Interview with Prof. Mike Rennie

Prof. Mike J. Rennie

Q Question. A Answer

Q. Mike, you had retired at the Postgraduate Medical School in Derby. What are you doing now? Are you still actively involved in research projects?

A. I hated being forced to retire – I was a month too old in August 2011 to benefit from new UK Gov regulations which would have forced the admin myrmidons to allow me to work for another two years.

It took me a while to adapt - not a good time. I keep busy by doing ad hoc jobs as a consultant to a number of universities (recently about the REF). I never get asked to meetings or to act as an external examiner and I get few requests to review grants or papers, though they do dribble along.

I also teach for the Open University – two second level 8 month courses, one on practical science in modern biology and another on sports conditioning and professional development. All on-line using tremendous presentation and conferencing tools.

I am a member of a DoH expert committee on mutagenesis in food and consumer products (as a lay member).

I continue to edit two journals on intensive care: the British Journal of Intensive Care and International Journal of Intensive Care. I also do freelance journalism and hope to increase my training/consultant activities as a grant doctor and publication (abstracts, papers, presentations) tutor for postgrad students.

My ex-colleagues are still getting round to producing paper drafts of work now completed, which we started a few years ago. I still can make some input here but only in correcting the drafts. Probably a few more yet to appear such as one where we explore the importance of microcirculation and muscle anabolism.

On a personal basis , I’ve recently made efforts to adopt a healthier lifestyle, and I have a wide range of cultural interests (and four grandchildren) which keep me sane.

Q. Looking back, what is your most important paper that you have published and why do you rank it your as your most important paper?

A. Oooh! Very hard to say. I was very proud of the early J Physiol paper on metabolic and hormonal changes during exercise which was undercited but rather novel in its scope.

The work I did with John Holloszy at WUMS St Louis on fat versus CHO (Biochem J, AJP) was highly cited but some found the results controversial.

I was very proud to be associated with influential work on stable isotope tracing of AA metabolism and muscle protein turnover in people: at rest with and without feeding, and the effects of exercise and Duchenne muscular dystrophy. These were published in FASEB J, Nature and Science. Great colleagues and good fortune helped tremendously.

This started an avalanche of work by others in this area and I produced quite a few papers showing that in many slow wasting conditions (including aging) depressed muscle protein synthesis was the major driver of wasting. This has continued to annoy many pacman enthusiasts... Of course its different in fast wasting.

The work I did with some great graduate students and postdocs in Dundee on muscle AA transport was interesting but turned out to be negative in demonstrating that transport is very unlikely to be a process regulating muscle anabolism (or catabolism) or fuel use.

Recent papers on insulin action and effects of PUFA were very satisfying.

The concept of anabolic resistance has been well received and so has the work on dose response and time dependency of anabolism after feeding are highlights of the last period when I felt my enthusiasm and insights were at a height. Sadly curtailed by crappy administrators.

Q. In the field of human skeletal muscle research, what is the most important paper where you were not involved and why do you rank it as the most important one?

A. I can’t think of single papers but the increase of the understanding of satellite cells will turn out to have been very important, as will the work just starting by others on epigenetic effects of training.

Q. Correct me if I am wrong but you frequently highlight (and data support that of course) that anabolic/mTOR signalling and protein synthesis do not always/rarely match in human muscle. Yet, rapamycin blocks amino acid and resistance exercise-induced increases in protein synthesis in human muscle and something must connect anabolic stimuli to protein synthesis. What is your current take on all of this?

A. Biology is full of examples of redundancy of pathways and also examples in which existence of a pathway does not mean that the pathway is important in regulation. One problem of the use of westerns and mRNA blots is that their interpretation ignores the fact that these are Vmax measures and not point of time activitity.

Q. Your have done a lot of research on sarcopenia and other forms of atrophy. Based on this what is your current practical recommendation for people with sarcopenia?

A. Choose your parents!

Q. Do you expect truly novel treatments to emerge in the next 20 years and if you had a guess what was the nature of these treatments?

A. There have been lots of false starts. I will be very interested to see if antibody based inhibition of catabolic proteins or receptors have any use. I doubt they will in ageing but might for cancer and critical care though the indications are equivocal. Drugs like SARMs look promising. HMB turns out to be interesting. I was sceptical for years but Phil Atherton has nailed its effects in stimulating MPS. And PUFA effects: fascinating.... anti-inflammatory or what? Puzzling.

Q. Do you have top tips for young researchers who wish to become active in the area of human skeletal muscle research?

A. First choose a good smart mentor who is interested in bright young people, who publishes prolifically, who has a well set up lab, and who gets grants to support novelty. I had two: John Holloszy and Richard Edwards. Be open to advice from clever colleagues at home and abroad and collaborate widely. Don’t follow fashion but use it to gain insights on stuff to avoid; anybody can think up an obvious hypothesis but then think “if that is true, then what...?”

The Hippo pathway in skeletal muscle: a self-interview

This blog entry is work in progress and I will correct or update it once I note errors or find more relevant information.  Q question, A answer.

Q What is the Hippo pathway?

A The Hippo pathway is a near-ubiquitously expressed signal transduction pathway which potently regulates the function and identity of embryonic and adult stem cells (Tremblay & Camargo, 2012), organ size (Pan, 2007), specific functions in adult organs and its dysregulation causes cancer (Harvey et al., 2013).

Q Why is it called ‘Hippo pathway’?

A Apparently the leading researchers in the field phoned each other and agreed on the suggestion by Georg Halder to name the pathway ‘Hippo pathway’ based on the name of one of the Hippo pathway kinases in the fly (information picked up at the bar during the second Hippo workshop in Rome...).

Q How does it work?

A The Hippo pathway is a mushrooming network of signalling proteins which can be sub-divided into the canonical Hippo pathway (Mst1/2 and Lats1/2 are the two canonical kinases), and non-canonical Hippo signalling mechanisms such as mechanotransduction which was shown by Stefano Piccolo's group (Dupont et al., 2011) and signalling via G protein-coupled receptors which was linked to the Hippo pathway by Kun-Liang Guan's team (Yu et al., 2012). Canonical and non-canonical Hippo signalling together regulate the activity of the transcriptional co-factors Yap (Huang et al., 2005;Sudol, 1994;Sudol et al., 1995) and Taz (Kanai et al., 2000) (Vglls are related factors). Yap and Taz can co-activate several transcriptional co-factors of which the Tead transcription factors are the most important ones (Zhao et al., 2008). Tead transcription factors bind to muscle C, A and T (Mar & Ordahl, 1988;Yoshida, 2008) CATTCC DNA motifs. Note that the MCAT elements are identical to GTIIC (Davidson et al., 1988) and Hippo response elements (Wu et al., 2008). Also sometimes the sequence of the other strand is given and no, this is not a plot to confuse non-Hippo reearchers. 

Figure 1. Hippo pathway. In the canonical Hippo pathway the kinases Mst1/2 and Lats1/2 inhibit the transcriptional co-factors Yap and Taz especially by phosphorylation of Ser127 and Ser89, respectively. Yap and Taz co-activate Tead1-4 and other transcription factors. Tead transcription factors regulate gene expression by binding to so-called muscle C, A, and T (MCAT) motifs which have a CATTCC DNA sequence motif. Such MCAT motifs are found in the promoters of many regulatory and functional skeletal muscle genes. Yap and Taz can also be regulated by a plethora of other mechanisms including mechanosensors (exericse physiologists should listen up) and agents such as adrenaline (again this should interest exercise physiologists) which signal via G protein-coupled receptors. This is termed the non-canonical Hippo pathway.

Q If it is so important, why is it only studied now?

A Good question, next question. Also the question is incorrect as the elements of the Hippo pathway were studied since the late 1980s but the real importance for mammals became only apparent when it was shown by Duojia Pan, Fernando Camargo and Rudolf Jaenisch that an over expression of constitutively active Yap increased liver size by ≈4-fold (Dong et al., 2007;Camargo et al., 2007). Currently Hippo pathway research is expanding at a great rate as it seems to have major roles in almost all organs.

Q How was it discovered?

A The discovery of the Hippo pathway can be traced back to two strands of research.

Strand 1: The major discoveries were made by searching for tumour suppressor genes in the fly (Harvey & Tapon, 2007). The knockout of such tumour suppressor genes by Nic Tapon, Kieran Harvey, Iswar Hariharan, Georg Halder, Duojia Pan and others led to organ overgrowth due to increased proliferation and reduced apoptosis. This way the canonical Hippo pathway was discovered. Later it was shown that the canonical Hippo pathway worked by suppressing the activity of the transcriptional co-factors Yap and Taz (Huang et al., 2005). The final link to transcription was made by showing that Yap and Taz co-activate mainly Tead transcription factors (Zhao et al., 2008).

Strand 2: Several important discoveries were made by muscle-focussed researchers who found already in 1988 that the Hippo pathway-targeted MCAT motif was required for the expression of cardiac troponin T in skeletal muscle (Mar & Ordahl, 1988). Janet Mar, Charles Ordahl, Iain Farrance, Irwin Davidson, Alexandre Stewart, Thomas Braun and their groups then studied the regulation of these MCAT elements and characterised Teads, Vglls but not Yap and Taz which are the most potent Hippo  downstream pathway members. Yap was first characterised by Marius Sudol (Sudol, 1994;Sudol et al., 1995). This shows that muscle was on the Hippo radar even before the Hippo pathway was recognised and named as such.

Q What is the function of the Hippo pathway in skeletal muscle?

A We have shown that all major elements of the Hippo pathway are expressed in skeletal muscle (Watt et al., 2010). Moreover, genes with functionally important MCAT elements include α-actin, type I myosin heavy chain and several other genes that regulate muscle development or myogenesis (Yoshida, 2008). We found that Yap is generally active in activated satellite cells (Judson et al., 2012) and myoblasts (Watt et al., 2010) (proliferating muscle cells) and becomes deactivated by transcriptional down-regulation and increased inhibitory Ser127 phosphorylation when these cells fuse to differentiate into muscle fibres. We also found that the expression of activated Yap drives the proliferation but inhibits the differentiation of myoblasts and activated satellite cells which is consistent with the function of Yap in other organs (Judson et al., 2012). Thus active Yap increases satellite cell numbers.

When over expressing Yap in adult muscle (not including satellite cells) then the muscle deteriorates, muscle fibres become nectrotic and some fibres resemble fibres found in patients with centronuclear dystrophy (Judson et al., 2013). This is a puzzling finding but suggests to us that Yap does not co-activate MCAT elements in genes such as α-actin and type I myosin heavy chain which are required for the normal function of differentiated skeletal muscle. There are caveats though as Yap may only be activated briefly in differentiated muscle.

Another study suggests that Tead1 over expression in adult muscle leads to a partial fast-to-slow fibre type shift (Tsika et al., 2008). However, a careful interpretation of this finding is needed as Tead transcription factors without co-factor activation generally repress genes (Koontz et al., 2013).

Q. Phew, that was a lot. Can you just summarise?

A. Of course. The Hippo pathway comprises the canonical Mst1/2-Lats1/2 kinases and the non-canonical pathway includes mechanotransduction and signalling via G-protein coupled receptors. The Hippo pathway is expressed in skeletal muscle and has incompletely understood functions in satellite cells and differentiated muscle fibres. In satellite cells Yap becomes activated during satellite cell activation and when active it increases satellite cell numbers by promoting their proliferation.

The role in differentiated muscle is incompletely understood. Key differentiated muscle genes such as α-actin and type I myosin heavy chain have MCAT elements in their regulatory regions and should be responsive to Hippo signalling. However, our results suggest that high Yap activity is detrimental which does not exclude that short periods of Yap activation maydrive the expression of key genes in differentiated muscle. Thus it is incompletely understood how MCAT motifs in key muscle genes are targeted and whether the Hippo pathwa is responsive to exercise or nutrition or other stimuli. Still plenty of work to do!

Q. Is there a review on the Hippo pathway in skeletal muscle?

A. We are currently writing one. I’ll keep you posted.  

Reference list

Camargo FD, Gokhale S, Johnnidis JB, Fu D, Bell GW, Jaenisch R, & Brummelkamp TR (2007). YAP1 increases organ size and expands undifferentiated progenitor cells. Curr Biol 17, 2054-2060.

Davidson I, Xiao JH, Rosales R, Staub A, & Chambon P (1988). The HeLa cell protein TEF-1 binds specifically and cooperatively to two SV40 enhancer motifs of unrelated sequence. Cell 54, 931-942.

Dong J, Feldmann G, Huang J, Wu S, Zhang N, Comerford SA, Gayyed MF, Anders RA, Maitra A, & Pan D (2007). Elucidation of a universal size-control mechanism in Drosophila and mammals. Cell 130, 1120-1133.

Dupont S, Morsut L, Aragona M, Enzo E, Giulitti S, Cordenonsi M, Zanconato F, Le DJ, Forcato M, Bicciato S, Elvassore N, & Piccolo S (2011). Role of YAP/TAZ in mechanotransduction. Nature 474, 179-183.

Harvey K & Tapon N (2007). The Salvador-Warts-Hippo pathway - an emerging tumour-suppressor network. Nat Rev Cancer 7, 182-191.

Harvey KF, Zhang X, & Thomas DM (2013). The Hippo pathway and human cancer. Nat Rev Cancer 13, 246-257.

Huang J, Wu S, Barrera J, Matthews K, & Pan D (2005). The Hippo signaling pathway coordinately regulates cell proliferation and apoptosis by inactivating Yorkie, the Drosophila Homolog of YAP. Cell 122, 421-434.

Judson RN, Gray SR, Walker C, Carroll AM, Itzstein C, Lionikas A, Zammit PS, De BC, & Wackerhage H (2013). Constitutive expression of yes-associated protein (yap) in adult skeletal muscle fibres induces muscle atrophy and myopathy. PLoS ONE 8, e59622.

Judson RN, Tremblay AM, Knopp P, White RB, Urcia R, De BC, Zammit PS, Camargo FD, & Wackerhage H (2012). The Hippo pathway member Yap plays a key role in influencing fate decisions in muscle satellite cells. J Cell Sci 125, 6009-6019.

Kanai F, Marignani PA, Sarbassova D, Yagi R, Hall RA, Donowitz M, Hisaminato A, Fujiwara T, Ito Y, Cantley LC, & Yaffe MB (2000). TAZ: a novel transcriptional co-activator regulated by interactions with 14-3-3 and PDZ domain proteins. EMBO J 19, 6778-6791.

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Tremblay AM & Camargo FD (2012). Hippo signaling in mammalian stem cells. Semin Cell Dev Biol 23, 818-826.

Tsika RW, Schramm C, Simmer G, Fitzsimons DP, Moss RL, & Ji J (2008). Overexpression of TEAD-1 in Transgenic Mouse Striated Muscles Produces a Slower Skeletal Muscle Contractile Phenotype. J Biol Chem 283, 36154-36167.

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