By GRETA FRIAR
As a neuron develops, the axon grows from the central cell body like an arm with many fingers reaching out to explore the surroundings. These fingers, known as growth cones, travel long distances seeking out specific other neurons to form synapses, connective links that create the circuitry in the brain controlling sensation, movement, thinking, and behavior.
The axon and its extensions have typically been thought of like simple cables carrying information between cell bodies, with the cell bodies directing all growth and movement. In fact, axons have minds of their own—so to speak—and they operate more independently and with much less ongoing control from the cell body than has been thought, says Jeffrey Macklis, the Max and Anne Wien Professor of Life Sciences in the Department of Stem Cell and Regenerative Biology, and Center for Brain Science, at Harvard.
Macklis has received a $5.9 million Pioneer Award from the National Institutes of Health Office of the Director to study diverse and subtype-specific growth cones, the tips of developing axons, as they move, make “decisions” at critical junctures, and use signals and cells along their distant trajectories to seek other neurons to make synaptic connections in mice.
The Pioneer Award supports high risk, high reward exploratory science. Macklis’ ambitious proposal to study growth cones aims to redefine how we think about neurons, improve our understanding of neurological and psychiatric diseases, and eventually lead to more targeted therapies to treat such diseases.
Although the idea that growth cones are semi-autonomous may seem radical, it makes sense, Macklis says. Because axons grow over long distances, it takes too much time—from a neuron’s perspective—for signals from the growth cone to reach the nucleus in the cell body and vice versa in order for the nucleus to instruct the growth cone on where to grow or what specific circuitry to build. The overall neuron spans so many distinct micro-environments simultaneously that no singular centralized set of instructions will work across the whole cell. Growth cones have to “make decisions” on their own about how to respond to their immediate environments.
“It’s like a federal system with both central and independent local government,” says Macklis.
More comprehensive analysis of growth cones at distinct developmental stages and in distinct neuron types could lead to better understanding of neurological and psychiatric diseases and disorders, Macklis says. Mechanisms contributing to neuron degeneration, autism spectrum disorders, psychiatric illness, developmental diseases, and potential regeneration could lie in the axons and growth cones of affected neuronal circuitry—yet molecular analysis of neurons has mostly been limited to the cell bodies.
“By looking only at the neuronal cell body, we have been missing a lot of the functions of the whole neuron and its circuitry,” Macklis says.
Macklis’ lab spent five years, recently with seed funding from the Allen Frontiers Group as an Allen Distinguished Investigator, creating an approach to analyze the molecular make-up of growth cones—specifically, their proteins and RNAs—in distinct neuron and axon types as the growth cones move and operate. In the lab’s proof-of-concept investigations, Macklis and his lab found that growth cones have a molecular make-up different from that of their own cell bodies. Additionally, growth cones of distinct neuron and axon types have distinct molecular machinery, which is much more rich and complex than previously assumed. The growth cones have the machinery to make, process, and degrade a very large set of their own proteins, using RNAs that are uncommon or even undetected in the cell body.
“We found independent molecular machinery,” Macklis says.
Macklis and his team analyzed growth cones in part by refining and modifying a decades-old method for separating cell parts that Macklis compares to twisting a balloon to create separate segments. They combined this with modern genetics and molecular labeling approaches to isolate and purify growth cones of multiple distinct types of neurons and axons at specific stages of their development. They used custom-developed modifications of fluorescent sorting to turn growth cones a different color than their own cell bodies. Then they treated the growth cones to ensure that the molecules they identified inside belonged to the growth cone and not to surrounding cells.
Macklis believes that neurobiologists will increasingly come to think of individual neurons as complex systems rather than as uniform cells with axon cables, and of growth cones as diverse, distinct, semi-autonomous elements within those systems.
Furthermore, a better understanding of how neurons can contain separate regions with somewhat independent operation could lead to insights into how other polarized cells—cells that show asymmetry—function. Though neurons are the extreme example of cellular asymmetry, polarized cells exist throughout the body.
Analyzing growth cones apart from neuron cell bodies could also lead to more targeted medical therapies for certain diseases. If the faulty molecular machinery involved in a disease is located in the growth cone and its later form as a functioning synapse, and not the cell body, then treatments could be designed to go to that specific part of the neuron instead of dosing the entire neuron and brain, Macklis says.
Over the next five years and beyond, Macklis’ lab will focus much of its energy on the project of learning more about growth cones, their diversity, and how they function over time, with the hope that their discoveries will lead to biological and medical advances.
“We need to look at neurons in a whole different way,” Macklis says, “and that will unlock new ways of thinking about the brain.”