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NASHVILLE, Tenn. ñ A new genetic model for a motor disorder that
confines an estimated 10,000 people in the United States to walkers and
wheelchairs indicates that instability in the microscopic scaffolding
within a key set of nerve cells is the cause of this devastating
disability.
The study, which is published in the July 13 issue of the journal
Current Biology, provides a provocative new insight into the molecular
basis of the disease called hereditary spastic paraplegia (HSP) and
suggests a new way to treat the inherited genetic disorder.
HSPóalso known as familial spastic paraparesis and Strumpell-Lorrain
syndromeócauses the ends of the nerves that control muscle activity to
deteriorate. These nerve cells run from the brain’s cerebral cortex to
the spinal cord where they connect to "downstream" nerve cells that
excite muscles throughout the body to control coordinated movement. HSP
causes weakness, spasms and loss of function in the muscles in the
lower extremities.
More than 20 genes have been linked to HSP. However, more than 40
percent of all cases have been traced to a single gene (SPG4) that
produces an enzyme called spastin. Previous studies have shown that
this enzyme interacts with microtubules, the tiny protein tubes that
provide structural support and transport avenues within most cells.
Microtubules are dynamic structures, continually growing and shrinking,
and their stability is closely regulated by a number of associated
proteins. In nerve cells, microtubules carry cellular components to
distant regions of the cell, regulate the growth of cellular branches
and provide a substrate for important protein interactions. All of
these functions are critically dependent on dynamic changes in
microtubule stability.
Researchers in the laboratories of Kendal Broadie at Vanderbilt
University and Andrea Daga at the University of Padova, Italy,
collaborated on the first studies of the role that spastin plays in
nerve communication in a living organism. They have discovered that
spastin is highly enriched in nerve cells, especially in the
synapsesóthe junctions between pairs of nerve cells and between nerve
and muscle cells. Using advanced genetic engineering methods to
manipulate gene dosage, their study showed that either too much or too
little of the enzyme disrupts microtubule stability, degrading the
nerve cells’ ability to function. They also found that pharmacological
treatments that counteracted these destabilizing effects restored
normal nerve function.
"These findings suggest that HSP is caused primarily through impaired
regulation of the nerve cell cytoskeleton," says Broadie, professor of
biological sciences and pharmacology and an investigator at the Kennedy
Center for Research on Human Development. "Spastin, as well as other
HSP-linked genes, appears to mediate this common function. This
suggests that treatment of this common defect may be effective for
many, perhaps most, HSP patients."
The organism that the researchers used in the study is the fruit fly,
Drosophila melanogaster, the "lab rat" of genetics research.
"Drosophila is the perfect organism to study complex genetic diseases
like HSP," says Vanderbilt graduate student Nick Trotta, who is the
first author on the paper. "All we need to know is what gene is
involved and we can change the way the gene is expressed within a few
weeks. That lets us replicate the conditions associated with the
disease so we can learn more about how it works."
At the molecular level, there is very little difference between a Drosophila neuron and a human neuron, Trotta says.
For example, fruit flies have a protein called D-Spastin that performs
the same functions as human spastin. The two enzymes are 48 percent
identical and 60 percent similar at the amino acid level.
"It may seem surprising that a fly can be used to uncover the molecular
basis of human disease, but it has been shown over and over again in
the last several decades that insights gained from Drosophila studies
provide immediate insight into the human condition. There is a long
history of Drosophila research pioneering major breakthroughs in our
understanding of fundamental biological problems," says Broadie.
To study how spastin functions in living neurons, the researchers
designed transgenic flies with altered amounts of D-Spastin protein in
their neurons and analyzed the effects.
Animals with lower levels of D-Spastin were weak, exhibited
coordination defects and died prematurely. At the cellular level, the
researchers found that the reduction of D-Spastin caused synapses to
fill with ultra-stable microtubules. As a consequence, synapses were
structurally reduced in size and showed profoundly altered
communication properties.
Flies with heightened levels of D-Spastin were also sick. High levels
of over-expression caused embryonic death. Lower levels caused nerve
cell death (neurodegeneration.) Even very low levels of over-expression
caused adults to be slow and weak. Within the nerve cells, the
researchers found that too much D-Spastin caused stable microtubules to
disappear altogether. At the synapse, elevated levels of spastin also
caused a large drop in synaptic signal strength, impairing nerve
communication.
These studies showed that spastin concentrates in synapses, where it
acts locally to destabilize microtubules. Moreover, the researchers
found that specific drugs that alter microtubule stability remedy the
defects that occur in synaptic function as a result of changes in
spastin levels. In both loss and over-expression cases, the researchers
found that treatment with drugs that correct microtubule stability
defects caused the synaptic signal strength to rebound to normal
levels. This result suggests that drugs that stabilize microtubule
activity might provide a new approach for treating HSP.
The Vanderbilt researchers were funded by the National Institutes of
Health and the Italian researchers by the Telethon Foundation in Rome.
Additional authors of the paper are Genny Orso and Maria Giovanna
Rossetto from the University of Padova.
For more news about Vanderbilt research, visit Exploration,
Vanderbilt’s online research magazine, at www.exploration.vanderbilt.edu.
Media contact: David F. Salisbury, (615) 343-6803
david.salisbury@vanderbilt.edu