Targeted Microtubule-Based Transport Facilitates Ribbon Synapse Assembly in Developing Cochlear Inner Hair Cells

Core Concepts

Ribbon precursors in inner hair cells utilize the polarized microtubule cytoskeleton and the anterograde motor Kif1a for targeted trafficking and structural plasticity to enable proper ribbon synapse assembly and maturation.

Abstract

The study investigates the developmental formation of ribbon synapses in cochlear inner hair cells (IHCs). Ribbon synapses are specialized presynaptic structures that facilitate the temporally-precise release of synaptic vesicles. The key findings are: IHCs exhibit a highly polarized and acetylated microtubule (MT) cytoskeleton oriented along the apico-basal axis, providing structural support and enabling targeted intracellular transport. Live-cell imaging reveals that a significant fraction of ribbon precursors translocate along MTs, exhibiting a range of directed and non-directed motility modes reminiscent of "slow" axonal transport in neurons. Pharmacological disruption of the MT cytoskeleton impairs ribbon precursor mobility, structural plasticity (fusion/fission events), and volume accumulation. Genetic disruption of the anterograde motor Kif1a leads to reduced ribbon volumes and impaired hearing, indicating its essential role in ribbon synapse assembly and maturation. The data suggest that MT-based and Kif1a-dependent transport of ribbon precursors is a conserved mechanism facilitating the rapid establishment and structural refinement of ribbon synapses in sensory hair cells.

Stats

Ribbon precursors in control conditions exhibited a mean velocity of 0.0995 ± 0.0006 μm/s when not associated with MTs, compared to 0.0055 ± 0.0003 μm/s for MT-associated precursors. Nocodazole treatment reduced the frequency of ribbon precursor fusion events from 6.4 ± 1.2 to 2.9 ± 0.7 events/h, and fission events from 7.4 ± 1.0 to 3.2 ± 0.8 events/h. Kif1a mutant mice displayed elevated auditory brainstem response thresholds and reduced ribbon volumes compared to wild-type littermates.

Quotes

"Ribbon precursors utilize the apico-basal microtubular (MT) cytoskeleton for targeted trafficking to the presynapse, in a process reminiscent of slow axonal transport in neurons." "Genetic disruption of the anterograde molecular motor Kif1a impaired ribbon volume accumulation during developmental maturation." "Our data thus indicate an essential role of the MT cytoskeleton and Kif1a in adequate ribbon synapse formation and structural maintenance."

Key Insights Distilled From

Slow kinesin-dependent microtubular transport facilitates ribbon synapse assembly in developing cochlear inner hair cells

by Voorn,R. A.,... at www.biorxiv.org 04-16-2024

https://www.biorxiv.org/content/10.1101/2024.04.12.589153v1

Deeper Inquiries

How do ribbon precursors interact with and associate with the microtubule cytoskeleton at the molecular level?

Ribbon precursors interact with the microtubule (MT) cytoskeleton in a highly dynamic and regulated manner. The MT network in inner hair cells (IHCs) is polarized along the apico-basal axis, with the majority of MT strands anchored at the apical cell pole and growing towards the basolateral compartment. Ribbon precursors utilize this polarized MT network for targeted trafficking to the presynaptic active zone (AZ). The interaction between ribbon precursors and MTs likely involves molecular motors, with the kinesin-3 family member Kif1a being a potential candidate for anterograde transport. Kif1a is known to facilitate the transport of synaptic vesicle precursors in neurons and may play a similar role in ribbon precursor transport in IHCs. In live-cell imaging experiments, it was observed that a significant fraction of ribbon precursors translocated along MT tracks. Three main types of trajectory patterns were identified: saltatory, gradual/continuous, and confined motion. These patterns suggest a combination of interrupted motor-based transport, slow anterograde movement, and non-directional confined motion. The slow anterograde transport of ribbon precursors likely involves transient associations with the MT cytoskeleton, leading to a 'stop-and-go' processivity with alternating stationary and transport phases. Overall, ribbon precursors interact with the MT cytoskeleton through a complex interplay of molecular motors, structural proteins, and regulatory factors, enabling their targeted translocation to the developing presynaptic AZ in IHCs.

How do ribbon precursors interact with and associate with the microtubule cytoskeleton at the molecular level?

Compensatory mechanisms that may partially offset the loss of Kif1a function in ribbon synapse assembly could involve other molecular motors and transport pathways. While Kif1a is implicated in anterograde transport of ribbon precursors along the MT cytoskeleton, there are other kinesin motors, such as Kif3a, that could potentially compensate for the loss of Kif1a. Kif3a is known to associate with RIBEYE in ribbon-bearing sensory systems and may play a role in ribbon precursor transport in the absence of functional Kif1a. Additionally, retrograde motors of the dynein family could contribute to the transport of ribbon precursors in the absence of Kif1a. Dynein motors move towards the minus (-) ends of MTs and could facilitate the retrograde transport of ribbon precursors or play a role in the structural maintenance of ribbon synapses. These compensatory mechanisms may help maintain some level of ribbon synapse assembly and function in the absence of Kif1a, although the overall efficiency and specificity of transport may be compromised. Further studies investigating the interplay between different molecular motors and their roles in ribbon precursor transport will provide insights into the compensatory mechanisms that come into play when Kif1a function is impaired.

What is the role of the actin cytoskeleton and myosin-based transport in the local redistribution and structural refinement of ribbon synapses during hair cell maturation?

The actin cytoskeleton and myosin-based transport likely play important roles in the local redistribution and structural refinement of ribbon synapses during hair cell maturation. While the focus has been on the MT-based transport of ribbon precursors, the actin cytoskeleton and myosin motors are essential components of the cellular transport machinery and are involved in various intracellular processes. The actin cytoskeleton forms a dynamic network of filaments that can regulate cell shape, motility, and intracellular transport. In the context of ribbon synapse assembly, the actin cytoskeleton may provide structural support and facilitate the movement of ribbon precursors within the IHC cytoplasm. Myosin motors, which are actin-based molecular motors, can generate force and drive the movement of cellular components along actin filaments. During hair cell maturation, the actin cytoskeleton and myosin motors may be involved in the local redistribution of detached ribbons and the structural refinement of ribbon synapses. This process may include the transport of ribbons to adjacent active zones, the maintenance of synaptic contacts, and the regulation of synaptic plasticity. The actin cytoskeleton could also contribute to the formation of diffusion barriers that control synaptic vesicle exocytosis and regulate the release of neurotransmitters at the synapse. Overall, the actin cytoskeleton and myosin-based transport likely play complementary roles to the MT-based transport system in the local redistribution and structural refinement of ribbon synapses during hair cell maturation. Further research is needed to elucidate the specific mechanisms and interactions involved in these processes.