Whether a neuron responds to extracellular signals such as guidance molecules, neurotrophins, and neurotransmitters depends on the amount and location of receptors for the signals in the neuron’s plasma membrane. These are regulated by ongoing endocytosis, recycling, and exocytosis. Endocytosis of AMPA receptors, for example, reduces responses to presynaptic glutamate release, whereas reinserting the receptors via exocytosis increases synaptic strength. After insertion into the plasma membrane, receptors can either diffuse rapidly within the membrane (called “burst” exocytosis) or remain clustered at the insertion point (“display” exocytosis). Using pH-sensitive fluorescent molecules to track protein movements, Jullié et al. found that transferrin, glutamate, and adrenergic receptors underwent both types of exocytosis. Moreover, receptors that underwent display exocytosis were often locally reinternalized within a few seconds, suggesting the fusion pore rapidly opened and closed. Reinternalized receptors often remained near the plasma membrane for several seconds before they were exocytosed in either burst or display events or their fluorescence faded as the endosome acidified.
Figure: Top, Principle of pH changes to detect internalized receptors. Acidic solution (pH 5.5) is yellow, fluorescent receptors are green, non-fluorescent receptors are black. At pH 7.4, receptors are fluorescent after exocytosis (1), but if they remain on the plasma membrane they will not be fluorescent at pH 5.5 (2). Alternatively, if they are internalized again at pH 7.4 (3), they will remain fluorescent when extracellular pH is switched to pH 5.5 (4). If they are internalized at pH 5.5, they will remain invisible (5). Bottom, Alternating the extracellular pH reveals the time of fusion pore opening (green arrow) and closing (yellow arrow). In this example the fusion pore open time is 5.8 s.
Damien Jullié, Daniel Choquet & David Perrais
David Perrais, tel. 05 57 57 56 92