An Overview of The Globozoospermia as A Multigenic Identified Syndrome

Casein kinase IIα' polypeptide (Csnk2a2) was the first introduced protein whose gene was associated with globozoospermia. This protein is a kind of serine-threonine kinase which relates to nuclear matrix. Multiple forms of acrosome imperfection like complete lack of acrosome, indented/detached acrosome from nucleus, and acrosomal remnants were recognized in spermatozoa of Csnk2a2-deficient mice. In other words, mice lacking the Csnk2a2 gene demonstrated aberrant development in both nucleus and acrosome (14).

β-Glucosidase 2 (GBA2) is a glycolipid hydrolase resident in ER and its relation to globozoospermia was first recognized in glycolipid storage disease due to deficiency of Gba2 in male mice with reduced fecundity. Glucosylceramides are normally transferred from developing germ cells to Sertoli cells for subsequent breakdown. Loss of the GBA2 results in accumulation of glucosylceramide in Sertoli cells and disrupts transport of glycolipid from germ cells which in turn interrupts normal Sertoli-germ cell interactions. Therefore, this defect leads to formation of abnormal sperm (Fig .1A, D). Unlike in mice, mutational assessments for GBA2 in 3 unrelated families, originating from Britain, Canada, and Germany, have been unfruitful to show an association with globozoospermia in man (15).

Spermatogenesis-associated 16 (SPATA16), also known as NYD-SP12, is a human testis specific protein and its ortholog encoding gene is expressed in mouse spermatocyte and spermatids. SPATA16 has a subcellular localization in Golgi apparatus and pro-acrosmal vesicles being transported to acrosome. Its function is sorting and modification of acrosomal enzymes in Golgi network (16). This protein also interacts with other proteins involved in acrosomal biogenesis including GOPC and Hrb (Fig .1B). SPATA16 was the first gene which was shown to contribute to human globozoospermia with an autosomal dominant pattern of inheritance (6).

Hrb, formerly known as human Rev-binding/interacting protein (hRIP), is the cofactor of HIV-1 Rev protein, involved in shuttling of proteins between nucleus and cytoplasm. Hrb interacts with proteins involved in nucleo-cytoplasmic trafficking (17). Based on these functions, Hrb mice knockout model revealed that, Hrb is involved in vesicle to vesicle docking, fusion of pro-acrosmal vesicles with acrosome and thereby acrosomal biogenesis (Fig .1B). Therefore, its absence was associated with globozoospermia (18). Further analysis of Hrb-/- mice revealed a second role for Hrb in formation of acroplaxome plague. Acroplaxome is encompassed by 3 proteins including: F-actin, Sak57 (an ortholog of keratin5) and myosin Va. In Hrb deficient mice, keratin 5 filament bundle in acroplaxome is missing and the strength of acrosome vesicle in binding to nucleus is reduced which its outcome is manifested as globozoospermia (19).

Protein interacting with C kinase 1 (PICK1) was initially found in brain. It plays an important role in protein trafficking of neurons. Although the PICK1-/- mice were produced to study the brain function but these mice were infertile. PICK1 like GOPC has a postsynaptic density 95, discs large, and zonula occludens-1 (PDZ) domain which is involved in PAG trafficking (Fig .1B) (20). So far one mutation in this gene has been reported to be associated with globozoospermia (9).

GOPC gene, encodes Golgi-associated PDZ and coiled-coil motif containing protein (GOPC). GOPC protein has 5 domains including: one PDZ domain, two coiled-coil motifs, and two conserved domains with unknown function (21). GOPC is involved in PAG transport from Golgi network to acrosome and its absence (GOPC-/) is associated with globozoospermia (Fig .1B). In addition to lack of acrosome, other deformities associated with this defect, are lack of post-acrosomal sheath or peri-nuclear theca (22) and coiled-coil tail (23).

ZP binding protein 1 (ZPBP1) or Sp38 or Iam 38 and its paralog, ZPBP2, were described as acrosomal proteins in mice and human. ZPBP1- deficient male mice are sterile and present round-head spermatozoa due to disrupted acrosome biogenesis. Zpbp1 is an intra-acrosomal protein and Zpbp1-deficient spermatids demonstrate defective protein matrix assembly and results in fragmentation of the abnormal acrosomes (Fig .1B) (24).

Considering candidate genes responsible for abnormal sperm head morphology, heterozygous mutation in ZPBP1 were described in patients with aforementioned condition, however direct involvement of ZPBP1 in the onset of such conditions remains to be clarified (25).

Sperm acrosome associated 1 (SPACA1) or SAMP32 (sperm acrosomal membrane-associated protein 32) is a testis-specific transmembrane protein involved in sperm-egg interaction. During elongation stage of developing spermatozoa, this protein is localized in inner acrosomal membrane (Fig .1B) (26) and no role has been envisaged in acrosome reaction. The role of this protein in globozoospermia was initially recognized when this protein was absent in Gopc- and Zpbp1-disrupted mouse line. However, later studies revealed that “disruption of Gopc caused a significant decrease in SPACA1 and ZPBP1” while “disruption of Zpbp1 caused loss of SPACA1whereas GOPC was unaffected” and “disruption of Spaca1 did not affect the amounts of GOPC and ZPBP1 in the testis”. Thereby, suggesting that Spaca1 is likely downstream of these two genes. Spaca1 deficiency leads to failure of acrosome thinning, coinciding with instability/or loss of acroplaxome and nuclear plate (27) and unlike most of aforementioned proteins, it has no role in protein transport in golgi network or in acrosome formation.

Heat shock protein 90b1 (Hsp90b1), a member of heat shock protein 90 family, is a testis specific endoplasmic chaperone involving in entire folding, activation and/or degradation of ER proteins (Fig .1B). It was shown that Hsp90b1- null sperm cells are round and not able to fertilize the oocyte. Therefore, absence of this protein showed a potential role in the incidence of globozoospermia (28). Recent study has hypothesized that phosphorylation of Hsp90b1 along with other chaperon proteins during sperm capacitation leads to the formation of ZP -recognized protein complexes and/or the translocation of these complexes to the surface of spermatozoa (29).

Vps54 is a protein apparently involved in tethering of vesicles from endosomes to the trans-golgi sacs (13). This is an alternative pathway in acrosome biogenesis as mentioned earlier. The role of this protein in acrosomal biogenesis was found when wobbler mouse with Vps54(L967Q) mutation were found to cause sterility. The protein codified by the Vps54 gene, has an active role in vesicular retrograde trafficking and like Hrb gene affects proacrosomal vesicle coalesces with acrosome (Fig .1B) (30).

SPAG4L and its isoform, are testis specific proteins, belong to SUN domain proteins. These transmembrane proteins are located on inner nuclear membrane (INM). By interacting with their partner on outer nuclear membrane (ONM), known as KASH domain can anchor or create linkage to nucleo- and cytoskeleton complex (LINC complex) (Fig .1C) (31).

Different members of this anchoring system have been discovered but their role in acrosomal biogenesis remains to be determined. Among these proteins, absence of SPAGL4/4L-2 has been associated with globozoospermia. SPAG4L/4L-2 is localized on apical side of nuclear membrane of developing spermatid and it may have a function in docking of acrosome vesicle to nuclear membrane (31).

DPY19L2, similar to SPAGL4/4L-2, is a transmembrane protein with 8-10 predicted domains in inner nuclear membrane. The expression of this protein is restricted to testis and like SPAGL4 (or SUN5) is involved in anchorage of cytoskeleton to nuclear membrane (Fig .1C). Therefore, its absence leads to instability and dissociation of the layered structure of acroplaxome, the nuclear/acrosome bridging region. Thereby, its absence results in formation of round head spermatozoa (32). ElInati et al. (10) revealed that DPY19L2 gene has an inevitable relationship with globozoospermia. They have shown that DPY19L2 is one of the main genes responsible for globozoospermia. In this regard, a wide spectrum of plausible mutations for this gene has been detected in globozoospermic individuals such as: deletion of the whole locus, nonsense, missense, splicing mutations, partial deletion in different regions of the gene encompassing exons 8, 9, 11, 15, 21 and intron 11 (10, 33-36).

Calicin is one of the subacrosomal cytoskeletal proteins involved in acrosomal biogenesis which its absence results in globozoospermia (8).