**6.4 PKA and motility**

*Innovations in Assisted Reproduction Technology*

Calcium plays a key role in sperm function by different aspects. Recent studies have been demonstrated that in knockout mice there are at least four components participate in the intracellular regulation of calcium level and initiation of sperm motility. These are CatSper1, CatSper2, Cav2.3 and PMCA4. CatSper1 are localized in the principle piece of sperm and it is a voltage gated Ca2+ channels of the testis. Lacking or any mutation in CatSper1 gene reduces the progressive motility and causes infertility. A sperm cell that lacks the CatSper1 showed progressive motility but failed to develop hyperactivated motility [25]. CatSper2 present in flagellum shows similarity to CatSper1 and it is also a voltage-gated ion channel. Sperm of mice having knockout CatSper2 gene depict decreased flagellar amplitude and also failed to develop hyperactivated motility [26]. Disruption of gene for PMCA4, that have Ca2+/calmodulin dependent ATPase activity involve in efflux of Ca2+, also causes infertility in men. In developing sperm cells and sperm flagellum the cyclic nucleotide gated Ca2+ channels are present. The role of these channels is to regulate

During sperm motility regulation, cAMP is the second key messenger. Adenylate cyclase converts the ATP into cAMP. Thus, the level of cAMP increases and in turn activates the cAMP dependent kinase A (PKA) which phosphorylates the serine and threonine residues in the flagellum, which ultimately causes the phosphorylation of tyrosine residues in the proteins [27, 28]. In most cells the adenylyl cyclase is activated by G protein in response to external stimuli. In mouse sperm the plasma membrane bounds (mACs) activated by G protein take a part in the acrosome reaction, and in chemotaxis and hyperactivation in human sperm [29]. It was

the influx of calcium in various micro domains of the flagella [26].

**6.2 Role of calcium in motility**

**6.3 cAMP and motility**

**30**

**Figure 3.**

*Signalling pathway showing regulation of motility of sperm in mammals.*

PKA causes the phosphorylation of tyrosine residue of flagellar proteins. The proteins anchoring with PKA site (AKAP3, AKAP4 and TAKAP-80) in the fibrous sheath, point out that the main role of this structure is to bind PKA in the principle piece of flagellum [31]. Regulatory and catalytic subunits are present in PKA holoenzyme. Four genes (RIα, RIβ, RIIα and RIIβ) are present in regulatory subunits (R subunit) in human and mouse; three catalytic (C subunit) Cα, Cβ and Cγ in human, and two C subunit Cα and Cβ in mice. The cAMP binding site are present in R and C subunits. C subunits is released when cAMP binds to R subunits and their catalytic site is activated by cAMP. The R and C subunits are involved in the motility of sperm (**Figure 3**).
