Effect of actinomycin D on the expression of nuclear antigen SS-B/La

All small nuclear RNAs (snRNAs) of the [U4/U6.U5] Tri-snRNP localize to nucleoli; Identification of the nucleolar localization element of U6 snRNA

Previously, we showed that spliceosomal U6 small nuclear RNA (snRNA) transiently passes through the nucleolus. Herein, we report that all individual snRNAs of the [U4/U6.U5] tri-snRNP localize to nucleoli, demonstrated by fluorescence microscopy of nucleolar preparations after injection of fluorescein-labeled snRNA into Xenopus oocyte nuclei. Nucleolar localization of U6 is independent from [U4/U6] snRNP formation since sites of direct interaction of U6 snRNA with U4 snRNA are not nucleolar localization elements. Among all regions in U6, the only one required for nucleolar localization is its 3' end, which associates with the La protein and subsequently during maturation of U6 is bound by Lsm proteins. This 3'-nucleolar localization element of U6 is both essential and sufficient for nucleolar localization and also required for localization to Cajal bodies. Conversion of the 3' hydroxyl of U6 snRNA to a 3' phosphate prevents association with the La protein but does not affect U6 localization to nucleoli or Cajal bodies.

Passively acquired autoimmunity and the maternal fetal dyad in systemic lupus erythematosus

Neonatal lupus syndromes consist of transient cutaneous and hematologic abnormalities and permanent cardiac disorders all of which result from a common pathogenetic mechanism, the passive transfer of maternal autoantibodies. Detrimental antibodies such as SSA/Ro and SSB/La gain access to the fetal circulation via the normal active transport system of the trophoblast tissue which is operative after 20 weeks gestation. Despite functional maturation of the cardiac conduction system by 16 weeks gestation, fetal bradycardias are most often encountered after this time. Several lines of evidence are advanced in this review to support the role of myocarditis as the initial consequence of autoantibody attack on the fetal heart. The end result of this inflammatory insult is permanent fibrosis manifest as complete congenital heart block (CCHB). Despite the clearly demonstrated presence of SSA/Ro and SSB/La in fetal tissues as well as their fluctuation in quantity during the cell cycle, the precise accessibility of these antigens to their respective autoantibodies in unknown at the present time. However, ultraviolet light is reported to induce cell surface expression of SSA/Ro on cultured keratinocytes. The recognition of CCHB by fetal echocardiogram is presented. The rationale for the use of dexamethasone which crosses the placenta in an active form to treat fetal immune effector functions is discussed. Intense maternal plasmapheresis in an attempt to rapidly decrease maternal autoantibodies may provide another approach to the management of CCHB. Through increasing knowledge of this model of "passively acquired pure" systemic lupus erythematosus, insights into mechanisms of tissue injury and strategies for treatment will emerge.