Mitochondrial fusion protein MFN2 interacts with the mitostatin-related protein MNS1 required for mouse sperm flagellar structure and function.

BACKGROUND: Cilia and the sperm flagellum share many structural properties. Meiosis-specific nuclear structural 1 (MNS1) is a recently characterized protein that is abundantly expressed in post-meiotic spermatids and is required for proper flagellar and motile cilia formation. To explore the possible functions of MNS1, we performed a BLAST search and determined it is homologous to the conserved domain pfam13868, exemplified by mitostatin. This protein interacts with mitofusin 2 (MFN2), a protein that participates in regulating mitochondrial associations to subcellular organelles. We hypothesized that an association between MFN2 and MNS1 in the sperm is involved in flagellar biogenesis and function. RESULTS: In the studies reported here, MFN2 was found in murine reproductive and somatic tissues high in ciliary content while MNS1 was present as two closely migrating bands in reproductive tissues. Interestingly, mitostatin was also present in reproductive tissues. Similar to Mns1 and mitostatin, Mfn2 was expressed in the testis as detected by RT-PCR. In addition, Mfn2 and Mns1 decreased in expression from pachytene spermatocytes to condensing spermatids as assessed by quantitative RT-PCR. Co-immunoprecipitation demonstrated an association between MFN2 and MNS1 in spermatogenic cells. Indirect immunofluorescence indicated that MFN2 and MNS1 co-localized to the sperm flagellum in freshly collected cauda epididymal sperm. MFN2 associated with the midpiece while MNS1 was present throughout the sperm tail in caput and cauda epididymal sperm. In spermatogenic cells, MFN2 was seen in the mitochondria, and MNS1 was present throughout the cell cytoplasm. MFN2 and MNS1 were present in detergent-resistant flagellar structures of the sperm. CONCLUSIONS: These results demonstrate that MFN2 and MNS1 are present in spermatogenic cells and are an integral part of the sperm flagellum, indicating they play a role in flagellar biogenesis and/or function.

Adenine nucleotide metabolism and a role for AMP in modulating flagellar waveforms in mouse sperm.

While most ATP, the main energy source driving sperm motility, is derived from glycolysis and oxidative phosphorylation, the metabolic demands of the cell require the efficient use of power stored in high-energy phosphate bonds. In times of high energy consumption, adenylate kinase (AK) scavenges one ATP molecule by transphosphorylation of two molecules of ADP, simultaneously yielding one molecule of AMP as a by-product. Either ATP or ADP supported motility of detergent-modeled cauda epididymal mouse sperm, indicating that flagellar AKs are functional. However, the ensuing flagellar waveforms fueled by ATP or ADP were qualitatively different. Motility driven by ATP was rapid but restricted to the distal region of the sperm tail, whereas ADP produced slower and more fluid waves that propagated down the full flagellum. Characterization of wave patterns by tracing and superimposing the images of the flagella, quantifying the differences using digital image analysis, and computer-assisted sperm analysis revealed differences in the amplitude, periodicity, and propagation of the waves between detergent-modeled sperm treated with either ATP or ADP. Surprisingly, addition of AMP to the incubation medium containing ATP recapitulated the pattern of sperm motility seen with ADP alone. In addition to AK1 and AK2, which we previously demonstrated are present in outer dense fibers and mitochondrial sheath of the mouse sperm tail, we show that another AK, AK8, is present in a third flagellar compartment, the axoneme. These results extend the known regulators of sperm motility to include AMP, which may be operating through an AMP-activated protein kinase.

Male mice express spermatogenic cell-specific triosephosphate isomerase isozymes.

Triosephosphate isomerase 1 (TPI1) is a member of the glycolytic pathway, which is a critical source of energy for motility in mouse sperm. By immunoblotting, we detected two male, germ line-specific TPI1 bands (Mr 33,400 and 30,800) as well as the somatic-type band (Mr 27,700). Although all three bands were observed in spermatogenic cells, somatic-type TPI1 disappeared from sperm during epididymal maturation. In vitro dephosphorylation analysis suggested that the two male, germ line-specific TPI1 bands were not the result of phosphorylation of the 27,700 Mr TPI1 band. The Mr 33,400; 30,800; and 27,700 TPI1 bands corresponded to the respective sizes of the proteins predicted to use the first, second, and third possible initiation codons of the Tpi1 cDNA. We performed immunofluorescence on epididymal sperm and determined that TPI1 specifically localized in the principal piece. The antibody staining was stronger in cauda epididymal sperm than in caput epididymal sperm, a finding consistent with the identification of TPI1 as a cauda epididymal sperm-enriched protein. Immunofluorescence with sodium dodecyl sulfate (SDS)-insoluble flagellar accessory structures showed a strong TPI1 signal only in the principal piece, indicating that TPI1 is a component of the fibrous sheath. Northern blot hybridization detected longer Tpi1 transcripts (1.56 kb) in mouse testis, whereas somatic tissues had shorter transcripts (1.32 kb). As there is only one triosephosphate isomerase gene in the mouse genome, we conclude that the three variants we see in sperm result from the use of alternative translation start codons in spermatogenic cells.