A Case of Mistaken Identity

We are fascinated by mermaids, centaurs, and other mythical chimeras at least in part because they challenge our notion that identity is exclusive. Within the nervous system, a neuron's identity is defined by its function. For example, motor neurons (MNs) relay information between the central nervous system (CNS) and muscle, whereas interneurons relay information solely within the CNS. But are these features mutually exclusive, or, like a mermaid or centaur, could a neuron express features of both MN and interneuron identities? Apparently so, since this is precisely the phenotype of MNs in mice lacking function of the Hb9 homeobox gene, as described in two papers in this issue of Neuron (2xArber, S., Han, B., Mendelsohn, M., Smith, M., Jessell, T.M., and Sockanathan, S. Neuron. 1999; 23: 659–674Abstract | Full Text | Full Text PDF | PubMed | Scopus (361)See all References, 11xThaler, J., Harrison, K., Sharma, K., Lettieri, K., Kehrl, J., and Pfaff, S.L. Neuron. 1999; 23: 675–687Abstract | Full Text | Full Text PDF | PubMed | Scopus (233)See all References).Despite the clear functional differences between MNs and interneurons, evidence from chicks, mice, and zebrafish suggest that these cells share an early developmental program. Lineage-tracing studies in chick and zebrafish show that a single progenitor can generate both interneurons and MNs (6xLeber, S.M., Breedlove, S.M., and Sanes, J.R. J. Neurosci. 1990; 10: 2451–2462PubMedSee all References, 5xKimmel, C.B., Warga, R.M., and Kane, D.A. Development. 1994; 120: 265–276PubMedSee all References). In mouse, progenitors expressing the transcription factors Lhx3, Lhx4 (also called Lim3 and Gsh4), and Pax6 generate both MNs with ventrally projecting axons (v-MNs) and a specific type of ventrally located interneuron (V2) (Sharma et al. 1998xSharma, K., Sheng, H.Z., Lettieri, K., Li, H., Karavanov, A., Potter, S., Westphal, H., and Pfaff, S.L. Cell. 1998; 95: 817–828Abstract | Full Text | Full Text PDF | PubMed | Scopus (314)See all ReferencesSharma et al. 1998). Mice lacking function of Islet1 (Isl1), a gene required for MN development, not only lack MNs, but also lack ventrally located V1 interneurons (Pfaff et al. 1996xPfaff, S.L., Mendelsohn, M., Stewart, C.L., Edlund, T., and Jessell, T.M. Cell. 1996; 84: 309–320Abstract | Full Text | Full Text PDF | PubMed | Scopus (546)See all ReferencesPfaff et al. 1996). Together, these studies reveal a close relationship between MNs and ventrally located interneurons and raise the question of what directs a cell toward one or the other identity.Identity choice might be regulated by selector genes that promote one identity or suppress another identity. In addition to the transcription factors expressed in common by their progenitors, V2 interneurons and v-MNs express cell type–specific transcription factors that are candidate selector genes. MNR2 is a candidate selector gene for MN identity. In chicks, this gene is expressed in progenitors of somatic MNs. Ectopic expression of MNR2 is sufficient to direct some spinal cord cells to become MNs and to prevent them from becoming interneurons (Tanabe et al. 1998xTanabe, Y., William, C., and Jessell, T.M. Cell. 1998; 95: 67–80Abstract | Full Text | Full Text PDF | PubMed | Scopus (269)See all ReferencesTanabe et al. 1998). Interestingly, mice seem to lack MNR2, but they do have a closely related gene, Hb9 (2xArber, S., Han, B., Mendelsohn, M., Smith, M., Jessell, T.M., and Sockanathan, S. Neuron. 1999; 23: 659–674Abstract | Full Text | Full Text PDF | PubMed | Scopus (361)See all References, 11xThaler, J., Harrison, K., Sharma, K., Lettieri, K., Kehrl, J., and Pfaff, S.L. Neuron. 1999; 23: 675–687Abstract | Full Text | Full Text PDF | PubMed | Scopus (233)See all References). This gene is also found in chick and shares with MNR2 the ability to direct some spinal cord cells to become MNs when it is ectopically expressed (Tanabe et al. 1998xTanabe, Y., William, C., and Jessell, T.M. Cell. 1998; 95: 67–80Abstract | Full Text | Full Text PDF | PubMed | Scopus (269)See all ReferencesTanabe et al. 1998). Thus, in some contexts Hb9 is sufficient to promote v-MN identity, making it a candidate v-MN selector gene in mice.If HB9 acts as a selector between interneuron and MN identities, one would predict that absence of HB9 function would lead to loss of the MN identity and that cells would become V2 interneurons instead. Thaler, Arber, and their colleagues used knockout strategies in mice to eliminate HB9 function. They found that in the absence of HB9 protein, v-MNs transiently express the Chx10 transcription factor characteristic of V2 interneurons, and they prematurely downregulate expression of the Isl1 transcription factor characteristic of MNs. However, and quite surprisingly, these MNs don't become interneurons. Instead, they behave like MNs, extending axons to muscle targets and making something at least approximating normal endplates, as judged by expression of choline acetyltransferase in the MNs and clustering of acetylcholine receptors on the muscle surface. Thus, in the absence of HB9 function, MNs are phenotypic hybrids that transiently express some molecular properties of V2 interneurons but also express the morphological characteristics of v-MNs. However, although these hybrid cells appear more MN-like than interneuron-like, they are not normal. MNs typically have a precise correspondence between soma position, gene expression pattern, and axonal trajectory (12xTsuchida, T., Ensini, M., Morton, S.B., Baldassare, M., Edlund, T., Jessell, T.M., and Pfaff, S.L. Cell. 1994; 79: 957–970Abstract | Full Text PDF | PubMed | Scopus (768)See all References, 1xAppel, B., Korzh, V., Glasgow, E., Thor, S., Edlund, T., Dawid, I.B., and Eisen, J.S. Development. 1995; 121: 4117–4125PubMedSee all References, 9xTanabe, Y. and Jessell, T.M. Science. 1996; 274: 1115–1122Crossref | PubMed | Scopus (583)See all References, 8xSharma, K., Sheng, H.Z., Lettieri, K., Li, H., Karavanov, A., Potter, S., Westphal, H., and Pfaff, S.L. Cell. 1998; 95: 817–828Abstract | Full Text | Full Text PDF | PubMed | Scopus (314)See all References, 3xEisen, J.S. Trends Neurosci. 1999; 22: 321–326Abstract | Full Text | Full Text PDF | PubMed | Scopus (46)See all References), which is lost in the absence of HB9 function. These studies show that HB9 protein is not required for specification of MN identity per se. However, it is required to prevent MNs from expressing molecular characteristics of V2 interneurons, and for later stages of MN development that result in the stereotyped patterning of these cells.These studies raise intriguing questions about neuronal function and development. Hybrid MNs in embryos lacking HB9 function project axons to muscle targets and form motor endplates. Can these MNs activate muscle? How does the activity pattern relate to that of wild types? These hybrid MNs lack the normal relationships between soma position, gene expression, and axonal trajectory. How are these features of MN development related to the choice between MN and interneuron identities? Answers to these questions should provide insights into the features of MN identity required for proper pathfinding and formation of appropriate synapses with pre- and postsynaptic partners.We are used to thinking of selector genes acting as binary switches that cause a cell to differentiate with characteristics of one or another cell type. However, these studies show that, like mermaids and centaurs, neurons can simultaneously express characteristics of two different identities. This situation is not unique to mouse v-MNs; loss-of-function studies in other animals provide additional examples. For instance, in zebrafish lacking function of the floating head gene, notochord precursor cells transiently express molecular properties both of notochord and of muscle and ultimately differentiate into muscle cells (Halpern et al. 1995xHalpern, M.E., Thisse, C., Ho, R.K., Thisse, B., Riggleman, B., Trevarrow, B., Weinberg, E.S., Postlethwait, J.H., and Kimmel, C. Development. 1995; 121: 4257–4264PubMedSee all ReferencesHalpern et al. 1995). Thus, the common view of how cell identity is acquired is too simplistic. We must modify it to account for the new level of complexity revealed by loss-of-function studies of genes that are sufficient to promote acquisition of a particular cell identity.