Our sensory systems are pretty amazing - they can translate the world around us into signals
that our brain can understand.
And even though we’ve learned a lot about how our brains do it, there’s a lot that
scientists are still trying to figure out. New discoveries are being made all the time!
Case in point: a couple of weeks ago, scientists identified a new kind of brain cell that seems
related to our sense of smell - and they’re still trying to figure out what it does!
Hey there! I’m Alie Astrocyte, and on this episode of Neuro Transmissions, we’ve got
some new olfaction action - and I’m here to tell you all about it!
In a previous video, we talked about how our brains translate odorants in the air into
smells in our brain using the olfactory system. Check out that video for a more detailed explanation
of how it works.
A study published in Cell this month could totally change the way we think about
olfaction!
To briefly recap, in our video I talked about the main olfactory system and how, inside
your nose, olfactory sensory neurons, or OSNs, can detect microscopic particles of odorants,
thanks to specialized olfactory receptors.
The discovery of these G-protein coupled receptors was considered so important that it won the
2004 Nobel Prize in Physiology and Medicine, for helping to clarify the mechanisms of olfaction.
Each cell only has one kind of receptor on it, so each OSN is unique. It can only respond
to certain kinds of chemical stimuli.
All of the OSNs with the same receptor send their axons to the same glomeruli within the
olfactory bulb.
The accessory olfactory system, which is mostly used for detecting pheromones, also uses the
same sort of GPC receptors.
So up until now, we kind of assumed that all olfaction relied on this kind of signaling
- that GPC receptors are the receptor of choice for smell, and that this one-receptor-per-neuron
pattern is really crucial for discriminating different odors.
But last month’s research has discovered a third group of olfactory neurons in mice
- called necklace neurons - that seem to use a totally different mechanism.
These necklace neurons are found in little recesses of the olfactory epithelium, dubbed
cul-de-sacs by the researchers, and they project to a set of glomeruli circling the back part
of the olfactory bulb like a necklace - hence the name.
We’ve known about these neurons for a while, but it wasn’t clear what they were doing.
Researchers from Harvard decided to answer this question by analyzing these neurons directly.
They used a process called FACS - fluorescence activated cell sorting.
FACS lets scientists mark particular types of cells with fluorescent antibodies, binding
only to the cell of interest in a solution containing many different cells.
This solution is then passed through a narrow channel, where a light and a computer track
each cell and separate the cells based on the fluorescent antibodies attached to them.
So the Harvard scientists were able to isolate the necklace neurons separately from the rest
of the olfactory epithelium, and then examine their gene expression to see how it
was different from the ordinary OSNs.
It turns out that the necklace neurons don’t use GPC receptors, the way other olfactory
neurons do.
Instead, they found that necklace neurons have membrane-spanning 4-pass A proteins.
Also called MS4PA proteins, it appears that these proteins have some of the same characteristics
as chemosensory receptors, leading the researchers to believe it may be a new kind of olfactory receptor.
Even more surprising, it turns out that individual necklace neurons express more than one kind
of MS4PA receptor at a time - which turns that whole one-receptor-per-neuron standard
on its head!
The study found that these receptors respond to chemical stimuli from fatty acids - like
those given off by seeds and nuts - and a particular pheromone that’s known to be
unpleasant to mice.
Their wonky expression pattern has led the researchers to hypothesize that these cells
might tell an animal that there’s something important out there in the environment. But
without the highly-specific organization seen in other OSNs, it probably doesn’t help
the animal pinpoint it precisely. So it’s like Spidey sense!
It turns out that these MS4PA proteins are expressed in all kinds of tissue - not just
in the olfactory epithelium - and might have evolved as an early chemosensory system, long
before they became specialized for a sense of smell.
And MS4PA proteins are found in a lot of different mammals - including humans - but researchers
don’t know if they’re used for olfaction across species.
So the next question to answer is whether or not these proteins act as olfactory receptors
for humans and other mammals - and how do we activate them?
In the meantime, it’s pretty amazing to know that scientists are still learning brand
new things about the way our sensory systems work - who knows what they’ll find out next?
Thanks for joining us for this week’s episode of Neuro Transmissions. If you liked this
video, please give it a thumbs up, share it with friends, and hit subscribe to become
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support helps keep us going.
I’m Alie Astrocyte, and until our next transmission - Over and Out!