Complexes of Vertebrate TMC1/2 and CIB2/3 Proteins Form Hair-Cell Mechanotransduction Cation Channels

Core Concepts

Calcium and integrin-binding proteins CIB2 and CIB3 form essential complexes with the pore-forming subunits TMC1 and TMC2 to enable mechanotransduction in vertebrate hair cells.

Abstract

The content describes the functional role of CIB2 and CIB3 proteins in forming complexes with TMC1 and TMC2, the pore-forming subunits of the mechano-electrical transduction (MET) apparatus in vertebrate hair cells.

Key highlights:

• CIB2 and CIB3 can form heteromeric complexes with TMC1 and TMC2, and are integral for MET function in mouse cochlea, vestibular end organs, and zebrafish inner ear and lateral line.
• Structural modeling and NMR experiments suggest that CIB2 and CIB3 can simultaneously interact with at least two cytoplasmic domains of TMC1 and TMC2, stabilizing them to form cation channels.
• In mice, loss of both CIB2 and CIB3 leads to profound hearing and vestibular deficits, while single mutants show functional redundancy.
• In zebrafish, Cib2 and Cib3 are both required for normal mechanotransduction in lateral line hair cells, with Cib2 playing a more critical role in specific hair cell subtypes.
• The findings demonstrate that intact CIB2/3 and TMC1/2 complexes are essential for hair-cell MET function across vertebrate mechanosensory systems.

Stats

Compared to controls, the probability of acoustic startle response was significantly reduced in zebrafish cib2 mutants (control: 0.71 ± 0.04; cib2: 0.28 ± 0.06, p = 0.005).
In zebrafish cib2 mutants, a significantly higher percentage of posterior responsive lateral line hair cells labeled with FM 1-43FX compared to anterior responsive hair cells (posterior flow: 60.5%, anterior flow: 39.5%, n = 26 neuromasts).
In zebrafish cib2 mutants, many short hair cells in the crista of the inner ear failed to label with FM 4-64, indicating a requirement for Cib2 in this hair cell subtype.

Quotes

"Calcium and integrin-binding protein 2 (CIB2) and CIB3 can form heteromeric complexes with TMC1 and TMC2 and are integral for MET function in mouse cochlea and vestibular end organs as well as in zebrafish inner ear and lateral line."
"Our AlphaFold 2 models suggest that vertebrate CIB proteins can simultaneously interact with at least two cytoplasmic domains of TMC1 and TMC2 as validated using nuclear magnetic resonance spectroscopy of TMC1 fragments interacting with CIB2 and CIB3."
"Molecular dynamics simulations of TMC1/2 complexes with CIB2/3 predict that TMCs are structurally stabilized by CIB proteins to form cation channels."

Key Insights Distilled From

Complexes of vertebrate TMC1/2 and CIB2/3 proteins form hair-cell mechanotransduction cation channels

by Giese,A. P. ... at www.biorxiv.org 05-30-2023

https://www.biorxiv.org/content/10.1101/2023.05.26.542533v2

Deeper Inquiries

How might the differential requirement for Cib2 and Cib3 in specific hair cell subtypes in the zebrafish inner ear and lateral line relate to their distinct mechanotransduction properties?

The differential requirement for Cib2 and Cib3 in specific hair cell subtypes in the zebrafish inner ear and lateral line is closely linked to the distinct mechanotransduction properties of these hair cells. In the primary posterior lateral line (pLL), hair cells that respond to posterior flow rely on Tmc2b, while those that respond to anterior flow can utilize both Tmc2b and Tmc2a. The study indicates that Cib2 is essential for mechanosensitive function in hair cells that express Tmc2a, which are oriented to detect anterior flow. Conversely, Cib3 appears to compensate for Cib2 in hair cells that primarily express Tmc2b and are oriented for posterior flow. This suggests that Cib2 and Cib3 may have evolved to fulfill specific roles in the mechanotransduction process, with Cib2 being crucial for the function of hair cells that require a broader range of Tmc proteins for mechanosensitivity. The ability of Cib3 to partially substitute for Cib2 in certain contexts highlights the functional redundancy and evolutionary conservation of these proteins, which may be critical for maintaining the integrity of the mechanotransduction apparatus across different hair cell types.

What are the potential implications of the CIB-TMC complex architecture for the regulation of mechanotransduction channel activity and ion selectivity?

The architecture of the CIB-TMC complex has significant implications for the regulation of mechanotransduction channel activity and ion selectivity. The structural model suggests that CIB proteins, such as CIB2 and CIB3, interact with multiple cytoplasmic domains of TMC proteins, including TMC1 and TMC2. This interaction likely stabilizes the TMC channels, facilitating their proper assembly and function as cation channels. The presence of CIB proteins may influence the conformational dynamics of TMC proteins, thereby modulating their gating properties in response to mechanical stimuli. Furthermore, the specific interactions between CIB proteins and TMC channels could play a role in determining the ion selectivity of the mechanotransduction channels. For instance, the stabilization of the TMC pore structure by CIB proteins may enhance the permeability of specific ions, such as calcium, which is crucial for the transduction of mechanical signals into electrical responses in hair cells. Understanding this complex architecture could lead to insights into how mechanotransduction is fine-tuned in different sensory contexts, potentially informing strategies to manipulate channel activity for therapeutic purposes.

Could the insights into the evolutionary conservation of CIB-TMC complexes across vertebrates inform the development of therapeutic strategies for hearing and balance disorders?

Insights into the evolutionary conservation of CIB-TMC complexes across vertebrates could significantly inform the development of therapeutic strategies for hearing and balance disorders. The functional redundancy observed between CIB2 and CIB3 in zebrafish and mice suggests that targeting these proteins could provide a pathway for therapeutic intervention in cases of genetic hearing loss or vestibular dysfunction. For instance, if one CIB protein is non-functional due to a mutation, enhancing the expression or activity of the other could potentially restore mechanotransduction function in hair cells. Additionally, understanding the conserved mechanisms of CIB-TMC interactions may facilitate the design of small molecules or gene therapies aimed at stabilizing these complexes, thereby improving channel function in patients with hearing loss. Furthermore, the evolutionary perspective highlights the potential for cross-species applications of therapeutic strategies, as similar mechanisms may be exploited in both mammals and non-mammalian vertebrates. This could lead to innovative approaches in regenerative medicine, such as the development of biomimetic scaffolds or gene editing techniques to repair or replace defective components of the mechanotransduction machinery in sensory cells.