Metabolic-scale gene activation screens identify SLCO2B1 as a heme transporter that enhances cellular iron availability.

Iron is the most abundant transition metal essential for numerous cellular processes. Although most mammalian cells acquire iron through transferrin receptors, molecular players of iron utilization under iron restriction are incompletely understood. To address this, we performed metabolism-focused CRISPRa gain-of-function screens, which revealed metabolic limitations under stress conditions. Iron restriction screens identified not only expected members of iron utilization pathways but also SLCO2B1, a poorly characterized membrane carrier. SLCO2B1 expression is sufficient to increase intracellular iron, bypass the essentiality of the transferrin receptor, and enable proliferation under iron restriction. Mechanistically, SLCO2B1 mediates heme analog import in cellular assays. Heme uptake by SLCO2B1 provides sufficient iron for proliferation through heme oxygenases. Notably, SLCO2B1 is predominantly expressed in microglia in the brain, and primary Slco2b1-/- mouse microglia exhibit strong defects in heme analog import. Altogether, our work identifies SLCO2B1 as a microglia-enriched plasma membrane heme importer and provides a genetic platform to identify metabolic limitations under stress conditions.

SLC25A39 is necessary for mitochondrial glutathione import in mammalian cells.

Glutathione (GSH) is a small-molecule thiol that is abundant in all eukaryotes and has key roles in oxidative metabolism1. Mitochondria, as the major site of oxidative reactions, must maintain sufficient levels of GSH to perform protective and biosynthetic functions2. GSH is synthesized exclusively in the cytosol, yet the molecular machinery involved in mitochondrial GSH import remains unknown. Here, using organellar proteomics and metabolomics approaches, we identify SLC25A39, a mitochondrial membrane carrier of unknown function, as a regulator of GSH transport into mitochondria. Loss of SLC25A39 reduces mitochondrial GSH import and abundance without affecting cellular GSH levels. Cells lacking both SLC25A39 and its paralogue SLC25A40 exhibit defects in the activity and stability of proteins containing iron-sulfur clusters. We find that mitochondrial GSH import is necessary for cell proliferation in vitro and red blood cell development in mice. Heterologous expression of an engineered bifunctional bacterial GSH biosynthetic enzyme (GshF) in mitochondria enables mitochondrial GSH production and ameliorates the metabolic and proliferative defects caused by its depletion. Finally, GSH availability negatively regulates SLC25A39 protein abundance, coupling redox homeostasis to mitochondrial GSH import in mammalian cells. Our work identifies SLC25A39 as an essential and regulated component of the mitochondrial GSH-import machinery.

GRIK5 Genetically Regulated Expression Associated with Eye and Vascular Phenomes: Discovery through Iteration among Biobanks, Electronic Health Records, and Zebrafish.

Although the use of model systems for studying the mechanism of mutations that have a large effect is common, we highlight here the ways that zebrafish-model-system studies of a gene, GRIK5, that contributes to the polygenic liability to develop eye diseases have helped to illuminate a mechanism that implicates vascular biology in eye disease. A gene-expression prediction derived from a reference transcriptome panel applied to BioVU, a large electronic health record (EHR)-linked biobank at Vanderbilt University Medical Center, implicated reduced GRIK5 expression in diverse eye diseases. We tested the function of GRIK5 by depletion of its ortholog in zebrafish, and we observed reduced blood vessel numbers and integrity in the eye and increased vascular permeability. Analyses of EHRs in >2.6 million Vanderbilt subjects revealed significant comorbidity of eye and vascular diseases (relative risks 2-15); this comorbidity was confirmed in 150 million individuals from a large insurance claims dataset. Subsequent studies in >60,000 genotyped BioVU participants confirmed the association of reduced genetically predicted expression of GRIK5 with comorbid vascular and eye diseases. Our studies pioneer an approach that allows a rapid iteration of the discovery of gene-phenotype relationships to the primary genetic mechanism contributing to the pathophysiology of human disease. Our findings also add dimension to the understanding of the biology driven by glutamate receptors such as GRIK5 (also referred to as GLUK5 in protein form) and to mechanisms contributing to human eye diseases.

Dynamic Glycosylation Governs the Vertebrate COPII Protein Trafficking Pathway.

The COPII coat complex, which mediates secretory cargo trafficking from the endoplasmic reticulum, is a key control point for subcellular protein targeting. Because misdirected proteins cannot function, protein sorting by COPII is critical for establishing and maintaining normal cell and tissue homeostasis. Indeed, mutations in COPII genes cause a range of human pathologies, including cranio-lenticulo-sutural dysplasia (CLSD), which is characterized by collagen trafficking defects, craniofacial abnormalities, and skeletal dysmorphology. Detailed knowledge of the COPII pathway is required to understand its role in normal cell physiology and to devise new treatments for disorders in which it is disrupted. However, little is known about how vertebrates dynamically regulate COPII activity in response to developmental, metabolic, or pathological cues. Several COPII proteins are modified by O-linked ß-N-acetylglucosamine (O-GlcNAc), a dynamic form of intracellular protein glycosylation, but the biochemical and functional effects of these modifications remain unclear. Here, we use a combination of chemical, biochemical, cellular, and genetic approaches to demonstrate that site-specific O-GlcNAcylation of COPII proteins mediates their protein-protein interactions and modulates cargo secretion. In particular, we show that individual O-GlcNAcylation sites of SEC23A, an essential COPII component, are required for its function in human cells and vertebrate development, because mutation of these sites impairs SEC23A-dependent in vivo collagen trafficking and skeletogenesis in a zebrafish model of CLSD. Our results indicate that O-GlcNAc is a conserved and critical regulatory modification in the vertebrate COPII-dependent trafficking pathway.

In vivo cell biology in zebrafish - providing insights into vertebrate development and disease.

Over the past decades, studies using zebrafish have significantly advanced our understanding of the cellular basis for development and human diseases. Zebrafish have rapidly developing transparent embryos that allow comprehensive imaging of embryogenesis combined with powerful genetic approaches. However, forward genetic screens in zebrafish have generated unanticipated findings that are mirrored by human genetic studies: disruption of genes implicated in basic cellular processes, such as protein secretion or cytoskeletal dynamics, causes discrete developmental or disease phenotypes. This is surprising because many processes that were assumed to be fundamental to the function and survival of all cell types appear instead to be regulated by cell-specific mechanisms. Such discoveries are facilitated by experiments in whole animals, where zebrafish provides an ideal model for visualization and manipulation of organelles and cellular processes in a live vertebrate. Here, we review well-characterized mutants and newly developed tools that underscore this notion. We focus on the secretory pathway and microtubule-based trafficking as illustrative examples of how studying cell biology in vivo using zebrafish has broadened our understanding of the role fundamental cellular processes play in embryogenesis and disease.

Histone butyrylation in the mouse intestine is mediated by the microbiota and associated with regulation of gene expression.

Post-translational modifications (PTMs) on histones are a key source of regulation on chromatin through impacting cellular processes, including gene expression1. These PTMs often arise from metabolites and are thus impacted by metabolism and environmental cues2-7. One class of metabolically regulated PTMs are histone acylations, which include histone acetylation, butyrylation, crotonylation and propionylation3,8. As these PTMs can be derived from short-chain fatty acids, which are generated by the commensal microbiota in the intestinal lumen9-11, we aimed to define how microbes impact the host intestinal chromatin landscape, mainly in female mice. Here we show that in addition to acetylation, intestinal epithelial cells from the caecum and distal mouse intestine also harbour high levels of butyrylation and propionylation on lysines 9 and 27 of histone H3. We demonstrate that these acylations are regulated by the microbiota and that histone butyrylation is additionally regulated by the metabolite tributyrin. Tributyrin-regulated gene programmes are correlated with histone butyrylation, which is associated with active gene-regulatory elements and levels of gene expression. Together, our study uncovers a regulatory layer of how the microbiota and metabolites influence the intestinal epithelium through chromatin, demonstrating a physiological setting in which histone acylations are dynamically regulated and associated with gene regulation.

Rgp1 contributes to craniofacial cartilage development and Rab8a-mediated collagen II secretion.

Rgp1 was previously identified as a component of a guanine nucleotide exchange factor (GEF) complex to activate Rab6a-mediated trafficking events in and around the Golgi. While the role of Rgp1 in protein trafficking has been examined in vitro and in yeast, the role of Rgp1 during vertebrate embryogenesis and protein trafficking in vivo is unknown. Using genetic, CRISPR-induced zebrafish mutants for Rgp1 loss-of-function, we found that Rgp1 is required for craniofacial cartilage development. Within live rgp1-/- craniofacial chondrocytes, we observed altered movements of Rab6a+ vesicular compartments, consistent with a conserved mechanism described in vitro. Using transmission electron microscopy (TEM) and immunofluorescence analyses, we show that Rgp1 plays a role in the secretion of collagen II, the most abundant protein in cartilage. Our overexpression experiments revealed that Rab8a is a part of the post-Golgi collagen II trafficking pathway. Following loss of Rgp1, chondrocytes activate an Arf4b-mediated stress response and subsequently respond with nuclear DNA fragmentation and cell death. We propose that an Rgp1-regulated Rab6a-Rab8a pathway directs secretion of ECM cargoes such as collagen II, a pathway that may also be utilized in other tissues where coordinated trafficking and secretion of collagens and other large cargoes is required for normal development and tissue function.

GeneMAP: A discovery platform for metabolic gene function.

Organisms maintain metabolic homeostasis through the combined functions of small molecule transporters and enzymes. While many of the metabolic components have been well-established, a substantial number remains without identified physiological substrates. To bridge this gap, we have leveraged large-scale plasma metabolome genome-wide association studies (GWAS) to develop a multiomic Gene-Metabolite Associations Prediction (GeneMAP) discovery platform. GeneMAP can generate accurate predictions, even pinpointing genes that are distant from the variants implicated by GWAS. In particular, our work identified SLC25A48 as a genetic determinant of plasma choline levels. Mechanistically, SLC25A48 loss strongly impairs mitochondrial choline import and synthesis of its downstream metabolite, betaine. Rare variant testing and polygenic risk score analyses have elucidated choline-relevant phenomic consequences of SLC25A48 dysfunction. Altogether, our study proposes SLC25A48 as a mitochondrial choline transporter and provides a discovery platform for metabolic gene function.

Autoregulatory control of mitochondrial glutathione homeostasis.

Mitochondria must maintain adequate amounts of metabolites for protective and biosynthetic functions. However, how mitochondria sense the abundance of metabolites and regulate metabolic homeostasis is not well understood. In this work, we focused on glutathione (GSH), a critical redox metabolite in mitochondria, and identified a feedback mechanism that controls its abundance through the mitochondrial GSH transporter, SLC25A39. Under physiological conditions, SLC25A39 is rapidly degraded by mitochondrial protease AFG3L2. Depletion of GSH dissociates AFG3L2 from SLC25A39, causing a compensatory increase in mitochondrial GSH uptake. Genetic and proteomic analyses identified a putative iron-sulfur cluster in the matrix-facing loop of SLC25A39 as essential for this regulation, coupling mitochondrial iron homeostasis to GSH import. Altogether, our work revealed a paradigm for the autoregulatory control of metabolic homeostasis in organelles.

Phenome-based approach identifies RIC1-linked Mendelian syndrome through zebrafish models, biobank associations and clinical studies.

Discovery of genotype-phenotype relationships remains a major challenge in clinical medicine. Here, we combined three sources of phenotypic data to uncover a new mechanism for rare and common diseases resulting from collagen secretion deficits. Using a zebrafish genetic screen, we identified the ric1 gene as being essential for skeletal biology. Using a gene-based phenome-wide association study (PheWAS) in the EHR-linked BioVU biobank, we show that reduced genetically determined expression of RIC1 is associated with musculoskeletal and dental conditions. Whole-exome sequencing identified individuals homozygous-by-descent for a rare variant in RIC1 and, through a guided clinical re-evaluation, it was discovered that they share signs with the BioVU-associated phenome. We named this new Mendelian syndrome CATIFA (cleft lip, cataract, tooth abnormality, intellectual disability, facial dysmorphism, attention-deficit hyperactivity disorder) and revealed further disease mechanisms. This gene-based, PheWAS-guided approach can accelerate the discovery of clinically relevant disease phenome and associated biological mechanisms.

Metabolic coessentiality mapping identifies C12orf49 as a regulator of SREBP processing and cholesterol metabolism.

Coessentiality mapping has been useful to systematically cluster genes into biological pathways and identify gene functions1-3. Here, using the debiased sparse partial correlation (DSPC) method3, we construct a functional coessentiality map for cellular metabolic processes across human cancer cell lines. This analysis reveals 35 modules associated with known metabolic pathways and further assigns metabolic functions to unknown genes. In particular, we identify C12orf49 as an essential regulator of cholesterol and fatty acid metabolism in mammalian cells. Mechanistically, C12orf49 localizes to the Golgi, binds membrane-bound transcription factor peptidase, site 1 (MBTPS1, site 1 protease) and is necessary for the cleavage of its substrates, including sterol regulatory element binding protein (SREBP) transcription factors. This function depends on the evolutionarily conserved uncharacterized domain (DUF2054) and promotes cell proliferation under cholesterol depletion. Notably, c12orf49 depletion in zebrafish blocks dietary lipid clearance in vivo, mimicking the phenotype of mbtps1 mutants. Finally, in an electronic health record (EHR)-linked DNA biobank, C12orf49 is associated with hyperlipidaemia through phenome analysis. Altogether, our findings reveal a conserved role for C12orf49 in cholesterol and lipid homeostasis and provide a platform to identify unknown components of other metabolic pathways.

Correlation of pelvic fractures and associated injuries: An analysis of 471 pelvic trauma patients.

BACKGROUND: In most respects, the vast majority of pelvic injuries is not of a life-threatening status, but co-presence of other injuries needs to be diagnosed. This study aims to evaluate associated pelvic and extra-pelvic visceral organ injuries of the patients with closed pelvic fractures. METHODS: This retrospective study was conducted with 471 adult patients who had been admitted to our Emergency Service with the diagnosis of pelvic fractures. Type of fractures, accompanying visceral organ injuries, the demographic data, type of operation, mortality rates were recorded and analysed statistically. RESULTS: The rate of operations carried out by the general surgery clinic or other surgical clinics in each type of fracture according to AO classification did not differ (p=0.118). In patients with A2, A3 and B1 types of fractures, the operation rate of general surgery clinic did not show a significant difference. However, most of the patients who had extrapelvic surgery were in the mild severity pelvic trauma, such as AO A2 and A3. A total of 31 patients were ex-patients, 17 of whom had AO-A2 type of fractures. The findings showed that there was a significant difference between abdominal ultrasonography outcome that was normal and non-orthopedic surgery types (p<0.001). There was no significant difference between the types of surgery performed and Abdominal CT outcome, which was normal (p=0.215). CONCLUSION: In the management of patients with pelvic fractures irrespective of its type or grade, the findings suggests that greater attention should be paid to not to overlook the associated injuries. Early blood and imaging tests are encouraged after the patient's hemodynamic status is stabilized.

Zebrafish Developmental Models of Skeletal Diseases.

The zebrafish skeleton shares many similarities with human and other vertebrate skeletons. Over the past years, work in zebrafish has provided an extensive understanding of the basic developmental mechanisms and cellular pathways directing skeletal development and homeostasis. This review will focus on the cell biology of cartilage and bone and how the basic cellular processes within chondrocytes and osteocytes function to assemble the structural frame of a vertebrate body. We will discuss fundamental functions of skeletal cells in production and secretion of extracellular matrix and cellular activities leading to differentiation of progenitors to mature cells that make up the skeleton. We highlight important examples where findings in zebrafish provided direction for the search for genes causing human skeletal defects and also how zebrafish research has proven important for validating candidate human disease genes. The work we cover here illustrates utility of zebrafish in unraveling molecular mechanisms of cellular functions necessary to form and maintain a healthy skeleton.

Evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes.

The evolutionary origins of the hypoxia-sensitive cells that trigger amniote respiratory reflexes - carotid body glomus cells, and 'pulmonary neuroendocrine cells' (PNECs) - are obscure. Homology has been proposed between glomus cells, which are neural crest-derived, and the hypoxia-sensitive 'neuroepithelial cells' (NECs) of fish gills, whose embryonic origin is unknown. NECs have also been likened to PNECs, which differentiate in situ within lung airway epithelia. Using genetic lineage-tracing and neural crest-deficient mutants in zebrafish, and physical fate-mapping in frog and lamprey, we find that NECs are not neural crest-derived, but endoderm-derived, like PNECs, whose endodermal origin we confirm. We discover neural crest-derived catecholaminergic cells associated with zebrafish pharyngeal arch blood vessels, and propose a new model for amniote hypoxia-sensitive cell evolution: endoderm-derived NECs were retained as PNECs, while the carotid body evolved via the aggregation of neural crest-derived catecholaminergic (chromaffin) cells already associated with blood vessels in anamniote pharyngeal arches.

Trafficking mechanisms of extracellular matrix macromolecules: insights from vertebrate development and human diseases.

Cellular life depends on protein transport and membrane traffic. In multicellular organisms, membrane traffic is required for extracellular matrix deposition, cell adhesion, growth factor release, and receptor signaling, which are collectively required to integrate the development and physiology of tissues and organs. Understanding the regulatory mechanisms that govern cargo and membrane flow presents a prime challenge in cell biology. Extracellular matrix (ECM) secretion remains poorly understood, although given its essential roles in the regulation of cell migration, differentiation, and survival, ECM secretion mechanisms are likely to be tightly controlled. Recent studies in vertebrate model systems, from fishes to mammals and in human patients, have revealed complex and diverse loss-of-function phenotypes associated with mutations in components of the secretory machinery. A broad spectrum of diseases from skeletal and cardiovascular to neurological deficits have been linked to ECM trafficking. These discoveries have directly challenged the prevailing view of secretion as an essential but monolithic process. Here, we will discuss the latest findings on mechanisms of ECM trafficking in vertebrates.