Chemogenetics

The term chemogenetics has been used to describe the processes by which macromolecules can be engineered to interact with previously unrecognized small molecules. Chemogenetics as a term was originally coined to describe the observed effects of mutations on chalcone isomerase activity on substrate specificities in the flowers of Dianthus caryophyllus.[1] This Method is very similar to Optogenetics however, it uses chemically engineered molecules and ligands instead of light and light-sensitive channels known as Opsins.

In recent research projects, chemogenetics has been widely used to understand the relationship between brain activity and behavior. Prior to chemogenetics, researchers used methods such as transcranial magnetic stimulation (TMS) and deep brain stimulation(DBS) to study the relationship between neuronal activity and behavior.[2]

Differences and similarities between optogenetics and chemogenetics

Optogenetics and Chemogenetics are the more recent and popular methods used to study this relationship. Both of these methods target specific brain circuits and cell population to influence cell activity. However, they use different procedures to accomplish this task. Optogenetics uses light-sensitive channels and pumps that are virally introduced into neurons. Cells' activity, having these channels, can then be manipulated by light. Chemogenetics, on the other hand, uses chemically engineered receptors and exogenous molecules specific for those receptors, to affect the activity of those cells. The engineered macromolecules used to design these receptors include nucleic acid hybrids,[3] kinases[4] , variety of metabolic enzymes,[5][6] and G-protein coupled receptors (GPCRs) such as DREADDs.[7][8][9]

DREADDs are the most common GPCRs used in chemogenetics. These receptors solely get activated by the drug of interest (inert molecule) and influence physiological and neural processes that take place within and outside of the central nervous system.

Chemogenetics has recently been favored over Optogenetics, and it avoids some of the challenges of Optogenetics. Chemogenetics does not require the expensive light equipment, and therefore, is more accessible. The resolution in Optogenetic declines due to light scattering and illuminance declined levels as the distance between the subject and the light source increases.These factors, therefore, don’t allow for all cells to be affected by light and lead to a lower spatial resolution. Chemogenetics, however, does not require light usage and therefore can achieve a higher spatial resolution.[10]

Uses

GPCRs' usage and chemogenetics are nowadays the targets for many of the pharmaceutical companies to cure and alleviate symptoms of diseases that involve all tissues of the body.[11] More specifically, DREADDs have been used to explore treatment options for various neurodegenerative and psychological conditions such as Parkinson’s disease, depression, anxiety, and addiction. These aforementioned conditions involve processes that occur within and outside of the nervous system involving neurotransmitters such as GABA and Glutamate.[12] Chemogenetics has therefore been used in pharmacology to adjust the levels of such neurotransmitters in specific neuron while minimizing the side effects of treatment. To treat and relieve the symptoms of any disease using the DREADDs, these receptors are delivered to the area of interest via viral transduction.

Recently some studies have considered using a new method called retro DREADDs. This method allows specific neuronal pathways to be studied under cellular resolution. Unlike classic DREADDs, this method is usually used in wild type animals, and these receptors are given to the targeted cells via injection of two viral vectors.[2]

Animal Models

DREADDS have been used in many animal models (e.g., mice and other non-primate animals) to target and influence the activity of various cells . Chemogenetics used in animals assists with demonstrating human disease models such as Parkinson's disease. Having this information allows scientists understand whether viral expression of DREADD proteins, both in-vivo enhancers and inhibitors of neuronal function can be used to bidirectionally affect the behaviors and the activity of the involved neurons. Recent studies have shown that DREADDs were successfully used to treat the motor deficits of rats modeling Parkinson's disease.[13] Other studies have had successes linking the usage of DREADDs and influencing drug seeking and drug sensitization behavior.[12]

The progression of Chemogenetics from rodents to non-human primates has been slow due to increased demand in time and expense surrounding these projects.However, some recent studies in 2016 have been able to demonstrate successes showing that silencing the activity of neurons in the Orbitofrontal cortex along with the removal of Rhinal cortex, restricted the reward task performance in macaques.[14]

Limitation of Use and Future Direction

Chemogenetics and usage of DREADDs have allowed researchers to advance in biomedical research areas including many neurodegenerative and psychiatric conditions. Chemogenetics have been used in these fields to induce specific and reversible brain lesions and therefore, study specific activities of neuron population. Although Chemogenetics offers specificity and high spatial resolution, it still faces some challenges when used in investigating neuropsychiatric disorders. Neuropsychiatric disorders usually have a complex nature where lesions in the brain have not been identified as the main cause. Chemogenetics has been used to reverse some of the deficits revolving such conditions however, it hasn’t been able to identify the main cause of neuropsychiatric diseases and cure these conditions completely due to complex nature of these conditions.

References

  1. ↑ Forkmann, G; Dangelmayr, B (1980). "Genetic control of chalcone isomerase activity in flowers of Dianthus caryophyllus". Biochem Genet. 18: 519–27. doi:10.1007/bf00484399.
  2. 1 2 Dobrzanski, Grzegorz; Kossut, Małgorzata (2017-04-01). "Application of the DREADD technique in biomedical brain research". Pharmacological Reports. 69 (2): 213–221. doi:10.1016/j.pharep.2016.10.015.
  3. ↑ Strobel, SA; Ortoleva-Donnelly, L; Ryder, SP; Cate, JH; Moncoeur, E (1998). "Complementary sets of noncanonical base pairs mediate RNA helix packing in the group I intron active site". Nat Struct Biol. 5: 60–66. doi:10.1038/nsb0198-60.
  4. ↑ Bishop, AC; Shah, K; Liu, Y; Witucki, L; Kung, C; Shokat, KM (1998). "Design of allele-specific inhibitors to probe protein kinase signaling". Curr Biol. 8: 257–66. doi:10.1016/s0960-9822(98)70198-8.
  5. ↑ Collot, J; Gradinaru, J; Humbert, N; Skander, M; Zocchi, A; Ward, TR (2003). "Artificial metalloenzymes for enantioselective catalysis based on biotin-avidin". J Am Chem Soc. 125: 9030–1. doi:10.1021/ja035545i.
  6. ↑ Haring, D; Distefano, MD (2001). "Enzymes by design: chemogenetic assembly of transamination active sites containing lysine residues for covalent catalysis". Bioconjug Chem. 12: 385–90. doi:10.1021/bc000117c.
  7. ↑ Strader, CD; Gaffney, T; Sugg, EE; Candelore, MR; Keys, R; et al. (1991). "Allele-specific activation of genetically engineered receptors". J Biol Chem. 266: 5–8.
  8. ↑ Coward, P; Wada, HG; Falk, MS; Chan, SD; Meng, F; et al. (1998). "Controlling signaling with a specifically designed Gi-coupled receptor". Proc Natl Acad Sci U S A. 95: 352–7. PMC 18222 Freely accessible. PMID 9419379. doi:10.1073/pnas.95.1.352.
  9. ↑ Westkaemper, R; Glennon, R; Hyde, E; Choudhary, M; Khan, N; Roth, B (1999). "Engineering in a region of bulk tolerance into the 5-HT2A receptor". Eur J Med Chem. 34: 441–47. doi:10.1016/s0223-5234(99)80094-4.
  10. ↑ Montgomery, Kate L.; Yeh, Alexander J.; Ho, John S.; Tsao, Vivien; Mohan Iyer, Shrivats; Grosenick, Logan; Ferenczi, Emily A.; Tanabe, Yuji; Deisseroth, Karl (2015-10-01). "Wirelessly powered, fully internal optogenetics for brain, spinal and peripheral circuits in mice". Nature Methods. 12 (10): 969–974. ISSN 1548-7091. doi:10.1038/nmeth.3536.
  11. ↑ Smith, RS; Hu, R; DeSouza, A; Eberly, CL; Krahe, K; Chan, W; Araneda, RC (29 July 2015). "Differential Muscarinic Modulation in the Olfactory Bulb.". The Journal of neuroscience : the official journal of the Society for Neuroscience. 35 (30): 10773–85. PMC 4518052 Freely accessible. PMID 26224860. doi:10.1523/JNEUROSCI.0099-15.2015. Retrieved 6 August 2015.
  12. 1 2 Volkow, Nora D.; Koob, George F.; McLellan, A. Thomas (2016-01-27). "Neurobiologic Advances from the Brain Disease Model of Addiction". New England Journal of Medicine. 374 (4): 363–371. doi:10.1056/nejmra1511480.
  13. ↑ Pienaar, Ilse S.; Gartside, Sarah E.; Sharma, Puneet; Paola, Vincenzo De; Gretenkord, Sabine; Withers, Dominic; Elson, Joanna L.; Dexter, David T. (2015-09-23). "Pharmacogenetic stimulation of cholinergic pedunculopontine neurons reverses motor deficits in a rat model of Parkinson’s disease". Molecular Neurodegeneration. 10 (1): 47. ISSN 1750-1326. PMC 4580350 Freely accessible. PMID 26394842. doi:10.1186/s13024-015-0044-5.
  14. ↑ Galvan, Adriana; Caiola, Michael J.; Albaugh, Daniel L. (2017-02-25). "Advances in optogenetic and chemogenetic methods to study brain circuits in non-human primates". Journal of Neural Transmission: 1–17. ISSN 0300-9564. doi:10.1007/s00702-017-1697-8.

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