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Dunaway LS, Loeb SA, Petrillo S, Tolosano E, Isakson BE. Heme metabolism in nonerythroid cells. J Biol Chem 2024; 300:107132. [PMID: 38432636 PMCID: PMC10988061 DOI: 10.1016/j.jbc.2024.107132] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2023] [Revised: 01/31/2024] [Accepted: 02/23/2024] [Indexed: 03/05/2024] Open
Abstract
Heme is an iron-containing prosthetic group necessary for the function of several proteins termed "hemoproteins." Erythrocytes contain most of the body's heme in the form of hemoglobin and contain high concentrations of free heme. In nonerythroid cells, where cytosolic heme concentrations are 2 to 3 orders of magnitude lower, heme plays an essential and often overlooked role in a variety of cellular processes. Indeed, hemoproteins are found in almost every subcellular compartment and are integral in cellular operations such as oxidative phosphorylation, amino acid metabolism, xenobiotic metabolism, and transcriptional regulation. Growing evidence reveals the participation of heme in dynamic processes such as circadian rhythms, NO signaling, and the modulation of enzyme activity. This dynamic view of heme biology uncovers exciting possibilities as to how hemoproteins may participate in a range of physiologic systems. Here, we discuss how heme is regulated at the level of its synthesis, availability, redox state, transport, and degradation and highlight the implications for cellular function and whole organism physiology.
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Affiliation(s)
- Luke S Dunaway
- Robert M. Berne Cardiovascular Research Center, University of Virginia School of Medicine, Charlottesville, Virginia, USA
| | - Skylar A Loeb
- Robert M. Berne Cardiovascular Research Center, University of Virginia School of Medicine, Charlottesville, Virginia, USA; Department of Molecular Physiology and Biophysics, University of Virginia School of Medicine, Charlottesville, Virginia, USA
| | - Sara Petrillo
- Deptartment Molecular Biotechnology and Health Sciences and Molecular Biotechnology Center "Guido Tarone", University of Torino, Torino, Italy
| | - Emanuela Tolosano
- Deptartment Molecular Biotechnology and Health Sciences and Molecular Biotechnology Center "Guido Tarone", University of Torino, Torino, Italy
| | - Brant E Isakson
- Robert M. Berne Cardiovascular Research Center, University of Virginia School of Medicine, Charlottesville, Virginia, USA; Department of Molecular Physiology and Biophysics, University of Virginia School of Medicine, Charlottesville, Virginia, USA.
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2
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Li Z, Li Y, Chu X, Du K, Tang Y, Xie Z, Yu M, Deng J, Lv H, Zhang W, Wang Z, Meng L, Yuan Y. Novel mutations in FLVCR1 cause tremors, sensory neuropathy with retinitis pigmentosa. Neuropathology 2024; 44:87-95. [PMID: 37469134 DOI: 10.1111/neup.12936] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2022] [Revised: 07/03/2023] [Accepted: 07/05/2023] [Indexed: 07/21/2023]
Abstract
The mutations of the feline leukemia virus subgroup C receptor-related protein 1 (FLVCR1) cause ataxia with retinitis pigmentosa. Recent studies indicated a large variation in the phenotype of FLVCR1-associated diseases. In this report, we describe an adult male who manifested first with tremors in his third decade, followed by retinitis pigmentosa, sensory ataxia, and sensory neuropathy in his fourth decade. While retinitis pigmentosa and sensory ataxia are well-recognized features of FLVCR1-associated disease, tremor is rarely described. Whole-exome sequencing revealed novel compound heterozygous pathogenic FLVCR1 variants: c.498 G > A; p.(Trp166*) and c.369 T > G; p.(Phe123Leu). In addition, we have highlighted the ultrastructural abnormalities of the sural biopsy in this patient.
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Affiliation(s)
- Zhenyu Li
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Yize Li
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Xujun Chu
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Kang Du
- Department of Neurology, Qujing First People's Hospital, Qujing, China
| | - Yuwei Tang
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Zhiying Xie
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Meng Yu
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Jianwen Deng
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - He Lv
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Wei Zhang
- Department of Neurology, Peking University First Hospital, Beijing, China
| | - Zhaoxia Wang
- Department of Neurology, Peking University First Hospital, Beijing, China
- Beijing Key Laboratory for Neurovascular Disease Discovery, Peking University First Hospital, Beijing, China
- Key Laboratory for Neuroscience, National Health Commission of the People's Republic of China, Peking University, Beijing, China
| | - Lingchao Meng
- Department of Neurology, Peking University First Hospital, Beijing, China
- Beijing Key Laboratory for Neurovascular Disease Discovery, Peking University First Hospital, Beijing, China
- Key Laboratory for Neuroscience, National Health Commission of the People's Republic of China, Peking University, Beijing, China
| | - Yun Yuan
- Department of Neurology, Peking University First Hospital, Beijing, China
- Beijing Key Laboratory for Neurovascular Disease Discovery, Peking University First Hospital, Beijing, China
- Key Laboratory for Neuroscience, National Health Commission of the People's Republic of China, Peking University, Beijing, China
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3
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Petrillo S, De Giorgio F, Bertino F, Garello F, Bitonto V, Longo DL, Mercurio S, Ammirata G, Allocco AL, Fiorito V, Chiabrando D, Altruda F, Terreno E, Provero P, Munaron L, Genova T, Nóvoa A, Carlos AR, Cardoso S, Mallo M, Soares MP, Tolosano E. Endothelial cells require functional FLVCR1a during developmental and adult angiogenesis. Angiogenesis 2023; 26:365-384. [PMID: 36631598 PMCID: PMC10328904 DOI: 10.1007/s10456-023-09865-w] [Citation(s) in RCA: 7] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Accepted: 01/06/2023] [Indexed: 01/13/2023]
Abstract
The Feline Leukemia Virus Subgroup C Receptor 1a (FLVCR1a) is a transmembrane heme exporter essential for embryonic vascular development. However, the exact role of FLVCR1a during blood vessel development remains largely undefined. Here, we show that FLVCR1a is highly expressed in angiogenic endothelial cells (ECs) compared to quiescent ECs. Consistently, ECs lacking FLVCR1a give rise to structurally and functionally abnormal vascular networks in multiple models of developmental and pathologic angiogenesis. Firstly, zebrafish embryos without FLVCR1a displayed defective intersegmental vessels formation. Furthermore, endothelial-specific Flvcr1a targeting in mice led to a reduced radial expansion of the retinal vasculature associated to decreased EC proliferation. Moreover, Flvcr1a null retinas showed defective vascular organization and loose attachment of pericytes. Finally, adult neo-angiogenesis is severely affected in murine models of tumor angiogenesis. Tumor blood vessels lacking Flvcr1a were disorganized and dysfunctional. Collectively, our results demonstrate the critical role of FLVCR1a as a regulator of developmental and pathological angiogenesis identifying FLVCR1a as a potential therapeutic target in human diseases characterized by aberrant neovascularization.
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Affiliation(s)
- Sara Petrillo
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy.
| | - F De Giorgio
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - F Bertino
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - F Garello
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - V Bitonto
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - D L Longo
- Institute of Biostructures and Bioimaging (IBB), Italian National Research Council (CNR), Via Nizza, 52, 10126, Turin, Italy
| | - S Mercurio
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - G Ammirata
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - A L Allocco
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - V Fiorito
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - D Chiabrando
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - F Altruda
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - E Terreno
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
| | - P Provero
- Department of Molecular Biotechnology and Health Sciences, and GenoBiToUS, Genomics and Bioinformatics Service, University of Torino, Turin, Italy
- Center for Translational Genomics and Bioinformatics, San Raffaele Scientific Institute IRCCS, Milan, Italy
| | - L Munaron
- Department of Life Sciences and Systems Biology, University of Torino, Via Accademia Albertina 13, 10123, Turin, Italy
| | - T Genova
- Department of Life Sciences and Systems Biology, University of Torino, Via Accademia Albertina 13, 10123, Turin, Italy
| | - A Nóvoa
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
| | - A R Carlos
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
| | - S Cardoso
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
| | - M Mallo
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
| | - M P Soares
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
| | - E Tolosano
- Department of Molecular Biotechnology and Health Sciences, Molecular Biotechnology Center (MBC) "Guido Tarone", University of Torino, Via Nizza, 52, 10126, Turin, Italy
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Li X, Li X, Xiang C, Ye F. Lead exposure represses mitochondrial metabolism by activation of heme-binding protein BACH1 in differentiated SH-SY5Y cell. THE SCIENCE OF THE TOTAL ENVIRONMENT 2022; 853:158665. [PMID: 36096218 DOI: 10.1016/j.scitotenv.2022.158665] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/19/2022] [Revised: 09/02/2022] [Accepted: 09/07/2022] [Indexed: 06/15/2023]
Abstract
Exposure to lead (Pb), a known toxin causing developmental neurotoxicity, can impair neurogenesis and oxidative phosphorylation (OXPHOS), but the mechanism is not clarified. In the current study, we aim to explore the effects of Pb on the differentiation of SH-SY5Y cells and investigate the role of heme and heme-binding protein BACH1 during differentiation. We found that Pb exposure caused a shift from OXPHOS to glycolysis, resulting in neurogenesis impairment by decreasing neurite growth and downregulation of PSD95 and Synapsin-1 in differentiated SH-SY5Y cells. Heme reduction mediated this mitochondria metabolism repression caused by Pb depending on BACH1 activation. Hemin supplement alleviated Pb-induced OXPHOS damage and adenosine triphosphate (ATP) reduction in differentiated SH-SY5Y cells, and further protected for Pb-induced damage of synapse. Heme binding factor BACH1 was negatively regulated by heme content and BACH1 knockout rescued the Pb-induced transcription and expression decline of genes related to OXPHOS and abrogated Pb-induced growth inhibition of axon promotion and synapse formation. Collectively, the present study demonstrates that heme deficiency mediates OXPHOS damage caused by Pb through BACH1 activation, resulting in neurogenesis impairment.
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Affiliation(s)
- Xiaoyi Li
- Center for Translational Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China.
| | - Xintong Li
- Department of Occupational and Environmental Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China; Key Laboratory of Environment and Health, Ministry of Education & Ministry of Environmental Protection, and State Key Laboratory of Environmental Health (Incubating), School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China
| | - Cui Xiang
- Department of Occupational and Environmental Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China; Key Laboratory of Environment and Health, Ministry of Education & Ministry of Environmental Protection, and State Key Laboratory of Environmental Health (Incubating), School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China
| | - Fang Ye
- Department of Occupational and Environmental Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China; Key Laboratory of Environment and Health, Ministry of Education & Ministry of Environmental Protection, and State Key Laboratory of Environmental Health (Incubating), School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430030, China.
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5
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Lischka A, Lassuthova P, Çakar A, Record CJ, Van Lent J, Baets J, Dohrn MF, Senderek J, Lampert A, Bennett DL, Wood JN, Timmerman V, Hornemann T, Auer-Grumbach M, Parman Y, Hübner CA, Elbracht M, Eggermann K, Geoffrey Woods C, Cox JJ, Reilly MM, Kurth I. Genetic pain loss disorders. Nat Rev Dis Primers 2022; 8:41. [PMID: 35710757 DOI: 10.1038/s41572-022-00365-7] [Citation(s) in RCA: 15] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Accepted: 05/10/2022] [Indexed: 01/05/2023]
Abstract
Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.
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Affiliation(s)
- Annette Lischka
- Institute of Human Genetics, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany
| | - Petra Lassuthova
- Department of Paediatric Neurology, 2nd Faculty of Medicine, Charles University in Prague and Motol University Hospital, Prague, Czech Republic
| | - Arman Çakar
- Neuromuscular Unit, Department of Neurology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey
| | - Christopher J Record
- Centre for Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, UK
| | - Jonas Van Lent
- Peripheral Neuropathy Research Group, Department of Biomedical Sciences, University of Antwerp, Antwerp, Belgium.,Laboratory of Neuromuscular Pathology, Institute Born Bunge, Antwerp, Belgium
| | - Jonathan Baets
- Laboratory of Neuromuscular Pathology, Institute Born Bunge, Antwerp, Belgium.,Translational Neurosciences, Faculty of Medicine and Health Sciences, University of Antwerp, Antwerp, Belgium.,Neuromuscular Reference Centre, Department of Neurology, Antwerp University Hospital, Antwerp, Belgium
| | - Maike F Dohrn
- Department of Neurology, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany.,Dr. John T. Macdonald Foundation, Department of Human Genetics and John P. Hussman Institute for Human Genomics, University of Miami, Miller School of Medicine, Miami, FL, USA
| | - Jan Senderek
- Friedrich-Baur-Institute, Department of Neurology, Ludwig-Maximilians-University, Munich, Germany
| | - Angelika Lampert
- Institute of Physiology, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany
| | - David L Bennett
- Nuffield Department of Clinical Neuroscience, Oxford University, Oxford, UK
| | - John N Wood
- Molecular Nociception Group, Wolfson Institute for Biomedical Research, University College London, London, UK
| | - Vincent Timmerman
- Peripheral Neuropathy Research Group, Department of Biomedical Sciences, University of Antwerp, Antwerp, Belgium.,Laboratory of Neuromuscular Pathology, Institute Born Bunge, Antwerp, Belgium
| | - Thorsten Hornemann
- Department of Clinical Chemistry, University Hospital Zurich, University of Zurich, Zurich, Switzerland
| | - Michaela Auer-Grumbach
- Department of Orthopedics and Trauma Surgery, Medical University of Vienna, Vienna, Austria
| | - Yesim Parman
- Neuromuscular Unit, Department of Neurology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey
| | | | - Miriam Elbracht
- Institute of Human Genetics, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany
| | - Katja Eggermann
- Institute of Human Genetics, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany
| | - C Geoffrey Woods
- Cambridge Institute for Medical Research, Keith Peters Building, Cambridge Biomedical Campus, Cambridge, UK
| | - James J Cox
- Molecular Nociception Group, Wolfson Institute for Biomedical Research, University College London, London, UK
| | - Mary M Reilly
- Centre for Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, UK
| | - Ingo Kurth
- Institute of Human Genetics, Medical Faculty, Uniklinik RWTH Aachen University, Aachen, Germany.
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Endothelial Heme Dynamics Drive Cancer Cell Metabolism by Shaping the Tumor Microenvironment. Biomedicines 2021; 9:biomedicines9111557. [PMID: 34829786 PMCID: PMC8615489 DOI: 10.3390/biomedicines9111557] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2021] [Revised: 10/24/2021] [Accepted: 10/25/2021] [Indexed: 01/06/2023] Open
Abstract
The crosstalk among cancer cells (CCs) and stromal cells within the tumor microenvironment (TME) has a prominent role in cancer progression. The significance of endothelial cells (ECs) in this scenario relies on multiple vascular functions. By forming new blood vessels, ECs support tumor growth. In addition to their angiogenic properties, tumor-associated ECs (TECs) establish a unique vascular niche that actively modulates cancer development by shuttling a selected pattern of factors and metabolites to the CC. The profile of secreted metabolites is strictly dependent on the metabolic status of the cell, which is markedly perturbed in TECs. Recent evidence highlights the involvement of heme metabolism in the regulation of energy metabolism in TECs. The present study shows that interfering with endothelial heme metabolism by targeting the cell membrane heme exporter Feline Leukemia Virus subgroup C Receptor 1a (FLVCR1a) in TECs, resulted in enhanced fatty acid oxidation (FAO). Moreover, FAO-derived acetyl-CoA was partly consumed through ketogenesis, resulting in ketone bodies (KBs) accumulation in FLVCR1a-deficient TECs. Finally, the results from this study also demonstrate that TECs-derived KBs can be secreted in the extracellular environment, inducing a metabolic rewiring in the CC. Taken together, these data may contribute to finding new metabolic vulnerabilities for cancer therapy.
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Palmaers NE, Wiegand SB, Herzog C, Echtermeyer FG, Eberhardt MJ, Leffler A. Distinct Mechanisms Account for In Vitro Activation and Sensitization of TRPV1 by the Porphyrin Hemin. Int J Mol Sci 2021; 22:ijms221910856. [PMID: 34639197 PMCID: PMC8509749 DOI: 10.3390/ijms221910856] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2021] [Revised: 10/03/2021] [Accepted: 10/04/2021] [Indexed: 12/02/2022] Open
Abstract
TRPV1 mediates pain occurring during sickling episodes in sickle cell disease (SCD). We examined if hemin, a porphyrin released during intravascular hemolysis modulates TRPV1. Calcium imaging and patch clamp were employed to examine effects of hemin on mouse dorsal root ganglion (DRG) neurons and HEK293t cells expressing TRPV1 and TRPA1. Hemin induced a concentration-dependent calcium influx in DRG neurons which was abolished by the unspecific TRP-channel inhibitor ruthenium red. The selective TRPV1-inhibitor BCTC or genetic deletion of TRPV1 only marginally impaired hemin-induced calcium influx in DRG neurons. While hTRPV1 expressed in HEK293 cells mediated a hemin-induced calcium influx which was blocked by BCTC, patch clamp recordings only showed potentiated proton- and heat-evoked currents. This effect was abolished by the PKC-inhibitor chelerythrine chloride and in protein kinase C (PKC)-insensitive TRPV1-mutants. Hemin-induced calcium influx through TRPV1 was only partly PKC-sensitive, but it was abolished by the reducing agent dithiothreitol (DTT). In contrast, hemin-induced potentiation of inward currents was not reduced by DTT. Hemin also induced a redox-dependent calcium influx, but not inward currents on hTRPA1. Our data suggest that hemin induces a PKC-mediated sensitization of TRPV1. However, it also acts as a photosensitizer when exposed to UVA-light used for calcium imaging. The resulting activation of redox-sensitive ion channels such as TRPV1 and TRPA1 may be an in vitro artifact with limited physiological relevance.
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