MDC Researchers Discover New Function of Cardiac
Sodium Pump
The heart can only beat if electrically charged
particles (ions) are transported back and forth across the plasma
membrane of the heart cells. The sodium-potassium pump conducts this
transport activity by pumping potassium ions into the cell interior
and allowing sodium ions to flow out of the cell. Indirectly, it also
regulates the concentration of calcium ions, which in turn control
heartbeat. Patients with cardiac insufficiency receive drugs that
affect the sodium pump in order to stabilize their heartbeat. Now,
scientists at the Max Delbrück Center for Molecular Medicine (MDC)
Berlin-Buch have discovered a new function of the sodium pump. During
embryonic heart morphogenesis, the pump generates a change in the ion
concentration within and outside the cells (ion gradient).
The scientists were able to demonstrate that the ion gradient
determines the top (apical) and bottom (basal) of heart cell surfaces.
Scientists call this cell polarity, a phenomenon vital for heart
function. The sodium pump is also involved in cell junction
maintenance, thus ensuring that the heart cells stay connected with
one another. Without this sodium pump, the heart never develops. A
report of the findings of PhD student Elena Cibrián-Uhalte and Dr.
Salim Abdelilah-Seyfried (both MDC) and researchers of the University
of California Los Angeles, USA has now been published in the Journal
of Cell Biology.
To date, cell biologists have identified a number of signal pathways
which regulate organ development. "What is new and very surprising
about this discovery is that ion gradients generated by the sodium
pump regulate these fundamental cell biological processes, namely cell
polarization and cell junction maintenance," said Dr. Seyfried,
developmental biologist at MDC. He directs a laboratory that studies
vertebrate development by using the zebrafish among other model
systems. Zebrafish are only a few centimeters in length and their
embryos are transparent, enabling researchers to observe each cell
under the microscope.
Dr. Seyfried is especially interested in how epithelial cells on the
outer and inner surfaces of the body polarize during organ development.
This cell polarity is essential for organ functioning. "The most
important finding of our present study is that the ion gradient
regulates cell polarity," he said. "But we still need to find out
exactly how ion gradients affect the polarization of myocardial cells
during their early development. Moreover," he added, "for technical
reasons, it is not possible for us to measure the ion concentrations
in tiny (zebrafish) hearts that are only a few micrometers across."
The scientists in Berlin suspect that the defects in the sodium pump
not only affect myocardial cell junction maintenance but also the
development of the blood-brain barrier and the retina. If this
hypothesis is confirmed, this means that the sodium pump plays a
central role in the development of different types of epithelial cells
not only in the heart but throughout the entire organism.
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