Figure 8.
Diminished QKI expression results in demyelination and disruption of paranodal structures in the adult AN. a–d, Ultrastructural features of AN fibers (a), a paranodal junction (b), axon initial segments at the Hab opening (c), and SGNs in cochleae from control QKIFL/FL;- mice 4 weeks after OHT injections (d). e, Low-magnification image of a control AN highlighting the location of RC, OSL, and Hab. f–j, Abnormalities in myelin morphology around axons and type I (I) SGNs and paranodal structures in QKIFL/FL;PLPCreERT mice 4 weeks after OHT injections. Pathological manifestations associated with diminished QKI expression included missing nerve fibers (f; *), myelin enfolding (f, white arrows), thinning myelin or reduced myelin lamellae (f, i, j, black arrows), a loss of the myelin sheath at the axon initial segment (h; *), and cytoplasmic vacuolization in glial cells (i, black arrowheads). The image in g shows disruptions in a paranode (black arrowheads) of a mutant mouse. Terminal myelin loops of the paranodal junctions flanking the nodes of Ranvier in the mutant mouse are disorganized, with abnormal spaces between the lamellae, and loop heads disconnected from the axolemma. Myelin lamellae ensheathing the soma of a mutant mouse are less compact and decreased in number (j, black arrow). k, G ratio analysis revealed hypomyelination in the AN fibers, in particular the large-caliber fibers (with diameters of >3 μm), in QKIFL/FL;PLPCreERT mice. l, m, Immunostaining for paranodal Cntn1 and the neuronal marker neurofilament 200 showed that while numbers of NF+ nerve fibers (m, white arrows) remained similar, the expression of paranodal Cntn1 was decreased greatly (i, m, white arrows) in mutant mice. n–p, Immunostaining for Cntn1 and the nodal marker NrCAM revealed the disappearance of Cntn1+ paranodal flanks in heminodes at the Hab (n, white arrows) the nodes of Ranvier along fibers in the OSL (o), and in RC nodes (p; arrowheads). q, r, ABR measurements from control and mutant mice showed no significant difference in ABR wave I thresholds (q) or amplitudes (r) between the two groups at 8 and 16 kHz. s, In contrast, ABR wave I latencies were significantly increased in the mutant mice (*p < 0.05 by Student's unpaired t test; Figure 8-1, for exact statistical output). All data were presented as the mean ± SEM. Scale bars: a, f, 2 μm; b, g, 400 nm; c, d, h, i, 2 μm; j, 800 nm; l, m, 8 μm; p (for n–p), 8 μm.