Photo by Michelle Hruby
Thirteen years ago, when Dr. Ed. Giniger contemplated a path of study for his postdoctoral research, he was not looking for the easy or the obvious.
Having just completed graduate work on yeast gene regulation at Harvard while living in a household of artificial intelligence researchers, he felt the emergence of an easy choice: genetic analysis of how the brain is wired.
"The hard part was finding an adviser who was willing to let me work on that," said Giniger, who joined the Hutch's Basic Sciences Division faculty in 1993. "The dogma of classical neuroscience at that time was that nervous-system patterning was not determined by genetics but was dependent primarily on experience."
Giniger did manage to find a lab willing to host his genetics project, that of Yuh Nung and Lily Jan at the University of California at San Francisco, and he has devoted his research career to understanding the genes and molecules that influence how nerve cells, called neurons, set up a functional network in the fruit fly Drosophila.
His work has implications for understanding devastating neurological disorders that result from nerve-cell degeneration, including Alzheimer's disease and amyotrophic lateral sclerosis, also known as Lou Gehrig's disease.
The development of the nervous system - akin to an intricate tangle of telephone wires that convey messages between the brain or spinal cord to remote parts of the body - arguably faces more challenges than any other organ system in the body.
During development, neurons are born from cell divisions in the early embryo. But the network is not complete until individual neurons stretch and grow from the central nervous system and contact their targets, muscles that receive stimuli to move or sensory organs that perceive environmental signals.
The process is invariant, meaning that every normal organism of its species forms virtually the same pattern of connections.
The question, Giniger said, is how does each neuron take the correct path to its target cells? And how is such a complex series of steps coordinately regulated for accuracy?
Typical spherical architecture
To reach their targets, nerve cells must elongate, which gives them a peculiar shape unlike any other cell type. Their architecture consists of a relatively typical-looking spherical cell body with a long extension called an axon, which, depending on the animal, might be up to several feet in length.
"You can imagine how far an axon might need to grow in a human - or a giraffe," Giniger said.
Axons elongate at their tips, called growth cones, with the help of specialized receptors that respond to molecules produced by both the neuron's target and by tissue along the neuron's path of elongation. After the receptors receive the proper growth signals, they trigger an internal process of cellular reorganization, mobilizing proteins that form the cell skeleton to the growth cone for elongation.
The entire process is remarkable for its accuracy and complexity, and several years ago Giniger wondered whether axon patterning might be overseen by a master regulatory system that checks for and corrects errors at each step of the process.
Insurance of accuracy
Such a proofreading or checkpoint mechanism is not unique in biology. It is the system that ensures accuracy as cells progress through the cell cycle before dividing. Mutations that compromise normal cell-cycle progression can lead to cancer.
Important components of the cell-cycle checkpoint include a group of proteins called Cdk's - cyclin-dependent kinases - that have been found in cells from yeast to man.
Over coffee with Hutch cell-cycle researchers, Giniger discovered that one member of the Cdk family, Cdk5, was found specifically in nerve cells.
"We wondered, could Cdk5 play a role in the fidelity of axon patterning analogous to the cell-cycle checkpoint?" he said. "It seemed there had to be a mechanism to prevent mistakes."
To test this hypothesis, Dr. Lisa Connell-Crowley, a postdoc in Giniger's lab, used genetically altered fruit flies with a defective Cdk5 and examined axon patterning in the developing embryo. Duc Vo, a technician, and Dr. Maude LeGAll, another postdoc, collaborated on this project.
While neurons still elongate in the mutant flies, patterning is disrupted.
"We found reproducible, subtle defects in axon patterning in these flies, including neurons stopping short of their target or growing past their target," Connell-Crowley said, "so it does look like Cdk5 is important for the fidelity of patterning."
Giniger speculated that Cdk5 could act as an error-surveillance system in one of two ways.
Growth in real time
"It may be that Cdk5 tells neurons when they are moving along the wrong pathway, causing them to shift direction," he said. "To study this, we'd like to be able to see axon growth in real time - a difficult experiment because embryos are so small. The other possibility is that Cdk5 helps cells maintain a balance of factors so that cells stay on the right track in the first place."
The group now hopes to understand how Cdk5 interacts with components of the elongation pathway, including molecules involved in axon guidance.
Other projects in the lab include studies of the receptor protein Notch, important for growth cone elongation. Notch, involved in many cell processes including blood cell development, has recently been implicated in some forms of brain degeneration.
Giniger's lab also studies a protein called Lola, which regulates the production of a suite of genes that assist Notch in guiding axons to their proper targets.
Lola, Notch and Cdk5 provide three complementary perspectives on a single problem. For the nervous system to be 'wired-up' properly, Notch must work in balance with a host of other receptors. Lola establishes that balance and Cdk5 oversees the neuron growth machinery to ensure that it remains in balance.
By viewing brain development from all three of these perspectives, Giniger hopes to understand why neural circuits develop as they do, and what causes those circuits to stop working and die in neurodegenerative diseases.