de-adheres from the adherens junction. Movies of the process show
that to de-adhere, the cells stretch, mechanically, pulling away from
the adherens junction before that junctional connection is lost.
Release from the adherens junction occurs late in the EMT, and
as soon as the cell “tail” is released, the membrane containing
cadherin is endocytosed while membrane bearing mesenchymal
determinants are inserted via exocytosis. The cell, now in the
interstitial spaces, rapidly demonstrates a mesenchymal phenotype
and expresses very different cell surface markers relative to its earlier
state as an epithelial cell. The big questions then are to ask how all
of this is controlled, how does each of the cell biological events
work, and how they are so nicely coordinated with one another.
The approach of my lab over the years has changed with the
technologies available but with each iteration, the question has
always been: How does it work?
Our first observation was made 35 years ago. We had begun a
study of sea urchin skeletogenic cells which can be seen to undergo
an EMT beginning about 9 h after fertilization. The sea urchin
embryo is transparent, and the skeletogenic cells are the first cells to
engage in gastrulation movements so the EMT is easy to see.
Methods had been developed to separate the skeletogenic cells
from the other cells of the embryo, and we noticed that at 9 h
post fertilization the skeletogenic precursor cells, now in culture,
began to move. They changed shape and began to crawl on the
substrate. In other words, they behaved as if they were still part of
the embryo and they began those movements autonomously, at the
same time as skeletogenic cells in vivo started EMT. We developed a
quantitative adhesion assay using a centrifuge to measure the force
needed to remove the cells from a substrate. Rachel Fink, now a
professor at Mt. Holyoke, and I observed that during the 45 min
period of the EMT the skeletogenic cells lost their affinity for other
cells and gained an affinity for extracellular matrix [1]. In other
words, the EMT was accompanied by a dramatic and quantifiable
adhesion change.
The next obvious question was to ask what molecules were
responsible for those adhesion changes? At the time the field was
busy identifying cell-cell and cell substrate adhesion molecules.
Masatoshi Takeichi had discovered cadherins [2], and Richard
Hynes, Clayton Buck, and others had identified integrins [3, 4],
so our efforts shifted to identifying as many of those molecules as
possible. The field also was embracing molecular biology so all of
my graduate students from that time onwards became familiar with
molecular technologies. We identified cadherins, integrins, a number of basement membrane proteins and put each of them to the
test in an effort to learn which molecules participated in the EMT
change. Those efforts were productive and showed that as the
skeletogenic cells went through EMT they lost an adhesive affinity
8
David R. McClay
that to de-adhere, the cells stretch, mechanically, pulling away from
the adherens junction before that junctional connection is lost.
Release from the adherens junction occurs late in the EMT, and
as soon as the cell “tail” is released, the membrane containing
cadherin is endocytosed while membrane bearing mesenchymal
determinants are inserted via exocytosis. The cell, now in the
interstitial spaces, rapidly demonstrates a mesenchymal phenotype
and expresses very different cell surface markers relative to its earlier
state as an epithelial cell. The big questions then are to ask how all
of this is controlled, how does each of the cell biological events
work, and how they are so nicely coordinated with one another.
The approach of my lab over the years has changed with the
technologies available but with each iteration, the question has
always been: How does it work?
Our first observation was made 35 years ago. We had begun a
study of sea urchin skeletogenic cells which can be seen to undergo
an EMT beginning about 9 h after fertilization. The sea urchin
embryo is transparent, and the skeletogenic cells are the first cells to
engage in gastrulation movements so the EMT is easy to see.
Methods had been developed to separate the skeletogenic cells
from the other cells of the embryo, and we noticed that at 9 h
post fertilization the skeletogenic precursor cells, now in culture,
began to move. They changed shape and began to crawl on the
substrate. In other words, they behaved as if they were still part of
the embryo and they began those movements autonomously, at the
same time as skeletogenic cells in vivo started EMT. We developed a
quantitative adhesion assay using a centrifuge to measure the force
needed to remove the cells from a substrate. Rachel Fink, now a
professor at Mt. Holyoke, and I observed that during the 45 min
period of the EMT the skeletogenic cells lost their affinity for other
cells and gained an affinity for extracellular matrix [1]. In other
words, the EMT was accompanied by a dramatic and quantifiable
adhesion change.
The next obvious question was to ask what molecules were
responsible for those adhesion changes? At the time the field was
busy identifying cell-cell and cell substrate adhesion molecules.
Masatoshi Takeichi had discovered cadherins [2], and Richard
Hynes, Clayton Buck, and others had identified integrins [3, 4],
so our efforts shifted to identifying as many of those molecules as
possible. The field also was embracing molecular biology so all of
my graduate students from that time onwards became familiar with
molecular technologies. We identified cadherins, integrins, a number of basement membrane proteins and put each of them to the
test in an effort to learn which molecules participated in the EMT
change. Those efforts were productive and showed that as the
skeletogenic cells went through EMT they lost an adhesive affinity
8
David R. McClay