Chowdhury S. Abdullah

E pigenetic enzyme- r esponsive nano p articles (ERPs), which destabilize under the influence of histone deacetylase 8 (HDAC8), have been characterized in terms of their physicochemical, nanomechanical, and in vitro attributes. These nanoparticles were synthesized from amphiphilic block copolymers composed of poly(acetyl l-lysine) as the hydrophobic and poly(ethylene glycol) as the hydrophilic block. Upon optimizing the particle assembly process, we demonstrated that the nanoscale properties of ERPs, including hydrodynamic diameter (D_H), size distribution, and stiffness, are selectively influenced by the HDAC8 enzyme. Using ¹H NMR and circular dichroism spectroscopy, we identified that deacetylation of the lysine residues located within the hydrophobic block acts as a molecular driver for the enzyme-induced destabilization of ERPs. Comprehensive in vitro studies demonstrated that ERPs are biocompatible and noncytotoxic and can accumulate in perinuclear and several cellular organelles, including lysosomes and mitochondria. Collectively, this study attempts to establish the mechanistic background of ERP functions, providing the baseline for designing epigenetic-enzyme-sensitive nanoscale platforms.

The incidence of metabolic dysfunction-associated steatohepatitis (MASH) is on the rise, and with limited pharmacological therapy available, identification of new metabolic targets is urgently needed. Oxalate is a terminal metabolite produced from glyoxylate by hepatic lactate dehydrogenase (LDHA). The liver-specific alanine-glyoxylate aminotransferase (AGXT) detoxifies glyoxylate, preventing oxalate accumulation. Here we show that AGXT is suppressed and LDHA is activated in livers from patients and mice with MASH, leading to oxalate overproduction. In turn, oxalate promotes steatosis in hepatocytes by inhibiting peroxisome proliferator-activated receptor-α (PPARα) transcription and fatty acid β-oxidation and induces monocyte chemotaxis via C–C motif chemokine ligand 2. In male mice with diet-induced MASH, targeting oxalate overproduction through hepatocyte-specific AGXT overexpression or pharmacological inhibition of LDHA potently lowers steatohepatitis and fibrosis by inducing PPARα-driven fatty acid β-oxidation and suppressing monocyte chemotaxis, nuclear factor-κB and transforming growth factor-β targets. These findings highlight hepatic oxalate overproduction as a target for the treatment of MASH.

Doxorubicin (DOX)-induced cardiomyopathy constitutes dose-dependent cardiac toxicity, culminating in fatal heart failure progression. Cardiac toxicity limits effective and subsequent use of DOX in chemotherapy regimens in pediatric, adult, and recurrent cancer patients. DOX-induced profound alterations in mitochondrial morphology, dynamics, and defects in mitochondrial energy metabolism in the heart comprise key stressors in DOX-induced cardiotoxicity. Hence, the discovery of novel molecular targets and therapeutics to mitigate DOX-induced mitochondrial dysfunctions are imperative. Herein, we provided two laboratory protocols to monitor DOX-induced alterations in mitochondrial morphology and respiration in isolated primary neonatal rat cardiomyocytes. Neonatal rat cardiomyocytes are extensively used to monitor signaling mechanisms regulating cardiomyopathy in vitro. Therefore, these protocols will help researchers study the effects of novel pharmacological and genetic manipulations against DOX-induced alterations in mitochondrial morphology and energy metabolism in cardiomyocytes.

Sigmar1 is a widely expressed, multitasking molecular chaperone protein playing functional roles in several cellular processes. Mutations in the Sigmar1 gene have been reported to associate with several motor neuropathies, including amyotrophic lateral sclerosis, distal hereditary motor neuropathy, silver-like syndrome, and frontotemporal lobar degeneration. All these human mutations associated with motor neuropathies show strong manifestation in skeletal muscle with phenotypes like muscle wasting and atrophy. However, the physiological function of Sigmar1 in skeletal muscle remains unexplored. Here, we determined the physiological role of Sigmar1 in skeletal muscle structure and function in five different skeletal muscles: gastrocnemius (Gastro), quadriceps (Quad), soleus (Sol), extensor digitorum longus (EDL), and tibialis anterior (TA). Quantification of myofiber cross-sectional area (CSA) showed altered muscle mass and a slow-to-fast fiber-type switch in the skeletal muscle fibers of the Sigmar1^−/− mice. Interestingly, ultrastructural analysis by transmission electron microscopy showed the presence of tubular aggregates in the type I myofibers (Gastro, Quad, and TA) of Sigmar1^−/− mice. Immunostaining also showed derangements in dystrophin localization in skeletal muscles from Sigmar1^−/− mice. Additionally, skeletal muscle myopathy in Sigmar1^−/− mice was associated with an increased number of central nuclei, increased collagen deposition, and fibrosis. Functional studies also showed reduced endurance and exercise capacity in the Sigmar1^−/− mice. Overall, our studies demonstrated, for the first time, a potential physiological function of Sigmar1 in the skeletal muscle in maintaining healthy muscle structure and function.

Background: The mutated α‐B‐Crystallin (CryAB ^R120G ) mouse model of desmin‐related myopathy (DRM) shows an age‐dependent onset of pathologic cardiac remodeling and progression of heart failure. CryAB ^R120G expression in cardiomyocytes affects the mitochondrial spatial organization within the myofibrils, but the molecular perturbation within the mitochondria in the relation of the overall course of the proteotoxic disease remains unclear. Methods and Results: CryAB ^R120G mice show an accumulation of electron‐dense aggregates and myofibrillar degeneration associated with the development of cardiac dysfunction. Though extensive studies demonstrated that these altered ultrastructural changes cause cardiac contractility impairment, the molecular mechanism of cardiomyocyte death remains elusive. Here, we explore early pathological processes within the mitochondria contributing to the contractile dysfunction and determine the pathogenic basis for the heart failure observed in the CryAB ^R120G mice. In the present study, we report that the CryAB ^R120G mice transgenic hearts undergo altered mitochondrial dynamics associated with increased level of dynamin‐related protein 1 and decreased level of optic atrophy type 1 as well as mitofusin 1 over the disease process. In association with these changes, an altered level of the components of mitochondrial oxidative phosphorylation and pyruvate dehydrogenase complex regulatory proteins occurs before the manifestation of pathologic adverse remodeling in the CryAB ^R120G hearts. Mitochondria isolated from CryAB ^R120G transgenic hearts without visible pathology show decreased electron transport chain complex activities and mitochondrial respiration. Taken together, we demonstrated the involvement of mitochondria in the pathologic remodeling and progression of DRM‐associated cellular dysfunction. Conclusions: Mitochondrial dysfunction in the form of altered mitochondrial dynamics, oxidative phosphorylation and pyruvate dehydrogenase complex proteins level, abnormal electron transport chain complex activities, and mitochondrial respiration are evident on the CryAB ^R120G hearts before the onset of detectable pathologies and development of cardiac contractile dysfunction.

Methamphetamine-associated cardiomyopathy is the leading cause of death linked with illicit drug use. Here we show that Sigmar1 is a therapeutic target for methamphetamine-associated cardiomyopathy and defined the molecular mechanisms using autopsy samples of human hearts, and a mouse model of “binge and crash” methamphetamine administration. Sigmar1 expression is significantly decreased in the hearts of human methamphetamine users and those of “binge and crash” methamphetamine-treated mice. The hearts of methamphetamine users also show signs of cardiomyopathy, including cellular injury, fibrosis, and enlargement of the heart. In addition, mice expose to “binge and crash” methamphetamine develop cardiac hypertrophy, fibrotic remodeling, and mitochondrial dysfunction leading to contractile dysfunction. Methamphetamine treatment inhibits Sigmar1, resulting in inactivation of the cAMP response element-binding protein (CREB), decreased expression of mitochondrial fission 1 protein (FIS1), and ultimately alteration of mitochondrial dynamics and function. Therefore, Sigmar1 is a viable therapeutic agent for protection against methamphetamine-associated cardiomyopathy.

Mitochondria are the key to properly functioning energy generation in the metabolically demanding cardiomyocytes and thus essential to healthy heart contractility on a beat-to-beat basis. As the central organelle for cellular metabolism and signaling in the heart, dysfunction of the mitochondria leads to cardiovascular disease. The healthy mitochondrial functioning critical to maintaining cardiomyocyte viability and contractility is accomplished by adaptive changes in the dynamics, biogenesis, and degradation of the mitochondria to ensure cellular proteostasis. Recent compelling evidence suggests that the classical protein quality control system in cardiomyocytes is also under constant mitochondrial control, either directly or indirectly. Impairment of cytosolic protein quality control may affect the position of the mitochondria in relation to other organelles, as well as mitochondrial morphology and function, and could also activate mitochondrial proteostasis. Despite a growing interest in the mitochondrial quality control system, very little information is available about the molecular function of mitochondria in cardiac proteostasis. In this review, we bring together current understanding of the adaptations and role of the mitochondria in cardiac proteostasis and describe the adaptive/maladaptive changes observed in the mitochondrial network required to maintain proteomic integrity. We also highlight the key mitochondrial signaling pathways activated in response to proteotoxic stress as a cellular mechanism to protect the heart from proteotoxicity. A deeper understanding of the molecular mechanisms of mitochondrial adaptations and their role in cardiac proteostasis will help to develop future therapeutics to protect the heart from cardiovascular diseases.