Thomas Deller

The Epilepsy-Linked Lgi1 Protein Assembles into Presynaptic Kv1 Channels and Inhibits Inactivation by Kvβ1

The voltage-gated potassium (Kv) channel subunit Kv1.1 is a major constituent of presynaptic A-type channels that modulate synaptic transmission in CNS neurons. Here, we show that Kv1.1-containing channels are complexed with Lgi1, the functionally unassigned product of the leucine-rich glioma inactivated gene 1 (LGI1), which is causative for an autosomal dominant form of lateral temporal lobe epilepsy (ADLTE). In the hippocampal formation, both Kv1.1 and Lgi1 are coassembled with Kv1.4 and Kvbeta1 in axonal terminals. In A-type channels composed of these subunits, Lgi1 selectively prevents N-type inactivation mediated by the Kvbeta1 subunit. In contrast, defective Lgi1 molecules identified in ADLTE patients fail to exert this effect resulting in channels with rapid inactivation kinetics. The results establish Lgi1 as a novel subunit of Kv1.1-associated protein complexes and suggest that changes in inactivation gating of presynaptic A-type channels may promote epileptic activity.

Bergmann glia expression of polyglutamine-expanded ataxin-7 produces neurodegeneration by impairing glutamate transport

Non-neuronal cells may be pivotal in neurodegenerative disease, but the mechanistic basis of this effect remains ill-defined. In the polyglutamine disease spinocerebellar ataxia type 7 (SCA7), Purkinje cells undergo non-cell-autonomous degeneration in transgenic mice. We considered the possibility that glial dysfunction leads to Purkinje cell degeneration, and generated mice that express ataxin-7 in Bergmann glia of the cerebellum with the Gfa2 promoter. Bergmann glia–specific expression of mutant ataxin-7 was sufficient to produce ataxia and neurodegeneration. Expression of the Bergmann glia–specific glutamate transporter GLAST was reduced in Gfa2-SCA7 mice and was associated with impaired glutamate transport in cultured Bergmann glia, cerebellar slices and cerebellar synaptosomes. Ultrastructural analysis of Purkinje cells revealed findings of dark cell degeneration consistent with excitotoxic injury. Our studies indicate that impairment of glutamate transport secondary to glial dysfunction contributes to SCA7 neurodegeneration, and suggest a similar role for glial dysfunction in other polyglutamine diseases and SCAs.

Synaptopodin-deficient mice lack a spine apparatus and show deficits in synaptic plasticity

The spine apparatus is a cellular organelle that is present in many dendritic spines of excitatory neurons in the mammalian forebrain. Despite its discovery >40 years ago, the function of the spine apparatus is still unknown although calcium buffering functions as well as roles in synaptic plasticity have been proposed. We have recently shown that the 100-kDa protein synaptopodin is associated with the spine apparatus. Here, we now report that mice homozygous for a targeted deletion of the synaptopodin gene completely lack spine apparatuses. Interestingly, this absence of the spine apparatus is accompanied by a reduction in hippocampal long-term potentiation (LTP) in the CA1 region of the hippocampus and by an impairment of spatial learning in the radial arm maze test. This genetic analysis points to a role of the spine apparatus in synaptic plasticity.

Mitochondrial dysfunction, peroxidation damage and changes in glutathione metabolism in PARK6

Oxidative stress and protein aggregation are biochemical hallmarks of Parkinsons disease (PD), a frequent sporadic late-onset degenerative disorder particularly of dopaminergic neurons in the substantia nigra, resulting in impaired spontaneous movement. PARK6 is a rare autosomal-recessively inherited disorder, mimicking the clinical picture of PD with earlier onset and slower progression. Genetic data demonstrated PARK6 to be caused by mutations in the protein PINK1, which is localized to mitochondria and has a serine-threonine kinase domain. To study the effect of PINK1 mutations on oxidative stress, we used primary fibroblasts and immortalized lymphoblasts from three patients homozygous for G309D-PINK1. Oxidative stress was evident from increases in lipid peroxidation and in antioxidant defenses by mitochondrial superoxide dismutase and glutathione. Elevated levels of glutathione reductase and glutathione-S-transferase were also observed. As a putative cause of oxidation, a mild decrease in complex I activity and a trend to superoxide elevation were detectable. These data indicate that PINK1 function is critical to prevent oxidative damage and that peripheral cells may be useful for studies of progression and therapy of PARK6.

Invasion of Hematopoietic Cells into the Brain of Amyloid Precursor Protein Transgenic Mice

The significance of the peripheral immune system in Alzheimers disease pathogenesis remains controversial. To study the CNS invasion of hematopoietic cells in the course of cerebral amyloidosis, we used a green fluorescence protein (GFP)-bone marrow chimeric amyloid precursor protein transgenic mouse model (APP23 mice). No difference in the number of GFP-positive invading cells was observed between young APP23 mice and nontransgenic control mice. In contrast, in aged, amyloid-depositing APP23 mice, a significant increase in the number of invading ameboid-like GFP-positive cells was found compared with age-matched nontransgenic control mice. Interestingly, independent of the time after transplantation, only a subpopulation of amyloid deposits was surrounded by invading cells. This suggests that not all amyloid plaques are a target for invading cells or, alternatively, all amyloid plaques attract invading cells but only for a limited time, possibly at an early stage of plaque evolution. Immunological and ultrastructural phenotyping revealed that macrophages and T-cells accounted for a significant portion of these ameboid-like invading cells. Macrophages did not show evidence of amyloid phagocytosis at the electron microscopic level, and no obvious signs for T-cell-mediated inflammation or neurodegeneration were observed. The observation that hematopoietic cells invade the brain in response to cerebral amyloidosis may hold an unrecognized therapeutic potential.

TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice.

Alzheimers disease (AD) is characterized by extracellular deposits of amyloid‐β protein which attract dense clusters of microglial cells. Here, we analyzed amyloid plaque‐associated areas in aged APP23 transgenic mice, an animal model of AD, by combining laser microdissection with microarray analysis and quantitative RT‐PCR (qPCR). By comparing gene expression profiles, we found that 538 genes (1.3% of a total of 41,234 analyzed genes) were differentially expressed in plaque‐associated versus plaque‐free tissue of aged APP23 transgenic mice. One of these genes is the microglia‐associated triggering receptor expressed on myeloid cells (TREM2) which enhances phagocytosis, but abrogates cytokine production as well as TLR and Fc receptor‐mediated induction of TNF secretion. Western Blot analysis demonstrated an upregulation of TREM2 protein in APP23 transgenic compared with nontransgenic mice. Confocal imaging studies, furthermore, confirmed colocalization of TREM2 protein with microglia. Thus, when TREM2 is induced on microglia in plaque‐loaded brain areas the respective signaling may prevent inflammation‐induced bystander damage of neurons. At the same time, TREM2 signaling may also account for the failure to sufficiently eliminate extracellular amyloid with the help of a systemic immune response.

Transgenic mice expressing mutant A53T human alpha-synuclein show neuronal dysfunction in the absence of aggregate formation

Alpha-synuclein was implicated in Parkinsons disease when missense mutations in the alpha-synuclein gene were found in autosomal dominant Parkinsons disease and alpha-synuclein was shown to be a major constituent of protein aggregates in sporadic Parkinsons disease and other synucleinopathies. We have generated transgenic mice expressing A53T mutant and wild-type human alpha-synuclein. The mutant transgenic protein was distributed abnormally to the axons, perikarya, and dendrites of neurons in many brain areas. In electron microscopic immunogold studies, no aggregation of alpha-synuclein was found in these mice. However, behavior analysis showed a progressive reduction of spontaneous vertical motor activity in both mutant lines correlating with the dosage of overexpression. In addition, deficits of grip strength, rotarod performance, and gait were observed in homozygous PrPmtB mice. Transgenic animals expressing mutant alpha-synuclein may be a valuable model to assess specific aspects of the pathogenesis of synucleinopathies.

AnkyrinG is required to maintain axo-dendritic polarity in vivo

Neurons are highly polarized cells that extend a single axon and several dendrites. Studies with cultured neurons indicate that the proximal portion of the axon, denoted as the axon initial segment (AIS), maintains neuronal polarity in vitro. The membrane-adaptor protein ankyrinG (ankG) is an essential component of the AIS. To determine the relevance of ankG for neuronal polarity in vivo, we studied mice with a cerebellum-specific ankG deficiency. Strikingly, ankG-depleted axons develop protrusions closely resembling dendritic spines. Such axonal spines are enriched with postsynaptic proteins, including ProSAP1/Shank2 and ionotropic and metabotropic glutamate receptors. In addition, immunofluorescence indicated that axonal spines are contacted by presynaptic glutamatergic boutons. For further analysis, double mutants were obtained by crossbreeding ankG−/− mice with L7/Purkinje cell-specific promoter 2 (PCP2) mice expressing enhanced green fluorescent protein (EGFP) in Purkinje cells (PCs). This approach allowed precise confocal microscopic mapping of EGFP-positive spiny axons and their subsequent identification at the electron microscopic level. Ultrastructurally, axonal spines contained a typical postsynaptic density and established asymmetric excitatory synapses with presynaptic boutons containing synaptic vesicles. In the shaft of spiny axons, typical ultrastructural features of the AIS, including the membrane-associated dense undercoating and cytoplasmic bundles of microtubules, were absent. Finally, using time-lapse imaging of organotypic cerebellar slice cultures, we demonstrate that nonspiny PC axons of EGFP-positive/ankG−/− mice acquire a spiny phenotype within a time range of only 3 days. Collectively, these findings demonstrate that axons of ankG-deficient mice acquire hallmark features of dendrites. AnkG thus is important for maintaining appropriate axo-dendritic polarity in vivo.

Clinical features, neurogenetics and neuropathology of the polyglutamine spinocerebellar ataxias type 1, 2, 3, 6 and 7

The spinocerebellar ataxias type 1 (SCA1), 2 (SCA2), 3 (SCA3), 6 (SCA6) and 7 (SCA7) are genetically defined autosomal dominantly inherited progressive cerebellar ataxias (ADCAs). They belong to the group of CAG-repeat or polyglutamine diseases and share pathologically expanded and meiotically unstable glutamine-encoding CAG-repeats at distinct gene loci encoding elongated polyglutamine stretches in the disease proteins. In recent years, progress has been made in the understanding of the pathogenesis of these currently incurable diseases: Identification of underlying genetic mechanisms made it possible to classify the different ADCAs and to define their clinical and pathological features. Furthermore, advances in molecular biology yielded new insights into the physiological and pathophysiological role of the gene products of SCA1, SCA2, SCA3, SCA6 and SCA7 (i.e. ataxin-1, ataxin-2, ataxin-3, α-1A subunit of the P/Q type voltage-dependent calcium channel, ataxin-7). In the present review we summarize our current knowledge about the polyglutamine ataxias SCA1, SCA2, SCA3, SCA6 and SCA7 and compare their clinical and electrophysiological features, genetic and molecular biological background, as well as their brain pathologies. Furthermore, we provide an overview of the structure, interactions and functions of the different disease proteins. On the basis of these comprehensive data, similarities, differences and possible disease mechanisms are discussed.

New insights into the pathoanatomy of spinocerebellar ataxia type 3 (Machado-Joseph disease).

Purpose of reviewThis review summarizes recent neuropathological findings in spinocerebellar ataxia type 3 and discusses their relevance for clinical neurology. Recent findingsThe extent of the spinocerebellar ataxia type 3 related central nervous neurodegenerative changes has been recently systematically investigated in a series of pathoanatomical studies. These studies showed that the extent of the central nervous degenerative changes of spinocerebellar ataxia type 3 has been underestimated so far. The newly described pattern of central nervous neurodegeneration includes the visual, auditory, vestibular, somatosensory, ingestion-related, dopaminergic and cholinergic systems. These pathological findings were correlated with clinical findings and explain a variety of the spinocerebellar ataxia type 3 symptoms observed in clinical practice. SummarySystematic pathoanatomical analysis of spinocerebellar ataxia type 3 brains helps to understand the structural basis of this neurodegenerative disease and offers explanations for a variety of disease symptoms. This better understanding of the neuropathology of the condition has implications for the treatment of spinocerebellar ataxia type 3 patients and represents a basis for further biochemical and molecular biological studies aimed at deciphering the pathomechanisms of this progressive ataxic disorder.