More than two decades ago, Samuel Pfaff, PhD, got his first taste of developmental biology when he watched a one-cell frog embryo become a swimming tadpole overnight. Since then, the molecular biologist, now a professor in the Gene Expression Laboratory at the Salk Institute in La Jolla, CA, has been traveling the developmental road of neurons to figure out how to repopulate damaged cells on the heels of spinal cord injury and amyotrophic lateral sclerosis (ALS). He is trying to recapitulate development in a lab dish. His hope is to generate the right kinds of cell types and figure out how to wire them up to repair the CNS. In his studies, Dr. Pfaff, who is also a Howard Hughes Medical Institute investigator, is targeting astrocytes.
“We have excellent ways to generate motor neurons and even better ways to develop astrocytes,” he told Neurology Today. “If you ask us today what we would do to successfully stop the death of motor neurons in ALS, this is it. We will target the bad guy in the system and see if we can overwhelm it with healthy astrocytes.”
Dr. Pfaff spoke to Neurology Today about his work, which is providing a new way to grow new neurons in the damaged adult brain.
HOW DOES UNDERSTANDING NEURONAL DEVELOPMENT HELP IN CONSIDERING NEW WAYS TO TREAT SEVERE CONDITIONS LIKE ALS?
Only in the last decade have scientists taken embryonic stem cells and used the information we know about normal development to capture in a lab dish what is going on inside the human body. We can begin to understand how the fate of a single neuron is regulated and how changing a step in the normal process can lead to a totally different fate. For instance, if embryonic stem cells are exposed to the right concentration of a protein called sonic hedgehog, the cells become motor neurons. Using this model might make it possible to create motor neurons to model diseases affecting these cells and uncover the pathways that control normal development and function of these cells.
WHAT ARE SOME OF THE HURDLES TO THIS APPROACH?
There are many hurdles to overcome. We need to determine how to make populations of motor neurons and deliver them to sites where they need to be and to make connections. When motor neurons are transplanted into the spinal cord, the environment is much different in the adult than in the embryo. When immature motor neurons are engrafted into an adult CNS, it is out of context, like dropping people into a strange city, where they lack clues for knowing where they are or what they should do. In the embryo, motor neurons grow an axon that seeks a muscle. In the adult, the motor neurons do not naturally regrow through a foreign environment. It may be necessary to use genes to enhance or alter the growth of characteristics of the cells.
WHAT IS POSSIBLE ONCE WE UNDERSTAND HOW TO DELIVER HEALTHY MOTOR NEURONS WHERE THEY BELONG?
We know that motor neurons are lost in ALS but we don't know how this happens. Does the problem originate within the motor neurons? In the SOD [superoxide dismutase] animal model of ALS, the evidence leads to defects in other cell types. Motor neurons are exposed normally to different neurotrophic factors. Glial cells and astrocytes maintain the motor neurons. When astrocytes carry the mutation in SOD, they secrete an unidentified substance that kills motor neurons. We can watch as they grow and introduce cells and see how it changes the structure and function of the cell.
HOW WILL IT WORK?
Once we have healthy astrocytes we will transplant them into the spinal cord of mutant SOD mice, especially cervical regions that regulate breathing. Ultimately, we need to target the entire system. The collaborative effort includes Don Cleveland, PhD, and his colleagues at the University of California-San Diego, who are testing antisense oligonucleotides that are taken up by cells to block the production of SOD. In the ALS models where mutant SOD is abnormally folded and alters cell function, blocking this process saves the motor neurons from damage and death.
We are continuing to use embryonic stem cells to model other motor neuron diseases including spinal muscular atrophy, a leading cause of childhood death. This recessively inherited neuromuscular disorder leads to degeneration of motor neurons in the spinal cord. We know the mutant gene is SMN1 [survival motor neuron 1] and we can culture the defective motor neurons carrying this mutation it in a dish. We are not far away from looking for compounds that can spare the loss of SMN1-deficient motor neuron.
WHAT EXCITES YOU ABOUT UNRAVELING THE PUZZLE OF ALS?
My colleagues and I believe that understanding the fetal development of the spinal cord will help in designing treatments for spinal muscular atrophy, ALS, spinal cord injury, and other conditions that target motor neurons. We are watching how motor neurons develop and branch out to make connections between the spinal cord and muscles in the body. These connections are critical to our every move. We are looking for genes that control every part of this developmental process – from the development of the neuron to how the axon navigates to the muscle and lays down its connections.
HOW FAR HAVE WE COME FROM THAT FATEFUL DAY WHEN YOU WATCHED A FERTILIZED FROG EMBRYO BECOME A SWIMMING TADPOLE?
I spent 24 hours in the lab watching this embryo become a swimming tadpole and wondering how the different cell types received a specific message to grow in a specific way. If we can understand these fundamental developmental principles that control cells of the spinal cord and how they are involved in movement we have a chance at reversing or halting the death of motor neurons in many conditions.