of Synaptic Activity - The Physiology of GluTs
Open PostDoc Position
Glutamate (Glu) is the most excitatory neurotransmitter in the brain of all vertebrates and many advanced
invertebrates, where it mediates critical functions in development,
normal physiology, and complex behavior. However, several factors
make Glu an unlikely signaling molecule. Most importantly, though Glu
is made in large amounts by all cells, the handling of Glu is in animals’
brain is very dangerous, because excessive Glu signaling is highly
toxic to many neurons. Therefore, in most of the evolutionary tree,
animals do not adopt Glu as their neurotransmitter of choice till
later in evolution, when they acquire very effective protective
barriers that insolate the synapses. Surprisingly, nematodes like C.
elegans are a clear exception to this rule. They juggle Glu
around more than 50% of their neurons, without the presence of
traditional protective barriers such as isolation of synapses by glia. Hoping to learn from this exception
more about the rule, we ask how do nematodes do it?
C. elegans, like many other
animals, use Glu Transporters (GluTs) to clear excess Glutamate from
synaptic connections. Indeed, our previous work showed that similarly
to mammalian GluTs, some of the nematode’s GluTs (e.g., GLT-1 &
GLT-4) are located close to the synapses in the animal’s “brain”
– the nerve-ring. The nerve-ring GluTs also seem to be very similar
to mammalian GluTs in their molecular structure. However, many other
worm GluTs (GLT-3, GLT-6, and GLT-7) are expressed on an elongated
tubular cell called the canal cell. From this
unusual location (which is some
distance away from the nerve-ring synapses), the canal-cell GluTs
exert powerful control over synaptic activity in the nerve-ring.
Adding to the mystery of the function of the C. elegans
canal-cell GluTs are some subtle but very significant modifications
seen in their molecular structure, not seen in any GluT in any other
organism (except other nematodes).
We use a microfluidic chip
to trap intact animals and stimulate specific circuits. We use
transgenic expression of the Ca2+ -sensitive fluorescence reporter
GCaMP, expressed under specific promoters, to record postsynaptic
responses to these stimuli. We determine the effect of knockout of
different GluTs on the activity of these circuits, and follow changes
such as spillover between circuits to determine the physiology of
glutamate clearance and circuit isolation in the absence of anatomical
separation between circuits.
We suggest that these unique molecular
properties hint that canal cell GluTs might have a special
physiological role. We are asking what are the functional
consequences of these modifications. Do these unusual GluTs provide
the basis to the amazing ability of nematodes to handle as a risky
transmitter as Glutamate?