Caspase 3 exhibits a yeast metacaspase proteostasis function that protects mitochondria from toxic TDP43 aggregates

Caspase 3 activation is a hallmark of cell death and there is a strong correlation between elevated protease activity and evolving pathology in neurodegenerative disease, such as amyotrophic lateral sclerosis (ALS). At the cellular level, ALS is characterized by protein aggregates and inclusions, comprising the RNA binding protein TDP-43, which are hypothesized to trigger pathogenic activation of caspase 3. However, a growing body of evidence indicates this protease is essential for ensuring cell viability during growth, differentiation and adaptation to stress. Here, we explored whether caspase 3 acts to disperse toxic protein aggregates, a proteostasis activity first ascribed to the distantly related yeast metacaspase ScMCA1. We demonstrate that human caspase 3 can functionally substitute for the ScMCA1 and limit protein aggregation in yeast, including TDP-43 inclusions. Proteomic analysis revealed that disrupting caspase 3 in the same yeast substitution model resulted in detrimental TDP-43/mitochondrial protein associations. Similarly, suppression of caspase 3, in either murine or human skeletal muscle cells, led to accumulation of TDP-43 aggregates and impaired mitochondrial function. These results suggest that caspase 3 is not inherently pathogenic, but may act as a compensatory proteostasis factor, to limit TDP-43 protein inclusions and protect organelle function in aggregation related degenerative disease.

Chromatin Reorganization during Myoblast Differentiation Involves the Caspase-Dependent Removal of SATB2

The induction of lineage-specific gene programs are strongly influenced by alterations in local chromatin architecture. However, key players that impact this genome reorganization remain largely unknown. Here, we report that the removal of the special AT-rich binding protein 2 (SATB2), a nuclear protein known to bind matrix attachment regions, is a key event in initiating myogenic differentiation. The deletion of myoblast SATB2 in vitro initiates chromatin remodeling and accelerates differentiation, which is dependent on the caspase 7-mediated cleavage of SATB2. A genome-wide analysis indicates that SATB2 binding within chromatin loops and near anchor points influences both loop and sub-TAD domain formation. Consequently, the chromatin changes that occur with the removal of SATB2 lead to the derepression of differentiation-inducing factors while also limiting the expression of genes that inhibit this cell fate change. Taken together, this study demonstrates that the temporal control of the SATB2 protein is critical in shaping the chromatin environment and coordinating the myogenic differentiation program.

Caspase Cleavage of Gelsolin Is an Inductive Cue for Pathologic Cardiac Hypertrophy

Background

Cardiac hypertrophy is an adaptive remodeling event that may improve or diminish contractile performance of the heart. Physiologic and pathologic hypertrophy yield distinct outcomes, yet both are dependent on caspase‐directed proteolysis. This suggests that each form of myocardial growth may derive from a specific caspase cleavage event(s). We examined whether caspase 3 cleavage of the actin capping/severing protein gelsolin is essential for the development of pathologic hypertrophy.

Methods and Results

Caspase targeting of gelsolin was established through protein analysis of hypertrophic cardiomyocytes and mass spectrometry mapping of cleavage sites. Pathologic agonists induced late‐stage caspase‐mediated cleavage of gelsolin. The requirement of caspase‐mediated gelsolin cleavage for hypertrophy induction was evaluated in primary cardiomyocytes by cell size analysis, monitoring of prohypertrophy markers, and measurement of hypertrophy‐related transcription activity. The in vivo impact of caspase‐mediated cleavage was investigated by echo‐guided intramyocardial injection of adenoviral‐expressed gelsolin. Expression of the N‐terminal gelsolin caspase cleavage fragment was necessary and sufficient to cause pathologic remodeling in isolated cardiomyocytes and the intact heart, whereas expression of a noncleavable form prevents cardiac remodeling. Alterations in myocardium structure and function were determined by echocardiography and end‐stage cardiomyocyte cell size analysis. Gelsolin secretion was also monitored for its impact on naïve cells using competitive antibody trapping, demonstrating that hypertrophic agonist stimulation of cardiomyocytes leads to gelsolin secretion, which induces hypertrophy in naïve cells.

Conclusions

These results suggest that cell autonomous caspase cleavage of gelsolin is essential for pathologic hypertrophy and that cardiomyocyte secretion of gelsolin may accelerate this negative remodeling response.

Cardiotrophin 1 stimulates beneficial myogenic and vascular remodeling of the heart

The post-natal heart adapts to stress and overload through hypertrophic growth, a process that may be pathologic or beneficial (physiologic hypertrophy). Physiologic hypertrophy improves cardiac performance in both healthy and diseased individuals, yet the mechanisms that propagate this favorable adaptation remain poorly defined. We identify the cytokine cardiotrophin 1 (CT1) as a factor capable of recapitulating the key features of physiologic growth of the heart including transient and reversible hypertrophy of the myocardium, and stimulation of cardiomyocyte-derived angiogenic signals leading to increased vascularity. The capacity of CT1 to induce physiologic hypertrophy originates from a CK2-mediated restraining of caspase activation, preventing the transition to unrestrained pathologic growth. Exogenous CT1 protein delivery attenuated pathology and restored contractile function in a severe model of right heart failure, suggesting a novel treatment option for this intractable cardiac disease.

Temporal activation of XRCC1-mediated DNA repair is essential for muscle differentiation

Transient DNA strand break formation has been identified as an effective means to enhance gene expression in living cells. In the muscle lineage, cell differentiation is contingent upon the induction of caspase-mediated DNA strand breaks, which act to establish the terminal gene expression program. This coordinated DNA nicking is rapidly resolved, suggesting that myoblasts may deploy DNA repair machinery to stabilize the genome and entrench the differentiated phenotype. Here, we identify the base excision repair pathway component XRCC1 as an indispensable mediator of muscle differentiation. Caspase-triggered XRCC1 repair foci form rapidly within differentiating myonuclei, and then dissipate as the maturation program proceeds. Skeletal myoblast deletion of Xrcc1 does not have an impact on cell growth, yet leads to perinatal lethality, with sustained DNA damage and impaired myofiber development. Together, these results demonstrate that XRCC1 manages a temporally responsive DNA repair process to advance the muscle differentiation program.

Abstract 420: Delineating the Caspase-dependent Targets and Signal Pathways That Promote Pathological Cardiac Hypertrophy

Although cardiac hypertrophy is initially an adaptive response, chronic stress on the heart is a maladaptive process that inevitably leads to end-stage heart failure. Interestingly, this pathological process is also characterized by cell behaviors associated with apoptosis. We previously demonstrated the essential role of the intrinsic cell death pathway during cardiac hypertrophy; however, the caspase-dependent pathways and cleavage targets remain elusive. To this aim, we evaluated a myocyte enhancer factor 2 (MEF2) transcription factor inhibitor, histone deacetylase 3 (HDAC3), and gelsolin as potential caspase cleavage substrates involved in the induction and/or maintenance of cardiac hypertrophy. In vitro cleavage assays were completed with effector caspase and recombinant substrate protein which demonstrated caspase-dependent cleavage. HDAC3 cleavage was observed during early stages of hypertrophy and reduced in the presence of a caspase inhibitor. Luciferase assays demonstrated that the transcriptional activity of MEF2 is dependent on intact caspase function suggesting caspase-directed HDAC3 cleavage may serve as a novel regulatory mechanism to alleviate MEF2 suppression to engage the hypertrophy gene expression program. Unlike HDAC3, caspase mediated gelsolin cleavage occurs at latter stages and is coincident with the cytoskeletal alterations that occur during this process. As gelsolin is a potent actin capping/severing enzyme, we hypothesize that caspase-mediated gelsolin activation acts as a key regulatory step in the structural rearrangements that allow for hypertrophy to occur. We have generated adenoviral vectors containing caspase cleavage mutants and cleaved forms of HDAC3 and gelsolin and will discuss the impact of these modified substrates on the hypertrophy process in vitro and in vivo. Collectively, this work suggests that caspase signalling acts to engage both the transcriptional program and cytoskeletal accommodations that characterize cardiac hypertrophy. Importantly, these observations suggest that identification of inhibitors that suppress caspase activity and/or activity of its cognate substrates may offer novel therapeutic targets to limit the development of pathological hypertrophy.

The role of Yca1 in proteostasis. Yca1 regulates the composition of the insoluble proteome

Proteostasis, the process of balancing protein production with protein degradation is vital to normal cell function. Defects within the mechanisms that control proteostasis lead to increased content of a specialized insoluble protein fraction that forms dense aggregates within the cell. We have previously implicated the Saccharomyces cerevisiae metacaspase Yca1 as an active participant in maintaining proteostasis, whereby Yca1 acts to limit aggregate content. Here, we further characterized the proteostasis role of Yca1 by conducting proteomic analysis of the insoluble protein fraction in wildtype and Yca1 knockout cells, under normal and heat stressed conditions. Our findings suggest that the composition of insoluble protein fraction is non-specific and comprises a wide array of protein species rather than a limited repertoire of aggregate susceptible proteins or peptides. Interestingly, the loss of Yca1 led to a significant decrease of proteins that control ribosome biogenesis and protein synthesis within the insoluble fraction, indicating that the cell may invoke a compensatory mechanism to limit protein production during stress, a feature dependent on Yca1 activity. Finally, we noted that protein degradation factors such as Cdc48 co-localize with Yca1 to the insoluble fraction, supporting the hypothesis that Yca1 may act primarily to dissolve or reduce accumulated aggregates. This article is part of a Special Issue entitled: From protein structures to clinical applications.

Bin1 SRC homology 3 domain acts as a scaffold for myofiber sarcomere assembly

In skeletal muscle development, the genes and regulatory factors that govern the specification of myocytes are well described. Despite this knowledge, the mechanisms that regulate the coordinated assembly of myofiber proteins into the functional contractile unit or sarcomere remain undefined. Here we explored the hypothesis that modular domain proteins such as Bin1 coordinate protein interactions to promote sarcomere formation. We demonstrate that Bin1 facilitates sarcomere organization through protein-protein interactions as mediated by the Src homology 3 (SH3) domain. We observed a profound disorder in myofiber size and structural organization in a murine model expressing the Bin1 SH3 region. In addition, satellite cell-derived myogenesis was limited despite the accumulation of skeletal muscle-specific proteins. Our experiments revealed that the Bin1 SH3 domain formed transient protein complexes with both actin and myosin filaments and the pro-myogenic kinase Cdk5. Bin1 also associated with a Cdk5 phosphorylation domain of titin. Collectively, these observations suggest that Bin1 displays protein scaffold-like properties and binds with sarcomeric factors important in directing sarcomere protein assembly and myofiber maturation.

Neural stem cell differentiation is dependent upon endogenous caspase 3 activity

Caspase proteases have become the focal point for the development and application of anti-apoptotic therapies in a variety of central nervous system diseases. However, this approach is based on the premise that caspase function is limited to invoking cell death signals. Here, we show that caspase-3 activity is elevated in nonapoptotic differentiating neuronal cell populations. Moreover, peptide inhibition of protease activity effectively inhibits the differentiation process in a cultured neurosphere model. These results implicate caspase-3 activation as a conserved feature of neuronal differentiation and suggest that targeted inhibition of this protease in neural cell populations may have unintended consequences.

A novel high throughput screening assay for HCV NS3 helicase activity

A novel assay for measurement of Hepatitis C virus (HCV) NS3 helicase activity was developed using Flashplate technology. This assay involves the use of a DNA duplex substrate and recombinant HCV NS3 produced in Escherichia coli. The DNA duplex consisted of a pair of oligonucleotides, one biotinylated, the other radiolabeled at their respective 5' termini. This DNA duplex was immobilized, via the biotin molecule, on the surface of a neutravidin-coated SMP103 Flashplate (NEN Life Science Products). Helicase activity results in the release of the radiolabeled oligonucleotide, which translates in signal reduction with respect to control wells. Biochemical characterization of the HCV NS3 helicase activity was performed using this assay. We demonstrated that the NS3-mediated unwinding is proportional to both the amount of DNA substrate in the well, and to the NS3 concentration in the reaction. Most of the NS3-mediated unwinding was achieved in the initial 60 min of incubation. As expected the reactions were ATP-dependent and found to be affected by the concentration of MgCl(2), MnCl(2), KCl, EDTA, and by pH. We found this assay to be highly reproducible since only slight variation was observed when a total of 68 helicase reactions were performed on one plate. Therefore, this Flashplate helicase assay is fast, convenient and reproducible. These criteria make it suitable for high throughput screening of potential NS3 helicase inhibitors.