Molecular coding and physiological roles of excitatory neurons in motor control

Locomotion is a complex motor action that provides humans and other animals with the ability to move through the environment. In vertebrate locomotion, supraspinal centers convey the initiating or terminating command signals to the spinal cord which in turn generates the rhythm and pattern of muscle activities underlying locomotor activity. In supraspinal centers, the intermingled configuration of neuronal populations has made it difficult to identify cell populations responsible for locomotor initiating and terminating signals with standard electrophysiological methods. These questions can be addressed, however, with the combinatorial use of mouse genetics to manipulate discrete groups of neurons and electrophysiological and behavioral studies to address their function in motor control. Although a lot of progress has been made in deciphering the organization of the mammalian spinal locomotor networks through the use of early developmental markers, the present molecular classification of interneurons does not capture rhythm-generating neurons. It has become apparent that the interneuron composition of the spinal cord is quite complex and that the cardinal classes of interneurons are actually comprised of a highly diverse set of transcriptionally distinct neuronal types that cover diverse physiological functions. There is, therefore, a strong need for the identification of fine- grained molecular markers for spinal interneurons overall and glutamatergic spinal interneurons in particular since glutamatergic neurons are thought to be the drive for rhythmic motor output. The work in this thesis addresses these questions and attempts at either ascribing functions to specific groups of neurons or providing a molecular database for future in- depth investigations. In Paper IV of this thesis, we studied the roles of a subset of glutamatergic neurons in the supraspinal control of locomotion. We identified brainstem V2a neurons as a glutamatergic excitatory descending pathway that is involved in the arrest of ongoing locomotion. In Paper I, we investigated the mechanisms underlying the abnormal locomotor pattern observed in mice with a disrupted EphA4 signaling pathway. We linked the hopping-like locomotor phenotype observed in EphA4 signaling mutants to the aberrant crossing of spinal glutamatergic neurons. In Paper II, we investigated the role of a subset of glutamatergic interneurons in rhythm generation in an attempt to elucidate the identity of rhythm-generating neurons. We showed that, although it is unlikely they are the sole rhythm-generating neurons, glutamatergic Hb9::Cre-derived interneurons contribute to rhythm generation in the mouse. In Paper III, we took it a step further and investigated the transcriptome profile of spinal glutamatergic neurons with the aim of identifying discrete molecular populations to which we can ascribe some of the locomotor functions that still remain elusive such as rhythm generation. Our findings provide a comprehensive overview of the transcription factors, ion channels and metabotropic receptors expressed in spinal glutamatergic neurons. Overall, the work in each of the constituent papers of this thesis has broadened our understanding of glutamatergic neurons, their molecular and functional diversity and at the same time brought us a step closer to deciphering the functional organization of locomotor networks in the mouse