Glucagon Receptor Antagonist-Stimulated α-Cell Proliferation Is Severely Restricted With Advanced Age.

Glucagon-containing α-cells potently regulate glucose homeostasis, but the developmental biology of α-cells in adults remains poorly understood. Although glucagon receptor antagonists (GRAs) have great potential as antidiabetic therapies, murine and human studies have raised concerns that GRAs might cause uncontrolled α-cell growth. Surprisingly, previous rodent GRA studies were only performed in young mice, implying that the potential impact of GRAs to drive α-cell expansion in adult patients is unclear. We assessed adaptive α-cell turnover and adaptive proliferation, administering a novel GRA (JNJ-46207382) to both young and aged mice. Basal α-cell proliferation rapidly declined soon after birth and continued to drop to very low levels in aged mice. GRA drove a 2.4-fold increase in α-cell proliferation in young mice. In contrast, GRA-induced α-cell proliferation was severely reduced in aged mice, although still present at 3.2-fold the very low basal rate of aged controls. To interrogate the lineage of GRA-induced α-cells, we sequentially administered thymidine analogs and quantified their incorporation into α-cells. Similar to previous studies of ß-cells, α-cells only divided once in both basal and stimulated conditions. Lack of contribution from highly proliferative "transit-amplifying" cells supports a model whereby α-cells expand by self-renewal and not via specialized progenitors.

Incretin Therapies Do Not Expand ß-Cell Mass or Alter Pancreatic Histology in Young Male Mice.

The impact of incretins upon pancreatic ß-cell expansion remains extremely controversial. Multiple studies indicate that incretin-based therapies can increase ß-cell proliferation, and incretins have been hypothesized to expand ß-cell mass. However, disagreement exists on whether incretins increase ß-cell mass. Moreover, some reports indicate that incretins may cause metaplastic changes in pancreatic histology. To resolve these questions, we treated a large cohort of mice with incretin-based therapies and carried out a rigorous analysis of ß-cell turnover and pancreatic histology using high-throughput imaging. Young mice received exenatide via osmotic pump, des-fluoro-sitagliptin, or glipizide compounded in diet for 2 weeks (short-term) on a low-fat diet (LFD) or 4.5 months (long-term) on a LFD or high-fat diet (HFD). Pancreata were quantified for ß-cell turnover and mass. Slides were examined for gross anatomical and microscopic changes to exocrine pancreas. Short-term des-fluoro-sitagliptin increased serum insulin and induced modest ß-cell proliferation but no change in ß-cell mass. Long-term incretin therapy in HFD-fed mice resulted in reduced weight gain, improved glucose homeostasis, and abrogated ß-cell mass expansion. No evidence for rapidly dividing progenitor cells was found in islets or pancreatic parenchyma, indicating that incretins do not induce islet neogenesis or pancreatic metaplasia. Contrasting prior reports, we found no evidence of ß-cell mass expansion after acute or chronic incretin therapy. Chronic incretin administration was not associated with histological abnormalities in pancreatic parenchyma; mice did not develop tumors, pancreatitis, or ductal hyperplasia. We conclude that incretin therapies do not generate ß-cells or alter pancreatic histology in young mice.

Interactions between the Coxiella burnetii parasitophorous vacuole and the endoplasmic reticulum involve the host protein ORP1L.

Coxiella burnetii is a gram-negative intracellular bacterium that forms a large, lysosome-like parasitophorous vacuole (PV) essential for bacterial replication. Host membrane lipids are critical for the formation and maintenance of this intracellular niche, yet the mechanisms by which Coxiella manipulates host cell lipid metabolism, trafficking and signalling are unknown. Oxysterol-binding protein-related protein 1 long (ORP1L) is a mammalian lipid-binding protein that plays a dual role in cholesterol-dependent endocytic trafficking as well as interactions between endosomes and the endoplasmic reticulum (ER). We found that ORP1L localized to the Coxiella PV within 12 h of infection through a process requiring the Coxiella Dot/Icm Type 4B secretion system, which secretes effector proteins into the host cell cytoplasm where they manipulate trafficking and signalling pathways. The ORP1L N-terminal ankyrin repeats were necessary and sufficient for PV localization, indicating that ORP1L binds a PV membrane protein. Strikingly, ORP1L simultaneously co-localized with the PV and ER, and electron microscopy revealed membrane contact sites between the PV and ER membranes. In ORP1L-depleted cells, PVs were significantly smaller than PVs from control cells. These data suggest that ORP1L is specifically recruited by the bacteria to the Coxiella PV, where it influences PV membrane dynamics and interactions with the ER.