Post-synaptic specialization of the neuromuscular junction: junctional folds formation, function, and disorders

Abstract

Post-synaptic specialization is critical to the neurotransmitter release and action potential conduction. The neuromuscular junctions (NMJs) are the synapses between the motor neurons and muscle cells and have a more specialized post-synaptic membrane than synapses in the central nervous system (CNS). The sarcolemma within NMJ folded to form some invagination portions called junctional folds (JFs), and they have important roles in maintaining the post-synaptic membrane structure. The NMJ formation and the acetylcholine receptor (AChR) clustering signal pathway have been extensively studied and reviewed. Although it has been suggested that JFs are related to maintaining the safety factor of neurotransmitter release, the formation mechanism and function of JFs are still unclear. This review will focus on the JFs about evolution, formation, function, and disorders. Anticipate understanding of where they are coming from and where we will study in the future.

Keywords: Development; Disease; Formation mechanism; Function; Junctional folds; Neuromuscular junction; Post-synaptic specialization.

Fig. 1

Morphological changes of the mouse JFs during development. A Before E14, the axon terminal is not attached to the muscle fiber, and the AChRs are pre-clustering in the post membrane. BL materials already exist around the muscle fibers. B At the E18-P0 stage, multi-nerves target the muscle fiber and induce the post-membrane to formate a shallow gutter. Few pit-like incipient JFs are not aligned opposite to the AZs at this stage. C During the first 5 days, excessive axon terminals were eliminated, and pit-like JFs were the dominant structure. D Before the 14 days, JFs elongated into the subneural sarcoplasm. Sarcoplasmic without axonal terminals covered are upheaved and matured to the sarcoplasmic protrusions. E At the 28 days, almost all JFs matured to the slit-like shape, and with the secondly JFs. The entire post-synaptic apparatus have been raised above the surface of the muscle fiber and termed the “endplate”. F At the aging JFs, axon terminals are denervation gradually. Few second JFs degeneration could be found, but the primary JFs was always preserved. Unopposed JFs to the AZs are prevalent in aging

Fig. 2

Phylogenetic tree of JFs evolution. Grey images represent representative species. The representative morphology of each animal JF is shown with a schematic diagram. The green dots represent critical events in the evolutionary history of JF. Protosynaptic proteins emerged in the Choanoflagellates and Spongia; the first NMJ emerged in the Annelida; glial cells participated in the NMJ structure in the Mollusca and Arthoropods; the first JFs of post-membrane depression emerged in the Chaeogmaths and Chordata; deeper and branched JFs were found in the Cartilagineous and Teleost fish; higher densities of JFs was found in Amphibians; the most complex of JFs structure existed in the mammalian animals

Fig. 3

Hypothesized mechanisms for JFs formation. A Exocytosis model modified from the previous hypothesis. E18.5-P0, beneath the depression areas, numerous caveolae with coated or uncoated were found. As these caveolae fused to the post-membrane opposed sites to the active zone, incipient JFs emerged on the stage. Accompanying more new membranes were inserted at P7-P28, the post-synaptic membrane investigated deeper into the cytoplasm and formed the maturely JFs [10]. However, this hypothesis can not explain how the BL are inserted into the JFs (question mark), especially why the secondly JFs formation. B The diaphragm of mice at P1 was observed by transmission electron microscope. Although Schwann cells (green) encapsulate multiple axons (red) in a single NMJ, only a single JF was observed beneath the NMJ. The incipient JF showed apparent invagination features, and the BL (arrow) within the JF maintained continuity with the outside. The inset figure is an enlarged view. Note that the active zone (arrowhead) is not opposed to the JF. C Invagination model was proposed by this reviewer. At E18-P0, the AZs were preceded by the formation of JFs, and they were not aligned with the JFs [13]. Initiated single from the axon terminal induced cytoskeleton pulled the sarcolemma investigated into the cytoplasm. In the first week after birth, the incipient JFs aligned the active zone to the opposed site with the interaction with BL, and the VGSC integrated into the valley of JFs. In the first 4 weeks after birth, the second JFs formed, which keep this mature structure to the adult stage. S: Schwann cell, M: Muscle fiber, A: Axon. The scale bar in B is 500 nm

Fig. 4

Diagram of AP conduction within the NMJ. A In the normal NMJ, (1) a nerve AP arrives at the nerve terminal and produces rapid depolarization, (2) VGCCs opening and Ca²⁺ entry, (3) the transmission vesicles fusion to the pre-membrane and release ACh into the cleft, (4) ACh bind to their post-synaptic receptors and generate a localized EPP at the crest of JFs, (5) EPP arrive the valleys of JFs and produces rapid depolarization, (6) VGSCs opening and generate of a muscle AP, (7) VGSCs on the neighbor JFs induced opening by voltage gating, resulting saltatory conduction happening, (8) Orthogonally aligned JFs propagate the AP along the long axis and drive the muscle fiber contraction. B. In VGSCs deficient JFs, muscle AP is not initiated by the Na⁺ inflow of VGSCs. To spread the AP throughout the whole NMJ region, it needs to activate more VGCCs and release more ACh vesicle quanta. Figures C and D are the plane views of A and B, respectively