τ Is Widely Expressed in Rat Tissues

The microtubule-associated protein tau, a major component of paired helical filaments in Alzheimer's disease, had been thought to be a neuron-specific protein. We investigated various rat tissues using both reverse transcriptase-coupled polymerase chain reaction and immunoblotting. tau was found to be widely expressed in many tissues besides the nervous system: at relatively high levels in the heart, skeletal muscle, lung, kidney, and testis and at low levels in the adrenal gland, stomach, and liver. In terms of the tau isoform expression, tissues fall into three classes: those expressing predominantly small tau, those expressing predominantly big tau, and those expressing both at comparable levels. The phosphorylation state of tau varied among the tissues, as shown by differences in the extents of changes in the reactivities with Tau 1 and electrophoretic mobilities after dephosphorylation. It is notable that tau in many nonneural tissues was highly phosphorylated at Ser396 (according to the numbering of the 441-residue human tau isoform). Thus, tau is widely expressed in rat tissues.