Axoplasmic transport and its signal transduction mechanism

Neuron requires a continual supply of materials synthesized in the cell body, for example a wide range of soluble proteins, membranous components, and various organelles. The transported materials are needed to replace constituents that turn over in the membrane and organelles of the fiber and also are needed to bring substances participating in energy metabolism. Other transported components are neurotransmitters or transmitter-related components supplied to the nerve terminals for the release and subsequent excitation of postsynaptic cells. Moreover, neurotropic substances and modulators are released from the nerve terminals to affect the functional state of the neuron. Conversely, some materials are conveyed back to the cell body. These include organelles, lysosomes, nerve growth factor, and selected small molecules such as adenosine, Ca2+, and some neurotransmitters. Axoplasmic transport is thought to be fundamental for a variety of neuronal cell functions. Thus it may be considered that axoplasmic transport relates to the dynamic physiological activity of neurons; in other words, axoplasmic transport is supposed to express the physiological activity of neurons. In turn, as in the case for many other physiological functions, axoplasmic transport is possibly controlled by neuronal, hormonal, and immunological systems. Since axoplasmic transport supplies neuron materials toward the synapses and back to the cell body, a feedback system of regulatory mechanisms of a variety of neuronal functions might be operated through axoplasmic transport pathways. Although axoplasmic transport is the important neuronal function, its regulation is poorly understood. In this review, we focus on the dynamics of organelle transport and its regulatory mechanisms mediated by neurotransmitters.