Muscle Bio Week 10.docx

***Week 10, Module 10: Acute Molecular Responses to Endurance Exercise***   - Repeated exercise is a strong stimulus for physiological adaptations. - The physiological stress is specific to the type of work being performed. - the next question, then, is how do muscle cells ***sense...

***Week 10, Module 10: Acute Molecular Responses to Endurance Exercise***   - Repeated exercise is a strong stimulus for physiological adaptations. - The physiological stress is specific to the type of work being performed. - the next question, then, is how do muscle cells ***sense*** this physiological stress, and then translate this into an ***appropriate response*** that will help the muscle cell ***adapt** *to better cope with future disturbances in homeostasis? To understand the cellular basis for how muscles adapt to exercise, we must recognise how physiological stresses (known as 'primary signals') are sensed during exercise, how this signal is then communicated within the muscle cell (via cell signalling pathways) leading to an adaptive response in the cell (changes in gene and therefore protein expression)   *Communicating the Endurance Exercise \'Signal\'* - Muscle contraction-induces changes in intracellular ***ATP***, ***calcium*** (Ca2+), ***oxygen***, ***reactive oxygen species*** (ROS), ***redox state*** (NAD+:NADH ratio), and ***mechanical stress***. - These changes, known as '***primary signals***', can all be sensed within the cell, and can lead to the activation of cell signalling pathways that control skeletal muscle adaptation to exercise.  - This provides a framework - known as ***excitation-transcription coupling*** - for the link between the cell signalling events activated in muscle by exercise, and the expression of genes that control skeletal muscle adaptation to exercise. - There are indeed a number of key sensors and signalling pathways that detect changes in primary signals altered by exercise, and communicate this signal within the cell to generate an appropriate response within the cell. There are 4 main pathways: PATHWAYS 1. AMP-activation protein kinase (AMPK) signalling:\ serine/threonine kinase, it acts by phosphorylating other proteins on serine or threonine amino acid sites. It\'s activated in response to cellular energy stress, which is indicated by an increase in the AMP:ATP ratio (more AMP, les ATP).\ The cellular energy state is rapidly influenced by muscle contraction, AMPK is a key signal transducer, sensing disruptions to cellular homeostasis and elicits a reaction.\ AMPK has been shown to be activated by exercise in an intensity-dependent manner, reflecting the effect of exercise intensity on cellular AMP and ATP levels.\ When activated, AMPK acts to conserve cellular energy (ATP) by inhibiting processes that consume energy (anabolic pathways), and activating processes that release energy (known as catabolic pathways).\ Cellular processes that are inhibited by AMPK activation include those that require energy (ATP) to occur, including protein and glycogen synthesis, whereas processes that help generate ATP, such as glucose transport into the cell and lipid metabolism, are positively influenced by AMPK.\ Importantly, AMPK can also influence metabolic gene transcription and mitochondrial biogenesis, in part by altering the ability of transcription factors (e.g., NRF-1, MEF2, and HDACs) to bind to DNA and activate gene expression 2. Calcium/Calmodulin-activated Protein Kinase (CaMK) Signalling:\ changes in intracellular calcium, which are crucial for cross bridge cycling, also represent an important primary signal for exercise adaptations.\ Intracellular Ca2+ concentrations are sensed by family of enzymes known as ***calcium/calmodulin-dependent protein kinases (CaMKs)***, of which ***CaMKII*** is the dominant type (isoform) in skeletal muscle. Like AMPK, increases in CaMKII phosphorylation occur in an intensity-dependent manner.\ Once active, the CaMKs can influence the uptake of both glucose and lipids, and can interact with transcription factors that control the expression of genes related to mitochondrial biogenesis (PGC-1α) and glucose uptake  3. Mitogen-activated Protein Kinase (MAPK) Signalling:\ The activity of the MAPK family of proteins is influenced by growth factors, cytokines (important cell signalling proteins), reactive oxygen species (ROS), and cellular stress. Exercise can activate all three main MAPK subfamilies, including ***ERK1/2*** (extracellular-regulated kinase 1/2), ***JNK*** (c-jun N-terminal kinase), and ***p38 MAPK***. When active, the MAPKs can regulate a diverse array of cellular processes by phosphorylating a variety of transcription factors and co-activators, including those for the PGC-1α gene (MEF2 and ATF2) 4. Cellular Redox Balance Signalling:\ In response to exercise and other stimuli such as fasting, dynamic changes in the cellular ***NAD+:NADH ratio*** occur. Within the cell, NAD+ functions as an important electron carrier that plays an important role in energy metabolism. During the breakdown of metabolic fuels via glycolysis and the Krebs/TCA cycle, NAD+ gathers electrons (being reduced to NADH in the process) and shuttles these electrons to the electron transport chain.\ A family of proteins, known as the sirtuins (SIRT), are sensitive to elevations in the cellular concentrations of NAD+ and increases in the NAD+/NADH ratio. \ In contrast to protein kinases, which phosphorylate other proteins, the SIRT proteins are ***deacetylases***, which means they remove acetyl groups from other proteins. This is another form of post-translational protein modification. \ During exercise, there is initially a decrease in the NAD+/NADH ratio (because NAD+ is reduced to NADH when gathering electrons during glycolysis and the Krebs/TCA cycle), which is reversed during the post-exercise recovery period. The SIRT proteins are therefore activated later into exercise recovery when there is an increase in the cellular NAD+/NADH ratio.\ Increased activity of the SIRT family of proteins, including ***SIRT1*** and ***SIRT3***, are associated with positive adaptations to exercise. When active, the SIRT proteins can de-acetylate important transcriptional regulators (including PGC-1α) and mitochondrial enzymes, leading to mitochondrial biogenesis and improved mitochondrial function.   *Regulation of Mitochondrial Biogenesis* - Mitochondrial biogenesis is the term used to describe an increase in the number and volume of mitochondria within a cell, and is a primary adaptation to endurance (aerobic) training in skeletal muscle. - A complex and highly-regulated process, mitochondrial biogenesis requires the coordinated expression of genes located within both the nucleus and also the mitochondria themselves (known as mitochondrial DNA or mtDNA). Specifically, mitochondrial biogenesis requires:  1. The transcription of genes within the nucleus that encode mitochondrial proteins 1. Translation of these mRNAs into mitochondrial proteins that are then transported into the mitochondria 1. Transcription and translation of mitochondrial genes encoded within the mtDNA, and 1. The biosynthesis of various mitochondrial components and enzyme complexes (such as those that make up the electron transport chain \[ETC\]).   The complex process of mitochondrial biogenesis is, in part, coordinated by an important protein that is sometimes known as the 'master regulator of mitochondrial biogenesis': **PGC-1α**.   PGC-1**α:** Master Regulator of Endurance Related Adaptations - A transcriptional co-activator: a protein that increases transcription without binding to DNA. - Regulates gene expression by influencing the activity of various transcription factors, including: NRF-1 and 2, EERa and PPARs. - When it influences transcriptional activity, it leads to: increased mitochondria (by interacting with NRF-1, NRF-2), increased fat oxidation proteins (by interacting with the PPARs), and increased blood vessels (by interacting with ERRα). - When levels of PGC-1α are artificially increased in animal muscle, known as overexpression, this increases mitochondrial biogenesis and makes the muscle resemble aerobically-trained muscle. Therefore, in many ways, PGC-1α induces most (if not all) of the muscular adaptations to endurance exercise. - PGC-1α is itself highly regulated by multiple post-translational modifications, including phosphorylation and de-acetylation. These modifications can influence the ability of PGC-1α to move into the nucleus, where transcription occurs, or influence its ability to interact with its binding partners. Specifically, PGC-1α is more active when it is ***more phosphorylated*** and ***less acetylated***.  - Another way to increase the activity of PGC-1α activity is to make more of the protein itself. The transcription of the PGC-1α gene is controlled by transcription factors that bind to three important regions in the PGC-1α gene (known as a promoter region): these include ***MEF2*** (myocyte enhancer factor 2), ***Ebox*** (enhancer boc), and ***CRE*** (c-AMP response element).  - Both AMPK and CaMKII signalling can increase PGC-1α gene transcription by activating MEF2. This is achieved by releasing MEF2 from its inhibitor - the ***HDAC*** (class II histone deacetylase) proteins. When phosphorylated by either AMPK or CaMKII, HDAC is exported from the nucleus and can no longer negatively interact with MEF2, thereby increasing PGC-1α expression