Traced on the Timeline: Discovery of Acetylcholine and the Components of the Human Cholinergic System in a Primitive Unicellular Eukaryote Acanthamoeba spp.

Acetylcholine (ACh) is the neurotransmitter of cholinergic signal transduction that affects the target cells via muscarinic (mAChR) and nicotinic (nAChR) cholinergic receptors embedded in the cell membrane. Of the cholinergic receptors that bind to ACh, the mAChRs execute several cognitive and metabolic functions in the human central nervous system (CNS). Very little is known about the origins and autocrine/paracrine roles of the ACh in primitive life forms. With the recent report of the evidence of an ACh binding mAChR1 like receptor in Acanthamoeba spp., it was tempting to investigate the origin and functional roles of cholinergic G-Protein coupled receptors (GPCRs) in the biology of eukaryotes. We inferred the presence of ACh, its synthetic, degradation system, and a signal transduction pathway in an approximately ～2.0 billion year old primitive eukaryotic cell Acanthamoeba castellanii. Bioinformatics analysis, ligand binding prediction, and docking methods were used to establish the origins of enzymes involved in the synthesis and degradation of ACh. Notably, we provide evidence of the presence of ACh in A. castellanii by colorimetric analysis, which to date is the only report of its presence in this primitive unicellular eukaryote. We show the evidence for the presence of homology of evolutionary conserved key enzymes of the cholinergic system like choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in A. castellanii spp., which were found to be near identical to their human counterparts. Tracing the origin, functions of ACh, and primeval mAChRs in primitive eukaryotic cells has the potential of uncovering covert cholinergic pathways that can be extended to humans in order to understand the states of cholinergic deficiency in neurodegenerative diseases (ND)