Extracellular matrix and vascular dynamics in the kidney of a murine model for Marfan syndrome

**Background:** Marfan syndrome (MFS) is an autosomal dominant connective tissue disorder caused by mutations in the FBN1 gene encoding fibrillin-1. While the kidney is not classically considered a primary affected organ, case reports describe glomerular disease in MFS patients. Fibrillin-1 is a structural component of the kidney's glomerulus and peritubular tissue. This study aimed to characterize extracellular matrix (ECM) changes and vascular hemodynamics in the kidney using the mgΔ^lpn^ mouse model of MFS.

**Methods:** Twenty-eight 6-month-old male C57Bl/6 mice were used (14 WT, 14 MFS heterozygotes). Histological analysis (PAS staining) was performed on cortical kidney sections to measure glomerular, capillary, and urinary space areas. Transmission electron microscopy (TEM) with tannic acid and orcein staining was used to evaluate fibers of the elastic fiber system (FEFS). Serial block-face scanning electron microscopy (SBF-SEM) provided 3D ultrastructural reconstructions. Immunofluorescence quantified fibrillin-1, fibronectin, MMP-9, collagen types I and III, α-smooth muscle actin, and connexin-43. Immunohistochemistry assessed angiotensin II (Ang II). Renal artery and vein blood flow were measured using ultrasound flow probes. Microvascular density and velocity were assessed using sidestream dark-field (SDF) video microscopy. Serum creatinine and urea were measured as kidney function biomarkers.

**Key Results:** MFS mice showed significant reductions in glomerular area (WT 1423 μm² ± 655.6; MFS 681.7 μm² ± 113.7), capillary space (WT 1142 μm² ± 397.5; MFS 606 μm² ± 108.9), and urinary space (WT 218 μm² ± 250.7; MFS 138.7 μm² ± 240.4) (p<0.05 to p<0.005). Fibrillin-1 immunofluorescence intensity was significantly reduced (WT 0.86 ± 0.61; MFS 0.47 ± 0.67 intensity/pixel), as was fibronectin (WT 2.01 ± 1.22; MFS 0.71 ± 0.36 intensity/pixel) (p<0.001). MMP-9 intensity was significantly increased in MFS (WT 2.51 ± 1.62; MFS 5.27 ± 3.11 intensity/pixel) (p<0.001). Collagen type I (WT 0.61 ± 0.54; MFS 2.34 ± 1.64 intensity/pixel) and type III (WT 1.28 ± 0.93; MFS 1.80 ± 1.30 intensity/pixel) were significantly increased in MFS (p<0.001). α-smooth muscle actin was significantly increased (WT 12.61 ± 4.71; MFS 16.92 ± 5.11 intensity/pixel), while connexin-43 was significantly reduced (WT 1.05 ± 1.19; MFS 0.49 ± 0.59 intensity/pixel). TEM revealed fragmented FEFS, increased collagen fibers, basement membrane duplication, and amorphous material in the glomerular capillary of MFS mice. Cluster analysis of FEFS thickness identified three WT subgroups (A: 1-12nm, B: 12-29nm, C: 29-280nm); the MFS group (30.51 nm ± 44.95) differed significantly from all WT subgroups. Renal artery elastic fiber integrity was reduced (WT 0.89 ± 0.18; MFS 0.54 ± 0.28) and tunica media thickness was decreased (WT 15.98 μm ± 2.34; MFS 14.28 μm ± 1.78). Renal artery blood flow was significantly lower (WT 0.30 mL/min ± 0.05; MFS 0.1 mL/min ± 0.03), as was renal vein blood flow (WT 0.31 mL/min ± 0.04; MFS 0.07 mL/min ± 0.01) (p<0.005 to p<0.001). Aortic blood flow was also reduced (WT 3.18 mL/min ± 0.41; MFS 2.07 mL/min ± 0.69). Microvascular density was significantly increased in MFS for vessels 4-8 μm diameter, while blood flow velocity was significantly reduced across all diameters analyzed (WT 362.5 μm/s ± 128.5; MFS 141.1 μm/s ± 78.6). Serum creatinine (WT 0.60 ± 0.12; MFS 0.42 ± 0.08) and urea (WT 61.09 mg/dL ± 11.54; MFS 54.66 mg/dL ± 8.77) showed no significant differences. Angiotensin II expression in kidney tissue was significantly reduced in MFS (WT 166.6 ± 10.68; MFS 152.7 ± 12.41 intensity/pixel) (p<0.001).

**Clinical Implications:** This study demonstrates that MFS in the mgΔ^lpn^ mouse model leads to significant structural ECM remodeling in the kidney, including glomerular hypoplasia, fibrillin-1 and fibronectin reduction, increased collagen deposition, and MMP-9 upregulation. These changes are associated with altered macro- and micro-hemodynamics, including reduced renal artery and vein blood flow, increased microvascular density, and decreased microvascular velocity. Despite these pathological changes, standard renal function markers (creatinine, urea) remained normal, suggesting that structural and vascular alterations precede functional decline. The findings indicate that kidney involvement in MFS may contribute to hypertension and cardiovascular worsening through increased vascular resistance, even in the absence of overt renal dysfunction. This has potential implications for clinical management of MFS patients, suggesting that renal vascular assessment may be valuable even when standard kidney function tests are normal.