Plectin-containing, centrally-localized focal adhesions exert traction forces in primary lung epithelial cells

Receptor clustering upon cell-substratum attachment induces assembly of cytoplasmic protein complexes termed focal adhesions (FAs), which connect, albeit indirectly, extracellular matrix to the cytoskeleton. A subset of cultured primary alveolar epithelial cells (AEC) display a unique pattern of vinculin/paxillin/talin-rich FAs in two concentric circles when cultured on glass and micro-patterned substrates: one ring of FAs located at the cell periphery (pFAs), another FA ring located centrally in the cell (cFAs). Unusually, cFAs associate with an aster-like actin array as well as keratin bundles. Moreover, cFAs show rapid paxillin turnover rates following fluorescence recovery after photobleaching and exert traction forces similar to those generated by FAs at the cell periphery. The plakin protein plectin localizes to cFAs and is primarily absent from pFAs, whereas tensin, a marker of mature/fibrillar adhesions, is found in both cFAs and pFAs. In primary AEC in which plectin expression is depleted, cFAs are largely absent with an attendant reorganization of both the keratin and actin cytoskeletons. We suggest that the mechanical environment in the lung gives rise to the assembly of unconventional FAs in AEC. These FAs not only show a distinctive arrangement, but also possess unique compositional and functional properties.