Reducing CXCR4-mediated nociceptor hyperexcitability reverses painful diabetic neuropathy
Nirupa D. Jayaraj, … , Richard J. Miller, Daniela M. Menichella
Nirupa D. Jayaraj, … , Richard J. Miller, Daniela M. Menichella
Published March 13, 2018
Citation Information: J Clin Invest. 2018;128(6):2205-2225. https://doi.org/10.1172/JCI92117.
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Reducing CXCR4-mediated nociceptor hyperexcitability reverses painful diabetic neuropathy

Abstract

Painful diabetic neuropathy (PDN) is an intractable complication of diabetes that affects 25% of patients. PDN is characterized by neuropathic pain and small-fiber degeneration, accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability and loss of their axons within the skin. The molecular mechanisms underlying DRG nociceptor hyperexcitability and small-fiber degeneration in PDN are unknown. We hypothesize that chemokine CXCL12/CXCR4 signaling is central to this mechanism, as we have shown that CXCL12/CXCR4 signaling is necessary for the development of mechanical allodynia, a pain hypersensitivity behavior common in PDN. Focusing on DRG neurons expressing the sodium channel Nav1.8, we applied transgenic, electrophysiological, imaging, and chemogenetic techniques to test this hypothesis. In the high-fat diet mouse model of PDN, we were able to prevent and reverse mechanical allodynia and small-fiber degeneration by limiting CXCR4 signaling or neuronal excitability. This study reveals that excitatory CXCR4/CXCL12 signaling in Nav1.8-positive DRG neurons plays a critical role in the pathogenesis of mechanical allodynia and small-fiber degeneration in a mouse model of PDN. Hence, we propose that targeting CXCR4-mediated DRG nociceptor hyperexcitability is a promising therapeutic approach for disease-modifying treatments for this currently intractable and widespread affliction.

Authors

Nirupa D. Jayaraj, Bula J. Bhattacharyya, Abdelhak A. Belmadani, Dongjun Ren, Craig A. Rathwell, Sandra Hackelberg, Brittany E. Hopkins, Herschel R. Gupta, Richard J. Miller, Daniela M. Menichella

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Figure 11

Chemogenetic activation of hM3Dq excitatory DREADD receptors in Nav1.8-positive DRG neurons leads to increased neuronal excitability.

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Chemogenetic activation of hM3Dq excitatory DREADD receptors in Nav1.8-p...
(A) The Nav1.8-Cre;RC::L-hM3Dq construct used in these experiments was designed so that Nav1.8-positive DRG neurons expressed m-Cherry–fused hM3Dq excitatory DREADD receptors, whereas all other cells expressed EGFP. (B) Representative images of DRGs (top and middle) and spinal cords (bottom) showing Nav1.8-positive DRG neurons expressing m-Cherry–fused hM3Dq excitatory DREADD receptors, whereas all other cells expressed EGFP. Original magnification, ×20, ×100, and ×10. Scale bars: 150 μm (top), 10 μm (middle), and 150 μm (bottom). (C–E) DRG neurons were cultured from hM3Dq excitatory DREADD mice and subjected to Fura-2–based [Ca2+]i imaging. Only cells expressing the hM3Dq DREADD receptors had [Ca2+]i responses to CNO (7.5 μM) (red), whereas all other EGFP-expressing cells did not respond (green). (D) Percentage of neurons responding to CNO (84.042% ± 1.9%). ****P < 0.0001, by Mann-Whitney U test (n = 94). (E) Representative image of the neurons used for [Ca2+]i imaging. Red indicates m-Cherry. Green indicates EGFP. Scale bar: 50 μm. (F–J) DRG primary cultures were prepared from these hM3Dq excitatory DREADD mice and m-Cherry–expressing cells were recorded. (G) Treatment with CNO (7.5 μM) along with a depolarizing current step led to increased AP frequency compared with (F) the current step alone or (H) after washing. (I) Changes in membrane voltage and (J) the AP frequency were quantified for various concentrations of CNO. These same experiments were performed in Nav1.8 td-Tomato DRG neurons that did not express DREADDs (Nav1.8-Cre;Ai9 mice). (I and J) Nav1.8-positve DRG neurons expressing the hM3Dq excitatory DREADD (Nav1.8-Cre;RC::L-hM3Dq) had significantly higher voltage membrane (Vm) and action potential frequencies compared to control Nav1.8-positve DRG neurons non-expressing the excitatory DREADD (Nav1.8-Cre;Ai9). *P < 0.05 and ***P < 0.001, by 1-way ANOVA with Tukey’s post hoc test (n = 16). Values are expressed as the mean ± SEM.