Chronically silencing cortical projection neurons leads to neuroanatomical and motor degeneration

Thesis

Background: The cerebral cortex is exceptionally complex. Computational capacities enabled by cytoarchitectural diversity are established during neurodevelopment which, if dysfunctional, can lead to neurological and psychiatric disorders. Neurons in deep cortical layers 5-6 (L5-6) are of particular interest, presenting as substrates where prenatal insults may provoke postnatal deficits. Studies using mouse-models show that L6b cells—which resemble human whitematter-interstitial-neurons (WMINs)—are remnants of the neurodevelopmentally-critical subplate; subplate axons pioneer corticofugal pathways which L5 axons later descend and dominate. Through adulthood, subplate-remnant abnormalities are associated with diseases of cortical hyperexcitability, yet anatomical and morphometric ramifications are incompletely characterised. Dysfunctional L5-projections can lead to neuromotor degeneration, however pathophysiological trajectories remain unknown.

Hypotheses: Genetically-targeting mouse L5- 6b cells elicits widespread neuroanatomical and functional changes. Particularly, L6- ‘augmentation’ may yield neuron hypertrophy resembling that of WMINs in postmortem tissues from human autism and epilepsy patients, while L5-‘silencing’ may spur progressive neuromotor decline supporting top-down theories of motor neuron degeneration. Investigations across functional, histological, and neuroimaging methods may derive complimentary network insights exceeding those which any technique would provide in isolation.

Methods: I developed and validated a novel mouse home-cage-monitoring system (SnackerTracker-TracKIT) and computational enhancements for a whole-brain cell analysis pipeline (r-QUINT) to enhance behavioural and anatomical outputs, respectively. r-QUINT described ramifications of genetic L6-‘silencing’/‘augmentation’, and a longitudinal study featuring behavioural, anatomical, and multimodal MRI experiments characterised consequences of L5-‘silencing’.

Results: Chapter-3 introduces the SnackerTracker device and TracKIT analysis platform. Chapter-4 describes rQUINT refinements, which reveal changes following L6b perturbations, while morphometric comparisons support L6b-WMIN analogy. Chapter-5 presents convergent evidence of neuromotor deterioration upon L5-silencing, where pathological trajectories resemble those of upper-motor-neuron diseases. Neuroimaging predicts—and histology validates—where and when deficits emerge.

Conclusions: This thesis presents methodological developments and multimodal analyses which characterise effects of silencing deep-layer cortical neurons, thereby enhancing knowledge surrounding associated disease-relevant networks.

Authors: Mueller, MH

• DOI: 10.5287/ora-0ze8mrwzm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cortex; neuroscience; multimodal neuroimaging; neurodevelopment; subplate; motor degeneration; layer 5
• Subjects: Neurosciences