Plectin ensures intestinal epithelial integrity and protects colon against colitis

Plectin, a highly versatile cytolinker protein, provides tissues with mechanical stability through the integration of intermediate filaments (IFs) with cell junctions. Here, we hypothesize that plectin-controlled cytoarchitecture is a critical determinant of the intestinal barrier function and homeostasis. Mice lacking plectin in an intestinal epithelial cell (IEC; PleΔIEC) spontaneously developed colitis characterized by extensive detachment of IECs from the basement membrane (BM), increased intestinal permeability, and inflammatory lesions. Moreover, plectin expression was reduced in the colons of ulcerative colitis (UC) patients and negatively correlated with the severity of colitis. Mechanistically, plectin deficiency in IECs led to aberrant keratin filament (KF) network organization and the formation of dysfunctional hemidesmosomes (HDs) and intercellular junctions. In addition, the hemidesmosomal α6β4 integrin (Itg) receptor showed attenuated association with KFs, and protein profiling revealed prominent downregulation of junctional constituents. Consistent with the effects of plectin loss in the intestinal epithelium, plectin-deficient IECs exhibited remarkably reduced mechanical stability and limited adhesion capacity in vitro. Feeding mice with a low-residue liquid diet that reduced mechanical stress and antibiotic treatment successfully mitigated epithelial damage in the PleΔIEC colon.


INTRODUCTION
The intestinal epithelium is composed of a single layer of tightly linked intestinal epithelial cells (IECs), forming a selective physical barrier that is critical for gut homeostasis. A breach in the intestinal barrier, referred to as "leaky gut" 1 , results in excessive exposure to luminal microbiota and in a concomitant innate immune response. Subsequent dysregulation of the finelytuned interplay among gut microbiota, IECs, and immune cells accounts for uncontrolled inflammation and pathogenesis of intestinal disorders such as inflammatory bowel disease (IBD) and colorectal cancer (CRC) 2,3 .
The epithelial barrier function is secured by cell junctions that seal intercellular spaces and interlink IECs with the underlying basement membrane (BM) into a structural and functional continuum. Alterations in junctional proteins and BM components may lead to a breakdown of the barrier, and genetic studies identified multiple links between junction/BM-associated genes and the development of IBD [4][5][6] . While apical tight junctions (TJs) and subjacent adherens junctions (AJs) confer paracellular transport selectivity, desmosomes (Ds) together with BM-linked hemidesmosomes (HDs) provide the intestinal epithelium with resilience to mechanical stress generated by intestinal peristalsis 7 . It is noteworthy that recently reported mouse models demonstrate the protective role of Ds and HDs in the context of both intestinal inflammation 8,9 and colitis-associated CRC 8 . Accumulating evidence suggests that fundamental features of Ds and HDs (such as stability, dynamics, and mechanotransduction capacity) heavily rely on their interconnection with keratin filament (KF) networks 10,11 . This places plakins 12 , a family of cytolinker proteins mediating physical linkage between KFs and cell junctions, at the very center of the processes controlling epithelial homeostasis.
Plectin, a highly versatile member of the plakin protein family, crosslinks intermediate filaments (IFs) of different types and anchors them at cellular junctions, including HDs and Ds of epithelial cells 13 . Multiple mutations in the plectin gene have been identified in epidermolysis bullosa (EB) 14 , a disorder characterized by excessive blister formation in skin 15,16 with reported cases of concurrent IBD 17,18 . Previous studies have shown that plectin ablation disrupts highly organized epithelial KF networks and alters the structure and functionality of cell junctions [19][20][21][22] . For example, tissue-specific deletion of plectin in the mouse biliary epithelium has adverse effects on the formation of TJs, AJs, and Ds, with deleterious consequences for epithelial stability under cholestasis 22 . Likewise, analysis of knock-in mice recapitulating dominant EB simplex suggests that HD stability in basal keratinocytes depends on plectin-mediated recruitment of KFs 20 . Mechanistically, dysfunctional HDs account for epithelial fragility and lesional defects 23 which resemble those seen in patients with IBD 24 . Although these observations suggest a linkage between plectin dysfunction and intestinal pathologies, plectin's role in the intestinal epithelium remains unaddressed.
In this study, we found that plectin expression was reduced in patients with active ulcerative colitis (UC) and that plectin expression levels negatively correlated with the severity of colitis. To study the underlying molecular mechanisms, we generated two new mouse lines: one constitutive (Ple ΔIEC ) and the other with tamoxifen (TMX)-inducible (Ple ΔIEC-ERT2 ) plectin ablation in IECs. The phenotypic characterization of these mice demonstrated that loss of plectin leads to spontaneous development of a colitic phenotype characterized by extensive detachment of IECs from the BM, increased intestinal permeability, and formation of inflammatory lesions. These results demonstrate the absolute indispensability of plectin for the maintenance of intestinal epithelium integrity, and moreover that both mouse lines provide a useful model system for investigating disease etiology and testing palliative therapies.

Suppression of plectin in human patients with UC
To examine the role of plectin in the pathogenesis of UC, we screened for potential alterations of plectin expression in a cohort of~100 UC patients. The analysis of immunolabeled biopsy samples taken from patients and healthy controls revealed discontinuous and rather patchy plectin staining patterns in UC biopsies. The gaps in the plectin staining pattern coincided with goblet cell openings heavily loaded with mucus. In healthy controls, plectin decorated both apical and basal membranes of IECs evenly (Figs. 1A and S1A), resembling plectin localization in mouse intestinal sections ( Fig. S1B and published previously 25 ). In addition, mRNA profiling showed significantly reduced expression levels of plectin in biopsies from patients with active UC (Fig. 1B). Histological analysis revealed that low mRNA levels of plectin were associated with higher inflammation (Fig. 1C) and higher C reactive protein levels in serum (not shown).
IEC-specific plectin-deficient mice develop a colitic phenotype due to intestinal barrier dysfunction To explore the role of plectin in the intestinal epithelium in greater detail, we generated IEC-specific plectin knockout (Ple ΔIEC ) mice. Successful ablation of plectin in IECs was confirmed by immunofluorescence microscopy (Fig. S1C). The newly generated Ple ΔIEC mice had a considerably lower bodyweight (Figs. 1D and S1D), suffered from persistent diarrhea with occasional rectal bleeding (Fig. S1E, and not shown), and frequently developed rectal prolapse (Fig. 1E). As the onset and progression of UC correlate with defects in the intestinal barrier function 26,27 , we assessed barrier integrity either by ex vivo measurements of intestinal transepithelial electrical resistance (TEER) or by in vivo orogastric gavage of FITC-dextran. We observed significantly lower TEER in the proximal and distal colon regions of 12-week-old Ple ΔIEC compared to Ple fl/fl mice. Moreover, TEER values in Ple fl/fl mice were three-times higher in their distal parts than in their proximal parts; by contrast, TEER values in Ple ΔIEC mice were equally low in distal and proximal colon segments (Fig. 1F). Compared to Ple fl/fl mice, Ple ΔIEC mice consistently displayed a higher penetration rate of FITC-dextran into blood already at 4 weeks, and this difference became even more apparent in older animals (Fig. 1G).
In addition, a histological inspection of hematoxylin-eosin (H&E)-stained colon sections revealed extensive translocation of luminal bacteria into Ple ΔIEC mucosa in 30-week-old mice (Fig. 1H). Given the hampered barrier function in the Ple ΔIEC intestine, we screened Ple fl/fl and Ple ΔIEC mice for signs of inflammation. Chemiluminescence-based whole body imaging 28 showed positive abdominal areas in Ple ΔIEC mice (Fig. 1I), which correlated strongly with significantly higher myeloperoxidase activity (MPO; Fig. 1J). Moreover, mild inflammation of the Ple ΔIEC colon was confirmed by increased immune cell infiltration, extent (or intensity) of acute/chronic inflammation, and lymphatic follicle size (Figs. 1K, L and S2A), and a higher percentage of edema and ulceration indicated higher epithelial damage (Fig. S2B). Together, these results suggest that plectin is critical for the maintenance of the intestinal barrier and thus could be directly linked to the onset and progression of UC.
Loss of plectin leads to hyperproliferation and aberrant differentiation of IECs Further histological inspection of H&E-stained colonic sections revealed thickening of the colonic mucosa and significant crypt damage with excessive sloughing of IECs detached from the subjacent BM in plectin-deficient specimens ( Fig. 2A). In addition, the colon of Ple ΔIEC mice showed a higher rate of proliferation as determined from Ki-67-stained sections (Fig. 2B). Consistently, an increase in the number of proliferating transit-amplifying IECs in the crypts of Ple ΔIEC animals was evident from BrdU incorporation assessed 2, 24, and 48 h after a BrdU pulse (Fig. 2C). Interestingly, TUNEL staining indicated a minimal degree of spontaneous apoptosis in both Ple fl/fl and Ple ΔIEC mice (Fig. S3A). In parallel with the prominent hyperplasia, the Ple ΔIEC colon contained a higher proportion of PAS-positive goblet cells (Fig. 2D), corresponding to a higher mucus discharge (Fig. 2E). Immunolabeled Ple ΔIEC colonic sections also showed a lower percentage of chromogranin A (ChgA)-positive enteroendocrine cells (Fig. S3B) and an extended keratin 20 (K20)-positive zone (Fig. S3C). Similar, albeit less pronounced, trends were observed in the Ple ΔIEC small intestine (Fig. S4). Plectin deficiency thus results in hyperproliferation and aberrant differentiation of IECs, affecting the spatiotemporal organization of the intestinal epithelium.
Plectin-deficient IECs form aberrant cell junctions and disordered KF networks As the structural and functional integrity of epithelia is secured by cell junctions 2,29 , we compared the morphology of cell-ECM (HDs) and cell-cell (TJs, AJs, and Ds) adhesions formed by Ple fl/fl and Ple ΔIEC IECs, using transmission electron microscopy (TEM). A quantitative analysis of the HD size revealed an extended crosssectional length of seemingly less electrodense HD plaques in the Ple ΔIEC colon; furthermore, the space between HDs and the BM was significantly dilated (Fig. 3A). Similar to HDs, we also found significantly dilated intercellular spaces of TJs, AJs, and Ds between adjacent Ple ΔIEC IECs (Fig. 3A). These morphological alterations coincided with generally lower expression levels of the hemidesmosomal constituents Itgα6 and Itgβ4 and the following cell-cell junctional markers: zonula occludens 1 (ZO-1; TJs), Ecadherin (E-cad; AJs), desmoglein 2 (Dsg2; Ds), and desmoplakin 1/2 (Dsp1/2; also Ds) at both mRNA and protein levels ( Fig. 3B-E).
These results clearly show that plectin deficiency leads to the formation of aberrant intestinal junctional complexes, which likely accounts for breached epithelial barrier integrity.
In previous studies, we showed that plectin controls cell junctions through anchorage of IF networks 20,22,30 . Therefore, we next compared the appearance of KFs in Ple fl/fl and Ple ΔIEC colon sections using immunofluorescence microscopy. Although the general appearance of K8 and K19 networks did not significantly differ in the two cell types (Fig. S5), super-resolution microscopy of pan-K-labeled sections revealed less pronounced apical staining of Ple ΔIEC IECs (Fig. 4A). Moreover, in Ple ΔIEC IECs, pan-K positive filaments formed less-ordered and rather coarse meshworks, while differentiated Caco-2 cells 16 days after the confluency were used. In wild-type (WT) cells, the KF network was densely packed around the cell center, from which individual KFs extended towards the cell periphery delineated by clearly defined desmoplakin-positive Ds (Fig. 4D). In contrast, KO cells showed tangled KFs, which were evenly distributed throughout the cytoplasm and seemingly overlapped with rather continuous desmoplakin-positive structures at the cell-cell borders (Fig. 4D). Similar to Ple ΔIEC IECs, actin organization in KO cells appeared inconspicuous (Fig. S6B).
Collectively, these findings indicate that plectin ablation in IECs results in altered keratin network organization and aberrant KF anchorage to desmosomal junctions.
Plectin preserves intestinal epithelial integrity through HD stabilization Plectin-mediated attachment of the keratin network to Itgα6β4containing HDs plays a crucial role in stabilizing the adhesion of keratinocytes to the matrix and hence imparts mechanical stability to the skin 20,31 . To examine whether the Ple ΔIEC intestine phenotypically follows the same paradigm, we scrutinized colon and small intestine sections immunolabeled for K8 and Itgα6 Braces (white) indicate hemidesmosomes (HD), tight junctions (TJ), adherens junctions (AJ), and desmosomes (Ds). Scale bar, 500 nm. Graphs show quantitative analyses of junctional complex widths (measured as the distance from IEC to BM (HD) or distance from IEC to IEC membrane (TJ, AJ, and Ds)). Five to fifteen junctions were measured (two mice per genotype). B Relative mRNA levels of integrin (Itg) α6 and β4 in scraped mucosa from Ple fl/fl and Ple ΔIEC distal colons, n = 4-5. C Quantification of Itgβ4 and Itgα6 in scraped distal colon mucosa from Ple fl/fl and Ple ΔIEC mice by immunoblotting. GAPDH, loading control. The graph shows relative band intensities normalized to average Ple fl/fl values, n = 3. D Relative mRNA levels of ZO-1, E-cadherin (E-cad), desmoglein 2 (Dsg2), and desmoplakin 1/2 (Dsp1/2) in Ple fl/fl and Ple ΔIEC distal colons, n = 5. E Quantification of ZO-1, E-cad, and Dsg2 in Ple fl/fl and Ple ΔIEC colon mucosa by immunoblotting. GAPDH, loading control. The graph shows relative band intensities normalized to average Ple fl/fl values, n = 3. Data are presented as mean ± SEM, n.s. not significant, *P < 0.05, **P < 0.01, † P < 0.001. To address the effects of plectin ablation biochemically, we prepared keratin-enriched cell fractions 19 from human WT and KO IEC Caco-2 lines and compared their integrin (Itg) content by immunoblotting using antibodies to Itgβ4. As expected, such cell fractions were highly enriched in keratins 8 (Fig. 5B), 18, and 19 (not shown). Although Itgβ4 levels were comparable in cell lysates, the Itgβ4 content of insoluble keratin fractions was significantly reduced in KO cells compared to WT cells (Fig. 5B). These observations correlated well with the histological data (Fig. 5A).
To assess whether plectin deficiency affects the biomechanical properties of IECs, we performed a series of quantitative assays with human IEC lines Caco-2 and hCC. Monitoring cell viability under mechanical stress on a stretched flexible membrane (uniaxial cyclic stretch) revealed the higher mechanical vulnerability of both KO cell lines, as the proportion of PI-positive (dead) cells significantly increased with stretch amplitudes ranging from 10% to 50% (Figs. 5C and S9A). Reduced mechanical resistance of KO cells was confirmed by fluid shear stress assay using a spinning disc device. When exposed to constant radial flow, KO cells displayed death rates about twice as high as that of their WT counterparts (Figs. 5D and S9B). Moreover, the fraction of detached Caco-2 (but not hCC) cells was higher for KO than WT cells. This suggests that plectin ablation weakens their adhesion to the underlying substratum.
To confirm this hypothesis, we quantified adhesion strength between ECM-coated superparamagnetic beads and cell adhesions using magnetic tweezers. We applied increasing forces of up to 15 nN to magnetic beads (force increase at 1 nN/s) and recorded the force at which each bead detached from the cell. From a total of >100 detachment events for each cell type, we calculated the cumulative detachment probability as a function of pulling force and report the force at which 50% of the beads detached from the cells (Figs. 5E and S9C). We measured lower detachment forces in both Caco-2 and hCC KO compared to WT cells, which confirms our hypothesis of weaker Itg-mediated adhesions in plectin-deficient cells. Hence, like for skin type I HDs 20 , plectin loss is deleterious for the stability of type II HDs present in the intestine, leading to compromised mechanical resilience of IECs and intestinal epithelia.
IEC-specific plectin deficiency exacerbates experimental colitis The spontaneous colitic phenotype in Ple ΔIEC mice ( Fig. 1) suggests that plectin deletion can contribute substantially to the pathogenesis of UC. To assess whether loss of plectin increases the susceptibility to colitis, we induced experimental colitis in Ple fl/fl and Ple ΔIEC mice. Even a short exposure (3-4 days) to low DSS doses (1.5-2%) resulted in a dramatic bodyweight loss of Ple ΔIEC mice, in sharp contrast to similarly treated Ple fl/fl which experienced only insignificant weight losses (Fig. 6A). The weight loss of mutant mice was associated with a higher disease activity index (DAI; Fig. 6A), a decreased survival rate (not shown), and a significant reduction in colon length (Fig. 6B). The severity of induced colitis in Ple ΔIEC mice coincided with larger inflamed areas at days 4 and 6 after the initiation of DSS-treatment (Fig. 6C), corresponding to a higher influx of MPO-positive neutrophils and intestinal epithelial injury. Further, histological evaluation of "Swiss rolls" of the entire colon confirmed these results and revealed clear signs of inflammation and epithelial damage in all DSStreated animals. However, large regions with heavy ulceration, crypt damage, and inflammatory response in Ple ΔIEC mice were in striking contrast to fewer lesions in Ple fl/fl mice (Figs. 6D and S10A).
Since gut microbial dysbiosis is a typical finding in UC patients [32][33][34] , we compared the composition of fecal microbiota in unchallenged Ple fl/fl and Ple ΔIEC mice at the ages of 4, 12, and 20 weeks. Surprisingly, despite the impaired intestinal barrier and concomitant inflammation phenotype of Ple ΔIEC mice (Fig. 1F-L), we observed no significant differences in alpha (Fig. S10B) and beta (Fig. 6E) diversities between both genotypes. In all animals, bacterial microbiota was dominated by bacteria belonging to families S24-7 (bacteroidetes), lactobacillaceae (firmicutes), and lachnospiraceae (firmicutes) (Fig. 6F). Together, these data show higher susceptibility of Ple ΔIEC mice to DSS-induced colitis, accompanied by severe epithelial damage and inflammation in the absence of microbial dysbiosis.
Reduced mechanical stability of epithelia accounts for intestinal injury in Ple ΔIEC mice To identify the onset and time course of intestinal injury in Ple ΔIEC mice, we assessed intestinal epithelial damage scores in newborn, 21-day-old, and 12-week-old mice (Figs. S11A and 7A-C). While newborn mice were histologically inconspicuous, the colon and the small intestine displayed first signs of damage in 21-day-old Ple ΔIEC mice (see also Fig. S12B), which coincided with weaning and transition to solid chow. The onset of the epithelial breach was accompanied by the subsequent development of inflammatory response as shown by increased immune cell infiltration, extent (or intensity) of inflammation, and lymphatic follicle number/size (Fig. S12A).  ). B Cell lysates, cytosol fractions, and keratin-enriched high salt extracts (HSE) were prepared from WT and KO Caco-2 cells and subjected to immunoblotting with antibodies to Itgβ4 and K8. GAPDH, loading control. Graphs show relative band intensities normalized to average Ple fl/fl values, n = 4-6. C Viability of WT and KO Caco-2 cells exposed to uniaxial cyclic stretch presented as a percentage of dead (PI-positive; PI + ) cells, n = 9-11. D Quantification of WT and KO Caco-2 cell viability (left) and adhesion (right) under radial shear flow shown as a percentage of dead and detached cells, respectively, n = 6. Boxplot data represent median, 25th, and 75th percentile with whiskers reaching the last data point. E Adhesion strength between ECM-coated superparamagnetic beads and WT and KO Caco-2 cells was quantified using magnetic tweezers that generated forces ramps at a speed of 1 nN/s up to a maximum force of 15 nN. Image and schematic depict magnetic tweezer setup. Arrowhead, paramagnetic bead; asterisk, magnetic tweezer tip; dotted circular line, cell border. Scale bar, 20 μm. The graph shows the percentage of beads (n = 103 WT, 109 KO cells) that remained adherent at a given pulling force. The boxplot shows the distribution of the median detachment force (calculated from bootstrapping by sampling with replacement, n = 1000 runs) and its distribution (25th, and 75th percentile with whiskers reaching the minimum and maximum sampled values). Bar graph data in all other subplots represent mean ± SEM, n.s. not significant, *P < 0.05, † P < 0.001.
To gain better control over plectin inactivation timing, we generated TMX-inducible IEC-specific plectin knockout (Ple ΔIEC-ERT2 ) mice. Three consecutive applications of TMX in 9-week-old kept on solid chow or provided with a low-residue liquid diet 6 days before TMX administration. The liquid diet significantly attenuated epithelial damage in the colon of Ple ΔIEC-ERT2 mice; however, the histological score indicates more severe injury of the small intestine (Figs. 7D-F and S11B). The beneficial effects of the liquid diet also manifested as less prominent colon swelling (not shown).
As previous studies linked the severity of colitis and intestinal injury with commensal microbiota 8,35 , next we treated Ple fl/fl and Ple ΔIEC-ERT2 mice with well-established broad-spectrum antibiotics 8 . In our experimental setup, the apparent milder colitic phenotype consistently coincided with lower epithelial damage in the colon of Ple ΔIEC-ERT2 mice. This treatment however did not affect epithelial injury of the Ple ΔIEC-ERT2 small intestine (Figs. 7G-I and S11C). Collectively, these data support the notion that the increased susceptibility of the plectin-deficient intestinal epithelium to mechanical strain impinged by luminal content is caused by a lack of KF attachment to Itg clusters and destabilization of intestinal HDs. Further, the fact that antibiotics also partially alleviate epithelial damage suggests that luminal bacteria significantly contribute to intestinal injury in Ple ΔIEC-ERT2 mice.

DISCUSSION
The intestinal epithelium faces substantial mechanical stress 36 , which is inextricably linked to gut physiology. Although several studies suggest the importance of intestinal KF networks 35,37,38 and KF-associated cell junctions (Ds and HDs) 8,9 for protection against intestinal inflammation and CRC, the contribution of altered epithelial mechanics to observed phenotypes remain unexplored. Here, we focus on the role of KF-cell junction linker plectin in the maintenance of intestinal homeostasis, and we provide a comprehensive analysis of molecular mechanisms governing the mechanical stability of intestinal epithelia.
The most notable phenotype of both plectin-deficient mouse models (Ple ΔIEC and Ple ΔIEC-ERT2 ) is the detachment of IECs from the   10 , which proposed that plectin (along with BPAG, another plakin family member) fortifies HD plaques both horizontally (by a lateral association of Itgβ4) and vertically (by interlinking Itgβ4 with KFs). Accordingly, ablation of plectin, the only plakin present in intestinal-type II HD 39 , would fully abrogate a functional link between KFs and HDs and would result in their overall destabilization. In line with this hypothesis, we in vitro show a trend towards higher detachment (paralleled with a higher death rate) of plectin-deficient IECs exposed to a uniaxial cyclic stretch and a constant radial flow compared to their WT counterparts. As both Ple fl/fl and Ple ΔIEC IECs display a minimal degree of spontaneous apoptosis in vivo, the observed excessive cell death in our experiments in vitro can likely be attributed to incomparable force magnitude and cell context under these two conditions. Consistent with our results from cell stretching and radial shear assays, we also determine significantly lower adhesion strength between ECM-coated superparamagnetic beads and plectin-deficient IECs using magnetic tweezers. Hence, by combining in vivo and in vitro approaches, we provide evidence that plectin is essential for the stability of intestinal HD type II, a structure preventing colitis 8 and presumably also the risk of colitisassociated CRC 8,40,41 .
Previous studies demonstrated that the deletion of plectin has adverse effects on the formation of intercellular junctions, with consequences for the epithelial barrier function 21,22 . It has been shown that plectin-deficient cholangiocytes form dysfunctional Ds and fail to upregulate some desmosomal proteins, such as desmoplakin, a putative binding partner of plectin 42 , in response to bile stasis 22 . This failure results in mechanical weakening of the biliary epithelium and contributes to plectin-related familial intrahepatic cholestasis 43 . In terms of mechanistic parallels between plectin-deficient biliary and intestinal epithelia, we found that apart from destabilizing HDs, plectin deficiency also leads to the prominent broadening of Ds, AJs, and TJs. Moreover, Ple ΔIEC IECs exhibit downregulation of corresponding junctional constituents (ZO-1, E-cad, Dsg2, and Dsp1/2). Although the resulting dilatation of intercellular spaces would per se suffice to explain the observed increase in intestinal permeability and bacterial penetration, the "leaky gut" in Ple ΔIEC mice seems ultimately rooted in the less firm IEC/BM connection, given the extent of IEC detachment. On the other hand, proper anchorage of KFs (determining cell mechanics) to Ds (ensuring intercellular cohesion) is known to provide load-bearing tissues with mechanical stability 11 . Showing altered KF cytoarchitecture and aberrant D formation in both in vitro plectin-deficient IEC systems and Ple ΔIEC mice, our results suggest that D-keratin complex abnormality substantially contributes to the compromised mechanics of the Ple ΔIEC intestinal epithelium.
We propose that the lack of functional plectin at HDs (in combination with its effects on KFs and Ds) and the resulting mechanical epithelial fragility favor an impaired intestinal barrier function and are ultimately responsible for colitis. Importantly, comparable mucosal deterioration was observed upon plectin ablation during development and in the adult intestine with its fully mature immune system. Furthermore, we also demonstrate that loss of plectin can exacerbate experimental colitis in mice. Consistent with these results, lower expression levels of plectin correlate with UC development in human patients, suggesting that defects in cytoskeleton coordination mediated through plectin contribute to IBD pathogenesis in humans by affecting IEC/BM adhesion, IEC cohesion, and mechanical properties. However, it is well recognized that properly organized KF networks 38,44 , HDs 8 , and intercellular junctions 2,3,9 exert numerous non-mechanical functions, providing the intestinal epithelium with protection against microbial infection and uncontrolled inflammation. Our data do not rule out similar functions in the Ple ΔIEC intestine. Further studies will be required to investigate how plectin deficiency affects cell-autonomous (barrier functionindependent) mechanisms involved in the interplay between IECs, gut microbiota, and immune cells.
We observed a prominent hyperproliferation of plectin-deficient IECs paralleled by a dramatically higher proportion of PAS-positive goblet cells. This phenotype closely resembles the situation in mice lacking hemidesmosomal α6 integrin 8 or K8 45,46 . Together, these findings suggest that the keratin/plectin/integrin axis is essential for balanced proliferation and differentiation of IECs. Interestingly, intestinal K8 and K18 were shown to promote Notch1 signaling, a major pathway of colonic cell fate specification 46 . Since Notch-mediated signal transduction depends on cytoskeletal tension 47,48 , it is tempting to speculate that intact plectin-controlled KF cytoarchitecture facilitates mechanoregulation of intestinal cell fate. Moreover, plectin anchors the cytoskeleton to the nuclear envelope via interaction with nesprin-3 49 and mediates transmission of mechanical stimuli directly to the nucleus. Multiple studies provide evidence that loss of plectin results in nuclear phenotypes, including altered nuclear positioning 22 , nuclear deformations 50,51 , chromatin modification, and gene expression 51 . To elucidate whether and how plectin regulates specific transcriptional programs in IECs and to find their contribution to the proper spatiotemporal proliferation/differentiation pattern within colonic crypts is a goal of our ongoing studies.
The rapid deterioration of the Ple ΔIEC intestinal mucosa following weaning (i.e., a switch to a solid diet and the amplification of muscle contractions) indicates that the origin of colitis in the absence of plectin is primarily associated with a reduced capacity of IECs to resist mechanical stress. In addition, the most severe epithelial injury was found in the distal colon, which is the region most intensely subjected to such stress. Comparably devastating epithelial instability has been well documented for epidermal layers in EB patients 15,16 . As there is no causal therapy for EB available 15 , the current treatment focuses mainly on the prevention of tissue destruction. Following the same rationale, we demonstrate that a low-residue liquid diet significantly attenuates colonic epithelial damage, thus protecting its barrier function. Surprisingly, this approach aggravates IEC detachment in the Ple ΔIEC small intestine, which might suggest augmented susceptibility of the small intestine to plectin loss. Therefore, future studies should investigate whether differential expression of plectin along the gastrointestinal tract might have an impact on regional differences in disease manifestations in patients. In line with the previous observations 8, 35 , antibiotic treatments markedly decrease not only mucosal inflammation but, intriguingly, also epithelial damage, which implies that hostmicrobiota interactions contribute to excessive IEC sloughing in the Ple ΔIEC intestine. Although dietary factors can likely ameliorate only less extensive trauma, our results suggest that a low-residue liquid diet combined with antibiotic treatment might be a useful palliative modality. To translate our findings into clinical medicine: it remains to be determined whether such a strategy (i) is suitable for long-term treatment and (ii) is effective with respect to systemic disease manifestation.

Patients
Colon biopsy samples were collected from patients diagnosed with UC (n = 97) and from healthy controls (n = 20) admitted to the Hepatogastroenterology Department at the Institute for Clinical and Experimental Medicine (Prague, Czech Republic) for a colonoscopy from July 2016 to May 2019. Subjects were assigned to the healthy control group only after all clinical examinations excluded any signs of autoimmune disease, inflammatory disease, and colon cancer. All UC patients with concurrent primary sclerosing cholangitis (PSC) were excluded from the study. Endoscopic UC activity at the time of a standard optical colonoscopy was categorized according to the Mayo endoscopic subscore and confirmed by histology examinations of the grade of inflammation. Clinical characteristics of patients are shown in Supplementary Table S1. Standard endoscopic biopsies were extracted from the inflamed non-dysplastic mucosa of the left colon (rectum) and immediately placed in an RNAlater solution. Total RNA was extracted according to the manufacturer's instructions.

Mice
Plectin flox/flox (Ple fl/fl ) mice 23 were crossed with villin-Cre transgenic mice (MGI 2448639) to generate Ple fl/fl /villin-Cre mice (Ple ΔIEC ) and with villin-creERT2 transgenic mice (MGI 3053826; both Cre strains were kindly provided by S. Robine 52 ) to generate Ple fl/fl /villin-creERT2 mice (Ple ΔIEC-ERT2 ). Age-matched littermate male mice were used in all experiments. Unless stated otherwise, mice were 12-14 weeks old. Animals were housed under specific pathogenfree conditions with regular access to chow and drinking water and a 12 h light/12 h dark regime.
Cells and CRISPR-mediated targeting of plectin Caco-2 cells were grown in Dulbecco's modified Eagle medium (DMEM) supplemented with 20% fetal bovine serum (FBS) in a 5% CO 2 /air humidified atmosphere at 37°C. Human colonic cells (hCC; T0570, Applied biological materials, Inc.) were cultured in DMEM supplemented with 10% FBS in 5% CO 2 /air humidified atmosphere at 37°C. Plectin knockout (KO) cell lines were generated by targeting genomic sequences of intron 25-26 and exon 31 of Plectin using CRISPR/Cas9 plasmid pX330 Cas9-Venus (a kind gift of B. Schuster, IMG CAS, Prague, Czech Republic) as described previously 22 . The potential off-target sites were predicted using CRISPOR (http://crispor.tefor.net/). The four top-ranking potential off-target sites for each guide RNA were selected for validation. The genomic DNA sequences surrounding the potential off-target sites were amplified by PCR using gene-specific primers (Supplementary Table 3). PCR products were analyzed by direct sequencing (Figs. S12 and S13).

Statistics
All results are presented as mean ± SEM. All normally distributed parametric data were analyzed by two-tailed unpaired Student t test. Comparisons of multiple groups to controls were performed using two-tailed one-way ANOVA. Comparisons of frequency distributions of BrdU-positive cells were analyzed with Mann-Whitney test. Survival curves were analyzed by Mantel-Cox test. Statistical analyses were performed using GraphPad Prism 5 (GraphPad Software, Inc., La Jolla, CA). Comparisons of detachment forces were done with bootstrapping (sampling with replacement) with 1000 replicates. Statistical significance was determined at the levels of *P < 0.05, **P < 0.01, † P < 0.001; n values are specified in the figure legends.
Study approval This study was approved by the Ethics Committee of the Institute for Clinical and Experimental Medicine and Thomayer Hospital with Multi-Center Competence (G16-06-25). Written informed consent was obtained from all subjects before the study. All animal studies were performed in accordance with European Directive 2010/63/EU and were approved by the Czech Central Commission for Animal Welfare (48/2014 and 23/2020).