Crucial role of posttranslational modifications of integrin α3 in interstitial lung disease and nephrotic syndrome

Genotype-Based Molecular Mechanisms in Alport Syndrome

Alport syndrome is an inherited disorder characterized by kidney disease, sensorineural hearing loss, and ocular abnormalities. Alport syndrome is caused by pathogenic variants in COL4A3 , COL4A4 , or COL4A5 , which encode the α 3, α 4, and α 5 chains of type 4 collagen that forms a heterotrimer expressed in the glomerular basement membrane. Knowledge of its genetic basis has informed the development of different models in dogs, mice, and rats that reflect its autosomal and X-linked inheritance patterns as well as different mutation types, including protein-truncating and missense variants. A key difference between these two types is the synthesis of α 3 α 4 α 5(IV), which is not made in autosomal Alport syndrome (two pathogenic variants in trans or biallelic) or male patients with X-linked Alport syndrome due to protein-truncating variants. By contrast, α 3 α 4 α 5(IV) is synthesized in Alport syndrome because of missense variants. For missense variants, in vitro studies suggest that these cause impaired type 4 collagen trafficking and endoplasmic reticulum stress. For protein-truncating variants, knockout models suggest that persistence of an immature α 1 α 1 α 2(IV) network is associated with biomechanical strain, which activates endothelin-A receptors leading to mesangial filopodia formation. Moreover, studies suggest that activation of collagen receptors, integrins and discoidin domain receptor 1, play a role in disease propagation. In this review, we provide an overview of how these genotype-phenotype mechanisms are key for a precision medicine-based approach in the future.

Roles for epithelial integrin α3β1 in regulation of the microenvironment during normal and pathological tissue remodeling

Integrin receptors for the extracellular matrix activate intracellular signaling pathways that are critical for tissue development, homeostasis, and regeneration/repair, and their loss or dysregulation contributes to many developmental defects and tissue pathologies. This review will focus on tissue remodeling roles for integrin α3β1, a receptor for laminins found in the basement membranes (BMs) that underlie epithelial cell layers. As a paradigm, we will discuss literature that supports a role for α3β1 in promoting ability of epidermal keratinocytes to modify their tissue microenvironment during skin development, wound healing, or tumorigenesis. Preclinical and clinical studies have shown that this role depends largely on ability of α3β1 to govern the keratinocyte's repertoire of secreted proteins, or the "secretome," including 1) matrix proteins and proteases involved in matrix remodeling and 2) paracrine-acting growth factors/cytokines that stimulate other cells with important tissue remodeling functions (e.g., endothelial cells, fibroblasts, inflammatory cells). Moreover, α3β1 signaling controls gene expression that helps epithelial cells carry out these functions, including genes that encode secreted matrix proteins, proteases, growth factors, or cytokines. We will review what is known about α3β1-dependent gene regulation through both transcription and posttranscriptional mRNA stability. Regarding the latter, we will discuss examples of α3β1-dependent alternative splicing (AS) or alternative polyadenylation (APA) that prevents inclusion of cis-acting mRNA sequences that would otherwise target the transcript for degradation via nonsense-mediated decay or destabilizing AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). Finally, we will discuss prospects and anticipated challenges of exploiting α3β1 as a clinical target for the treatment of cancer or wound healing.

Metabolism-driven glycosylation represents therapeutic opportunities in interstitial lung diseases

Metabolic changes are coupled with alteration in protein glycosylation. In this review, we will focus on macrophages that are pivotal in the pathogenesis of pulmonary fibrosis and sarcoidosis and thanks to their adaptable metabolism are an attractive therapeutic target. Examples presented in this review demonstrate that protein glycosylation regulates metabolism-driven immune responses in macrophages, with implications for fibrotic processes and granuloma formation. Targeting proteins that regulate glycosylation, such as fucosyltransferases, neuraminidase 1 and chitinase 1 could effectively block immunometabolic changes driving inflammation and fibrosis, providing novel avenues for therapeutic interventions.

Integrin α3 negative podocytes: A gene expression study

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New cell model to investigate the impact of loss of integrin α3 in podocytes.

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In this novel model, genes of the extracellular matrix and adhesome are mostly downregulated.

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Loss of integrin α3 results in changes of cell adhesion and spreading.

Integrin α3β1 is a cell adhesion receptor widely expressed in epithelial cells. Pathogenic variants in the gene encoding the integrin α3 subunit ITGA3 lead to a syndrome including interstitial lung disease, nephrotic syndrome, and epidermolysis bullosa (ILNEB). Renal involvement mainly consists of glomerular disease caused by loss of adhesion between podocytes and the glomerular basement membrane.

The aim of this study was to characterize the impact of loss of integrin α3 on human podocytes.

ITGA3 was stably knocked-out in the human podocyte cell line AB8/13, designated as Podo^A3−, and in human proximal tubule epithelial cell line HK2 using the targeted genome editing technique CRISPR/Cas9. Cell clones were characterized by Sanger sequencing, quantitative PCR, Western Blot and immunofluorescence staining. RNASeq of integrin α3 negative cells and controls was performed to identify differential gene expression patterns.

Differentiated Podo^A3− did not substantially change morphology and adhesion under standard culture conditions, but displayed significantly reduced spreading and adhesion when seed on laminin 511 in serum free medium. Gene expression studies demonstrated a distinct dysregulation of the adhesion network with downregulation of most integrin α3 interaction partners. In agreement with this, biological processes such as “extracellular matrix organization” and “cell differentiation” as well as KEGG pathways such as “ECM-receptor interaction”, “focal adhesion” and the “PI3K-Akt signaling pathway” were significantly downregulated in human podocytes lacking the integrin α3 subunit.

Biallelic Missense Mutations in the Integrin Alpha 3 Gene Causes Skin Fragility Without Structural Defects in Lungs and Kidneys

Abstract is missing (Short communication)

The extracellular matrix in development

As the crucial non-cellular component of tissues, the extracellular matrix (ECM) provides both physical support and signaling regulation to cells. Some ECM molecules provide a fibrillar environment around cells, while others provide a sheet-like basement membrane scaffold beneath epithelial cells. In this Review, we focus on recent studies investigating the mechanical, biophysical and signaling cues provided to developing tissues by different types of ECM in a variety of developing organisms. In addition, we discuss how the ECM helps to regulate tissue morphology during embryonic development by governing key elements of cell shape, adhesion, migration and differentiation.

Summary: This Review discusses our current understanding of how the extracellular matrix helps guide developing tissues by influencing cell adhesion, migration, shape and differentiation, emphasizing the biophysical cues it provides.

Clinical practice guidelines for laboratory diagnosis of epidermolysis bullosa

Linked Comment: https://doi.org/10.1111/bjd.18377.

https://doi.org/10.1111/bjd.18829 available online

Update on Genetic Conditions Affecting the Skin and the Kidneys

Genetic conditions affecting the skin and kidney are clinically and genetically heterogeneous, and target molecular components present in both organs. The molecular pathology involves defects of cell-matrix adhesion, metabolic or signaling pathways, as well as tumor suppressor genes. This article gives a clinically oriented overview of this group of disorders, highlighting entities which have been recently described, as well as the progress made in understanding well-known entities. The genetic bases as well as molecular cell biological mechanisms are described, with therapeutic applications.

Monoallelic Mutations in the Translation Initiation Codon of KLHL24 Cause Skin Fragility

The genetic basis of epidermolysis bullosa, a group of genetic disorders characterized by the mechanically induced formation of skin blisters, is largely known, but a number of cases still remain genetically unsolved. Here, we used whole-exome and targeted sequencing to identify monoallelic mutations, c.1A>G and c.2T>C, in the translation initiation codon of the gene encoding kelch-like protein 24 (KLHL24) in 14 individuals with a distinct skin-fragility phenotype and skin cleavage within basal keratinocytes. Remarkably, mutation c.1A>G occurred de novo and was recurrent in families originating from different countries. The striking similarities of the clinical features of the affected individuals point to a unique and very specific pathomechanism. We showed that mutations in the translation initiation codon of KLHL24 lead to the usage of a downstream translation initiation site with the same reading frame and formation of a truncated polypeptide. The pathobiology was examined in keratinocytes and fibroblasts of the affected individuals and via expression of mutant KLHL24, and we found mutant KLHL24 to be associated with abnormalities of intermediate filaments in keratinocytes and fibroblasts. In particular, KLHL24 mutations were associated with irregular and fragmented keratin 14. Recombinant overexpression of normal KLHL24 promoted keratin 14 degradation, whereas mutant KLHL24 showed less activity than the normal molecule. These findings identify KLHL24 mutations as a cause of skin fragility and identify a role for KLHL24 in maintaining the balance between intermediate filament stability and degradation required for skin integrity.

Viable phenotype of ILNEB syndrome without nephrotic impairment in siblings heterozygous for unreported integrin alpha3 mutations

BACKGROUND

Integrin α3 (ITGA3) gene mutations are associated with Interstitial Lung disease, Nephrotic syndrome and Epidermolysis bullosa (ILNEB syndrome). To date only six patients are reported: all carried homozygous ITGA3 mutations and presented a dramatically severe phenotype leading to death before age 2 years, from multi-organ failure due to interstitial lung disease and congenital nephrotic syndrome. The involvement of skin and cutaneous adnexa was variable with sparse hair and nail dysplasia combined or not to skin lesions ranging from skin fragility to epidermolysis bullosa-like blistering.

RESULTS

We report on two siblings of 13 and 9 years born to non-consanguineous healthy parents, who display growth delay, severe pulmonary fibrosis with fatigue, dyspnea on exertion and wheezing, atrophic skin with erythematosus lesions, rare eyelashes/eyebrows and pachyonychia. By exome sequencing, we identified two unreported ITGA3 missense mutations, c.373G>A (p.(G125R)) in exon 3 and c.821G>A (p.(R274Q)) in exon 6, affecting highly conserved residues in the integrin α3 extracellular N-terminal β-propeller domain. Homology modelling of α3β1 heterodimer fragment, encompassing the mutation sites, showed that G125 plays a pivotal structural role in the β-propeller, while R274 might prevent the interaction between integrin and urokinase complex.

CONCLUSION

We report a variant of ILNEB syndrome in two siblings differing from the previously reported patients in the lack of nephrotic impairment and survival beyond childhood. Our siblings are the first reported compound heterozygous for ITGA3 mutations; this state as well as the hypomorphic nature of their p.(R274Q) mutation likely account for their survival.

Recently Identified Forms of Epidermolysis Bullosa

Epidermolysis bullosa (EB) comprises a collection of clinically diverse inherited blistering diseases that affect the skin and, in some subtypes, mucous membranes and other organs. Currently classified into four main subtypes (EB simplex, junctional EB, dystrophic EB, and Kindler syndrome, mainly based on the level of skin cleavage), the spectrum of EB extends to more than 30 clinical subtypes with pathogenic mutations in at least 18 distinct genes. This review focuses on three recent additions to variants of EB: all are autosomal recessive, and result from mutations in either DST-e (coding for epidermal dystonin, also known as the 230 kDa bullous pemphigoid antigen, BP230), EXPH5 (coding for exophilin-5, also known as Slac2-b), or ITGA3 (coding for the integrin alpha-3 subunit). Each of these new forms of EB is reviewed with respect to the initial gene discovery, clinical features, the current mutation database, and skin pathology. Awareness of these recently described forms of EB is helpful in the clinical evaluation of patients with EB and in defining genotype-phenotype correlation for inherited blistering skin diseases.