Ultrastructural analysis and molecular screening of the neuronal cytoplasmic inclusion, "the stigmoid body"

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The stigmoid body (STB), which was a distinct non-membrane-bound neurocytoplasmic inclusion originally detected by immunohistochemistry for human placental antigen X-P2S (hPAX-P2S), also contains huntingtin-associated protein 1 (HAP1A/B), and the HAP1A-cDNA transfection induces STB-like inclusions in cultured cells. A striking number of the STBs are preferentially distributed in the limbic and hypothalamic regions, where neurodegeneration rarely occurs, rather than the targets of neurodegeneration including the striatum, thalamus and neocortex. The current research first provided clear evidence that HAP1 interacts with androgen receptor (AR) derived from spinal-and-bulbar-muscular-atrophy (SBMA) via its ligand-binding domain in a polyQ-length-dependent manner and forms prominent inclusions sequestering polyQ-AR, and that addition of dihydrotestosterone reduces the association strength of HAP1 with ARQ25 more dramatically than that with ARQ65. Furthermore, SBMA-mutant-ARQ65-induced apop … More tosis was suppressed by cotransfection with HAP1, supporting "the HAP1/STB protection hypothesis" that the HAP1/STB plays a protective role against neurodegeneration.In addition, the anti-hPAX-P2S antiserum was first demonstrated to recognize HAPI^<474-577> (XP2S domain) near C-terminuses of HAP1A/B in Western blotting, and hPAX-P2S-immunoreactions of the brain STB and HAP1A-induced inclusions were shown to be eliminated by pre-adsorption with HAP1^<474-577>. These findings clearly indicated that hPAX-P2S is identical with STB-constituted HAP1 and that the HAP1-induced/immunoreactive inclusions correspond to the hPAX-P2S-immunoreactive STBs. In addition, the anti-hPAX-P2S antiserum far weakly immunostained HAP1B-induced/immunoreactive diffuse structures, suggesting that the XP2S domain is covered by some molecule in the diffuse structures and disclosed in the STB and have an important clue to elucidate mechanism of the STB formation. In immuno-electron microscopy, HAP1 is not only localized to the STB but also associated with endoplasmic-reticulum-like tubular structures adjacent to the STB, but the STB is not detected so far by any antibodies to organelle-markers including KDEL, GMP130, LAMP1, EEA1 or Bcl2. The organelle origin has yet to be determined. Less