A rat mutation producing demyelination (dmy) maps to chromosome 17

Positional cloning of rat mutant genes reveals new functions of these genes

The laboratory rat (Rattus norvegicus) is a key model organism for biomedical research. Rats can be subjected to strict genetic and environmental controls. The rat's large body size is suitable for both surgical operations and repeated measurements of physiological parameters. These advantages have led to the development of numerous rat models for genetic diseases. Forward genetics is a proven approach for identifying the causative genes of these disease models but requires genome resources including genetic markers and genome sequences. Over the last few decades, rat genome resources have been developed and deposited in bioresource centers, which have enabled us to perform positional cloning in rats. To date, more than 100 disease-related genes have been identified by positional cloning. Since some disease models are more accessible in rats than mice, the identification of causative genes in these models has sometimes led to the discovery of novel functions of genes. As before, various mutant rats are also expected to be discovered and developed as disease models in the future. Thus, the forward genetics continues to be an important approach to find genes involved in disease phenotypes in rats. In this review, I provide an overview the development of rat genome resources and describe examples of positional cloning in rats in which novel gene functions have been identified.

Downregulation of aspartoacylase during the progression of myelin breakdown in the dmy mutant rat with mitochondrial magnesium channel MRS2 defect

OBJECTIVE

The dmy rat is an autosomal recessive mutant that exhibits severe rapid myelin breakdown throughout the central nervous system at 7-8 weeks of age. The dmy rat has a point mutation in Mrs2 gene, which encodes an essential component of the major electrophoretic Mg²⁺ influx system in the mitochondria. However, it remains unknown how mitochondrial dysfunction leads to the myelin breakdown.

METHODS

We focused on the aspartoacylase (ASPA) and mitochondrion-related metabolites to clarify the mechanism of myelin pathology in dmy rats. Aspa mRNA was significantly decreased in both the gray matter and the ventral white matter of spinal cord in the dmy rats from 4 to 8 weeks of age. Very faint immunohistochemical expression for ASPA was noted in the gray and white matter of the affected dmy rats at 8 weeks. Liquid chromatography mass spectrometry revealed no different amount of N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A (CoA) in neurons, in the brain and spinal cord between the dmy and control rats.

CONCLUSION

Our results indicated that the pyruvate dehydrogenase activity might be reduced due to the loss of Mg²⁺ transport activity in the mitochondria of the dmy rats, suggesting acetyl CoA production might be reduced. The number of oligodendrocytes was well preserved until 7 weeks. It is intriguing that prior to the myelin destruction at 7-8 weeks, disrupted expression of Aspa mRNA and ASPA protein undergoes from early stage of myelinogenesis. These data indicate that ASPA expression would be a useful index to evaluate a function of oligodendrocyte in the dmy rat.

Enhanced Expression of Trib3 during the Development of Myelin Breakdown in dmy Myelin Mutant Rats

The demyelination (dmy) rat exhibits hind limb ataxia and severe myelin breakdown in the central nervous system. The causative gene of dmy rats is the MRS2 magnesium transporter gene. Tribbles homolog 3 (Trib3) is a pseudokinase molecule that modifies certain signal pathways, and its expression is increased in response to various stresses. Here we sought to clarify the mechanism of myelin breakdown by focusing Trib3, which is remarkably up-regulated in dmy rats. The expression of Trib3 mRNA was significantly increased at 4, 5, 6, 7 and 8 weeks of age in the dmy rats, prior to the prominent myelin breakdown between 7 and 10 weeks of age. The expression level of Trib3 was increased concurrently with the progression of the clinical and pathological conditions in the dmy rats. Double immunofluorescence demonstrated that TRIB3 was mainly expressed in neurons and oligodendrocytes and localized in the Golgi apparatus. Our findings indicate that Trib3 may be associated with the pathogenic mechanism of dmy rats.

Inherited and acquired disorders of myelin: The underlying myelin pathology

Remyelination is a major therapeutic goal in human myelin disorders, serving to restore function to demyelinated axons and providing neuroprotection. The target disorders that might be amenable to the promotion of this repair process are diverse and increasing in number. They range primarily from those of genetic, inflammatory to toxic origin. In order to apply remyelinating strategies to these disorders, it is essential to know whether the myelin damage results from a primary attack on myelin or the oligodendrocyte or both, and whether indeed these lead to myelin breakdown and demyelination. In some disorders, myelin sheath abnormalities are prominent but demyelination does not occur. This review explores the range of human and animal disorders where myelin pathology exists and focusses on defining the myelin changes in each and their cause, to help define whether they are targets for myelin repair therapy.

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We reviewed myelin disorders of the CNS in humans and animals.

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Myelin damage results from primary attack on the oligodendrocyte or myelin sheath.

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All major categories of disease can affect CNS myelin.

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Myelin vacuolation is common, yet does not always result in demyelination.

A mutation in the gene encoding mitochondrial Mg²+ channel MRS2 results in demyelination in the rat

The rat demyelination (dmy) mutation serves as a unique model system to investigate the maintenance of myelin, because it provokes severe myelin breakdown in the central nervous system (CNS) after normal postnatal completion of myelination. Here, we report the molecular characterization of this mutation and discuss the possible pathomechanisms underlying demyelination. By positional cloning, we found that a G-to-A transition, 177 bp downstream of exon 3 of the Mrs2 (MRS2 magnesium homeostasis factor (Saccharomyces cerevisiae)) gene, generated a novel splice acceptor site which resulted in functional inactivation of the mutant allele. Transgenic rescue with wild-type Mrs2-cDNA validated our findings. Mrs2 encodes an essential component of the major Mg²⁺ influx system in mitochondria of yeast as well as human cells. We showed that the dmy/dmy rats have major mitochondrial deficits with a markedly elevated lactic acid concentration in the cerebrospinal fluid, a 60% reduction in ATP, and increased numbers of mitochondria in the swollen cytoplasm of oligodendrocytes. MRS2-GFP recombinant BAC transgenic rats showed that MRS2 was dominantly expressed in neurons rather than oligodendrocytes and was ultrastructurally observed in the inner membrane of mitochondria. Our observations led to the conclusion that dmy/dmy rats suffer from a mitochondrial disease and that the maintenance of myelin has a different mechanism from its initial production. They also established that Mg²⁺ homeostasis in CNS mitochondria is essential for the maintenance of myelin.

The myelin sheath that surrounds the axon of a neuron acts as a biological insulator. Its major function is to increase the speed at which impulses propagate along myelinated fibers in the central nervous system, as well as the peripheral nervous system. Alterations or damage affecting this structure (demyelination) result in the disruption of signals between the brain and other parts of the body. In the rat, mutations producing demyelination have been frequently identified and characterized and have contributed to a better understanding of the genetics of myelin development, physiology, and pathology. This paper reports the molecular characterization of a recessive allele responsible for the progressive disruption of myelin that was initially observed in mutant rats, previously named demyelination (dmy). This mutation generates an additional splicing acceptor site in an intron of the mitochondrial Mg²⁺ transporter gene (Mrs2), resulting in the insertion of a 83-bp genomic DNA segment into the Mrs2 transcript and complete functional inactivation of the mutant allele. We firstly defined the biological function of MRS2 in mammals and demonstrated the crucial and unexpected role of MRS2 in myelin physiology. Our findings might be helpful in the development of new therapeutic strategies for demyelinating syndromes.

Immunohistochemical and morphometrical studies on myelin breakdown in the demyelination (dmy) mutant rat

The demyelination (dmy) rat is a unique mutant exhibiting severe myelin breakdown in the central nervous system (CNS). In this study, we conducted immunohistochemical and morphometrical investigations in the dmy rat. From around 6 weeks of age, the affected rats developed ataxia especially in the hindlimbs. Afterwards, ataxia worsened rapidly, resulting in complete paralysis of the hindlimbs and recumbency. Histopathology at 7 to 10 weeks of age revealed myelin destruction throughout the white matter of the CNS in the dmy rats. The most severely affected lesions were distributed in the corpus callosum, capsula interna, striatum, subcortical white matter, cerebellar peduncle, and ventral and lateral parts of the spinal cord. Immunohistochemistry demonstrated prominent astrogliosis and many ED-1 positive macrophages in the myelin-destructed areas. Until the 4th week, no significant differences in myelin thickness and fiber diameter were found between dmy and control rats. However, from 5 weeks of age, myelin thickness of residual myelinated fibers in dmy rats became significantly less than that in controls. These data indicated that the dmy phenotype shows a prolonged period of myelin destruction, suggesting that dmy mutation affects the adequate maintenance of myelin.