The biochemical dynamics of the glycogen phosphatase laforin directly impact brain metabolism

TL;DR

This study explores how the glycogen phosphatase laforin affects brain metabolism and its role in a fatal childhood disease.

Contribution

The study reveals that laforin's phosphatase activity is crucial for normal brain metabolism and disease prevention.

Findings

LCS protein shows higher phosphate and glucan binding compared to wild-type laforin.

LCS mice exhibit distinct and significant global metabolic perturbations in the brain.

Altered laforin dynamics lead to aberrant protein retention in the brain of LCS knock-in mice.

Abstract

Laforin is the only known glycogen phosphatase. Mutations in the laforin gene lead to the fatal childhood dementia and progressive myoclonic epilepsy known as Lafora disease (LD). A hallmark of LD is aberrant, cytoplasmic, glycogen-like aggregates known as Lafora bodies. Surprisingly, recent reports indicate that overexpression of a phosphatase-deficient laforin mutant, with the catalytic cysteine mutated to serine (LCS), prevented the formation of Lafora bodies in a laforin KO mouse model. This finding led to questions regarding the biological relevance of laforin phosphatase activity and its role in LD etiology. In this study, we defined the in vitro and in vivo effects of the LCS mutation. LCS protein lacks catalytic activity but exhibits significantly higher binding to phosphate and long glucan chains compared with WT laforin. In addition, LCS exhibits altered dynamics via hydrogen–deuterium exchange mass spectrometry and interacts more robustly with its binding partners malin and protein targeting to glycogen. We demonstrate that these altered dynamics result in aberrant retention of the LCS protein in the brain of the LCS knock-in mouse model, compared with laforin levels in WT mice. To examine the metabolic consequences of these biophysical changes, we compared the brain metabolomic phenotypes of LCS mice to WT and laforin KO mice. Furthermore, LCS mice display a distinct and significant global perturbation in metabolism. These results indicate a key signaling role for glycogen phosphorylation in glycogen metabolism, revealing an important biological role for laforin catalytic phosphatase activity.

Figures