Intrasarcoplasmic Amyloidosis Impairs Proteolytic Function of Proteasomes in Cardiomyocytes by Compromising Substrate Uptake

Abstract

Abstract-The presence of increased ubiquitinated proteins and amyloid oligomers in failing human hearts strikingly resembles the characteristic pathology in the brain of many neurodegenerative diseases. The ubiquitin-proteasome system (UPS) is responsible for degradation of most cellular proteins and plays essential roles in virtually all cellular processes. UPS impairment by aberrant protein aggregation was previously shown in cell culture but remains to be demonstrated in intact animals. Mechanisms underlying the impairment are poorly understood. We report here that UPS proteolytic function is severely impaired in the heart of a mouse model of intrasarcoplasmic amyloidosis caused by cardiac-restricted expression of a human desmin-related myopathy-linked missense mutation of ␣B-crystallin (CryAB R120G ). The UPS impairment was detected before cardiac hypertrophy, and failure became discernible, suggesting that defective protein turnover likely contributes to cardiac remodeling and failure in this model. Further analyses reveal that the impairment is likely attributable to insufficient delivery of substrate proteins into the 20S proteasomes, and depletion of key components of the 19S subcomplex may be responsible. The derangement is likely caused by aberrant protein aggregation rather than loss of function of the CryAB gene because UPS malfunction was not evident in CryAB-null hearts and inhibition of aberrant protein aggregation by Congo red or a heat shock protein significantly attenuated CryAB R120G -induced UPS malfunction in cultured cardiomyocytes. Because of the central role of the UPS in cell regulation and the high intrasarcoplasmic amyloidosis prevalence in failing human hearts, our data suggest a novel pathogenic process in cardiac disorders with abnormal protein aggregation. Key Words: proteasome Ⅲ ubiquitin Ⅲ protein aggregation Ⅲ ␣B-crystallin Ⅲ desmin-related cardiomyopathy Ⅲ amyloidosis Ⅲ transgenic mice M ost cellular proteins are degraded through the ubiquitin-proteasome system (UPS). UPS-mediated proteolysis includes 2 major steps: attachment of a chain of ubiquitin to the target protein molecule through a process known as ubiquitination and degradation of the ubiquitinated proteins by the 26S proteasome. The latter consists of a barrel-shaped 20S core and the 19S cap on 1 or both ends of the 20S. The actual proteolytic activity resides in the interior of the 20S, whereas the 19S plays a critical role in channeling ubiquitinated protein molecules into the 20S. 1 Ubiquitinated proteins accumulate in the cell when the proteasome is inhibited. 1 Aberrant protein aggregation is a common process in many neural degenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases. 2,3 Remaining to be demonstrated in intact animals, this process was shown in cell culture to impair UPS-mediated proteolysis. 4 Therefore, UPS malfunction is considered an important pathogenic mechanism in neurodegeneration. 5 Recent studies revealed that abnormal protein aggregation, intrasarcoplasmic amyl...

Tubulin hyperacetylation is adaptive in cardiac proteotoxicity by promoting autophagy

“…As a result, CryAB R120G leads to accumulation of CryABand desmin-containing aggregates in the sarcoplasm, which leads to disruption of myofibrillar structure and eventually to cardiomyocyte death and heart failure (9, 10, 14-16). We and others have described deficits in protein degradation resulting from the CryAB R120G mutation, including reductions in degradation through the ubiquitin-proteasome system (UPS) and autophagy (17)(18)(19).There is an urgent need for new therapeutic interventions for treating cardiac proteotoxic disorders, as heart failure arising from diverse etiologies can lead to misfolded protein accumulations (16). As the UPS cannot eliminate large aggregated proteins from the cytoplasm due to the size constraints of the proteasome, we have recently focused on using the autophagylysosome pathway to eliminate cellular inclusion bodies, damaged organelles, and aggresomes.…”

“…As a result, CryAB R120G leads to accumulation of CryABand desmin-containing aggregates in the sarcoplasm, which leads to disruption of myofibrillar structure and eventually to cardiomyocyte death and heart failure (9, 10, 14-16). We and others have described deficits in protein degradation resulting from the CryAB R120G mutation, including reductions in degradation through the ubiquitin-proteasome system (UPS) and autophagy (17)(18)(19).…”

Nuclear factor erythroid-2 related factor 2 Nrf2 -mediated protein quality control in cardiomyocytes

“…In the past 15 years, a missense (R120G) mutation of alpha B-crystallin (CryAB R120G ), a bona fide misfolded protein linked to human disease (60), has been extensively used to study cardiac cytosolic PQC (10,20,(61)(62)(63)(64)(65). Inadequate PQC, as manifested by UPS and ALP functional insufficiency (10,20,61,62,65), has been experimentally demonstrated to play a major pathogenic role in CryAB R120G -induced cardiomyopathy in both cardiomyocyte cultures and transgenic mice (7).…”

“…Inadequate PQC, as manifested by UPS and ALP functional insufficiency (10,20,61,62,65), has been experimentally demonstrated to play a major pathogenic role in CryAB R120G -induced cardiomyopathy in both cardiomyocyte cultures and transgenic mice (7). Notably, a reductive stress hypothesis had been proposed by Rajasekaran and colleague to explain pertubed PQC and resultant cardiac pathology caused by increased expresion of a misfolded cytosolic chaperone protein, based on their studies on a transgenic mouse model of cardiac overexpression of a human CryAB R120G (hCryAB R120G ) (66).…”

“…Hence, here we propose an alternative model which takes the findings by Kannan et al into consideration along with most, if not all, other important findings from studying a similar mCryAB R120G mouse model. First, although both proteasome activity and macroautophagy are adaptively upregulated in CryAB R120G overexpressing hearts (62,67), these upregulations are inadequate and both proteasome functional insufficiency and ALP insufficiency have been demonstrated to play an essential role in mCryAB R120G -based cardiomyopathy (10,65). Second, the ALP insufficiency can in turn impede the degradation of ubiquitinated proteins by proteasome via accumulation of an ALP substrate protein p62 (22,23).…”