A retrospective study of 103 adults with biopsy-diagnosed focal segmental glomerulosclerosis (FSGS) found that glomerular staining for transient receptor potential canonical 5 (TRPC5) varied markedly among patients and was associated with poorer treatment response and more kidney failure events. The analysis, reported in PRIME-PATH I, characterized TRPC5 and the actin-associated scaffold protein Palladin in archived kidney-biopsy specimens. TRPC5 immunoreactivity was present in 70 biopsies (68.0%) and absent in 33 (32.0%), while Palladin showed either diffuse or discontinuous capillary-granular staining in every evaluable specimen. The findings establish a tissue-level phenotype, but they do not demonstrate that TRPC5 is functionally active, causally involved in FSGS, or capable of predicting which treatment will work.

The study included adults whose diagnostic biopsies were recorded between 2010 and 2020 at Gazi University Faculty of Medicine. Of 182 patients screened, 103 met the criteria for the analysis: adequate formalin-fixed paraffin-embedded tissue, at least two evaluable glomeruli on immunostained sections, and at least 24 months of follow-up. The cohort had a median age of 40 years, median estimated glomerular filtration rate (eGFR) of 68 mL/min/1.73 m² and median baseline urinary protein excretion of 3,986 mg per day. Most biopsies were from native kidneys, while nine came from kidney allografts. Median follow-up was 48 months. The investigators did not consistently have the clinical, genetic and ultrastructural information needed to classify FSGS as primary, genetic, maladaptive, secondary or undetermined.

Two renal pathologists, blinded to clinical outcomes, jointly classified the immunostaining by light microscopy. TRPC5 was scored as absent when there was nuclear-only staining or no reproducible capillary-granular signal, discontinuous when capillary-granular staining was patchy, and continuous or diffuse when it was consistently distributed. The latter two categories were combined as TRPC5-positive. Because every biopsy contained glomerular Palladin immunoreactivity, Palladin was analyzed as a staining-distribution phenotype rather than as a positive-versus-negative marker. Slides were excluded from classification if globally sclerotic glomeruli, purely extraglomerular signal or nonreproducible background prevented evaluation. The antibody and laboratory procedures had positive controls, but the archival series did not include an orthogonal assay confirming that the observed signal represented quantitative TRPC5 protein abundance.

At 24 months, 30 patients (29.1%) had a complete response to treatment, defined as urinary protein below 0.5 g per day, and 19 (18.4%) had a partial response, defined as at least a 50% reduction from baseline. The remaining 54 patients (52.4%) met neither criterion and were classified as poor responders. During follow-up, 33 patients (32.0%) developed kidney failure, defined as maintenance dialysis or kidney transplantation. TRPC5 positivity was substantially more frequent among poor responders than responders, occurring in 45 of 54 poor responders (83.3%) and 25 of 49 responders (51.0%). In direct comparisons, poor response occurred in 45 of 70 TRPC5-positive patients (64.3%) and nine of 33 TRPC5-negative patients (27.3%). Kidney failure occurred in 30 of 70 TRPC5-positive patients (42.9%) and three of 33 TRPC5-negative patients (9.1%).

In the primary multivariable model, which accounted for Palladin distribution, age, sex, baseline eGFR and log-transformed baseline proteinuria, TRPC5 positivity was associated with poor response with an odds ratio of 3.28 (95% confidence interval 1.13–9.52) and with kidney failure with an odds ratio of 5.51 (1.25–24.23). Adding the treatment category used during the first six months after biopsy did not materially change the estimates: the odds ratio was 3.26 (1.03–10.29) for poor response and 6.12 (1.15–32.43) for kidney failure. The wide confidence intervals, especially for kidney failure, indicate substantial uncertainty around the magnitude of these associations. Treatment was not randomized and differed between groups: 71.4% of TRPC5-positive patients received some immunosuppression within six months, compared with 42.4% of TRPC5-negative patients, leaving possible confounding by disease severity or clinical selection.

Chronic structural injury offered a partial alternative explanation. TRPC5-positive patients had a lower baseline eGFR and a higher median interstitial fibrosis and tubular atrophy grade than TRPC5-negative patients, although the difference in eGFR was statistically significant and the fibrosis difference was based on the 70 patients with available data. Poor responders also had more global glomerulosclerosis and more severe fibrosis. After adjustment for the percentage of globally sclerotic glomeruli in the full cohort, TRPC5 remained associated with poor response, with an odds ratio of 3.42 (1.15–10.20), and with kidney failure, with an odds ratio of 5.54 (1.25–24.63). In contrast, when interstitial fibrosis and tubular atrophy were added in the 70-patient complete-case analysis, the TRPC5 estimates became attenuated and statistically imprecise: 2.27 (0.52–9.97) for poor response and 4.12 (0.55–31.12) for kidney failure. The investigators therefore could not determine whether TRPC5 supplied prognostic information independent of chronic injury.

Palladin distribution did not independently distinguish outcomes in the adjusted analysis. Diffuse staining, present in 75 biopsies (72.8%), was associated with complete response in 17 of 75 patients (22.7%), compared with 13 of 28 patients (46.4%) in the discontinuous group, but it was not significantly associated with the binary poor-response outcome or kidney failure. Adding Palladin to the adjusted models did not improve fit for poor response, and it supplied no convincing incremental prognostic information. The result is consistent with the compartment-dependent biology of Palladin: glomerular distribution is not interchangeable with total tissue abundance, podocyte-specific expression or interstitial fibrosis-associated staining. Rather, the pattern offered descriptive information about the glomerular cytoskeletal context in which FSGS was observed, without establishing a second prognostic marker.

The biological interpretation remains deliberately narrow. TRPC5 is a calcium-permeable, nonselective transient receptor potential canonical channel that has been linked in experimental systems to Rac1-associated signaling, podocyte actin remodeling, synaptopodin destabilization and albuminuric injury. Some genetic or pharmacological inhibition studies have reported protection, while other experiments have found inconsistent effects of TRPC5 activation or evidence of Rac1-mediated injury independent of the channel. The retrospective staining patterns cannot distinguish increased protein abundance from channel activation, calcium influx, Rac1 signaling or causal dependence on TRPC5. Nor did the study colocalize the signal with podocyte markers such as nephrin, podocin, synaptopodin or WT1, so the signal cannot be assigned definitively to podocytes. The most defensible conclusion is that TRPC5-directed immunoreactivity identifies heterogeneous tissue states associated with clinical outcomes, not that it defines a new form of FSGS.

Several additional limitations constrain the analysis. Seventy-nine of the 182 screened patients were excluded, principally because evaluable tissue or 24-month outcome data were unavailable, and the direction of potential selection bias could not be estimated. Etiologic classification was incomplete, nine allograft biopsies broadened the population beyond native-kidney disease, IFTA scores were missing for 33 patients, and exact kidney-failure dates were not uniformly available. Exact event timing also prevented formal time-to-event analysis. Treatment exposures were heterogeneous, staining was semiquantitative, and independent pre-consensus pathology scores were not retained, so interobserver agreement could not be calculated retrospectively. The single-center design and 33 kidney-failure events further limit precision and generalizability. These constraints make the observed associations hypothesis-generating rather than ready for clinical application.

The investigators propose that a next-generation study should combine etiologic classification, standardized treatment exposure, digital or quantitative tissue phenotyping, podocyte-specific localization, independent protein validation and functional measurements of signaling activity. Such a design could test whether TRPC5 heterogeneity mainly reflects chronic damage, marks a distinct biological state or identifies a subgroup with concordant target engagement and therapeutic relevance. For now, the central advance is descriptive: within morphologically defined FSGS, the same segmental-sclerosis pattern can coexist with sharply different glomerular TRPC5-directed staining distributions. Connecting that tissue phenotype to cell identity, pathway activity, disease cause and treatment response will require prospective evidence rather than inference from a retrospective staining-outcome association.

Subject of Research: Glomerular TRPC5 and Palladin staining patterns and exploratory clinical associations in FSGS biopsy tissue

Article Title: Glomerular TRPC5 and Palladin immunohistochemical profiling in focal segmental glomerulosclerosis: tissue heterogeneity and exploratory clinical associations (PRIME-PATH I)

Article References: Ural, Z., Ogüt, B., Isık Gönül, I., & Güz, G. (2026). Glomerular TRPC5 and Palladin immunohistochemical profiling in focal segmental glomerulosclerosis: tissue heterogeneity and exploratory clinical associations (PRIME-PATH I). Frontiers in Medicine, 13, Article 1924319. https://doi.org/10.3389/fmed.2026.1924319

Image Credits: AI Generated

DOI: 10.3389/fmed.2026.1924319

Keywords: TRPC5, focal segmental glomerulosclerosis, FSGS, kidney biopsy, immunohistochemistry, podocytes, Palladin, proteinuria, kidney failure, precision nephrology, Glomerular, immunohistochemical