Revised role of interstitial cells of Cajal in cholinergic neurotransmission in the gut

We read with interest the recent paper by Bhetwal and colleagues (Bhetwal et al. 2013) reporting that interstitial cells of Cajal (ICC)-deficient, W/Wv mutant mice exhibit enhanced cholinergic neuromuscular responses, contrary to previous reports (Ward et al. 2000; Sanders et al. 2010). The authors propose an intriguing model of “ICC as intermediaries in cholinergic neuromuscular transmission in the gut”, which could alternatively be interpreted as an expendable role for ICC in this process. This model contends that normally, intramuscular ICC (ICC-IM) transduce cholinergic neurotransmission, but in their absence direct neuromuscular cholinergic neurotransmission takes place. This model is based on the assumption that ICC-IM control the accessibility of the neurally released acetylcholine (ACh) to different pools of muscarinic receptors on the ICC and smooth muscle cells. (1) The classical view of cholinergic neurotransmission maintains that neurally released ACh acts directly on muscarinic receptors located on smooth muscle cells to produce contraction. However, in 2000, Ward and colleagues (Ward et al. 2000) reported that ICC-IM-deficient W/Wv mice have impaired cholinergic neurotransmission that challenged the classical view and suggested that ICC-IM transduce cholinergic neural signals to the smooth muscles. Muscarinic receptors on smooth muscles were considered not to be important in this process. Involvement of the ICC in cholinergic neurotransmission had changed the paradigm used for defining the pathogenesis of disorders of gastrointestinal motility and their treatment (Sanders, 2006). Bhetwal et al. (2013) report that ICC-deficient W/Wv mice have preserved contractions to cholinergic neurotransmitter excitation. These current findings are similar to those reported earlier by Zhang and colleagues in ICC-deficient Ws/Ws rats, and support the classical model of direct cholinergic neuromuscular transmission (Zhang et al. 2011). (2) Bhetwal et al. (2013) now propose a model that includes direct neuromuscular transmission but also preserves ICC-mediated cholinergic neurotransmission. Accordingly, normally in the presence of ICC-IM, the ICC transduce cholinergic signals to the smooth muscles (indirect neurotransmission), but when the ICC are absent, direct neuromuscular transmission takes place. The physiological rationale of the proposed ‘situational’ role of ICC in cholinergic neurotransmission is unclear and is based on many assumptions which requires careful scrutiny. The underlying assumptions for this model are that: (a) ICC are exclusively innervated by the enteric motor nerves, and that they can restrict accessibility of the neurally released ACh to muscarinic receptors present on ICC-IM at the exclusion of the muscarinic receptors on smooth muscles; (b) activation of muscarinic receptors on ICC generate a depolarizing potential in the ICC that is then conducted to smooth muscles and which cause muscle contraction; and (c) stimulation of muscarinic receptors on ICC-IM activate different signalling pathways from those activated by the stimulation of muscarinic receptors on the smooth muscles. The morphological evidence has not demonstrated that the enteric motor nerves exclusively innervate ICCs at the exclusion of the smooth muscles and that neuro-ICC junctions have features that can limit the accessibility of the released ACh to focal areas of muscarinic receptors on the ICC. Evidence for selective innervation of the ICC is mostly based on studies that lack comparison with innervation of the smooth muscles, using microscopic techniques that have a resolution of only 200 nm from which to make conclusions about junctions <20 nm (Mitsui & Komuro, 2002; Goyal & Chaudhury, 2013). Tedious electron microscopic evaluation of thin serial sections of an entire varicosity would be needed to define all contacts of a varicosity with effector cells in 3-dimensions (Hirst et al. 1992). Such studies have not been performed to investigate neuro-ICC junctions. Moreover, in order for these junctions to restrict the flow of the neurotransmitters outside the area of focal contact, they should have a synapse-like cavity surrounded by a barrier created by certain highly specialized adhesion molecules similar to that seen in the synapses. The focal synaptic cavities are not present at the neuro-ICC junctions (Goyal & Chaudhury, 2013). No data currently exist that show a focal high concentration profile of the neurally released ACh at the neuro-ICC junctions. It is assumed that stimulation of muscarinic receptors on the ICC generates a large depolarizing potential that is then conducted to smooth muscles. However, efficient conduction of such potentials from ICC to smooth muscles via gap junctions has not been documented (Daniel et al. 2007). Bhetwal et al. conclude that the recruitment of the ROCK-mediated MYPT1 signalling pathway by neurally released ACh in the ICC-deficient W/Wv mutants and not in ICC-containing WT mice is indicative of the action of the released ACh on muscarinic receptors on the smooth muscles in W/Wv mice and on the muscarinic receptors on the ICC in the control mice. However, this conclusion is narrow as it fails to consider alternative explanations such as alterations in intracellular signalling and receptor expression in the smooth muscles due to c-kit deficiency that are independent of morphological changes in the ICC (Zhang et al. 2011; Bautista-Cruz & Paterson, 2011). In summary, this is an important study that reverses the prevailing, albeit controversial, view that the ICC were essential in transducing cholinergic signals to the smooth muscles in the gut, and supports the classical view of direct neuromuscular transmission. In addition, it raises an intriguing possibility that ICC play a role in cholinergic neurotransmission only when they are present, but they are not necessary for the neurotransmission.