Evidence for forebrain cholinergic neuronal loss in congenital ornithine transcarbamylase deficiency

Impairment of cognitive function in ornithine transcarbamylase deficiency is global rather than domain-specific and is associated with disease onset, sex, maximum ammonium, and number of hyperammonemic events

Beginning in 2006, the Urea Cycle Disorders Consortium (UCDC) has conducted a longitudinal study of eight inherited deficiencies of enzymes and transporters of the urea cycle, including 444 individuals with ornithine transcarbamylase deficiency (OTCD), of whom 300 (67 males, 233 females) received psychological evaluation. In a cross-sectional study (age range, 3-71 years), analysis of covariance (ANCOVA) determined the association between outcomes in five cognitive domains (global intelligence, executive functions, memory, visuomotor integration, visual perception) and sex, age at testing and timing of disease onset defined as early onset (≤28 days; EO), late onset (LO), or asymptomatic (AS). The dataset of 183 subjects with complete datasets (31 males, 152 females) revealed underrepresentation of EO subjects (2 males, 4 females), who were excluded from the ANCOVA. Although mean scores of LO and AS individuals were within 1 SD of the population norm, AS subjects attained significantly higher scores than LO subjects and males higher scores than females. Correlations between cognitive domains were high, particularly intelligence proved to be a distinguished indicator for cognitive functioning. Maximum plasma ammonium concentration and intelligence correlated significantly higher in EO (r = -0.47) than in LO subjects (r = 0.04). Correlation between the number of hyperammonemic events and intelligence scores were similar for EO (r = -0.30) and LO (r = -0.26) individuals. The number of clinical symptoms was significantly associated with intelligence (r = -0.28) but not with scores in other domains. Results suggest that OTCD has a global impact on cognitive functioning rather than a specific effect on distinct cognitive domains.

In human brain ornithine transcarbamylase (OTC) immunoreactivity is strongly expressed in a small number of nitrergic neurons

There is recent evidence for ornithine transcarbamylase (OTC) expression in adult human brain. We decided to immunocytochemically map OTC throughout brain, and to further characterize OTC-immunopositive neurons. By using double immunolabeling technique for OTC and neuronal nitric oxide synthase (nNOS) OTC protein expression was revealed in a small subset of nitrergic (nNOS) neurons. Since citrulline (the reaction product of OTC) enhances the bioavailability of L-arginine, the substrate of nNOS, it is conceivable that OTC activity supports NO production in nitrergic neurons.

Cognitive outcome in urea cycle disorders

Despite treatment, cognitive and motor deficits are common in individuals with inherited urea cycle disorders. However, the extent to which the deficits involve specific cognitive or sensorimotor domains is unknown. Furthermore, little is known about the neurochemical basis of cognitive impairment in these disorders. This paper reviews studies of cognitive and motor dysfunction in urea cycle disorders, and discusses potential venues for investigation of the underlying neural basis that may elucidate these defects. Such methods of investigation may serve as a model for studying the relationship between genes, biochemical markers, brain function, and behavior in other metabolic diseases.

Ornithine transcarbamylase deficiency: a urea cycle defect

The symptoms and signs of ornithine transcarbamylase deficiency are discussed. When the condition occurs among males in the neonatal period it is likely to be lethal. Pathological findings are non-specific. The diagnosis should be considered if coma with cerebral oedema and respiratory alkalosis occurs for no obvious reason. When hyperammonaemia is found, enzyme assay on a liver biopsy should be considered. A useful clue in an asymptomatic patient is a voluntary adoption of a vegetarian diet. Provocative tests, such as the allopurinol test can be used, but the method most frequently applied is mutation analysis. In the case of prenatal diagnosis this is possible on a chorionic villus sample. The prognosis of ornithine transcarbamylase deficiency is better for those with an onset after infancy, but morbidity from brain damage does not appear to be linked to the number of episodes of hyperammonaemia that have occurred. The syndrome results from a deficiency of the mitochondrial enzyme ornithine transcarbamylase which catalyses the conversion of ornithine and carbamoyl phosphate to citrulline. The gene responsible for this enzyme is located on Xp21.1, and is expressed in the liver and gut. Mutations can be divided into two groups: those with neonatal onset with all enzyme activity abolished, and those with later onset with partial and varying enzyme deficiency. There can be a variety of precipitating causes, for example sodium valproate. Treatment can be given with a low protein diet, and with alternate pathway drugs such as sodium benzoate and phenylbutyrate. Liver transplant can be considered when symptoms are life-threatening, although there may be severe complications.Gene replacement therapy is the hope of the future.

Reversal of biochemical and behavioral parameters of brain aging by melatonin and acetyl L-carnitine

The potential utility of dietary supplementation in order to prevent some of the oxidative and inflammatory changes occurring in the brain with age, has been studied. The cerebral cortex of 27-month-old male B6C3F1 mice had elevated levels of nitric oxide synthase 1 (EC 1.14.13.39) (nNOS) and peptide nitrotyrosine relative to cortices of younger (4-month-old) animals. After 25-month-old mice received basal diet together with 300 mg/l acetyl L-carnitine in the drinking water for 8 weeks, these levels were fully restored to those found in younger animals. A partial restoration was found when old animals received basal diet supplemented with 200 ppm melatonin in the diet. Levels of mRNA (messenger RNA) for nNOS were unchanged following these treatments implying translational regulation of nNOS activity. Behavioral indices indicative of exploratory behavior were also depressed in aged animals. Dietary supplementation with melatonin or acetyl L-carnitine partially reversed these changes. These findings suggest that dietary supplementation cannot merely arrest but indeed reverse some age-related increases in markers of oxidative and inflammatory events occurring with the cortex.

Changes in Ca(2+) channel expression upon differentiation of SN56 cholinergic cells

The SN56 cell line, a fusion of septal neurons and neuroblastoma cells, has been used as a model for central cholinergic neurons. These cells show increased expression of cholinergic neurochemical features upon differentiation, but little is known about how differentiation affects their electrophysiological properties. We examined the changes in Ca(2+) channel expression that occur as these cells undergo morphological differentiation in response to serum withdrawal and exposure to dibutyryl-cAMP. Undifferentiated cells expressed a T-type current with biophysical and pharmacological properties similar, although not identical, to those reported for the current generated by the alpha(1H) (CaV3.2) Ca(2+) channel subunit. Differentiated cells expressed, in addition to this T-type current, high voltage activated currents which were inhibited 38% by the L-type channel antagonist nifedipine (5 microM), 37% by the N-type channel antagonist omega-conotoxin-GVIA (1 microM), and 15% by the P/Q-type channel antagonist omega-agatoxin-IVA (200 nM). Current resistant to these inhibitors accounted for 15% of the high voltage activated current in differentiated SN56 cells. Our data demonstrate that differentiation increases the expression of neuronal type voltage gated Ca(2+) channels in this cell line, and that the channels expressed are comparable to those reported for native basal forebrain cholinergic neurons. This cell line should thus provide a useful model system to study the relationship between calcium currents and cholinergic function and dysfunction.