CHANGES IN ACTIVITIES OF THYMIDYLATE SYNTHASE AND DIHYDROFOLATE REDUCTASE IN SEA URCHIN EGGS AFTER FERTILIZATION

Area of Science:

• Developmental Biology
• Biochemistry
• Molecular Biology

Background:

• Thymidylate synthase activity in sea urchin eggs shows dynamic changes post-fertilization, correlating with the cell cycle.
• Dihydrofolate reductase activity remains stable in early sea urchin development.

Purpose of the Study:

• To investigate the role of dihydrofolate reductase in sea urchin egg cleavage.
• To determine the effects of aminopterin, a dihydrofolate reductase inhibitor, on early sea urchin development.

Main Methods:

• Enzyme activity assays for thymidylate synthase and dihydrofolate reductase.
• Exposure of sea urchin eggs (Clypeaster, Pseudocentrotus, Anthocidaris) to aminopterin during fertilization.
• Assessment of developmental arrest at different cell stages.
• Rescue experiments using dihydrofolate and various deoxyribonucleosides.

Main Results:

• Aminopterin inhibits dihydrofolate reductase and arrests sea urchin egg development at specific stages (32-64 cell or morula).
• Co-administration of dihydrofolate or thymidine effectively counteracts aminopterin-induced developmental arrest.
• Other deoxyribonucleosides do not prevent aminopterin's inhibitory effects, except for deoxyadenosine at high concentrations.

Conclusions:

• Dihydrofolate reductase is essential for normal sea urchin egg cleavage and embryonic development.
• Thymidine plays a critical role in supporting development when dihydrofolate reductase is inhibited.
• Aminopterin serves as a valuable tool for studying folate metabolism and cell cycle regulation in sea urchin embryos.