Biology Reference
In-Depth Information
receptor accumulation with cell polarization does not discriminate cause from consequence.
In the keratinocyte experiments described above, cathodal accumulation of EGF receptors
does not take place if the activity of those receptors is blocked by PD158780. This small mol-
ecule would not be predicted to have a dramatic effect on the charge or electrophoretic
mobility of EGF receptors, so its ability to block their cathodal clustering suggests that
EGF receptor activity is essential for clustering to take place. This may be because activation
of EGF receptors causes them to move from bulk membrane to lipid rafts,
16
which may alter
their mobility, or it may be because the movement of the receptors is a
response
to the setting
up of a cathodally directed internal migration orientation system rather than a cause of it. It
does not depend on polarization of the actin cytoskeleton, though, because active EGF recep-
tors and the Erk MAP kinase through which they signal polarize in electric fields even when
actin polymerization is inhibited.
11
In experiments where the charge of cell surface glycopro-
teins was reversed using fairly non-specific chemical conjugation of surface proteins to biotin
followed by coupling to avidin
d
a treatment that would affect a large range of proteins
d
the
proteins clustered at the opposite end of the cell in an electric field compared with their
normal cathode-ward clustering; yet the crawl direction was still towards the cathode.
17
This again suggests that electrophoretic clustering of proteins towards one end of the cell
is not itself a driver of directional migration; although, in this experiment, it was always
possible that the electrophoretic migration of an unidentified but critically important key
protein was unaffected by the chemical treatment.
Myxamoebae of
Dictostelium discoideum
will also undergo galvanotaxis, but attempts to
analyse this genetically argue strongly that galvanotaxis does not operate simply by modu-
lating the chemotactic systems of the cell. Null mutants in the cAMP receptor, the G
a
protein
and the G
protein, all of which are essential for chemotaxis up-gradients of cAMP, still
undergo galvanotaxis although its efficiency is reduced in the G
b
mutants. The basic
machinery of locomotion, such as protrusive actin polymerization at the leading edge, is
apparently identical in myxamoebae migrating by chemotaxis and galvanotaxis. Overall,
these data suggest that electric fields are sensed by a mechanism quite distinct from that
used for chemotaxis, and that the pathways converge on the locomotory machinery of the
cell. Experiments on mammalian neutrophils and keratinocytes have shown that electric
fields can activate PI-3-kinase and that Akt, the principle target for this kinase, is present
mainly in the leading edge of the cell that is undergoing galvanotaxis. Furthermore, PI-3-
kinase activity is necessary for galvanotaxis.
18
Putting these observations together
d
and
accepting for a moment the risk that amoebae and mammals may not undergo galvanotaxis
in the same way
d
the implication is that electric fields act downstream of chemotaxis sensors
but upstream of PI-3-kinase activation and polarization. In a system known to involve feed-
back, such as that between PI-3-kinase and PTEN (Chapter 9), 'upstream' is a nebulous
concept and it is possible that the primary effect of the field is to define the trailing end of
the cell, which will then have the effect of defining the leading end (Chapter 9).
The search for a mechanism of galvanotaxis has therefore generated several models and, at
the moment, some confusion. Simple
in vivo
experiments suggest that action by electrophor-
esis of a chemoattractant may be possible. Detailed studies of receptors suggest that biasing
of receptor distribution may be a mechanism, although it is not clear whether this bias is
a cause or a consequence of cell polarization in the electric field, and genetic evidence
suggests that cells can orientate themselves in an electric field independently of their
b
