CHAPTER

1 The Interactions between Biomaterials and Tissues

1.3 Cells and Cell Injury

• Cells are composed of nucleic acids, proteins, and other molecules.

• They are held together by cell-to-cell junctions to form tissues comprising 4 general types; – Epithelium, Connective tissue, Muscle, and Nerve

• Organs are assembled from these tissues, glued together by ECM synthesized by cells.

• Perform the various functions; circulation, respiration, digestion, excretion, movement, and reproduction.

• In the following chapter, concept of structure-function correlation beyond cells to ECM and complex tissues, technologies examined normal and abnormal tissues, physiological responses to environmental stimuli, mechanism of cell injury, cell-material interactions are studied.

• In this chapter, general characteristics and function of cells, compartmentalization of regionally specialized function by membranes, regulation and coordination of cell function, response of cells to injury, cell death.

Normal Cell • Most essential cell attributes are ;

• Intracellular constituents; – Microcosm of water, ions, sugars, and cytosol or

cytoplasm (small Mw molecules, ATP etc) – Plasma membrane (phospholipid bilayer) is

impermeable to charged and/or polar molecules because of its hydrophobic inner core.

– However, it is rendered selectively permeable to ions and amino acid etc. by channel or transport proteins inserted through it.

– Cells also have capacity to internalize material from outside environment by capturing bits in invaginated folds of plasma membrane called vesicles.

• Depending on volume and size of ingested material, – Phagocytosis (eating) and pinocytosis (drinking) – Transcytosis is movement of vesicles from one side

of cell to another, and play an important role in mediating the increased vascular permeability that occurs around tumors and at sites of inflammation.

• Expressing a variety of specific surface molecules that facilitate interactions with other cells, soluble ligands (e.g. insulin), and/or with ECM (communication).

• Normal housekeeping functions are compartmentalized within intercellular organelles : unique environment as pH, calcium, enzyme conc., etc

• Rough endoplasmic reticulum(RER)

• Golgi apparatus; secretion protein synthesis

• Ribosome; cytosol protein synthesis

• Smooth endoplasmic reticulum (SER); steroid hormone & lipoprotein synthesis

• Lysosome; catabolism, degradation

• Peroxisome; generating H2O2

• The architecture of cell is maintained by a scaffolding of intracellular proteins called cytoskeleton. – Cell movement by rearrangment of cytoskeleton – Cell polarity (difference in structure & function at

the top of a cell vs its side or base) – Specific functions; reflected by relative amount of

organelles, ex., kidney tubular epithelial cells (reabsorb Na & Cl) and cardiac myocytes (contract 50-100 times/min.) have generous complement of mitochondria. Pancreatic islet cell (synthesized insulin) or plasma cell (produced antibody) have a well-developed RER.

Plasma Membrane • Protection, nutrient acquisition, communication • Amphipathic phospholipids; Dynamic, fluid,

inhomogeneous lipid bilayer embedded proteins • To form 2D sheet with hydrophilic head groups facing

toward aqueous cytoplasm or extracellular fluid, and hydrophobic lipid tails interacting to form a central core. (Ch.3.3: Fig.2)

• Resistant to movement of large or polar molecules, • Requires protein channels or transport mechanisms

• Inserted membrane proteins have different solubilities. The membrane lipid inhomogeneities result in functionally distinct islands. This has significance in terms of cell-cell and cell-matrix interactions, as well as in intracellular signaling.

• The asymmetry of inner and outer leaflets of plasma membrane has functional significance in that negative charged gangliosides and glycolipids – both of which are on the outer face of bilayer – are important for cell-cell and cell-matrix interactions, local electrostatic effects, and creation of barriers to infection. Inositol phospholipids, on inner face, are for intracellular signaling.

Ganglioside is a molecule composed

of a glycosphingolipid with one or more

sialic acids (NANA) linked on the sugar chain.

• How proteins associate with membranes reflects their function. Most inserted proteins have one or more relatively hydrophobic segments that traverse the lipid bilayer.

• Proteins involved in forming pores or transporting other molecules will typically be transmembrane.

• Large complex is employed to translate ligand-surface receptor binding into intracellular response, and also form the basis for intercellular connection as tight junctions and homotypic interaction on adjacent cells.

Passage through plasma membrane • Small nonpolar molecules (O2, CO2) readily dissolve in

lipid bilayers and rapidly diffuse across them. • Large hydrophobic molecules (steroid hormone) also

readily cross lipid bilayers. • Even polar molecules, if sufficiently small (water, etOH,

and urea at Mw of 18, 46, 60 daltons) rapidly cross membranes.

• In contrast, glucose (Mw 180) excluded, • Completely impermeable to ions, regardless of size.

• To facilitate the entrance or disposal, specific transport proteins are required.

• For small Mw molecules (ions, glucose, nucleotides, amino acid up to app. 1000 da), there are 2 main categories, carrier proteins and channel proteins

• For larger molecules or particles, uptake is mediated by specific receptors, internalized via endocytosis.

• Large molecules destined for export are packaged in secretory vacuoles and expelled their contents in a process called exocytosis

Unique transporter • Each type of transported molecules (ion, sugar,

nucleotide) requires a unique membrane transport protein, typically exhibit strong specificity. – will move glucose, but not galactose – will move potassium, but not sodium

• Carrier proteins bind their specific ligand and undergo a series of conformational changes to transfer it across membrane; transport is relatively slow.

• In comparison, channel proteins create hydrophilic pores; these permit rapid movement of selected solutes.

• In most cases, conc. and/or electrical gradient between inside and outside of cell drives solute movement. (with inside of a cell negative relative to the outside)

• In some cases, active transport is accomplished by carrier molecules and requires energy expenditure.

• e.g. Multidrug resistance(MDR) protein pumps polar compounds out of cell and render cancer cells more resistant to treatment.

• Similar transport mechanism also regulate intracellular and intraorganellar pH; most cytosolic enzymes prefer to work at pH 7.2, whereas lysosomes function best at pH 5 or less.

Osmosis • Water will move freely into or out of cells along its conc.

gradient. • Extracellular salt in excess of that seen in cytoplasm

(hypertonicity) will cause a net movement of water out of cells; conversely hypotonicity will cause a net movement of water into cells.

• Since intracellular environment is rich in charged molecules, cells need to constantly actively regulate intracellular osmolarity by pumping out small inorganic ions(Na, Cl) and attracting charged counterions.

• Loss of ability to generate energy (in cell injured by toxins or lacking oxygen) results in swollen, and eventually ruptured, cell.

Endocytosis • Uptake and metabolism of large extracellular

molecules requires vesicle and membrane recycling. • Proteins, large carbohydrates, and macromolecules

cannot enter cells by either channels or carriers. • Instead, they are internalized by endosytosis (Ch.3.3:

Fig.3). • Endosytosis begins at a specialized region of plasma

membrane called clathrin-coated pit, which rapidly invaginates and pinches off to form clathrin-coated vesicle (about 2500/min. in typical fibroblast)

Receptor-meditated endocytosis • Clathrin is a hexamer of proteins that spontaneously

assemble into basket-like lattice to drive budding process of endocytosis. Trapped within vesicle will be a minute gulp of extracellular milieu, as well as any molecules specifically bound to receptors on the internalized bit.

• ex) This is pathway by which cells internalize iron from circulation: bound to protein called transferrin, interacts with cell surface transferrin receptors.

Endosome • Vesicles rapidly lose their clathrin coat and then fuse

with acidic intracellular structure called endosome. • Where they discharge their contents for digestion

and further passage to lysosome (Ch.3.3: Fig.4) • After release of bound ligand, receptors can return to

plasma membrane for another cycle (e.g. transferrin receptor) or may be degraded (LDL receptor).

• Degradation of receptor after internalization provides an important control for receptor expression and signaling.

Transcytosis & Exocytosis • Endocytosis can also deliver material completely

across a cell, i.e., from apical surface to basolateral face (transcytosis) , forming the basis for transport of nutrients from gastrointestinal tract to blood stream.

• Endocytosis is an ongoing process, with constant recycling of vesicles back to plasma membrane (exocytosis). Exocytosis must be tightly coupled with endocytosis,

• since a cell will internalize 10~20% of its own cell volume each hour, or about 1~2% of its plasma membrane each minute.

Cell Communication • is critical in multicellular organisms. Extracellular signals

may determine whether a cell lives or dies. • Intercellular signaling is critical in developing embryo in

order that cells appear in correct quantity and location, and in maintaining tissue.

• Intercellular signaling is also important in intact organism, ensuring that all tissues act in appropriate concert in response to stimuli as digestive as food or threat to life.

• Loss of communication may be reflected in congenital structural defect or in unregulated cell growth (cancer) or ineffective response to extrinsic stress.

• Adjacent cells may communicate via gap junctions, which are narrow hydrophilic channels that effectively connect the 2 cells’ cytoplasm.

• The channels permit movement of small ions, metabolites, and potential second messenger molecules, but not larger macromolecules.

• Since most signaling molecules are present at extremely low conc. (<10-8 M), binding to appropriate target cell receptor is typically a high-affinity and exquisitely specific interaction.

Extracellular signaling, via soluble mediators occurs in 3 different forms:

1. Paracrine, where affects cells only in immediate vicinity. There can be only minimal diffusion, with signal rapidly degraded, taken up by other cells, or trapped in ECM.

2. Synaptic, where activated neural tissue secretes neurotransmitters at a specialized cell junction (synapse) onto target cells.

3. Endocrine, where regulatory substance, as hormone, is released into blood stream and acts on target cells at a distance.

• Autocrine: signaling between the same kind of cell • Juxtacrine: signaling through a plasma membrane

Intracellular & cell-surface Receptors • may be on cell surface or intracellular, • In the intracellular case, ligands must be sufficiently

hydrophobic to enter the cell (e.g., vitamin D, steroid, thyroid hormone).

• For intracellular receptors, ligands binding leads to formation of receptor-ligand complex that directly associates with nuclear DNA and subsequently either activates or turns off gene transcription

• For cell-surface receptors, ligands binding can (1) opens ion channels, (2) activates associated GTP binding regulatory protein (G protein), or (3) activates an associated enzyme.

• Secondary, intracellular downstream events frequently involve phosphorylation or de-phosphorylation of target molecules, with subsequent changes in enzymatic activity.

• In the absence of appropriate exogenous ligand, they may undergo a form of cellular suicide called apoptosis, or programmed cell death.

Cytoskeleton • The ability of cells to adopt a particular shape, maintain cell

polarity, organize relationship of intracellular organelles, and move depends on intracellular scaffolding of proteins called cytoskeleton.

• There are 3 major classes of cytoskeletal proteins:

– 6 ~ 8 nm ф actin microfilaments, – 10 nm ф intermediate filaments, – 25 nm ф microtubules

• These proteins impart structure (the intermediate) and dynamic (actin & microtubules) used to movement and cellular contraction.

http://www.uic.edu/classes/bios/bios100/lectures/cells.htm

Actin • Globular protein actin (G-actin, 43,000 Da) is major

subunit of microfilaments and is most abundant cytosolic protein in cells.

• The monomers polymerize into long double-stranded helical filaments (F-actin) with defined polarity (one end is stable, the other end grows or shrinks)

• In muscle cells, filamentous protein myosin binds to actin and moves along it, driven by ATP hydrolysis.

• In non-muscle cells, F-actin and assortment of actin-binding proteins form well-organized bundles and networks that control cell shape and movements.

Intermediate Filaments • comprise large and heterogeneous family of closely

related structural proteins. • These ropelike fibers are found predominantly in stable

polymerized form within cells; – expression in specific cell types, – not actively reorganizing like actin and microtubules – impart strength and carry mechanical stress:

• In epithelia connected spot desmosomes • Form major structural proteins of skin & hair (i.e.

keratin)

Microtubules • consist of polymerized dimers of α- and β-tubulin arrayed

in constantly elongating or shrinking hollow tubes. • have a defined polarity with ends designated “+ “or “-”;

– In most cells, “-” end is embedded in microtubule organizing center(centrosome) that lies near nucleus.

– “+” end elongates or recedes in response to stimuli by addition or subtraction of tubulin dimers.

• may serve as mooring lines for protein “molecular motor” that hydrolyze ATP to move vesicles, organelles, or other.

• Polarity of microtubules allows cells to direct whether attached structures are coming or going relative to nucleus.

• In neurons, microtubules are critical for delivery of molecules synthesized in nuclear area to far-flung reaches of cytosol of axon, which may be as far away as 10,000 times width of cell (some motor neurons in spinal cord have axons extending to muscles of big toe over 1 m away; axon growth rate: 1~5mm/day)

• used to move chromosomes apart during mitosis, and play a basic role in cellular proliferation.

• harnessed to facilitate cellular mobility with associated cellular motors; cilia to move mucus & dust out of airway, and flagella to propel sperm.

• Maintaining cellular and tissue integrity requires cell-cell and cell-ECM interactions, mediated through plasma membrane and translated into cytoskeleton.

• Organization of tissues requires attaching cells together and to the underlying ECM scaffolding, of which surface attachments are connected via transmembrane proteins to cytoskeletal elements.

• Extracellular perturbations may be translated into intracellular events.

• External face of cell membrane is diffusely studded with carbohydrate-modified (glycosylated) proteins and lipids.

• This coat (glycocalyx) functions primarily as chemical and mechanical barrier, but also serves important role in cell-cell and cell-ECM interactions; sperm-egg attachment, blood clotting, lymphocyte recirculation, inflammatory response.