Promoting Neuronal Tolerance of Diabetic Stress: Modulating Molecular Chaperones

Abstract

The etiology of diabetic peripheral neuropathy (DPN) involves an interrelated series of metabolic and vascular insults that ultimately contribute to sensory neuron degeneration. In the quest to pharmacologically manage DPN, small-molecule inhibitors have targeted proteins and pathways regarded as "diabetes specific" as well as others whose activity are altered in numerous disease states. These efforts have not yielded any significant therapies, due in part to the complicating issue that the biochemical contribution of these targets/pathways to the progression of DPN does not occur with temporal and/or biochemical uniformity between individuals. In a complex, chronic neurodegenerative disease such as DPN, it is increasingly appreciated that effective disease management may not necessarily require targeting a pathway or protein considered to contribute to disease progression. Alternatively, it may prove sufficiently beneficial to pharmacologically enhance the activity of endogenous cytoprotective pathways to aid neuronal tolerance to and recovery from glucotoxic stress. In pursuing this paradigm shift, we have shown that modulating the activity and expression of molecular chaperones such as heat shock protein 70 (Hsp70) may provide translational potential for the effective medical management of insensate DPN. Considerable evidence supports that modulating Hsp70 has beneficial effects in improving inflammation, oxidative stress, and glucose sensitivity. Given the emerging potential of modulating Hsp70 to manage DPN, the current review discusses efforts to characterize the cytoprotective effects of this protein and the benefits and limitations that may arise in drug development efforts that exploit its cytoprotective activity.

Keywords: Bioenergetics; Diabetic neuropathy; Heat shock proteins; Hsp70; Inflammation; Molecular chaperones; Novologues.

Fig. 1. Modulating Hsp70 with KU-596 decreased the expression of txnip in diabetic DRG in an Hsp70-dependent manner

Wild type (A) and Hsp70 KO (B) mice were rendered diabetic with streptozotocin and after 12 weeks of diabetes, treated with vehicle or weekly doses of KU-596 at 20 mg/kg for 4 weeks. mRNA was harvested from the lumbar DRG and used for RNA-Seq analysis. Shown are representative alignments to the mouse txnip gene from non-diabetic, diabetic and diabetic animals that received KU-596 therapy. Dark blue bars indicate sequences that mapped to the forward strand while dark red bars are sequences that mapped to the reverse strand of the 8 exons of the txnip gene. See Ma et al., (2015) for additional details.

Fig. 2. Modulating Hsp70 with KU-32 decreased neuregulin-induced c-jun expression and demyelination in an Hsp70-dependent manner

Myelinated DRG explants were prepared from wild type or Hsp70 KO mice. The cultures were treated with vehicle or 1 μM KU-32 overnight prior to inducing demyelination with 200 ng/ml neuregulin 1. Cultures were stained for myelin basic protein and the degeneration of the myelinated segments was quantified (A, B). Other cultures were prepared for immunoblot analysis of c-jun and the effects of the treatments on c-jun and phospho-c-jun expression quantified (C–E). Modified with permission from Li et al., (2012), ASN Neuro published by Portland Press.

Fig. 3. Summary of potential impact of iHsp70 and eHsp70 on hyperglycemia-mediated dysfunction

(I) Hyperglycemia manipulates multiple metabolic pathways to induce both intra- and extracellular stresses to the cell. These events converge at three points affecting mitochondrial dysfunction, oxidative stress and pro-inflammatory signaling to facilitate the progression of DPN. (II) iHsp70 inhibits hyperglycemia induced dysfunctions through multiple mechanisms including mitochondrial protein shuttling, up/downregulation of anti/pro-oxidant proteins respectively, repair or degradation of damaged and mis-folded protein, and inhibition of pro-inflammatory signaling via NF-κ B. (III) eHsp70 may play a complicated role since if internalized, it can bolster iHsp70 stores and actions while external binding to one of multiple purported receptors can antagonize the action of iHsp70 by initiating or exacerbating NF-κB mediated inflammatory signaling.