In a forever busy pharmaceutical market, the need for analgesic and anti-inflammatory drugs is still in high demand. Various conditions such as rheumatoid arthritis, diabetes or viral infections are frequently associated with persistent and unalleviated pain, therefore uncovering novel pharmacological targets is deemed necessary.
Store-operated calcium entry (SOCE), a complex mechanism orchestrated by calcium release activated calcium (CRAC) channels, has been recently associated with neuropathic and inflammatory pain. Moreover, several attempts to describe the presence of CRAC channel molecular components in pain-sensing DRG and dorsal horn neurons have been made. Yet, the exact identity of CRAC channel complex components, the function of junctional proteins in CRAC channel assembly and the exact role of SOCE within primary afferent neurons remains poorly understood. Here I hypothesized that SOCE functions as a facilitator of Ca2+ signalling in inflammatory pain conditions and that junctional proteins offer a supportive role of the CRAC channels assembly in DRG neurons.
To test this, I characterized the localization pattern of CRAC components within DRG neurons by immunohistrochemistry. STIM (1 and 2) and Orai (1 and 3) isoforms were all expressed within primary afferent neurons of different sensory modalities. STIM1 and Orai1 were expressed to a higher degree as compared to their family members and co-localized into puncta upon SOCE activation as demonstrated by proximity ligation assay (PLA). Interestingly, STIM2 revealed a higher preference in large-diameter (presumed mechanosensory) neurons, as compared to STIM1.
To confirm the presence of SOCE, the effect of CRAC inhibitors YM58483 and Synta66 in DRG neurons was also investigated. Both compounds significantly reduced SOCE in DRG neurons; the efficacy of inhibition was dramatically increased by pre-incubation suggesting that these CRAC inhibitors may not act as direct ion channel pore blockers but rather interfere with the CRAC complex functional assembly.
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CRAC channel complex assembly relies on the close proximity of the cellular plasma membrane and endoplasmic reticulum, yet, nothing is known thus far about how these membrane junctions are maintained in DRG. This thesis brings evidence of the expression of Junctophilin (JPH) proteins within primary sensory neurons. Amongst the family members, JPH4 was expressed the highest, followed by JPH1 and JPH3, while no expression of JPH2 was observed.
Moreover, experiments using immunocytochemistry revealed that JPH4 is co-expressed with Orai1 and STIM1 in pain-sensing neurons, while PLA demonstrated that both Orai1 and STIM1 co-localize with JPH4 at endoplasmic reticulum (ER)-plasma membrane (PM) junctions upon bradykinin (BK) -induced SOCE.
To test whether JPH4 is a SOCE facilitator, several experiments were performed. Thus, it was demonstrated that STIM1 directly interacts with JPH4 upon ER Ca2+ store depletion using immunoprecipitation. Furthermore, knocking-down JPH4 disrupted the Orai1-STIM1 clustering and coupling at the ER-PM junctions as observed by PLA. Interestingly, silencing JPH4 in DRG neurons inhibited SOCE without altering the basal Ca2+ pool levels and impaired the subsequent ER Ca2+ store refill, as demonstrated by calcium imaging experiments. These observations taken together strengthen the role of JPH4 in facilitating SOCE in DRG neurons.
CRAC channels have been previously considered attractive targets for treatment of inflammatory conditions and more pharmaceutical companies showed an increased interest in developing CRAC modulators in the past decade. The high expression of JPH4 in peripheral sensory neurons, its importance in promoting the CRAC channel assembly and its role on the oscillatory ER Ca2+ refill provide evidence for considering JPH proteins as putative targets for treating inflammatory pain.
