Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Area of Science:

• Cell Biology
• Protozoology
• Biochemistry

Background:

• The nucleus, traditionally viewed as a passive organelle, is increasingly recognized for its dynamic properties.
• Understanding nuclear mechanics is crucial for comprehending cellular processes like division and response to stimuli.

Purpose of the Study:

• To investigate the presence and mechanism of Ca2+-dependent contractility within the nucleus of Actinophrys sol.
• To elucidate the role of nuclear structures in Ca2+-induced nuclear shape changes.

Main Methods:

• Isolation and detergent-extraction of nuclei from Actinophrys sol.
• Controlled addition of calcium ions (Ca2+) and EGTA to induce and reverse nuclear contraction.
• Electron microscopy to visualize nuclear structural changes.
• Testing the effects of Mg2+, colchicine, and cytochalasin B on nuclear contractility.

Main Results:

• Isolated nuclei exhibited Ca2+-dependent contraction, reducing diameter from 16.5±1.7 µm to 11.0±1.3 µm.
• The threshold for Ca2+-induced contraction was 2.9 x 10(-7) M; Mg2+ did not induce contraction.
• Nuclear contraction was reversible with EGTA, allowing repeated cycles of contraction and expansion.
• Electron microscopy revealed Ca2+-induced aggregation and transformation of filamentous nuclear structures.

Conclusions:

• Actinophrys sol nuclei possess Ca2+-dependent contractile capabilities.
• Nuclear contraction is mediated by Ca2+-induced transformations of internal filamentous structures, suggesting a nucleoskeletal component.
• These findings highlight the dynamic nature of the protozoan nucleus and its response to ionic stimuli.