**2. Types of NMSC**

#### **2.1 Remak Schwann cells**

Robert Remak first described the unmyelinated nerve fibers using the nerve fiber teasing technique in 1838 [6], so, in his honor, they were named "Remak fibers."

In the PNS most nerve fibers are unmyelinated [1], formed by RSCs accommodating a variable number of small-caliber axons (less than 1 μm diameter) (**Figure 2B**).

RSCs do not produce myelin, but they are essential for normal PNS development and functioning.

During PNS formation, pockets with multiple axons within a single mesaxon can be encountered. This aspect occurs only occasionally in normal adult Remak fibers where the small diameter axons of C nerve fibers (sensory/afferent), postganglionic sympathetic fibers, and some preganglionic sympathetic or parasympathetic fibers are accommodated in separate grooves of longitudinally interconnected RSCs forming the Remak bundles. Each RSC surrounds many axons, during radial sorting, forming a mesaxon for each axon. It is uncommon for an axon to be in direct contact with the basement membrane of the Schwann cell [4].

The number of axons surrounded by a RSC varies depending on the type of nerve fibers or a particular region along them. Thus, there is a higher number of axons exiting the dorsal root ganglion than in the distal segments of the peripheral nerve. In the cutaneous nerves, the number of axons per RSC decreases as they approach the skin [7], suggesting the existence of specific mechanisms regulating RSC-axons association as they approach their target. Moreover, the distribution of the axons within the Remak bundles varies along the peripheral nerve, with multiple axons within one pocket of the RSC toward the dorsal root and completely isolated axons in the distal segments [8].

There are studies reporting the presence of few short, myelinated internodes along a unmyelinated fiber especially in older animals [9].

Thus, it appears that the "ensheathment fate" of axons to either become myelinated or unmyelinated fibers relies on local/environmental cues. One of the most

#### *Demyelination Disorders*

extensively studied is the neuregulin 1 type III signaling through ErbB receptors, an axolemmal myelin-inducing factor [3] that promotes the formation of a mesaxon for each unmyelinated axon as well as SC differentiation into myelinating cells, depending on the expression level [10].

Another feature of unmyelinated nerve fibers is that axons may switch between neighboring Remak bundles along the nerve.

Moreover, a RSC can surround axons with different functions, for example, both sensitive and sympathetic axons, both axons expressing TrkA (tropomyosin receptor kinase A) receptors with a high affinity for nerve growth factor (NGF) and axons expressing RET (rearranged during transfection) receptors that respond to glial cell line-derived neurotrophic factor (GDNF) and artemin or axons derived from different dorsal ganglia [1].

## *2.1.1 Remak Schwann cell differentiation*

The RSCs differentiation is governed, at least in part, by neuronal cues, especially by the signaling pathway neuregulin 1 type III (Nrg1-III)/ErbB2/ErbB3 receptor cascades. However, a number of cell-autonomous genes also contribute to SCs differentiation toward RSCs, one of which is gamma-aminobutyric acid type B1 receptor (GABBR1) [4].

SCs derive from the neural crest cells, after contacting nascent nerves during embryogenesis. Neural crest cells give rise to SCP, in early embryonic nerves, which further differentiate into iSCs, in late embryonic and perinatal nerves. Postnatally, iSCs will further differentiate either toward myelinating cells or non-myelinating cells according to axon-derived signals. The myelinating cells form the myelin sheath of large axons (larger than 1 μm diameter). The non-myelinating cells ensheath small axons forming unmyelinated fibers, called Remak bundles, or they migrate toward the neuromuscular junctions, where they become terminal/perisynaptic/teloglia Schwann cells (**Figure 3**).

#### *2.1.1.1 Neuregulin*

There are four distinct genes for neuregulins, but neuregulin 1 NRG1 is the best studied. NRG1, also known as glial growth factor (GGF), is a growth factor with EGF domain homology known to induce growth, differentiation, and migration of Schwann cells throughout development [10, 11]. NRG1 has three isoforms out of which type III is considered to be the most important signaling molecule for SC-axon interactions. NRG1 type III is produced by neurons and is released from axons by proteases, such as BACE1, or may remain anchored to the axonal membrane. NR1-III interacts with high-affinity tyrosine kinase receptors ErbB2/ ErbB3 heterodimers, triggering the activation of downstream pathways, such as Ras/MAPK and PI3K/Akt SCs. Stimulation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) cascade was proven to lead to the suppression of myelinating state [12] through ErbB2 and ErbB3 receptors that are expressed in Schwann cells [13]. The NRG1-ErbB signaling pathway seems to play a crucial role in the SCs lineage for both myelinating and non-myelinating SCs and promotes SCP precursor survival after birth as well as during in vitro culturing [10, 14].

However, recent studies showed that in transgenic animal models where NRG1 is conditionally ablated during postnatal life, there is no reduction in the number of sensory axons but larger, unordered Remak bundles with polyaxonal pockets, where axons are not separated by SC processes, are formed, and some large-diameter

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

#### **Figure 3.**

*Schwann cell development and maturation: their role in the evolution of myelinated and unmyelinated peripheral nerve fibers. Schwann cell precursors differentiate into immature Schwann cells which start the process of "radial sorting". A pro-myelinating Schwann cell envelops a large axon and becomes a myelinating Schwann cell. An immature Schwann cell which ensheaths many small axons becomes mature non-myelinating Schwann cell, forming a Remak bundle.*

axons lose the myelin sheath. Only the sensory function was affected, without changing the survival and axonal maintenance of the neurons [15]. However, after nerve injury, RSCs re-establish normal Remak bundles, suggesting that during adulthood, after the basal lamina was established, axonal sorting is no more required [16].

Another experimental in vivo study on mouse sciatic nerve showed that NRG1 type III Erb2/Erb3 signaling regulates the morphological changes of the SCs. The study used a NRG1 type III knockout mouse model (+/−) with a low expression of NRG1 type III, which produced Remak bundles with a higher number of axons and smaller spaces between axons [17].

#### *2.1.1.2 Genes acting cell-autonomously in Schwann cells*

A number of studies have shown that there are certain genes that control SCs fate [4, 12, 18, 19] and that they act cell-autonomously in SCs. There are genes that can trigger upregulation of NRG1 during differentiation after injury, thus stimulating remyelination and redifferentiation of SCs [20].

An important genetically determining factor during SCs development is the gene for gamma-aminobutyric acid type B1 receptor (GABBR1), which is active mainly in RSCs as compared to myelinating SCs [21]. An in vivo experimental research showed that the absence of *GABBR1* in embryonic SCs leads to an increased number of small-caliber axons and Remak bundles and a decreased number of the largecaliber axons [19]. Furthermore, NRG1-III expression was decreased in GABBR1 mutant animals, in correlation with lower mean diameter axons along with a compensatory gene overexpression and protein levels of ErbB2 and ErbB3. Further studies are needed to analyze the requirement and the mechanism of these cellautonomous genes in SC fate decision.

#### *2.1.2 Remak Schwann cell maturation*

During maturation, RSCs extend cell processes that individually encircle each axon with the plasmatic membrane and cytoplasm, separating it from surrounding axons. Naked axons, which were not completely surrounded by RSC cytoplasm and which come into direct contact with other axons, demonstrate failed RSC maturation after nerve injury [22]. Recent studies have shown that the expression level of a protein that is highly expressed in non-myelinating SCs, neuropathy target esterase (Nte), is correlated with SC developmental maturation and remyelination after neuronal injury. However, this protein is not involved in myelination [23].

Other proteins, such as mTOR [24–26], or G-G protein-coupled receptor Gpr126/Adgrg6, through laminin-211 and collagen type IV interactions, are required for both myelinating and non-myelinating SCs growth and function, during developmental stages as well as after nerve injury. Gpr126 controls radial sorting, myelination, SC-axon interactions, as well as Remak bundle formation [27–30].

In SCs, both deletion and overexpression of mTOR complex I adapter (Raptor) disrupts Remak bundle formation by increasing the number of axons in Remak bundles, with many naked axons [26], or decreasing the number of axons in Remak bundles and aberrant wrapping of multiple membrane-layered axons by RSCs, respectively [24, 31].

#### *2.1.3 Role of Remak Schwann cells*

The absence of myelin gives Remak fibers a certain plasticity, sprouting, and growth abilities that exceed that of myelinated fibers. That is why they are found especially in PNS, where the risk of physical injuries is much higher than in the CNS.

Although Remak fibers are found mainly in the PNS, they are also found in the CNS, associated with unmyelinated fibers in the parallel fiber system of the cerebellum and nigrostriatal pathway [1, 32].

#### *2.1.3.1 RSCs as immune competent cells*

NMSCs, like other SCs, can also function as immunocompetent cells playing an essential contribution in mounting and modulating of immune response in certain conditions, by antigen presentation and cytokine secretion, as well as by their direct interaction with immune cells. Moreover, NMSCs express specific pattern recognition receptors (PRR) for the detection of pathogens, such as Toll-like receptors (TLRs) and the nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family [33–35].

The crosstalk between immune- and peripheral nerve SCs through a large array of molecules either expressed or recognized by SCs build up the base for nervousimmune system interactions. The subject was extensively reviewed by Tzekova et al. [34]. Moreover, Hu et al. showed that NMSCs located in the thymus develop correlations with thymocytes, lymphocytes, and dendritic cells under normal and pathological conditions. They concluded that NMSCs are highly suitable for studying the local interactions of the PNS and primary lymphoid tissues or organs [36]. The same observations were made by Ma et al. studying the mouse spleen and the interactions between NMSCs and leukocytes [37].

Another role for NMSCs was concluded by the study of Yamazaki et al. which showed that NMSCs maintain hematopoietic stem cell hibernation in the bone marrow niche. They demonstrated that NMSCs proved responsible for activation of TGF-beta latent form. These glial cells, ensheathing autonomic nerves, get in

contact with hematopoietic stem cells and maintain them in hibernation by regulating activation of latent TGF-beta [38].
