P-Rex1 is a novel substrate of the E3 ubiquitin ligase Malin associated with Lafora disease

Glial Alterations in the Glutamatergic and GABAergic Signalling Pathways in a Mouse Model of Lafora Disease, a Severe Form of Progressive Myoclonus Epilepsy

AIMS

Lafora disease (LD; OMIM#254780) is a rare form of progressive myoclonus epilepsy characterised by the accumulation of insoluble deposits of glycogen in the brain and peripheral tissues. In mouse models of LD, we have identified neuroinflammation as a secondary hallmark of the disease, characterised by increased levels of reactive astrocytes and activated microglia. Our previous work demonstrated that the TNF and IL-6 inflammatory signalling pathways are the primary drivers of this neuroinflammatory phenotype. In this work, we aimed to investigate whether TNF and IL-6 pathway activation contributes to alterations in the glutamatergic and GABAergic signalling pathways.

METHODS

We performed immunofluorescence and western blot analyses on the hippocampus of a mouse model of LD to evaluate potential changes in proteins associated with glutamatergic and GABAergic signalling pathways.

RESULTS

Our findings reveal dysregulation in the expression of subunits of excitatory glutamatergic receptors (phospho-GluN2B and GluK2), as well as an increase in the levels of the GABA transporter GAT1. In addition, we detected activated forms of the Src and Lyn protein kinases in the hippocampus. More importantly, these alterations predominantly occur in nonneuronal cells, such as reactive astrocytes and microglia, underscoring the critical involvement of glial cells in the pathophysiology of LD.

CONCLUSIONS

The observed upregulation of glutamatergic receptor subunits likely amplifies excitatory glutamatergic signalling, whereas the increased expression of GAT1 may reduce the inhibitory GABAergic tone. These changes contribute to the characteristic hyperexcitability of LD.

Glial alterations in the glutamatergic and GABAergic signaling pathways in a mouse model of Lafora disease, a severe form of progressive myoclonus epilepsy

Lafora disease (LD; OMIM#254780) is a rare form of progressive myoclonus epilepsy characterized by the accumulation of insoluble deposits of aberrant glycogen (polyglucosans), named Lafora bodies (LBs), in the brain but also in peripheral tissues. It is assumed that the accumulation of LBs is related to the appearance of the characteristic pathological features of the disease. In mouse models of LD, we and others have reported an increase in the levels of reactive astrocytes and activated microglia, which triggers the expression of the different pro-inflammatory mediators. Recently, we have demonstrated that the TNF and IL-6 inflammatory signaling pathways are the main mediators of the neuroinflammatory phenotype associated with the disease. In this work, we present evidence that the activation of these pathways produces a dysregulation in the levels of different subunits of the excitatory ionotropic glutamatergic receptors (phopho-GluN2B, phospho-GluA2, GluK2) and also an increase in the levels of the GABA transporter GAT1 in the hippocampus of the Epm2b-/- mice. In addition, we present evidence of the presence of activated forms of the Src and Lyn protein kinases in this area. These effects may increase the excitatory glutamatergic signaling and decrease the inhibitory GABAergic tone, leading to hyper-excitability. More importantly, the enhanced production of these subunits occurs in non-neuronal cells such as activated microglia and reactive astrocytes, pointing out a key role of glia in the pathophysiology of LD.

Impaired malin expression and interaction with partner proteins in Lafora disease

Lafora disease (LD) is an autosomal recessive myoclonus epilepsy with onset in the teenage years leading to death within a decade of onset. LD is characterized by the overaccumulation of hyperphosphorylated, poorly branched, insoluble, glycogen-like polymers called Lafora bodies. The disease is caused by mutations in either EPM2A, encoding laforin, a dual specificity phosphatase that dephosphorylates glycogen, or EMP2B, encoding malin, an E3-ubiquitin ligase. While glycogen is a widely accepted laforin substrate, substrates for malin have been difficult to identify partly due to the lack of malin antibodies able to detect malin in vivo. Here we describe a mouse model in which the malin gene is modified at the C-terminus to contain the c-myc tag sequence, making an expression of malin-myc readily detectable. Mass spectrometry analyses of immunoprecipitates using c-myc tag antibodies demonstrate that malin interacts with laforin and several glycogen-metabolizing enzymes. To investigate the role of laforin in these interactions we analyzed two additional mouse models: malin-myc/laforin knockout and malin-myc/LaforinCS, where laforin was either absent or the catalytic Cys was genomically mutated to Ser, respectively. The interaction of malin with partner proteins requires laforin but is not dependent on its catalytic activity or the presence of glycogen. Overall, the results demonstrate that laforin and malin form a complex in vivo, which stabilizes malin and enhances interaction with partner proteins to facilitate normal glycogen metabolism. They also provide insights into the development of LD and the rescue of the disease by the catalytically inactive phosphatase.

1H and 31P magnetic resonance spectroscopy reveals potential pathogenic and biomarker metabolite alterations in Lafora disease

Lafora disease is a fatal teenage-onset progressive myoclonus epilepsy and neurodegenerative disease associated with polyglucosan bodies. Polyglucosans are long-branched and as a result precipitation- and aggregation-prone glycogen. In mouse models, downregulation of glycogen synthase, the enzyme that elongates glycogen branches, prevents polyglucosan formation and rescues Lafora disease. Mouse work, however, has not yet revealed the mechanisms of polyglucosan generation, and few in vivo human studies have been performed. Here, non-invasive in vivo magnetic resonance spectroscopy (1H and 31P) was applied to test scan feasibility and assess neurotransmitter balance and energy metabolism in Lafora disease towards a better understanding of pathogenesis. Macromolecule-suppressed gamma-aminobutyric acid (GABA)-edited 1H magnetic resonance spectroscopy and 31P magnetic resonance spectroscopy at 3 and 7 tesla, respectively, were performed in 4 Lafora disease patients and a total of 21 healthy controls (12 for the 1H magnetic resonance spectroscopy and 9 for the 31PMRS). Spectra were processed using in-house software and fit to extract metabolite concentrations. From the 1H spectra, we found 33% lower GABA concentrations (P = 0.013), 34% higher glutamate + glutamine concentrations (P = 0.011) and 24% lower N-acetylaspartate concentrations (P = 0.0043) in Lafora disease patients compared with controls. From the 31P spectra, we found 34% higher phosphoethanolamine concentrations (P = 0.016), 23% lower nicotinamide adenine dinucleotide concentrations (P = 0.003), 50% higher uridine diphosphate glucose concentrations (P = 0.004) and 225% higher glucose 6-phosphate concentrations in Lafora disease patients versus controls (P = 0.004). Uridine diphosphate glucose is the substrate of glycogen synthase, and glucose 6-phosphate is its extremely potent allosteric activator. The observed elevated uridine diphosphate glucose and glucose 6-phosphate levels are expected to hyperactivate glycogen synthase and may underlie the generation of polyglucosans in Lafora disease. The increased glutamate + glutamine and reduced GABA indicate altered neurotransmission and energy metabolism, which may contribute to the disease's intractable epilepsy. These results suggest a possible basis of polyglucosan formation and potential contributions to the epilepsy of Lafora disease. If confirmed in larger human and animal model studies, measurements of the dysregulated metabolites by magnetic resonance spectroscopy could be developed into non-invasive biomarkers for clinical trials.

Deciphering the Polyglucosan Accumulation Present in Lafora Disease Using an Astrocytic Cellular Model

Lafora disease (LD) is a neurological disorder characterized by progressive myoclonus epilepsy. The hallmark of the disease is the presence of insoluble forms of glycogen (polyglucosan bodies, or PGBs) in the brain. The accumulation of PGBs is causative of the pathophysiological features of LD. However, despite the efforts made by different groups, the question of why PGBs accumulate in the brain is still unanswered. We have recently demonstrated that, in vivo, astrocytes accumulate most of the PGBs present in the brain, and this could lead to astrocyte dysfunction. To develop a deeper understanding of the defects present in LD astrocytes that lead to LD pathophysiology, we obtained pure primary cultures of astrocytes from LD mice from the postnatal stage under conditions that accumulate PGBs, the hallmark of LD. These cells serve as novel in vitro models for studying PGBs accumulation and related LD dysfunctions. In this sense, the metabolomics of LD astrocytes indicate that they accumulate metabolic intermediates of the upper part of the glycolytic pathway, probably as a consequence of enhanced glucose uptake. In addition, we also demonstrate the feasibility of using the model in the identification of different compounds that may reduce the accumulation of polyglucosan inclusions.