The roles of Ca2+- influx and PLC for pemphigus vulgaris pathogenesis
Ca2+ flux-dependent signalling plays an important role in pemphigus pathogenesis. Inhibiting Ca2+ signalling blocks PV-IgG-induced loss of keratinocyte adhesion in vitro and blistering in human skin ex vivo (Figure 5). Binding of PV-IgG toDSG 1 [1], associated with PI4K and PLC , activates PLC and releases IP3 [2]. IP3 activates IP3Rthereby releasing Ca2+ from the ER into the cytosol [3]. Low ER Ca2+ concentration activatesSTIM1 at the ER membrane. STIM1 contacts ORAI1 by forming the CRAC and thereby causing Ca2+ influx from the extracellular space [4]. With the IP3R still active, this further increases the Ca2+ concentration in the cytosol. Cytosolic Ca2+ finally activates PKC[5], which destabilizes the desmosome by keratin retraction and depletion of DSG 1 and DSG 3 [6].
The complete mechanism of action of PKC is still unknown. However, it presumably affects desmosome turn-over on several levels. Upon PV-IgG treatment PKC has been proposed to translocate to the desmosomal plaque . Cytoskeletal components such as keratin 8 and 18 are substrates of PKC possibly inducing keratin filament retraction. Similarly, PKC affects the actin cytoskeleton, mostly causing disassembly and reduced anchorage and may also modulate desmosome turn-over by modulating the actin-binding adducin . Moreover, PKCphosphorylates DP which is involved in destabilizing the desmosome , and caused DSG 3 fragmentation and acantholysis in pemphigus passive-immune-transfer mouse models which supports the observations that inhibiting PKC ameliorated PV-IgG-induced loss of adhesion in vitro .
Ca2+ has very profound effects on desmosome regulation. The stability of newly formed desmosomes is dependent on extracellular Ca2+ . After several days, desmosomes become hyper-adhesive and independent of extracellular Ca2+ . Hyper-adhesive desmosomes are also less sensitive to PV-IgG . Inhibiting PKC causes a rapid transformation from Ca2+-dependent to Ca2+-independent desmosomes . Active PKC is associated with DSG 1, which is known to be important for regulation of epidermal differentiation . It is thus conceivable thatDSG 1 controls desmosome adhesion and skin differentiation at least in part by controlling Ca2+ levels in keratinocytes.
Loss of cell adhesion in response to AK23, which did not cause Ca2+ influx and disrupts DSG 3 but notDSG 1 was ameliorated significantly by inhibiting PLC . This indicates that PKC , which was observed to be involved inDSG 3 depletion in vivo and in vitro may be activated independently of Ca2+ or baseline activity is sufficient. It was reported, that PV-IgG against thyroid peroxidase and other targets might also be able to activate Ca2+flux possibly via PLC . This indicates that these non-desmoglein antibodies might play a crucial role for disease severity and relapse.
It is unlikely that PKC is the sole downstream pathway activated by antibodies against DSG 1. In human epidermis, inhibition ofPKC alone under conditions which were effective in vitroand in mice in vivo, was not enough to block blistering in human skin ex vivo . In contrast, inhibition of p38MAPK , was protective in murine skin in vivo and human epidermis but not in mucosa ex vivo . Since we observed that p38MAPK is associated with both DSG 1 and DSG 3 , these data are compatible with the hypothesis that antibodies against DSG 1 andDSG 3 or other targets are required for skin blistering.PLC might also modulate other events regulated by p38MAPKsuch as actin remodelling via RHOA .
This demonstrates that PV pathology is dependent on the antibody profiles, different anti-DSG 1/DSG 3 ratios as well as different target epitopes might play an important role . This might influence the clinical phenotype by inducing specific signalling responses in keratinocytes . More studies are required to delineate the functional interplay between the complex mechanisms involved in pemphigus pathogenesis. Nevertheless, the data presented here can explain for the first time why autoantibodies against DSG 1 but not DSG 3 are required for epidermal involvement in PV. SincePI4K upstream of Ca2+ signalling specifically interacts with DSG 1 but not with DSG 3, this knowledge may allow a new strategy to develop DSG 1-specific treatment options in pemphigus.