Mitochondrial aminoacyl-tRNA synthetase disorders: an emerging group of developmental disorders of myelination

Background

The mitochondrial aminoacyl-tRNA synthetase proteins (mt-aaRSs) are a group of nuclear-encoded enzymes that facilitate conjugation of each of the 20 amino acids to its cognate tRNA molecule. Mitochondrial diseases are a large, clinically heterogeneous group of disorders with diverse etiologies, ages of onset, and involved organ systems. Diseases related to mt-aaRS mutations are associated with specific syndromes that affect the central nervous system and produce highly characteristic MRI patterns, prototypically the DARS2, EARS, and AARS2 leukodystrophies, which are caused by mutations in mitochondrial aspartyl-tRNA synthetase, mitochondria glutamate tRNA synthetase, and mitochondrial alanyl-tRNA synthetase, respectively.

Body

The disease patterns emerging for these leukodystrophies are distinct in terms of the age of onset, nature of disease progression, and predominance of involved white matter tracts. In DARS2 and EARS2 disorders, earlier disease onset is typically correlated with more significant brain abnormalities, rapid neurological decline, and greater disability. In AARS2 leukodystrophy cases reported thus far, there is nearly invariable progression to severe disability and atrophy of involved brain regions, often within a decade. Although most mutations are compound heterozygous inherited in an autosomal recessive fashion, homozygous variants are found in each disorder and demonstrate high phenotypic variability. Affected siblings manifest disease on a wide spectrum.

Conclusion

The syndromic nature and selective vulnerability of white matter tracts in these disorders suggests there may be a shared mechanism of mitochondrial dysfunction to target for study. There is evidence that the clinical variability and white matter tract specificity of each mt-aaRS leukodystrophy depend on both canonical and non-canonical effects of the mutations on the process of mitochondrial translation. Furthermore, different sensitivities to the mt-aaRS mutations have been observed based on cell type. Most mutations result in at least partial retention of mt-aaRS enzyme function with varied effects on the mitochondrial respiratory chain complexes. In EARS2 and AARS2 cells, this appears to result in cumulative impairment of respiration. Mt-aaRS mutations may also affect alternative biochemical pathways such as the integrated stress response, a homeostatic program in eukaryotic cells that typically confers cytoprotection, but can lead to cell death when abnormally activated in response to pathologic states. Systematic review of this group of disorders and further exploration of disease mechanisms in disease models and neural cells are warranted.

Dendrimer-N-acetyl-L-cysteine modulates monophagocytic response in adrenoleukodystrophy

OBJECTIVE

X-linked adrenoleukodystrophy (ALD) is a neurodegenerative disorder due to mutations in the peroxisomal very long-chain fatty acyl-CoA transporter, ABCD1, with limited therapeutic options. ALD may manifest in a slowly progressive adrenomyeloneuropathy (AMN) phenotype, or switch to rapid inflammatory demyelinating cerebral disease (cALD), in which microglia have been shown to play a pathophysiological role. The aim of this study was to determine the role of patient phenotype in the immune response of ex vivo monophagocytic cells to stimulation, and to evaluate the efficacy of polyamidoamine dendrimer conjugated to the antioxidant precursor N-acetyl-cysteine (NAC) in modulating this immune response.

METHODS

Human monophagocytic cells were derived from fresh whole blood, from healthy (n = 4), heterozygote carrier (n = 4), AMN (n = 7), and cALD (n = 4) patients. Cells were exposed to very long-chain fatty acids (VLCFAs; C24:0 and C26:0) and treated with dendrimer-NAC (D-NAC).

RESULTS

Ex vivo exposure to VLCFAs significantly increased tumor necrosis factor α (TNFα) and glutamate secretion from cALD patient macrophages. Additionally, a significant reduction in total intracellular glutathione was observed in cALD patient cells. D-NAC treatment dose-dependently reduced TNFα and glutamate secretion and replenished total intracellular glutathione levels in cALD patient macrophages, more efficiently than NAC. Similarly, D-NAC treatment decreased glutamate secretion in AMN patient cells.

INTERPRETATION

ALD phenotypes display unique inflammatory profiles in response to VLCFA stimulation, and therefore ex vivo monophagocytic cells may provide a novel test bed for therapeutic agents. Based on our findings, D-NAC may be a viable therapeutic strategy for the treatment of cALD. Ann Neurol 2018;84:452-462.

Defining a role for calcium in the resumption and progression of meiosis in the pig oocyte

The role of calcium (Ca++) during spontaneous meiotic maturation of pig oocytes was examined. The hypothesis that elevations of endogenously derived intracellular Ca++ are prerequisite for germinal vesicle breakdown (GVBD) and progression through meiosis was tested. In addition, investigations were carried out to determine whether GVBD and meiotic progression were dependent upon extracellular Ca++ influx. Elevation of endogenously derived Ca++ was inhibited directly by loading cells with BAPTA, a specific Ca++-chelator, or indirectly with neomycin. Extracellular Ca++ influx was prevented by culturing oocytes in Ca(++)-deficient medium, with EGTA, or in the presence of the Ca++ channel blocker verapamil. Pretreatment with BAPTA/AM and subsequent culture in the absence of added exogenous Ca++ resulted in a similar inhibition of GVBD (1 microM BAPTA/AM vs. untreated, P < 0.01). After 4 h following follicular release, oocytes were no longer sensitive to BAPTA/AM treatment. Neomycin also significantly inhibited GVBD (0.5 mM neomycin vs. untreated, P < 0.05) as well as meiotic progression past metaphase I (0.25 mM neomycin vs. untreated, P < 0.05). The incidence of GVBD was not significantly affected when oocytes were simply cultured in Ca++ deficient medium or when cultured in the presence of EGTA or verapamil. However, progression of meiosis past GVBD to metaphase II was suppressed by reducing levels of Ca++ in the culture medium (0.68 mM Ca++ vs. 1.7 mM Ca++, P < 0.05) and by treatment with verapamil (0.25 mM verapamil vs. untreated, P < 0.05). Meiotic progression was completely blocked at metaphase I in the presence of 0.85 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear degeneration and meiotic aberrations observed in human oocytes matured in vitro: analysis by light microscopy

OBJECTIVE

To apply an improved air-dry procedure for the light microscopic identification of both degenerate and aberrant meiotic configurations in cultured human oocytes. MATERIAL AND DESIGN: Meiotically immature, normal appearing human oocytes retrieved after oophorectomy were placed into culture for 9 to 46 hours. Subsequently, oocytes were assessed morphologically and then air dried for light microscopic examination of chromatin configurations.

MAIN OUTCOME MEASURES

Oocyte chromatin configurations were identified as normal, degenerate, or aberrant and then classified according to meiotic stage. Retrospective analyses were conducted to determine if [1] normal meiotic configurations were associated with morphologically viable oocytes and [2] degenerate or aberrant meiotic configurations were always associated with degenerate oocytes. In addition, for each meiotic stage, the proportion of oocytes exhibiting either degenerate or aberrant chromatin configurations was calculated.

RESULTS

Of 101 oocyte chromatin configurations analyzed, 71.3% were normal, 11.9% were degenerate, and 16.8% displayed meiotic aberrations. Retrospective analyses revealed that the majority of both normal and aberrant chromatin configurations were associated with morphologically viable oocytes (93.1% and 88.2%, respectively), whereas all of the degenerate chromatin configurations were associated with morphologically degenerate oocytes. When assessed by stage, nuclear degeneration was observed exclusively at the germinal vesicle and diakinesis stages, whereas meiotic aberrations occurred most frequently after chromosome condensation. These aberrations were manifested either as clumped metaphase I configurations or as two distinct groups of bivalents that appeared to result from bivalent migration along the meiotic spindle without homologue segregation.

CONCLUSIONS

Slightly > 25% of human oocytes recovered after oophorectomy were incapable of undergoing normal meiotic maturation in culture. The majority of these abnormal oocytes appeared morphologically normal and yet possessed meiotic aberrations. These observations indicate that caution should be taken when using oocytes matured in vitro for application in assisted reproductive technology programs.

Chromosomal analysis of meiotic stages of human oocytes matured in vitro: benefits of protease treatment before fixation

OBJECTIVE

To modify Tarkowski's air-dry technique for mouse oocytes to develop a rapid, consistent procedure for human oocytes that enables accurate scoring of meiotic stage.

DESIGN, SETTING, AND PATIENTS

Meiotically immature human oocytes, obtained after oophorectomy, were cultured for various periods and then subjected to Tarkowski's air-dry procedure (n = 104) or to our modified procedure (n = 175) that used a brief exposure to protease (20 to 40 seconds) before fixation.

MAIN OUTCOME MEASURES

Air-dried oocytes were assessed for readability and for whether they contained overspread or overlapping chromosomes. In addition, discrete meiotic stages in human oocytes were identified.

RESULTS

Our protease procedure significantly increased readability of air-dried oocytes (96% versus 79% readable for protease versus Tarkowski, respectively; P less than 0.001) by significantly reducing the number of preparations with either overscattered (0.7% versus 3.4% for protease versus Tarkowski, respectively, P less than 0.05) or overlapping (1.3% versus 18% for protease versus Tarkowski, respectively, P less than 0.001) chromosomes.

CONCLUSIONS

Protease exposure of oocytes, combined with a modification of Tarkowski's procedure, resulted in high quality air-dries of human oocytes. This rapid and reliable procedure should have clinical application in in vitro fertilization programs for meiotic assessment of oocytes failing to fertilize.