He has worked extensively on intracellular signaling by the cGMP-dependent protein kinases (PKGs). PKGs are key regulators of smooth muscle cell contractility, platelet aggregation, memory/learning, and bone homeostasis. They are involved in regulating cell growth, differentiation, and apoptosis, and in opposing the pathophysiology of pulmonary and systemic hypertension, cardiac hypertrophy, vascular injury/restenosis, and osteoporosis. Dr. Casteel’s studies have focused on PKG-protein interactions, including the identification of novel PKGI interacting proteins and the elucidation of a common PKGI interaction motif. He has collaborated with groups that have solved PKG crystal structures, and he has used deuterium exchange mass spectrometry to study the role of protein dynamics in regulating PKG activity.