Homer regulates calcium signalling in growth cone turning.
BACKGROUND
Homer proteins are post-synaptic density proteins with known functions in receptor trafficking and calcium homeostasis. While they are key mediators of synaptic plasticity, they are also known to function in axon guidance, albeit by mechanisms that are yet to be elucidated. Homer proteins couple extracellular receptors - such as metabotropic glutamate receptors and the transient receptor potential canonical family of cation channels - to intracellular receptors such as inositol triphosphate and ryanodine receptors on intracellular calcium stores and, therefore, are well placed to regulate calcium dynamics within the neural growth cone. Here we used growth cones from dorsal root ganglia, a well established model in the field of axon guidance, and a growth cone turning assay to examine Homer1 function in axon guidance.


RESULTS
Homer1 knockdown reversed growth cone turning from attraction to repulsion in response to the calcium-dependent guidance cues brain derived neurotrophic factor and netrin-1. Conversely, Homer1 knockdown had no effect on repulsion to the calcium-independent guidance cue Semaphorin-3A. This reversal of attractive turning suggested a requirement for Homer1 in a molecular switch. Pharmacological experiments confirmed that the operational state of a calcium-calmodulin dependent protein kinase II/calcineurin phosphatase molecular switch was dependent on Homer1 expression. Calcium imaging of motile growth cones revealed that Homer1 is required for guidance-cue-induced rise of cytosolic calcium and the attenuation of spontaneous cytosolic calcium transients. Homer1 knockdown-induced calcium transients and turning were inhibited by antagonists of store-operated channels. In addition, immunocytochemistry revealed the close association of Homer1 with the store-operated proteins TRPC1 and STIM1 within dorsal root ganglia growth cones.


CONCLUSIONS
These experiments provide evidence that Homer1 is an essential component of the calcium signalling repertoire within motile growth cones, regulating guidance-cue-induced calcium release and maintaining basal cytosolic calcium.
Gasperini
Robert
R
School of Medicine, University of Tasmania, Hobart, 7001, Tasmania, Australia. robert.gasperini@utas.edu.au
Choi-Lundberg
Derek
D
Thompson
Michael J W
MJ
Mitchell
Camilla B
CB
Foa
Lisa
L
D000818:Animals; D019208:Brain-Derived Neurotrophic Factor / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D002478:Cells, Cultured; D004622:Embryo, Mammalian; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005727:Ganglia, Spinal / Q000166:cytology; D020439:Growth Cones / Q000187:drug effects / Q000502:physiology*; D000072233:Homer Scaffolding Proteins; D009414:Nerve Growth Factors / Q000494:pharmacology; D000075388:Netrin-1; D020319:Oligodeoxyribonucleotides, Antisense / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D040121:Semaphorin-3A / Q000494:pharmacology; D011984:Sensory Receptor Cells / Q000166:cytology*; D013997:Time Factors; D025521:Tumor Suppressor Proteins / Q000494:pharmacology
D019208:Brain-Derived Neurotrophic Factor; D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D000072233:Homer Scaffolding Proteins; C000606624:Homer1 protein, rat; D009414:Nerve Growth Factors; C000617354:Ntn1 protein, rat; D020319:Oligodeoxyribonucleotides, Antisense; D040121:Semaphorin-3A; D025521:Tumor Suppressor Proteins; D000075388:Netrin-1; D002118:Calcium