Biology subjects

Christie, J. M.

Publications and source records attributed to Christie, J. M..

6 recordsLinked to original sources

bioRxiv2023

Molecular layer disinhibition unlocks climbing-fiber-instructed motor learning in the cerebellum

Climbing fibers supervise cerebellar learning by providing signals to Purkinje cells (PCs) that instruct adaptive changes to mistakenly performed movements. Yet, climbing fibers are regularly active, even during well performed movements, suggesting that a mechanism dynamically regulates the ability of climbing fibers to induce corrective plasticity in response to motor errors. We found that molecular layer interneurons (MLIs), whose inhibition of PCs powerfully opposes climbing-fiber-mediated excitation, serve this function. Optogenetically suppressing the activity of floccular MLIs in mice during the vestibulo-ocular reflex (VOR) induces a learned increase in gain despite the absence of performance errors. Suppressing MLIs when the VOR is mistakenly underperformed reveled that their inhibitory output is necessary to orchestrate gain-increase learning by conditionally permitting climbing fibers to instruct plasticity induction during ipsiversive head turns. Ablation of an MLI circuit for PC disinhibition prevents gain-increase learning during VOR performance errors which was rescued by re-imposing PC disinhibition through MLI activity suppression. Our findings point to a decisive role for MLIs in gating climbing-fiber-mediated learning through their context-dependent inhibition of PCs.

neuroscience↗

bioRxiv2022

Cerebellar motor memory expression requires learned alterations to the activity of inhibitory molecular layer interneurons

Procedural memories formed in the cerebellum in response to motor errors depend on changes to Purkinje cell (PC) spiking patterns that correct movement when the erroneous context is repeated. Because molecular layer interneurons (MLIs) inhibit PCs, learning-induced changes to MLI output may participate in reshaping PC spiking patterns. Yet, it remains unclear whether error-driven learning alters MLI activity and whether such changes are necessary for the memory engram. We addressed this knowledge gap by measuring and manipulating MLI activity in the flocculus of both sexes of mice before and after vestibulo-ocular reflex (VOR) adaptation. We found that MLIs are activated during vestibular stimuli and that their population response exhibits a phase shift after the instantiation of gain-increase VOR adaptation, a type of error-driven learning requiring climbing-fiber-mediated instructive signaling. Although acute optogenetic suppression of MLI activity did not affect baseline VOR performance, it negated the expression of gain-increase learning, demonstrating a specific causal role of MLI activity changes in motor memory expression. This effect was transitory; after a multi-day consolidation period, the expression of VOR gain-increase learning was no longer sensitive to MLI activity suppression. Together, our results indicate that error-driven alteration of MLI activity is necessary for labile, climbing-fiber-induced motor memory expression. Significance StatementIn the cerebellum, motor learning induces an associative memory of the sensorimotor context of an erroneous movement that, when recalled, results in a new pattern of output that improves subsequent trials of performance. Our study shows that error-driven motor learning induces changes to the activity pattern of cerebellar molecular layer interneurons and that this new pattern of activity is required to express the corrective motor memory.

neuroscience↗

bioRxiv2021

The cerebellum encodes and influences the initiation and termination of discontinuous movements

The cerebellum is hypothesized to represent timing information important for organizing salient motor events during periodically performed discontinuous movements. To provide functional evidence validating this idea, we measured and manipulated Purkinje cell (PC) activity in the lateral cerebellum of mice trained to volitionally elicit periodic bouts of stereotyped licking for regularly allocated water rewards. Overall, PC simple spiking modulated during task performance, ramping prior to both lick-bout initiation and termination, two important motor events delimiting movement cycles. The ramping onset occurred earlier for the initiation of un-cued exploratory licking that anticipated water availability relative to licking that was reactive to water allocation, suggesting that the cerebellum is engaged differently depending on the movement context. In a subpopulation of PCs, climbing-fiber-evoked responses also increased during lick-bout initiation, but not termination, highlighting differences in how cerebellar input pathways represent task-related information. Optogenetic perturbation of PC activity disrupted the behavior in both initiating and terminating licking bouts and reduced the ability of animals to finely time predictive action around reward delivery, confirming a causative role in movement organization. Together, these results substantiate that the cerebellum contributes to the control of explicitly timed repeated motor actions.

neuroscience↗

bioRxiv2021

Regulation of Plant Phototropic Growth by NPH3/RPT2-like Substrate Phosphorylation and 14-3-3 Binding

Polarity underlies all plant physiology and directional growth responses such as phototropism. Yet, our understanding of how plant tropic responses are established is far from complete. The plasma-membrane associated BTB-containing protein, NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key determinant of phototropic growth which is regulated by AGC kinases known as the phototropins (phots). However, the mechanism by which phots initiate phototropic signalling via NPH3, and other NPH3/RPT2-like (NRL) members, has remained unresolved. Here we demonstrate that NPH3 is directly phosphorylated by phot1 both in vitro and in vivo. Light-dependent phosphorylation within a conserved consensus sequence (RxS) located at the extreme C-terminus of NPH3 is necessary to promote its functionality for phototropism and petiole positioning in Arabidopsis. Phosphorylation of this region by phot1 also triggers 14-3-3 binding combined with changes in NPH3 phosphorylation and localisation status. Seedlings expressing mutants of NPH3 that are unable to bind or constitutively bind 14-3-3s show compromised functionality that is consistent with a model where signalling outputs arising from a gradient in NPH3 RxS phosphorylation/localisation across the stem are a major contributor to phototropic responsiveness. Our current findings provide further evidence that 14-3-3 proteins are instrumental components regulating auxin-dependent growth and show for the first time that NRL proteins are direct phosphorylation targets for plant AGC kinases. Moreover, the C-terminal phosphorylation site/14-3-3-binding motif of NPH3 is conserved in several members of the NRL family, suggesting a common mechanism of regulation.

plant biology↗

bioRxiv2018

Conversion of graded presynaptic climbing fiber activity into graded postsynaptic Ca2+ signals by Purkinje cell dendrites

The brain must make sense of external stimuli to generate relevant behavior. We used a combination of in vivo approaches to investigate how the cerebellum processes sensory-related information. We found that the inferior olive encodes contexts of sensory-associated external cues in a graded manner, apparent in the presynaptic activity of their axonal projections in the cerebellar cortex. Further, individual climbing fibers were broadly responsive to different sensory modalities but relayed sensory-related information to the cortex in a lobule-dependent manner. Purkinje cell dendrites faithfully transformed this climbing fiber activity into dendrite-wide Ca2+ signals without a direct contribution from the mossy fiber pathway. These results demonstrate that the size of climbing fiber-evoked Ca2+ signals in Purkinje cell dendrites is largely determined by the firing level of climbing fibers. This coding scheme emphasizes the overwhelming role of the inferior olive in generating salient signals useful for instructing plasticity and learning.

neuroscience↗

bioRxiv2018

Native Mass Spectrometry Reveals the Conformational Diversity of the UVR8 Photoreceptor

UVR8 is a plant photoreceptor protein that regulates photomorphogenic and protective responses to UV light. The inactive, homodimeric state absorbs UV-B light resulting in dissociation into monomers, which are considered to be the active state and comprise a {beta}-propeller core domain and intrinsically disordered N- and C-terminal tails. The C-terminus is required for functional binding to signalling partner COP1. To date, however, structural studies have only been conducted with the core domain where the terminal tails have been truncated. Here, we report structural investigations of full-length UVR8 using native ion mobility mass spectrometry adapted for photo-activation. We show that, whilst truncated UVR8 photo-converts from a single conformation of dimers to a single monomer conformation, the full-length protein exist in numerous conformational families. The full-length dimer adopts both a compact state and an extended state where the C-terminus is primed for activation. In the monomer the extended C-terminus destabilises the core domain to produce highly extended yet stable conformations, which we propose are the fully active states that bind COP1. Our results reveal the conformational diversity of full-length UVR8. We also demonstrate the potential power of native mass spectrometry to probe functionally important structural dynamics of photoreceptor proteins throughout nature.\n\nTOC Graphic\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=141 SRC=\"FIGDIR/small/371658_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (60K):\norg.highwire.dtl.DTLVardef@177c6f9org.highwire.dtl.DTLVardef@a845c9org.highwire.dtl.DTLVardef@17db14forg.highwire.dtl.DTLVardef@103d4f8_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗