Journal of Neuroscience, Vol 15, 6679-6687, Copyright © 1995 by Society for Neuroscience
Altered nerve terminal arborization and synaptic transmission in Drosophila mutants of cell adhesion molecule fasciclin I
Y Zhong and J Shanley
Cold Spring Harbor Laboratory, New York 11724, USA.
This work investigates the role of cell adhesion molecules in development
of synaptic connections and functions through a genetic approach. Fasciclin
I (Fas I) is an insect glycoprotein capable of mediating homophilic cell
adhesion. It has been shown that Fas I is expressed in motor nerve axons
and terminals that innervate larval body- wall muscles in Drosophila.
Immunohistochemical analysis of these motor nerve terminals has revealed
that nerve terminal arborization, quantified by the numbers of the nerve
terminal branches and varicosities, is enhanced in the null mutant fas ITE.
In contrast, the number of branches and varicosities are reduced in larvae
that overexpress the Fas I molecule resulting from additional copies of the
fas I transgene in P(fas I+) or the chromosome duplication in Dp(fas I)
mutants. Although arborization is altered, the overall stereotypical
pattern of nerve terminal innervation of the body-wall muscle fibers is
preserved in all the Fas I mutants examined. The voltage-clamp analysis of
excitatory junctional currents (ejcs) at the neuromuscular junction
indicates that the amplitude of ejcs is reduced in fas ITE, but increased
in P(fas I+) and Dp(fas I) compared to that in wild-type larvae. Further
electrophysiological analysis shows that the quantal content and the evoked
frequency-dependent response are affected in these mutants, indicating a
defective presynaptic function in addition to the anatomic abnormality.
Therefore, the cell adhesion molecule Fas I may not be essential for target
recognition and synaptogenesis at the larval neuromuscular junction, but
may play a role in fine-turning nerve terminal arborization and possibly in
modifying, directly or indirectly, development of presynaptic functions.
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