Chapter 1

INTRODUCTION

 

 

 

 

The effect of shear on gene expression is a well-understood topic in today’s context. Several observations have been made which suggest that up-regulation and down-regulation of expression can be caused by stress applied to the cell walls.

 

The most basic example of such an effect can be seen on the up-regulation and down-regulation of gene expression in the blood vessels. The blood flowing through the vessels applies a shear, which causes the modulation of the various vascular functions. Also, various studies have been made on the effect of shear on the bone matrix. Various molecular level events take place when shear is applied to the matrix due to physical causes.

 

The up-regulation and down-regulation of gene expression in the vitally important blood stream and other body parts, is of utmost interest to scientists in medicine. By controlling the gene expression using shear, diseases can be cured, and new methods may be devised to replace drugs, which are commonly used. Hence, the present study gains a lot of importance with a large potential in the field of medicine. In the Chemical Engineering context, the productivities of the bio-reactors can be optimized using the results of the study of shear on expression. Using the results we can design bio-reactors with greater yields and productivities than ordinary reactors.

 

Keeping the aforementioned in mind, the long term of the present study will be to ascertain the changes that occur in genetic expression when various kinds of shears are applied, and to exploit the results for designing bio-reactors that give enhanced productivity. To this end the short-term goal of the present study is to device methods to predict the extent of up-regulation and down-regulation in gene expression when exposed to shear in different ways.

 

The first stage report is a Literary survey on the effect of shear (applied in different ways) on various kinds of sequences. The breakthroughs achieved in the field and the applications have been compiled in the present report. Also, a molecular level insight has been provided, using the various mechanisms that have been proposed till date. The present survey is primarily on the effects seen inside the human body. The results however can be used to design reactors that would be used to manufacture the proteins artificially.

 

The field of the present study seems to be very promising, with potentially great applications in Chemical Reactor design as well as surgery and medical research. An intent study is planned in the next stages to fully understand the behavior of genes when subjected to shear, so that the results can be fully exploited for reactor design procedures.

 

 

 

 

 

 

 

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                            Chapter 2

                                    Effect of Shear on the Regulation of Gene Expression.

 

 

2.1 Effect on the production of Endothelial derived products

 

Fluid shear causes simultaneous differential regulation of endothelial-derived products. It has been reported that using bovine aortic endothelium monolayers, such a differential regulation can be achieved (Malek and Izumo). In the study it has also been reported that the gene expression in all the studied cases are shear stress rather than shear rate dependent.

 

The mechanical environment to which a blood vessel is exposed is known to regulate its structure during development (Langille, 1993) and in response to iatrogenic changes such as those induced by arterio-venous shunts and vessel constriction (Kamiya and Togawa, 1980). Studies have lead to a conclusion that the blood vessel is no longer considered to be simply a non-thrombogenic passive conduit for blood flow, but a continually adapting, physically and chemically interdependent network of elements with the common goal of maintaining optimal function in response to constantly changing hemodynamic and metabolic conditions (Davies and Tripathi, 1993; Langille, 1993; Ross, 1993).

 

 

 

 

 

 

Figure 2.1:

Vessel Wall constituents and hemodynamic environment (Malek and Izumo, 1995)

The endothelium secretes a large variety of substances with both autocrine and paracrine actions on a number of target cells. The activity of these substances includes growth regulation, vasoactive and mitogenic control, fibrinolysis/thrombolysis and matrix remodeling and cell adhesion/activation.

 

 

Endothelial Products

Actions and Properties

Target

ET-1

Vasoconstrictor, paracrine mitogen.

Smooth muscle, fibroblast.

NO

Vasodilator, growth/protein-synthesis inhibitor

Smooth muscle, fibroblast, platelet endothelium.

BPGF

Matrix bound autocrine/paracrine mitogen

Endothelium, smooth muscle, fibroblast

PGDF-A & PGDF-B

Soluble paracrine Mitogen

Smooth muscle, fibroblast

TGF-b

Soluble autocrine/paracrine growth inhibitor

Endothelium, smooth muscle, fibroblast.

TM

Surface bound antirthrombotic/antifibrinolytic

Thrombin, Protein C cascade

PAI-1

Inhibits palsminogen activators t-PA, u-PA

Plasmin action.

MCP-1

Monocyte chemotactic protein

Monocyte

VCAM-1

Monocyte/Lymphocyte surface-bound receptor

Monocyte, lymphocyte and leucocyte

ICAM-1

Monocyte/Lymphocyte surface-bound receptor

Monocyte, lymphocyte and leucocyte

 

Table 2.1: Endothelial derived products. Actions and target sites of activity.

 

 

Physiological steady laminar fluid shear stress (6 h duration) regulate s the expression of a number of endothelial products in a specific fashion. ET-1 and PGDF-B factors, which cause both vasoconstriction and increase smooth muscle and fibroblast growth (Yangisawa et al., 1988; D’Amore and Smith, 1993), show decreased expression under steady shear stress in a magnitude dependant fashion (15 and 36 dyne/cm2). In contrast, elevated shear (36 dyne/cm2) increases production of bFGF (Basic Fibroblast Growth Factor), a heparin-binding factor usually stored in the matrix underlying endothelial cells and is though to induce growth of both endothelial cells and smooth cells (Rifkin and Moscatelli, 1989). Similarly, differential regulation is seen in response to shear with thrombomodulin and t-PA, with the former anticoagulant receptor under going decreased and the latter fibrinolytic substance sustaining increased expression under shear. The magnitude of shear stress also plays a crucial role: Although shear decreases ET-1, PGDF-B and TM mRNA at both 15 and 36 dyn/cm2, it increases bFGF and t-PA mRNA levels to a significant extent only at magnitudes greater than 15 dyn/cm2.

 

 

Figure 2.2: Effect of shear stress on endothelial gene expression in a time and magnitude dependant manner. Northern blot analysis revealing the pattern of endothelial messenger (mRNA) expression changes on a number of functional products induced by physiological levels of steady laminar fluid shear stress in BAE monolayers at 6h following onset of flow.

 

 

Fig. 2.3: Effect of steady laminar shear of 15 dyn/cm2 on ET-1 mRNA content.

 

The effect of a steady laminar shear of a magnitude of 15 dyn/cm2 on the mRNA content of ET-1 (Fig. 2.3) reveals that a small 20-40% increase occurs between 0.5-1.0 hours following the onset of shear application. This is followed by a 5-10 folds decrease after 3 hours. Referring to Fig. 2.4, it has been observed that mRNA levels of ET-1 show a shear magnitude dependant decrease after 4 hours.

 

Fig 2.4: mRNA level falls in a shear magnitude dependant manner.

 

 

 

The expression in the ET-1 cells depends on the shear magnitude, and not on the shear rate. This result is clearly seen in Fig. 2.5. We see that the expression is magnitude dependant. The curves flatten out in the same manner showing that there is no dependency on the shear rate.

Fig. 2.5: Expression in endothelium is not dependant on the shear rate.

 

 

 

Fig. 2.6: Down regulation of expression in the endothelium is a reversible process.

 

 

 

From the figure above the fact that the expression in endothelium is a reversible process is clear. We see that as soon as the shear application is stopped the upregulation of expression occurs. 

 

Table 2.2: Characterization of response to dynamic response stimuli

 

Gene Product

Response to dynamic Stimuli.

c-fos

Higher increase with pulsatile compared to steady flow.

c-myc

No difference between pulsatile and steady flow.

c-fun

No difference between pulsatile and steady flow

ET-1

Downregulation similar under pulsatile, steady and turbulent flow.

NOS

Upregulation more pronounced in periodic on-off-on flow.

PGDF-A

Upregulation higher in pulsatile vs steady state flow.

PGDF-B

Upregulation higher in steady flow, down regulation same as ET-1.

bFGF

Upregulation sensitive to mean shear magnitude rather than the dynamic character.

t-PA

Upregulation similar in response in both steady, pulsatile flow

TM

Downregulation similar in response in steady, pulsatile and turbulent flow.

 

 

2.2 Influence on Gene Expression in the bone matrix due to shear stress

 

The shape of bone changes as a result of bone remodeling corresponding to physical circumstances such as mechanical stress. The tissue which receives the loaded mechanical stress most efficiently is the bone matrix. Loaded mechanical stress is converted to a series of biochemical reactions, and finally activates osteoclasts and osteoblasts to cause bone resorption and formation. Biological and molecular biology studies have recently resulted in the identification of the gene of which expression level is changed by mechanical stress. Nitric Oxide (NO) and camp is secreted in response to mechanical stress in the immediate early stage.

 

Genes encoding enzymes such as glutamate/ aspartate transporter (GLAST). Nitric Oxide Synthatase (NOS) and prostaglandin G/H synthatase (PGHS-2) are identified as mechanical stress responsive. It has been observed that the expression level of IGF-I is enhanced under the control of PTH/PTHrP. The expression of c-fos is increased by loading of mechanical stress. AP1, a heterodimer of c-FOS/c-JUN, functions as a transcription factor of downstream gene(s). The enhanced expression of osteopontin (OPN) in the osteocytes of bone resorption sites was demonstrated by in situ hybridization and immunohistochemistry and transdifferentiation of chondrocytes with the abundant expression of BMP-2 and –4 in the process of distraction osteogenesis was observed.

 

2.3 Influence of the Application of Pressure in addition to shear on Gene        Expression

 

The vascular endothelium is continuously exposed to three major types of fluid dynamic forces: shear stress, compressive and circumferential stress force. Acute elevation of intraluminal (within the tube) pressure is known to induce myogenic response in the vascular wall. It has been observed (Gan et al., 2000) that during high pressure condition, eNOS mRNA was upregulated after 3 hours and leveled off after 6 hours of perfusion, while Et-1 mRNA was elevated after 6 h of perfusion. Significant vasodilation was also observed after 3 hours in the high pressure system. In the study, high and low pressure levels (39.9 and 20.00 mm of Hg respectively) were applied. The mean shear stress was maintained at identical levels in both the systems (10.00 dyn/cm-2). The average flow rate was kept at 79 and 52.5 ml/min in the high and low-pressure systems, respectively.

 

 

  

 

 

Fig. 2.6 A                                                                 Fig. 2.6 B

a)      Increased immuno-reactivity of ET-1 at elevated pressure levels.

b)      Increased immuno-reactivity of eNOS at elevated pressure levels.

The results of the experiments showed that the sub acute elevation of pressure in the presence of physiological shear stress increases the expression of ET-1 in the vascular endothelium and induces a transient up-regulation of eNOS expression.

 

2.4 Influence of Mechanical Loading and Unloading on mRNA levels

 

In a study  (Sun and Yokota, 2001),  the cellular morphology and mRNA levels of matrix metalloproeinase-13 (MMP-13) genes under mechanical stress in human MH7A cells were investigated. The cells were isolated from the knee joint of a rheumatoid arthritis patient. A loading and unloading procedure was deviced and is described with the help of Fig. 2.7.

 

Fig. 2.7: Loading and unloading

              (Sun and Yokota, 2001)

 

 

 

 

 

 

 

Loading was done using a reciprocal shaker. The shaker was devised to generate a constant sliding motion along a prescribed axis at a frequency of 80 oscillations per minute. The peak-to-peak stroke was taken as 20 mm. Unloading was done using a clinostat. It was a horizontal rotor with a rotating shaft of 10 mm diameter. A constant angular velocity of 6 rpm was applied.

 

It was observed that loading transiently decreased the level of MMP-13  mRNA and unloading increased its mRNA level. The unloaded cells appeared to be rounded and displayed poorly developed track of peripheral fibres. The cells under loading tended to align to shear flow and were elongated. It was also observed that altering the oscillatory direction of mechanical loads contributed to a further reduction in mRNA expression of MMP-13.

 

Fig: 2.8: a) Control Cells b) Cells rounded after loading c) Cells elongated after unloading. (Sun and Yokota, 2001)

 

The results show the role that mechanical loading and unloading can play in the mRNA level regulation and the potential value of physical therapy for arthritic joints.

 

2.5 Influence of prolonged application of shear stress on expression

 

In a study (Bongrazio et al., 2000) the variation of gene expression by prolonged shear stress was investigated. Human umbilical vein endothelial cells (HUVECs) were exposed to laminar shear stress (6 dyn/cm–2; 24 hours) and analyzed by differential display (DDRT-PCR). Flow modulation of differentially expressed genes and in human cardiac microvascular endothelial cells (HCMECs; 24 hours) was analyzed. The analysis displayed that there were 13 down and 20 up regulated products in response to flow.

 

Four known genes were identified. Amongst them, Angiopoietin-2, a protein, was progressively downregulated from 4 to 48 hours of shear stress. It was observed that growth arrest specific mRNA 3(gas3) and calpactin 1 light chain (p11) were upregulated only on prolonged exposure (24 – 48 hours.).

 

Fig. 2.8: Time dependant modulation of angiopoietin-2, METH-1, gas3 and p11 by                                           shear stress exposure, for 0, 4, 24 and 48 hours respectively. (Bongrazio et al., 2000).

 

Hence we see that some proteins show regulation effects on large exposure times to shear stress.

 

 

 

2.6 Effect of cyclic strain on Gene Expression

 

Studies on the effect of cyclic strain on the induction of transcriptional factors like AP-1. c-AMP etc. were done ( Du et al., 1995). EC were exposed to 10% average strain of 60cpm for up to 24 hours. At varying time points, nuclear protein was extracted and analyzed for production of AP-1, CRE and NF-kB by electro-mobility shift assay. The results demonstrated that EC exposure to cyclic strain leads to a significant induction of AP-1 CRE and NF-kB in HAEC and HUVEC, but not in BAEC.

 

 

                Fig. 2.9:  Effect of cyclic strain on AP-1 induction and CRE induction.

                                EC were exposed to 10% 60 cpm average strain in a cyclic

                                manner. (Du et al., 1995)  

 

The unit that was used to apply cyclic strain on EC comprised of a vacuum unit regulated by a solenoid valve and a computer program. Cells are seeded on flexible-bottomed culture plates with a hydrophilic collagen surface. The plates are maintained in the vacuum manifold of the stretch apparatus. When a vacuum is applied to the culture plate, the bottoms deform to a known percentage elongation. When the vacuum is released, the plate bottoms return to their original conformation. The magnitude, duration, and frequency of the applied force can be varied in this system. For the experiments, the membranes were subjected to deformation with 150 mm of Hg of vacuum at a frequency of  60 cpm (0.5 s of deformation alternating with 0.5 s of deformation) for up to 24 hours.

 

AP-1 and NF-kB peaked at 4 hours, whereas CRE increased in a biphasic manner at 15 minutes and 24 hours. Fig. 2.9 shows the effect of cyclic strain on theproduction of EC derived products.

 

 

 

                  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                    Chapter 3

                                                Applications and Conclusion

 

 

The studies of the effect of shear strain have been done in vitro and in vivo. The results show clearly that the expression of genes to various proteins, especially in the Endothelial Cells, are affected by shear application. Also, the expression depends on the nature of shear force, the duration and on some other parameters.

 

It has been observed that the expression levels are different for different genes, and also the nature of the curve of expression level and magnitude of strain, as well as duration of strain have different effects. Such data can be of vital importance to the Biochemical and the Pharmaceutical Industry. The data can be used to increase the yield and selectivity of proteins produced artificially by optimizing the designs of bioreactors.  Apart from these, the results can be used for drug discovery.

 

A pathway for  gene and drug discovery, using the study of shear effect has been described in the work by Topper and Gimbrone, 1999. Powerful molecular biology techniques can be used to probe the patterns of endothelial gene expression induced by defined fluid mechanical forces in vitro. In this strategy, cultured endothelial cells are exposed to distinct regimes of flow like: a) Laminar Shear Stress (LSS) or b) turbulent Shear Stress (TSS), in a specialized con-plated apparatus.  The resulting alternations in gene expression are analyzed by techniques such as differential display and transcriptional profiling. The expression patterns of multiple endothelial cells can thus be discerned.  Examples of individual mRNA transcripts differentially expressed in response to LSS, TSS or cytokine simulation are shown in Fig. 3.1

Fig. 3.1: Strategy for gene discovery, (Topper and Gimbrone, 1999)

 

Hence, we can conclude that the study of shear effects can be of great advantage to the Bio-Chemical industry. Also, it can help in gene discovery and eventually lead to better and cheaper drugs for various diseases.

 

Future Work: Keeping the long-term goal of designing reactors with higher yields and selectivity, the future work (for the next stage) should be the study of the effect of strain of various kinds in vitro on the production of proteins, and to find the strain magnitude and duration of application where the expression of a particular drug is maximized or minimized. This can lead to the design of better reactors. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

APPENDIX: Glossary of important terms.

 

Adhesion Molecule: From the Latin adhaerere = "to stick to". The term adhesion molecule refers to a glycoprotein (thus oligosaccharide) molecular "chain" that protrudes from the surface membrane of certain cells, and causes cells (possessing "matching" adhesion molecules) to adhere to each other. For example, in 1952 Aaron Moscona observed that (harvesting enzyme-separated) chicken embryo cells did not remain separated, but instead coalesced again into an (embryo) aggregate. In 1955, Philip Townes and Johannes Holtfreter showed that "like" amphibian (e.g., frog) neuron cells will rejoin together after being physically separated (e.g., with a knife blade); but "unlike" cells remain segregated (apart).

Adhesion molecules also play a crucial role in guiding monocytes to sources of infection (e.g., pathogens) because adhesion molecules in the walls of blood vessels (after activation caused by pathogen invasion of adjacent tissue) adhere to like adhesion molecules in the membranes of monocytes in the blood. The monocytes pass through the blood vessel walls, become macrophages, and fight the pathogen infection (e.g., triggering tissue inflammation, etc.).

Anticoagulant: Any substance that prevents blood clotting. Those drugs administered for  treatment of thromboembolic disorders are heparin, which inactivates thrombin and several other clotting factors and which must be administered

Autocrine: Secretion of a substance, such as a growth factor, that stimulates the secretory cell itself. One route to independence of growth control is by autocrine growth factor production.

 

 

B-Lymphocytes: A class of white blood cells originating in the bone marrow and found in blood, spleen, and lymph nodes. They are the precursors of (blood) plasma cells (B cells) that secrete antibodies (IgG) directed against invading antigens (e.g., of pathogenic bacteria). Via a complex "gene splicing" process, the B-cells of the human body are able to produce more than one billion different IgG antibodies (i.e., able to bind-onto and neutralize a billion different antigens).

 

Bone matrix: The intercellular substance of bone tissue consisting of collagen fibres, ground substance, and inorganic bone salts.

 

Continuous-Perfusion: A type of cell culture in which the cells (either mammalian or otherwise) are immobilized in a part of the system, and nutrients/oxygen are allowed to flow through the stationary cells, thus effecting nutrient/waste exchange. Ideally the system incorporates features that retard the activity of proteolytic enzymes, and reduce the need for anti- infective agents (e.g., antibiotics) and fetal bovine serum, which are required by most other cell culture systems. Continuous perfusion is used because, among other things, it eliminates the need to separate the cells from the culture medium when fresh medium is exchanged for old.

 

Chondrocytes: Mature cartilage cells embedded in lacunae within the cartilage matrix.

 

Endothelial Cells: These are the flat, sort of plate-shaped cells that line the surface of all blood vessels, heart, and lymphatics within the body. Endothelial cells possess transmembrane (i.e., through the cell membrane) molecules known as adhesion molecules, which selectively allow the passage (from bloodstream to tissues) of some molecules (e.g., leukocytes, monocytes, hormones, etc.). Endothelial cells are packed much tighter together in the capillaries that provide blood to the brain. This tighter packing limits the size and kind of molecules that can pass into the brain. This blood-brain barrier serves to protect the sensitive brain tissue from pathogens or harmful molecules (e.g., toxins).

 

Endothelium: The layer of epithelial cells that line blood vessels throughout the body. The layer selectively allows the passage (from bloodstream to tissues) of nutrients, hormones, and other molecules that are essential for tissue growth and function. The endothelium is involved in the recovery and recycling of old red blood cells. It also produces:

Fibroblasts: Cells that are precursors to the connective tissue cells found in the skin. They make structural proteins like collagen, which gives skin its strength. Because fibroblasts do not express antigens on their cell surfaces (free standing, separated), fibroblasts possess potential for use in making artificial organs (e.g., artificial pancreas for diabetics), since recipient immune system cannot recognize the fibroblast cells as foreign.

 

Fibroblast Growth Factor (FGF): It is a protein that stimulates the formation/development of blood vessels and fibroblasts (precursors to collagen, the connective tissue "glue" that holds cells together). FGF also is mitogenic (causes cells to divide and multiply) for both fibroblasts and endothelial cells, and attracts those two cell types (i.e., is chemotactic). Basic FGF is ten times more "potent" than acidic FGF in most bioassays.

 

Gene Expression: Conversion of the genetic information within a gene, into an actual protein (or cell process). Certain proteins (i.e., when present in relevant cells) regulate the expression (e.g., increase/decrease/timing) of some genes.

 

Growth Factor: A specific substance that must be present in the organism's tissues (when in vivo) or growth medium (when in vitro) in order for the growth-factor-specific cells to grow/multiply.

Hemodynamics: The study of the forces involved in the circulation of blood.

Iatrogenic: Induced in a patient by a physician's activity, manner, or therapy. Used especially of an infection or other complication of treatment.

 

In situ: In the natural or original position (e.g., inside the body).

 

In vitro: In an unnatural position (e.g., outside the body, in the test tube). "In vitro" is Latin for "in glass." For example, the testing of a substance, or the experimentation in (using) a "dead" cell-free system

 

In vivo: Latin for "in living" (e.g., the testing of a new pharmaceutical substance or experimentation in (using) a living, whole organism. An in vivo test is one in which an experimental substance is injected into an animal such as a rat in order to ascertain its effect on the organism

 

Lymphocyte: A type of cell found in the blood, spleen, lymph nodes, etc. of higher animals. They are formed very early in fetal life, arising in the liver by the sixth week of human gestation. There exist two subclasses of lymphocytes: B lymphocytes and T lymphocytes. B lymphocytes make antibodies (immunoglobins) of which there are five classes: IgM, IgA, IgG, IgD and IgE. The antibodies circulate in the bloodstream. T lymphocytes recognize and reject foreign tissue, modulate B cell activity, kill tumor cells, and kill host cells infected with virus. T-lymphocytes are also called T cells.

 

Mitogenic: Causing mitosis or transformation.

 

Mitogen: A substance (e.g., growth factor, hormone, etc.) that initiates cell division within the body. For example, most Angiogenic Growth Factors (e.g., fibroblast growth factor) stimulate cell division of the endothelial cells which line blood vessel walls

 

Monocytes: Also called monocyte macrophages. The round-nucleated cells that circulate in the blood. In summary they engulf and kill microorganisms, present antigen to the lymphocytes, kill certain tumor cells, and are involved in the regulation of inflammation. These cells are often the first to encounter a foreign substance or pathogen or normal cell debris in the body. When they do, the material is taken up (engulfed) and degraded by means of oxidative and hydrolytic enzymatic attack. Peptides that result from the degradation of foreign protein are then bound to a monocyte protein called class II MHC (major histocompatibility complex) and this self-foreign complex then migrates to the surface of the cell where it is embedded into the cell membrane in such a way as to present the peptide to the outside of the cell. This positioning allows T lymphocytes to recognize (inspect) the peptide. Whereas self-peptides derived from normal cellular debris are ignored, foreign peptides activate precursors of helper T cells to further mature into active, lymphokine-secreting helper T lymphocytes, also known as TH cells. When monocytes move out of the bloodstream and into the tissues they are then called macrophages.

 

Osteocytes: Osteoblasts that have become embedded within the bone matrix, occupying a flat oval cavity and sending, through the canaliculi, slender cytoplasmic processes that make contact with processes of other osteocytes.

 

Osteoclasts: A large multinuclear cell associated with the absorption and removal of bone. An odontoclast, also called cementoclast, is cytomorphologically the same as an osteoclast and is involved in cementum resorption.

 

Osteoblasts: Cells that arise from fibroblasts and which, as they mature, are associated with the production of bone.

 

Osteopontin: Bone specific sialoprotein (57 kD: probably two similar peptides) that links cells and the hydroxyapatite of mineralised matrix, has RGD sequence. Found only in calcified bone, probably produced by osteoblasts.

Paracrine: Form of signalling in which the target cell is close to the signal releasing cell. Neurotransmitters and neurohormones are usually considered to fall into this category.

 

Reperfusion:  The restoration of blood flow to an occluded (i.e., blocked) blood vessel. May be done biochemically (e.g., via tissue plasminogen activator) or via surgery.

 

Resorption: The loss of substance through physiologic or pathologic means, such as loss of dentin and cementum of a tooth.  Origin: L. Resorbere = to swallow again

 

Tissue Plasminogen Activator (tPA): A glycoprotein that possesses thrombolytic (i.e., blood clot-dissolving) activity. It is used as a drug to dissolve clots and acts by first binding to fibrin (clots). It then activates (i.e., proteolytically cleaves) plasminogen (molecules) to yield plasmin, a bloodborne enzyme that itself cleaves molecular bonds in the fibrin clot. The plasmin molecules diffuse through the fibrin clot and cause the clot to dissolve rapidly. With the dissolution of the clot, blood flow to the formerly blocked blood vessel (e.g., the heart) is restored

 

 

Thrombin: The key to thrombus (blood clot) formation. Thrombin is a proteolytic enzyme that cleaves fibrinogen into (molecular) pieces, which then spontaneously assemble themselves into fibrin, which forms a clot.

 

Thrombus: The blood clot itself. The mass of blood coagulated in situ in the heart or other blood vessel. For example, such a clot causes a heart attack when the coagulation occurs in the vessels feeding the heart.

 

Thrombosis: The intravascular (i.e., inside of blood vessel) formation of a blood clot.

 

Thrombomodulin: A cell surface protein found on endothelial cells that plays a key role in modulating the final step in the coagulation process. After thrombin binds to thrombomodulin, thrombin loses its ability to cleave fibrinogen to form fibrin. In addition, once thrombin binds to thrombomodulin, thrombin's activation of protein C is increased 200-fold and this activated protein C then degrades factors Va and VIIIa which are both required for the production of thrombin from prothrombin. Hence, thrombomodulin modulates the activity of the enzyme thrombin causing a cessation of full-blown clotting activity.

 

Transcription Factors: Proteins and/or other chemical compounds that interact with each other, and with regulatory sequences within DNA (when immediately adjacent to the DNA in a cell), to either facilitate (i.e., "turn on") or inhibit (i.e., "turn off ") the activity (i.e., coding for proteins) of that DNA's genes. Transcription factors hold potential to:

Some transcription factors are an integral component in certain gene expression cascades. For example, a gene expression cascade is initiated by the first gene causing expression of a transcription factor, when then itself interacts with cell's DNA to either cause or speed-up yet another gene expression. The protein resulting from that second gene expression is yet another transcription factor which triggers another (i.e., third) gene expression, and so on.

Vasoconstriction: The diminution of the calibre of vessels, especially constriction of arterioles leading to decreased blood flow to a part. Parenterally and the oral anticoagulants  which inhibit the hepatic synthesis of vitamin K dependent clotting factors.

 

Vasodilation: The increase in the internal diameter of a blood vessel that results from relaxation of smooth muscle within the wall of the vessel. This causes an increase in blood flow, but a decrease in systemic vascular resistance

 

Vascular Endothelial Growth Factor (VEGF):  A human growth factor (GF) that causes growth/proliferation of blood vessels/endothelium and endothelial cells

 

Vasoactive: Exerting an effect upon the calibre of blood vessels.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REFERENCES

 

 

·        Adel M. Malek, Seigo Izumo, J., Control of Endothelial Cell Gene Expression by Flow, J. Biomechanics, Vol: 28, 1995, pp 1515-1528.

 

·        Davies, P.F. and Tripathi, S.C., Mechanical stress mechanisms and the cell, an endothelial paradigm. Circ. Res., Vol: 72, 1993, pp230-245.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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