Looking back on his career, Venkatesh (Venki) Murthy sees no direct path or single-minded passion. “I find there’s joy on the side roads,” explains the newly tenured Biophysics/Neurobiology Professor in the Molecular and Cellular Biology (MCB) department about his “scattered” CV. He does see, however, a theme in his meanderings from his undergraduate studies in mechanical engineering at the Indian Institute of Technology in Chennai (Madras), near his hometown in Southern India, through graduate work in physiology and biophysics, to postdoctoral and faculty studies in neurobiology.
Along the way, he applied his analytical and engineering skills to an understudied form of synaptic plasticity, and to an often-overlooked type of neuron and a frequently ignored part of the synaptic junction between neurons. Now he’s turning to live animals and incorporating behavioral studies to examine synapses in their native environment. He’s also leaving behind the hippocampus, the site of learning and memory where synaptic plasticity is famously important, to join several neuroscientists at MCB in studying the sense of smell. But Murthy focuses on the under-investigated “middle ground” between the lower-level molecular biology within olfactory neurons and the higher-level processing of odor perception. This all makes sense, as he tells the story.
The Meandering Path
“I was always interested in both math and biology, but had to decide between them because of the exam system in India. I chose engineering,” Murthy recalls. Then, he veered towards University of Washington to get a MS in bioengineering in 1988. Intrigued that science allowed him to ask his own questions rather than solving other people’s problems, he stayed for a PhD in physiology and biophysics in 1994 with Eberhard Fetz, a physicist-turned-neuroscientist. While in Seattle, Murthy not only got hooked on science, but also hooked up with his future wife Meredith.
During post-doctoral work at the Salk Institute with Charles Stevens and Terrence Sejnowski, he switched to the cellular and sub-cellular level to understand how synaptic connections give rise to the collective behavior of circuits. He delved into how the pre-synaptic neuron’s vesicles, the bubbles that contain neurotransmitters, break apart to release these brain chemicals and then stitch themselves back up. Murthy continued exploring vesicle trafficking when he joined MCB in 1999.
Homeostatic Plasticity: Just Loud Enough
In a “natural transition,” Murthy began looking at the pre-synaptic neuron’s role in maintaining so-called homeostatic plasticity in the hippocampus. “Most people study excitatory plasticity, which increases receptors on the post-synaptic cell and strengthens circuits. But a compensatory plasticity balances excitation with inhibition.” He theorized that a feedback mechanism must keep the circuit’s chatter in a normal range: not too loud and not too quiet. Somehow, the network looks at its own activity.
In a 2001 paper in Neuron, he tested that theory by inhibiting the excitatory glutamate circuit. Puzzlingly, that inhibition strengthened synapses, contradicting the well-accepted Hebbian theory of “If you don’t fire, you don’t get wired” into a circuit. Equally novel, the synapses strengthened because the pre-synaptic cells made more vesicles and so, on average, released more excitatory neurotransmitters.
“When the network is silent,” Murthy speculates, “the pre-synaptic cells shout louder, as if saying ‘Hey, guys, I’m here. Listen to me.’ But if instead of inhibiting the network you excite it, the opposite occurs. Inhibitory neurons tone down the network to restore balance. But how does the network know to do that?”
A provocative explanation appeared a year later in Nature. Murthy managed to deafen just one neuron so it couldn’t hear excitatory signals and so wouldn’t fire. Surprisingly, that one neuron acted autonomously to influence the pre-synaptic neurons. “The deaf neuron says ‘Guys, I can’t hear!’ But rather than turn up its own receiver by making more receptors, it sends a retrograde signal to other neurons to talk louder and send more neurotransmitters.”
However, the opposite thing happened in a developing circuit. If one neuron didn’t fire, it got lost, obeying the Hebbian theory of how circuits get wired by firing in response to stimulation.
When Murthy then blocked the whole network’s excitatory signals, the inhibitory process subsided significantly. “The neurons packaged fewer inhibitory neurotransmitters in their vesicles, because if the network is already silent, inhibition is pointless.” But unlike how deafening just one neuron to excitatory signals affected that neuron’s excitatory inputs, depriving that neuron’s activity had no effect on its inhibitory inputs.
Alive and Sniffing
“I’d been studying networks in a dish with patch clamp electrodes to measure their firing, but I wanted to tie these discoveries to what happens in the real brain,” Murthy says of his next transition. But the hippocampus is too deep for optical imaging, too far from sensory and motor systems, and too much a topographical terra incognita to tell which neurons do what. The olfactory system, however, has none of those problems.
“Many people study olfactory receptors in the nose and how they map to precise points, called glomeruli, on the brain’s olfactory bulb. Others study which areas of the brain interpret those signals to create perceptions. But the middle ground of how the synaptic circuits form and what connects to what is a poorly populated field. But that’s what I do. It’s also well known that the sense of smell evokes powerful memories, so it must be plastic as well.”
At the beginning, Murthy collaborated with MCB’s Markus Meister, Professor of Biology, to learn how odors are represented on the olfactory bulb. “I was very lucky to have such good mentors in this department. I didn’t have to seek them out, and they were very supportive.”
Putting his engineering to use, Murthy helped devise a multi-colored contraption of tubes to deliver 100 distinct odors sequentially to a mouse’s nostrils. He also built a two-photon microscope to track where each odor activates the olfactory bulb.
He then asked: are the glomeruli’s connections hard-wired from birth, or plastic and driven by activity? By closing off one nostril and depriving it of sensory experience, he found that the glomeruli’s deprived neurons strengthened their synapses to become more sensitive to the odors they somehow knew they were missing. “We’ve known that the olfactory system can tune its sensitivity, because when exposed long-term to an odor, you become insensitive to it. But to find this plasticity at the very first stage was a big surprise.”
Almost all live-animal olfactory studies require anesthetizing the animal so researchers can image their brains as they lie still. “But olfaction is active,” Murthy contends. “The animal breathes in and out and sniffs. Are the odor maps in awake and behaving animals the same as those observed in anesthetized animals?”
To image the brain of a mouse scurrying around a cage, Murthy’s lab, teaming up with Meister again, devised a meter-long flexible fiber optic cable with 20,000 individual elements, each covering 10 microns, about the size of a neuron. Together, the bundle spans 50 glomeruli. Using this method, Murthy is starting at the first synapse and will “march” his way up the circuit to learn where the first, if any, difference occurs between anesthetized mice and those running around freely.
Next, he will teach mice to recognize very similar odorants and see where changes occur in a learned task. Then he’ll move to the cortex. He might also take another side road, figuring out what all the back-projections from the brain are doing in the olfactory bulb. “The olfactory bulb is not just a relay station, passing signals on. What else does it do?”
Exposing Others to Other Worlds
As professor, Murthy’s gives his cellular neuroscience undergraduates a taste of the techniques and fundamental principles they’ll need later. “It’s important to understand electrical noise in patch clamp amplifiers, the signal-to-noise ratio in optical microscopy, and the physics of multi-photon microscopes.”
If it sounds like he’s merging the best of both Harvard and his Indian education, he is – with his other “baby” (besides his two young children), the Harvard-Bangalore Science Initiative. He compares Bangalore to Boston’s 128 Corridor, with very cutting edge science.
“I grew up not knowing the excitement of doing science, so I want to expose technically-oriented students in India to that world.” This summer, five Harvard undergraduates went to three participating institutions in Bangalore, and he hopes graduates and post-docs will follow. He also hopes Indian students will come to Harvard. They might enjoy the side roads, too.