Wolfram syndrome: structural and functional analyses of mutant and wild-type wolframin, the WFS1 gene product

Abstract

Mutations of the WFS1 gene are responsible for Wolfram syndrome, a rare, recessive disorder characterized by early-onset, non-autoimmune diabetes mellitus, optic atrophy and further neurological and endocrinological abnormalities. The WFS1 gene encodes wolframin, a putative multispanning membrane glycoprotein of the endoplasmic reticulum. The function of wolframin is completely unknown. In order to characterize wolframin, we have generated polyclonal antibodies against both hydrophilic termini of the protein. Wolframin was found to be ubiquitously expressed with highest levels in brain, pancreas, heart and insulinoma beta-cell lines. Analysis of the structural features provides experimental evidence that wolframin contains nine transmembrane segments and is embedded in the membrane in an N(cyt)/C(lum) topology. Wolframin assembles into higher molecular weight complexes of approximately 400 kDa in the membrane. Pulse-chase experiments demonstrate that during maturation wolframin is N-glycosylated but lacks proteolytical processing. Moreover, N-glycosylation appears to be essential for the biogenesis and stability of wolframin. Here we investigate, for the first time, the molecular mechanisms that cause loss-of-function of wolframin in affected individuals. In patients harboring nonsense mutations complete absence of the mutated wolframin is caused by instability and rapid decay of WFS1 nonsense transcripts. In a patient carrying a compound heterozygous missense mutation, R629W, we found markedly reduced steady-state levels of wolframin. Pulse-chase experiments of mutant wolframin expressed in COS-7 cells indicated that the R629W mutation leads to instability and strongly reduced half-life of wolframin. Thus, the Wolfram syndrome in patients investigated here is caused by reduced protein dosage rather than dysfunction of the mutant wolframin.

Increased insulin demand promotes while pioglitazone prevents pancreatic beta cell apoptosis in Wfs1 knockout mice

“…Beta cells are selectively lost from the pancreatic islets and this loss is genetically programmed [10]. Our group and others previously identified the Wolfram syndrome gene, designating it WFS1 [11] or wolframin [12] and showing that it is localised primarily in the ER [13,14]. Homozygous Wfs1 knockout mice developed glucose intolerance.…”

Wfs1 gene deletion causes growth retardation in mice and interferes with the growth hormone pathway

“…It is membrane bound (9 -11 transmembrane segments) and located in endoplasmic reticulum (ER) (8). There is evidence that this protein play a role in the regulation of ER Ca 2ϩ levels (18,29).…”

Impact of polymorphisms in WFS1 on prediabetic phenotypes in a population-based sample of middle-aged people with normal and abnormal glucose regulation

“…WFS1 encodes an 890 amino acid membrane glycoprotein located in the endoplasmic reticulum (ER). It is ubiquitously expressed in many organs including the brain and pancreas, with weak signals detected in liver, skeletal muscle and kidney [4,5]. Mutations in WFS1 are known to cause the Wolfram syndrome, which is an autosomal recessive disorder clinically defined by diabetes insipidus, non-autoimmune diabetes mellitus with juvenile onset, optic atrophy and deafness [5,6].…”