June 14, 2004 — A University of Utah study indicates that genes involved in embryo development must work at both ends of a nerve before the nerve is able to link the brain to each body part it controls.
The study found that when a development gene named Hoxb1 worked in a mouse’s brain but not in the developing facial tissues, nerves extended outward from the brain but failed to reach the facial muscles, leaving the mouse unable to blink its eyes, flatten its ears or make other facial movements.
“The question is how the nerves form a circuit from the brain to the target tissue and back to the brain” as an embryo develops, says Mario Capecchi, professor and co-chair of human genetics at the University of Utah School of Medicine and an investigator for the Howard Hughes Medical Institute.
The new study “is the first time it has been shown that the same genes involved in making the nerves that go from the brain to the face also are found in the cells making the tissues of the face,” Capecchi says. “That might be a way nerves know where to go and then recognize the target tissue” within facial muscles.
Because mice are so similar genetically to other mammals, the findings in mice likely hold true in humans, he adds.
Benjamin Arenkiel, a human genetics doctoral student who conducted the research under Capecchi’s supervision, says the study shows “Hoxb1 is involved in generating the nerves that go to the face as well as the facial tissues themselves. Hoxb1 helps the brain get wired to and control the muscles of the face.”
Arenkiel adds: “If this gene controls the nerves and the targets – facial tissues – this simple genetic code also might work to wire up the body from head to foot. … Other Hox genes may act along the length of the body to control nerve circuits at each particular level of the body.”
Capecchi agrees: “You are using the same gene outside and inside the brain to allow nerves to make the appropriate circuits. This may be true not only for nerves in the face, but for other Hox genes that help form nerves that control the body from head to foot.”
The study is being published in the June 15 online edition of the journal Genes & Development and in the journal’s July 1 printed issue. Capecchi and Arenkiel co-authored the scientific paper with Gary Gaufo and Petr Tvrdik [Petr Tvrdik is correct], both of whom are postdoctoral fellows in human genetics at the University of Utah
Wiring the brain to the face
The study is the latest in a series in which Capecchi and his colleagues have examined the workings of homeobox or Hox genes, which act like conductors to orchestrate the operation of other genes, turning those genes on and off at the appropriate times as an embryo develops.
All mammals have 13 groups of Hox genes with two to four genes per group, for a total of 39 Hox genes. These genes help ensure various parts of the body form in the correct place during embryo development.
Researchers already knew that Hoxb1 was “expressed” or active – meaning it produces a protein to carry out its genetic instructions – within the embryonic brain. In 1996, Capecchi and British scientists showed that the Hoxb1 gene helped promote development of nerve fibers that eventually extend to facial muscles and control them. Those muscles allow mice to wiggle their whiskers, blink their eyes and pull back their ears. The same muscles in humans allow people to smile, frown, cry, pucker their lips and make other facial expressions.
In the new study, Arenkiel, Capecchi and colleagues showed that for the seventh cranial nerves (one on each side of the face) to develop properly, Hoxb1 must be active or “expressed” at both ends of the brain-to-face circuit.
When the researchers disabled the Hoxb1 gene in the face but not in the brain of a developing mouse embryo, the nerves extending from the brain did not reach the facial muscles, but instead withered and died, leaving the facial muscles paralyzed. When harmless puffs of air were blown in the mutant mouse’s face, it was unable to close its eyes and scrunch up its face like a normal mouse.
That demonstrated that Hoxb1 must be active at both ends of a developing nerve circuit, somewhat akin to wiring in a home working properly only if an electrician hooks it up to both the power supply and to outlets, lights and appliances.
Some human children suffer a disease similar to the mutant mice: a rare birth defect named Mobius syndrome, in which the sixth and seventh cranial nerves are missing or underdeveloped, leaving facial muscles paralyzed.
“These kids can’t open and close their eyes, they can’t form tears, they don’t have facial expressions so they can’t smile, they can’t frown,” Capecchi says. “The problem is similar in that the nerves don’t innervate facial muscles.”
Arenkiel says it is not yet known if a mutation of the Hoxb1 gene plays a role in Mobius syndrome, but the mutant gene and the syndrome cause nearly identical symptoms.
Where the Hoxb1 gene works in the face
The researchers showed that in facial tissues, Hoxb1 was active not within muscle cells, but within glia, which are cells that form an insulating sheath around nerves – like coating on an electrical wire – that innervate the facial muscles.
“That was a surprise because we thought it would be in the muscles themselves,” Arenkiel says. “Through this work, we think glia not only play a passive role in protecting a circuit, but also may act in establishing the circuit” by guiding the developing nerve into the facial muscle as both develop together.
Because Hoxb1 turns other genes on and off, it may guide developing nerves to facial muscles by acting on genes such as Ephs and Ephrins, which already were known to emit chemical signals that help guide nerves to the tissues they control, says Arenkiel.
“Ben has shown what’s happening outside the brain is also important for how those nerves behave once they emerge from the brain,” Capecchi says. “The important part of this paper is the same thing could be happening along the whole body.”
Capecchi is well-known for developing a method known as gene targeting that allows the breeding of “knockout mice,” which are mice in which a gene has been disabled or knocked out so scientists can see what goes wrong without the gene and thus learn the gene’s normal function. In the new study, however, a different method had to be used so the Hoxb1 gene was disabled only in developing facial tissues, not in the brain.
Capecchi’s role in gene targeting has resulted in numerous awards, including the National Medal of Science, the Albert Lasker Award for Basic Medical Research, the Wolf Prize in Medicine, the Kyoto Prize and many others.