A role for PS integrins in morphological growth and synaptic function at the postembryonic neuromuscular junction of Drosophila

A family of three position-specific (PS) integrins are expressed at the Dro sophila neuromuscular junction (NMJ): a beta subunit (beta PS), expressed i n both presynaptic and postsynaptic membranes, and tao alpha subunits (alph a PS1, alpha PS2), expressed at least in the postsynaptic membrane. PS inte grins appear at postembryonic NMJs coincident with the onset of rapid morph ological growth and terminal type-specific differentiation, and are restric ted to type I synaptic boutons, which mediate fast, excitatory glutamatergi c transmission. We show that two distinctive hypomorphic mutant alleles of the beta subunit gene myospheroid (mys(b9) and mys(ts1)), differentially af fect beta PS protein expression at the synapse to produce distinctive alter ations in NMJ branching, bouton formation, synaptic architecture and the sp ecificity of synapse formation on target cells. The mys(b9) mutation alters beta PS localization to cause a striking reduction in NMJ branching, bouto n size/number and the formation of aberrant 'mini-boutons', which may repre sent a developmentally arrested state. The mys(ts1) mutation strongly reduc es beta PS expression to cause the opposite phenotype of excessive synaptic sprouting and morphological growth. NMJ function in these mutant condition s is altered in line with the severity of the morphological aberrations. Co nsistent with these mutant phenotypes, transgenic overexpression of the bet a PS protein with a heat-shock construct or tissue-specific GAL4 drivers ca uses a reduction in synaptic branching and bouton number. We conclude that beta PS integrin at the postembryonic NMJ is a critical determinant of morp hological growth and synaptic specificity. These data provide the first gen etic evidence for a functional role of integrins at the postembryonic synap se.