Nobel Prize focused on life-giving cellular cargo delivery system

A Nobel Prize was recently awarded to three researchers who discovered bubbles within bubbles that have tentacles. Award winners James Rothman, Randy Schekman and Thomas Südhof were pioneers in the study of vesicles, which are like bubbles inside human cells whose outer layers are made of the same stuff that separates cell insides from the outside world. Because the bubbles’ insides are kept separate from the rest of the cell’s interior, they can store, organize and deliver highly reactive biochemicals and proteins, releasing them only when and where they’re needed.

Electron microscope images of vesicles near Golgi complex in a cell.

Furthermore, the outer membranes of vesicles can fuse to outer cell membranes or to the membranes surrounding other cellular machines. This lets them take in contents from one compartment, move them to another walled-off area, and deposit them there. It also lets a cell start manufacturing a hormone or an antibody in one spot, truck it somewhere else for finishing, move it to the cell's surface, and secrete it to do a job elsewhere in the body.

To ensure that it dumps its contents into the right compartment, each vesicle has “tentacles,” squiggly proteins that “taste” the surface of other vesicles to make sure they have reached the right destination. Without vesicle formation and fusion, cells could neither live nor signal to each other as part of complex tissues. Vesicles are so central to cellular function that they are most likely involved in almost every disease when things go wrong, although we don’t yet know their role.

For his share of the prize, Dr. Rothman, a professor at Yale, discovered a protein complex that lets vesicles fuse with membranes to deliver their molecular cargo. He has a UAB connection in his former student, Dr. Elizabeth Sztul, Ph.D., professor in the UAB Department of Cell, Developmental and Integrative Biology and a vesicle expert in her own right. Dr. Sztul said she was “thrilled” to see vesicle research be recognized in the form of a Nobel Prize, and we thought to ask her why her field deserves notice.

Show notes from the podcast

1:56  A Nobel Prize win for vesicle research underscores just how vital basic scientific research is, along with the fundamental importance of vesicles in human life, said Dr. Sztul. Cells are made of compartments, and the prize went to the three-scientist team for discovering how proteins and genes work together to move key substances from one compartment to another.

3:02  Vesicular traffic has implication for all of life. Genetic mistakes that occur in genes that control this delivery system mean that an embryo does not often survive, and if so, with severe disabilities. The Nobel Prize winners designed tests that identified the cogs (proteins) that make possible the machinery behind vesicle formation and transport.

4:15 Specifically, Dr. Rothman, a biochemist, identified the proteins required for vesicle transport and then re-created vesicle trafficking in a test tube. Dr. Schekman identified some of the genes that control vesicle transport, and that have done so throughout evolution. He works one-celled organisms like yeast that share vesicle pathways with human cells. Dr Sudhof, a neurobiologist, showed how vesicles deliver proteins, not just to the right place, but also with perfect timing, to make cellular life possible. Together, the pioneers outlined how a vesicle "knows" where to go and when to fuse.

5:47  Genetic mutations, random changes in that occur in genes as they constantly get copied, are usually fatal within a few days when they occur within the central machinery proteins of vesicle trafficking in a human embryo. Dr. Stzul describes one key vesicle protein type as snares, which help a vesicle grab on to the outside of another compartment they want to fuse with. Genetic defects in snares are fatal, but people are born with genetic changes in less essential machinery related to vesicles and survive.

6:22 For instance, proteins on the sides of vesicles that are nicknamed tethers or tentacles, touch and "sample" the outer membranes of surrounding vesicles to determine which they should target for fusion.  A mutation in one these protein tentacles causes a very serious, rare disease called congenital disorder of glycosylation. Tethers and snares represent two ways that any vesicle chooses a specific vesicle that it will deliver its cargo into.

8:40 There are many types of payloads delivered by vesicle in human cells. The immune system uses them to swallow invading bacteria, and then to deliver chemicals to that vesicle that destroy the bacteria.  Nerve cells use them to deliver signaling molecules to the next cell in line as a nerve message runs along a nerve pathway. After you eat a meal, cells in your pancreas packs digestive enzymes into vesicles and ship them off to the gut.

10:38 Vesicle delivery of proteins by cells is tightly regulated and very precise, Dr. Sztul said. Even small genetic errors in the genes that make the vesicle proteins can cause disease, including one called craniofacial disorder. These patients have bones that don't form properly because the structural protein collagen, which makes the lion's share of skin and bones, is not delivered properly by vesicles.

12:37  Dr. Sztul's lab is trying to figure out all of the steps needed to move all important proteins in the cell from where they are made, through every required vesicle stop along the way and to a final destination.  Each stop in this journey involves a web of interacting proteins, and Dr. Sztul would like to map all of these interactions precisely in time and space. She believe that this map, once complete, will reveal links between mutations in vesicle trafficking genes and many diseases, both common and rare. These patterns will emerge, she said, as more and more patients routinely get their DNA sequenced as part of personalized medicine.

15:00 With the map in place, researchers will have a basis for the design of treatments that compensate for problems with specific vesicle proteins. At least theoretically, every protein that regulates trafficking will become targets for drug design. In cancer for instance, vesicles may be used by tumors to deliver proteins that cause cancer cells to spread or that encourage the growth of blood vessels that feed tumors. Drugs might be designed to keep vesicles from making these harmful deliveries.

18:15 The field is working to invent imaging technologies that can track the movement and action in a living cell, not just of a couple of interacting proteins at a time, but that can watch perhaps 60 vesicle trafficking proteins at work during one stage of trafficking. Also on the horizon, an in-depth understanding of the interaction between vesicle trafficking and outer key cellular actions. How do hormones effect vesicle trafficking in cells that secrete hormones? How vesicles in immune cells become filled with antibodies when the body senses that it has been infected with a bacterium or a virus, or in response to a vaccine?

Goal: keep your transplanted organ permanently

Unless you have an identical twin, needing an organ transplant comes with a serious problem even beyond the fact that you need a transplant. Assuming the surgery goes well, the minute the new organ is grafted into your body, your immune system will recognize it as foreign, akin to invading bacteria, and seek to destroy it.

Taking the kidney for an example, there was a time 25 years ago when half of kidney transplant recipients lost their transplant due to immune rejection. The field of transplant immunology has in recent years become very good at preventing this during the first year after transplant using drugs that turn down the immune response, but long-term rejection remains commonplace.

The immune systems of many organ recipients eventually destroy transplanted kidneys over ten to 15 years. Worse yet, patients live through those years with a suppressed immune system; making them vulnerable to viral infections, some of which cause cancer.

Research efforts to solve these thorny, remaining problems in transplant immunology continue, but the field is under duress thanks to cuts in federal research funding, says UAB's Rosyln Mannon, M.D., director of research at the UAB Comprehensive Transplant Institute and a kidney transplant specialist. She was among the organizers of a recent transplant immunology symposium held by the institute.

Dr. Mannon sat down with The Mix to talk about research frontiers in transplantation, including efforts to design drugs that precisely turn down the activity of immune cells involved in transplant rejection, while ignoring those that fight infection.


Show notes for the podcast:

1:05  As we develop in the womb, special proteins are built on the surfaces of all our cells that serve as tags that say "self," and thus keep our immune cells from attacking them.  A transplanted organ obviously has different cell-surface, protein labels.

1:31  When surgeons put in a transplanted organ, the proteins labels on the organ surfaces are picked up and carried by immune cells to nearby lymph nodes, where they trigger the building of an army of cells designed specifically to attack the new organ. Several sets of immune cells are swept up into the effort to destroy the transplanted organ (also called a graft), including T cells and antibodies, two workhorse cells of the adaptive immune system.

1:47 Thus, the response to a transplanted organ that immunologists must deal with when preventing transplant rejection includes a mix of proteins, including antibodies that glom onto and remove foreign cells,    and cells that swarm to the transplant site and release destructive chemicals (e.g. cytokines).

3:15 The field of transplant immunology has been "incredibly successful" at preventing acute transplant rejection in the first year after the transplant using a subtle, powerful mix of drugs that damp down the immune system to protect transplanted organs.

3:39  The average person who receives a kidney this year from a diseased patient can expect the graft to last ten years. If the organ came from a living donor, the transplant may continue to function for 15 years, and especially if the organ came from a well matched family member. Despite these advances, all patients see their transplants fail eventually. About half of them "fail" because the patient dies, some from heart disease. Others organs fail because the medicines taken to suppress immune systems leave patients vulnerable to infection.

4:20 Physicians typically take a biopsy of a failing kidney to see why it has failed after working well for so long. In some cases, the slides will reveal that the immune system finally overcame immunosuppressive drugs to recognize the transplant as foreign and attack it. Interestingly, sometimes it will be one part of the immune system that finally tracks down the organ (e.g. antibodies), and sometimes another (T cells).  In still other cases, the biopsy may reveal that a longtime, undetected viral infection has destroyed the organ, or maybe it was fibrosis, the wear-and-tear scarring that comes with age.

5:15 The failure of organ transplants many years after implantation for these varied reasons is the central, remaining problem facing transplant immunologists and their patients.

5:43 The drugs used to suppress the immune system on the way to protecting a transplanted organ have evolved. In the old days, transplant recipients received steroids like prednisone (an anti-allergy drug) and drugs called anti-metabolites. In the mid-1980s, a set of drugs called calceneurin inhibitors arrived, including cyclosporin and then later Prograf. Most patients in those days got large doses of drugs like these, some of which themselves scar the kidneys. Other risks of such therapy included knocking the immune system thoroughly enough to encourage viral infections like the Epstein-Barr virus, cytomegalovirus and related cancers viruses.  The latter make random genetic changes in the cells they infect, some of which accidentally cause the abnormal growth seen in cancer. Presentations at the recent symposium talked about ways of fine-tuning immunosuppressive treatments to minimize damage related to their use.

7:47 Newer FDA-approved treatments appear to have fewer side effects, but still have the same problem as older drugs: they knock down the immune system broadly instead just the cells attacking the new organ. All physicians can do is gradually reduce the dose of immune suppressing medications over time under the assumption that the immune system has come to see the transplant as self, but doing so may result in the late-stage rejections currently observed five and ten years down the line.

8:53 Frontiers in the field include research efforts to design therapies that influence only the subsets of immune cells most associated with transplant rejection. Certain kinds of immune cells "remember" they have encountered a foreign protein, for instance.  Therapies may destroy most of those cells, but those that remain eventually become capable of re-launching the attack on the transplant.  On the other hand, one subset of T cells, called Tregs, are known to damp down the immune response in a careful way. What if engineers were able to deploy a person's own Tregs to damp down response to specific proteins on the surfaces of transplanted tissue?

10:11  Research efforts looking suppressing specific immune mechanisms, while the future of the field, are still in the early phases. Dr. Mannon hopes they suppress more surgically than the global suppression seen with older drugs.

10:45  The recent UAB transplant immunology symposium was timely, said Dr. Mannon, because UAB has been working to establish a collaborative consortium of transplant centers in the Southeast. UAB is one of the largest clinical transplant centers in the country, as is its partner in this symposium, the Emory Transplant Institute. Vanderbilt and the Medical College of South Carolina also have a strong interests in this area. The symposium was the first forum to identify and discuss the central, remaining problems in transplant immunology, and to launch joint efforts to solve them.

11:15 Among the research frontiers discussed was how best to arm patients with the ability to fight off infections without jeopardizing their transplants. One way may be to harness the bacteria that live in the human gut. What role do the bacteria that permanently colonize the body have in the immune system and transplant rejection? Can the interaction between our gut bugs and antibiotics for instance be manipulated to improve long-term outcomes of transplant patients by damping down system-wide inflammation?

14:24  Dr. Mannon is the newly elected president of the American Society of Transplantation, the second president in a row to come from UAB after Dr. Robert Gaston, M.D.  UAB has for years been recognized across the Southeast for the large volume of clinical transplant procedures done here, but having the presidency sends a message to the nation about the strength of the basic and translational research underway.

15:45 The society has been very active on Capitol Hill in terms of lobbying for the support of related research, and that stance will continue during Dr. Mannon's term. As for the group's legislative agenda, they have been trying for 12 years to get a bill passed that would provide coverage for immunosuppressive therapy for the life of those with kidney transplants. Currently, Medicare pays for such medications for three years after the transplant, after which medications the become prohibitively expensive for those without private insurance. Another bill would ensure that those who donate a kidney to another person cannot have their coverage dropped by an insurer after they give the gift of life.