Bowel dysfunction in Wolfram syndrome

MCT1-dependent energetic failure and neuroinflammation underlie optic nerve degeneration in Wolfram syndrome mice

Wolfram syndrome 1 (WS1) is a rare genetic disorder caused by mutations in the WFS1 gene leading to a wide spectrum of clinical dysfunctions, among which blindness, diabetes, and neurological deficits are the most prominent. WFS1 encodes for the endoplasmic reticulum (ER) resident transmembrane protein wolframin with multiple functions in ER processes. However, the WFS1-dependent etiopathology in retinal cells is unknown. Herein, we showed that Wfs1 mutant mice developed early retinal electrophysiological impairments followed by marked visual loss. Interestingly, axons and myelin disruption in the optic nerve preceded the degeneration of the retinal ganglion cell bodies in the retina. Transcriptomics at pre-degenerative stage revealed the STAT3-dependent activation of proinflammatory glial markers with reduction of the homeostatic and pro-survival factors glutamine synthetase and BDNF. Furthermore, label-free comparative proteomics identified a significant reduction of the monocarboxylate transport isoform 1 (MCT1) and its partner basigin that are highly enriched on retinal glia and myelin-forming oligodendrocytes in optic nerve together with wolframin. Loss of MCT1 caused a failure in lactate transfer from glial to neuronal cell bodies and axons leading to a chronic hypometabolic state. Thus, this bioenergetic impairment is occurring concurrently both within the axonal regions and cell bodies of the retinal ganglion cells, selectively endangering their survival while impacting less on other retinal cells. This metabolic dysfunction occurs months before the frank RGC degeneration suggesting an extended time-window for intervening with new therapeutic strategies focused on boosting retinal and optic nerve bioenergetics in WS1.

Wolfram syndrome 1 and Wolfram syndrome 2

PURPOSE OF REVIEW

Wolfram syndrome 1 (WS1) is an autosomal recessive disorder characterized by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness (DI DM OA D syndrome) associated with other variable clinical manifestations. The causative gene for WS1 (WFS1) encoding wolframin maps to chromosome 4p16.1. Wolframin has an important function in maintaining the homeostasis of the endoplasmic reticulum (ER) in pancreatic β cells. Recently, another causative gene, CISD2, has been identified in patients with a type of Wolfram syndrome (WS2) resulting in early optic atrophy, diabetes mellitus, deafness, decreased lifespan, but not diabetes insipidus. The CISD2-encoded protein ERIS (endoplasmic reticulum intermembrane small protein) also localizes to ER, but does not interact directly with wolframin. ERIS maps to chromosome 4q22.

RECENT FINDINGS

Numerous studies have shown an interesting similarity between WFS1 and CISD2 genes. Experimental studies demonstrated that the Cisd2 knockout (Cisd2) mouse shows premature aging and typical symptoms of Wolfram syndrome. These researches provide interesting insight into the relation of neurodegenerative diseases, mitochondrial disorders, and autophagy and are useful for the pathophysiological understanding of both Wolfram syndrome and mitochondrial-mediated premature aging.

SUMMARY

The knowledge of WS1 and WS2 pathogenesis, and of the interactions between WFS1 and CISD2 genes, is useful for accurate diagnostic classification and for diagnosis of presymptomatic individuals.