Distinct Populations of Layer 5B Pyramidal Neurons in the Primary Motor Complex
A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy
The ability of motor cortex to plan, execute, and refine different movements depends on the coordinated activity of many neurons found across its laminar structure. Layer 5b (L5b), a deep cortical layer that drives output signals from the cortex, contains excitatory pyramidal neurons that innervate many subcortical areas of the brain. In the motor cortex, L5b is thicker and contains more pyramidal neurons than L5b of other cortical areas. Electrophysiological, anatomical, and RNA-Seq profiling of neurons in the motor cortex suggests there are diverse pyramidal neuron types within L5b. However, the precise identities of these distinct populations and their defining traits have been difficult to assess. Determining the cell type-specific properties of distinct L5b pyramidal neurons will not only help in understanding how the motor cortex is able to execute its varied functions, but may also reveal how selective vulnerability is established in neurodegenerative diseases that affect the motor cortex, such as Amyotrophic Lateral Sclerosis (ALS). Despite being expressed in all cells of the body, mutations associated with ALS lead to specific loss of LMNs in the brainstem and the ventral horn of the spinal cord, and UMNs in L5b of the motor cortex. For this reason, it is important to characterize the unique molecular profiles that may underlie an increased vulnerability of these cell Ph.D. types to the ubiquitously expressed mutations. In the motor cortex, this requires us to determine the characteristics that differentiate vulnerable L5b cells from other resistant cell types in the same area, and understand how these features may contribute to their death in ALS. This study aims to understand how anatomical traits and molecular properties, defined at the level of gene expression, vary across subpopulations of L5b pyramidal neurons in the motor cortex. We show that there are two distinct, but closely related, pyramidal neuron subtypes in mouse primary motor cortex which occupy discrete sublayers of L5b. In the SOD1-G93A mouse model of ALS, we observe loss of only one of these cell types, establishing the other as an analogous resistant L5b population. Using TRAP (Translating Ribosome Affinity Purification) with RNA-Seq, we show that these cells have important baseline differences in gene expression in healthy tissues, and that they display differential molecular responses to SOD1-G93A expression. Together, these findings reveal that the gene expression differences between the distinct L5b populations not only reflect their diverse cortical and subcortical anatomy, but may also establish selective vulnerability in ALS.