Of the 77 screened studies, 13 were selected, scrutinized, and included in this review. Several studies determined the potential utility of Aβ42 and tau as salivary biomarkers, but other compounds including acetylcholinesterase, lactoferrin, trehalose, and metabolites were investigated as well.
Salivary acetylcholinesterase (AchE) levels were analyzed in two studies. Ellman’s colorimetric method was implemented for both of the studies. Bakhtiari et al. tested saliva samples from 15 AD subjects and 15 control subjects. Higher levels of AchE was reported, but statistical significance was not established. Sayer et al. tested saliva samples from 47 volunteers (22 AD cases, 14 AD nonresponder cases, and 11 control cases). They found an overall negative correlation between age and AchE levels, as the r-value was -0.768 (with p<0.001). It was also reported that AD subjects had 73% lower levels of AchE in comparison to control subjects (with p<0.005) .
Salivary Aβ42 levels were analyzed in four studies, and all of them utilized an enzyme-linked immunosorbent-type assay (ELISA) with the exception of one. Bermejo-Pareja et al. tested 126 saliva samples from both AD and control cases, in addition to 51 saliva samples from Parkinson’s patients. They concluded that salivary Aβ42 levels were significantly greater in patients suffering from mild to moderate AD, but not for patients with severe AD. Their results did not conclude a significant difference between Parkinson’s patients and controls. Lee et al. analyzed the expression of Aβ42 in both saliva and other tissues. 37 volunteers participated in the study, including 27 non-AD and 7 AD cases. They reported a mean of 22.060.41 pg/mL of salivary Aβ42 for the non-AD cases and a mean of 59.076.33 pg/mL for the AD cases. Kim et al. utilized an immunoassay containing nanobeads to detect salivary Aβ42 levels for 45 individuals (28 AD cases and 17 normal controls). Their results concluded higher levels of salivary Aβ42 for the AD cases vs. control cases, but their study did not have a p-value. Sabbagh et al. analyzed salivary Aβ42 levels from 15 AD patients and 7 normal controls. They reported a mean of 21.1 0.3 pg/mL for the normal controls and a mean of 51.71.6 pg/mL for AD cases, with p<0.001.
Salivary tau levels were analyzed in three studies. Ashton et al. tested 213 saliva samples from both AD and control cases, in addition to 68 saliva samples from individuals with aMCI. This study used the single-molecule array (SIMOA) technique to analyze total tau levels. Increased t-tau levels in AD patients were observed, but statistical significance was not established. Shi et al. utilized an ELISA to analyze p-tau and t-tau levels. 59 volunteers participated in this study, which included 21 AD cases and 38 control cases. Increased p-tau/t-tau ratio levels were reported for AD patients, with p<0.05. Pekeles et al. obtained unstimulated saliva in order to analyze the p-tau/t-tau ratio at different phosphorylation sites (S396,S400,S404,T403,T404). This study implemented the Western Blot analysis method to quantify their findings. 337 volunteers participated throughout the two clinical studies conducted by Pekeles et al., including 87 AD subjects and 167 control subjects. Their first study included 55 aMCI subjects as well, and their second study included 16 FTD subjects and an additional 12 neurological patients that did not suffer from dementia. Their findings indicated a significantly higher p-tau/t-tau ratio at the S396 site for AD patients in comparison to the elderly control individuals. However, they reported no correlation between elevated salivary tau levels and both CSF tau and hippocampal volume. There was also significant variation for salivary tau levels in AD subjects, which may pose a limitation towards implementation of tau as a legitimate AD biomarker.
Trehalose and Lactoferrin
Both trehalose and lactoferrin levels in saliva were analyzed in two studies. Lau et al. utilized an extended gate ion-sensitive field-effect transistor (EG-ISFET) biosensor to analyze salivary trehalose. 60 saliva samples were tested, including 20 AD subjects, 20 PD subjects, and 20 control subjects. Higher salivary trehalose levels were found in the AD subjects, but statistical significance was not established. Carro et al. used an ELISA to detect salivary lactoferrin levels. The objective was to determine if decreased lactoferrin levels could serve as an indicator of AD. 365 individuals participated throughout the two clinical studies conducted by Carro et al., including 116 AD subjects, 59 aMCI subjects, and 131 control subjects. Their first study also included 59 aMCI subjects. Mass spectrometry was implemented to confirm that this protein could be detected in saliva before further experimentation. This study also analyzed salivary lactoferrin levels for aMCI and PD subjects. Carro et al. concluded (with p<0.001) significantly lower levels for both AD and aMCI subjects in comparison with the control subjects, but PD subjects had significantly higher levels in comparison with control subjects. 7.43 μg/mL was the established cutoff value between AD/MCI subjects and controls in this study.
Salivary metabolites were analyzed in two studies, which totaled 285 AD subjects, 35 aMCI subjects, and 263 control subjects. Huan et al. used liquid chromatography mass spectrometry (LC-MS) to assess the following metabolites: alanylphenylalanine, aminobytyric acid + H2, amino-dihydroxybenzene, choline-cytidine, glucosyl-galactosyl-hydroxylysine * (H2O), histidylphenylalanine, methylguanosine, phenylalanylphenylalanine, phenylalanylproline, and urocanic acid. Their work featured two clinical studies to further confirm their findings. Between AD and control subjects, there was a significant difference for the following metabolites (with p<0.01): choline-cytidine, histidylphenylalanine, methylguanosine, phenylalanylphenylalanine, phenylalanylproline, and urocanic acid. Between AD and aMCI subjects, there was a significant difference for the following metabolites (with p<0.01): alanylphenylalanine, aminobytyric acid + H2, amino-dihydroxybenzene, glucosyl-galactosyl-hydroxylysine * (H2O), and phenylalanylproline. Q. Liang et al. implemented ultraperformance liquid chromatography mass spectrometry (UPLC-MS) to assess the following metabolites: inosine, ornithine, phenyllactic acid, and spinganine-1-phosphate. They concluded significantly elevated levels of spinganine-1-phosphate and ornithine for AD subjects in comparison to control subjects and significantly lower levels of inosine for AD subjects in comparison to control subjects (with p<0.01).