Copathology is an important factor in the multifaceted and heterogeneous course of AD [2]. However, coexistence does not imply causality, nor does it necessarily reflect the existence of a mechanistic link between the two processes. In the present work we addressed the role of VRFs to disentangle their effects on AD-specific neurobiological changes, BBB permeability and cognitive decline according to age of onset.

Age of onset moderates the effects of vascular risk factors on AD biomarkers and blood-brain-barrier permeability

As a composite score, VS reflects a holistic view of how cumulative VRFs affect vascular health and has recently been applied to investigate the relationship between vascular risk and AD biomarkers [24]. In our cohort, some of the VRFs (Hypertension, Smoke, DM type 2 and Dyslipidemia) contributed more to the total VS than others, but overall the entity of their impact was similar across all groups regardless of age of onset.

Considering cumulative vascular risk, we first found that in our cohort VS was not associated with AD neuropathological burden as measured by CSF p-tau/Aβ42 [25], regardless of age of onset. On the other hand, higher VS was associated with higher CSF levels of t-tau, reflecting greater neuronal damage, in both the extreme ages of onset (EOAD and vLOAD) but not in cLOAD.

Other previous studies reported that vascular health has a significant impact on neurodegeneration but not on amyloid deposition [26]. Our results add new and interesting insights on how this impact could be modified by the age of onset, hinting at the presence of different age-dependent susceptibility to neurodegeneration, likely sustained by distinguished drivers of vulnerability or better resilience to vascular injuries in each group. At raw data observation, EOAD and vLOAD seem to share a positive association between VRFs and CSF t-tau levels. In EOAD, this could reflect the failure to bring into play defensive strategies against disease progression, due to accentuated vulnerability. Notably, individuals with EOAD often have shorter relative survival time and steeper cognitive decline [27], supported by a greater pathological burden [28]. Moreover, since AD neurobiological changes start developing up to 20 years before symptoms onset [29], it is interesting to speculate that the earlier clinical debut in EOAD could be at least partly driven by this less resilience to damage accumulation. Indeed, a more damaged brain could be more vulnerable to the presence of aggravating external factors, such as VRFs, or, from an opposite point of view, the advent of VRFs themselves could alter homeostasis and precipitate the AD cascade toward the development of clinical symptoms. Conversely, aging is a well-known risk factor for both AD and VRFs, and findings of a different impact of midlife vs. late-life VRFs on AD biological changes has been reported in older cognitively unimpaired subjects [20, 30]. In this study, we highlight that increasing age-related frailty could have additive damaging effects, boosting vulnerability to VRFs in LOAD, which is reflected in the presence of an association between VS and CSF t-tau only in vLOAD but not in cLOAD.

Moreover, we analyzed the role of those VRFs that had a higher weight within the VS. In both the groups in which VS was positively associated with higher CSF t-tau, EOAD and vLOAD, a concomitant association with Dyslipidemia was always confirmed. On the other hand, Hypertension was additionally linked with higher CSF t-tau only in EOAD, while Smoke had a detrimental role only in vLOAD. Of note, even though the effects of single VRFs seems to play out differently in different clinical contexts, their potential relationship with CSF t-tau is always captured by a background association with VS. Thus, while a more granular analysis could help to verify and to dissect the role of each individual VRF, VS could be trusted as a reliable tool to seize the cumulative effect of VRFs.

Our study also addressed the influence of age of onset on the relationship between VRFs and BBB integrity [31]. Specifically, VS does not seem to influence Qalb in either EOAD or vLOAD, but higher VS was linked with higher Qalb values in both All AD and in cLOAD, with this association being mainly supported by Hypertension and DM type 2 among the individual VRFs. Moreover, our mediation analysis in all AD shows that BBB permeability plays an indirect mediating effect in the relationship between VRFs and CSF t-tau, that is also retrievable as a trend selectively in the cLOAD subgroups while being absent in EOAD and vLOAD. This may indicate that the lack of association between vascular risk and t-tau in cLOAD could partially be due to a potential uncoupling mediated by the modulation of BBB integrity. Indeed, solid data demonstrate that VRFs and vascular injury can alter BBB integrity and Qalb values during the course of AD [15]. At the same time, previous literature shows that BBB permeability affects CSF t-tau levels via an inverse relationship [32, 33], so that higher Qalb values correspond to lower CSF t-tau and this finding is also confirmed in our own results [see Fig. 2]. Thus, while VRFs are able to directly support the progression of neurodegeneration in EOAD and vLOAD, it is possible to speculate the existence of some degree of resilient response to vascular stress in cLOAD, involving the modulation of BBB permeability.

Cognitive decline is influenced by blood-brain-barrier and total tau but not by vascular risk factors

The longitudinal analysis showed the absence of a statistical association between individual vascular risk and worsening cognitive functions in our cohort of AD patients. While this finding is supported by other observational studies in the literature [34], to our knowledge, this is the first study to consider patients with a biologically based diagnosis of AD. Previous research reported differential effects of CVP and of some risk factors on the cognitive trajectories of EOAD and LOAD [30, 35, 36] supporting the use of a stratification based on the age of onset to identify patients with different vulnerability to aggravating factors. Our results highlight that cumulative VRFs do not impact the rate of cognitive decline regardless of age of onset. However, it is not possible to exclude that some specific factors, e.g., hypertension or diabetes, could have specific repercussions in some categories of patients, and further studies are needed to corroborate this hypothesis.

On the other hand, our results highlight that both higher CSF t-tau and increased BBB permeability are associated with higher rates of cognitive decline when considering the whole AD sample [14, 37]. Nevertheless, by stratifying patients into subgroups according to age of onset we observed differences in the extent to which cognitive decline is affected by the variables in our model, as well as in the type of associated variables. First, our regression model showed a higher goodness-of-fit in EOAD and vLOAD than in cLOAD, accounting respectively for 57% and 44% of ΔMMSE variability vs. 10% in the latter. Second, we retrieved different apparent vulnerability to risk factors, since the association with CSF t-tau is observed, again, selectively in EOAD and vLOAD, while higher BBB permeability seems to be the only determinant of cognitive worsening in cLOAD.

The relevance of BBB permeability in determining cognitive decline is thoroughly reported, despite the mechanisms lying behind this relationship are still not entirely cleared yet [13]. Indeed, the regulation of BBB permeability seems to be engaged by complex mechanisms that can favor cognitive worsening, including the development of brain capillary damage in the hippocampus, irrespective of amyloid and tau pathology [12]. At the same time, under certain circumstances, BBB modulation has been associated with regulatory or even homeostatic effects in response to disease progression [32, 38].

Our findings shed light on a new element of complexity related to the age of onset, as the importance of BBB permeability appears to be particularly significant in cLOAD. In this subgroup higher Qalb values, which increase in response to VRFs, are associated with steeper cognitive decline but also with lower levels of CSF t-tau. However, this ambivalent and eluding role of BBB integrity seems to be downsized by the limited effects of Qalb on ΔMMSE in this cohort, as shown by low goodness-of-fit in our regression analysis. Thus, it is possible that a harmful increase in Qalb could be an acceptable cost to be paid to mitigate neurodegeneration. In contrast, this buffer-like effect seems less effective in categories such as EOAD and vLOAD, in which compensatory events, including BBB modulation, could be hindered by faster degeneration and aging. Notably, in these subgroups, VRFs directly impact CSF t-tau levels, which in turn weigh on clinical course.

In summary, these results show that cumulative vascular risk does not seem to directly impact cognitive decline in AD, but rather, it could support the progression of neurodegeneration in both younger and very old patients with AD. Instead, in patients debuting in the classic age range – 65 to 75 - the effects of VRFs seem to induce increased BBB permeability, which is a determinant of cognitive worsening while also partially acting as a buffer against neurodegeneration.

Limitations

This study has limitations, such as the lack of a more detailed neuropsychological follow-up. Indeed, previous literature reported summative effects of amyloid plaques and CVP on longitudinal executive functions [39], and expanding our analysis to explore this matter further will be important. Also, verifying the presence of these associations on more selected groups of patients, such as carriers of APOE ε3 over ε4 genotype, and exploring the differential effects of strictly vascular (i.e., hypertension, hypercholesterolemia) and more metabolic (i.e., diabetes, BMI) risk factors could be relevant [40]. Moreover, the paucity of our sample size– especially for some of the subgroups in the longitudinal study (i.e. EOAD and vLOAD) – did not allow us to fully address the impact of each individual VRFs on cognitive decline.

In addition, the effects of other vascular changes, such as white matter hyperintensities, lacunes, and microhemorrhages, have not been discussed in this study. Widening the analysis to a more extended cohort including multimodal measurements of neurodegenerative processes, such as neurofilament light chains or MRI measurements, will be crucial both to reinforce and expand our findings.