Susann Boretius

Glycolytic oligodendrocytes maintain myelin and long-term axonal integrity.

Oligodendrocytes, the myelin-forming glial cells of the central nervous system, maintain long-term axonal integrity. However, the underlying support mechanisms are not understood. Here we identify a metabolic component of axon–glia interactions by generating conditional Cox10 (protoheme IX farnesyltransferase) mutant mice, in which oligodendrocytes and Schwann cells fail to assemble stable mitochondrial cytochrome c oxidase (COX, also known as mitochondrial complex IV). In the peripheral nervous system, Cox10 conditional mutants exhibit severe neuropathy with dysmyelination, abnormal Remak bundles, muscle atrophy and paralysis. Notably, perturbing mitochondrial respiration did not cause glial cell death. In the adult central nervous system, we found no signs of demyelination, axonal degeneration or secondary inflammation. Unlike cultured oligodendrocytes, which are sensitive to COX inhibitors, post-myelination oligodendrocytes survive well in the absence of COX activity. More importantly, by in vivo magnetic resonance spectroscopy, brain lactate concentrations in mutants were increased compared with controls, but were detectable only in mice exposed to volatile anaesthetics. This indicates that aerobic glycolysis products derived from oligodendrocytes are rapidly metabolized within white matter tracts. Because myelinated axons can use lactate when energy-deprived, our findings suggest a model in which axon–glia metabolic coupling serves a physiological function.

Reduced social interaction and ultrasonic communication in a mouse model of monogenic heritable autism.

Autism spectrum conditions (ASCs) are heritable conditions characterized by impaired reciprocal social interactions, deficits in language acquisition, and repetitive and restricted behaviors and interests. In addition to more complex genetic susceptibilities, even mutation of a single gene can lead to ASC. Several such monogenic heritable ASC forms are caused by loss-of-function mutations in genes encoding regulators of synapse function in neurons, including NLGN4. We report that mice with a loss-of-function mutation in the murine NLGN4 ortholog Nlgn4, which encodes the synaptic cell adhesion protein Neuroligin-4, exhibit highly selective deficits in reciprocal social interactions and communication that are reminiscent of ASCs in humans. Our findings indicate that a protein network that regulates the maturation and function of synapses in the brain is at the core of a major ASC susceptibility pathway, and establish Neuroligin-4-deficient mice as genetic models for the exploration of the complex neurobiological disorders in ASCs.

Neuroligin-3-deficient mice: model of a monogenic heritable form of autism with an olfactory deficit.

Autism spectrum disorder (ASD) is a frequent neurodevelopmental disorder characterized by variable clinical severity. Core symptoms are qualitatively impaired communication and social behavior, highly restricted interests and repetitive behaviors. Although recent work on genetic mutations in ASD has shed light on the pathophysiology of the disease, classifying it essentially as a synaptopathy, no treatments are available to date. To develop and test novel ASD treatment approaches, validated and informative animal models are required. Of particular interest, in this context are loss‐of‐function mutations in the postsynaptic cell adhesion protein neuroligin‐4 and point mutations in its homologue neuroligin‐3 (NL‐3) that were found to cause certain forms of monogenic heritable ASD in humans. Here, we show that NL‐3‐deficient mice display a behavioral phenotype reminiscent of the lead symptoms of ASD: reduced ultrasound vocalization and a lack of social novelty preference. The latter may be related to an olfactory deficiency observed in the NL‐3 mutants. Interestingly, such olfactory phenotype is also present in a subgroup of human ASD patients. Tests for learning and memory showed no gross abnormalities in NL‐3 mutants. Also, no alterations were found in time spent in social interaction, prepulse inhibition, seizure propensity and sucrose preference. As often seen in adult ASD patients, total brain volume of NL‐3 mutant mice was slightly reduced as assessed by magnetic resonance imaging (MRI). Our findings show that the NL‐3 knockout mouse represents a useful animal model for understanding pathophysiological events in monogenic heritable ASD and for developing novel treatment strategies in this devastating human disorder.

Elevated phosphatidylinositol 3,4,5-trisphosphate in glia triggers cell-autonomous membrane wrapping and myelination.

In the developing nervous system, constitutive activation of the AKT/mTOR (mammalian target of rapamycin) pathway in myelinating glial cells is associated with hypermyelination of the brain, but is reportedly insufficient to drive myelination by Schwann cells. We have hypothesized that it requires additional mechanisms downstream of NRG1/ErbB signaling to trigger myelination in the peripheral nervous system. Here, we demonstrate that elevated levels of phosphatidylinositol 3,4,5-trisphosphate (PIP3) have developmental effects on both oligodendrocytes and Schwann cells. By generating conditional mouse mutants, we found that Pten-deficient Schwann cells are enhanced in number and can sort and myelinate axons with calibers well below 1 μm. Unexpectedly, mutant glial cells also spirally enwrap C-fiber axons within Remak bundles and even collagen fibrils, which lack any membrane surface. Importantly, PIP3-dependent hypermyelination of central axons, which is observed when targeting Pten in oligodendrocytes, can also be induced after tamoxifen-mediated Cre recombination in adult mice. We conclude that it requires distinct PIP3 effector mechanisms to trigger axonal wrapping. That myelin synthesis is not restricted to early development but can occur later in life is relevant to developmental disorders and myelin disease.

High-resolution 3D MRI of mouse brain reveals small cerebral structures in vivo

This work demonstrates technical approaches to high-quality magnetic resonance imaging (MRI) of small structures of the mouse brain in vivo. It turns out that excellent soft-tissue contrast requires the reduction of partial volume effects by using 3D MRI at high (isotropic) resolution with linear voxel dimensions of about 100-150 microm. The long T(2)* relaxation times at relatively low magnetic fields (2.35 T) offer the benefit of a small receiver bandwidth (increased signal-to-noise) at a moderate echo time which together with the small voxel size avoids visual susceptibility artifacts. For measuring times of 1-1.5 h both T(1)-weighted (FLASH) and T(2)-weighted (Fast Spin-Echo) 3D MRI acquisitions exhibit detailed anatomical insights in accordance with histological sections from a mouse brain atlas. Preliminary applications address the identification of neuroanatomical variations in different mouse strains and the use of Mn(2+) as a T(1) contrast agent for neuroaxonal tracing of fiber tracts within the mouse visual pathway.

In vivo 3D MRI staining of mouse brain after subcutaneous application of MnCl2

Follow‐up T1‐weighted 3D gradient‐echo MRI (2.35 T) of murine brain in vivo (N = 5) at 120 μm isotropic resolution revealed spatially distinct signal increases 6–48 hr after subcutaneous application of MnCl2 (20 mg/kg). The effects result from a shortening of the water proton T1 relaxation time due to the presence of unchelated paramagnetic Mn2+ ions, which access the brain by systemic circulation and crossing of the blood–brain barrier (BBB). A pronounced Mn2+‐induced signal enhancement was first seen in structures without a BBB, such as the choroid plexus, pituitary gland, and pineal gland. Within 24 hr after administration, Mn2+ contrast highlighted the olfactory bulb, inferior colliculi, cerebellum, and the CA3 subfield of the hippocampus. The affinity of Mn2+ to various brain systems suggests the neuronal uptake of Mn2+ ions from the extracellular space and subsequent axonal transport. Thus, at least part of the Mn2+ contrast reflects a functional brain response of behaving animals, for example, in the olfactory system. In vivo MRI staining of the brain by systemic administration of MnCl2 may contribute to phenotyping mutant mice with morphologic and functional alterations of the central nervous system. Magn Reson Med 48:852–859, 2002.

Chromatin Regulation by BAF170 Controls Cerebral Cortical Size and Thickness

Increased cortical size is essential to the enhanced intellectual capacity of primates during mammalian evolution. The mechanisms that control cortical size are largely unknown. Here, we show that mammalian BAF170, a subunit of the chromatin remodeling complex mSWI/SNF, is an intrinsic factor that controls cortical size. We find that conditional deletion of BAF170 promotes indirect neurogenesis by increasing the pool of intermediate progenitors (IPs) and results in an enlarged cortex, whereas cortex-specific BAF170 overexpression results in the opposite phenotype. Mechanistically, BAF170 competes with BAF155 subunit in the BAF complex, affecting euchromatin structure and thereby modulating the binding efficiency of the Pax6/REST-corepressor complex to Pax6 target genes that regulate the generation of IPs and late cortical progenitors. Our findings reveal a molecular mechanism mediated by the mSWI/SNF chromatin-remodeling complex that controls cortical architecture.

Assessment of lesion pathology in a new animal model of MS by multiparametric MRI and DTI

Magnetic resonance imaging (MRI) is the gold standard for the detection of multiple sclerosis (MS) lesions. However, current MRI techniques provide little information about the structural features of a brain lesion with inflammatory cell infiltration, demyelination, gliosis, acute axonal damage and axonal loss. To identify methods for a differentiation of demyelination, inflammation, and axonal damage we developed a novel mouse model combining cuprizone-induced demyelination and experimental autoimmune encephalomyelitis. MS-like brain lesions were assessed by T1-weighted, T2-weighted, and magnetization transfer MRI as well as by diffusion tensor imaging (DTI). T2-weighted MRI differentiated control and diseased mice, while T1-weighted MRI better reflected the extent of inflammation and axonal damage. In DTI, axonal damage and cellular infiltration led to a reduction of the axial diffusivity, whereas primary demyelination after cuprizone treatment was reflected by changes in radial but not axial diffusivity. Importantly, alterations in radial diffusivity were less pronounced in mice with demyelination, inflammation, and acute axonal damage, indicating that radial diffusivity may underestimate demyelination in acute MS lesions. In conclusion, the combined information from different DTI parameters allows for a more precise identification of solely demyelinated lesions versus demyelinated and acutely inflamed lesions. These findings are of relevance for offering individualized, stage-adapted therapies for MS patients.

In vivo 3D MRI staining of the mouse hippocampal system using intracerebral injection of MnCl2

The morphology and function of the hippocampal system of C57BL/6J mice (n = 8) was studied in vivo using T1-weighted 3D magnetic resonance imaging (MRI) (117 microm isotropic resolution) after bilateral injection of MnCl(2) (0.25 microl, 5 or 200 mM) into the posterior hippocampal formation. The neuronal uptake of the T1-shortening Mn(2+) ions resulted in a pronounced MRI signal enhancement within the CA3 subfield and dentate gyrus with milder increases in CA1 and subiculum. This finding is in line with differences in the excitability of hippocampal neurons previously reported using electrophysiologic recordings. The subsequent axonal transport of Mn(2+) highlighted the principal extrinsic projections from the posterior hippocampal formation via the fimbria and the precommissural fornix to the dorsal part of the lateral septal nucleus. A strong MRI signal enhancement was also observed in the ventral hippocampal commissure. A time-course analysis revealed unsaturated conditions of Mn(2+) accumulation at about 2 h after injection and optimal contrast-to-noise ratios at about 6 h after injection. The present results using Mn(2+)-enhanced 3D MRI open new ways for studying the role of the hippocampal system in specific aspects of learning and memory in normal and mutant mice.

Model‐based nonlinear inverse reconstruction for T2 mapping using highly undersampled spin‐echo MRI

To develop a model‐based reconstruction technique for T2 mapping based on multi‐echo spin‐echo MRI sequences with highly undersampled Cartesian data encoding.