RNA-seq analysis of gene expression profiles in isolated stria vascularis from wild-type and Alport mice reveals key pathways underling Alport strial pathogenesis

Previous work demonstrates that the hearing loss in Alport mice is caused by defects in the stria vascularis. As the animals age, progressive thickening of strial capillary basement membranes (SCBMs) occurs associated with elevated levels of extracellular matrix expression and hypoxia-related gene and protein expression. These conditions render the animals susceptible to noise-induced hearing loss. In an effort to develop a more comprehensive understanding of how the underlying mutation in the COL4A3 gene influences homeostasis in the stria vascularis, we performed vascular permeability studies combined with RNA-seq analysis using isolated stria vascularis from 7-week old wild-type and Alport mice on the 129 Sv background. Alport SCBMs were found to be less permeable than wild-type littermates. RNA-seq and bioinformatics analysis revealed 68 genes were induced and 61 genes suppressed in the stria from Alport mice relative to wild-type using a cut-off of 2-fold. These included pathways involving transcription factors associated with the regulation of pro-inflammatory responses as well as cytokines, chemokines, and chemokine receptors that are up- or down-regulated. Canonical pathways included modulation of genes associated with glucose and glucose-1-PO4 degradation, NAD biosynthesis, histidine degradation, calcium signaling, and glutamate receptor signaling (among others). In all, the data point to the Alport stria being in an inflammatory state with disruption in numerous metabolic pathways indicative of metabolic stress, a likely cause for the susceptibility of Alport mice to noise-induced hearing loss under conditions that do not cause permanent hearing loss in age/strain-matched wild-type mice. The work lays the foundation for studies aimed at understanding the nature of strial pathology in Alport mice. The modulation of these genes under conditions of therapeutic intervention may provide important pre-clinical data to justify trials in humans afflicted with the disease.

MO033THE DUAL ETAR/AT1R BLOCKER SPARSENTAN SLOWS RENAL DISEASE, IMPROVES LIFESPAN, AND ATTENUATES HEARING LOSS IN ALPORT MICE: COMPARISON WITH LOSARTAN

Background and Aims

In Alport syndrome (AS), endothelin type A receptor (ETAR) activation plays a role in both renal and inner ear pathologies. Currently, angiotensin converting enzyme inhibitors or angiotensin II type 1 receptor (AT1R) blockers are the standard of care for patients with AS; however, these drugs have not been shown to improve hearing. Previously, we showed that sparsentan (SP), a dual ETAR/AT1R inhibitor, prevented increases in proteinuria, fibrosis, glomerulosclerosis, and glomerular basement membrane dysmorphology. The AS mouse model lifespan was extended when SP treatment was started at 3 weeks (W) of age (pre-proteinuric). Here we compare the effect of SP and the AT1R blocker losartan (LS) on lifespan and proteinuria in AS mice treated from 4 W, and on hearing loss and associated inner ear pathology in AS mice treated from 3 W to 8.75 W.

Method

Wild type (WT) and AS mice were treated daily with vehicle (V), 60, 120, or 200 mg/kg SP (SP60, SP120, or SP200) by oral gavage, or 20 mg/kg LS by daily oral gavage from 3-4 W of age and in drinking water at 10 mg/kg from 4-8.75 W. Three studies were conducted: early intervention for hearing from 3-8.75 W (V, SP120, and LS, n=5) or for assessment of laminin α2, laminin α5, and collagen IVα1 expression in stria vascularis from 3-7 W (V, SP200, and LS, n=7), and for lifespan with treatment from 3 W (V, n=10) or from 4 W (LS, SP60, or SP120, n=5). Urinary protein/creatinine (UP/C) was assessed weekly during the lifespan study. In the hearing study, the auditory brainstem response (ABR) assessed hearing ability and sensitivity to noise at 8-9 W (n=5). The cochleae were preserved and transmission electron microscopy was used to assess severity of strial pathology and to measure strial capillary basement membrane (SCBM) thickness (n=5-7). Accumulation of extracellular matrix in SCBM was determined in mice treated with V, SP200, or LS by immunofluorescence microscopy using antibodies to laminin α2, laminin α5, or collagen IVα1.

Results

In AS mice treated with SP from 4 W, SP120 significantly (P<0.05) increased median lifespan by 34 days compared to LS-treated mice, and by 50.5 days compared to V-treated mice (Figure 1). AS mice treated with SP60 or LS initiated at 4 W did not have a significant difference in lifespan compared to V. At 8 W, SP120 but not SP60 or LS significantly (P<0.05) attenuated the increase in UP/C compared to V (UP/C mg/mg mean±SD 8 W: 47±16 V; 31±6 LS; 61±44 SP60; 20±3 SP120). Notably, development of UP/C at 11 W in AS-SP120 mice was significantly attenuated (P<0.05) compared to that in AS-LS mice (UP/C mg/mg mean±SD 11 W: 58±8 LS; 77±51 SP60; 18±10 SP120). In hearing studies, although both SP120 and LS significantly (P<0.05) prevented SCBM thickening compared to AS-V mice (mean SCBM width±SD nm: 57.8±2.1 WT-V; 67.6±5.5 AS-V; 54.7±2.4 AS-SP120; 55.0±5.9 AS-LS), post-noise ABR thresholds at 16 kHz were significantly improved only in AS-SP120 mice compared to AS-V mice (*P<0.05; Figure 2). AS-LS post-noise hearing did not differ from that of AS-V mice. Moreover, SP200 tended to ameliorate increases in expression levels of laminin α2, laminin α5, and collagen IVα1 to a greater extent than LS in the SCBM of AS mice. Finally, dysmorphology of the stria vascularis was noted in LS but not SP120-treated AS mice.

Conclusion

SP120 provided significant nephroprotection in AS mice when treatment was started at 4 W, significantly extending lifespan beyond that of mice treated with LS while delaying the development of proteinuria. SP120 treatment in AS mice reduced the SCBM width. Moreover, SP120 treatment significantly attenuated noise-induced hearing loss in AS mice whereas LS was without effect. These pre-clinical studies, if successfully translated to the clinic, may suggest that sparsentan offers a novel, dual-therapeutic approach in AS by reducing both renal injury and hearing loss.

Pericyte abnormalities precede strial capillary basement membrane thickening in Alport mice

In 129 Sv autosomal Alport mice, the strial capillary basement membranes (SCBMs) progressively thicken between 5 and 9 weeks of age resulting in a hypoxic microenvironment with metabolic stress and induction of pro-inflammatory cytokines and chemokines. These events occur concomitant with a drop in endocochlear potential and a susceptibility to noise-induced hearing loss under conditions that do not permanently affect age/strain-matched littermates. Here we aimed to gain an understanding of events that occur before the onset of SCBM thickening. Alport stria has normal thickness and shows levels of extracellular matrix (ECM) molecules in the SCBMs commensurate with wild-type mice. Hearing thresholds in the 3-week Alport mice do not differ from those of wild-type mice. We performed RNAseq analysis using RNA from stria vascularis isolated from 3-week Alport mice and wild type littermates. Data was processed using Ingenuity Pathway Analysis software and further distilled using manual procedures. RNAseq analysis revealed significant dysregulation of genes involved in cell adhesion, cell migration, formation of protrusions, and both actin and tubulin cytoskeletal dynamics. Overall, the data suggested changes in the cellular architecture of the stria might be apparent. To test this notion, we performed dual immunofluorescence analysis on whole mounts of the stria vascularis from these same animals stained with anti-isolectin gs-ib4 (endothelial cell marker) and anti-desmin (pericyte marker) antibodies. The results showed evidence of pericyte detachment and migration as well as the formation of membrane ruffling on pericytes in z-stacked confocal images from Alport mice compared to wild type littermates. This was confirmed by TEM analysis. Earlier work from our lab showed that endothelin A receptor blockade prevents SCBM thickening and ECM accumulation in the SCBMs. Treating cultured pericytes with endothelin-1 induced actin cytoskeletal rearrangement, increasing the ratio of filamentous to globular actin. Collectively, these findings suggest that the change in type IV collagen composition in the Alport SCBMs results in cellular insult to the pericyte compartment, activating detachment and altered cytoskeletal dynamics. These events precede SCBM thickening and hearing loss in Alport mice, and thus constitute the earliest event so far recognized in Alport strial pathology.

Lysyl oxidase like-2 contributes to renal fibrosis in Col4α3/Alport mice

Lysyl oxidase like-2 (LOXL2) is an amine oxidase with both intracellular and extracellular functions. Extracellularly, LOXL2 promotes collagen and elastin crosslinking, whereas intracellularly, LOXL2 has been reported to modify histone H3, stabilize SNAIL, and reduce cell polarity. Although LOXL2 promotes liver and lung fibrosis, little is known regarding its role in renal fibrosis. Here we determine whether LOXL2 influences kidney disease in COL4A3 (-/-) Alport mice. These mice were treated with a small molecule inhibitor selective for LOXL2 or with vehicle and assessed for glomerular sclerosis and fibrosis, albuminuria, blood urea nitrogen, lifespan, pro-fibrotic gene expression and ultrastructure of the glomerular basement membrane. Laminin α2 deposition in the glomerular basement membrane and mesangial filopodial invasion of the glomerular capillaries were also assessed. LOXL2 inhibition significantly reduced interstitial fibrosis and mRNA expression of MMP-2, MMP-9, TGF-β1, and TNF-α. LOXL2 inhibitor treatment also reduced glomerulosclerosis, expression of MMP-10, MMP-12, and MCP-1 mRNA in glomeruli, and decreased albuminuria and blood urea nitrogen. Mesangial filopodial invasion of the capillary tufts was blunted, as was laminin α2 deposition in the glomerular basement membrane, and glomerular basement membrane ultrastructure was normalized. There was no effect on lifespan. Thus, LOXL2 plays an important role in promoting both glomerular and interstitial pathogenesis associated with Alport syndrome in mice. Other etiologies of chronic kidney disease are implicated with our observations.

X-Linked Alport Dogs Demonstrate Mesangial Filopodial Invasion of the Capillary Tuft as an Early Event in Glomerular Damage

BACKGROUND

X-linked Alport syndrome (XLAS), caused by mutations in the type IV collagen COL4A5 gene, accounts for approximately 80% of human Alport syndrome. Dogs with XLAS have a similar clinical progression. Prior studies in autosomal recessive Alport mice demonstrated early mesangial cell invasion as the source of laminin 211 in the glomerular basement membrane (GBM), leading to proinflammatory signaling. The objective of this study was to verify this process in XLAS dogs.

METHODS

XLAS dogs and WT littermates were monitored with serial clinicopathologic data and kidney biopsies. Biopsies were obtained at set milestones defined by the onset of microalbuminuria (MA), overt proteinuria, onset of azotemia, moderate azotemia, and euthanasia. Kidney biopsies were analyzed by histopathology, immunohistochemistry, and electron microscopy.

RESULTS

XLAS dogs showed progressive decrease in renal function and progressive increase in interstitial fibrosis and glomerulosclerosis (based on light microscopy and immunostaining for fibronectin). The only identifiable structural abnormality at the time of microalbuminuria was ultrastructural evidence of mild segmental GBM multilamination, which was more extensive when overt proteinuria developed. Co-localization studies showed that mesangial laminin 211 and integrin α8β1 accumulated in the GBM at the onset of overt proteinuria and coincided with ultrastructural evidence of mild cellular interpositioning, consistent with invasion of the capillary loops by mesangial cell processes.

CONCLUSION

In a large animal model, the induction of mesangial filopodial invasion of the glomerular capillary loop leading to the irregular deposition of laminin 211 is an early initiating event in Alport glomerular pathology.

Endothelin-1 mediated induction of extracellular matrix genes in strial marginal cells underlies strial pathology in Alport mice

Alport syndrome, a type IV collagen disorder, manifests as glomerular disease associated with hearing loss with thickening of the glomerular and strial capillary basement membranes (SCBMs). We have identified a role for endothelin-1 (ET-1) activation of endothelin A receptors (ET_ARs) in glomerular pathogenesis. Here we explore whether ET-1 plays a role in strial pathology. Wild type (WT) and Alport mice were treated with the ET_AR antagonist, sitaxentan. The stria vascularis was analyzed for SCBM thickness and for extracellular matrix (ECM) proteins. Additional WT and Alport mice were exposed to noise or hypoxia and the stria analyzed for hypoxia-related and ECM genes. A strial marginal cell line cultured under hypoxic conditions, or stimulated with ET-1 was analyzed for expression of hypoxia-related and ECM transcripts. Noise exposure resulted in significantly elevated ABR thresholds in Alport mice relative to wild type littermates. Alport stria showed elevated expression of collagen α1(IV), laminin α2, and laminin α5 proteins relative to WT. SCBM thickening and elevated ECM protein expression was ameliorated by ET_AR blockade. Stria from normoxic Alport mice and hypoxic WT mice showed upregulation of hypoxia-related, ECM, and ET-1 transcripts. Both ET-1 stimulation and hypoxia up-regulated ECM transcripts in cultured marginal cells. We conclude that ET-1 mediated activation of ET_ARs on strial marginal cells results in elevated expression of ECM genes and thickening of the SCBMs in Alport mice. SCBM thickening results in hypoxic stress further elevating ECM and ET-1 gene expression, exacerbating strial pathology.