Peripheral nervous system and grounds for the neural insult in leprosy

Abstracts

O mecanismo de interação entre o Mycobacterium leprae e as células neurais não está esclarecido até o momento. Não há interpretação satisfatória do tropismo da bactéria ao sistema nervoso periférico, em particular. O presente estudo é uma revisão da microfisiologia da estrutura do aparelho extracelular, ligado às células de Schwann, assim como a descrição das unidades morfológicas, provavelmente envolvidas no processo de ligação à parede celular da bactéria.

Carcinoma de células escamosas; Carcinoma de células escamosas; Incidência; Neoplasias cutâneas

REVIEW

Peripheral nervous system and grounds for the neural insult in leprosy

Jorge João Chacha^I; Miriam N. Sotto^II; Lothar Peters^III; Silvia Lourenço^IV; Evandro A. Rivitti^V; Petr Melnikov^VI

^IPh.D., Discipline of Dermatology - Faculdade de Medicina da Universidade Federal de Mato Grosso do Sul (UFMS) – Campo Grande (MS), Brazil

^IIPh.D., Medical School, Universidade de São Paulo (USP) – São Paulo (SP), Brazil

^IIIMaster, Department of Pharmacy, Universidade Federal de Mato Grosso do Sul (UFMS) – Campo Grande (MS), Brazil

^IVPh.D., Faculdade de Medicina da Universidade de São Paulo (USP) – São Paulo (SP), Brazil

^VPh.D., Faculdade de Medicina da Universidade de São Paulo (USP) – São Paulo (SP), Brazil

^VIPh.D., Faculdade de Medicina da Universidade Federal de Mato Grosso do Sul (UFMS) – Campo Grande (MS), Brazil

^{Mailing Address}

ABSTRACT

The mechanism of interaction between Mycobacterium leprae and neural cells has not been elucidated so far. No satisfactory interpretation exists as to the bacterium tropism to the peripheral nervous system in particular. The present study is a review of the micro-physiology of the extracellular apparatus attached to Schwann cells, as well as on the description of morphological units probably involved in the process of the binding to the bacterial wall.

Keywords: Antigens, CD29; Dystroglycans; Laminin; Peripheral nervous system; Schwann cells

INTRODUCTION

Leprosy is a chronic disease caused by Mycobacterium leprae with clinical polymorphic characteristics ¹, relevant to the destruction of peripheral nerves, resulting in irreversible disabilities ², causing suffering that goes beyond pain and embarrassment strictly related with the physical damage, with great social and psychological impact, justifying the advances in multidisciplinary approach of patients as the healthcare actions required to control the disease ³.

It is estimated today that there are 3,000,000 people with the disease in the world with permanent type 2 disability, and in Brazil, 55,000 have it, despite complete polychemotherapy (PCT) ⁴. It is known that 30% of the destruction of nerve fibers is required to produce clinical manifestations. The nervous aggression of leprosy may be defined into two ways: an initial one, in which there is absence of inflammatory cells, and a common one in which both paucibacillary and multibacillary forms occur, and late when the process become inflammatory, the later manifestations may developed auto-immunity ^5,6 cytotoxicity ^7,8 and presence of ROI (reactive oxygen intermediates)⁹ and NO (nitrous oxide) ¹⁰, fibroblasts ¹¹, NGF-R (Nerve growth factor) ¹², NgCAM (Neural Glial Cell Adhesion Molecule), interferon-gamma and TGF- beta – (Transforming growth factor beta),¹³ TNF-alfa – TNF-beta (Tumor necrosis factors), IL-6, IL-8, IL-12 and IL-10 (Interleukins)^14,15 and matrix metalloproteinase ¹⁶.. The exact nature of the genetic components, such as HLA and non-HLA, MICA and MICB, NRA MP1 and others, especially the exact number of involved genes, their biological functions and the genetic variations of these genes responsible for the observed effects, are still completely unknown ¹⁷.

In the present paper, we focused on the role of these structures that are present in the peripheral nervous system and their relation with Mycobacterium leprae.

THE MAIN CHARACTERISTICS OF THE PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system is comprised of nervous fiber formed by one or more axons involved by Schwann cells maintained by the endoneurum, plus amorphous material of extracellular matrix, capillaries, fibroblasts and mast cells, as well as epineurum and perineurum ¹⁸.

Considering that the perineural sheath of axons and Schwann cells are involved by a basal lamina, in figure 1 (line B) we can see the special organization of extracellular matrix (line A), comprising many molecules secreted by multiple cells, among which Schwann cells ¹⁹ (line C).

The basal lamina, through interactions with cellular membrane receptors, participates in cell metabolism, organization of proteins of plasma membrane, cell migration during embryogenesis and cell differentiation ¹⁹. Moreover, it influences axonal regeneration serving as a guide of nervous fibers in this process and it has structural and signaling functions ²⁰. Laminin (Figure 1-B1), one of the components of basal lamina, is a large and flexible molecule formed by three long chains of polypeptides (γ, γ and γ) forming an asymmetrical cross, maintained together by disulfide bridges. To present, a total of 15 isoforms of laminins are known ²¹.

Basal lamina and laminins are important for the peripheral nervous system. Before birth, they support the correct development of the neural system, whereas after birth they guarantee the integrity and interaction with components of the extracellular matrix, in addition to programming myelin synthesis by Schwann cells ^21,22.

In patients with muscle dystrophies and peripheral neuropathies, we can observe absence of one of the isoforms of laminin, laminin-2, causing discontinuation of basal lamina, demyelization and reduction of stimuli conduction. In the absence of laminin-2, Schwann cells become incapable of controlling these processes inside the axon, and the number of cells go down and proliferation gets slower ^21,23.

Laminins in extracellular matrix binds to collagen IV, perlecan and entactins and cell surface receptors, especially integrins (Figure 1-C1). They are formed by two subunits γ (18) and γ (8), which work as heterodimeric transmembrane receptors. Subunits combine among each other, giving approximately 24 variations described to present ^21,24.

Integrins work to maintain the morphological contacts between the extracellular matrix and the cytoskeleton to preserve the cell architecture and the balance of the internal medium through multiple signaling functions ^21,25,26. They are located in sites named "areas of focal adhesion", where the intracytoplasm portion maintains contact with the cytoskeleton, providing a communication channel between the intracellular and the extracellular medium. They activate signaling paths that impact the cell behavior ²⁷. Extracellularly, they act primarily as receptors of collagen, laminin and fibronectin whose ligants may bind to different types of integrins. At the same time, different binders activate specific signalization paths to the intracellular medium ^26,28-30.

Group γ1 comprises 12 integrins, each one of them with different affinities for binding to components of extracellular matrix ^22,24,31. In the nerve, unit γ1 of integrins can form dimers with many γ units, forming receptors to the many components of the extracellular matrix, including laminin present on the basal lamina ^22,24.

Studies analyzing the expression of integrins in experimental autoimmune neuritis in rats and in Guillain – Barré syndrome in humans evidenced failures in the expression of many integrins ³².

The abnormalities of their expressions, laminins and integrins, are of such importance that they can be used to characterize the progression and severity of cases of neuropathy in humans and in rats ^32-34.

Among the other molecules expressed in the peripheral nervous system, there is protein S-100 (Figure 1-C2), which comprises²¹ different isoforms. The first visualization of protein S-100 in sensorial corpuscles was demonstrated by Iwanaga³⁵ in 1982. Later, it was confirmed that isoforms S-100 a and S-100 β were the main forms present in the nervous tissue ³⁶. It is a highly acid water-soluble dimer with molecular weight of 21kDa, which shows strong affinity for calcium ³⁷37. It belongs to the family of modulating proteins expressed exclusively in vertebrates. It is present in glial cells, Schwann cells, melanocytes, Langherans cells, dermal dendrocytes, histiocytes, adipocytes, myoepithelial cells and some gland epithelial cells.

This protein participates in intra and extracellular regulating activities of multifunctional character. It inhibits phosphorilation and regulates the stability of the cytoskeleton by modifying its metabolism through enzymatic activities that include calcium homeostasis ³⁸.

As to extracellular functions, there are influences on inflammatory cells, neurons, astrocytes, glial cells, as well as endothelial and epithelial cells ³⁹. It has been shown in humans and guinea pigs that after compression or section of the nerve, expression of S 100 in corpuscles of Meissner and Paccini is irregular ^40,41,42. The expression of protein S-100 has been used in studies involving leprosy, especially to better diagnose the disease, in special in undetermined and tuberculoid forms with negative bacilloscopy.

Fleury⁴³ in 1987 detected abnormalities to protein S-100 in 8 cases (88.8%) out of 9 cases with non-specific histopathological granuloma without bacillus. Singh in 1994 ⁴⁴ studied granulomatous dermatoses and suggested the use of protein S-100 in diagnostic elucidation of leprosy.

Thomas in 199845 employed the method of protein S-100 also in granulomatous diseases, concluding that it was useful in the exclusion of leprosy precisely when nervous endings presented as normal.

Another important receptor of Schwann cell is dystroglycan (Figure 1-C3). It was originally isolated in skeletal muscles. It is subdivided into two polypeptides, a and β, in which the alpha part is related with extracellular environment and the beta part with the cytoskeleton (Figure 1-C4) of the cells ⁴⁶.

Dystroglycan is a receptor of laminin-2. It maintains the adhesion of extracellular matrix of basal lamina with cell membrane of Schwann cells. There are evidences of the participation of dystroglycan in the myelinization of Schwann cells. Mice with absence of this molecule present abnormalities in myelinization and defects in nervous conduction ⁴⁷. Its involvement is suggested in the pathogenesis of muscle dystrophies ⁴⁸. By correlating Mycobacterium leprae and laminin ², it is possible to assume that dystroglycan participates in the process of action of demyelinization and axonal depression of leprosy ⁴⁹ with similar action mechanism of integrins, especially β1.⁵⁰

INTERACTION OF MYCOBACTERIUM LEPRAE WITH PERIPHERAL NERVOUS SYSTEM

The affinity of Mycobacterium leprae by peripheral nerves is known since the first references by Danielssen and Boeck in 1848.⁵¹ Later, evidences were gathered towards having Schwann cells in the peripheral nerves as the target of the bacillus ⁵². Studies have demonstrated that the binding of Mycobacterium leprae to Schwann cells induces to demyelinization ⁵⁶.

Schwann cells from the peripheral nervous system present 2 phenotypes, myelinized and non-myelinized, which present different response to Mycobacterium leprae.⁵⁷ Even though it binds to both phenotypes of Schwann cells, the binds of non-myelinized cells are stronger. The cells in the second phenotype are the natural niche for the multiplication of bacteria, enabling the microorganism to be protected from immune responses of the host, providing an extremely favorable site for the proliferation and survival in the peripheral nervous system ⁵⁸. There are evidences that the binding between Mycobacterium leprae and Schwann cells take place at the extracellular matrix basal lamina ⁵⁹.

Therefore, in the case of leprosy, basal lamina does not represent a protective barrier against the entry of mycobacteria. To the contrary, it is through its structural components that occurs the invasion. There are "in vivo" and "in vitro" studies demonstrating that at molecular level the binding between Mycobacterium leprae and Schwann cells is made through laminin ⁶⁰.

In fact, the cell wall of Mycobacterium leprae has an external electron-transparent layer which is mainly formed by fitiocerol dimerocerosic acid and glycolipids, especially phenolic glycolipid 1 (PGL-1). It is specific of this mycobacterium and contains a specific trisaccharide not found on the wall of any other mycobacteria ^58,60. As shown, in studies with purified PGL-1, glycolipid is specifically binding to γ2 chain of laminin-2. In tissue culture, this binding is mediated by the specific trisaccharide that has been mentioned ^61,62.

There is still another protein of molecular mass 21 kDa, associated with the mycobacteria wall named Mycobacterium leprae laminin binding protein (ML-LBP21), capable of binding to laminin-2. Differently from PGL-1, it is similar to proteins found in mycobacteria which are unable to invade Schwann cells, and do not seem to have the same importance ⁶³.

CONCLUSION

The action mechanisms of Mycobacterium leprae are not sufficiently understood and totally accepted yet. The problem is its predilection for the peripheral nervous system, whereas other mycobacterium are morphologically similar and do not present this specific mechanism.

The integrity of peripheral nervous system fibers is maintained by adhesion or binding of basal lamina of Schwann cells. The aggression of Mycobacterium leprae on the extracellular matrix causes the disruption between the basal lamina and Schwann cells, causing abnormalities in physiological and neuronal functions, which are sometime irreversible.

The knowledge of molecular interactions between substances of bacterial cellular wall and the components of extracellular complex – Schwann cells is getting more comprehensive. To present, the key known structures involved in the pathogenesis, such as laminins, integrins, proteins S-100, dystroglycans and others, will certainly enable early diagnosis and provide new therapeutic options.

REFERENCES

Conflict of interest: None

Financial funding: None

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Publication Dates

• Publication in this collection
  07 Jan 2010
• Date of issue
  Oct 2009