Pharmacologic rescue of an enzyme-trafficking defect in primary hyperoxaluria 1

Efficient and safe in vivo treatment of primary hyperoxaluria type 1 via LNP-CRISPR-Cas9-mediated glycolate oxidase disruption

Primary hyperoxaluria type 1 (PH1) is a severe genetic metabolic disorder caused by mutations in the AGXT gene, leading to defects in enzymes crucial for glyoxylate metabolism. PH1 is characterized by severe, potentially life-threatening manifestations due to excessive oxalate accumulation, which leads to calcium oxalate crystal deposits in the kidneys and, ultimately, renal failure and systemic oxalosis. Existing substrate reduction therapies, such as inhibition of liver-specific glycolate oxidase (GO) encoded by HAO1 using siRNA or CRISPR-Cas9 delivered by adeno-associated virus, either require repeated dosing or have raised safety concerns. To address these limitations, our study employed lipid nanoparticles (LNPs) for CRISPR-Cas9 delivery to rapidly generate a PH1 mouse model and validate the therapeutic efficacy of LNP-CRISPR-Cas9 targeting the Hao1 gene. The LNP-CRISPR-Cas9 system exhibited efficient editing of the Hao1 gene, significantly reducing GO expression and lowering urinary oxalate levels in treated PH1 mice. Notably, these effects persisted for 12 months with no significant off-target effects, liver-induced toxicity, or substantial immune responses, highlighting the approach's safety and specificity. Furthermore, the developed humanized mouse model validated the efficacy of our therapeutic strategy. These findings support LNP-CRISPR-Cas9 targeting HAO1 as a promising and safer alternative for PH1 treatment with a single administration.

Mutation Characteristics of Primary Hyperoxaluria in the Chinese Population and Current International Diagnosis and Treatment Status

Background

Primary hyperoxaluria (PH) is a rare autosomal recessive disorder, mainly due to the increase in endogenous oxalate production, causing a series of clinical features such as kidney stones, nephrocalcinosis, progressive impairment of renal function, and systemic oxalosis. There are three common genetic causes of glycolate metabolism anomalies. Among them, PH type 1 is the most prevalent and severe type, and early end-stage renal failure often occurs.

Summary

This review summarizes PH through pathophysiology, genotype, clinical manifestation, diagnosis, and treatment options. And explore the characteristics of Chinese PH patients.

Key Messages

Diagnosis of this rare disease is based on clinical symptoms, urinary or blood oxalate concentrations, liver biopsy, and genetic testing. Currently, the main treatment is massive hydration, citrate inhibition of crystallization, dialysis, liver and kidney transplantation, and pyridoxine. Recently, RNA interference drugs have also been used. In addition, technologies such as gene editing and autologous liver cell transplantation are also being developed. C.815_816insGA and c.33_34insC mutation in the AGXT gene could be a common variant in Chinese PH1 population. Mutations at the end of exon 6 account for approximately 50% of all Chinese HOGA1 mutations. Currently, the treatment of PH in China still relies mainly on symptomatic and high-throughput dialysis, with poor prognosis (especially for PH1 patients).

Navigating the Evolving Landscape of Primary Hyperoxaluria: Traditional Management Defied by the Rise of Novel Molecular Drugs

Primary hyperoxalurias (PHs) are inherited metabolic disorders marked by enzymatic cascade disruption, leading to excessive oxalate production that is subsequently excreted in the urine. Calcium oxalate deposition in the renal tubules and interstitium triggers renal injury, precipitating systemic oxalate build-up and subsequent secondary organ impairment. Recent explorations of novel therapeutic strategies have challenged and necessitated the reassessment of established management frameworks. The execution of diverse clinical trials across various medication classes has provided new insights and knowledge. With the evolution of PH treatments reaching a new milestone, prompt and accurate diagnosis is increasingly critical. Developing early, effective management and treatment plans is essential to improve the long-term quality of life for PH patients.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

In vivo base editing rescues primary hyperoxaluria type 1 in rats

Primary hyperoxaluria type 1 (PH1) is a childhood-onset autosomal recessive disease, characterized by nephrocalcinosis, multiple recurrent urinary calcium oxalate stones, and a high risk of progressive kidney damage. PH1 is caused by inherent genetic defects of the alanine glyoxylate aminotransferase (AGXT) gene. The in vivo repair of disease-causing genes was exceedingly inefficient before the invention of base editors which can efficiently introduce precisely targeted base alterations without double-strand DNA breaks. Adenine base editor (ABE) can precisely convert A·T to G·C with the assistance of specific guide RNA. Here, we demonstrated that systemic delivery of dual adeno-associated virus encoding a split-ABE8e could artificially repair 13% of the pathogenic allele in AgxtQ84X rats, a model of PH1, alleviating the disease phenotype. Specifically, ABE treatment partially restored the expression of alanine-glyoxylate-aminotransferase (AGT), reduced endogenous oxalate synthesis and alleviated calcium oxalate crystal deposition. Western blot and immunohistochemistry confirmed that ABE8e treatment restored AGT protein expression in hepatocytes. Moreover, the precise editing efficiency in the liver remained stable six months after treatment. Thus, our findings provided a prospect of in vivo base editing as a personalized and precise medicine for PH1 by directly correcting the mutant Agxt gene.

The Interaction and Effect of a Small MitoBlock Library as Inhibitor of ALR Protein-Protein Interaction Pathway

MIA40 and ALR of the MIA pathway mediate the import of protein precursors that form disulfides into the mitochondrial intermembrane space. This import pathway is suggested to be a linear pathway in which MIA40 first binds to the precursor via a disulfide linkage and oxidizes it. Subsequently, ALR re-oxidizes MIA40 and then ALR transfers electrons to terminal electron acceptors. However, the precise mechanism by which ALR and MIA40 coordinate translocation is unknown. With a collection of small molecule modulators (MB-5 to MB-9 and MB-13) that inhibit ALR activity, we characterized the import mechanism in mitochondria. NMR studies show that most of the compounds bind to a similar region in ALR. Mechanistic studies with small molecules demonstrate that treatment with compound MB-6 locks the precursor in a state bound to MIA40, blocking re-oxidation of MIA40 by ALR. Thus, small molecules that target a similar region in ALR alter the dynamics of the MIA import pathway differently, resulting in a set of probes that are useful for studying the catalysis of the redox-regulated import pathway in model systems.

Lipid-Centric Design of Plasma Membrane-Mimicking Nanocarriers for Targeted Chemotherapeutic Delivery

The plasma membranes (PM) of mammalian cells contain diverse lipids, proteins, and carbohydrates that are important for systemic recognition and communication in health and disease. Cell membrane coating technology that imparts unique properties of natural plasma membranes to the surface of encapsulated nanoparticles is thus becoming a powerful platform for drug delivery, immunomodulation, and vaccination. However, current coating methods fail to take full advantage of the natural systems because they disrupt the complex and functionally essential features of PMs, most notably the chemical diversity and compositional differences of lipids in two leaflets of the PM. Herein, a new lipid coating approach is reported in which the lipid composition is optimized through a combination of biomimetic and systematic variation approaches for the custom design of nanocarrier systems for precision drug delivery. Nanocarriers coated with the optimized lipids offer unique advantages in terms of bioavailability and efficiency in tumor targeting, tumor penetration, cellular uptake, and drug release. This pilot study provides new insight into the rational design and optimization of nanocarriers for cancer chemotherapeutic drugs and lays the foundation for further customization of cell membrane-mimicking nanocarriers through systematic incorporation of other components.

The MitoLuc Assay System for Accurate Real-Time Monitoring of Mitochondrial Protein Import Within Mammalian Cells

Mitochondrial protein import is critical for organelle biogenesis, bioenergetic function, and health. The mechanism of which is poorly understood, particularly of the mammalian system. To address this problem we have established an assay to quantitatively monitor mitochondrial import inside mammalian cells. The reporter is based on a split luciferase, whereby the large fragment is segregated in the mitochondrial matrix and the small complementary fragment is fused to the C-terminus of a purified recombinant precursor protein destined for import. Following import the complementary fragments combine to form an active luciferase-providing a sensitive, accurate and continuous measure of protein import. This advance allows detailed mechanistic examination of the transport process in live cells, including the analysis of import breakdown associated with disease, and high-throughput drug screening. Furthermore, the set-up has the potential to be adapted for the analysis of alternative protein transport systems within different cell types, and multicellular model organisms.

Clinical practice recommendations for primary hyperoxaluria: an expert consensus statement from ERKNet and OxalEurope

Primary hyperoxaluria (PH) is an inherited disorder that results from the overproduction of endogenous oxalate, leading to recurrent kidney stones, nephrocalcinosis and eventually kidney failure; the subsequent storage of oxalate can cause life-threatening systemic disease. Diagnosis of PH is often delayed or missed owing to its rarity, variable clinical expression and other diagnostic challenges. Management of patients with PH and kidney failure is also extremely challenging. However, in the past few years, several new developments, including new outcome data from patients with infantile oxalosis, from transplanted patients with type 1 PH (PH1) and from patients with the rarer PH types 2 and 3, have emerged. In addition, two promising therapies based on RNA interference have been introduced. These developments warrant an update of existing guidelines on PH, based on new evidence and on a broad consensus. In response to this need, a consensus development core group, comprising (paediatric) nephrologists, (paediatric) urologists, biochemists and geneticists from OxalEurope and the European Rare Kidney Disease Reference Network (ERKNet), formulated and graded statements relating to the management of PH on the basis of existing evidence. Consensus was reached following review of the recommendations by representatives of OxalEurope, ESPN, ERKNet and ERA, resulting in 48 practical statements relating to the diagnosis and management of PH, including consideration of conventional therapy (conservative therapy, dialysis and transplantation), new therapies and recommendations for patient follow-up.

Improving Treatment Options for Primary Hyperoxaluria

The primary hyperoxalurias are three rare inborn errors of the glyoxylate metabolism in the liver, which lead to massively increased endogenous oxalate production, thus elevating urinary oxalate excretion and, based on that, recurrent urolithiasis and/or progressive nephrocalcinosis. Frequently, especially in type 1 primary hyperoxaluria, early end-stage renal failure occurs. Treatment possibilities are scare, namely, hyperhydration and alkaline citrate medication. In type 1 primary hyperoxaluria, vitamin B6, though, is helpful in patients with specific missense or mistargeting mutations. In those vitamin B6 responsive, urinary oxalate excretion and concomitantly urinary glycolate is significantly decreased, or even normalized. In patients non-responsive to vitamin B6, RNA interference medication is now available. Lumasiran® is already available on prescription and targets the messenger RNA of glycolate oxidase, thus blocking the conversion of glycolate into glyoxylate, hence decreasing oxalate, but increasing glycolate production. Nedosiran blocks liver-specific lactate dehydrogenase A and thus the final step of oxalate production. Similar to vitamin B6 treatment, where both RNA interference urinary oxalate excretion can be (near) normalized and plasma oxalate decreases, however, urinary and plasma glycolate increases with lumasiran treatment. Future treatment possibilities are on the horizon, for example, substrate reduction therapy with small molecules or gene editing, induced pluripotent stem cell-derived autologous hepatocyte-like cell transplantation, or gene therapy with newly developed vector technologies. This review provides an overview of current and especially new and future treatment options.

In vivo CRISPR-Cas9 inhibition of hepatic LDH as treatment of primary hyperoxaluria

Genome-editing strategies, especially CRISPR-Cas9 systems, have substantially increased the efficiency of innovative therapeutic approaches for monogenic diseases such as primary hyperoxalurias (PHs). We have previously demonstrated that inhibition of glycolate oxidase using CRISPR-Cas9 systems represents a promising therapeutic option for PH type I (PH1). Here, we extended our work evaluating the efficacy of liver-specific inhibition of lactate dehydrogenase (LDH), a key enzyme responsible for converting glyoxylate to oxalate; this strategy would not be limited to PH1, being applicable to other PH subtypes. In this work, we demonstrate a liver-specific inhibition of LDH that resulted in a drastic reduction of LDH levels in the liver of PH1 and PH3 mice after a single-dose delivery of AAV8 vectors expressing the CRISPR-Cas9 system, resulting in reduced urine oxalate levels and kidney damage without signs of toxicity. Deep sequencing analysis revealed that this approach was safe and specific, with no off-targets detected in the liver of treated animals and no on-target/off-tissue events. Altogether, our data provide evidence that in vivo genome editing using CRISPR-Cas9 systems would represent a valuable tool for improved therapeutic approaches for PH.

Mitochondrial protein import determines lifespan through metabolic reprogramming and de novo serine biosynthesis

Sustained mitochondrial fitness relies on coordinated biogenesis and clearance. Both processes are regulated by constant targeting of proteins into the organelle. Thus, mitochondrial protein import sets the pace for mitochondrial abundance and function. However, our understanding of mitochondrial protein translocation as a regulator of longevity remains enigmatic. Here, we targeted the main protein import translocases and assessed their contribution to mitochondrial abundance and organismal physiology. We find that reduction in cellular mitochondrial load through mitochondrial protein import system suppression, referred to as MitoMISS, elicits a distinct longevity paradigm. We show that MitoMISS triggers the mitochondrial unfolded protein response, orchestrating an adaptive reprogramming of metabolism. Glycolysis and de novo serine biosynthesis are causatively linked to longevity, whilst mitochondrial chaperone induction is dispensable for lifespan extension. Our findings extent the pro-longevity role of UPR^mt and provide insight, relevant to the metabolic alterations that promote or undermine survival and longevity.

Mitochondrial function is linked to lifespan. Here the authors show that inhibition of mitochondrial protein import leads to a reduction in mitochondrial abundance and extends lifespan in Caenorhabditis elegans via activation of glycolysis and de novo serine biosynthesis.

Generating stable isolated mitochondrial recipient clones in mammalian cells using MitoPunch mitochondrial transfer

This protocol describes the assembly and use of MitoPunch to deliver mitochondria containing mitochondrial DNA (mtDNA) into cells lacking mtDNA (ρ0 cells). MitoPunch generates stable isolated mitochondrial recipient clones with restored mtDNA and recovered respiration, enabling investigation of mtDNA mutations and mtDNA-nuclear DNA interactions in a range of cell types. For complete details on the use and execution of this protocol, please refer to Sercel et al. (2021) and Patananan et al. (2020).

Perspectives in primary hyperoxaluria - historical, current and future clinical interventions

Primary hyperoxalurias are a devastating family of diseases leading to multisystem oxalate deposition, nephrolithiasis, nephrocalcinosis and end-stage renal disease. Traditional treatment paradigms are limited to conservative management, dialysis and combined transplantation of the kidney and liver, of which the liver is the primary source of oxalate production. However, transplantation is associated with many potential complications, including operative risks, graft rejection, post-transplant organ failure, as well as lifelong immunosuppressive medications and their adverse effects. New therapeutics being developed for primary hyperoxalurias take advantage of biochemical knowledge about oxalate synthesis and metabolism, and seek to specifically target these pathways with the goal of decreasing the accumulation and deposition of oxalate in the body.

Primary hyperoxalurias are a devastating family of diseases that eventually lead to end-stage renal disease. In this Review, Shee and Stoller discuss current treatment paradigms for primary hyperoxalurias, new therapeutics and their mechanisms of action, and future directions for novel research in the field.

Primary hyperoxalurias (PHs) are a devastating family of rare, autosomal-recessive genetic disorders that lead to multisystem oxalate deposition, nephrolithiasis, nephrocalcinosis and end-stage renal disease.

Traditional treatment paradigms are limited to conservative management, dialysis and inevitably transplantation of the kidney and liver, which is associated with high morbidity and the need for lifelong immunosuppression.

New therapeutics being developed for PHs take advantage of biochemical knowledge about oxalate synthesis and metabolism to specifically target these pathways, with the goal of decreasing the accumulation and deposition of plasma oxalate in the body.

New therapeutics can be divided into classes, and include substrate reduction therapy, intestinal oxalate degradation, chaperone therapy, enzyme restoration therapy and targeting of the inflammasome.

Lumasiran, a mRNA therapeutic targeting glycolate oxidase, was the first primary hyperoxaluria-specific therapeutic approved by the European Medicines Agency and the FDA in 2020.

Future work includes further clinical trials for promising therapeutics in the pipeline, identification of biomarkers of response to PH-directed therapy, optimization of drug development and delivery of new therapeutics.

Novel therapeutic approaches for the primary hyperoxalurias

Loss-of-function mutations in three genes, involved in the metabolic pathway of glyoxylate, result in increased oxalate production and its crystallization in the form of calcium oxalate. This leads to three forms of primary hyperoxaluria-an early-onset inherited kidney disease with wide phenotypic variability ranging from isolated kidney stone events to stage 5 chronic kidney disease in infancy. This review provides a description of metabolic processes resulting in oxalate overproduction and summarizes basic therapeutic approaches. Unfortunately, current treatment of primary hyperoxaluria does not allow the prevention of loss of kidney function or to substantially diminish other symptoms in most patients. However, latest breakthroughs in biotechnology provide new promising directions for drug development. Some of them have already progressed to the level of clinical trials; others are just at the stage of proof of concept. Here we review the most advanced technologies including those that have been harnessed as possible therapeutic modalities.

Small Molecule-Based Enzyme Inhibitors in the Treatment of Primary Hyperoxalurias

Primary hyperoxalurias (PHs) are a group of inherited alterations of the hepatic glyoxylate metabolism. PHs classification based on gene mutations parallel a variety of enzymatic defects, and all involve the harmful accumulation of calcium oxalate crystals that produce systemic damage. These geographically widespread rare diseases have a deep impact in the life quality of the patients. Until recently, treatments were limited to palliative measures and kidney/liver transplants in the most severe forms. Efforts made to develop pharmacological treatments succeeded with the biotechnological agent lumasiran, a siRNA product against glycolate oxidase, which has become the first effective therapy to treat PH1. However, small molecule drugs have classically been preferred since they benefit from experience and have better pharmacological properties. The development of small molecule inhibitors designed against key enzymes of glyoxylate metabolism is on the focus of research. Enzyme inhibitors are successful and widely used in several diseases and their pharmacokinetic advantages are well known. In PHs, effective enzymatic targets have been determined and characterized for drug design and interesting inhibitory activities have been achieved both in vitro and in vivo. This review describes the most recent advances towards the development of small molecule enzyme inhibitors in the treatment of PHs, introducing the multi-target approach as a more effective and safe therapeutic option.

Stable transplantation of human mitochondrial DNA by high-throughput, pressurized isolated mitochondrial delivery

Generating mammalian cells with specific mitochondrial DNA (mtDNA)-nuclear DNA (nDNA) combinations is desirable but difficult to achieve and would be enabling for studies of mitochondrial-nuclear communication and coordination in controlling cell fates and functions. We developed 'MitoPunch', a pressure-driven mitochondrial transfer device, to deliver isolated mitochondria into numerous target mammalian cells simultaneously. MitoPunch and MitoCeption, a previously described force-based mitochondrial transfer approach, both yield stable isolated mitochondrial recipient (SIMR) cells that permanently retain exogenous mtDNA, whereas coincubation of mitochondria with cells does not yield SIMR cells. Although a typical MitoPunch or MitoCeption delivery results in dozens of immortalized SIMR clones with restored oxidative phosphorylation, only MitoPunch can produce replication-limited, non-immortal human SIMR clones. The MitoPunch device is versatile, inexpensive to assemble, and easy to use for engineering mtDNA-nDNA combinations to enable fundamental studies and potential translational applications.

Transplantation for Primary Hyperoxaluria Type 1: Designing New Strategies in the Era of Promising Therapeutic Perspectives

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease caused by the functional defect of alanine-glyoxylate aminotransferase that results in the overproduction of oxalate. It can be devastating especially for kidneys, leading to end-stage renal disease (ESRD) during the first 2 to 3 decades of life in most patients. Consequently, many PH1 patients need kidney transplantation. However, because PH1 is caused by a liver enzyme deficiency, the only cure of the metabolic defect is liver transplantation. Thus, current transplant strategies to treat PH1 patients with ESRD include dual liver-kidney transplantation. However, the morbidity and mortality associated with liver transplantation make these strategies far from optimal. Fortunately, a therapeutic revolution is looming. Indeed, innovative drugs are being currently tested in clinical trials, and preliminary data show impressive efficacy to reduce the hepatic overproduction of oxalate. Hopefully, with these therapies, liver transplantation will no longer be necessary. However, some patients with progressing renal disease or those who will be diagnosed with PH1 at an advanced stage of chronic kidney disease will ultimately need kidney transplantation. Here we review the current knowledge on this subject and discuss the future of kidney transplant management in PH1 patients in the era of novel therapies.

Pressure-Driven Mitochondrial Transfer Pipeline Generates Mammalian Cells of Desired Genetic Combinations and Fates

Generating mammalian cells with desired mitochondrial DNA (mtDNA) sequences is enabling for studies of mitochondria, disease modeling, and potential regenerative therapies. MitoPunch, a high-throughput mitochondrial transfer device, produces cells with specific mtDNA-nuclear DNA (nDNA) combinations by transferring isolated mitochondria from mouse or human cells into primary or immortal mtDNA-deficient (ρ0) cells. Stable isolated mitochondrial recipient (SIMR) cells isolated in restrictive media permanently retain donor mtDNA and reacquire respiration. However, SIMR fibroblasts maintain a ρ0-like cell metabolome and transcriptome despite growth in restrictive media. We reprogrammed non-immortal SIMR fibroblasts into induced pluripotent stem cells (iPSCs) with subsequent differentiation into diverse functional cell types, including mesenchymal stem cells (MSCs), adipocytes, osteoblasts, and chondrocytes. Remarkably, after reprogramming and differentiation, SIMR fibroblasts molecularly and phenotypically resemble unmanipulated control fibroblasts carried through the same protocol. Thus, our MitoPunch "pipeline" enables the production of SIMR cells with unique mtDNA-nDNA combinations for additional studies and applications in multiple cell types.

Lipid-Bicelle-Coated Microfluidics for Intracellular Delivery with Reduced Fouling

Innovative technologies for intracellular delivery are ushering in a new era for gene editing, enabling the utilization of a patient's own cells for stem cell and immunotherapies. In particular, cell-squeezing platforms provide unconventional forms of intracellular delivery, deforming cells through microfluidic constrictions to generate transient pores and to enable effective diffusion of biomolecular cargo. While these devices are promising gene-editing platforms, they require frequent maintenance due to the accumulation of cellular debris, limiting their potential for reaching the throughputs necessary for scalable cellular therapies. As these cell-squeezing technologies are improved, there is a need to develop next-generation platforms with higher throughput and longer lifespan, importantly, avoiding the buildup of cell debris and thus channel clogging. Here, we report a versatile strategy to coat the channels of microfluidic devices with lipid bilayers based on noncovalent lipid bicelle technology, which led to substantial improvements in reducing cell adhesion and protein adsorption. The antifouling properties of the lipid bilayer coating were evaluated, including membrane uniformity, passivation against nonspecific protein adsorption, and inhibition of cell attachment against multiple cell types. This surface functionalization approach was applied to coat constricted microfluidic channels for the intracellular delivery of fluorescently labeled dextran and plasmid DNA, demonstrating significant reductions in the accumulation of cell debris. Taken together, our work demonstrates that lipid bicelles are a useful tool to fabricate antifouling lipid bilayer coatings in cell-squeezing devices, resulting in reduced nonspecific fouling and cell clogging to improve performance.

Effect of alanine supplementation on oxalate synthesis

The Primary Hyperoxalurias (PH) are rare disorders of metabolism leading to excessive endogenous synthesis of oxalate and recurring calcium oxalate kidney stones. Alanine glyoxylate aminotransferase (AGT), deficient in PH type 1, is a key enzyme in limiting glyoxylate oxidation to oxalate. The affinity of AGT for its co-substrate, alanine, is low suggesting that its metabolic activity could be sub-optimal in vivo. To test this hypothesis, we examined the effect of L-alanine supplementation on oxalate synthesis in cell culture and in mouse models of Primary Hyperoxaluria Type 1 (Agxt KO), Type 2 (Grhpr KO) and in wild-type mice. Our results demonstrated that increasing L-alanine in cells decreased synthesis of oxalate and increased viability of cells expressing GO and AGT when incubated with glycolate. In both wild type and Grhpr KO male and female mice, supplementation with 10% dietary L-alanine significantly decreased urinary oxalate excretion ~30% compared to baseline levels. This study demonstrates that increasing the availability of L-alanine can increase the metabolic efficiency of AGT and reduce oxalate synthesis.

Recovery From Dialysis in Patients With Primary Hyperoxaluria Type 1 Treated With Pyridoxine: A Report of 3 Cases

Primary hyperoxaluria type 1 (PH1) is a genetic disorder characterized by overproduction of oxalate and eventual kidney failure. Kidney failure is usually irreversible in PH1. However, in patients with PH1 homozygous for the G170R mutation (in which the glycine at amino acid 170 is replaced by an arginine), pyridoxine is an enzyme cofactor and decreases urinary oxalate excretion by reducing hepatic oxalate production. We report recovery from dialysis in 3 patients with PH1 homozygous for the G170R mutation in response to pharmacologic-dose pyridoxine treatment. Median age at initiation or resumption of pyridoxine treatment was 37 (range, 20-53) years, and median daily pyridoxine dose was 8.8 (range, 6.8-14.0) mg per kilogram of body weight. Duration of hemodialysis before recovery of kidney function was 10 (range, 5-19) months. Plasma oxalate concentration improved after recovery of kidney function. At a median of 3 (range, 2-46) months following discontinuation of hemodialysis, estimated glomerular filtration rate was 34 (range, 23-52) mL/min/1.73m², plasma oxalate concentration was 8.8 (range, 4.6-11.3) μmol/L, and urinary oxalate excretion was 0.93 (range, 0.47-1.03) mmol/d. Kidney function was maintained during a median of 3.2 (range, 1.3-3.8) years of follow-up. These observations suggest that kidney failure may be reversible in a subset of patients with PH1 homozygous for the G170R mutation treated with pharmacologic-dose pyridoxine.

Stable retention of chloramphenicol-resistant mtDNA to rescue metabolically impaired cells

The permanent transfer of specific mtDNA sequences into mammalian cells could generate improved models of mtDNA disease and support future cell-based therapies. Previous studies documented multiple biochemical changes in recipient cells shortly after mtDNA transfer, but the long-term retention and function of transferred mtDNA remains unknown. Here, we evaluate mtDNA retention in new host cells using ‘MitoPunch’, a device that transfers isolated mitochondria into mouse and human cells. We show that newly introduced mtDNA is stably retained in mtDNA-deficient (ρ0) recipient cells following uridine-free selection, although exogenous mtDNA is lost from metabolically impaired, mtDNA-intact (ρ+) cells. We then introduced a second selective pressure by transferring chloramphenicol-resistant mitochondria into chloramphenicol-sensitive, metabolically impaired ρ+ mouse cybrid cells. Following double selection, recipient cells with mismatched nuclear (nDNA) and mitochondrial (mtDNA) genomes retained transferred mtDNA, which replaced the endogenous mutant mtDNA and improved cell respiration. However, recipient cells with matched mtDNA-nDNA failed to retain transferred mtDNA and sustained impaired respiration. Our results suggest that exogenous mtDNA retention in metabolically impaired ρ+ recipients depends on the degree of recipient mtDNA-nDNA co-evolution. Uncovering factors that stabilize exogenous mtDNA integration will improve our understanding of in vivo mitochondrial transfer and the interplay between mitochondrial and nuclear genomes.

Investigational Therapies for Primary Hyperoxaluria

Recent years have brought exciting new insights in the field of primary hyperoxaluria (PH), both on a basic research level as well as through the progress of novel therapeutics in clinical development. To date, very few supportive measures are available for patients suffering from PH, which, together with the severity of the disorder, make disease management challenging. Basic and clinical research and development efforts range from correcting the underlying gene mutations, preventing calcium oxalate crystal-induced kidney damage, to the administration of probiotics favoring the intestinal secretion of excess oxalate. In this review, current advances in the development of those strategies are presented and discussed.

Disease-Associated Genetic Variation in Human Mitochondrial Protein Import

Mitochondrial dysfunction has consequences not only for cellular energy output but also for cellular signaling pathways. Mitochondrial dysfunction, often based on inherited gene variants, plays a role in devastating human conditions such as mitochondrial neuropathies, myopathies, cardiovascular disorders, and Parkinson and Alzheimer diseases. Of the proteins essential for mitochondrial function, more than 98% are encoded in the cell nucleus, translated in the cytoplasm, sorted based on the presence of encoded mitochondrial targeting sequences (MTSs), and imported to specific mitochondrial sub-compartments based on the integrated activity of a series of mitochondrial translocases, proteinases, and chaperones. This import process is typically dynamic; as cellular homeostasis is coordinated through communication between the mitochondria and the nucleus, many of the adaptive responses to stress depend on modulation of mitochondrial import. We here describe an emerging class of disease-linked gene variants that are found to impact the mitochondrial import machinery itself or to affect the proteins during their import into mitochondria. As a whole, this class of rare defects highlights the importance of correct trafficking of mitochondrial proteins in the cell and the potential implications of failed targeting on metabolism and energy production. The existence of this variant class could have importance beyond rare neuromuscular disorders, given an increasing body of evidence suggesting that aberrant mitochondrial function may impact cancer risk and therapeutic response.

A High-Resolution Luminescent Assay for Rapid and Continuous Monitoring of Protein Translocation across Biological Membranes

Protein translocation is a fundamental process in biology. Major gaps in our understanding of this process arise due the poor sensitivity, low time resolution and irreproducibility of translocation assays. To address this, we applied NanoLuc split-luciferase to produce a new strategy for measuring protein transport. The system reduces the timescale of data collection from days to minutes and allows for continuous acquisition with a time resolution in the order of seconds, yielding kinetics parameters suitable for mechanistic elucidation and mathematical fitting. To demonstrate its versatility, we implemented and validated the assay in vitro and in vivo for the bacterial Sec system and the mitochondrial protein import apparatus. Overall, this technology represents a major step forward, providing a powerful new tool for fundamental mechanistic enquiry of protein translocation and for inhibitor (drug) screening, with an intensity and rigor unattainable through classical methods.

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Conventional methods for monitoring protein translocation are laborious and discontinuous and lack kinetic detail.

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A split NanoLuc system was adapted for real-time monitoring of protein translocation through the bacterial Sec system and the mitochondrial Tim23 complex.

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The new assay reduces the timescale of data acquisition from days to minutes.

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It produces high-quality results suitable for kinetic fitting and model derivation.

Biochemical properties and oxalate-degrading activity of oxalate decarboxylase from bacillus subtilis at neutral pH

Oxalate decarboxylase (OxDC) from Bacillus subtilis is a Mn-dependent hexameric enzyme that converts oxalate to carbon dioxide and formate. OxDC has greatly attracted the interest of the scientific community, mainly due to its biotechnological and medical applications in particular for the treatment of hyperoxaluria, a group of pathologic conditions caused by oxalate accumulation. The enzyme has an acidic optimum pH, but most of its applications involve processes occurring at neutral pH. Nevertheless, a detailed biochemical characterization of the enzyme at neutral pH is lacking. Here, we compared the structural-functional properties at acidic and neutral pH of wild-type OxDC and of a mutant form, called OxDC-DSSN, bearing four amino acid substitutions in the lid (Ser161-to-Asp, Glu162-to-Ser, Asn163-toSer, and Ser164-to-Asn) that improve the oxalate oxidase activity and almost abolish the decarboxylase activity. We found that both enzymatic forms do not undergo major structural changes as a function of pH, although OxDC-DSSN displays an increased tendency to aggregation, which is counteracted by the presence of an active-site ligand. Notably, OxDC and OxDC-DSSN at pH 7.2 retain 7 and 15% activity, respectively, which is sufficient to degrade oxalate in a cellular model of primary hyperoxaluria type I, a rare inherited disease caused by excessive endogenous oxalate production. The significance of the data in the light of the possible use of OxDC as biological drug is discussed. © 2019 IUBMB Life, 1-11, 2019.

CRISPR/Cas9-mediated glycolate oxidase disruption is an efficacious and safe treatment for primary hyperoxaluria type I

CRISPR/Cas9 technology offers novel approaches for the development of new therapies for many unmet clinical needs, including a significant number of inherited monogenic diseases. However, in vivo correction of disease-causing genes is still inefficient, especially for those diseases without selective advantage for corrected cells. We reasoned that substrate reduction therapies (SRT) targeting non-essential enzymes could provide an attractive alternative. Here we evaluate the therapeutic efficacy of an in vivo CRISPR/Cas9-mediated SRT to treat primary hyperoxaluria type I (PH1), a rare inborn dysfunction in glyoxylate metabolism that results in excessive hepatic oxalate production causing end-stage renal disease. A single systemic administration of an AAV8-CRISPR/Cas9 vector targeting glycolate oxidase, prevents oxalate overproduction and kidney damage, with no signs of toxicity in Agxt1^−/− mice. Our results reveal that CRISPR/Cas9-mediated SRT represents a promising therapeutic option for PH1 that can be potentially applied to other metabolic diseases caused by the accumulation of toxic metabolites.

Substrate reduction therapies (SRT) are a promising therapeutic approach for monogenic inherited metabolic diseases. Here the authors evaluate the therapeutic potential of an in vivo CRISPR/Cas9-mediated SRT to treat primary hyperoxaluria type I and demonstrate its safety and efficacy.

Novel therapeutic approaches in primary hyperoxaluria

Introduction: Currently, three types of primary hyperoxaluria (PH I-III) are known, all based on different gene-mutations affecting the glyoxylate metabolism in the liver. Disease hallmark is an increased endogenous oxalate production and thus massively elevated urinary excretion of oxalate and other type-specific metabolites. Hyperoxaluria induces the formation of calcium-oxalate kidney stones and/or nephrocalcinosis. In addition to that, a chronic inflammasome activation by hyperoxaluria per se, often leads to an early deterioration of kidney function, regularly resulting in end-stage renal disease (ESRD) at least in patients with type I PH. Except for vitamin B6 treatment in PH I, therapeutic regimen nowadays consists only of supportive measures, like significantly increased fluid intake and medication increasing the urinary solubility like alkaline citrate. Areas covered: Disease burden can be severe, and both clinicians and scientist are eager in finding new therapeutic approaches. The currently ongoing clinical studies and promising research in this field are reported in this paper. To present a complete overview, we searched electronic databases, like Clinical trial gov, National Center for Biotechnology Information PubMed, congress reports, press releases and personal information acquired at congresses and conventions. Searches were conducted using the following medical headings: (primary) hyperoxaluria, PH, therapy, treatment and research. Expert opinion: There is light on the horizon that new treatment options will be available in due time, as there are several promising therapeutic agents currently under investigation, some being at the first levels of drug development, but some already in ongoing clinical trials (phase I-III).

Structural and functional insights on the roles of molecular chaperones in the mistargeting and aggregation phenotypes associated with primary hyperoxaluria type I

To carry out their biological function in cells, proteins must be folded and targeted to the appropriate subcellular location. These processes are controlled by a vast collection of interacting proteins collectively known as the protein homeostasis network, in which molecular chaperones play a prominent role. Protein homeostasis can be impaired by inherited mutations leading to genetic diseases. In this chapter, we focus on a particular disease, primary hyperoxaluria type 1 (PH1), in which disease-associated mutations exacerbate protein aggregation in the cell and mistarget the peroxisomal alanine:glyoxylate aminotransferase (AGT) protein to mitochondria, in part due to native state destabilization and enhanced interaction with Hsp60, 70 and 90 chaperone systems. After a general introduction of molecular chaperones and PH1, we review our current knowledge on the structural and energetic features of PH1-causing mutants that lead to these particular pathogenic mechanisms. From this perspective, and in the context of the key role of molecular chaperones in PH1 pathogenesis, we present and discuss current and future perspectives for pharmacological treatments for this disease.

Molecular basis of primary hyperoxaluria: clues to innovative treatments

Primary hyperoxalurias (PHs) are rare inherited disorders of liver glyoxylate metabolism, characterized by the abnormal production of endogenous oxalate, a metabolic end-product that is eliminated by urine. The main symptoms are related to the precipitation of calcium oxalate crystals in the urinary tract with progressive renal damage and, in the most severe form named Primary Hyperoxaluria Type I (PH1), to systemic oxalosis. The therapies currently available for PH are either poorly effective, because they address the symptoms and not the causes of the disease, or highly invasive. In the last years, advances in our understanding of the molecular bases of PH have paved the way for the development of new therapeutic strategies. They include (i) substrate-reduction therapies based on small-molecule inhibitors or the RNA interference technology, (ii) gene therapy, (iii) enzyme administration approaches, (iv) colonization with oxalate-degrading intestinal microorganisms, and, in PH1, (v) design of pharmacological chaperones. This paper reviews the basic principles of these new therapeutic strategies and what is currently known about their application to PH.

Efficacy of Hydroxy-L-proline (HYP) analogs in the treatment of primary hyperoxaluria in Drosophila Melanogaster

Background

Substrate reduction therapy with analogs reduces the accumulation of substrates by inhibiting the metabolic pathways involved in their biosynthesis, providing new treatment options for patients with primary hyperoxalurias (PHs) that often progress to end-stage renal disease (ESRD). This research aims to evaluate the inhibition efficacy of Hydroxy-L-proline (HYP) analogs against calcium oxalate (CaOx) crystal formation in the Drosophila Melanogaster (D. Melanogaster) by comparing them with Pyridoxine (Vitamin B6).

Methods

Three stocks of Drosophila Melanogaster (W¹¹⁸, CG3926 RNAi, and Act5C-GAL4/CyO) were utilized. Two stocks (CG3926 RNAi and Act5C-GAL4 /CyO) were crossed to generate the Act5C > dAGXT RNAi recombinant line (F₁ generation) of D. Melanogaster which was used to compare the efficacy of Hydroxy-L-proline (HYP) analogs inhibiting CaOx crystal formation with Vitamin B₆ as the traditional therapy for primary hyperoxaluria.

Results

Nephrolithiasis model was successfully constructed by downregulating the function of the dAGXT gene in D. Melanogaster (P-Value = 0.0045). Furthermore, the efficacy of Hydroxy-L-proline (HYP) analogs against CaOx crystal formation was demonstrated in vivo using D. Melanogaster model; the results showed that these L-Proline analogs were better in inhibiting stone formation at very low concentrations than Vitamin B₆ (IC₅₀ = 0.6 and 1.8% for standard and dietary salt growth medium respectively) compared to N-acetyl-L-Hydroxyproline (IC₅₀ = 0.1% for both standard and dietary salt growth medium) and Baclofen (IC₅₀ = 0.06 and 0.1% for standard and dietary salt growth medium respectively). Analysis of variance (ANOVA) also showed that Hydroxy-L-proline (HYP) analogs were better alternatives for CaOx inhibition at very low concentration especially when both genetics and environmental factors are intertwined (p < 0.0008) for the dietary salt growth medium and (P < 0.063) for standard growth medium.

Conclusion

Addition of Hydroxy-L-Proline analogs to growth medium resulted in the reduction of CaOx crystals formation. These analogs show promise as potential inhibitors for oxalate reduction in Primary Hyperoxaluria.

Translation inhibition corrects aberrant localization of mutant alanine-glyoxylate aminotransferase: possible therapeutic approach for hyperoxaluria

Primary hyperoxaluria type 1 is a severe kidney stone disease caused by abnormalities of the peroxisomal alanine-glyoxylate aminotransferase (AGT). The most frequent mutation G170R results in aberrant mitochondrial localization of the active enzyme. To evaluate the population of peroxisome-localized AGT, we developed a quantitative Glow-AGT assay based on the self-assembly split-GFP approach and used it to identify drugs that can correct mislocalization of the mutant protein. In line with previous reports, the Glow-AGT assay showed that mitochondrial transport inhibitors DECA and monensin increased peroxisomal localization of the mutant. Here, we demonstrate that prolonged treatment with the translation elongation inhibitor emetine, a medicinal alkaloid used in treatment of amoebiasis, corrected G170R-AGT mislocalization. Furthermore, emetine reduced the augmented oxalate level in culture media of patient-derived hepatocytes bearing the G170R mutation. A distinct translation inhibitor GC7 had a similar effect on the mutant Glow-AGT relocalization indicating that mild translation inhibition is a promising therapeutic approach for primary hyperoxaluria type 1 caused by AGT misfolding/mistargeting.

KEY MESSAGES

• There is no effective conservative treatment to decrease oxalate production in PH1 patients. • Chemical chaperones rescue mislocalization of mutant AGT and reduce oxalate levels. • We have developed an assay for precise monitoring of the peroxisomal AGT. • Inhibition of translation by emetine reroutes the mutant protein to peroxisome. • Mild translation inhibition is a promising cure for conformational disorders.

Nephrolithiasis and Nephrocalcinosis in Childhood-Risk Factor-Related Current and Future Treatment Options

Nephrolithiasis, urolithiasis, and nephrocalcinosis (NC) have become common causes of hospitalization and referral to pediatric outpatient clinics. It is of utmost importance to start with diagnostic evaluation directly after the first passage of a kidney stone, or if NC is diagnosed, in each pediatric patient. This is necessary, as in about 80% of children a metabolic reason for stone disease is detected. Current treatment options are scarce and mainly include general measures like an increased fluid intake or elevating the solubility of a lithogenic substance. According to the given lithogenic risk factor(s), specific treatment options are available and are being summarized in this review. Furthermore, an outlook on potential future treatment options, including innovative strategies such as mRNA-based or recombinant enzyme substitution therapy, is given.

Molecular therapy of primary hyperoxaluria

During the last few decades, the molecular understanding of the mechanisms involved in primary hyperoxalurias (PHs) has set the stage for novel therapeutic approaches. The availability of PH mouse models has facilitated preclinical studies testing innovative treatments. PHs are autosomal recessive diseases where the enzymatic deficit plays a central pathogenic role. Thus, molecular therapies aimed at restoring such deficit or limiting the consequences of the metabolic derangement could be envisioned, keeping in mind the specific challenges posed by the cell-autonomous nature of the deficiency. Various molecular approaches like enzyme replacement, substrate reduction, pharmacologic chaperones, and gene and cell therapies have been explored in cells and mouse models of disease. Some of these proof-of-concept studies have paved the way to current clinical trials on PH type 1, raising hopes that much needed treatments will become available for this severe inborn error of metabolism.

Epidemiology of paediatric renal stone disease: a 22-year single centre experience in the UK

Background

Whilst still rare, the incidence of paediatric stone disease is increasing in developed countries and it is important to evaluate the aetiology. We set up a dedicated renal stone service for children combining medical and surgical expertise in 1993 and now have a large case series of children to investigate the epidemiology.

Methods

A retrospective hospital note review of children presenting with kidney stones during the last 22 years (1993–2015) was conducted. All patients had a comprehensive infective and metabolic screen and were classified as metabolic, infective or idiopathic stone disease.

Results

Five hundred eleven patients (322 male) were reviewed. The median age of presentation was 4.4y for males (1 m-16.6y) and 7.3y (1–18.5y) for females with a median height and weight on the 25th centile for male and on 10th and 25th for female, respectively. One hundred seventy five (34%) had an underlying metabolic abnormality, 112 (22%) had infective stones and 224 (44%) were classified as idiopathic.

Of the 175 patients with a metabolic abnormality: 91 (52%) had hypercalciuria (76 persistent and 15 transient), 37 (21%) hyperoxaluria, 38 (22%) cystinuria, 3 (2%) abnormalities in the purine metabolism and the remainder other metabolic abnormalities. Bilateral stones occurred in 27% of the metabolic group compared to 16% in the non-metabolic group (OR 0.2, p < 0.05). Urinary tract infection was a common complication (27%) in the metabolic group.

Conclusions

In this paper, we present the largest cohort of paediatric stone disease reported from a developed country giving details on both, clinical and laboratory data. We show that in the majority of the patients there is an identifiable underlying metabolic and/or infective aetiology emphasizing the importance of a full work up to provide adequate treatment and prevent recurrence. Moreover, we show that stone disease in children, in contrast to the adult population, does not seem to be associated with obesity, as children have a weight below average at presentation.

Drug Library Screening for the Identification of Ionophores That Correct the Mistrafficking Disorder Associated with Oxalosis Kidney Disease

Primary hyperoxaluria is the underlying cause of oxalosis and is a life-threatening autosomal recessive disease, for which treatment may require dialysis or dual liver-kidney transplantation. The most common primary hyperoxaluria type 1 (PH1) is caused by genetic mutations of a liver-specific enzyme alanine:glyoxylate aminotransferase (AGT), which results in the misrouting of AGT from the peroxisomes to the mitochondria. Pharmacoperones are small molecules with the ability to modify misfolded proteins and route them correctly within the cells, which may present an effective strategy to treat AGT misrouting in PH1 disorders. We miniaturized a cell-based high-content assay into 1536-well plate format and screened ~4200 pharmacologically relevant compounds including Food and Drug Administration, European Union, and Japanese-approved drugs. This assay employs CHO cells stably expressing AGT-170, a mutant that predominantly resides in the mitochondria, where we monitor for its relocation to the peroxisomes through automated image acquisition and analysis. The miniaturized 1536-well assay yielded a Z' averaging 0.70 ± 0.07. Three drugs were identified as potential pharmacoperones from this pilot screen, demonstrating the applicability of this assay for large-scale high-throughput screening.

High throughput cell-based assay for identification of glycolate oxidase inhibitors as a potential treatment for Primary Hyperoxaluria Type 1

Glycolate oxidase (GO) and alanine:glyoxylate aminotransferase (AGT) are both involved in the peroxisomal glyoxylate pathway. Deficiency in AGT function causes the accumulation of intracellular oxalate and the primary hyperoxaluria type 1 (PH1). AGT enhancers or GO inhibitors may restore the abnormal peroxisomal glyoxylate pathway in PH1 patients. With stably transformed cells which mimic the glyoxylate metabolic pathway, we developed an indirect glycolate cytotoxicity assay in a 1,536-well plate format for high throughput screening. This assay can be used to identify compounds that reduce indirect glycolate-induced cytotoxicity by either enhancing AGT activity or inhibiting GO. A pilot screen of 4,096 known compounds identified two membrane permeable GO inhibitors: dichromate salt and colistimethate. We also developed a GO enzyme assay using the hydrogen peroxide-Amplex red reporter system. The IC₅₀ values of potassium dichromate, sodium dichromate, and colistimethate sodium were 0.096, 0.108, and 2.3 μM in the GO enzyme assay, respectively. Further enzyme kinetic study revealed that both types of compounds inhibit GO activity by the mixed linear inhibition. Our results demonstrate that the cell-based assay and GO enzyme assay developed in this study are useful for further screening of large compound libraries for drug development to treat PH1.

A Five-Gene Expression Signature Predicts Clinical Outcome of Ovarian Serous Cystadenocarcinoma

Ovarian serous cystadenocarcinoma is a common malignant tumor of female genital organs. Treatment is generally less effective as patients are usually diagnosed in the late stage. Therefore, a well-designed prognostic marker provides valuable data for optimizing therapy. In this study, we analyzed 303 samples of ovarian serous cystadenocarcinoma and the corresponding RNA-seq data. We observed the correlation between gene expression and patients' survival and eventually established a risk assessment model of five factors using Cox proportional hazards regression analysis. We found that the survival time in high-risk patients was significantly shorter than in low-risk patients in both training and testing sets after Kaplan-Meier analysis. The AUROC value was 0.67 when predicting the survival time in testing set, which indicates a relatively high specificity and sensitivity. The results suggest diagnostic and therapeutic applications of our five-gene model for ovarian serous cystadenocarcinoma.

Effects of alanine:glyoxylate aminotransferase variants and pyridoxine sensitivity on oxalate metabolism in a cell-based cytotoxicity assay

The hereditary kidney stone disease primary hyperoxaluria type 1 (PH1) is caused by a functional deficiency of the liver-specific, peroxisomal, pyridoxal-phosphate-dependent enzyme, alanine:glyoxylate aminotransferase (AGT). One third of PH1 patients, particularly those expressing the p.[(Pro11Leu; Gly170Arg; Ile340Met)] mutant allele, respond clinically to pharmacological doses of pyridoxine. To gain further insight into the metabolic effects of AGT dysfunction in PH1 and the effect of pyridoxine, we established an "indirect" glycolate cytotoxicity assay using CHO cells expressing glycolate oxidase (GO) and various normal and mutant forms of AGT. In cells expressing GO the great majority of glycolate was converted to oxalate and glyoxylate, with the latter causing the greater decrease in cell survival. Co-expression of normal AGTs and some, but not all, mutant AGT variants partially counteracted this cytotoxicity and led to decreased synthesis of oxalate and glyoxylate. Increasing the extracellular pyridoxine up to 0.3μM led to an increased metabolic effectiveness of normal AGTs and the AGT-Gly170Arg variant. The increased survival seen with AGT-Gly170Arg was paralleled by a 40% decrease in oxalate and glyoxylate levels. These data support the suggestion that the effectiveness of pharmacological doses of pyridoxine results from an improved metabolic effectiveness of AGT; that is the increased rate of transamination of glyoxylate to glycine. The indirect glycolate toxicity assay used in the present study has potential to be used in cell-based drug screening protocols to identify chemotherapeutics that might enhance or decrease the activity and metabolic effectiveness of AGT and GO, respectively, and be useful in the treatment of PH1.

Inhibition of Glycolate Oxidase With Dicer-substrate siRNA Reduces Calcium Oxalate Deposition in a Mouse Model of Primary Hyperoxaluria Type 1

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive, metabolic disorder caused by mutations of alanine-glyoxylate aminotransferase (AGT), a key hepatic enzyme in the detoxification of glyoxylate arising from multiple normal metabolic pathways to glycine. Accumulation of glyoxylate, a precursor of oxalate, leads to the overproduction of oxalate in the liver, which accumulates to high levels in kidneys and urine. Crystalization of calcium oxalate (CaOx) in the kidney ultimately results in renal failure. Currently, the only treatment effective in reduction of oxalate production in patients who do not respond to high-dose vitamin B₆ therapy is a combined liver/kidney transplant. We explored an alternative approach to prevent glyoxylate production using Dicer-substrate small interfering RNAs (DsiRNAs) targeting hydroxyacid oxidase 1 (HAO1) mRNA which encodes glycolate oxidase (GO), to reduce the hepatic conversion of glycolate to glyoxylate. This approach efficiently reduces GO mRNA and protein in the livers of mice and nonhuman primates. Reduction of hepatic GO leads to normalization of urine oxalate levels and reduces CaOx deposition in a preclinical mouse model of PH1. Our results support the use of DsiRNA to reduce liver GO levels as a potential therapeutic approach to treat PH1.

α-MHC MitoTimer mouse: In vivo mitochondrial turnover model reveals remarkable mitochondrial heterogeneity in the heart

In order to maintain an efficient, energy-producing network in the heart, dysfunctional mitochondria are cleared through the mechanism of autophagy, which is closely linked with mitochondrial biogenesis; these, together with fusion and fission comprise a crucial process known as mitochondrial turnover. Until recently, the lack of molecular tools and methods available to researchers has impeded in vivo investigations of turnover. To investigate the process at the level of a single mitochondrion, our laboratory has developed the MitoTimer protein. Timer is a mutant of DsRed fluorescent protein characterized by transition from green fluorescence to a more stable red conformation over 48 h, and its rate of maturation is stable under physiological conditions. We fused the Timer cDNA with the inner mitochondrial membrane signal sequence and placed it under the control of a cardiac-restricted promoter. This construct was used to create the alpha-MHC-MitoTimer mice. Surprisingly, initial analysis of the hearts from these mice demonstrated a high degree of heterogeneity in the ratio of red-to-green fluorescence of MitoTimer in cardiac tissue. Further, scattered solitary mitochondria within cardiomyocytes display a much higher red-to-green fluorescence (red-shifted) relative to other mitochondria in the cell, implying a block in import of newly synthesized MitoTimer likely due to lower membrane potential. These red-shifted mitochondria may represent older, senescent mitochondria. Concurrently, the cardiomyocytes also contain a subpopulation of mitochondria that display a lower red-to-green fluorescence (green-shifted) relative to other mitochondria, indicative of germinal mitochondria that are actively engaged in import of newly-synthesized mito-targeted proteins. These mitochondria can be isolated and sorted from the heart by flow cytometry for further analysis. Initial studies suggest that these mice represent an elegant tool for the investigation of mitochondrial turnover in the heart.

Development of a phenotypic high-content assay to identify pharmacoperone drugs for the treatment of primary hyperoxaluria type 1 by high-throughput screening

Primary hyperoxaluria is a severe disease for which the best current therapy is dialysis or organ transplantation. These are risky, inconvenient, and costly procedures. In some patients, pyridoxine treatment can delay the need for these surgical procedures. The underlying cause of particular forms of this disease is the misrouting of a specific enzyme, alanine:glyoxylate aminotransferase (AGT), to the mitochondria instead of the peroxisomes. Pharmacoperones are small molecules that can rescue misfolded proteins and redirect them to their correct location, thereby restoring their function and potentially curing disease. In the present study, we miniaturized a cell-based assay to identify pharmacoperone drugs present in large chemical libraries to selectively correct AGT misrouting. This assay employs AGT-170, a mutant form of AGT that predominantly resides in the mitochondria, which we monitor for its relocation to the peroxisomes through automated image acquisition and analysis. Over the course of a pilot screen of 1,280 test compounds, we achieved an average Z'-factor of 0.72±0.02, demonstrating the suitability of this assay for HTS.

Progress toward improved therapies for inborn errors of metabolism

Because of their prevalence, severity and lack of effective treatments, inborn errors of metabolism need novel and more effective therapeutic approaches. The opportunity for an early treatment coming from expanded newborn screening has made this need even more urgent. To meet this demand, a growing number of novel treatments are entering in the phase of clinical development. Strategies to overcome the detrimental consequences of the enzyme deficiencies responsible for inborn errors of metabolism have been focused on multiple fronts at the levels of the gene, RNA, protein and whole cell. These strategies have been accomplished using a wide spectrum of approaches ranging from small molecules to enzyme replacement therapy, cell and gene therapy. The applications of new technologies in the field of inborn errors of metabolism, such as genome editing, RNA interference and cell reprogramming, along with progress in pre-existing strategies, such as gene therapy or cell transplantation, have tremendous potential for clinical translation.

Molecular recognition of PTS-1 cargo proteins by Pex5p: implications for protein mistargeting in primary hyperoxaluria

Peroxisomal biogenesis and function critically depends on the import of cytosolic proteins carrying a PTS1 sequence into this organelle upon interaction with the peroxin Pex5p. Recent structural studies have provided important insights into the molecular recognition of cargo proteins by Pex5p. Peroxisomal import is a key feature in the pathogenesis of primary hyperoxaluria type 1 (PH1), where alanine:glyoxylate aminotransferase (AGT) undergoes mitochondrial mistargeting in about a third of patients. Here, we study the molecular recognition of PTS1 cargo proteins by Pex5p using oligopeptides and AGT variants bearing different natural PTS1 sequences, and employing an array of biophysical, computational and cell biology techniques. Changes in affinity for Pex5p (spanning over 3–4 orders of magnitude) reflect different thermodynamic signatures, but overall bury similar amounts of molecular surface. Structure/energetic analyses provide information on the contribution of ancillary regions and the conformational changes induced in Pex5p and the PTS1 cargo upon complex formation. Pex5p stability in vitro is enhanced upon cargo binding according to their binding affinities. Moreover, we provide evidence that the rational modulation of the AGT: Pex5p binding affinity might be useful tools to investigate mistargeting and misfolding in PH1 by pulling the folding equilibria towards the native and peroxisomal import competent state.

Liver peroxisomal alanine:glyoxylate aminotransferase and the effects of mutations associated with Primary Hyperoxaluria Type I: An overview

Liver peroxisomal alanine:glyoxylate aminotransferase (AGT) (EC 2.6.1.44) catalyses the conversion of l-alanine and glyoxylate to pyruvate and glycine, a reaction that allows glyoxylate detoxification. Inherited mutations on the AGXT gene encoding AGT lead to Primary Hyperoxaluria Type I (PH1), a rare disorder characterized by the deposition of calcium oxalate crystals primarily in the urinary tract. Here we describe the results obtained on the biochemical features of AGT as well as on the molecular and cellular effects of polymorphic and pathogenic mutations. A complex scenario on the molecular pathogenesis of PH1 emerges in which the co-inheritance of polymorphic changes and the condition of homozygosis or compound heterozygosis are two important factors that determine the enzymatic phenotype of PH1 patients. All the reported data represent relevant steps toward the understanding of genotype/phenotype correlations, the prediction of the response of the patients to the available therapies, and the development of new therapeutic approaches. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

A time to reap, a time to sow: mitophagy and biogenesis in cardiac pathophysiology

Balancing mitophagy and mitochondrial biogenesis is essential for maintaining a healthy population of mitochondria and cellular homeostasis. Coordinated interplay between these two forces that govern mitochondrial turnover plays an important role as an adaptive response against various cellular stresses that can compromise cell survival. Failure to maintain the critical balance between mitophagy and mitochondrial biogenesis or homeostatic turnover of mitochondria results in a population of dysfunctional mitochondria that contribute to various disease processes. In this review we outline the mechanics and relationships between mitophagy and mitochondrial biogenesis, and discuss the implications of a disrupted balance between these two forces, with an emphasis on cardiac physiology. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".