Nerve Cell Navigation Decoded
Newly launched nerve cells in a growing embryo must chart their course to distant destinations, and many of the means they use to navigate have yet to surface. In a study published in the current issue of the journal Neuron, scientists at the Salk Institute for Biological Studies have recovered a key signal that guides motor neurons — the nascent cells that extend from the spinal cord and must find their way down the length of limbs such as arms, wings and legs.
Motor nerves growing toward their muscle targets are detected with fluorescent protein in transgenic mouse embryos. A normal (wild type) embryo is shown in green and different mutants isolated in a genetic screen are shown in yellow (Columbus), red (Magellan), and blue (DeLeon). A variety of motor neuron growth and development defects are apparent in the mutants. (Credit: Joe Lewcock)
The Salk study, led by Samuel Pfaff, Ph.D, a professor in the Gene Expression Laboratory, identifies a mutation they christened Magellan, after the Portuguese mariner whose ship Victoria was first to circumnavigate the globe. The Magellan mutation occurs in a gene that normally pilots motor neurons on the correct course employing a newly discovered mechanism, their results demonstrate.
In the mutants, growing neurons can be seen leaving the spinal cord normally but then appear to lose direction. The elongating cells develop “kinks” and sometimes fold back on themselves or become entwined in a spiral, forming coils outside the spinal cord. “They appear to become lost in a traffic roundabout,” described Pfaff, who observed the growing neurons with fluorescent technology.
Understanding how motor neurons reach the appropriate targets is necessary for the implementation of novel therapies, including embryonic stem cell replacement for the treatment of presently incurable disorders such as Lou Gehrig’s disease, in which motor neurons undergo irreversible decay.
“Embryonic studies provide useful insights on how to replicate the system in an adult,” said Pfaff. And, as he also pointed out, the mechanisms used by motor neurons are likely to be similar to those used in other parts of the central nervous system, such as the brain. The Magellan mutation discovered by Pfaff’s group was found in mice, but the affected gene, called Phr1, has also been identified in other model systems, including fruit flies and the worm species C. elegans.
A growing nerve bears at its bow a structure called the growth cone, a region rich in the receptor molecules whose job is to receive cues from the environment, much as ancient mariners who observed the stars and set their course accordingly. During development, the growth cone continuously pushes forward, while the lengthening neuron behind it matures into the part of the cell called the axon. Once the growing cell “lands” at its target in a muscle cell, it is the axon that will relay the messages that allow an animal to control and move its limbs at will.
In Magellan mutants, Pfaff’s team discovered that the growth cone becomes disordered. Rather than forming a distinct “cap” on the developing neuron, the cone is dispersed in pieces along both the forward end and the axon extending behind it.
“The defect is found in the structure of the neuron itself,” said Pfaff, noting that the fundamental pieces, such as the receptors capable of reading cues, all seem to be present. Without the correct orientation of receptors, however, signals cannot be read accurately, resulting in growth going off course.
“A precise gradient normally exists across the cone,” said Pfaff, “which is disrupted in the Magellan mutants.” As a result, cells lose their polarity. They literally do not know the front end from the back end, according to Pfaff. This sense of polarity is a universal feature common to all growing neurons. Therefore, “Phr1 is likely to play a role in most growing neurons to ensure their structure is retained at the same time they are growing larger,” he said.
Pfaff and his group identified Magellan using a novel system they had developed, in which individual motor neurons and axons can be visualized fluorescently. They were able to screen more than a quarter of a million mutations, and the mutations of interest were rapidly mapped to known genes as a result of the availability of the sequenced mouse genome — a byproduct of the effort to sequence entire genomes such as that in the human.
The Magellan mutation is located in a gene known as Phr1, which is also active in other parts of the nervous system, indicating that it most likely functions to steer other types of neurons, such as those that enervate sensory organs or connect different regions of the brain. Studies of Magellan may therefore shed light on how a variety of neurological disorders might be treated with cell replacement strategies.
Lead author on the study is Joseph W. Lewcock, formerly a postdoctoral fellow in Pfaff’s laboratory and currently at Genentech, Inc. Additional Salk authors include postdoctoral fellow Nicolas Genoud and senior research assistant Karen Lettieri.
Neuron. 2007 Nov 22;56(4):604-20.
The Ubiquitin Ligase Phr1 Regulates Axon Outgrowth through Modulation of Microtubule Dynamics.
Gene Expression Laboratory, The Salk Institute for Biological Studies, La Jolla, CA 92037, USA.
To discover new genes involved in axon navigation, we conducted a forward genetic screen for recessive alleles affecting motor neuron pathfinding in GFP reporter mice mutagenized with ENU. In Magellan mutant embryos, motor axons were error prone and wandered inefficiently at choice points within embryos, but paradoxically responded to guidance cues with normal sensitivity in vitro. We mapped the Magellan mutation to the Phr1 gene encoding a large multidomain E3 ubiquitin ligase. Phr1 is associated with the microtubule cytoskeleton within neurons and selectively localizes to axons but is excluded from growth cones. Motor and sensory neurons from Magellan mutants display abnormal morphologies due to a breakdown in the polarized distribution of components that segregate between axons and growth cones. The Magellan phenotype can be reversed by stabilizing microtubules with taxol or inhibiting p38MAPK activity. Thus, efficacious pathfinding requires Phr1 activity for coordinating the cytoskeletal organization that distinguishes axons from growth cones.