Functional dissection of cerebellar output circuits that orchestrate limb motor control

Project Summary
The cerebellum is essential for coordinating motor behavior through rapid adjustments of ongoing movements.
To refine movement, the cerebellum processes motor and sensory information, and transmits output that
ultimately modulates motor neuron activity to ensure successful execution. The path through which the
cerebellum can influence limb movement is through output circuits in the cerebellar nuclei (CN). Yet little is known
about how CN circuits are organized and whether discrete pathways are dedicated to specific motor functions
for limb control. The recent identification of a molecularly distinct subset of CN neurons that project to the cerebral
cortex via the thalamus and can affect forelimb movement begins to reveal a broader neural subtype logic to
cerebellar output. Further exploration indicates that there are also CN neurons that directly innervate the cervical
spinal cord. These descending projections could provide a more direct route for influencing motor output, but the
specific roles they have in movement execution and refinement remain poorly understood. Based on preliminary
evidence, the major hypothesis of this proposal is that anatomically and molecularly distinct subsets of CN
neurons have discrete contributions to forelimb motor control. Specifically, cerebellar-spinal circuits play a critical
role in rapid online correction, and their functional output is needed to prevent forelimb ataxia. Three Aims will
explore how spinal-projecting CN circuits differ from the more heavily studied CN circuits that project to the
thalamus. Aim 1 defines specific subtypes of neurons within the cerebellar nuclei by delineating their input and
output connectivity and molecular identities. The distinctions in efferent and afferent connectivity of spinal- and
thalamus-projecting CN neurons will be defined using combinatorial genetic and viral circuit tracing tools in mice.
In addition, molecular distinctions between these two classes of cerebellar output neurons will be identified using
single nuclei RNA-sequencing and multiplexed in situ hybridization. Aim 2 sets out to establish the functional
connectivity between these distinct CN subpopulations and forelimb muscles and examines how spinal- and
thalamus-projecting cerebellar output pathways influence goal-directed forelimb movements. Selective
optogenetic perturbation, electromyographic (EMG) recording, and high-resolution kinematic analysis will be
used to determine how spinal- and thalamus-projecting CN neurons differentially affect muscle activity and online
refinement of behavior. Aim 3 explores how the activity of discrete CN output pathways correlates with forelimb
online correction and endpoint precision. This goal will be accomplished by recording from spinal- and thalamus-
projecting CN populations and forelimb muscles during dexterous reaching behaviors, and by applying
generalized linear models to determine if neural activity predicts EMG and kinematic movement features. By
defining the organization of two major cerebellar output pathways and identifying the ways in which they influence
dexterous movements, this work will provide insight into how diverse circuits differentially participate in motor
control, and clarify how injury and disease of cerebellar circuits can lead to motor impairments in humans.

Grant Number: 4R01NS128898-02
NIH Institute/Center: NIH
Principal Investigator: EIMAN AZIM