The Acroplaxome Is the Docking Site of Golgi-Derived Myosin Va/Rab27a/b-Containing Proacrosomal Vesicles in Wild-Type and Hrb Mutant Mouse Spermatids1

Abstract

Acrosome biogenesis involves the transport and fusion of Golgi-derived proacrosomal vesicles along the acroplaxome, an F-actin/keratin 5-containing cytoskeletal plate anchored to the spermatid nucleus. A significant issue is whether the acroplaxome develops in acrosomeless mutant mice. Male mice with a Hrb null mutation are infertile and both spermatids and sperm are round-headed and lack an acrosome. Hrb, a protein that contains several NPF motifs (Asn-Pro-Phe) and interacts with proteins with Eps15 homology domains, is regarded as critical for the docking and/or fusion of Golgi-derived proacrosomal vesicles. Here we report that the lack of an acrosome in Hrb mutant spermatids does not prevent the development of the acroplaxome. Yet the acroplaxome in the mutant contains F-actin but is deficient in keratin 5. We also show that the actin-based motor protein myosin Va and its receptor, Rab27a/b, known to be involved in vesicle transport, are present in the Golgi and Golgi-derived proacrosomal vesicles in wild-type and Hrb mutant mouse spermatids. In the Hrb mutant, myosin-Va-bound proacrosome vesicles tether to the acroplaxome, where they flatten and form a flat sac, designated pseudoacrosome. As spermiogenesis advances, round-shaped spermatid nuclei of the mutant display several nuclear protrusions, designated nucleopodes. Nucleopodes are consistently found at the acroplaxome- pseudoacrosome site. Our findings support the interpretation that the acroplaxome provides a focal point for myosin-Va/ Rab27a/b-driven proacrosomal vesicles to accumulate, coalesce, and form an acrosome in wild-type spermatids and a pseudoacrosome in Hrb mutant spermatids. We suggest that nucleopodes develop at a site where a keratin 5-deficient acroplaxome may not withstand tension forces operating during spermatid nuclear shaping.

Failure to assemble the peri-nuclear structures in GOPC deficient spermatids as found in round-headed spermatozoa

“…In step 1-8 spermatids, the nuclear shape was round and not deformed yet, but fragmented acrosome-like vesicles, previously called pseudoacrosomes (Kierszenbaum et al, 2004), were found near the nucleus in appreciable sizes (Fig.1).…”

“…The marginal ring has recently been characterized and is thought to be essential for the acrosomal vesicles to be anchored to the inner membrane of nuclear envelope or nuclear lamina (Kierszenbaum et al, 2004). The marginal ring is generally formed at the descending edge of the acrosome, and consists of F-actin and keratin 5 in addition to some other unknown molecule(s) in wild-type spermatids ( Fig.…”

“…As to the chemical component of the acroplaxome, Hrb -/ -spermatids lack keratin 5, thus forming a defective acroplaxome plaque (Kierszenbaum et al, 2004). The weak scaffolding function may be due to the deficiency of keratin 5.…”

The cellular HIV-1 Rev cofactor hRIP is required for viral replication

“…First, hRIP activity is highly specific and is required for the proper cytoplasmic localization of RRE-containing RNAs but not typical cellular mRNAs (20). Second, hRIP is localized at the perinuclear region, where the machinery for vesicular trafficking is situated (35,41,42). Third, hRIP contains an N-terminal zinc finger motif that shares a high degree of sequence identity with the catalytic domain of ADP-ribosylation factor GTPase-activating proteins (Arf GAPs) (43).…”

“…Taken together, these findings suggest that the zinc finger motif of hRIP, and possibly a catalytic activity similar to that of Arf GAPs, may mediate release of Rev-RNA complexes from the perinuclear region. Fourth, hRIP is required for correct localization and function of acrosomal components during spermatogenesis, a process that depends upon vesicular trafficking (41,42). Finally, recent studies have shown that murine leukemia virus RNA is transported to the plasma membrane through a vesicular trafficking pathway (45).…”