Following the leader: how new blood vessels choose which cell goes first

Cells compete to lead the tip of growing blood vessels, a team of European and American scientists has discovered. Their study, published recently in Nature Communications, also revealed how the leading cell is chosen.

Cells with a low level of neuropilin (marked green) do not form tip-cells if they compete with normal cells with a higher level of neuropilin (red). Picture by Emma Gordon, Yale/Uppsala

Understanding how the body regulates angiogenesis – the formation of new blood vessels including veins, arteries and capillaries – is relevant to many diseases.

“The healthy body needs to regulate angiogenesis, because too many or too few blood vessels can cause disease,” explains MDC group leader Prof Dr Holger Gerhardt, who led the study.

Excess angiogenesis is involved in cancer, where tumours fuel their growth with new blood vessels; and diabetic retinopathy, an eye disease where blood vessels block vision. In contrast chronic wounds often have too little blood flow. Therapies that increase or decrease angiogenesis can treat these diseases.

Our body regulates angiogenesis using protein signals. Imagine the proteins as a series of switches that can be turned on or off. If the wrong switches are on, the body makes too many or too few blood vessels, causing disease.

If we had drugs to turn these protein switches on or off, we could stop blood vessels growing in tumours, or stimulate them in chronic wounds. But before we can target the switches, we need to identify them and which part of angiogenesis they control.

Angiogenesis starts when new blood vessels sprout. The new vessels form a simple network, which later develops into capillaries, arteries and veins. Sprouting blood vessels have a leading tip cell followed by stalk cells that grow in the direction set by the tip. Without a tip, new vessels can't sprout.

Gerhardt's team and their collaborators, Anne Eichmann and her team at Yale, studied sprouting blood vessels and found there is a competition between cells to determine which one becomes the tip. “It's like a social situation where everyone wants to be the leader,” Gerhardt explains, “but not all cells have leadership qualities.”

Gerhardt's team revealed that a protein called neuropilin is a key leadership quality – it allows cells to lead the tip of a sprouting blood vessel. They worked this out using a series of mosaic cell experiments.

In these experiments, the researchers either increased or decreased the levels of neuropilin in one set of cells, then mixed them with another group of cells with a normal neuropilin level. Each group of cells was labelled with a different colour. After mixing the cells, they waited for new blood vessels to sprout and used the colours to track which cells became tips and stalks.

They found that cells with more neuropilin took the tip position, while cells with less neuropilin became stalks. This means neuropilin allows cells to form tips so that new blood vessels can sprout, making it a key switch in angiogenesis. Disrupting it would be a way to stop angiogenesis, making neuropilin an interesting new drug target.

Once the mosaic experiments revealed that neuropilin allows cells to become tips, Gerhardt and his team went on to find out how it interacts with other proteins that are involved in switching cells between being tips or stalks.

“The interesting thing about neuropilin,” according to Gerhardt, “is that it links two signalling pathways that we previously thought were separate, notch and TGFβ.”

Scientists knew that notch blocked angiogenesis and tip cells, but didn't know how it worked. Gerhardt's team discovered that notch acts by reducing the amount of neuropilin. Without neuropilin cells can't become tips.

The study found that neuropilin promotes tip formation by reducing a separate pathway, the TGFβ pathway. If this pathway is on, cells become stalk cells and no new blood vessels sprout. It's only when neuropilin blocks the TGFβ pathway that tip cells form, spearheading new blood vessels.

This finding could be relevant to the genetic disease HHT (Hereditary Hemorrhagic Telangiectasia), where problems with angiogenesis lead to fragile blood vessels and bleeding. Some HHT patients have mutations to genes in the TGFβ pathway, so understanding neuropilin signalling in this context is important.

Understanding HHT is the basis of Gerhardt's collaboration with Eichmann at Yale which has revealed how neuropilin activates the tips of sprouting blood vessels. Their cooperation takes advantage of the Yale researchers' knowledge of HHT genetics and the Gerhardt lab's expertise in the cellular biology of blood vessels.

The neuropilin study was completed during Gerhardt's previous role at the London Research Institute, but the collaboration has continued since his arrival at the MDC. He is also establishing new links with doctors and clinical researchers in Berlin.

Gerhardt's research group will work with the Charité – Universitätsmedizin Berlin to investigate diabetic retinopathy. Their studies of this eye disease will combine the MDC's capability in cellular and molecular research with the tissue imaging facilities and patient studies at the Charité. This work is supported by the Berlin Institute of Health. Gerhardt will also collaborate with cardiologists from the Deutsches Zentrum für Herz-Kreislauf-Forschung (DZHK ) to investigate the role of angiogenesis in heart disease.

Highlight Reference:

Aspalter IM, Gordon E, Dubrac A, Ragab A, Narloch J, Vizán P, Geudens I, Collins RT, Franco CA, Abrahams CL, Thurston G, Fruttiger M, Rosewell I, Eichmann A, Gerhardt H. Alk1 and Alk5 inhibition by Nrp1 controls vascular sprouting downstream of Notch. Nat Commun. 2015 Jun 17;6:7264. doi: 10.1038/ncomms8264


Featured Image: Mosaik experiments with blood vessels show: cells with a low level of neuropilin (marked green) do not form tip-cells if they compete with normal cells with a higher level of neuropilin (red). Picture courtesy of Emma Gordon, Yale/Uppsala.