Photo by Theresa Naujack
In the intricate sequence of steps that transform a fertilized egg into an adult animal or plant, no cell is an island.
"A lot of what happens during development relies on cell-to-cell communication," said Dr. Phil Soriano, developmental biologist in the Basic Sciences Division. "Cells have to receive signals to carry out their proper roles."
Determining the identity of those signals, and how they are transmitted and received, is a problem that Soriano's lab tackles to unravel one of the central mysteries in biology: How does a single cell mature into a complex organism?
While scientists have not decoded the blueprints for building an adult animal, Soriano's lab has characterized some of the key players in the development of both the mammalian vascular system, the network of vessels that transports blood throughout the body, and the skeleton.
The lab's studies on two important receptor proteins have implications for understanding vascular diseases that affect the kidney and the retina, as well as birth defects such as spina bifida.
One foot in, one foot out
Receptors with a role in developmental signaling are large proteins perched on cell surfaces that keep one foot inside the cell and one foot outside. The balancing act allows them to receive information from the outside, typically in the form of substances called growth factors, and transmit signals to the machinery inside the cell that turns subsets of genes on or off.
Receptors are like busy telephone operators - processing multiple incoming calls and having to decide quickly where to transfer them. A given receptor might be a target for multiple signaling molecules that come from the outside, and each signal triggers a different event within the cell.
Given the seemingly overwhelming tangle of signals and messages, making sense of this network requires posing careful questions.
The Soriano lab focuses on two receptors, called platelet-derived growth factor receptors alpha and beta (PDGFR-a and PDGFR-b), whose structures and properties have been particularly well studied. PDGFR-a and PDGFR-b are members of a family of proteins called receptor tyrosine kinases, which receive and transmit many types of cellular signals.
"These two receptors are the kings of signaling," said Dr. Richard Klinghoffer, a postdoctoral fellow in Soriano's lab. "They've been the subjects of extensive biochemical analysis since the 1970s, and a lot is known about the signaling molecules that bind to them. That allows us to look at how these receptors affect discrete signaling pathways."
Soriano's lab uses a genetic approach to identify the developmental pathways PDGFR-a and PDGFR-b help to trigger.
Like most geneticists, the group learns a lot about function by studying variants or mutant forms of the receptors and then comparing the effects to a pristine, or wild-type version.
Earlier lab work examined mice with complete deficiencies in PDGFR-a or PDGFR-b, but because each receptor affects a multitude of cellular processes, a finer-scale analysis was needed, said Dr. Michelle Tallquist, also a postdoc in the laboratory.
"There are so many things that these receptors can do," she said. "We need to dissect them to understand their functions. What we really want know is, what are the downstream signaling pathways affected by PDGFR-a and PDGFR-b? By studying specific mutations in each of the receptors, we've learned that pathways are distinguishable and separable."
Tallquist and Klinghoffer have used this kind of mutational analysis to discover that mutations in PDGFR-b cause defects in mice in the cells that compose the smooth muscle of the vascular system, the intricate system of vessels that transport blood through the body.
The vascular defect is particularly apparent in the kidney, where it impairs the function of the smallest of these vessels, known as capillaries. The kidney defect observed in mice is similar to a human condition known as glomerular sclerosis, a condition in which the tissue in the glomerulus, the kidney's main filtration device, becomes clogged with proteins that normally remain outside the cell.
Mutations in PDGFR-b also can affect proper development of the eye, Klinghoffer said.
"Mice can develop retinopathy - damage to the retina - similar to what can cause blindness in diabetics," he said. "This is due to the impaired smooth muscle of the vascular system in the eye."
Mutations in PDGFR-a cause a different subset of developmental abnormalities, primarily impairment of the skeleton and central nervous system.
One mutation in PDGFR-a in mice results in a cleft palate and improper closure of the spine, a condition known in humans as spina bifida. Spina bifida is a permanently disabling birth defect that typically requires surgery soon after birth and can result in varying degrees of paralysis.
Other PDGFR-a mutations affect cells called oligodendrocytes, which are responsible for coating nerves with myelin, a substance important for nerve transmission.
Proteins related to PDGFR-a and PDGFR-b, the so-called receptor tyrosine kinases, are found in a wide variety of organisms, making for an interesting study in evolutionary biology, said Dr. Guy Hamilton, a postdoc in Soriano's lab.
Hamilton studies whether receptor tyrosine kinases from organisms as seemingly unrelated as fruit flies and mice can compensate for each other, to test the theory that the signaling mechanisms have been conserved throughout evolution.