Alkaline phosphatase is increased in both brain and plasma in Alzheimer's disease

Tissue non-specific alkaline phosphatase (TNAP) has been shown to promote the neurotoxicity of extracellular tau which contributes to the spread of pathology in Alzheimer's disease (AD).

To investigate changes in TNAP activity in the hippocampus in both sporadic and familial AD, and to examine whether changes in neuronal TNAP are reflected systemically by looking at changes in plasma TNAP activity in AD.

We measured the activity of TNAP in the hippocampus in sporadic AD, familial AD and appropriate age-matched controls, and in an ageing series (age: 25-88 years) of brains. In addition, we measured TNAP activity in plasma from 110 AD and 110 non-demented control participants.

TNAP activity was significantly increased in the hippocampus in sporadic (by 56%; p = 0.038) and familial AD (by 121%; p = 0.042) compared with the age-matched controls. However, there was no correlation of TNAP activity with age. Furthermore, plasma TNAP activity was increased in AD (by 13%; p = 0.018) and inversely correlated with cognitive function (r(s) = -0.211; p = 0.027).

Together, these data indicate that TNAP is increased in both sporadic and familial AD but not in the aged brain, indicating that the increase is likely a consequence of AD-associated changes in the brain. The neuronal change in TNAP is reflected in an increase in plasma TNAP in AD and is inversely correlated with cognitive function.