Tunneling nanotube–like structures regulate distant cellular interactions during heart formation

Amazing stuff!

From the perspective abstract:

"The heart is the first organ to become functional during embryonic development in vertebrates. The process begins with the formation of the heart tube, which consists of an inner endothelial lining (endocardium) and a layer of muscle cells (myocardium), separated by a thick extracellular matrix called the cardiac jelly. Subsequent development of the heart ventricles involves the formation of trabeculae—muscular ridges lined by the endocardium. How signals pass between the endocardium and myocardium across the cardiac jelly to ensure normal development is not fully understood. ...

report that, in mice, the myocardium and endocardium of the early heart tube communicate directly through tiny membrane-enclosed projections called tunneling nanotube–like structures (TNTLs). Loss of TNTLs disrupts trabecular formation and thus ventricular development and embryonic viability."

From the editor's summary and abstract:

"Editor’s summary

During cardiac development, heart muscle cells (the myocardium) and inner lining cells (the endocardium) are separated by a space filled with cardiac jelly. ... discovered that despite this physical separation, the two cell types communicate with each other through tiny structures called tunneling nanotube-like microstructures (TNTLs) ... TNTLs extend across the cardiac jelly, allowing direct cell-cell contact, signal transduction, and selective protein transfer. Disrupting TNTLs leads to abnormal heart wall formation, highlighting the importance of TNTLs in cardiac development. ...

Structured Abstract

INTRODUCTION

Heart development is a highly orchestrated process dependent on dynamic interactions between the myocardium and the endocardium. The two layers are separated by a noncellular matrix called cardiac jelly and communicate through signaling pathways involving membrane-bound receptors and ligands. However, the mechanisms enabling such signaling interaction over physical distances remain poorly understood.

In this work, we characterized tunneling nanotube–like structures (TNTLs), which we found physically connecting cardiomyocytes (CMs) in the myocardium to endocardial cells (ECs) in the endocardium. These structures likely help to facilitate long-distance intercellular communication essential for heart formation.

RATIONALE

Heart formation relies on precise signaling interactions between the myocardium and endocardium, particularly during trabecular development. Signaling pathways, such as Notch1, Vegf, and Nrg1, have ligands and receptors segregated across these two cardiac layers. The mechanisms enabling these long-distance interactions across the intervening cardiac jelly are unclear.

We hypothesized that TNTL structures exist between the cardiac layers and could mediate intercellular long-distance communication in the developing heart, allowing for the transport of signaling molecules and cytoplasmic proteins between them.

RESULTS

We used genetic labeling, contact-tracing techniques, and advanced imaging to demonstrate the existence of TNTLs in mouse embryonic hearts. These TNTLs extended from CMs to ECs across the cardiac jelly, establishing direct connections that enable signal transduction and cytoplasmic protein transfer.

The TNTLs were identified in the heart through the genetic labeling of cellular protrusions. During mouse development TNTLs were shown to form between CMs and ECs as early as embryonic day 8.0. The filamentous structures inside TNTL were characterized by three-dimensional imaging and the reconstruction of the electron microscopy (EM) and cryo-EM images.

The TNTLs contained actin filaments, and TNTL formation depended on actin polymerization. The presence of actin filaments in TNTLs was confirmed in a transgenic mouse line that could label actin filaments with a fluorescent marker. Inhibiting actin polymerization chemically or by ablating the small guanosine triphosphatase, Cdc42, eliminated TNTLs.

The TNTLs were involved in regulating Notch1 signaling and other signaling pathways. TNTLs were sufficient to activate Notch1 signaling in ECs, with ligands from CMs transported through these microstructures to ECs.

Loss of TNTLs resulted in reduced Notch signaling and other signaling pathways. TNTLs were able to transport signaling molecules, cytoplasmic proteins, and trafficking vesicles, underscoring their role as conduits for intercellular communication.

The TNTLs were essential in cardiac morphogenesis. Disruption of TNTLs in embryonic hearts resulted in impaired ventricular wall morphogenesis, evidenced by loss of trabeculae and defective myocardial growth.

CONCLUSION

In this work, we identified TNTLs as a critical mechanism for long-distance intercellular communication during heart development. These actin-rich structures physically bridge the myocardium and endocardium, allowing for the efficient exchange of signaling molecules necessary for trabecular formation and ventricular wall morphogenesis.

Disruption of TNTLs compromises these interactions, highlighting their essential role in heart patterning. This work provides insights into mechanisms of cellular communication and suggests that TNTL formation might help cells to regulate long-distance cell-cell communication and modulate tissue patterning in mammalian systems. ..."

Tunneling through cardiac jelly (no public access) "Membrane projections from muscle cells enable signaling in the developing mouse heart"

Tunneling nanotube–like structures regulate distant cellular interactions during heart formation | Science (no public access)

Microstructure regulates signaling interaction and cytoplasmic proteins transfer between CMs and ECs.