Beta Cell Mates: (left to right) symposium chair Sushil Rane, NIDDK; Jurgen Wess, NIDDK; Marvin Gershengorn, NIDDK; Josephine Egan, NIA; and David Harlan, NIDDK

For some, the quest is to increase the progenitor pool of pancreatic beta cells, to derive stem cells that can be controlled in culture and serve as replacements for damaged or lost beta cells; for others, the quest is for new treatments.

Stem Cell Studies

Typically, cultured human beta cells do not proliferate well or retain the mature phenotype, noted Marvin Gershengorn, chief of the Clinical Endocrinology Branch and scientific director, NIDDK, who has been exploring the optimization of hIPCs (human islet cell–derived precursor cells) for about five years.

Gershengorn’s lab has established that hIPCs are a special type of mesenchymal stromal cells that can be induced to differentiate into adipocytes, chondrocytes, and osteocytes, as well as cells of the endocrine pancreas. "We can change the culture conditions that result in epithelial or endocrine-like cell clusters; we can upregulate the insulin transcript level, generating clusters of C peptide–expressing cells," he said, noting that his team has transplanted these cells into mice and demonstrated in vivo hIPC functionality.

The lab of Sushil Rane, of the Regenerative Biology Section in the NIDDK Diabetes Branch, has been examining the role of the cell-cycle regulator CDK4 in beta cell regeneration.

Hyperglycemic and hypoinsulin-emic, CDK4-knockout mice have clearly lost beta cell mass, Rane said; conversely, a knocked-in point mutation yields "huge islets and increased beta cell mass."

The knock-in cohort demonstrates increased regeneration potential via differentiation of stem cell–like progenitors in the pancreatic duct, increased beta cell proliferation, and accompanying functionality reflected in superior glucose tolerance, insulin secretion in response to glucose, and protection against streptozotocin-induced diabetes.

The team intends to explore the regenerative potential of other cell-cycle proteins as well, Rane said.

Rough Measures

Accurate measurement of pancreatic beta cell mass, a key indicator of beta cell function that could elucidate diabetes progression and assess response to treatment, is in a "sorry state," said Dave Harlan, chief of the Diabetes Branch, NIDDK. He apologized for a talk that would highlight questions eluding answers and somewhat disappointing research results.

PET imaging using dihydrotetrabenzene (DTBZ), the radioactive ligand for the vesicular monoamine transporter-2 found in pancreatic beta cells, has been getting a lot of attention as a potential noninvasive method to measure beta cell mass, Harlan said.

He noted that PET images obtained from pancreas-transplant recipients reveal a bright signal from the transplanted organ, while the individual’s native pancreas sends out a low signal; even so, results from individuals with long-standing type 1 diabetes have been inconsistent–as have results in monkey studies. For instance, autopsy findings show that the ligand binding is not specific in monkey pancreatic beta cells.

"We conclude that DTBZ PET scan signals do not accurately measure pancreatic beta mass," Harlan said, adding that though beta cell regeneration may occur in vivo—at least in mice—improved function has not yet been shown in humans.

Pathways to Treatment

Introducing her research into islet biology to discern new approaches to treat type 2 diabetes, Josephine Egan observed that the workings of the pancreatic islet cells are only inferred from changes in hormone levels in response to stimulants.

The disordered responses that trumpet diabetes onset provide clues to treatment strategies, said Egan, senior investigator and chief of the Diabetes Section of the Laboratory of Clinical Investigation, NIA.

These derangements include decreased insulin secretion; the loss of the pulsatility of insulin release, a reflection of beta cell dysfunction; blunted response of the gut protein glucose-dependent insulinotropic polypeptide (GIP); increased glucagon secretion; and increased pancreatic polypeptide secretion.

Among agents Egan has been exploring is GLP-1, another gut hormone, which, unlike GIP, releases insulin and normalizes blood sugar in patients with diabetes and appears to correct imbalances more effectively than exogenous insulin.

Jürgen Wess and his colleagues have generated a series of mutant mouse models to better understand the role of M3 muscarinic acetylcholine receptors in beta cell function. The selective overexpression of M3 receptors in beta cells leads to enhanced insulin release and improved glucose tolerance, suggesting that "therapies aimed at enhancing this pathway might be useful in the treatment of type 2 diabetes," said Wess, chief of the Molecular Signaling Section, Laboratory of Bioorganic Chemistry, NIDDK.

The team has also created a series of mutant M3 muscarinic receptors that are unable to bind acetylcholine (the endogenous neurotransmitter) but can activate different classes of G proteins when treated with the pharmacologically inert compound clozapine-N-oxide (CNO). Wess discussed findings in transgenic mice that selectively express these receptors in their pancreatic beta cells.

CNO treatment of different transgenic mouse lines resulted in the selective activation of either Gq- or Gs-type G proteins in pancreatic beta cells. In both cass, acute CNO administration led to a significant increase in insuin release and improved glucose tolerance.

Key questions that remain to be addressed, Wess, said, are: What are the chronic effects of activating these different signaling pathways—or a mixture of both—on insulin synthesis and release and beta cell mass and survival, and how can these findings guide the development of novel therapeutic agents?