The role of an endogenous PKA inhibitor, PKl alpha, in organizing left-right axis formation

Protein kinase inhibitor (PKI) is an endogenous inhibitor of cAMP-dependent protein kinase A (PKA). We have found that the cr-isoform of PKI (PKI alph a) is asymmetrically expressed along the left-right (L-R) axis in chick emb ryos, At stage 6, PKI alpha is expressed on the right side of the node, and this asymmetric expression continues until stage 7+. After stage 8, PKI al pha expression returns symmetric. Treatment of embryos with antisense PKI a lpha oligonucleotides increased the incidence of reversed heart looping. An tisense oligonucleotides also induced ectopic expression of the left-specif ic genes Nodal and Pitx2, and suppressed the expression of the right-specif ic gene SnR in the right lateral plate mesoderm. Similarly, treatment with PEA activators forskolin and Sp-cAMPs resulted in both reversed heart loopi ng and bilateral expression of Nodal. Ectopic activin induced PKI alpha on the left side of the node, while ectopic Shh and anti-Shh antibody had no e ffect on PKI alpha expression. Taken together, these data suggest that PKI alpha induced by an activin-like molecule, through the inhibition of PKA ac tivity, suppresses the Nodal-Pitx2 pathway on the right side of the body.