Human Stem Cells Elucidate the Mechanisms of Beta-Cell Failure in Diabetes


Wolfram syndrome is a rare form of diabetes characterized by high blood sugar levels that result from insufficient levels of the hormone insulin.  The chronically high blood sugar levels cause degeneration of the optic nerve, leading to progressive vision loss (optic atrophy).  Wolfram syndrome patients often also have abnormal pituitary glands that release abnormally low levels of the hormone vasopressin (also known as antidiuretic hormone or ADH), which causes hearing loss caused by changes in the inner ear (sensorineural deafness), urinary tract problems, reduced amounts of the sex hormone testosterone in males (hypogonadism), or neurological or psychiatric disorders.

Diabetes mellitus is typically the first symptom of Wolfram syndrome, usually diagnosed around age 6. Nearly everyone with Wolfram syndrome who develops diabetes mellitus requires insulin replacement therapy. Optic atrophy is often the next symptom to appear, usually around age 11. The first signs of optic atrophy are loss of color vision and peripheral (side) vision. Over time, the vision problems get worse, and people with optic atrophy are usually blind within approximately 8 years after signs of optic atrophy first begin.

Mutations in the WFS1 gene cause more than 90 percent of the cases of Wolfram syndrome type 1.  The WFS1 gene encodes a protein called wolframin that regulates the amount of calcium in cells.  A proper calcium balance is important for a whole host of cellular processes, including cell-to-cell communication, the tensing (contraction) of muscles, and protein processing.  Wolframin protein is found in many different tissues, such as the pancreas, brain, heart, bones, muscles, lung, liver, and kidneys.  Inside cells, wolframin is in the membrane of a cell structure called the endoplasmic reticulum that is involved in protein production, processing, and transport. Wolframin is particularly important in the pancreas, where it helps process proinsulin into mature hormone insulin, the hormone that helps control blood sugar levels.

WFS1 gene mutations lead to the production of a sub-functional versions of wolframin.  As a result, calcium levels within cells are not properly regulated and the endoplasmic reticulum does not work correctly.  When the endoplasmic reticulum does not have enough functional wolframin, the cell triggers its own cell death (apoptosis).  In the pancreas, the cells that make insulin (beta cells) die off, which causes diabetes mellitus.  The gradual loss of cells along the optic nerve eventually leads to blindness, and the death of cells in other body systems likely causes the various signs and symptoms of Wolfram syndrome type 1.

A certain mutation in the CISD2 gene also causes Wolfram syndrome type 2. The CISD2 gene provides instructions for making a protein that is in the outer membrane of cell structures called mitochondria,the energy-producing centers of cells.  Even though the function of the CISD2 protein is unknown, CISD2 mutations produce nonfunctional CISD2 protein that causes mitochondria to eventually break down. This accelerates the onset of cell death.  Cells with high energy demands such as nerve cells in the brain, eye, or gastrointestinal tract are most susceptible to cell death due to reduced energy, and people with mutations in the CISD2 gene have ulcers and bleeding problems in addition to the usual Wolfram syndrome features.

Some people with Wolfram syndrome do not have an identified mutation in either the WFS1 or CISD2 gene. The cause of the condition in these individuals is unknown.

Now that you have a proper introduction to Wolfram syndrome, scientists from the New York Stem Cell Foundation and Columbia University Medical Center have produce induced pluripotent stem cells (iPSCs) from skin samples provided by Wolfram syndrome patients.  All of the patients who volunteered for this study were recruited from the Naomi Berrie Diabetes Center and had childhood onset diabetes and required treatment with injected insulin, and all had vision loss.  Control cell lines that did not have mutations in WFS1 were obtained from Coriell Research for Medical Research.

These skin samples contained cells known as fibroblasts and these were reprogrammed into induced pluripotent stem cells.  In order to show that these cells were truly iPSCs, this group implanted them underneath the kidney capsule of immuno-compromised mice, and they formed the teratoma tumors so characteristic of these cells.

When these iPSCs were differentiated into insulin-secreting pancreatic beta cells, Linshan Shang and her colleagues discovered that the beta cells made from cells that did not come from Wolfram syndrome patients secreted normal levels of insulin.  However, those beta cells made from iPSCs derived from Wolfram patients failed to secrete normal quantities of insulin either in culture or when transplanted into the bodies of laboratory animals.  Further investigations of these cells showed these beta cells showed elevated levels of stress in the endoplasmic reticulum as a result of an accumulation of unfolded proteins.

What on earth is endoplasmic reticulum protein-folding stress?  First some cell biology.  When the cell needs to make a protein that will be secreted, embedded in a membrane or vesicle. that protein begins its life on ribosomes (protein synthesis factories of the cell) in the cytoplasm, but later those ribosomes are dragged to a cellular structure called the endoplasmic reticulum.  While on the surface of the endoplasmic reticulum, the ribosome completes the synthesis of the protein and extrudes the protein into the interior of the endoplasmic reticulum or embeds the protein into the endoplasmic reticulum membrane.  From there, the protein is trafficked in a vesicle to another subcellular structure called the Golgi apparatus, were it undergoes further modification, and from the Golgi apparatus, the protein goes to the membrane, secretory vesicle or other places.

If the proteins in the endoplasmic reticulum cannot fold properly, they clump and build up inside the endoplasmic reticulum, and this induces the ERAD or Endoplasmic Reticulum-Associated Protein Degradation response.  The players in the ERAD response are shown below.  As you can see, this response is rather complicated, but if it fails to properly clear the morass of unfolded proteins in the endoplasmic reticulum, then the cell will undergo programmed cell death.

ERAD Response

However, this research team did not stop there.  When they treated the cultured beta cells made from cells taken from Wolfram syndrome patients with a chemical called 4-phenyl butyric acid, the stress on the cells was relieved and the cells survived.  This experiment shows that relieving this unfolded protein stress is a potential target for clinical intervention.

“These cells represent an important mechanism that causes beta-cell failure in diabetes.  This human iPS cell model represents a significant step forward in enabling the study of this debilitating disease and the development of new treatments,” said Dieter Egli, the principal investigator of the study, and senior research fellow at the New York Stem Cell Foundation.

Because all forms of diabetes mellitus ultimately result from an inability of the pancreatic beta cells to provide sufficient quantities of insulin in response to a rise in blood sugar concentrations, this Wolfram patient stem cell model enables an analysis of a more specific pathway that leads to beta-cell failure in more prevalent forms of diabetes.  Furthermore, this strategy enables the testing of strategies to restore beta-cell function that may be applicable to all types of diabetes.

Susan L. Solomon of the New York Stem Cell Foundation, said, “Using stem cell technology, we were able to study a devastating condition to better understand what causes the diabetes syndromes as well as discover possible new drug targets.”

Rudolph L. Leibel, a professor of diabetes research and co-author of this study, said, “This report highlights again the utility of close examination of rare disorders as a path to elucidating more common ones.  Our ability to create functional insulin-producing cells using stem cell techniques on skin cells from patients with Wolfram’s syndrome has helped to uncover the role of ER stress in the pathogenesis of diabetes.  The use of drugs that reduce such stress may prove useful in the prevention and treatment of diabetes.”

The ERAD response seems to play a role in the survival of insulin-producing beta cells in both type 1 and type 2 diabetes.  The ERAD response opposes the stress of the immune assault in type 1 diabetes and the metabolic stress of high blood glucose levels in both types of diabetes.  When the ERAD response fails, cell death ensues and this reduces the number of insulin-producing cells.

Advertisements

Published by

mburatov

Professor of Biochemistry at Spring Arbor University (SAU) in Spring Arbor, MI. Have been at SAU since 1999. Author of The Stem Cell Epistles. Before that I was a postdoctoral research fellow at the University of Pennsylvania in Philadelphia, PA (1997-1999), and Sussex University, Falmer, UK (1994-1997). I studied Cell and Developmental Biology at UC Irvine (PhD 1994), and Microbiology at UC Davis (MA 1986, BS 1984).

2 thoughts on “Human Stem Cells Elucidate the Mechanisms of Beta-Cell Failure in Diabetes”

Comments are closed.