Lipid Molecule Plays Key Role in Neurotransmission

[Guest guest]

From Medical Institute Research News September 23, 2004

Lipid Molecule Plays Key Role in Neurotransmission

New studies indicate that a specific type of lipid molecule plays a

critical role in controlling the behavior of vesicles that store

neurotransmitters within neurons. Neurotransmitters are the chemical

messengers that neurons release to communicate with one another.

The identification of a regulatory role for the molecule,

phosphatidylinositol 4,5 biphosphate, or PtdIns(4,5)P2, provides a new

view of the operation of the machinery that produces and recycles

synaptic vesicles.

“More and more it is being appreciated that the chemistry of lipids is

important for membrane dynamics, in this case in a specialized area of

the synapse.” (Pietro De Camilli)

Led by Medical Institute (HHMI) investigator Pietro De

Camilli, the researchers published their findings in the September 23,

2004, issue of the journal Nature. De Camilli, HHMI investigator

Flavell, Reiko Fitzsimonds, and their colleagues at Yale University

School of Medicine collaborated on the studies with A. and

researchers from the Weill Medical College of Cornell University.

Synaptic vesicles are initially loaded with neurotransmitter molecules

in the interior of neurons. They then secrete their cargo at synapses —

the junctions between neurons. At the synapse, the vesicles undergo a

process called exocytosis, in which they fuse with the synaptic plasma

membrane and unload their neurotransmitters. Afterward, they are drawn

back into the neuron in the process of endocytosis.

According to De Camilli, there had been indirect evidence from cell-free

systems, pharmacological and transfection studies that PtdIns(4,5)P2

played a role in controlling this process. However, there had not been

enough genetic evidence to establish that this lipid molecule played a

regulatory role both in synaptic-vesicle fusion and endocytic recovery.

“We think that this paper provides conclusive support to the hypothesis

that PtdIns(4,5)P2 plays an important role in exocytosis, at least in

part via its binding to proteins of the exocytic machinery, and in

endocytosis via its binding to the clathrin adaptors,” he said.

In their experiments, De Camilli and his colleagues knocked out the gene

encoding an enzyme that produces PtdIns(4,5)P2 in mice. When they

deleted this enzyme, called PIP kinase type 1 gamma, they saw greatly

reduced levels of PtdIns(4,5)P2 in the brain and a deficiency in

neurotransmitter secretion. Mice lacking both copies of the gene died

shortly after birth.

They then examined synaptic transmission in cultures of neurons from

these neonatal mice, and therefore deficient in PtdIns(4,5)P2, finding a

smaller recycling pool of vesicles and a smaller population of vesicles

ready for fusion. The researchers then studied the influence of

PtdIns(4,5)P2 on the speed of vesicle recycling after fusion. They used

a fluorescent tracer to follow the process by which empty vesicles are

internalized and then reused in another round of secretion, finding that

the cells lacking PtdIns(4,5)P2 had a delayed recycling.

To study endocytosis in greater detail, they also transfected cultured

neurons with a chimeric protein that is targeted to the synaptic vesicle

membrane and that fluoresces brightly when the lumen of the vesicles is

exposed to conditions of low acidity, which happens during exocytosis.

While held inside the vesicles, where conditions are acidic, the protein

exhibits only very dim fluorescence. This experiment allowed the

scientists to monitor opening and closing of the vesicles during the

processes of exocytosis and endocytosis. The experiments revealed that

the neurons lacking PtdIns(4,5)P2 showed a slowing of endocytosis

relative to exocytosis when they were electrically stimulated to trigger

neurotransmitter release. Further electron microscopy studies using

endocytic tracers revealed that such neurons also showed an impaired

clathrin-dependent endocytosis.

According to De Camilli, the studies of the role of PtdIns(4,5)P2

represent the beginning of exploration into the regulatory role of the

class of lipids called phosphoinositides in membrane traffic within

neurons. The reversible phosphorylation of their inositol rings makes

them powerful regulators of interactions between the membrane bilayer

and protein modules present in the cytosol. “This field is in its

infancy,” he said. “Although much has been learned about

phosphoinositides and membrane traffic in other systems, we still know

very little about the regulatory role of phosphoinositides in synaptic

physiology. We have begun to explore the role of PtdIns(4,5)P2, but we

would also like to explore the role of other phosphoinositides.”

Basic studies such as those conducted by De Camilli and his colleagues

could well lead to insights into diseases that involve abnormal

phosphoinositide metabolism. For example, people with Down syndrome have

an extra copy of the gene encoding synaptojanin 1, a brain enriched

enzyme that degrades PtdIns(4,5)P2, and patients with Lowe syndrome, who

also have mental retardation, lack another PI(4,5)P2 degrading enzyme.

Other diseases that can result from abnormal metabolism of

phosphoinositides include cancer and diabetes.

“In general, this work helps to emphasize the role of metabolism of

membrane lipids in the regulation of membrane processes,” said De

Camilli. “Typically, studies of membrane processes focus on the role of

proteins, with lipids traditionally thought of as primarily structural

components. But more and more it is being appreciated that the chemistry

of lipids is important for membrane dynamics, in this case in a

specialized area of the synapse,” he said. “Advances in the field of

lipids biology may offer new targets for therapeutic intervention in

human diseases.”

http://www.hhmi.org//news/decamilli.html