Identification of molecular controls over corticospinal motor neuron segmental target specificity

Molecular mechanisms over differentiation and differential axonal targeting of distinct neuron subtypes in the cerebral cortex are beginning to be elucidated. These studies have focused on controls that specifically distinguish one subtype of neocortical projection neurons, e.g. corticospinal motor neurons (CSMN), from closely related corticothalamic projection neurons (CThPN) or intracortical callosal projection neurons (CPN). CSMN are located in layer V of the neocortex and make synaptic connections to motor output circuitry in the spinal cord and brainstem. CSMN axons form the corticospinal tract (CST), which is the major motor output pathway from the motor cortex and critically controls voluntary movement. CSMN somatotopically and precisely target specific segments along the rostrocaudal axis of the spinal cord, the molecular basis for which remains unknown. We used microarrays to examine gene expression differences between two CSMN subpopulations that target different levels of the spinal cord - CSMN-C (which extend axons to the brainstem and cervical spinal cord) and CSMN-L (which preferrrentially extend axons to the thoracic and lumbar spinal cord). We compared CSMN-C vs CSMN-L gene expression at 3 critical developmental time points (previously described in Arlotta et a., 2005) All CSMN were retrogradely labeled with fluorescent microespheres injected into either the cerebral peduncle at P0 or the cervical spinal cord at P3 and P5.5. Rostrolateral (for CSMN-C) and caudomedial (for CSMN-L) cortices were microdissected and the labeled CSMN subpopulations in these distinct cortical areas were FACS purified at 3 developmental stages for RNA extraction and hybridization on Affymetrix microarrays.