Protein kinase A acts at the basal body of the primary cilium to prevent Gli2 activation and ventralization of the mouse neural tube

Abstract

Protein kinase A (PKA) is an evolutionarily conserved negative regulator of the hedgehog (Hh) signal transduction pathway. PKAis known to be required for the proteolytic processing event that generates the repressor forms of the Ci and Gli transcriptionfactors that keep target genes off in the absence of Hh. Here, we show that complete loss of PKA activity in the mouse leads tomidgestation lethality and a completely ventralized neural tube, demonstrating that PKA is as strong a negative regulator of thesonic hedgehog (Shh) pathway as patched 1 (Ptch1) or suppressor of fused (Sufu). Genetic analysis shows that although PKA isimportant for production of the repressor form of Gli3, the principal function of PKA in the Shh pathway in neural developmentis to restrain activation of Gli2. Activation of the Hh pathway in PKA mutants depends on cilia, and the catalytic and regulatorysubunits of PKA are localized to a compartment at the base of the primary cilia, just proximal to the basal body. The data showthat PKA does not affect cilia length or trafficking of smoothened (Smo) in the cilium. Instead, we find that there is a significantincrease in the level of Gli2 at the tips of cilia of PKA-null cells. The data suggest a model in which PKA acts at the base of thecilium after Gli proteins have transited the primary cilium; in this model the sequential movement of Gli proteins betweencompartments in the cilium and at its base controls accessibility of Gli proteins to PKA, which determines the fates of Gli proteinsand the activity of the Shh pathway. © 2011.