Purine-related metabolites and their converting enzymes are altered in frontal, parietal and temporal cortex at early stages of Alzheimer's disease pathology

Abstract

Adenosine, hypoxanthine, xanthine, guanosine and inosine levels were assessed by HPLC, and the activity of related enzymes 5'-nucleotidase (5'-NT), adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) measured in frontal (FC), parietal (PC) and temporal (TC) cortices at different stages of disease progression in Alzheimer's disease (AD) and in age-matched controls. Significantly decreased levels of adenosine, guanosine, hypoxanthine and xanthine, and apparently less inosine, are found in FC from the early stages of AD; PC and TC show an opposing pattern, as adenosine, guanosine and inosine are significantly increased at least at determinate stages of AD whereas hypoxanthine and xanthine levels remain unaltered. 5'-NT is reduced in membranes and cytosol in FC mainly at early stages but not in PC, and only at advanced stages in cytosol in TC. ADA activity is decreased in AD when considered as a whole but increased at early stages in TC. Finally, PNP activity is increased only in TC at early stages. Purine metabolism alterations occur at early stages of AD independently of neurofibrillary tangles and β-amyloid plaques. Alterations are stage dependent and region dependent, the latter showing opposite patterns in FC compared with PC and TC. Adenosine is the most affected of the assessed purines.

Keywords: Alzheimer's disease; adenosine; cerebral cortex; purine metabolism.

Figure 1

Purine metabolic pathway. E‐NTPDases: ecto‐nucleoside tri‐phosphate diphosphohydrolases; PD, ectophosphodiesterase; ADA: adenosine deaminase; SAHH: S‐adenosyl homocysteine hydrolase; SHMT: S‐adenosyl methyl transferase; PNP: purine nucleoside phosphorylase; HGPRT: hypoxanthine–guanine phosphoribosyl transferase [modified from 57].

Figure 2

Adenosine, guanosine, hypoxanthine, xanthine and inosine levels in frontal cortex (FC), parietal cortex (PC) and temporal cortex (TC) in control (C) and cases with AD‐related pathology (AD) at early (AD I–II), intermediate (AD III–IV) and advanced (AD V–VI) stages of NFT pathology. Values are expressed as mean ± SEM. *P < 0.05, **P < 0.01 and ***P < 0.001 according to ANOVA–Fisher's LSD test.

Figure 3

Contribution of alkaline phosphatase to measured 5′‐nucleotidase (5′‐NT) activity. Presence of 100 µM levamisole, a selective alkaline phosphatase inhibitor, during the assay determining 5′‐NT activity did not modify the activity of 5′‐NT neither in membranes nor cytosolic fraction.

Figure 4

5′‐Nucleotidase (5′‐NT) activity in membranes and in cytosol in the frontal cortex (FC), parietal cortex (PC) and temporal cortex (TC) in control (C) and cases with AD‐related pathology (AD) at early (AD I–II), intermediate (AD III–IV) and advanced (AD V–VI) stages of NFT pathology. 5′‐NT activity was determined by fitting a Michaelis–Menten model using a nonlinear regression. Values are expressed as mean ± SEM. *P < 0.05 and **P < 0.01 significantly different from the control value according to ANOVA–Fisher's LSD test.

Figure 5

Correlation analysis between 5′‐nucleotidase activity and postmortem delay values. 5′‐NT activity in membranes and in cytosol from the frontal, parietal and temporal cortices of control and cases with AD‐related pathology (Figure 4). r: Pearson's correlation coefficient. P: P value. Straight line: linear regression fit of 5′‐NT activity value.

Figure 6

Adenosine deaminase analysis. A. ADA activity in the frontal cortex (FC), parietal cortex (PC) and temporal cortex (TC) in control (C) and cases with AD‐related pathology (AD) at early (AD I–II), intermediate (AD III–IV) and advanced (AD V–VI) stages of NFT pathology. Data are mean ± SEM. *P < 0.05 significantly different from the control value, according to ANOVA–Fisher's LSD test. B. Correlation analysis between ADA activity and postmortem delay values. r: Pearson's correlation coefficient. P: P value. Straight line: linear regression fit of ADA activity value.

Figure 7

Purine nucleotide phosphorylase analysis. A. PNP activity in the frontal cortex (FC), parietal cortex (PC) and temporal cortex (TC) in control (C) and cases with AD‐related pathology (AD) at early (AD I–II), intermediate (AD III–IV) and advanced (AD V–VI) stages of NFT pathology. Data are mean ± SEM. *P < 0.05 significantly different from the control value, according to ANOVA–Fisher's LSD test. B. Correlation analysis between PNP activity and postmortem delay values. r: Pearson's correlation coefficient. P: P value. Straight line: linear regression fit of PNP activity value.