DREADDs for clinical translation

Drugs are the mainstay of treatment for a very wide range of neurological and psychiatric disorders, but their effectiveness is often low. They act relatively indiscriminately on nerve cells involved in different aspects of brain and spinal cord function. Although much progress has been made in identifying which nerve cells are over- or underactive in many disorders, it is often not possible to correct these imbalances with medication. Gene therapy potentially overcomes this barrier: the excitability of different populations of nerve cells can be altered by inserting DNA into them, causing them to become intrinsically more or less excitable depending on the nature of the protein encoded by the DNA. The therapy can be targeted to specific nerve cells in discrete regions of the brain or spinal cord by the use of viruses that have been made unable to replicate. Although promising, gene therapy is hampered by its irreversible nature: once the DNA has been inserted, the genetic modification cannot be undone. However, a major breakthrough was the invention of 'chemogenetics', whereby the DNA encodes a receptor for a drug. The presence of the receptor has no effect on the nerve cell's properties until the drug that activates the receptor is administered. In this way the therapeutic effect can be adjusted or used on-demand. We have pioneered the application of this technology to an especially severe form of epilepsy that cannot be treated satisfactorily with conventional medication. Our proposal is aimed at bringing chemogenetic treatment closer to the clinic, specifically by ensuring that it can be used with a drug that has an excellent safety profile. We have designed a receptor that can be activated by diphenhydramine, a very safe drug that is used in over-the-counter remedies for motion sickness and allergies. We call the receptor an inhibitory GRANPA (G-protein coupled Receptor Activated by a Non-Prescription Agent). We will verify that it can be used to suppress seizures on demand, and use the same principles to design an excitatory GRANPA to achieve an adjustable increase in nerve cell activity. Finally, because the effects of some receptors wear off with prolonged drug activation, we will further modify the inhibitory and excitatory GRANPAs to minimise their tendency to lose effect. Although we will validate our tools in models of epilepsy, which have the advantage of rapid read-out and proximity to the clinic, they potentially have applications in a wide range of other neurological and psychiatric diseases that account for an enormous burden to society and to affected individuals.

Abnormalities of neuronal circuit excitability account for a very large disease burden. Although much progress has been made in understanding the circuit basis of such disorders, treatment has lagged behind, mainly because of the very poor specificity of small molecules for defined neuronal circuits. Of the advanced therapies that have been proposed, gene therapy is closest to the clinic, because it allows the excitability of specific neuronal circuits to be modulated. Chemogenetics has the added advantage of allowing such modulation to be adjusted or used on demand. Our proposal aims to lower barriers to clinical translation of treatment using muscarinic Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). We have extensively screened FDA/EMA-approved drugs that could be used as agonists of the inhibitory DREADD hM4D(Gi). Although we identified olanzapine as such a drug, its antagonism at H1, muscarinic dopaminergic and serotonergic receptors contributes to sedation, weight gain, and a lowering of seizure threshold in some people. However, by introducing additional point mutations to hM4D(Gi) we have made it highly sensitive to the over-the-counter antihistamine drug diphenhydramine, whilst preserving its coupling to intracellular signalling cascades, and without conferring constitutive activity. We refer to the modified receptor as a G-Protein-Coupled Receptor Activated by Non Prescription Agents (GRANPA). We will validate the use of the inhibitory GRANPA in a rodent model of epilepsy. In addition, we will introduce the same mutations to the excitatory DREADD derived from the human M3 muscarinic receptor (hM3Dq). This will broaden the application of GRANPAs to the treatment of neurological diseases where an increase in neuronal excitability is desired. Finally, we will design inhibitory and excitatory GRANPAs where intracellular sequences have been mutated to reduce internalization and desensitization, to optimise their long-term use.