Introduction
Chronic kidney disease occurs commonly in aged domestic cats.1–3 The term CKD defines an alteration of the renal function, structure, or both that has been present for several months, irrespective of the cause.4,5 Regardless of its primary underlying cause, CKD is irreversible and slowly progressive.4 Molecular mechanisms of the progression of CKD in models for people and rodents have been reported6–9; however, few studies10–12 have evaluated these mechanisms in cats.
Tubulointerstitial inflammation and fibrosis, typical histologic features of end-stage renal disease in people,13,14 are observed in most cats with CKD, including early stages of CKD.1,15 Prior studies16–21 assessed a limited number of specific profibrotic and proinflammatory mediators in cats with CKD. For example, compared with clinically normal cats, cats with CKD have higher urine concentrations of TGF-β1 and interleukin-8 and lower urine concentration of VEGF-A,18,19 and these findings suggest that these mediators may influence the progression of CKD.
Our group recently described a model of ischemia-induced CKD in cats in which a single 90-minute episode of transient, unilateral RI induces chronic tubulointerstitial inflammation and fibrosis,22,23 which are changes that mimic those of naturally occurring kidney disease in cats and people.1,15,24,25 Our findings indicate that there was an upregulation of transcription of specific profibrotic genes (including TGF-β1, MMP-2, MMP-7, MMP-9, and TIMP-1) and downregulation of VEGFA that persisted for 6 months following the ischemic episode.20 A subsequent study21 of cats with naturally occurring CKD shows similar patterns of differential regulation. Importantly, unilateral RI caused changes in gene transcription of TIMP-1 and VEGF-A that affected both kidneys.20 This information supports the role of renal hypoxia in the intrinsic progression of renal disease26–28 and the notion that acute kidney injury and CKD are interconnected syndromes.29,30
Comprehensive analysis of the renal transcriptome following RI may identify novel biomarkers, therapeutic targets, or both for cats with CKD.31 The aim of the present study was to use RNAseq to characterize renal transcriptional activities of genes associated with proinflammatory and profibrotic pathways in ischemia-induced CKD in cats. We hypothesized that renal tissues from cats with experimentally induced CKD would have greater genetic expression of molecular pathways associated with inflammation and fibrosis than would renal tissues from control cats and that the genetic expression of these pathways would differ for renal tissue samples obtained of both the IKs and CNIKs from cats with experimentally induced CKD versus those obtained from control cats.
Materials and Methods
Samples
We used banked tissue samples of left and right kidneys from cats that underwent 90 minutes of uni-lateral (right) RI 6 months before tissue collection (RI group; n = 6) and banked tissue samples of right kidneys (control kidneys) from healthy cats (control group; 9), of which all tissues samples except those from 1 control cat had been evaluated in previous studies.20,23 The kidney tissue samples from the cats in the RI group were further subgrouped on the basis of whether they were from the directly affected IK versus the CNIK.
Clinicopathologic analyses reflective of renal function (including serum and urine biochemical analyses) had been performed within 4 days before euthanasia; for analysis purposes, urine specific gravity measurements > 1.060 were assigned a value of 1.061. In those studies,20,23 the cats were euthanized by IV administration of pentobarbital, and the kidneys were harvested immediately thereafter. All kidneys were grossly and histologically evaluated by 2 board-certified veterinary pathologists (CAB and DRR), as described.20,23
Sample processing and RNAseq
Sample processing and total RNA extraction were performed as previously reported.20 Briefly, each kidney was sectioned, and then one-fourth to one-half was minced and immediately placed in RNA stabilization solution.a Following overnight incubation at 4°C, samples preserved in RNA stabilization solution were decanted, individually homogenized with the use of a mortar and pestle, divided into aliquots of 30 mg, and stored at −80°C until analysis. Total RNA was extracted from 1 aliquot of renal homogenate per sample with the use of a commercially available RNA extraction kit,b then quantified with a spectrophotometer.c Integrity of the isolated RNA was assessed first by visualization of 18S and 28S ribosomal bands on 1.2% agarose gels, then by analysis with a bioanalyzer systemd that assigns an RNA integrity number ranging from 1 to 10 to each sample. Samples with an RNA integrity number ≥ 5.5 were used in the RNAseq analysis.
The Georgia Genomics and Bioinformatics Core performed the RNAseq library preparation and the RNAseq. Briefly, total RNA samples were poly(A) enriched before cDNA synthesis to minimize rRNA in the final libraries. The cDNA samples were converted to barcoded, stranded RNAseq libraries with the use of a commercially available kite per the manufacturer's instructions. A quantitative PCR assay library kitf and fragment analysis kitg were used to assess the RNAseq libraries for concentration and size distribution, respectively. Equimolar amounts of all libraries were pooled in a final RNAseq pool. The concentration and size distribution of the final pool were assessed, and then samples were sequencedh with the use of a paired-end, 75-cycle sequencing protocol.
Statistical analyses
Statistical analyses of demographic and clinical data were performed with available software.i For each cat group (RI and control cats), distributions of data for continuous variables were examined for normality with the Shapiro-Wilk test and by visual assessment of histograms and normal Q-Q plots. Data were compared between groups with the Mann-Whitney U test.
Bioinformatic analyses— Sequencing data were demultiplexed with the use of available softwarej and on the basis of the sample-barcodes map to assign reads to their samples. The RNAseq reads from each sample were quality-trimmed with standard soft-warek to remove low-quality bases, homopolymers, and artifacts.
The trimmed RNAseq data were analyzed as described.32 Briefly, standard software was used to mapl quality reads to the reference feline genome,m then identify splice junctions and isoforms and develop the per-gene count matrix.n Next, 2 algorithmso,p were used as described33,34 to perform normalization and identify differentially expressed genes among the samples. In the differential expression analysis, the pairwise comparisons used were IK versus CNIK, IK versus control kidneys, and CNIK versus control kidneys. For each algorithm, the appropriate design matrix for each comparison was used to account for independence (control kidneys vs IKs or CNIKs) and nonindependence (IKs vs CNIKs) of the observations. To account for multiple testing errors, an FDR of ≤ 0.05 was used. Multiplicity correction was performed by applying the Benjamini-Hochberg method on values of P to control for the FDR.35,q
The resultant lists of significantly (FDR ≤ 0.05) upregulated genes from the pairwise comparisonso were used for gene ontology analysis with a classification systemr as described.36 A tool within the classification system was used to identify gene ontology classifications that were overrepresented in these lists of significantly upregulated genes, compared with the reference feline genome. This was performed with the use of the Fisher exact test to determine the probability that the number of genes observed in each category occurred by chance (randomly), as determined by the reference feline genome and by applying the Benjamini-Hochberg method to control for FDR < 0.05.
Coexpression clustering analysis— A single list without duplicative inclusion of significantly differentially expressed genes was compiled with the use of a union operation to combine the 2 gene sets yielded from the comparisons analyses algorithms.o,p Raw read counts were obtained from the RNAseq mapping data and normalized with the use of RNAseq library size and gene length count to obtain the transcripts-per-million–scaled matrix. Next, cluster analysis on the transcripts-per-million–scaled matrix was performed with an available software package,s as described,37 and because only a subset of genes was expected to have been assigned to biologically interpretable groups, the Tmax was used in this analysis to assess for cluster stability. Genes assigned to clusters with a Tmax value greater than the threshold (≥ 0.8) were considered and subsequently visualized, and the Tmax values (range, 0 to 1) implied the level of confidence (low to high, respectively) in the cluster membership. The gene coexpression clustering criteria parameters selected were a K-means algorithm for the cluster method, a cluster range to test of 2 to 25 clusters, and a transformation model of log centered log ratio; a minimum cutoff of 5 for a normalized count (removing genes with low expression from the analysis); the number of starts set at 1,000; and the iteration maximum of 10,000.
Results
Samples
Clinical information was collected from the medical records for cats in the RI group (6 spayed females) and the control group (9 sexually intact females). The median age and body weight were significantly (P = 0.002 and P = 0.006, respectively) greater for the RI group (520 days and 4.72 kg) versus the control group (270 days and 3.59 kg), and the median urine protein-to-creatinine ratio was significantly (P = 0.004) lower for the RI group (0.10; reference range, < 0.5) versus the control group (0.14). The median urine specific gravity and concentrations of serum creatinine and SUN did not substantially differ for cats in the RI group (1.056, 1.4 mg/dL, and 24 mg/dL, respectively) versus the control group (1.055, 1.0 mg/dL, and 28 mg/dL, respectively). Data for all 6 cats in the RI group and 8 of the 9 cats in the control group had been reported previously.20,23 The cats in the RI group had undergone ovariohysterectomy during the same anesthetic event as their surgically induced RI, and a progressive decline in glomerular filtration rate and an increase in serum creatinine concentration were documented for cats of the RI group during the original 6-month study.23 Additionally, histologic examination of the IKs from cats in the RI group revealed lesions that mimicked those of naturally occurring CKD (variable numbers of obsolescent glomeruli, tubulointerstitial inflammation, and fibrosis), whereas no abnormalities were identified on histologic reexaminations of the CNIKs from cats in the RI group or the kidneys from cats in the control group.
Differentially expressed genes
Adequate RNA integrity was confirmed for all but 1 CNIK sample; therefore, samples from 6 IKs, 5 CNIKs, and 9 control kidneys were evaluated. The differential expression analysis performed with the use of 2 different algorithms to conduct pairwise comparisons between IK, CNIK, and control kidneys revealed a set of 4,739o and a set of 5,529p differentially expressed genes.
In the data set obtained with the algorithmo preferred for experiments with fewer than 12 biological replicates,38 the IKs had 1,743 upregulated and 1,196 downregulated genes, compared with those of the control kidneys; the CNIK had 209 upregulated and 291 downregulated genes, compared with those of the control kidneys; and the IKs had 849 upregulated and 451 downregulated genes, compared with those of the CNIKs.
Findings from RNAseq indicated that the MMP-2 gene was upregulated in samples from IKs, compared with CNIKs; the MMP-7 and MMP-9 genes were up-regulated in samples from IKs, compared with both CNIKs and control kidneys; the TIMP-1 gene was up-regulated in samples from both the IKs and CNIKs, compared with control kidneys; and the TGF-β1 gene was upregulated in samples from IKs, compared with control kidneys. When these findings were compared with those of a previous study20 that used reverse transcription PCR assays to evaluate differential transcription of genes in tissue samples from these same kidneys, similar patterns of regulation were identified (Supplementary Table S1, available at: avmajournals.avma.org/doi/suppl/10.2460/ajvr.82.7.589).
Gene ontology analysis
Gene ontology analysis of the differentially expressed genes that were upregulated in the IKs versus control kidneys revealed an overrepresentation of several proinflammatory and profibrotic pathways (Table 1). When overrepresented and upregulated genes in the IKs, compared with control kidneys, were considered by gene ontology category (ie, biological processes, molecular function, or cellular component) and pathway, notable genes identified and related to biological processes included CXCL9, CXCL10, CXCL11, and CXCL16 (pathway, T-cell chemotaxis); T-cell–specific surface glycoproteins CD28 and CD5 (pathway, T-cell costimulation); TGF-β1 and TGF-β3, and vimentin (pathway, positive regulation of collagen biosynthetic process); CD74 (pathway, positive regulation of neutrophil chemotaxis); and platelet-derived growth factor receptor-α (pathway, platelet degranulation). Within the molecular function category, notable genes identified included TGF-β1, TGF-β2, TGF-β3, fibroblast growth factor-10 and -11, insulin-like growth factor-1, VEGF-C, hepatocyte growth factor, and macrophage colony-stimulating factor 1 (pathway, growth factor activity); MMP-7, MMP-9, MMP-11, MMP-13, MMP-16, MMP-23B, and MMP-28 (pathway, metalloendopeptidase activity); SPP1 (pathway, extracellular matrix binding); and connective tissue growth factor, fibronectin, vascular cell adhesion protein-1, and intercellular adhesion molecule-1 (pathway, integrin binding). As for the cellular component category, pertinent genes included TIMP-1, MGP, and fibulin-1 (pathway, extra-cellular matrix). Likewise, when overrepresented and upregulated genes in the IKs, compared with the CNIKs, were considered by gene ontology category and pathway, notable genes included MMP-2, MMP-7, MMP-9, MMP-13, and MMP-19 (pathway, metalloendopeptidase activity); SPP1 (pathway, extracellular matrix binding); and MGP (pathway, extracellular matrix; Table 2).
Select overrepresented pathways, grouped by gene ontology category, that were upregulated in banked tissue samples of the IKs from 6 cats that had undergone 90 minutes of unilateral (right) RI to experimentally induce CKD 6 months before tissue collection (RI group), compared with results for tissue samples of the right kidneys (control kidneys) from 9 healthy cats (control group).
| GO category | Overrepresented pathway* | Fold enrichment | Raw P value†‡ | FDR‡ |
|---|---|---|---|---|
| Biological process | Dendritic cell chemotaxis (GO:0002407) | 11.55 | 1.70E-03 | 3.49E-02 |
| T-cell chemotaxis (GO:0010818) | 10.83 | 1.39E-04 | 4.63E-03 | |
| Positive regulation of macrophage chemotaxis (GO:0010759) | 10.5 | 1.20E-05 | 5.59E-04 | |
| Platelet degranulation (GO:0002576) | 9.63 | 2.68E-03 | 4.91E-02 | |
| T-helper I-type immune response (GO:0042088) | 9.02 | 9.38E-04 | 2.23E-02 | |
| T-cell migration (GO:0072678) | 8.89 | 2.88E-05 | 1.19E-03 | |
| T-cell costimulation (GO:0031295) | 8.02 | 1.38E-03 | 2.94E-02 | |
| Positive regulation of collagen biosynthetic process (GO:0032967) | 7.22 | 1.96E-03 | 3.86E-02 | |
| Positive regulation of epithelial to mesenchymal transition (GO:0010718) | 6.08 | 2.22E-04 | 6.81E-03 | |
| Positive regulation of granulocyte chemotaxis (GO:0071624) | 5.95 | 6.13E-04 | 1.58E-02 | |
| Positive regulation of neutrophil chemotaxis (GO:0090023) | 5.95 | 6.13E-04 | 1.57E-02 | |
| Regulation of T-cell proliferation (GO:0042129) | 4.29 | 1.55E-07 | 1.13E-05 | |
| T-cell activation (GO:0042110) | 4.28 | 3.87E-II | 4.88E-09 | |
| Positive regulation of angiogenesis (GO:0045766) | 3.08 | 2.26E-04 | 6.87E-03 | |
| Molecular function | Toll-like receptor binding (GO:0035325) | 9.63 | 2.20E-04 | 1.90E-02 |
| Collagen binding (GO:0005518) | 6.8 | 5.28E-08 | 1.96E-05 | |
| Chemokine binding (GO:0019956) | 6.74 | 3.38E-04 | 2.41E-02 | |
| Extracellular matrix binding (GO:0050840) | 5.87 | 3.45E-06 | 6.09E-04 | |
| Integrin binding (GO:0005178) | 4.38 | 3.10E-06 | 6.38E-04 | |
| Chemokine activity (GO: 0008009) | 4.51 | 2.74E-04 | 2.12E-02 | |
| Heparin binding (GO:0008201) | 3.51 | 2.14E-05 | 2.65E-03 | |
| Growth factor activity (GO:0008083) | 2.94 | 2.05E-05 | 2.62E-03 | |
| Metalloendopeptidase activity (GO:0004222) | 2.49 | 5.88E-04 | 3.89E-02 | |
| Cellular component | T-cell receptor complex (GO:0042101) | 9.63 | 2.20E-04 | 6.49E-03 |
| Integrin complex (GO:0008305) | 7.47 | 5.28E-08 | 2.50E-06 | |
| Major histocompatibility complex class II protein complex (GO:0042613) | 7.22 | 6.89E-04 | 1.84E-02 | |
| Extracellular matrix (GO:0031012) | 3.68 | 9.26E-21 | 3.83E-18 |
Reported as the pathway and the 7-digit unique identifier prefix with GO.
The raw P value was determined with the Fisher exact test.
Reported as the exponential notation for powers of 10 (eg, 1.70E-4 means 1.70 × 10−4).
E = Exponential notation. GO = Gene ontology.
Select overrepresented pathways, grouped by gene ontology category, that were upregulated in tissue samples of the IKs (n = 6) versus the CNIKs (5) from the 6 cats in the IR group described in Table 1. Adequate RNA integrity was not confirmed for the remaining CNIK sample; therefore, it was not included in the analysis.
| GO category | Overrepresented pathway* | Fold enrichment | Raw P value†‡ | FDR‡ |
|---|---|---|---|---|
| Biological process | Establishment of T-cell polarity (GO:0001768) | 21.92 | 1.14E–03 | 4.04E-02 |
| Positive regulation of interleukin-2 biosynthetic process (GO:0045086) | 16.7 | 3.26E-04 | 1.49E-02 | |
| T-cell costimulation (GO:0031295) | 16.24 | 6.10E-05 | 3.82E-03 | |
| T-cell chemotaxis (GO:0010818) | 14.61 | 4.76E-04 | 2.03E-02 | |
| Positive regulation of macrophage chemotaxis (GO:0010759) | 10.63 | 1.21E-03 | 4.16E-02 | |
| Positive regulation of neutrophil migration (GO:1902624) | 10.31 | 8.00E-05 | 4.78E-03 | |
| Positive regulation of granulocyte chemotaxis (GO:0071624) | 10.31 | 8.00E-05 | 4.76E-03 | |
| Positive regulation of interleukin-10 production (GO:0032733) | 10.31 | 8.00E-05 | 4.74E-03 | |
| Platelet activation (GO:0030168) | 5.84 | 4.14E-04 | 1.83E-02 | |
| Molecular function | Cytokine receptor activity (GO:0004896) | 6.94 | 5.23E-10 | 6.47E-07 |
| Cellular component | T-cell receptor complex (GO:0042101) | 16.24 | 6.10E-05 | 3.17E-03 |
| Major histocompatibility complex class II protein complex (GO:0042613) | 14.61 | 1.69E-05 | 1.04E-03 | |
| Integrin complex (GO:0008305) | 6.05 | 9.18E-04 | 3.04E-02 | |
| Extracellular matrix (GO:0031012) | 3.18 | 1.09E-08 | 2.26E-06 |
See Table 1 for key.
For the CNIKs versus control kidneys, there were fewer differentially expressed genes. Results of gene ontology analysis indicated that overrepresented pathways pertained, once again, to the regulation of extracellular matrix deposition (Table 3). Notable genes included in the overrepresented pathways included TIMP-1, TIMP-2, connective tissue growth factor, thrombospondin-1, MGP, MMP-28, and fibulin-1 (cellular component category; pathway, extracellular matrix).
Select overrepresented pathways upregulated in tissue samples of the CNIKs (n = 5) versus the control kidneys described in Tables 1 and 2.
| GO category | Overrepresented pathway* | Fold enrichment | Raw P value†‡ | FDR‡ |
|---|---|---|---|---|
| Biological process | Regulation of cell migration (GO:0030334) | 4.19 | 1.94E-05 | 5.04E-02 |
| Molecular function | Protein complex binding (GO:0032403) | 3.86 | 2.23E-05 | 4.14E-02 |
| Cellular component | Extracellular matrix (GO:0031012) | 6.64 | 1.63E-09 | 2.70E-06 |
See Table 1 for key.
Gene coexpression clustering analysis
A union list of 4,801 differentially expressed genes was compiled. After analysis with various transformation and cluster models, the selected number of clusters was 20, which explained most of the variances among samples (Figure 1). Genes of interest, including SPP1 and MGP, were identified in cluster 2 (Table 4) and cluster 10 (Table 5), respectively, with greater expression detected in tissue samples from IKs versus CNIKs or control kidneys and in samples from CNIKs versus control kidneys.
Cluster plots of coexpressed genes in banked tissue samples of the IKs (n = 6) and CNIKs (5) from 6 cats that had undergone 90 minutes of unilateral (right) RI to experimentally induce CKD 6 months before tissue collection (RI group) and in banked tissue samples of the right kidneys (control kidneys; 9) from 9 healthy cats (control group), with each plot representing 1 of the 20 gene clusters that explained most of the variances among samples. In each plot, the y-axis represents transformed, normalized gene expression, the x-axis represents the 20 individual renal tissue samples (CNIKs, sample Nos. 1 through 5; IKs, sample Nos. 6 through 11; and control kidneys, sample Nos. 12 through 20), the red lines represent genes that had a cluster membership Tmax value < 0.8, and the black lines represent genes that had a cluster membership Tmax value ≥ 0.8. Adequate RNA integrity was not confirmed for the remaining CNIK sample; therefore, it was not included in the analysis.
Citation: American Journal of Veterinary Research 82, 7; 10.2460/ajvr.82.7.589
List of genes identified in cluster 2 as coexpressed in tissue samples of the IKs (n = 6), CNIKs (5), and control kidneys (9) described in Tables 1 and 2.
| Gene symbol or locus | Description* |
|---|---|
| SPP1 | Secreted phosphoprotein-1 |
| PLET1 | Placenta expressed transcript 1 |
| LOC109493266 | Undetermined |
| LOC105261057 | Undetermined |
| LOC105260308 | Undetermined |
| LOC105260274 | Undetermined |
| LOC105259930 | Undetermined |
| LOC101098498 | Predicted: Neutrophil gelatinase-associated lipocalin-like |
| LOC101090949 | Undetermined |
| LOC101086711 | Undetermined |
| JCHAIN | Joining chain of multimeric IgA and IgM |
Genes described as undetermined were unknown in the reference annotation.
List of genes identified in cluster 10 as coexpressed in tissue samples of the IKs (n = 6), CNIKs (5), and control kidneys (9) described in Tables 1 and 2.
| Gene symbol or location | Description* |
|---|---|
| CCL21 | C-C motif chemokine ligand 21 |
| CD53 | Leukocyte surface antigen CD53 |
| CD74 | Leukocyte surface antigen CD74 |
| CFD | Complement factor D |
| CH13L1 | Chitinase 3 like I |
| CLDN3 | Claudin 3 |
| CLEC2D | C-type lectin domain family 2 member D |
| CORO1A | Coronin IA |
| DRA | HLA class II histocompatibility antigen, DR a chain-like |
| IL7R | Interleukin 7 receptor |
| IRF8 | Interferon regulatory factor 8 |
| KRT7 | Keratin 7 |
| LCP1 | Lymphocyte cytosolic protein I |
| LOC101098301 | DLA class II histocompatibility antigen, DR-I P chain |
| LOC102899262 | Undetermined |
| LOC109494204 | Undetermined |
| LTF | Lactotransferrin |
| MFAP4 | Microfibril-associated protein 4 |
| MGP | Matrix Gla protein |
| P116 | Peptidase inhibitor 16 |
| P13 | Peptidase inhibitor 3 |
| PTPRC | Protein tyrosine phosphatase, receptor type C |
| SLC26A3 | Solute carrier family 26 member 3 |
| SLC34A2 | Solute carrier family 34 member 2 |
| TACSTD2 | Tumor-associated calcium signal transducer 2 |
Genes described as undetermined were unknown in the reference annotation.
DLA = Dog leukocyte antigen. HLA = Human leukocyte antigen.
Discussion
The results of the present study identified an extensive list of proinflammatory and profibrotic pathways upregulated in tissue samples of the kidneys from cats that 6 months previously underwent transient, unilateral RI to experimentally induce CKD. Although 6 months had elapsed between the ischemic event and harvest of renal tissues, a union set of 4,801 genes, many of which are involved in the regulation of inflammatory cell chemotaxis, cytokine and growth factor activities, and deposition of extracellular matrix, was identified as differentially expressed in pairwise comparisons of results for the IKs, CNIKs, and control kidneys. Although the direct ischemic injury was induced in only the right kidneys (the IKs) of the cats in the RI group, proinflammatory and profibrotic pathways were also upregulated in the CNIKs, compared with results for control kidneys. Findings of the present study together with those of a prior study,20 support acute kidney injury and CKD as closely related syndromes.29,30 Further, the present study identified additional cytokines that may be implicated in the progression of CKD in cats.
Fibulin-1, an extracellular matrix protein prominently expressed in blood vessels,39 was upregulated in renal tissue samples from cats in the RI group versus the control group. Relatedly, in people, high plasma fibulin-1 concentration has been detected in individuals with various types of renal disease (glomerulonephritis, diabetic nephropathy, obstructive uropathy, and analgesic abuse nephropathy); therefore, this protein has been suggested as a marker for renal impairment.40 Also in people, a study41 shows that plasma concentrations of fibulin-1 inversely correlate with glomerular filtration rate and positively correlate with systemic blood pressure and other hemodynamic markers of cardiovascular risk in CKD and diabetes. Investigation of fibulin-1 plasma concentration is warranted to assess this protein's potential as a biomarker for renal disease in cats.
In the present study, SPP1, which is an extracellular matrix protein and proinflammatory cytokine also known as osteopontin, was upregulated in both kidneys from the RI group, compared with control kidneys. In rodents and people, SPP1 is highly expressed in renal tubular epithelial cells and has been suggested as an important factor in the initial inflammatory responses involving natural killer cell activity in RI reperfusion injury.42 In experimental models of glomerulonephritis in rodents, expression of SPP1 correlates with the severity of the associated tubulointerstitial injury.43 Findings in people and rodents suggest that SPP1 also contributes to the pathogenesis of diabetic nephropathy.44 In people with diabetes, the plasma concentration of SPP1 is independently associated with the presence and severity of nephropathy and coronary heart disease.45 Although data regarding SPP1 in cats with CKD is lacking, gene coexpression clustering analysis in the present study revealed that the SPP1 gene is coexpressed with a predicted locus for neutrophil gelatinase–associated lipocalin. Because urinary neutrophil gelatinase–associated lipocalin has gained attention as a marker for the prediction of progression of CKD in cats,46 SPP1 may similarly offer promise as a biomarker for progressive CKD in cats.
Our results indicated that MGP, an important vitamin K–dependent local inhibitor of vascular calcification,47 was also upregulated in tissue samples from the IKs and CNIKs, compared with those from the control kidneys. Circulating concentrations of an inactive form of MGP were positively and independently associated with aortic calcification in people with various stages of CKD and proposed as a biomarker of vascular calcification in people with renal disease.48 Also in people, expression of MGP correlates with interstitial fibrosis, tubular atrophy, acute tubular injury, and interstitial inflammation and is positively associated with an increased risk of end-stage renal disease.49 In our study, MGP was coexpressed with several genes that encode for proteins involved in inflammatory processes, such as lactotransferrin (a mediator of immune response to injury),50 interleukin-7 receptors, and interferon regulatory factor 8; therefore, further investigation is warranted regarding the potential use of MGP as a marker for ongoing renal inflammation and fibrosis in cats.
There were limitations to the present study. First, because banked tissues were available from only a small number of cats with experimentally induced CKD,23 the number of biological replicates in each group was relatively small. Second, a sham-operated control group was not available. Therefore, it was possible that some changes identified for the CNIKs, compared with the control kidneys, may have been from global decreases in renal perfusion under anesthesia, rather than a result of the intentional ischemia induced in the IK. Additionally, cats in the RI group were older and all spayed, whereas the control group consisted of younger and sexually intact female cats. Such differences in age and reproductive status may have influenced gene transcription results; however, it appeared unlikely that the transcriptional regulation of renal inflammation and fibrosis would have been impacted by these factors, particularly given the fact that the upregulation of several of the identified proinflammatory and profibrotic pathways was for the IKs versus the CNIKs from cats in the RI group. Overall, the cats from which the renal tissues were obtained were young, and the question remains as to whether aged cats would have similar patterns of transcriptional activities following RI. Another limitation was that because tissues were collected at a single, relatively late, time point after the ischemic event, potential transient changes in the transcription of genes that promote inflammation and fibrosis earlier in the renal inflammatory processes may not have been detected. Further, the present study was at the transcriptional level and may not have reflected changes in protein expression in the kidneys or changes that could occur in the kidneys of cats with naturally occurring CKD.
Findings from the present study supported our hypotheses that renal tissues from cats with experimentally induced CKD would have greater genetic expression of molecular pathways associated with inflammation and fibrosis than would renal tissues from control cats and that the genetic expression of these pathways would differ for tissue samples obtained from both the IKs and CNIKs from cats with experimentally induced CKD versus those from control cats. Additionally, our results suggested that acute, transient ischemic renal injury leads to persistent and wide-ranging changes in transcription of those genes that promote inflammation and fibrosis. Fibulin-1, SPP1, and MGP may prove to be useful biomarkers of active kidney injury in cats, as they are in people. Future evaluation of fibulin-1, SPP1, and MGP in renal tissues, plasma, or both from cats with spontaneous CKD is warranted to explore their potential as renal biomarkers in cats. Additionally, future prospective studies may include a larger cohort, age- and gender-matched controls, and samples collected at multiple and earlier time points following RI.
Acknowledgments
This work was performed at the University of Georgia College of Veterinary Medicine and was internally funded. Dr. Bianca Lourenço was the recipient of a Boehringer Ingelheim Postdoctoral Scholarship.
The authors declare that there were no conflicts of interest. Presented in part as an oral abstract at the 28th European College of Veterinary Internal Medicine - Companion Animals Annual Congress, Rotterdam, Netherlands, September 2018.
Abbreviations
| CKD | Chronic kidney disease |
| CNIK | Contralateral nonischemic kidney |
| CXCL | C-X-C motif chemokine ligand |
| FDR | False discovery rate |
| IK | Ischemic kidney |
| MGP | Matrix Gla protein |
| MMP | Matrix metalloproteinase |
| RI | Renal ischemia |
| RNAseq | RNA sequencing |
| SPP1 | Secreted phosphoprotein-1 |
| TGF-β | Transforming growth factor-β |
| TIMP | Tissue inhibitor of metalloproteinase |
| Tmax | Maximum conditional probability |
| VEGF | Vascular endothelial growth factor |
Footnotes
RNAlater stabilization solution, Quiagen, Valencia, Calif.
RNeasy Plus Mini Kit, Quiagen, Valencia, Calif.
NanoDrop spectrophotometer, Thermo Fisher Scientific, Waltham, Mass.
2100 Bioanalyzer system, Agilent Technologies, Palo Alto, Calif.
KAPA mRNA HyperPrep Kit, Kapa Biosystems, Woburn, Mass.
KAPA qPCR library quantification kit, Kapa Biosystems, Wo-burn, Mass.
High Sensitivity NGS Fragment Analysis kit, Advanced Analytical Technologies, Ankeny, Iowa.
NextSeq 500 platform, Illumina Inc, San Diego, Calif.
JMP Pro, version 14.1.0, SAS Institute Inc, Cary, NC.
bcl2fastq, version 2, Illumina Inc, San Diego, Calif.
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TopHat, version 2.1.1, Center for Computational Biology, Johns Hopkins University, Baltimore, Md. Available at: ccb.jhu.edu/software/tophat/index.shtml.
Felis_catus_8.0, GCF_000181335.2_Felis_catus_8.0_ge nomic.gff. Available at: www.ncbi.nlm.nih.gov/assembly/GCF_000181335.2/. Accessed Apr 15, 2017.
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