Molecular Exercise Physiology: An Introduction

Book Preface

‘ Molecular exercise physiology’ is a discipline within exercise physiology and a shortened version of the term ‘ molecular and cellular exercise physiology’ which was used, amongst others, by Frank W. Booth
( 1), a pioneer in this area ( see Box 1.1). In the first part of this chapter, we define molecular exercise physiology, distinguish it from exercise biochemistry and trace its roots in molecular biology and exercise physiology.

Origins and definition of molecular exercise physiology

We define molecular exercise physiology as ‘ the study of the molecular responses to exercise and underlying mechanisms that lead to physiological adaptation following exercise’. The field is focused particularly at the level of the molecule and cell, and investigates how the microscopic make- up of our cells responds to exercise, ultimately leading to adaptation at the cellular, tissue and system levels. The discipline combines the use of molecular biology wet- laboratory techniques with physiological methods to examine the molecular make- up and responses of our cells and tissues to exercise. Therefore, molecular exercise physiologists are concerned with four overarching areas of study: ( 1) The role of heritable genetic traits ( variation in the genetic code found within our DNA) and their associated influence on the physiological response and adaptation to exercise. More recently with a sharper focus on the role of ( 2) ‘ epi’- genetics ( meaning ‘ above’ genetics), where both the environmental ‘ stressor’ of exercise and underlying genetics interact to influence molecular responses and adaptation to exercise, for example, by molecular modifications to our genetic code at the DNA level that subsequently affect how our genes are turned on and off following exercise. In terms of the molecular responses to exercise, exercise physiologists also want to understand: ( 3) How the environmental ‘ stressor’ of exercise modulates the abundance and activity of molecular ‘ signal- transduction’ networks, leading to the turning on or off of genes and therefore the resulting protein levels, culminating in changes/ adaptation at the cellular or tissue level. Of course, these molecular mechanisms are also applicable across other exercise physiology sub- disciplines, such as sports and exercise nutrition, different environmental conditions ( e.g. hot, cold conditions, at altitude and different time zones), ageing and diseases ( e.g. cancer, diabetes and obesity). Therefore, molecular exercise physiology has diversified rapidly to incorporate the use of molecular biology to study exercise in aligned sport and exercise fields across performance and health- related disciplines. The majority of these regulatory and molecular mechanisms are studied in blood and skeletal muscle tissue and/ or isolated satellite cells ( regenerative cell found in skeletal muscle) and sometimes adipose tissue, albeit to a lesser extent than muscle tissue. This is due to the relative ease of sampling of blood and the ever- increasing obtainability of skeletal muscle biopsies in human participants and/ or patients under various exercise conditions. Finally, molecular exercise physiologists have also been interested in ( 4) the role of satellite cells and their role in the repair and regeneration of muscle after exercise, and more recently its role as a molecular ‘ communicator’ in skeletal muscle.

Book overview and chapter outline

After providing a short history of the evolution of molecular exercise physiology as a field ( Chapter 1), the book will start by introducing the main methods ( Chapter 2) employed by molecular exercise physiologists. This will provide an important understanding of the main methods that are employed in the research that is then discussed in the later theoretical chapters. Chapter 3 will introduce sport and exercise genetics and the ‘ central dogma’ of molecular biology, DNA to RNA to protein, and how this relates to exercise physiology. Importantly, it will cover how variations in inherited information found in our genetic code may contribute to changes in physiology that could affect exercise and physiological performance. Chapter 4 will then discuss the genetics of muscle mass and strength, and in Chapter 5, we go on to cover the genetics of endurance exercise. Chapter 6 will discuss how exercise can ‘ epigenetically’ modify our inherited genetic code and lead to altered molecular responses involved in exercise adaptation. Because epigenetic modifications to DNA can be retained over longer periods, this chapter will also discuss the concept of epigenetic ‘ muscle memory’ and how this paradigm might be an important consideration across the exercise and sport sciences. Chapter 7 will introduce the ‘ signal transduction’ theory of molecular exercise responses and how these lead to adaptation at the physiological level. Chapters 8 and 9 will look at these molecular responses in both resistance exercise and endurance exercise, respectively. Then, the book will move on to discuss molecular exercise physiology in the context of altered nutrition ( Chapter 10), different environmental conditions, e.g. hot and cold conditions, at altitude and in different time zones
and the influence of circadian rhythms ( Chapter 11), and with disease such as cancer
( Chapter 12). Finally, we will address the role of satellite cells in molecular exercise physiology
( Chapter 13).

A short hIstory of the evolutIon of molecular exercIse physIology as a fIeld

How did molecular exercise physiology evolve? In exercise physiology, as in other life sciences in the 20th century, the trend was to move the frontier of research from the whole- body level and organ systems towards cells and molecules ( 2). This does not mean that molecular studies now replace organ systems research. Instead, the molecular research complements and often explains what occurs at the organ and whole- body levels. Molecular studies enable researchers to answer questions that previous generations of exercise physiologists were unable to answer due to the lack of suitable methods. Thus, molecular exercise physiology is an extension of, and in complement to, classical exercise physiology. The era that preceded and prepared the ground for molecular exercise physiology was the biochemistry of exercise era. While the whole- body and organ- level exercise physiologists frequently used non- invasive methods, exercise biochemists begun to use invasive measures and made wet lab research their main analytical method of study. Consequently, physiologists needed to learn wet lab methods and had to equip their laboratories with the necessary consumables and equipment. This included, for example, chemicals, pipettes, pH meters, centrifuges, spectrophotometers and microscopes. Human exercise physiologists with a skeletal muscle focus also needed to learn the muscle biopsy technique, which was introduced to the field by Jonas Bergstrom, and/ or had to use animal models to derive skeletal muscle tissue. This was necessary because tissue samples, especially from skeletal muscle, were needed for subsequent biochemical or histochemical analysis ( see Chapter 2 for a summary of histochemical analysis of muscle tissue).

The first use of human skeletal muscle biopsy techniques to study exercise adaptation in humans at the biochemical level was by Jonas Bergstrom in 1962, publishing the paper ‘ Electrolytes in Man’ ( 3). Then in 1966, Bergstrom and Hultman were the first to measure glycogen synthesis in human muscle after exercise to exhaustion ( 4, 5). Bergstrom and Saltin in 1967 also assessed muscle glycogen in the quadriceps of both trained and untrained males, and after different starting nutritional backgrounds ( e.g. high carbohydrate/ high protein) ( 6, 7). This work was followed by Costill in 1971, who extended biopsy measurements for muscle glycogen to the gastrocnemius and soleus ( as well as quadriceps) after exhaustive running and following multiple bouts of exercise ( 8, 9). Exercise biochemistry was therefore the first discipline that attempted to understand changes in some of the important biochemical parameters following acute and repeated exercise bouts. In this era, exercise biochemistry was primarily the study of metabolites and protein activity using exciting novel methodologies that are now taken for granted such as enzyme- based assays, spectrophotometric and fluorometric analyses together with
histological measurements. However, even prior to this work in human muscle, Gollnick’s seminal work in 1961 applied exercise biochemistry in rodents to investigate the regulation of adenosine triphosphate ( ATP) and lactate dehydrogenase ( LDH) activity in heart and skeletal muscle tissues following exercise
( 10, 11). John Holloszy also pioneered the early exercise biochemistry field by measuring biochemical enzyme activity associated with mitochondrial adaption to aerobic exercise in rats ( 12). Frank Booth (see Box 1.1), together with Hollozsy ( 13– 15), extended this earlier work in considerably more detail over the ensuing decade, studying in detail the biochemical adaptation to exercise ( 16– 19). Pertinent examples, which were probably the first bona fide use of molecular biology in exercise physiology, came from Watson, Stein and Booth who undertook the first work into changes of gene/ messenger RNA
( mRNA) expression ( how much a gene turns on or off) investigating alpha- actin mRNA after muscle immobilization that simulates disuse and cessation of physical activity in the muscle ( 20). Even more impressively, this research used techniques for gene expression analysis that predated the future Nobel
Prize- winning method of polymerase chain reaction ( PCR), described below. They subsequently went on to investigate alpha- actin and cytochrome C mRNA expression following both exercise and immobilization in rats ( 17, 21– 23). Frank Booth also undertook some of the first work on muscle protein synthesis ( 24) and degradation ( 25), later combining these with adaptation at the biochemical and gene expression levels ( 26, 27). It was here that early exercise biochemists laid the basis for a field that would soon emerge, leading Booth to his postulation within the literature of a specific field called
‘ cellular and molecular exercise physiology’ ( 1, 28– 30) ( see Box 1.1). The advent of PCR to amplify RNA and DNA templates by Kerry Mullis in the late 1980s, leading to the Nobel Prize in chemistry in 1993, made gene expression analysis more sensitive, rapid, accurate and enabled higher throughput within the molecular biology field ( 31, 32). Importantly, Booth’s lab was one of the first to simultaneously undertake direct gene transfer into the skeletal muscle of live rodents ( 33, 34), allowing them to alter the level of gene expression of specific genes enabling a powerful model to mechanistically ratify ( or even refute) the role of individual genes in exercise adaptation. These gene ‘ knock- down’ and gene ‘ knock- in’ ( or overexpression) models are discussed below, and the theory for these models in the context of sport and exercise covered in more detail in Chapter 3.