NDUFS4, miR-27b and Cardiac Hypertrophy
Norma is majoring in Biological Sciences and minoring in Global Health Studies. She has been interested in research since high school and previously worked on investigating the genetic basis of obesity and its association with non alcoholic fatty liver disease. Currently, she is working with others on a project to develop a prostate cancer risk calculator in African American men.
In a nutshell, what is your research topic?
Cardiac hypertrophy, the enlargement of the heart, is often implicated in heart failure and other complications. Although it can be physiological, pathological hypertrophy is often associated with obesity, high blood pressure, and other preexisting conditions. Mitochondria is especially important in the functioning of the heart because it provides the energy required for the heart to pump blood normally. There is a protein, NDUFS4, that encodes for an important subunit in the mitochondria. There is also mi-R27b, a microRNA that inhibits expression of certain genes, that is often upregulated in heart failure. My research topic is about the genetic links between the mitochondria, NDUFS4, miR-27b, and cardiac hypertrophy.
How did you come to your research topic?
My research came about after a systems genetics review that attempts to associate certain traits and phenotypes with the genes that might code for them. About 100 inbred strains of mice and their genetic diversity was used to study metabolic traits present in humans such as obesity, diabetes, and heart failure. Over 800 proteins from 3 chromosomes were mapped and NDUFS4 was closely matched to an area in chromosome 13 that also included one of the complexes that are important for efficient cellular respiration.
Where do you see the future direction of this work leading? How might future researchers build on your work, or what is left to discover in this field?
Hopefully I see this leading into using miR-27b as a biomarker for metabolic diseases. More research would further define and establish the connection between miR-27b, NDUFS4 and cardiac hypertrophy. I hope from there miR-27b can also be used to diagnose many metabolic diseases without invasive biopsies.
Where are you heading to after graduation?
After graduation I hope to head to graduate school and investigate similar topics to find the genetic relationships within diseases. I also hope to do this on a more social sense and investigate factors such as race and socioeconomic factor on diseases as well.
Cardiac hypertrophy, the enlargement of the heart, is often implicated in heart failure and other complications. Although it can be physiological, pathological hypertrophy is often associated with obesity, high blood pressure, and other preexisting conditions. Mitochondria is especially important in the functioning of the heart because it provides the energy required for the heart to pump blood normally. Using systems genetics, NDUFS4, which encodes for a complex I subunit, was identified. It is known that the deletion of NDUFS4 leads to cardiac hypertrophy; however, the genetic association between the two is yet to be identified. Dysregulation of microRNAs (miRNAs) could alter the signaling pathways of cardiomyocytes, leading to cardiac hypertrophy and heart failure. The miR-27b is upregulated in heart failure and new methods of targeting and detection can serve as a treatment and/or as a biomarker for heart failure. This review presents and discusses the genetic link between the mitochondria, NDUFS4, miR-27b, and cardiac hypertrophy.
The heart is one of the most demanding muscles of the body. Because the heart requires a lot of energy to function, mutations in the development or functioning of mitochondria, are also implicated in heart failure, which is the inability of the heart to pump enough blood to satisfy the body’s metabolic demands.
The mitochondria has been suggested to play an important role in heart failure due to its role in energy production, oxidative stress, calcium homeostasis, and metabolism.1 Disruption to mitochondrial complex I is commonly seen in the failing heart.2 Since it acts as the entry point for NADH in the respiratory chain and generates the membrane potential for oxidative phosphorylation, damage to complex I would create lasting damage to cardiomyocytes with the activation of apoptosis signaling pathways.3
Cardiac hypertrophy also precedes heart failure. In response to external and internal stress on the body, cardiomyocytes increase in size in order to compensate for the increased demands of the body.4 Although this tendency adapts in response to physical activity or pregnancy, chronic pressure overload in the heart can lead to hypertrophy and eventually heart failure in which the left ventricle cannot contract normally and cardiomyocytes are unable to return back to their original size, resulting in insufficient blood circulation.5
Using a systems genetics approach, NDUFS4, a protein critical for complex I assembly, was identified as a protein to affect heart pathophysiology. Although it is known that the loss of NDUFS4 leads to cardiac hypertrophy, the mechanism behind this process is still not completely known.
Cardiac hypertrophy is an adaptive response to stress in which cardiomyocytes will increase in size to match the demands of the body during those conditions. There are two types: pathological and physiological. Physiological hypertrophy involves cardiomyocytes maintaining cardiac function over time with cardiomyocytes eventually returning to their original size.6 Pathological cardiac hypertrophy is not really as adaptive because it is also associated with cell death, fibrosis, and mitochondrial dysfunction.7 It usually arises before heart failure, which prevents the body from getting enough blood for metabolic function. The response is usually characterized by an increase in heart mass in the left ventricle.
Hypertrophy develops depending on the type of stress that the heart is exposed to. Physiological hypertrophy is a response to outside stressors such as exercise, pregnancy, and other activities which include the movement of large muscle groups. A different signaling pathway leads to physiological hypertrophy and the heart is also able to return to its original size.8 Pathological hypertrophy arises as a result of oxidative stressors such as cell death, regulation of Ca2+ handling proteins and mitochondrial dysfunction. There is a different, non-reversible signaling pathway for pathological hypertrophy.9
Oxidative stress refers to the imbalance between reactive oxidative species (ROS) and defense molecules. ROS are chemical species such as free radicals, hydroxyls, and non-radicals capable of generating free radicals that are highly reactive. They can damage DNA, membranes, proteins, and other molecules. NADPH oxidases have been suggested to be a key source of ROS.10
There is greater evidence suggesting that redox sensitive pathways are implicated in the development in cardiac hypertrophy, either in response to neurohumoral stimuli or chronic pressure overload. In modified mice lacking the Nox2 subunit of NADPH oxidase that were also injected with angiotensin II to induce hypertrophy, cardiac hypertrophy was inhibited in Nox2 knockout mice.11 This indicates that hypertrophy induced by angiotensin II is dependent on Nox2 and possibly NADPH.
MITOCHONDRIA IN HEART FAILURE
The mitochondria is a double membrane bound organelle that is present in almost all eukaryotic cells.12 In addition to playing a huge role in ATP synthesis and the release of energy into the cell, mitochondria also play huge roles in cellular function and dysfunction, such as in apoptosis, the storage of calcium ions, and the production of heme. They also carry their own genome known as mtDNA which encodes for 13 essential proteins for respiration. They are composed of an inner and outer membrane, the intermembrane space, and the matrix.13
The electron transport chain contains a series of protein complexes embedded within the mitochondrial inner membrane. It consists of a series of electron carriers arranged by redox potentials. During oxidative phosphorylation, electrons from NADH and FADH2 combine with oxygen to form water. At the same time protons are pumped into the intermembrane space to produce an electrochemical gradient that drives the synthesis of ATP.
Mitochondrial complex I is the largest subunit of the electron transport chain. It consists of 45 subunits, for which their dysfunction is associated with mitochondrial disease and eventually heart failure.14 Of those 45 sub units, there are 14 conserved core submits (Figure 1) for energy transduction and 31 proteins as supernumerary subunits.15 There is little existing research on the function of these supernumerary submits, but when mutated, they are known to cause diseases. Independently, they can play particular roles in apoptosis but they can also play nonspecific roles in regulation and assembly. Complex I is L shaped with one hydrophilic matrix arm and one hydrophobic arm.16 The hydrophobic arm has an N module at the end of it, which is associated with the oxidation of NADH. The Q module bridges the arms and is involved in the transport of electrons.17
In the electron transport chain, complex I uses two electrons to form the oxidation of NADH to reduce ubiquinone to ubiquinol in the inner mitochondrial membrane. This provides electrons for the reduction of O2 to water. The potential energy released from the reduction is also used to transport protons across the inner membrane to provide the force necessary to drive the production of ATP using ATP synthase.18
SYSTEMS GENETICS APPROACH TO IDENTIFY CANDIDATE GENES
The Hybrid Mouse diversity panel consists of about 100 inbred strains of mice whose diversity allows for the study of metabolic traits present in human disease including obesity, diabetes, heart failure, immune regulation, and fatty liver disease. It was developed in response to the greater need of increased resolution for genetic mapping and to create a platform for genomic, metabolomic, proteomic, and transcriptomic data integration. One of the advantages of the HMDP resource is prioritization of candidate genes that will be further characterized using in vitro and in vivo approaches.
To this end, using an integrative proteomics approach with HMDP, a whole heart proteomic analysis was conducted. 840 proteins were mapped to investigate mitochondrial pathways and heart function (Figure 2). Hotspots were identified in loci located in chromosomes (chr) 13, 17, and 7. SNPs located within these loci allowed for the identification of candidate genes to study. NDUFS4 closely mapped to a SNP at a chr 13 locus rich in complex I. It was mapped outside the locus of linkage disequilibrium (Figure 3).
NDUFS4 is a protein critical for complex I assembly.19 This protein is especially found in areas that are metabolically active such as the heart, brain, liver, and skeletal muscle. In a heart specific knockout mouse with the deletion of NDUFS4, neither ATP supply nor ROS production in the ETC is impaired in unstressed conditions. However, during oxidative stress, complex I deficiency as a result of the deletion led to increased NADH accumulation and a decreased NAD+/NADH ratio, resulting in protein acetylation. This acetylation made the mice more susceptible to stress and accelerated the development of heart failure.20
In a whole body knockout mouse, mutations in the NDUFS4 gene also lead to a leigh-like phenotype in which mice would die within 50 days after birth.21 NDUFS4 knockout mitochondria would have partial activity due to mitochondrial complex III stabilizing complex I to provide partial activity by forming a supercomplex.22
Micro RNAs (miRNAs) are endogenous noncoding RNAs that inhibit the expression of specific genes by either tearing down target mRNAs or by inhibiting their expression.23 As chronic stress and obesity can induce hypertrophic growth in the heart leading to cardiovascular disease, it is also shown that miRNAs are also upregulated in heart failure.24 In response to physiological or pathological stress, miRNAs can influence a hypertrophic response through different signaling cascades.25
The miR-27b acts as an angiogenic switch by promoting endothelial tip cell fate and sprouting and is upregulated in Smad deficient hearts (Figure 4).26 Overexpression of miR-27b results in cardiac hypertrophy both in vitro and in vivo.27 Therefore, downregulating miR-27b could act as a target for disease.28 After subjecting mice to elevated miR-27b levels in a transverse aortic constriction model to induce hypertrophy, antagomir-27b treatments significantly reversed left ventricle mass and wall thickening, and normalized fractional shortening.29 This indicates that miR-27b could be a potential biomarker for cardiac hypertrophy.30
Diagnosing cardiac hypertrophy is usually done with methods such as electrocardiography, magnetic resonance imagery, and computed tomography.31 However, these methods are limited by their complexity and sensitivity, only proving effective when patients are experiencing more severe hypertrophy to warrant more detailed examination. However, with miR-27b elevated in hypertensive patients with left ventricular hypertrophy,32 this RNA could be used to distinguish hypertensive patients with LVH from those who don’t, acting as an early biomarker for treatment.
In summary, we hypothesize that the peak SNP found in chromosome 13 regulates miR-27b, and thus NDUFS4. The rationale for this is that miR-27b, a known regulator for NDUFS4, is located within the locus. This would be tested in future experiments to further highlight the connection between miR-27b, NDUFS4, and cardiac hypertrophy. Furthermore, based on our systems genetics approach and other investigators’ work, we propose NDUFS4 and miR-27b as two potential key factors in the development of cardiac hypertrophy. The next step must be to identify their exact roles; for example, NDUFS4 and miR-27 might influence cardiac hypertrophy through a signaling pathway or other mechanism. This could involve identifying whether the miRNA or protein is directly responsible. Hypertrophied cardiomyocytes could be treated to knock down miR-27b expressions in order to investigate whether it would protect the cells from hypertrophy. This could also involve an overexpression of NDUFS4 in order to determine whether it can reverse or prevent hypertrophy from occurring.
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Calvaruso, M. A., Willems, P., van den Brand, M., Valsecchi, F., Kruse, S., Palmiter, R., Smeitink, J. & Nijtmans, L. (2012). Mitochondrial complex III stabilizes complex I in the absence of NDUFS4 to provide partial activity. Human Molecular Genetics, 21(1), 115–120. https://doi.org/10.1093/hmg/ddr446
Chouchani, E. T., Methner, C., Buonincontri, G., Hu, C.-H., Logan, A., Sawiak, S. J., Murphy, M. P. & Krieg, T. (2014). Complex I Deficiency Due to Selective Loss of Ndufs4 in the Mouse Heart Results in Severe Hypertrophic Cardiomyopathy. PLoS ONE, 9(4), e94157. https://doi.org/10.1371/journal.pone.0094157
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Nakamura, M. & Sadoshima, J. (2018). Mechanisms of physiological and pathological cardiac hypertrophy. Nature Reviews Cardiology, 15(7), 387–407. https://doi.org/10.1038/s41569-018-0007-y
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Wang, J. & Yang, X. (2012). The function of miRNA in cardiac hypertrophy. Cellular and Molecular Life Sciences, 69(21), 3561–3570. https://doi.org/10.1007/s00018-012-1126-y
Wang, Y., Chen, S., Gao, Y. & Zhang, S. (2017). Serum MicroRNA-27b as a Screening Biomarker for Left Ventricular Hypertrophy. Texas Heart Institute Journal, 44(6), 385–389. https://doi.org/10.14503/THIJ-16-5955
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Figure 1. The structure of Complex I from Thermus Thermophilus.
Structure of Complex I of an L shape containing 14 core subunits and 2 supernumerary units in white. Three orientations are directed with the hydrophilic domain protein structure removed in the bottom figure (Adapted from Hirst, 2013).
Figure 2. Identification of Candidate Genes for Heart failure.
Mapping of 840 heart mitochondrial proteins rom 72 HMDP strains to identify pQTL networks. Associations between protein expression and genetic variants not within 1Mb of respective gene locations shown on the diagonal were considered as trans-pQTLs. Three trans-pQTL hotspots were identified by arrows at chromosomes 13,17, and 7.
Figure 3. Circos, Manhattan and Regional, and Genotype Distribution Plots for Chromosome 13.
Figure 4. Cardiac Hypertrophy Signaling Pathways.
Multiple signaling pathways for cardiac hypertrophy as a pathological response. MiR-27b is regulated by a signaling pathway involving TGF-β and the blocking of Smad (Adapted from Wang and Yang, 2012).