MRTF specifies a muscle-like contractile module in Porifera.

Area of Science:

• Evolutionary biology
• Cell biology
• Zoology

Background:

• Animal movement relies on muscle tissues, but their evolutionary origins remain unclear.
• Sponges, though considered muscle-less, exhibit whole-body contractions for water canal clearance.
• The cellular basis, regulation, and development of sponge contractile tissues are largely unknown.

Purpose of the Study:

• To investigate the cellular and molecular mechanisms underlying coordinated contractions in sponges.
• To characterize the contractile module, its regulation, and developmental pathways in sponges.
• To explore the evolutionary implications of sponge contractile tissues for the origin of animal muscles.

Main Methods:

• Ultrastructural analysis of contractile tissues in the sponge *Ephydatia muelleri*.
• Biochemical assays to identify key contractile proteins (actin, myosin II, transgelin).
• Investigation of calcium signaling pathways and transcription factor involvement (MLCK, MRTF).

Main Results:

• Identified a tissue-wide contractile module comprising actin, striated-muscle myosin II, and transgelin.
• Demonstrated contraction regulation via internal calcium release upstream of the myosin-light-chain-kinase (MLCK) pathway.
• Found evidence for myocardin-related transcription factor (MRTF) involvement in contractile tissue development and environmental responsiveness.

Conclusions:

• Sponge contractile tissues share homologous components with animal muscles, suggesting a common ancestral tissue.
• The identified contractile module provides insights into the early evolution of contractile systems in animals.
• MRTF acts as a force-sensor, enabling dynamic remodeling of sponge canal tissues and regeneration from stem cells.