Highlights d Phox2a is transiently expressed in embryonic anterolateral system (AS) spinal neurons d Phox2a AS neuron development reflects AS neuron diversity d Spinal Phox2a knockout causes aberrant AS connectivity and nociceptive defects d Human and mouse embryonic spinal Phox2a neurons are similar

The mechanisms that activate some genes while silencing others are critical to ensure precision in lineage specification as multipotent progenitors become restricted in cell fate. During neurodevelopment, these mechanisms are required to generate the diversity of neuronal subtypes found in the nervous system. Here we report interactions between basic helix-loop-helix (bHLH) transcriptional activators and the transcriptional repressor PRDM13 that are critical for specifying dorsal spinal cord neurons. PRDM13 inhibits gene expression programs for excitatory neuronal lineages in the dorsal neural tube. Strikingly, PRDM13 also ensures a battery of ventral neural tube specification genes such as Olig1, Olig2 and Prdm12 are excluded dorsally. PRDM13 does this via recruitment to chromatin by multiple neural bHLH factors to restrict gene expression in specific neuronal lineages. Together these findings highlight the function of PRDM13 in repressing the activity of bHLH transcriptional activators that together are required to achieve precise neuronal specification during mouse development.

Proprioception, the sense of limb and body position, generates a map of the body that is essential for proper motor control, yet we know little about precisely how neurons in proprioceptive pathways develop and are wired. Proprioceptive and cutaneous information from the periphery is sent to secondary neurons in the spinal cord that integrate and relay this information to the cerebellum either directly or indirectly through the medulla. Defining the anatomy of these direct and indirect pathways is fundamental to understanding how proprioceptive circuits function. Here, we use genetic tools in mice to define the developmental origins and unique anatomical trajectories of these pathways. Developmentally, we find that Clarke's column (CC) neurons, a major contributor to the direct spinocerebellar pathway, derive from the Neurog1 progenitor domain. By contrast, we find that two of the indirect pathways, the spino-lateral reticular nucleus (spino-LRt) and spino-olivary pathways, are derived from the Atoh1 progenitor domain, despite previous evidence that Atoh1-lineage neurons form the direct pathway. Anatomically, we also find that the mossy fiber terminals of CC neurons diversify extensively with some axons terminating bilaterally in the cerebellar cortex. Intriguingly, we find that CC axons do not send axon collaterals to the medulla or cerebellar nuclei like other mossy fiber sources. Altogether, we conclude that the direct and indirect spinocerebellar pathways derive from distinct progenitor domains in the developing spinal cord and that the proprioceptive information from CC neurons is processed only at the level of granule cells in the cerebellum.

Proprioception, the sense of limb and body position, generates a map of the body that is essential for proper motor control, yet we know little about precisely how neurons in proprioceptive pathways are wired. Defining the anatomy of secondary neurons in the spinal cord that integrate and relay proprioceptive and potentially cutaneous information from the periphery to the cerebellum is fundamental to understanding how proprioceptive circuits function. Here, we define the unique anatomic trajectories of long-range direct and indirect spinocerebellar pathways as well as local intersegmental spinal circuits using genetic tools in both male and female mice. We find that Clarke's column neurons, a major contributor to the direct spinocerebellar pathway, has mossy fiber terminals that diversify extensively in the cerebellar cortex with axons terminating bilaterally, but with no significant axon collaterals within the spinal cord, medulla, or cerebellar nuclei. By contrast, we find that two of the indirect pathways, the spino-lateral reticular nucleus and spino-olivary pathways, are in part, derived from cervical Atoh1-lineage neurons, whereas thoracolumbar Atoh1-lineage neurons project mostly locally within the spinal cord. Notably, while cervical and thoracolumbar Atoh1-lineage neurons connect locally with motor neurons, no Clarke's column to motor neuron connections were detected. Together, we define anatomic differences between long-range direct, indirect, and local proprioceptive subcircuits that likely mediate different components of proprioceptive-motor behaviors.

Peripheral somatosensory input is modulated in the dorsal spinal cord by a network of excitatory and inhibitory interneurons. PTF1A is a transcription factor essential in dorsal neural tube progenitors for specification of these inhibitory neurons. Thus, mechanisms regulating Ptf1a expression are key for generating neuronal circuits underlying somatosensory behaviors. Mutations targeted to distinct cis-regulatory elements for Ptf1a in mice, tested the in vivo contribution of each element individually and in combination. Mutations in an autoregulatory enhancer resulted in reduced levels of PTF1A, and reduced numbers of specific dorsal spinal cord inhibitory neurons, particularly those expressing Pdyn and Gal. Although these mutants survive postnatally, at ∼3-5 wk they elicit a severe scratching phenotype. Behaviorally, the mutants have increased sensitivity to itch, but acute sensitivity to other sensory stimuli such as mechanical or thermal pain is unaffected. We demonstrate a requirement for positive transcriptional autoregulatory feedback to attain the level of the neuronal specification factor PTF1A necessary for generating correctly balanced neuronal circuits.