Background Polycystic kidney disease (PKD) is the leading genetic cause of kidney failure, resulting in the accumulation of fluid-filled cysts and gross enlargement of the kidney. Mutations in PKD1 or PKD2, which encode ciliary polycystin proteins, are the most common cause of PKD. These proteins function in a cilia-dependent cyst activation (CDCA) pathway-one that requires cilia for its procystic function-yet the molecular driver(s) of this pathway are unknown. ARL13B is a regulatory GTPase enriched in cilia and links to cystogenesis in the kidneys. ARL13B possesses guanine nucleotide exchange factor (GEF) activity for ARL3, another ADP-ribosylation factor-like family GTPase with links to cilia.Methods We used 2 distinct Arl13b mouse alleles to investigate whether ARL13B is a component of the CDCA pathway: Arl13b V358A encodes for enzymatically normal ARL13B that is undetectable in cilia, and Arl13b R79Q encodes for cilia-localized ARL13B lacking a residue critical for its ARL3 GEF activity. We used these alleles in a Pkd1-deficient adult mouse model and investigated kidney morphology (hematoxylin and eosin and cystic index analysis), physiology (BUN measurements), kidney fibrosis (picrosirius staining and alpha-smooth muscle actin levels), kidney injury (SRY box transcription factor 9 immuno-fluorescent staining and quantification), and Wnt signaling (beta-catenin and cyclin D1 protein levels).Results We found that loss of ciliary ARL13B or mutation of a single residue critical for its ARL3 GEF activity suppressed Pkd1-dependent cysts. We observed reductions in kidney size, cystic index, and BUN. We also observed suppression of kidney fibrosis, kidney injury, and beta-catenin and cyclin D1 protein levels.Conclusions Our results identified a subcellular location and mechanism driving Pkd1-dependent cystogenesis in the kidneys. We demonstrated that expression of a critical residue for ARL13B's GEF activity specifically in cilia is a key mechanism of the CDCA pathway driving cystogenesis in the kidneys.
Background Diabetic kidney disease (DKD) is driven by complex pathologic mechanisms including lipid accumulation, chronic inflammation, and fibrosis, contributing to progressive kidney dysfunction. Macrophages are key contributors to DKD pathogenesis, yet their functional heterogeneity in the diabetic kidney remains poorly defined. Although triggering receptor expressed on myeloid cells 2 (TREM2+) macrophages have been identified within the diabetic microenvironment, the relationship between their developmental origin, anatomic localization, and functional specialization remains unclear.Methods In this study, we integrated single-cell RNA sequencing, spatial transcriptomic analysis of diabetic OVE26 kidneys, and reciprocal bone marrow transplantation in a high-fat diet plus streptozotocin-induced diabetic model to examine how TREM2+ macrophages are distributed across renal compartments and how TREM2 deficiency affects DKD injury.Results Global TREM2 deficiency markedly exacerbated tubulointerstitial injury and glomerular hypertrophy in DKD, establishing TREM2 as a protective factor. Single-cell RNA sequencing and spatial mapping revealed that TREM2 induction in diabetic kidneys occurs in 2 spatially distinct macrophage populations: Mrc1+ kidney-resident macrophages localized to the periglomerular niche and Ccr2+ monocyte-derived macrophages infiltrating the tubulointerstitium. Transcriptomic and functional analyses were consistent with a role of TREM2 in macrophage phagocytic and lipid-handling programs. Its loss led to the downregulation of phagosome and endocytic pathways, resulting in failed clearance of apoptotic debris and lipid aggregates, which was particularly pronounced in the tubulointerstitium. Reciprocal bone marrow transplant experiments suggest that recipient-resident TREM2 was more associated with preservation of glomerular structure, whereas donor-derived hematopoietic TREM2 was more strongly associated with limiting tubular lipid accumulation, apoptotic tubular epithelial cells, and tubulointerstitial injury.Conclusions These findings suggested that TREM2-expressing macrophages contributed to compartment-specific protective responses in DKD. TREM2 deficiency was associated with greater diabetic glomerular and tubular injury, with distinct phenotypes observed in donor hematopoietic and recipient compartments.
