Loaded with Love – and Other Signals

Gazing at the face of our newborn or into the eyes of our sweetheart we might feel a sudden rush of joy – thanks to the hormone oxytocin, which has already been locked and loaded into special dispatch stations found on neurons in our brains. Weizmann Institute of Science researchers have now revealed the mechanism for restocking the oxytocin in these dedicated quick-release sites. These findings, which were recently published in eLife, may hold important clues for research into such disorders as autism that are thought to be tied to oxytocin levels and its release into the body.

Oxytocin is not just a love hormone. Among other things, it plays a role in normal social interaction, childbirth and lactation, as well as in numerous physiological functions including regulation of stress and appetite. Since both timing and amounts are crucial, the body maintains a store of oxytocin at its release sites so that it can be applied upon specific commands from the brain.

To understand this arrangement, the group of Prof. Gil Levkowitz of the Institute’s Molecular Cell Biology Department investigated how oxytocin is transported and stored – in the brains of tiny, transparent zebrafish larvae. “In zebrafish, as in humans, the hormone is packed in special lipid capsules, or vesicles, which make their way into nodes called synapses. These synapses are distributed along the length and at the ends of the axons –  the long thin nerve cell extensions,” he says. 

Led by postdoctoral resear cher Dr. Savani Anbalagan and Staff Scientist Dr. Janna Blechman, the research group combined cutting-edge microscopy with genetic engineering methods to track precisely what happens inside the synapses. “It’s really convenient that we can see straight into the zebrafish brain, down to the level of a single nerve cell. This enables us to monitor, in a living organism, the journey of this hormone from its initial production site to that of its secretion – its release into the body,” says Levkowitz.

First, the scientists made sure hormone levels have a measurable base level within the vesicles; then they introduced salt water into the fish’s tanks. In mammals, oxytocin is released when dehydration threatens, telling the kidneys to conserve water. Since zebrafish live in fresh water, adding salt to their environment produced an effect similar to dehydration. The researchers found that in a saline environment, oxytocin was rapidly released from the vesicles in the synapses, and it was replenished when the salt was washed out.

The research group next decided to follow a lead suggesting the surprising involvement of a ubiquitous protein called actin – mostly known as a structural material in the cell.

They developed a method to interfere with the chain-building of the long actin molecules in the vesicles, and to observe and measure the dynamics of this process, known as polymerization, under the microscope. The researchers observed that the polymerized actin appeared to be surrounding the vesicles within the synapses. These experiments revealed that it is the dynamics of the actin as it breaks down (depolymerization) and reforms that ensures constant amounts of oxytocin-containing vesicles within each synapse. Observing under the microscope, the scientists realized this was very similar to the way in which the actin in developing axons continually breaks down and regrows as these are guided toward their end targets. “Once the axon is fully grown, the growth mechanism is repurposed for oxytocin accumulation and storage,” says Levkowitz.

Using this insight, the group conducted further experiments to clarify how this process is controlled. They uncovered several nerve-cell molecules involved in regulating the dynamics of the actin, and thus the cycle of transport, release and recycling of the hormones in the vesicles. Existing genetics research pointing to the actions of these molecules helped the researchers put together the scenario of how these work together to keep the readily available hormone amounts at ideal levels. 

“We think the polymerization and depolymerization of actin is something like controlling the amount of luggage on a baggage-claim carousel in an airport – one that works most efficiently if it has an optimal speed. If the carousel moves too fast, the luggage handlers can’t keep up, while if it is too slow, the passengers will quickly empty it. Either way the amount on the carousel we be depleted, making the process less than optimal. Even small changes in efficiency can add up, over time, to larger faults in the system,” says Levkowitz.

 The scientists realized this was very similar to the way in which the actin in developing axons continually breaks down and regrows

“The players we identified are likely to be involved in regulating other hormones that are released in the brain. And,” he adds, “since there are indications that abnormalities in oxytocin’s action – or its lack – are involved in autism, these findings will hopefully provide new avenues for research into this disorder.”

Also participating in this research were Michael Gliksberg and Dr. Ludmila Gordon of Levkowitz’s lab, Dr. Ron Rotkopf of the Bioinformatics Units, and Drs. Tali Dadosh and Eyal Shimoni of the Super-Resolution and Electron Microscopy Units of the Chemical Research Support Department, all of the Weizmann Institute of Science.   

Prof. Gil Levkowitz's research is supported by the Nella and Leon Benoziyo Center for Neurological Diseases; the Richard F. Goodman Yale/Weizmann Exchange Program; and the estate of Emile Mimran. Prof.  Levkowitz is the incumbent of the Elias Sourasky Professorial Chair.