The Effects of Arrhythmogenic Right Ventricular Cardiomyopathy-Causing Proteins on the Mechanical and Signaling Properties of Cardiac Myocytes

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by a high incidence of lethal ventricular arrhythmias, fibrofatty replacement of myocardium, and can account for up to 20% of sudden cardiac death (SCD) cases in the young. Typically involving autosomal dominant transmission, germline mutations in genes encoding desmosomal proteins have been identified as a cause of ARVC, although the pathogenesis of the disease is still unclear. While early detection and treatment can provide a normal life expectancy for the majority of patients, with less than 10% progressing to overt right ventricular failure, low genetic penetrance and epigenetic modifiers (such as endurance exercise) can make the condition difficult to diagnose. Addressing this clinical challenge requires a better understanding of the defective molecular mechanisms that underlie the disease. To that end, the goal of this dissertation is to provide insight into the effects of ARVC-causing mutant proteins on the mechanical and signaling properties of cardiac myocytes. Using elastography and histological techniques, we begin by characterizing the structural and mechanical properties of the native right ventricular myocardium, particularly the right ventricular apex (RVA). Because the RVA is a key site for development of arrhythmias and a potential pacing target, a careful characterization of its structure and mechanical properties are essential for understanding its role in cardiac physiology. In the first section of this dissertation, we perform a systematic analysis of the structural features and mechanical strains in the heart, focusing on the RVA region. More than half of ARVC patients exhibit one or more mutations in genes encoding desmosomal proteins. This has led many investigators to suggest that ARVC is a "disease of the desmosome" in which defective cell-cell adhesion plays a critical pathogenic role, although direct evidence for this hypothesis is lacking. To gain greater insights into potential mechanisms by which desmosomal mutations cause ARVC, we next characterize biomechanical properties and responses to shear stress (motivated by our results in the previous section) in neonatal rat ventricular myocytes expressing two distinct mutant forms of the desmosomal protein plakoglobin which have been linked to ARVC in patients. We show that ARVC-causing mutations in plakoglobin lead to altered cellular distribution of plakoglobin, without alterations in cell mechanical properties or certain early signaling pathways. The identification of defective molecular mechanisms that are common across ARVC-patients remains a strategic area of research. Specifically, recent studies have investigated the mechanistic basis for different ARVC-causing mutations in hopes of identifying common defects in a signaling pathway - information that could be used to develop diagnostic tests or identify therapeutic targets. In the last section of this dissertation, we investigate the effects of mutant plakophilin-2 expression, and repeat key experiments performed in the previous section to identify common defects in mechanical and signaling properties. We identify a common, underlying defect in ARVC pathogenesis. Specifically, we show that disease-causing mutations across different desmosomal proteins can cause the cell to respond abnormally to mechanical shear stress with respect to plakoglobin trafficking

Folliculin, the Product of the Birt-Hogg-Dube Tumor Suppressor Gene, Interacts with the Adherens Junction Protein p0071 to Regulate Cell-Cell Adhesion

Birt-Hogg-Dube (BHD) is a tumor suppressor gene syndrome associated with fibrofolliculomas, cystic lung disease, and chromophobe renal cell carcinoma. In seeking to elucidate the pathogenesis of BHD, we discovered a physical interaction between folliculin (FLCN), the protein product of the BHD gene, and p0071, an armadillo repeat containing protein that localizes to the cytoplasm and to adherens junctions. Adherens junctions are one of the three cell-cell junctions that are essential to the establishment and maintenance of the cellular architecture of all epithelial tissues. Surprisingly, we found that downregulation of FLCN leads to increased cell-cell adhesion in functional cell-based assays and disruption of cell polarity in a three-dimensional lumen-forming assay, both of which are phenocopied by downregulation of p0071. These data indicate that the FLCN-p0071 protein complex is a negative regulator of cell-cell adhesion. We also found that FLCN positively regulates RhoA activity and Rho-associated kinase activity, consistent with the only known function of p0071. Finally, to examine the role of Flcn loss on cell-cell adhesion in vivo, we utilized keratin-14 cre-recombinase (K14-cre) to inactivate Flcn in the mouse epidermis. The K14-Cre-Bhdflox/flox mice have striking delays in eyelid opening, wavy fur, hair loss, and epidermal hyperplasia with increased levels of mammalian target of rapamycin complex 1 (mTORC1) activity. These data support a model in which dysregulation of the FLCN-p0071 interaction leads to alterations in cell adhesion, cell polarity, and RhoA signaling, with broad implications for the role of cell-cell adhesion molecules in the pathogenesis of human disease, including emphysema and renal cell carcinoma

Folliculin regulates cell–cell adhesion, AMPK, and mTORC1 in a cell‐type‐specific manner in lung‐derived cells

Abstract Germline loss‐of‐function BHD mutations cause cystic lung disease and hereditary pneumothorax, yet little is known about the impact of BHD mutations in the lung. Folliculin (FLCN), the product of the Birt–Hogg–Dube (BHD) gene, has been linked to altered cell–cell adhesion and to the AMPK and mTORC1 signaling pathways. We found that downregulation of FLCN in human bronchial epithelial (HBE) cells decreased the phosphorylation of ACC, a marker of AMPK activation, while downregulation of FLCN in small airway epithelial (SAEC) cells increased the activity of phospho‐S6, a marker of mTORC1 activation, highlighting the cell type–dependent functions of FLCN. Cell–cell adhesion forces were significantly increased in FLCN‐deficient HBE cells, consistent with prior findings in FLCN‐deficient human kidney‐derived cells. To determine how these altered cell–cell adhesion forces impact the lung, we exposed mice with heterozygous inactivation of Bhd (similarly to humans with germline inactivation of one BHD allele) to mechanical ventilation at high tidal volumes. Bhd+/− mice exhibited a trend (P = 0.08) toward increased elastance after 6 h of ventilation at 24 cc/kg. Our results indicate that FLCN regulates the AMPK and mTORC1 pathways and cell–cell adhesion in a cell type–dependent manner. FLCN deficiency may impact the physiologic response to inflation‐induced mechanical stress, but further investigation is required. We hypothesize that FLCN‐dependent effects on signaling and cellular adhesion contribute to the pathogenesis of cystic lung disease in BHD patients

Electrophysiological characterization of drug response in hSC-derived cardiomyocytes using voltage-sensitive optical platforms

Introduction: Voltage-sensitive optical (VSO) sensors offer a minimally invasive method to study the time course of repolarization of the cardiac action potential (AP). This Comprehensive in vitro Proarrhythmia Assay (CiPA) cross-platform study investigates protocol design and measurement variability of VSO sensors for preclinical cardiac electrophysiology assays. Methods: Three commercial and one academic laboratory completed a limited study of the effects of 8 blinded compounds on the electrophysiology of 2 commercial lines of human induced pluripotent stem-cell derived cardiomyocytes (hSC-CMs). Acquisition technologies included CMOS camera and photometry; fluorescent voltage sensors included di-4-ANEPPS, FluoVolt and genetically encoded QuasAr2. The experimental protocol was standardized with respect to cell lines, plating and maintenance media, blinded compounds, and action potential parameters measured. Serum-free media was used to study the action of drugs, but the exact composition and the protocols for cell preparation and drug additions varied among sites. Results: Baseline AP waveforms differed across platforms and between cell types. Despite these differences, the relative responses to four selective ion channel blockers (E-4031, nifedipine, mexiletine, and JNJ 303 blocking IKr, ICaL, INa, and IKs, respectively) were similar across all platforms and cell lines although the absolute changes differed. Similarly, four mixed ion channel blockers (flecainide, moxifloxacin, quinidine, and ranolazine) had comparable effects in all platforms. Differences in repolarisation time course and response to drugs could be attributed to cell type and experimental method differences such as composition of the assay media, stimulated versus spontaneous activity, and single versus cumulative compound addition. Discussion: In conclusion, VSOs represent a powerful and appropriate method to assess the electrophysiological effects of drugs on iPSC-CMs for the evaluation of proarrhythmic risk. Protocol considerations and recommendations are provided toward standardizing conditions to reduce variability of baseline AP waveform characteristics and drug responses

Cell-Cell Contact Preserves Cell Viability via Plakoglobin

Control over cell viability is a fundamental property underlying numerous physiological processes. Cell spreading on a substrate was previously demonstrated to be a major factor in determining the viability of individual cells. In multicellular organisms, cell-cell contact is likely to play a significant role in regulating cell vitality, but its function is easily masked by cell-substrate interactions, thus remains incompletely characterized. In this study, we show that suspended immortalized human keratinocyte sheets with persisting intercellular contacts exhibited significant contraction, junctional actin localization, and reinforcement of cell-cell adhesion strength. Further, cells within these sheets remain viable, in contrast to trypsinized cells suspended without either cell-cell or cell-substrate contact, which underwent apoptosis at high rates. Suppression of plakoglobin weakened cell-cell adhesion in cell sheets and suppressed apoptosis in suspended, trypsinized cells. These results demonstrate that cell-cell contact may be a fundamental control mechanism governing cell viability and that the junctional protein plakoglobin is a key regulator of this process. Given the near-ubiquity of plakoglobin in multicellular organisms, these findings could have significant implications for understanding cell adhesion, modeling disease progression, developing therapeutics and improving the viability of tissue engineering protocols

Electromechanical properties of suspended Graphene Nanoribbons

Graphene nanoribbons present diverse electronic properties ranging from semiconducting to half-metallic, depending on their geometry, dimensions and chemical composition. Here we present a route to control these properties via externally applied mechanical deformations. Using state-of-the-art density functional theory calculations combined with classical elasticity theory considerations, we find a remarkable Young's modulus value of ~7 TPa for ultra-narrow graphene strips and a pronounced electromechanical response towards bending and torsional deformations. Given the current advances in the synthesis of nanoscale graphene derivatives, our predictions can be experimentally verified opening the way to the design and fabrication of miniature electromechanical sensors and devices based on ultra-narrow graphene nanoribbons.Comment: 12 pages, 6 figure

An ancient dental gene network regulates development and continuous regeneration of teeth in sharks

The appearance of toothed vertebrates has proven a major determinant of the overall success of this lineage. This is most apparent in sharks and rays (elasmobranchs), which further retain the capacity for life-long tooth regeneration. Given their comparatively basal phylogenetic position, elasmobranchs therefore offer the opportunity for crucial insights into putative ancestral characters of tooth development, yet despite their evolutionary significance this remains poorly understood. Using the established chondrichthyan model, the catshark (Scyliorhinus sp.), we identified the expression of genes representative of conserved signaling pathways during stages of early dental competence, tooth initiation and regeneration. The expression patterns of β-catenin, shh, bmp4, pax9, pitx1/2, and the stem cell marker Sox2, characterise an ancestrally conserved gene set deployed during initiation of the elasmobranch dentition, suggesting that all vertebrate dentitions are defined by the expression of this core set of genes. These findings provide novel evidence to support the conservation in deep evolutionary time of a core set of dental patterning genes, therefore further defining the evolutionary trajectory of tooth development. We show how these genes facilitate the emergence of the shark dentition and offer insights into their deployment during development of the dental lamina, a sheet of dental epithelial cells that are responsible for continuous tooth regeneration. This study further promotes a specific experimental agenda to further characterise the roles of these core developmental genes during vertebrate tooth development, and importantly dental regeneration