The Protein Kinase A Regulatory Subunit R1A (Prkar1a) Plays Critical Roles in Peripheral Nerve Development

Abstract

Signaling through cAMP has been implicated in Schwann cell (SC) proliferation and myelination, but the signaling pathway components downstream of cAMP required for SC function remain unknown. Protein kinase A (PKA) is a potential downstream effector of cAMP. Here, we induced loss of Prkar1a, the gene encoding the type 1A regulatory subunit of PKA, in SC to study its role in nerve development; loss of Prkar1a is predicted to elevate PKA activity. Conditional Prkar1a knock-out in mouse SC (Prkar1a-SCKO) resulted in a dramatic and persistent axonal sorting defect, and unexpectedly decreased SC proliferation in Prkar1a-SCKO nerves in vivo. Effects were cell autonomous as they were recapitulated in vitro in Prkar1a-SCKO SC, which showed elevated PKA activity. In the few SCs sorted into 1:1 relationships with axons in vivo, SC myelination was premature in Prkar1a-SCKO nerves, correlating with global increase in the cAMPregulated transcription factor Oct-6 and expression of myelin basic protein. These data reveal a previously unknown role of PKA in axon sorting, an unexpected inhibitory role of PKA on SC cell proliferation in vivo and define the importance of Prkar1a in peripheral nerve development.

How Schwann Cells Sort Axons

“…Signaling from the basal lamina converge onto other signaling cascades that control the remodeling of actin cytoskeleton, and regulation of cyclic AMP and Protein kinase A (PKA) (Guo and others, 2013a; Guo and others, 2012; Montani and others, 2014). The remodeling of actin cytoskeleton and cell polarity is also regulated by Rho GTPase family members, and several Rho GTPases are required for radial sorting (Benninger and others, 2007; Guo and others, 2013b; Nodari and others, 2007).…”

“…However, we do not know if low levels of Neuregulins are enough to determine the non-myelination state, or if there are also specific inhibitory molecules expressed on small axons. Recent data indicate that several signaling pathways in Schwann cells, such as excessive PKA (Guo and others, 2013a), GSK3-beta activation (da Silva and others, 2014), and ILK-RhoA-ROCK signaling, (da Silva and others, 2014) all negatively regulate radial sorting. Are low levels of Neuregulin or other specific ligands activating these negative signals in Schwann cells?…”

The stellate vascular smooth muscle cell phenotype is induced by IL-1β via the secretion of PGE2 and subsequent cAMP-dependent protein kinase A activation

“…As mentioned above, the disruption of PKAR1α in vivo is associated with an increase in the catalytic activity of PKA [30][31][32], and this increase in catalytic activity (which is lethal in the embryo due to the failure to establish mesoderm-derived structures) is partially rescued by eliminating the PKA catalytic subunit α [42], providing evidence for a causal relationship between the elimination of PKAR1α and the increase in PKA catalytic activity. Embryonic fibroblasts from PKAR1α −/− mouse embryos display similar changes in morphology to VSMCs in which R1α is silenced, with a decrease in activated Rac levels [32].…”

“…Based on these data, we decided to silence R1α to confirm the involvement of PKA in this process. In many cell types and in vivo, the disruption of PKAR1α is associated with an increase in the catalytic activity of PKA [30][31][32]. Consistent with a role for PKA in stellate cell Table 3 Levels of mRNAs encoding inflammatory markers in transdifferentiated VSMCs.…”

“…Conversely, the pharmacological inhibition of Rac1 or the expression of a dominant negative form of Rac1 is sufficient to induce the transition to a stellate phenotype [20]. Finally, PKAR1α disruption in Schwann cells is associated with a decrease in the formation of lamellopodia [31], a process involving Rac1 activation [43]. This suggests a role for Rac1 in the PGE 2 /cAMP-dependent development of stellate morphology in VSMCs.…”