CIN
Brain miRNA’s as markers of drug induced changes in neural integrity
Testing mechanistic and placebo-controlled studies in which we will administer a psychedelic drug (i.e. psilocybin) that is known to affect neural integrity
Introduction
MicroRNAs (miRNAs) are small RNA molecules of about 22 nucleotides in length. Since the first miRNA was discovered in 1993 in Caenorhabditis elegans, 2588 human miRNAs have been identified. The primary function of miRNAs is RNA silencing and post-transcriptional regulation of gene expression. Consequently, miRNAs have been found to exhibit unique expression patterns as an early response to internal and external conditions. Recently, miRNAs have also been detected in the extracellular fraction of the blood. These circulating miRNAs (cmiRNAs) were found to be stable even under conditions as harsh as boiling, extreme pH, long-time storage at room temperature, and multiple freeze-thaw cycles.
The stability of cmiRNAs in the circulation, the accessibility through minimally invasive “liquid biopsies” and the advantage that cmiRNAs can be detected in a quantitative manner by relatively simple methods such as a real-time PCR, makes them a very promising new class of biomarkers for interrogating organ and brain pathologies. In fact, circulating miRNAs are currently explored in search for reliable and highly specific biomarkers of drug-induced efficacy and/or injury. We recently have shown that a pattern of extracellular miRNAs, including several liver enriched cmiRNAs, was able to discriminate acetaminophen-induced liver injury from liver cirrhosis, acute hepatitis B and type 2 diabetes mellitus. Moreover, the pattern of extracellular miRNAs that was specific for acetaminophen induced liver-injury comprised of, next to liver-enriched, also brain-enriched miRNAs, which were known to play essential roles in brain function. This finding was in correspondence to known CNS toxicity induced by acetaminophen in rodents (8), and consequently prompted us to hypothesize that acetaminophen might also induce neurotoxicity.
Hypothesis
This finding of organ-enriched miRNAs in the circulation upon drug-induced organ damage led to the hypothesis that also profiles of brain-borne miRNAs assessed by RNA sequencing are detectable in the circulation and would allow interrogating the brain condition by minimally-invasive liquid biopsies, including estimation of the biological mechanism and potential therapeutic effect of specific psychoactive drugs. The brain expresses more distinct and larger numbers of miRNAs than any other tissue in vertebrates. Neural miRNAs are involved at various stages of synaptic development, including dendritogenesis, synapse formation and synapse maturation. miRNAs have emerged as master regulators of gene expression in the nervous system where they contribute to (maladaptive) neuronal network plasticity. We therefore hypothesize that drug induced changes in neural integrity (i.e. either neurotoxicity or neuroplasticity) can be predicted from drug induced changes in miRNA.
Research
We will test the hypothesis in mechanistic, placebo-controlled studies in which we will administer a psychedelic drug (i.e. psilocybin) that is known to affect neural integrity. Recent clinical trials have demonstrated that single dose administrations of psilocybin produce an instant reduction of depressive symptoms in depressed patients who are resistant to traditional antidepressant treatment. Moreover, the instant relief of depressive symptoms pertains for days or weeks suggesting that these drugs induce an enduring reset of the brain (10). Molecular and cellular studies in rodent models demonstrate that psychedelic drugs such as psilocybin rapidly increase synaptogenesis, including increased density and function of spine synapses, in the prefrontal cortex (PFC) and hippocampus and thereby cause a functional reconnection of neurons that underlies the rapid behavioral responses. However, exposure to high doses of psilocybin or related psychedelics have also been shown to produce neurotoxicity in drug abusers. Single administrations of low and high doses of psilocybin are thus expected to cause neurogenesis and neurotoxicity respectively and are perfectly suited to test our hypothesis that drug induced changes in neural integrity can be demonstrated through miRNA profiling.
In addition, if changes in neural integrity caused by psychedelics cause corresponding changes in miRNA, then it also follows that the latter can be prevented or reversed when blocking the mechanism of action of psychedelics. Psilocybin exerts is major action through agonism of 5HT2A and 5HT1A receptors. The 5HT2A receptor is primarily a cortical receptor. In humans, expression of the 5-HT2A receptor is considerably higher in the cortex than in subcortical structures such as the thalamus, basal ganglia, and hippocampus with minimal/negligible expression in the cerebellum and brainstem. The 5-HT1A receptor is highly expressed on serotonergic neurons in the raphe nuclei where it functions as a presynaptic autoreceptor – exerting a strong homeostatic control over 5-HT neuron firing rates and thus, 5-HT efflux in the forebrain. The majority of 5-HT1A receptors are expressed postsynaptically in many brain regions, particularly the limbic system and cortex. Given that 5HT1A and 5HT2A receptors are exclusively located in the brain, we expect that blockade of these receptors will automatically enable us to identify brain miRNAs that are involved in the psychedelic response. The profile of brain-related cmiRNAs that respond to psilocybin challenges will alter during combined administration of psilocybin and a 5HT2A antagonist or 5HT1 agonist. Profiles of brain-derived cmiRNA in serum will offer the opportunity to evaluate early responses of the brain to administration of psychoactive drugs, which can be used as easy and sensitive biomarkers of neurotoxicity or neuroplasticity.