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26 | PhD Summer selection 15
SASHA MENDJAN
Stem Cells and Human Mesodermal
Organogenesis
[email protected]
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Key organs and tissues like the heart, kidney, fat, blood, musculoskeletal and vascular
systems are derived from the embryonic germ layer called mesoderm (Figure 1). The
fascinating developmental journey from early mesodermal precursors to functioning
organs is not well understood, especially not for human development and developmental
disorders.
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Stem cells have revolutionised developmental biology and opened new avenues
to regenerative medicine. The extraordinary qualities of pluripotent stem cells
include self-renewal in vitro, amenability to genetic modifications and the potential
to differentiate into many specialised cell types and tissues. This makes them
a powerful tool to i) study the molecular control of human development and
disease, ii) discover and test drugs, and iii) use in transplantation therapy (Figure 2).
Our group is using stem cells to decipher molecular mechanisms that control
human mesodermal organogenesis and pathogenesis. Based on these mechanistic
insights, we are developing novel approaches to generate and study functional
mesodermal tissues and organ-like structures (organoids) in a dish.
We are particularly interested in the following questions:
1. How do gene regulatory networks control cardiac, lateral and somitic
mesoderm specification and pathogenesis?
Human mesodermal congenital disorders and other pathologies arise early in
development due to mutations and deregulation of key genes. These factors
operate in gene regulatory networks that control mesoderm induction, patterning and specification of mesodermal subtypes (e.g. cardiac, lateral plate
and somitic) into distinct progenitors and tissues. We can mimic aspects of
mesoderm development in vitro and differentiate human pluripotent stem cells
into specialised mesodermal cell types like cardiomyocytes and adipocytes by
using defined signals and specific timing (Figure 3).
Our aim is to explore the signalling, epigenetic and transcription factor networks
in cardiac mesoderm that distinguish atrial, ventricular and pacemaker cardiomyocytes. Analogously, we are interested in the lateral and somitic mesoderm
networks that specify white and brown adipocytes. We use powerful genetic,
imaging, proteomic and computational methods to elucidate how specification
works and when does it go wrong in human heart disease and in dysfunctional
fat cells. Finding answers to some of these fundamental questions will have huge
implications for our understanding of human development and for regenerative
medicine.
PhD Summer selection 15 | 27
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Figure 1:
The Mesoderm germ layer gives rise to key organs and tissues.
Figure 2: The potential of human pluripotent stem cells in basic and translational research
Figure 3: The induction of mesoderm with different signalling gradients results in patterning into major mesodermal subtypes. These restricted precursors further
specify into more specialised mesoderm-derived cell types.
Figure 4: i):The derivation (upper left) of gut organoids from pluripotent stem cells (PSC); ii) and iii) combining PSC-derived mesodermal progenitors that are
fluorescently labelled. Illustration of the signalling crosstalk resulting in fluorescent reporter gene activation between assembled tissues.
2. How do tissues interact in human cardiac and adipogenic
organoids?
In the vertebrate embryo, signalling and interactions between
different progenitor tissues drive organogenesis. For instance,
the heart is developing from several interacting lateral plate
mesoderm tissues while growing fat depots depend on interactions between (pre)adipocytes and vascular cells. This
regulatory crosstalk of different cell types has been implicated
in the derivation, self-organisation and growth of organ-like
structures (e.g. gut organoids) in vitro. We therefore hypothesise
that inter-tissue gene networks that control organ formation
will drive the derivation of cardiac and adipogenic organoids
(Figure 4).
Our goal is to assemble a simple human heart model by combining distinct lateral plate mesoderm precursors (myocardial,
endocardial and epicardial), as they would interact during early
cardiac development. Similarly, we aim to mingle adipocyte and
vascular progenitors to derive fat organoids. Our approach is
to combine cell layers expressing fluorescent reporters of key
marker genes allowing the identification, physical separation
and molecular analysis of interacting tissues (Figure 4). The
integration of imaging, phosphoproteomic, metabolomic,
epigenetic and gene expression data will reveal how an organ forms, grows and matures in a dish. Expandable human
mesodermal organoids will be invaluable for effective drug
discovery and disease modelling.
FURTHER READING
Mendjan S, Mascetti VL, Ortmann D, Ortiz M, Karjosukarso D, Ng Y,
Moreau T & Pedersen RA. 2014 Sep 4;15(3):310-25. Epub 2014 Jul 18.
Cell Stem Cell. NANOG and CDX2 Pattern Distinct Subtypes of Human
Mesoderm during Exit from Pluripotency
Pedersen RA, Mascetti VL, Mendjan S. Cell Stem Cell. 2012 Jun
14;10(6):646-7. Synthetic organs for regenerative medicine
Alessandro Bertero, Pedro Madrigal, Antonella Galli, Nina C. Hubner,
Deborah Burks, Roger Pedersen, Daniel Gaffney, Mendjan S*, Siim
Pauklin*, and Ludovic Vallier*. 2015 Apr 1;29(7):702-17, Genes &
Development. Activin/Nodal signaling and NANOG orchestrate
human embryonic stem cell fate decisions by controlling the H3K4me3
chromatin mark.
Sasai Y. Cell Stem Cell. 2013 May 2;12(5):520-30. Next-generation
regenerative medicine: organogenesis from stem cells in 3D culture.
Arnold SJ1, Robertson EJ. Nat Rev Mol Cell Biol. 2009 Feb;10(2):91-103.
Making a commitment: cell lineage allocation and axis patterning in
the early mouse embryo.
PHD SUMMER SELEC TION 15