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A pulmonary alveolus, also called an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where pulmonary gas exchange takes place. Oxygen is exchanged for carbon dioxide at the blood–air barrier between the alveolar air and the pulmonary capillary. Alveoli make up the functional tissue of the mammalian lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.
Pulmonary surfactant is a surface-active complex of phospholipids and proteins formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorbing to the air-water interface of alveoli, with hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of the surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension.
Dipalmitoylphosphatidylcholine (DPPC) is a phospholipid (and a lecithin) consisting of two C16 palmitic acid groups attached to a phosphatidylcholine head-group.
Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca2+-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their function is to bind to oligosaccharide structure or lipids that are on the surface of microorganisms. Like other PRRs they bind pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) of oligosaccharide origin. Binding of collectins to microorganisms may trigger elimination of microorganisms by aggregation, complement activation, opsonization, activation of phagocytosis, or inhibition of microbial growth. Other functions of collectins are modulation of inflammatory, allergic responses, adaptive immune system and clearance of apoptotic cells.
Surfactant protein D, also known as SP-D, is a lung surfactant protein part of the collagenous family of lectins called collectin. In humans, SP-D is encoded by the SFTPD gene and is part of the innate immune system. Each SP-D subunit is composed of an N-terminal domain, a collagenous region, a nucleating neck region, and a C-terminal lectin domain. Three of these subunits assemble to form a homotrimer, which further assemble into a tetrameric complex.
Surfactant protein A is an innate immune system collectin. It is water-soluble and has collagen-like domains similar to SP-D. It is part of the innate immune system and is used to opsonize bacterial cells in the alveoli marking them for phagocytosis by alveolar macrophages. SP-A may also play a role in negative feedback limiting the secretion of pulmonary surfactant. SP-A is not required for pulmonary surfactant to function but does confer immune effects to the organism.
Surfactant protein B is an essential lipid-associated protein found in pulmonary surfactant. Without it, the lung would not be able to inflate after a deep breath out. It rearranges lipid molecules in the fluid lining the lung so that tiny air sacs in the lung, called alveoli, can more easily inflate.
Surfactant protein C (SP-C), is one of the pulmonary surfactant proteins. In humans this is encoded by the SFTPC gene.
Collagen XVII, previously called BP180, is a transmembrane protein which plays a critical role in maintaining the linkage between the intracellular and the extracellular structural elements involved in epidermal adhesion, identified by Diaz and colleagues in 1990.
Phosphatidylglycerol is a glycerophospholipid found in pulmonary surfactant and in the plasma membrane where it directly activates lipid-gated ion channels.
Multidrug resistance-associated protein 1 (MRP1) is a protein that in humans is encoded by the ABCC1 gene.
Surfactant protein A1(SP-A1), also known as Pulmonary surfactant-associated protein A1(PSP-A) is a protein that in humans is encoded by the SFTPA1 gene.
NK2 homeobox 1 (NKX2-1), also known as thyroid transcription factor 1 (TTF-1), is a protein which in humans is encoded by the NKX2-1 gene.
Coatomer subunit gamma is a protein that in humans is encoded by the COPG gene. It is one of seven proteins in the COPI coatomer complex that coats vesicles as they bud from the Golgi complex.
RhoD is a small signaling G protein, and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOD.
Surfactant protein A2(SP-A2), also known as Pulmonary surfactant-associated protein A2(PSP-A2) is a protein that in humans is encoded by the SFTPA2 gene.
Surfactant metabolism dysfunction is a condition where pulmonary surfactant is insufficient for adequate respiration. Surface tension at the liquid-air interphase in the alveoli makes the air sacs prone to collapsing post expiration. This is due to the fact that water molecules in the liquid-air surface of alveoli are more attracted to one another than they are to molecules in the air. For sphere-like structures like alveoli, water molecules line the inner walls of the air sacs and stick tightly together through hydrogen bonds. These intermolecular forces put great restraint on the inner walls of the air sac, tighten the surface all together, and unyielding to stretch for inhalation. Thus, without something to alleviate this surface tension, alveoli can collapse and cannot be filled up again. Surfactant is essential mixture that is released into the air-facing surface of inner walls of air sacs to lessen the strength of surface tension. This mixture inserts itself among water molecules and breaks up hydrogen bonds that hold the tension. Multiple lung diseases, like ISD or RDS, in newborns and late-onsets cases have been linked to dysfunction of surfactant metabolism.
In molecular biology, Pulmonary surfactant protein D (SP-D) is a protein domain predominantly found in lung surfactant. This protein plays a special role; its primary task is to act as a defence protein against any pathogens that may invade the lung. It also plays a role in lubricating the lung and preventing it from collapse. It has an interesting structure as it forms a triple-helical parallel coiled coil, helps the protein to fold into a trimer.
Ficolins are pattern recognition receptors that bind to acetyl groups present in the carbohydrates of bacterial surfaces and mediate activation of the lectin pathway of the complement cascade.
Surfactant therapy is the medical administration of pulmonary surfactant that is derived from outside of the body. Pulmonary surfactant is a soap-like chemical synthesized by type II alveolar pneumocytes and is of various lipids. This biological fluid reduces surface tension and lines the aqueous layer covering the alveolar surface of the lung. For more details, see Pulmonary surfactant.
References
- ↑ "Erika Crouch | Medical Scientist Training Program" . Retrieved 2024-05-22.
- ↑ "Erika Crouch and SP-D | Erika Crouch Lab". wupathlabs.wustl.edu. Retrieved 2023-02-28.
- ↑ "Erika Crouch, MD, PhD | Pathology & Immunology | Washington University in St. Louis". pathology.wustl.edu. Retrieved 2023-02-28.
- ↑ Crouch, Erika C. (1998-08-01). "Collectins and Pulmonary Host Defense". American Journal of Respiratory Cell and Molecular Biology. 19 (2): 177–201. doi:10.1165/ajrcmb.19.2.140. ISSN 1044-1549. PMID 9698590.
- ↑ Persson, Anders; Rust, Kevin; Chang, Donald; Moxley, Michael; Longmore, William; Crouch, Edmound (1988-11-15). "CP4: a pneumocyte-derived collagenous surfactant-associated protein. Evidence for heterogeneity of collagenous surfactant proteins". Biochemistry. 27 (23): 8576–8584. doi:10.1021/bi00423a011. ISSN 0006-2960. PMID 3219363.
- ↑ Crouch, E.; Persson, A.; Chang, D.; Heuser, J. (1994). "Molecular structure of pulmonary surfactant protein D (SP-D)". Journal of Biological Chemistry. 269 (25): 17311–17319. doi: 10.1016/s0021-9258(17)32556-5 . ISSN 0021-9258. PMID 8006040.
- ↑ Crouch, Erika C. (1998-11-19). "Structure, biologic properties, and expression of surfactant protein D (SP-D)". Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease. 1408 (2): 278–289. doi:10.1016/S0925-4439(98)00073-8. ISSN 0925-4439.
- ↑ Hanson, Phyllis I.; Stahl, Philip D. (2004). "From the neuromuscular junction to cellular architecture and beyond – commentary on 30 years of imaging by John E. Heuser". European Journal of Cell Biology. 83 (6): 229–242. doi:10.1078/0171-9335-00398. ISSN 0171-9335. PMID 15511079.
- ↑ Hartshorn, K L; Crouch, E C; White, M R; Eggleton, P; Tauber, A I; Chang, D; Sastry, K (1994-07-01). "Evidence for a protective role of pulmonary surfactant protein D (SP-D) against influenza A viruses". Journal of Clinical Investigation. 94 (1): 311–319. doi:10.1172/JCI117323. ISSN 0021-9738. PMC 296311 . PMID 8040272.
- ↑ Seaton, Barbara A.; Crouch, Erika C.; McCormack, Francis X.; Head, James F.; Hartshorn, Kevan L.; Mendelsohn, Richard (2010). "Review: Structural determinants of pattern recognition by lung collectins". Innate Immunity. 16 (3): 143–150. doi:10.1177/1753425910368716. ISSN 1753-4259. PMID 20423923.