Macrophage Accumulation and Cyst Expansion in Pkd2 , Ift88 , and Double Mutant Mouse Models.

Abstract

Macrophage accumulation in cystic kidney disease is not directly regulated by the cilia-dependent cyst activation pathway. Macrophage accumulation and cytokine expression are not driving cyst initiation but rather parallel cyst expansion and contribute to cyst progression. Kidney cyst formation occurs due to loss of cilia-localized polycystin proteins (<i>e.g</i>., <i>Pkd1</i> or <i>Pkd2</i>) or ciliary structure (<i>e.g</i>., <i>Ift88</i> or <i>Kif3a</i>). However, cyst progression is more rapid in polycystin mutant mice compared with cilia mutant mice, and loss of cilia in the polycystin mutant background (<i>e.g</i>., <i>Pkd2</i> and <i>Kif3a</i> mutation) greatly attenuates cyst development. This led to the proposal that the polycystins function to repress a cyst-promoting pathway that is dependent on an intact cilium, this is referred to as the cilia-dependent cyst activation pathway. Renal macrophages are also involved in regulating cyst progression, but it is unknown whether this occurs through the cilia-dependent cyst activation or separate pathway. To examine whether macrophage accumulation was regulated through a cilia-dependent pathway, we compared macrophage accumulation and cytokine expression levels in <i>Pkd2</i> mutant kidneys with or without intact cilia (<i>Ift88</i> mutants). To avoid the impact of cyst-induced damage on macrophage accumulation, we conducted comparisons after standardizing the samples for cystic indices between the <i>Pkd2</i>, <i>Ift88</i>, and <i>Pkd2;Ift88</i> double mutants. Disruption of <i>Ift88</i> in <i>Pkd2</i> mutants reduced cyst burden and attenuated macrophage accumulation and cytokine expression levels. However, when the mutants were standardized based on cystic indices, no significant differences in macrophage number or cytokine expression were evident between the <i>Pkd2</i>, <i>Ift88</i>, and <i>Pkd2;Ift88</i> double mutants at either early or advanced stage of cyst progression, and pathway analysis revealed that the macrophage populations were similar between groups based on single-cell RNAseq data. These data indicated that macrophage accumulation and cytokine expression did not drive cyst initiation but rather paralleled cyst expansion regardless of the genotype or rate of disease progression through a cilia-independent pathway.