*2.2.1 PSC phenotype and distribution*

PSC, also known as teloglia or terminal Schwann cell, is a type of non-myelinating Schwann cell which is found above the presynaptic nerve terminal at the level of the NMJ. Louis-Antoine Ranvier described in 1878 the presence of a type of cell in the NMJ distinct from the axon terminal or the muscle fiber. He named this cells "arborization nuclei" because of their widespread projections along the NMJs. Later on, with improved histology techniques and in the era of electron microscopy, several studies identified the presence of this specific type of cell in the NMJ (**Figure 2C**).

PSCs express several markers that are used to highlight them in situ. The most common approach used is anti-S100b immunolabeling [46]. S100b is a nonspecific marker for all types of SCs, either myelinating or non-myelinating ones. In amphibians only, to distinguish PSCs from myelinating SCs, two specific antibodies are used, peanut agglutinin (PNA) [47] and 2A12 monoclonal antibody [48], which mark the extracellular matrix and the cells' surface, respectively. Interestingly, PSCs express several myelin proteins such as myelin-associated glycoprotein (MAG), galactocerebroside, protein zero (P0), and 2′,3′-cyclic nucleotide 3′-phosphodiesterase [49]. The cells are not involved in the process of myelination, though the presence of these proteins proves the common origin of the two types of Scs. Additionally, PSCs express on their surface several receptors such as acetylcholine receptors, ATP, purinergic receptors, and L-type voltage-dependent calcium channels that usually take part in the synaptic transmission [50–53] supporting the hypothesis that PSCs play an active role in the NMJ rather than having only a structural role.

#### *Non-Myelinating Schwann Cells in Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.91930*

Several studies determined that the number of PSCs gradually increase after birth [54]. Adult NMJ may contain one up to five PSCs [55–57], and their number is modulated by PSC-muscle cross talk through neurotrophins [58].

PSCs tend to be positioned at the presynaptic side, on top of the motor axon terminal, without the intervention of a basal lamina [55, 56]. Recently a new population of fibroblast-like cells named kranocytes—NMJ-capping cells—was detected on the other side, above the basal lamina of the PSC, covering all other cells of the NMJ. They are thought to have important roles in the NMJ repair after nerve injury [59, 60]. Kranocytes appear to communicate with PSCs via neuregulin signaling pathway to act synergistically after nerve damage [59].

Most studies about PSCs were performed either on amphibian (frog) or rodent (mouse) samples [53]. A peculiarity of the frog's NMJ, where the unmyelinated nerve terminal is completely surrounded by PSCs and does not form dilated terminal buttons and the synaptic contact is formed all along, is that PSCs send finger-like projections into the synaptic cleft, on the presynaptic side, which separate, at a regular distance, active areas where the neurotransmitters are released from covered areas [52, 61]. These active areas correspond on the opposite side to the folds of the sarcolemma, the postsynaptic element of the NMJ, which are rich in nicotinic acetylcholine receptors [52, 61]. In mammals, PSC projections do not reach the synaptic cleft (**Figure 2D**).

### *2.2.2 PSC roles in the formation and function of the neuromuscular junction*

PSCs are involved in the growth and maintenance of the NMJ during development.

Although these cells do not take part in the initial formation of the axon-muscle junction, PSCs have key contributions in the next stages of NMJ development. In animal models lacking SCs, the axon reaches the muscle in the initial step of the NMJ formation, but only for a brief time [62, 63]. In the absence of SCs, the NMJ gets disrupted, suggesting the vital role of PSCs in the NMJ maintenance during development [64]. Soon after the contact between the axon and the muscle, PSCs intensively divide, sprout, and are primarily involved in the growth of the synapse [64].

PSCs are also involved in the physiological processes of polyneuronal innervation and synapse elimination. PSCs are involved in the multiple innervation process of the muscles and suffer a regression in parallel with the axonal withdrawal [1, 65, 66]. After the process of axonal withdrawal, PSCs are engaged in the removal of nerve debris, through phagocytosis [67].

The signaling pathway which facilitates the survival and growth of PSCs and the tight communication between PSCs and motor axons is the neuregulin1-ErbB pathway [1].

PSCs have important roles in the maintenance of the NMJ during the adult life as the structural support. Ablation in PSCs on the adult NMJ does not impede the immediate structure and function of the synapse, but after a period of time, the motor axon terminals retract, and the NMJ collapses [64, 68]. Thus PSCs have a significant contribution to the structural maintenance of the synapse under the action of physical factors such as the intense tractions between the nerve and the muscle [53].

These cells dynamically participate in the process of synaptic transmission of information between the motor axons and the muscles, having an important role in the modulation of NMJ activity [53, 57, 69]. Not only PSCs can alter the synaptic transmission, but PSC activity can also be modified by synaptic transmission. Or, as some authors like to say, PSCs can both "talk" and "listen" in the synapse [53, 69].

When the nerve terminal increases its firing rate and a large amount of neurotransmitter is released in the synaptic cleft, a simultaneous increase of intracellular calcium occurs in PSCs [70, 71]. A similar effect is obtained by applying exogenous acetylcholine and ATP, molecules normally released by the synaptic vesicles, to PSCs [51]. Moreover, the levels of intracellular calcium vary depending on the type of the nerve firing rate, either burst or continuous [72]. These events do not occur in the myelinating SCs and emphasize the detection of synaptic activity by PSCs and the modification of their cellular behavior secondary to the synaptic transmission [69]. This is similar to a decoding process of the synaptic activity. Thereby, the events correspond to the "listening" ability of PSCs in the synapse.

The increase of the PSC intracellular calcium levels does not play only a "decoder" role. This transient raising modulates the synapse by intensifying the neuromuscular transmission. PSCs are expressed on the surface of several G protein-coupled receptors with contributions in the modulation of the synapse activity [73]. Evidences suggest that different ligands of these G protein-coupled receptors determine different changes in the neuromuscular transmission, as follows: a GTP analogue decreased the neurotransmitter release, while a GDP analogue reduced the synaptic depression [73]. These events correspond to the "talking" ability of PSCs in the synapse.

Therefore, PSCs are not only a structural, passive component of the NMJ, but an active one. These evidences confirm that the NMJ is a tripartite synapse.

#### *2.2.3 Roles in pathology*

PSCs induce and guide the growth of nerve sprouts to re-establish the NMJ after nerve injury.

All the actions that PSCs perform in an attempt to regain the activity of the NMJ appear to be mediated by neuregulin1-ErbB signaling pathway [74].

First of all, after nerve degeneration, PSCs develop phagocytic traits for the clearance of the debris from the nerve terminals [75].

Second of all, PSCs are involved in the guiding of reinnervation. A few days after the nerve injury, PSCs from the altered NMJ begin to abundantly sprout, and these new processes reach adjacent undamaged synapses [76]. In this manner, "bridges" are established between the innervated and the dennervated NMJs. The role of the newly formed bridges is to facilitate the nerve pathway to find the altered NMJ and to regenerate the synapse more rapidly [69, 76]. However, satellite NMSCs seem to play a role in nerve regeneration after insult as well and might be involved in pathogenic pathways of neuropathic pain [77].

Miller Fisher syndrome is a Guillain-Barré syndrome variant with antibodies against GQ1b ganglioside that is clinically characterized by ataxia, ophthalmoplegia, and areflexia. Studies on mouse models revealed that PSCs represent an important target of the autoimmune process, the cellular destruction is complement dependent, and this pathogenic mechanism might be relevant for the human disease [68, 78].

Amyotrophic lateral sclerosis (ALS) is a challenge for both the clinician and the researcher due to the obscure pathological mechanisms that are still not completely understood. The role of glial cells in the pathophysiology of the disease is not clear yet. Most probably the SC modifications are a consequence of the neurodegeneration process. However in human patients with ALS, PSCs have abnormal features with cellular processes that extend into the synaptic cleft [79]. Additionally, in ALS mouse models, PSCs have abnormal intracellular levels of calcium, causing a flaw in the synaptic "decoding" function [80].

Another neuromuscular disease in which PSCs appear to be involved is spinal muscular atrophy (SMA). In an ultrastructural study on SMA mouse models, PSCs in the diaphragmatic muscle show changes in their morphology such as vacuole-like translucent profiles and an electron-dense cytoplasm [81]. Another study on SMA mouse models revealed that in the evolution of the disease, there is a progressive loss of PSCs, leading to an improperly remodeling and regeneration of the NMJ [82].
