Lafora disease (LD) is a fatal, autosomal recessive neurodegenerative disorder that results in progressive myoclonus epilepsy. A hallmark of LD is the accumulation of insoluble, aberrant glycogen-like structures called Lafora bodies. LD is caused by mutations in the gene encoding the E3 ubiquitin ligase malin or the glucan phosphatase laforin. Although LD was first described in 1911, its symptoms are still lacking a consistent molecular explanation and consequently a cure is far from being achieved. Some data suggest that malin forms a functional complex with laforin. This complex promotes the ubiquitination of proteins involved in glycogen metabolism and misregulation of pathways involved in this process results in Lafora body formation. In addition, recent results obtained from both cell culture and LD mouse models have highlighted a role of the laforin-malin complex in the regulation of ER-stress and protein clearance pathways. These results suggest that LD should be considered as a novel member of the group of protein clearance diseases such as Parkinson’s, Huntington’s, or Alzheimer’s, in addition to being a glycogen metabolism disease. Herein, we review the latest results concerning the role of malin in LD and attempt to decipher its function.
Laforin; malin; glucan phosphatase; Lafora disease; Lafora bodies; glycogen; autophagy; ER stress; Ubiquitination; E3-ubiquitin ligase
The AMPKβ2 subunit can be modified by sumoylation carried out by the E3-SUMO ligase PIASy, which attaches SUMO2 but not SUMO1 moieties. This posttranslational modification is specific to AMPKβ2 and enhances the activity of the AMPK complex. Sumoylation of AMPKβ2 is antagonistic and competes with ubiquitination of the same subunit.
AMP-activated protein kinase (AMPK) is a sensor of cellular energy status. It is a heterotrimer composed of a catalytic α and two regulatory subunits (β and γ). AMPK activity is regulated allosterically by AMP and by the phosphorylation of residue Thr-172 within the catalytic domain of the AMPKα subunit by upstream kinases. We present evidence that the AMPKβ2 subunit may be posttranslationally modified by sumoylation. This process is carried out by the E3-small ubiquitin-like modifier (SUMO) ligase protein inhibitor of activated STAT PIASy, which modifies the AMPKβ2 subunit by the attachment of SUMO2 but not SUMO1 moieties. Of interest, AMPKβ1 is not a substrate for this modification. We also demonstrate that sumoylation of AMPKβ2 enhances the activity of the trimeric α2β2γ1 AMPK complex. In addition, our results indicate that sumoylation is antagonist and competes with the ubiquitination of the AMPKβ2 subunit. This adds a new layer of complexity to the regulation of the activity of the AMPK complex, since conditions that promote ubiquitination result in inactivation, whereas those that promote sumoylation result in the activation of the AMPK complex.
Laforin, encoded by a gene that is mutated in Lafora Disease (LD, OMIM 254780), is a modular protein composed of a carbohydrate-binding module and a dual-specificity phosphatase domain. Laforin is the founding member of the glucan-phosphatase family and regulates the levels of phosphate present in glycogen. Multiple reports have described the capability of laforin to form dimers, although the function of these dimers and their relationship with LD remains unclear. Recent evidence suggests that laforin dimerization depends on redox conditions, suggesting that disulfide bonds are involved in laforin dimerization. Using site-directed mutagenesis we constructed laforin mutants in which individual cysteine residues were replaced by serine and then tested the ability of each protein to dimerize using recombinant protein as well as a mammalian cell culture assay. Laforin-Cys329Ser was the only Cys/Ser mutant unable to form dimers in both assays. We also generated a laforin truncation lacking the last three amino acids, laforin-Cys329X, and this truncation also failed to dimerize. Interestingly, laforin-Cys329Ser and laforin-Cys329X were able to bind glucans, and maintained wild type phosphatase activity against both exogenous and biologically relevant substrates. Furthermore, laforin-Cys329Ser was fully capable of participating in the ubiquitination process driven by a laforin-malin complex. These results suggest that dimerization is not required for laforin phosphatase activity, glucan binding, or for the formation of a functional laforin-malin complex. Cumulatively, these results suggest that cysteine 329 is specifically involved in the dimerization process of laforin. Therefore, the C329S mutant constitutes a valuable tool to analyze the physiological implications of laforin’s oligomerization.
Lafora disease; autophagy; glycogen metabolism; laforin; malin; neurodegeneration
Lafora progressive myoclonus epilepsy (Lafora disease; LD) is a fatal autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in either the EPM2A gene, encoding the dual specificity phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Previously, we and others showed that laforin and malin form a functional complex that regulates multiple aspects of glycogen metabolism, and that the interaction between laforin and malin is enhanced by conditions activating AMP-activated protein kinase (AMPK). Here, we demonstrate that laforin is a phosphoprotein, as indicated by two-dimensional electrophoresis, and we identify Ser25 as the residue involved in this modification. We also show that Ser25 is phosphorylated both in vitro and in vivo by AMPK. Lastly, we demonstrate that this residue plays a critical role for both the phosphatase activity and the ability of laforin to interact with itself and with previously established binding partners. Our data suggest that phosphorylation of laforin-Ser25 by AMPK provides a mechanism to modulate the interaction between laforin and malin. Regulation of this complex is necessary to maintain normal glycogen metabolism. Importantly, Ser25 is mutated in some Lafora disease patients (S25P), and our results begin to elucidate the mechanism of disease in these patients.
Laforin; AMPK; phosphorylation; alanine scanning mutagenesis; protein-protein interaction; glucan-phosphatase
Lafora Disease (LD) is a fatal neurodegenerative epileptic disorder that presents as a neurological deterioration with the accumulation of insoluble, intracellular, hyperphosphorylated carbohydrates called Lafora bodies (LBs). LD is caused by mutations in either the gene encoding laforin or malin. Laforin contains a dual specificity phosphatase domain and a carbohydrate-binding module, and is a member of the recently described family of glucan phosphatases. In the current study, we investigated the functional and physiological relevance of laforin dimerization. We purified recombinant human laforin and subjected the monomer and dimer fractions to denaturing gel electrophoresis, mass spectrometry, phosphatase assays, protein-protein interaction assays, and glucan binding assays. Our results demonstrate that laforin prevalently exists as a monomer with a small dimer fraction both in vitro and in vivo. Of mechanistic importance, laforin monomer and dimer possess equal phosphatase activity, and they both associate with malin and bind glucans to a similar extent. However, we found differences between the two states' ability to interact simultaneously with malin and carbohydrates. Furthermore, we tested other members of the glucan phosphatase family. Cumulatively, our data suggest that laforin monomer is the dominant form of the protein and that it contains phosphatase activity.
Malin is an E3-ubiquitin ligase that is mutated in Lafora disease, a fatal form of progressive myoclonus epilepsy. In order to perform its function, malin forms a functional complex with laforin, a glucan phosphatase that facilitates targeting of malin to its corresponding substrates. While laforin phylogeny has been studied, there are no data on the evolutionary lineage of malin.
After an extensive search for malin orthologs, we found that malin is present in all vertebrate species and a cephalochordate, in contrast with the broader species distribution previously reported for laforin. These data suggest that in addition to forming a functional complex, laforin and perhaps malin may also have independent functions. In addition, we found that malin shares significant identity with the E3-ubiquitin ligase TRIM32, which belongs to the tripartite-motif containing family of proteins. We present experimental evidence that both malin and TRIM32 share some substrates for ubiquitination, although they produce ubiquitin chains with different topologies. However, TRIM32-specific substrates were not reciprocally ubiquitinated by the laforin-malin complex.
We found that malin and laforin are not conserved in the same genomes. In addition, we found that malin shares significant identity with the E3-ubiquitin ligase TRIM32. The latter result suggests a common origin for malin and TRIM32 and provides insights into possible functional relationships between both proteins.
AMPK; malin; TRIM32; E3 ubiquitin ligase; phylogeny; Lafora disease
Lafora disease is an autosomal recessive form of progressive myoclonus epilepsy with no effective therapy. Although the outcome is always unfavorable, onset of symptoms and progression of the disease may vary. We aimed to identify modifier genes that may contribute to the clinical course of Lafora disease patients with EPM2A or EPM2B mutations. We established a list of 43 genes coding for proteins related to laforin/malin function and/or glycogen metabolism and tested common polymorphisms for possible associations with phenotypic differences using a collection of Lafora disease families. Genotype and haplotype analysis showed that PPP1R3C may be associated with a slow progression of the disease. The PPP1R3C gene encodes protein targeting to glycogen (PTG). Glycogen targeting subunits play a major role in recruiting type 1 protein phosphatase (PP1) to glycogen-enriched cell compartments and in increasing the specific activity of PP1 toward specific glycogenic substrates (glycogen synthase and glycogen phosphorylase). Here, we report a new mutation (c.746A>G, N249S) in the PPP1R3C gene that results in a decreased capacity to induce glycogen synthesis and a reduced interaction with glycogen phosphorylase and laforin, supporting a key role of this mutation in the glycogenic activity of PTG. This variant was found in one of two affected siblings of a Lafora disease family characterized by a remarkable mild course. Our findings suggest that variations in PTG may condition the course of Lafora disease and establish PTG as a potential target for pharmacogenetic and therapeutic approaches.
Lafora disease is a fatal autosomal recessive form of progressive myoclonus epilepsy. Patients manifest myoclonus and tonic–clonic seizures, visual hallucinations, intellectual, and progressive neurologic deterioration beginning in adolescence. The two genes known to be involved in Lafora disease are EPM2A and NHLRC1 (EPM2B). The EPM2A gene encodes laforin, a dual-specificity protein phosphatase, and the NHLRC1 gene encodes malin, an E3-ubiquitin ligase. The two proteins interact with each other and, as a complex, are thought to regulate glycogen synthesis. Here, we report three Lafora families with two novel pathogenic mutations (C46Y and L261P) and two recurrent mutations (P69A and D146N) in NHLRC1. Investigation of their functional consequences in cultured mammalian cells revealed that malinC46Y, malinP69A, malinD146N, and malinL261P mutants failed to downregulate the level of R5/PTG, a regulatory subunit of protein phosphatase 1 involved in glycogen synthesis. Abnormal accumulation of intracellular glycogen was observed with all malin mutants, reminiscent of the polyglucosan inclusions (Lafora bodies) present in patients with Lafora disease.
Epilepsy; Mendelian disorder; Genetics; Autosomal recessive
A functional laforin–malin complex promotes the ubiquitination of AMP-activated protein kinase (AMPK), a sensor of cellular energy status. The laforin–malin complex promotes the formation of K63-linked ubiquitin chains, which are not involved in proteasome degradation but could regulate the subcellular localization of substrate proteins.
Lafora progressive myoclonus epilepsy is a fatal neurodegenerative disorder caused by defects in the function of at least two proteins: laforin, a dual-specificity protein phosphatase, and malin, an E3-ubiquitin ligase. In this study, we report that a functional laforin–malin complex promotes the ubiquitination of AMP-activated protein kinase (AMPK), a serine/threonine protein kinase that acts as a sensor of cellular energy status. This reaction occurs when any of the three AMPK subunits (α, β, and γ) are expressed individually in the cell, and it also occurs on AMPKβ when it is part of a heterotrimeric complex. We also report that the laforin–malin complex promotes the formation of K63-linked ubiquitin chains, which are not involved in proteasome degradation. On the contrary, this modification increases the steady-state levels of at least AMPKβ subunit, possibly because it leads to the accumulation of this protein into inclusion bodies. These results suggest that the modification introduced by the laforin–malin complex could affect the subcellular distribution of AMPKβ subunits.
Lafora disease (LD) is a progressive, lethal, autosomal recessive, neurodegenerative disorder that manifests with myoclonus epilepsy. LD is characterized by the presence of intracellular inclusion bodies called Lafora bodies (LB), in brain, spinal cord and other tissues. More than 50 percent of LD is caused by mutations in EPM2A that encodes laforin. Here we review our recent findings that revealed that laforin regulates autophagy. We consider how autophagy compromise may predispose to LB formation and neurodegeneration in LD, and discuss future investigations suggested by our data.
autophagy; glycogen metabolism; Lafora disease; laforin; malin; neurodegeneration
Lafora disease (LD) is an autosomal recessive, progressive myoclonus epilepsy, which is characterized by the accumulation of polyglucosan inclusion bodies, called Lafora bodies, in the cytoplasm of cells in the central nervous system and in many other organs. However, it is unclear at the moment whether Lafora bodies are the cause of the disease, or whether they are secondary consequences of a primary metabolic alteration. Here we describe that the major genetic lesion that causes LD, loss-of-function of the protein laforin, impairs autophagy. This phenomenon is confirmed in cell lines from human patients, mouse embryonic fibroblasts from laforin knockout mice and in tissues from such mice. Conversely, laforin expression stimulates autophagy. Laforin regulates autophagy via the mammalian target of rapamycin kinase-dependent pathway. The changes in autophagy mediated by laforin regulate the accumulation of diverse autophagy substrates and would be predicted to impact on the Lafora body accumulation and the cell stress seen in this disease that may eventually contribute to cell death.
Lafora progressive myoclonus epilepsy (Lafora disease; LD) is a fatal autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in either the EPM2A gene, encoding the dual specificity phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Previously, we and others have shown that both proteins form a functional complex that regulates glycogen synthesis by a novel mechanism involving ubiquitination and proteasomal degradation of at least two proteins, glycogen synthase and R5/PTG. Since laforin and malin localized at the endoplasmic reticulum (ER) and their regulatory role likely extend to other proteins unrelated to glycogen metabolism, we postulated that their absence may also affect the ER-unfolded protein response pathway.
Here, we demonstrate that siRNA silencing of laforin in Hek293 and SH-SY5Y cells increases their sensitivity to agents triggering ER-stress, which correlates with impairment of the ubiquitin-proteasomal pathway and increased apoptosis. Consistent with these findings, analysis of tissue samples from a LD patient lacking laforin, and from a laforin knockout (Epm2a-/-) mouse model of LD, demonstrates constitutive high expression levels of ER-stress markers BIP/Grp78, CHOP and PDI, among others.
We demonstrate that, in addition to regulating glycogen synthesis, laforin and malin play a role protecting cells from ER-stress, likely contributing to the elimination of unfolded proteins. These data suggest that proteasomal dysfunction and ER-stress play an important role in the pathogenesis of LD, which may offer novel therapeutic approaches for this fatal neurodegenerative disorder.
Mammalian AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that acts as a sensor of cellular energy status. AMPK is a heterotrimer of three different subunits, i.e. α, β and γ, with α being the catalytic subunit and β and γ having regulatory roles. Although several studies have defined different domains in α and β involved in the interaction with the other subunits of the complex, little is known about the regions of the γ subunits involved in these interactions. To study this, we have made sequential deletions from the N-termini of the γ subunit isoforms and studied the interactions with α and β subunits, both by two hybrid analysis and by co-immunoprecipitation. Our results suggest that a conserved region of 20-25 amino acids in γ1, γ2 and γ3, immediately N-terminal to the Bateman domains, is required for the formation of a functional, active αβγ complex. This region is required for the interaction with the β subunits. The interaction between the α and γ subunits does not require this region and occurs instead within the Bateman domains of the γ subunit, although the α-γ interaction does appear to stabilize the β-γ interaction. In addition, sequential deletions from the C-termini of the γ subunits indicate that deletion of any of the CBS motifs prevents the formation of a functional complex with the α and β subunits.
AMP-activated protein kinase; gamma subunits, protein-protein interaction; two-hybrid analysis; co-immunoprecipitation; Bateman domain
Protein phosphatase 1, comprising the regulatory subunit Reg1 and the catalytic subunit Glc7, has a role in glucose repression in Saccharomyces cerevisiae. Previous studies showed that Reg1 regulates the Snf1 protein kinase in response to glucose. Here, we explore the functional relationships between Reg1, Glc7, and Snf1. We show that different sequences of Reg1 interact with Glc7 and Snf1. We use a mutant Reg1 altered in the Glc7-binding motif to demonstrate that Reg1 facilitates the return of the activated Snf1 kinase complex to the autoinhibited state by targeting Glc7 to the complex. Genetic evidence indicated that the catalytic activity of Snf1 negatively regulates its interaction with Reg1. We show that Reg1 is phosphorylated in response to glucose limitation and that this phosphorylation requires Snf1; moreover, Reg1 is dephosphorylated by Glc7 when glucose is added. Finally, we show that hexokinase PII (Hxk2) has a role in regulating the phosphorylation state of Reg1, which may account for the effect of Hxk2 on Snf1 function. These findings suggest that the phosphorylation of Reg1 by Snf1 is required for the release of Reg1-Glc7 from the kinase complex and also stimulates the activity of Glc7 in promoting closure of the complex.
The HXK2 gene is required for a variety of regulatory effects leading to an adaptation for fermentative metabolism in Saccharomyces cerevisiae. However, the molecular basis of the specific role of Hxk2p in these effects is still unclear. One important feature in order to understand the physiological function of hexokinase PII is that it is a phosphoprotein, since protein phosphorylation is essential in most metabolic signal transductions in eukaryotic cells. Here we show that Hxk2p exists in vivo in a dimeric-monomeric equilibrium which is affected by phosphorylation. Only the monomeric form appears phosphorylated, whereas the dimer does not. The reversible phosphorylation of Hxk2p is carbon source dependent, being more extensive on poor carbon sources such as galactose, raffinose, and ethanol. In vivo dephosphorylation of Hxk2p is promoted after addition of glucose. This effect is absent in glucose repression mutants cat80/grr1, hex2/reg1, and cid1/glc7. Treatment of a glucose crude extract from cid1-226 (glc7-T152K) mutant cells with λ-phosphatase drastically reduces the presence of phosphoprotein, suggesting that CID1/GLC7 phosphatase together with its regulatory HEX2/REG1 subunit are involved in the dephosphorylation of the Hxk2p monomer. An HXK2 mutation encoding a serine-to-alanine change at position 15 [HXK2 (S15A)] was to clarify the in vivo function of the phosphorylation of hexokinase PII. In this mutant, where the Hxk2 protein is unable to undergo phosphorylation, the cells could not provide glucose repression of invertase. Glucose induction of HXT gene expression is also affected in cells expressing the mutated enzyme. Although we cannot rule out a defect in the metabolic state of the cell as the origin of these phenomena, our results suggest that the phosphorylation of hexokinase is essential in vivo for glucose signal transduction.