Meet Danio rerio - Keeper of the secret to self-healing

By observing how an embryo forms a new body, developmental biologists in Melbourne are laying the foundation for a new branch of medicine that gives damaged bodies the capacity to regenerate.

Story by Gio Braidotti

Professor Peter Currie

A fascination with animal forms has prompted Professor Peter Currie’s
breakthroughs on stem cells, blood cancers and degenerative
muscle diseases.

Within the penguin’s wobbly waddle or the mesmerising elegance of slithering snakes – locked within all of nature’s galloping, burrowing, hop-skip-and-jumping spectacle – are bodies with remarkable abilities but distinctly different forms, or ‘morphologies’.

It takes an almost naive intelligence to question the origin of body forms, be they of a panda, shark or human. Professor Peter Currie, winner of the 2015 Australian Museum Eureka Prize for Scientific Research, is susceptible to this kind of curiosity.

His fascination for the form of animals has led to a series of breakthroughs that are remarkable for their ability to advance human medicine. Included in this is the case of Danio rerio, the humble Himalayan zebrafish, which is able to regenerate a severed spinal cord – and may hold the secret to enabling other creatures, including humans, to do the same.

Animals such as Danio rerio have already allowed Professor Currie to identify the underlying causes of degenerative muscle diseases, to artificially make in a dish the blood stem cells needed by hemophiliacs or leukemia patients in lieu of bone marrow transplants, and to prevent the proliferation of these cells as a way to treat blood cancers.

And more novel findings are brewing away in his laboratory – especially about how muscles form, grow and regenerate.

The key to these breakthroughs is knowledge gained by watching how a single cell – the fertilised egg – of different animals across evolution creates a distinctly styled body.

“Given the right signals, embryonic stem cells undergo distinctive patterns of growth that generate all the various tissues and organs, while also providing the adult body with the stem cells it needs for growth and regeneration,” he says.

The adult human body unfortunately retains little regenerative ability, he adds. But it’s there, locked into the code that specified the body in the first place. He wants to unlock the body’s regenerative secrets and, for the first time in human history, apply that knowledge to advance medical science.

Professor Currie explains that evolution worked out how to make the essential features of an animal’s body one time. That’s right – just the once. Evolution then proceeded to conserve this core set of instructions – the ‘body plan’ – but learnt to tweak it to create myriad variations, much as the same instruments in an orchestra can play different symphonies.

Applying the body plan

From Professor Currie’s perspective, that has three great advantages. First, it means simple and well-characterised species – typically fruit flies, zebrafish or mice – can be used to do the grunt work of decoding the essential nature of the body plan. The use of these ‘model’ species generates knowledge that’s broadly applicable across evolution, including humans.

For example, a mutation in a fruit fly ‘body-plan gene’ that causes malformed eyes was found to have a counterpart in the mouse and human genome that causes congenital blindness. That discovery was crucial to convincing an initially sceptical scientific community in the 1990s that the same body-plan genes are conserved in species that look radically different.

This highlights the second advantage: the ability to use body-plan science to model human diseases. In Professor Currie’s case, he has a particular focus on the muscular dystrophies that weaken skeletal muscle.

Third, the animal world presents Professor Currie with a kaleidoscope of tweaked diversity in how the body plan is expressed. He uses the contrast provided by different species — especially at critical moments in evolution, such as the transition from fins to limbs – to gain insights into the essential nuances that determine the ultimate form and function of tissues.

Zebrafish have the most exquisite embryology. We can literally watch a new zebrafish form under the microscope over 24 hours, with the same transparency available to us when developmental processes go wrong and disease arises.

Professor Currie’s particular talent relates to bringing together these strands of developmental biology, disease modelling and evolutionary developmental biology.

As such, he uses the well-characterised laboratory species, but adds the occasional ‘non-model’ species, such as the strikingly beautiful epaulette shark (Hemiscyllium ocellatum), to maintain momentum in his discoveries. “I must admit I’m a bit of magpie,” he says. “I’m interested in a lot of things. Pounding away at one question or one gene is not enough for me. I prefer to ask interesting questions and follow wherever my investigations lead.”

The body project

Professor Currie’s marriage of morphological studies and medicine takes place at the A$153 million Australian Regenerative Medicine Institute (ARMI), the research centre he heads at Monash University in the Melbourne suburb of Clayton, the site of an extensive science and technology precinct. Central to the work undertaken by ARMI scientists is a basement housing a near-industrial-scale cluster of aquariums, for it’s the zebrafish and other species that drive the biological discoveries fuelling ARMI’s medical breakthroughs.

Zebrafish are able to regenerate a severed spinal cord in six weeks. This ability gave Professor Currie’s laboratory the contrast needed to identify why the human spinal cord fails to regenerate following injury. Included in these discoveries are differences in the way scar tissue forms, and the need for regeneration-inducing molecules that are now finding clinical applications.

“It may seem strange, but zebrafish are a really good human disease model,” Professor Currie says.

“For muscular dystrophies, a zebrafish becomes ill in the same way as a human. The same cells are affected and the same breakdown of the muscle occurs.”

However, the feature that gets developmental biologists especially excited is that once fertilised, the zebrafish egg becomes almost completely transparent.

"If you’ve ever looked down a microscope at a zebrafish embryo you’d understand why we work on them,” Professor Currie says. “They just have the most exquisite embryology. We can literally watch a new zebrafish form under the microscope over 24 hours, with the same transparency available to us when developmental processes go wrong and disease arises."

zebrafish

Danio rerio, the zebrafish, is a tropical freshwater species
native to the streams of the south-east Himalayan region.
They grow up to 6.4 centimetres in length and have a lifespan
of about three years, with a generation time of three months.
In addition, they have an easy genome to analyse and manipulate,
along with powerful regenerative abilities.

Stem cells are us

The marriage of developmental, evolutionary and regenerative science is especially present in Professor Currie's research on skeletal muscle. While his laboratory does ask the fundamental developmental question- how different muscle types are formed and patterned in the embryo - his curiosity doesn't stop there.

His colony of epaulette sharks, for example, allows him to compare muscle development across evolution. The contrast they provide helps him understand nuances of muscle development that can alter form and function, thereby revealing the key cellular and molecular pathways that regulate development.

Ultimately, he’s seeking the deep principles that regulate skeletal muscle formation and function. This requires investigations into an area rarely visited by developmental biology: the muscle’s ability to grow once formed and to regenerate following injury.

“This is important, because eventually the ability to grow new muscle stops in humans, causing sarcopenia, the debilitating muscle wasting responsible for the frailty, weakness and loss of quality of life in the elderly,” he explains.

In recent discoveries, he's identified that muscle growth and regeneration involve stem cells with entirely different embryonic origins. He's made important headway understanding the stem cells responsible for growth. Included is the realisation that stem cells for muscle growth operate through clonal drift which means just one or two stem cells in a bundle of muscle drift through the tissue, producing the cells needed to form new layers of muscle. He traced the origin of these muscle-growth stem cells to their source in the embryo and made a surprising discovery that the director of the Walter and Eliza Hall Institute, Professor Douglas Hilton, says is a major stride for stem cell research globally.

By combining genetics, lineage tracing and live imaging, Peter and his team discovered a novel population of cells [the endotome] within a compartment of the embryo called the somites, and a new gene called meox1 that acts like a molecular switch in the induction of blood versus muscle stem cells," Professor Hilton says.

Blood stem cells are monumentally important to medicine, as they resupply the body with both oxygen-carrying red blood cells and the white blood cells of the immune system. They’re among the most studied of all stem cells, but no-one has been able to make true self-renewing blood stem cells in a dish, until Professor Currie identified the new cell type and its ability to ‘nurse’ blood stem cell creation.

“This is scientific research at the highest level. It shows originality, creativity in the conceptual approach, and insightful deductions that will lead the way solving yet unidentified stem cell formation pathways in other tissues,” Professor Hilton says.

This is the work that won Professor Currie, his PhD student, Phong Nguyen, and Dr Georgina Hollway of the Garvan Institute the Eureka Prize for Scientific Research, and it has many important medical implications.

"You have no control over what questions you end up asking and therefore the kind of technology you need to answer those questions,” Professor Currie says of his approach to research. “Because of that, we work collaboratively with researchers with complementary expertise. For us, the collective intellect of our  collaborators is an important attribute to solving our questions. In return, we help them."

New blood
Making self-renewing blood stem cells in a dish could see leukemia patients bypass the need for bone marrow transplants following irradiation of diseased cells. Instead of bone marrow, the re-seeding could be achieved using blood stem cells nursed into being in a dish, using this discovery by the Australian Regenerative Medicine Institute and Professor Peter Currie.

For the blood discoveries, for instance, he worked with five ARMI teams. International partnerships are also important, with ARMI maintaining formal links with US and UK institutes, but also hosting the only node of the prestigious European Molecular Biology Laboratory (EMBL) located outside of Europe.

“The whole concept of the EMBL Australia initiative is to nurture young scientists,” he says. “We provide resources, mentorship and money for the brightest young talent to come and work here, start independent laboratories in a mentored environment, and become the next generation of professors.”

Ultimately it will take all the different ingredients the zebrafish, the collaborators, the non-model species, and a new generation of scientists to realise the dream of learning to tap evolution’s own medicine cabinet.

“We call it the fish tank to bedside approach’,”  he quips.

“After all, from evolution’s perspective, we’re just modified fish.”