Amyloid beta and tau proteins in Alzheimer’s disease and Down syndrome

Alzheimer's disease (AD) is one of the commonest dementing illnesses, affecting about four million people in the USA. Brain extracellular fluid (ECF) is in direct contact with cerebrospinal fluid (CSF) via patent ependymal and pial surfaces along the ventriculo-subarachnoid space, and the molecules entering brain ECF will eventually diffuse into the CSF. The protein composition of CSF is likely to reflect that of the brain intercellular spaces, and therefore the CSF examination provides a way to sample the microenvironment of the brain. The levels are not dependent upon blood-brain barrier dysfunction. It has been suggested that increased levels of CSF tau reflect the neurofibrillary pathology in brain tissue. In a pilot study, CSF p-tau₂₃₁ distinguished between individuals with dementia in Alzheimer's disease (DAD) and those with other neurological disorders, with a sensitivity of 85% and a specificity of 97%. Tau protein has been combined with neuronal thread protein and with the soluble interleukin-6 receptor complex.