Proinsulin maturation, misfolding, and proteotoxicity.
As a tool to explore proinsulin (PI) trafficking, a human PI cDNA has been constructed with GFP fused within the C peptide. In regulated secretory cells containing appropriate prohormone convertases, the hProCpepGFP construct undergoes endoproteolytic processing to CpepGFP and native human insulin, which are specifically detected and cosecreted in parallel with endogenous insulin. Expression of C(A7)Y mutant PI results in autosomal dominant diabetes in Akita mice. We directly identify the misfolded PI in Akita islets and also show that C(A7)Y mutant PI, either in the context of the hProCpepGFP chimera or not, engages directly in protein complexes with nonmutant PI, impairing the trafficking and recovery of nonmutant PI. This trapping mechanism decreases insulin production in beta cells. Thereafter we observe a loss of beta cell viability. The data imply that PI misfolding leading to impaired endoplasmic reticulum exit of nonmutant PI may be a key early step in a chain reaction of beta cell dysfunction and demise leading to onset and progression of diabetes.
Liu
Ming
M
Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical Center, Ann Arbor, MI 48109, USA.
Hodish
Israel
I
Rhodes
Christopher J
CJ
Arvan
Peter
P
D002460:Cell Line; D018076:DNA, Complementary / Q000235:genetics; D017930:Genes, Reporter; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007668:Kidney; D050506:Mutant Chimeric Proteins / Q000378:metabolism; D011384:Proinsulin / Q000235:genetics / Q000378:metabolism* / Q000633:toxicity*; D017510:Protein Folding; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism
D018076:DNA, Complementary; D050506:Mutant Chimeric Proteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D011384:Proinsulin