Apoptosis inducing factor deficiency causes reduced mitofusion 1 expression and patterned Purkinje cell degeneration

Alteration in mitochondrial dynamics has been implicated in many neurodegenerative diseases. Mitochondrial apoptosis inducing factor (AIF) plays a key role in multiple cellular and disease processes. Using immunoblotting and flow cytometry analysis with Harlequin mutant mice that have a proviral insertion in the AIF gene, we first revealed that mitofusion 1 (Mfn1), a key mitochondrial fusion protein, is significantly diminished in Purkinje cells of the Harlequin cerebellum. Next, we investigated the cerebellar pathology of Harlequin mice in an age-dependent fashion, and identified a striking process of progressive and patterned Purkinje cell degeneration. Using immunohistochemistry with zebrin II, the most studied compartmentalization marker in the cerebellum, we found that zebrin II-negative Purkinje cells first started to degenerate at 7 months of age. By 11 months of age, almost half of the Purkinje cells were degenerated. Subsequently, most of the Purkinje cells disappeared in the Harlequin cerebellum. The surviving Purkinje cells were concentrated in cerebellar lobules IX and X, where these cells were positive for heat shock protein 25 and resistant to degeneration. We further showed that the patterned Purkinje cell degeneration was dependent on caspase but not poly(ADP-ribose) polymerase-1 (PARP-1) activation, and confirmed the marked decrease of Mfn1 in the Harlequin cerebellum. Our results identified a previously unrecognized role of AIF in Purkinje cell degeneration, and revealed that AIF deficiency leads to altered mitochondrial fusion and caspase-dependent cerebellar Purkinje cell loss in Harlequin mice. This study is the first to link AIF and mitochondrial fusion, both of which might play important roles in neurodegeneration.

Research Highlights
► This study is the first report to link the deficiency of mitochondrial apoptosis inducing factor (AIF) and the mitochondrial fusion protein mitofusion 1 (Mfn1) and to show that AIF deficiency leads to down-regulation of Mfn1.
► This study is also the first report to show the massive Purkinje cell degeneration in Harlequin mutant mice, which occurs in a striking progressive and patterned process.
► We further demonstrate that the Purkinje cell degeneration is dependent on caspase but not poly(ADP-ribose) polymerase-1 (PARP-1) activation, and that Purkinje neurons positive for heat shock protein 25 are resistant to degeneration.
► Our experimental results are unexpected, but also exciting because they suggest that AIF is involved in regulating mitochondrial fusion and neurodegeneration.