New insights into ion channel‐dependent signalling during left‐right patterning

Abstract

The left-right organizer (LRO) in the mouse consists of pit cells within the depression, located at the end of the developing notochord, also known as the embryonic node and crown cells lining the outer periphery of the node. Cilia on pit cells are posteriorly tilted, rotate clockwise and generate leftward fluid flow. Primary cilia on crown cells are required to interpret the directionality of fluid movement and initiate flow-dependent gene transcription. Crown cells express PC1-L1 and PC2, which may form a heteromeric polycystin channel complex on primary cilia. It is still only poorly understood how fluid flow activates the ciliary polycystin complex. Besides polycystin channels voltage gated channels like HCN4 and KCNQ1 have been implicated in establishing asymmetry. How this electrical network of ion channels initiates left-sided signalling cascades and differential gene expression is currently only poorly defined.Rajeev Tajhya received his PhD from Baylor College of Medicine. He worked on characterizing the role of potassium channels that affected myogenesis in patients with myotonic dystrophy type 1 and tested the stability and potency of a drug that is currently in a clinical trial for treatment of autoimmune diseases. He then worked as a postdoctoral fellow in the Delling lab to identify differences in the genetic map of the left and right sides of mouse embryonic node with a focus on ion channel genes and genes involved in congenital heart defects. Markus Delling obtained his PhD at the ZMNH at the University of Hamburg, Germany. He did his postdoctoral training with David Clapham at the Children's Hospital Boston, where he started working on ion channels in primary cilia. He now has his own lab at the University of California San Francisco where he continues to study the function of ion channels in primary cilia signalling. Abstract figure legendMotile cilia on pit cells (blue) generate a leftward directed flow within the LRO. Primary cilia on crown cells (brown) lining the outer perimeter of the node presumably function as flow detectors as they are required to initiate left-sided asymmetric gene expression (insert). The initial asymmetry generated within the LRO spreads out into the surrounding mesoderm (green) and thus defines the left side of the embryo. The polycystin members PC1-L1 and PC2 reside on primary cilia of crown cells and genetic evidence strongly suggests that this ion channel complex is required for flow-induced gene expression (insert). Yet the activation mechanism by which this ion channel complex is activated by flow on primary cilia still remains poorly understood. Besides ciliary polycystin channels other membrane localized voltage-gated channels (insert) have been implicated in establishing asymmetry, suggesting that they may function as signal amplifiers.

Phosphorylation and Ubiquitylation Regulate Protein Trafficking, Signaling, and the Biogenesis of Primary Cilia

“…Despite a large variety of receptors, far fewer mediators are commonly used in cellular signaling processes. Cyclic nucleotides or calcium ions are second messengers, the concentrations of which are interpreted by specific enzymes to further transmit signals via PTMs (Hilgendorf et al, 2019;Sherpa et al, 2019;Tajhya and Delling, 2020). To communicate with the rest of the cell, effectors are transported into and out of cilia in a dynamic fashion, which allows their modification according to the signaling status (Niewiadomski et al, 2019).…”

Understanding embryonic development for cell‐based therapies of intervertebral disc degeneration: Toward an effort to treat disc degeneration subphenotypes

“…During early embryogenesis, the notochord emerges by convergent extension from the Spemann‐Mangold organizer, known as the “shield” in Zebrafish and Hensen's “node” in the chick and mouse, which contributes to the patterning of the body axes: anterior‐posterior, dorsal‐ventral, and left‐right. Pit cells of the node help coordinate patterning by rotating motile cilia in a clockwise direction, creating a leftward flow of fluid that regulates flow‐dependent gene expression and circulates morphogens 37 . At this stage, transcription of cilia marker genes including Foxj1 , Rfx3 , Lrd , Polaris , and Kif3a can be identified in the node and persist in the nascent notochord 38 .…”

“…Pit cells of the node help coordinate patterning by rotating motile cilia in a clockwise direction, creating a leftward flow of fluid that regulates flow-dependent gene expression and circulates morphogens. 37 At this stage, transcription of cilia marker genes including Foxj1, Rfx3, Lrd, Polaris, and Kif3a can be identified in the node and persist in the nascent notochord. 38 NODAL, belonging to the TGF-β superfamily, is required for the formation of the primitive streak, initiation of gastrulation, and determination of the left-right axis.…”

Insights into the Regulation of Ciliary Disassembly

“…In general, cilia are thought to act as sensors of different biochemical and mechanical cues that mediate cellular processes such as proliferation, differentiation, and cell migration [ 4 , 5 ]. They mediate such function(s) by housing a wide variety of ion channels [ 6 , 7 , 8 ] and receptors essential for multiple signaling pathways. Some of the most common signaling pathways studied in association with cilia are Hedgehog [ 9 , 10 ], WNT [ 11 , 12 ], NOTCH [ 13 ], HIPPO [ 14 , 15 ], TGF, and growth factor signaling pathways [ 15 ].…”