Inefficient translocation of preproinsulin contributes to pancreatic β cell failure and late-onset diabetes.
Among the defects in the early events of insulin biosynthesis, proinsulin misfolding and endoplasmic reticulum (ER) stress have drawn increasing attention as causes of β cell failure. However, no studies have yet addressed potential defects at the cytosolic entry point of preproinsulin into the secretory pathway. Here, we provide the first evidence that inefficient translocation of preproinsulin (caused by loss of a positive charge in the n region of its signal sequence) contributes to β cell failure and diabetes. Specifically, we find that, after targeting to the ER membrane, preproinsulin signal peptide (SP) mutants associated with autosomal dominant late-onset diabetes fail to be fully translocated across the ER membrane. The newly synthesized, untranslocated preproinsulin remains strongly associated with the ER membrane, exposing its proinsulin moiety to the cytosol. Rather than accumulating in the ER and inducing ER stress, untranslocated preproinsulin accumulates in a juxtanuclear compartment distinct from the Golgi complex, induces the expression of heat shock protein 70 (HSP70), and promotes β cell death. Restoring an N-terminal positive charge to the mutant preproinsulin SP significantly improves the translocation defect. These findings not only reveal a novel molecular pathogenesis of β cell failure and diabetes but also provide the first evidence of the physiological and pathological significance of the SP n region positive charge of secretory proteins.
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105.
Huan
H
Guo
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105.
Yi
Y
Xiong
the Division of Organ Transplantation, University of Chicago, Chicago, Illinois 60637.
Piotr
P
Witkowski
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105, the Division of Metabolism, Tianjin Medical University General Hospital, Tianjin, China 300052.
Jingqing
J
Cui
the Division of Organ Transplantation, University of Chicago, Chicago, Illinois 60637.
Ling-jia
LJ
Wang
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105.
Jinhong
J
Sun
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105, the Department of Research in Biochemistry, National Institute of Respiratory Diseases "Ismael Cosío Villegas", Mexico City 14080, Mexico, and.
Roberto
R
Lara-Lemus
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105.
Leena
L
Haataja
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105.
Kathryn
K
Hutchison
the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 sshan@caltech.edu.
Shu-ou
SO
Shan
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105, parvan@umich.edu.
Peter
P
Arvan
From the Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105, the Division of Metabolism, Tianjin Medical University General Hospital, Tianjin, China 300052, mingli@umich.edu.
Ming
M
Liu
D000595:Amino Acid Sequence; D000818:Animals; D003920:Diabetes Mellitus / Q000378:metabolism* / Q000473:pathology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000737:chemistry / Q000378:metabolism*; D007515:Islets of Langerhans / Q000378:metabolism* / Q000473:pathology; D051379:Mice; D008969:Molecular Sequence Data; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D017386:Sequence Homology, Amino Acid
D007328:Insulin; D011498:Protein Precursors; C013737:preproinsulin
Cytosolic Protein Accumulation; Diabetes; Insulin Synthesis; Mutant; Preproinsulin; Proinsulin; Protein Translocation; β Cell