Table 2.
Non-mammalian Reporter Genes and Their Corresponding Imaging Tracers
| Reporter |
Imaging Agent |
Refs. | ||||
|---|---|---|---|---|---|---|
| Class | Name | Properties | aaa | Modality | Properties | |
| Enzyme | β-galactosidase | glycoside hydrolase enzyme; product of LacZ gene and isolated from E. coli | 1,021 | optical CL: near-infrared dioxetane luminophores (emission λ = 690 nm); MRI: EgadMeb; PET: 2-(4-[123I]iodophenyl)ethyl-1-thio-β-d-galactopyranoside, 3-(2′-[18F]fluoroethoxy)-2-nitrophenyl-β-d-galactopyranoside, 3-[11C]methoxy-2-nitrophenyl-β-d-galactopyranoside; [18F]FPyGal; SPECT: 5-[125I]iodoindol-3-yl-β-d-galactopyranoside ([125I]IBDG); 4-chloro-3-bromoindole-galactose (X-gal) | cellular toxicity depending on the substrates; lack of sensitivity and high background; rapid renal clearance of [125I]IBDG impedes intratumoral availability if systemically administered | 62, 63, 64, 65, 66, 67, 68 |
| E. coli dihydrofolate reductase (eDHFR) | catalyzes NADPH-dependent reduction of folate; inhibited by highly specific small molecule trimethoprim | 159 | PET: [11C]trimethoprim, [18F]trimethoprim (TMP) | rapid renal clearance and hepatobiliary metabolism | 69,70 | |
| HSV1-tk and mutants | kinase causing cellular tracer | 376 | PET: [124I]FIAU, [18F]FEAU, [18F]FHBG | tracers do not cross the blood-brain barrier | 71, 72, 73, 74, 75 | |
| emerald luciferase (ELuc) and mutants | catalyzes oxygenation of d-luciferin to oxyluciferin; emits strongest luminescence among beetle luciferases; from click beetle (Pyrearinus termitilluminans) | 543 | optical BL: d-luciferin/ATP (emission λ = 534–626 nm; dependent on wild-type [WT]/mutant used) | lack of signal in the brain, as the substrate cannot cross the BB barrier; low thermostability and low light intensity | 76, 77, 78, 79 | |
| firefly luciferase (fLuc) and mutants | catalyzes the oxygenation of d-luciferin to oxyluciferin; derived from the North American firefly (Photinus pyralis) | 550 | optical BL: d-luciferin/ATP (emission λ = 550–615 nm; depending on WT/mutant used) | depending on the type used; high thermolability and exhibits a bathochromic shift at >30°C and pH levels <7.8 | 80,81,82 | |
| Gaussia luciferase (GLuc) and mutants | from Gaussia princeps; one of the smallest luciferases cloned so far; catalyzes the oxidative decarboxylation of coelenterazine to produce luminescence | 185 | optical BL: coelenterazine (emission λ = 480–513 nm; depending on WT/mutant used) | no clinical use; background auto-luminescence | 83, 84, 85, 86 | |
| green click beetle luciferase and mutants | derived from Pyrophorus plagiophthalamus | 542 | optical BL: luciferin (emission λ = 543) | no clinical use | 87 | |
| NanoLuc | derived from Oplophorus gracilorostris (deep sea shrimp) | 171 | optical BL: imidazopyrazinone substrate (furimazine) (emission λ = 456 nm) | signal is heavily attenuated in tissues | 88, 89, 90, 91 | |
| Renilla luciferase (RLuc) and mutants | derived from Renilla reniformis (sea pansy) | 311 | optical BL: coelenterazine (emission λ = 475–535 nm; depending on the variant) | WT RLuc suffers from low stability in serum and thermolability at >30°C | 81,92,93 | |
| Transporter | MS-1 magA | putative ion transport protein from magnetotatic bacteria (Magnetospirillum sp. strain AMB-1) | 434 | MRI: endogenous or exogenous Fe | delay of change in signal, which is dependent on Fe availability | 94, 95, 96 |
| sodium-taurocholate cotransporting polypeptide (NTCP) | 349 | MRI: indocyanine green (ICG) | 97 | |||
| Artificial protein | lysine-rich protein | frequency-selective contrast, based on transfer of radiofrequency labeling from the reporter’s amide protons to water protons | 200 | MRI: chemical exchange saturation transfer (CEST) MRI | 98,99 | |
| Fluorescent proteins | mNeptune | fluorescent protein chromophore; derived from Entacmaea quadricolor | 244 | optical FL: (emission λ = 650 nm) | no clinical use | 100 |
| mPlum | fluorescent protein chromophore; derived from DsRed of Discosoma (sea anemone) | 226 | optical FL: (emission λ = 649 nm) | no clinical use; low acid sensitivity | 101 | |
| mTagRFP | fluorescent protein chromophore; derived from Entacmaea quadricolor | 238 | optical FL: emission λ = 584 nm | no clinical use | 102 | |
| E2-Crimson | derived from DsRed-Express2 | 225 | optical FL: emission λ = 543 nm | no clinical use | 103,104 | |
| NIR fluorescent protein | iFP1.4 | requires exogenously added biliverdin as a co-factor; derived from Deinococcus radiodurans | 328 | optical FL: emission λ = 708 nm | no clinical use | 105,106 |
| iRFP 670 | endogenous biliverdin sufficient as a co-factor; derived from Rhodopseudomonas palustris (CGA009) | 312 | optical FL: emission λ = 670 nm | no clinical use | 105,107,108 | |
| iRFP 713 | endogenous biliverdin sufficient as a co-factor; derived from Rhodopseudomonas palustris | 317 | optical FL: emission λ = 713 nm | no clinical use | 105,107, 108, 109 | |
| iRFP 720 | endogenous biliverdin sufficient as a co-factor; derived from Rhodopseudomonas palustris | 317 | optical FL: emission λ = 720 nm | no clinical use | 110,111 | |
| Gas-filled protein complex | gas vesicle structural protein A/gas vesicle protein C | gas vesicles generate contrast; gas vesicles occupy more than 10% of the volume of transduced cells | GvpA: 71 | ultrasound: 2.7–4.7 MPa insonation | 112,113 | |
| GvpA; derived from Dolichospermum lemmermannii; GvpC; derived from Dolichospermum flosaquae | GvpC: 193 | |||||
| mammalian acoustic reporter gene (mARG) | gas vesicles generate contrast | 2,500 | ultrasound: 3.2 MPa insonation | 112 | ||
CL, chemiluminescence; BL, bioluminescence; FL, fluorescence imaging.
a
Amino acid chain length as an indication of reporter molecular weight (MW; not accounting for posttranslational modifications); wild-type reporter MWs are indicated.
b
EgadMe: 1-(2-(β-galactopyranosyloxy)propyl)-4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane)gadolinium(III).