Defective PKA Signaling in Cushing's Syndrome

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Summary/Abstract Molecular and cellular endocrine responses often evoke the mobilization of signal transduction cascades. This fundamental process proceeds through macromolecular assemblies of signaling enzymes sequestered with preferred substrates. The relay of information through these solid-state signaling units ensures initiation (or termination) of molecular events at defined intracellular locations. Recent evidence suggests that disruption of protein-protein interactions that sustain local signal complexes is linked to disease. This proposal will test if mutations affecting the subcellular distribution of the catalytic (C) subunit of protein kinase A (PKA) contribute to the etiology of ACTH-independent Cushing's syndrome. Cushing's syndrome is an endocrine disorder diagnosed by excessive cortisol levels in the blood, mid-section weight gain, diabetes and hypertension. ACTH-dependent Cushing's disease often occurs as a consequence of pituitary tumors that overproduce adrenocorticotropic hormone (ACTH), which stimulates excess cortisol release from the adrenal glands. However, ACTH-independent forms of the disease are linked to mutations in genes encoding the catalytic subunits of protein kinase A PKA holoenzymes exist as heterotetramers consisting of two regulatory (R) and two catalytic (C) subunits. A traditional view infers R and C subunits of PKA dissociate upon activation by the second messenger cAMP. Recent discoveries have redefined our understanding of how this configuration operates. Last year we showed that active C subunits are not released from type II PKA holoenzymes when cells are stimulated with hormones. Under this new paradigm, active C subunits remain associated with RII subunits, and constrained within subcellular “signaling islands” by A-kinase anchoring proteins (AKAPs). Consequently, active kinase remains sequestered within a few microns of substrates. These findings have forged a testable hypothesis that mislocalization of active PKA is responsible for ACTH- independent Cushing's syndrome. Preliminary studies imply that 1) recruitment to AKAP signaling islands is the key determinant for type II PKA substrate selectivity, and 2) mutations that prohibit C subunit interaction with anchored R subunits lead to mislocalized and unregulated kinase in Cushing's syndrome. Two specific aims are proposed. Aim1 will integrate structural, live-cell imaging and chemical-biology strategies to establish the spatial parameters of the type I PKA isozyme. Aim 2 will combine CRISPR/Cas9 gene-editing in adrenal cells with live-cell imaging and cortisol profiling to investigate if recently identified mutations in PKA C subunits linked to Cushing's syndrome preclude recruitment into AKAP signaling islands to drive this endocrine disorder.