Ribonucleosides from tRNA in hyperglycemic mammalian cells and diabetic murine cardiac models

AIMS

Cardiomyopathy is a diabetic comorbidity with few molecular targets. To address this, we evaluated transfer RNA (tRNA) modifications in the diabetic heart because tRNA modifications have been implicated in diabetic etiologies.

MAIN METHODS

tRNA was isolated from aorta, apex, and atrial tissue of healthy and diabetic murine hearts and related hyperglycemic cell models. tRNA modifications and canonical ribonucleosides were quantified by liquid-chromatography tandem mass spectrometry (LC-MS/MS) using stable isotope dilution. Correlations between ribonucleosides and diabetic comorbidity pathology were assessed using statistical analyses.

KEY FINDINGS

Total tRNA ribonucleoside levels were analyzed from cell types and healthy and diabetic murine heart tissue. Each heart structure had characteristic ribonucleoside profiles and quantities. Several ribonucleosides were observed as significantly different in hyperglycemic cells and diabetic tissues. In hyperglycemic models, ribonucleosides N4-acetylcytidine (ac4C), 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U), 5-methylcytidine (m5C), and N1-methylguanosine (m1G) were anomalous. Specific tRNA modifications known to be on murine tRNAIni(CAU) were higher in diabetic heart tissue which suggests that tRNA modifications could be regulating translation in diabetes.

SIGNIFICANCE

We identified tRNA ribonucleosides and tRNA species associated with hyperglycemia and diabetic etiology.

mRNA therapy for myocardial infarction: A review of targets and delivery vehicles

Cardiovascular diseases are the leading cause of death in the world. This is partly due to the low regenerative capacity of adult hearts. mRNA therapy is a promising approach under development for cardiac diseases. In mRNA therapy, expression of the target protein is modulated by delivering synthetic mRNA. mRNA therapy benefits cardiac regeneration by increasing cardiomyocyte proliferation, reducing fibrosis, and promoting angiogenesis. Because mRNA is translated in the cytoplasm, the delivery efficiency of mRNA into the cytoplasm and nucleus significantly affects its therapeutic efficacy. To improve delivery efficiency, non-viral vehicles such as lipid nanoparticles have been developed. Non-viral vehicles can protect mRNA from enzymatic degradation and facilitate the cellular internalization of mRNA. In addition to non-viral vehicles, viral vectors have been designed to deliver mRNA templates into cardiac cells. This article reviews lipid nanoparticles, polymer nanoparticles, and viral vectors that have been utilized to deliver mRNA into the heart. Because of the growing interest in lipid nanoparticles, recent advances in lipid nanoparticles designed for cardiac mRNA delivery are discussed. Besides, potential targets of mRNA therapy for myocardial infarction are discussed. Gene therapies that have been investigated in patients with cardiac diseases are analyzed. Reviewing mRNA therapy from a clinically relevant perspective can reveal needs for future investigations.

Multiplexed microfluidic chip for cell co-culture

Paracrine signaling is challenging to study in vitro, as conventional culture tools dilute soluble factors and offer little to no spatiotemporal control over signaling. Microfluidic chips offer potential to address both of these issues. However, few solutions offer both control over onset and duration of cell-cell communication, and high throughput. We have developed a microfluidic chip designed to culture cells in adjacent chambers, separated by valves to selectively allow or prevent exchange of paracrine signals. The chip features 16 fluidic inputs and 128 individually-addressable chambers arranged in 32 sets of 4 chambers. Media can be continuously perfused or delivered by diffusion, which we model under different culture conditions to ensure normal cell viability. Immunocytochemistry assays can be performed in the chip, which we modeled and fine-tuned to reduce total assay time to 1 h. Finally, we validate the use of the chip for co-culture studies by showing that HEK293Ta cells respond to signals secreted by RAW 264.7 immune cells in adjacent chambers, only when the valve between the chambers is opened.

Injectable Extracellular Matrix Microparticles Promote Heart Regeneration in Mice with Post-ischemic Heart Injury

Ischemic heart injury causes permanent cardiomyocyte loss and fibrosis impairing cardiac function. Tissue derived biomaterials have shown promise as an injectable treatment for the post-ischemic heart. Specifically, decellularized extracellular matrix (dECM) is a protein rich suspension that forms a therapeutic hydrogel once injected and improves the heart post-injury response in rodents and pig models. Current dECM-derived biomaterials are delivered to the heart as a liquid dECM hydrogel precursor or colloidal suspension, which gels over several minutes. To increase the functionality of the dECM therapy, an injectable solid dECM microparticle formulation derived from heart tissue to control sizing and extend stability in aqueous conditions is developed. When delivered into the infarcted mouse heart, these dECM microparticles protect cardiac function promote vessel density and reduce left ventricular remodeling by sustained delivery of biomolecules. Longer retention, higher stiffness, and slower protein release of dECM microparticles are noted compared to liquid dECM hydrogel precursor. In addition, the dECM microparticle can be developed as a platform for macromolecule delivery. Together, the results suggest that dECM microparticles can be developed as a modular therapy for heart injury.

Microenvironment Stiffness Amplifies Post-ischemia Heart Regeneration in Response to Exogenous Extracellular Matrix Proteins in Neonatal Mice

The cardiogenesis of the fetal heart is absent in juveniles and adults. Cross-transplantation of decellularized extracellular matrix (dECM) can stimulate regeneration in myocardial infarct (MI) models. We have previously shown that dECM and tissue stiffness have cooperative regulation of heart regeneration in transiently regenerative day 1 neonatal mice. To investigate underlying mechanisms of mechano-signaling and dECM, we pharmacologically altered heart stiffness and administered dECM hydrogels in non-regenerative mice after MI. The dECM combined with softening exhibits preserved cardiac function, LV geometry, increased cardiomyocyte mitosis and lowered fibrosis while stiffening further aggravated ischemic damage. Transcriptome analysis identified a protein in cardiomyocytes, CLCA2, confirmed to be upregulated after MI and downregulated by dECM in a mechanosensitive manner. Synthetic knock-down of CLCA2 expression induced mitosis in primary rat cardiomyocytes in the dish. Together, our results indicate that therapeutic efficacy of extracellular molecules for heart regeneration can be modulated by heart microenvironment stiffness in vivo.

Accounting for Material Changes in Decellularized Tissue with Underutilized Methodologies

Tissue decellularization has rapidly developed to be a practical approach in tissue engineering research; biological tissue is cleared of cells resulting in a protein-rich husk as a natural scaffold for growing transplanted cells as a donor organ therapy. Minimally processed, acellular extracellular matrix reproduces natural interactions with cells in vitro and for tissue engineering applications in animal models. There are many decellularization techniques that achieve preservation of molecular profile (proteins and sugars), microstructure features such as organization of ECM layers (interstitial matrix and basement membrane) and organ level macrofeatures (vasculature and tissue compartments). While structural and molecular cues receive attention, mechanical and material properties of decellularized tissues are not often discussed. The effects of decellularization on an organ depend on the tissue properties, clearing mechanism, chemical interactions, solubility, temperature, and treatment duration. Physical characterization by a few labs including work from the authors provides evidence that decellularization protocols should be tailored to specific research questions. Physical characterization beyond histology and immunohistochemistry of the decellularized matrix (dECM) extends evaluation of retained functional features of the original tissue. We direct our attention to current technologies that can be employed for structure function analysis of dECM using underutilized tools such as atomic force microscopy (AFM), cryogenic electron microscopy (cryo-EM), dynamic mechanical analysis (DMA), Fourier-transform infrared spectroscopy (FTIR), mass spectrometry, and rheometry. Structural imaging and mechanical functional testing combined with high-throughput molecular analyses opens a new approach for a deeper appreciation of how cellular behavior is influenced by the isolated microenvironment (specifically dECM). Additionally, the impact of these features with different decellularization techniques and generation of synthetic material scaffolds with desired attributes are informed. Ultimately, this mechanical profiling provides a new dimension to our understanding of decellularized matrix and its role in new applications.

Microenvironment stiffness requires decellularized cardiac extracellular matrix to promote heart regeneration in the neonatal mouse heart

The transient period of regeneration potential in the postnatal heart suggests molecular changes with maturation influence the cardiac response to damage. We have previously demonstrated that injury and exercise can stimulate cardiomyocyte proliferation in the adult heart suggesting a sensitivity to exogenous signals. Here, we consider whether exogenous fetal ECM and mechanically unloading interstitial matrix can drive regeneration after myocardial infarction (MI) surgery in low-regenerative hearts of day5 mice. Compared to controls, exogenous fetal ECM increases cardiac function and lowers fibrosis at 3 weeks post-injury and this effect can be augmented by softening heart tissue. In vitro experiments support a mechano-sensitivity to exogenous ECM signaling. We tested potential mechanisms and observed that fetal ECM increases nuclear YAP localization which could be enhanced by pharmacological stabilization of the cytoskeleton. Blocking YAP expression lowered fetal ECM effects though not completely. Lastly we observed mechanically unloading heart interstitial matrix increased agrin expression, an extracellular node in the YAP signaling pathway. Collectively, these data support a combined effect of exogenous factors and mechanical activity in altering agrin expression, cytoskeletal remodeling, and YAP signaling in driving cardiomyocyte cell cycle activity and regeneration in postnatal non-regenerative mice. STATEMENT OF SIGNIFICANCE: With the purpose of developing regenerative strategies, we investigate the influence of the local niche on the cardiac injury response. We conclude tissue stiffness, as anticipated in aging or disease, impairs regenerative therapeutics. Most novel, mechanical unloading facilitates enhanced cardiac regeneration only after cells are pushed into a permissive state by fetal biomolecules. Specifically, mechanical unloading appears to increase extracellular agrin expression that amplifies fetal-stimulation of nuclear YAP signaling which correlates with observed increases of cell cycle activity in cardiomyocytes. The results further suggest the cytoskeleton is critical to this interaction between mechanical unloading and independently actived YAP signaling. Using animal models, tissue explants, and cells, this work indicates that local mechanical stimuli can augment proliferating-permissive cardiomyocytes in the natural cardiac niche.

Exercise induces new cardiomyocyte generation in the adult mammalian heart

Loss of cardiomyocytes is a major cause of heart failure, and while the adult heart has a limited capacity for cardiomyogenesis, little is known about what regulates this ability or whether it can be effectively harnessed. Here we show that 8 weeks of running exercise increase birth of new cardiomyocytes in adult mice (~4.6-fold). New cardiomyocytes are identified based on incorporation of ¹⁵N-thymidine by multi-isotope imaging mass spectrometry (MIMS) and on being mononucleate/diploid. Furthermore, we demonstrate that exercise after myocardial infarction induces a robust cardiomyogenic response in an extended border zone of the infarcted area. Inhibition of miR-222, a microRNA increased by exercise in both animal models and humans, completely blocks the cardiomyogenic exercise response. These findings demonstrate that cardiomyogenesis can be activated by exercise in the normal and injured adult mouse heart and suggest that stimulation of endogenous cardiomyocyte generation could contribute to the benefits of exercise.

The adult mammalian heart has a limited cardiomyogenic capacity. Here the authors show that intensive exercise leads to a 4.6-fold increase in murine cardiomyocyte proliferation requiring the expression of miR-222, and that exercise induces an extended cardiomyogenic response in the murine heart after infarction.

Loss of white adipose hyperplastic potential is associated with enhanced susceptibility to insulin resistance

Fat mass expansion occurs by adipocyte hypertrophy or recruitment of differentiating adipocyte progenitors, the relative balance of which may impact systemic metabolism. We measured adipogenesis in murine subcutaneous (sWAT) and visceral white adipose tissue (vWAT) using stable isotope methodology and then modeled adipocyte turnover. Birth and death rates were similar within depots; however, turnover was higher in vWAT relative to sWAT. In juvenile mice, obesity increased adipogenesis, but in adults, this was only seen in vWAT after prolonged high-fat feeding. Statistical modeling suggests differentiation of adipocyte progenitors without an accompanying self-renewing division step may partially explain the age-dependent decline in hyperplastic potential. Additional metabolic interrogation of obese mice demonstrated an association between adipocyte turnover and insulin sensitivity. These data therefore identify adipocyte hypertrophy as the dominant mechanism of adult fat mass expansion and support the paradoxical concept that metabolic disease ensues due to a failure of adipose tissue plasticity.

Cardiac regeneration based on mechanisms of cardiomyocyte proliferation and differentiation

Cardiomyocyte proliferation and progenitor differentiation are endogenous mechanisms of myocardial development. Cardiomyocytes continue to proliferate in mammals for part of post-natal development. In adult mammals under homeostatic conditions, cardiomyocytes proliferate at an extremely low rate. Because the mechanisms of cardiomyocyte generation provide potential targets for stimulating myocardial regeneration, a deep understanding is required for developing such strategies. We will discuss approaches for examining cardiomyocyte regeneration, review the specific advantages, challenges, and controversies, and recommend approaches for interpretation of results. We will also draw parallels between developmental and regenerative principles of these mechanisms and how they could be targeted for treating heart failure.

Myocyte remodeling in response to hypertrophic stimuli requires nucleocytoplasmic shuttling of muscle LIM protein

CSRP3 or muscle LIM protein (MLP) is a nucleocytoplasmic shuttling protein and a mechanosensor in cardiac myocytes. MLP regulation and function was studied in cultured neonatal rat myocytes treated with pharmacological or mechanical stimuli. Either verapamil or BDM decreased nuclear MLP while phenylephrine and cyclic strain increased it. These results suggest that myocyte contractility regulates MLP subcellular localization. When RNA polymerase II was inhibited with alpha-amanitin, nuclear MLP was reduced by 30%. However, when both RNA polymerase I and II were inhibited with actinomycin D, there was a 90% decrease in nuclear MLP suggesting that its nuclear translocation is regulated by both nuclear and nucleolar transcriptional activity. Using cell permeable synthetic peptides containing the putative nuclear localization signal (NLS) of MLP, nuclear import of the protein in cultured rat neonatal myocytes was inhibited. The NLS of MLP also localizes to the nucleolus. Inhibition of nuclear translocation prevented the increased protein accumulation in response to phenylephrine. Furthermore, cyclic strain of myocytes after prior NLS treatment to remove nuclear MLP resulted in disarrayed sarcomeres. Increased protein synthesis and brain natriuretic peptide expression were also prevented suggesting that MLP is required for remodeling of the myofilaments and gene expression. These findings suggest that nucleocytoplasmic shuttling MLP plays an important role in the regulation of the myocyte remodeling and hypertrophy and is required for adaptation to hypertrophic stimuli.

Stimulus interval, rate and direction differentially regulate phosphorylation for mechanotransduction in neonatal cardiac myocytes

The effect of interval, direction and rate of strain on mechanotransduction in neonatal rat cardiomyocytes is determined for focal adhesion kinase (Y397pFAK), extracellular signal-regulated kinase ERK1/2 (Thr(183)/Tyr(185)) and paxillin (pY31) and phosphorylation time courses to 10% strain assessed. Cells are non-responsive at 5 min but recover at 15 min (P<0.03) with FAK nuclear translocation by 30 min. Cyclic biaxial strain increased phosphorylation from slower to faster rates (P<0.05). Uniaxial strain to groove-aligned myocytes increased FAK and ERK1/2 phosphorylation transversely more than longitudinally (P<0.05). Mechanotransduction may have a refractory period of 5 min and differentiate directions and rates of strain.