Background & Aims
Neuropathic pain conditions are frequent and hard to treat in clinics. For example, neuropathic pain associated with peripheral neuropathy induced by diabetes (DPN) or chemotherapy drugs (CIPN) severely reduces quality of life of related patients. However, the current treatment for neuropathic pain is inadequate. Ion channels are major mechanisms and targets for neuropathic pain. Compared to plasma membrane ion channels, intracellular ion channels have not been well studied. For instance, the role of a major lysosomal ion channel (TRPML1) has not been studied in any pain conditions. The current abstract aims to study the role of lysosomal ion channel TRPML1 in peripheral neuropathic pain conditions. Specifically, we aim to study the effects of 1) global knockout of TRPML1; 2) conditional knockout of TRPML1 in dorsal root ganglion (DRG) neurons, microglia, and astrocytes; and 3) TRPML1 modulators on neuropathic pain behaviors in mouse models of DPN and CIPN.
Methods
Animal: a global TRPML1 knockout line (B6.Cg-Mcoln1tm1Sasl/J, Jackson) was used. For conditional knockout of TRPML1, Mcoln1-flox (B6/JGpt-Mcoln1em1Cflox/Gpt, GemPharmatech), Avil-Cre (C57BL/6JSmoc-Avilem1(Cre)Smoc, Model Organisms), Cx3cr1-iCre (Cyagen), GFAP-Cre (Cyagen) were used.
Pain model: Type 1 diabetes (T1D) model was generated by STZ injection (Bierhaus et al., 2012; Agarwal et al., 2018). Type 2 pre-diabetes (T2D) model was induced by high-fat diet (HFD) (Cooper et al., 2017; George et al., 2022) . Oxaliplatin model was described previously (Su et al., 2020; Wu et al., 2021).
Behavioral test: Mechanical sensitivity was assessed by the von Frey assay of “simplified up-down” method (Su et al., 2020; Wu et al., 2021). Heat sensitivity was examined by Hargreaves’ test (Wu et al., 2021). Cold sensitivity was evaluated by acetone test (Su et al., 2020; Wu et al., 2021). Open-field test was used to assess the emotional component of pain behaviors.
Results
Global knockout of TRPML1: partially or fully prevented decreased paw withdrawal threshold (PWT) to von Frey filaments in male and/or female animals in peripheral neuropathic pain models induced by STZ, HFD, and oxaliplatin.
Conditional knockout of TRPML1: AvilCre:Mcoln1fl/fl mice partially or fully prevented oxaliplatin-induced decrease in PWT and/or increase in paw withdrawal score (PWS) to acetone in male and/or female groups. In contrast, CX3CR1Cre:Mcoln1fl/fl mice did not prevent oxaliplatin-induced any changes in PWT or PWS in either male or female groups. Interestingly, GFAPCre:Mcoln1fl/fl mice selectively reduced oxaliplatin-induced change in PWT (but not PWS) in male (but not female) group.
Behavioral pharmacology: A TRPML1 agonist (ML-SA1, 2 µg , ith) decreased PWT to von Frey filaments in WT but not TRPML1 gKO mice. A TRPML1 inhibitor (ML-SI3, 4µg, ith) reversed decreased PWT induced by oxaliplatin or STZ.
Conclusions
Overall, our results suggest that activation of lysosomal TRPML1 induces while inhibition of TRPML1 prevents and reverses neuropathic pain associated with DPN and CIPN. TRPML1 expressed in DRG neurons plays a critical role in the development of neuropathic pain induced by CIPN. TRPML1 expressed in astrocytes is important for CIPN-induced mechanical allodynia in male animals. Therefore, TRPML1 may be an important mechanism and potential target for peripheral neuropathic pain conditions.
References
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