Div. of Neurobiology and Howard Hughes Med. Inst., 519 LSA, University of California, Berkeley, CA, 94720, USA
During synapse formation pre- and postsynaptic cells must align
morphological and functional features, such as the presynaptic machinery
of transmitter release and the postsynaptic receptor field. To date, most
attention has focussed on postsynaptic development. Here we examine the
mechanisms of presynaptic maturation at the Drosophila neuromuscular
junction (NMJ). We analyse three mutations that affect the development of
the somatic muscles at different levels, and describe the extent to which
presynaptic structures can form in the presence of defective postsynaptic
targets.
Late stage 17 wild type embryos exhibit NMJs characterised by a
typically shaped synaptic cleft and presynaptic active zones with a T-bar
and clustered vesicles. In twist mutant embryos that lack mesoderm
motorneurons exit the central nervous system and form normal presynaptic
active zones facing either neural cells or the hemolymph. Thus, both
induction of the genes coding for the components of active zones and the
assembly of active zones do not require postsynaptic target muscles.
Embryos mutant for the mef2 gene form mononucleate specified
myoblasts which fail to differentiate into muscles. These immature
postsynaptic targets attract nerve terminals but during subsequent
development presynaptic active zones fail to localise at neuromuscular
contacts. Thus, NMJ differentiation can be genetically separated from the
process of target recognition and requires mef2 dependent properties in
the differentiating muscle.
Embryos mutant for the myoblast city gene lack myoblast fusion,
but otherwise form fully differentiated mononucleate muscles which are
correctly targeted by motorneurons and form functional neuromuscular
synapses with correctly localised presynaptic active zones. However, NMJs
are smaller and excess active zones form away from the muscle. This
suggests that the mef2 dependent muscle properties required for NMJ
differentiation are limited in the mononucleate muscles of myoblast city
mutant embryos.
We will propose a model for NMJ formation as a basis for future work.