By Joe Bonner
In today's issue of Science, scientists at Rockefeller University and New York University School of Medicine report that the nerve damage that leads to a loss of sensation and disability of people with leprosy occurs in the early stages of infection.
The nerve damage, a hallmark of leprosy previously thought to be a byproduct of the immune system's response to the leprosy bacteria, now seems to be a direct result of the leprosy bug attaching itself to specialized nerve cells called Schwann cells, the glial, or supporting, cells of the peripheral nervous system (PNS).
The findings suggest that the body's immune response does not play a significant role in the early stage of neurological injury.
The damage is characterized by the disruption of the myelin sheath, the insulation on nerve cell connections that helps transmit rapid signals between the brain and the peripheral organs, for example, skin and muscles. Damage to myelin causes loss of sensation, disability and paralysis.
Using laboratory cell tissue cultures and mice genetically manipulated to lack two key immune system cells, the research team, led by Rockefeller University microbiologist/cell biologist Anura Rambukkana, Ph.D., showed that Mycobacterium leprae (M. leprae), the bacterium that causes leprosy, destroys the protective myelin sheath that surrounds nerve fibers and then hides out in the supporting cells that enclose non-myelin nerve fibers, poised to initiate later attacks.
"What we show here is a novel mechanism of inducing demyelination by a bacterial pathogen," says Rambukkana. "This may unravel clues for early molecular events of neurodegeneration processes in other diseases, such as multiple sclerosis, which we currently know little about."
"By using this bacteria we will be able to obtain novel insight not only into the mechanism of the early demyelination process, but also how the complex molecular architecture of the demyelinated fiber is disrupted," adds Rambukkana.
Using a "co-culture" system developed by co-author James Salzer, M.D., Ph.D., at the NYU School of Medicine, in which myelinated nerves form normally in cell culture, the researchers found that M. leprae produced significant damage to the myelin sheaths 24 hours after attaching to the nerves.
Myelin damage is the earliest effect observed, followed by the degeneration of nerve axon that carries the nerve impulses. M. leprae does not harm any other parts of the cell, and causes no signs of cell death.
The researchers found that, unexpectedly, M. leprae does not need to enter the cell to cause degeneration of the myelin sheath.
"This suggests that binding of M. leprae to the surface of the myelin sheath is sufficient to induce myelin breakdown, presumably by activating signals inside the cell," says Salzer. "Such signals could also be activated in other diseases that cause demyelination."
The researchers also showed that M. leprae does not need to be alive to demyelinate nerve cells. Similar results were obtained after cultured Schwann cells were exposed to bacteria that had been killed with radiation and to fractions of the bacterium's cell wall.
Previous research by Rambukkana, Salzer and their colleagues implicated a major component of the bacterium's cell wall called PGL-1 in its predilection for peripheral nerves see this URL), and they now propose that PGL-1 is a crucial cell wall molecule directly involved in nerve damage in leprosy.
The mouse model the researchers studied, called a "Rag-1 knockout," is genetically altered to lack mature B and T cells (B cells are responsible for producing the infection-fighting proteins called antibodies, while T cells help regulate the body's immune response).
As in the cell culture model, direct administration of both M. leprae and its cell wall to the peripheral nerves of Rag-1 mice caused significant myelin damage, providing clues for early nerve injury in patients before the immune system comes into play.
Schwann cells also play a significant secondary role in providing support for the various growth factors during the development of the spinal motor neurons and sensory neurons. Thus, Schwann cells play a wider role not only in the normal development of the nerve cells in the PNS, but also in their regeneration.
Nerve cells in the PNS are able to regenerate after injury, unlike their counterparts in the central nervous system (CNS). Examination of myelinated and non-myelinated nerve cells infected with M. leprae revealed an interesting survival strategy of this bacterium.
As the damaged myelinated nerve cells repair themselves by generating new nonmyelinating Schwann cells after attack, M. leprae sequesters itself in these nonmyelinated nerve cells, waiting for a chance to attack again once they multiply and escape from these cells.
Rambukkana hypothesizes that this phenomenon accounts for the lapsing/remitting characteristic of leprosy.
M. leprae is known to cause debilitating neurological injury in humans, but the clinical manifestation occurs years after a slow infectious process. According to Rambukkana, information about the pre-clinical mechanisms in M. leprae-induced demyelination may allow researchers to develop therapeutics and common diagnostic tests for early detection of demylenating diseases of both infectious origin and unknown etiology, such as multiple sclerosis and Guillain-Barré syndrome.
The PNS consists of all the nerves that fan out from the CNS and innervate the muscles, skin and internal organs. The myelin sheath enables the axon to greatly improve the reliability and speed of the electric impulse, much like insulation on electrical wires. When myelin is damaged, the nerve fibers are no longer insulated and nerve impulses cannot be conducted efficiently.
Therefore, the knowledge gained by such M. leprae-induced myelin damage in the early infectious process provides valuable insights into the pathologic mechanisms of early neurodegenerative diseases in general.
Rambukkana's and Salzer's co-authors are George Zanazzi, B.S., at NYU and Nikos Tapinos, M.D., Ph.D., at Rockefeller.
This research was funded in part by the National Institute of Allergy and Infectious Diseases (NIAID) and the National Institute of Neurological Diseases and Stroke (NINDS), both part of the federal government's National Institutes of Health, and the UNDP/World Bank/WHO Special Program for Research in Tropical Diseases.