Ekrem Dere

Ablation of Neuronal Ceramide Synthase 1 in Mice Decreases Ganglioside Levels and Expression of Myelin-associated Glycoprotein in Oligodendrocytes

Background: Ceramide synthase 1 catalyzes the synthesis of C18 ceramide and is mainly expressed in neurons of the brain. Results: Ablation of ceramide synthase 1 decreases ganglioside levels and expression of oligodendrocytic myelin-associated glycoprotein in motor-impaired mice. Conclusion: CerS1-derived C18 gangliosides are essential for cerebellar development and neurodevelopmentally regulated behavior in mice. Significance: Neuronal gangliosides regulate expression of myelin-associated glycoprotein in oligodendrocytes. Ceramide synthase 1 (CerS1) catalyzes the synthesis of C18 ceramide and is mainly expressed in the brain. Custom-made antibodies to a peptide from the C-terminal region of the mouse CerS1 protein yielded specific immunosignals in neurons but no other cell types of wild type brain, but the CerS1 protein was not detected in CerS1-deficient mouse brains. To elucidate the biological function of CerS1-derived sphingolipids in the brain, we generated CerS1-deficient mice by introducing a targeted mutation into the coding region of the cers1 gene. General deficiency of CerS1 in mice caused a foliation defect, progressive shrinkage, and neuronal apoptosis in the cerebellum. Mass spectrometric analyses revealed up to 60% decreased levels of gangliosides in cerebellum and forebrain. Expression of myelin-associated glycoprotein was also decreased by about 60% in cerebellum and forebrain, suggesting that interaction and stabilization of oligodendrocytic myelin-associated glycoprotein by neuronal gangliosides is due to the C18 acyl membrane anchor of CerS1-derived precursor ceramides. A behavioral analysis of CerS1-deficient mice yielded functional deficits including impaired exploration of novel objects, locomotion, and motor coordination. Our results reveal an essential function of CerS1-derived ceramide in the regulation of cerebellar development and neurodevelopmentally regulated behavior.

Pathologic and Phenotypic Alterations in a Mouse Expressing a Connexin47 Missense Mutation That Causes Pelizaeus-Merzbacher–Like Disease in Humans

Gap junction channels are intercellular conduits that allow diffusional exchange of ions, second messengers, and metabolites. Human oligodendrocytes express the gap junction protein connexin47 (Cx47), which is encoded by the GJC2 gene. The autosomal recessive mutation hCx47M283T causes Pelizaeus-Merzbacher–like disease 1 (PMLD1), a progressive leukodystrophy characterized by hypomyelination, retarded motor development, nystagmus, and spasticity. We introduced the human missense mutation into the orthologous position of the mouse Gjc2 gene and inserted the mCx47M282T coding sequence into the mouse genome via homologous recombination in embryonic stem cells. Three-week-old homozygous Cx47M282T mice displayed impaired rotarod performance but unchanged open-field behavior. 10-15-day-old homozygous Cx47M282T and Cx47 null mice revealed a more than 80% reduction in the number of cells participating in glial networks after biocytin injections into oligodendrocytes in sections of corpus callosum. Homozygous expression of mCx47M282T resulted in reduced MBP expression and astrogliosis in the cerebellum of ten-day-old mice which could also be detected in Cx47 null mice of the same age. Three-month-old homozygous Cx47M282T mice exhibited neither altered open-field behavior nor impaired rotarod performance anymore. Adult mCx47M282T expressing mice did not show substantial myelin alterations, but homozygous Cx47M282T mice, additionally deprived of connexin32, which is also expressed in oligodendrocytes, died within six weeks after birth and displayed severe myelin defects accompanied by astrogliosis and activated microglia. These results strongly suggest that PMLD1 is caused by the loss of Cx47 channel function that results in impaired panglial coupling in white matter tissue.

Heterozygous Ambra1 deficiency in mice: A genetic trait with autism-like behavior restricted to the female gender.

Autism-spectrum disorders (ASD) are heterogeneous, highly heritable neurodevelopmental conditions affecting around 0.5% of the population across cultures, with a male/female ratio of approximately 4:1. Phenotypically, ASD are characterized by social interaction and communication deficits, restricted interests, repetitive behaviors, and reduced cognitive flexibility. Identified causes converge at the level of the synapse, ranging from mutation of synaptic genes to quantitative alterations in synaptic protein expression, e.g., through compromised transcriptional or translational control. We wondered whether reduced turnover and degradation of synapses, due to deregulated autophagy, would lead to similar phenotypical consequences. Ambra1, strongly expressed in cortex, hippocampus, and striatum, is a positive regulator of Beclin1, a principal player in autophagosome formation. While homozygosity of the Ambra1 null mutation causes embryonic lethality, heterozygous mice with reduced Ambra1 expression are viable, reproduce normally, and lack any immediately obvious phenotype. Surprisingly, comprehensive behavioral characterization of these mice revealed an autism-like phenotype in Ambra1+/− females only, including compromised communication and social interactions, a tendency of enhanced stereotypies/repetitive behaviors, and impaired cognitive flexibility. Reduced ultrasound communication was found in adults as well as pups, which achieved otherwise normal neurodevelopmental milestones. These features were all absent in male Ambra1+/− mice. As a first hint explaining this gender difference, we found a much stronger reduction of Ambra1 protein in the cortex of Ambra1+/− females compared to males. To conclude, Ambra1 deficiency can induce an autism-like phenotype. The restriction to the female gender of autism-generation by a defined genetic trait is unique thus far and warrants further investigation.

The role of gap junctions in the brain in health and disease

Gap junctions connect the cytosolic compartments of adjacent cells for direct electrotonic and metabolic cell-to-cell communication. Gap junctions between glial cells or neurons are ubiquitously expressed in the brain and play a role in brain development including cell differentiation, cell migration and survival, tissue homeostasis, as well as in human diseases including hearing loss, skin disease, neuropathies, epilepsy, brain trauma, and cardiovascular disease. Furthermore, gap junctions are involved in the synchronization and rhythmic oscillation of hippocampal and neocotical neuronal ensembles which might be important for memory formation and consolidation. In this review the accumulated evidence from mouse mutant and pharmacological studies using gap junction blockers is summarized and the progress made in dissecting the physiological, pathophysiological and behavioral roles of gap junction mediated intercellular communication in the brain is discussed.

Revisiting adult neurogenesis and the role of erythropoietin for neuronal and oligodendroglial differentiation in the hippocampus

Recombinant human erythropoietin (EPO) improves cognitive performance in neuropsychiatric diseases ranging from schizophrenia and multiple sclerosis to major depression and bipolar disease. This consistent EPO effect on cognition is independent of its role in hematopoiesis. The cellular mechanisms of action in brain, however, have remained unclear. Here we studied healthy young mice and observed that 3-week EPO administration was associated with an increased number of pyramidal neurons and oligodendrocytes in the hippocampus of ~20%. Under constant cognitive challenge, neuron numbers remained elevated until >6 months of age. Surprisingly, this increase occurred in absence of altered cell proliferation or apoptosis. After feeding a 15N-leucine diet, we used nanoscopic secondary ion mass spectrometry, and found that in EPO-treated mice, an equivalent number of neurons was defined by elevated 15N-leucine incorporation. In EPO-treated NG2-Cre-ERT2 mice, we confirmed enhanced differentiation of preexisting oligodendrocyte precursors in the absence of elevated DNA synthesis. A corresponding analysis of the neuronal lineage awaits the identification of suitable neuronal markers. In cultured neurospheres, EPO reduced Sox9 and stimulated miR124, associated with advanced neuronal differentiation. We are discussing a resulting working model in which EPO drives the differentiation of non-dividing precursors in both (NG2+) oligodendroglial and neuronal lineages. As endogenous EPO expression is induced by brain injury, such a mechanism of adult neurogenesis may be relevant for central nervous system regeneration.

Preexisting Serum Autoantibodies Against the NMDAR Subunit NR1 Modulate Evolution of Lesion Size in Acute Ischemic Stroke

Background and Purpose— Recently, we reported high seroprevalence (age-dependent up to >19%) of N-methyl-D-aspartate-receptor subunit NR1 (NMDAR1) autoantibodies in both healthy and neuropsychiatrically ill subjects (N=4236). Neuropsychiatric syndrome relevance was restricted to individuals with compromised blood–brain barrier, for example, apolipoprotein E4 (APOE4) carrier status, both clinically and experimentally. We now hypothesized that these autoantibodies may upon stroke be protective in individuals with hitherto intact blood–brain barrier, but harmful for subjects with chronically compromised blood–brain barrier. Methods— Of 464 patients admitted with acute ischemic stroke in the middle cerebral artery territory, blood for NMDAR1 autoantibody measurements and APOE4 carrier status as indicator of a preexisting leaky blood–brain barrier was collected within 3 to 5 hours after stroke. Evolution of lesion size (delta day 7–1) in diffusion-weighted magnetic resonance imaging was primary outcome parameter. In subgroups, NMDAR1 autoantibody measurements were repeated on days 2 and 7. Results— Of all 464 patients, 21.6% were NMDAR1 autoantibody–positive (immunoglobulin M, A, or G) and 21% were APOE4 carriers. Patients with magnetic resonance imaging data available on days 1 and 7 (N=384) were divided into 4 groups according to NMDAR1 autoantibody and APOE4 status. Groups were comparable in all stroke-relevant presenting characteristics. The autoantibody+/APOE4− group had a smaller mean delta lesion size compared with the autoantibody−/APOE4- group, suggesting a protective effect of circulating NMDAR1 autoantibodies. In contrast, the autoantibody+/APOE4+ group had the largest mean delta lesion area. NMDAR1 autoantibody serum titers dropped on day 2 and remounted by day 7. Conclusions— Dependent on blood–brain barrier integrity before an acute ischemic brain injury, preexisting NMDAR1 autoantibodies seem to be beneficial or detrimental.

Episodic Memories in Anxiety Disorders: Clinical Implications

The aim of this review is to summarize research on the emerging role of episodic memories in the context of anxiety disorders (AD). The available literature on explicit, autobiographical, and episodic memory function in AD including neuroimaging studies is critically discussed. We describe the methodological diversity of episodic memory research in AD and discuss the need for novel tests to measure episodic memory in a clinical setting. We argue that alterations in episodic memory functions might contribute to the etiology of AD. We further explain why future research on the interplay between episodic memory function and emotional disorders as well as its neuroanatomical foundations offers the promise to increase the effectiveness of modern psychological treatments. We conclude that one major task is to develop methods and training programs that might help patients suffering from AD to better understand, interpret, and possibly actively use their episodic memories in a way that would support therapeutic interventions and counteract the occurrence of symptoms.

A critical appraisal of the what-where-when episodic-like memory test in rodents: Achievements, caveats and future directions.

During the last decade the what, where and when (WWWhen) episodic-like memory (ELM) task, which is based on the object recognition paradigm, has been utilized for the cognitive phenotyping of mouse mutants and transgenic mouse models of neuropsychiatric diseases. It was also widely used to identify the neuroanatomical, electrophysiological and pharmacological foundations of ELM formation, retention and retrieval. Findings from these studies have helped to increase our understanding of the neurobiology and neuropathology of episodic memory in the context of neurodegenerative and neuropsychiatric diseases. Pharmacological studies identified novel targets that might facilitate episodic memory formation in patients with memory problems. In this review, we attempt to delineate the cognitive operations and processes that might underlie rodent performance in the WWWhen/ELM task. We discuss major issues of the object recognition paradigm, including the problem of familiarity vs. recollection-based object recognition, the problem of novel object-induced neophobia, and propose novel methodological solutions to these issues. In conclusion, the WWWhen/ELM task has proven to be a useful tool in the fields of behavioral and translational clinical neuroscience and has the potential to be further refined to address major problems in animal memory research.

The histamine H1 receptor and recollection-based discrimination in a temporal order memory task in the mouse.

The histaminergic system in the central nervous system is involved in a variety of physiological, pathological and behavioral processes. There is now substantial evidence for an important role of histaminergic neurotransmission in learning and memory related processes. The histamine H1 receptor (H1R) is the most abundant histamine receptor in the mammalian brain. We have recently demonstrated that the genetic inactivation of the H1R in mice impairs episodic-like memory, defined as the ability to remember previous experiences with respect to their content and their temporal and spatial context. The ability to encode and retrieve the temporal order of unique events, that is its temporal context, is a core feature of episodic memory. Here we asked whether episodic-like memory deficits of H1R-KO mice are possibly due to changes in the processing, encoding or maintenance of temporal or sequence information which is critical for episodic-like memory formation. H1R-KO mice were tested in the temporal object memory (TOM) task with different inter-trial intervals (ITIs). H1R-KO mice showed impaired TOM when being tested under both, short and longer ITIs. Another aim of the study was to determine whether temporal order discrimination is based on either familiarity or recollection-based memory processes. The performance of wild type (WT) animals in the TOM task suggests that they used recollection-like discrimination strategies.

Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a 5-HT2A/C receptor agonist.

The neuronal tetraspan proteins, M6A (Gpm6a) and M6B (Gpm6b), belong to the family of proteolipids that are widely expressed in the brain. We recently reported Gpm6a deficiency as a monogenetic cause of claustrophobia in mice. Its homolog proteolipid, Gpm6b, is ubiquitously expressed in neurons and oligodendrocytes. Gpm6b is involved in neuronal differentiation and myelination. It interacts with the N-terminal domain of the serotonin transporter (SERT) and decreases cell-surface expression of SERT. In the present study, we employed Gpm6b null mutant mice (Gpm6b(-/-)) to search for behavioral functions of Gpm6b. We studied male and female Gpm6b(-/-) mice and their wild-type (WT, Gpm6b(+/+)) littermates in an extensive behavioral test battery. Additionally, we investigated whether Gpm6b(-/-) mice exhibit changes in the behavioral response to a 5-HT2A/C receptor agonist. We found that Gpm6b(-/-) mice display completely normal sensory and motor functions, cognition, as well as social and emotionality-like (anxiety, depression) behaviors. On top of this inconspicuous behavioral profile, Gpm6b(-/-) mice of both genders exhibit a selective impairment in prepulse inhibition of the acoustic startle response. Furthermore, in contrast to WT mice that show the typical locomotion suppression and increase in grooming activity after intraperitoneal administration of DOI [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], Gpm6b(-/-) mice demonstrate a blunted behavioral response to this 5-HT2A/C receptor agonist. To conclude, Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a serotonergic challenge.