Monitoring gene expression in a single Xenopus oocyte using multiple cytoplasmic collections and quantitative RT-PCR

Multienzyme activity profiling for evaluation of cell‐to‐cell variability of metabolic state

In solid organs, cells of the same “type” can vary in their molecular phenotype. The basis of this state variation is being revealed by characterizing cell features including the expression pattern of mRNAs and the internal distribution of proteins. Here, the variability of metabolic state between cells is probed by enzyme activity profiling. We study individual cells of types that can be identified during the post‐mitotic phase of oogenesis in Xenopus laevis. Whole‐cell homogenates of isolated oocytes are used for kinetic analysis of enzymes, with a focus on the initial reaction rate. For each oocyte type studied, the activity signatures of glyceraldehyde 3‐phosphate dehydrogenase (GAPDH) and malate dehydrogenase 1 (MDH1) vary more between the homogenates of single oocytes than between repeat samplings of control homogenates. Unexpectedly, the activity signatures of GAPDH and MDH1 strongly co‐vary between oocytes of each type and change in strength of correlation during oogenesis. Therefore, variability of the kinetic behavior of these housekeeping enzymes between “identical” cells is physiologically programmed. Based on these findings, we propose that single‐cell profiling of enzyme kinetics will improve understanding of how metabolic state heterogeneity is related to heterogeneity revealed by omics methods including proteomics, epigenomics, and metabolomics.

Reconstitution of Intracellular Calcium Signaling in Xenopus Egg Extracts

Generation of calcium signal in the cytoplasm of fertilized or parthenogenetically activated eggs has been extensively studied in the intact eggs of several biological species. Calcium transient was found to elicit a plethora of biochemical and cellular events in these cells. Remarkably, intracellular calcium signaling can also be reconstituted in cell-free environment. In this chapter, we describe the methods that allow reconstitution, detection, and quantification of the calcium signal in cell-free extracts of Xenopus oocytes and eggs.

Global decay of mRNA is a hallmark of apoptosis in aging Xenopus eggs

Cytoplasmic mRNAs are specifically degraded in somatic cells as a part of early apoptotic response. However, no reports have been presented so far concerning mRNA fate in apoptotic gametes. In the present study, we analyzed the content of various cytoplasmic mRNAs in aging oocytes and eggs of the African clawed frog, Xenopus laevis. To circumvent large gene expression variation among the individual oocytes and eggs, single-cell monitoring of transcript levels has been implemented, using multiple cytoplasmic collections and reverse transcriptase quantitative PCR. It was found that numerous cytoplasmic mRNAs, coding for proteins classified in different functional types, are robustly degraded in apoptotic Xenopus eggs, but not in aging oocytes. mRNA degradation becomes evident in the eggs after meiotic exit at the time of cytochrome c release. A strong correlation between the length of PCR amplicon and specific transcript content was observed, suggesting endonucleolytic cleavage of mRNA. In addition, it was found that mRNA deadenylation also contributes to apoptotic mRNA degradation. Altogether, these findings indicate that the global decay of mRNA represents a hallmark of apoptosis in aging Xenopus eggs. To our knowledge, this is the first description of mRNA degradation in apoptotic gamete cells.