ABSTRACT
The heart of ascidians (marine invertebrate chordates) has a tubular structure, and heartbeats propagate from one end to the other. The direction of pulsation waves intermittently reverses in the heart of ascidians and their relatives; however, the underlying mechanisms remain unclear. We herein performed a series of experiments to characterize the pacemaker systems in isolated hearts and their fragments, and applied a mathematical model to examine the conditions leading to heart reversals. The isolated heart of Ciona robusta autonomously generated pulsation waves at ∼20 to 25 beats min−1 with reversals at ∼1 to 10 min intervals. Experimental bisections of isolated hearts revealed that independent pacemakers resided on each side and also that their beating frequencies periodically changed as they expressed bimodal rhythms, which comprised an ∼1.25 to 5.5 min acceleration/deceleration cycle of a beating rate of between 0 and 25 beats min−1. Only fragments including 5% or shorter terminal regions of the heart tube maintained autonomous pulsation rhythms, whereas other regions did not. Our mathematical model, based on FitzHugh–Nagumo equations applied to a one-dimensional alignment of cells, demonstrated that the difference between frequencies expressed by the two independent terminal pacemakers determined the direction of propagated waves. Changes in the statuses of terminal pacemakers between the excitatory and oscillatory modes as well as in their endogenous oscillation frequencies were sufficient to lead to heart reversals. These results suggest that the directions of pulsation waves in the Ciona heart reverse according to the changing rhythms independently expressed by remotely coupled terminal pacemakers.
Footnotes
Author contributions
Conceptualization: Y.F., K. Fukuda, K. Fujimoto, A.S.N.; Methodology: Y.F., K. Fukuda, K.M., Y.I., K. Fujimoto, A.S.N.; Software: K. Fukuda, K.M., Y.I., K. Fujimoto; Validation: Y.F., K. Fukuda, K.M., Y.I., K. Fujimoto, A.S.N.; Formal analysis: Y.F., K. Fukuda, K.M., A.S.N.; Investigation: Y.F., K. Fukuda, A.S.N.; Resources: Y.F., K. Fukuda, K.M., Y.I., K. Fujimoto, A.S.N.; Data curation: Y.F., K. Fukuda, K.M., K. Fujimoto, A.S.N.; Writing - original draft: Y.F., K. Fukuda, A.S.N.; Writing - review & editing: Y.F., K. Fukuda, K.M., Y.I., K. Fujimoto, A.S.N.; Visualization: Y.F., K. Fukuda, K. Fujimoto, A.S.N.; Supervision: K.M., K. Fujimoto, A.S.N.; Project administration: K. Fujimoto, A.S.N.; Funding acquisition: Y.F., A.S.N.
Funding
This research was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grants to A.S.N. (20K06713, 22H04827, 23K05843). Y.F. was supported by the UGAS Student Research Grant Project of The United Graduate School of Agricultural Sciences, Iwate University, and also by the JSPS Research Fellowship for Young Scientists (23KJ0080). This research was also supported by Grants-in-Aid from Hirosaki University to A.S.N. (Grant for Exploratory Research by Young Scientists during 2012-2015 and 2017, Institutional Research Grant for Young Investigators 2017-2019, and Interdisciplinary Collaborative Research Grant for Young Scientists in 2018 and 2019), and also in part by The Yamada Science Foundation, The Sumitomo Foundation, and The Sekisui Integrated Research Foundation to A.S.N.
Data availability
All experimental data can be found within the article and its supplementary information. The source code developed for the model simulation was deposited in GitHub (https://github.com/fujimotokoichi/ciona_heartbeat24).