This SciShow video is supported by Brilliant!

As a SciShow viewer, you can  keep building your STEM skills for 20% off an annual premium  subscription at Brilliant.org/SciShow. [♪ INTRO] So when you hear the word  “leprosy”, your brain may or may not conjure up an accurate  picture of Hansen’s disease: a bacterial infection that causes  skin lesions and nerve damage. Though you might correctly recall the enormous social stigma that persists  against people who are infected.

Either way, your mind probably  doesn’t come up with any advantages. Like why would it? Well, according to some  scientists, studying the bacteria that cause Hansen’s disease could provide  incredible benefits to liver research… all thanks to one funky little mammal.

Hansen’s disease is an infectious  disease caused by a bacterium called Mycobacterium leprae, or ML for short. In humans, these bacteria infect Schwann  cells, which are cells that protect the peripheral nervous system; that is, the  parts outside your brain and spinal cord. Schwann cells provide insulation  for the long axons of nerve cells.

Some of them form the myelin  sheath, which helps to transmit electrical impulses that  carry messages between nerve cells. When nerve cells are damaged  from an injury or infection, Schwann cells receive signals to  revert to a relatively immature state. That reversion is called de-differentiation.

As cells mature, they tend to split off into more specialized states; they differentiate. When that specialization gets put in  reverse, that’s de-differentiation. The cells become less mature and less specialized.

Under typical circumstances, these  de-differentiated Schwann cells help axons regenerate, and that allows the peripheral nervous system to  recover from illness or injury. And at the end of the nerve repair process, the Schwann cells grow back up  into their differentiated states. And when a person contracts Hansen’s disease, the ML bacteria use this system to their benefit.

ML bacteria reprogram Schwann  cells not just into an immature, de-differentiated state, but even further  into something resembling a stem cell. Stem cells can differentiate into  lots of different types of cells, but that’s sort of all that they do. A Schwann cell that’s de-differentiated this far isn’t good at being a Schwann cell any more.

The de-differentiation process in  cells infected with ML bacteria causes damage to axons by  breaking down the myelin sheath. Cells without the physical barrier of  myelin are much easier for the bacteria to invade, so this helps the  bacteria evade the immune system. The result is chaos for those nerves  that needed that regenerative system.

Without nerves that have the  ability to heal themselves, nerve damage in people with Hansen’s  disease can lead to loss of feeling or sensation, muscle weakness,  and sometimes even blindness. Strangely enough, however, there is a  scenario where ML infection has the nearly the opposite effect, promoting regeneration  in a very different kind of tissue. Liver tissue.

The liver tissue… of nine-banded armadillos. ML infects not just humans,  but also some armadillos, native to North and South America. A 2022 study from researchers  at the University of Edinburgh has shown that nine-banded armadillos  infected with the ML bacteria grow bigger livers that are healthy  and functional in all respects.

And it’s kind of similar to what the  bacteria are doing in human nerve cells: borrowing the host’s regenerative  processes for their own ends. In armadillos that carry ML bacteria, the  bacteria increase the expression of genes that code for proteins that are  typically associated with growth, wound-healing, and increased  metabolic responses in the liver. Even more fascinating, these are very  similar to genes and proteins that are expressed in human liver cells during  fetal development and adult growth.

This de-differentiation that ML causes in  armadillo livers is temporary, allowing the cells to proliferate and then re-differentiate  into healthy specialized liver cells. This is less damaging than the  de-differentiation that ML causes in human nerve cells, where their  reversion to a stem-cell like state trashes their ability to heal themselves  since they cannot re-differentiate. Which shows how very similar  effects at the cellular level can lead to wildly different effects in  different tissues and different organisms.

Now, tissue regeneration is an active  area of study for a couple of reasons. In humans, we want to keep  organs healthy as people age, and we want to understand  how to heal damaged tissue without cells growing out of  control and becoming cancerous. Scientists can revert mature cells to a  more stem cell-like state in rodent models.

But… those cells tend to become cancerous. Which makes that method no  good for healing damaged organs in humans without some serious further study. Well, guess what armadillos infected  with ML bacteria DON’T have?

They don’t have liver cancer! The researchers did not find any masses, tumors, or other tissue abnormalities in  any of the infected armadillos. And the ML bacteria’s little reprogramming act has little effect on genes that  encourage tumor growth.

This reprogramming is likely  beneficial to the bacteria themselves, because by causing cell proliferation  and increased liver size, the bacteria get a nicer,  more nutrient-packed habitat. The bacteria also protect themselves  by keeping the host healthy, so it’s in their interest to not cause  any real harm to the host’s liver. And this isn’t just, like, ok, well  very lucky for armadillos I guess, because the researchers in the study  think this makes armadillos a great way to study how liver tissue regenerates  and apply that knowledge to humans.

Liver disease causes millions of  deaths around the world every year. The human liver is typically  able to heal itself when needed. But that can be impaired in cases  like liver failure or cirrhosis.

The researchers are hopeful that using  armadillos as a model of liver regeneration could help us develop tools to  heal damaged livers in humans, or maybe even grow brand new livers  to order when a transplant is needed. Which is a pretty unexpected upside  to one of the most hated diseases in human history interacting with  one of my favorite little animals. I’d like to think that it could  even help break down some stigma against people living with Hansen’s.

Sometimes, nature creates silver  linings in the oddest of places. Researchers can find solutions in strange places when they look at problems from new angles. Sometimes it’s a matter of understanding  the variables you’re working with.

Sometimes you need to work  backwards to find a solution. And those are all skills that you can learn from fundamental pre-algebra courses at Brilliant.org. Brilliant is an online learning platform with courses in science, computer science, and math.

With Brilliant’s help, you  can learn stuff like algebra through interactive diagrams and puzzles. Brilliant supports your  growth and learning through over 60 Brilliant courses, and  by supporting this SciShow video! Their course on Solving  Equations has 21 lessons that help you understand both sides of an equation.

And we could all use practice with that! To get started with 20% off an annual  premium Brilliant subscription, you can click the link in  the description down below or go to Brilliant.org/SciShow.  Thanks for watching! [♪ OUTRO]