Biology textbooks might require updates, according to researchers from Johns Hopkins Medicine, who have uncovered new evidence suggesting that an arm-like feature of mammalian brain cells could be shaped differently than previously believed for over a century.
Biology textbooks might require updates, according to scientists at Johns Hopkins Medicine, who have unveiled new evidence indicating that an arm-like structure of mammalian brain cells may be shaped differently than scientists have thought for more than a century.
Their research on mouse brain cells demonstrates that the axons — the arm-like components that communicate with other brain cells — are not the cylindrical tubes typically depicted in textbooks and online resources. Instead, they resemble pearls strung together.
The findings were published online on December 2 in Nature Neuroscience.
“Understanding axon structure is crucial for grasping how brain cells send signals,” states Shigeki Watanabe, Ph.D., an associate professor of cell biology and neuroscience at Johns Hopkins University School of Medicine. “Axons serve as the conduits that link our brain tissue, facilitating learning, memory, and other essential functions.”
Researchers have been aware that bead-like formations in axons, known as axon beading, can appear in deteriorating brain cells and in individuals suffering from Parkinson’s and other neurodegenerative disorders due to compromised membrane and structural integrity in neurons.
In normal circumstances, axons are believed to be tube-shaped with a predominantly constant diameter, featuring sporadic bulging areas (synaptic varicosities that store neurotransmitters, enabling communication with other brain cells).
Watanabe first observed recurrent axon pearling in the nervous system of worms and grew more intrigued after discussing the structures with Swiss scientist Graham Knott, Ph.D. In 2012, a research team from Harvard University had published findings identifying repeated “skeletal” elements in axons, leading Watanabe and Knott to explore experiments aimed at removing the axon skeleton to see if the pearl formations would vanish.
Johns Hopkins graduate student and first author of the study, Jacqueline Griswold, tested this concept but discovered that axon pearling persisted.
Following this, Watanabe and Griswold collaborated with theoretical biophysicist Padmini Rangamani, Ph.D., a professor of pharmacology at the University of California San Diego School of Medicine, to investigate the physical properties of axons more closely.
To visualize axons on brain cells (neurons) that are 100 times thinner than a human hair, the researchers employed high-pressure freezing electron microscopy. This method, unlike standard electron microscopy, which outlines cellular structures by directing electron beams at them, involved freezing mouse neurons to maintain the original shape of the structures.
“With standard electron microscopy, we have to fix and dehydrate tissues, but freezing preserves their shape—much like freezing a grape instead of drying it into a raisin,” explains Watanabe.
The team examined three types of mouse neurons: those cultivated in the lab, those collected from adult mice, and those acquired from mouse embryos. The neurons studied were nonmyelinated (lacking the myelin sheath that usually surrounds axons).
The researchers identified the bubbly, pear-shaped axons across tens of thousands of images captured from the tissue samples.
The scientists designated the swelling areas of the axon as “non-synaptic varicosities.”
“These results contest a century of established knowledge regarding axon structure,” Watanabe remarks.
The researchers also employed mathematical modeling to examine whether the axon membrane affected the shape or existence of the pearl-like structures. Their findings indicated that straightforward mechanical models could effectively describe these structures.
Additionally, experiments incorporating the mathematical model and mouse brain samples revealed that increasing sugar concentration in the solution surrounding the axon or diminishing tension in the axonal membranes led to a reduction in the size of the pearl structures.
In a separate experiment, the scientists removed cholesterol from the neuron’s membrane to increase its fluidity. In this condition, they observed a decrease in pearling in both the mathematical models and mouse neurons, along with a compromised ability of the axon to convey electrical signals.
“A broader space within the axons promotes quicker passage of ions [chemical particles] and reduces congestion,” Watanabe states.
The research team also applied high-frequency electrical stimulation to the mouse neurons, resulting in a swelling of the pearled structures along axons to be, on average, 8% longer and 17% wider for at least 30 minutes post-stimulation, which boosted the speed of electrical transmission. Conversely, when cholesterol was removed from the membrane, the axon’s pearls reverted from their swollen condition, and the speed of electrical signals showed no change.
The research team intends to investigate axonal “arms” in human brain tissue, obtained with consent from patients undergoing brain surgery and those who have succumbed to neurodegenerative diseases. This research provided the foundation for a recently awarded Multiple Principal Investigator grant to Watanabe and Rangamani from the National Institute of Mental Health.
Funding for this study was provided by the Johns Hopkins University School of Medicine, the Marine Biological Laboratory Whitman Fellowship, the Chan Zuckerberg Initiative Collaborative Pair Grant and Supplement Award, the Brain Research Foundation Scientific Innovations Award, a Helis Foundation award, as well as various National Institutes of Health grants, the Air Force Office of Scientific Research, the Alfred P. Sloan Research Fellowship, a McKnight Foundation scholarship, a Klingenstein-Simons Fellowship Award in Neuroscience, a Vallee Foundation scholarship, the National Science Foundation, and the Kavli Institutes at Johns Hopkins and UC San Diego.
Other contributors to this research included Chintan Patel, Renee Pepper, Sumana Raychaudhuri, Quan Gan, Sarah Syed, and Brady Maher from Johns Hopkins University, as well as Mayte Bonilla-Quintana, Christopher Lee, Cuncheng Zhu, and Miriam Bell from UC San Diego, Siyi Ma from the Marine Biology Laboratory, Mitsuo Suga and Yuuki Yamaguchi from JEOL in Tokyo, and Ronan Chéreau along with U. Valentin Nägerl from the Université de Bordeaux in France.
