Immunocytochemical detection of phosphatidylinositol 4,5-bisphosphate localization sites within the nucleus

Phosphatidylinositol 4,5-bisphosphate {PIP₂) is a key element of signal transduction, being the preferential substrate of specific phospholipase that produce second messengers such as inositol trisphosphate (IP3) and diacylglycerol (DG). Because PIP₂ has been cytochemically identified by monoclonal antibodies not only in the cytoplasmic membranes but also in the nuclear envelope and within the nucleus, we performed a study by immunoblotting and by confocal and electron microscopic immunocytochemistry to identify the nuclear sites of PIP₂ localization and to exclude any cross- reactivity of the antibody with other nuclear molecules. The results confirm the specificity of the immunolabeling and indicate that PIP₂ is localized at precise intranuclear sites both in in situ and in isolated nuclei. They also show that a significant amount of the phospholipid is retained by the cytoskeleton and by the inner nuclear matrix in in situ matrix preparations. Moreover the sensitivity of the immunocytochemical reaction is capable of detecting quantitative variations of PIP₂ nuclear content induced by agonists that modulate the signal transduction system at the nuclear level.