Colocalization and fluorescence resonance energy transfer between cdk5 and AT8 suggests a close association in pre-neurofibrillary tangles and neurofibrillary tangles

Cyclin-dependent kinase 5 (cdk5) is a serine/threonine kinase that, when activated, induces neurite outgrowth. Recent in vitro studies have shown that cdk5 phosphorylates tau at serine 199, serine 202, and threonine 205 and that p25, an activator of cdk5, is increased in Alzheimer disease (AD). Since tau is hyperphosphorylated at these sites in neurofibrillary tangles, we examined brain tissue from patients with AD and normal elderly control cases to determine whether cdk5 and these phosphoepitopes colocalize in neurofibrillary tangles. Adjacent temporal lobe sections were double immunostained with a polyclonal anti-cdk5 and monoclonal AT8 (which recognizes phosphorylated serine 199, serine 202, and threonine 205 in tau) antibodies. A subset of AT8 phosphotau-positive neurons was immunoreactive for cdk5 in entorhinal (area 28) and perirhinal (area 35) cortices and CA1 of the hippocampus. We assessed the ratio of cdk5-positive cells to AT8-positive cells and found that there is a higher degree of colocalization in pre-neurofibrillary tangles as opposed to intraneuronal and extraneuronal neurofibrillary tangles. We further examined colocalization using fluorescence resonance energy transfer. This suggests a close, stable intermolecular association between cdk5 and phosphorylated tau, consistent with phosphorylation of tau by cdk5 in AD brain.