Emerging roles of planar cell polarity proteins in glutamatergic synapse formation, maintenance and function in health and disease

The local signaling mechanism which directly assembles and maintains glutamatergic synapses has not been well understood. Glutamatergic synapses are made of presynaptic and postsynaptic compartments with distinct sets of proteins. The planar cell polarity (PCP) pathway is highly conserved and responsible for establishing and maintaining the cell and tissue polarity along the tissue plane. The six core PCP proteins form antagonizing complexes within the cells and asymmetric intercellular complexes across neighboring cells which regulate cell-cell interactions during planar polarity signaling. Accumulating evidence suggests that the PCP proteins play essential roles in glutamatergic synapse assembly, maintenance and function in the brain. This review summarizes the key evidence that PCP proteins may be responsible for the formation and stability of the vast majority of the glutamatergic synapses in hippocampus and medial prefrontal cortex, the progress in understanding the mechanisms of how PCP proteins assemble and maintain glutamatergic synapses and initial insights on how disruption of the function of the PCP proteins can lead to neurodegenerative, neurodevelopmental and neuropsychiatric disorders. The PCP proteins may be the missing pieces of a long-standing puzzle and filling this gap of knowledge may provide the basis for understanding many unsolved questions in synapse biology.

The Fragile X Messenger Ribonucleoprotein 1 Participates in Axon Guidance Mediated by the Wnt/Planar Cell Polarity Pathway

The Planar cell polarity (PCP) pathway is known to mediate the function of the Wnt proteins in growth cone guidance. Here, we show that the PCP pathway may directly influence local protein synthesis within the growth cones. We found that Fragile X Messenger Ribonucleoprotein 1 (FMRP) interacts with Fzd3. This interaction is negatively regulated by Wnt5a, which induces FMRP phosphorylation. Knocking down FMRP via electroporating shRNAs into the dorsal spinal cord lead to a randomization of anterior-posterior turning of post-crossing commissural axons, which could be rescued by a FMRP rescue construct. Using RNAscope, we found that some of the FMRP target mRNAs encoding PCP components, PRICKLE2 and Celsr2, as well as regulators of cytoskeletal dynamics and components of cytoskeleton, APC, Cfl1, Map1b, Tubb3 and Actb, are present in the commissural neuron growth cones. Our results suggest that PCP signaling may regulate growth cone guidance, at least in part, by regulating local protein synthesis in the growth cones through via an interaction between Frizzled3 and FMRP.

Celsr3 Inactivation in the Brainstem Impairs Rubrospinal Tract Development and Mouse Behaviors in Motor Coordination and Mechanic-Induced Response

Inactivation of Celsr3 in the forebrain results in defects of longitudinal axonal tracts such as the corticospinal tract. In this study, we inactivated Celsr3 in the brainstem using En1-Cre mice (Celsr3 cKO) and analyzed axonal and behavioral phenotypes. Celsr3 cKO animals showed an 83% reduction of rubrospinal axons and 30% decrease of corticospinal axons in spinal segments, associated with increased branching of dopaminergic fibers in the ventral horn. Decreases of spinal motoneurons, neuromuscular junctions, and electromyographic signal amplitude of the biceps were also found in mutant animals. Mutant mice had impaired motor coordination and defective response to heavy mechanical stimulation, but no disability in walking and food pellet handling. Transsynaptic tracing demonstrated that rubrospinal axons synapse on spinal neurons in the deep layer of the dorsal horn, and mechanical stimulation of hindpaws induced strong calcium signal of red nuclei in control mice, which was less prominent in mutant mice. In conclusion, Celsr3 regulates development of spinal descending axons and the motor network in cell and non-cell autonomous manners, and the maturation of the rubrospinal system is required for motor coordination and response to mechanical stimulation.

Evidence for opposing roles of Celsr3 and Vangl2 in glutamatergic synapse formation

The signaling mechanisms that choreograph the assembly of the highly asymmetric pre- and postsynaptic structures are still poorly defined. Using synaptosome fractionation, immunostaining, and coimmunoprecipitation, we found that Celsr3 and Vangl2, core components of the planar cell polarity (PCP) pathway, are localized at developing glutamatergic synapses and interact with key synaptic proteins. Pyramidal neurons from the hippocampus of Celsr3 knockout mice exhibit loss of ∼50% of glutamatergic synapses, but not inhibitory synapses, in culture. Wnts are known regulators of synapse formation, and our data reveal that Wnt5a inhibits glutamatergic synapses formed via Celsr3. To avoid affecting earlier developmental processes, such as axon guidance, we conditionally knocked out Celsr3 in the hippocampus 1 week after birth. The CA1 neurons that lost Celsr3 also showed a loss of ∼50% of glutamatergic synapses in vivo without affecting the inhibitory synapses assessed by miniature excitatory postsynaptic current (mEPSC) and electron microscopy. These animals displayed deficits in hippocampus-dependent behaviors in adulthood, including spatial learning and memory and fear conditioning. In contrast to Celsr3 conditional knockouts, we found that the conditional knockout of Vangl2 in the hippocampus 1 week after birth led to a large increase in synaptic density, as evaluated by mEPSC frequency and spine density. PCP signaling is mediated by multiple core components with antagonizing functions. Our results document the opposing roles of Celsr3 and Vangl2 in glutamatergic synapse formation.

Plasticity of motor network and function in the absence of corticospinal projection

Despite the obvious clinical interest, our understanding of how developmental mechanisms are redeployed during degeneration and regeneration after brain and spinal cord injuries remains quite rudimentary. In animal models of spinal cord injury, although spontaneous regeneration of descending axons is limited, compensation by intact corticospinal axons, descending tracts from the brainstem, and local intrinsic spinal networks all contribute to the recovery of motor function. Here, we investigated spontaneous motor compensation and plasticity that occur in the absence of corticospinal tract, using Celsr3|Emx1 mice in which the corticospinal tract is completely and specifically absent as a consequence of Celsr3 inactivation in the cortex. Mutant mice had no paresis, but displayed hyperactivity in open-field, and a reduction in skilled movements in food pellet manipulation tests. The number of spinal motoneurons was reduced and their terminal arbors at neuromuscular junctions were atrophic, which was reflected in electromyography deficits. Rubrospinal projections, calretinin-positive propriospinal projections, afferent innervation of motoneurons by calretinin-positive segmental interneurons, and terminal ramifications of monoaminergic projections were significantly increased. Contrary to control animals, mutants also developed a severe and persistent disability of forelimb use following the section of the rubrospinal tract at the C4 spinal level. These observations demonstrate for the first time that the congenital absence of the corticospinal tract induces spontaneous plasticity, both at the level of the motor spinal cord and in descending monoaminergic and rubrospinal projections. Such compensatory mechanisms could be recruited in case of brain or spinal cord lesion or degeneration.