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2005; Schaefer et al., 2000; Yan et al., 2009; Zhen et al., 2000
). Such regulatory
mechanisms were later shown to be conserved from Drosophila to mammals (
Jin
and Garner, 2008
).
Another highlight of synapse formation research is the increasing findings on
molecular control of synapse specificity. In C. elegans, most neurons form en
passant synapses in restricted regions of axons. An interesting question is how
neurons know where to form synapses. Using single-cell synapse labeling,
new cell surface molecules have been identified to control specific synapse
position (
Shen and Bargmann, 2003
). Additionally, several reports have revealed
that classical axon guidance clues and their receptors, such as WNT/Frizzle,
UNC-6/UNC-5, and UNC-6/UNC-40, function in cell-type specific synapse pat-
tern formation (
Colon-Ramos et al., 2007; Klassen and Shen, 2007; Poon et al.,
2008
). Moreover, the interactions between cognate receptor and ligand at synap-
ses differ from those in axon guidance.
D. Maintenance of Neuronal Architecture
C. elegans undergoes four larval stages with adapted behaviors. Recent studies
show that there is tremendous plasticity in the maintenance of cell-body and process
position. For example, the L1/ neuroglian-related transmembrane protein SAX-7/
LAD-1 is required for maintenance of cell-body and axon position during develop-
ment (
Sasakura et al., 2005; Wang et al., 2005
). In sax-7 loss of function mutants,
most axons and processes are normally positioned in newly hatched larva and early
larva stages and become misplaced in later larva stages. SAX-7 works both in
neurons and the epidermis to maintain neuronal position. Another example is the
FGF receptor EGL-15, which is expressed in the epidermis adjacent to neuron but
not in neurons themselves (
Bulow et al., 2004
). These studies show that maintenance
of neuron position in different developmental stages depends on cell-cell or cell-
ECM (extra cellular matrix) adhesion. In addition to these adhesion molecules, the
environmental insults are also important for maintaining of neuronal architecture,
For example, hypoxia can upregulate the Eph receptor VAB-1 through HIF-1 (hyp-
oxia-inducible factor 1) to affect axon guidance and neuronal migration (
Pocock and
Hobert, 2008
).
VI. Outlook
In summary, the defined anatomy of C. elegans and genetic manipulations have
made C. elegans a favorite model organism for examining the development and
function of the nervous system. The invention of fluorescent proteins for labeling
individual neurons or organelles has revolutionized the way to analyze development
of neuronal structures and connectivity. New methods such as machine-based auto-
mated screening, neuronal-specific RNAi knockdown, and whole genome
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