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The testis-expressed chaperone protein, HspA2 (previously creatine kinase M isoform) was established as a measure of human sperm cellular maturity, function and fertility. The presence of HspA2 in the synaptonemal complex is likely to link low HspA2 expression and increased frequency of chromosomal aneuploidies in arrested-maturity spermatozoa. A relationship also exists between HspA2 expression in elongating spermatids and the associated spermatogenetic events, including plasma membrane remodelling and the formation of zona pellucida and hyaluronic acid (HA) binding sites. The HA receptor of mature spermatozoa, when coupled with HA-coated slides and/or Petri dishes, allows visual observation of sperm-HA binding, providing a basis for sperm maturity testing, a major improvement in semen evaluation, and selection of mature spermatozoa for intracytoplasmic sperm injection (ICSI). Thus, in HA-selected spermatozoa the frequency of chromosomal disomy and diploidy is reduced 4- to 6-fold compared with semen sperm fractions. This reduction is similar to the increase in numerical chromosomal aberrations in ICSI children. Combined studies of sperm shape and chromosome probes demonstrated that sperm morphology does not aid selection of haploid spermatozoa. The HA-mediated sperm selection is a novel and efficient technique that may alleviate potential problems related to ICSI fertilization with visually selected spermatozoa.

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We previously described a putative creatine kinase M isoform in human sperm that is developmentally regulated and expressed during late spermiogenesis, simultaneous with cytoplasmic extrusion. We have now identified this protein as the testis-expressed 70-kDa heat shock protein chaperone known as HspA2 (the human homologue of mouse Hsp70-2). We have isolated and characterized HspA2 (formerly CK-M) by amino acid sequencing and have localized it by immunocytochemistry to spermatocytes at low levels, to spermatids, and in the tail of mature sperm. The specificity of the CK-M/HspA2 antiserum to HspA2 was demonstrated on immunoblots of one- and two-dimensional SDS-PAGE. In agreement with our earlier biochemical data, immunocytochemistry of testicular tissue indicated that HspA2 is selectively expressed in mature spermatids and in sperm about to be released in the seminiferous tubuli. The identity of HspA2 has been further confirmed by cross-absorption of the mouse HSP70-2 antibody by the HspA2/CK-M fraction, and by identical immunostaining patterns of human testicular tissue using either the anti-CK-M/HspA2 or an anti-mouse Hsp70-2 antisera. During spermiogenesis, both cytoplasmic extrusion and plasma membrane remodeling, which facilitate the formation of the zona pellucida binding site, involve major intrasperm protein transport, which may be chaperoned by HspA2. Accordingly, in immature human sperm, which fail to express HspA2, there is cytoplasmic retention and lack of zona pellucida binding. The present findings provide the biological rationale for the role of the human HspA2 as an objective biochemical marker of sperm function and male fertility, which we have established in earlier clinical studies.