Dopamine D1 receptor-mediated aggregation of N-terminal fragments of mutant huntingtin and cell death in a neuroblastoma cell line

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by abnormal CAG repeat expansion in the IT15 gene encoding huntingtin protein (htt). Mutated htt is predicted to acquire toxic properties in specific brain regions. For instance, striatal neurons expressing dopamine receptors predominantly degenerate in HD patients. Although the basis of this specific vulnerability remains unclear, a great deal of evidence has documented the ability of the dopamine system to modulate the toxicity of expanded htt. To investigate the relationship between dopamine receptors and expanded htt, we transfected enhanced green fluorescent proteins (EGFP) tagged to normal (25 CAG) or mutant (103 CAG) htt in SK-N-MC neuroblastoma cells that endogenously express D1 receptors. Forming nuclear and cytoplasmic aggregates, mutant htt-EGFP was toxic to cells beyond 24 h post-transfection. Remarkably, low doses of a selective D1 receptors agonist or forskolin, an activator of adenylate cyclase, accelerated the formation of mutant htt nuclear aggregates, whereas the number of cytoplasmic aggregates was decreased. These effects were associated with a minor increase in cell death. Understanding the functional bases of these effects may further elucidate the role of dopamine receptors signaling in the complex pathophysiology of HD.