Johns Hopkins biomedical engineers, working with colleagues in
Korea, have produced a laboratory chip with nanoscopic grooves and
ridges capable of growing cardiac tissue that more closely
resembles natural heart muscle.
Surprisingly, heart cells cultured in this way used a
“nanosense” to collect instructions for growth and function solely
from the physical patterns on the nanotextured chip and did not
require any special chemical cues to steer the tissue development
in distinct ways. The scientists say this tool could be used to
design new therapies or diagnostic tests for cardiac disease.
The device and experiments using it were described in this
week’s online Early Edition of Proceedings of the National
Academy of Sciences. The work, a collaboration with Seoul
National University, represents an important advance for
researchers who grow cells in the lab to learn more about cardiac
disorders and possible remedies.
“Heart muscle cells grown on the smooth surface of a Petri dish,
would possess some, but never all, of the same physiological
characteristics of an actual heart in a living organism,” said
Andre Levchenko, a Johns Hopkins associate professor of biomedical
engineering in the Whiting School of Engineering. “That’s because
heart muscle cells — cardiomyocytes — take cues from the highly
structured extracellular matrix, or ECM, which is a scaffold made
of fibers that supports all tissue growth in mammals. These cues
from the ECM influence tissue structure and function, but when you
grow cells on a smooth surface in the lab, the physical signals can
be missing. To address this, we developed a chip whose surface and
softness mimic the ECM. The result was lab-grown heart tissue that
more closely resembles the real thing.”
Levchenko added that when he and his colleagues examined the
natural heart tissue taken from a living animal, “we immediately
noticed that the cell layer closest to the extracellular matrix
grew in a highly elongated and linear fashion. The cells orient
with the direction of the fibers in the matrix, which suggests that
ECM fibers give structural or functional instructions to the
myocardium, a general term for the heart muscle.” These
instructions, Levchenko said, are delivered on the nanoscale,
activity at the scale of one-billionth of a meter and a thousandth
of the width of a human hair.
Levchenko and his Korean colleagues, working with Deok-Ho Kim, a
biomedical engineering doctoral student from Levchenko’s lab and
the lead author of the PNAS article, developed a two-dimensional
hydrogel surface simulating the rigidity, size and shape of the
fibers found throughout a natural ECM network. This bio-friendly
surface made of nontoxic polyethylene glycol displays an array of
long ridges resembling the folded pattern of corrugated cardboard.
The ridged hydrogel sits upon a glass slide about the size of a
U.S. dollar coin. The team made a variety of chips with ridge
widths spanning from 150 to 800 nanometers, groove widths ranging
from 50 to 800 nanometers, and ridge heights varying from 200 to
500 nanometers. This allowed researchers to control the surface
texture over more than five orders of magnitude of length.
“We were pleased to find that within just two days, the cells
became longer and grew along the ridges on the surface of the
slide,” Kim said. Furthermore, the researchers found improved
coupling between adjacent cells, an arrangement that more closely
resembled the architecture found in natural layers of heart muscle
tissue.
 |
||||
Cells grown on smooth, unpatterned hydrogels, however, remained
smaller and less organized with poorer cell-to-cell coupling
between layers.
“It was very exciting to observe engineered heart cells behave
on a tiny chip in two dimensions like they would in the native
heart in three dimensions,” Kim said.
Collaborating with Leslie Tung, a professor of biomedical
engineering at the Johns Hopkins School of Medicine, the
researchers found that, after a few more days of growth, cells on
the nanopatterned surface began to conduct electric waves and
contract strongly in a specific direction, as intact heart muscle
would.
“Perhaps most surprisingly, these tissue functions and the
structure of the engineered heart tissue could be controlled by
simply altering the nanoscale properties of the scaffold. That
shows us that heart cells have an acute ‘nanosense,'” Levchenko
said.
“This nanoscale sensitivity was due to the ability of cells to
deform in sticking to the crevices in the nanotextured surface and
probably not because of the presence of any molecular cue,”
Levchenko said. “These results show that the ECM serves as a
powerful cue for cell growth, as well as a supporting structure,
and that it can control heart cell function on the nanoscale
separately in different parts of this vital organ. By mimicking
this ECM property, we could start designing better engineered heart
tissue.”
Looking ahead, Levchenko anticipates that engineering surfaces
with similar nanoscale features in three dimensions, instead of
just two, could provide an even more potent way to control the
structure and function of cultured cardiac tissue.
