7th World Biomaterials Congress: Take heart from new research

One of the highlights of the congress was the presentation by plenary speaker Prof Sir Magdi Yacoub, from the Imperial College London, Heart Science Centre in the UK, brought to Australia by Sydney-based company Ventracor.

Introduced as someone who "has carried out more heart transplants than anyone on this planet", Yacoub's passion for heart research was obvious as he spoke about the many aspects of his team's work in tissue engineering aortic heart valves and regenerating heart tissue.

The mortality rate in humans from heart disease is massive worldwide, and the two major killers are valve disease and heart failure. Yacoub said valve replacement and heart transplantation are both success stories, but other strategies are needed.

Valve replacement does improve survival, but the driving force for Yacoub's team in tissue engineering heart valves is not just to produce durable valves but also to reproduce the sophisticated functions of a valve, which translates into more clinically relevant outcomes.

Heart valves have a complex morphology and are capable of withstanding large amounts of continuous pressure. They are made up of three parts, or cusps, which, in turn, are made up of a number of different cell types -- endothelial cells, interstitial cells, stem cells, dendritic cells and passenger cells, as well as an extracellular matrix component (ECM).

The aortic valve is of major importance as it allows blood to flow from the left ventricle of the heart into the aorta -- the largest artery that distributes oxygen-rich blood throughout the body -- closing after each heartbeat to prevent blood leaking back into the ventricle.

"We know the different parts of the heart valve are extremely sophisticated, in that each component part would change its size and shape during one cardiac cycle. It is not just a passive piece of tissue, as many of us used to think," said Yacoub.

By studying the structure, function and regulation of the aortic valve, Yacoub's team is gaining insights into how the valve functions in its dynamic state in order to then engineer the valve itself.

They have found that the valve stem cells have some plasticity and, along with the endothelial cells, induce endothelial-to-mesenchymal transformation (a process required for proper heart development) to form interstitial cells. The researchers are now conducting studies using these cells and the endothelial-to-mesenchymal transformation process to form cells with appropriate genotypes and phenotypes required for tissue engineering a valve.

In other studies, the ECM of valve tissue has been found to contain a number of factors, such as collagen, elastic proteoglycans, and various polysaccharide glycosaminoglycans, that interact or cross-talk with valve cells, playing a major role in their function. Analysis of gene expression in interstitial cells has shown that these cells produce and secrete a number of molecules, for instance the skeletal muscle-specific transcription factor, myogenin, which is involved in switching on the transcription of many muscle specific genes.

Yacoub's team is harnessing these processes to take their work on tissue engineering heart valves further. In vitro studies, using three-dimensional scaffolds made of collagen type 1 to construct valves, have shown that human interstitial cells grow very well on this collagen matrix.

The researchers then subjected these interstitial cells to different mechanical forces to induce phenotypic changes in them and determine their suitability to act as valve cells.

"When subjected to pressure they respond in a very specific manner, not like fibroblasts in the skin for example," said Yacoub. "Valve interstitial cells keep increasing tension in themselves as time passes, and if you apply tension they respond by changing shape as well as mechanical properties."

When these cells respond to tension they start secreting collagen and they can also produce different components of the ECM, which has major implications for tissue engineering since the molecules in the ECM can modify the behaviour of valve cells.

Heart transplants are a big operation, limited by the availability of hearts, so Yacoub's team is focusing on regenerating the heart muscle cells, or myocardium, of the heart as an alternative.

In describing this work, which is focused on reviving, or causing heart muscle cells to grow again after an individual has had a heart attack, Yacoub referred to studies on individuals who were in severe heart failure with no hearts were available for transplantation.

At least 90 per cent of cells die within the first day of someone having a heart attack, which has a significant influence on the mechanical and electrical functions of the heart.

A left ventricular assist device (LVAD), which is used as a bridge to transplantation or a bridge to recovery in certain patients, was implanted into the heart of these patients to assess the heart by monitoring intraventricular blood flow and relating this to an individual's electrocardiogram (ECG). The LVAD also enables researchers to study haemodynamic factors (blood pressure, blood flow, vascular volumes, physical properties of the blood, heart rate, ventricular function), as well as the effects of chemical factors.

Yacoub's team is looking for markers to help them determine changes that are relevant to enhancing survival of heart cells. The electrophysiological properties of individual muscle cells, and the factors they express, are critical in monitoring recovery.

For example, after a heart attack heart muscle cells express a massive amount of proinflammatory cytokines that reduce with recovery. "The combination of biology with engineering is important in this work," said Yacoub. "Combining LVAD support with clenbuterol, a pharmacological agent studied on skeletal and cardiac muscle, produced a shift of pressure volume indicating it enhances the efficacy of the LVAD," explained Yacoub. "Clenbuterol induces myofibrillogenesis [growth of muscle cells] which could be very relevant."

Describing the results as "almost like magic", Yacoub said two thirds of patients had recovered enough to be able to get up and exercise normally, where previously they were confined to bed. Improvements continued after the device was explanted and individuals had "excellent quality of life", he said.

Another area of this work involves introducing cells into the damaged heart via an intracoronary infusion, although Yacoub emphasised "we need to know more before we rush to the clinic". This research is focusing first on in vitro work, looking at the capacity of cells to survive, proliferate, differentiate and integrate. For example, the researchers have found that over-expression of connexin 43 can enhance the interaction of myocardial cells with one another.

The transdifferentiation of bone marrow derived cells into the myocardium has not been successful, Yacoub said: "Haemopoietic stem cells or unfractionated stem cells do not transform into myocardial cells, they are lost in translation."

Finishing his talk with a reminder about the Advances in Tissue Engineering and Biology of Heart Valves Conference coming up in Florence in September (www.florenceheartvalve.org.uk), Yacoub said he "wants to create something as beautiful as the birth of Venus by Botticelli, the original of which is in the venue where the conference will be held."

Advances in imaging

If there's one tool that can cross over all the different disciplines that make up biomaterials, it's probably imaging. Steve Goodman, of US firm Imago Scientific Instruments, gave a keynote talk at the congress about the latest advances technologies for imaging molecules in biomaterials and biomedical samples.

Arguing that microscopy is "the key evaluation tool for biomaterials, tissue engineering, drug delivery, and biomedical science as it provides direct information on molecular, cellular, and subcellular spatial relationships within living systems, and between living systems and biomaterials", Goodman gave a fast-paced overview of some of the analytical imaging technologies currently available with which to access the 3D molecular world.

Some of these include entirely new imaging modalities such as scanning probe microscopies (atomic force, tunnelling, near field optical); confocal light microscopy; multi-photon excitation light microscopy, which enables 3D reconstruction of living cells using excitation that does minimal damage to tissue; fluorescence lifetime imaging (FLIM), based on the decay of fluorescence varying with environment -- light or oxygen in the nucleus or cytoplasm; diffusion tension MRI, using the diffusion of fluid to obtain high quality images of brain and muscle tissue; and the list goes on.

Goodman said other technologies included new functional labelling methods; second harmonic optical imaging to enable 3D structural and functional mapping of biomacromolecular tissue and scaffold structures deep within tissues without the use of stains or labels; and optical coherence tomography, an ultrasound technique that uses light instead of sound waves to permit non-invasive examination of interfaces such as extracellular matrices and functional measurement of blood flow deep within living tissues.

Goodman's company specialises in 3D atomic probe microscopes, a combination of a projection microscope and an imaging mass spectrometer, providing for atomic-scale analysis -- it resolves millions of individual atom positions in three dimensions then identifies them by time-of-flight mass spectrometry.

"These microscopes are for understanding the structure of matter at a very small scale," said Goodman. "They provide the eyes to the nanoworld. We can analyse polymers and metals at this stage [they can determine the 3D atomic structure and complete elemental composition], but we can't do it in biological specimens yet."

Goodman said one of those instruments would be sold to the University of Sydney in the near future.