Small projections of dendritic spines known as spinules are unexpectedly dynamic, while a stable subgroup may form multi-synaptic spine connections, according to the first detailed study of their behavior.
Aberrant spinules have been observed in a variety of neurological conditions, so learning more about their function could be the key to a better understanding of disease, according to Colleen Zaccard, PhD, a postdoctoral fellow and lead author of the study published in Neuron.
“The fact that their nanoscale has caused spinules to be overlooked in so many data sets for so long, and that they can potentially form connectivity-altering secondary synapses, makes this a very exciting discovery,” said Zaccard, who is also chair of the Chicago Women in STEM Initiative.
Dendrites are branched extensions of postsynaptic neurons that receive electrochemical stimulation from presynaptic neurons. Protrusions of dendrites, called dendritic spines, can each receive inputs from a single presynaptic terminal, forming a typical synapse. Changes at synapses are thought to underlie learning and memory, as well as many neuropsychiatric disorders. Spines can alter their shape in response to synaptic activity and form smaller protrusions of their own, known as spinules.
Typically less than one micron in length, these fine protrusions are most prevalent on large, active mushroom-shaped spines. Spinules were previously thought to project into presynaptic terminals and facilitate spine remodeling, but there had been little investigation into how their activity relates to function due to technological constraints, according to Zaccard.
“Up until this study, most of what we knew about spinules was based on fixed electron microscopy data from decades ago,” Zaccard said.
Recent improvements in the acquisition speed of live-cell three-dimensional super-resolution imaging opened the door to studying spinules further, but the technology was available only at a select few facilities, including the Howard Hughes Medical Institute’s Janelia campus in Virginia.
Zaccard spent a month there, studying microglia-neuron interactions in an unsuccessful series of experiments, but there was a silver lining.
“At one point I thought I had come home empty-handed, but then we noticed these thin membrane protrusions on spines that were unexpectedly dynamic, extending and retracting quite rapidly” Zaccard said.
Zaccard and her collaborators continued analyzing data and conducting experiments at Northwestern’s Center for Advanced Microscopy, finding that most of the spinules were very short lived — lasting only seconds — and seemed to be exploring their environment.
In contrast, a small sub-population was long lived, lasting for 60 seconds or more, at times exceeding the 10-15 minute imaging duration. These long lasting spinules were captured trafficking fragments of PSD95, a postsynaptic protein marker, and displaying heightened local calcium signaling, an indicator of synaptic transmission.
“We hypothesize that the spinule repeatedly extends and retracts until it senses a nearby active synapse, and extends towards it to form a stable secondary connection,” Zaccard said.
This two-stage model may be how multi-synaptic spines — which are associated with motor learning and fear response in rodents — are formed.
“Spinules could also help to guide synapses toward the correct partners, to make the right connections,” said Peter Penzes, PhD, the Ruth and Evelyn Dunbar Professor of Psychiatry and Behavioral Sciences, professor of Physiology and Pharmacology and director of the Center for Autism and Neurodevelopment, and the senior author of the study. “This opens up a completely new dimension of understanding how brain connections are made and change.”
Further, alterations in spinules have been noted in a variety of diseases, raising the possibility that spinule-driven changes in connectivity may play a role in certain neurological conditions, according to Zaccard.
“An excess of spinules may contribute to a hyper-excitable network, such as in epilepsy,” Zaccard said. “We also need future studies to investigate the role of spinules in disorders like Parkinson’s disease and amyotrophic lateral sclerosis, where they’ve found altered spinules in affected areas.”
This work was supported by National Institutes of Health grants R01MH071316 and R01MH107182.
