Pathogenesis of Lafora Disease: Transition of Soluble Glycogen to Insoluble Polyglucosan

Abstract

Lafora disease (LD, OMIM #254780) is a rare, recessively inherited neurodegenerative disease with adolescent onset, resulting in progressive myoclonus epilepsy which is fatal usually within ten years of symptom onset. The disease is caused by loss-of-function mutations in either of the two genes EPM2A (laforin) or EPM2B (malin). It characteristically involves the accumulation of insoluble glycogen-derived particles, named Lafora bodies (LBs), which are considered neurotoxic and causative of the disease. The pathogenesis of LD is therefore centred on the question of how insoluble LBs emerge from soluble glycogen. Recent data clearly show that an abnormal glycogen chain length distribution, but neither hyperphosphorylation nor impairment of general autophagy, strictly correlates with glycogen accumulation and the presence of LBs. This review summarizes results obtained with patients, mouse models, and cell lines and consolidates apparent paradoxes in the LD literature. Based on the growing body of evidence, it proposes that LD is predominantly caused by an impairment in chain-length regulation affecting only a small proportion of the cellular glycogen. A better grasp of LD pathogenesis will further develop our understanding of glycogen metabolism and structure. It will also facilitate the development of clinical interventions that appropriately target the underlying cause of LD.

Keywords: chain length distribution; glycogen phosphorylation; lafora disease; laforin; malin; polyglucosan body.

Figure 1

Lafora disease (LD) in the context of glycogen metabolism. Synthesis of glycogen molecules in the cytosol (top row) essentially includes autoglucosylation of glycogenin, elongation of chains by glycogen synthase (GS), and introduction of branching points by glycogen branching enzyme (GBE). The regularly branched and water-soluble glycogen molecules are degraded by glycogen phosphorylase (GP) and glycogen debranching enzyme (AGL, indirect debranching) in the cytosol or by lysosomal enzymes such as acid α-glucosidase (GAA). Functional laforin and malin essentially inhibit the formation of Lafora bodies and prevent LD as indicated by the inhibitory arrow. Mutations leading to non-functional laforin or malin (protein domains explained in the text) result in formation and accumulation of Lafora bodies as exemplarily shown in brain sections of the laforin-deficient mouse model (bottom right, scale bar equals 100 µm). In humans, LD manifests in an eventually fatal progression of clinical symptoms (bottom left).

Figure 2

Prevention of LBs depends on the presence of malin but not on laforin’s phosphatase activity. The phenotypic features (C6 phosphate and chain length distribution, CLD) of brain glycogen as well as LB occurrence in the hippocampus are summarized for different LD-related mouse lines. Overexpression of phosphatase inactive laforin (C266S mutation in the DSP domain) leads to glycogen hyperphosphorylation but prevents both CLD abnormality and LB accumulation provided malin is functional. Hyperphosphorylation per se does not cause LB formation and accumulation.

Figure 3

Branching structure of amylopectin and glycogen. (A) Chemical basis of a branched α-glucan. R1, glucan chain at the reducing end; R2 and R3, chains at the nonreducing ends; (B,C) Segments of glycogen and amylopectin, respectively. In glycogen, branching points are essentially distributed evenly; in amylopectin, they are clustered. In areas with a low abundance of branching points, double-helices form. Modified version from [20].

Figure 4

Role of phosphate esters during degradation of plant amylopectin. Degradation of starch granules in higher plants is facilitated by cycles of phosphorylation and dephosphorylation. Tightly packed, semi-crystalline exterior glucan chains of amylopectin undergo phase transition (specified in the text) through incorporation of phosphate esters by glucan water dikinase (GWD) and phosphoglucan water dikinase (PWD), both utilizing ATP. Exo-acting β-amylases (BAM) remove maltose units from the non-reducing ends, their action being obstructed by phosphate esters and branching points. The glucan phosphatases SEX4 (starch-excess 4) and LSF2 (Like-SEX4 2) remove phosphate esters and enable further BAM-mediated degradation. Isoamylase (ISA) is required for direct debranching in the amorphous layer of amylopectin before GWD-mediated phosphorylation initiates degradation of the subsequent semi-crystalline layer. Figure modified from [47,78].

Figure 5

Mechanistic model for Lafora body formation and accumulation. (A) Glycogen as a disperse mixture of molecules that possess different contents of long glucan chains and according risks of precipitation. The minor proportion of molecules (blue circle) actually subject to precipitation (grey arrow) may be modified by local reduction of chain elongation that is achieved by a well-supported mechanism explained in (C). This modification increases the relative branching frequency and decreases the risk to precipitate; (B) If glycogen precipitation is not (entirely) avoided, precipitated glycogen is accumulating over time (y-axis) as seen in Lafora disease, unless cells are able to mobilize and remove precipitated glycogen (e.g., hypothetically (question mark) through autophagy) at a rate that is higher than that of precipitation; (C) The mechanism by which a functional laforin-malin complex can reduce GS activity as supported by experimental data obtained in cell-culture [30,58]. Negative effects are indicated by inhibitory arrows while normal arrows indicate a positive effect. By targeting GS and PTG to degradation, the laforin-malin complex reduces GS activity and relatively increases branching frequency. It is proposed that this process may not significantly affect the entire GS population but is occurring locally, i.e., predominantly at a minor proportion of glycogen molecules with a high risk of precipitation (blue circle in (A)).