In developing mammalian telencephalon, the loss of adherens junctions and cell cycle exit represent crucial steps in the differentiation of neuroepithelial cells into neurons, but the relationship between these cellular events remains obscure. Atypical protein kinase C (aPKC) is known to contribute to junction formation in epithelial cells and to cell fate determination for Drosophila neuroblasts. To elucidate the functions of aPKCλ, one out of two aPKC members, in mouse neocortical neurogenesis, a Nestin-Cre mediated conditional gene targeting system was employed. In conditional aPKCλ knockout mice, neuroepithelial cells of the neocortical region lost aPKCλ protein at embryonic day 15 and demonstrated a loss of adherens junctions, retraction of apical processes and impaired interkinetic nuclear migration that resulted in disordered neuroepithelial tissue architecture. These results are evidence that aPKCλ is indispensable for the maintenance of adherens junctions and may function in the regulation of adherens junction integrity upon differentiation of neuroepithelial cells into neurons. In spite of the loss of adherens junctions in the neuroepithelium of conditional aPKCλ knockout mice, neurons were produced at a normal rate. Therefore, we concluded that, at least in the later stages of neurogenesis, regulation of cell cycle exit is independent of adherens junctions.

Overexpression of a TPA‐insensitive PKC member, an atypical protein kinase C (aPKClambda), results in an enhancement of the transcriptional activation of TPA response element (TRE) in cells stimulated with epidermal growth factor (EGF) or platelet‐derived growth factor (PDGF). EGF or PDGF also caused a transient increase in the in vivo phosphorylation level and a change in the intracellular localization of aPKClambda from the nucleus to the cytosol, indicating the activation of aPKClambda in response to this growth factor stimulation. These immediate signal‐dependent changes in aKPClambda were observed for a PDGF receptor add‐back mutant (Y40/51) that possesses only two of the five major autophosphorylation sites and binds PI3‐kinase, and were inhibited by wortmannin, an inhibitor of PI3‐kinase. Furthermore, an N‐terminal fragment of the catalytic subunit of PI3‐kinase, p110alpha, inhibited aPKClambda‐dependent activation of TRE in Y40/51 cells stimulated with PDGF. Overexpression of p110alpha resulted in an enhancement of TRE expression in response to PDGF and the regulatory domain of aPKClambda inhibited this TRE activation in Y40/51 cells. These results provide the first in vivo evidence supporting the presence of a novel signalling pathway from receptor tyrosine kinases to aPKClambda through PI3‐kinase.

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Protein kinase C (PKC), a major cellular receptor for tumor-promoting phorbol esters and diacylglycerols (DGs), appears to be involved in On the basis of findings obtained from brain preparations of protein kinase C (PKC) (a mixture of "conventional" PKCs) that showed that diacylglycerol (DG) activates PKC (1) and that tumor-promoting phorbol esters such as phorbol 12-tetradecanoate 13-acetate (TPA) bind and activate PKC (2), in combination with experiments done in vivo using PKC activators such as membrane-permeant DG and TPA, Nishizuka and coworkers (3) proposed the now widely accepted hypothesis that PKC acts as a receptor for DG and is involved in receptor-mediated processes that are associated with inositolphospholipid breakdown. Subsequent experiments carried out in many laboratories using the above-mentioned PKC activators have showed the significance of the DG/PKC pathway in a variety of cellular functions (4). Unfortunately, however, there has been no experimental system in which to demonstrate directly in vivo that a certain cell surface receptor activates a certain PKC molecule through the phospholipase C (PLC) pathway.In addition to DG, stimulation of cell surface receptors triggers the generation of a variety of lipid metabolites (4,5).

Modulation of gene expression by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) is thought to be mediated by protein kinase C (PKC), a major cellular receptor for TPA. We confirm this by showing that the overexpression of PKC delta enhances the TPA induction of the TRE‐tk‐CAT reporter gene in NIH3T3 cells. To investigate the mutual relationship between PKC delta‐ and Ras‐dependent signal transduction pathways to a TRE binding transcription factor, AP1/Jun, we constructed constitutively active and dominant negative mutants of PKC delta. Activated Ras induced reporter gene expression in collaboration with overexpressed c‐Jun or JunD, and this induction was insensitive to the dominant negative PKC delta. On the other hand, reporter gene expression induced by the constitutively active PKC delta was severely inhibited by dominant negative Ras, as well as by the dominant negative PKC delta. Thus, Ras activation must be indispensable for PKC delta to activate AP1/Jun. In the absence of overexpressed c‐Jun or JunD, activated Ras was, however, clearly less effective than constitutively active PKC delta which showed full activation of reporter gene expression by itself. This suggests the presence of an additional, Ras‐independent, signaling pathway downstream of PKC delta to activate AP1/Jun. In spite of the remarkable ability of constitutively active PKC delta to activate TRE‐tk‐CAT expression, this mutant suppressed cell growth.