Partnerships, Puzzles and Paradigms-the Joy of a 40 Year Scientific Collaboration

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Joseph Goldstein and I met in June 1966 when we both joined the Massachusetts General Hospital in Boston as medical interns. Joe had graduated from Southwestern Medical School in Dallas, Texas, one of the youngest medical schools in the UnitedStates. I graduated from the University of Pennsylvania School of Medicine, the oldest medical school in the United States. We came from different backgrounds. Joe was born and raised in a tiny rural community in the Deep South. I was born in Brooklyn, New York, and I grew up big-city Philadelphia, Pennsylvania. Despite these differences, we were drawn together by our common interests in medical science. We each had a burning desire to solve medical puzzles, and hopefully to help develop more powerful treatments.

After finishing our residencies, both of us moved to the National Institutes of Health, just outside of Washington D.C. There we worked with profound basic scientists. Joe worked in the laboratory of Marshall Nirenburg, who would later receive the Nobel Prize for solving the genetic code. I trained with Earl Stadtman, one of the world's leading enzymologists. There I developed a fascination with metabolic regulation that was to last for my whole life.

As a medical student, Joe had made a commitment to return to the Department of Medicine at Southwestern Medical School after his training. I was initially very skeptical about Dallas. Texas was a very conservative state where political views were substantially different from my own. My wife, Alice, shared this concern.

At this point, Joe introduced me to Donald Seldin, the chairman of medicine at Southwestern. I was deeply impressed. Seldin had the broadest, and also the deepest, knowledge of medical science that I had ever seen in one individual. He was building a department filled with physician-scientists, all of whom took care of patients, and all of whom supervised laboratories designed to solve the puzzles of human disease.

After much consideration, I decided to join Southwestern Medical College. While at the NIH, Dr. Goldstein and I helped to care for two children who were suffering from multiple heart attacks caused by extremely high levels of plasma Low Density Lipoprotein (LDL) cholesterol. In Dallas we obtained skin biopsies from similar patients and studied their cholesterol homeostasis in cell culture. This work led to the discovery of the LDL receptor and the process of receptor-mediated endocytosis. Later many other proteins were shown to enter cells by this process. Receptor-mediated endocytosis was our first paradigm.

We early found that the production of LDL receptors is regulated. Cells produce abundant LDL receptors when they need cholesterol. When cholesterol accumulates, the gene for the LDL receptor is silenced and the number of receptors declines. We found the secret to this control mechanism when we isolated a protein called Sterol Regulatory Element Binding Protein (SREBP) that activates transcription, not only of the LDL receptor, but also of all the genes that are necessary to produce cholesterol and fatty acids in cells. We discovered that SREBPs are synthesized as membrane-bound proteins. The active fragment must be liberated proteolytically from the membrane in order to activate transcription. When cholesterol accumulates in membranes, SREBP proteolysis is blocked and the transcription of all target genes declines. We called this process regulated intramembrane proteolysis (RIP), and it turned out to be our second paradigm. Several other transcription factors are now known to be synthesized as membrane-bound proteins and to be liberated proteolytically just like SREBP.

The third paradigm emerged from our studies of a human disease called Niemann-Pick C Disease. In this disease, LDL cholesterol enters the cell through the LDL receptor. The cholesterol is delivered to lysosomes, but there is a block in the removal of cholesterol from lysosomes. As a result, cholesterol accumulates inlysosomes of cells throughout the body. Affected children die from progressive malfunction of the brain, liver and lung.

Two genes had been identified as the cause of NPC disease. One encodes NPC-2, a soluble protein that floats in the lumen of lysosomes. The other encodes NPC-1, a complex polytopic membrane protein with 13 membrane-spanning helices. Homozygous mutations in either gene cause the identical syndrome. Using purified NPC-2 and NPC-1 proteins, we showed that NPC-2 binds cholesterol immediately after it is released from LDL. The NPC-2 transports the cholesterol to the membrane where NPC-1 resides. There, NPC-2 transfers cholesterol directly to the amino-terminal domain of NPC-1. The transfer is carried out such that the hydrophobic cholesterol is never exposed to the water phase. The transfer to NPC-1 occurs by a direct sliding mechanism. We call it the hydrophobic handoff and this is the third paradigm. We expect that hydrophobic handoffs maybe a general mechanism by which water-insoluble lipids are transferred from one protein or one organelle to another.

Throughout the 40 years of our scientific collaboration, Joe Goldstein and I have thoroughly enjoyed sharing our discoveries. Neither of us takes separate credit for the discoveries that have emerged from this partnership. All of our experiments are conducted only after the two of us agree upon the approach. Although other scientists have collaborated as intensely as we do, our collaboration, spanning over four decades may be the lengthiest, at least among biologists who have been recognized with the Nobel prize.