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Review
. 2021 Aug 2:15:694292.
doi: 10.3389/fncel.2021.694292. eCollection 2021.

Innate Immunity to Spiral Ganglion Neuron Loss: A Neuroprotective Role of Fractalkine Signaling in Injured Cochlea

Affiliations
Review

Innate Immunity to Spiral Ganglion Neuron Loss: A Neuroprotective Role of Fractalkine Signaling in Injured Cochlea

Andrew Rigel Stothert et al. Front Cell Neurosci. .

Abstract

Immune system dysregulation is increasingly being attributed to the development of a multitude of neurodegenerative diseases. This, in large part, is due to the delicate relationship that exists between neurons in the central nervous system (CNS) and peripheral nervous system (PNS), and the resident immune cells that aid in homeostasis and immune surveillance within a tissue. Classically, the inner ear was thought to be immune privileged due to the presence of a blood-labyrinth barrier. However, it is now well-established that both vestibular and auditory end organs in the inner ear contain a resident (local) population of macrophages which are the phagocytic cells of the innate-immune system. Upon cochlear sterile injury or infection, there is robust activation of these resident macrophages and a predominant increase in the numbers of macrophages as well as other types of leukocytes. Despite this, the source, nature, fate, and functions of these immune cells during cochlear physiology and pathology remains unclear. Migration of local macrophages and infiltration of bone-marrow-derived peripheral blood macrophages into the damaged cochlea occur through various signaling cascades, mediated by the release of specific chemical signals from damaged sensory and non-sensory cells of the cochlea. One such signaling pathway is CX3CL1-CX3CR1, or fractalkine (FKN) signaling, a direct line of communication between macrophages and sensory inner hair cells (IHCs) and spiral ganglion neurons (SGNs) of the cochlea. Despite the known importance of this neuron-immune axis in CNS function and pathology, until recently it was not clear whether this signaling axis played a role in macrophage chemotaxis and SGN survival following cochlear injury. In this review, we will explore the importance of innate immunity in neurodegenerative disease development, specifically focusing on the regulation of the CX3CL1-CX3CR1 axis, and present evidence for a role of FKN signaling in cochlear neuroprotection.

Keywords: CX3CR1; fractalkine; macrophages; neuroprotection; ribbon synapses; sensorineural hearing loss; spiral ganglion neurons.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Schematic depicting the structural components of fractalkine (FKN; CX3CL1). FKN is a large (373aa) transmembrane protein found exclusively on neurons in the central nervous system (CNS)/peripheral nervous system (PNS), and on SGNs and inner hair cells in the cochlea. Structural components that form FKN include the extracellular chemokine domain (76aa), extracellular mucin stalk (241aa), the transmembrane domain (19aa), and the intracellular cytoplasmic tail (37aa).
Figure 2
Figure 2
Schematic depicting CX3CL1-CX3CR1 signaling. CX3CL1 of FKN (ligand) is constitutively expressed by neurons in the CNS, PNS and by SGNs and IHCs in the mature cochlea. The CX3CR1 (receptor) is present on immune cells, including monocytes, macrophages, and microglia. In response to damage, CX3CL1 is proteolytically cleaved from the extracellular side of the neuronal cell membrane via ADAMs 17/10 or Cathespin-s. This cleavage produces the soluble form of CX3CL1 (containing the chemokine signaling domain and mucin like stalk). Soluble CX3CL1 binds to its unique receptor CX3CR1, resulting in macrophage chemotaxis to sites of injury. The membrane bound form of CX3CL1 plays a role in leukocyte adhesion. This signaling, when intact, induces neuroprotection during neuroinflammation and neurodegeneration.
Figure 3
Figure 3
The proposed model for fractalkine-mediated neuroprotection after cochlear injury. Following cochlear damage there is a sustained increase in the numbers of macrophages in the spiral ganglion. This increase in numbers is likely due to infiltration of blood-derived macrophages from the vasculature. (A) Following cochlear trauma, CX3CL1 is proteolytically cleaved from SGNs/IHCs and soluble CX3CL1 binds to its unique receptor CX3CR1 expressed by macrophages. Activation of fractalkine signaling induces an immune response characterized by recruitment of anti-inflammatory and pro-healing macrophages from the vasculature into the injured cochlea, regulation of cochlear inflammation, and preserved SGNs. (B) When FKN signaling is disrupted, i.e., due to loss of function in humans carrying CX3CR1 polymorphisms, there is a lack of macrophage recruitment and a sustained proinflammatory state following trauma. This results in SGN death, due to unregulated inflammation and phagocytosis by macrophages.

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