Mitochondrial Enzymes of the Urea Cycle Cluster at the Inner Mitochondrial Membrane

Elucidating the Role of Human ALAS2 C-terminal Mutations Resulting in Loss of Function and Disease

The conserved enzyme aminolevulinic acid synthase (ALAS) initiates heme biosynthesis in certain bacteria and eukaryotes by catalyzing the condensation of glycine and succinyl-CoA to yield aminolevulinic acid. In humans, the ALAS isoform responsible for heme production during red blood cell development is the erythroid-specific ALAS2 isoform. Owing to its essential role in erythropoiesis, changes in human ALAS2 (hALAS2) function can lead to two different blood disorders. X-linked sideroblastic anemia results from loss of ALAS2 function, while X-linked protoporphyria results from gain of ALAS2 function. Interestingly, mutations in the ALAS2 C-terminal extension can be implicated in both diseases. Here, we investigate the molecular basis for enzyme dysfunction mediated by two previously reported C-terminal loss-of-function variants, hALAS2 V562A and M567I. We show that the mutations do not result in gross structural perturbations, but the enzyme stability for V562A is decreased. Additionally, we show that enzyme stability moderately increases with the addition of the pyridoxal 5'-phosphate (PLP) cofactor for both variants. The variants display differential binding to PLP and the individual substrates compared to wild-type hALAS2. Although hALAS2 V562A is a more active enzyme in vitro, it is less efficient concerning succinyl-CoA binding. In contrast, the M567I mutation significantly alters the cooperativity of substrate binding. In combination with previously reported cell-based studies, our work reveals the molecular basis by which hALAS2 C-terminal mutations negatively affect ALA production necessary for proper heme biosynthesis.

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

Novel Mitochondria-Targeted Amphiphilic Aminophosphonium Salts and Lipids Nanoparticles: Synthesis, Antitumor Activity and Toxicity

The creation of mitochondria-targeted vector systems is a new tool for the treatment of socially significant diseases. Phosphonium groups provide targeted delivery of drugs through biological barriers to organelles. For this purpose, a new class of alkyl(diethylAmino)(Phenyl) Phosphonium halides (APPs) containing one, two, or three diethylamino groups was obtained by the reaction of alkyl iodides (bromides) with (diethylamino)(phenyl)phosphines under mild conditions (20 °C) and high yields (93-98%). The structure of APP was established by NMR and XRD. A high in vitro cytotoxicity of APPs against M-HeLa, HuTu 80, PC3, DU-145, PANC-1, and MCF-7 lines was found. The selectivity index is in the range of 0.06-4.0 μM (SI 17-277) for the most active APPs. The effect of APPs on cancer cells is characterized by hyperproduction of ROS and depolarization of the mitochondrial membrane. APPs induce apoptosis, proceeding along the mitochondrial pathway. Incorporation of APPs into lipid systems (liposomes and solid lipid nanoparticles) improves cytotoxicity toward tumor cells and decrease toxicity against normal cell lines. The IC50s of lipid systems are lower than for the reference drug DOX, with a high SI (30-56) toward MCF-7 and DU-145. APPs exhibit high selective activity against Gram-positive bacteria S. aureus 209P and B. segeus 8035, including methicillin-resistant S. aureus (MRSA-1, MRSA-2), comparable to the activity of the fluoroquinolone antibiotic norfloxacin. A moderate in vivo toxicity in CD-1 mice was established for the lead APP.

Hypoosmosis alters hepatocyte mitochondrial morphology and induces selective release of carbamoyl phosphate synthetase 1

Carbamoyl phosphate synthetase 1 (CPS1) is the most abundant hepatocyte mitochondrial matrix protein. Hypoosmotic stress increases CPS1 release in isolated mouse hepatocytes without cell death. We hypothesized that increased CPS1 release during hypoosmosis is selective and associates with altered mitochondrial morphology. Both ex vivo and in vivo models were assessed. Mouse hepatocytes and livers were challenged with isotonic or hypoosmotic (35 mosM) buffer. Mice were injected intraperitoneally with water (10% body weight) with or without an antidiuretic. Mitochondrial and cytosolic fractions were isolated using differential centrifugation, then analyzed by immunoblotting to assess subcellular redistribution of four mitochondrial proteins: CPS1, ornithine transcarbamylase (OTC), pyrroline-5-carboxylate reductase 1 (PYCR1), and cytochrome c. Mitochondrial morphology alterations were examined using electron microscopy. Hypoosmotic treatment of whole livers or hepatocytes led to preferential or increased mitochondrial release, respectively, of CPS1 as compared with two mitochondrial matrix proteins (OTC/PYCR1) and with the intermembrane space protein, cytochrome c. Mitochondrial apoptosis-induced channel opening using staurosporine in hepatocytes led to preferential CPS1 and cytochrome c release. The CPS1-selective changes were accompanied by dramatic alterations in ultrastructural mitochondrial morphology. In mice, hypoosmosis/hyponatremia led to increased liver vascular congestion and increased CPS1 in bile but not blood, coupled with mitochondrial structural alterations. In contrast, isotonic increase of intravascular volume led to a decrease in mitochondrial size with limited change in bile CPS1 compared with hypoosmotic conditions and absence of the hypoosmosis-associated histological alterations. Taken together, hepatocyte CPS1 is selectively released in response to hypoosmosis/hyponatremia and provides a unique biomarker of mitochondrial injury.NEW & NOTEWORTHY Exposure of isolated mouse livers, primary cultured hepatocytes, or mice to hypoosmosis/hyponatremia conditions induces significant mitochondrial shape alterations accompanied by preferential release of the mitochondrial matrix protein CPS1, a urea cycle enzyme. In contrast, the intermembrane space protein, cytochrome c, and two other matrix proteins, including the urea cycle enzyme ornithine transcarbamylase, remain preferentially retained in mitochondria. Therefore, hepatocyte CPS1 manifests unique mitochondrial stress response compartmentalization and is a sensitive sensor of mitochondrial hypoosmotic/hyponatremic injury.

Expression of Synj2bp in mouse liver regulates the extent of wrappER-mitochondria contact to maintain hepatic lipid homeostasis

BACKGROUND

In mouse liver hepatocytes, nearly half of the surface area of every mitochondrion is covered by wrappER, a wrapping-type of ER that is rich in fatty acids and synthesizes lipoproteins (VLDL) (Anastasia et al. in Cell Rep 34:108873, 2021; Hurtley in Science (80- ) 372:142-143, 2021; Ilacqua et al. in J Cell Sci 135:1-11, 2021). A disruption of the ultrastructure of the wrappER-mitochondria contact results in altered fatty acid flux, leading to hepatic dyslipidemia (Anastasia et al. 2021). The molecular mechanism that regulates the extent of wrappER-mitochondria contacts is unknown.

METHODS

We evaluated the expression level of the mitochondrial protein Synj2bp in the liver of normal and obese (ob/ob) mice. In addition, we silenced its expression in the liver using an AAV8 vector. We coupled quantitative EM morphometric analysis to proteomics and lipid analyses on these livers.

RESULTS

The expression level of Synj2bp in the liver positively correlates with the extent of wrappER-mitochondria contacts. A 50% reduction in wrappER-mitochondria contacts causes hepatic dyslipidemia, characterized by a gross accumulation of lipid droplets in the liver, an increased hepatic secretion of VLDL and triglycerides, a curtailed ApoE expression, and an increased capacity of mitochondrial fatty acid respiration.

CONCLUSION

Synj2bp regulates the extent of wrappER-mitochondria contacts in the liver, thus contributing to the control of hepatic lipid flux.

Cord blood metabolomics reveals gestational metabolic disorder associated with anti-thyroid peroxidase antibodies positivity

Background

Thyroid disease is one of the common endocrine disorders affecting the pregnant women, in which thyroid autoimmunity can alter the progress and the outcome of pregnancy. Women with euthyroid status but anti-thyroid peroxidase (anti-TPO) antibodies positivity before pregnancy are prone to subclinical gestational hypothyroidism. However, the connections between anti-TPO antibodies positivity and gestational hypothyroidism remain largely unknown. The aim of the present study is to investigate the differences of fetal metabolic profile at birth according to maternal anti-TPO status.

Methods

We performed 1H-NMR metabolomics on cord blood of a nested case control cohort of 22 pregnant women with matched thyroid hormone levels and demographic data, including 11 women with euthyroid status but anti-thyroid antibodies positivity (into the anti-TPO antibodies positivity group) and 11 matched women as controls with euthyroid status and negative anti-thyroid antibodies (into the control group).

Results

Distinct metabolic profiles were observed between the anti-TPO antibody positivity group and the nested control group, from which a total of 10 metabolites with between-group altered abundances were structurally identified. Five out of the 10 metabolites were up-regulated in the anti-TPO antibodies positivity group, including D-Glucose, L-Glutamine, 3-Hydroxybutyric acid, Myo-Inositol, Creatinine. The other 5 metabolites were down-regulated in the anti-TPO antibodies positivity group, including L-Leucine, L-Lysine, L-Glutamic acid, L-Tyrosine, and L-Phenylalanine. All the 10 metabolites have been previously reported to be correlated with hypothyroidism. Metabolite set enrichment analysis and pathway analysis suggested that amino acid metabolism pathways (especially the phenylalanine metabolism) were associated with anti-TPO antibodies positivity.

Conclusion

The results of this study suggested that fetal metabolic disorder is correlated with anti-TPO antibodies positivity, representing by abundance alteration of hypothyroidism associated metabolites and the related disturbance of amino acid metabolism pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-022-04564-8.

Serum Metabolomic Analysis of Male Patients with Cannabis or Amphetamine Use Disorder

Studies have demonstrated that chronic consumption of abused drugs induces alterations in several proteins that regulate metabolism. For instance, methamphetamine exposure reduces glucose levels. Fatty and amino acid levels were altered in groups exposed to abused drugs. Therefore, in our study, we investigated the serum metabolomic profile of patients diagnosed with cannabis and/or amphetamine use disorders. Blood was obtained from subjects (control, amphetamine, and cannabis). Detection of serum metabolites was performed using gas chromatography. The ratio peak areas for metabolites were analyzed across the three groups. Both cannabis and amphetamine groups showed higher d-erythrotetrafuranose, octadecanoic acid, hexadecenoic acid, trans-9-octadecanoic acid, lactic acid and methyl thio hydantoin metabolites compared with the control group. Moreover, cannabis patients were found to possess higher glycine, 9,12 octadecanoic acid malonic acid, phosphoric acid and prostaglandin F1a than controls. Our analysis showed that the identified metabolic profile of cannabis or amphetamine use disorder patients was different than control group. Our data indicated that chronic exposure to cannabis or amphetamine dysregulated metabolites in the serum. Future studies are warranted to explore the effects of these abused drugs on the metabolic proteins.

Assessing Protein Interactions for Clustering of Mitochondrial Urea Cycle Enzymes

Enzyme clustering is a phenomenon that involves partitioning of proteins that function together in a common subcellular or sub-organellar compartment. Traditional genetic, biochemical, and biophysical approaches for studying protein-protein interactions in complexes with defined stoichiometry yield inconclusive results when applied to clustered proteins. This chapter describes a combination of approaches to study clustered proteins including co-immunoprecipitation, biochemical co-localization in purified mitochondria, and super resolution imaging of endogenous proteins in situ. These approaches can be used to study interactions among proteins that form clusters. We will illustrate this approach by using the urea cycle enzymes that localize in the mitochondrial matrix, and form clusters at the inner mitochondrial membrane.

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.