Morphological and functional characterizations of Schwann cells stimulated with Mycobacterium leprae

What happens when Schwann cells are exposed to Mycobacterium leprae - A systematic review

Mycobacterium leprae, the pathogen that causes human leprosy, has a unique affinity for infecting and persisting inside Schwann cells, the principal glia of the peripheral nervous system. Several studies have focused on this intricate host-pathogen interaction as an attempt to advance the current knowledge of the mechanisms governing nerve destruction and disease progression. However, during the chronic course of leprosy neuropathy, Schwann cells can respond to and internalize both live and dead M. leprae and bacilli-derived antigens, and this may result in divergent cellular pathobiological responses. This may also distinctly contribute to tissue degeneration, failure to repair, inflammatory reactions, and nerve fibrosis, hallmarks of the disease. Therefore, the present study systematically searched for published studies on M. leprae-Schwann cell interaction in vitro to summarize the findings and provide a focused discussion of Schwann cell dynamics following challenge with leprosy bacilli.

Phenolic glycolipid-1 of Mycobacterium leprae is involved in human Schwann cell line ST8814 neurotoxic phenotype

Leprosy is a chronic infectious disease caused by Mycobacterium leprae infection in Schwann cells. Axonopathy is considered a hallmark of leprosy neuropathy and is associated with the irreversible motor and sensory loss seen in infected patients. Although M. leprae is recognized to provoke Schwann cell dedifferentiation, the mechanisms involved in the contribution of this phenomenon to neural damage remain unclear. In the present work, we used live M. leprae to infect the immortalized human Schwann cell line ST8814. The neurotoxicity of infected Schwann cell-conditioned medium (SCCM) was then evaluated in a human neuroblastoma cell lineage and mouse neurons. ST8814 Schwann cells exposed to M. leprae affected neuronal viability by deviating glial ¹⁴ C-labeled lactate, important fuel of neuronal central metabolism, to de novo lipid synthesis. The phenolic glycolipid-1 (PGL-1) is a specific M. leprae cell wall antigen proposed to mediate bacterial-Schwann cell interaction. Therefore, we assessed the role of the PGL-1 on Schwann cell phenotype by using transgenic M. bovis (BCG)-expressing the M. leprae PGL-1. We observed that BCG-PGL-1 was able to induce a phenotype similar to M. leprae, unlike the wild-type BCG strain. We next demonstrated that this Schwann cell neurotoxic phenotype, induced by M. leprae PGL-1, occurs through the protein kinase B (Akt) pathway. Interestingly, the pharmacological inhibition of Akt by triciribine significantly reduced free fatty acid content in the SCCM from M. leprae- and BCG-PGL-1-infected Schwann cells and, hence, preventing neuronal death. Overall, these findings provide novel evidence that both M. leprae and PGL-1, induce a toxic Schwann cell phenotype, by modifying the host lipid metabolism, resulting in profound implications for neuronal loss. We consider this metabolic rewiring a new molecular mechanism to be the basis of leprosy neuropathy.

The formation of lipid droplets favors intracellular Mycobacterium leprae survival in SW-10, non-myelinating Schwann cells

Leprosy is a chronic infectious disease that is caused by the obligate intracellular pathogen Mycobacterium leprae (M.leprae), which is the leading cause of all non-traumatic peripheral neuropathies worldwide. Although both myelinating and non-myelinating Schwann cells are infected by M.leprae in patients with lepromatous leprosy, M.leprae preferentially invades the non-myelinating Schwann cells. However, the effect of M.leprae infection on non-myelinating Schwann cells has not been elucidated. Lipid droplets (LDs) are found in M.leprae-infected Schwann cells in the nerve biopsies of lepromatous leprosy patients. M.leprae-induced LD formation favors intracellular M.leprae survival in primary Schwann cells and in a myelinating Schwann cell line referred to as ST88-14. In the current study, we initially characterized SW-10 cells and investigated the effects of LDs on M.leprae-infected SW-10 cells, which are non-myelinating Schwann cells. SW-10 cells express S100, a marker for cells from the neural crest, and NGFR p75, a marker for immature or non-myelinating Schwann cells. SW-10 cells, however, do not express myelin basic protein (MBP), a marker for myelinating Schwann cells, and myelin protein zero (MPZ), a marker for precursor, immature, or myelinating Schwann cells, all of which suggests that SW-10 cells are non-myelinating Schwann cells. In addition, SW-10 cells have phagocytic activity and can be infected with M. leprae. Infection with M. leprae induces the formation of LDs. Furthermore, inhibiting the formation of M. leprae-induced LD enhances the maturation of phagosomes containing live M.leprae and decreases the ATP content in the M. leprae found in SW-10 cells. These facts suggest that LD formation by M. leprae favors intracellular M. leprae survival in SW-10 cells, which leads to the logical conclusion that M.leprae-infected SW-10 cells can be a new model for investigating the interaction of M.leprae with non-myelinating Schwann cells.

Leprosy is a chronic infectious disease that is caused by the obligate intracellular pathogen Mycobacterium leprae (M.leprae). Leprosy is the leading cause of all non-traumatic peripheral neuropathies worldwide. Both myelinating and non-myelinating Schwann cells are infected by M.leprae in lepromatous leprosy, but the non-myelinating Schwann cells show greater susceptibility to M.leprae invasion. However, the effect of M.leprae infection on non-myelinating Schwann cells has not been elucidated. Our results show that SW-10 cells are non-myelinating Schwann cells. Infection with M. leprae induces lipid droplet (LD) formation. Furthermore, inhibition of M. leprae-induced LD formation enhances the maturation of phagosomes containing live M.leprae and decreases the ATP content of M. leprae in SW-10 cells, suggesting that LD formation by M. leprae favors M. leprae survival in SW-10 cells. Based on these findings, it should be clear that M.leprae-infected SW-10 cells can serve as a new model for investigating the interaction of M.leprae with non-myelinating Schwann cells.

Transforming growth factor-β1 may be a key mediator of the fibrogenic properties of neural cells in leprosy

Fibrosis is the main cause of irreversible nerve damage in leprosy. Phenotypic changes in Mycobacterium leprae (ML)-infected Schwann cells (SCs) have been suggested to mediate this process. We found that SC line cultures stimulated with ML upregulated transforming growth factor-β1 (TGF-β1), and that TGF-β1 or ML induced increased numbers of α-smooth muscle actin (α-SMA)-positive cells with characteristic stress fibers. Mycobacterium leprae and TGF-β1 also induced increased type I collagen and fibronectin mRNA and secretion and augmented mRNA levels of SOX9 and ZEB1, which are involved in the epithelial-mesenchymal transition. These effects could be inhibited by the TGF-β1 type I receptor (ALK5) inhibitor, SB-431542. In nerve biopsies from leprosy-infected patients with varying grades of fibrosis (n = 11), type I and III collagen and fibronectin were found in the endoneurium and perineurium, α-SMA-positive cells filled the fibrotic perineurium but not the endoneurium, and CD34-positive fibroblasts predominated in the endoneurium. Results of transcriptional studies of 3 leprosy nerves and 5 controls were consistent with these data, but α-SMA and other mRNA levels were not different from those in the control samples. Our findings suggest that TGF-β1 may orchestrate events, including reprogramming of the SC phenotype, leading to transdifferentiation, connective tissue cell expansion, and fibrogenesis in the evolution of leprosy nerve lesions during some evolutionary stages.