Intracellular processing and toxicity of the truncated androgen receptor: Nuclear congophilia-associated cell death

The pathogenesis of the selective motor neuron death in spinal bulbar muscu lar atrophy (SBMA) is not fully understood. Similar to observations with ot her mutant polyglutamine (poly Q) expanded proteins, truncated androgen rec eptor (AR) with expanded poly Q tract cause intracellular aggregates; howev er, the precise relationship between aggregates and disease pathogenesis is unresolved. In order to have a better understanding of the cellular proces sing and toxicity of the mutant AR, we focused on a short N-terminal portio n of AR containing normal or expanded poly Q repeats, and have carried out biochemical, immunocytochemical, cytochemical and ultrastructural studies o f BHK cells at different intervals after transfection. In cells expressing mutant truncated AR, using an anti-AR N-terminal antibody, we observed no i mmune staining in the nucleus and identified immune negative aggregates sur rounded by immunopositive material in the cytoplasm. Congo red staining ide ntified a component of aggregates with a P-pleated secondary structure in b oth cytosol and nucleus, while electron microscopy revealed a fibrillary-gr anular material as the ultrastructural correlate. In addition. acid phospha tase staining and ubiquitin immunocytochemistry demonstrated that in transf ected cells, both lysosomal and nonlysosomal degradation systems are active ly involved in handling the mutant truncated AR. The temporal relationship of nuclear congophilia to a subsequent massive cell death suggests that ent ry of proteolytic cleavage products into the nucleus, perhaps the expanded poly Q stretch itself, may play an important role in cell toxicity.