Cody McKee - PhD Candidate, Neuroscience Graduate Program
Protein Phosphatase 1 (PP1) is a major Serine (Ser)/Threonine (Thr) phosphatase responsible for more than half of all Ser/Thr dephosphorylation events in eukaryotic cells. Three genes encode the three major isoforms of PP1 (α, β, and γ). While PP1α and PP1γ are considered major players in synaptic physiology, the neuronal function of PP1β is unknown. Recently, de novo mutations in PP1β have been linked to intellectual developmental disabilities in children, suggesting a critical role for PP1β in the central nervous system. While correlations between PP1 and various other neurodevelopmental/neurodegenerative diseases have been suggested, a causative role for PP1 in many of these contexts has yet to be established. The current study seeks to investigate the neuronal role of PP1β in vivo, and to uncover potential mechanisms by which PP1β influences neuronal function.
A Thy1-Cre mouse line was used to generate neuron specific PP1β conditional KO (PP1β cKO) mice. These mice exhibit a failure to thrive and typically die by 2-3 postnatal weeks. Hippocampal slice recordings demonstrated increased paired-pulse facilitation, suggesting impaired neurotransmitter release. In agreement with studies suggesting activity influences myelination within specific brain regions, we found significantly lower levels of myelin basic protein in the cortex of PP1β cKO mice. Furthermore, to assess the influence of PP1β on myelin function in a predominately activity-independent context, we measured compound action potentials (CAPs) along the optic nerve. Deficits in CAP recordings suggested impaired optic nerve myelination. However, analysis of the electron micrographs failed to detect a significant difference in myelinated axons. Using immunofluorescence, we then uncovered significantly fewer nodes of Ranvier in PP1β cKO mice that could potentially explain the CAP recordings. This deficit in nodes coincided with an increase in phosphorylation of PP1β-specific substrate, myosin light chain, which localizes to nodes of Ranvier. These data suggest a potential role for PP1β in nodal structure that could influence action potential propagation.
To then study the role of PP1β in adolescent mice, we generated a neuron specific inducible PP1β KO mouse line (iKO). These iKO mice exhibit progressive deterioration of hind limb functionality and premature demise at ~4 weeks post recombination. We then uncovered significant changes in various respiratory parameters suggesting a potential mechanism to explain the premature demise. However, while no morphological changes were observed within neuromuscular junctions in the diaphragm, it is possible that neurotransmitter release at these synapses is abrogated, and this will be investigated in the future.
These data support the hypothesis that PP1β alters action potential propagation in a way that disrupts downstream functionality. These results shed light on the role of PP1β and potential mechanisms that could be disrupted by PP1β in pathological states. Future studies will seek to uncover the molecular substrates underlying these effects and provide potential therapeutic targets for diseases in which PP1β functionality may be altered.
May 10, 2024 @ 2:00 p.m.
Medical Center | K-207 (2-6408)
Host: Advisor: Houhui (Hugh) Xia