A brief history of hair cell regeneration research and speculations on the future.

Millions of people worldwide suffer from hearing and balance disorders caused by loss of the sensory hair cells that convert sound vibrations and head movements into electrical signals that are conveyed to the brain. In mammals, the great majority of hair cells are produced during embryogenesis. Hair cells that are lost after birth are virtually irreplaceable, leading to permanent disability. Other vertebrates, such as fish and amphibians, produce hair cells throughout life. However, hair cell replacement after damage to the mature inner ear was either not investigated or assumed to be impossible until studies in the late 1980s proved this to be false. Adult birds were shown to regenerate lost hair cells in the auditory sensory epithelium after noise- and ototoxic drug-induced damage. Since then, the field of hair cell regeneration has continued to investigate the capacity of the auditory and vestibular epithelia in vertebrates (fishes, birds, reptiles, and mammals) to regenerate hair cells and to recover function, the molecular mechanisms governing these regenerative capabilities, and the prospect of designing biologically-based treatments for hearing loss and balance disorders. Here, we review the major findings of the field during the past 25 years and speculate how future inner ear repair may one day be achieved.

Reduction of mutant ataxin-7 expression restores motor function and prevents cerebellar synaptic reorganization in a conditional mouse model of SCA7.

Spinocerebellar ataxia type 7 (SCA7) is a dominantly inherited neurodegenerative disorder caused by a CAG - polyglutamine (polyQ) repeat expansion in the ataxin-7 gene. In polyQ disorders, synaptic dysfunction and neurodegeneration may develop prior to symptom onset. However, conditional expression studies of polyQ disease models demonstrate that suppression of gene expression can yield complete reversal of established behavioral abnormalities. To determine if SCA7 neurological and neurodegenerative phenotypes are reversible, we crossed PrP-floxed-SCA7-92Q BAC transgenic mice with a tamoxifen-inducible Cre recombinase transgenic line, CAGGS-Cre-ER™. PrP-floxed-SCA7-92Q BAC;CAGGS-Cre-ER™ bigenic mice were treated with a single dose of tamoxifen 1 month after the onset of detectable ataxia, which resulted in ~50% reduction of polyQ-ataxin-7 expression. Tamoxifen treatment halted or reversed SCA7 motor symptoms, reduced ataxin-7 aggregation in Purkinje cells (PCs), and prevented loss of climbing fiber (CF)-PC synapses in comparison to vehicle-treated bigenic animals and tamoxifen-treated PrP-floxed-SCA7-92Q BAC single transgenic mice. Despite this phenotype rescue, reduced ataxin-7 expression did not result in full recovery of cerebellar molecular layer thickness or prevent Bergmann glia degeneration. These results demonstrate that suppression of mutant gene expression by only 50% in a polyQ disease model can have a significant impact on disease phenotypes, even when initiated after the onset of detectable behavioral deficits. The findings reported here are consistent with the emerging view that therapies aimed at reducing neurotoxic gene expression hold the potential to halt or reverse disease progression in afflicted patients, even after the onset of neurological disability.

Spinocerebellar ataxia type 7 cerebellar disease requires the coordinated action of mutant ataxin-7 in neurons and glia, and displays non-cell-autonomous bergmann glia degeneration.

Spinocerebellar ataxia type 7 (SCA7) is a dominantly inherited disorder characterized by cerebellum and brainstem neurodegeneration. SCA7 is caused by a CAG/polyglutamine (polyQ) repeat expansion in the ataxin-7 gene. We previously reported that directed expression of polyQ-ataxin-7 in Bergmann glia (BG) in transgenic mice leads to ataxia and non-cell-autonomous Purkinje cell (PC) degeneration. To further define the cellular basis of SCA7, we derived a conditional inactivation mouse model by inserting a loxP-flanked ataxin-7 cDNA with 92 repeats into the translational start site of the murine prion protein (PrP) gene in a bacterial artificial chromosome (BAC). The PrP-floxed-SCA7-92Q BAC mice developed neurological disease, and exhibited cerebellar degeneration and BG process loss. To inactivate polyQ-ataxin-7 expression in specific cerebellar cell types, we crossed PrP-floxed-SCA7-92Q BAC mice with Gfa2-Cre transgenic mice (to direct Cre to BG) or Pcp2-Cre transgenic mice (which yields Cre in PCs and inferior olive). Excision of ataxin-7 from BG partially rescued the behavioral phenotype, but did not prevent BG process loss or molecular layer thinning, while excision of ataxin-7 from PCs and inferior olive provided significantly greater rescue and prevented both pathological changes, revealing a non-cell-autonomous basis for BG pathology. When we prevented expression of mutant ataxin-7 in BG, PCs, and inferior olive by deriving Gfa2-Cre;Pcp2-Cre;PrP-floxed-SCA7-92Q BAC triple transgenic mice, we noted a dramatic improvement in SCA7 disease phenotypes. These findings indicate that SCA7 disease pathogenesis involves a convergence of alterations in a variety of different cell types to fully recapitulate the cerebellar degeneration.

A simple composite phenotype scoring system for evaluating mouse models of cerebellar ataxia.

We describe a protocol for the rapid and sensitive quantification of disease severity in mouse models of cerebella ataxia. It is derived from previously published phenotype assessments in several disease models, including spinocerebellar ataxias, Huntington s disease and spinobulbar muscular atrophy. Measures include hind limb clasping, ledge test, gait and kyphosis. Each measure is recorded on a scale of 0-3, with a combined total of 0-12 for all four measures. The results effectively discriminate between affected and non-affected individuals, while also quantifying the temporal progression of neurodegenerative disease phenotypes. Measures may be analyzed individually or combined into a composite phenotype score for greater statistical power. The ideal combination of the four described measures will depend upon the disorder in question. We present an example of the protocol used to assess disease severity in a transgenic mouse model of spinocerebellar ataxia type 7 (SCA7).