Investigation of juxtaglomerular structure and function
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Renin is the initiator and rate-limiting factor of the renin-angiotensin system, a major mechanism of blood pressure regulation. The synthesis and secretion of this active circulatory enzyme is confined exclusively to the dense core granules of kidney juxtaglomerular (JG) cells where its precursor prorenin is packaged, cleaved to the active form and stored for release on a regulated pathway. Given its importance, surprisingly little is known about this process, in part due to the difficulty in culturing primary JG cells in vitro and the lack of reliable cell lines. The initial aim of the current work was to investigate renin-containing granule dynamics in live JG cells. To achieve this, I attempted to derive novel cell lines from triple transgenic mouse models comprising immortalised granulated or non-granulated JG cells. Due to the nature of JG cells in culture, the use of these cell lines to investigate granulation was not feasible; therefore the culture of primary JG cell culture was modified and enhanced to visualise granule dynamics in live, primary JG cells for the first time. By isolating cells using a Percoll gradient and plating them on fibronectin-coated dishes, rapid and full adhesion of JG cells was achieved, as well as prolonged expression of renin from 3 days to up to 8 days post-isolation. Using this protocol to isolate JG cells from RenGFP renin reporter mice and identifying granules using the acidotropic fluorophore Lysotracker, granule dynamics were investigated in primary JG cells. High resolution, rapid image acquisition was performed using widefield and total internal reflection microscopy, showing that dense core granules respond dynamically to the β-adrenergic agonist isoproterenol, a known renin secretory stimulus. Two different pools of granules of varying granule diameters and dynamic parameters were identified optically, suggesting that separate granule pools were being identified. Mice null for the Ren-1d gene lack renin storage granules in their JG cells, however granulation was restored in Ren1d-null mice carrying a transgene encompassing the human renin (hRen) locus. Therefore in order to investigate the relationship between renin expression and the amount of granulation in JG cells, mice expressing human renin were used. To dissect the granulation phenotype in detail, 2D electron micrographs were taken of JG cells, which were immunogold stained to confirm renin content, and reconstructed in 3D. Female hRen mice showed a significantly higher volume of granulation and an increased granule number compared to males, a finding consistent with the sexually dimorphic expression of the transgene, supporting the hypothesis that granulation in JG cells is dependent on the level of renin expression. The macula densa (MD) is a critical sensor of flow and salt content in the blood; through extensive tubulo-vascular crosstalk known as tubuloglomerular feedback (TGF), it releases key signalling factors stimulating and inhibiting renin synthesis and secretion from JG cells. Ren-1d-/- mice showed a hypercellular and columnar MD plaque, which was not restored by the introduction of the hRen transgene, indicating that TGF may be impaired in these mice. Using an isolated, perfused juxtaglomerular apparatus model it was shown that high salt- and increased flow-induced TGF functioned effectively in Ren1d-/- and huRen+/-Ren1d-/- mice, although animals on a Ren1d-/- background showed decreased sensitivity of glomerular tuft contraction and abnormal calcium signalling within macula densa cells. In conclusion, an appropriate in vitro model was developed for investigating granule dynamics in JG cells, using which granule motion was visualised and quantified for the first time in these cells. Although JG cell granulation is required for normal MD morphology, it was shown to not affect JGA function.