Inspired by Sea Creatures : designing biomaterials for regenerative medicine

10 February 2012 Sea urchins have a clear advantage over humans, at least when it comes to their connective tissue and its stateshifting abilities. With the knowledge of how they do it, Ana Lopes Ribeiro, wants to design biomaterials for…

10 February 2012
Sea urchins have a clear advantage over humans, at least when it comes to their connective tissue and its stateshifting
abilities. With the knowledge of how they do it, Ana Lopes Ribeiro, wants to design biomaterials for regenerative
medicine.

The sea is full of strange and wonderful creatures. Take the echinoderms, for example. This phylum consists of a very diverse set of animals including sea urchins, sea stars and sea cucumbers. Although quite different at first glance, they share many similar physiological features.

Miracle tissue
One of the coolest is their connective tissues’ ability to reversibly change its state from solid to liquid in a matter of seconds. That is, for example, how the sea urchin stiffens the base of its spines in defense when someone’s foot gets too close; or how the sea star can one moment be firmly stuck to a rock and soften up and dangle its arm at the next; or how sea cucumbers can “pour” their entire bodies into a hole when escaping a predator and then stiffen back up when danger is gone. This special kind of connective tissue is popularly called catch collagen or, more academically, mutable collagenous tissue (MCT).

MCT isn’t confined to
echinoderms. It is present
in humans as well – more
precisely, in pregnant
women. The connective
tissue that ties the pelvis
together is stiff before and
during the first months of
pregnancy, but gets much
softer towards the end, allowing
the opening of the
pelvis during childbirth. It
then slowly returns to its
stiffer state. The process
is very similar to the stiffening
and softening that happens in sea urchins,
stars and cucumbers, although many
magnitudes of order slower.
MCT’s main roles are locomotion, the
energy-efficient maintenance of posture
(e.g. being stuck to a rock for a long time)
and defense. There is evidence that MCT is
also responsible for the remarkable regenerative
capabilities found in echinoderms.
MCT’s amazing properties have caught
the eye of materials scientists, who are trying
to understand the mechanisms behind
its transformation and use this knowledge
to engineer new biomaterials. One of the
groups involved in MCT research is the
group headed by Mário Barbosa, based
at the Institute of biomedical engineering
(INEB) in Porto, Portugal.
Going fishing
Ana Lopes Ribeiro is the first author of
the group’s latest paper. She has chosen the
field of biomaterials because it lies at the intersection
between materials science and
biology. “Research in the biomaterials field
was very attractive for me because of the
potential for practical application in medicine
and its multi-disciplinary nature”, she
says. And what better place to undertake interdisciplinary
biomedical research than at
INEB, which has a mission to improve human
life through engineering. She joined
the lab of her supervisor Mário Barbosa,
whose passion for the sea turned his interest
towards sea urchin MCT.
“My thesis advisor became fascinated by
sea urchins while he was fishing from the
rocky beaches of northern Portugal. Fish is
not very abundant there, so he had plenty
of time to look for sea life around him. He
became intrigued by the burrows created by
the teeth of sea urchins in the hard granite
rocks that are typical of our coast. He decided
to visit Daniela Candia Carnevali, an expert
in sea urchins and other echinoderms
and a professor at the University of Milano.
They talked about starting a joint project
and the unique properties of mutable collagenous
tissue came up as a totally new approach
in the field of biomaterials for tissue
regeneration”, Ana recollects.
Something to chew on
And that is how Ana’s project in sea urchin
mutable collagenous tissues was born.
She is currently finishing her PhD at the
INEB, as a part of the MIMESIS (Marine
Invertebrate Models & Engineered Substrates
for Innovative bio-Scaffolds)
project.
“At the moment, my research
is based on the fundamental
mechanisms of MCT
mutability and its biochemistry
and biomechanics. I’m looking
for new concepts for the design
and engineering of dynamic
bio materials for tissue regeneration”,
says Ana.
In their latest PLoS ONE paper,
Ana and her colleagues
have identified new morphological
specifics involved in mutability
(PLoS ONE 6(9): e24822).
They used the compass depressor ligament
(CDL), isolated from the sea urchin Paracentrorus
lividus, as a model MCT tissue.
The CDL is a part of the sea urchin chewing
system (the mechanisms around the
mouth opening at the bottom of the sea ur-

chin). The stiffening and softening of the
CDL controls the movement of the mouth
and its five calcified teeth – these teeth are
responsible for the burrows that first fascinated
Ana’s boss and turned his interest towards
sea urchins. The structure of the CDL
is similar to other MCTs – it consists of parallel
collagen fibres and some microfibres,
which actively participate in the stiffening/
softening mechanism.
The paper itself presents detailed, stateof-
the-art microscopic analysis of the CDL
ligament. They have studied the morphology
of the three mechanical states of the
CDL – stiff, standard and soft, and found
that collagen fibres get much closer when
the collagen stiffens, while the distance
doesn’t change between the standard and
soft states. They have also identified the
microstructure of the ligament in all three
states and identified the relationships between
MCT’s main components – collagen
fibres, fibrilin microfibrils and the proteoglycan-
rich gooey matrix.
Fighting scars
Additionally, Ana and her colleagues
analysed the morphology of the juxtaligamental
cells, which are present in all MCTs,
and correlated them with the ligament’s
stiffness. These cells are rich in granules,
which probably contain the effectors that
mediate the stiffening of the MCTs.
They divided the granules into two
types according to their electron density
– the darker, mature ones and the lighter
ones. When the CDL was in its normal state,
cells contained more of the mature granules,
while after changing to either soft or
stiff forms the mature granules became depleted.
They concluded that there are probably
two types of morphologically indistinguishable
granules, containing either stiffening
or loosening factors, which regulate
the transition from normal to one of the
other two states.
“Our results help to define the functional
role of this specialised extracellur matrix,
which is strikingly similar to mammalian
collagen found in human tendons, ligaments,
cornea, skin and blood vessels”, Ana
says. The main difference is that when collagen
fibres break or cross-link in the human
collagen tissues, the process can’t be
reversed. That is why tendons reach only
a portion of their strength after injury and
why skin collagen tends to lose its strength
and elasticity with time.
In contrast, the bonds between collagen
fibrils in the MCT are constantly being
broken and re-established with the help of
stiffening and loosening agents. Knowing
exactly what these are will be the key in engineering
biomaterial tissues based on collagen
with tunable mechanical properties,
which is the group’s ultimate goal.
This artificial tissue could be widely
used in human medicine. “Possible applications
of these dynamic biomaterials could
be regeneration of soft connective tissues
or therapeutic de-stiffening of scar tissue,
which tends to contract”, Ana explains.
When a tendon (e.g. Achilles tendon)
is torn, surgeons connect the broken collagen
fibres but a side-effect is the formation
of scar tissue, which makes the tendon
weaker and less elastic. Doctors could apply
the treatment based on MCT biomaterials
to torn tendons and with modulation
using stiffening and loosening effectors it
could form a scaffold for a healthy, scarfree
recovery.
Having a facial
The MCT biomaterial would also have
limitless possibilities in the cosmetic industry
– just imagine a treatment that could return
elasticity to facial collagen in a matter
of minutes! But there is still a lot of work
to be done before these treatments hit the
stores.
Multidisciplinary science, according
to Ana, is what Portugal does well. It has
good universities and research institutes,
which are very well connected and often
share equipment and infrastructure. The
position of young scientists in common with
the rest of Europe, is quite tough, “There are
not many postdoc grants offered by the national
foundation of science and technology.
Also, the chances for a young scientist to
get a permanent research position are quite
slim – many laboratories don’t have permanent
positions for investigators at all.”
But Ana has a dream: “My dream is to
apply the knowledge from my PhD to develop
tunable biomaterials inspired by MCTs,
which would be used in regenerative medicine.
These materials will be able to change
their mechanical and structural properties
in response to external manipulation. In
the long run, we hope to build a biomaterial
which would be a dynamic scaffold and
would change according to the physiological
needs and molecular signals of the surrounding
tissue.”
She believes that, with the strong team
that they have and the joint efforts of both
biologists and engineers, they will make it.
Fingers crossed!
Irena Hreljac