Mechanical stretch activates piezo1 in caveolae of alveolar type I cells to trigger ATP release and paracrine stimulation of surfactant secretion from alveolar type II cells

Secretion of pulmonary surfactant in the alveoli of the lungs is essential to maintain lung function. Stretching of alveoli during lung inflation is the main trigger for surfactant secretion. Yet, the molecular mechanisms how mechanical distension of alveoli results in surfactant secretion are still elusive. The alveolar epithelium consists of alveolar epithelial type I (ATI) and surfactant secreting type II (ATII) cells. ATI, but not ATII cells, express caveolae, small plasma membrane invaginations that can respond to plasma membrane stresses and serve mechanotransductive roles. Within this study, we investigated the role of caveolae as mechanosensors in the alveolus. We generated a human caveolin-1 knockout ATI cell (hAELVi^cav-/- ) using CRISPR/Cas9. Wildtype (hAELVi^wt ) and hAELVi^cav-/- cells grown on flexible membranes responded to increasing stretch amplitudes with rises in intracellular Ca²⁺ . The response was less frequent and started at higher stretch amplitudes in hAELVi^cav-/- cells. Stretch-induced Ca²⁺ -signals depended on Ca²⁺ -entry via piezo1 channels, localized within caveolae in hAELVi^wt and primary ATI cells. Ca²⁺ -entry via piezo1 activated pannexin-1 hemichannels resulting in ATP release from ATI cells. ATP release was reduced in hAELVi^cav-/- cells. In co-cultures resembling the alveolar epithelium, released ATP stimulated Ca²⁺ signals and surfactant secretion from neighboring ATII cells when co-cultured with hAELVi^wt but not hAELVi^cav-/- cells. In summary, we propose that caveolae in ATI cells are mechanosensors within alveoli regulating stretch-induced surfactant secretion from ATII cells.

A role for polycystin-1 and polycystin-2 in neural progenitor cell differentiation

Polycystin-1 (PC1) and polycystin-2 (PC2) are transmembrane proteins encoded by the Pkd1 and Pkd2 genes, respectively. Mutations in these genes are causative for the development of autosomal-dominant polycystic kidney disease. A prominent feature of this disease is an unbalanced cell proliferation. PC1 and PC2 physically interact to form a complex, which localizes to the primary cilia of renal epithelial cells. Recently, PC1 and PC2 have also been described to be present in primary cilia of radial glial cells (RGCs) and to contribute to the planar cell polarity of late RGCs and E1 ependymal cells. As neural progenitor cells (NPCs), early RGCs have to balance proliferation for expansion, or for self-renewal and differentiation to generate neurons. It is not known whether the polycystins play a role in this process. Here, we show that PC1 and PC2 are expressed in RGCs of the developing mouse cerebral cortex during neurogenesis. Loss-of-function analysis and cell-based assays reveal that a reduction of PC1 or PC2 expression leads to increased NPC proliferation, while the differentiation to neurons becomes impaired. The increased NPC proliferation is preceded by enhanced Notch signaling and accompanied by a rise in the number of symmetric cell divisions. The transcription factor STAT3 seems to be mechanistically important for polycystin signaling in NPCs as either STAT3 knockdown or inhibition of STAT3 function abrogates the increased proliferation driven by reduced polycystin expression. Our findings indicate that PC1 and PC2 are critical for maintaining a balance between proliferation and differentiation of NPCs.