MITOCHONDRIAL SHEATH MOVEMENT AND DETACHMENT IN MAMMALIAN, BUT NOT NONMAMMALIAN, SPERM INDUCED BY DISULFIDE BOND REDUCTION

The successful completion of the fertilization process requires the pr operly choreographed unsheathing of the tightly packaged sperm once it has been fully incorporated into the egg's cytoplasm. The nuclear and accessory structures of mammalian sperm become stabilized by disulfid e bonds (S-S) during epididymal maturation. This stabilization is reve rsed during fertilization by the reduction of S-S cross-linking, but l ittle is known about the effect of S-S reduction on individual disulfi de-hardened structures such as the sperm's connecting piece, fibrous s heath, and mitochondria. Here, we demonstrate the action of the S-S-re ducing environment on the mitochondrial sheath of mammalian sperm, vis ualized by the vital fluorescent probe Mito Tracker and by electron mi croscopy. In both human and bull sperm, mitochondria form a compact he lix (mitochondrial sheath) wrapped around the midpiece and connecting piece that can be fluorescently labelled by a short incubation with 10 0 nM MitoTracker. Exposure of bull sperm to 0.1-10 mM dithiothreitol ( DTT; a disulfide bond-reducing agent) induced a time and dose-dependen t sliding of the mitochondrial sheath down the axoneme, accompanied by the excision of the sperm tail and decondensation of the sperm nucleu s. Increasing the concentration of DTT to 100 mM accelerated mitochond rial movement, causing a completed stripping of sperm mitochondria and partial disassembly of the connecting piece. Likewise, human sperm re sponded to DTT treatment by the sliding or removal of the mitochondria l sheath and decondensation of the sperm chromatin. These events were not observed in the sperm of lower vertebrates and invertebrates (Xeno pus laevis and Lytechinus pictus, respectively) exposed to an excess o f DTT. Thus the sensitivity of sperm mitochondria to the S-S reducing environment seems to be an exclusive feature of mammalian sperm. The m ovement of sperm mitochondria induced by S-S reduction may be an initi al critical step in the disassembly of the mammalian sperm tail during fertilization. (C) 1997 Wiley-Liss, Inc.