In the developing spinal cord, the dorsal neural tube gives rise to two populations of somatosensory interneurons — association and relay neurons. Little is known about the factors that specify these different neuronal subtypes, but two groups now report in Neuron that the transcription factor Lbx1 seems to have an important role.

Two classes of neuronal progenitors, which Müller et al. designate as class A and class B, emerge from the dorsal spinal cord. The class A cells are initially positioned dorsal to the class B cells, although the two populations swap positions later in development. The class A cells, which do not express Lbx1, give rise to relay interneurons. The class B cells, which express Lbx1, give rise to association interneurons that go on to colonize the substantia gelatinosa.

Gross et al. and Müller et al. used loss- and gain-of-function approaches to examine the role of Lbx1 in dorsal interneuron specification. They found that knocking out the Lbx1 gene causes respecification of the class B cells to a class A identity, which is manifested in a loss of dorsal horn association interneurons. Misexpression of Lbx1, on the other hand, causes class A progenitors to adopt a class B fate. The authors conclude that Lbx1 is required for the generation of dorsal association interneurons through the specification of class B progenitors.

The authors also gained an insight into when Lbx1 acts during neurogenesis. Lbx1 expression appears in class B cells only after they have left the proliferative ventricular zone, indicating that it acts on postmitotic neurons. This implies that neuronal progenitor cells in the dorsal spinal cord retain some developmental plasticity after they have stopped dividing, and that their fate can be altered by manipulating the expression of Lbx1.

It has been known for some time that the dorsal neural tube consists of several progenitor domains, each of which expresses a distinct combination of transcription factors. However, Lbx1 is the first factor to be assigned a specific role in the specification of dorsal neuronal subtypes.