Friday, March 1, 2019

Exploration of DREADD


Written by: Destinee Chaidez

During Dr. Jelena Radulovic’s talk about processing stress related memories in the hippocampal cortical circuits, she mentioned a recently developed chemogenetic technique being implemented throughout neurological studies known as DREADD. I had heard of optigentics before, but had no idea what this designer receptor process entailed. Due to this growing curiosity I decided to find an article explaining what this technique was and its utilizations.

Image result for DREADDDREADD stands for Designer Receptor Exclusively Activated by Designer Drug. Because of the lack of ability to manipulate GPCR that are so critical to the systematic workings of our CNS, DREADDs were invented. They are genetically engineered recombinant class of GPCR that do not react with endogenous ligands. This allows researchers to control the activation levels of these receptors.  This tool is used to remotely controlled cell signaling and behavior. Activation of a designer receptor is dependent on the inert ligand, which is almost always clozapine-N-oxide (CNO). This activation can either silence or enhance neural firing, as explained by Dr.Radulovic. These DREADDs are transmitted into tissues of desire using viral vectors. I found out that this simulation happens in vivo, meaning freely moving animal preparations!  I was still a confused as to how this “designer compound” targets and activates. It wasn’t until I read the article “Chemogenetics revealed: DREADD occupancy and activation via converted clozapine”, that things started to make sense. In this article scientist address the mechanisms to which DREADD essentially manifest by and its misconceptions.

Juan L. GomezJordi BonaventuraWojciech Lesniak et al. investigated the degree to which   CNO enters the brain to occupy DREADD , as it was never confirmed in past studies. They also tested the hypothesis that CNO did not cross blood brain barrier. In this experiment the cortex of rats was injected into the right or left hemisphere PET images were than taken and coregistered with simultaneous MRIs. Later they injected DREADD into the striatum of rats and then harvested organs and brains to examine uptake of radioligand.  Results indicated that CNO does not enter the brain after systemic drug injections and shows low affinity for DREADDs. In vivo CNO rapidly converts to Clozapine and shows high DREADD affinity and potency. Upon systemic CNO injections, converted clozapine readily enters the brain and occupies central nervous system-expressed DREADDs, whereas systemic subthreshold clozapine injections induce preferential DREADD-mediated behaviors (Gomez et al. , 2017). Essentially the findings of this study conclude that DREADDs expressed in the brain are not activated by the designer compound CNO. This honestly made more sense compared to my initial perception of the tool. I learned that DREADDs activated by the CNO metabolite clozapine, a drug with a variation of endogenous targets.

This is important because it may have significant implications for the interpretation of results obtained from this promising technology. I do believe that these advancements are one step closer to translatable therapeutic gene transfers for humans. However, I would like to note that a limiting factor of the use of CNO in clinical practice of DREADD can cause controversy due to the known conversion to clozapine in vivo. Also its important to note that dependence of DREADD technology on the same inert ligand CNO limits its effectiveness for variety in chemo genetic control of neuronal activity.



                                   References:

Video about DREADD

Article
Gomez, J., Bonaventura, J., Lesniak, W., Mathews, W., Sysa-Shah, P., Rodriguez, L., . . . Michaelides, M. (2017). Chemogenetics revealed: DREADD occupancy and activation via converted clozapine. Science (New York, N.Y.),357(6350), 503-507.

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