Prosomes form sarcomere-like banding patterns in skeletal, cardiac, and smooth muscle cells

Role of proteases in the pathophysiology of cardiac disease

Cardiovascular disease is a major cause of death and thus a great deal of effort has been made in salvaging the diseased myocardium. Although various factors have been identified as possible causes of different cardiac diseases such as heart failure and ischemic heart disease, there is a real need to elucidate their role for the better understanding of the cardiac disease pathology and formulation of strategies for developing newer therapeutic interventions. In view of the intimate involvement of different types of proteases in maintaining cellular structure, the role of proteases in various cardiac diseases has become the focus of recent research. Proteases are present in the cytosol as well as are localized in a number of subcellular organelles in the cell. These are known to use extracellular matrix, cytoskeletal, sarcolemmal, sarcoplasmic reticular, mitochondrial and myofibrillar proteins as substrates. Work from different laboratories using a wide variety of techniques has shown that the activation of proteases causes alterations of a number of specific proteins leading to subcellular remodeling and cardiac dysfunction. Inhibition of protease action by different drugs and agents, therefore, has a clinical relevance and is expected to form a part of new treatment paradigm for improving heart function. This review examines the biochemistry and localization of some of the proteases in the cardiac tissue in addition to identification of the sites of action of some protease inhibitors. (Mol Cell Biochem 263: 241-256, 2004).

Membrane-myofibril cross-talk in myofibrillogenesis and in muscular dystrophy pathogenesis: lessons from the zebrafish

Striated muscle has a highly ordered structure in which specialized domains of the cell membrane involved in force transmission (costameres) and excitation-contraction coupling (T tubules) as well as the internal membranes of the sarcoplasmic reticulum are organized over specific regions of the sarcomere. Optimal muscle function is dependent on this high level of organization but how it established and maintained is not well understood. Due to its ex utero development and transparency, the zebrafish embryo is an excellent vertebrate model for the study of dynamic relationships both within and between cells during development. Transgenic models have allowed the delineation of cellular migration and complex morphogenic rearrangements during the differentiation of skeletal myocytes and the assembly and organization of new myofibrils. Molecular targeting of genes and transcripts has allowed the identification of key requirements for myofibril assembly and organization. With the recent advances in gene editing approaches, the zebrafish will become an increasingly important model for the study of human myopathies and muscular dystrophies. Its high fecundity and small size make it well suited to high-throughput screenings to identify novel pharmacologic and molecular therapies for the treatment of a broad range of neuromuscular conditions. In this review, we examine the lessons learned from the zebrafish model regarding the complex interactions between the sarcomere and the sarcolemma that pattern the developing myocyte and discuss the potential for zebrafish as a model system to examine the pathophysiology of, and identify new treatments for, human myopathies and muscular dystrophies.

μ-Calpain is involved in the postmortem proteolysis of gizzard smooth muscle

Postmortem changes in proteins that have been implicated in affecting muscle integrity were examined in goose (GG) and duck (DG) gizzard smooth muscle stored at 5°C. GG and DG smooth muscles were sampled at 0, 1, 3 and 7 day of storage. The pH was approximately 7 in both GG and DG samples during postmortem storage. Casein zymograms showed that 0-day μ-calpain activity was higher (p<0.05) in GG than in DG samples. As postmortem time progressed, μ-calpain was activated and autolyzed more extensively in GG than in DG samples. However, μ/m-calpain remained relatively stable in both samples. Western blots indicated that postmortem desmin degradation was more rapid in GG than in DG samples. In contrast, α-actinin remained nearly unchanged in both samples. Therefore, our results suggest that μ-calpain has an important role in the postmortem proteolysis of gizzard smooth muscle.

Novel interactions between actin and the proteasome revealed by complex haploinsufficiency

Saccharomyces cerevisiae has been a powerful model for uncovering the landscape of binary gene interactions through whole-genome screening. Complex heterozygous interactions are potentially important to human genetic disease as loss-of-function alleles are common in human genomes. We have been using complex haploinsufficiency (CHI) screening with the actin gene to identify genes related to actin function and as a model to determine the prevalence of CHI interactions in eukaryotic genomes. Previous CHI screening between actin and null alleles for non-essential genes uncovered ∼240 deleterious CHI interactions. In this report, we have extended CHI screening to null alleles for essential genes by mating a query strain to sporulations of heterozygous knock-out strains. Using an act1Δ query, knock-outs of 60 essential genes were found to be CHI with actin. Enriched in this collection were functional categories found in the previous screen against non-essential genes, including genes involved in cytoskeleton function and chaperone complexes that fold actin and tubulin. Novel to this screen was the identification of genes for components of the TFIID transcription complex and for the proteasome. We investigated a potential role for the proteasome in regulating the actin cytoskeleton and found that the proteasome physically associates with actin filaments in vitro and that some conditional mutations in proteasome genes have gross defects in actin organization. Whole-genome screening with actin as a query has confirmed that CHI interactions are important phenotypic drivers. Furthermore, CHI screening is another genetic tool to uncover novel functional connections. Here we report a previously unappreciated role for the proteasome in affecting actin organization and function.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

Gene and genon concept: coding versus regulation. A conceptual and information-theoretic analysis of genetic storage and expression in the light of modern molecular biology

We analyse here the definition of the gene in order to distinguish, on the basis of modern insight in molecular biology, what the gene is coding for, namely a specific polypeptide, and how its expression is realized and controlled. Before the coding role of the DNA was discovered, a gene was identified with a specific phenotypic trait, from Mendel through Morgan up to Benzer. Subsequently, however, molecular biologists ventured to define a gene at the level of the DNA sequence in terms of coding. As is becoming ever more evident, the relations between information stored at DNA level and functional products are very intricate, and the regulatory aspects are as important and essential as the information coding for products. This approach led, thus, to a conceptual hybrid that confused coding, regulation and functional aspects. In this essay, we develop a definition of the gene that once again starts from the functional aspect. A cellular function can be represented by a polypeptide or an RNA. In the case of the polypeptide, its biochemical identity is determined by the mRNA prior to translation, and that is where we locate the gene. The steps from specific, but possibly separated sequence fragments at DNA level to that final mRNA then can be analysed in terms of regulation. For that purpose, we coin the new term "genon". In that manner, we can clearly separate product and regulative information while keeping the fundamental relation between coding and function without the need to introduce a conceptual hybrid. In mRNA, the program regulating the expression of a gene is superimposed onto and added to the coding sequence in cis - we call it the genon. The complementary external control of a given mRNA by trans-acting factors is incorporated in its transgenon. A consequence of this definition is that, in eukaryotes, the gene is, in most cases, not yet present at DNA level. Rather, it is assembled by RNA processing, including differential splicing, from various pieces, as steered by the genon. It emerges finally as an uninterrupted nucleic acid sequence at mRNA level just prior to translation, in faithful correspondence with the amino acid sequence to be produced as a polypeptide. After translation, the genon has fulfilled its role and expires. The distinction between the protein coding information as materialised in the final polypeptide and the processing information represented by the genon allows us to set up a new information theoretic scheme. The standard sequence information determined by the genetic code expresses the relation between coding sequence and product. Backward analysis asks from which coding region in the DNA a given polypeptide originates. The (more interesting) forward analysis asks in how many polypeptides of how many different types a given DNA segment is expressed. This concerns the control of the expression process for which we have introduced the genon concept. Thus, the information theoretic analysis can capture the complementary aspects of coding and regulation, of gene and genon.

Heart failure and protein quality control

The heart is constantly under mechanical, metabolic, and thermal stress, even at baseline physiological conditions, and cardiac stress may increase as a result of environmental or intrinsic pathological insults. Cardiomyocytes are continuously challenged to efficiently and properly fold nascent polypeptides, traffic them to their appropriate cellular locations, and keep them from denaturing in the face of normal and pathological stimuli. Because deployment of misfolded or unfolded proteins can be disastrous, cells, in general, and cardiomyocytes, in particular, have developed a multilayered protein quality control system for maintaining proper protein conformation and for reorganizing and removing misfolded or aggregated polypeptides. Here, we examine recent data pointing to the importance of protein quality control in cardiac homeostasis and disease.

Ca(2+)-dependent proteolysis in muscle wasting

Skeletal muscle wasting is a prominent feature of cachexia, a complex systemic syndrome that frequently complicates chronic diseases such as inflammatory and autoimmune disorders, cancer and AIDS. Muscle wasting may also develop as a manifestation of primary or neurogenic muscular disorders. It is now generally accepted that muscle depletion mainly arises from increased protein catabolism. The ubiquitin-proteasome system is believed to be the major proteolytic machinery in charge of such protein breakdown, yet there is evidence suggesting that Ca(2+)-dependent system, lysosomes and, in some conditions at least, even caspases are involved as well. The role of Ca(2+)-dependent proteolysis in skeletal muscle wasting is reviewed in the present paper. This system relies on the activity of calpains, a family of Ca(2+)-dependent cysteine proteases, whose regulation is complex and not completely elucidated. Modulations of Ca(2+)-dependent proteolysis have been associated with muscle protein depletion in various pathological contexts and particularly with muscle dystrophies. Calpains can only perform a limited proteolysis of their substrates, however they may play a critical role in initiating the breakdown of myofibrillar protein, by releasing molecules that become suitable for further degradation by proteasomes. Some evidence would also support a role for lysosomes and caspases in muscle wasting. Thus it cannot be excluded that different intracellular proteolytic systems may coordinately concur in shifting muscle protein turnover towards excess catabolism. Many different signals have been proposed as potentially involved in triggering the enhanced protein breakdown that underlies muscle wasting. How they are transduced to initiate the hypercatabolic response and to activate the proteolytic pathways remains largely unknown, however.

Proteasomes are tightly associated to myofibrils in mature skeletal muscle

Proteasomes are the major actors of nonlysosomal cytoplasmic protein degradation. In particular, these large protein complexes (about 2500 kDa) are considered to be responsible for muscular degradation during skeletal muscle atrophy. Despite their unusual and important size, they are widely described as soluble and mobile in the cytoplasm. In mature skeletal muscle, we have previously observed a sarcomeric distribution of proteasomes, as revealed by the distribution of alpha1/p27K, a subunit of the 20S core-particle (prosome) of proteasome. Here, we extend these observations at the electron microscopic level in vivo. We also show that this sarcomeric pattern is dependent of the extension of the sarcomere. Using isolated myofibrils, we demonstrate that proteasomes are still attached to the myofibrils after the isolation procedure, and reproduce the observations made in vivo. In addition, the extraction of actin by gelsolin largely removes proteasomes from isolated myofibrils, but some of them are held in place after this extraction, showing a sarcomeric disposition in the absence of any detectable actin, and suggesting the existence of another molecular partner for these interactions. From these results, we conclude that most of detectable 20S proteasomes in skeletal muscle cells is tightly attached to the myofibrils.

Anthracyclines: Molecular Advances and Pharmacologic Developments in Antitumor Activity and Cardiotoxicity

The clinical use of anthracyclines like doxorubicin and daunorubicin can be viewed as a sort of double-edged sword. On the one hand, anthracyclines play an undisputed key role in the treatment of many neoplastic diseases; on the other hand, chronic administration of anthracyclines induces cardiomyopathy and congestive heart failure usually refractory to common medications. Second-generation analogs like epirubicin or idarubicin exhibit improvements in their therapeutic index, but the risk of inducing cardiomyopathy is not abated. It is because of their janus behavior (activity in tumors vis-à-vis toxicity in cardiomyocytes) that anthracyclines continue to attract the interest of preclinical and clinical investigations despite their longer-than-40-year record of longevity. Here we review recent progresses that may serve as a framework for reappraising the activity and toxicity of anthracyclines on basic and clinical pharmacology grounds. We review 1) new aspects of anthracycline-induced DNA damage in cancer cells; 2) the role of iron and free radicals as causative factors of apoptosis or other forms of cardiac damage; 3) molecular mechanisms of cardiotoxic synergism between anthracyclines and other anticancer agents; 4) the pharmacologic rationale and clinical recommendations for using cardioprotectants while not interfering with tumor response; 5) the development of tumor-targeted anthracycline formulations; and 6) the designing of third-generation analogs and their assessment in preclinical or clinical settings. An overview of these issues confirms that anthracyclines remain "evergreen" drugs with broad clinical indications but have still an improvable therapeutic index.

Intracellular localization of proteasomes

Proteasomes are present in the cytoplasm and in the nuclei of all eukaryotic cells, however their relative abundance within those compartments is highly variable. In the cytoplasm, proteasomes associate with the centrosomes, cytoskeletal networks and the outer surface of the endoplasmic reticulum (ER). In the nucleus, proteasomes are present throughout the nucleoplasm but are void from the nucleoli. Sometimes they associate with discrete subnuclear domains called the PML nuclear bodies (POD domains). PML bodies in the nucleus, and the pericentrosomal area of the cytoplasm may function as proteolytic centers of the cell, since they are enriched in components of the proteasome system. Under conditions of impaired proteolysis proteasomes and ubiquitinated proteins further accumulate at these locations, forming organized aggregates. In case of the pericentrosomal area those aggregates have been termed "aggresomes". Once formed, aggresomes can impair the function of the proteasome system, which may promote apoptosis. Under favorable conditions they can be cleared, probably by autophagy.

Proteasome dynamics during cell cycle in rat Schwann cells

The proteasome is responsible for most of the protein degradation that takes place in the cytoplasm and nucleus. Immunofluorescence and electron microscopy are used to study proteasome dynamics during the cell cycle in rat Schwann cells. During interphase, the proteasome is present in the nucleus and cytoplasm and shows no colocalization with cytoskeletal components. Some cytoplasmic proteasomes always localize in the centrosome both in interphase and in mitotic cells and only associate with microtubules during mitosis. The proteasome exits the nucleus during prophase. In anaphase, the proteasome becomes prominent in the region between the two sets of migrating chromosomes and in association with interzonal microtubules and stem bodies. In telophase, the proteasome begins to reenter the nucleus and is prominent in the midbody region until the end of cytokinesis. The proteasome does not colocalize with actin or vimentin during mitosis, except for colocalization with actin in the sheet-like lamellipodia, which serve as substrate attachments for the cell during mitosis. During S phase, nuclear proteasomes colocalize with foci of BrdU incorporation, but this association changes with time: maximal at early S phase and declining as S phase progresses to the end. These results are discussed in relation to the biochemical pathways involved in cell cycle progression.