Neuronal axons succumb, disintegrating due to absence of crucial external protein; synaptic links deteriorate as a result.
Flexing the Neuron's Framework: Perlecan's Role in Axonal Resilience
The axon, a long extension of a neuron, journeys far from its cell body to connect with other neurons and muscles. A study recently conducted by researchers at MIT's Picower Institute for Learning and Memory sheds light on the importance of a surrounding protein called perlecan in ensuring the structural integrity of this delicate projection. It reveals that perlecan maintains the extracellular matrix, a flexible and supportive environment, hindering the axon from fracturing during development.
In the absence of perlecan in Drosophila fruit flies, the study found that axonal segments can divide and the synapses that they form begin to wither away. Perlecan facilitates the formation of a stable and flexible extracellular matrix, assisting cells in developing and functioning in a supportive yet pliable environment.
"We discovered that the extracellular matrix surrounding nerves was being altered, ultimately leading the nerves to snap completely," says study senior author Troy Littleton, the Menicon Professor in MIT's departments of Biology and Brain and Cognitive Sciences.
Perlecan is essential for human survival postnatally, and mutations that decrease perlecan can lead to Schwartz-Jampel syndrome, characterized by neuromuscular problems and skeletal distortions. This research may help explain how neurons are affected in the condition, Littleton explains, while also expanding our understanding of the extracellular matrix's role in supporting axon and neural circuit development.
Ellen Guss, who recently defended her doctoral thesis on the work, led the research published June 8 in eLife.
At first, the team didn't anticipate their study to provide new insights into axon resilience during development. Instead, they aimed to investigate whether perlecan might help arrange some protein components in synapses that fruit fly nerves use to connect with muscles. However, after knocking out the gene responsible for perlecan encoding in flies, they observed that neurons appeared to "shrink back" many synapses during a late stage of larval development. Proteins on the muscle side of the synaptic connection remained, but the neuron side withered away, suggesting a larger role for perlecan than initially thought.
The researchers discovered that perlecan wasn't particularly concentrated around synapses but rather in a structure called the neural lamella, surrounding axon bundles and safeguarding the structure's integrity. This finding indicated that perlecan's absence might not be an issue at the synapse but instead causes problems along axons due to its absence in the extracellular matrix surrounding nerve bundles.
The team developed a daily imaging technique for observing fly neural development, allowing them to watch axons and synapses evolve over a four-day span. They observed that initially, axons and synapses developed normally, but eventually, not only synapses but also entire segments of axons started to fade away.
They also saw that axon segments located further from the fly's brain were more likely to break apart, indicating that these segments were becoming increasingly vulnerable as they extended. When they examined individual segments, they determined that locations where axons were breaking down were followed by synapse loss, suggesting that axon breakage caused the synapse retraction.
"Breakages were happening in a segment-wide manner," Littleton says. "In some segments, the nerves would snap, and in others, they wouldn't. Whenever there was a breakage event, you would see all the neuromuscular junctions (synapses) across all muscles in that segment retract."
Comparing the structure of the lamella in mutant versus healthy flies, they found that the lamella was thinner and defective in the mutants. Moreover, where the lamella was weakened, axons were prone to break, and microtubule structures that run the length of the axon began to become misaligned, forming tangled bundles at sites of severed axons.
In another key finding, the team demonstrated that perlecan's essential role depended on its secretion from multiple cells, rather than just neurons. Blocking the protein in a single cell type or another did not cause the same problems produced by total knockdown, and enhancing secretion from just neurons was not enough to offset its deficiency from other sources.
Overall, the evidence points to a scenario where the absence of perlecan secretion results in a thin and defective neural lamella, with the extracellular matrix becoming too rigid. As the animal moves, the further axon bundles extend from the brain, the more likely they are to break as movement stresses impinge upon the weakened regions. The misalignment of microtubules within the axons then leads to synapse loss as the cells can no longer sustain the synapses.
"When you don't have the flexibility, although the extracellular matrix is still present, it becomes very rigid and tight, leading to breakage as the animal moves and pulls on those nerves over time," Littleton explains. "This suggests that the extracellular matrix is functional early on but lacks the appropriate properties to sustain some key functions over time as the animal begins to move and navigate around. The loss of flexibility becomes crucial."
In addition to Littleton and Guss, other paper authors include Yulia Akbergenova and Karen Cunningham.
The study was supported by the National Institutes of Health, as well as The Picower Institute for Learning and Memory and The JPB Foundation.
- The study conducted by researchers at MIT's Picower Institute for Learning and Memory, published in eLife, focuses on the protein perlecan's role in maintaining the health of axons, suggesting its influence on learning and genetics.
- Perlecan ensures the structural integrity of axons, aiding in their resilience during development, by maintaining the extracellular matrix, a flexible and supportive environment, preventing axons from fracturing.
- In the absence of perlecan, as observed in Drosophila fruit flies, axonal segments divide and the synapses they form begin to wither away, indicating a key role in health-and-wellness and neurobiology.
- Mutations that decrease perlecan can lead to Schwartz-Jampel syndrome, a condition characterized by neuromuscular problems and skeletal distortions, demonstrating its importance in human health research.
- The research expands our understanding of the extracellular matrix's role in supporting axon and neural circuit development, highlighting its significance in ventures concerning science, health, and biology.
- The findings may help explain how neurons are affected in Schwartz-Jampel syndrome and contributes to ongoing research in the field of science, health-and-wellness, and health ventures.
