Laforin, a dual specificity phosphatase involved in Lafora disease, regulates insulin response and whole-body energy balance in mice

Laforin is a dual specificity protein phosphatase involved in Lafora disease (LD), a fatal form of progressive myoclonus epilepsy characterized by neurodegeneration and the presence of intracellular polyglucosan inclusions (Lafora bodies) in different tissues. In this work, we describe that mice lacking laforin (epm2a-/-) have enhanced insulin response leading to altered whole-body energy balance. This enhanced insulin response overactivates the Akt pathway which increases glucose uptake in the heart, resulting in increased glycogen levels and the formation of polyglucosan inclusions. In addition, enhanced insulin response resulted in increased liver lipid biosynthesis, resulting in hepatic steatosis. On the contrary, overexpression in rat hepatoma FTO2B cells of native laforin but not of a form lacking phosphatase activity (C266S) resulted in attenuation of insulin signaling. These results define laforin as a new regulator of insulin sensitivity, which provides novel insights into LD pathogenesis and identifies this phosphatase as a potential novel component of the insulin signaling cascade.     

Laforin, the dual-phosphatase responsible for Lafora disease, interacts with R5 (PTG), a regulatory subunit of protein phosphatase-1 that enhances glycogen accumulation

Progressive myoclonus epilepsy of Lafora type (LD, MIM 254780) is a fatal autosomal recessive disorder characterized by the presence of progressive neurological deterioration, myoclonus, epilepsy and polyglucosan intracellular inclusion bodies, called Lafora bodies. Lafora bodies resemble glycogen with reduced branching, suggesting an alteration in glycogen metabolism. Linkage analysis and homozygosity mapping localized EPM2A, a major gene for LD, to chromosome 6q24. EPM2A encodes a protein of 331 amino acids (named laforin) with two domains, a dual-specificity phosphatase domain and a carbohydrate binding domain. Here we show that, in addition, laforin interacts with itself and with the glycogen targeting regulatory subunit R5 of protein phosphatase 1 (PP1). R5 is the human homolog of the murine Protein Targeting to Glycogen, a protein that also acts as a molecular scaffold assembling PP1 with its substrate, glycogen synthase, at the intracellular glycogen particles. The laforin-R5 interaction was confirmed by pull-down and co-localization experiments. Full-length laforin is required for the interaction. However, a minimal central region of R5 (amino acids 116-238), including the binding sites for glycogen and for glycogen synthase, is sufficient to interact with laforin. Point-mutagenesis of the glycogen synthase-binding site completely blocked the interaction with laforin. The majority of the EPM2A missense mutations found in LD patients result in lack of phosphatase activity, absence of binding to glycogen and lack of interaction with R5. Interestingly, we have found that the LD-associated EPM2A missense mutation G240S has no effect on the phosphatase or glycogen binding activities of laforin but disrupts the interaction with R5, suggesting that binding to R5 is critical for the laforin function. These results place laforin in the context of a multiprotein complex associated with intracellular glycogen particles, reinforcing the concept that laforin is involved in the regulation of glycogen metabolism.     

Laforin preferentially binds the neurotoxic starch-like polyglucosans, which form in its absence in progressive myoclonus epilepsy

Lafora disease (LD) is a fatal and the most common form of adolescent-onset progressive epilepsy. Fulminant endoplasmic reticulum (ER)-associated depositions of starch-like long-stranded, poorly branched glycogen molecules [known as polyglucosans, which accumulate to form Lafora bodies (LBs)] are seen in neuronal perikarya and dendrites, liver, skeletal muscle and heart. The disease is caused by loss of function of the laforin dual-specificity phosphatase or the malin E3 ubiquitin ligase. Towards understanding the pathogenesis of polyglucosans in LD, we generated a transgenic mouse overexpressing inactivated laforin to trap normal laforin's unknown substrate. The trap was successful and LBs formed in liver, muscle, neuronal perikarya and dendrites. Using immunogold electron microscopy, we show that laforin is found in close proximity to the ER surrounding the polyglucosan accumulations. In neurons, it compartmentalizes to perikaryon and dendrites and not to axons. Importantly, it binds polyglucosans, establishing for the first time a direct association between the disease-defining storage product and disease protein. It preferentially binds polyglucosans over glycogen in vivo and starch over glycogen in vitro, suggesting that laforin's role begins after the appearance of polyglucosans and that the laforin pathway is involved in monitoring for and then preventing the formation of polyglucosans. In addition, we show that the laforin interacting protein, EPM2AIP1, also localizes on the polyglucosan masses, and we confirm laforin's intense binding to LBs in human LD biopsy material.     

Lafora disease proteins malin and laforin are recruited to aggresomes in response to proteasomal impairment

Lafora disease (LD), an autosomal recessive neurodegenerative disorder, is characterized by the presence of cytoplasmic polyglucosan inclusions known as Lafora bodies in several tissues including the brain. Laforin, a protein phosphatase, and malin, an ubiquitin ligase, are two of the proteins that are known to be defective in LD. Malin interacts with laforin and promotes its polyubiquitination and degradation. Here we show that malin and laforin co-localize in endoplasmic reticulum (ER) and that they form centrosomal aggregates when treated with proteasomal inhibitors in both neuronal and non-neuronal cells. Laforin/malin aggregates co-localize with gamma-tubulin and cause redistribution of alpha-tubulin. These aggregates are also immunoreactive to ubiquitin, ubiquitin-conjugating enzyme, ER chaperone and proteasome subunits, demonstrating their aggresome-like properties. Furthermore, we show that the centrosomal aggregation of laforin and malin is dependent on the functional microtubule network. Laforin and malin form aggresome when expressed together or otherwise, suggesting that the two proteins are recruited to the centrosome independent of each other. Taken together, our results suggest that the centrosomal accumulation of malin, possibly with the help of laforin, may enhance the ubiquitination of its substrates and facilitate their efficient degradation by proteasome. Defects in malin or laforin may thus lead to increased levels of misfolded and/or target proteins, which may eventually affect the physiological processes of the neuron. Thus, defects in protein degradation and clearance are likely to be the primary trigger in the physiopathology of LD.     

Mechanism suppressing glycogen synthesis in neurons and its demise in progressive myoclonus epilepsy

Glycogen synthesis is normally absent in neurons. However, inclusion bodies resembling abnormal glycogen accumulate in several neurological diseases, particularly in progressive myoclonus epilepsy or Lafora disease. We show here that mouse neurons have the enzymatic machinery for synthesizing glycogen, but that it is suppressed by retention of muscle glycogen synthase (MGS) in the phosphorylated, inactive state. This suppression was further ensured by a complex of laforin and malin, which are the two proteins whose mutations cause Lafora disease. The laforin-malin complex caused proteasome-dependent degradation both of the adaptor protein targeting to glycogen, PTG, which brings protein phosphatase 1 to MGS for activation, and of MGS itself. Enforced expression of PTG led to glycogen deposition in neurons and caused apoptosis. Therefore, the malin-laforin complex ensures a blockade of neuronal glycogen synthesis even under intense glycogenic conditions. Here we explain the formation of polyglucosan inclusions in Lafora disease by demonstrating a crucial role for laforin and malin in glycogen synthesis.     

Regulation of glycogen synthesis by the laforin-malin complex is modulated by the AMP-activated protein kinase pathway

Lafora progressive myoclonus epilepsy (LD) is a fatal autosomal recessive neurodegenerative disorder characterized by the presence of glycogen-like intracellular inclusions called Lafora bodies. LD is caused by mutations in two genes, EPM2A and EPM2B, encoding respectively laforin, a dual-specificity protein phosphatase, and malin, an E3 ubiquitin ligase. Previously, we and others have suggested that the interactions between laforin and PTG (a regulatory subunit of type 1 protein phosphatase) and between laforin and malin are critical in the pathogenesis of LD. Here, we show that the laforin-malin complex downregulates PTG-induced glycogen synthesis in FTO2B hepatoma cells through a mechanism involving ubiquitination and degradation of PTG. Furthermore, we demonstrate that the interaction between laforin and malin is a regulated process that is modulated by the AMP-activated protein kinase (AMPK). These findings provide further insights into the critical role of the laforin-malin complex in the control of glycogen metabolism and unravel a novel link between the energy sensor AMPK and glycogen metabolism. These data advance our understanding of the functional role of laforin and malin, which hopefully will facilitate the development of appropriate LD therapies.     

Recent advances in the molecular basis of Lafora’s progressive myoclonus epilepsy

Laforas disease (LD) is an autosomal recessive and fatal form of progressive myoclonus epilepsy with onset in late childhood or adolescence. LD is characterised by the presence of intracellular polyglucosan inclusions, called Lafora bodies, in tissues including the brain, liver and skin. Patients have progressive neurologic deterioration, leading to death within 10 years of onset. No preventive or curative treatment is available for LD. At least three genes underlie LD, of which two have been isolated and mutations characterised: EPM2A and NHLRC1. The EPM2A gene product laforin is a protein phosphatase while the NHLRC1 gene product malin is an E3 ubiquitin ligase that ubiquitinates and promotes the degradation of laforin. Analyses of the structure and function of these gene products suggest defects in post-translational modification of proteins as the common mechanism that leads to the formation of Lafora inclusion bodies, neurodegeneration and the epileptic phenotype of LD. In this review, we summarise the available information on the genetic basis of LD, and correlate these advances with the rapidly expanding information about the mechanisms of LD gained from studies on both cell biological and animal models. Finally, we also discuss a possible mechanism to explain the locus heterogeneity observed in LD.     

Dysfunctions in endosomal-lysosomal and autophagy pathways underlie neuropathology in a mouse model for Lafora disease

Lafora progressive myoclonus epilepsy (also known as Lafora disease, LD) is an inherited and fatal form of a neurodegenerative disorder characterized by the presence of carbohydrate-rich inclusions called Lafora bodies. LD can be caused by defects in the laforin phosphatase or the malin ubiquitin ligase and the clinical symptoms resulting from these two defects are almost similar. In order to understand the molecular basis of LD pathogenesis and the role of Lafora bodies in neuropathology, we have studied the laforin-deficient mice as a model and show here that Lafora bodies recruit proteasomal subunit, endoplasmic reticulum chaperone GRP78/Bip, autophagic protein p62 and endosomal regulators Rab5 and Rab7. The laforin-deficient brain also reveals the proliferation of enlarged lysosomes, lipofuscin granules, amyloid-β peptides and increased levels of insoluble form of ubiquitinated protein, indicating a significant impairment in the cellular degradative pathway. Further, abnormal dendrites and increased gliosis, especially at the vicinity of Lafora bodies, were noted in the LD brain. Taken together, our study suggests that the neuropathology in LD is not limited to Lafora bodies, that some of the neuropathological changes in LD are likely to be secondary effects caused by Lafora bodies, and that impairment in the autophagy-endosomal-lysosomal pathways might underlie some of the symptoms in LD.     

Lafora progressive Myoclonus Epilepsy mutation database-EPM2A and NHLRC1 (EPM2B) genes

Progressive Myoclonus Epilepsy (PME) of the Lafora type is an autosomal recessive disease, which presents in teenage years with myoclonia and generalized seizures leading to death within a decade of onset. It is characterized by pathognomonic inclusions, Lafora bodies (LB), in neurons and other cell types. Two genes causing Lafora disease (LD), EPM2A on chromosome 6q24 and NHLRC1 (EPM2B) on chromosome 6p22.3 have been identified, and our recent results indicate there is at least one other gene causing the disease. The EPM2A gene product, laforin, is a protein tyrosine phosphatase (PTP) with a carbohydrate-binding domain (CBD) in the N-terminus. NHLRC1 encodes a protein named malin, containing a zinc finger of the RING type in the N-terminal half and 6 NHL-repeat domains in the C-terminal direction. To date 43 different variations in EPM2A and 23 in NHLRC1 are known, including missense, nonsense, frameshift, and deletions. We have developed a human LD mutation database using a new generic biological database cross-referencing platform. The database, which currently contains 66 entries is accessible on the World Wide Web (http://projects.tcag.ca/lafora). Entries can be submitted via the curator of the database or via a web-based form.     

Lafora progressive myoclonus epilepsy mutation database‐EPM2A and NHLRC1 (EMP2B) genes

Progressive Myoclonus Epilepsy (PME) of the Lafora type is an autosomal recessive disease, which presents in teenage years with myoclonia and generalized seizures leading to death within a decade of onset. It is characterized by pathognomonic inclusions, Lafora bodies (LB), in neurons and other cell types. Two genes causing Lafora disease (LD), EPM2A on chromosome 6q24 and NHLRC1 (EPM2B) on chromosome 6p22.3 have been identified, and our recent results indicate there is at least one other gene causing the disease. The EPM2A gene product, laforin, is a protein tyrosine phosphatase (PTP) with a carbohydrate‐binding domain (CBD) in the N‐terminus. NHLRC1 encodes a protein named malin, containing a zinc finger of the RING type in the N‐terminal half and 6 NHL‐repeat domains in the C‐terminal direction. To date 43 different variations in EPM2A and 23 in NHLRC1 are known, including missense, nonsense, frameshift, and deletions. We have developed a human LD mutation database using a new generic biological database cross‐referencing platform. The database, which currently contains 66 entries is accessible on the World Wide Web ( http://projects.tcag.ca/lafora ). Entries can be submitted via the curator of the database or via a web‐based form. © 2005 Wiley‐Liss, Inc.     

Lafora bodies and neurological defects in malin-deficient mice correlate with impaired autophagy

Lafora disease (LD), a fatal neurodegenerative disorder characterized by the presence of intracellular inclusions called Lafora bodies (LBs), is caused by loss-of-function mutations in laforin or malin. Previous studies suggested a role of these proteins in the regulation of glycogen biosynthesis, in glycogen dephosphorylation and in the modulation of the intracellular proteolytic systems. However, the contribution of each of these processes to LD pathogenesis is unclear. We have generated a malin-deficient (Epm2b-/-) mouse with a phenotype similar to that of LD patients. By 3-6 months of age, Epm2b-/- mice present neurological and behavioral abnormalities that correlate with a massive presence of LBs in the cortex, hippocampus and cerebellum. Sixteen-day-old Epm2b-/- mice, without detectable LBs, show an impairment of macroautophagy (hereafter called autophagy), which remains compromised in adult animals. These data demonstrate similarities between the Epm2a-/- and Epm2b-/- mice that provide further insights into LD pathogenesis. They illustrate that the dysfunction of autophagy is a consequence of the lack of laforin-malin complexes and a common feature of both mouse models of LD. Because this dysfunction precedes other pathological manifestations, we propose that decreased autophagy plays a primary role in the formation of LBs and it is critical in LD pathogenesis.     

Laforin and malin knockout mice have normal glucose disposal and insulin sensitivity

Lafora disease is a fatal, progressive myoclonus epilepsy caused in ~90% of cases by mutations in the EPM2A or EPM2B genes. Characteristic of the disease is the formation of Lafora bodies, insoluble deposits containing abnormal glycogen-like material in many tissues, including neurons, muscle, heart and liver. Because glycogen is important for glucose homeostasis, the aberrant glycogen metabolism in Lafora disease might disturb whole-body glucose handling. Indeed, Vernia et al. [Vernia, S., Heredia, M., Criado, O., Rodriguez de Cordoba, S., Garcia-Roves, P.M., Cansell, C., Denis, R., Luquet, S., Foufelle, F., Ferre, P. et al. (2011) Laforin, a dual-specificity phosphatase involved in Lafora disease, regulates insulin response and whole-body energy balance in mice. Hum. Mol. Genet., 20, 2571-2584] reported that Epm2a-/- mice had enhanced glucose disposal and insulin sensitivity, leading them to suggest that laforin, the Epm2a gene product, is involved in insulin signaling. We analyzed 3-month- and 6-7-month-old Epm2a-/- mice and observed no differences in glucose tolerance tests (GTTs) or insulin tolerance tests (ITTs) compared with wild-type mice of matched genetic background. At 3 months, Epm2b-/- mice also showed no differences in GTTs and ITTs. In the 6-7-month-old Epm2a-/- mice, there was no evidence for increased insulin stimulation of the phosphorylation of Akt, GSK-3 or S6 in skeletal muscle, liver and heart. From metabolic analyses, these animals were normal with regard to food intake, oxygen consumption, energy expenditure and respiratory exchange ratio. By dual-energy X-ray absorptiometry scan, body composition was unaltered at 3 or 6-7 months of age. Echocardiography showed no defects of cardiac function in Epm2a-/- or Epm2b-/- mice. We conclude that laforin and malin have no effect on whole-body glucose metabolism and insulin sensitivity, and that laforin is not involved in insulin signaling.     

Lafora progressive myoclonus epilepsy: NHLRC1 mutations affect glycogen metabolism

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 malin(C46Y), malin(P69A), malin(D146N), and malin(L261P) 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.     

Laforin, defective in the progressive myoclonus epilepsy of Lafora type, is a dual-specificity phosphatase associated with polyribosomes

The progressive myoclonus epilepsy of Lafora type is an autosomal recessive disorder caused by mutations in the EPM2A gene. EPM2A is predicted to encode a putative tyrosine phosphatase protein, named laforin, whose full sequence has not yet been reported. In order to understand the function of the EPM2A gene, we isolated a full-length cDNA, raised an antibody and characterized its protein product. The full-length clone predicts a 38 kDa laforin that was very close to the size detected in transfected cells. Recombinant laforin was able to hydrolyze phosphotyrosine as well as phosphoserine/threonine substrates, demonstrating that laforin is an active dual-specificity phosphatase. Biochemical, immunofluorescence and electron microscopic studies on the full-length laforin expressed in HeLa cells revealed that laforin is a cytoplasmic protein associated with polyribosomes, possibly through a conformation-dependent protein-protein interaction. We analyzed the intracellular targeting of two laforin mutants with missense mutations. Expression of both mutants resulted in ubiquitin-positive perinuclear aggregates suggesting that they were misfolded proteins targeted for degradation. Our results suggest that laforin is involved in translational regulation and that protein misfolding may be one of the molecular bases of the Lafora disease phenotype caused by missense mutations in the EPM2A gene.     

The Lafora disease gene product laforin interacts with HIRIP5, a phylogenetically conserved protein containing a NifU-like domain

Lafora disease is an autosomal recessive type of progressive myoclonus epilepsy caused by mutations in the EPM2A gene. The EPM2A gene-encoded protein laforin is a dual-specificity phosphatase that associates with polyribosomes. Because the cellular functions of laforin are largely unknown, we used the yeast-two hybrid system to screen for protein(s) that interact with laforin. We found that laforin interacts with a phylogenetically conserved protein HIRIP5 that harbors a NifU-like domain. Both in vitro and in vivo assay have shown that the interaction is specific and that laforin probably uses its N-terminal CBD-4 domain to interact with the C-terminal NifU-like domain of the HIRIP5 protein. HIRIP5 encodes a cytosolic protein and is expressed ubiquitously, perhaps reflecting a house-keeping function. The presence of a NifU-like domain in the HIRIP5 protein raises an interesting possibility that it may be involved in iron homeostasis. Although the significance of the interaction between HIRIP5 and laforin proteins is not yet fully known, because laforin dephosphorylated HIRIP5 in vitro, HIRIP5 promises to be an interesting laforin-binding partner and would contribute to the understanding of the molecular pathology of Lafora disease.     

Two novel mutations in the EPM2A gene in a Korean patient with Lafora's progressive myoclonus epilepsy

The progressive myoclonus epilepsy of the Lafora type (LD; MIM 254780) is a rare autosomal recessive disorder characterized by epilepsy, myoclonus, progressive neurological deterioration, and the presence of periodic acid-Schiff-positive polyglucosan inclusions (Lafora bodies). Mutations in the EPM2A gene have recently been found to cause LD and about 30 or more mutations have been reported thus far. LD is relatively common in countries of the Mediterranean Basin, the Middle East, India, and Pakistan. Although a few sporadic cases with the typical LD phenotype have also been reported in the Far East including Korea and Japan, a recent effort to find mutations in Japanese LD families was not successful. In the present study, we report two novel mutations in a Korean girl with LD; a 1-bp insertion mutation (c.223insC; G75fsX107) in exon 1 and a missense mutation (c.559A>G; T187A) in exon 3 of the EPM2A gene. To our knowledge, this is the first report of a genetically confirmed case of LD in Koreans and also in the Far East.     

Lafora disease in the Indian population: EPM2A and NHLRC1 gene mutations and their impact on subcellular localization of laforin and malin

Lafora disease (LD) is a fatal form of teenage-onset autosomal recessive progressive myoclonus epilepsy. LD is more common among geographic isolates and in populations with a higher rate of consanguinity. Mutations in two genes, EPM2A encoding laforin phosphatase, and NHLRC1 encoding malin ubiquitin ligase, have been shown to cause the LD. We describe here a systematic analysis of the EPM2A and the NHLRC1 gene sequences in 20 LD families from the Indian population. We identified 12 distinct mutations in 15 LD families. The identified novel mutations include 4 missense mutations (K140N, L310W, N148Y, and E210 K) and a deletion of exon 3 for EPM2A, and 4 missense mutations (S22R, L279P, L279P, and L126P) and a single base-pair insertional mutation (612insT) for NHLRC1. The EPM2A gene is known to encode two laforin isoforms having distinct carboxyl termini; a major isoform localized in the cytoplasm, and a minor isoform that targeted the nucleus. We show here that the effect of the EPM2A gene mutation L310W was limited to the cytoplasmic isoform of laforin, and altered its subcellular localization. We have also analyzed the impact of NHLRC1 mutations on the subcellular localization of malin. Of the 6 distinct mutants tested, three targeted the nucleus, one formed perinuclear aggregates, and two did not show any significant difference in the subcellular localization as compared to the wild-type malin. Our results suggest that the altered subcellular localization of mutant proteins of the EPM2A and NHLRC1 genes could be one of the molecular bases of the LD phenotype.     

Novel glycogen synthase kinase 3 and ubiquitination pathways in progressive myoclonus epilepsy

Lafora progressive myoclonus epilepsy, caused by defective laforinor malin, insidiously present in normal teenagers with cognitivedecline, followed by rapidly intractable epilepsy, dementiaand death. Pathology reveals neurodegeneration with neurofibrillarytangle formation and Lafora bodies (LBs). LBs are deposits ofstarch-like polyglucosans, insufficiently branched and henceinsoluble glycogen molecules resulting from glycogen synthase(GS) overactivity relative to glycogen branching enzyme activity.We previously made the unexpected observation that laforin,in the absence of which polyglucosans accumulate, specificallybinds polyglucosans. This suggested that laforin's role is todetect polyglucosan appearances during glycogen synthesis andto initiate mechanisms to downregulate GS. Glycogen synthasekinase 3 (GSK3) is the principal inhibitor of GS. Dephosphorylationof GSK3 at Ser 9 activates GSK3 to inhibit GS through phosphorylationat multiple sites. Glucose-6-phosphate is a potent allostericactivator of GS. Glucose-6-phosphate levels are high when theamount of glucose increases and its activation of GS overridesany phospho-inhibition. Here, we show that laforin is a GSK3Ser 9 phosphatase, and therefore capable of inactivating GSthrough GSK3. We also show that laforin interacts with malinand that malin is an E3 ubiquitin ligase that binds GS. We proposethat laforin, in response to appearance of polyglucosans, directstwo negative feedback pathways: polyglucosan–laforin–GSK3–GSto inhibit GS activity and polyglucosan–laforin–malin–GSto remove GS through proteasomal degradation.