Abstract P3072: Human Induced Pluripotent Stem Cell Derived Cardiomyocytes (iPSC-CMs) Respond To Inflammatory Stimuli Through Innate Immune Responses

Introduction: Cells respond to inflammatory stimuli in settings of sterile and pathologic cardiac inflammation. Nuclear factor Kappa B (NFKB) is a transcriptional mediator of innate immune responses. Lipopolysaccharide (LPS) promotes NFKB activation through toll-like receptors (TLR) to initiate cell autonomous processes that resist infection. High-mobility box group 1 (HMGB1) is a chromatin associated protein released upon nuclear rupture that similarly promotes cytokine release in response to injury. Few studies have interrogated multiple inflammatory stimuli and their impacts on iPSC-CM function. We examined innate immune activation using distinct TLR agonists and assessed functional responses in both 2D iPSC-CM monoculture and 3D collagen-based engineered heart tissues.

Methods and Results: We measured electric field potential duration (Nanion CardioExcyte 96) of iPSC-CM monolayers after exposure to LPS and HMGB1. We observed a four fold increase in downstroke velocity after exposure to HMGB1 (512.7±126.9 V/s; p=0.0002 n=12) and a 30% decrease in downstroke velocity after LPS treatment (-188.9±61.4 V/s; p=0.004 n=12). This response was associated with nuclear localization of phosphorylated NFKB. Collagen-based engineered heart tissues demonstrated contractile defects in response to 48h of exposure to TLR agonists. A 50% reduction in fractional shortening was observed following HMGB1 treatment (-0.0060±0.0009; p<0.0001 n=6) and an 80% reduction was associated with LPS exposure (-0.0088±0.0006; p<0.0001 n=6). To identify gene expression regulatory regions associated with this response, we examined transcriptional activity of three candidate enhancer regions implicated in cytokine production in iPSC-CMs. These regions demonstrated a 0.5- to 20-fold increase in enhancer activity in iPSC-CMs. One enhancer demonstrated a 22% increase in activity following 48h of LPS, consistent with a direct role for NFKB in innate immune activation in human iPSC-CMs; this enhancer regulates a gene with increased expression in dilated cardiomyopathy.

Conclusion: These data indicate that cardiomyocytes respond directly to inflammatory stimuli and demonstrate the utility of 2D and 3D human tissue culture models for cardiac inflammation.

Distinct pathological signatures in human cellular models of myotonic dystrophy subtypes

Myotonic dystrophy (DM) is the most common autosomal dominant muscular dystrophy and encompasses both skeletal muscle and cardiac complications. DM is nucleotide repeat expansion disorder in which type 1 (DM1) is due to a trinucleotide repeat expansion on chromosome 19 and type 2 (DM2) arises from a tetranucleotide repeat expansion on chromosome 3. Developing representative models of DM in animals has been challenging due to instability of nucleotide repeat expansions, especially for DM2, which is characterized by nucleotide repeat expansions often greater than 5,000 copies. To investigate mechanisms of human DM, we generated cellular models of DM1 and DM2. We used regulated MyoD expression to reprogram urine-derived cells into myotubes. In this myogenic cell model, we found impaired dystrophin expression, in the presence of muscleblind-like 1 (MBNL1) foci, and aberrant splicing in DM1 but not in DM2 cells. We generated induced pluripotent stem cells (iPSC) from healthy controls and DM1 and DM2 subjects, and we differentiated these into cardiomyocytes. DM1 and DM2 cells displayed an increase in RNA foci concomitant with cellular differentiation. iPSC-derived cardiomyocytes from DM1 but not DM2 had aberrant splicing of known target genes and MBNL sequestration. High-resolution imaging revealed tight association between MBNL clusters and RNA foci in DM1. Ca2+ transients differed between DM1- and DM2 iPSC-derived cardiomyocytes, and each differed from healthy control cells. RNA-sequencing from DM1- and DM2 iPSC-derived cardiomyocytes revealed distinct misregulation of gene expression, as well as differential aberrant splicing patterns. Together, these data support that DM1 and DM2, despite some shared clinical and molecular features, have distinct pathological signatures.

Ferlin proteins in myoblast fusion and muscle growth

Myoblast fusion contributes to muscle growth in development and during regeneration of mature muscle. Myoblasts fuse to each other as well as to multinucleate myotubes to enlarge the myofiber. The molecular mechanisms of myoblast fusion are incompletely understood. Adhesion, apposition, and membrane fusion are accompanied by cytoskeletal rearrangements. The ferlin family of proteins is implicated in human muscle disease and has been implicated in fusion events in muscle, including myoblast fusion, vesicle trafficking and membrane repair. Dysferlin was the first mammalian ferlin identified and it is now known that there are six different ferlins. Loss-of-function mutations in the dysferlin gene lead to limb girdle muscular dystrophy and the milder disorder Miyoshi Myopathy. Dysferlin is a membrane-associated protein that has been implicated in resealing disruptions in the muscle plasma membrane. Newer data supports a broader role for dysferlin in intracellular vesicular movement, a process also important for resealing. Myoferlin is highly expressed in myoblasts that undergoing fusion, and the absence of myoferlin leads to impaired myoblast fusion. Myoferlin also regulates intracellular trafficking events, including endocytic recycling, a process where internalized vesicles are returned to the plasma membrane. The trafficking role of ferlin proteins is reviewed herein with a specific focus as to how this machinery alters myogenesis and muscle growth.

Nuclear sequestration of δ-sarcoglycan disrupts the nuclear localization of lamin A/C and emerin in cardiomyocytes

Sarcoglycan is a membrane-associated protein complex found at the plasma membrane of cardiomyocytes and skeletal myofibers. Recessive mutations of delta-sarcoglycan that eliminate expression, and therefore function, lead to cardiomyopathy and muscular dystrophy by producing instability of the plasma membrane. A dominant missense mutation in the gene encoding delta-sarcoglycan was previously shown to associate with dilated cardiomyopathy in humans. To investigate the mechanism of dominantly inherited cardiomyopathy, we generated transgenic mice that express the S151A delta-sarcoglycan mutation in the heart using the alpha-myosin heavy-chain gene promoter. Similar to the human delta-sarcoglycan gene mutation, S151A delta-sarcoglycan transgenic mice developed dilated cardiomyopathy at a young age with enhanced lethality. Instead of placement at the plasma membrane, delta-sarcoglycan was found in the nucleus of S151A delta-sarcoglycan cardiomyocytes. Retention of delta-sarcoglycan in the nucleus was accompanied by partial nuclear sequestration of beta- and gamma-sarcoglycan. Additionally, the nuclear-membrane-associated proteins, lamin A/C and emerin, were mislocalized throughout the nucleoplasm. Therefore, the S151A delta-sarcoglycan gene mutation acts in a dominant negative manner to produce trafficking defects that disrupt nuclear localization of lamin A/C and emerin, thus linking together two common mechanisms of inherited cardiomyopathy.

Genetic background influences muscular dystrophy

Mutations in the genes encoding dystrophin and its associated proteins, the sarcoglycans, lead to muscular dystrophy in humans and in mouse models. In the presence of identical gene mutations, the muscular dystrophy phenotype can be highly variable. Using a mouse model of limb girdle muscular dystrophy engineered with a null allele of gamma-sarcoglycan, we bred the identical gamma-sarcoglycan mutation into four different genetic backgrounds. We found that the gamma-sarcoglycan mutation is least severe in the129SV/J (129) strain and most severe on the DBA 2J JAX (DBA) strain using quantitative measures of Evan's blue dye uptake, as a marker of membrane permeability defects, and hydroxyproline content, as a marker of fibrosis. In addition we show that the DBA mice are most severely affected regardless of gender and age. The enhanced phenotype observed in the DBA strain was not caused by exercise as the DBA mice scored the lowest in a voluntary activity test. The milder phenotype seen in the 129SV/J and C57B6/J strains suggests that these backgrounds contain modifier loci that partially suppress the muscular dystrophy phenotype. Identification of these modifier genes and the associated pathways may lead to novel therapeutic strategies.