20. Cells in the pancreas of a 1-week-old mouse that express the Ptf1a transcription factor gene are revealed in this photograph. The Ptf1a gene has been genetically engineered to express a bacterial enzyme that produces a dark blue color. In both mice and humans, Ptf1a is essential for formation of the entire pancreas, including insulin-secreting beta cells. By tracing the “cell lineage,” or family history, of Ptf1a-expressing cells, scientists hope to learn more about how to maintain—or restore—the function of beta cells. At top right is the sausage-shaped spleen (light orange), and at bottom is the duodenum. Photo by Fong Cheng Pan, Ph.D., research fellow, Department of Cell & Developmental Biology, Vanderbilt University. Courtesy of Christopher V. E. Wright, D.Phil.
61. Looking good. The pancreas of a mouse after it was transplanted with human beta cells ( left ) looks similar to that of an animal that produces insulin normally ( right ). CREDIT: Narushima et al., Nature Biotechnology Brimming with b's. Newfound cells in the pancreas give rise to neurons (red) and insulin-producing b cells (green). CREDIT: SEABERG ET AL. , NATURE BIOTECHNOLOGY The full picture. Human ES cells can eventually give rise to cells that resemble pancreatic beta cells (labeled β).
Blood vessels are the body's plumbing, supplying food and oxygen and removing waste. Now two papers published online this week by Science show that blood vessels play a more active role than previously believed: Even before they start to function, blood vessels help the liver and pancreas develop. In 1995, cancer researchers found the first evidence that there's more to blood vessels than meets the eye. They reported that endothelial cells, which make up blood vessel walls, produce growth factors, indicating they may play some role in development. The new studies build on that work. The first team, led by Douglas Melton of Harvard, began scrutinizing blood vessels after noticing that the endoderm--embryonic tissue fated to become the pancreas and other organs--touches a major blood vessel, the dorsal aorta. Because the developed pancreas monitors blood vessels, measuring glucose levels and producing insulin accordingly, the researchers wondered whether the pancreas and dorsal aorta also communicated during development. To test this, they performed experiments such as growing mouse endoderm in culture with and without embryonic dorsal aortae. Only alongside the blood vessel did the tissue produce pancreas-specific markers, including insulin. The second group, led by Kenneth Zaret of the Fox Chase Cancer Center in Philadelphia, took a different approach. They examined how mouse livers develop when a gene called flk-1 , which encodes a receptor for VEGF, is mutated so that no endothelial cells develop in the liver. When they grew these liver cells in petri dishes, the flk-1 culture grew to the same size as the normal culture but contained far less liver tissue, growing connective tissue instead. Surprisingly, the endothelial cells' influence arose well before the cells fully formed into vessels, suggesting that the cells themselves--and not some component of the blood--are sending the growth signal. Understanding how cells define themselves in an embryo is critical to designing stem-cell-based treatments for disease. "If we're going to induce organs to form, we have to have a thorough understanding of how the embryo develops them," says organ replacement biologist Michael Longaker of Stanford University School of Medicine in Palo Alto, California. "We will never do it in a more elegant way than the embryo."
Insulin is labelled here in green, glucagon in red, and the nuclei in blue
The process by which insulin is released from beta cells, in response to changes in blood glucose concentration, is a complex and interesting mechanism that illustrates the intricate nature of insulin regulation. Type 2 glucose transporters (GLUT2) mediate the entry of glucose into beta cells (see panel 2). As the raw fuel for glycolysis, the universal energy-producing pathway, glucose is phosphorylated by the rate-limiting enzyme glucokinase. This modified glucose becomes effectively trapped within the beta cells and is further metabolized to create ATP, the central energy molecule. The increased ATP:ADP ratio causes the ATP-gated potassium channels in the cellular membrane to close up, preventing potassium ions from being shunted across the cell membrane. The ensuing rise in positive charge inside the cell, due to the increased concentration of potassium ions, leads to depolarization of the cell. The net effect is the activation of voltage-gated calcium channels, which transport calcium ions into the cell. The brisk increase in intracellular calcium concentrations triggers export of the insulin-storing granules by a process known as exocytosis. The ultimate result is the export of insulin from beta cells and its diffusion into nearby blood vessels. Extensive vascular capacity of surrounding pancreatic islets ensures the prompt diffusion of insulin (and glucose) between beta cells and blood vessels. Insulin release is a biphasic process. The initial amount of insulin released upon glucose absorption is dependent on the amounts available in storage. Once depleted, a second phase of insulin release is initiated. This latter release is prolonged since insulin has to be synthesized, processed, and secreted for the duration of the increase of blood glucose. Furthermore, beta cells also have to regenerate the stores of insulin initially depleted in the fast response phase.
A single gene may explain the vast size difference between that tiny terrier yapping in the park and the massive mastiff ignoring the din. Nate Sutter, a geneticist at the National Human Genome Research Institute in Bethesda, Maryland, wanted to know the reason why big dogs, such as Irish wolfhounds, can grow up to 50 times larger than other members of their own species, such as chihuahuas. So he started out looking at large and small dogs of one breed — the Portuguese water dog. Scientists on the team took X-rays of 500 Portuguese water dogs and made 91 measurements of their skeletons. Based on these data, the researchers classified the water dogs as either big or small for their own breed. They then looked for differences in DNA between the large and small water dogs. This is a relatively easy job: a consortium of scientists including Sutter published the DNA sequence of the dog genome last December, and have mapped out the places where there is a lot of variation between individuals in a given breed. There are fewer of these places of variation in purebred dogs than there are in humans. The team found that one of the few differences in these Portuguese water dogs occurred in a gene called 'insulin-like growth factor 1', or Igf-1. This is one of many genes already known to influence the size of mice: when Igf-1 is knocked out, the animals grow up to be mini-mice. So the team wondered whether this gene was responsible for dog body size. Great pomeranians? To answer this question, scientists closely analysed the Igf-1 genes in 75 Portuguese water dogs and 350 other dogs of very large and very small breeds — from pomeranians and Yorkshire terriers up to great Danes and St Bernards. They also examined the gene in wild dogs, such as wolves and foxes, who are distantly related to domestic dogs. They found that almost all of the 18 small breeds carried the identical variant of the gene as small Portuguese water dogs. But almost none of the 15 giant breeds carried this gene variant. That suggested that the gene plays a major role in controlling dog body size, Sutter said on 11 October at the annual meeting of the American Society of Human Genetics in New Orleans, Louisiana. If researchers want to make a giant chihuahua, they now know where to start. The gene seems to work by setting how much of the growth factor dogs make. In Portuguese water dogs, smaller animals make less of the growth factor than big ones. The 'small' version of Igf-1 seems to have formed long ago, Sutter says. When humans began breeding tiny dogs, they inadvertently selected for this version of the gene, and over time the breeding process fixed the 'small' variant into tiny dog breeds. Man's best friend The study proves how useful genetic studies in dogs can be, Sutter says. Because dog breeders know the history of individual dogs in a breed, and because the dogs are purebred — meaning they have lost a lot of their genetic variation — it is easier to uncover the genetic causes of traits such as body size than it is in people. ADVERTISEMENT Other members of Sutter's group, led by Elaine Ostrander, are also looking for genes that cause diseases including cancer. Sutter says he hopes that they will find similar success. "The power in dog populations is that they can deliver a simple genetic story about a precise genetic trait," Sutter says. "I think we're also going to find this with other complex traits Action Its primary action is mediated by binding to specific IGF receptors present on many cell types in many tissues. The signal is transduced by intracellular events. IGF-1 is one of the most potent natural activators of the AKT signaling pathway , a stimulator of cell growth and multiplication and a potent inhibitor of programmed cell death . Almost every cell in the human body is affected by IGF-1, especially cells in muscle , cartilage , bone , liver , kidney , nerves , skin , and lungs . In addition to the insulin -like effects, IGF-1 can also regulate cell growth and development, especially in nerve cells, as well as cellular DNA synthesis. [ edit ] IGF-2 and Insulin; related growth factors IGF-1 is closely related to a second protein called " IGF-2 ". IGF-2 also binds the IGF-1 Receptor. However, IGF-2 alone binds a receptor called the "IGF II Receptor" (also called the Mannose-6 phosphate receptor). The insulin growth factor-II receptor (IGF2R) lacks signal transduction capacity, and its main role is to act as a sink for IGF-2 and make less IGF-2 available for binding with IGF-1R. As the name "insulin-like growth factor 1" implies, IGF-1 is structurally related to insulin, and is even capable of binding the insulin receptor, albeit at lower affinity than insulin.
A woman in Japan has had her diabetes reversed by a transplant of insulin-producing cells from her mother. The procedure has given strikingly fast results and marks a departure from previous operations, which relied on cadaver organs as a source of the cells. The first successful transplantation of such cells, called islet cells, from the pancreas of a non-living donor to a diabetic patient was performed in 2000. Since then, about 100 people have had their diabetic condition reversed by the procedure. But waiting for a suitable donor can be a problem, particularly in countries such as Japan, where traditional beliefs against removal of organs from the deceased means that donors are in short supply. For this reason, a team led by Shinichi Matsumoto of Kyoto University Hospital decided to investigate the possibility of extracting islet cells from a live donor. Their first patient was a 27-year-old woman who had become dependent on daily insulin shots after suffering inflammation of the pancreas at a young age. Her 56-year-old mother was the donor. Delicate procedure In a day-long operation, the team transplanted about 10mL of tissue from the pancreas of the mother to the daughter. The procedure was a tricky one, since islet cells are notoriously delicate. "It's difficult to extract them and keep them healthy," explains islet transplantation expert Stephanie Amiel of King's College London, UK. She adds that the cells sometimes form clots after the operation. Matsumoto says that transplants taken from live donors make for healthier cells. Both mother and daughter fared well, he says, and 22 days after the surgery, the young woman no longer needed insulin injections to regulate her blood sugar. "From our experience, this patient has more than double the blood insulin level compared with patients who received one cadaveric islet transplantation," says Matsumoto. The researchers say that people who receive cells from non-living donors tend to become insulin independent only after two or three such procedures. ADVERTISEMENT Amiel says that the daughter's speedy recovery from the recent operation is remarkable given that she only received islets from a portion of the pancreatic tissue; most procedures involve transplanting cells from the entire organ. But Amiel also adds that because the surgery took place in January, the long-term benefits are unclear. "These are quite early days," he says. Matsumoto says he plans to conduct a further 10 such operations this year.
There is promising news today for those who hope to turn the potential of undifferentiated stem cells into medical miracles: Researchers are reporting a way to produce insulin-producing cells from mouse embryonic stem cells. Millions of diabetes patients could benefit if researchers can achieve such alchemy with human cells. ILLUSTRATION: CAMERON SLAYDEN Doctors have reported promising results in transplanting pancreatic cells from cadavers into diabetic patients, enabling a handful of recipients to stop insulin injections indefinitely. But the demand for cells is far greater than the supply, and an unlimited source of cells that could produce insulin would be a hot commodity. So far, success at growing such cells from stem cells has been limited. Ron McKay and his colleagues at the National Institute of Neurological Disease and Stroke in Bethesda, Maryland, usually focus on brain development, but they were intrigued by recent papers reporting that some pancreas cells express nestin, a protein typical of developing neural cells. The scientists already knew how to encourage mouse embryonic stem cells to express nestin, and they wondered if they could coax their nestin-positive cells to take on more characteristics of pancreas cells. When they briefly exposed nestin-positive cells to a growth factor, the cells differentiated not only into neural cells but also into clusters that resemble the insulin-producing islets in the pancreas. The clusters' inner cells produced insulin, while outer cells produced glucagon and somatostatin, two proteins typical of pancreas cells, the team reports in a paper published online today by Science . "The percentage of cells that become insulin positive is remarkable and way above what others have reported," says developmental biologist Palle Serup, who studies pancreas development at the Hagedorn Research Institute in Gentofte, Denmark. Yet important caveats remain. The clusters produce only about 2% as much insulin as normal islets and failed to make insulin in response to a glucose level that typically triggers a response in normal cells. That does not discourage researchers like molecular biologist Ken Zaret of the Fox Chase Cancer Center in Philadelphia. "The glass is 1/50th full," says Zaret, who predicts that refinements in the culture technique or drug manipulations will boost insulin production. Stem Cell Letdown in Pancreas Insulin-producing cells in the pancreas of adult mice apparently don't develop from stem cells, an experiment has shown. Instead, they derive from the reproduction of existing cells--the kind that are destroyed in type I diabetes. The find, published in this week's issue of Nature , suggests that if scientists can find ways to boost the proliferation of these cells it might be useful for treating type I diabetes. In type I diabetes, a misdirected immune system apparently attacks and kills pancreatic islet cells, called B cells. These cells respond to glucose levels in the blood by producing insulin. When they die, patients must inject themselves with insulin. Preliminary studies have suggested that transplanting donor B cells into patients can free recipients of the need to inject insulin. But the supply of transplantable cells is limited; each transplant requires cells from several cadavers. Scientists hoping to find a plentiful source for B cells have been searching for pancreatic stem cells. In an effort to pin down the source of new B cells in the body, Douglas Melton of Harvard University and his colleagues designed transgenic mice in which insulin-producing cells could be prompted to produce a protein, called HPAP, that is detectable with a blue stain. When the mice were 6 to 8 weeks old, the team turned on the HPAP gene. (This labeled about a third of the insulin-producing B cells.) Once the HPAP gene is turned on, B cells will pass the gene on to any daughter cells. But if new B cells instead come from stem cells, which presumably don't make insulin, they should not be labeled by the stain, the team reasoned. The scientists allowed some of the mice to live up to 12 months--midlife for a mouse--before sacrificing them and examining the pancreas. If stem cells had been active, they would have produced unstained B cells, upping their abundance relative to the stained cells. But instead, the percentage of blue stained cells was higher in the year-old mice than in the 6-week-old mice. This suggests that B cells replicate themselves, Melton's team says, and that the pancreas is unlikely to harbor stem cells that produce large numbers of new B cells. "The experiment is an elegant demonstration that B cells can themselves proliferate," says Ronald McKay of National Institute of Neurological Disorders and Stroke in Bethesda, Maryland. The trick now is finding the factors that regulate that proliferation of B cells so scientists might be able to grow the cells in culture, he says.
