Anti-human placental antigen complex X-P2 (hPAX-P2) anti-serum recognizes C-terminus of huntingtin-associated protein 1A common to 1B as a determinant marker for the stigmoid body.

The anti-serum against an unknown human placental antigen complex X-P2 (hPAX-P2) immunohistochemically recognizes three putative molecules (hPAX-P2S, hPAX-P2N, and hPAX-P2R), each of which is associated with the stigmoid bodies (STBs), necklace olfactory glomeruli (NOGs), or reticulo-filamentous structures (RFs) in the rat brain. The STBs also contain huntingtin-associated protein 1 (HAP1), and the HAP1-cDNA transfection induces STB-like inclusions in cultured cells. In order to clarify the relationship between hPAX-P2S and HAP1 isoforms (A/B), we performed Western blotting, immuno-histo/cytochemistry for light- and electron-microscopy and pre-adsorption tests with HAP1 deletion fragments. The results showed that the anti-hPAX-P2 anti-serum recognizes HAP1(474-577) of HAP1A/B in Western blotting and strongly immunostains HAP1A-induced STB-like inclusions but far weakly detects HAP1B-induced diffuse structures in HAP1-transfected HEK 293 cells. In the rat brain, immunoreactivity of the anti-hPAX-P2 anti-serum for the STBs was eliminated by pre-adsorption with HAP1(474-577), whereas no pre-adsorption with any different HAP1 fragments can suppress immunoreactivity for the NOGs and RFs, which were not immunoreactive to anti-HAP1 anti-serum. These findings indicate that hPAX-P2S, which is distinct from hPAX-P2N and hPAX-P2R, is identical with STB-constituted HAP1 and that the HAP1-induced/immunoreactive inclusions correspond to the hPAX-P2-immunoreactive STBs previously identified in the brain.