Posts Tagged ‘biochemistry’
exploring spermatogenesis
Canto: So If Charles Darwin was alive today, he’d be gobsmacked at the facts derived from the ‘random variation’ end of his theory of natural selection from random variation. I’m talking about genes, DNA, genetic recombination and all that we know about meiosis and mitosis, spermatogenesis and oogenesis, genomics and epigenetics, mitochondrial DNA, ribosomes, mRNA, proteins and the like, none of which I’m particularly knowledgeable about – but surely even what I know about it all would make Darwin’s head explode.
Jacinta: Yes, and of course Darwin did all his studies on phenotypes, a term he would never have heard. He studied pigeons, finches, barnacles, fossils and a wide variety of plants. But he was never able to ‘crack the code’ of random variation. Why did offspring differ from parents? Why did those offspring vary from the utterly dysfunctional to the super-functional? For a time he considered pangenesis, his coinage, as a solution. This involved ‘gemmules’ inherited from both parents, blended together and somehow modified by the environment, presumably in a Lamarckian way. So Darwin never quite cracked the code of inheritance as we understand it today, but the work with plants which occupied his last years – allowing him to avoid the acrimony around human origins surrounding the publication of On the origin of species – produced important results for the understanding of plant reproductive biology. Take this quote from the Smithsonian magazine:
Darwin designed highly rigorous experiments and made predictions—which turned out to be correct—using his theory of natural selection. For example, he predicted that the myriad floral adaptations he saw existed to ensure that flowers were outcrossed, or fertilized by individuals other than themselves. He then tested this hypothesis with over a decade of pollination experiments and found that self-pollination leads to lower fitness and higher sterility. Inbred plants, like inbred animals, don’t fare well, at least over time—a phenomenon that’s now known as inbreeding depression.
Canto: Right, but let’s not get bogged down in the history of reproductive biology and the birth of genetics here, as it’s hard enough for me to comprehend meiosis and mitosis, gametes and zygotes and all the rest, as we understand it all today. We’ve previously written about meiosis, but I want to understand, or to begin to understand, in this post, how the process of producing gametes is so different in male and female mammals.
Jacinta: Okay, so we’re talking about gametogenesis. The male gametes are called sperm, the female gametes are called eggs, and so have two forms of gametogenesis, spermatogenesis and oogenesis. In this post I’ll focus on the male, saving the best for another post. So sperm is formed in the testes…
Canto: The ballsacks?
Jacinta: Uhh, well, the sack is just the sack, also known as the scrotum. Inside, you’ll find a testicle, hopefully. And as you well know there are, ideally, two of them. That is, two sacks, each with its testicle. And a testicle is about as complex as any other piece of biological machinery – a lifetime’s learning worth. Take this illustration, courtesy of ken hub.com:
Note the seminiferous tubules above. That’s where the sperm is formed, first by the mitotic division of a spermatogonial stem cell…
Canto: Eh what? How did they get in there?
Jacinta: Okay let me try to understand this for myself, but I may get more and more bogged down. It all begins at the beginning, during the early stages of male foetal development. The primordial germ cells differentiate in the testis, in these seminiferous tubules… But let me first fast forward to the end of the process and describe a complete, mature sperm cell or spermatozoon. That’s an active, motile sperm – plural spermatozoa, or just plain sperm. It’s divided into three parts, essentially, the head, the midpiece and the tail. At the head we find the acrosome and the tightly packed nucleus. The midpiece contains the mitochondria. which provides energy for the sperm’s motility, and the tail is essentially the flagellum, the sperm’s outboard motor, so to speak.
Canto: Okay, so that’s the end product – get back to the spermatogonial stem cells and the seminiferous tubules.
Jacinta: Fine. Spermatogonia are undifferentiated male germ cells, or sperm cells. It’s hard to find a simplified, but not overly simplified, explanation of how pluripotent or totipotent stem cells become germ cells, or any other cells for that matter, but it begins in the embryo. A cell signalling process in the embryo induces a small, transient proportion of the cell mass, the primitive streak, to become primordial germ cells (PGCs), along with other cells. This process is called gastrulation, in which the embryo begins to differentiate into distinct cell lineages. For the PGCs, according to a paper cited in Wikipedia, ‘The specification of primordial germ cells in mammals is mainly attributed to the downstream functions of two signaling pathways; the BMP signaling pathway and the canonical Wnt/β-catenin pathway’. This is essentially about regulatory proteins, I think.
Canto: This is getting too complicated for me. How come that second pathway is canonical?
Jacinta: See, you are paying attention. That Wnt/beta-catenin pathway gets a lot of attention in scientific papers, because we know that its deregulation is a problem in serious diseases and cancers. Basically these pathways are essential for embryonic development. The terms ‘canonical’ and ‘noncanonical’ are terms of art used to describe the standard production of Wnt proteins for development or homeostasis, and less well-known, or later-discovered pathways. I think. Anyway, let’s get back to spermatogonia, of which there are three types – A dark, A pale and B. The A dark spermatogonia are the reserves, and they don’t generally go through the mitosis process – they remain dormant. The A pale cells (so called because they have pale nuclei compared to the A dark cells) undergo mitosis to become the type B cells, which grow and develop to become primary spermatocytes, a process called spermatocytogenesis, truly. All of this occurs, as mentioned, in the seminiferous tubules of the testes, and begins at puberty.
Canto: Okay so how do these primary spermatocytes differ from spermatozoa, or how do they become spermatozoa?
Jacinta: The primary spermatocytes are diploid cells, so they need to undergo meiosis to become gametes. After meiosis 1, two haploid cells are formed, called secondary spermatocytes. And of course, being diploid cells undergoing that first process of meiosis, there’s this crossing over or recombination that occurs, shuffling the deck so to speak. And this is followed by meiosis 2, replicating the haploid cells, and so forth. But you ask how the spermatozoa are formed as an end product, so I need to take us back to those tubules in the testes. They’re packed with particular cells called Sertoli cells, and just outside the tubules are Leydig cells, which produce testosterone. Anyway, once these sperm cells have developed further they travel up to the epididymis via the rete testis, where they continue to mature, ready for ejaculation. They reach the rete testis, and presumably also the epididymis, by means of peristalsis, which you’ll know about from the intestines and other parts of the body.
Canto: Sort of. You think you know about stuff until you find out what you don’t know, which is overwhelmingly vast. Mais, continue..
Jacinta: So the last transformations, making them those mobile little tadpole-like critters, occur in the epididymis. But returning to those tubules. There are lots of Sertoli cells in there, and the sperm is developed in the gaps between them, strangely enough, but they acquire nutrients from those cells to help them along. Their journey between the cells takes them from the outer membrane of the tubule to the lumen. At the beginning of this journey they’re called spermatogonia. They’re going to go through this differentiating process to finally become spermatozoa. Now I’ve already partially described the first step, when a spermatogonium divides by mitosis, into two cells, one of which is kept in reserve, the Ad or ‘dark’ cell. The Ap or ‘pale’ cells continue on the pathway between the Sertoli cells towards the lumen, somehow becoming B cells – don’t know how that happens, but it involves mitosis, perhaps with nutrients from the Sertoli cells. I think, because the process of mitosis is continuous, those reserve cells are left behind all along the pathway. Or maybe not. But that pathway is obstructed along the way by ‘tight junctions’ between the Sertoli cells, which create separate compartments as they open and close before and behind the sperm cells (which are now called primary spermatocytes) like locks in a canal. Now these compartments, called basal and lumenal compartments, aren’t empty, they’re full of chemicals, signalling proteins and such, a different mix for each compartment, which add to the spermatocyte’s development. So the sperm grows as it travels along this pathway, accumulating more cytoplasm. And the junctions close very tightly after the sperm moves through, to prevent leakage into the next chemical environment. Now, somewhere along this pathway between the Sertoli cells, the primary spermatocyte is ready to divide into two secondary spermatocytes via meiosis, a very different form of cell division from mitosis.
Canto: Yes, meiosis has those two parts, ending with four haploid cells from one diploid cell, and genetic recombination to make us all unique.
Jacinta; Okay, moving right along, so to speak, those four haploid cells are now called spermatids, and they continue to mature in the lumen. They’re still not motile, they’re rounded cells at first, but they go through lots of changes, to the conformation of the DNA, for example, with histone proteins being replaced by protamines. We’re now entering the final processes, known as spermiogenesis, which I think occurs after transportation to the epididymis. The cytoplasm is removed, the acrosomal cap is formed, and the other structures I mentioned at the outset, the mitochondrial spiral and the fibres that form the flagellum, all take shape. This whole process, from spermatogonia to spermatozoa, takes about 65 days.
Canto: Okay, that’s enough of all that, I don’t particularly want to learn about seminal fluids and ejaculation at this point, fascinating though that might be – I’m more interested in the female stuff, the generation of eggs, known as oogenesis.
Jacinta: So that for you to detail in a future post.
References
https://embryo.asu.edu/pages/charles-darwins-theory-pangenesis
https://sciencing.com/difference-female-mammals-male-mammals-8092368.html
Spermatogenesis | Reproductive system physiology | NCLEX-RN | Khan Academy (video)
https://en.wikipedia.org/wiki/Germline_development#Germ_line_development_in_mammals
Covid 19: some stuff on remdesivir
Canto: So there’s this promising new antiviral drug that researchers are working on. Remdesivir. Terrible name. Why not something more hard-hitting like rambovir or rockyvir?
Jacinta: Well I’m not sure it’s an American drug, and I don’t think it’s new. It’s new for Covid-19. Everything’s new for Covid-19. And here we should repeat the standard caveat: ‘No specific agent has yet been demonstrated to be clinically effective in the management of Covid-19’.
Canto: Well done. So I’m reading this online article from a week or so ago – and a week’s a long time in Covid-19 – on the website of the Medical Journal of Australia, and it tells me that the antimicrobial remdesivir is ‘an investigational nucleotide prodrug’ – glad it’s not one of them antidrugs – and was used on the first diagnosed Covid-19 sufferer in the US. So maybe it is American. The article doesn’t say anything about its effect on the patient, but apparently it was first developed as a potential therapy for Ebola, and there’s some laboratory evidence that it can inhibit the replication of SARS-CoV-2.
Jacinta: That’s right, so four clinical trials have already begun in the US to test the effects of remdesivir, and another two are registered in China.
Canto: Well according to this media release only yesterday (April 17) from the National Institutes of Health (NIH) in the USA, they’ve already been testing the drug on poor old rhesus macaques…
Jacinta: They infected em? Bastards.
Canto: History is written by the victors my friend. And also by those who can actually write. Anyway, they responded very well to early treatment with reduced clinical signs and lung damage in a study designed to simulate treatment procedures for human patients in a hospital setting…
Jacinta: That’s nice. They got to sleep in real beds, like middle-class macaques.
Canto: Maybe. Of course, none of this has been peer-reviewed yet, but it’s very promising. But let me give you the total lowdown. You know that there have to be control groups, right?
Jacinta: Uhhh – uh-o. So… Let me see, they were all infected with the virus, but only some got the remdesivir, right?
Canto: Well of course they had to make the comparison. So they had two groups of six rhesus macaques, and they infected both groups with SARS-CoV-2. Then 12 hours later the treatment group received an injection of remdesivir. Sorry about the other group. After that the treatment group received a booster injection every day for the next six days. The initial treatment was timed to more or less coincide with the animals’ highest projected viral load. They first examined the animals 12 hours after initial treatment, and the treatment seemed pretty effective, only one still showed some mild symptoms, while in the control group they all displayed ‘rapid and difficult breathing’ …
Jacinta: Called dyspnoea in medical lingo.
Canto: Thank you. So the study continued for seven days, and over that time the treated monkeys were found to have significantly less virus in, and damage to, their lungs than the untreated.
Jacinta: So what happened to the untreated monkeys after that?
Canto: I might say ‘don’t ask, don’t tell’, but I think it’s reasonable to assume that after seven days they were treated with remdesivir and recovered. And that they chose a short, seven-day testing period so as not to endanger any monkey lives?
Jacinta: Hmmm. I don’t know too much about monkey business… Anyway, this remdesivir is obviouly promising and we must watch out for the results of other trials. But what is this remdesivir? What exactly is an antiviral, or a ‘nucleotide prodrug’, and do they all act in the same way? I know they’re not vaccines, they don’t induce antibodies, so how do they suppress the infection?
Canto: Okay, so our first stop on our info crawl is Wikipedia. Think of antivirals as a counterpart to antibiotics, aimed at viruses rather than bacterial pathogens, except that, unlike most antibiotics, their aim is to suppress rather than to kill the pathogen.
Jacinta: Really? Why not aim to kill the virus?
Canto: I don’t know, perhaps that’s not so easy with viruses. Anyway, while most antivirals target specific viruses, some are broad-spectrum, and I suppose remdesivir is one of those, since it was also successful against MERS, another coronavirus, and was first developed to combat Ebola virus, which isn’t a coronavirus as far as I know.
Jacinta: Remdesivir was earlier described as a nucleotide prodrug. A nucleotide is the basic structural unit of a nucleic acid such as RNA. A prodrug is by definition an inactive biological or pharmacological compound that can be converted within the body to have active drug properties. So the field of antiviral drug research has developed a lot, especially as a result of the HIV epidemic, and those that followed. All of this has expanded our knowledge of how viruses enter hosts and proliferate. SARS-CoV-2 is a set of RNA nucleotides surrounded by a protein capsid, or capsule, over which is a lipid envelope. It enters the host via the spike protein, and through this membrane fusion it infects host T lymphocytes – white blood cells that form a part of our immune system.
Canto: Yes, and trying to describe it all in lay terms – so that we understand it – is damn difficult. We know remdesivir has been somewhat effective for a broad spectrum of action against RNA viruses, and I note in this abstract that it’s ‘a nucleotide analog inhibitor of RNA-dependent RNA polymerases (RdRps)’ My guess is this means it acts like a nucleotide, inhibiting these RDRps. An RNA polymerase, I’m learning, is an enzyme (a type of protein) that’s ‘responsible for copying a DNA sequence into an RNA sequence, during the process of transcription’. But maybe an RNA-dependent RNA polymerase works on RNA, in the absence of DNA. So presumably remdesivir inhibits this essential enzyme from carrying out the transcription process that replicates the virus.
Jacinta: Maybe. By the way, as we travel the net on our info crawl, we’ve discovered some amazing stuff, such as this Covid-19 pandemic series of ongoing videos from a source called MedCram that began in late January and traces the spread, and the drama. The series begins with these words: ‘one of the things that’s in the news and hopefully goes away real soon is the coronavirus epidemic from 2019…’ That, to me, was more compelling than any advertising hook I’ve ever read.
Canto: Yes I’m keen to watch the whole series. Anyway, I believe remdesivir, also called RDV, has been used in an unauthorised way on human subjects already, and news from this Chemical and Engineering News website is that, understandably, interest in the drug and in scaling up production is reaching fever pitch, with a lot of pressure on Gilead, the company that presumably has a patent on RDV.
Jacinta: Of course, as we’ve already pointed out, this is exactly not the time for one private company to get precious about its rights to profit. Scaling up, assuming the drug’s effectiveness can be confirmed, should involve multiple labs in multiple countries. Having said that, producing a drug like RDV, described as a ‘medium complexity project’ compared to an apparently simpler drug such as the antimalarial drug hydrochloroquine, already involves a chain of companies and suppliers in a multi-step process. Every step in the process would need to be efficient, to prevent bottlenecks. Scaling-up also raises questions – remember Tamiflu? Our government stockpiled it in vast amounts in spite of damning analyses by the Cochrane Collaboration and others about its limited effectiveness and problematic side-effects. We don’t yet have proper analysis of RDV’s effectiveness, and we don’t know how much of it might be required, because nobody can predict the eventual course of this pandemic.
Canto: All true, but right now people are dying, and this is clearly the worst pandemic in more than a century. There are of course candidates other than RDV, it would be unwise to focus on just one, but public and private resources should be combined to bring any possible effective treatment to fruition. That’s what I reckon.
References
https://www.mja.com.au/journal/2020/clinical-presentation-and-management-covid-19
https://en.wikipedia.org/wiki/Antiviral_drug
https://ama.com.au/ausmed/govt-stands-tamiflu-despite-damning-findings
A DNA dialogue 2: the double helix
Canto: Ok we talked about base pairs at the end of dialogue 1. A (nucleo)base pair is, duh, a pairing of nucleobases. There are four types of base in DNA – adenine and thymine, which always pair together, and the other pairing, cytosine and guanine.
Jacinta: Please explain – what’s a nucleobase, what do they do, and why do they come in pairs?
Canto: Well, let’s see, how do we begin… DNA stands for deoxyribonucleic acid…
Jacinta: So it’s an acid. But bases are like the opposite of acids aren’t they? So how can an acid be constructed of its opposite?
Canto: Look, I can’t answer that right now – I haven’t a clue – but let’s keep investigating the structure and function, and the answers might come. So, you’ll know that there was a battle in the 1950s to elucidate the structure of DNA, and it was found to form a double helix two strands of – I don’t know what – connected to each other in a twisted sort of way by, I think, those base pairs connected by hydrogen bonds. Anyway, here’s a fairly typical explanation, from Nature Education, which we’ll try to make sense of:
The double helix describes the appearance of double-stranded DNA, which is composed of two linear strands that run opposite to each other, or anti-parallel, and twist together. Each DNA strand within the double helix is a long, linear molecule made of smaller units called nucleotides that form a chain. The chemical backbones of the double helix are made up of sugar and phosphate molecules that are connected by chemical bonds, known as sugar-phosphate backbones. The two helical strands are connected through interactions between pairs of nucleotides, also called base pairs. Two types of base pairing occur: nucleotide A pairs with T, and nucleotide C pairs with G.
Jacinta: So I think I have a problem with this description. I think I need a picture, fully labelled. So the two strands themselves are made up of nucleotides, and the connections between them are made up of bonded sugar and phosphate molecules? But the strands are connected, via sugar and phosphate, in particular ways – ‘through interactions’ – which only allow A to pair with T, and C to pair with G?.
Canto: I think that’s right. Maybe we can find a picture.
Jacinta: Ok, so we got it completely wrong. The backbone, of sugar-phosphate, is the outer, twisted strand, or two of them, like the vertical bars of a twisted ladder, or the toprails of a spiral staircase, and the base pairs are like the stairs themselves, made of two separate parts, the bases, bonded together by hydrogen…
Canto: Forget the description, the picture above is worth all our words. It also tells us that the DNA molecule is around 2 nanometres wide. That’s two billionths of a metre. And 3.4 nanometres long for a full twist of the double helix, I think.
Jacinta: Whateva. There’s also this claim that the two strands are ‘anti-parallel’. It looks to me as if they’re simply parallel, but twisted. What does this mean? Is it significant?
Canto: I don’t know – maybe we’ll find out next time. I’m already exhausted.
Jacinta: …….
epigenetics and imprinting 4: the male-female thing
Gametes are gametes because of epigenetic modifications in their pro-nuclei, but they have to lose these modifications, or transform them, when they come together to form zygotes. The male pro-nucleus DNA methylation is stripped away immediately after sperm penetrates egg. The egg pronucleus undergoes the same process, but more gradually. It’s like a wiping away of epigenetic memory, creating totipotency, which becomes a more limited pluripotency as the blastocyst, with its inner cell mass (ICM), forms.
The ICM cells begin differentiating through the regulation of some key genes. For example, a gene codes for a protein that switches on a set of genes, which code for proteins in a cascading effect. But it’s not quite a matter of switching genes on or off, it’s rather more complex. The process is called gene reprogramming, and it’s of course done effortlessly during every reproductive cycle. Artificial reprogramming of the kind carried out by Yamanaka and others, an essential part of cloning, hasn’t come close to this natural process that goes on in mammals and other species every day.
Clearly, though the epigenetic reprogramming for the female pronucleus is different from that carried out more swiftly in the male. As Carey puts it, ‘the pattern of epigenetic modifications in sperm is one that allows the male pronucleus to be reprogrammed relatively easily.’ Human researchers haven’t been particularly successful in reprogramming an adult nucleus by various methods, such as transferring it to a fertilised egg or treating it with the four genes isolated by Yamanaka. The natural process of gene reprogramming eliminates most of the epigenetic effects accumulated in the parent genes, but as the reprogramming is a different process in the male and female pro-nucleus, this shows that they aren’t functionally equivalent. There is a ‘parent-of-origin effect’. Experiments done on mice to explore this effect found that DNA methylation, an important form of chromatin modification (and the first one discovered), was passed on to offspring by the female parent. That’s to say, DNA from the female was more heavily methylated than that from the male. Carey describes the DNA as ‘bar-coded’ as coming from the male or the female. The common term for this is imprinting, and it’s entirely epigenetic.
Imprinting has been cast by Carey, and no doubt others, as an aspect of the ‘battle of the sexes’. This battle may well be imprinted in the pronuclei of the fertilised egg. Here’s how Carey puts the two opposing positions:
Male: This pregnant female is carrying my genes in the form of this foetus. I may never mate with her again. I want my foetus to get as big as possible so that it has the greatest chance of passing on my genes.
Female: I want this foetus to pass on my genes. But I don’t want it to be at the cost of draining me so much that I never reproduce again. I want more than this one chance to pass on my genes.
So there’s a kind of balance that has developed in we eutherian mammals, in a battle to ensure that neither sex gains the upper hand. Further experiments on mice in recent times have explored how this battle is played out epigenetically. I’ll look at them in the next post in this series.
Reference
The Epigenetics Revolution, by Nessa Carey, 2011
epigenetics and imprinting 2: identical genes and non-identical phenotypes
I’ve now listened to a talk given by Nessa Carey (author of The epigenetic revolution) at the Royal Institution, but I don’t think she even mentioned imprinting, so I may not mention it again in this post, but I’ll get back to it.
The talk was of course easier to follow than the book, and it didn’t really teach me anything new, but it did hammer home some points that I should’ve mentioned at the outset, and that is that it’s obvious that genetics isn’t the whole story of our inheritance and development because it doesn’t begin to explain how, from one fertilised egg – the union of, or pairing of, two sets of chromosomes – we get, via divisions upon divisions upon divisions, a complex being with brain cells, blood cells, skin cells, liver cells and so forth, all with identical DNA. It also doesn’t explain how a maggot becomes a fly with the same set of genes (or a caterpillar becomes a butterfly, to be a little more uplifting). These transformations, which maintain genetic inheritance while involving massive change, must be instigated and shaped by something over and above genetics but intimately related to it – hence epigenetics. Other examples include whether a crocodile hatchling will turn out male or female – determined epigenetically via the temperature during development, rather than genetically via the Y chromosome in mammals.
So, to add to the description I gave last time, the histone proteins that the DNA wraps itself round come in batches or clusters of eight. The DNA wraps around one cluster, then another, and so on with millions of these histone clusters (which have much-studied ‘tails’ sticking out of them). And I should also remind myself that our DNA comes in a four-letter code strung together, out of which is constructed 3 billion or so letters.
The detailed description here is important (I hope). One gene will be wrapped around multiple histone clusters. Carey, in her talk, gave the example of a gene that breaks down alcohol faster in response to consumption over time. As Carey says, ‘[the body] has switched on higher expression of the gene that breaks down alcohol’. The response to this higher alcohol consumption is that signals are generated in the liver which induce modifications in the histone tails, which drive up gene expression. If you then reduce your alcohol consumption over time, further modifications will inhibit gene expression. And it won’t necessarily be a matter of off or on, but more like less or more, and the modifications may relate to perhaps an endless variety of other stimuli, so that it can get very complicated. We’re talking about modifications to proteins but there can also be modifications to DNA itself. These modifications are more permanent, generally. This is what creates specialised cells – it’s what prevents brain cells from creating haemoglobin, etc. Those genes are ‘tightened up’ or compacted in neurons by the modifying agents, so that, for example, they’re permanently unable to express the haemoglobin-creating function.
All of this is extremely fascinating and complex, of course, but the most fascinating – the most controversial and headline-creating stuff – has to do with carrying epigenetic changes to the next generation. The inheritance of acquired characteristics, no less. Next time.
References
What is epigenetics? with Nessa Carey – The Royal Institution (video)
The Epigenetics Revolution, by Nessa Carey, 2011
How statins work 1 – stuff about cholesterol, saturated fats and lipoproteins…
filched from Wikipedia – don’y worry, I don’t understand it either – at least not yet
Statins are HMG-CoA reductase inhibitors, according to Wikipedia’s first sentence on the topic. HMG-CoA reductase is an enzyme – a macromolecule that accelerates or catalyses chemical reactions in cells. The enzyme works in the mevalonate pathway, which produces cholesterol and other terpenoids (terpenoids are very common, varied and useful forms of hydrocarbon).
So what does HMG-CoA stand for, and what’s a reductase?
3 hydroxy -3 methyl-glutaryl coenzyme A, which may be explained later. A reductase is an enzyme which catalyses a reduction reaction, and I’m not sure if that refers to redox reactions, in which case reduction involves the gaining of electrons…
But let’s look at cholesterol, which statins are used against. Sterols are lipid molecules with a polar OH component, and ‘chole’, meaning bile, comes from the liver. So cholesterol is a type of lipid molecule produced largely by the liver or hepatic cells of vertebrate animals. Cholesterol is essential for life, and it’s synthesised in the cell via a complex 37-step process (the mevalonate pathway makes up the first 18 of these). It makes up about 30% of our cell membranes, and its continual production is necessary to maintain cell membrane structure and fluidity. In high food-intake countries such as Australia and the US, we ingest about 300mg of cholesterol a day on average. We also have an intake of phytosterols, produced by plants, which might vary from 200-300mgs. Of course, this is massively dependent on individual diets (increased phytosterol intake may reduce LDL cholesterol, but it comes with its own quite serious problems).
The (very basic) structure of cholesterol is shown below.
The body of the molecule (centre) contains 4 rings of carbon and hydrogen – A, B and C are 6-carbon rings, while D has 5. The bonds between rings A and B, and C and D, represent methyl groups. On the left is a hydroxyl group, which is hydrophilic and polar, though the massive body of the molecule is extremely hydrophobic, which is reinforced by the cholesterol tail connected to the D ring. The hydroxyl polarity creates a binding site, which builds structure as the molecule binds to others.
Interestingly, the need for cholesterol synthesis varies with temperature, or climate. This has to do with fluidity and melting points. People who live in colder climates require less cholesterol production because, in cold weather, solid structure remains intact. Hotter climates cause greater fluidity and increased entropy, so more cholesterol needs to be synthesised to create and maintain structure.
So now to the 18-step mevalonate pathway, by which the liver produces lanosterol, the precursor to cholesterol. Well, on second thoughts, maybe not… It’s fiendishly complex and Nobel Prizes have been deservedly won for working it all out and I’m currently thinking that physics is easy-peasy compared to biochemistry (or maybe not). What I’m coming up against is the interconnectivity of everything and the need to be thorough. For example, in order to understand statins we need to understand cholesterol, and in order to understand cholesterol we need to understand lipids, lipoproteins, the liver, the bloodstream, the digestive system… So I sometimes feel overwhelmed but also annoyed at the misinformation everywhere, with chiropractors or ‘MDs’ announcing the ‘truth’ about statins, cholesterol or whatever in 500-word screeds or 5-minute videos.
Anyway, back to work. Cholesterol is a lipid molecule, and lipids are generally hydrophobic (they don’t mix with water, or to be more exact they’re not very soluble in water), but cholesterol has a hydrophilic hydroxyl side to it. Lipids that have this hydrophilic/hydrophobic mix are called amphipathic. Phospholipids in cell membranes are an example. and they interact with cholesterol in the ‘phospholipid bilayer’. As an indication of the complexity involved, here’s a quote from an abstract of a biochemical paper on this very topic:
Mammalian cell membranes are composed of a complex array of glycerophospholipids and sphingolipids that vary in head-group and acyl-chain composition. In a given cell type, membrane phospholipids may amount to more than a thousand molecular species. The complexity of phospholipid and sphingolipid structures is most likely a consequence of their diverse roles in membrane dynamics, protein regulation, signal transduction and secretion. This review is mainly focused on two of the major classes of membrane phospholipids in eukaryotic organisms, sphingomyelins and phosphatidylcholines. These phospholipid classes constitute more than 50% of membrane phospholipids. Cholesterol is most likely to associate with these lipids in the membranes of the cells.
Anyway, perhaps for now at least I won’t explore the essential role of cholesterol in cell structure and function, but the role of ingested cholesterol, the difference between LDL and HDL cholesterol, and how it relates to saturated fats and heart disease, particularly atherosclerosis. As Gregory Roberts explains it in a Cosmos article, saturated fats (found in butter, meat and palm oil) definitely raise total cholesterol…
But what is saturated fat, as opposed to polyunsaturated or mono-unsaturated fat? Most of us have heard of these terms but do we really know what they mean? Here comes Wikipedia to the rescue (because there’s a lot of bullshit out there):.
A saturated fat is a type of fat in which the fatty acid chains have all or predominantly single bonds. A fat is made of two kinds of smaller molecules: glycerol and fatty acids. Fats are made of long chains of carbon (C) atoms. Some carbon atoms are linked by single bonds (-C-C-) and others are linked by double bonds (-C=C-). Double bonds can react with hydrogen to form single bonds. They are called saturated, because the second bond is broken and each half of the bond is attached to (saturated with) a hydrogen atom. Most animal fats are saturated. The fats of plants and fish are generally unsaturated. Saturated fats tend to have higher melting points than their corresponding unsaturated fats, leading to the popular understanding that saturated fats tend to be solids at room temperatures, while unsaturated fats tend to be liquid at room temperature with varying degrees of viscosity (meaning both saturated and unsaturated fats are found to be liquid at body temperature).
Saturated Fat, Wikipedia. I’ve removed links and notes – they’re just too much of a good thing! Apologies for the lengthy quote but I think this is essential reading in this context.
Various fats contain different proportions of saturated and unsaturated fat. Examples of foods containing a high proportion of saturated fat include animal fat products such as cream, cheese, butter, other whole milk dairy products and fatty meats which also contain dietary cholesterol. Certain vegetable products have high saturated fat content, such as coconut oil and palm kernel oil. Many prepared foods are high in saturated fat content, such as pizza, dairy desserts, and sausage.
Guidelines released by many medical organizations, including the World Health Organization, have advocated for reduction in the intake of saturated fat to promote health and reduce the risk from cardiovascular diseases. Many review articles also recommend a diet low in saturated fat and argue it will lower risks of cardiovascular diseases, diabetes, or death. A small number of contemporary reviews have challenged these conclusions, though predominant medical opinion is that saturated fat and cardiovascular disease are closely related.
High density lipoprotein (HDL) cholesterol can be a problem if your levels are low. HDL absorbs cholesterol and carries it back to the liver, from where it’s removed from the body. So generally high levels of HDL will reduce your chances of heart attack and stroke.
As Roberts notes, from the 1950s, heart disease has risen to be a major problem. Heart attack victims have been regularly found to have arteries clogged with ‘waxy plaques filled with cholesterol’. Further proof that cholesterol was to blame came with studies of people with a genetic disease – familial hypercholesterolemia (FH) – which meant that they had some five times the normal levels of blood cholesterol, and suffered heart attacks even as children or teenagers. Also, the rise in blood cholesterol levels and the rise in heart attacks, and heart disease generally, were correlated. This was unlikely to be coincidental.
But what’s a lipoprotein and why the different densities? Here we get into another area of extraordinary complexity. Lipoproteins are vehicles for transporting hydrophobic lipid molecules such as cholesterol, triglycerides and phospholipids through the watery bloodstream or the watery extracellular fluid (blood plasma – the yellowish liquid through which haemoglobin and lipoproteins etc are transported – is a proportion of that fluid). They act as emulsifiers, ‘encapsulating’ the lipids so that they can mix with and move through the fluid. Lipoproteins don’t just come in HD and LD forms – we classify them in terms of their density much as we classify colours in the light (electromagnetic) spectrum. According to that density classification we recognise five major types of lipoprotein in the bloodstream.
Cholesterol arrives in the blood via endogenous (internal) and exogenous (external) pathways. Some 70% of our cholesterol is produced by the liver, so, though diet is an important facet of changing cholesterol levels, finding ways of modifying or blocking liver production was clearly another option. Through studying the way fungi produced chemicals such as penicillin that break down cell walls (a large part of which are cholesterol), Akira Endo was the first to produce a statin from a mould in oranges – mevastatin. That was the beginning of the statin story.
References
https://en.wikipedia.org/wiki/Mevalonate_pathway
https://cosmosmagazine.com/society/will-statin-day-really-keep-doctor-away
Cholesterol metabolism, part one – video by Ben1994 (excellent)
Cholesterol structure, part 1/2, by Catalyst University
https://en.wikipedia.org/wiki/Cholesterol
https://en.wikipedia.org/wiki/Statin