Endocrine cells rely on efficient prohormone synthesis, folding, and elimination of misfolded peptides for proper function. Endoplasmic reticulum-associated degradation (ERAD) and autophagy are two primary mechanisms of cellular protein quality control, and each are required for normal beta cell function. However, their role in alpha cells has largely been unexplored. ERAD is mediated by two highly conserved ER membrane proteins: Hrd1, a ubiquitin ligase that retrotranslocates and ubiquitinates misfolded ER proteins for proteasomal degradation, and Sel1L, an adaptor protein required for Hrd1 stability and function. Autophagy is a conserved mechanism for recycling ER and cytosolic products via lysosomal degradation and requires a precise signaling sequence for activation, involving key autophagosome protein Atg7. To investigate the role of ERAD and autophagy in alpha cells, we targeted deletion of Sel1L and/or Atg7 with the Cre-lox system, using mice expressing Cre recombinase under the proglucagon promoter. Sel1LΔGcg, Atg7ΔGcg, and Sel1L;Atg7ΔGcg mice all showed normal growth and oral glucose tolerance. Sel1LΔGcg mice had dilated ER and reduced glucagon content in alpha cells following ERAD inactivation, but glucagon secretory capacity in vivo was largely maintained. In contrast, disruption of autophagy in Atg7ΔGcg mice led to alpha cell hypertrophy and vacuolization, and reduced glucagon secretion in vivo following insulin-induced hypoglycemia and 2-deoxyglucose-mediated glucoprivation. These secretory defects were exacerbated in Sel1L;Atg7ΔGcg mice, which also had reduced glucagon secretion after a 24 hour fast and a 77% reduction in pancreatic glucagon content. By immunohistochemistry, very few alpha cells were present in islets of adult Sel1L;Atg7ΔGcg mice. Our data demonstrate that alpha cells are more susceptible to defective autophagy rather than defective Sel1L-Hrd1 ERAD, but that these pathways act synergistically to maintain functional alpha cell mass.
R.B. Reinert: None. X. Cui: None. N. Shrestha: None. L. Qi: None.
National Institutes of Health (T32DK007245)