Nonneural nuclear inclusions of androgen receptor protein in spinal and bulbar muscular atrophy

Abstract

Spinal and bulbar muscular atrophy is an X-linked motor neuronopathy caused by the expansion of an unstable CAG repeat in the coding region of the androgen receptor (AR) gene. Nuclear inclusions of the mutant AR protein have been shown to occur in the spinal motor neurons of spinal and bulbar muscular atrophy (Li M, Kobayashi Y, Merry D, Tanaka F, Doyu M, Hashizume Y, Fischbeck KH, Sobue G: Nuclear inclusions in spinal and bulbar muscular atrophy. Ann Neurol 1998 (in press)). In this study, we demonstrate the tissue-specific distribution, immunochemical features, and fine structure of nuclear inclusions of spinal and bulbar muscular atrophy. Nuclear inclusions were observed in affected spinal and brainstem motor neurons, but not in other, nonaffected neural tissues. Similar nuclear inclusions occurred in nonneural tissues including scrotal skin, dermis, kidney, heart, and testis, but not in the spleen, liver, and muscle. These inclusions had similar epitope features detectable by antibodies that recognize a small portion of the N-terminus of the AR protein only, and they were ubiquitinated. Electron microscopic immunohistochemistry showed dense aggregates of AR-positive granular material without limiting membrane, both in the neural and nonneural inclusions. These findings indicate that nuclear inclusions of AR protein are present in selected nonneural tissues as well as in neurons that degenerate in spinal and bulbar muscular atrophy, suggesting that a common mechanism underlies in the formation of neural and nonneural nuclear inclusions.

Figure 1.

Locations of the epitopes of Abs in the human AR.

Figure 2.

Immunohistochemistry of the AR protein in nuclear inclusions in the central nervous system of SBMA patients. AR staining is observed in motor neurons of the medulla oblongata (A and B) and pons (C and D). A and B are stained with PG-21, and C and D are stained with AR(N-20). Magnification: A to D, 600×.

Figure 3.

Nuclear inclusions in nonneural tissues of SBMA patients. AR staining is observed in epithelial cells in the proximal tubules of the kidney (A), scrotal skin epidermal cells (C), and testis (E). PG-21 was used as the first Ab. These inclusions all stained positively for ubiquitin (B, D and F). Magnification: A to F, 840×.

Figure 4.

Electron microscopic immunohistochemical features of the nuclear inclusions in motor neurons and scrotal skin epidermal cells of SBMA patients. Dense granular aggregation of AR-immunoreactive material without a limiting membrane was found in both motor neurons (A to C) and scrotal skin epidermal cells (D to F). Nucleoli, which are larger than the inclusions, are clearly distinguished. The same inclusions observed light microscopically (A and D) are demonstrated electron microscopically (B, C, E, and F). AR(N-20) was used as the first Ab. Magnification: A, 720×; D, 1,000×; B and E, 2,000×; C, 10,000×; F, 15,000×.