Vascular Biology Lab - Angiogenesis

Angiogenesis, is the term used to describe the growth of new capillary blood vessels in the body. It is an important natural process, used for healing and reproduction. The body controls angiogenesis by producing a precise balance of growth and inhibitory factors in healthy tissues. The fundamental structure of blood vessels is a thin inner layer of endothelial cells. Therefore, the building block of angiogenesis is endothelial tubes. We investigate the behaviour of endothelial cells upon treatment with angiogenesis active pharmacological compounds and proteins. We study both pro-angiogenic and anti-angiogenic events to understand the underlying signaling mechanism of the actives.

Angio means, “ of blood or lymph vessel” and genesis means, “beginning an origin”

Briefly, it’s a physiological process involving the growth of new blood vessels from pre-existing vessels. It is a normal process in growth and development, as well as in wound healing. However, this is also a fundamental step in the transition of tumors from a dormant state to a malignant state

Though there has been some debate over the terminology of Angiogenesis, broadly we have 2 different process of blood vessel formation:

Vasculogenesis, a term used for spontaneous blood-vessel formation.

Intussusception, a term for new blood vessel formation by splitting off existing ones.

Types of angiogenesis

Sprouting angiogenesis

Angiogenic factors activate receptors on endothelial cells of the existing blood vessels. In response to which proteases are secreted that degrade basement membrane and the endothelial cells are set free to proliferate in the surrounding matrix forming solid sprouts attaching neighboring blood vessels. These sprouts extend towards the angiogenic stimulus, in the process they are transformed into full- fledged vessels as pericytes and other cells migrate to the site

Intussusceptive angiogenesis

It is also known as splitting angiogenesis, because during this the vessel wall extends into the lumen to split a single vessel into two. First, the two opposing capillary walls establish a zone of contact. Second, the endothelial cell junctions are reorganized and the vessel bilayer is perforated to allow growth factors and cells to penetrate into the lumen. Third, a core is formed between the two new vessels at the zone of contact that is filled with pericytes and myofibroblasts. These cells begin laying collagen fibers into the core to provide an extra cellular matrix for growth of the vessel lumen. Finally, the core is fleshed out with no alterations to the basic structure. Intussusception is important because it is a reorganization of existing cells. It allows a vast increase in the number of capillaries without a corresponding increase in the number of endothelial cells. This is especially important in embryonic development, as there are not enough resources to create a rich microvasculature with new cells every time a new vessel develops.

Therapeutic angiogenesis

Micro CT scans of cirrhotic mouse liver

Reconstruction Voxel: 20 m m; Display Voxel: 40 m m

Therapeutic angiogenesis is the application of specific compounds, which may inhibit or induce the creation of new blood vessels in the body in order to combat disease. The presence of blood vessels where there should be none may affect the mechanical properties of a tissue, increasing the likelihood of failure. The absence of blood vessels in a repairing or otherwise metabolically active tissue may retard repair or some other function. Several diseases (eg. ischemic chronic wounds) are the result of failure or insufficient blood vessel formation and may be treated by a local expansion of blood vessels, thus bringing new nutrients to the site, facilitating repair. Other diseases, such as age-related macular degeneration, may be created by a local expansion of blood vessels, interfering with normal physiological processes.

Factors that stimulate Angiogenesis

Mechanical stimulation

Mechanical stimulation of angiogenesis is not well characterized. There is a significant amount of controversy with regard to shear stress acting on capillaries to cause angiogenesis, although current knowledge suggests that increased muscle contractions may increase angiogenesis. This may be due to an increase in the production of nitric oxide during exercise.

Chemical stimulation

Various different chemical stimulants have been identified, characterized and worked upon that stimulate the process of angiogenesis and vasculogenesis. Few of the chemical stimulants comprise; VEGF, MMP, FGF, DII4.

VEGF or Vascular endothelial growth factor, as the name suggests mostly works on the endothelial cells and promotes blood vessel formation by catering to both vasculogenesis and angiogenesis.

MMP or Matrix metalloproteinase are secreted in response to different pro-angiogenic signals. They help in the degradation of extra cellular matrix and there by facilitate the migration of endothelial cells and pericytes, an important step in initiation of angiogenesis.

FGF or Fibroblast growth factors are heparin-binding proteins. They stimulate the growth and proliferation of endothelial cells. They also aid in the physical arrangement of endothelial cells in tube structures.

PDGF or Platelet derived growth factors are growth factor proteins derived from platelets and aid in the growth and proliferation of endothelial cells.

Recently some new pro-angiogenic factors have been identified like DII4, which stimulates angiogenesis in ways similar to VEGF.

Vasculogenesis

The Debate over Angiogenesis and Vasculogenesis

For long there has been debate over the terminology to be used for the process of blood vessel formation. Different schools of thoughts classify the process differently and depending on the source of blood vessel formation the process has been categorized in Vasculogenesis and Angiogenesis.

Vasculogenesis is the process of blood vessel formation occurring by a de novo production of endothelial cells. Though it is similar to angiogenesis, the two are different in one aspect:

The term angiogenesis denotes the formation of new blood vessels from pre-existing ones, while vasculogenesis is the term used for the formation of new blood vessels when there are no pre-existing ones.

Interestingly, if a monolayer of endothelial cells begins sprouting to form capillaries, Angiogenesisis occurring.

While, Vasculogenesis, in contrast is the term, when endothelial precursor cells migrate and differentiate in response to local cues (such as growth factors and extra cellular matrix) to form new blood vessels.

First, believed to occur only during embryologic development, but recently, however, it was realized that vasculogenesis can also occur in the adult organism. Circulating endothelial precursor cells (derivatives of stem cells) have been identified and reportedly able to contribute, albeit to varying degrees, to neovascularization, such as during tumor growth, or to the revascularization process following trauma, e.g. after cardiac ischemia.

Angiogenesis-current developments

Tip cells ; the directional behavior of vessel growth during sprouting angiogenesis.

In sprouting angiogenesis, specialized endothelial tip cells lead the outgrowth of blood vessel sprouts towards gradients of vascular endothelial growth factor (VEGF). These cells are different from endothelial cells at the base in respect to their response to VEGF. The tip cells respond to VEGF-A only by guided migration; the proliferative response to VEGF-A occurs in the sprout stalks. These two cellular responses are both mediated by agonistic activity of VEGF-A on VEGF receptor 2. Whereas tip cell migration depends on a gradient of VEGF-A, proliferation is regulated by its concentration. Thus, vessel patterning during retinal angiogenesis depends on the balance between two different qualities of the extra cellular VEGF-A distribution, which regulate distinct cellular responses in defined populations of endothelial cells. VEGF-A is also essential for the induction of endothelial tip cells, but it is not known how single tip cells are selected to lead each vessel sprout, and how tip-cell numbers are determined. There have been reports of delta-like 4 (Dll4)–Notch1 signalling regulates the formation of appropriate numbers of tip cells to control vessel sprouting and branching in the mouse retina.

Prima-facie objective of our lab is to study the physiology of Angiogenesis. We are looking at the basic science of Angiogenesis for a better understanding of the subject and also at the same time trying to apply our understanding in application to present new avenues and inventions. Keeping our main focus on Angiogenesis we are trying to explore all the possible avenues of the subject. Our research can be divided into:

Basic Science and Applicative Research

We are constantly striving to find the basics of Angiogenesis and the factors affecting it. Presently we are working on the effect of Cadmium toxicity and its effect on Angiogenesis. Another interesting area that promises immense potential is, Shear stress and its effect on Angiogenesis. The subject, even though interesting has not been explored much, is gaining attention lately. We are working in the area as our approach to enlighten the area of Basic Angiogenesis.

“Inventions are incomplete without application.” Driven by the core thought we always strive to find applications to our present knowledge. We are trying to apply our knowledge of Angiogenesis to regulate the growth of blood vessels, which can be applied in treatment of various anomalies involving blood vessels including Cancer. We are working to find new therapeutic agents for controlling Angiogenesis. We have been working a potential anti-angiogenic protein Ang-001 that promises to be of great potential in combating malignancy and tumors.

Another major applicative area we are working on is the growth of blood vessels in-vitro. We are working on the prospects of Micro-gravity and its effects on Angiogenesis. Many reports have shown that tissue growth is enhanced under zero gravity and micro gravity conditions. We are trying to understand the effect of micro-gravity environment on Angiogenesis and the applications of the field.