The Signaling Pathways Mediated by P2Y Nucleotide Receptors in the Formation and Maintenance of the Skeletal Neuromuscular Junction

Abstract

The motor neuron, the Schwann cell and the muscle cell are highly specialized at the vertebrate skeletal neuromuscular junction (NMJ). The muscle cell surface contains a high local density of acetylcholine (ACh) receptors (AChRs), acetylcholinesterase (AChE) and their interacting macromolecules at the NMJ, forming the postsynaptic specializations. During the early stages of development, the incoming nerve terminal induces the formation of these postsynaptic specializations; the nerve secretes agrin and neuregulin (NRG), which are known to aggregate existing AChRs and to increase the expression of AChR at the synaptic region, respectively. In addition, adenosine 5′-triphosphate (ATP) is stored at the motor nerve terminals and is coreleased with ACh during muscle contraction. Recent evidence suggests that ATP can play a role in forming and maintaining the postsynaptic specializations by activating its corresponding receptors. In particular, one of the nucleotide receptor subtypes, the P2Y₁ receptor, is specifically localized at the NMJs. The gene expression of AChR and AChE is upregulated after the activation of P2Y₁ receptors. Thus, the synaptic ATP together with agrin and NRG can act as a synapse-organizing factor to induce the expression of postsynaptic functional effectors.

ATP Potentiates Agrin-induced AChR Aggregation in Cultured Myotubes

“…ATP potentiates the response to the applied ACh in nerve-muscle co-culture (see references in Ref. 23), which is in line with the activity-dependence of the reshaping of synaptic architecture at the nmjs (4). On the other hand, the application of P2Y 1 receptor agonists activated the membrane-bound RhoA and stimulated the actin cytoskeleton organization in cultured vascular myocytes (28).…”

“…This high concentration of ATP at the cleft is maintained whenever the contraction of muscle is triggered. The response of synaptic ATP at the nmjs is strongly supported by the existence of P2Y receptors at the post-synaptic muscle, which in particular has a high level of expression in adult skeletal muscles (23,24,26). Together with previous studies, we hypothesize that ATP could have functional roles in affecting the neuromuscular transmission in short-and long-term aspects.…”

“…ATP is an additional potential trophic factor at the nmjs (23,24). In the synaptic vesicles at vertebrate nmjs (25), ATP stabilizes acetylcholine (ACh) and is co-released quantally with it in a ratio of about 1 ATP to 5 ACh.…”

“…There is much evidence that the native P2Y 1 receptor in tissues, as well as in muscle, so far exam-ined is linked to the formation of inositol triphosphate and to intracellular Ca 2ϩ mobilization (see references in Refs. 23,26). ATP potentiates the response to the applied ACh in nerve-muscle co-culture (see references in Ref.…”

Muscle Induces Neuronal Expression of Acetylcholinesterase in Neuron-Muscle Co-culture

“…ATP is co-stored in and constantly co-released quantally with acetylcholine from the nerve terminals or released by the muscle at the nmjs (49). Under normal physiological conditions, ATP concentration in the synaptic cleft could be in a range of 0.1-1 mM; these concentrations are adequate to obtain nearly maximal to maximal effects on its receptors in in vivo preparations.…”