Reducing CXCR4-mediated nociceptor hyperexcitability reverses painful diabetic neuropathy
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
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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Research Article Neuroscience

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 8

Chemogenetic inhibition of Nav1.8-positive DRG neurons expressing the inhibitory DREADD receptor PDi is G-protein mediated.

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Chemogenetic inhibition of Nav1.8-positive DRG neurons expressing the in...
(A) Current-clamp recordings from inhibitory PDi–expressing Nav1.8-positive neurons in primary cultures isolated from Nav1.8-Cre;Ai9;RC::PDi mice fed a RD (blue). (B) Application of CNO (2.5 μM) reduced the AP frequency, and (C) washing out the CNO partially restored the firing rate. (D–F) Overnight incubation of RD DRG cultures with pertussis toxin (PTX, green) abolished the inhibitory effect of CNO. (G) In RD Nav1.8-positive DRG neurons expressing DREADD receptors, a significant decrease in AP frequency after application of CNO at both the ×1 and ×2 rheobase was observed. ***P < 0.001 and ****P < 0.0001 (n = 7 and 9, respectively). (H) The same mice fed a HFD also showed a decrease in AP frequency after application of CNO.***P < 0.001 and ****P < 0.0001 (n = 9 for both groups). (I) Overnight incubation of DRG cultures with pertussis toxin abolished the inhibitory effects of CNO. There was no difference in AP frequency after preincubation with PTX and application of CNO at either the ×1 or ×2 rheobase (n = 4 and 12, respectively). (J) [Ca2+]i responses in DRG explants from Nav1.8-Cre;RC::PDi GCaMP6 mice showed that [Ca2+]i responses after addition of capsaicin (2 μM) were inhibited during incubation with CNO (8 μM for 5 min). After washing, Nav1.8-positive DRG neurons showed restored [Ca2+]i transients to capsaicin (2 μM) and HK (10 mM) (n = 120 neurons; 10 explants). (K) The responses to lower concentrations of capsaicin were quantified as the responses to capsaicin as a percentage of the total number of HK-responsive neurons. *P < 0.05. Values are expressed as the mean ± SEM. P values were calculated using a Mann-Whitney U test.