Identification of a novel family of laminin N-terminal alternate splice isoforms: structural and functional characterization

Protein retention in the endoplasmic reticulum rescues Aβ toxicity in Drosophila

Amyloid β (Aβ) accumulation is a hallmark of Alzheimer's disease. In adult Drosophila brains, human Aβ overexpression harms climbing and lifespan. It's uncertain whether Aβ is intrinsically toxic or activates downstream neurodegeneration pathways. Our study uncovers a novel protective role against Aβ toxicity: intra-endoplasmic reticulum (ER) protein accumulation with a focus on laminin and collagen subunits. Despite high Aβ, laminin B1 (LanB1) overexpression robustly counters toxicity, suggesting a potential Aβ resistance mechanism. Other laminin subunits and collagen IV also alleviate Aβ toxicity; combining them with LanB1 augments the effect. Imaging reveals ER retention of LanB1 without altering Aβ secretion. LanB1's rescue function operates independently of the IRE1α/XBP1 ER stress response. ER-targeted GFP overexpression also mitigates Aβ toxicity, highlighting broader ER protein retention advantages. Proof-of-principle tests in murine hippocampal slices using mouse Lamb1 demonstrate ER retention in transduced cells, indicating a conserved mechanism. Though ER protein retention generally harms, it could paradoxically counter neuronal Aβ toxicity, offering a new therapeutic avenue for Alzheimer's disease.

Phenotypic variability in LAMA3-associated amelogenesis imperfecta

OBJECTIVE

Amelogenesis imperfecta (AI) is defined as inherited enamel malformations. LAMA3 (laminin alpha-3) encodes a critical protein component of the basement membrane (laminin-332). Individuals carrying heterozygous LAMA3 mutations have previously been shown to have localized enamel defects. This study aimed to define clinical phenotypes and to discern the genetic etiology for four AI kindreds.

MATERIALS AND METHODS

Whole-exome analyses were conducted to search for sequence variants associated with the disorder, and micro-computed tomography (μCT) to characterize the enamel defects.

RESULTS

The predominant enamel phenotype was generalized thin enamel with defective pits and grooves. Horizonal bands of hypoplastic enamel with chalky-white discoloration and enamel hypomineralization were also observed and demonstrated by μCT analyses of affected teeth. Four disease-causing LAMA3 mutations (NM_198129.4:c.3712dup; c.5891dup; c.7367del; c.9400G > C) were identified. Compound heterozygous MMP20 mutations (NM_004771.4:c.539A > G; c.692C > T) were also found in one proband with more severe enamel defects, suggesting a mutational synergism on disease phenotypes. Further analyses of the AI-causing mutations suggested that both α3A (short) and α3B (long) isoforms of LAMA3 are essential for enamel formation.

CONCLUSIONS

Heterozygous LAMA3 mutations can cause generalized enamel defects (AI1A) with variable expressivity. Laminin-332 is critical not only for appositional growth but also enamel maturation.

Laminin N-terminus (LaNt) proteins, laminins and basement membrane regulation

Basement membranes (BMs) are structured regions of the extracellular matrix that provide multiple functions including physical support and acting as a barrier, as a repository for nutrients and growth factors, and as biophysical signalling hubs. At the core of all BMs is the laminin (LM) family of proteins. These large heterotrimeric glycoproteins are essential for tissue integrity, and differences between LM family members represent a key nexus in dictating context and tissue-specific functions. These variations reflect genetic diversity within the family, which allows for multiple structurally and functionally distinct heterotrimers to be produced, each with different architectures and affinities for other matrix proteins and cell surface receptors. The ratios of these LM isoforms also influence the biophysical properties of a BM owing to differences in their relative ability to form polymers or networks. Intriguingly, the LM superfamily is further diversified through the related netrin family of proteins and through alternative splicing leading to the generation of non-LM short proteins known as the laminin N-terminus (LaNt) domain proteins. Both the netrins and LaNt proteins contain structural domains involved in LM-to-LM interaction and network assembly. Emerging findings indicate that one netrin and at least one LaNt protein can potently influence the structure and function of BMs, disrupting the networks, changing physical properties, and thereby influencing tissue function. These findings are altering the way that we think about LM polymerisation and, in the case of the LaNt proteins, suggest a hitherto unappreciated form of LM self-regulation.

Laminin N-terminus α31 expression during development is lethal and causes widespread tissue-specific defects in a transgenic mouse model

Laminins (LMs) are essential components of all basement membranes where they regulate an extensive array of tissue functions. Alternative splicing from the laminin α3 gene produces a non‐laminin but netrin‐like protein, Laminin N terminus α31 (LaNt α31). LaNt α31 is widely expressed in intact tissue and is upregulated in epithelial cancers and during wound healing. In vitro functional studies have shown that LaNt α31 can influence numerous aspects of epithelial cell behavior via modifying matrix organization, suggesting a new model of laminin auto‐regulation. However, the function of this protein has not been established in vivo. Here, a mouse transgenic line was generated using the ubiquitin C promoter to drive inducible expression of LaNt α31. When expression was induced at embryonic day 15.5, LaNt α31 transgenic animals were not viable at birth, exhibiting localized regions of erythema. Histologically, the most striking defect was widespread evidence of extravascular bleeding across multiple tissues. Additionally, LaNt α31 transgene expressing animals exhibited kidney epithelial detachment, tubular dilation, disruption of the epidermal basal cell layer and of the hair follicle outer root sheath, and ~50% reduction of cell numbers in the liver, associated with depletion of hematopoietic erythrocytic foci. These findings provide the first in vivo evidence that LaNt α31 can influence tissue morphogenesis.

Laminin N-terminus α31 is upregulated in invasive ductal breast cancer and changes the mode of tumour invasion

Laminin N-terminus α31 (LaNt α31) is an alternative splice isoform derived from the laminin α3 gene. The LaNt α31 protein is enriched around the terminal duct lobular units in normal breast tissue. In the skin and cornea the protein influences epithelial cell migration and tissue remodelling. However, LaNt α31 has never been investigated in a tumour environment. Here we analysed LaNt α31 in invasive ductal carcinoma and determined its contribution to breast carcinoma invasion. LaNt α31 expression and distribution were analysed by immunohistochemistry in human breast tissue biopsy sections and tissue microarrays covering 232 breast cancer samples. This analysis revealed LaNt α31 to be upregulated in 56% of invasive ductal carcinoma specimens compared with matched normal tissue, and further increased in nodal metastasis compared with the tumour mass in 45% of samples. 65.8% of triple negative cases displayed medium to high LaNt α31 expression. To study LaNt α31 function, an adenoviral system was used to induce expression in MCF-7 and MDA-MB-231 cells. 2D cell migration and invasion into collagen hydrogels were not significantly different between LaNt α31 overexpressing cells and control treated cells. However, LaNt α31 overexpression reduced the proliferation rate of MCF-7 and MDA-MB-231 cells. Moreover, LaNt α31 overexpressing MDA-MB-231 cells displayed a striking change in their mode of invasion into laminin-containing Matrigel; changing from multicellular streaming to individual cellular-invasion. In agreement with these results, 66.7% of the tumours with the highest LaNt α31 expression were non-cohesive. Together these findings indicate that breast cancer-associated changes in LaNt α31 expression could contribute to the processes involved in tumour invasion and may represent a new therapeutic target.

Laminin Polymerization and Inherited Disease: Lessons From Genetics

The laminins (LM) are a family of basement membranes glycoproteins with essential structural roles in supporting epithelia, endothelia, nerves and muscle adhesion, and signaling roles in regulating cell migration, proliferation, stem cell maintenance and differentiation. Laminins are obligate heterotrimers comprised of α, β and γ chains that assemble intracellularly. However, extracellularly these heterotrimers then assemble into higher-order networks via interaction between their laminin N-terminal (LN) domains. In vitro protein studies have identified assembly kinetics and the structural motifs involved in binding of adjacent LN domains. The physiological importance of these interactions has been identified through the study of pathogenic point mutations in LN domains that lead to syndromic disorders presenting with phenotypes dependent on which laminin gene is mutated. Genotype-phenotype comparison between knockout and LN domain missense mutations of the same laminin allows inferences to be drawn about the roles of laminin network assembly in terms of tissue function. In this review, we will discuss these comparisons in terms of laminin disorders, and the therapeutic options that understanding these processes have allowed. We will also discuss recent findings of non-laminin mediators of laminin network assembly and their implications in terms of basement membrane structure and function.

Organization of the laminin polymer node

Laminin polymerization is a key step of basement membrane assembly that depends on the binding of α, β and γ N-terminal LN domains to form a polymer node. Nodal assembly can be divided into two steps consisting of β- and γ-LN dimerization followed by calcium-dependent addition of the α-LN domain. The assembly and structural organization of laminin-111 LN-LEa segments was examined by size-exclusion chromatography (SEC) and electron microscopy. Triskelion-like structures were observed in negatively-stained images of purified α1/β1/γ1 LN-LEa trimers. Image averaging of these revealed a heel-to-toe organization of the LN domains with angled outward projections of the LEa stem-like domains. A series of single-amino acid substitutions was introduced into the polymerization faces of the α1, β1 and γ1 LN domains followed by SEC analysis to distinguish between loss of β-γ mediated dimerization and loss of α-dependent trimerization (with intact β-γ dimers). Dimer-blocking mutations were confined to the γ1-toe and the β1-heel, whereas the trimer-only-blocking mutations mapped to the γ1-heel, β1-toe and the α1-toe and heel. Thus, in the polymer node the γ1-toe pairs with the β1-heel, the β1-toe pairs with the α1-heel, and the α1-toe pairs with the γ1-heel.

Netrins: Evolutionarily Conserved Regulators of Epithelial Fusion and Closure in Development and Wound Healing

Epithelial remodelling plays a crucial role during development. The ability of epithelial sheets to temporarily lose their integrity as they fuse with other epithelial sheets underpins events such as the closure of the neural tube and palate. During fusion, epithelial cells undergo some degree of epithelial-mesenchymal transition (EMT), whereby cells from opposing sheets dissolve existing cell-cell junctions, degrade the basement membrane, extend motile processes to contact each other, and then re-establish cell-cell junctions as they fuse. Similar events occur when an epithelium is wounded. Cells at the edge of the wound undergo a partial EMT and migrate towards each other to close the gap. In this review, we highlight the emerging role of Netrins in these processes, and provide insights into the possible signalling pathways involved. Netrins are secreted, laminin-like proteins that are evolutionarily conserved throughout the animal kingdom. Although best known as axonal chemotropic guidance molecules, Netrins also regulate epithelial cells. For example, Netrins regulate branching morphogenesis of the lung and mammary gland, and promote EMT during Drosophila wing eversion. Netrins also control epithelial fusion during optic fissure closure and inner ear formation, and are strongly implicated in neural tube closure and secondary palate closure. Netrins are also upregulated in response to organ damage and epithelial wounding, and can protect against ischemia-reperfusion injury and speed wound healing in cornea and skin. Since Netrins also have immunomodulatory properties, and can promote angiogenesis and re-innervation, they hold great promise as potential factors in future wound healing therapies.

Laminin N-terminus α31 protein distribution in adult human tissues

Laminin N-terminus α31 (LaNt α31) is a netrin-like protein derived from alternative splicing of the laminin α3 gene. Although LaNt α31 has been demonstrated to influence corneal and skin epithelial cell function, its expression has not been investigated beyond these tissues. In this study, we used immunohistochemistry to characterise the distribution of this protein in a wide-array of human tissue sections in comparison to laminin α3. The data revealed widespread LaNt α31 expression. In epithelial tissue, LaNt α31 was present in the basal layer of the epidermis, throughout the epithelium of the digestive tract, and in much of the epithelium of the reproductive system. LaNt α31 was also found throughout the vasculature of most tissues, with enrichment in reticular-like fibres in the extracellular matrix surrounding large vessels. A similar matrix pattern was observed around the terminal ducts in the breast and around the alveolar epithelium in the lung, where basement membrane staining was also evident. Specific enrichment of LaNt α31 was identified in sub-populations of cells of the kidney, liver, pancreas, and spleen, with variations in intensity between different cell types in the collecting ducts and glomeruli of the kidney. Intriguingly, LaNt α31 immunoreactivity was also evident in neurons of the central nervous system, in the cerebellum, cerebral cortex, and spinal cord. Together these findings suggest that LaNt α31 may be functionally relevant in a wider range of tissue contexts than previously anticipated, and the data provides a valuable basis for investigation into this interesting protein.

Skin proteomics - analysis of the extracellular matrix in health and disease

INTRODUCTION

The skin protects the human body from external insults and regulates water and temperature homeostasis. A highly developed extracellular matrix (ECM) supports the skin and instructs its cell functions. Reduced functionality of the ECM is often associated with skin diseases that cause physical impairment and also have implications on social interactions and quality of life of affected individuals.

AREAS COVERED

With a focus on the skin ECM we discuss how mass spectrometry (MS)-based proteomic approaches first contributed to establishing skin protein inventories and then facilitated elucidation of molecular functions and disease mechanisms.

EXPERT OPINION

MS-based proteomic approaches have significantly contributed to our understanding of skin pathophysiology, but also revealed the challenges in assessing the skin ECM. The numerous posttranslational modifications of ECM proteins, like glycosylation, crosslinking, oxidation, and proteolytic maturation in disease settings can be difficult to tackle and remain understudied. Increased ease of handling of LC-MS/MS systems and automated/streamlined data analysis pipelines together with the accompanying increased usage of LC-MS/MS approaches will ensure that in the coming years MS-based proteomic approaches will continue to play a vital part in skin disease research. They will facilitate the elucidation of molecular disease mechanisms and, ultimately, identification of new druggable targets.

CRISPR-Cas9-mediated labelling of the C-terminus of human laminin β1 leads to secretion inhibition

Objectives

The laminins (LM) are a family of basement membranes glycoproteins with essential roles in supporting epithelia, endothelia, nerves and muscle adhesion, and in regulating a range of processes including cell migration, stem cell maintenance and differentiation. However, surprisingly little is known about the mechanisms of turnover and remodelling of LM networks due to lack of appropriate tools to study these processes at the necessary resolution. Recently, the nematode C. elegans ortholog of human the LMβ1 chain was labelled at the C-terminus with the photoconvertible fluorophore Dendra2. Here we used genome editing to establish a similar system in a mammalian cell line as proof of concept for future mammalian models.

Results

CRISPR-Cas9 was used to introduce the Dendra2 sequence at the C-terminus of LMβ1 in the human lung adenocarcinoma cell line A549. Despite expression of the tagged protein within cells, no detectable LMβ1-Dendra2 protein was deposited to the extracellular matrices or conditioned media of edited cells. Moreover, the edited cells displayed reduced proliferation rates. Together, these data suggest that, in humans, addition of C-terminal Dendra2 tag to LMβ1 inhibits LM secretion, and is not a viable approach for use in animal models.

Differential Distribution of Laminin N-Terminus α31 Across the Ocular Surface: Implications for Corneal Wound Repair

Purpose

Laminin N-terminus (LaNt) α31 is a relatively unstudied protein derived from the laminin α3 gene but structurally similar to netrins. LaNt α31 has, to date, been investigated only in two-dimensional (2D) keratinocyte culture where it influences cell migration and adhesion, processes integral to wound repair. Here we investigated LaNt α31 distribution in ocular surface epithelium, during limbal stem cell activation, and corneal wound healing.

Methods

Human, mouse, and pig eyes, ex vivo limbal explant cultures, and alkali burn wounds were processed for immunohistochemistry with antibodies against LaNt α31 along with progenitor cell-associated proteins. LaNt α31 expression was induced via adenoviral transduction into primary epithelial cells isolated from limbal explants, and cell spreading and migration were analyzed using live imaging.

Results

LaNt α31 localized to the basal layer of the conjunctival, limbal, and corneal epithelial cells. However, staining was nonuniform with apparent subpopulation enrichment, and some suprabasal reactivity was also noted. This LaNt α31 distribution largely matched that of keratin 15, epidermal growth factor receptor, and transformation-related protein 63α (p63α), and displayed similar increases in expression in activated limbal explants. During active alkali burn wound repair, LaNt α31 displayed increased expression in limbal regions and loss of basal restriction within the cornea. Distribution returned to predominately basal cell restricted once the wounded epithelium matured. Cultured corneal epithelial cells expressing LaNt α31 displayed increased 2D area and reduced migration, suggesting a functional link between this protein and key wound repair activities.

Conclusions

These data place LaNt α31 in position to influence laminin-dependent processes including wound repair and stem cell activation.

Laminins: Roles and Utility in Wound Repair

Significance: Laminins are complex extracellular macromolecules that are major players in the control of a variety of core cell processes, including regulating rates of cell proliferation, differentiation, adhesion, and migration. Laminins, and related extracellular matrix components, have essential roles in tissue homeostasis; however, during wound healing, the same proteins are critical players in re-epithelialization and angiogenesis. Understanding how these proteins influence cell behavior in these different conditions holds great potential in identifying new strategies to enhance normal wound closure or to treat chronic/nonhealing wounds. Recent Advances: Laminin-derived bioactive peptides and, more recently, laminin-peptide conjugated scaffolds, have been designed to improve tissue regeneration after injuries. These peptides have been shown to be effective in decreasing inflammation and granulation tissue, and in promoting re-epithelialization, angiogenesis, and cell migration. Critical Issues: Although there is now a wealth of knowledge concerning laminin form and function, there are still areas of some controversy. These include the relative contribution of two laminin-based adhesive devices (focal contacts and hemidesmosomes) to the re-epithelialization process, the impact and implications of laminin proteolytic processing, and the importance of laminin polymer formation on cell behavior. In addition, the roles in wound healing of the laminin-related proteins, netrins, and LaNts are still to be fully defined. Future Directions: The future of laminin-based therapeutics potentially lies in the bioengineering of specific substrates to support laminin deposition for ex vivo expansion of autologous cells for graft formation and transplantation. Significant recent advances suggest that this goal is within sight.

Epidermal growth-factor-induced transcript isoform variation drives mammary cell migration

Signal-induced transcript isoform variation (TIV) includes alternative promoter usage as well as alternative splicing and alternative polyadenylation of mRNA. To assess the phenotypic relevance of signal-induced TIV, we employed exon arrays and breast epithelial cells, which migrate in response to the epidermal growth factor (EGF). We show that EGF rapidly – within one hour – induces widespread TIV in a significant fraction of the transcriptome. Importantly, TIV characterizes many genes that display no differential expression upon stimulus. In addition, similar EGF-dependent changes are shared by a panel of mammary cell lines. A functional screen, which utilized isoform-specific siRNA oligonucleotides, indicated that several isoforms play essential, non-redundant roles in EGF-induced mammary cell migration. Taken together, our findings highlight the importance of TIV in the rapid evolvement of a phenotypic response to extracellular signals.

Laminin 332 deposition is diminished in irradiated skin in an animal model of combined radiation and wound skin injury

Skin exposure to ionizing radiation affects the normal wound healing process and greatly impacts the prognosis of affected individuals. We investigated the effect of ionizing radiation on wound healing in a rat model of combined radiation and wound skin injury. Using a soft X-ray beam, a single dose of ionizing radiation (10-40 Gy) was delivered to the skin without significant exposure to internal organs. At 1 h postirradiation, two skin wounds were made on the back of each rat. Control and experimental animals were euthanized at 3, 7, 14, 21 and 30 days postirradiation. The wound areas were measured, and tissue samples were evaluated for laminin 332 and matrix metalloproteinase (MMP) 2 expression. Our results clearly demonstrate that radiation exposure significantly delayed wound healing in a dose-related manner. Evaluation of irradiated and wounded skin showed decreased deposition of laminin 332 protein in the epidermal basement membrane together with an elevated expression of all three laminin 332 genes within 3 days postirradiation. The elevated laminin 332 gene expression was paralleled by an elevated gene and protein expression of MMP2, suggesting that the reduced amount of laminin 332 in irradiated skin is due to an imbalance between laminin 332 secretion and its accelerated processing by elevated tissue metalloproteinases. Western blot analysis of cultured rat keratinocytes showed decreased laminin 332 deposition by irradiated cells, and incubation of irradiated keratinocytes with MMP inhibitor significantly increased the amount of deposited laminin 332. Furthermore, irradiated keratinocytes exhibited a longer time to close an artificial wound, and this delay was partially corrected by seeding keratinocytes on laminin 332-coated plates. These data strongly suggest that laminin 332 deposition is inhibited by ionizing radiation and, in combination with slower keratinocyte migration, can contribute to the delayed wound healing of irradiated skin.

Laminin deposition in the extracellular matrix: a complex picture emerges

Laminins are structural components of basement membranes. In addition, they are key extracellular-matrix regulators of cell adhesion, migration, differentiation and proliferation. This Commentary focuses on a relatively understudied aspect of laminin biology: how is laminin deposited into the extracellular matrix? This topic has fascinated researchers for some time, particularly considering the diversity of patterns of laminin that can be visualized in the matrix of cultured cells. We discuss current ideas of how laminin matrices are assembled, the role of matrix receptors in this process and how laminin-associated proteins modulate matrix deposition. We speculate on the role of signaling pathways that are involved in laminin-matrix deposition and on how laminin patterns might play an important role in specifying cell behaviors, especially directed migration. We conclude with a description of new developments in the way that laminin deposition is being studied, including the use of tagged laminin subunits that should allow the visualization of laminin-matrix deposition and assembly by living cells.