Axonal degeneration is mediated by the mitochondrial permeability transition pore - PubMed (original) (raw)

Axonal degeneration and mitochondrial swelling are delayed by CsA. Representative electron micrographs of control (A, B), CsA-treated (C), and JNK inhibitor-treated (D) wild-type sciatic nerves from explants cultured for 3 d (A–D). For comparison, representative images from distal nerve at 3 d in vivo axotomy are shown (F–K). A–D, Nerves were processed immediately for EM or after 3 d incubation in vehicle, CsA (20 μ

m

), or the JNK inhibitor SP600125 (60 μ

m

). Top row, WT nerves at low magnification; bottom row, corresponding mitochondria at high magnification. A, At 0 d the top shows that axons are rounded, Schmidt-Lanterman incisures are visible, and the tissue is well organized. Examples of the morphology of mitochondria from day 0 axons are shown at high magnification in the bottom. B, After incubation for 3 d in vehicle solution, the tissue is disorganized, axonal degeneration is extensive, and myelin sheaths are collapsed. In the high-magnification images at the bottom, mitochondria of axons with conserved axoplasm are clearly increased in diameter. C, After 3 d exposure to CsA, the nerve tissue is less altered compared with A, and more axons show a preserved axoplasm; the high-magnification images show mitochondria comparable those in A. D, After exposure to SP600125, the overall picture is similar to that after CsA, including the appearance of mitochondria. Scale bars: top, 10 μm; bottom, 300 nm. E, Average diameter of mitochondria in WT axons measured in EM transverse sections of axons with preserved axoplasm is shown with error bars for SEM. At 3 d of incubation, the diameter of mitochondria is 225% of the control value. This swelling is prevented substantially by CsA, the JNK inhibitor SP600125, DIDS, R.Red, and BAPTA-AM (n ≥ 45 mitochondria/nerve over 3 separate experiments; # p < 0.05 by Student's t test compared with 0 d; *p < 0.05 by Student's t test compared with 3 d vehicle). F–K, Axons undergoing degeneration in vivo display swollen mitochondria comparable to those in explants. Clusters of mitochondria are a characteristic feature of degenerating axons in explants (data not shown) and in vivo (F, G). Other abnormalities include remodeled mitochondrial cristae (H–K) and rupture of the outer mitochondrial membrane (arrow in K, which represents a higher magnification of a region in J). Accumulation of electron-dense material is usually associated with swollen mitochondria (F–I, arrowhead in I); the nature of this dense and disorganized material is not clear, but an interesting possibility is that it represents aggregated cytoskeletal proteins in the process of degeneration. Scale bars: (F, G), 1 μm; (H, J), 200 nm; (I), 300 nm. L, Mitochondrial swelling precedes axonal degeneration. Wild-type sciatic nerve explants were incubated in vehicle solution for the indicated durations. Nerve explants were fixed and processed for electron microscopy. The left _y_-axis shows mean mitochondrial diameter in axons (black circles, solid black line) vs Schwann cells of cross-sectioned sciatic nerves (black triangles, dashed black line), and the right _y_-axis shows axon degeneration expressed as percentage of degenerated axons (right _y_-axis, red triangles, solid red line). Axonal mitochondrial diameters but not those in the Schwann cells show a rapid increase after 6 h. In contrast, axonal degeneration is not apparent until after 24 h (n = 3 per each time point, between 30 and 100 mitochondria measured per n; error bars indicate SEM).