In the early years of my medical training, many years ago, it was hugely exciting to learn the basic science of how the body functioned, before we learnt about the pathological processes affecting body function, and how to treat these. Two visual memories have always stuck in my mind from the lectures I attended back then. These were both of some brilliant diagrams drawn on the blackboard during these basic physiology and anatomy lectures by two charismatic teachers. The first was of the neural pathways flowing to and from the brain, and these complex but organized information directing structures were drawn with such clarity that they entranced me, to the extent that they perhaps in part led to me becoming an integrative neuroscience and system regulation researcher for much of my career. The second blackboard drawing I remember was of the metabolic pathways of a cell, which made me feel a sense of awe regarding the complexity and sheer volume of processes and structures present in a cell when I saw them for the first time as line drawings on the blackboard way back then. Being throughout my life more of a systems and processes, rather than a specifics and detail, type of thinker and person, these cell pathway pictures made me feel a touch ‘queasy’, no matter how beautiful they were drawn, and this ‘queasiness’ I felt then perhaps moved me ‘away’ from molecular or cellular biology as a career choice, because intuitively I knew even then I would never have the capacity, or interest, to learn each different enzyme or DNA or protein structure in the cell, and how each of these ‘worked’ to provide the basic energy needed to sustain life as we know it. A couple of days ago, Professor Craig Sale, one of the UK’s foremost Physiology and Exercise Science researchers, and surely one of the nicest persons on the planet, posted a fascinating review article on Twitter examining basic cellular metabolism changes related to exercise to rest transitions, and reading about the associated changes in ATP, NADH, and CrP, amongst other cellular substrates and enzymes described in this review article, reminded me of those flow diagrams of my medical training days, and got me thinking about the basic function of the cell, and how the flux of all these basic cellular substrates is managed in the microscopic cellular environment.
The cell is the basic structural and functional unit of any organism, including us humans. One’s entire body is made up of cells, and it is thought that each person consists of around 10 trillion cells (a number so vast I can’t get my head around it). While the cells making up different structures in the body like skin, muscle, bone have some differences in their structure and function, the basic ‘makeup’ of all cells is almost identical, and fascinatingly from an evolutionary perspective works similarly across all species and indeed most living structures. Each cell is surrounded by a cell membrane, and has a number of organelles in it that are important for its survival and optimal function, including a nucleus (where the cell’s DNA is stored which is important for replication), mitochondria (the energy creating ‘powerhouses’ of the cell), and several storage and structure creating entities known as the Golgi apparatus and endoplasmic reticulum, amongst others. The cell membrane is important not just for structural integrity, but also for controlling the movement of substrates, fuels, metabolites and signalling molecules into the cell, using enzyme related receptor mechanisms. The cell’s interior consists of a fluid substance known as cytoplasm, in which enzymes and cofactors either break down carbohydrates, fats and proteins into basic energy products (the basic energy molecule in the cell is adenosine triphosphate – ATP) in a process known as catabolism (this process also occurs at a rapid, energetically efficient process in the mitochondria), or builds up structural components required to repair damage in the cell, or to create a second cell when the cell replicates, in a process known as anabolism.
In either the cytoplasm of the cell, or the mitochondria, are the numerous metabolic ‘pathways’ which were so elegantly drawn for us in our student days, and as a ‘cascade’ through numerous sequential intermediary products in a process managed by sequential enzymes and co-enzymes, fuels such as carbohydrates and fats are catabolised into basic energy products such as ATP. For example in the cytoplasm of the cell, the ‘glycolytic’ pathway occurs, while in the mitochondria the ‘Krebs cycle’ and ‘citric acid cycle’ are different catabolic pathways. Oxygen is a necessary requirement of the mitochondrial basic energy producing processes which are therefore defined as ‘aerobic’ pathways, while the cytoplasmic glycolytic processes do not require oxygen, and are defined as ‘anaerobic’ pathways. The anaerobic glycolytic pathways are not as efficient in producing basic energy as the aerobic pathways, and produce metabolic by-products which need to be removed from the system, such as lactic acid. Because lactic acid builds up during high intensity exercise like sprints or other ballistic / maximal activity, it is thought that during high intensity exercise the cells become oxygen deprived or ‘anaerobic’, and the glycolytic pathways are use preferentially and as a final energy ‘reserve’, though whether cells are ever completely oxygen deprived and therefore rely on ‘anaerobic’ mechanisms to produce fuel during activities of daily living or during exercise is still controversial. There are an enormous amount of different pathways in a cell, and seemingly each month, a new enzyme, co-factor, intermediate product, or metabolic by-product is discovered, which creates an even more complicated ‘picture’ of the working environment in each cell.
The regulation of these complex cellular activities and structures has focussed principally on metabolic flux and enzyme kinetic processes. Metabolic flux is defined as the rate of turnover of molecules through a specific metabolic pathway, and describes the ‘movement’ of substrates or intermediary products ‘through’ a specific pathway. Metabolic flux is related to the ‘need’ of both the general body and specific cell environment for energy, and is increased when there is greater need (for example when one exercises), or when there is greater substrate present (for example after a meal). One of the most amazing things in science is how each of the different ‘steps’ of each pathway increases sequentially and in a temporally co-ordinated way when increased need or increased substrate availability occurs, to ensure that the pathways work correctly and are not ‘overwhelmed’ whenever there is the need for increased activity in all its component parts. This coordinated increase in activity in an entire metabolic pathway is thought to be controlled by increased and optimized enzyme function at each step of the specific pathways processes. Enzymes are protein molecules that can ‘manipulate’ and therefore control other molecules and substrates, and enzyme kinetics is defined as the study of the chemical reactions that are regulated by enzymes. When there is increased energy requirements, the function of the pathway’s enzymes is up-regulated, in order to ‘deal with’ the increased demand. The function of an enzyme can be plotted (for those technically minded folk an example of this is the Michaelis-Menten function equation) as substrate concentration increases, and generally at the start of a period of increased ‘need’, enzyme activity at each different step of a metabolic pathway is rapidly increased to compensate for the increased requirement. Subsequent to this initial rapid increase in enzyme activity, enzyme activity ‘levels out’ as the absolutely maximal activity capacity of the enzyme is reached. The enzyme kinetic / cell regulation researcher folk suggest that the rate limiting capacity of any pathway (and therefore the energy creating ‘controller’) is that of the enzyme with the ‘lowest’ functional capacity – in other words, the enzyme in a pathway that can least up-regulate its function in a time of increased energy demand or increased energy fuel supply, is the factor that controls the metabolic properties and activity of that particular cell. In this paradigm, therefore, the human body’s physical functional capacity is related to these cellular-level rate limiting enzymes, together with the quantity of energy fuels available that can be used by the enzymes.
All this knowledge of basic cell structure and function, that is still increasing incrementally (perhaps even exponentially) as yet another cellular regulatory molecule, enzyme or membrane signalling / transduction regulatory protein is discovered, still fills me with as much awe today as it did nigh on thirty years ago when I first learnt about it as a first year medical student. But, I do believe that a lot more research is needed in the field of cellular metabolic regulation for us to have a clearer understanding of its regulatory processes. Indeed, the wonderful ‘pictures’ drawn of the metabolic pathways may, in a paradoxical way given that they are so complex, be describing cellular regulatory mechanism in a too simplistic manner, and we perhaps have a long way to go still to fully understand regulatory control mechanisms at the cellular level. For example, hundreds, if not thousands of different metabolic pathways are actively catabolizing substrate fuels, or synthesizing new structural molecules, at any one point in time. Likewise, thousands, if not millions of different individual molecules are being acted upon, or are acting upon other molecules, in any one cell at any single point in time. How the integrity and fidelity of each metabolic pathway is maintained in the face of all this co-existing ‘other’ metabolic activity has still not been determined. How each molecule ‘knows’ where it ‘has to go’, and where in the cell it will be acted upon, at a single point in time, let alone in the required temporally appropriate manner, is still pretty much unknown. Equally, how the function of individual cells is harmoniously regulated as a component of the gestalt millions and billions of cells in a specific organ, which all must be similarly up-regulated in time of need or increased substrate concentration, and then down-regulated at time of work-rest transition, is not understood at all. Whether different specific cells have different efficiencies and metabolic milieus compared to that of their neighbours, has also not been determined, and for us to have knowledge of this conundrum will need spectacular new laboratory techniques to be developed. How the afferent ‘messages’ from each cell become a gestalt ‘message’ to the brain which ‘suggest’ a requirement for an initiation of behavioural change, when for example fuel supplies are depleting, is also completely unknown, as is how each different cell receives similar efferent information to either increase anabolic or catabolic need as it is required. Furthermore, the relationship between the ‘physical’ control processes such as enzyme kinetic control process or metabolic flux determinants, and electrical / electromagnetic energy, is not clear. All active metabolic control mechanisms are underpinned by electrical activity changes, or at least electrical activity can be detected in cells whenever physical chemical changes occur in the cell. One of the most interesting research papers I have ever read described a study of NADPH activity in macrophage cells. When interleukin-6, a humoral (blood / fluid related) signalling / regulatory molecule was added to the cells, the concentrations of NADPH increased. When an electrical current was supplied to the cells along with interleukin-6, the concentration of NADPH increased even more than when just the interleukin-6 was added. How this ‘piezo-electric’ electrical / chemical interaction works at the cellular level is still not clear, as is whether electrical, or electromagnetic activity, are subsidiary or integral components of cells and their metabolic regulation.
A beautiful picture or line drawing of a particular metabolic pathway of a cell gives us therefore a ‘snapshot’ of the processes involved in that particular pathway, but does not give us the full picture of what must be ‘dizzying’ real life / real time activity occurring as a hugely complex, interactive, always changing, process and environment in any one cell, let alone in an aggregation of cells. How control processes occurs in not just one cell but similarly in many cells is another problem of an order of magnitude greater than we can perhaps currently understand with our available research techniques and conceptual frameworks we use to understand such function, which usually involve breaking down such dynamic processes into its composite parts to allow easier explanation. By reducing the complexity such in order to do so, we perhaps lose our capacity to understand the ‘gestalt’ control processes and mechanisms in the cell. A good scientist will always be humbled by the awareness of how much activity, and how much regulatory control, is required for even a single, ‘simple’ cell, which is the basic building block of all physical life processes and structures we know of and are made of. An integrative systems scientist like myself will always admire and respect the work done by the scientist folk who work at solving the ‘detail’ that exists in each cell and its individual component. But the big questions of cellular function and metabolic regulation are still surely ahead of us to be answered, and beautiful flow diagrams of cellular metabolic regulatory pathways, as those which will always be engraved in my mind from those seminal days of my academic youth were, will never be sufficient to allow understanding of the ‘place’ of the regulatory processes in the cell in the bigger picture of the regulation of life as we know it.
Perhaps using a three dimensional high-tech video clip of real time cellular activity, some charismatic lecturer, way in the future, will be able to explain to some young medical students how it really does all work, when I have long been resting in my pine box, and my cells are of the earth and being used as energy for another generation of cells in another future scientists body. Time will tell. But, for now, I will have to be satisfied with the memory of those beautiful drawn cellular pathways, and keep trying to remember the names of each substrate, enzyme and intermediate metabolite associated with them. And if I could go back in time, I would tell those two great lecturers that their drawing skills and passion for their subject matter were still remembered thirty years down the line, and inspired a career choice for me!