Summary: The cilia on neurons play a key role in ensuring dopamine receptor 1 signals are correctly received.
Source: Ohio State University
A historically overlooked rod-like projection present on nearly every cell type in the human body may finally be getting its scientific due: A new study has found that these appendages, called cilia, on neurons in the brain have a key role in ensuring a specific dopamine receptor’s signals are properly received.
The research was conducted in mouse models of a disorder called Bardet-Biedl syndrome, and applies to one of five proteins that regulate dopamine signaling, called dopamine receptor 1. In certain regions of the brain, this receptor can be thought of as an “on” switch that initiates motivated behavior—basically any behavior linked to pursuit of a goal.
The study showed that if the receptor either gets stuck on cilia or never has a chance to localize to these cell “antennae,” messages telling the body to move are reduced.
“There’s something about dopamine receptor 1 needing to get to and from neuronal cilia that’s required for proper signaling,” said lead author Kirk Mykytyn, associate professor of biological chemistry and pharmacology in The Ohio State University College of Medicine.
“This is the first demonstration that cilia are important for dopamine receptor 1 signaling.”
The study is published in the Journal of Neuroscience.
Bardet-Biedl syndrome (BBS) is part of a class of human diseases called ciliopathies—caused by dysfunctional cilia on a range of cell types—and is characterized by multiple organ system defects, adult blindness, obesity and intellectual disabilities.
Though the syndrome involves cilia dysfunction throughout the body, Mykytyn’s lab studies the neural component of BBS to determine the role of primary cilia in the brain. Primary cilia are single appendages projecting from each neuronal cell body.
Though they don’t get a lot of attention, some cilia functions are well-known: Primary cilia in the olfactory system give humans a sense of smell, and outer segments of photoreceptors, which are modified primary cilia, allow us to see.
“These appendages are present on nearly every cell type in the human body and are ubiquitous throughout the brain,” he said. “Interestingly, we’re just now beginning to understand what their role is.”
Mykytyn previously found that the deletion of BBS proteins in mouse models of Bardet-Biedl syndrome hindered the movement of specific types of receptors to and from primary cilia. The implications could be far-reaching: These G protein-coupled receptors, as a group, are the largest family of cell-surface receptors in mammals, and are the most common family of proteins to be targeted by drugs.
“We wanted to investigate what the consequence might be of disrupting trafficking of a particular G protein-coupled receptor to and from cilia,” he said.
Dopamine receptor 1 is one of those receptors.
In this work, researchers engineered mice whose dopamine receptor 1-expressing neurons either lacked a BBS protein or the machinery needed to develop cilia. As expected, the missing BBS protein caused the receptor to accumulate on these neurons’ cilia, and, surprisingly, the mice became obese.
The team also discovered that mice lacking cilia—meaning the receptor had no chance to do whatever it does on cilia—had the same outcome: Interestingly, the mice did not become obese because they overate—instead, the obesity was associated with a significant reduction in activity. They were sedentary, and they got fat. Control mice, by comparison, behaved normally and did not gain weight.
“That tendency to move around less and become obese is consistent with a reduction in dopamine receptor 1 signaling,” Mykytyn said. “The striatum, a region of the brain where these neurons are located, is known to be involved in motivated behavior and movement.
“We knew animals with BBS become obese, and it appeared that this was due to overeating. But what we found here was that when we disrupted BBS proteins just in dopamine receptor 1-expressing neurons, mice got obese not because of overeating, but because they had a significant reduction in motivated behavior,” he said.
“The ‘go’ signal is not as high as it should be, and you get these mice that don’t move around as much.”
The electrophysiology lab of co-lead author Candice Askwith, associate professor of neuroscience at Ohio State, determined that these neurons maintained their excitability—further evidence that the diminished receptor signaling did not overtly damage neuronal circuits to cause the behavior outcomes in the mice.
“The idea that cilia are novel targets to modify dopamine-dependent neuronal signaling and motivated behaviors is extremely exciting,” Askwith said.
Beyond providing a potential explanation for obesity in Bardet-Biedl syndrome, the findings suggest primary cilia in the brain are a critical player in proper signaling within the dopaminergic system, which helps regulate motor control, motivation, reward and cognitive function.
Disruption of dopamine receptor 1 localization to primary cilia impairs signaling in striatal neurons
A rod-shaped appendage called a primary cilium projects from the soma of most central neurons in the mammalian brain. The importance of cilia within the nervous system is highlighted by the fact that human syndromes linked to primary cilia dysfunction, collectively termed ciliopathies, are associated with numerous neuropathologies, including hyperphagia-induced obesity, neuropsychiatric disorders, and learning and memory deficits.
Neuronal cilia are enriched with signaling molecules, including specific G protein-coupled receptors (GPCRs) and their downstream effectors, suggesting they act as sensory organelles that respond to neuromodulators in the extracellular space.
We previously showed that GPCR ciliary localization is disrupted in neurons from mouse models of the ciliopathy Bardet-Biedl syndrome (BBS). Based on this finding we hypothesized that mislocalization of ciliary GPCRs may impact receptor signaling and contribute to the BBS phenotypes.
Here, we show that disrupting localization of the ciliary GPCR dopamine receptor 1 (D1) in male and female mice, either by loss of a BBS protein or loss of the cilium itself, specifically in D1-expressing neurons, results in obesity.
Interestingly, the weight gain is associated with reduced locomotor activity, rather than increased food intake. Moreover, loss of a BBS protein or cilia on D1-expressing neurons leads to a reduction in D1-mediated signaling.
Together, these results indicate that cilia impact D1 activity in the nervous system and underscore the importance of neuronal cilia for proper GPCR signaling.