Beyond stemness?
The view that stem cells are not fixed in their proliferative
behaviors reinforces suggestions by others - mostly
motivated by the problems that arise when trying to define
stem cells in terms of potency - that the term stem cell
properly applies only to ‘condition’, not ‘character’ [18,19].
If this is really the case, what does it say about the many
attempts over the years to define stemness in terms of
molecular properties?
Above all, it suggests that stemness is a property of
systems, rather than cells, with the relevant system being,
at minimum, a cell lineage, and more likely a lineage plus
an environment. A system with stemness is typically one
that can achieve a controlled size, maintain itself homeostatically,
and regenerate when necessary. Moreover, it
most probably does so by exploiting basic principles of
feedback control.
If stemness is a system-level property, then the concept of
stem cell is really fundamentally different from that of, say,
gene. A more similar concept might be something like
‘rate-limiting enzyme’, which also defines a class of
tangible, physical objects, but only does so in terms of their
functional roles within a system, not their intrinsic
biochemical properties.
The assertion that the stem cell concept cannot be reduced
to the molecular properties of individual cells is more than
just an esoteric philosophical stance; it has important
practical ramifications. For one thing, it suggests that the
kind of molecular understanding that researchers most
urgently need to pursue is of basic cellular phenomena
that, while not unique to stem cells, are critical for stem
cell function: for example, the ability of daughter cells to
take on fates different from their mothers; the ability of
sister cells to take on fates different from each other; and
the ability of external cues to regulate both of these
properties.
For another, the observation that stem cell behaviors can
emerge as a consequence of feedback control calls attention
to the fact that stem cell systems are, fundamentally,
dynamical systems. Their behaviors can be complex and
counterintuitive, yet ultimately still understandable,
especially with the help of modeling or simulation. Given
that lineage relationships and feedback configurations can
be far more elaborate than those shown in Figure 1,
concerted efforts are needed to elucidate the classes of
dynamical behaviors of which stem cell systems are
capable. For example, in the case of cancers that are stem
cell driven, it is not clear that we actually have grounds to
assume that the specific chemotherapeutic targeting of
cancer stem cells will necessarily stop tumors in their
tracks. Indeed, if feedback and lineage progression continue
to take place in cancerous tissues, we might observe
that, under different conditions - different stages of
tumorigensis, different parts of a tumor, different amounts
of tumor cells - different cell types will assume the role of
'cancer stem cells'. The therapeutic implications of this
possibility are clearly substantial.
In summary, it would seem that the concept of stem cell
indeed has the potential to hold us back - especially if we
focus on demanding from it things it cannot give. But if we
can re-fashion our thinking at a different level - in which
systems relationships and dynamics take the place of
molecular signatures and simple gene regulatory circuits then
there is a chance that the concept of stem cell will
continue to light the path toward biological understanding.