Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 118

3.3. Intrafilopodial Traffic
As discussed in Section 2.4 , mature chemical synapses are sites of active
membrane turnover. On the presynaptic side, vesicles fuse with the mem-
brane and are endocytosed and recycled during the release of neurotrans-
mitter into the synaptic cleft. Similarly, neurotransmitter receptors on the
postsynaptic membrane undergo controlled insertion into the membrane
and recycling. Recent work has identified that membrane traffic and
turnover also occurs within dendritic, axonal and growth cone filopodia.
Importantly, molecules associated with the cytoplasmic side of vesicles also
contribute to the dynamics of filopodia.
Initial cues that growth cones secrete neurotransmitters, and may have
active exocytotic mechanisms similar to those of mature presynaptic sites,
were obtained from studies of developing motoneurons in vitro ( Young and
Poo, 1983 ). The formation of the synapse between a motoneuron growth
cone and a nonneuronal muscle cell, termed the neuromusclar junction , rep-
resents the best understood instance of synapse formation to date. These
synapses are fundamental to the ability of the central nervous system to
control patterns of skeletal muscle activity. The establishment of synaptic
responses by muscle cells in response to the arrival of a motoneuron growth
cone is rapid, indicating that the presynaptic machinery is operative. Neu-
romuscular junctions utilize acetylcholine as a neurotransmitter. In a set
of elegant experiments, the release of acetylcholine was monitored from
growth cones in vitro through the use of a patch of outside-out embry-
onic muscle membrane affixed with a gigaohm seal at the tip of a record-
ing micropipette ( Young and Poo, 1983 ). The membrane at the tip of the
pipette thus contained properly oriented muscle membrane acetylcholine
receptors and allowed the investigators to monitor the release of acetylcho-
line from growth cones. Placement of this electrophysiological probe within
3-50 µm of the growth cone demonstrated activation of the muscle mem-
brane acetylcholine receptors, revealing the release of acetylcholine from
the growth cone. Consistent with these studies, synaptic vesicles have been
visualized undergoing bidirectional transport and membrane fusion within
growth cone filopodia ( Chang and De Camilli, 2001 ; Sabo and McAllister,
2003 ; Alberts et al., 2006 ).
The presynaptic localization of neurotransmitter vesicle release machin-
ery is also correlated with the formation and stabilization of axonal filopo-
dia and branches ( Cohen-Cory and Lom, 2004 ). As an example, in vivo dual
color imaging of Xenopus retinal ganglion cell axons undergoing branching