Sensory Neurons Derived from Diabetic Rats Have Diminished Internal Ca Stores Linked to Impaired Re-uptake by the Endoplasmic Reticulum

Abstract

Distal symmetrical sensory neuropathy in diabetes involves the dying back of axons, and the pathology equates with axonal dystrophy generated under conditions of aberrant Ca 2+ signalling. Previous work has described abnormalities in Ca 2+ homoeostasis in sensory and dorsal horn neurons acutely isolated from diabetic rodents. We extended this work by testing the hypothesis that sensory neurons exposed to long-term Type 1 diabetes in vivo would exhibit abnormal axonal Ca 2+ homoeostasis and focused on the role of SERCA (sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase). DRG (dorsal root ganglia) sensory neurons from age-matched normal and 3–5-month-old STZ (streptozotocin)-diabetic rats (an experimental model of Type 1 diabetes) were cultured. At 1–2 days in vitro an array of parameters were measured to investigate Ca 2+ homoeostasis including (i) axonal levels of intracellular Ca 2+ , (ii) Ca 2+ uptake by the ER (endoplasmic reticulum), (iii) assessment of Ca 2+ signalling following a long-term thapsigargin-induced blockade of SERCA and (iv) determination of expression of ER mass and stress markers using immunocytochemistry and Western blotting. KCl- and caffeine-induced Ca 2+ transients in axons were 2-fold lower in cultures of diabetic neurons compared with normal neurons indicative of reduced ER calcium loading. The rate of uptake of Ca 2+ into the ER was reduced by 2-fold ( P <0.05) in diabetic neurons, while markers for ER mass and ER stress were unchanged. Abnormalities in Ca 2+ homoeostasis in diabetic neurons could be mimicked via long-term inhibition of SERCA in normal neurons. In summary, axons of neurons from diabetic rats exhibited aberrant Ca 2+ homoeostasis possibly triggered by suboptimal SERCA activity that could contribute to the distal axonopathy observed in diabetes.