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Levin-Kravets O, Kordonsky A, Shusterman A, Biswas S, Persaud A, Elias S, Langut Y, Florentin A, Simpson-Lavy KJ, Yariv E, Avishid R, Sror M, Almog O, Marshanski T, Kadosh S, Ben David N, Manori B, Fischer Z, Lilly J, Borisova E, Ambrozkiewicz MC, Tarabykin V, Kupiec M, Thaker M, Rotin D, Prag G. Split Chloramphenicol Acetyl-Transferase Assay Reveals Self-Ubiquitylation-Dependent Regulation of UBE3B. J Mol Biol 2021; 433:167276. [PMID: 34599943 DOI: 10.1016/j.jmb.2021.167276] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2021] [Revised: 09/20/2021] [Accepted: 09/22/2021] [Indexed: 10/20/2022]
Abstract
Split reporter protein-based genetic section systems are widely used to identify and characterize protein-protein interactions (PPI). The assembly of split markers that antagonize toxins, rather than required for synthesis of missing metabolites, facilitates the seeding of high density of cells and selective growth. Here we present a newly developed split chloramphenicol acetyltransferase (split-CAT) -based genetic selection system. The N terminus fragment of CAT is fused downstream of the protein of interest and the C terminus fragment is tethered upstream to its postulated partner. We demonstrate the system's advantages for the study of PPIs. Moreover, we show that co-expression of a functional ubiquitylation cascade where the target and ubiquitin are tethered to the split-CAT fragments results in ubiquitylation-dependent selective growth. Since proteins do not have to be purified from the bacteria and due to the high sensitivity of the split-CAT reporter, detection of challenging protein cascades and post-translation modifications is enabled. In addition, we demonstrate that the split-CAT system responds to small molecule inhibitors and molecular glues (GLUTACs). The absence of ubiquitylation-dependent degradation and deubiquitylation in E. coli significantly simplify the interpretation of the results. We harnessed the developed system to demonstrate that like NEDD4, UBE3B also undergoes self-ubiquitylation-dependent inactivation. We show that self-ubiquitylation of UBE3B on K665 induces oligomerization and inactivation in yeast and mammalian cells respectively. Finally, we showcase the advantages of split-CAT in the study of human diseases by demonstrating that mutations in UBE3B that cause Kaufman oculocerebrofacial syndrome exhibit clear E. coli growth phenotypes.
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Affiliation(s)
- Olga Levin-Kravets
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Alina Kordonsky
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Anna Shusterman
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Sagnik Biswas
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Avinash Persaud
- Cell Biology Program, The Hospital for Sick Children and Biochemistry Department, University of Toronto, Toronto, ON, Canada
| | - Sivan Elias
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Yael Langut
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Amir Florentin
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Kobi J Simpson-Lavy
- The Shmunis School of Biomedicine and Cancer Research, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Elon Yariv
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Reut Avishid
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Mor Sror
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Ofir Almog
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Tal Marshanski
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel. https://twitter.com/@TalMarsh
| | - Shira Kadosh
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Nicole Ben David
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Bar Manori
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Zohar Fischer
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Jeremiah Lilly
- Novartis Institutes for Biomedical Research, 250 Massachusetts Ave., Cambridge, MA 02139, USA
| | - Ekaterina Borisova
- Institute of Medical Genetics, Tomsk National Research Medical Center Neuroscience, Lobachevsky University of the Russian Academy of Sciences Nizhny Novgorod, pr. Gagarina 24, Nizhny Novgorod, Russia; Institute of Cell Biology and Neurobiology, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Charitéplatz 1, 10117 Berlin, Germany
| | - Mateusz C Ambrozkiewicz
- Institute of Cell Biology and Neurobiology, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Charitéplatz 1, 10117 Berlin, Germany. https://twitter.com/@MAmbrozkiewicz
| | - Victor Tarabykin
- Institute of Cell Biology and Neurobiology, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Charitéplatz 1, 10117 Berlin, Germany
| | - Martin Kupiec
- The Shmunis School of Biomedicine and Cancer Research, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| | - Maulik Thaker
- Novartis Institutes for Biomedical Research, 250 Massachusetts Ave., Cambridge, MA 02139, USA
| | - Daniela Rotin
- Cell Biology Program, The Hospital for Sick Children and Biochemistry Department, University of Toronto, Toronto, ON, Canada
| | - Gali Prag
- School of Neurobiology, Biochemistry & Biophysics, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel; Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
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Deubiquitylating enzymes in neuronal health and disease. Cell Death Dis 2021; 12:120. [PMID: 33483467 PMCID: PMC7822931 DOI: 10.1038/s41419-020-03361-5] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2020] [Revised: 12/15/2020] [Accepted: 12/16/2020] [Indexed: 12/11/2022]
Abstract
Ubiquitylation and deubiquitylation play a pivotal role in protein homeostasis (proteostasis). Proteostasis shapes the proteome landscape in the human brain and its impairment is linked to neurodevelopmental and neurodegenerative disorders. Here we discuss the emerging roles of deubiquitylating enzymes in neuronal function and survival. We provide an updated perspective on the genetics, physiology, structure, and function of deubiquitylases in neuronal health and disease. ![]()
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E. coli-Based Selection and Expression Systems for Discovery, Characterization, and Purification of Ubiquitylated Proteins. Methods Mol Biol 2018. [PMID: 30242709 DOI: 10.1007/978-1-4939-8706-1_11] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
Abstract
Ubiquitylation is an eukaryotic signal that regulates most cellular pathways. However, four major hurdles pose challenges to study ubiquitylation: (1) high redundancy between ubiquitin (Ub) cascades, (2) ubiquitylation is tightly regulated in the cell, (3) the transient nature of the Ub signal, and (4) difficulties to purify functional ubiquitylation apparatus for in vitro assay. Here, we present systems that express functional Ub cascades in E. coli, which lacks deubiquitylases, Ub-dependent degradations, and control mechanisms for ubiquitylation. Therefore, expression of an ubiquitylation cascade results in the accumulation of stable ubiquitylated protein that can be genetically selected or purified, thus circumventing the above challenges. Co-expression of split antibiotic resistance protein fragments tethered to Ub and ubiquitylation targets along with ubiquitylation enzymes (E1, E2, and E3) gives rise to bacterial growth on selective media. We show that ubiquitylation rate is highly correlated with growth efficiency. Hence, genetic libraries and simple manipulations in the selection system facilitate the identification and characterization of components and interfaces along Ub cascades. The bacterial expression system also facilitates the detection of ubiquitylated proteins. Furthermore, the expression system allows affinity chromatography-based purification of milligram quantities of ubiquitylated proteins for downstream biochemical, biophysical, and structural studies.
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Shen J, Li P, Shao X, Yang Y, Liu X, Feng M, Yu Q, Hu R, Wang Z. The E3 Ligase RING1 Targets p53 for Degradation and Promotes Cancer Cell Proliferation and Survival. Cancer Res 2017; 78:359-371. [PMID: 29187402 DOI: 10.1158/0008-5472.can-17-1805] [Citation(s) in RCA: 43] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2017] [Revised: 10/11/2017] [Accepted: 11/21/2017] [Indexed: 11/16/2022]
Abstract
As a component of the transcriptional repression complex 1 (PRC1), the ring finger protein RING1 participates in the epigenetic regulation in cancer. However, the contributions of RING1 to cancer etiology or development are unknown. In this study, we report that RING1 is a critical negative regulator of p53 homeostasis in human hepatocellular and colorectal carcinomas. RING1 acts as an E3 ubiquitin (Ub) ligase to directly interact with and ubiquitinate p53, resulting in its proteasome-dependent degradation. The RING domain of RING1 was required for its E3 Ub ligase activity. RING1 depletion inhibited the proliferation and survival of the p53 wild-type cancer cells by inducing cell-cycle arrest, apoptosis, and senescence, with only modest effects on p53-deficient cells. Its growth inhibitory effect was partially rescued by p53 silencing, suggesting an important role for the RING1-p53 complex in human cancer. In clinical specimens of hepatocellular carcinoma, RING1 upregulation was evident in association with poor clinical outcomes. Collectively, our results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer.Significance: These results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer. Cancer Res; 78(2); 359-71. ©2017 AACR.
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Affiliation(s)
- Jiajia Shen
- Department of Biochemistry, Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China
| | - Pengyu Li
- Qilu Hospital of Shandong University, Jinan, China
| | - Xuejing Shao
- Zhejiang Province Key Laboratory of Anti-Cancer Drug Research, Institute of Pharmacology and Toxicology, School of Pharmaceutical Sciences, Zhejiang University, Hangzhou, China
| | - Yang Yang
- State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences, Shanghai, China
| | - Xiujun Liu
- Department of Biochemistry, Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China
| | - Min Feng
- Cancer Therapeutics and Stratified Oncology, Genome Institute of Singapore, Agency for Science, Technology, and Research (A*STAR), Biopolis, Singapore
| | - Qiang Yu
- Cancer Therapeutics and Stratified Oncology, Genome Institute of Singapore, Agency for Science, Technology, and Research (A*STAR), Biopolis, Singapore
| | - Ronggui Hu
- State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences, Shanghai, China.
| | - Zhen Wang
- Department of Biochemistry, Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China.
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Attali I, Tobelaim WS, Persaud A, Motamedchaboki K, Simpson-Lavy KJ, Mashahreh B, Levin-Kravets O, Keren-Kaplan T, Pilzer I, Kupiec M, Wiener R, Wolf DA, Rotin D, Prag G. Ubiquitylation-dependent oligomerization regulates activity of Nedd4 ligases. EMBO J 2017; 36:425-440. [PMID: 28069708 DOI: 10.15252/embj.201694314] [Citation(s) in RCA: 42] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2016] [Revised: 11/25/2016] [Accepted: 12/06/2016] [Indexed: 11/09/2022] Open
Abstract
Ubiquitylation controls protein function and degradation. Therefore, ubiquitin ligases need to be tightly controlled. We discovered an evolutionarily conserved allosteric restraint mechanism for Nedd4 ligases and demonstrated its function with diverse substrates: the yeast soluble proteins Rpn10 and Rvs167, and the human receptor tyrosine kinase FGFR1 and cardiac IKS potassium channel. We found that a potential trimerization interface is structurally blocked by the HECT domain α1-helix, which further undergoes ubiquitylation on a conserved lysine residue. Genetic, bioinformatics, biochemical and biophysical data show that attraction between this α1-conjugated ubiquitin and the HECT ubiquitin-binding patch pulls the α1-helix out of the interface, thereby promoting trimerization. Strikingly, trimerization renders the ligase inactive. Arginine substitution of the ubiquitylated lysine impairs this inactivation mechanism and results in unrestrained FGFR1 ubiquitylation in cells. Similarly, electrophysiological data and TIRF microscopy show that NEDD4 unrestrained mutant constitutively downregulates the IKS channel, thus confirming the functional importance of E3-ligase autoinhibition.
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Affiliation(s)
- Ilan Attali
- Department of Biochemistry and Molecular Biology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - William Sam Tobelaim
- Department of Physiology & Pharmacology, Sackler Tel Aviv University, Tel Aviv, Israel
| | - Avinash Persaud
- Cell Biology Program, The Hospital for Sick Children and Biochemistry Department, University of Toronto, Toronto, ON, Canada
| | - Khatereh Motamedchaboki
- Tumor Initiation & Maintenance Program and NCI Cancer Centre Proteomics Facility, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, CA, USA
| | - Kobi J Simpson-Lavy
- Department of Molecular Microbiology and Biotechnology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Bayan Mashahreh
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University-Hadassah Medical School, Jerusalem, Israel
| | - Olga Levin-Kravets
- Department of Biochemistry and Molecular Biology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Tal Keren-Kaplan
- Department of Biochemistry and Molecular Biology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Inbar Pilzer
- Department of Biochemistry and Molecular Biology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Martin Kupiec
- Department of Molecular Microbiology and Biotechnology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Reuven Wiener
- Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University-Hadassah Medical School, Jerusalem, Israel
| | - Dieter A Wolf
- Tumor Initiation & Maintenance Program and NCI Cancer Centre Proteomics Facility, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, CA, USA.,School of Pharmaceutical Sciences, Xiamen University, Xiamen, China
| | - Daniela Rotin
- Cell Biology Program, The Hospital for Sick Children and Biochemistry Department, University of Toronto, Toronto, ON, Canada
| | - Gali Prag
- Department of Biochemistry and Molecular Biology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel .,Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
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Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism. Nat Commun 2016; 7:12960. [PMID: 27698474 PMCID: PMC5059453 DOI: 10.1038/ncomms12960] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2015] [Accepted: 08/19/2016] [Indexed: 01/01/2023] Open
Abstract
Ubiquitin receptors decode ubiquitin signals into many cellular responses. Ubiquitin receptors also undergo coupled monoubiquitylation, and rapid deubiquitylation has hampered the characterization of the ubiquitylated state. Using bacteria that express a ubiquitylation apparatus, we purified and determined the crystal structure of the proteasomal ubiquitin-receptor Rpn10 in its ubiquitylated state. The structure shows a novel ubiquitin-binding patch that directs K84 ubiquitylation. Superimposition of ubiquitylated-Rpn10 onto electron-microscopy models of proteasomes indicates that the Rpn10-conjugated ubiquitin clashes with Rpn9, suggesting that ubiquitylation might be involved in releasing Rpn10 from the proteasome. Indeed, ubiquitylation on immobilized proteasomes dissociates the modified Rpn10 from the complex, while unmodified Rpn10 mainly remains associated. In vivo experiments indicate that contrary to wild type, Rpn10-K84R is stably associated with the proteasomal subunit Rpn9. Similarly Rpn10, but not ubiquitylated-Rpn10, binds Rpn9 in vitro. Thus we suggest that ubiquitylation functions to dissociate modified ubiquitin receptors from their targets, a function that promotes cyclic activity of ubiquitin receptors. Ubiquitin (Ub) receptors are responsible for the recognition of ubiquitylated proteins. Here the authors describe the crystal structure of the ubiquitylated form of the Ub-receptor Rpn10, which suggest that ubiquitylation of Rpn10 promotes its dissociation from the proteasome.
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A bacterial genetic selection system for ubiquitylation cascade discovery. Nat Methods 2016; 13:945-952. [DOI: 10.1038/nmeth.4003] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2016] [Accepted: 08/01/2016] [Indexed: 12/11/2022]
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Yang K, Li G, Gong P, Gui W, Yuan L, Zhuang Z. Chemical Protein Ubiquitylation with Preservation of the Native Cysteine Residues. Chembiochem 2016; 17:995-8. [PMID: 27113245 DOI: 10.1002/cbic.201600042] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2016] [Indexed: 12/22/2022]
Abstract
We report a cysteine-based ligation strategy for generating a monoubiquitylated protein while preserving the native cysteine residues on the acceptor protein. In monoubiquitylation of proliferating cell nuclear antigen (PCNA) this method circumvents the need to mutate the native cysteine residues on PCNA. The chemically ubiquitylated PCNA contains a noncleavable linkage of the same length as the native isopeptide linkage. It also retains the normal function of the native Ub-PCNA in stimulating the ATPase activity of replication factor C (RFC) and lesion bypass synthesis by Polη. This method may be adapted for chemical ubiquitylation of other proteins and for site-specific modification of a target protein at a specific site through sulfhydryl chemistry.
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Affiliation(s)
- Kun Yang
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA
| | - Guorui Li
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA
| | - Ping Gong
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA
| | - Weijun Gui
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA
| | - Libo Yuan
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA
| | - Zhihao Zhuang
- Department of Chemistry and Biochemistry, University of Delaware, 214A Drake Hall, Newark, DE, 19716, USA.
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The challenge of producing ubiquitinated proteins for structural studies. Cells 2014; 3:639-56. [PMID: 24926903 PMCID: PMC4092866 DOI: 10.3390/cells3020639] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2014] [Revised: 05/27/2014] [Accepted: 05/28/2014] [Indexed: 12/14/2022] Open
Abstract
Protein ubiquitination is an important post-translational modification involved in several essential signalling pathways. It has different effects on the target protein substrate, i.e., it can trigger the degradation of the protein in the proteasome, change the interactions of the modified protein with its partners, or affect its localization and activity. In order to understand the molecular mechanisms underlying the consequences of protein ubiquitination, scientists have to face the challenging task of producing ubiquitinated proteins for structural characterization with X-ray crystallography and/or nuclear magnetic resonance (NMR) spectroscopy. These techniques require milligrams of homogeneous samples of high purity. The strategies proposed so far for the production of ubiquitinated proteins can be divided into two groups, i.e., chemical (or non-enzymatic) and enzymatic methodologies. In this review, we summarize the still very sparse examples available in the literature that describe successful production of ubiquitinated proteins amenable for biochemical and structural studies, and discuss advantages and disadvantages of the techniques proposed. We also give a perspective of the direction in which the field might evolve.
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