Sumo E2 Ligase UBC9 is Required for Efficient Protein Quality Control in Cardiomyocytes
Rationale: Impairment of proteasomal function is pathogenic in a number of cardiac proteinopathies and can eventually lead to heart failure. Loss of proteasomal activity often results in the accumulation of large protein aggregates. The ubiquitin proteasome system (UPS) is primarily responsible for cellular protein degradation and, while the role of ubiquitination in this process is well studied, the function of an ancillary post-translational modification, SUMOylation, in protein quality control (PQC) is not fully understood.
Objective: To determine the role of UBC9, a SUMO-conjugating enzyme, in cardiomyocyte PQC.
Methods and Results: Gain- and loss-of-function approaches were used to determine the importance of UBC9. Overexpression of UBC9 enhanced UPS function in cardiomyocytes while knockdown of UBC9 by siRNA caused significant accumulations of aggregated protein. UPS function and relative activity was analyzed using a UPS reporter protein, GFPu. Subsequently, UBC9's effects on UPS function were tested in a proteotoxic model of desmin-related cardiomyopathy, caused by cardiomyocyte specific expression of a mutated alpha B crystallin, CryABR120G. CryABR120G expression leads to aggregate formation and decreased proteasomal function. Co-infection of UBC9-adenovirus with CryABR120G virus reduced the proteotoxic sequelae, decreasing overall aggregate concentrations. Conversely, knockdown of UBC9 significantly decreased UPS function in the model and resulted in increased aggregate levels.
Conclusions: UBC9 plays a significant role in cardiomyocyte PQC and its activity can be exploited to reduce toxic levels of misfolded or aggregated proteins in cardiomyopathy.
- Received July 5, 2014.
- Revision received July 28, 2014.
- Accepted August 4, 2014.