Nic Tapon: Growth Control by the Hippo Pathway

Nic Tapon is one of the few scientists who have made major contributions to the discovery of a new signal transduction pathway. As a result of tumour suppressor screens in the fruit fly he and others identified a set of genes/proteins that regulate development, organ growth/size, stem cell function and whose dysregulation is involved in the development of many types of cancer. This regulatory system is now known as the Hippo pathway. 

Nic Tapon & fruit fly, the key model of his research

http://www.london-research-institute.org.uk/research/nic-tapon

Recent research has connected Hippo members to many other regulatory systems including AMPK and mTOR (a hint to exercise physiologists) and Nic's team has contributed important discoveries. When giving a talk at Aberdeen Nic was willing to answer some questions related to his research. 

1) How did you become involved in the tumour suppressor screens and why did you use the fruit fly as a model?

My PhD was a cell culture-based project, but when I wrote the introduction to my thesis, I became fascinated by the fly and worm genetics literature and the power of screens. The possibility of doing complex genetic manipulations very rapidly was also part of the attraction. I was lucky to be offered a postdoc position in Iswar Hariharan’s lab at a time when genetic mosaicism (the ability to genetically manipulate small patches of tissue in an otherwise wild type organism) was revolutionising fly genetics. This created a gold rush of phenotype-based screens to study all sorts of processes, from tumour suppression to neurodegeneration. At the same time, the fly genome was being sequenced, which made mapping the mutations much easier. The discovery of the Hippo pathway was part of this genetic gold rush. These were exciting times!

2) When and how did you note that the tumour suppressor genes were not independent genes but constitute a pathway?

During the course of my screen, I identified mutations in two genetic loci with similar phenotypes, salvador and dali (long story…). So it was the similarity between the two phenotypes (more proliferation, less apoptosis) that told us these two genes might function in a common pathway. dali turned out to be the previously identified tumour suppressor kinase warts/lats, which had been discovered by Peter Bryant and Tian Xu. Marius Sudol had identified the WW domain as a small protein-protein interaction motif, and Salvador has two WW domains, while Warts has several WW domain-binding motif. Kieran Harvey then showed that Salvador and Warts can physically interact in vitro, and we went from two orphan tumour suppressors to a (modest) pathway!

3) How did people in the field agree on the term ‘Hippo’?

Georg Halder had called his mutants hippo because the head of mutant flies had a puckered appearance reminiscent of hippopotamus skin. This seemed more fun than Drosophila Mammalian Sterile 20-like, so the rest of us adopted that name! DJ Pan started to use the term “Hippo pathway” in his 2003 Cell paper (Wu et al), and the rest is history…

4) Why do you think was the Hippo pathway was overlooked for so long despite its ubiquitous expression and importance?

The core kinase cassette was actually discovered in yeast as the Mitotic Exit Network/Septation Initiation Network before the Hippo pathway was identified in flies. The connection with tissue growth was made thanks to advances in genetic mosaicism, which allowed phenotype-based screens for tumour suppressor genes. So I think it was technological advances that were needed to kick-start the field.

5) The Tead transcription factors, which are now seen as the key downstream regulators of the pathway, were first characterised often in the context of skeletal muscle. Were you surprised by this (Hippo was mainly studied in epithelial cells) and how did this affect your thinking about the Hippo pathway?

All major signalling pathways are used reiteratively in many processes. Although it was initially identified in the context of growth control, the Hippo pathway is no different. So no, not surprised.

6) What are your next (non-confidential) research plans?

That would be telling! Understanding the wildtype function of the Hippo pathway (see Lawrence and Casal, Dev Biol 2013).

First Aberdeen Myology Meeting, 25.4.2014

On the 25^th of April, 2014 we organised the First Aberdeen Myology Meeting with support from the MRC grant that is shared between Pete Zammit, Cosimo De Bari and Henning Wackerahge. For the meeting the speakers were:

Prof. Pete Zammit, King's College London

Prof. Mike Rennie, University of Nottingham

Prof. Steve Harridge, King's College London

Dr. Henning Wackerhage, University of Aberdeen

In order to make it suitable for a wider audience we asked the speaker to give the first half of the talk in a way which is suitable for the general public followed by a second half which is specific to their research. 

Pete Zammit started the afternoon with a basic introduction to skeletal muscle and genetic disorders which worked very well as it reminded the muscle researchers of some basic facts (human muscle fibres can be up to 20 cm long and probably have tens of thousands of nuclei) and at the same time introduced the non-muscle researchers and general public to the topic. 

He then went on to talk about his work on his teams work on laminopathies. These are genetic diseases involving mutations in genes that encode proteins which are in the nuclear lamina and in some cases cause muscular dystrophy. He also presented data on exon skipping, a promising treatment to turn dysfunctional genes into genes that result in a moderately affected protein and thereby result in treatment. This is promising research as exon skipping has been tried with success in patients with muscular dystrophy. 

Figure 1. Prof. Pete Zammit during his talk. 

Mike Rennie gave a talk introducing sarcopenia, the loss of muscle mass during normal ageing, and the concept of 'anabolic resistance' which refers to the observation that older human muscles respond with less protein synthesis to anabolic stimuli such as amino acids than young muscle. Mike has led human muscle protein synthesis in the field and thus it was an excellent review of the work which spanned several decades. Towards the end he has presented a reinterpretation of his data which he is currently presenting at various meetings.

Steve Harridge started by introducing Masters athletes and his studies on Masters athletes as a model for ageing. The world records of Masters athletes show that at least some individuals can maintain a very high level or aerobic capacity and strength into old age. He then presented work by Chibeza Agley who was an oustanding with Steve and who is now a Postdoc in Cambridge: 

http://www.stemcells.cam.ac.uk/researchers/researchersbygroup/silvalabmembers/chibeza-agley

Bezza, as he is known, optimised the methods for isolating myoblasts and fibroblasts from human muscle biopsies and then differentiated these cells. This research showed that fibroblasts but not myoblasts can differentiate into fat cells: http://jcs.biologists.org/content/early/2013/09/23/jcs.132563.abstract

 Figure 2. Prof. Steve Harridge during his talk

At the end I introduced our research on the Hippo pathway in rhabdomyosarcoma which just got accepted by Cancer Cell. This work was led by Fernando Camargo and Annie Tremblay at Harvard who did the mouse models whilst we worked on human cancer cells and stained human tissues. Edoardo Missiaglia, Pete Zammit and Janet Shipleys were other collaborators on this large scale project. 

In the first part I was introducing the hallmarks of cancer (sustained proliferation etc) and how mutations in cancer cells or the germline drive these hallmarks. In the second part I presented the data showing that started with the discovery that the Hippo protein Yap can drive the proliferation of satellite cells. We then asked the question whether persistent Yap hyperactivity would lead to cancer and found that it did but only in activated but not quiescent satellite cells. These cancers in mice mimicked human embryonal rhabdomyosarcoma (ERMS), which is the most common form of this cancer. When Yap is reduced in human ERMS cells then these cells differentiate into normal, differentiated muscle tissue. We also studied the molecular mechanisms by which Yap alone can drive this cancer and many other aspects. This will be covered in another blog entry. 

Figure 3. Dr. Henning Wackerhage during his talk on Cancer and Muscle. 

So overall it was good to see many colleagues, students and researchers attending the myology day. I think the combination of an introductory part and an actual research part worked well, giving non-muscle research participants the chance to learn.

Last but not least the acknowledgements. Thanks to all the speakers for travelling to Aberdeen! Thanks to Rosa, Claire, Vanessa, Roby, Abdalla and Raphael for organising the day which involved advertisment, communication, catering, transport, accommodation and so on. You have done a fantastic job!

VDM, AM, HW

Scottish Hippo workshops

The annual Scottish Hippo Workshop

The Hippo pathway is a recently identified, ubiquitously expressed signal transduction pathway which has major functions during development, in stem cells, organ size regulation and cancer. In Scotland, groups at the Universities of St. Andrews, Edinburgh and Aberdeen have started to work on this pathway. To foster collaboration we have invited each other for talks (Dr Marius Sudol and Prof Junichi Sadoshima gave talks in Scotland) and we have started to organize Scottish Hippo workshops where we combine Hippo talks with a joint meal afterwards. The aim of this blog entry is to briefly introduce the Scottish Hippo scene and the Scottish Hippo workshops.

First Scottish Hippo workshop, University of Aberdeen, 5.12.2012
The keynote speaker during the first workshop was Dr. Paul Reynolds from the University of St. Andrews and his talk was entitled ‘Adventures upstream with Hippos: investigating the Hippo pathway in mammals upstream of the core kinase cassette’. It featured the role of Willin/Frmd6 and Kibra upstream in the Hippo pathway which are major contributions by the St. Andrews team to the pathway architecture and the role of the Hippo pathway in breast cancer (Figure 1).

Figure 1 Paul Reynolds speaking at the Institute of Medical Sciences, Aberdeen December 2012.

Second Scottish Hippo Workshop, University of St Andrews, 13.12.2013
The second Scottish Hippo Workshop was held at the University of St Andrews and organised by Dr. Paul Reynolds. The conference was kick started by Professor Frank Gunn-Moore, where he spoke about the involvement of the Hippo Pathway in Neuroscience. The conference finished with a wrap up by Dr Henning Wackerhage from the University of Aberdeen who gave a talk entitled ‘When Hippo met MCAT: How Hippo links to stem cells, cancer and dystrophy in skeletal muscle’. Dr. Anke Roelofs gave a talk on Hippo in cartilage followed by shorter PhD student talks.

Figure 2 Frank Gunn-Moore giving his talk on Willin/FRMD6 and the peripheral nervous system

Figure 3 Henning Wackerhage talking on ‘When Hippo met MCAT. How Hippo links to stem cells, cancer and dystrophy in skeletal muscle’.

So if you are interested in joining the Scottish Hippo scene then please e-mail Henning (h.wackerhage@abdn.ac.uk) and we will keep you in the loop.

Vanessa De Mello (PhD student at Aberdeen with an interest in science journalism)

Henning Wackerhage