Tau-Induced Pathology in Epilepsy and Dementia: Notions from Patients and Animal Models.

Patients with dementia present epilepsy more frequently than the general population. Seizures are more common in patients with Alzheimer's disease (AD), dementia with Lewy bodies (LBD), frontotemporal dementia (FTD) and progressive supranuclear palsy (PSP) than in other dementias. Missense mutations in the microtubule associated protein tau (MAPT) gene have been found to cause familial FTD and PSP, while the P301S mutation in MAPT has been associated with early-onset fast progressive dementia and the presence of seizures. Brains of patients with AD, LBD, FTD and PSP show hyperphosphorylated tau aggregates, amyloid-ß plaques and neuropil threads. Increasing evidence suggests the existence of overlapping mechanisms related to the generation of network hyperexcitability and cognitive decline. Neuronal overexpression of tau with various mutations found in FTD with parkinsonism-linked to chromosome 17 (FTDP-17) in mice produces epileptic activity. On the other hand, the use of certain antiepileptic drugs in animal models with AD prevents cognitive impairment. Further efforts should be made to search for plausible common targets for both conditions. Moreover, attempts should also be made to evaluate the use of drugs targeting tau and amyloid-ß as suitable pharmacological interventions in epileptic disorders. The diagnosis of dementia and epilepsy in early stages of those diseases may be helpful for the initiation of treatments that could prevent the generation of epileptic activity and cognitive deterioration.

Host sphingomyelin increases West Nile virus infection in vivo.

Flaviviruses, such as the dengue virus and the West Nile virus (WNV), are arthropod-borne viruses that represent a global health problem. The flavivirus lifecycle is intimately connected to cellular lipids. Among the lipids co-opted by flaviviruses, we have focused on SM, an important component of cellular membranes particularly enriched in the nervous system. After infection with the neurotropic WNV, mice deficient in acid sphingomyelinase (ASM), which accumulate high levels of SM in their tissues, displayed exacerbated infection. In addition, WNV multiplication was enhanced in cells from human patients with Niemann-Pick type A, a disease caused by a deficiency of ASM activity resulting in SM accumulation. Furthermore, the addition of SM to cultured cells also increased WNV infection, whereas treatment with pharmacological inhibitors of SM synthesis reduced WNV infection. Confocal microscopy analyses confirmed the association of SM with viral replication sites within infected cells. Our results unveil that SM metabolism regulates flavivirus infection in vivo and propose SM as a suitable target for antiviral design against WNV.

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.

Hyperexcitability and epileptic seizures in a model of frontotemporal dementia.

Epileptic seizures are more common in patients with Alzheimer disease than in the general elderly population. Abnormal forms of hyperphosphorylated tau accumulate in Alzheimer disease and other tauopathies. Aggregates of tau are also found in patients with epilepsy and in experimental models of epilepsy. We report here the analysis of epileptic activity and neuropathological correlates of a transgenic line over-expressing human mutant tau, a model of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The FTDP-17 model displays spontaneous epileptic activity and seizures with spike-wave complexes in the EEG, and a higher sensitivity to the GABAA receptor antagonist pentylenetetrazol (PTZ) when compared to age-matched controls, showing a notably increased seizure length and a shorter latency to develop severe seizures. FTDP-17 human tau mutants also display lower convulsive thresholds and higher lethality after PTZ injections. Astrocytosis and activated microglia are prominent in the hippocampus and other brain regions of young FTDP-17 mice where the human mutant tau transgene is expressed, before the appearance of hyperphosphorylated tau aggregates in these structures. FTDP-17 human mutant tau over-expression produces epilepsy and increased GABAA receptor-mediated hyperexcitability in the absence of Aß pathology. Although aggregates of hyperphosphorylated tau have been observed in patients with epilepsy and in different chemically and electrically generated models of epilepsy, the FTDP-17 tau mutant analyzed here is the first model of genetically modified tau that presents with epilepsy. This model may represent a valuable tool to assay novel treatments in order to reduce tau pathology, a potential factor which may be involved in the development of epileptic seizures in dementia and other neurodegenerative diseases.

Hyperphosphorylated tau aggregates in the cortex and hippocampus of transgenic mice with mutant human FTDP-17 Tau and lacking the PARK2 gene.

Mutations in the PARK2 gene encoding parkin cause autosomal recessive juvenile parkinsonism, but have also been found in patients diagnosed with certain tauopathies. Conversely, mutations in the MAPT gene encoding tau are present in some types of parkinsonism. In order to investigate the possible relationship between these two proteins, we generated a double mutant mouse that is deficient in PARK2 and that over-expresses the hTauVLW transgene, a mutant form of the tau protein present in FTDP-17. Independent deletion of PARK2 or over-expression of the hTauVLW transgene produces mild phenotypic alterations, while a substantial increase in parkin expression is observed in hTauVLW transgenic mice. However, double mutant mice present memory and exploratory deficits, and accumulation of PHF-1 and AT8 hyperphosphorylated tau epitopes in neurons. These phenomena are coupled with reactive astrocytosis, DNA fragmentation, and variable cerebral atrophy. Here, we show that cortical and hippocampal neurons of double mutant mice develop argyrophilic Gallyas-Braak aggregates of phosphorylated tau from 3 months of age. Their number decreases in old animals. Moreover, numerous phosphorylated tau aggregates were identified with the conformation-dependent Alz-50 antibody and the S-Thioflavin staining. Ventral motor nuclei of the spinal cord also present Alz-50, AT8, and PHF1 hyperphosphorylated tau aggregates when parkin is deleted in mice over-expressing the hTauVLW transgene, begining at early ages. Thus, the combination of PARK2 gene deletion with hTauVLW over-expression in mice produces abnormal hyperphosphorylated tau aggregates, similar to those observed in the brain of patients diagnosed with certain tauopathies. In the light of these changes, these mice may help to understand the molecular processes responsible for these diseases, and they may aid the development of new therapeutic strategies to treat neurodegenerative diseases related to tau and parkin proteins.

Pharmacological Interventions to Ameliorate Neuropathological Symptoms in a Mouse Model of Lafora Disease.

Lafora disease (LD, OMIM 254780) is a rare fatal neurodegenerative disorder that usually occurs during childhood with generalized tonic-clonic seizures, myoclonus, absences, drop attacks, or visual seizures. Unfortunately, at present, available treatments are only palliatives and no curative drugs are available yet. The hallmark of the disease is the accumulation of insoluble polyglucosan inclusions, called Lafora bodies (LBs), within the neurons but also in heart, muscle, and liver cells. Mouse models lacking functional EPM2A or EPM2B genes (the two major loci related to the disease) recapitulate the Lafora disease phenotype: they accumulate polyglucosan inclusions, show signs of neurodegeneration, and have a dysregulation of protein clearance and endoplasmic reticulum stress response. In this study, we have subjected a mouse model of LD (Epm2b-/-) to different pharmacological interventions aimed to alleviate protein clearance and endoplasmic reticulum stress. We have used two chemical chaperones, trehalose and 4-phenylbutyric acid. In addition, we have used metformin, an activator of AMP-activated protein kinase (AMPK), as it has a recognized neuroprotective role in other neurodegenerative diseases. Here, we show that treatment with 4-phenylbutyric acid or metformin decreases the accumulation of Lafora bodies and polyubiquitin protein aggregates in the brain of treated animals. 4-Phenylbutyric acid and metformin also diminish neurodegeneration (measured in terms of neuronal loss and reactive gliosis) and ameliorate neuropsychological tests of Epm2b-/- mice. As these compounds have good safety records and are already approved for clinical uses on different neurological pathologies, we think that the translation of our results to the clinical practice could be straightforward.

Enhanced sensitivity of laforin- and malin-deficient mice to the convulsant agent pentylenetetrazole.

Lafora disease is a rare form of inherited progressive myoclonus epilepsy caused by mutations in the EPM2A gene encoding laforin, or in the EPM2B gene, which encodes malin. It is characterized by the presence of polyglucosan inclusion bodies (Lafora bodies) in brain and other tissues. Genetically engineered mice lacking expression of either the laforin (Epm2a(-/-) ) or malin (Epm2b(-/-) ) genes display a number of neurological and behavioral abnormalities that resemble those found in patients suffering from Lafora disease; of these, both Epm2a(-/-) and Epm2b(-/-) mice have shown altered motor activity, impaired motor coordination, episodic memory deficits, and different degrees of spontaneous epileptic activity. In this study, we analyze the sensitivity of Epm2a(-/-) and Epm2b(-/-) mice to the convulsant drug pentylenetetrazol (PTZ), an antagonist of the γ-aminobutyric acid type A (GABAA) receptor, commonly used to induce epileptic tonic-clonic seizures in laboratory animals. PTZ-induced epileptic activity, including myoclonic jerks and tonic-clonic seizures, was analyzed in 2 age groups of mice comprising representative samples of young adult and aged mice, after administration of PTZ at sub-convulsive and convulsive doses. Epm2a(-/-) and Epm2b(-/-) mice showed a lower convulsive threshold after PTZ injections at sub-convulsive doses. A lower convulsive threshold and shorter latencies to develop epileptic seizures were observed after PTZ injections at convulsive doses. Different patterns of generalized seizures and of discharges were observed in Epm2a(-/-) and Epm2b(-/-) mice. Epm2a(-/-) and Epm2b(-/-) mice present an increased sensitivity to the convulsant agent PTZ that may reflect different degrees of increased GABAA receptor-mediated hyperexcitability.

Laforin and malin deletions in mice produce similar neurologic impairments.

Lafora disease is a progressive myoclonus epilepsy caused by mutations in the EPM2A gene encoding laforin or in the EPM2B gene encoding malin. It is characterized by the presence of polyglucosan intracellular inclusion bodies (Lafora bodies) in brain and other tissues. Targeted disruption of Epm2a or Epm2b genes in mice produced widespread neuronal degeneration and accumulation of Lafora bodies in neuronal and nonneuronal tissues. Here we analyzed the neurologic alterations produced by disruption of the laforin gene in Epm2a mice and compared them to those in malin-deficient mice. Both Epm2a and Epm2b mice showed altered motor activity, impaired motor coordination, abnormal hind limb clasping, and episodic memory deficits. Epm2a mice also had tonic-clonic seizures, whereas both Epm2a and Epm2b mice had spontaneous single spikes, spike-wave, polyspikes, and polyspike-wave complexes with correlated myoclonic jerks. Neurologic alterations observed in the mutants were comparable and correlated with the accumulation of abundant Lafora bodies in the cerebral cortex, the hippocampus, the basal ganglia, the cerebellum, and the brainstem, suggesting that these inclusions could cause cognitive and behavioral deterioration. Thus, both Epm2a and Epm2b mice exhibit many pathologic aspects seen in patients with Lafora disease and may be valuable for the study of this disorder.

Malin knockout mice support a primary role of autophagy in the pathogenesis of Lafora disease.

Lafora disease (LD), a fatal neurodegenerative disorder characterized by intracellular inclusions called Lafora bodies (LBs), is caused by recessive loss-of-function mutations in the genes encoding either laforin or malin. Previous studies suggested a role of these proteins in regulating glycogen biosynthesis, in glycogen dephosphorylation and in the modulation of intracellular proteolytic systems. However, the contribution of each of these processes to LD pathogenesis is unclear. Here we review our recent finding that dysfunction of autophagy is a common feature of both laforin- and malin-deficient mice, preceding other pathological manifestations. We propose that autophagy plays a primary role in LD pathogenesis and is a potential target for its treatment.