| Choline acetyltransferase (ChAT; EC 2.3.1.6) is the biosynthetic enzyme for the neurotransmitter acetylcholine and serves as the most specific biochemical marker of cholinergic neurons in the central and peripheral nervous systems, where it is synthesized in the perikaryon and transported to nerve terminals in both soluble and non‑ionic membrane‑associated forms to support continuous transmitter production at synapses. ChAT is a single‑chain globular protein that belongs to the choline/carnitine acyltransferase family and is organized into two domains forming an interfacial active‑site tunnel; structural analysis combined with mechanistic studies shows that conserved residues such as His334, Tyr95, Pro108 and Ser550 coordinate catalysis by acting as a general base, stabilizing the unprotonated histidine, and supporting oxyanion stabilization during tetrahedral intermediate formation, enabling the reversible transfer of an acetyl group from acetyl‑CoA to choline with high specificity. The ChAT gene has a distinctive genomic architecture in which its first intron encodes the vesicular acetylcholine transporter (VAChT), and multiple mRNA species (R‑, N‑ and M‑type) produced by alternative promoters and splicing yield different transcript forms that in humans can generate both large and small ChAT proteins, while rodents produce a single protein form; these arrangements allow coordinated transcriptional regulation of ChAT and VAChT so that cytosolic acetylcholine synthesis and vesicular loading remain tightly matched in cholinergic neurons. Immunohistochemical and in situ hybridization mapping identifies ChAT‑positive neurons in basal forebrain regions such as the medial septal nucleus, diagonal band of Broca and nucleus basalis of Meynert, in striatal structures including caudate nucleus, putamen and nucleus accumbens, in brainstem nuclei such as the pedunculopontine and laterodorsal tegmental nuclei and medial habenula, and in spinal anterior horn motor neurons, where their widely projecting axons create an extensive cholinergic network that participates in learning, memory, arousal, sleep and movement. At the mechanistic level, ChAT catalysis proceeds via deprotonation of choline by His334, nucleophilic attack on the acetyl‑CoA carbonyl to form a tetrahedral oxyanion intermediate stabilized by Ser550, collapse of this intermediate with release of CoA and formation of acetylcholine, and choline acetyltransferase operates as a reversible acyltransferase whose kinetic properties, including substrate affinities for choline and acetyl‑CoA, determine acetylcholine synthesis rates under physiological and pathological conditions. Mutations in CHAT that alter active‑site residues, substrate binding pockets or domain interfaces reduce catalytic efficiency or destabilize the enzyme and cause congenital myasthenic syndromes with episodic apnea and fatigable weakness, highlighting that intact ChAT structure and function are essential for neuromuscular transmission and that mutation distribution across the 3D structure explains disease severity and response to cholinergic therapies. In neurodegenerative disease, ChAT activity is significantly reduced in multiple cortical and hippocampal regions in Alzheimer’s disease, especially in early‑onset cases, and enzyme activity correlates with dementia severity and neurofibrillary tangle burden rather than plaque counts, supporting the cholinergic hypothesis that basal forebrain cholinergic dysfunction contributes to cognitive decline and providing a quantitative marker of cholinergic pathway integrity in human tissue and animal models. ChAT abnormalities have also been reported in amyotrophic lateral sclerosis, schizophrenia and sudden infant death syndrome, where disturbed cholinergic innervation is thought to influence clinical manifestations, and recent work on primate‑specific large ChAT isoforms indicates that increased expression of an 82‑kDa variant can attenuate progression of amyloid pathology and cholinergic dysfunction in Alzheimer‑like mouse models, suggesting that ChAT isoform composition and regulation may have therapeutic relevance. |
