Reversible phosphorylation of key proteins, such as ion channels and neurotransmitter-synthesizing enzymes, regulates synaptic transmission and other aspects of neuronal physiology and development. While the enzymes that add phosphates to proteins, protein kinases, have been studied extensively, much less is known about the equally important enzymes that catalyze the reverse reaction, protein phosphatases

PP2A is one of the major classes of Ser/Thr phosphatases with likely thousands of phospho-protein substrates. The predominant form of PP2A is a heterotrimer of catalytic, scaffolding, and variable regulatory subunits. These variable subunits define substrate specificity, subcellular localization, and regulation of PP2A by second messengers and reversible phosphorylation. Our research focuses on two brain-specific PP2A regulatory subunits with relevance to neurodegenerative disorders, B'β and Bβ2. B'β acts as a gatekeeper in the regulation of tyrosine hydroxylase (TH) activity and catecholamine synthesis. Inhibitors of PP2A enzymes containing this subunit may therefore have therapeutic potential for Parkinson's disease. Bβ2 is derived from a gene mutated in spinocerebellar ataxia type 12 (SCA12) and recruits PP2A to the outer mitochondrial membrane to promote mitochondrial fission or inhibit mitochondrial fusion. Mitochondrial restructuring by Bβ2 sensitizes neurons to a variety of insults. Bβ2 inhibitors may thus form the basis for novel neuroprotective therapies. The kinase opposing PP2A/Ββ2's effect on mitochondrial morphology and survival is cAMP-dependent protein kinase (PKA) anchored to mitochondria via A kinase anchoring protein (AKAP) 1. We have thus far identified phosphorylation sites in two of the dynamin-family GTPases that sculpt mitochondria and are exploring their involvement in the morphogenetic and survival modulating activities of PP2A/Bβ2 and PKA/AKAP1.

Current projects:

1. Outer mitochondrial PP2A and PKA in neuronal survival and synapse formation. The balance of mitochondria-associated kinase (PKA/AKAP1) and phosphatase (PP2A/Bβ2) activities regulates apoptosis and other aspects of mitochondrial function. Using gene modified mice and primary hippocampal cultures, we are characterizing the interplay between these outer mitochondrial PP2A and PKA holoenzymes in ischemic and excitotoxic injury, as well as in mitochondrial transport and synapse formation.
2. Regulation of the mitochondrial fission/fusion machinery by reversible phosphorylation. Mutations in the large dynamin-related GTPases that restructure mitochondria can cause neurodegenerative diseases and severe birth defects. Because outer mitochondrial PP2A and PKA control neuronal survival through the mitochondrial fission/fusion equilibrium, we are exploring phosphorylation of mitochondria-shaping enzymes as a novel checkpoint in the cell death program.
3. Role of PP2A in nerve growth factor signaling and neuronal differentiation. Neurotrophins signal through the Trk family of receptor tyrosine kinases to promote neuronal differentiation and survival. We have identified specific PP2A holoenzymes that either inhibit or stimulate TrkA-mediated nerve growth factor signaling and are in the process of characterizing their mechanism of action in PC12 cells and primary neurons.
4. Regulation of catecholamine biosynthesis by PP2A. Tyrosine hydroxylase (TH), the rate limiting enzyme in the synthesis of dopamine and (nor)epinephrine, is dephosphorylated and inactivated by a neuron-specific PP2A holoenzyme containing the B'β subunit. We are exploring the regulation of TH by PP2A/B'β in vitro, in PC12 cells, and in dopaminergic neurons with the eventual goal of developing new Parkinson's disease therapies.

Recent publications