to the pathogenesis of AD. We have recently analyzed some markers of vascular damage, in particular we have demonstrated that mean plasma levels of TF (Tissue Factor) and TFPI (TF Pathway Inhibitor) are both correlated with Hcy and they are significantly higher in AD and MCI patients than in healthy subjects (Piazza 2007). Moreover, the measurement of immunologically defined "circulating endothelial cells" (CECs) has been used to assess vascular integrity and the amount of microparticles (MP) has been reported elevated in a number of conditions where vascular dysfunction, thrombosis and inflammation are relevant. However, the identification of relevant biological markers for the state of brain microvessels in demented patients is still lacking. Such biomarkers, together with known risk factors common to AD and VD, can be used to better understand the involvement of cerebrovascular implications in the pathophysiology of dementia, with possible therapeutic interventions. In conclusion, it is possible that the initial endothelial damage in AD brains can trigger Abeta deposits which, in turn, may fuel monocytes infiltration through damaged BBB and microglia activation. Abeta deposits and inflammation can lead to the production of superoxide radicals, exacerbating endothelial injury. Moreover, all these processes can support previous findings of the generalized peripheral and CNS oxidative stress that typically defines AD.