Extracellular pH Controls NHE1 Expression in Epidermis and Keratinocytes: Implications for Barrier Repair

Abstract

We have previously shown that the Na+/H+ antiporter (NHE1) is an essential endogenous pathway responsible for stratum corneum (SC) acidification. Since the epidermis must re-establish its epidermal barrier after acute barrier perturbations, we asked whether the NHE1 was, in turn, regulated by changes in barrier status. We found that in vivo epidermal NHE1 expression was upregulated within hours of barrier disruption. We next asked whether NHE1 was regulated by barrier status per se, or by the SC alkalinization that accompanies barrier perturbation. NHE1 was upregulated by alkalinizing SC pH, whereas this antiporter was downregulated by acidifying SC pH, independent of changes in barrier status. Moreover, acidifying SC pH overrode the effects of barrier break in regulating NHE1 expression, suggesting that SC alkalinization is the major stimulus for increased NHE1 expression. Finally, we confirmed that the keratinocyte NHE1 antiporter is regulated by extracellular pH independent of barrier status, by demonstrating that NHE1 was upregulated in cultured keratinocytes exposed to pH 8.3 medium and downregulated in cultured keratinocytes exposed to pH 6.3 medium. These data suggest that the keratinocyte NHE1 is regulated by extracellular pH. SC barrier break also upregulates NHE1 expression, but this response seems to be mediated by concomitant changes in SC pH.

Basic structural and functional characteristics of the epidermal barrier in wild mammals living in different habitats and climates

“…This information is supported by our results of a likewise continuous production of the antiporter NHE-1 that regulates the pH conditions for barrier formation. NHE-1 controls the acidification of extracellular microdomains that are essential for activation of pH-sensitive enzymes to guarantee, most essentially, undisturbed repair of the epidermal permeability barrier (Behne et al 2002;Hachem et al 2005). We could also show, for example, that NHE-1 not only accumulates in the stratum granulosum, as demonstrated for laboratory rats (e.g.…”

“…In the keratinocytes, NHE-1 is essential for acidifying the lipid microdomains (lipid rafts; Wenneckes et al 2009) and very important for barrier repair or recovery (e.g. Behne et al 2002;Hachem et al 2005).…”

Focal Adhesion Kinase Controls pH-Dependent Epidermal Barrier Homeostasis by Regulating Actin-Directed Na+/H+ Exchanger 1 Plasma Membrane Localization

“…In response to acute alkalinization as in barrier disruption, NHE1 upregulation allows a quick response to restore epidermal permeability barrier homeostasis. 32,33 Therefore, alkalization of extracellular space might explain the compensatory up-regulation of NHE1 protein levels that we observed in SC of FAK K5 KO mice. Yet, even higher total NHE1 protein levels did not suffice to bring pH down to normal acidic values, because NHE1 failed to translocate to the plasma membrane in terminally differentiated FAK Ϫ/Ϫ keratinocytes.…”

“…NHE1 must be localized at the plasma membrane of terminally differentiated keratinocytes to regulate extracellular pH. 33,36 Immunolocalization studies confirmed that very little of NHE1 translocated from the cytosol to the plasma membrane in the upper SG of FAK K5 KO mice ( Figure 6B levels of terminal differentiation, assessed as expression levels of early (involucrin) and late (loricrin) differentiation markers ( Figure 7B). As in our in vivo studies presented above, NHE1 translocation was defective in differentiated keratinocytes lacking functional FAK, whereas NHE1 production was not significantly affected.…”

“…36 The histidase pathway is not essential for bulk SC acidification because adult his/his mice still display normal acidic pH of SC even though they have lost Ͼ90% of histidase function. 33 sPLA2 represent a large family of low-molecular weight, Ca 2ϩ -requiring secretory enzymes that catalyze the hydrolysis of phospholipids at the sn-2 position, leading to the generation of free fatty acids and lysophospholipids. We compared levels of epidermal sPLA2 isoforms in FAK loxP/ϩ and FAK K5 KO mice using real-time PCR ( Figure 5).…”