For humans to think and feel, nerve cells in the brain have to communicate with each other. To do so, a neuron packages special signaling molecules, called neurotransmitters, into small pouches that merge with the cell's outer membrane, where the load is released into the synapse, the space between the cells. A second neuron then picks up the neurotransmitters. To be able to quickly repeat the process, the first neuron recycles the used membranes and re-forms new pouches within the cell, where they pick up their next batch of neurotransmitters.
For the past 30 years, Pietro De Camilli has been studying in meticulous detail the steps involved in the release of neurotransmitters via the pouches, or synaptic vesicles, and the re-formation of the vesicles inside nerve endings in a process called endocytosis. He has characterized key molecules, including proteins and lipids, that are important in the formation and movement of synaptic vesicles, and he showed how these processes are regulated. Although De Camilli's focus on neurons has provided fundamental insights about how nerve cells communicate, his findings are applicable to other cells, which also communicate by releasing chemicals from vesicles and also recycle their membranes.
De Camilli's interest in how cells discharge chemicals, such as hormones or digestive enzymes, began in medical school in Milan in the late 1960s. He pursued medicine because he felt the degree would provide him with the best training to do biological research. At that time, Ph.D. programs didn't exist in Italy. He became excited about the then current research on the movement of membrane-bound vesicles carrying materials in and out of cells because it was the first attempt at describing intracellular membrane dynamics. In his first research project, for his medical school thesis, he studied the role in secretion of a class of lipids called phosphoinositides, a topic to which he returned later in his career.
De Camilli continued investigating secretion after receiving his M.D. and a postgraduate degree in endocrinology, the medical discipline specializing in the release of hormones. These are molecules that transmit signals through the bloodstream.
Interested in how secretion is controlled, De Camilli moved to the United States in 1978 for a postdoctoral fellowship with Paul Greengard, then at Yale. Greengard's laboratory had found a nerve cell protein, subsequently called synapsin, which undergoes a chemical modification when the cell is stimulated by neurotransmitters. It was believed that synapsin regulated synaptic transmission, but it was unknown how. De Camilli showed that synapsin is a major component of the surface of synaptic vesicles that contain neurotransmitters, thus revealing this protein's role in neurosecretion.
"Synapsin was the first synaptic vesicle protein to be extensively characterized and its identification was the foundation for improved synaptic vesicle purification and the discovery of all other components in these vesicles," says De Camilli, who was recruited as an independent researcher to Yale in 1979 by George Palade, a leader in the field of secretion.
De Camilli feels that a key contribution of his early studies of synapsin and synaptic vesicles was to bring the perspective of a cell biologist, which he considers himself, to the neurobiology field. At the time, neurons were studied primarily by "electrophysiologists, who analyzed currents across nerve cell membranes and neurochemists who studied cell extracts," De Camilli says. "As a cell biologist, I brought new tools (such as advanced microscopy) to study the action of proteins in their native cellular context."
During the next 10 years, De Camilli and others characterized the many components of synaptic vesicles and some of their associated proteins. He found that some proteins in synaptic vesicles are shared by vesicles of cells that secrete hormones, such as the beta cells in the pancreas that release insulin. One such protein, he found, can become a target of autoimmunity in humans, a finding that led to a test for diabetes type 1, which is caused by an autoimmune disease that destroys beta cells.
By the mid-1990s, De Camilli felt that, as the inventory of synaptic vesicle proteins was being completed, the next key question was how synaptic vesicles during endocytosis are assembled and recycled inside the cell after each round of secretion. He soon found that a specific phosphoinositide in the cell's membrane, called PI(4,5)P2, regulates vesicle formation during endocytosis and that a phosphate protruding from this lipid binds endocytic factors. Once the vesicle is formed, an enzyme called synaptojanin, which De Camilli identified, removes the phosphate and allows shedding of endocytic factors.
He subsequently identified other proteins, endophilin and amphiphysin, that bind and penetrate the cell's membrane and showed how these proteins help generate the curvature that enables endocytic vesicles to form.
De Camilli considers his findings about the membrane's lipid components in synaptic vesicle formation as his most seminal work. They are significant he says, because they highlighted lipids' central role in endocytic traffic in neurons and in other cells. He muses, though, that had he not done his phosphoinositide research in medical school, he might not have been primed for his later findings. "What makes the career of a scientist unique and defines his or her ability to approach a problem is the collection of experiences and topics that he or she has researched," De Camilli says. "My early research with phosphoinositides gave me a perspective no one else had."
Although De Camilli acknowledges he wasn't interested in clinical medicine during medical school, he now appreciates its value. Biologists, he says, create mutations or give drugs to a cell or animal to see how the disturbance affects the system. Patients with diseases, De Camilli explains, allow scientists to learn from the many perturbations that nature spontaneously produces.
Today, De Camilli continues to study membrane traffic at synapses and remains fascinated by how synaptic vesicles form. He also is returning to his medical roots: He is studying how disruption in membrane traffic may cause diseases, including neurodegenerative diseases, such as Alzheimer's and Lowe syndrome, a condition caused by abnormal phosphoinositide metabolism that leads to pathology in the eyes, kidneys, and brain. "The intersection of basic science and medicine promises to be very fruitful," he says.