From a Blank Sheet to a Heartbeat

Investigating the Growth and Differentiation of Heart Cells

Differentiation, the process through which a cell becomes more specialized, is key in the development of the inner workings of the heart. By studying this mechanism, student researchers are furthering the unique approach of countering heart diseases and other cardiac conditions by starting from heart-making itself.

By Alisha Jain, David Linderman, Taruc Alvarez, Yasaman Pirahanchi | staff writers | UTS Vol. 3 (2012-2013)

According to philosopher John Locke, individuals are born as “blank slates.” Human knowledge comes from experience and perception because we are essentially made of the accumulation of various experiences throughout our lives. This theory of “tabula rasa” is not all that different from the various molecular triggers and markers of cell differentiation in stem cells of the developing embryo. These stem cells have the potential to become any specialized cell in the body.

Differentiation is the process by which cells use signals to adopt a specific fate. The development of every organ in our bodies can be attributed to this process. As cells gather various cellular signals, they gradually “learn” and are programmed to attain their fate, losing their “blank slate” identity. Student researchers here at UCSD set out to answer how exactly heart cells “learn” to become heart cells through research on various signaling molecules and growth factors that transform cardiac stem cells from “tabula rasae” into their respective differentiated cells.

The Proteins that Make our Heart Ache

One undergraduate researcher, Tina Vajdi, investigated differentiation in zebrafish heart tissue in Dr. Deborah Yelon’s lab in UCSD’s Division of Biology. The cardiac inflow tract is a crucial structure in the heart, and how it forms is not well-understood. Formation of this tract has been correlated with expression of the signaling molecule BMP4, a member of the bone morphogenetic protein (BMP) family of growth factors. Tina examined the developmental stages of the inflow tract by labeling embryos at each stage of growth by staining the cells and imaging them with a microscope. This works through “in situ hybridization,” where a small fragment of RNA, called a probe, is hybridized with the RNA in the zebrafish in order to detect the sequence coding for the protein in question. Thus, at each stage of growth, Tina tracked down specific sequences, each of which codes for specific proteins necessary for proper zebrafish embryo development. The signal transduction pathway of BMP is important for heart muscle cells in the atrium. Through a decrease in BMP signaling, there is a decrease in the number of these cells and this causes a visible decrease in the size of the atrium. Although the mechanism of the impact of the atrium on the inflow tract growth and vice versa is not completely known, Tina’s studies indicate that the cardiac inflow tract also decreased in size when there is a decrease in BMP signaling.

Another molecular marker for differentiation used along with bmp4 was islet1, which indicates progression of inflow tract development. In order to assess the significance of certain genes, the genes must be inhibited in some way to examine the effect on the organism. Tina used drug treatment with LDN and Dorsomorphin to permanently inhibit BMP signaling in order to examine its effect in this pathway. Other methods to determine the importance of BMP included the lost-a-fin mutation, which caused mild reduction of BMP signaling, and a heat-activated mutation, caused by heat-shocking the zebrafish, which decreased BMP signaling. Each of these treatments caused reductions in BMP signaling but the amount of this reduction changed depending on which of the three methods was chosen. The decrease in BMP signaling caused a decrease in atrial size, and this in turn decreased the size of the inflow tract. Thus, by looking at the BMP receptor, more severe impacts of BMP signaling on the inflow tract can be observed. Insights into inflow tract development from Tina’s research could have potential for treatment of congenital heart disease in humans, since zebrafish have hearts with one atrium and one ventricle. This two-chamber system is a simplified model of the human four-chamber system and allows scientists to induce mutations in the model system that could mimic potential genetic mutations underlying the mechanisms of congenital heart disease.

Repairing the Heart Through Regeneration

Gino Chesini, a Master’s researcher, performed research on G-protein coupled receptors (GPCRs), which are involved in detecting external signals at the surface of cells in Dr. Joan Hellar Brown’s lab in UCSD’s Department of Pharmacology. He found that different GPCRs are preferentially expressed in cardiac progenitor cells (CPCs) as compared to mature cardiac cells. PCR analysis revealed which of these G-proteins played the most active role in the heart stem cells, thereby allowing identifiable targets for differentiation. A two year phase one clinical trial by Dr. Roberto Bolli of the University of Louisville’s group was just completed that attempted to promote cardiac repair using these cells. CPCs were isolated from patients suffering from heart failure and were injected back into the patients. Administration of these stem cells resulted in partial cardiac regeneration and recovery of cardiac function. A better understanding of the cellular signaling mechanisms in CPCs may help to coax these stem cells to differentiate properly and heal the heart more effectively, a prospect Gino believes will be promising for further research.

Big Hearts, Big Problems

Daniel McDonald, a Master’s researcher in the lab of Dr. Paul Insel of UCSD’s Department of Pharmacology, has done extensive research uncovering new molecular targets to treat high blood pressure in the lungs, which is a condition that can lead to heart failure.

Increased pressure in the lung circulation leads to the narrowing of the blood vessel that feeds deoxygenated blood to the lungs, restricting blood flow and causing shortness of breath.

Moreover, it causes the heart to swell up and become extremely large, forcing it to work harder with every pump. Therefore, there is a real need to find an effective heart-shrinking treatment for these patients. Daniel’s research last year focused on finding molecular “pumps” in the cells of the heart that could be blocked by drugs to reduce their activity. This would ultimately inhibit their proliferation so the heart stays limber and reduce tension in the muscles where these cells are located.

Each of our cells starts out life as a blank slate without a specific purpose and needs to be specified in order to work with the rest of our bodies. Differentiation could in fact be seen as one of the most important biological mechanisms in history because without this mechanism of assigning meaning to each cell, no complex living organism could ever exist! As UCSD researchers have found, these key molecular markers in the cell during heart development are true heart-makers!

& YASAMAN PIRAHANCHI. Alisha Jain is a Human Biology major from
Thurgood Marshall College. She will be graduating in 2016. David Linderman is
a Biochemistry & Cell Biology and Psychology double major from Revelle College.
He will be graduating in 2013. Taruc Alvarez is a Biochemistry & Cell Biology
major from Eleanor Roosevelt College. He will be graduating in 2016. Yasaman
Pirahanchi is a Biochemistry & Cell Biology major from Revelle College. She will
be graduating in 2014.