Fig 2: Munc18-1 is exclusively located in the presynaptic component of the NMJ. Multiple-immunofluorescence-stained muscles visualized at the confocal microscope. (A) Munc18-1 colocalizes with AChR from a NMJ en face view. (B) Munc18-1 colocalizes with AChR from a NMJ en side view. (C,D) Munc18-1 colocalizes with syntaxin and AChR from en face view. (E) Munc18-1 colocalizes with syntaxin and AChR from en side view. (F) Munc18-1 colocalizes with S-100 and AChR from en face view. (G) Munc18-1 colocalizes with S-100 and AChR from a NMJ en side view. Scale bars = 10 µm.

Fig 3: Summary. Munc18-1 is expressed and phosphorylated in basal conditions in the skeletal muscle. It is placed in the motor nerve terminals and absent in the Schwann cells and muscle cells. Munc18-1 phosphorylation at the residue Ser-313 occurs in response to extracellular Ca2+ increase and to PKC activation, either pharmacologic (PMA) or physiologically induced. Presynaptic stimulation induces calcium influx, which promotes the activation of cPKCßI. This isoform downregulates the nPKCe phosphorylation whereas nPKCe isoform enhances cPKCßI. Both nPKCe and cPKCßI isoforms contribute to regulate Munc18-1 phosphorylation during synaptic activity. (A) Representation of how BDNF/TrkB signaling, nPKCe, and cPKCßI modulate Munc18-1 under presynaptic stimulation without contraction. In stimulation protocols, synaptic activity results in pnPKCe enhancing Munc18-1 phosphorylation and cPKCßI decreasing it, maybe through negatively regulating the action of pnPKCe. Thus, the balance between the activities of these two isoforms can be a relevant cue in the regulation of the exocytotic apparatus. pnPKCe activity and pMunc18-1 level are positively related and negatively modulated by the TrkB receptor. TrkB would enhance pcPKCßI activity to prevent the synaptic activity-induced Munc18-1 phosphorylation mediated by nPKCe. (B) Representation of how BDNF/TrkB signaling, nPKCe and cPKCßI modulate Munc18-1 under presynaptic stimulation with contraction. Muscle contraction prevents the synaptic activity–induced Munc18-1 phosphorylation through a mechanism that decreases the nPKCe/cPKCßI/TrkB signaling.

Fig 4: Munc18-1 and pMunc18-1 in adult skeletal muscle under basal conditions. (A) Representative Western blot bands from diaphragm showing specificity for the antibodies Munc18-1 and pMunc18-1 (Ser-313). (B) Representative Western blot bands and quantification of membrane and cytosol distribution of Munc18-1 and pMunc18-1. Both are mainly located in the membrane but pMunc18-1 is barely detectable in the cytosolic fraction. GAPDH is exclusively in the cytosol fraction and the Na+/K+-ATPase is in the membrane. (C) Representative Western blot bands and quantification of Munc18-1 and pMunc18-1 after PMA treatment in diaphragm, brain and spinal cord. (D) Representative Western blot bands and quantification of Munc18-1 and pMunc18-1 after different calcium concentrations. High Ca2+ concentration increases pMunc18-1 levels while decreases Munc18-1. Data are mean percentage ± SEM, *p < 0.05 (n = 5).

Fig 5: nPKCe and cPKCßI modulate Munc18-1 phosphorylation under muscle contraction conditions. (A) Representative Western blot bands and quantification show that Munc18-1 phosphorylation and the pMunc18-1/Munc18-1 ratio are significantly decrease after stimulation resulting in contraction in nPKCe inhibitor peptide eV1-2 preincubated muscles. (B) Representative Western blot bands and quantification show that the pMunc18-1/Munc18-1 ratio are significantly decreases after stimulation resulting in contraction in cPKCßI inhibitor peptide ßIV5-3 preincubated muscles. (C) Representative Western blot bands and quantification show that the phosphorylation of nPKCe and the pnPKCe/nPKCe ratio significantly increases after stimulation resulting in contraction in cPKCßI inhibitor peptide ßIV5-3 preincubated muscles. Data are mean percentage ± SEM, *p < 0.05 (n = 5).