Structure, genetics and function of the pulmonary associated surfactant proteins A and D: The extra-pulmonary role of these C type lectins

The roles of collectins in renal diseases and transplantation

The collectins are soluble C-type lectins and a group of proteins characterized with common structural features: a collagen-like domain and a carbohydrate-binding domain. These proteins are essential components of the innate immune system, pivotal for recognizing and eliminating pathogens to protect against infections. Over recent decades, research has significantly advanced our understanding of collectins. Beyond their fundamental role in host defense, collectins have been emerged as multifunctional proteins involved in modulating inflammatory and immune responses, facilitating the clearance of cellular debris, and even stimulating cell proliferation. These diverse roles are critical for maintaining physiological balance and hold substantial implications in various disease processes, particularly in renal diseases and transplantation. Here, we review the roles of collectins in renal diseases and transplantation focusing on four prominent members of the collectin family: mannose-binding lectin (MBL), surfactant proteins (SP-A and SP-D), and collectin-11 (CL-11). These proteins have gained considerable attention in current research due to their roles in renal diseases and transplantation, shedding light on their impact beyond traditional immune defense mechanisms. Understanding their involvement in these contexts is crucial for exploring potential therapeutic avenues and interventions aimed at mitigating renal pathology and improving outcomes in transplantation settings.

Advanced MicroRNA delivery for lung inflammatory therapy: surfactant protein A controls cellular internalisation and degradation of extracellular vesicles

INTRODUCTION

Alveolar macrophages (AMs) are the first line of defence against pathogens that initiate an inflammatory response in the lungs and exhibit a strong affinity for surfactant protein A (SP-A). Extracellular vesicles (EVs) have emerged as a promising drug delivery platform due to their minimal cytotoxicity. However, precise targeting of specific cell types and the rapid lysosomal degradation of EVs within recipient cells remain persistent challenges.

METHOD

In this study, we explored the biological significance of SP-A-EVs as novel drug delivery systems for combating lung inflammation. We first verified that respiratory EVs express SP-A receptor (SP-R210), facilitating the conjugation of SP-A with EVs. The delivery efficiency, cellular internalisation pathways and therapeutic effects were evaluated using an in vivo mouse model.

RESULTS

SP-A-EVs were robustly internalised into AMs both in vitro and in vivo. Furthermore, our investigation revealed that the toll-like receptor 4-mediated endocytosis pathway was employed for the uptake of SP-A-EVs, significantly delaying their degradation compared with natural EVs, which primarily followed the conventional lysosomal degradation pathway within AMs. In a functional study, we successfully loaded anti-inflammatory microRNA (let-7b) into SP-A-EVs, leading to the suppression of AM activation and the alleviation of lung inflammation induced by lipopolysaccharide.

CONCLUSION

These findings underscore the potential of SP-A-EVs as highly effective drug delivery systems for targeted therapeutics in lung-related disorders, capitalising on the strong affinity between AMs and SP-A and the modulation of cellular internalisation.

Revisiting surfactant protein D: an immune surveillance molecule bridging innate and adaptive immunity

Surfactant protein D (SP-D) is a C-type lectin that was originally discovered as a lung surfactant associated phospholipid recognising protein. It was originally shown to be of great importance in surfactant turnover and homeostasis in conjunction with another hydrophilic surfactant protein i.e. SP-A. In addition, it was found to agglutinate bacteria in suspension and likely a key defence molecule in the lungs. Since its early days of characterization in 1990s, SP-D has turned out to be a central player in the mucosal immunity as pulmonary as well as extrapulmonary innate immune molecule. The most exciting development has been characterization of its C-type lectin or carbohydrate recognition domain (CRDs) that exists in a homotrimeric form in native as well as recombinant versions. SP-D has a range of strategies to recognise pathogen-associated molecular patterns (PAMPs) and thus act as a soluble PAMP-recognizing receptor (PRR), and subsequent destruction of the pathogens directly, or indirectly via phagocytic cells. SP-D also recognizes a range of allergens, competes out with specific IgE antibodies, and downregulates histamine release by basophils and mast cells. These anti-microbial and anti-allergic properties of SP-D have been validated by in vivo murine models of infection and allergy. The SP-D gene deficient mice exhibit remarkable phenotypes where lungs are leaky, showing features of fibrosis and emphysema. One of the seminal discoveries in the field has been the observation that activated eosinophils (and other immune cells) can be induced into apoptotic pathways by SP-D. This raised the possibility that SP-D can be an innate immune surveillance molecule. Studies have revealed the ability of a recombinant fragment of human SP-D containing homotrimeric neck and CRD region to induce apoptosis via intrinsic as well as extrinsic pathways; in addition, it also seems capable of interfering with epithelial-to-mesenchymal transition. These studies have opened up enormous possibilities for setting up pre-clinical and clinical trials.

Constructing the cGAMP-Aluminum Nanoparticles as a Vaccine Adjuvant-Delivery System (VADS) for Developing the Efficient Pulmonary COVID-19 Subunit Vaccines

The cGAMP-aluminum nanoparticles (CAN) are engineered as a vaccine adjuvant-delivery system to carry mixed RBD (receptor-binding domain) of the original severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its new variant for developing bivalent pulmonary coronavirus disease 2019 (COVID-19) vaccines (biRBD-CAN). High phosphophilicity/adsorptivity made intrapulmonary CAN instantly form the pulmonary ingredient-coated CAN (piCAN) to possess biomimetic features enhancing biocompatibility. In vitro biRBD-CAN sparked APCs (antigen-presenting cells) to mature and make extra reactive oxygen species, engendered lysosome escape effects and enhanced proteasome activities. Through activating the intracellular stimulator of interferon genes (STING) and nucleotide-binding domain and leucine-rich repeat and pyrin domain containing proteins 3 (NALP3) inflammasome pathways to exert synergy between cGAMP and AN, biRBD-CAN stimulated APCs to secret cytokines favoring mixed Th1/Th2 immunoresponses. Mice bearing twice intrapulmonary biRBD-CAN produced high levels of mucosal antibodies, the long-lasting systemic antibodies, and potent cytotoxic T lymphocytes which efficiently erased cells displaying cognate epitopes. Notably, biRBD-CAN existed in mouse lungs and different lymph nodes for at least 48 h, unveiling their sustained immunostimulatory activity as the main mechanism underlying the long-lasting immunity and memory. Hamsters bearing twice intrapulmonary biRBD-CAN developed high resistance to pseudoviral challenges performed using different recombinant strains including the ones with distinct SARS-CoV-2-spike mutations. Thus, biRBD-CAN as a broad-spectrum pulmonary COVID-19 vaccine candidate may provide a tool for controlling the emerging SARS-CoV-2 variants.

The Role of Pulmonary Collectins, Surfactant Protein A (SP-A) and Surfactant Protein D (SP-D) in Cancer

Surfactant proteins A and D (SP-A and SP-D) belong to the collectin subfamily of C-type oligomeric lectins. They are pattern-recognition molecules (PRMs), able to recognise pathogen- or danger-associated molecular patterns (PAMPs, DAMPs) in the presence of Ca2+ cations. That property enables opsonisation or agglutination of non-self or altered/abnormal self cells and contributes to their clearance. Like other collectins, SP-A and SP-D are characterised by the presence of four distinct domains: a cysteine-rich domain (at the N-terminus), a collagen-like region, an α-helical neck domain and a globular carbohydrate-recognition domain (CRD) (at the C-terminus). Pulmonary surfactant is a lipoprotein complex, preventing alveolar collapse by reducing surface tension at the air-liquid interface. SP-A and SP-D, produced by type II alveolar epithelial cells and Clara cells, are not only pattern-recognition molecules but also contribute to the surfactant structure and homeostasis. Moreover, they are expressed in a variety of extrapulmonary sites where they are involved in local immunity. The term "cancer" includes a variety of diseases: tumours start from uncontrolled growth of abnormal cells in any tissue which may further spread to other sites of the body. Many cancers are incurable, difficult to diagnose and often fatal. This short review summarises anti- and pro-tumorigenic associations of SP-A and SP-D as well as perspectives of their usefulness in cancer diagnosis and therapy.

Polymorphisms in Immune Genes and Their Association with Tuberculosis Susceptibility: An Analysis of the African Population

Tuberculosis remains a global health concern, with substantial mortality rates worldwide. Genetic factors play a significant role in influencing susceptibility to tuberculosis. This review examines the current progress in studying polymorphisms within immune genes associated with tuberculosis susceptibility, focusing on African populations. The roles of various proteins, including Toll-like receptors, Dendritic Cell-Specific Intercellular Adhesion Molecule-3 Grabbing Non-Integrin, vitamin D nuclear receptor, soluble C-type lectins such as surfactant proteins A and D, C-type Lectin Domain Family 4 Member E, and mannose-binding lectin, phagocyte cytokines such as Interleukin-1, Interleukin-6, Interleukin-10, Interleukin-12, and Interleukin-18, and chemokines such as Interleukin-8, monocyte chemoattractant protein 1, Regulated upon activation, normal T-cell expressed and secreted are explored in the context of tuberculosis susceptibility. We also address the potential impact of genetic variants on protein functions, as well as how these findings align with the genetic polymorphisms not associated with tuberculosis. Functional studies in model systems provide insights into the intricate host-pathogen interactions and susceptibility mechanisms. Despite progress, gaps in knowledge remain, highlighting the need for further investigations. This review emphasizes the association of Single Nucleotide Polymorphisms with diverse aspects of tuberculosis pathogenesis, including disease detection and Mycobacterium tuberculosis infection.

Collectins and ficolins in neonatal health and disease

The immune system starts to develop early in embryogenesis. However, at birth it is still immature and associated with high susceptibility to infection. Adaptation to extrauterine conditions requires a balance between colonization with normal flora and protection from pathogens. Infections, oxidative stress and invasive therapeutic procedures may lead to transient organ dysfunction or permanent damage and perhaps even death. Newborns are primarily protected by innate immune mechanisms. Collectins (mannose-binding lectin, collectin-10, collectin-11, collectin-12, surfactant protein A, surfactant protein D) and ficolins (ficolin-1, ficolin-2, ficolin-3) are oligomeric, collagen-related defence lectins, involved in innate immune response. In this review, we discuss the structure, specificity, genetics and role of collectins and ficolins in neonatal health and disease. Their clinical associations (protective or pathogenic influence) depend on a variety of variables, including genetic polymorphisms, gestational age, method of delivery, and maternal/environmental microflora.

Circulating Surfactant Protein D: A Biomarker for Acute Lung Injury?

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening lung diseases in critically ill patients. The lack of prognostic biomarkers has halted detection methods and effective therapy development. Quantitative biomarker-based approaches in the systemic circulation have been proposed as a means of enhancing diagnostic strategies as well as pharmacotherapy in a patient-specific manner. Pulmonary surfactants are complex mixtures made up of lipids and proteins, which are secreted into the alveolar space by epithelial type II cells under normal and pathological conditions. In this review, we summarize the current knowledge of SP-D in lung injury from both preclinical and clinical studies. Among surfactant proteins, surfactant protein-D (SP-D) has been more widely studied in ALI and ARDS. Recent studies have reported that SP-D has a superior discriminatory ability compared to other lung epithelial proteins for the diagnosis of ARDS, which could reflect the severity of lung injury. Furthermore, we shed light on recombinant SP-D treatment and its benefits as a potential drug for ALI, and we encourage further studies to translate SP-D into clinical use for diagnosis and treatment.

Pathogenic bacteria defense and complement activation function analysis of Collectin-10 from Hexagrammos otakii

With the tremendous success of the artificial breeding of Hexagrammos otakii, the yield has been substantially improved. However, intensive farming often results in bacterial diseases; hence it is imperative to find new antimicrobial molecules. In the present study, we identified a homologous cDNA fragment of collectin-10 from H. otakii, designated as HoCL-10. The cDNA length is 899 bp, which contains an open reading frame (ORF) of 816 bp encoding a secreted protein with 272 amino acid residues. The peptide of HoCL-10 contains an N-terminal collagen domain, a neck region, and a C-terminal carbohydrate recognition domain. The qRT-PCR results revealed that HoCL-10 mRNA was highest expressed in the liver and skin and was significantly induced post-LPS stimulation. The sugar and bacteria binding assay suggested that the recombinant HoCL-10 (rHoCL-10) could recognize various pathogen-associated molecular patterns (PAMPs) and bacteria. For effect on cells, rHoCL-10 enhanced the phagocytosis and migration ability of the macrophage indicated using pro-phagocytosis assay and trans-well assay. To determine the role of HoCL-10 in the complement system, the interaction between HoCL-10 and mannose-binding lectin associated serine protease 1, 2 (MASP-1, 2) were analyzed and demonstrated using ELISA and Far-western. And in vivo, the concentration of membrane-attack complex (MAC) in fish plasma was significantly down-regulated post-injection with HoCL-10 antibody. Finally, the bacteria challenge experiment was performed, implying that HoCL-10 may assist the host in defending against microbial invasion. The findings suggest that HoCL-10 may play crucial roles in host defense against microorganisms, possibly through opsonizing pathogens and activating the complement system.

Association of Surfactant Protein D Single Nucleotide Polymorphisms rs721917, rs2243639, rs3088308 with Recurrent Aphthous Stomatitis in Pakistani Population

Recurrent aphthous stomatitis (RAS) is a benign ulcerative condition, defined by the recurrent formation of non-contagious mucosal ulcers. Surfactant protein D (SP-D) is secreted frequently at surfaces exposed directly to body fluids. This study aims to investigate the association of SP-D single nucleotide polymorphisms (SNPs) with the onset of RAS. Blood samples from 212 subjects (106 cases/controls each) were collected during 2019 and genotyped for SP-D SNPs (rs721917, rs2243639, rs3088308) by polymerase chain reaction and restriction fragment length polymorphism followed by 12% polyacrylamide gel electrophoresis. Minor aphthous (75.5%) was the commonly observed ulcer type as compared to herpetiform (21.7%) and major aphthous ulcers (2.8%). A family history of RAS was reported in 70% of cases. RAS was found significantly associated with rs3088308 genotypes T/A (95% (Cl): 1.57-5.03, p = 0.0005), A/A (95% (Cl): 1.8-6.7, p = 0.0002), T-allele (95% (Cl): 1.09-2.36, p = 0.01), A-allele (95% (Cl): 1.42-3.91, p = 0.01), rs721917 genotype T/T (95% (Cl): 1.15-25.35, p = 0.03), and T-allele (95% (Cl): 1.28-3.10, p = 0.002). Female gender and obese body mass index (BMI) were significantly associated with rs3088308 genotypes T/A (95% (CI): 1.89-15.7, p = 0.001), T/T (95% (Cl): 1.52-11.9, p = 0.005), A-allele (95% (Cl): 1.65-7.58, p < 0.001), and T-allele (95% (Cl): 1.4-10.1, p <0.001) and rs721917 genotype T/T (95% (CI) = 1.3-33, p = 0.02), respectively. This study describes the association of SP-D SNPs (rs721917, rs3088308) with RAS in the Pakistani population.

Surfactant protein a attenuates generalized and localized neuroinflammation in neonatal mice

Surfactant protein A (SP-A) has important roles in innate immunity and modulation of pulmonary and extrapulmonary inflammation. Given SP-A has been detected in rat and human brain, we sought to determine if SP-A has a role in modulating inflammation in the neonatal mouse brain. Neonatal wildtype (WT) and SP-A-deficient (SP-A-/-) mice were subjected to three models of brain inflammation: systemic sepsis, intraventricular hemorrhage (IVH) and hypoxic-ischemic encephalopathy (HIE). Following each intervention, RNA was isolated from brain tissue and expression of cytokine and SP-A mRNA was determined by real-time quantitative RT-PCR analysis. In the sepsis model, expression of most cytokine mRNAs was significantly increased in brains of WT and SP-A-/- mice with significantly greater expression of all cytokine mRNA levels in SP-A-/- mice compared to WT. In the IVH model, expression of all cytokine mRNAs was significantly increased in WT and SP-A-/- mice and levels of most cytokine mRNAs were significantly increased in SP-A-/- mice compared to WT. In the HIE model, only TNF-α mRNA levels were significantly increased in WT brain tissue while all pro-inflammtory cytokine mRNAs were significantly increased in SP-A-/- mice, and all pro-inflammatory cytokine mRNA levels were significantly higher in SP-A-/- mice compared to WT. SP-A mRNA was not detectable in brain tissue of adult WT mice nor in WT neonates subjected to these models. These results suggest that SP-A-/- neonatal mice subjected to models of neuroinflammation are more susceptible to both generalized and localized neuroinflammation compared to WT mice, thus supporting the hypothesis that SP-A attenuates inflammation in neonatal mouse brain.

Interaction between alveolar macrophages and epithelial cells during Mycoplasma pneumoniae infection

Mycoplasma pneumoniae, as one of the most common pathogens, usually causes upper respiratory tract infections and pneumonia in humans and animals. It accounts for 10% to 40% of community-acquired pneumonia in children. The alveolar epithelial cells (AECs) are the first barrier against pathogen infections, triggering innate immune responses by recruiting and activating immune cells when pathogens invade into the lung. Alveolar macrophages (AMs) are the most plentiful innate immune cells in the lung, and are the first to initiate immune responses with pathogens invasion. The cross-talk between the alveolar epithelium and macrophages is necessary to maintain physiological homeostasis and to eradicate invaded pathogen by regulating immune responses during Mycoplasma pneumoniae infections. This review summarizes the communications between alveolar macrophages and epithelial cells during Mycoplasma pneumoniae infections, including cytokines-medicated communications, signal transduction by extracellular vesicles, surfactant associated proteins-medicated signal transmission and establishment of intercellular gap junction channels.

Towards an artificial human lung: modelling organ-like complexity to aid mechanistic understanding

Respiratory diseases account for over 5 million deaths yearly and are a huge burden to healthcare systems worldwide. Murine models have been of paramount importance to decode human lung biology in vivo, but their genetic, anatomical, physiological and immunological differences with humans significantly hamper successful translation of research into clinical practice. Thus, to clearly understand human lung physiology, development, homeostasis and mechanistic dysregulation that may lead to disease, it is essential to develop models that accurately recreate the extraordinary complexity of the human pulmonary architecture and biology. Recent advances in micro-engineering technology and tissue engineering have allowed the development of more sophisticated models intending to bridge the gap between the native lung and its replicates in vitro Alongside advanced culture techniques, remarkable technological growth in downstream analyses has significantly increased the predictive power of human biology-based in vitro models by allowing capture and quantification of complex signals. Refined integrated multi-omics readouts could lead to an acceleration of the translational pipeline from in vitro experimental settings to drug development and clinical testing in the future. This review highlights the range and complexity of state-of-the-art lung models for different areas of the respiratory system, from nasal to large airways, small airways and alveoli, with consideration of various aspects of disease states and their potential applications, including pre-clinical drug testing. We explore how development of optimised physiologically relevant in vitro human lung models could accelerate the identification of novel therapeutics with increased potential to translate successfully from the bench to the patient's bedside.

Surfactant proteins and innate immunity of otitis media

Otitis media (OM) is the most common disease among young children and one of the most frequent reasons to visit the pediatrician. Development of OM requires nasopharyngeal colonization by a pathogen which must gain access to the tympanic cavity through the eustachian tube (ET) along with being able to overcome the defense mechanisms of the immune system and middle ear mucosa. OM can be caused by viral or bacterial infection. The three main bacterial pathogens are Streptococcus pneumoniae, nontypeable Haemophilus influenzae (NTHi), and Moraxella catarrhalis. Innate immunity is important in OM resolution as the disease occurs in very young children before the development of specific immunity. Elements of innate immunity include natural barriers and pattern recognition receptors such as Toll like receptors (TLRs), and Nod like receptors (NLRs). Surfactant proteins A (SP-A) and D (SP-D) act as pattern recognition receptors and are found in the lung and many other tissues including the ET and the middle ear where they probably function in host defense. Surfactant has a potential for use in the treatment of OM due to surface tension lowering function in the ET, and the possible immune functions of SP-D and SP-A in the middle ear and ET.

Transient Receptor Potential (TRP) Channels in Airway Toxicity and Disease: An Update

Our respiratory system is exposed to toxicants and pathogens from both sides: the airways and the vasculature. While tracheal, bronchial and alveolar epithelial cells form a natural barrier in the airways, endothelial cells protect the lung from perfused toxic compounds, particulate matter and invading microorganism in the vascular system. Damages induce inflammation by our immune response and wound healing by (myo)fibroblast proliferation. Members of the transient receptor potential (TRP) superfamily of ion channel are expressed in many cells of the respiratory tract and serve multiple functions in physiology and pathophysiology. TRP expression patterns in non-neuronal cells with a focus on TRPA1, TRPC6, TRPM2, TRPM5, TRPM7, TRPV2, TRPV4 and TRPV6 channels are presented, and their roles in barrier function, immune regulation and phagocytosis are summarized. Moreover, TRP channels as future pharmacological targets in chronic obstructive pulmonary disease (COPD), asthma, cystic and pulmonary fibrosis as well as lung edema are discussed.

Role of surfactant protein-D in ocular bacterial infection

Purpose

Our review explains the role of surfactant protein D (SP-D) in different kinds of bacterial infection based on its presence in different ocular surface tissues. We discuss the potential role of SP-D against invasion by pathogens, with the aim of identifying new prospects for the possible mechanism of SP-D-mediated immune processes, and the diagnosis, prognosis, or treatment of ocular bacterial infection.

Methods

We reviewed articles about the role of SP-D in various ocular bacterial infections or infection-related ocular diseases through PubMed, Google Scholar, and the Web of Science databases.

Results

SP-D acts as an important immune factor that can resemble molecules in different polymerization states and that defends against pathogen invasion. The increased SP-D production and secretion in tear fluid and the cornea after ocular bacterial infections such as Staphylococcus aureus, Pseudomonas aeruginosa keratitis, and infection-related ocular diseases, was shown to have potential anti-inflammatory effects. The mechanisms of SP-D’s action against ocular bacterial infections include presenting, aggregating, opsonizing, and phagocytizing antigens, as well as regulating anti-bacterial immunity processes, including toll-like receptor-5 (TLR-5) pathway and IL-8 effect, TLR-4 and TLR-2 pathways and other possible ways remained to be elucidated in more detail. The findings demonstrate the potential of SP-D as an important clinical diagnostic biomarker prognosis predictor, and target for ocular immunotherapy.

Conclusion

SP-D participates in invasion by different ocular bacteria and infection-related ocular diseases through multiple immune mechanisms. This finding provides new prospects for the diagnosis, prognosis and treatment of ocular bacterial infection.

Human epididymis protein 4 and fetal lung maturity

OBJECTIVES

To document the maternal and fetal cord blood levels of human epididymis protein 4 (HE-4) in term and preterm newborns in order to investigate the possible physiological role of HE-4 in fetal lung development.

METHODS

This cross-sectional study was conducted in a university-affiliated hospital between April 2018 and September 2018. The study population consisted of cesarean section (C-section) deliveries after 24 weeks of pregnancy. Both maternal and umbilical cord HE-4 levels (mHE-4 and uHE-4, respectively) were measured using chemiluminescent microparticle immunoassay. Amniotic fluid was sampled from each case to determine the lamellar body count (LBC) as the gold standard test for lung maturation. All the parameters, including the uHE-4 levels, were compared between the term delivery (≥37 weeks) (n=52) and preterm delivery (24-37th weeks) (n=30) groups. The best cut-off value of uHE-4 was calculated for fetal lung maturity.

RESULTS

There were no statistically significant differences between the groups regarding the demographic data. The mHE-4 levels did not statistically significantly differ between the groups (p>0.05) whereas the uHE-4 level of the preterm newborns was significantly higher than that of the term newborns (p<0.05). There was a significant negative association between the uHE-4 level and LBC (r=-0.389; p<0.001). The uHE-4 level was the only statistically significant fetal parameter indicating fetal lung maturity confirmed by LBC. At a cut-off value of 281 pmol/L, uHE-4 had 96.8% sensitivity, 45% specificity, 84.5% positive predictive value, and 81.8% negative predictive value for fetal lung maturity.

CONCLUSIONS

Although the exact physiological role of HE-4 has not yet been elucidated, this preliminary study supports the idea that HE-4 plays a role in fetal lung maturation to some extent.

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

The Potential of Lung Epithelium Specific Proteins as Biomarkers for COVID-19-Associated Lung Injury

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection was first reported in Wuhan, China, and was declared a pandemic by the World Health Organization (WHO) on 20 March 2020. The respiratory system is the major organ system affected by COVID-19. Numerous studies have found lung abnormalities in patients with COVID-19, including shortness of breath, respiratory failure, and acute respiratory distress syndrome. The identification of lung-specific biomarkers that are easily measurable in serum would be valuable for both clinicians and patients with such conditions. This review is focused on the pneumoproteins and their potential to serve as biomarkers for COVID-19-associated lung injury, including Krebs von den Lungen-6 (KL-6), surfactant proteins (SP-A, SP-B, SP-C, SP-D), and Clara cell secretory protein (CC16). The current findings indicate the aforementioned pneumoproteins may reflect the severity of pulmonary manifestations and could serve as potential biomarkers in COVID-19-related lung injury.

Pulmonary surfactant: a unique biomaterial with life-saving therapeutic applications

Pulmonary surfactant is a complex lipoprotein mixture secreted into the alveolar lumen by type 2 pneumocytes, which is composed by tens of different lipids (approximately 90% of its entire mass) and surfactant proteins (approximately 10% of the mass). It is crucially involved in maintaining lung homeostasis by reducing the values of alveolar liquid surface tension close to zero at end-expiration, thereby avoiding the alveolar collapse, and assembling a chemical and physical barrier against inhaled pathogens. A deficient amount of surfactant or its functional inactivation is directly linked to a wide range of lung pathologies, including the neonatal respiratory distress syndrome. This paper reviews the main biophysical concepts of surfactant activity and its inactivation mechanisms, and describes the past, present and future roles of surfactant replacement therapy, focusing on the exogenous surfactant preparations marketed worldwide and new formulations under development. The closing section describes the pulmonary surfactant in the context of drug delivery. Thanks to its peculiar composition, biocompatibility, and alveolar spreading capability, the surfactant may work not only as a shuttle to the branched anatomy of the lung for other drugs but also as a modulator for their release, opening to innovative therapeutic avenues for the treatment of several respiratory diseases.

Increased Alveolar Heparan Sulphate and Reduced Pulmonary Surfactant Amount and Function in the Mucopolysaccharidosis IIIA Mouse

Mucopolysaccharidosis IIIA (MPS IIIA) is a lysosomal storage disease with significant neurological and skeletal pathologies. Respiratory dysfunction is a secondary pathology contributing to mortality in MPS IIIA patients. Pulmonary surfactant is crucial to optimal lung function and has not been investigated in MPS IIIA. We measured heparan sulphate (HS), lipids and surfactant proteins (SP) in pulmonary tissue and bronchoalveolar lavage fluid (BALF), and surfactant activity in healthy and diseased mice (20 weeks of age). Heparan sulphate, ganglioside GM3 and bis(monoacylglycero)phosphate (BMP) were increased in MPS IIIA lung tissue. There was an increase in HS and a decrease in BMP and cholesteryl esters (CE) in MPS IIIA BALF. Phospholipid composition remained unchanged, but BALF total phospholipids were reduced (49.70%) in MPS IIIA. There was a reduction in SP-A, -C and -D mRNA, SP-D protein in tissue and SP-A, -C and -D protein in BALF of MPS IIIA mice. Captive bubble surfactometry showed an increase in minimum and maximum surface tension and percent surface area compression, as well as a higher compressibility and hysteresis in MPS IIIA surfactant upon dynamic cycling. Collectively these biochemical and biophysical changes in alveolar surfactant are likely to be detrimental to lung function in MPS IIIA.

Molecular Mechanisms of Maternal Diabetes Effects on Fetal and Neonatal Surfactant

Respiratory distress is a significant contributor to newborn morbidity and mortality. An association between infants of diabetic mothers (IDMs) and respiratory distress syndrome (RDS) has been well recognized for decades. As obesity and diabetes prevalence have increased over the past several decades, more women are overweight and diabetic in the first trimester, and many more pregnant women are diagnosed with gestational diabetes. Glycemic control during pregnancy can be challenging due to the maternal need for higher caloric intake and higher insulin resistance. Surfactant is a complex molecule at the alveolar air-liquid interface that reduces surface tension. Impaired surfactant synthesis is the primary etiology of RDS. In vitro cell line studies, in vivo animal studies with diabetic rat offspring, and clinical studies suggest hyperglycemia and hyperinsulinemia can disrupt surfactant lipid and protein synthesis, causing delayed maturation in surfactant in IDMs. A better understanding of the molecular mechanisms responsible for surfactant dysfunction in IDMs may improve clinical strategies to prevent diabetes-related complications and improve neonatal outcomes.

Genetic disorders of the surfactant system: focus on adult disease

Genes involved in the production of pulmonary surfactant are crucial for the development and maintenance of healthy lungs. Germline mutations in surfactant-related genes cause a spectrum of severe monogenic pulmonary diseases in patients of all ages. The majority of affected patients present at a very young age, however, a considerable portion of patients have adult-onset disease. Mutations in surfactant-related genes are present in up to 8% of adult patients with familial interstitial lung disease (ILD) and associate with the development of pulmonary fibrosis and lung cancer.High disease penetrance and variable expressivity underscore the potential value of genetic analysis for diagnostic purposes. However, scarce genotype-phenotype correlations and insufficient knowledge of mutation-specific pathogenic processes hamper the development of mutation-specific treatment options.This article describes the genetic origin of surfactant-related lung disease and presents spectra for gene, age, sex and pulmonary phenotype of adult carriers of germline mutations in surfactant-related genes.

Exploring the human lacrimal gland using organoids and single-cell sequencing

The lacrimal gland is essential for lubrication and protection of the eye. Disruption of lacrimal fluid production, composition, or release results in dry eye, causing discomfort and damage to the ocular surface. Here, we describe the establishment of long-term 3D organoid culture conditions for mouse and human lacrimal gland. Organoids can be expanded over multiple months and recapitulate morphological and transcriptional features of lacrimal ducts. CRISPR-Cas9-mediated genome editing reveals the master regulator for eye development Pax6 to be required for differentiation of adult lacrimal gland cells. We address cellular heterogeneity of the lacrimal gland by providing a single-cell atlas of human lacrimal gland tissue and organoids. Finally, human lacrimal gland organoids phenocopy the process of tear secretion in response to neurotransmitters and can engraft and produce mature tear products upon orthotopic transplantation in mouse. Together, this study provides an experimental platform to study the (patho-)physiology of the lacrimal gland.

The plate body: 3D ultrastructure of a facultative organelle of alveolar epithelial type II cells involved in SP-A trafficking

Plate bodies are facultative organelles occasionally described in the adult lungs of various species, including sheep and goat. They consist of multiple layers of plate-like cisterns with an electron dense middle bar. The present study was performed to elucidate the three-dimensional (3D) characteristics of this organelle and its presumed function in surfactant protein A (SP-A) biology. Archived material of four adult goat lungs and PFA-fixed lung samples of two adult sheep lungs were used for the morphological and immunocytochemical parts of this study, respectively. 3D imaging was performed by electron tomography and focused ion beam scanning electron microscopy (FIB-SEM). Immuno gold labeling was used to analyze whether plate bodies are positive for SP-A. Transmission electron microscopy revealed the presence of plate bodies in three of four goat lungs and in both sheep lungs. Electron tomography and FIB-SEM characterized the plate bodies as layers of two up to over ten layers of membranous cisterns with the characteristic electron dense middle bar. The membranes of the plates were in connection with the rough endoplasmic reticulum and showed vesicular inclusions in the middle of the plates and a vesicular network at the sides of the organelle. Immuno gold labeling revealed the presence of SP-A in the vesicular network of plate bodies but not in the characteristic plates themselves. In conclusion, the present study clearly proves the connection of plate bodies with the rough endoplasmic reticulum and the presence of a vesicular network as part of the organelle involved in SP-A trafficking.

Post-correlation on-lamella cryo-CLEM reveals the membrane architecture of lamellar bodies

Lamellar bodies (LBs) are surfactant-rich organelles in alveolar cells. LBs disassemble into a lipid-protein network that reduces surface tension and facilitates gas exchange in the alveolar cavity. Current knowledge of LB architecture is predominantly based on electron microscopy studies using disruptive sample preparation methods. We established and validated a post-correlation on-lamella cryo-correlative light and electron microscopy approach for cryo-FIB milled cells to structurally characterize and validate the identity of LBs in their unperturbed state. Using deconvolution and 3D image registration, we were able to identify fluorescently labeled membrane structures analyzed by cryo-electron tomography. In situ cryo-electron tomography of A549 cells as well as primary Human Small Airway Epithelial Cells revealed that LBs are composed of membrane sheets frequently attached to the limiting membrane through “T”-junctions. We report a so far undescribed outer membrane dome protein complex (OMDP) on the limiting membrane of LBs. Our data suggest that LB biogenesis is driven by parallel membrane sheet import and by the curvature of the limiting membrane to maximize lipid storage capacity.

Using the post-correlation on-lamella cryo-CLEM workflow, Klein, Wimmer et al. show that lamellar bodies (LBs) are composed of membrane sheets frequently attached to the limiting membrane through T-junctions in ABCA3 overexpressing cells and in primary human small airway epithelial cells. This study provides insights into LB biogenesis and membrane packing inside the LB.

Surfactant protein A reduces TLR4 and inflammatory cytokine mRNA levels in neonatal mouse ileum

Levels of intestinal toll-like receptor 4 (TLR4) impact inflammation in the neonatal gastrointestinal tract. While surfactant protein A (SP-A) is known to regulate TLR4 in the lung, it also reduces intestinal damage, TLR4 and inflammation in an experimental model of necrotizing enterocolitis (NEC) in neonatal rats. We hypothesized that SP-A-deficient (SP-A^-/-) mice have increased ileal TLR4 and inflammatory cytokine levels compared to wild type mice, impacting intestinal physiology. We found that ileal TLR4 and proinflammatory cytokine levels were significantly higher in infant SP-A^-/- mice compared to wild type mice. Gavage of neonatal SP-A^-/- mice with purified SP-A reduced ileal TLR4 protein levels. SP-A reduced expression of TLR4 and proinflammatory cytokines in normal human intestinal epithelial cells (FHs74int), suggesting a direct effect. However, incubation of gastrointestinal cell lines with proteasome inhibitors did not abrogate the effect of SP-A on TLR4 protein levels, suggesting that proteasomal degradation is not involved. In a mouse model of experimental NEC, SP-A^-/- mice were more susceptible to intestinal stress resembling NEC, while gavage with SP-A significantly decreased ileal damage, TLR4 and proinflammatory cytokine mRNA levels. Our data suggests that SP-A has an extrapulmonary role in the intestinal health of neonatal mice by modulating TLR4 and proinflammatory cytokines mRNA expression in intestinal epithelium.

Necrotizing enterocolitis intestinal barrier function protection by antenatal dexamethasone and surfactant-D in a rat model

BACKGROUND

Necrotizing enterocolitis (NEC) is the most common gastrointestinal disorder in premature neonates. Possible therapeutic approaches are centered on promoting maturation of the gastrointestinal mucosal barrier. Studies have demonstrated that antenatal administration of corticosteroids can decrease NEC incidence and mortality.

METHODS

Pregnant rat dams were administered dexamethasone 48 h prior to delivery. The pups were subjected to an experimental NEC-like injury protocol. Ileal tissues and sera were collected and evaluated for inflammatory cytokines, gut permeability and expressions and localizations of tight junction proteins, and surfactant protein-D by immunohistochemistry/immunofluorescent staining. Intestinal epithelial cells (IEC-6) were pretreated with SP-D to examine the effect of SP-D on tight junction protein expressions when challenged with platelet-activating factor and lipopolysaccharide to model proinflammatory insults.

RESULTS

Antenatal dexamethasone reduced systemic inflammation, preserved intestinal barrier integrity, and stimulated SP-D expression on the intestinal mucosal surface in pups exposed to NEC-like injury. Pretreatment of SP-D blocked platelet-activating factor/lipopolysaccharide-induced tight junction disruption in IEC-6 cells in vitro.

CONCLUSIONS

Antenatal dexamethasone preserves the development of intestinal mucosal barrier integrity and reduces incidence and morbidity from an experimental NEC-like injury model. Dexamethasone upregulation of intestinal SP-D-protective effects on tight junction proteins.

IMPACT

Antenatal administration of dexamethasone can function in concert with intestinal surfactant protein-D to decrease systemic inflammatory responses, and protect intestinal barrier integrity in a neonatal rat model of NEC. A novel role of intestinal SP-D in preserving tight junction protein structures under inflammatory conditions. We describe the intestinal SP-D-an overlooked role of antenatal dexamethasone in neonatal NEC?

Role of collectin-11 in innate defence against uropathogenic Escherichia coli infection

Classical collectins (surfactant protein A and D) play a significant role in innate immunity and host defence in uropathogenic Escherichia coli (UPEC)-induced urinary tract infection (UTI). However, the functions of collectin-11 (CL-11) with respect to UPEC and UTI remain largely unexplored. This study aimed to investigate the effect of CL-11 on UPEC and its role in UTI. We further examined its modulatory effect on inflammatory reactions in proximal tubular epithelial cells (PTECs). The present study provides evidence for the effect of CL-11 on the growth, agglutination, binding, epithelial adhesion and invasion of UPEC. We found increased basal levels of phosphorylated p38 MAPK and human cytokine homologue (keratinocyte-derived chemokine) expression in CL-11 knockdown PTECs. Furthermore, signal regulatory protein α blockade reversed the increased basal levels of inflammation associated with CL-11 knockdown in PTECs. Additionally, CL-11 knockdown effectively inhibited UPEC-induced p38 MAPK phosphorylation and cytokine production in PTECs. These were further inhibited by CD91 blockade. We conclude that CL-11 functions as a mediator of innate immunity via direct antibacterial roles as well as dual modulatory roles in UPEC-induced inflammatory responses during UTI. Thus, the study findings suggest a possible function for CL-11 in defence against UTI.

Interplay between C-type lectin receptors and microRNAs in cellular homeostasis and immune response

C-type lectin receptors (CLRs) belong to the family of pattern recognition receptors (PRRs). They have a critical role to play in the regulation of a range of physiological functions including development, respiration, angiogenesis, inflammation, and immunity. CLRs can recognize distinct and conserved exogenous pathogen-associated as well as endogenous damage-associated molecular patterns. These interactions set off downstream signaling cascades, leading to the production of inflammatory mediators, activation of effector immune cells as well as regulation of the developmental and physiological homeostasis. CLR signaling must be tightly controlled to circumvent the excessive inflammatory burden and to maintain the cellular homeostasis. Recently, MicroRNAs (miRNAs) have been shown to be important regulators of expression of CLRs and their downstream signaling. The delicate balance between miRNAs and CLRs seems crucial in almost all aspects of multicellular life. Any dysregulations in the miRNA-CLR axes may lead to tumorigenesis or inflammatory diseases. Here, we present an overview of the current understanding of the central role of miRNAs in the regulation of CLR expression, profoundly impacting upon homeostasis and immunity, and thus, development of therapeutics against immune disorders.

Exposure to traffic-related particle matter and effects on lung function and potential interactions in a cross-sectional analysis of a cohort study in west Sweden

OBJECTIVES

To investigate the long-term effects of source-specific particle matter (PM) on lung function, effects of Surfactant Protein A (SP-A) and glutathione S-transferase (GST) genes GSTP1 and GSTT1 gene variants and effect modification by single-nucleotide polymorphism (SNP) genotype.

DESIGN

Cohort study with address-based annual PM exposure assigned from annual estimates of size (PM₁₀, PM_2.5 and PM_BC) and source-specific (traffic, industry, marine traffic and wood burning) dispersion modelling.

SETTING

Gothenburg, Sweden.

PARTICIPANTS

The ADult-Onset asthma and NItric oXide Study had 6685 participants recruited from the general population, of which 5216 (78%) were included in the current study with information on all variables of interest. Mean age at the time of enrolment was 51.4 years (range 24-76) and 2427 (46.5%) were men.

PRIMARY AND SECONDARY OUTCOME MEASURES

The primary outcome was forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV₁). Secondary outcome measures were effects and gene-environment interactions of SP-A and GSTT1 and GSTP1 genotypes.

RESULTS

Exposure to traffic-related PM₁₀ and PM_2.5 was associated with decreases in percent-predicted (% predicted) FEV₁ by -0.48% (95% CI -0.89% to -0.07%) and -0.47% (95% CI -0.88% to -0.07%) per IQR 3.05 and 2.47 µg/m³, respectively, and with decreases in % predicted FVC by -0.46% (95% CI -0.83% to -0.08%) and -0.47% (95% CI -0.83% to -0.10%). Total and traffic-related PM_BC was strongly associated with both FEV₁ and FVC by -0.53 (95% CI -0.94 to -0.13%) and -0.43% (95% CI -0.77 to -0.09%) per IQR, respectively, for FVC, and similarly for FEV₁. Minor allele carrier status for two GSTP1 SNPs and the GSTT1 null genotype were associated with decreases in % predicted lung function. Three SP-A SNPs showed effect modification with exposure to PM_2.5 from industry and marine traffic.

CONCLUSIONS

PM exposure, specifically traffic related, was associated with FVC and FEV₁ reductions and not modified by genotype. Genetic effect modification was suggested for industry and marine traffic PM_2.5.

Pulmonary Surfactants: a New Therapeutic Target in Asthma

Purpose of Review

Lung tissues are highly susceptible to airway inflammation as they are inevitably exposed to inhaled pathogens and allergens. In the lungs, clearance of infectious agents and regulation of inflammatory responses are important for the first-line defense, where surfactants play a role in host defense mechanisms. In this review, clinical significance of pulmonary surfactants in asthma has been highlighted.

Recent Findings

Surfactants, such as surfactant protein A (SP-A) and SP-D released from alveolar epithelium, reduce pathogen infection and control immune-cell activation. Especially, SP-D directly binds to eosinophil surface, leading to inhibition of extracellular trap formation and reduction in airway inflammation. Production of surfactants is commonly determined by both genetic (single nucleotide polymorphisms) and environmental factors influencing processes involved in the development of asthma. In addition, nintedanib (an intracellular inhibitor of tyrosine kinases) could increase SP-D levels and is used in patients with idiopathic pulmonary fibrosis. These findings may provide a possible application of SP-D in asthma.

Summary

Surfactants are key players contributing to host defense through maintaining the immune system. As clinical implications of surfactants involved in asthma have been suggested, further translational studies are needed to apply surfactants as an effective therapeutic target in patients with asthma.

Alveolar Type 2 Epithelial Cells as Potential Therapeutics for Acute Lung Injury/Acute Respiratory Distress Syndrome

Acute lung injury/acute respiratory distress syndrome is a common clinical illness with high morbidity and mortality, which is still one of the medical problems urgently needed to be solved. Alveolar type 2 epithelial cells are an important component of lung epithelial cells and as a kind of stem cells, they can proliferate and differentiate into alveolar type 1 epithelial cells, thus contributing to lung epithelial repairment. In addition, they synthesize and secrete all components of the surfactant that regulates alveolar surface tension in the lungs. Moreover, alveolar type 2 epithelial cells play an active role in enhancing alveolar fluid clearance and reducing lung inflammation. In recent years, as more advanced approaches appear in the field of stem and progenitor cells in the lung, many preclinical studies have shown that the cell therapy of alveolar type 2 epithelial cells has great potential effects for acute lung injury/acute respiratory distress syndrome. We reviewed the recent progress on the mechanisms of alveolar type 2 epithelial cells involved in the damaged lung repairment, aiming to explore the possible therapeutic targets in acute lung injury/acute respiratory distress syndrome.

Structure and activity of human surfactant protein D from different natural sources

Surfactant protein D (SP-D) is a C-type lectin that participates in the innate immune defense of lungs. It binds pathogens through its carbohydrate recognition domain in a calcium-dependent manner. Human surfactant protein D (hSP-D) has been routinely obtained from bronchoalveolar lavage of patients suffering from pulmonary alveolar proteinosis (PAP) and from amniotic fluid (AF). As a consequence of the disease, hSP-D obtained from PAP is found in higher amounts and is mainly composed of higher order oligomeric forms. However, PAP-hSP-D has never been directly compared with nonpathological human protein in terms of structure and biological activity. Moreover, the quantitative distribution of the different hSP-D oligomeric forms in human protein obtained from a natural source has never been evaluated. In this work, we have determined the quantitative distribution of AF-hSP-D oligomers, characterized the sugars attached through the N-glycosylation site of the protein, and compared the activity of hSP-D from AF and PAP with respect to their ability to bind and agglutinate bacteria. We have found that fuzzy balls (40%) are the most abundant oligomeric form in AF-hSP-D, very closely followed by dodecamers (33%), with both together constituting 73% of the protein mass. The glycan attached to the N-glycosylation site was found to be composed of fucose, galactose, sialic acid, and N-acetylglucosamine. Finally, in the functional assays performed, hSP-D obtained from PAP showed higher potency, probably as a consequence of its higher proportion of large oligomers compared with hSP-D from AF.

[Surfactant proteins A and D: role in the pathogenesis of community-acquired pneumonia and possible predictive perspectives]

Community-acquired pneumonia is one of the most common infectious diseases and remains one of the leading causes of death in this group of diseases. Studies of community-acquired pneumonia are extremely relevant for modern clinical practice. One of the important role in the pathogenesis of bacterial, viral, fungal invasion in the system of a human lung system belongs to the pulmonary surfactant, in particular, its proteins SP-A and SP-D. This article reviews the well-known mechanisms of important biological properties of immunomodulatory activity of the proteins SP-A and SP-D in response to microbial infection in the lungs. The mechanisms of participation of surfactant proteins SP-A and SP-D in the cascade of reactions that lead to severe life-threatening complications in community-acquired pneumonia are considered. The use of serum levels of surfactant proteins SP-A and SP-D can help finding new diagnostic and prognostic approaches in patients with community-acquired pneumonia.

Functional characterization of the different oligomeric forms of human surfactant protein SP-D

Surfactant Protein D (SP-D) is a collectin protein that participates in the innate immune defense of the lungs. SP-D mediates the clearance of invading microorganisms by opsonization, aggregation or direct killing, which are lately removed by macrophages. SP-D is found as a mixture of trimers, hexamers, dodecamers and higher order oligomers, "fuzzy balls". However, it is unknown whether there are differences between these oligomeric forms in functions, activity or potency. In the present work, we have obtained fractions enriched in trimers, hexamers and fuzzy balls of full-length recombinant human (rh) SP-D by size exclusion chromatography, in a sufficient amount to perform functional assays. We have evaluated the differences in protein lectin-dependent activity relative to aggregation and binding to E. coli, one of the ligands of SP-D in vivo. Fuzzy balls are the most active oligomeric form in terms of binding and aggregation of bacteria, achieving 2-fold binding higher than hexamers and 50% bacteria aggregation at very short times. Hexamers, recently described as a defined oligomeric form of the protein, have never been isolated or tested in terms of protein activity. rhSP-D hexamers efficiently bind and aggregate bacteria, achieving 50-60% aggregation at final time point and high protein concentrations. Nevertheless, trimers are not able to aggregate bacteria, although they bind to them. Therefore, SP-D potency, in functions that relay on the C-lectin activity of the protein, is proportional to the oligomeric state of the protein.

Structural and Functional Determinants of Rodent and Human Surfactant Protein A: A Synthesis of Binding and Computational Data

Surfactant protein A (SP-A) provides surfactant stability, first line host defense, and lung homeostasis by binding surfactant phospholipids, pathogens, alveolar macrophages (AMs), and epithelial cells. Non-primates express one SP-A protein whereas humans express two: SP-A1 and SP-A2 with core intra- and inter-species differences in the collagen-like domain. Here, we used macrophages and solid phase binding assays to discern structural correlates of rat (r) and human (h) SP-A function. Binding assays using recombinant rSP-A expressed in insect cells showed that lack of proline hydroxylation, truncations of amino-terminal oligomerization domains, and site-directed serine (S) or alanine (A) mutagenesis of cysteine 6 (C6S), glutamate 195 (E195A), and glutamate 171 (E171A) in the carbohydrate recognition domain (CRD) all impaired SP-A binding. Replacement of arginine 197 with alanine found in hSP-A (R197A), however, restored the binding of hydroxyproline-deficient rSP-A to the SP-A receptor SP-R210 similar to native rat and human SP-A. In silico calculation of Ca⁺⁺ coordination bond length and solvent accessibility surface area revealed that the “humanized” R197A substitution alters topology and solvent accessibility of the Ca⁺⁺ coordination residues of the CRD domain. Binding assays in mouse AMs that were exposed to either endogenous SP-A or hSP-A1 (6A²) and hSP-A2 (1A⁰) isoforms in vivo revealed that mouse SP-A is a functional hybrid of hSP-A1 and hSP-A2 in regulating SP-A receptor occupancy and binding affinity. Binding assays using neonatal and adult human AMs indicates that the interaction of SP-A1 and SP-A2 with AMs is developmentally regulated. Furthermore, our data indicate that the auxiliary ion coordination loop encompassing the conserved E171 residue may comprise a conserved site of interaction with macrophages, and SP-R210 specifically, that merits further investigation to discern conserved and divergent SP-A functions between species. In summary, our findings support the notion that complex structural adaptation of SP-A regulate conserved and species specific AM functions in vertebrates.

Genetic causes of surfactant protein abnormalities

PURPOSE OF REVIEW

Mutations in genes encoding proteins critical for the production and function of pulmonary surfactant cause diffuse lung disease. Timely recognition and diagnosis of affected individuals is important for proper counseling concerning prognosis and recurrence risk.

RECENT FINDINGS

Involved genes include those encoding for surfactant proteins A, B, and C, member A3 of the ATP-binding cassette family, and for thyroid transcription factor 1. Clinical presentations overlap and range from severe and rapidly fatal neonatal lung disease to development of pulmonary fibrosis well into adult life. The inheritance patterns, course, and prognosis differ depending upon the gene involved, and in some cases the specific mutation. Treatment options are currently limited, with lung transplantation an option for patients with end-stage pulmonary fibrosis. Additional genetic disorders with overlapping pulmonary phenotypes are being identified through newer methods, although these disorders often involve other organ systems.

SUMMARY

Genetic disorders of surfactant production are rare but associated with significant morbidity and mortality. Diagnosis can be made invasively through clinically available genetic testing. Improved treatment options are needed and better understanding of the molecular pathophysiology may provide insights into treatments for other lung disorders causing fibrosis.

Rheologically Essential Surfactant Proteins of the CSF Interacting with Periventricular White Matter Changes in Hydrocephalus Patients - Implications for CSF Dynamics and the Glymphatic System

Surfactant proteins (SP) are multi-systemic proteins playing crucial roles in the regulation of rheological properties of physiological fluids, host defense, and the clearance of potentially harmful metabolites. Hydrocephalus patients suffer from disturbed central nervous system (CNS) fluid homeostasis and exhibit remarkably altered SP concentrations within the cerebrospinal fluid (CSF). A connection between CSF-SPs, CSF flow, and ventricular dilatation, a morphological hallmark of hydrocephalus, has been reported previously. However, currently there are no studies investigating the link between rheologically active SPs and periventricular white matter changes caused by impaired CSF microcirculation in hydrocephalic conditions. Thus, the aim of this study was to assess their possible relationships. The present study included 47 individuals (27 healthy subjects and 20 hydrocephalus patients). CSF specimens were analyzed for concentrations of SP-A, SP-C, and SP-D by using enzyme-linked immunosorbent assays (ELISAs). Axial T2w turbo inversion recovery magnitude (TIRM) magnetic resonance imaging was employed in all cases. Using a custom-made MATLAB-based tool for quantification of magnetic resonance signal intensities in the brain, parameters related to disturbed deep white matter CSF microcirculation were estimated (TIRM signal intensity (SI)-mean, minimum, maximum, median, mode, standard deviation, and percentiles, p10th, p25th, p75th, p90th, as well as kurtosis, skewness, and entropy of the SI distribution). Subsequently, statistical analysis was performed (IBM SPSS 24™) to identify differences between hydrocephalic patients and healthy individuals and to further investigate the connections between CSF-SP changes and deep white matter signal intensities. SP-A (0.38 ± 0.23 vs. 0.76 ± 0.49 ng/ml) and SP-C (0.54 ± 0.28 vs. 1.27 ± 1.09 ng/ml) differed between healthy controls and hydrocephalus patients in a statistically significant manner. Also, corresponding quantification of white matter signal intensities revealed statistically significant differences between hydrocephalus patients and healthy individuals: SImean (370.41 ± 188.15 vs. 222.27 ± 99.86, p = 0.001), SImax (1115.30 ± 700.12 vs. 617.00 ± 459.34, p = 0.005), SImedian (321.40 ± 153.17 vs. 209.52 ± 84.86, p = 0.001), SImode (276.55 ± 125.63 vs. 197.26 ± 78.51, p = 0.011), SIstd (157.09 ± 110.07 vs. 81.71 ± 64.94, p = 0.005), SIp10 (229.10 ± 104.22 vs. 140.00 ± 63.12, p = 0.001), SIp25 (266.95 ± 122.62 vs. 175.63 ± 71.42, p = 0.002), SIp75 (428.80 ± 226.88 vs. 252.19 ± 110.91, p = 0.001), SIp90 (596.47 ± 345.61 vs. 322.06 ± 176.00, p = 0.001), skewness (1.19 ± 0.68 vs. 0.43 ± 1.19, p = 0.014), and entropy (5.36 ± 0.37 vs. 4.92 ± 0.51, p = 0.002). There were no differences regarding SP-D levels in hydrocephalus patients vs. healthy controls. In the acute hydrocephalic subgroup, correlations were as follows: SP-A showed a statistically significant correlation with SImax (r = 0.670, p = 0.024), SIstd (r = 0.697, p = 0.017), SIp90 (r = 0.621, p = 0.041), and inverse correlation with entropy (r = - 0.700, p = 0.016). SP-C correlated inversely with entropy (r = - 0.686, p = 0.020). For the chronic hydrocephalus subgroup, the following correlations were identified: SP-A correlated with kurtosis of the TIRM histogram (r = - 0.746, p = 0.021). SP-C correlated with SImean (r = - 0.688, p = 0.041), SImax (r = - 0.741, p = 0.022), SImedian (r = - 0.716, p = 0.030), SImode (r = - 0.765, p = 0.016), SIstd (r = - 0.671, p = 0.048), SIp25 (r = - 0.740, p = 0.023), SIp75 (r = - 0.672, p = 0.048), and SIp90 (r = - 0.667, p = 0.050). SP-D apparently does not play a major role in CSF fluid physiology. SP-A and SP-C are involved in different aspects of CNS fluid physiology. SP-A appears to play an essential compensatory role in acute hydrocephalus and seems less involved in chronic hydrocephalus. In contrary, SP-C profile and white matter changes are remarkably connected in CSF of chronic hydrocephalus patients. Considering the association between CSF flow phenomena, white matter changes, and SP-C profiles, the latter may especially contribute to the regulation of paravascular glymphatic physiology.

Retinal degeneration mutation in Sftpa1tm1Kor/J and Sftpd -/- targeted mice

Surfactant proteins are important collectin immune molecules with a wide distribution throughout the body, including the ocular system. Mice with gene deletions for the surfactant protein genes Sftpa1 and Sftpd were observed to have visual impairment and thinning of the outer nuclear layers of the retina. We hypothesized that gene deletion of Sftpa1 and Sftpd (Sftpa1tm1Kor/J and Sftpd-/-) results in early retinal degeneration in these mice. Sftpa1tm1Kor/J and Sftpd-/- retinas were evaluated by histopathology and optical coherence tomography (OCT). Retinas from Sftpa1tm1Kor/J and Sftpd -/- mice showed early retinal degeneration with loss of the outer nuclear layer. After screening of mice for known retinal degeneration mutations, the mice were found to carry a previously unrecognized Pde6brd1 genotype which resulted from earlier breeding of the strain with Black Swiss mice during their generation. The mutation was outbred and the genotype of Sftpa1tm1Kor/J and Sftpd-/- was confirmed. Outbreeding of the Pde6brd1 mutation resulted in restoration of normal retinal architecture confirmed by in vivo and in vitro examination. We can therefore conclude that loss of Sftpa1 and Sftpd do not result in retinal degeneration. We have now generated retinal Sftpa1 and Sftpd targeted mice that exhibit normal retinal histology.

Three-step monitoring of glycan and galectin profiles in the anterior segment of the adult chicken eye

A histochemical three-step approach is applied for processing a panel of sections that covers the different regions of fixed anterior segment of the adult chicken eye. This analysis gains insight into the presence of binding partners for functional pairing by galectin/lectin recognition in situ. Glycophenotyping with 11 fungal and plant lectins (step 1) revealed a complex pattern of reactivity with regional as well as glycan- and cell-type-dependent differences. When characterizing expression of the complete set of the seven adhesion/growth-regulatory chicken galectins immunohistochemically (step 2), the same holds true, clearly demonstrating profiles with individual properties, even for the CG-1A/B paralogue pair. Testing this set of labeled tissue lectins as probes (step 3) detected binding sites in a galectin-type-dependent manner. The results of steps 2 and 3 reflect the divergence of sequences and argue against functional redundancy among the galectins. These data shape the concept of an in situ network of galectins. As consequence, experimental in vitro studies will need to be performed from the level of testing a single protein to work with mixtures that mimic the (patho)physiological situation, a key message of this report.

Collectins in urinary tract and kidney diseases

The innate immune system serves as the frontline defense against invading pathogens and initiates an inflammatory response to microorganisms. Collectins are C-type lectins that are structurally characterized by a collagen-like sequence and a carbohydrate recognition domain. Moreover, they are widely expressed throughout the body and are involved in the innate immunity against a variety of pathogens, regulating inflammation, and protecting the lungs from pathogens. Recently, two classical collectins, surfactant protein A (SP-A) and surfactant protein D (SP-D), as well as novel collectin 11, were found present in urinary tract tissues. They are increasingly recognized as key players in activating the humoral arm of innate immunity and host defense in urinary tract and kidney diseases, although their biological features, functions, and mechanisms in this regard remain largely unclear. In this review, we aim to integrate results reported by ourselves and others to summarize and gain a better understanding of the functions of collectins (SP-A, SP-D, and collectin 11) in urinary tract and kidney diseases.