Aberrations of ammonia metabolism in ornithine carbamoyltransferase-deficient spf-ash mice and their prevention by treatment with urea cycle intermediate amino acids and an ornithine aminotransferase inactivator

Corticosteroid suppresses urea-cycle-related gene expressions in ornithine transcarbamylase deficiency

Background

Ornithine transcarbamylase deficiency (OTCD) is most common among urea cycle disorders (UCDs), defined by defects in enzymes associated with ureagenesis. Corticosteroid administration to UCD patients, including OTCD patients, is suggested to be avoided, as it may induce life-threatening hyperammonemia. The mechanism has been considered nitrogen overload due to the catabolic effect of corticosteroids; however, the pathophysiological process is unclear.

Methods

To elucidate the mechanism of hyperammonemia induced by corticosteroid administration in OTCD patients, we analyzed a mouse model by administering corticosteroids to OTC^spf−ash mice deficient in the OTC gene. Dexamethasone (DEX; 20 mg/kg) was administered to the OTC^spf−ash and wild-type (WT) mice at 0 and 24 h, and the serum ammonia concentrations, the levels of the hepatic metabolites, and the gene expressions related with ammonia metabolism in the livers and muscles were analyzed.

Results

The ammonia levels in Otc^spf−ash mice that were administered DEX tended to increase at 24 h and increased significantly at 48 h. The metabolomic analysis showed that the levels of citrulline, arginine, and ornithine did not differ significantly between Otc^spf−ash mice that were administered DEX and normal saline; however, the level of aspartate was increased drastically in Otc^spf−ash mice owing to DEX administration (P < 0.01). Among the enzymes associated with the urea cycle, mRNA expressions of carbamoyl-phosphate synthase 1, ornithine transcarbamylase, arginosuccinate synthase 1, and arginosuccinate lyase in the livers were significantly downregulated by DEX administration in both the Otc^spf−ash and WT mice (P < 0.01). Among the enzymes associated with catabolism, mRNA expression of Muscle RING-finger protein-1 in the muscles was significantly upregulated in the muscles of WT mice by DEX administration (P < 0.05).

Conclusions

We elucidated that corticosteroid administration induced hyperammonemia in Otc^spf−ash mice by not only muscle catabolism but also suppressing urea-cycle-related gene expressions. Since the urea cycle intermediate amino acids, such as arginine, might not be effective because of the suppressed expression of urea-cycle-related genes by corticosteroid administration, we should consider an early intervention by renal replacement therapy in cases of UCD patients induced by corticosteroids to avoid brain injuries or fatal outcomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12876-022-02213-0.

Dietary Management, Clinical Status and Outcome of Patients with Citrin Deficiency in the UK

Background: Little is known about the optimal dietary treatment for citrin deficiency. Our aim is to describe the management of UK citrin deficiency patients. Methods: A longitudinal retrospective review was performed. Data were collected from medical records on presenting signs and symptoms, dietary management and clinical outcome. Results: data were collected on 32 patients from 21 families. 50% were females (16/32). Median age at diagnosis was 4 y (5 days–35 y) with 12 patients diagnosed in the neonatal period with neonatal intrahepatic cholestasis (NICCD), eight later in childhood (FTTDCD) and 12 by family screening based on index cases from five families. No patient had adult-onset type II citrullinemia. The patient age at the time of data collection was a median of 11 y (1–44 y). 91% (29/32) of patients had normal physical and neurological development, 47% (15/32) experienced recurrent unexplained abdominal pain and 9% (3/32) episodes of hypoglycaemia. Siblings had different phenotypes (5 families had > 1 affected patient). Most patients preferred high protein foods, limiting sugar-containing foods. Only 41% (13/32) were prescribed a low CHO, high protein, high fat diet (restriction varied) and two used medium chain triglyceride (MCT) supplements. No patient was prescribed drug therapy. Twenty-five per cent (8/32) of patients were underweight and 41% (13/32) had height <−1 z-scores. Conclusions: patients presented with various phenotypes, symptoms and suboptimal growth. Symptoms and biochemical markers improved with age, but height remained low in some. More research is necessary to assess the effectiveness of dietary approaches in improving clinical outcomes and symptoms in citrin deficiency.

Efficient in vivo editing of OTC-deficient patient-derived primary human hepatocytes

Background & Aims

Genome editing technology has immense therapeutic potential and is likely to rapidly supplant contemporary gene addition approaches. Key advantages include the capacity to directly repair mutant loci with resultant recovery of physiological gene expression and maintenance of durable therapeutic effects in replicating cells. In this study, we aimed to repair a disease-causing point mutation in the ornithine transcarbamylase (OTC) locus in patient-derived primary human hepatocytes in vivo at therapeutically relevant levels.

Methods

Editing reagents for precise CRISPR/SaCas9-mediated cleavage and homology-directed repair (HDR) of the human OTC locus were first evaluated against an OTC minigene cassette transposed into the mouse liver. The editing efficacy of these reagents was then tested on the native OTC locus in patient-derived primary human hepatocytes xenografted into the FRG (Fah^-/-Rag2^-/-Il2rg^-/-) mouse liver. A highly human hepatotropic capsid (NP59) was used for adeno-associated virus (AAV)-mediated gene transfer. Editing events were characterised using next-generation sequencing and restoration of OTC expression was evaluated using immunofluorescence.

Results

Following AAV-mediated delivery of editing reagents to patient-derived primary human hepatocytes in vivo, OTC locus-specific cleavage was achieved at efficiencies of up to 72%. Importantly, successful editing was observed in up to 29% of OTC alleles at clinically relevant vector doses. No off-target editing events were observed at the top 10 in silico-predicted sites in the genome.

Conclusions

We report efficient single-nucleotide correction of a disease-causing mutation in the OTC locus in patient-derived primary human hepatocytes in vivo at levels that, if recapitulated in the clinic, would provide benefit for even the most therapeutically challenging liver disorders. Key challenges for clinical translation include the cell cycle dependence of classical HDR and mitigation of unintended on- and off-target editing events.

Lay summary

The ability to efficiently and safely correct disease-causing mutations remains the holy grail of gene therapy. Herein, we demonstrate, for the first time, efficient in vivo correction of a patient-specific disease-causing mutation in the OTC gene in primary human hepatocytes, using therapeutically relevant vector doses. We also highlight the challenges that need to be overcome for this technology to be translated into clinical practice.

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Therapeutically relevant levels of single-nucleotide repair of the human OTC locus were achieved in vivo.

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Single-nucleotide editing of primary human hepatocytes was facilitated by a highly hepatotropic bioengineered AAV capsid.

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A novel human minigene platform proved highly effective for evaluation of human liver-specific genome editing reagents.

Ammonia and autophagy: An emerging relationship with implications for disorders with hyperammonemia

(Macro)autophagy/autophagy is a highly regulated lysosomal degradative process by which cells recycle their own nutrients, such as amino acids and other metabolites, to be reused in different biosynthetic pathways. Ammonia is a diffusible compound generated daily from catabolism of nitrogen-containing molecules and from gastrointestinal microbiome. Ammonia homeostasis is tightly controlled in humans and ammonia is efficiently converted by the healthy liver into non-toxic urea (through ureagenesis) and glutamine (through glutamine synthetase). Impaired ammonia detoxification leads to systemic hyperammonemia, a life-threatening condition resulting in detrimental effects on central nervous system. Here, we review current understanding on the role of ammonia in modulation of autophagy and the potential implications in the pathogenesis and treatment of disorders with hyperammonemia.

Acute metabolic decompensation due to influenza in a mouse model of ornithine transcarbamylase deficiency

The urea cycle functions to incorporate ammonia, generated by normal metabolism, into urea. Urea cycle disorders (UCDs) are caused by loss of function in any of the enzymes responsible for ureagenesis, and are characterized by life-threatening episodes of acute metabolic decompensation with hyperammonemia (HA). A prospective analysis of interim HA events in a cohort of individuals with ornithine transcarbamylase (OTC) deficiency, the most common UCD, revealed that intercurrent infection was the most common precipitant of acute HA and was associated with markers of increased morbidity when compared with other precipitants. To further understand these clinical observations, we developed a model system of metabolic decompensation with HA triggered by viral infection (PR8 influenza) using spf-ash mice, a model of OTC deficiency. Both wild-type (WT) and spf-ash mice displayed similar cytokine profiles and lung viral titers in response to PR8 influenza infection. During infection, spf-ash mice displayed an increase in liver transaminases, suggesting a hepatic sensitivity to the inflammatory response and an altered hepatic immune response. Despite having no visible pathological changes by histology, WT and spf-ash mice had reduced CPS1 and OTC enzyme activities, and, unlike WT, spf-ash mice failed to increase ureagenesis. Depression of urea cycle function was seen in liver amino acid analysis, with reductions seen in aspartate, ornithine and arginine during infection. In conclusion, we developed a model system of acute metabolic decompensation due to infection in a mouse model of a UCD. In addition, we have identified metabolic perturbations during infection in the spf-ash mice, including a reduction of urea cycle intermediates. This model of acute metabolic decompensation with HA due to infection in UCD serves as a platform for exploring biochemical perturbations and the efficacy of treatments, and could be adapted to explore acute decompensation in other types of inborn errors of metabolism.

Citrin deficiency and current treatment concepts

In this paper, we describe the historical aspects of citrin and citrin deficiency, characteristic food preference and food aversion of citrin-deficient subjects, and carbohydrate toxicity in relation to ureogenesis and issues of the conventional treatment procedures for hyperammonemia in citrin deficiency, leading to current treatment concepts for citrin deficiency. We also emphasize the importance of a citrin deficiency mouse model in elucidating the pathophysiology and developing novel therapeutics based on the pathophysiology, such as sodium pyruvate.

The acyl nitroso Diels-Alder (ANDA) reaction of sorbate derivatives: an X-ray and 15N NMR study with an application to amino-acid synthesis

We present a study of the acyl nitroso Diels-Alder (ANDA) reaction of sorbate esters and sorbic alcohol derivatives, using alkoxycarbonyl nitroso dienophiles. An optimisation of the reaction conditions for ethyl sorbate is first presented, and the product is used in an efficient synthesis of 5-methylornithine. Structure-reactivity trends in sorbic alcohol (E,E-2,4-hexadien-1-ol) and its acylated analogues are then discussed. We present single-crystal X-ray structural proof for key adducts in both series and present in detail a novel HMBC/HSQC ((1)H-(15)N) criterion for ready distinction of regioisomers arising from such ANDA reactions.

Treatment of a citrin-deficient patient at the early stage of adult-onset type II citrullinaemia with arginine and sodium pyruvate

Citrin deficiency is a common congenital metabolic defect not only in East Asian populations but also in other populations around the world. It has been shown that although liver transplantation is ultimately required in many patients to prevent neurological decompensation associated with hyperammonaemia, arginine is effective in lowering ammonia in hyperammonaemic patients, and a high-protein low-carbohydrate diet may provide some benefit to infants in improving failure to thrive. In the present study, the clinical symptoms and laboratory findings are reported for a 13-year-old citrin-deficient girl in the early stage of adult-onset type II citrullinaemia (CTLN2), and the therapeutic effect of orally administered arginine and sodium pyruvate was investigated. The patient complained of anorexia, lethargy, fatigue and poor growth, and showed laboratory findings typical of CTLN2; elevated levels of plasma citrulline, threonine-to-serine ratio, and serum pancreatic secretory trypsin inhibitor. Oral administration of arginine and sodium pyruvate for over 3 years improved her clinical symptoms and has almost completely normalized her laboratory findings. It is suggested that the administration of arginine and sodium pyruvate with low-carbohydrate meals may be an effective therapy in patients with citrin deficiency in order either to prolong metabolic normalcy or to provide a safer and more affordable alternative to liver transplantation.

Interaction between murine spf-ash mutation and genetic background yields different metabolic phenotypes

The spf-ash mutation in mice results in reduced hepatic and intestinal ornithine transcarbamylase. However, a reduction in enzyme activity only translates in reduced ureagenesis and hyperammonemia when an unbalanced nitrogen load is imposed. Six-week-old wild-type control and spf-ash mutant male mice from different genetic backgrounds (B6 and ICR) were infused intravenously with [(13)C(18)O]urea, l-[(15)N(2)]arginine, l-[5,5 D(2)]ornithine, l-[6-(13)C, 4,4,5,5, D(4)]citrulline, and l-[ring-D(5)]phenylalanine to investigate the interaction between genetic background and spf-ash mutation on ureagenesis, arginine metabolism, and nitric oxide production. ICR(spf-ash) mice maintained ureagenesis (5.5 +/- 0.3 mmol.kg(-1).h(-1)) and developed mild hyperammonemia (145 +/- 19 micromol/l) when an unbalanced nitrogen load was imposed; however, B6(spf-ash) mice became hyperammonemic (671 +/- 15 micromol/l) due to compromised ureagenesis (3.4 +/- 0.1 mmol.kg(-1).h(-1)). Ornithine supplementation restored ureagenesis and mitigated hyperammonemia. A reduction in citrulline entry rate was observed due to the mutation in both genetic backgrounds (wild-type: 128, spf-ash: 60; SE 4.0 micromol.kg(-1).h(-1)). Arginine entry rate was only reduced in B6(spf-ash) mice (B6(spf-ash): 332, ICR(spf-ash): 453; SE 20.6 micromol.kg(-1).h(-1)). Genetic background and mutation had an effect on nitric oxide production (B6: 3.4, B6(spf-ash): 2.8, ICR: 9.0, ICR(spf-ash): 4.6, SE 0.7 micromol.kg(-1).h(-1)). Protein breakdown was the main source of arginine during the postabsorptive state and was higher in ICR(spf-ash) than in B6(spf-ash) mice (phenylalanine entry rate 479 and 327, respectively; SE 18 micromol.kg(-1).h(-1)). Our results highlight the importance of the interaction between mutation and genetic background on ureagenesis, arginine metabolism, and nitric oxide production. These observations help explain the wide phenotypic variation of ornithine transcarbamylase deficiency in the human population.

Ornithine restores ureagenesis capacity and mitigates hyperammonemia in Otc(spf-ash) mice

We showed that Otc(spf-ash) mice, a model of ornithine transcarbamylase deficiency, were able to sustain ureagenesis at the same rate as control mice, despite reduced enzyme activity, when a complete mixture of amino acids was provided. An unbalanced amino acid mixture, however, resulted in reduced ureagenesis and hyperammonemia. To study the effect of ornithine supplementation [316 micromol/(kg.h)] on urea and glutamine kinetics in conscious Otc(spf-ash) mice under a glycine-alanine load [6.06 mmol/(kg.h)], a multiple tracer infusion protocol ([(13)C(18)O]urea, [5-(15)N]glutamine, [2,3,3,4,4 D(5)]glutamine and [ring-D(5)] phenylalanine) was conducted. Ornithine supplementation increased ureagenesis [3.18 +/- 0.88 vs. 4.56 +/- 0.51 mmol/(kg.h), P < 0.001], reduced plasma ammonia concentration (1125 +/- 621 vs. 193 +/- 94 micromol/L, P < 0.001), and prevented acute hepatic enlargement (P < 0.006) in Otc(spf-ash) mice. Ornithine supplementation also increased [96 +/- 20 vs. 120 +/- 16 micromol/(kg.h), P < 0.001] the transfer of (15)N from glutamine to urea, to values observed in the control mice [123 +/- 17 micromol/(kg.h)]. De novo amido-N glutamine flux was higher [1.57 +/- 0.37 vs. 3.04 +/- 0.86 mmol/(kg.h); P < 0.001] in Otc(spf-ash) mice, but ornithine supplementation had no effect (P < 0.56). The flux of glutamine carbon skeleton was affected by both genotype (P < 0.0001) and by ornithine (P 0. 036). In conclusion, ornithine supplementation restored ureagenesis, mitigated hyperammonemia, prevented liver enlargement, and normalized the transfer of (15)N from glutamine to urea. These data strongly suggest that ornithine has the potential for the biochemical correction of OTCD in Otc(spf-ash) mice.

Reduced ornithine transcarbamylase activity does not impair ureagenesis in Otc(spf-ash) mice

Mouse models for urea cycle disorders have been available for the past 30 y; however, until now, no measurements of urea production in vivo have been conducted. Urea entry rate was determined in Otc(spf-ash) and littermate controls employing a primed-continuous infusion of 15N15N urea. A saline infusion control, a complete mixture of amino acids (AA), or a glycine-alanine (GA) mixture was infused at 86 (AA1 and GA1) and 172 mg N.kg(-1).h(-1) (AA2 and GA2) to impose a defined nitrogen load on the urea cycle. Urea entry rate and plasma urea concentration increased (P < 0.001) as a consequence of the increase in the infusion rate of the complete mixture of amino acids, but the 2 genotypes did not differ (P = 0.96 and P = 0.44, respectively). The infusion of the GA mixture, however, decreased (P < 0.001) the plasma urea concentration and urea entry rate in Otc(spf-ash) mice compared with controls. At the highest level (GA2), urea entry rate was further depressed (P < 0.001), Otc(spf-ash) mice became hyperammonemic (1701 +/- 150 micromol/L), and hyperammonemic symptoms were evident. An acute hepatic enlargement (P < 0.001) was also evident in Otc(spf-ash) mice infused with GA2. These results show that despite vestigial OTC activity, Otc(spf-ash) mice were able to maintain ureagenesis at the same rate of control animals when a complete mixture of amino acids was infused. This implies that Otc(spf-ash) mice are able to dispose of ammonia, without apparent adverse effects, when a balance mixture of amino acids is provided, despite reduced enzyme activity.

Reconstruction of liver organoid using a bioreactor

AIM: To develop the effective technology for reconstruction of a liver organ in vitro using a bio-artificial liver.

METHODS: We previously reported that a radial-flow bioreactor (RFB) could provide a three-dimensional high-density culture system. We presently reconstructed the liver organoid using a functional human hepatocellular carcinoma cell line (FLC-5) as hepatocytes together with mouse immortalized sinusoidal endothelial cell (SEC) line M1 and mouse immortalized hepatic stellate cell (HSC) line A7 as non parenchymal cells in the RFB. Two x 10⁷ FLC-5 cells were incubated in the RFB. After 5 d, 2 x 10⁷ A7 cells were added in a similar manner followed by another addition of 10⁷ M1 cells 5 d later. After three days of perfusion, some cellulose beads with the adherent cells were harvested. The last incubation period included perfusion with 200 nmol/L swinholide A for 2 h and then the remaining cellulose beads along with adherent cells were harvested from the RFB. The cell morphology was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). To assess hepatocyte function, we compared mRNA expression for urea cycle enzymes as well as albumin synthesis by FLC-5 in monolayer cultures compared to those of single-type cultures and cocultures in the RFB.

RESULTS: By transmission electron microscopy, FLC-5, M1, and A7 were arranged in relation to the perfusion side in a liver-like organization. Structures resembling bile canaliculi were seen between FCL-5 cells. Scanning electron microscopy demonstrated fenestrae on SEC surfaces. The number of vesiculo-vacuolar organelles (VVO) and fenestrae increased when we introduced the actin-binding agent swinholide-A in the RFB for 2h. With respect to liver function, urea was found in the medium, and expression of mRNAs encoding arginosuccinate synthetase and arginase increased when the three cell types were cocultured in the RFB. However, albumin synthesis decreased.

CONCLUSION: Co-culture in the RFB system can dramatically change the structure and function of all cell types, including the functional characteristics of hepatocytes. Our system proves effective for reconstruction of a liver organoid using a bio-artificial liver.

Slc25a13-knockout mice harbor metabolic deficits but fail to display hallmarks of adult-onset type II citrullinemia

Adult-onset type II citrullinemia (CTLN2) is an autosomal recessive disease caused by mutations in SLC25A13, the gene encoding the mitochondrial aspartate/glutamate carrier citrin. The absence of citrin leads to a liver-specific, quantitative decrease of argininosuccinate synthetase (ASS), causing hyperammonemia and citrullinemia. To investigate the physiological role of citrin and the development of CTLN2, an Slc25a13-knockout (also known as Ctrn-deficient) mouse model was created. The resulting Ctrn-/- mice were devoid of Slc25a13 mRNA and citrin protein. Liver mitochondrial assays revealed markedly decreased activities in aspartate transport and the malate-aspartate shuttle. Liver perfusion also demonstrated deficits in ureogenesis from ammonia, gluconeogenesis from lactate, and an increase in the lactate-to-pyruvate ratio within hepatocytes. Surprisingly, Ctrn-/- mice up to 1 year of age failed to show CTLN2-like symptoms due to normal hepatic ASS activity. Serological measures of glucose, amino acid, and ammonia metabolism also showed no significant alterations. Nitrogen-loading treatments produced only minor changes in the hepatic ammonia and amino acid levels. These results suggest that citrin deficiency alone may not be sufficient to produce a CTLN2-like phenotype in mice. These observations are compatible, however, with the variable age of onset, incomplete penetrance, and strong ethnic bias seen in CTLN2 where additional environmental and/or genetic triggers are now suspected.