Presentation on theme: "Cell Interactions with Biomaterials"— Presentation transcript:
1Cell Interactions with Biomaterials Topics:Cell Structure and ComponentsProperties of Cell ComponentsInteraction of Cells with Extracellular Material (ECM)Cell Adhesion and Migration
2A successful biomaterial implant must support all the required functions of the attached (or neighboring) cells, includingViability All cell typesCommunication All cell typesProtein synthesis All cell typesProliferation Some cell typesMigration Some cell typesActivation/differentiation Some cell typesProgrammed cell death Some cell types
3Cell Structure 2 types of cells: Differentiated: perform specific tissue functionsUndifferentiated: progenitors for many different cell typesCell tasks are compartmentalized in various organelles. Organelles in all mammal cells include the plasma cell membrane, mitochondria, Golgi apparatus, cytoplasm, lysosome, cytoskeleton, nucleus, and smooth and rough endoplasmic reticulumThe Cell
4Other proteins target specific extracellular molecules The cell membrane separates the cytoplasm, or cell interior, from the aqueous external environment. It is a bilayered structure made up of phospholipids, or fatty acids with a polar (hydrophilic) head and nonpolar tail.The mitochondria produce the energy for cell functions via a process called oxidative phosphorization. Surrounded by a phospholipid membrane, the mitochondria contains enzymes that help break down moleculesTransmembrane proteins span the cell membrane to channel ions into and out of cell and maintain proper cell chemistryIn oxidative phosphorization ATP (adenosine triphosphate) is converted to ADP (adenosine diphosphate, an exothermic reaction that releases energy to drive the cellular process.Other proteins target specific extracellular moleculesPhospholipid
5The cytoskeleton is made up of three filaments: Actin microfibrils,~6-8 nm diameterIntermediate filaments ~10 nm diameterMicrotubules ~25 nm diameterThe Golgi apparatus modifies, sorts and packages proteins for their final destinationMade up of proteins, the cytoskeletonGives the cell shapeCan provide cell locomotionAids in separation/duplication of DNALysosomes are specialized vesicles with digestive enzymes
6The nucleus is the control center for the cell. It contains: DNA – (deoxyribonucleic acid) that is condensed into chromatin, a complex combination of DNA and protein that makes up chromosomesThe nuclear envelope, a bilayer of phospholipid membranes that surrounds the nucleusThe outer membrane of the nuclear envelope is contiguous with the endoplasmic reticulum and is connected with the inner membrane at specific locations called nuclear pores. The pores are composed of proteins that form gates to allow only specific molecules in and out of the nucleusRibosomes, minute round particles composed of RNA and protein that are found in the cytoplasm of living cells and catalyze reactions in which mRNA is used to synthesize proteins. Ribosomes are located in the nucleolus
7DNADNA (deoxyribonucleic acid) is a polymer of nucleic acid subunits. Each nucleic acid has a phosphate group, a sugar, and a base.DNA contains genes, and forms the template for all proteins synthesized by the cellWhen a gene is expressed, the cell is actively producing the protein encoded by the geneGenes contain codons which determine the structure of the proteinThe bases are directed toward the interior, where they form hydrogen bonds with other bases.Bases can be either double ring structures (e.g., purines Adenine (A) or guanine (G)), or single ring (pyrimidines, e.g., thymine (T) or cytosine (C))DNA
8DNA-RNA Protein Synthesis RNA participates in DNA synthesis and protein production.Three types of RNA:Messenger or mRNATransfer, or tRNARibosomal, or rRNARNA is similar to DNA, but the sugar in the backbone contains an additional O2, and thymine (T) in DNA has been replaced with uracil (U).RNA is single stranded , and does not form the helical structure of DNAmRNA: messenger RNA; DNA is unzipped, and mRNA strands are synthesized that are complementary to DNAtRNA: serves as an adaptor to combine mRNA strands in the rough ERrRNA: the central component of the ribosome, the function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translationDNA-RNA Protein Synthesis
9The outer membrane of the nucleus is connected to the endoplasmic reticulum (ER). The ER is the site of protein synthesis.The ER is made of long, flattened sheets of phospholipids, and may be either rough or smooth.The rough ER has ribosomes attached to the surface that act as catalysts for protein synthesis.The smooth ER is more tubular and does not contain ribosomes. It packages the proteins produced in the nucleolus and rough ER in phospholipids for delivery to the Golgi apparatus.Vesicles transport proteins from the ER to the Golgi, or from the Golgi to the target destination (exocytosis). Specialized vesicles (lyosomes) may also take in and digest particles from the ECM
10Schematic of the endoplasmic reticulum (ER), which is responsible for protein synthesis. The rough ER, which contains a large number of ribosomes, is attached to the nuclear envelope. The rough ER transforms into the smooth ER away from the nucleus. Pieces of the ER will then split off and transfer to the Golgi apparatus, where the proteins created in the ER are further modified and transported ot their final destinations.
11Interactions between a cell and its environment can result in cell spreading, migration, communication, differentiation and activation. This is called “outside-in” signaling.Conversely, a cell may secrete molecules or rearrange contacts to alter the ECM. This is called “Inside-out” signaling
12Types of cell contactsTight junctions: cells adhere fast to each other – no molecular transportGap junctions: small hydrophilic channels between cells and membranesDesmosomes: mechanical attachment between two cells.Cells can attach to ECM via hemidesmosomes or focal adhesion. These form strong adhesion to the ECM
13Cell membrane receptors & ligands Cellular interactions are facilitated by cell membrane receptors, each of which is specific for a small range of target molecules, or ligands.Common receptor molecules are:Cadherins: responsible for demosomes; a cadherin molecule on one cell binds to a cadherin on another cell. This is homophilic bindingSelectins: selectins are like cadherins, but bind to other types of receptors, or heterophilic bindingMucins: participate in heterophilic binding to selectins.Integrins: transmembrane proteins involved in both cell-cell and cell-matrix contacts.Location of cadherins in epithelial cells.. They link to each other to bind cells, and their cytoplasmic regions attach to intermediate filaments linking the ECM to the intracellular environment
14Cell membrane receptors & ligands Integrins have two distinct a and b subunits, and are called heterodimers.Variations in the composition of the a and b chains results in selective adhesion to different ligandsOther cell adhesion molecules (CAMs) comprise a large group of membrane proteins that mediate cell-cell interactions via both homophilic and heterophilic binding.An example of these other CAMs is the immunoglobulin (Ig) family in the picture at right.The receptors described consist mainly of protein with a small attached carbohydrate (sugar). Similar molecules with small protein and large sugar content are called proteoglycans.Various types of cell membrane receptors: mucins, integrins, selectins, and Ig-cell adhesion molecules (Ig-CAMs).
15Extracellular Matrix – ECM Environment Understanding the cell-ECM interaction and response is key to designing new biomaterialsThe ECM may be thought of as a fiber reinforced composite with fibers made of collagen or elastin, and a matrix made of glycoproteins and proteoglycansECM video
16X and Y molecules are often proline and hydroxyproline CollagenUpon secretion of procollagen from the cell into the ECM, small peptide sequences are cleaved to from the molecule to allow for more efficient packing of collagen molecules into fibrils ( nm). Individual fibers are then assembled into larger fibers (0.5 – 3 mm diameter)Collagen is the most abundant protein in mammals. It is responsible for tissue tensile strength.Collagen is made up of a-polypeptides, or amino acids in a gly-x-y pattern (above). Gly is a small molecule resulting in tight packingX and Y molecules are often proline and hydroxyprolineCell excretes procollagen molecules that self-assemble into fibers (right)Properties of collagen may be manipulated by controlling crosslinking of molecules
17ElastinsElastin is responsible for the resiliency and elasticity of the ECM.Elastin is made up of 85% hydrophobic amino acidsWhen relaxed, elastin molecules coil up. When a tensile load is applied they unfold into long chains. The chains are crosslinked to adjacent elastin molecules
18These molecules attract and interact strongly with water. Additional ECM molecules include proteoglycans and glycoproteins. These proteins are mainly carbohydrate, with some protein side chains.These molecules attract and interact strongly with water.Carbohydrates in proteoglycans form long chains of polysaccharides called glycosaminoglycans (GAGs)Proteoglycans have several GAGs attached to a protein core.An aqueous environment is favorable to transport and store bioactive molecules.Example of the bottle-brush structure of a large proteoglycan including areas of keratan sulfate and chrondroitin sulfate that are attached to a protein core
19Two molecules that represent glycoproteins are fibronectin and laminin. Each consists of peptide subunits held together by disulfide bonds and contain many sites for binding to various ECM molecules.Laminin (right) consists of three disulfide linked peptide chains in a loosely woven structure with multiple binding sitesGlycoproteins are important in blood clotting (coagulation) since they posses binding domains for heparin and fibrin
20Cell-Environment Interactions that Affect Cellular Functions Interactions between cells and cell/ECM can alter cells function and affect gene expression in the nucleusAlterations in gene expression affect four major functions:Cell viabilityProliferationDifferentiationProtein synthesis and communicationChanges in ECM can cause cell death via necrosis or apoptosis due to changes in the local chemistry (e.g. pH), factors that attach to cell membrane leading to cell death.Apoptosis: cell shrinkage followed by fragmentation into vesicles containing small groups of organelles. No inflammatory responseNecrosis: cell death from membrane permeability and enzyme leakage, leading to disintegration
21Mitosis: cell division Cell classificationsLabile: replicate continuouslyTerminal: terminally differentiatedStable: don’t change once differentiated but can be induced to proliferateThe cell cycle is divied into two phases: Mitosis (the M phase) and interphase (G1,S,G2)Mitosis: cell divisionInterphase: cellular DNA and organelles replicated in preparation for mitosisGo: quiescent phase of stable cellsG1: general cell growthS: DNA replicatedG2: proteins and structures enabling cell division ar assembled
22Mitosis is divided into several characteristic periods: Prophase Metaphase Anaphase TelophaseProphase: dissipation of the nucleolus and formation of the mitotic spindlesMetaphase: chromosomes are aligned between two mitotic spindlesAnaphase: chromosomes are pulled apart by the spindle microtubules and arrange themselves at the spindle polesTelophase: nuclear envelope begins to reform and the cell starts to undergo cytokinesisCytokinesis: division of cell cytoplasm
23Cell DifferentiationProgenitor or stem cells can form more than one type of cellThe cells produced may be committed or differentiated and can be labile, stable or permanentOrCreate additional pluripotent or totipotent cells (produce other or all cell types)Red blood cells generated from hematopoietic stem cell can either replicate itself of differentiate into various cell types. These cells are pluripotentEmbryonic stem cells are an example of a totipotent cell
24The commitment and progression of an MSC to and through a specific lineage involves the action of bioactive molecules such as growth factors and cytokines.In the process of differentiation and maturation the cell increases its production of tissue-specific molecules.Terminally differentiated cells may alter their levels of synthesis of matrix molecules to play an increased role in tissue maintenance and homeostasisMesenchymal stem cells (MSCs) can differentiate into bone, cartilage, muscle, tendon, ligament, and adipose tissue.Differentiation stages can be initiated and controlled by soluble and insoluble elements in their environment and is directly applicable to tissue engineering
26Models for Adhesion, Spreading and Migration The DLVO theory: (Derjaguin, Landau, Verway and Overbeek)Based on thermodynamicsParticles potential energy the sum of attractive and repulsive forces: U = UA + URParticles approaching a surface reduce their potential energy, and are loosely attached at the secondary minimum – long range electrostatic/van der Waals forcesParticles overcoming primary minimum become firmly attached through short range electrostatic forcesModel shortcomings: does not include steric repulsion, surface topography/roughness or ligand-receptor interactions
27Spreading & MigrationCell spreading: After attachment, cells extend finger-like pseudopodia along surface. The integrin receptors in the cell membrane interact with ligands on the material surface to firmly anchor the cell in place.Cell spreading includes cytoskeleton rearrangement and production/adsorption of adhesive proteins on surfaceCell migration: extension of the cell membrane in long pseudopodia is directed by polymerization of actin microfibrils near the leading edge of the cell. (b) the membrane then attaches to the substrate via integrin receptors. (c,d) After the pseudopodia are firmly adhered, there is generation of a contractile force along with a release of rear receptors, leading to forward motion. (e) the integrin receptors are recycled to the leading edge so they may be used again as the process continues.
28Tracking cell movements Plots of cell trajectory, such as the one at right, provide information on cell movement in the form of translocation speed (s) and persistence time (t).Translocation speed is the speed of cell movement over any straight-line portion of the graph, between changes in directionPersistence time is the length of time that the cell moves along the substrate without a drastic change in direction.Measurements of this type are used in mathematical models developed to model cell migrationTrajectories of bovine pulmonary artery endothelial cells migrating in a uniform environment. Symbols represent the location of the centroid of each cell at 30 minute intervals. Arrows indicate starting points.
30Protein SynthesisReceptor-ligand binding can change the function of committed cells which is associated with alterations in the amount of protein synthesizedTranscription: Chromatin becomes less compact; RNA “unzips” DNA and synthesizes mRNA strands that are complimentary to DNA then moves into the endoplasmic reticulum (ER)Translation: the process of converting codons from the mRNA to a polypeptide. This takes place in the ER through complex interaction with tRNA.Block diagram of the steps for the creation and modification of proteins.Post Translation: fully formed proteins are combined into various molecules
31Summary of the steps of collagen synthesis Summary of the steps of collagen synthesis. Transcription, translation, synthesis to create the a-chain and the joining of three of thee chains (post translational modification) to create the collagen triple helix occur within the cell. The procollagen molecule is then secreted and assembled into fibrils and finally fibers.