Dirk Montag

Mice deficient for the myelin-associated glycoprotein show subtle abnormalities in myelin

Using homologous recombination in embryonic stem cells, we have generated mice with a null mutation in the gene encoding the myelin-associated glycoprotein (MAG), a recognition molecule implicated in myelin formation. MAG-deficient mice appeared normal in motor coordination and spatial learning tasks. Normal myelin structure and nerve conduction in the PNS, with N-CAM overexpression at sites normally expressing MAG, suggested compensatory mechanisms. In the CNS, the onset of myelination was delayed, and subtle morphological abnormalities were detected in that the content of oligodendrocyte cytoplasm at the inner aspect of most myelin sheaths was reduced and that some axons were surrounded by two or more myelin sheaths. These observations suggest that MAG participates in the formation of the periaxonal cytoplasmic collar of oligodendrocytes and in the recognition between oligodendrocyte processes and axons.

Autistic-like behaviours and hyperactivity in mice lacking ProSAP1/Shank2

Autism spectrum disorders comprise a range of neurodevelopmental disorders characterized by deficits in social interaction and communication, and by repetitive behaviour. Mutations in synaptic proteins such as neuroligins, neurexins, GKAPs/SAPAPs and ProSAPs/Shanks were identified in patients with autism spectrum disorder, but the causative mechanisms remain largely unknown. ProSAPs/Shanks build large homo- and heteromeric protein complexes at excitatory synapses and organize the complex protein machinery of the postsynaptic density in a laminar fashion. Here we demonstrate that genetic deletion of ProSAP1/Shank2 results in an early, brain-region-specific upregulation of ionotropic glutamate receptors at the synapse and increased levels of ProSAP2/Shank3. Moreover, ProSAP1/Shank2−/− mutants exhibit fewer dendritic spines and show reduced basal synaptic transmission, a reduced frequency of miniature excitatory postsynaptic currents and enhanced N-methyl-d-aspartate receptor-mediated excitatory currents at the physiological level. Mutants are extremely hyperactive and display profound autistic-like behavioural alterations including repetitive grooming as well as abnormalities in vocal and social behaviours. By comparing the data on ProSAP1/Shank2−/− mutants with ProSAP2/Shank3αβ−/− mice, we show that different abnormalities in synaptic glutamate receptor expression can cause alterations in social interactions and communication. Accordingly, we propose that appropriate therapies for autism spectrum disorders are to be carefully matched to the underlying synaptopathic phenotype.

Glutaminyl cyclase inhibition attenuates pyroglutamate Aβ and Alzheimer's disease–like pathology

Because of their abundance, resistance to proteolysis, rapid aggregation and neurotoxicity, N-terminally truncated and, in particular, pyroglutamate (pE)-modified Aβ peptides have been suggested as being important in the initiation of pathological cascades resulting in the development of Alzheimers disease. We found that the N-terminal pE-formation is catalyzed by glutaminyl cyclase in vivo. Glutaminyl cyclase expression was upregulated in the cortices of individuals with Alzheimers disease and correlated with the appearance of pE-modified Aβ. Oral application of a glutaminyl cyclase inhibitor resulted in reduced Aβ3(pE)–42 burden in two different transgenic mouse models of Alzheimers disease and in a new Drosophila model. Treatment of mice was accompanied by reductions in Aβx–40/42, diminished plaque formation and gliosis and improved performance in context memory and spatial learning tests. These observations are consistent with the hypothesis that Aβ3(pE)–42 acts as a seed for Aβ aggregation by self-aggregation and co-aggregation with Aβ1–40/42. Therefore, Aβ3(pE)–40/42 peptides seem to represent Aβ forms with exceptional potency for disturbing neuronal function. The reduction of brain pE-Aβ by inhibition of glutaminyl cyclase offers a new therapeutic option for the treatment of Alzheimers disease and provides implications for other amyloidoses, such as familial Danish dementia.

Crucial Role for the Myelin‐associated Glycoprotein in the Maintenance of Axon‐Myelin Integrity

It has recently been shown that mice deficient in the gene for myelin‐associated glycoprotein develop normal myelin sheaths in the peripheral nervous system. Here we report that in mutant mice older than 8 months the maintenance of axon‐myelin units is disturbed, resulting in both axon and myelin degeneration. Morphological features include those typically seen in human peripheral neuropathies, where demyelination‐induced Schwann cell proliferation and remyelination lead to the formation of so‐called onion bulbs. Expression of tenascin‐C, a molecule indicative of peripheral nerve degeneration, was up‐regulated by axon‐deprived Schwann cells and regenerating axons were occasionally seen. Myelin‐associated glycoprotein thus appears to play a crucial role in the long‐term maintenance of the integrity of both myelin and axons.

Brevican-deficient mice display impaired hippocampal CA1 long- term potentiation but show no obvious deficits in learning and memory

ABSTRACT Brevican is a brain-specific proteoglycan which is found in specialized extracellular matrix structures called perineuronal nets. Brevican increases the invasiveness of glioma cells in vivo and has been suggested to play a role in central nervous system fiber tract development. To study the role of brevican in the development and function of the brain, we generated mice lacking a functional brevican gene. These mice are viable and fertile and have a normal life span. Brain anatomy was normal, although alterations in the expression of neurocan were detected. Perineuronal nets formed but appeared to be less prominent in mutant than in wild-type mice. Brevican-deficient mice showed significant deficits in the maintenance of hippocampal long-term potentiation (LTP). However, no obvious impairment of excitatory and inhibitory synaptic transmission was found, suggesting a complex cause for the LTP defect. Detailed behavioral analysis revealed no statistically significant deficits in learning and memory. These data indicate that brevican is not crucial for brain development but has restricted structural and functional roles.

Disruption of the Gene for the Myelin-Associated Glycoprotein Improves Axonal Regrowth along Myelin in C57BL/Wlds Mice

The myelin-associated glycoprotein (MAG) has been shown to be inhibitory for certain neurons in vitro (Mukhopadhyay et al., 1994; McKerracher et al., 1994). To investigate whether MAG is an inhibitory component in peripheral myelin in vivo, MAG-deficient mutant mice were cross-bred with C57BL/Wlds mice that have delayed lesion-induced myelin degeneration and axon regrowth. While in crushed nerves of C57BL/Wlds mice expressing MAG, only 16% of myelin sheaths were associated with regrowing axons, this number was doubled in MAG-deficient C57BL/Wlds mice. These observations suggest that the absence of MAG may contribute to the improved axonal regrowth in the double mutants. Therefore, degeneration of MAG-containing myelin might be an important prerequisite to optimize axonal regrowth after peripheral nerve injury.

Misguided Axonal Projections, Neural Cell Adhesion Molecule 180 mRNA Upregulation, and Altered Behavior in Mice Deficient for the Close Homolog of L1

ABSTRACT Cell recognition molecules are involved in nervous system development and participate in synaptic plasticity in the adult brain. The close homolog of L1 (CHL1), a recently identified member of the L1 family of cell adhesion molecules, is expressed by neurons and glia in the central nervous system and by Schwann cells in the peripheral nervous system in a pattern overlapping, but distinct from, the other members of the L1 family. In humans, CHL1 (also referred to as CALL) is a candidate gene for 3p- syndrome-associated mental impairment. In the present study, we generated and analyzed CHL1-deficient mice. At the morphological level, these mice showed alterations of hippocampal mossy fiber organization and of olfactory axon projections. Expression of the mRNA of the synapse-specific neural cell adhesion molecule 180 isoform was upregulated in adult CHL1-deficient mice, but the mRNA levels of several other recognition molecules were not changed. The behavior of CHL1-deficient mice in the open field, the elevated plus maze, and the Morris water maze indicated that the mutant animals reacted differently to their environment. Our data show that the permanent absence of CHL1 results in misguided axonal projections and aberrant axonal connectivity and alters the exploratory behavior in novel environments, suggesting deficits in information processing in CHL1-deficient mice.

Novelty-induced increased expression of immediate-early genes c-fos and arg 3.1 in the mouse brain.

The detection of novel stimuli is a memory-dependent process. The presented stimulus has to be compared with memory contents to judge its novelty. In addition, the novelty of stimuli activates attention-related processes that facilitate memory formation. To determine the involvement of limbic and neocortical brain structures in novelty detection, we exposed mice to a novel gustatory stimulus (0.5% saccharin) added to their drinking fluid. We then compared the novelty-induced expression of the two immediate-early genes (IEGs) c-fos and arg 3.1, with their expression in mice familiarized with the same stimulus or mice not exposed to that stimulus. Exposure to taste novelty increased expression of c-fos and arg 3.1 mRNA in the cingulate cortex and deep layers of the parietal cortex. In addition, c-fos mRNA expression was increased in the amygdala and arg 3.1 mRNA was increased in the dentate gyrus. Expression of c-fos and arg 3.1 was elevated 30 min after the exposure to novelty. For arg 3.1, a second peak of expression was found 4.5 h after presentation of the novel stimulus. Our results indicate that the amygdala, the dentate gyrus, and the cingulate and parietal cortices may be involved in novelty detection and associated cognitive events, and suggest that c-fos and arg 3.1 play distinct roles in these processes.

The close homologue of the neural adhesion molecule L1 (CHL1): patterns of expression and promotion of neurite outgrowth by heterophilic interactions

The close homologue of L1 (CHL1), a member of the L1 family of neural adhesion molecules, is first expressed at times of neurite outgrowth during brain development, and is detectable in subpopulations of neurons, astrocytes, oligodendrocyte precursors and Schwann cells of the mouse and rat. Aggregation assays with CHL1‐transfected cells show that CHL1 does not promote homophilic adhesion or does it mediate heterophilic adhesion with L1. CHL1 promotes neurite outgrowth by hippocampal and small cerebellar neurons in substrate‐bound and soluble form. The observation that CHL1 and L1 show overlapping, but also distinct patterns of synthesis in neurons and glia, suggests differential effects of L1‐like molecules on neurite outgrowth.

Structural features of a close homologue of L1 (CHL1) in the mouse: a new member of the L1 family of neural recognition molecules

We have identified a close homologue of L1 (CHL1) in the mouse. CHL1 comprises an N‐terminal signal sequence, six immunoglobulin (Ig)‐like domains, 4.5 fibronectin type III (FN)‐like repeats, a transmembrane domain and a C‐terminal, most likely intracellular domain of ˜100 amino acids. CHL1 is most similar in its extracellular domain to chicken Ng‐CAM (˜40% amino acid identity), followed by mouse L1, chicken neurofascin, chicken Nr‐CAM, Drosophila neuroglian and zebrafish L1.l (37‐28% amino acid identity), and mouse F3, rat TAG‐1 and rat BIG‐1 (˜27% amino acid identity). The similarity with other members of the Ig superfamily [e.g. neural cell adhesion molecule (N‐CAM), DCC, HLAR, rse] is 16‐11%. The intracellular domain is most similar to mouse and chicken Nr‐CAM, mouse and rat neurofascin (˜60% amino acid identity) followed by chicken neurofascin and Ng‐CAM, Drosophila neuroglian and zebrafish L1.l and L1.2 (˜40% amino acid identity). Besides the high overall homology and conserved modular structure among previously recognized members of the L1 family (mouse/human L1/rat NILE; chicken Ng‐CAM; chicken/mouse Nr‐CAM; Drosphila neuroglian; zebrafish L1.l and L1.2; chicken/mouse neurofascin/rat ankyrin‐binding glycoprotein), criteria characteristic of L1 were identified with regard to the number of amino acids between positions of conserved amino acid residues defining distances within and between two adjacent Ig‐like domains and FN‐like repeats. These show a collinearity in the six Ig‐like domains and four adjacent FN‐like repeats that is remarkably conserved between L1 and molecules containing these modules (designated the L1 family cassette), including the GPI‐linked forms of the F3 subgroup (mouse F3/chicken F1l/human CNTN1; rat BIG‐l/mouse PANG; rat TAG‐l/mouse TAX‐l/chicken axonin‐1). The colorectal cancer molecule (DCC), previously introduced as an N‐CAM‐like molecule, conforms to the L1 family cassette. Other structural features of CHL l shared between members of the L1 family are a high degree of N‐glycosidically linked carbohydrates (˜20% of its molecular mass), which include the HNK‐1 carbohydrate structure, and a pattern of protein fragments comprising a major 185 kDa band and smaller fragments of 165 and 125 kDa. As for the other L1 family members, predominant expression of CHL l is observed in the nervous system and at later developmental stages. In the central nervous system CHL l is expressed by neurons, but, in contrast to L1, also by glial cells. Our findings suggest a common ancestral L1‐like molecule which evolved via gene duplication to generate a diversity of structurally and functionally distinct yet similar molecules.