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SCHEDULE
8:00 a.m.
Registration & Continental breakfast
8:45 a.m.
Welcome:
William Linton
Carl E. Gulbrandson
Clive N. Svendsen (Moderator)
9:00 a.m. Allan
Spradling: Regulation of Stems Cells
by Chromatin and Competition
9:40 a.m.
Judith Kimble:
Molecular Controls of
Germline Stem Cells
in C. elegans
10:20-10:40 a.m. BREAK
10:40 a.m.
Janet Rossant:
Stem Cells
from the Mammalian
Blastocyst - Not All Stem Cells Are Alike
11:20 a.m.
Rick Young:
Transcriptional Regulatory Circuitry of
Human Embryonic Stem Cells
Noon –
1:00 p.m. LUNCH
1:00 p.m.
Alejandro Sánchez Alvarado:
Stem Cells and
Regeneration in the Planarian Schmidtea mediterranea
1:40 p.m.
Ken Poss:
Mechanisms Guiding
Organ
Regeneration in Zebrafish
2:20-2:40 p.m. BREAK
2:40 p.m.
Sean Morrison: Stem Cells and Cancer
3:20 p.m.
James A. Thomson:
Exiting the Pluripotent State
and Back Again
4:00-5:00 p.m. RECEPTION
Reception Sponsor: Quarles & Brady, LLP
ABSTRACTS
(in order of presentation):
Regulation of stem cells by chromatin
and competition
Todd Nystul, Michael Buszczak, Lucy
Morris, Don Fox and Allan Spradling, HHMI/Embryology, Carnegie
Institution, Baltimore MD 21218 USA
Adult stem cells hold a special status as relatively
undifferentiated, long-term tissue progenitors that can undergo
asymmetric, self-renewing divisions. We have identified a novel
Drosophila gene, scrawny, that encodes an H2B ubiquitin-specific
protease. Scrawny mutant animals prematurely lose many
kinds of stem cells, suggesting that H2B deubiquitination is a
widespread mechanism for suppressing premature stem cell
differentiation. Although stem cells maintain tissue structure
over the adult lifetime, many individual Drosophila stem cells
are regularly replaced within their niches by the daughters of
neighboring stem cells. We studied long distance replacement
using the follicle stem cells (FSCs), which are located
laterally in exactly two single-cell niches on opposite sides of
the germarium. FSC daughters frequently migrate laterally
across the width of the germarium where they target the opposite
FSC niche. Cross-migrating daughters usually fail to take up
niche residence, but instead differentiate and contribute to the
follicular epithelium. About 5% of the time, however, they
displace the resident FSC, remain in the niche and function as
active stem cells. Stem cell competition may be a common and
selectively advantageous adaptation that reduces the chance that
deleterious mutations will be maintained in stem cells and the
tissues cells they support. Our studies also show that
mutations can arise that cause mutant stem cells to
preferentially replace wild type stem cells. These
“hyper-replacer” mutations may be able to spread from one niche
to another until they comprise a substantial portion of tissue
and might represent an important new class of precancerous
lesion.
Nystul, T. and Spradling, A.C.
(2007). An epithelial niche in the Drosophila ovary undergoes
long range stem cell replacement. Cell Stem Cell 1, 277-285.
Molecular controls of C. elegans germline stem cells: Wnt
signaling, Notch signaling and an RNA/MAPK regulatory network
Judith Kimble,
Department of Biochemistry, University of Wisconsin-Madison
and HHMI, Madison, Wisconsin 53706.
The nematode C. elegans has proven to be a premier model
for discovery of fundamental regulatory mechanisms that are used
broadly throughout the animal kingdom, including humans. Well
known examples include regulators of cell death and RNAi. Stem
cell controls are no exception. The concept of a “stem cell
niche” was originally derived from studies of vertebrate blood
stem cells, but the first cellular candidate for a stem cell
niche was the C. elegans “distal tip cell”. The
distal tip cell (DTC) is a single mesenchymal cell that is both
necessary and sufficient for maintenance of germline stem cells.
The specification of DTCs relies on Wnt signaling during early
development, and the maintenance of stem cells by DTCs relies on
Notch signaling, both during developmental proliferation and
adult homeostasis. The DTC expresses the LAG-2 DSL ligand, and
germline stem cells (GSC) express the GLP-1 Notch receptor and
transcription factors dedicated to the Notch pathway. When Notch
signaling is eliminated, GSCs are lost; when Notch signaling is
unregulated, GSCs form a germline tumor. We are just beginning
to learn how Notch signaling controls stem cell maintenance.
Indeed, Notch signaling promotes the transcriptional activation
of two MAPK regulators. One such regulator is FBF-2, a PUF
RNA-binding protein that represses MAPK mRNA; the other is
LIP-1, a homolog of MKP/DSP dual specificity phosphatases that
inhibit MAPK activity. In vertebrates, Notch signaling has been
implicated in the control of neural and hematopoietic stem
cells, and MAPK has been implicated in controls of both growth
and differentiation. Our analysis of the regulatory network
controlling C. elegans GSCs is therefore likely to have
important parallels for stem cell controls broadly in the animal
kingdom.
Stem cells from the mammalian blastocyst- not all stem cells are
alike
Janet Rossant, SickKids Research Institute and the Department of
Molecular Genetics, University of Toronto
Three types
of permanent cell lines can be derived from the mouse blastocyst;
embryonic stem (ES) cells, trophoblast stem (TS) cells and
extraembryonic endoderm (XEN) cells. All three express markers
and show properties in chimeras consistent with their origin
from the epiblast, trophectoderm and primitive endoderm lineages
of the blastocyst respectively. Key lineage-specific
transcription factors, such as Oct4/Sox2, Cdx2 and GATA6/Sox7
determine their fate. Altered expression of these factors can
reprogram ES cells to TS or XEN cells. This direct parallel
between lineage specification in the embryo and stem cell
behaviour in vitro allows experimental cross-talk between
cell culture and embryonic development to provide new insights
into stem cell fate. To date, however, it has not proven
possible to derive all three permanent progenitor cell lines
from human blastocysts. Human ES cells seem to be less
lineage-restricted than mouse ES cells, producing some
trophoblast and extraembryonic endoderm upon differentiation
in vitro. We have developed a system to conditionally
express lineage-specific transcription factors in human ES cells
and show that human ES cells respond differently to mouse ES
cells, producing new progenitor cell lines with properties of
postimplantation germ layers rather than extraembryonic cell
types. These studies and other published reports lead us to
propose that there may be be more than one stable pluripotent
phenotype that can be derived from early embryos or germ cells.
Understanding the lineage origin of stem cells is important in
order to understand the starting point for driving their
differentiation into cell types of therapeutic importance.
Transcriptional Regulatory Circuitry of
Human Embryonic Stem Cells
Richard Young, Whitehead Institute and MIT
The capacity
of embryonic stem cells to self-renew and to give rise to
virtually all somatic lineages holds much promise for human
regenerative medicine. Recent studies have shown that somatic
cells can be reprogrammed into an embryonic stem cell-like
state. We are mapping the regulatory circuitry of these cells
by investigating how transcription factors, chromatin
regulators, small RNAs and signaling pathways control the gene
expression programs responsible for self-renewal and
pluripotency. We are also investigating how genome expression is
reprogrammed to produce new cell states. New insights into
global control mechanisms will be discussed.
Stem
Cells and Regeneration in the Planarian
Schmidtea mediterranea
Alejandro Sánchez Alvarado, Ph.D.
Howard Hughes Medical Institute, Dept. of Neurobiology & Anatomy
University of Utah School of Medicine
The problem of regeneration is fundamentally a problem of tissue
homeostasis that involves either the replacement of cells due to
normal “wear and tear” (cell turnover), or the replacement of
cells after injury. This is particularly significant for
organisms possessing relatively long life spans, in which
maintenance of all body parts and their functional integration
is required for many years in order for the individual to
thrive. Replacement of differentiated cells, therefore, is a
major challenge all multicellular organisms must face. Humans,
for example, with an average life span of 80 years must replace
billions of cells lost to cell turnover every day. Despite the
importance of tissue homeostatic processes to human biology and
health, relatively little is known about how adult tissue
homeostasis is controlled. Gaining mechanistic insight on these
problems requires the identification of a model organism in
which these issues can be easily dissected and rapidly
understood. Key molecular insights can be obtained by studying
simpler animals since cellular differentiation events are known
to be ancient evolutionary inventions and tissue replacement is
broadly distributed among multicellular life forms. Planarians
provide a unique and experimentally tractable system for
studying such homeostatic, regenerative processes: all tissues
are regulated in the adult, and tissue turnover is robust and
rapid (as little as 7-10 days).
Here, I
will discuss how the study of a simple metazoan, the planarian
Schmidtea mediterranea, is beginning to shed light on the
way adult animals regulate tissue homeostasis and the
replacement of body parts lost to injury.
Mechanisms guiding organ regeneration in zebrafish
Kenneth D. Poss, Ph.D. (Assistant Professor, Cell Biology, Duke
University)
Certain non-mammalian vertebrates like urodele amphibians and
teleost fish regenerate complex tissues much more effectively
than mammals, creating tantalizing examples of successful organ
regeneration. Zebrafish are extraordinarily regenerative,
equipped to renew amputated fins, injured retinae, a severed
spinal cord, and lost cardiac muscle; plus, they are amenable to
both forward and reverse genetic approaches. Current goals in
the field are to uncover the responsible cellular mechanisms,
and to develop and use tools for high-resolution interrogation
of regenerative events at the molecular level. Zebrafish heart
regeneration is particularly interesting, since the mammalian
heart shows little or no regeneration after injury. In our
analysis of zebrafish heart regeneration, we have found evidence
that undifferentiated progenitor cells help build new muscle
after mechanical injury, and that this activity is facilitated
by dynamic responses of the epicardial cell layer surrounding
the heart. We are continuing to pursue the cellular origin of
regenerated muscle, and to identify molecules synthesized in the
epicardium and other cardiac cell types that influence the
success of regeneration. Through these investigations, new
insights into heart regeneration will be revealed that have the
potential to impact regenerative medicine.
The loss of Nf1 transiently promotes self-renewal but not
tumorigenesis by neural crest stem cells
Nancy M. Joseph, Jack T. Mosher, and Sean
J. Morrison
Center for Stem Cell Biology, Howard Hughes Medical Institute,
and Life Sciences Institute, University of Michigan,
Ann Arbor, Michigan, 48109-2216
Cancers are
often proposed to arise from stem cells that have been
transformed by mutatations that inappropriately activate
self-renewal mechanisms. In the peripheral nervous system (PNS),
a congenital disorder called neurofibromatosis type I is caused
by the loss of the neurofibromin (Nf1) tumor suppressor,
leading to the formation of PNS tumors including neurofibromas
and malignant periheral nerve sheath tumors (MPNSTs). A
long-standing question has been whether these tumors arise from
neural crest stem cells (NCSCs) or differential glia. Germline
or conditional Nf1 deficiency caused a transient increase
in NCSC frequency and self-renewal in most regions of the fetal
PNS. However, Nf1-deficient NCSCs did not persist
postnatally in regions of the PNS that developed tumors, and
could not form tumors upon transplantation into adult nerves.
Adult
P0a-Cre+Nf1fl/-
mice developed neurofibromas, and Nf1+/-Ink4a/Arf-/-
and Nf1/p53+/- mice developed MPNSTs, but
NCSCs did not persist postnatally in affected locations in these
mice. Instead, MPNSTs and plexiform neurofibromas appeared to
arise from differentiated glia that began proliferating
inappropriately postnatally or during adulthood. Cancer and
benign tumors in the PNS can therefore arise from differentiated
glia.
James A. Thomson, Ph.D. [to be announced]
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