EIs like tacrine, galantamine, donepezil, rivastigmine and metrifonate, in the hopes of ameliorating the cholinergic deficit in the AD brain [416]. These drugs have been associated with the slowing of cognitive deterioration, but effects are quite heterogeneous. Donepezil, a common AChEI in the treatment of AD, has been shown to aid in the preservation of blood flow in several vulnerable brain regions in mild-to-moderate AD, following one year of treatment [417]. In a SPECT follow-up study of mild-to-moderate AD patients undergoing long-term AChEI therapy, it was observed that patients who saw clinical benefit from AChEI treatment exhibited stabilized perfusion profiles, whereas those who were resistant to AChEI therapy displayed perfusion deficits characteristic of AD [418]. This would seem to suggest that the preservation of cholinergic transmission in some patients has positive outcomes for perfusion and this may potentially contribute to improved outcomes. Rivastigmine was also reported to improve CBF in some brain regions in AD patients, in particular in the temporal cortex [419]. Recently, AChEIs were also shown to improve frontal lobe perfusion in AD patients following a year of treatment [420]. A study of the AChEI velnacrine revealed that a single dose was sufficient to improve regional perfusion and metabolism in AD patients and this was associated with slightly improved word recall [421]. Thus, the cognitive benefits provided by AChEI treatment in AD may be partially mediated by improved cerebral perfusion. Of course, AChEIs are not disease-modifying, as the loss of cholinergic innervation eventually becomes too great to compensate for. However, it should be noted that AChEIs are often prescribed following the clinical manifestation of the disorder, when pathology may be advanced. As discussed earlier, perfusion deficits may begin years to decades before the onset of clinical symptoms and the deleterious effects of these changes may guide the brain in a pathological direction. In addition, predisposing factors like family history of AD and APOE genotype may cause measurable changes in perfusion prior to the development of AD. The preservation of CBF and metabolism in the pre-AD brain may be of benefit in the prevention of metabolic and other deficits associated with reduced perfusion. Screening for AD risk factors in combination with diagnostic imaging and therapies to modulate CBF may provide some benefit at this stage, but this requires confirmation. In reality, such an early therapeutic intervention would depend on reliable screening techniques and the prescription of drugs to otherwise healthy individuals, which is impractical in the absence of factors associated with high individual risk. It is currently unclear whether very early interventions to prevent vascular perfusion deficits would be disease-modifying, and if not, whether they would provide any benefit in slowing symptomatic onset.
The therapeutic targeting of defective Aβ clearance mechanisms at the BBB may serve the dual purpose of reducing parenchymal Aβ load while reducing Aβ-associated pathology at the BBB including CAA and Aβ-mediated vascular inflammation. LRP1 is one of the most important mediators of Aβ clearance across the BBB. As discussed, LRP1 expression is significantly affected in the AD cerebral vasculature -in endothelial cells, vSMCs and pericytes. LRP1 expression also declines with age [422] and as described, APOE genotype plays a major role in the heterogeneities associated with LRP1 deficits in the AD vasculature. Thus, the rescue of LRP1 may potentially represent a novel therapeutic strategy in preventing Aβ build-up and deposition in early AD or in at-risk individuals. Alternatively, the inhibition of RAGE on endothelial cells may reduce the association of Aβ with endothelial cells and the entry of Aβ into the brain. It is currently unknown whether LRP1 augmentation is possible in the human cerebral vasculature. Pharmacological manipulation may involve the use of statins, which appear to increase LRP1 expression and amyloid clearance in in vitro BBB models [423] and to upregulate hepatic LRP1 in rats [424]. There is some evidence that statins may have beneficial effects in terms of lowering AD risk and improving cognition [425]. However, several clinical trials have failed to demonstrate efficacy and this is thus a controversial area in AD therapeutics [426]. Still, more research is required into the specific effects of statins on vascular LRP1 expression in vivo. LRP1 upregulation could also be a viable goal for gene therapy [427] and this is an area that is currently unexplored. Finally, as mentioned earlier, it has been shown that sLRP is also reduced in AD patients, potentially contributing to an increase in plasma Aβ and greater Aβ uptake into the brain. APP +/sw mice treated with recombinant LRP1 ligand-binding domain IV (LRP-IV) displayed a significant reduction in cortical and hippocampal Aβ load, both vascular and parenchymal, as well as free Aβ [256]. It has thus been suggested that sLRP supplementation or replacement may reduce the BBB uptake of Aβ through endothelial RAGE, which is significantly upregulated in the AD microvasculature. Similarly, soluble RAGE (sRAGE) may also serve a similar function. RAGE inhibitors have reached clinical trial and have been tested in animal models, but the results of these studies have been mixed regarding efficacy and safety [428]. Another approach could involve the modulation of enzymatic amyloid degradation pathways in the AD brain. Neprilysin, a key enzyme in brain Aβ degradation pathways, has received considerable interest in recent years as a regulator of brain Aβ load [429]. Neprilysin is downregulated in the AD cortex [430] and in the cerebral vasculature in particular, likely contributing to deficits in amyloid degradation within vSMCs [263]. Interestingly, the downregulation of neprilysin in the AD brain appears to begin at an early stage. Neprilysin activity is reduced in the blood serum of patients with MCI [431] and neprilysin levels were reported to decline in the CSF of patients with early AD and amnestic MCI progressing to AD [432]. Neprilysin polymorphisms have been associated with the appearance of CAA and are correlated with its severity [433]. Thus, this could represent a target for intervention in the prevention or reversal of Aβ pathology in early AD. Genetic modulation of neprilysin expression may be a viable route to achieve this. It has been demonstrated in neprilysin-deficient SH-SY5Y neuroblastoma cells that histone deacetylase (HDAC) inhibitors like valproic acid can induce the transcriptional upregulation of neprilysin [434] and valproic acid can increase neprilysin expression and reduce plaque load in APP/PS1 mice [435]. Although HDAC inhibitors would not be selective for cell type, they could aid in the preservation of neprilysin activity in vascular cells. However, it is important to note that HDACs are involved in the transcriptional regulation of a large number of other important genes. Future research could help to identify more specific regulators of neprilysin expression. Other pharmacological approaches may also be useful in upregulating neprilysin expression in cerebral vascular cells, but current approaches are not targeted towards the vasculature. For example, somatostatin receptor 4 (SSTR-4) agonists have been shown to upregulate neprilysin expression in APP/PS1 mice and reduce soluble Aβ 1-42 load [436]. Brain endothelial cells express all five subtypes of SSTRs and the vascular targeting of these receptors might be an option. Several other neprilysin-upregulating pharmacological treatments have shown similar effects on Aβ deposition in AD mouse models and other enzymes involved in amyloid degradation may potentially be targeted in a similar manner [437].
It has also been suggested that some pro-angiogenic treatments may be of benefit to AD vascular dysfunction, given the vessel degeneration that has been reported in the AD cortex. Previously, we discussed how VEGF expression is downregulated in the AD cortex and the pathological consequences of this. VEGF supplementation has been suggested as a way to rescue vascular deficits associated with impaired vascular proliferation. AD transgenic mice treated with VEGF display reductions in Aβ-induced endothelial cell apoptosis, stimulated endothelial proliferation, improved cognitive performance and reduced Aβ and p-tau load [152][153][154]. This may point to VEGF supplementation as a route by which vascular and other deficits may be treated in AD. Aβ-induced angiogenesis has been reported previously, as described earlier in this review. Given the role of aberrant angiogenesis in the remodeling of the vascular system and weakening of the BBB, as well as its likely contribution to perfusion deficits, strategies that prevent angiogenesis may theoretically have therapeutic benefit in AD. As described earlier, Aβ immunization in the Tg2576 AD mouse model has been shown to dramatically reduce hypervascularity and halt aberrant neoangiogenesis, likely through the abrogation of Aβ-mediated angiogenic pathways [303]. Indeed, Aβ immunization dramatically reduced Aβ plaque load, reduced the occurrence of activated microglia and improved vascular barrier function [302]. Thus, therapies that aim to directly reduce amyloid load in the AD brain may also be useful in the prevention of angiogenesis-related BBB deficits, as well as other vascular pathologies like CAA, Aβ-mediated vascular cell death and Aβ-mediated vasoactive effects. Several early Aβ immunization trials in AD patients were halted due to severe side-effects in some patients, including meningoencephalitis and intracerebral hemorrhage, but there is still interest in this therapeutic route and there are currently several such therapies in development and in clinical trials [438].
Aβ immunotherapy with anti-Aβ monoclonal antibodies (mABs) has emerged as a promising strategy in the reduction of brain Aβ levels without the severe side effects associated with Aβ vaccines-a few such anti-Aβ mAB immunotherapies are in clinical trial [439]. However, several recent trials have been discontinued after failing to meet primary endpoints. A murine homologue of aducanumab, a fully human anti-Aβ mAB, was shown to bind and reduce both soluble oligomeric and insoluble fibrillary