Cholesterol and apolipoproteins in Alzheimer's disease have become a rapidly developing-and sometimes confusing-area of investigation, but most of it suggests that lowering cholesterol might slow disease progression. A paper in tomorrow's Science presents a new, beneficial function for cholesterol in the brain that may help explain some previous data. Scientists led by Frank Pfrieger the Max-Planck/CNRS group in Strasbourg, France, report that neurons need cholesterol secreted by glial cells to form and maintain functional synapses.
"Our work introduces a new connection between synapse formation and cholesterol metabolism that was not realized before," Pfrieger said. "The main hypothesis from our findings is that CNS neurons are able to synthesize a certain amount of cholesterol that supports their survival and possibly their growth, but at the same time they also require additional cholesterol for synaptogenesis, which they cannot synthesize themselves. This needs to be imported. We think that the centers of synthesis are glial cells. It is known that astrocytes make ApoE and secrete lipoproteins, but until now nobody knew why," he continues.
This work began in 1997, when Pfrieger, then in Ben Barres' laboratory at Stanford University in California developed a culture system for purified retinal ganglion neurons. He observed that neurons formed functioning synapses only once astrocytes were added to the culture. Removing the astrocytes caused the neurons to lose those synapses again. Follow-up papers suggested that astrocytes control synapse formation by secreting a mysterious soluble factor (Ullian et al., Nagler et al.).
The present paper identifies this factor as cholesterol. The researchers treated cultured neurons with elution fractions from fractionated glia-conditioned medium, recorded spontaneous synaptic activity in these neurons, and further purified those fractions that induced increased activity. The researchers also used two-D gel electrophoresis and protein sequencing by nanospray mass spectrometry to identify a complex containing ApoE in the membranes of neurons treated with glial-conditioned medium. Their initial hunch that ApoE might be the elusive factor proved wrong but their next guess-cholesterol-proved right.
Further experiments showed that cholesterol increased the frequency of spontaneous postsynaptic currents and that reducing cholesterol with the drug mevastatin eliminated this effect, as did inhibiting lipoprotein uptake with RAP, a chaperone known to inhibit ligand (i.e., ApoE-cholesterol) binding by LDL receptors. Finally, the scientists show cholesterol increases by eightfold the number of autapses (those are synapses an individually cultured neurons makes with itself) and by 10-fold the quantal content of neurotransmitter release per neuron.
"The simplest explanation for these findings is that cholesterol bound to ApoE-containing lipoprotein particle is released by astrocytes and taken up by neurons, where it then promotes an increase in synapse number," write Barres and Stephen Smith in an accompanying Perspectives article.
Neurons probably require glial cholesterol as building material for the assembly of presynaptic components such as synaptic vesicles and release sites, but an additional role in signal transduction is also possible, Pfrieger said. These findings fit in with the successive waves of cell-type differentiation in the embryonic brain and might explain why synaptogenesis begins only after astrocytes have developed.
The relevance of this work to Alzheimer's remains to be investigated. Given that ApoE4 is associated with increased risk for late-onset AD, one experiment would be to test whether the different isoforms of ApoE affect synapse formation differently in this culture system. It is unknown whether adult neurons depend on glial-derived cholesterol for synapse maintenance or for the formation of new ones during learning and memory formation. Intriguingly, the LDL-receptor related protein has been implicated in synaptic plasticity in hippocampal slices (Zhuo et al.). In trying to understand this question, it is important to keep in mind that most brain cholesterol must be synthesized within the brain rather than being imported from the blood, says Pfrieger (Danik et al.).—Gabrielle Strobel
No Available References
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