Endoplasmic Reticulum-associated Degradation (ERAD) and Autophagy Cooperate to Degrade Polymerogenic Mutant Serpins

The serpinopathies are a family of diseases characterized by the accumulation of ordered polymers of mutant protein within the endoplasmic reticulum. They are a diverse group including alpha(1)-antitrypsin deficiency and the inherited dementia familial encephalopathy with neuroserpin inclusion bodies or FENIB. We have used transient transfection of COS7 cells and mouse embryonic fibroblasts, PC12 cell lines that conditionally express wild type and mutant neuroserpin and fly models of FENIB to assess the cellular handling of wild type and mutant serpins. By using a polymer-specific monoclonal antibody, we show that mutant neuroserpin forms polymers after a delay of at least 30 min and that polymers can be cleared in PC12 cell lines and from the brain in a fly model of FENIB. At steady state, the fractions of intracellular polymerogenic G392E mutant neuroserpin in the monomeric and polymeric states are comparable. Inhibition of the proteasome with MG132 reveals that both mutant neuroserpin and alpha(1)-antitrypsin are degraded predominantly by endoplasmic reticulum-associated degradation (ERAD). Pharmacological and genetic inhibitions demonstrate that autophagy is responsible for bulk turnover of wild type and mutant serpins, but can be stimulated by rapamycin to compensate for proteasome inhibition. The significance of these findings to the treatment of serpinopathies is discussed.