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Ghosh S, Finnemann SC, Vollrath D, Rothlin CV. In the Eyes of the Beholder-New Mertk Knockout Mouse and Re-Evaluation of Phagocytosis versus Anti-Inflammatory Functions of MERTK. Int J Mol Sci 2024; 25:5299. [PMID: 38791338 PMCID: PMC11121519 DOI: 10.3390/ijms25105299] [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: 03/16/2024] [Revised: 04/26/2024] [Accepted: 04/26/2024] [Indexed: 05/26/2024] Open
Abstract
Greg Lemke's laboratory was one of the pioneers of research into the TAM family of receptor tyrosine kinases (RTKs). Not only was Tyro3 cloned in his laboratory, but his group also extensively studied mice knocked out for individual or various combinations of the TAM RTKs Tyro3, Axl, and Mertk. Here we primarily focus on one of the paralogs-MERTK. We provide a historical perspective on rodent models of loss of Mertk function and their association with retinal degeneration and blindness. We describe later studies employing mouse genetics and the generation of newer knockout models that point out incongruencies with the inference that loss of MERTK-dependent phagocytosis is sufficient for severe, early-onset photoreceptor degeneration in mice. This discussion is meant to raise awareness with regards to the limitations of the original Mertk knockout mouse model generated using 129 derived embryonic stem cells and carrying 129 derived alleles and the role of these alleles in modifying Mertk knockout phenotypes or even displaying Mertk-independent phenotypes. We also suggest molecular approaches that can further Greg Lemke's scintillating legacy of dissecting the molecular functions of MERTK-a protein that has been described to function in phagocytosis as well as in the negative regulation of inflammation.
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Affiliation(s)
- Sourav Ghosh
- Department of Neurology, School of Medicine, Yale University, New Haven, CT 06520, USA
- Department of Pharmacology, School of Medicine, Yale University, New Haven, CT 06520, USA
| | - Silvia C. Finnemann
- Center for Cancer, Genetic Diseases and Gene Regulation, Department of Biological Sciences, Fordham University, Bronx, NY 10458, USA;
| | - Douglas Vollrath
- Department of Genetics, Stanford University School of Medicine, Stanford, CA 94305, USA;
| | - Carla V. Rothlin
- Department of Pharmacology, School of Medicine, Yale University, New Haven, CT 06520, USA
- Department of Immunobiology, School of Medicine, Yale University, New Haven, CT 06520, USA
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Brown GC. Cell death by phagocytosis. Nat Rev Immunol 2024; 24:91-102. [PMID: 37604896 DOI: 10.1038/s41577-023-00921-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/17/2023] [Indexed: 08/23/2023]
Abstract
Cells can die as a consequence of being phagocytosed by other cells - a form of cell death that has been called phagotrophy, cell cannibalism, programmed cell removal and primary phagocytosis. However, these are all different manifestations of cell death by phagocytosis (termed 'phagoptosis' for short). The engulfed cells die as a result of cytotoxic oxidants, peptides and degradative enzymes within acidic phagolysosomes. Cell death by phagocytosis was discovered by Metchnikov in the 1880s, but was neglected until recently. It is now known to contribute to developmental cell death in nematodes, Drosophila and mammals, and is central to innate and adaptive immunity against pathogens. Cell death by phagocytosis mediates physiological turnover of erythrocytes and other leucocytes, making it the most abundant form of cell death in the mammalian body. Immunity against cancer is also partly mediated by macrophage phagocytosis of cancer cells, but cancer cells can also phagocytose host cells and other cancer cells in order to survive. Recent evidence indicates neurodegeneration and other neuropathologies can be mediated by microglial phagocytosis of stressed neurons. Thus, despite cell death by phagocytosis being poorly recognized, it is one of the oldest, commonest and most important forms of cell death.
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Affiliation(s)
- Guy C Brown
- Department of Biochemistry, University of Cambridge, Cambridge, UK.
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Dorion MF, Yaqubi M, Senkevich K, Kieran NW, MacDonald A, Chen CXQ, Luo W, Wallis A, Shlaifer I, Hall JA, Dudley RWR, Glass IA, Stratton JA, Fon EA, Bartels T, Antel JP, Gan-or Z, Durcan TM, Healy LM. MerTK is a mediator of alpha-synuclein fibril uptake by human microglia. Brain 2024; 147:427-443. [PMID: 37671615 PMCID: PMC10834256 DOI: 10.1093/brain/awad298] [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: 03/08/2023] [Revised: 07/26/2023] [Accepted: 08/29/2023] [Indexed: 09/07/2023] Open
Abstract
Mer tyrosine kinase (MerTK) is a receptor tyrosine kinase that mediates non-inflammatory, homeostatic phagocytosis of diverse types of cellular debris. Highly expressed on the surface of microglial cells, MerTK is of importance in brain development, homeostasis, plasticity and disease. Yet, involvement of this receptor in the clearance of protein aggregates that accumulate with ageing and in neurodegenerative diseases has yet to be defined. The current study explored the function of MerTK in the microglial uptake of alpha-synuclein fibrils which play a causative role in the pathobiology of synucleinopathies. Using human primary and induced pluripotent stem cell-derived microglia, the MerTK-dependence of alpha-synuclein fibril internalization was investigated in vitro. Relevance of this pathway in synucleinopathies was assessed through burden analysis of MERTK variants and analysis of MerTK expression in patient-derived cells and tissues. Pharmacological inhibition of MerTK and siRNA-mediated MERTK knockdown both caused a decreased rate of alpha-synuclein fibril internalization by human microglia. Consistent with the non-inflammatory nature of MerTK-mediated phagocytosis, alpha-synuclein fibril internalization was not observed to induce secretion of pro-inflammatory cytokines such as IL-6 or TNF, and downmodulated IL-1β secretion from microglia. Burden analysis in two independent patient cohorts revealed a significant association between rare functionally deleterious MERTK variants and Parkinson's disease in one of the cohorts (P = 0.002). Despite a small upregulation in MERTK mRNA expression in nigral microglia from Parkinson's disease/Lewy body dementia patients compared to those from non-neurological control donors in a single-nuclei RNA-sequencing dataset (P = 5.08 × 10-21), no significant upregulation in MerTK protein expression was observed in human cortex and substantia nigra lysates from Lewy body dementia patients compared to controls. Taken together, our findings define a novel role for MerTK in mediating the uptake of alpha-synuclein fibrils by human microglia, with possible involvement in limiting alpha-synuclein spread in synucleinopathies such as Parkinson's disease. Upregulation of this pathway in synucleinopathies could have therapeutic values in enhancing alpha-synuclein fibril clearance in the brain.
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Affiliation(s)
- Marie-France Dorion
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Moein Yaqubi
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Konstantin Senkevich
- McGill Parkinson Program and Neurodegenerative Diseases Group, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Human Genetics, McGill University, Montreal H3A 0C7, Canada
| | - Nicholas W Kieran
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Adam MacDonald
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Carol X Q Chen
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Wen Luo
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Amber Wallis
- UK Dementia Research Institute, University College London, London WC1E 6BT, UK
| | - Irina Shlaifer
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Jeffery A Hall
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Roy W R Dudley
- Department of Pediatric Surgery, Division of Neurosurgery, Montreal Children's Hospital, McGill University Health Centers, Montreal H4A 3J1, Canada
| | - Ian A Glass
- Department of Pediatrics, University of Washington, Seattle, WA 98195, USA
| | | | - Jo Anne Stratton
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Edward A Fon
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- McGill Parkinson Program and Neurodegenerative Diseases Group, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Tim Bartels
- UK Dementia Research Institute, University College London, London WC1E 6BT, UK
| | - Jack P Antel
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Ziv Gan-or
- McGill Parkinson Program and Neurodegenerative Diseases Group, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Human Genetics, McGill University, Montreal H3A 0C7, Canada
| | - Thomas M Durcan
- Early Drug Discovery Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
| | - Luke M Healy
- Neuroimmunology Unit, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
- Department of Neurology and Neurosurgery, Montreal Neurological Institute-Hospital, McGill University, Montreal H3A 2B4, Canada
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Happonen KE, Burrola PG, Lemke G. Regulation of brain endothelial cell physiology by the TAM receptor tyrosine kinase Mer. Commun Biol 2023; 6:916. [PMID: 37673933 PMCID: PMC10482977 DOI: 10.1038/s42003-023-05287-y] [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: 02/21/2023] [Accepted: 08/25/2023] [Indexed: 09/08/2023] Open
Abstract
The receptor tyrosine kinase Mer (gene name Mertk) acts in vascular endothelial cells (ECs) to tighten the blood-brain barrier (BBB) subsequent to viral infection, but how this is achieved is poorly understood. We find that Mer controls the expression and activity of a large cohort of BBB regulators, along with endothelial nitric oxide synthase. It also controls, via an Akt-Foxo1 pathway, the expression of multiple angiogenic genes. Correspondingly, EC-specific Mertk gene inactivation resulted in perturbed vascular sprouting and a compromised BBB after induced photothrombotic stroke. Unexpectedly, stroke lesions in the brain were also reduced in the absence of EC Mer, which was linked to reduced plasma expression of fibrinogen, prothrombin, and other effectors of blood coagulation. Together, these results demonstrate that Mer is a central regulator of angiogenesis, BBB integrity, and blood coagulation in the mature vasculature. They may also account for disease severity following infection with the coronavirus SARS-CoV-2.
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Affiliation(s)
- Kaisa E Happonen
- Molecular Neurobiology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA, 92037, USA
| | - Patrick G Burrola
- Molecular Neurobiology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA, 92037, USA
| | - Greg Lemke
- Molecular Neurobiology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA, 92037, USA.
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