Lack of Fractalkine Receptor on Macrophages Impairs Spontaneous Recovery of Ribbon Synapses After Moderate Noise Trauma in C57BL/6 Mice.

Noise trauma causes loss of synaptic connections between cochlear inner hair cells (IHCs) and the spiral ganglion neurons (SGNs). Such synaptic loss can trigger slow and progressive degeneration of SGNs. Macrophage fractalkine signaling is critical for neuron survival in the injured cochlea, but its role in cochlear synaptopathy is unknown. Fractalkine, a chemokine, is constitutively expressed by SGNs and signals via its receptor CX3CR1 that is expressed on macrophages. The present study characterized the immune response and examined the function of fractalkine signaling in degeneration and repair of cochlear synapses following noise trauma. Adult mice wild type, heterozygous and knockout for CX3CR1 on a C57BL/6 background were exposed for 2 h to an octave band noise at 90 dB SPL. Noise exposure caused temporary shifts in hearing thresholds without any evident loss of hair cells in CX3CR1 heterozygous mice that have intact fractalkine signaling. Enhanced macrophage migration toward the IHC-synaptic region was observed immediately after exposure in all genotypes. Synaptic immunolabeling revealed a rapid loss of ribbon synapses throughout the basal turn of the cochlea of all genotypes. The damaged synapses spontaneously recovered in mice with intact CX3CR1. However, CX3CR1 knockout (KO) animals displayed enhanced synaptic degeneration that correlated with attenuated suprathreshold neural responses at higher frequencies. Exposed CX3CR1 KO mice also exhibited increased loss of IHCs and SGN cell bodies compared to exposed heterozygous mice. These results indicate that macrophages can promote repair of damaged synapses after moderate noise trauma and that repair requires fractalkine signaling.

Metformin therapy in a transgenic mouse model of Huntington's disease.

Huntington's disease (HD) is a hereditary neurodegenerative disease that leads to striatal degeneration and a severe movement disorder. We used a transgenic mouse model of HD (the R6/2 line with approximately 150 glutamine repeats) to test a new therapy for this disease. We treated HD mice with metformin, a widely used anti-diabetes drug, in the drinking water (0, 2 or 5mg/ml) starting at 5 weeks of age. Metformin treatment significantly prolonged the survival time of male HD mice at the 2mg/ml dose (20.1% increase in lifespan) without affecting fasting blood glucose levels. This dose of metformin also decreased hind limb clasping time in 11-week-old mice. The higher dose did not prolong survival, and neither dose of metformin was effective in female HD mice. Collectively, our results suggest that metformin may be worth further investigation in additional HD models.