LIM kinase 1 accumulates in presynaptic terminals during synapse maturation.

LIM kinase 1 (LIMK1) is a cytoplasmic protein kinase that is highly expressed in neurons. In transfected cells, LIMK1 binds to the cytoplasmic tail of neuregulins and regulates the breakdown of actin filaments. To identify potential functions of LIMK1 in vivo, we have determined the subcellular distribution of LIMK1 protein within neurons of the rat by using immunomicroscopy. At neuromuscular synapses in the adult hindlimb, LIMK1 was concentrated in the presynaptic terminal. However, little LIMK1 immunoreactivity was detected at neuromuscular synapses before the 2nd week after birth, and most motoneuron terminals were not strongly LIMK1 immunoreactive until the 3rd week after birth. Thus, LIMK1 accumulation at neuromuscular synapses coincided with their maturation. In contrast, SV2, like many other presynaptic terminal proteins, can be readily detected at neuromuscular synapses in the embryo. Similar to its late accumulation at developing synapses, LIMK1 accumulation at regenerating neuromuscular synapses occurred long after these synapses first formed. In the adult ventral spinal cord, LIMK1 was concentrated in a subset of presynaptic terminals. LIMK1 gradually accumulated at spinal cord synapses postnatally, reaching adult levels only after P14. This study is the first to implicate LIMK1 in the function of presynaptic terminals. The concentration of LIMK1 in adult, but not nascent, presynaptic terminals suggests a role for this kinase in regulating the structural or functional characteristics of mature synapses.