From FRET Imaging to Practical Methodology for Kinase Activity Sensing in Living Cells - Progress in Molecular Biology and Translational Science - page 204

protein-phosphatase activities of MKP (MAPK-specific phosphatases),
exerting key roles in the regulation of MAPK/Erk dynamics. 154 Inhibition
of MAPK/Erk activation using inhibitors against components of the MAPK
cascade has a survival effect on cells induced to die following chemical or
physical challenges. Forced MAPK/Erk cytosolic localization prevents the
survival or a mitogenic response but potentiates activities of specific pro-
apoptotic proteins such as DAPK (death-associated protein kinase), 155 Bik
(bcl-2-interacting killer), 156 and PEA-15 (astrocytic phosphoprotein-
15) 157 via the activation of the PI3K/Akt pathway . Depending on cell type
and stimulus, compartmentalized MAPK activity will mediate either cell
survival or cell death (see Ref. 3 for a review). Thus, spatial signatures for
the signal propagation of the MAPK/Erk signaling pathway have begun
to be explored both at the experimental and theoretical levels in many dif-
ferent cellular models, from gametes to tissues. 158-160
Scaffolding proteins and interactions of kinases with specific components
of the cell architecture also determine the spatial and temporal integration of
the cAMP/PKA signal. 161-163 In addition, the simple spatial organization of
the cell also determines the spatiotemporal organization of the signal, leading
to differential activation within the cell compartment. Indeed, in the case of
the cAMP/PKA signaling cascade, modeling studies 164 and imaging
experiments 165,146 have revealed the importance of cell morphology in
the particular situation of neurons, where submembrane domains display
faster and stronger responses than the bulk somatic cytosol. 166 It is
therefore of great importance to address the question of intracellular
signal integration, bearing in mind the multidimensional—space and
time—parameters that determine the physiological outcome of an

external signal.
Kinase activity reporter methodologies will definitely be able to generate
enough data to be considered for modeling approaches. Still, while record-
ing the dynamical pathway, which may exhibit variation from one cell to
another, one needs to identify within these apparently variable responses re-
liable marks or features that are common between experiments and invariant
to changes in cell morphologies or behaviors. These marks may be obtained
through the acute stimulation/inhibition of a pathway, 167 but a response hi-
erarchy may also be obtained from constitutive fluctuations in the activity of
the biosensors. 168 The latter approach is performed in a nonperturbed path-
way, under conditions closer to a physiological range than those in acute in-
hibition or activation. In concept, fluctuation analysis is quite scalable to a
large number of components and requires further developed mathematical
tools to ingrate this large set of component activities.