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1 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Clinical Institute, University of Aarhus, Aarhus, Denmark
2 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Institute of Anatomy, University of Aarhus, Aarhus, Denmark
3 NHLBI, LKEM, National Institutes of Health, Bethesda, MD, USA
* To whom correspondence should be addressed. E-mail: helle.praetorius{at}iekf.au.dk.
Since 
1 integrin is involved in sensing of fluid flow rate in endothelial cells, a function that in MDCK cells is confined to the primary cilium, we hypothesized 1 integrin to be an important part of the primary ciliary mechano-sensory apparatus in MDCK cells. We
observed that 1 integrin, 
3 and perhaps 5 integrin were localized to the primary cilium
of MDCK cells by combining lectin- and immuno-fluorescence confocal microscopy. 1 integrin was also co-localized with tubulin to the primary cilia of the rat renal collecting ducts, as well as to the cilia of proximal tubules and thick ascending limbs. Immunogoldelectron
microscopy confirmed the presence of 1 integrin on primary cilia of MDCK cells and rat collecting ducts.
Intracellular Ca2+ levels, monitored by fluorescence microscopy on Fluo-4 loaded MDCK cells, significantly increased upon addition of fibronectin, a 1 integrin ligand, to mature MDCK cells with an IC50 of 0.02 mg/l. In immature, non-ciliated cells or in deciliated mature cells, the IC50 was 0.40 mg/l. Blocking the fibronectin binding sites of 1 integrin with RGD peptide prevented the Ca2+ signal. Cross-linking of 1 integrins by Sambucus nigra agglutinin produced a Ca2+ response similar to the addition of fibronectin. Futhermore, the fibronectin-induced response was not dependent on flow or a flow-induced Ca2+ response. Finally, the flow-induced Ca2+ response was not prevented by the fibronectin-induced signal.
Although 1 integrin on the primary cilium greatly potentiates the fibronectin-induced Ca2+ signaling in MDCK cells, the flow-dependent Ca2+ signal is not mediated through activation of 1 integrin.
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