Case 4 - Skeletal Muscle and Heart Growth and Adaptation PDF

Case 4 – Skeletal muscle and heart growth and adapta7on 1. What is the structure of skeletal muscle (recap)? Macro-anatomy muscle A muscle consists of connec.ve.ssue, muscle fibers, nerves and blood vessels. The connec.ve.ssue is divided into three different sheaths: - Epimysium (dense irregular....

Case 4 – Skeletal muscle and heart growth and adapta7on 1. What is the structure of skeletal muscle (recap)? Macro-anatomy muscle A muscle consists of connec.ve.ssue, muscle fibers, nerves and blood vessels. The connec.ve.ssue is divided into three different sheaths: - Epimysium (dense irregular.ssue that sits around the whole muscle) - Perimysium and fascicles (muscle fibers are grouped into fascicles which are surrounded by the perimysium) - Endomysium (fine areolar connec.ve.ssue that wraps around each individual muscle fiber) Skeletal muscles are aDached to the connec.ve.ssues of the bone either direct to the bones or indirect via a tendon (aponeurosis) Micro-anatomy muscle A muscle consists of many muscle fibers. Each muscle fiber is a long cylindrical cell which has a lot of nuclei that lies just beneath the sarcolemma (plasma membrane). The cytosol of a muscle fiber cell is called sarcoplasm. Every muscle fiber consists out of thousands of myofibrils. Cell organelles lie between the myofibrils à mitochondria, sarcoplasmic re.culum (SR) and T-tubules. A myofibril contains the contrac.le elements à sarcomere which consists out of myofilaments (ac>n, >>n, nebulin and myosin). The sarcomeres are the work unit of the muscle, and they lie end-to-end with each other à connected by a Z-line. In the I-band only the thin ac.n filaments are present while they are anchored in the Z-disc. The A band consists of the H-zone and the M line. The H-zone only consists of the thick myosin filaments. The M line is in the H-zone. Here are thick myosin filaments connected with the protein myomesin. Myosin filaments consist out of many myosin molecules which have two globular heads and a tail made out of two intertwined polypep.de chains. The flexible hinge region is the place just before the heads which can connect with ATP and change their posi.on. When the heads are connected to the ac.n filaments = cross-bridge à the motor of force à low force (uncontracted) and high force (contracted). The ac.n filament is connected to a lot of proteins: tropomyosin which spirals round it to stabilize and can block the myosin-binding sites and troponin which consist of an inhibitory subunit which is bound to ac.n (Tnl), a subunit which binds to tropomyosin (TnT) to posi.on it properly and a calcium binding subunit (TnC). Elas.c filaments (>>n) lie all the way from the Z-disc to the M-line. They maintain the structure of the sarcomere and prevent it from overstretching.  Dystrophin is a protein that aDached the sarcomere from it basal lamina to the collagen fibers à provide structure. Nebulin is a long protein which is winded around the ac.n filaments. It controls the length of the thin filament and provides structure. The human body has approximately 650 muscles and 187 joints. 3 types of skeletal muscle Muscle types Type Func>on Appearance Loca>on Skeletal - Anchors tendons to bones - Typically, a muscle with a All over the body - Allows body to move tendon on each end. - PaDern: o Myonuclei à inside, at the sides of each fiber. o Satellite cells à lie on the muscle fibers Cardiac Contracts to pump blood Looser version of skeletal muscle heart Smooth Func.on depends on loca.on Not organized Organs like the intes.nes, blood vessels, uterus and bladder 2. What is the plas>city of skeletal muscle? Skeletal muscle plas.city refers to the ability of muscle.ssue to adapt in response to various external s.muli such as physical ac.vity, nutri.on, hormonal changes, and age. This adaptability includes both increases (hypertrophy) and decreases (atrophy) in muscle mass and changes in muscle fiber type composi.on, metabolic proper.es, and mitochondrial content. Muscle plas.city relies on dynamic adjustments in muscle protein turnover, which is a balance of protein synthesis and breakdown. Protein turnover (=synthesis and breakdown) (pathways) Protein synthesis Depending on how much degrada.on there is compared to the protein synthesis, the muscle grows and causes hypertrophy. Protein synthesis can be triggered by several factors: - Testosterone - Satellite cell ac.va.on  - Folista.n by inhibi.ng myosta.n - Physical training and nutrient-rich diet IGF-1 pathway IGF-1 is produced by either the liver or the muscle itself. It has a special receptor on the outside of muscle cell à IRS-1. When the IGF-1 binds to this receptor a whole cascade of events is going to happen. First PI3K is phosphorylated which is used as second messenger to phosphorylate AKT. AKT is one of the key elements within the IGF-1 cascade. It has three major effects on protein synthesis involving two proteins: - Inhibi>on of GSK3b à GSK3b is a protein kinase that inhibits the transla.on factor EIF-2 which is needed responsible for the recruitment of the first tRNA with the first amino to the small ribosomal subunit. By inhibi>ng the inhibitor eIF-2 is ac.vated and can induce transla>onal ini>a>on. - Ac>va>on of mTOR which will build mTORC1 and mTORC2. The mTORC2 has a posi.ve feedback loop on its own ac.va.on while mTORC1 has two major targets: 1. Inhibi>on of 4EBP1 à 4EBP1 is the inhibitor of eIL-4, which is needed for the recogni.on of the mRNA to the ribosome and to facilitate the binding of the ini.a.on complex. By inhibi.ng the inhibitor, eIL-4 is ac.vated and can induce transla.onal ini.a.on. 2. mTORC1 ac>vates S6K1 à s6K1 has a posi.ve influence on the transla>onal capacity. All three mechanism increase the amount of protein synthesis and therefore cause muscle hypertrophy. Counteract of IGF-1 pathway The IGF-1 pathway can be disturbed by myosta>n. Myosta.n is a protein which is necessary for muscle growth arrest when the muscle reached its desired size. Myosta.n breaks the IGF-1 cascade and therefore inhibits protein synthesis. It binds to the receptor ActRIIB on the outside of the muscle cells and ac.vates either Smad2 or Smad3. Those two molecules have the ability to inhibit the ac>va>on of AKT. No phosphorylated and phosphorylated and thereby ac.vated AKT means no ac>va>on of mTOR or inhibi.on of GSK3b. the transla.onal ini.a.on factors are nor ac.vated, and the protein synthesis is disabled. à myosta.n can be inhibited by Folista>n. Folista.n therefore promotes protein synthesis by inhibi.ng the inhibitor. Protein breakdown Protein degrada.on induces the shrinkage of muscle when it is in an imbalance to protein synthesis. It causes atrophy. Protein degrada.on can be triggered by different factors: - Ca2+ dependent pathway - Inflamma.on - Low-IGF-1 levels  - Myosta.n à explained above - Disuse of muscles and starva.on Inflamma>on Local inflamma.on causes an increase in cytokines. A special cytokine released during inflamma.on is TNF-a. When TNF-a enters the muscle cell it can bind to the transcrip.on factor NF-kB and therefore ac.vate it. Once NF-kB is ac.vated it upregulates the expression of the protein MuRF1. Low IGF-1 levels When the concentra.on of IGF-1 is low, AKT cannot be ac.vated by the cascade. AKT has another important func.on for protein synthesis à the inhibi.on of FoxO. When AKT cannot be ac.vated because of lacking IGF-1, then FoxO is no longer inhibited. FoxO is a transcrip.on factor which s.mulated the expression of MuRF1 and Atrogin-1. Another func.on of FoxO is the induc.on of the autophagy pathway via lysosomes. FoxO s.mulates lysosomes to ingest proteins which are then degraded into single amino acids. à single amino acid can in return ac.vate mTOR which would have nega>ve feedback for protein degrada.on by ac.va.ng protein synthesis. Mono-ubiqui.na.on UPS pathway UPS stand for ubiqui>n 26S-proteasome system. It is a protein breakdown pathway that depends on the ac.va.on of MuRF1 and atrogin-1. When inflamma.on or low-IGF-1 levels ini.ate the ac.va.on of MuRF1 and atrogin-1, those two molecules can act as ubiqui>n ligases. They tag the ubiqui.n onto proteins and therefore mark them for the proteasome. The proteasome will break them into smaller pep.des which will eventually be degraded into single amino acids. 3. What is the func>on of satellite cells (=stem cells) in muscle growth? Satellite cells Satellite cells are mononucleated stem cells with myogenic poten.al which are located under the basal lamina of myofibers but with their own plasma membrane. They present 3- 6% of all muscle nuclei and they can be recognized by special transcrip.on factor (e.g. Pax3 or pax7) or surface proteins. They are somite-derived myoblasts that have not fused and remain poten.ally available throughout adult life. They have the ability to upregulate MRF (e.g. MyoD or myogenin) or undergo asymmetric division leading to the forma.on of undifferen.ated cells which in return differen.ate into myoblast which will form new myofibers. The downregula.on of Pax7 within satellite cells cause the cells to differen.ate and become myotubes. Satellite stem cells express pax7 but not Myf-5. They can divide asymmetrically into another satellite stem cell or a satellite progenitor cell. The progenitor cells do express both Pax7 and Myf-5. The progenitor cell is the cell with the des.ny to differen.ate into a muscle cell. Myonuclear domain hypothesis This hypothesis describes the loss and gain of myofiber nuclei with the ac.va.on of satellite cells with the maintaining of the myonuclear domain (MD). Each nucleus is restricted to a maximal amount of cytoplasm which causes the muscle fiber to grow with increased myonuclei number but the myonuclear domain stays the same. Fibers only grow in thickness and not in length in postnatal muscle develop. Satellite cells cause prolifera.on when ac.vated. This causes a small amount to repopulate the muscle fiber as satellite cells, but the majority differen.ate and fuses with the exis.ng myofibers à myonuclear accre>on. Satellite cell ac>va>on Stem cells within muscle fibers are in a quiescent state just underneath the basal lamina of the muscle fiber. The satellite cells can be ac.vated by HGF and downregulated by myosta>n. Once you have an ac.vated satellite cell there are 2 pathways. Either the cell expresses Pax7 and goes to a state of self-renewal. This cell ends up in the quiescent state and does not undergo any differen.a.on. Or Pax7 is downregulated, and different other factors are released. Prolifera.on of a satellite cell is induced by the factors: - Notch and Delta - IGF-1 - MGFs - HGF - IL-6 The prolifera.ng cells will go into the state of differen.a.on. This is induced by IGF-1, suppressed by myosta>n and HGF, and can lead to two different outcomes: 1. The differen.a.ng cells will form a new myotube which is needed if the muscle is damaged and needs regenera.on. 2. The differen.a.ng cells fuse with exis.ng myofibers and cause muscle hypertrophy, myonuclear accre>on and satellite cell pool size expansion. 4. What are the cell type and their func>on in heart? Major cardiac cell types that make up the heart. The heart is a mul.cellular organ comprised of different cell types responsible for providing specific func.ons related to electrical conductance (macrophages, pacemaker, and Purkinje cells), mechanical work (cardiomyocytes), and.ssue remodeling (endothelial cells, fibroblasts, macrophages, and stem cells). Mechanical work Cardiomyocytes They are the contrac.le cells of the heart that generate the force required to pump blood throughout the body. These cells contain sarcomeres, the basic contracile units, and are rich in mitochondria to support the high energy demands of con.nuous contrac.on. Electrical conductance Macrophages Pacemaker cells These specialized cells (like sinatrial (SA) node cells, atrioventricular (AV) node cells, and Purkinje fibers) generate and transmit electrical impulses that regulate the heartbeat. They ensure that the heart beats in a coordinated and rhythmic manner. Tissue remodeling Endothelial cells These cells line the interior of blood vessels within the heart. They regulate blood flow, facilitate the exchange of oxygen and nutrients, and play a role in modula.ng inflamma.on and vascular tone in response to various s.muli. Fibroblasts Cardiac fibroblasts are responsible for producing the extracellular matrix (ECM), which provides structural support to the heart.ssue and maintainsn its elas.city. They are essen.al for maintaining the mechanical integrity of the heart and play a role in repairing.ssue aher injury by forming scar.ssue. Pericytes à func.on Motor neurons 5. Which factors influence cardiomyocyte growth and adapta>on? Cardiomyocyte growth and adapta.on are influenced by several factors involving mitochondrial biogenesis, transcrip.onal regula.on, and metabolic matura.on. Aher birth, extensive mitochondrial expansion occurs to meet energy demands, driven by transcrip.on factors like PPAR𝛼, PPAR𝛿 and PGC-1𝛼. These factors enhance faDy acid oxida.on (FAO), crucial for energy produc.on in adult cardiomyocytes. Addi.onal regulators, including thyroid hormone receptors (TRs), estrogen-related receptors (ERRs), and proteins like Perm1, coordinate mitochondrial and metabolic adapta.ons. The role of these transcrip.onal and signaling pathways ensures the heart’s capacity for efficient ATP genera.on and responsiveness to physiological demands. Cardiomyocyte growth and adapta.on are influenced by a combina.on of mechanical, hormonal, and metabolic factors: - Mechanical stress: cardiomyocytes respond to mechanical loading, such as increased blood pressure or volume overload, by hypertrophy. This process, known as mechanotransduc>on, involves detec.ng stretch or tension and ac.va.ng signaling pathways that promote growth. - Hormonal factor: IGF-1 promotes cardiomyocyte survival, growth, and hypertrophy by ac.va.ng the PI3K/AKT pathway, leading to beneficial cardiac adapta.ons. o Thyroid hormones: these hormones increase metabolic rate an influence heart rate, contrac.lity, and cardiomyocyte growth. Thyroid hormone receptors in cardiomyocytes help regulate genes involved in growth and metabolism. o Catecholamines (epinephrine and norepinephrine): released in response to stress, these hormones bind to adrenergic receptors in the heart, increasing heart rate and contrac.lity. Chronic s.mula.on, however, can lead to pathological hypertrophy and contribute to heart failure. - Nutri>onal and metabolic factors: cardiomyocytes rely on both glucose and faDy acids as fuel. Nutri.onal status, energy availability, and metabolic hormones influence cardiomyocyte growth and metabolic adapta.ons. Low oxygen levels (hypoxia) can s.mulate growth factors such as hypoxia-inducible factor-1 (HIF-1), promo.ng angiogenesis (forma.on of new blood vessels) and cardiomyocyte survival under stress. Factors that limit heart’s ability à Transcrip.on factors: - Gata-4 - MEF-2 - NKx2.5 6. What are the changes in metabolism? Aher birth, there is a significant increase in mitochondrial biogenesis in the heart to meet its high energy demands. This is driven by transcrip.onal ac.va.on of genes involved in mitochondrial expansion and energy produc.on pathways, allowing the heart to produce more ATP. In the fetal state, the heart primarily uses glucose and lactate, but aher birth, the heart becomes an “omnivore”, capable of oxidizing various substrates such as faDy acids (FA), glucose, lactate, pyruvate, and ketone bodies. The main fuel for the adult heart is faDy acids, which are processed through mitochondrial faDy acid oxida.on (FAO), a high- capacity pathway. Several transcrip.on factors and nuclear receptors play a crucial role in regula.ng postnatal cardiac metabolism. For instance, the nuclear receptors peroxisome proliferator-ac.vated receptor alpha (PPAR𝛼) and PPAR𝛿 are central to regula.ng the uptake and oxida.on of faDy acids. Other important regulators include PGC-1𝛼, which co-ac.vates PPAR𝛼, and thyroid hormone receptors (TRs), which also support mitochondrial biogenesis and func.on in the heart. Following mitochondrial biogenesis, the heart undergoes a matura.on process that further enhances its ability to oxidize faDy acids and maintain high rates of oxida.ve phosphoryla.on. This matura.on is essen.al for the heart to efficiently produce ATP under various physiological condi.ons. The regula.on of mitochondrial biogenesis and faDy acid oxida.on is influenced by endogenous lipid signals and feedback mechanisms, including the interac.on between PGC- 1𝛼 and other factors like ERRs (estrogen-related receptors), TRs, and various transcrip.on factors. These factors collec.vely drive the matura.on of cardiac mitochondria and metabolic pathways. Summary In summary, aher birth, there is a shih from reliance on glucose to faDy acid oxida.on for energy produc.on in the heart, and this transi.on is driven by complex transcrip.onal regula.on involving nuclear receptors, coac.vators, and other regulatory factors. These changes equip the heart with the capacity for high ATP produc.on necessary for its adult func.on. Switch to faDy acid oxida.on around 9 months. Know which factors regulate these adapta.ons. - PGC-1a - PPARa Adapta.on aher birth à important factor = blood vessels Aher birth à angiogenesis (induced by VEGs) 7. How does the composi>on of heart muscle change from infancy to adulthood? From infancy to adulthood, the heart muscle undergoes significant composi.onal changes, primarily through shihs in the types of contrac.le protein isoforms that support its func.on. Early on, the heart expresses “fetal’ isoforms such as Myh6 (𝛼-MHC), TNNI1 (cardia troponin 1), and the longer N2BA form of..n, which provide greater compliance and flexibility. As the heart matures, there is a transi.on to “adult” isoforms like Myh7 (β-MHC), TNNI3, and the shorter N2B..n, enhancing contrac.le efficiency and s.ffness for adult heart demands. These transi.ons include shihs in: - Myosin heavy chains - Troponin isoforms - Other structural proteins, which are cri.cal for the heart’s strength and endurance. This isoform switching is regulated by transcrip.on factors and is reinforced by hormonal influences, par.cularly thyroid hormone, which triggers adult isoform expression postnatally. Addi.onally, ion channels and Ca2+ handling proteins become more specialized, suppor.ng mature electrophysiological proper.es and efficient contrac.on. Difference between isoforms and why is it necessary Sarcomere increases in complex