Transcriptional, biochemical, and immunohistochemical analyses of CaMKKß/2 splice variants that co-localize with CaMKIV in spermatids.

Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) phosphorylates and activates downstream protein kinases, including CaMKI, CaMKIV, PKB/Akt, and AMPK; thus, regulates various Ca2+-dependent physiological and pathophysiological pathways. Further, CaMKKß/2 in mammalian species comprises multiple alternatively spliced variants; however, their functional differences or redundancy remain unclear. In this study, we aimed to characterize mouse CaMKKß/2 splice variants (CaMKKß-3 and ß-3x). RT-PCR analyses revealed that mouse CaMKKß-1, consisting of 17 exons, was predominantly expressed in the brain; whereas, mouse CaMKKß-3 and ß-3x, lacking exon 16 and exons 14/16, respectively, were primarily expressed in peripheral tissues. At the protein level, the CaMKKß-3 or ß-3x variants showed high expression levels in mouse cerebrum and testes. This was consistent with the localization of CaMKKß-3/-3x in spermatids in seminiferous tubules, but not the localization of CaMKKß-1. We also observed the co-localization of CaMKKß-3/-3x with a target kinase, CaMKIV, in elongating spermatids. Biochemical characterization further revealed that CaMKKß-3 exhibited Ca2+/CaM-induced kinase activity similar to CaMKKß-1. Conversely, we noted that CaMKKß-3x impaired Ca2+/CaM-binding ability, but exhibited significantly weak autonomous activity (approximately 500-fold lower than CaMKKß-1 or ß-3) due to the absence of C-terminal of the catalytic domain and a putative residue (Ile478) responsible for the kinase autoinhibition. Nevertheless, CaMKKß-3x showed the ability to phosphorylate downstream kinases, including CaMKIα, CaMKIV, and AMPKα in transfected cells comparable to CaMKKß-1 and ß-3. Collectively, CaMKKß-3/-3x were identified as functionally active and could be bona fide CaMKIV-kinases in testes involved in the activation of the CaMKIV cascade in spermatids, resulting in the regulation of spermiogenesis.