Resistance to trophic neurite outgrowth of sensory neurons exposed to insulin.
Insulin offers trophic support through receptors expressed widely on peripheral neurons. In this work, we studied whether peripheral sensory neurons demonstrate resistance to its trophic properties, a property relevant during type 2 diabetes mellitus or following supraphysiological therapy. Insulin receptors were not only localized to neuronal membranes and cytoplasm but also had a unique, previously unrecognized localization to neuronal nuclei. We confirmed that nanomolar doses increased neurite outgrowth of adult sensory neurons, but in response to micromolar doses of insulin, even following a brief 2-h exposure, survival and outgrowth of neurites were blunted. Neurons exposed to picomolar insulin concentrations in their media for 5 days had resistance to the impact of later nanomolar doses of insulin. Using a stripe assay seeded with insulin, neurites were more likely to reject higher doses of insulin. Insulin down-regulated mRNAs of the insulin receptor β subunit and up-regulated levels of GSK-3β, both potential mechanisms of insulin resistance, while down-regulating the protein expression of pAkt and pGSK-3β. Overall, these studies identify neuronal nuclear targeting of insulin and evidence for insulin-induced resistance to its trophic properties. The findings have implications for the understanding of the actions of insulin in the treatment of diabetes and neurological disorders.
Department of Clinical Neurosciences and the Hotchkiss Brain Institute, University of Calgary, Calgary, Alberta, Canada.
Bhagat
B
Singh
Yongqin
Y
Xu
Todd
T
McLaughlin
Vandana
V
Singh
Jose A
JA
Martinez
Anand
A
Krishnan
Douglas W
DW
Zochodne
D000818:Animals; D001369:Axons / Q000502:physiology; D015153:Blotting, Western; D002462:Cell Membrane / Q000187:drug effects; D002467:Cell Nucleus / Q000187:drug effects; D002478:Cells, Cultured; D003593:Cytoplasm / Q000187:drug effects; D005260:Female; D015870:Gene Expression / Q000187:drug effects; D038362:Glycogen Synthase Kinase 3 / Q000096:biosynthesis; D000071679:Glycogen Synthase Kinase 3 beta; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007150:Immunohistochemistry; D007328:Insulin / Q000494:pharmacology*; D007333:Insulin Resistance / Q000235:genetics / Q000502:physiology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D016501:Neurites / Q000187:drug effects*; D051058:Oncogene Protein v-akt / Q000096:biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D060888:Real-Time Polymerase Chain Reaction; D011972:Receptor, Insulin / Q000096:biosynthesis; D011984:Sensory Receptor Cells / Q000187:drug effects*
D007004:Hypoglycemic Agents; D007328:Insulin; D011972:Receptor, Insulin; D000071679:Glycogen Synthase Kinase 3 beta; C000605507:Gsk3b protein, mouse; C000605508:Gsk3b protein, rat; D051058:Oncogene Protein v-akt; D038362:Glycogen Synthase Kinase 3