ABSTRACT
Synaptic connections can be stably maintained for prolonged periods, yet can be rapidly disassembled during the developmental refinement of neural circuitry and following cytological insults that lead to neurodegeneration. To date, the molecular mechanisms that determine whether a synapse will persist versus being remodeled or eliminated remain poorly understood. Mutations in Drosophila stathmin were isolated in two independent genetic screens that sought mutations leading to impaired synapse stability at the Drosophila neuromuscular junction (NMJ). Here we demonstrate that Stathmin, a protein that associates with microtubules and can function as a point of signaling integration, is necessary to maintain the stability of the Drosophila NMJ. We show that Stathmin protein is widely distributed within motoneurons and that loss of Stathmin causes impaired NMJ growth and stability. In addition, we show that stathmin mutants display evidence of defective axonal transport, a common feature associated with neuronal degeneration and altered synapse stability. The disassembly of the NMJ in stathmin includes a predictable sequence of cytological events, suggesting that a common program of synapse disassembly is induced following the loss of Stathmin protein. These data define a required function for Stathmin during synapse maintenance in a model system in which there is only a single stathmin gene, enabling future genetic investigation of Stathmin function with potential relevance to the cause and progression of neuromuscular degenerative disease.
Subject(s)
Neuromuscular Junction/physiology , Stathmin/metabolism , Analysis of Variance , Animals , Animals, Genetically Modified , Axons/physiology , Drosophila , Drosophila Proteins/genetics , Drosophila Proteins/metabolism , Microscopy, Confocal , Mutation/genetics , Neuromuscular Junction/cytology , Neuromuscular Junction/genetics , Presynaptic Terminals/metabolism , RNA Interference/physiology , Stathmin/genetics , Vesicular Glutamate Transport Proteins/geneticsABSTRACT
Survival and maturation of dorsal root ganglia sensory neurons during development depend on target-derived neurotrophins. These target-derived signals must be transmitted across long distances to alter gene expression. Here, we address the possibility that long-range retrograde signals initiated by target-derived neurotrophins activate a specialized transcriptional program. The transcription factor MEF2D is expressed in sensory neurons; we show that expression of this factor is induced in response to target-derived neurotrophins that stimulate the distal axons. We demonstrate that MEF2D regulates expression of an anti-apoptotic bcl-2 family member, bcl-w. Expression of mef2d and bcl-w is stimulated in response to activation of a Trk-dependent ERK5/MEF2 pathway, and our data indicate that this pathway promotes sensory neuron survival. We find that mef2d and bcl-w are members of a larger set of retrograde response genes, which are preferentially induced by neurotrophin stimulation of distal axons. Thus, activation of an ERK5/MEF2D transcriptional program establishes and maintains the cellular constituents of functional sensory circuits.
Subject(s)
Gene Expression Regulation/drug effects , Nerve Growth Factors/pharmacology , Protein Tyrosine Phosphatases, Non-Receptor/metabolism , Proto-Oncogene Proteins c-bcl-2/metabolism , Sensory Receptor Cells/physiology , Animals , Cell Survival/drug effects , Cell Survival/physiology , Cells, Cultured , Chlorocebus aethiops , Embryo, Mammalian , Enzyme Inhibitors/pharmacology , Ganglia, Spinal/cytology , Gene Expression Regulation/physiology , Green Fluorescent Proteins/genetics , In Situ Nick-End Labeling/methods , Mitogen-Activated Protein Kinase 7/metabolism , Nerve Tissue Proteins/genetics , Nerve Tissue Proteins/metabolism , Protein Tyrosine Phosphatases, Non-Receptor/genetics , Proto-Oncogene Proteins c-bcl-2/genetics , RNA Interference/physiology , RNA, Messenger/metabolism , Rats , Sensory Receptor Cells/drug effects , Signal Transduction/drug effects , Signal Transduction/genetics , Transfection/methodsABSTRACT
Mutations that alter dynein function are associated with neurodegenerative diseases, but it is not known why defects in dynein-dependent transport impair neuronal survival. Here we show that dynein function in axons is selectively required for the survival of neurons that depend on target-derived neurotrophins. Stimulation of axon terminals with neurotrophins causes internalization of neurotrophin receptors (Trks). Using real-time imaging of fluorescently tagged Trks, we show that dynein is required for rapid transport of internalized, activated receptors from axon terminals to remote cell bodies. When dynein-based transport is inhibited, neurotrophin stimulation of axon terminals does not support survival. These studies indicate that defects in dynein-based transport reduce trafficking of activated Trks and thereby obstruct the prosurvival effect of target-derived trophic factors, leading to degeneration of target-dependent neurons.
Subject(s)
Axonal Transport/drug effects , Dyneins/metabolism , Nerve Growth Factors/pharmacology , Neurons/drug effects , Receptors, Nerve Growth Factor/metabolism , Animals , Axonal Transport/physiology , Cell Survival/drug effects , Cell Survival/physiology , Cells, Cultured , Dyneins/antagonists & inhibitors , Female , Ganglia, Spinal/drug effects , Ganglia, Spinal/metabolism , Neurons/metabolism , Pregnancy , Rats , Receptors, Nerve Growth Factor/agonistsABSTRACT
Neurotrophins were originally identified as target-derived factors that regulate the survival and differentiation of innervating neurons. However, neurotrophins can also be released by presynaptic cells to stimulate postsynaptic neurons. Recent studies indicate that differences exist between the signaling pathways activated by neurotrophin stimulation of nerve terminals (retrograde signaling) and neurotrophin stimulation of cell bodies. Retrograde signaling relies on the formation of signaling endosomes, vesicles containing activated Trk receptors and their ligands. Signaling endosomes travel from the nerve terminals to remote cell bodies, where they selectively activate a novel MAP kinase, Erk5, as well as PI3 kinase, and thereby stimulate neuronal survival. The differences in the signaling pathways activated by neurotrophins, which depends on the location of stimulation, provide a mechanism by which neurons can interpret the 'where' as well as the 'what' of growth factor stimulation.
