PTPN2 mutations cause epithelium-intrinsic barrier loss that synergizes with mucosal immune hyperactivation (original) (raw)

In this issue of the JCI, Marchelletta and Krishnan et al. focus on the role of TCPTP in intestinal barrier regulation (7). Disease-associated human PTPN2 polymorphisms result in loss of TCPTP function, making Ptpn2 WT, knockout, and heterozygous mice a useful system. Both heterozygous and knockout mice showed markedly increased permeability to 4 kDa dextran, relative to WT mice. In contrast, the barrier to 70 kDa dextran was maintained (7). These results implicate increased flux across the low-capacity paracellular leak pathway, accommodating macromolecules up to approximately 125 Å in diameter, and exclude epithelial damage as the source of barrier loss (8). The leak pathway is regulated by myosin light chain kinase, which phosphorylates myosin II regulatory light chain to trigger endocytosis of the tight junction protein occludin (9, 10). Consistent with this process, Marchelletta and Krishnan et al. found that myosin II regulatory light chain phosphorylation was increased and occludin was internalized in epithelial cells of _Ptpn2_-knockout mice (also referred to as _Tcptp_-deficient mice). Although not explored here, it is likely that myosin light chain kinase expression was also increased (11, 12).

Marchelletta and Krishnan et al. also demonstrated increased expression of the pore-forming tight junction protein claudin-2 in _Ptpn2_-knockout mice. Claudin-2 may contribute to the increased transmucosal conductance, i.e., reduced resistance, measured in the distal ileum and cecum of _Ptpn2_-knockout mice, but cannot explain increased 4 kDa dextran flux, as claudin-2 channels exclude molecules larger than water and monovalent cations. The observed claudin-2 upregulation may be secondary to increased mucosal IL-13 and IL-22, each of which promotes claudin-2 transcription, were increased within intestinal mucosae of _Ptpn2_-knockout mice. In addition to these cytokine-mediated effects, Marchelletta and Krishnan et al. considered the possibility that TCPTP loss in epithelial cells could directly amplify STAT signaling and claudin-2 expression. To pursue this possibility, the authors developed intestinal epithelial-specific _Ptpn2_-knockout mice (also called TCPTP-deficient mice). In the absence of exogenous insults, intestinal epithelial STAT signaling and claudin-2 expression were upregulated in these mice, supporting the conclusion that epithelial TCPTP loss contributes to disease (7).

The complex in vivo milieu makes it difficult to definitively demonstrate epithelium-intrinsic effects of TCPTP mutation. Marchelletta and Krishnan et al. turned to a stably expressed dominant-negative TCPTPC126S mutant in human intestinal epithelial cells. This mutant was sufficient to increase both claudin-2 expression and pore pathway permeability. siRNA blockade of claudin-2 expression partially corrected this transepithelial electrical resistance (TER) loss (7). TCPTP dysfunction is therefore sufficient to activate claudin-2 expression and increase permeability of the high-capacity, charge- and size-selective pore pathway via an epithelium-intrinsic process (8).

Analyses of tissues from intestinal epithelial-specific _Ptpn2_-knockout mice confirmed increased claudin-2 expression and reduced TER, but 4 kDa dextran permeability was unchanged. However, in vitro studies showed that both STAT1 activation and 4 kDa dextran permeability increases induced by IFN-γ were potentiated in TCPTPC126S-expressing epithelial cells, consistent with the role of TCPTP in extinguishing cytokine signaling (7). Hyperactivation of STAT and MAPK pathways in TCPTP-deficient epithelial cells is therefore likely to explain increased pore and leak pathway permeabilities by promoting transcription of claudin-2 and myosin light chain kinase, respectively (13, 14).