A loss-of-function mutation in human Oxidation Resistance 1 disrupts the spatial-temporal regulation of histone arginine methylation in neurodevelopment

BACKGROUND

Oxidation Resistance 1 (OXR1) gene is a highly conserved gene of the TLDc domain-containing family. OXR1 is involved in fundamental biological and cellular processes, including DNA damage response, antioxidant pathways, cell cycle, neuronal protection, and arginine methylation. In 2019, five patients from three families carrying four biallelic loss-of-function variants in OXR1 were reported to be associated with cerebellar atrophy. However, the impact of OXR1 on cellular functions and molecular mechanisms in the human brain is largely unknown. Notably, no human disease models are available to explore the pathological impact of OXR1 deficiency.

RESULTS

We report a novel loss-of-function mutation in the TLDc domain of the human OXR1 gene, resulting in early-onset epilepsy, developmental delay, cognitive disabilities, and cerebellar atrophy. Patient lymphoblasts show impaired cell survival, proliferation, and hypersensitivity to oxidative stress. These phenotypes are rescued by TLDc domain replacement. We generate patient-derived induced pluripotent stem cells (iPSCs) revealing impaired neural differentiation along with dysregulation of genes essential for neurodevelopment. We identify that OXR1 influences histone arginine methylation by activating protein arginine methyltransferases (PRMTs), suggesting OXR1-dependent mechanisms regulating gene expression during neurodevelopment. We model the function of OXR1 in early human brain development using patient-derived brain organoids revealing that OXR1 contributes to the spatial-temporal regulation of histone arginine methylation in specific brain regions.

CONCLUSIONS

This study provides new insights into pathological features and molecular underpinnings associated with OXR1 deficiency in patients.

Instability in NAD+ metabolism leads to impaired cardiac mitochondrial function and communication

Poly(ADP-ribose) polymerase (PARP) enzymes initiate (mt)DNA repair mechanisms and use nicotinamide adenine dinucleotide (NAD⁺) as energy source. Prolonged PARP activity can drain cellular NAD⁺ reserves, leading to de-regulation of important molecular processes. Here, we provide evidence of a pathophysiological mechanism that connects mtDNA damage to cardiac dysfunction via reduced NAD⁺ levels and loss of mitochondrial function and communication. Using a transgenic model, we demonstrate that high levels of mice cardiomyocyte mtDNA damage cause a reduction in NAD⁺ levels due to extreme DNA repair activity, causing impaired activation of NAD⁺-dependent SIRT3. In addition, we show that myocardial mtDNA damage in combination with high dosages of nicotinamideriboside (NR) causes an inhibition of sirtuin activity due to accumulation of nicotinamide (NAM), in addition to irregular cardiac mitochondrial morphology. Consequently, high doses of NR should be used with caution, especially when cardiomyopathic symptoms are caused by mitochondrial dysfunction and instability of mtDNA.

Visualization and Live Imaging of Oligodendrocyte Organelles in Organotypic Brain Slices Using Adeno-associated Virus and Confocal Microscopy

Neurons rely on the electric insulation and trophic support of myelinating oligodendrocytes. Despite the importance of oligodendrocytes, the advanced tools currently used to study neurons, have only partly been taken on by oligodendrocyte researchers. Cell type-specific staining by viral transduction is a useful approach to study live organelle dynamics. This paper describes a protocol for visualizing oligodendrocyte mitochondria in organotypic brain slices by transduction with adeno-associated virus (AAV) carrying genes for mitochondrial targeted fluorescent proteins under the transcriptional control of the myelin basic protein promoter. It includes the protocol for making organotypic coronal mouse brain slices. A procedure for time-lapse imaging of mitochondria then follows. These methods can be transferred to other organelles and may be particularly useful for studying organelles in the myelin sheath. Finally, we describe a readily available technique for visualization of unstained myelin in living slices by Confocal Reflectance microscopy (CoRe). CoRe requires no extra equipment and can be useful to identify the myelin sheath during live imaging.

Localisation of N-acetylaspartate in oligodendrocytes/myelin

The role of N-acetylaspartate in the brain is unclear. Here we used specific antibodies against N-acetylaspartate and immunocytochemistry of carbodiimide-fixed adult rodent brain to show that, besides staining of neuronal cell bodies in the grey matter, N-acetylaspartate labelling was present in oligodendrocytes/myelin in white matter tracts. Immunoelectron microscopy of the rat hippocampus showed that N-acetylaspartate was concentrated in the myelin. Also neuronal cell bodies and axons contained significant amounts of N-acetylaspartate, while synaptic elements and astrocytes were low in N-acetylaspartate. Mitochondria in axons and neuronal cell bodies contained higher levels of N-acetylaspartate compared to the cytosol, compatible with synthesis of N-acetylaspartate in mitochondria. In aspartoacylase knockout mice, in which catabolism of N-acetylaspartate is blocked, the levels of N-acetylaspartate were largely increased in oligodendrocytes/myelin. In these mice, the highest myelin concentration of N-acetylaspartate was found in the cerebellum, a region showing overt dysmyelination. In organotypic cortical slice cultures there was no evidence for N-acetylaspartate-induced myelin toxicity, supporting the notion that myelin damage is induced by the lack of N-acetylaspartate for lipid production. Our findings also implicate that N-acetylaspartate signals on magnetic resonance spectroscopy reflect not only vital neurons but also vital oligodendrocytes/myelin.

White matter lactate--does it matter?

About half of the human brain is white matter, characterized by axons covered in myelin, which facilitates the high speed of nerve signals from one brain area to another. At the time of myelination, the oligodendrocytes that synthesize myelin require a large amount of energy for this task. Conditions that deprive the tissue of energy can kill the oligodendrocytes. During brain development, the oligodendrocytes may use lactate as an alternative source of energy and material for myelin formation. Mature oligodendrocytes, however, can release lactate through the myelin sheath as nutrient for axons. In addition, lactate carries signals as a volume transmitter. Myelin thus seems to serve as a provider of substrates and signals for axons, and not as a mere insulator. We review the fluxes of lactate in white matter and their significance in brain function.

LOC689986, a unique gene showing specific expression in restricted areas of the rodent neocortex

Background

The neocortex is a highly specialised and complex brain structure, involved in numerous tasks, ranging from processing and interpretation of somatosensory information, to control of motor functions. The normal function linked to distinct neocortical areas might involve control of highly specific gene expression, and in order to identify such regionally enriched genes, we previously analysed the global gene expression in three different cortical regions (frontomedial, temporal and occipital cortex) from the adult rat brain. We identified distinct sets of differentially expressed genes. One of these genes, namely the hypothetical protein LOC689986 (LOC689986), was of particular interest, due to an almost exclusive expression in the temporal cortex.

Results

Detailed analysis of LOC689986 in the adult rat brain confirmed the expression in confined areas of parieto-temporal cortex, and revealed highly specific expression in layer 4 of the somatosensory cortex, with sharp borders towards the neighbouring motor cortex. In addition, LOC689986 was found to be translated in vivo, and was detected in the somatosensory cortex and in the Purkinje cells of the cerebellar cortex. The protein was present in neuronal dendrites and also in astrocyte cells. Finally, this unique gene is apparently specific for, and highly conserved in, the vertebrate lineage.

Conclusions

In this study, we have partially characterised the highly conserved LOC689986 gene, which is specific to the vertebrate linage. The gene displays a distinct pattern of expression in layer 4 of the somatosensory cortex, and areas of the parieto-temporal cortex in rodents.

Immunogold detection of L-glutamate and D-serine in small synaptic-like microvesicles in adult hippocampal astrocytes

Glutamate and the N-methyl-D-aspartate receptor ligand D-serine are putative gliotransmitters. Here, we show by immunogold cytochemistry of the adult hippocampus that glutamate and D-serine accumulate in synaptic-like microvesicles (SLMVs) in the perisynaptic processes of astrocytes. The estimated concentration of fixed glutamate in the astrocytic SLMVs is comparable to that in synaptic vesicles of excitatory nerve terminals (≈ 45 and ≈ 55 mM, respectively), whereas the D-serine level is about 6 mM. The vesicles are organized in small spaced clusters located near the astrocytic plasma membrane. Endoplasmic reticulum is regularly found in close vicinity to SLMVs, suggesting that astrocytes contain functional nanodomains, where a local Ca(2+) increase can trigger release of glutamate and/or D-serine.

N-methyl-D-aspartate receptor subunit dysfunction at hippocampal glutamatergic synapses in an animal model of attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most common neurobehavioural disorder among children. ADHD children are hyperactive, impulsive and have problems with sustained attention. These cardinal features are also present in the best validated animal model of ADHD, the spontaneously hypertensive rat (SHR), which is derived from the Wistar Kyoto rat (WKY). Current theories of ADHD relate symptom development to factors that alter learning. N-methyl-D-aspartate receptor (NMDAR) dependent long term changes in synaptic efficacy in the mammalian CNS are thought to represent underlying cellular mechanisms for some forms of learning. We therefore hypothesized that synaptic abnormality in excitatory, glutamatergic synaptic transmission might contribute to the altered behavior in SHRs. We studied physiological and anatomical aspects of hippocampal CA3-to-CA1 synapses in age-matched SHR and WKY (controls). Electrophysiological analysis of these synapses showed reduced synaptic transmission (reduced field excitatory postsynaptic potential for a defined fiber volley size) in SHR, whereas short-term forms of synaptic plasticity, like paired-pulse facilitation, frequency facilitation, and delayed response enhancement were comparable in the two genotypes, and long-term potentiation (LTP) of synaptic transmission was of similar magnitude. However, LTP in SHR was significantly reduced (by 50%) by the NR2B specific blocker CP-101,606 (10 microM), whereas the blocker had no effect on LTP magnitude in the control rats. This indicates that the SHR has a functional predominance of NR2B, a feature characteristic of early developmental stages in these synapses. Quantitative immunofluorescence and electron microscopic postembedding immunogold cytochemistry of the three major NMDAR subunits (NR1, NR2A; and NR2B) in stratum radiatum spine synapses revealed no differences between SHR and WKY. The results indicate that functional impairments in glutamatergic synaptic transmission may be one of the underlying mechanisms leading to the abnormal behavior in SHR, and possibly in human ADHD.

Subcellular localization of the glutamate transporters GLAST and GLT at the neuromuscular junction in rodents

The vertebrate neuromuscular junction (NMJ) is known to be a cholinergic synapse at which acetylcholine (ACh) is released from the presynaptic terminal to act on postsynaptic nicotinic ACh receptors. There is now growing evidence that glutamate, which is the main excitatory transmitter in the CNS and at invertebrate NMJs, may have a signaling function together with ACh also at the vertebrate NMJ. In the CNS, the extracellular concentration of glutamate is kept at a subtoxic level by Na(+)-driven high-affinity glutamate transporters located in plasma membranes of astrocytes and neurons. The glutamate transporters are also pivotal for shaping glutamate receptor responses at synapses. In order to throw further light on the potential role of glutamate as a cotransmitter at the NMJ we used high-resolution immunocytochemical methods to investigate the localization of the plasma membrane glutamate transporters GLAST (glutamate aspartate transporter) and GLT (glutamate transporter 1) in rat and mice NMJ regions. Confocal laser-scanning immunocytochemistry showed that GLT is restricted to the NMJ in rat and mouse skeletal muscle. Lack of labeling signal in knock-out mice confirmed that the immunoreactivity observed at the NMJ was specific for GLT. GLAST was also localized at the NMJ in rat but not detected in mouse NMJ (while abundant in mouse brain). Post-embedding electron microscopic immunocytochemistry and quantitative analyses in rat showed that GLAST and GLT are enriched in the junctional folds of the postsynaptic membrane at the NMJ. GLT was relatively higher in the slow-twitch muscle soleus than in the fast-twitch muscle extensor digitorum longus, whereas GLAST was relatively higher in extensor digitorum longus than in soleus. The findings show--together with previous demonstration of vesicular glutamate, a vesicular glutamate transporter and glutamate receptors--that mammalian NMJs contain the machinery required for synaptic release and action of glutamate. This indicates a signaling role for glutamate at the normal NMJ and provides a basis for the ability of denervated muscle to be reinnervated by glutamatergic axons from the CNS.