CHT1 (High-Affinity Choline Transporter)
The neurotransmitter acetylcholine (ACh) interacts with the nicotinic and muscarinic ACh receptors and is involved in a variety of physiological, behavioral, and cognitive functions in the central and peripheral nervous system. ACh is synthesized from choline and acetyl coenzyme A by choline acetyltransferase in the cholinergic presynaptic terminals. Although it has long been recognized from classic studies on ACh metabolism that exogenous choline is critical for ACh synthesis, it was not until the 1970s that the existence of high-affinity choline transporters in cholinergic neurons was postulated based on studies using isolated nerve ending particles (synaptosomes). Extracellular choline is actively transported into the presynaptic terminals by the Na+-dependent, high-affinity choline transporter in the plasma membrane and subsequently used for ACh synthesis. The high-affinity choline uptake is the rate-limiting step in ACh synthesis. Choline acetyltransferase is present in the cholinergic terminals in kinetic excess and therefore is not thought to be the rate-limiting step in ACh synthesis. The high-affinity choline transporter was first cloned in 2000 and designated as CHT1 (Okuda et al. 2000).
Structure and Functional Properties of CHT1
Regulation of CHT1 Expression/Trafficking
Immunohistochemical studies have established that CHT1 is specifically expressed in cholinergic neurons (Misawa et al. 2001). CHT1 is highly enriched in their presynaptic terminals as well as cell bodies. In the central nervous system, CHT1 is specifically expressed in regions known to contain cholinergic neurons, such as the basal forebrain, striatum, brainstem, and spinal cord. Since the cellular distribution of CHT1 correlates well with that of choline acetyltransferase or the vesicular ACh transporter, CHT1 can be considered as a cholinergic neuronal marker. In the nematode Caenorhabditis elegans, the expression of cholinergic marker genes is co-regulated by the COE (Collier, Olf, EBF)-type transcription factor UNC-3 (Kratsios et al. 2012). Several members of the COE transcription factor family are expressed in mammalian cholinergic neurons, and a conserved COE motif is found in the 5′ upstream regulatory region of the CHT1 gene. It is therefore highly likely that this transcriptional regulation is critical for cholinergic neuron-specific expression of CHT1. The expression of CHT1 gene is regulated at the transcriptional level by several factors, including nerve growth factor (NGF) and bone morphogenetic proteins (BMPs) (Berse et al. 2005; Madziar et al. 2008). In superior cervical ganglion neurons, it has been reported that an activity-dependent retrograde signal regulates the CHT1 expression (Krishnaswamy and Cooper 2009). This retrograde signal has not been identified and it is also unknown whether this regulation can be generalized to other cholinergic neurons.
Studies on CHT1 localization at the subcellular level have established that CHT1, despite functioning at the plasma membrane, is predominantly localized intracellularly at synaptic vesicles (Ferguson et al. 2003). In the rat neuromuscular junctions, more than 90% of CHT1 proteins are observed on synaptic vesicles (Nakata et al. 2004). CHT1 is constitutively internalized primarily via the clathrin-mediated endocytosis pathway (Ribeiro et al. 2003). A dileucine-like motif (Leu-531 and Val-532) located within the C-terminal cytoplasmic region of CHT1 is important for its constitutive endocytosis as demonstrated by heterologous expression systems (Ribeiro et al. 2005). Extracellular choline stimulates a dynamin-dependent internalization of CHT1, and the specific inhibitor HC-3 blocks it (Okuda et al. 2011). It has long been known that high-affinity choline uptake is acutely regulated by neuronal activity in the cholinergic nerve terminals (Simon and Kuhar 1975). In brain synaptosomes, depolarization induces a significant increase in the maximal uptake rate of choline or in the maximal binding capacity of HC-3, suggesting the increase in cell surface expression of CHT1. It is now understood that these observations are due to the fact that intracellular CHT1 residing in synaptic vesicles is translocated to the plasma membrane by exocytosis. It is considered that this neuronal activity-dependent trafficking of CHT1 is critical for sustaining ACh synthesis in cholinergic neurons.
Physiological Roles of CHT1
It has long been known that administration of HC-3 to animals causes lethal respiratory failure (Schueler 1955). CHT1 knockout mice were generated and characterized by Blakely and coworkers (Ferguson et al. 2004). The mice fail to survive the first hours of life as a result of hypoxia from a respiratory failure, and their phenotype suggests an impaired ability to sustain ACh synthesis. These results suggest that CHT1 is an essential protein to sustain cholinergic neurotransmission by transporting extracellular choline. In contrast, CHT1 heterozygous knockout mice appear normal and maintain normal levels of the high-affinity choline uptake, even though the expression levels of CHT1 protein are reduced by approximately half. However, they display deficits in endurance performance during a treadmill test (Bazalakova et al. 2007) and in attention-demanding cognitive tasks (Parikh et al. 2013). These deficits may be due to the limited intracellular pool of CHT1 available for neuronal activity-dependent vesicular trafficking that is required to sustain ACh synthesis in cholinergic neurons.
To date, there have been two reports regarding the relationship of CHT1 with human diseases, such as a distal hereditary motor neuropathy type VII and a congenital myasthenic syndrome with episodic apnea. In the former case, a dominantly acting mutation identified was a single-base deletion (c.1497delG), resulting in a near-complete deletion of the cytoplasmic C-terminal region of CHT1 (Barwick et al. 2012), and in the latter case, 11 recessive missense mutations were identified (Bauche et al. 2016). The high-affinity choline uptake activity in patients with these diseases was predicted to be diminished by mutations that impaired CHT1 function and/or trafficking.
There is a functionally relevant, nonsynonymous single nucleotide polymorphism in the coding region of the human CHT1 gene (c.265A>G, rs1013940) (Okuda et al. 2002). This polymorphism results in an isoleucine to valine substitution (p.Ile89Val, I89V) within transmembrane domain 3 of the protein. The I89V variant transporter shows a 40–50% decrease in the choline uptake rate with no change in the affinity for choline compared with the wild-type transporter, when expressed in cultured cells. This variant is considered to have a deficit in substrate translocation or reorientation of the protein and consequently has a lower transport rate. This polymorphism is reported to be associated with pediatric attention-deficit hyperactivity disorder (ADHD) (English et al. 2009) and the severity in unipolar major depressive disorder (Hahn et al. 2008).
Cholinergic neurons are endowed with the high-affinity choline transporter for ACh synthesis at the presynaptic terminals. CHT1 mediates the Na+-dependent, high-affinity choline uptake, which is the rate-limiting step in ACh synthesis. It is specifically expressed in cholinergic neurons. An important regulatory mechanism of CHT1 is neuronal activity-dependent trafficking, which is critical for sustaining ACh synthesis. A lethal impairment of ACh synthesis in CHT1 knockout mice underscores the physiological significance of CHT1 function in cholinergic neurotransmission.
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