The ubiquitin-proteasome system in cardiac dysfunction

Since proteins play crucial roles in all biological processes, the finely tuned equilibrium between their synthesis and degradation regulates cellular homeostasis. Controlling the quality of proteome informational content is essential for cell survival and function. After initial synthesis, membrane and secretory proteins are modified, folded, and assembled in the endoplasmic reticulum, whereas other proteins are synthesized and processed in the cytosol. Cells have different protein quality control systems, the molecular chaperones, which help protein folding and stabilization, and the ubiquitin-proteasome system (UPS) and lysosomes, which degrade proteins. It has generally been assumed that UPS and lysosomes are regulated independently and serve distinct functions. The UPS degrades both cytosolic, nuclear proteins, and myofibrillar proteins, whereas the lysosomes degrade most membrane and extracellular proteins by endocytosis as well as cytosolic proteins and organelles via autophagy. Over the last two decades, the UPS has been increasingly recognized as a major system in several biological processes including cell proliferation, adaptation to stress and cell death. More recently, activation or impairment of the UPS has been reported in cardiac disease and recent evidence indicate that autophagy is a key mechanism to maintain cardiac structure and function. This review mainly focuses on the UPS and its various components in healthy and diseased heart, but also summarizes recent data suggesting parallel activation of the UPS and autophagy in cardiac disease.     

p27Kip1 Metabolism: A Fascinating Labyrinth

The progression through the phases of cell division cycle is regulated by different cyclins and cyclin-dependent kinases (CDKs) complexes. Due to their key function, the activity of cyclin/CDK complexes is controlled by several mechanisms, including the inhibition by a number of proteins collectively defined CDK inhibitors or CKIs. Among the CKIs, p27Kip1 represents a protein of central activity for the control of several phenotypes, including proliferation, differentiation and malignant transformation. p27Kip1 belongs to the growing family of “natively unfolded”, “in¬trinsically disordered” or “intrinsically unstructured” proteins. The disorder proteins present a very large number of possible conformations that, after the binding, converge to a well-defined structure with an extraordinary affinity for the target. As matter of fact, the absence of a preexisting folding strongly facilitates p27Kip1 interaction with a number of targets. Until recently, p27Kip1 has been solely viewed as a nuclear protein with the function of modulating cyclin–CDK activity and hence, cell-cycle progression. However, exhaustive studies have now demonstrated that the protein plays additional roles outside of the nucleus, including, particularly, the control of cell motility. Thus, the cellular localization is of fundamental importance in p27Kip1 function. Accordingly, at least two different mechanisms of degradation, occurring either in the nucleus or in the cytosol, have been observed. Convincing evidences have demonstrated that p27Kip1 is a phosphoprotein showing at least 6-8 phosphorylatable residues. However, the precise functional roles of the phosphorylations and the identification of the kinases responsible for the post-synthetic modifications are still debated. In this brief review, we will report the Literature data that connect the post-synthetic modifications of p27Kip1 with its function, localization and metabolism. The picture that emerges demonstrates that several of the pieces of the CKI metabolism are still nebulous.     

Aging and regulated protein degradation: who has the UPPer hand?

In all cells, protein degradation is a constant, ongoing process that is critical for cell survival and repair. The ubiquitin/proteasome pathway (UPP) is the major proteolytic pathway that degrades intracellular proteins in a regulated manner. It plays critical roles in many cellular processes and diseases. Disruption of the UPP is particularly relevant to pathophysiological conditions that provoke the accumulation of aberrant proteins, such as in aging as well as in a variety of neurodegenerative disorders including Alzheimer's and Parkinson's diseases. For unknown reasons, most of these neurodegenerative disorders that include familial and sporadic cases exhibit a late onset. It is possible that these neurodegenerative conditions exhibit a late onset because proteasome activity decreases with aging. Aging‐dependent impairment in proteolysis mediated by the proteasome may have profound ramifications for cell viability. It can lead to the accumulation of modified, potentially toxic proteins in cells and can cause cell injury or premature cell death by apoptosis or necrosis. While it is accepted that aging affects UPP function, the question is why does aging cause a decline in regulated protein degradation by the UPP? Herein, we review some of the properties of the UPP and mechanisms mediating its age‐dependent impairment. We also discuss the relevance of these findings leading to a model that proposes that UPP dysfunction may be one of the milestones of aging.     

Post-translational regulation of the cellular levels of DAPK

Death associated protein kinase (DAPK) is a large, multi-domain ser/thr kinase whose activities converge upon multiple signaling pathways that regulate autophagy, caspase-dependent cell death, cell adhesion and migration. The cellular levels of DAPK are post-translationally regulated by the combined activities of two degradation systems, including the ubiquitin proteasome and an extra-lysosomal proteolysis pathway. At least three distinct E3 ubiquitin ligases target DAPK, including mindbomb1, the chaperone dependent ligase, CHIP (carboxy terminus of Hsp70-interacting protein) and a cullin RING ligase complex, KLHL20-Cul3-RBX1. In addition, it appears that the cellular levels of DAPK are also regulated by an extra-lysosomal protease, cathepsin B. While protein quality control and recycling clearly benefit cells by removal of misfolded or toxic proteins and recycling of their components, the finding that multiple surveillance systems target DAPK suggests that these protein degradation systems also act to fine tune DAPK expression levels in response to specific signaling pathways.     

Role of the calpain system in muscle growth.

Muscle protein degradation has an important role in rate of muscle growth. It has been difficult to develop procedures for measuring rate of muscle protein degradation in living animals, and most studies have used in vitro systems and muscle strips to determine rate of protein degradation. The relationship between results obtained by using muscle strips and rate of muscle protein turnover in living animals is unclear because these strips are in negative nitrogen balance and often develop hypoxic cores. Also, rate of protein degradation is usually estimated by release of labeled amino acids, which reflects an average rate of degradation of all cellular proteins and does not distinguish between rates of degradation of different groups of proteins such as the sarcoplasmic and the myofibrillar proteins in muscle. A number of studies have suggested that the calpain system initiates turnover of myofibrillar proteins, which are the major group of proteins in striated muscle, by making specific cleavages that release thick and thin filaments from the surface of the myofibril and large polypeptide fragments from some of the other myofibrillar proteins. The calpains do not degrade myofibrillar proteins to small peptides or to amino acids, and they cause no bulk degradation of sarcoplasmic proteins. Hence, the calpains are not directly responsible for release of amino acids during muscle protein turnover. Activity of the calpains in living cells is regulated by calpastatin and Ca2+, but the nature of this regulation is still unclear.     

自噬与泛素化蛋白降解途径的分子机制及其功能

细胞内所有的蛋白质和大多数的细胞外蛋白都在不断的进行更新,即它们在不断地被降解,并被新合成的蛋白质取代。细胞内蛋白的降解主要通过两个途径,即自噬和泛素蛋白酶体系统。自噬是一种由溶酶体介导的细胞内过多或异常蛋白质的降解机制。在细胞内主要有3种类型的自噬,即分子伴侣介导的自噬、微自噬和巨自噬。泛素蛋白酶体系统是由泛素介导的一种高度复杂的蛋白降解机制,它参与降解细胞内许多蛋白质并且这个过程具有高度特异性。细胞内蛋白质的降解参与调节许多细胞过程,包括细胞周期、DNA修复、细胞生长和分化、细胞质量的控制、病原生物的感染反应和细胞凋亡等。许多严重的人类疾病被认为是由于蛋白质降解系统的紊乱而引起的。文章综述了自噬和泛素化途径及其分子机制,以及蛋白质降解系统紊乱的病理学意义。     

Intracellular proteolysis: signals of selective protein degradation

Selective proteolysis is one of the mechanisms for the maintenance of cell homeostasis via rapid degradation of defective polypeptides and certain short-lived regulatory proteins. In prokaryotic cells, high-molecular-mass oligomeric ATP-dependent proteases are responsible for selective protein degradation. In eukaryotes, most polypeptides are attacked by the multicatalytic 26S proteasome, and the degradation of the majority of substrates involves their preliminary modification with the protein ubiquitin. The proteins undergoing the selective proteolysis often contain specific degradation signals necessary for their recognition by the corresponding proteases.     

Mitochondrial protein import: from transport pathways to an integrated network

Mitochondria, the powerhouses of the cell, import most of their proteins from the cytosol. It was originally assumed that mitochondria imported precursor proteins via a general pathway but recent studies have revealed a remarkable variety of import pathways and mechanisms. Currently, five different protein import pathways can be distinguished. However, the import machineries cooperate with each other and are connected to other systems that function in the respiratory chain, mitochondrial membrane organization, protein quality control and endoplasmic reticulum-mitochondria junctions. In this Opinion, we propose that mitochondrial protein import should not be seen as an independent task of the organelle and that a network of cooperating machineries is responsible for major mitochondrial functions.     

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

Variations on a theme: Combined molecular chaperone and proteolysis functions in Clp/HSP100 proteins

Most stress-inducible polypeptides are members of broader protein families that function either as molecular chaperones or constituents of proteolytic pathways. These systems control many aspects of protein structure and function throughout the cell under all types of growth regimes. The Clp/HSP1 00 protein family is a recently characterized representative, with constitutive and stress-inducible members found in many different organisms and various intracellular locations. Besides being regulators of energy-dependent proteolysis, Clp proteins may also function as molecular chaperones. Constitutive Clp proteins are involved foremost in cellular protein maintenance and repair, in cooperation with other chaperone and proteolytic systems. At high temperatures, additional Clp proteins are induced in response to rising levels of inactive polypeptides, resulting from either biosynthetic errors, thermal denaturation and aggregation. Clp proteins presumably help to stabilize selected polypeptides during severe thermal stress and enable resolubilization of non-functional protein aggregates, as well as promoting the degradation of irreversibly damaged polypeptides. The union of chaperone and proteolytic regulatory functions in one molecule suggests that certain Clp proteins play a decisive role in determining the destiny of proteins, not only during normal growth but also under conditions of extreme stress. This review briefly covers recent findings on the diversity of Clp proteins and their potential importance within the cell.     

Regulating the regulator: negative regulation of Cbl ubiquitin ligases

Cbl proteins are regulators of signal transduction through many pathways and, consequently, regulate cell function and development. They are ubiquitin ligases that ubiquitinate and target many signaling molecules for degradation. The Cbl proteins themselves are regulated by an increasingly complex network of interactions that fine-tune the effects that Cbl proteins have on signaling. The negative regulation of Cbl protein function can occur via cis-acting structural elements that prevent inappropriate ubiquitin ligase activity, degradation of the Cbl proteins, inhibition without degradation owing to interaction with other signaling proteins, deubiquitination of Cbl substrates, and regulation of assembly of the endosomal ESCRT-I complex. Defects in the regulatory mechanisms that control Cbl function are implicated in the development of immunological and malignant diseases.     

Mechanism of synaptic protein turnover and its regulation by neuronal activity

Neurons are long-lived cells with a complex architecture, in which synapses may be located far away from the cell body and are subject to plastic changes, thereby posing special challenges to the systems that maintain and dynamically regulate the synaptic proteome. These mechanisms include neuronal autophagy and the endolysosome pathway, as well as the ubiquitin/proteasome system, which cooperate in the constitutive and regulated turnover of presynaptic and postsynaptic proteins. Here, we summarize the pathways involved in synaptic protein degradation and the mechanisms underlying their regulation, for example, by neuronal activity, with an emphasis on the presynaptic compartment and outline perspectives for future research. Keywords: Synapse, Synaptic vesicle, Autophagy, Endolysosome, Proteasome, Protein turnover, Protein degradation, Endosome, Lysosome     

Adenovirus degradation of cellular proteins

Eukaryotic cells orchestrate constant synthesis and degradation of intracellular components, including soluble proteins and organelles. The two major intracellular degradation pathways are the ubiquitin/proteasome system and autophagy. Whereas ubiquitin/proteasome system is involved in rapid degradation of proteins, autophagy selectively removes protein aggregates and damaged organelles. Failure of these highly adjusted proteolytic systems to maintain basal turnover leads to altered cellular homeostasis. During evolution, certain viruses have developed mechanisms to exploit their functions to facilitate their own replication, prevent viral clearance and promote the outcome of infection. In this article, we summarize the current opinion on adenoviruses (Ad) and molecular host cell targets, extending on recent evidences for protein degradation pathways in infected cells. We describe recently identified connections between Ad-mediated proteolysis and viral replication with main emphasis on the function of certain Ad proteins.