Abnormal muscle spindle innervation and large-fiber neuropathy in diabetic mice.
OBJECTIVE
Large-fiber diabetic polyneuropathy (DPN) leads to balance and gait abnormalities, placing patients at risk for falls. Large sensory axons innervating muscle spindles provide feedback for balance and gait and, when damaged, can cause altered sensorimotor function. This study aimed to determine whether symptoms of large-fiber DPN in type 1 and type 2 diabetic mouse models are related to alterations in muscle spindle innervation. In addition, diabetic mice were treated with insulin to assess whether sensorimotor and spindle deficits were reversible.


METHODS
Behavioral assessments were performed in untreated and treated streptozotocin (STZ)-injected C57BL/6 mice to quantitate diabetes-induced deficits in balance and gait. Quantification of Ia axon innervation of spindles was carried out using immunohistochemistry and confocal microscopy on STZ-injected C57BL/6 and db/db mice.


RESULTS
STZ-injected C57BL/6 mice displayed significant and progressive sensorimotor dysfunction. Analysis of Ia innervation patterns of diabetic C57BL/6 spindles revealed a range of abnormalities suggestive of Ia axon degeneration and/or regeneration. The multiple abnormal Ia fiber morphologies resulted in substantial variability in axonal width and inter-rotational distance (IRD). Likewise, db/db mice displayed significant variability in their IRDs compared with db(+) mice, suggesting that damage to Ia axons occurs in both type 1 and type 2 diabetes models. Insulin treatment improved behavioral deficits and restored Ia fiber innervation in comparison with nondiabetic mice.


CONCLUSIONS
Similar to small fibers, Ia axons are vulnerable to diabetes, and their damage may contribute to balance and gait deficits. In addition, these studies provide a novel method to assay therapeutic interventions designed for diabetes-induced large-fiber dysfunction.
Muller
Karra A
KA
Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, Kansas 66160, USA.
Ryals
Janelle M
JM
Feldman
Eva L
EL
Wright
Douglas E
DE
D000818:Animals; D001369:Axons / Q000502:physiology; D003921:Diabetes Mellitus, Experimental / Q000188:drug therapy / Q000503:physiopathology*; D003929:Diabetic Neuropathies / Q000188:drug therapy / Q000503:physiopathology*; D004195:Disease Models, Animal; D007328:Insulin / Q000627:therapeutic use; D051379:Mice; D008810:Mice, Inbred C57BL; D009043:Motor Activity / Q000187:drug effects; D009470:Muscle Spindles / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000294:innervation*; D009412:Nerve Fibers / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology
D007328:Insulin