Damaged nerves of the brain, eye, and spinal cord cannot grow back.
Ephraim Trakhtenberg and colleagues at the University of Connecticut reported that ribosomal protein genes that are found at low levels in adult nerve cells may be very important to axon regeneration.
Three sections of optic nerve were injured by crushing (the white diamond on the far left of each nerve marks the crush point.) The lower two nerves each express genes (Rpl7 or Rpl7a) newly identified by the Trakhtenberg lab as promoting nerve axon regeneration. The axons carry the bright green dye. The insets to the right show how much more axon regrowth is occurring in the nerves that express the regeneration genes, and how no regrowth happens in the normal control (top). Credit: Trakhtenberg Lab/UConn Health.
Axons are the long arms of nerve cells that reach from our extremities to our spinal cord, and from our eye to our brain. Injuries that smash or sever axons—and often the large bundles of axons that we commonly call nerves—can cause paralysis, blindness, lack of sexual function, or other devastating outcomes. Ribosomes are tiny factories in the cell responsible for making proteins.
Protein production is key to all forms of growth, whether it’s muscle growth after hitting the gym, or the development of a tiny embryo into a fully grown individual. As embryonic nerve cells finish growing and mature, the demand for these ribosomes diminishes, and as a result the ribosomal genes become nearly inactive.
The researchers looked at two ribosomal protein genes, Rpl7 and Rpl7a, which had previously been found to be associated with neurodegenerative diseases. In mice, the researchers altered nerve cells so that instead of these genes staying quiet they would be robustly expressed in the adult cells. The researchers then crushed bundles of axons from these cells, modeling damage to the optic nerve (the nerve that connects the eye to the brain). Damaged axons of the optic nerves never grow back in adults, causing blindness. But the neurons that had been altered to keep producing Rpl7 and Rpl7a began growing axons again after injury.
The results suggest that the ~80 ribosomal proteins found at low levels in adult nerve cells may be very important to axon regeneration.