Transcription factor Hb9 is expressed in glial cell lineages in the developing mouse spinal cord

Abstract

Hb9 (Mnx1) is a transcription factor described as a spinal cord motor neuron (MN)-specific marker critical factor for the postmitotic specification of these cells. To date, expression of Hb9 in other cell types has not previously been reported. We performed a fate-mapping approach to examine distributions of Hb9-expressing cells and their progeny (‘Hb9-lineage cells’) within the embryonic and adult spinal cord of Hb9^cre;Ai14 mice. We found that Hb9-lineage cells are distributed in a gradient of increasing abundance throughout the rostrocaudal spinal cord axis during embryonic and postnatal stages. Furthermore, although the majority of Hb9-lineage cells at cervical spinal cord levels are MNs, at more caudal levels, Hb9-lineage cells include small-diameter dorsal horn neurons, astrocytes, and oligodendrocytes. In the peripheral nervous system, we observed a similar phenomenon with more abundant Hb9-lineage Schwann cells in muscles of the lower body versus upper body muscles. We cultured spinal cord progenitors in vitro and found that gliogenesis was increased by treatment with the caudalizing factor FGF-8B, while glial tdTomato expression was increased by treatment with both FGF-8B and GDF-11. Together, these observations suggest that early and transient expression of Hb9 in spinal cord neural progenitors may be induced by caudalizing factors such as FGF and GDF signaling. Furthermore, our work raises the possibility that early Hb9 expression may influence the development of spinal cord macroglia and Schwann cells, especially at caudal regions. Together, these findings highlight the importance of using caution when designing experiments using Hb9^cre mice to perform spinal cord MN-specific manipulations.

Significance Statement

The transcription factor Hb9 is key in postmitotic specification of spinal cord motor neurons, and thought to be expressed specifically in this cell population. We performed fate-mapping experiments and found that Hb9-lineage cells are not restricted to motor neurons, but also include spinal cord macroglia and Schwann cells. Strikingly, Hb9-lineage cells are present in an increasing rostrocaudal gradient in the CNS. Proportions of Hb9-lineage astrocytes in cultures of spinal cord progenitors could be manipulated by treatment with caudalizing factors FGF-8B and GDF-11. These findings highlight an interesting developmental phenomenon in which there is an increasing rostrocaudal gradient of molecularly-defined cell types. Further, these findings urge caution for researchers using Hb9^cre mice to study spinal motor neurons in isolation.