|[Frontiers in Bioscience 1, d19-29 March 1, 1996]|
MICROINJECTION STRATEGIES FOR THE STUDY OF MITOGENIC SIGNALING
IN MAMMALIAN CELLS
Ned J.C.Lamb, Cecile Gauthier-Rouviere and Anne Fernandez.
Cell Biology Unit, C R B M , CNRS-INSERM, 1919, Route de Mende, F-34033, Montpellier Cedex. France.
Received 15/12/95; Accepted 30/01/96; On-line 03/01/96
Entry into G1 phase involves switching from quiescence (G0) to a proliferative state, with the induction of "immediate-early" genes. Stimulation of these early genes does not necessarily require protein synthesis, implying that appropriate regulatory factors are present before growth factor stimulation (i. e. in G0) (2,19). This is the case for SRF, which is present in quiescent cells, and after the inductive signal has been received, induces the expression of the early genes, such as c-fos. The rapidity of proliferative activation is due to the presence of such proteins even in G0 phase, whose regulation is by virtue of post-translational modifications, and predominantly by phosphorylation. The potential role of different protein kinases or phosphatases activated by growth factors in the subsequent activation of transcription factors, has been studied by microinjection of different purified enzymes. In the case of c-fos gene expression, two kinases were studied : casein kinase II (CKII) and Ca+2/phospholipid-dependent protein kinase (C-kinase) (15,20). Microinjection of either purified CKII or C-kinase resulted in the induction of c-fos in quiescent cells (with similar kinetics as serum induction). CKII is a kinase which is recruited to the nucleus at the G0/G1 transition. This enzyme has different nuclear targets, suggesting its implication in the final step of mitogenic signaling i.e. phosphorylation of transcription factors. Following CKII microinjection into quiescent cells, SRF was phosophorylated (15), and such phosphorylation was shown to slightly increase the DNA binding affinity of SRF and to markedly increase the rate of SRE-SRF exchange in vitro (21,22). Moreover, the possibility of coinjecting oligonucleotides corresponding to different protein binding DNA sequences together with protein kinases, has allowed identification of whether a promoter sequence is involved in the transcriptional activation by a specific kinase pathway. The co-injection of SRE oligonucleotide with CKII or C-kinase showed that activation at the SRE site (previously shown to be necessary for serum-induced c-fos expression) is also absolutely required for c-fos expression induced by CKII and C-kinase (table 2).
This approach allows to rapidly and transiently increase the level of a kinase or phosphatase and follow the effect of this elevation either on substrate phosphorylation or on a particular physiologic process such as c-fos gene expression, cell shape modification, etc.). By direct injection of purified enzymes, one can specifically study the end result of activating a particular enzyme and its downstream effectors (see summary in table 1).
A complementary approach involves microinjection of a specific inhibitory peptide against kinases or phosphatases. This can be applied to studying the role of several different protein kinases (C-kinase, A-kinase, CKII), or phosphatases such as PP1. It may prove useful in situations where no inhibitory antibody is available. These peptides are synthesized based on the sequence of an endogenous inhibitory protein (for example PKI is a synthetic peptide analogous in sequence to a 20 amino acid proteolytic cleavage fragment which retains a high inhibitory activity for A-kinase and can be modified to increase its stability in vivo (24)). The peptide can be synthesized by inserting a pseudosubstrate site in the regulatory domain from a given protein. this strategy has been used in synthesizing C-PKI, a synthetic peptide which acts as a potent C-kinase substrate antagonist (25)). These two peptides are suitable as probes in living cells. In particular, microinjection of the C-PKI peptide has been used to show the requirement of a C-kinase activity for ras-induced c-fos expression (12). A-kinase inhibition by PKI inside living cells resulted in marked effects on chromatin structure and cytoskeletal organisation (26) resembling those that accompany mitotic entry. Indeed, A-kinase appears to be a specific antagonist of many mitotic pathways (26,27). Such a global loss of A-kinase activity which prevents basic cell functions, prevents probing specific aspects of the cell metabolism. For example microinjection of PKI results in the exclusion of most transcription factors from the nucleus. Indeed, using microinjection of PKI, it was recently demonstrated that A-kinase activity is required in the process of active nuclear import of all proteins, including SRF (28). Another recent study addressing the role of protein phosphatases, used microinjection of a plasmid coding for a constitutively active form of inhibitor 1, a specific inhibitor of protein phosphatase type 1 (PP1), (29). These experiments proved that the transcription factor (CREB) that binds and activates the cAMP Response Element (CRE) is dephosphorylated on its A-kinase site by PP1, thereby limiting the transcriptional activity of CREB (29). This example combines the strategy of applying inhibitory peptides to specifically inactivate an enzyme pathway, and the use of expression plasmid microinjection, which allows the sustained overexpression of a peptide or a protein, as detailed below.