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School of Medicine
Alex Kolodkin of Neuroscience and Howard Hughes Medical Institute investigator, on the importance of establishing connections
What do you tell people you do?
KOLODKIN: I tell people we’re really interested in how the brain is wired up: how circuits are assembled and organized. Our particular focus is understanding how connectivity develops. The brain works through the action of circuits. And circuits are formed by connections between different neurons. So how and where those connections form determine the activity of a circuit. We investigate the mechanisms behind the formation of circuits. The neurons my team studies can have thousands of synaptic connections with other neurons.
As a postdoctoral fellow, you discovered and cloned the first semaphorin gene in the grasshopper and, over the past 15 years, have studied this family of messenger molecules in numerous insect and animal models, including strains of genetically engineered mice. What’s so special about semaphorins?
KOLODKIN: Our latest research, conducted in collaboration with Dr. David Ginty here at Hopkins, reveals that certain semaphorins—proteins belonging to the large semaphorin family of proteins, many of which are found in the developing nervous system and can guide the filament-like neuronal processes, called axons and dendrites, from nerve cells to their appropriate targets during embryonic life—apparently perform different roles later on, once axons reach their targets. In postnatal development and adulthood, we find that a semaphorin called Sema3F appears to be regulating the development of synapses—the connections that provide the basis for signaling between nerve cells. This is a major step forward, we believe, in our understanding of the assembly of neural circuits that underlie behavior.
If a brain lacks the Sema3F guidance cue or its cell surface receptor, too many connections form; they are in the wrong places and distributed incorrectly, and this affects the overall output of that neuron — its ability to correctly relay communication from that neuron to another.
You started your investigation in grasshoppers, and your lab now works with both flies and mice. Why study semaphorins in insects?
KOLODKIN: When there are a small number of neurons extending their processes — and this is the beauty of the grasshopper system — you can actually watch these pathways form, extending towards and away from intermediated and final targets. Then the other neurons enter the picture and either follow these pioneers, or establish their own independent pathways. Given the simplicity of these systems, it is possible to watch this scaffold facilitate the assembly of an increasingly more complex nervous system. Of course, these insect systems also allow unique and powerful experimental manipulations to be performed, so you can develop and test models for how neurons wire the nervous system in the living animal.
Of course we all know that flies and grasshoppers are different beasts. And you can fit an entire fruit-fly embryo into two segments of a grasshopper embryonic nervous system at a relatively similar stage of development. Although flies and grasshoppers are significantly different, the overall architecture and organization of their nervous systems are remarkably similar. You can find a neuron in the grasshopper that takes a right turn at a specific location in the central nervous system and then extends toward the developing brain of the animal, and in miniature fly version you can find a neuron in the same location that does exactly the same thing. The fly neurons have the same cellular origin, express the same markers/proteins in their nuclei and on the cell surface, and the shape of their trajectories are the same. That realization was achieved at the cellular level. What I did as a postdoc, along with many colleagues, was to demonstrate that this similarity is conserved at the molecular level. What was really cool is we found that related molecules that guide axons in the grasshopper and the fly are also found in humans. Over the years, all of this work has resulted in the identification of phylogenetically conserved families of guidance cues and conserved families of receptors that respond to these cues: in worms, in insects and in mammals. Moreover, in all of these organisms how they work is in many ways the same.
You seem surprised about where your latest work has taken you—into the area of synapse formation. How’d you get here?
KOLODKIN: Studying synapse development and function is not an area we have traditionally been interested in. However, our work over the past 15 years has brought us to this direction as we have used the tools we have generated to look in more in depth at neural development, first during the embryonic stages, then at early postnatal neural development and in the adult nervous system. From this work we have begun to define roles of neuronal guidance cues and receptors— certain semaphorins and their neuropilin and plexin receptors. We are investigating not only how axons and dendrites extend to their targets but also what they do when they get there. These functions involve selectively regulating the elaboration of synaptic specializations along regions of the neuron that receive input from multiple sources. We are finding that our work on how these proteins function during embryogenesis is really helpful in understanding similar and different ways in which they regulate how synapses form and function in the adult nervous system.
You refer to yourself in the plural “we,” which apparently includes not only those working in your lab, but also colleagues with whom you’ve enjoyed close collaborations. Can you elaborate a bit on the importance of this?
KOLODKIN: When I came to Hopkins in January 1995, no one knew what the semaphorin receptors were, so that prompted the collaborative work I did, and still do, with David Ginty, also a professor here at Hopkins in the Neuroscience Department. We arrived at Hopkins within a week of each other and have been collaborating very closely ever since. Sol Snyder, who hired us, is a wonderful scientist and he had great vision for our department with respect to who he wanted to expand the scope of neuroscience research here at Hopkins—this has provided me with outstanding opportunities to both conduct and expand my research directions. The only thing he didn’t do was have our labs ready when he said he would, which turned out to be a real gift because David and I set up a joint temporary lab in Vernon Mountcastle’s old office and laboratory. We cobbled everything together while waiting for renovations, and during this time, David and I realized that we could pursue shared interests in neural development, including a search for the first semaphorin receptor, which we found in 1997.
Your lab has developed a new mutant mouse—with all due respect to grasshoppers and flies, the mouse seems to be an organism of choice now for much neuroscience research—to study the function of another semaphorin protein called Semaphorin5a during development. This work moves into yet another new direction for your group.
KOLODKIN: We made these mutant mice for studying how Sema5a functions during neural development, and it does play important roles in several neural systems, for example in the development of the retinal. However, it turns out that this mouse might be an interesting model for conducting behavioral analysis related to autism spectrum disorders (ASDs). We’re collaborating with Hongjun Song here at Hopkins to conduct behavioral assays that are thought to model certain aspects of autistic behavior.
Our reason for even thinking that this direction was relevant came from recent work performed in part here at Hopkins by Dr. Aravinda Chakravarti in the McKusick-Nathans Institute of Genetic Medicine. We were very surprised to see the Sema5a gene emerge from a genome-wide association study (GWAS) as a strong candidate risk factor for autism and realize that we have in hand a mouse model to study this potential connection to human disease. We’ve proposed a pilot program to look at these mice—their neuroanatomy, neurophysiology and the behavior to see if they might be a useful ASD model. We are very excited about this research direction and interested to learn whether this mutant mouse will provide a way to actually design experiments directed toward understanding autism.
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