Bone matters - researching the structure of bones

Bone matters - researching the structure of bones

  • Skeleton of a horse head
    Skeleton of a horse head

Nobody knows exactly what happened to Rio the horse to fill him with such raging fury against anything that entered his field. He chased walkers, trampled dogs, slammed himself against fences. But he did not just damage them – he damaged himself. It seemed as if he broke a leg every time. Rio’s rescuer, Melinda Duer, Professor of Biological and Biomedical Chemistry, wondered why. What made his bones so prone to breaking? She was sure it was a stupid question with an obvious answer, so she asked her friend, equine vet Dr Rachel Murray (Robinson 1984). “Every time Rio gets touched, he gets a leg fracture,” she said. “Why?” Murray laughed and said: “If I knew that, we’d both be rich.”

Duer looked through the literature and found that Murray was right. Whether animals or humans, nobody seemed to know why some bones are weaker than others. So Duer made it her business to find out – and, 10 years later, she believes her team have found the answer. Using a complex combination of techniques, including nuclear magnetic resonance spectroscopy, X-ray diffraction, and computer modelling and imaging, they have discovered that at a molecular level, bone mineral – which makes up the majority of bone – consists of layers. Layers upon nanoscopic layers of flat bone mineral crystals of calcium phosphate, trapping further gooey layers of water and citrate molecules between them. “It’s like glass plates with water and tiny ball bearings – the citrate molecules – in between,” says Duer. “Or millefeuille, with perfect, frictionless cream.”

Bones underpin everything we do, both literally and figuratively. We can’t walk without a spine, or breathe without a ribcage. Our language has evolved to recognise the importance of bones: we strive to get down to the “bare bones” of an issue, we “put flesh on the bones” of an idea. When our internal support system breaks, cracks, slips or softens, the consequences are catastrophic. They’re also common: osteoporosis, where the struts that make up a bone’s structure become fragile and therefore more liable to break, affects more than three million people in the UK alone. Hip fractures cost the NHS around £1.9bn a year. That’s just the cost of the hospital stay and doesn’t even include the social support needed afterwards. More people die from musculoskeletal disorders than of cancer.

So bone is a complex, yet vital, material. But the answer to Duer’s original “stupid question” about why some bones are weaker than others is both far more complex and potentially far more wide-ranging than it might first seem. To understand why, you need to know more about citrate molecules. This is how the process works: normal cell metabolism processes cause citrate molecules to arrive within the bone tissue. They have a dual role: the first is to deliver calcium phosphate to the bone’s calcium phosphate crystals. The molecules can get into the fine mesh that is bone tissue, make their delivery, and, if the tissue is healthy, they can’t get out. That’s where their second essential role comes in – forming layers to keep those calcium phosphate crystals apart, separate and so consequently able to move. This movement under pressure allows bones to have their extraordinary flexibility and strength: they are natural shock absorbers.

  • Professor Melinda Duer. Portrait by Anna Hux.
    Professor Melinda Duer. Portrait by Anna Hux.

A new way of looking at human tissue

When this system works correctly, it works brilliantly. But sometimes it goes wrong, and that is when bones become fragile. Unhealthy bone tissue, whether damaged by trauma or just the natural ageing process, has holes in the mesh – holes big enough for the citrate molecules to escape once they’ve delivered their calcium phosphate. Without the supporting layers of citrate molecules keeping them apart, the calcium phosphate crystals fuse together. They become bigger, with less flexibility and give – leading to weaker bones which are more likely to break.

This is all new territory. As Duer points out, when she posed her original question there were many theories, but ultimately, few conclusions about what ultimately causes bone weakness. “There are a huge number of studies out there, but more contradictions than answers. Maybe it was the lack of a mineral, too much of a mineral, maybe the mineral crystals were too big, maybe they were too small… but they weren’t doing anything wrong. Bone is just so difficult to study – so complex, so heterogeneous. It changes over time. Both the molecular level structure and the microscopic level structure matter. And the interplay of all those different structures and the molecules come together to make even bigger structures which tell us more about bigger structures, and all of those structures matter, too.”

Consequently, like the bones themselves, Duer’s work could now underpin a whole new way of looking at human tissue, by studying tissue behavior on a molecular level. Building on the tissue study techniques that her team pioneered, Duer now wants to answer one of the biggest questions of our time, and one that impacts a huge range of conditions from dementia to cancer: what makes human tissue work as it does? A tissue, in general, is made up of collagen and other proteins, on to which cells stick to form new tissue, she explains. How does a new bone cell, for example, know that it’s a bone cell and not a muscle cell? We know that the collagen and the other molecules in the tissue give it signals. But we don’t know what those signals are, or why they give them. And those signals don’t always have positive effects. Altered molecular structures can signal to cancer cells to move to other tissues, or to shut down brain cells when they sense the formation of plaque. Duer’s team are about to publish a study which has found similarities between bone formation and calcification of the arteries, which can lead to stroke and heart attack – the biggest cause of death in the developed world.

There’s no clue to the complexity of Duer’s work in her office. A former laboratory, it’s high ceilinged, bare and business-like, with only a large, round, bog-standard wood-veneer table standing out. It’s at this table, she says, that she’s had her most fulfilling times: swapping ideas with computer modellers, physicists, biologists, medics, and materials specialists, all of whom were involved in her decade-long quest for answers.

“The best ideas are when I say to one of my biological collaborators: why does this happen?” she says. “And they go ‘Oh, that’s a good question, never thought about that, I don’t know.’ It’s true of every discipline that you’re trained in a particular way, that this is how things are. I’m like that with chemistry. But when you bring different disciplines together, the people from outside those core disciplines are going to ask stupid questions – and every now and again someone will say: ‘Hmmm. We never questioned that. Perhaps we should.’”

Early indications suggest that too much sugar causes cells to die horribly, leaving bits of damaged DNA floating around

Another wacky question: sugar

Duer’s longing to ask the big questions started at Sir James Smith’s School, a small comprehensive school in North Cornwall and her science teacher, Ron Trevithick, a relative of Richard Trevithick, inventor of the steam engine. With Trevithick, there were no stupid questions and no boundaries: he encouraged Duer to apply to Cambridge, where she studied natural sciences at Emmanuel. She was the first in her family to go to university. Her parents both left school at 15, worked their way up, and encouraged her to do things differently. Carl Sagan’s groundbreaking science TV series, Cosmos, was a turning point for her.

“I enjoyed what I stupidly thought was the deterministic nature of science at that time – the fact that there was always an answer,” she remembers. “But he said a lot about what was known and then in the same breath described what wasn’t known. It was just obvious to a teenager that there was more that wasn’t known than what was. As I went on in science, I realised that yes, there was always an answer, but it wasn’t always obvious what it was – and there was always another question, and then another, on and on.”

Alongside her work on tissue, Duer is working on a three-year study funded by the Medical Research Foundation investigating another “wacky question” that occurred to her: does eating too much sugar damage cells? Early indications seem to suggest that it doesn’t just kill them, it causes them to die rather horribly, leaving what Duer unashamedly describes as “toxic, horrible and yucky” bits of chemically damaged DNA floating around in the tissue. These could do untold damage and hold the clue to the causes of osteoporosis and osteoarthritis.

“This is what gets me out of bed in the morning right now,” she says. “Finding out how tissues really work, on a molecular level. Ultimately, for things like cancer, I think a better way of treating it, rather than trying to kill every cancer cell, would be to change the tissue. Change what’s around it so that the tissue itself is telling cancer cells: ‘Don’t behave like a cancer cell’ or ‘Just do nothing’. At the very least, don’t tell other cells to misbehave or die. We’re just getting into that, in that we’ve realised the specific questions we need to be asking. It’s so exciting.

“I have learned a lot in the past 10 years but I am still so ignorant that I am going to keep on asking stupid questions. Because we don’t know the answers.”

This article first appeared in CAM - the Cambridge Alumni Magazine, edition 76Find out how to receive CAM.