Biomarkers for AD are highly needed in clinic, especially those based on samples of easy collection, such as blood (5, 10). Currently, validated AD biomarkers are represented by neuroimage measures and quantification of the βA peptide, t-tau and p-tau derived from CSF, both requiring specific equipment or invasive procedures, respectively. Attempts to validate the CSF biomarkers were also made in blood, although demanding high-performance analytical tools for their detection (24). The interassay variability and inconsistency of βA measurements in plasma are main factors that impair the interpretation of results and represent major obstacles to their clinical use (25). However, recently promising results have demonstrated that blood p-tau181 is capable to predict tau and βA pathologies and to differentiate AD from other neurodegenerative disorders (26), hence supporting this tissue as a useful source for AD biomarker investigations, aiming to develop simple, accessible, and scalable tests for screening and diagnosis of AD.
In previous studies, we and others have shown that levels of membrane-bound ADAM10 are reduced in platelets of patients with AD compared to cognitively healthy individuals (15, 16, 18) and that this reduction correlated with patients’ cognitive performance, as measured by the CDT (27) or MMSE (14) scores. Moreover, levels and platelet ADAM10 activity were shown to be increased throughout cognitively healthy aging, pointing to the possibility that ADAM10 might contribute to or is a prerequisite for cognitively healthy aging (28). On the other hand, ADAM plasma levels were found to be increased as early as in patients with mild cognitive impairment (MCI), as well as in AD, compared to healthy controls (19). We hypothesized that these higher plasmatic ADAM10 levels found in MCI and AD patients represent less active protein bound at the platelet’s membrane exerting the sheddase activity. This could also be the case of neuronal ADAM10, where inactive forms can be cleaved from the membrane and released in the CSF by other proteins.
In agreement with this hypothesis, ADAM10 itself can undergo shedding and be extracellularly released by other proteins from the ADAM family, ADAM9 and 15 (29), which can be the source of the plasmatic detection of this protein. In addition, recent findings of our group have demonstrated that in plasma and CSF samples of both healthy and AD patients, ADAM10 is unable to cleave a fluorogenic substrate, whereas in whole lysates of platelets and SH-SY5Y neuroblastoma cells, the protein is active (19).
The requirement of a membrane-bound form for ADAM10 activity was further highly supported by findings of a study showing that only the active form of this metalloproteinase is expressed at the surface of different cell types, including leukocytes derived from peripheral blood (30). Moreover, the negatively charged phospholipid phosphatidylserine (PS) translocation to the outer membrane leaflet is pivotal for ADAM10 to exert its sheddase function (31).
In previous studies, we demonstrated that the levels of ADAM10 in platelets had sensitivity and specificity of 80 and 91% respectively, to identify AD patients versus controls matched by sex and age. (14). These experiments were performed in platelets, where we have shown that the protein is active, as it is bound to the membrane. When considering plasmatic ADAM10, the protein achieved 72% sensitivity and 100% specificity, at the cut-off > 1765 pg/mL, to correctly differentiate among healthy controls versus MCI and AD patients (19).
Here, we used different models to investigate whether the plasmatic levels of ADAM10 would be efficient to predict cognitive declines in older adults after a 3-year follow-up period. We showed that the increase in ADAM10 plasma levels influences the decrease of the MMSE score values in the follow-up, and this seems to be more significant in those with normal MMSE at baseline, therefore proving that ADAM10 plasma levels can be a predictor of cognitive decline.
A systematic review and meta-analysis found six blood-based AD candidate biomarkers from different proteomic studies that exhibited a consistent pattern of regulation in three or more independent cohorts, namely alpha-2-macroglobulin (α2M), pancreatic polypeptide (PP), apolipoprotein A-1 (ApoA-1), afamin, insulin growth factor binding protein-2 (IGFBP-2) and fibrinogen-γ-chain (8). Most of these biomarkers are related to systemic inflammatory responses rather than with AD pathophysiology itself. This can bring into question the fact that inflammation per se is a response already found in several age-related diseases, common throughout aging.
It is important to highlight that MMSE is a screening tool for cognitive impairment that detects losses in the evolutionary follow-up of dementias (22). However, in some populations, individuals with lower educational levels perform worse than individuals from countries with high levels of education, but still have no cognitive decline. Regarding this, MMSE cut-offs were validated for each population, including the Brazilian one (23, 32). Hence, the results found here may not represent the general population and should be adapted for different specificities, such as the education level.
Other limitations of this work include the evaluation of a single AD blood biomarker candidate, instead of a panel or a signature that would be more representative of the longitudinal changes in cognition. Moreover, a lack of a complete battery including the application of a diverse set of instruments does not allow a detailed cognitive evaluation of the participants. Yet, this is the first longitudinal study investigating the effects of plasmatic ADAM10 level changes on cognition.