Cadherin-11 and the mechanobiology of the fibrogenic niche

Chronic kidney disease afflicts roughly a tenth of the world’s population, and across its many starting points it eventually converges on a single endpoint: progressive interstitial fibrosis. Cadherin-11 has been recognized as a noninvasive biomarker with urinary and tissue levels correlating with interstitial fibrosis, tubular atrophy, and progression to end-stage disease [2]. Cadherin-11 has already been implicated as a profibrotic factor in rheumatoid synovium, calcific aortic valve disease, myocardial infarction, and pulmonary fibrosis, but whether cadherin-11 is just a marker for fibrosis or in fact causes it had remained unresolved. In this issue of Kidney International, Huffstater and colleagues settle the question [1]. Across three mechanistically distinct injuries (unilateral ureteral obstruction, aristolochic acid nephropathy, and uninephrectomy with angiotensin II infusion), both genetic deletion of Cdh11 and a cadherin-11-blocking antibody reduce fibrosis, preserve renal function, attenuate circulating injury and inflammatory markers, and limit stiffening of the fibrogenic niche. The benefit even reaches the heart, attenuating angiotensin II-driven hypertrophy independently of blood pressure. Cadherin-11 emerges as a driver of fibroblast activation.

From the perspective of mechanobiology, this work is compelling in that it maps mechanics in space and ties tissue stiffening causally to a single molecule. Using tenascin-C to mark the fibrogenic niche [9], the authors show that, rather than scattering through the cortex, cadherin-11 resides within the fibrogenic niche. Moreover, by using atomic force microscopy coupled with immunolabeling, the authors show that peritubular stiffening requires overlap of cadherin-11 and tenascin-C. Regions positive for tenascin-C but lacking cadherin-11 are no stiffer than uninvolved tissue, and where cadherin-11 is absent the niche does not stiffen at all. A mosaic co-culture experiment further localizes the cadherin-11-dependent secretome primarily to fibroblasts rather than to macrophages, strengthening the conclusion that renal fibroblasts occupy a central position in coordinating both the mechanical and secretory signals that drive tissue remodeling. Cadherin-11 is therefore not simply a passive marker of fibrosis, but instead an active effector that builds the stiff microenvironment through which tissue injury propagates. This is reinforced by the fact that an antibody-mediated inhibition can attenuate even established injury, which highlights its potential as a therapeutic point of intervention within the fibrogenic niche itself.

From the mechanobiological perspective, these results are at first puzzling. Single-cell work has taught us to expect cadherin engagement to restrain the contractile program rather than drive it. Classical cadherins build cell-cell junctions, and confluent cells display contact inhibition as well as shuttle the mechanotransducer YAP out of the nucleus. By analogy with mesenchymal stem cells, where the competition is best characterized, N-cadherin ligation antagonizes integrin-based mechanosensing; we have quantified the relationship between cadherin and integrin-based mechanosensing as a force effect in which engaging cadherins lowers the integrin engagement rate, reduces cytoskeletal tension, relaxes the nucleus, and thereby returns YAP to the cytoplasm [6]. This is consistent with the broader principle that traction governs YAP by deforming the nucleus and opening its pores to nuclear import [8]. Therefore, if cadherins reduce cell activation, why should deleting caderin-11 ameliorate tissue fibrosis?

The resolution of this question is based on the fact that cadherin’s mechanical effect is not intrinsic to the molecule itself; instead, it is set by which adhesion compartment the cadherin engages. Cadherin-11 is unique amongst classical cadherins in that, rather than remaining restricted to cell-cell contacts, it partitions into focal adhesions and couples to the fibronectin co-receptor syndecan-4, physically joining the cell-matrix adhesion machinery [3]. Its incorporation is tension-dependent, and its engagement drives the contractile, αSMA-enriched, TGF-β1-secreting program that defines myofibroblast activation [4]. Cell-cell and cell-matrix adhesions are not independent inputs, since they compete for a shared and finite reservoir of actin for their cytoskeletal networks [7]. While N-cadherin draws actin toward cell-cell junctions and lowers traction, cadherin-11 attaches onto the integrin arm and reinforces it, raising the traction that the cell can sustain.

This gives the two opposite behaviors a single, dynamic reading. As a fibroblast activates, it switches its adherens-type junctions from N-cadherin to cadherin-11 [5]. Mechanobiologically, that switch causes the cell to traverse a signaling axis, trading the restraining N-cadherin for the reinforcing cadherin-11 as it raises traction (Figure 1). Functionally, inhibiting cadherin-11 reduces the ability of myofibroblasts embedded in collagen gels to contract, whereas inhibiting N-cadherin does not [5]: cadherin-11 localizes to help fibroblasts strengthen focal contacts and matrix tensioning, implying that its loss disrupts activation at an early stage. Consistent with this view, Huffstater et al. show that cadherin-11-null fibroblasts compact collagen weakly and fail to mount a strain response; the interstitial regions that stain for cadherin-11 are the stiff ones.

The mechanical interpretation offers a compelling foundation for potential therapeutic intervention. Antifibrotic approaches directed against soluble mediators such as TGF-β have generally produced limited clinical benefit, whereas matrix stiffness is itself a potent profibrotic stimulus that perpetuates disease through recursive mechanotransduction: elevated contractility stiffens the matrix, which cells sense and respond to with still more contraction. By targeting cadherin-11, it may be possible to interrupt this self-reinforcing circuit at the level of cellular force-generation. Reducing cadherin-11-dependent traction would be expected to limit matrix stiffening, thereby reducing the mechanical signals that further enhances cellular contractility. This targeting of a mechanical positive-feedback loop, rather than a single cytokine pathway, may underlie the efficacy of cadherin-11 inhibition in diverse forms of kidney injury, including obstruction-induced, toxin-induced, and hypertensive damage.

There is more to the story, however, and several questions remain open. Whether a cadherin reinforces or restrains activation appears to involve a context-dependent switch rather than an intrinsic property of the molecule itself. Cadherin-11 has been reported to suppress activation in some settings, suggesting that additional cytoskeletal and cytoplasmic factors can regulate cadherin behavior. Moreover, junctional cadherins regulate availability of β-catenin for transcriptional signaling, a mechanism whose relevance in the kidney remains largely unexplored. The current loss-of-function studies rely on global Cdh11 deletion, which cannot establish whether fibroblasts act autonomously; conditional, fibroblast-specific knockouts are the natural next step. In addition, cadherin-11 appears to serve two distinct functions simultaneously: it mediates homophilic fibroblast-fibroblast adhesion while also participating in a syndecan-4-linked cell-matrix adhesion complex. Because tissue remodeling depends on both intracellular force generation (i.e., contractility) and intercellular force transmission, disentangling these two roles will be essential to localize the mechanism proposed here, and to define which aspect of cadherin-11 function is most critical for fibrosis. Moreover, it remains an open question whether cadherin-11 is uniquely reinforcing among type II cadherins or whether the sign of a cadherin’s effect is determined more generally by how it is wired into the force-bearing machinery of the cell. Finally, although arterial pressure remained unchanged in these experiments, the observed protection against cardiac hypertrophy leaves an important question unresolved: does this benefit arise indirectly from improved renal function, or does it reflect a direct role for cadherin-11 in the heart? Resolving this distinction will be important for understanding the full therapeutic scope of cadherin-11 inhibition. What makes cadherin-11 an attractive target is also what initially made the results seem surprising. Unlike most cadherins, which are typically viewed as restraints on migration and mechanical activity, cadherin-11 functions as an amplifier of force generation and transmission; consequently, reducing its activity resets the fibrogenic niche at its mechanical source.