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 muscular atrophy (SBMA) is not fully understood. Similar to observations with other mutant polyglutamine (poly Q) expanded proteins, truncated androgen receptor (AR) with expanded poly Q tract cause intracellular aggregates; however, the precise relationship between aggregates and disease pathogenesis is unresolved. In order to have a better understanding of the cellular processing and toxicity of the mutant AR, we focused on a short N-terminal portion of AR containing normal or expanded poly Q repeats, and have carried out biochemical, immunocytochemical, cytochemical and ultrastructural studies of BHK cells at different intervals after transfection. In cells expressing mutant truncated AR, using an anti-AR N-terminal antibody, we observed no immune staining in the nucleus and identified immune negative aggregates surrounded by immunopositive material in the cytoplasm. Congo red staining identified a component of aggregates with a beta-pleated secondary structure in both cytosol and nucleus, while electron microscopy revealed a fibrillary-granular material as the ultrastructural correlate. In addition, acid phosphatase staining and ubiquitin immunocytochemistry demonstrated that in transfected cells, both lysosomal and nonlysosomal degradation systems are actively involved in handling the mutant truncated AR. The temporal relationship of nuclear congophilia to a subsequent massive cell death suggests that entry of proteolytic cleavage products into the nucleus, perhaps the expanded poly Q stretch itself, may play an important role in cell toxicity.