Downloaded from genesdev.cshlp.org on September 27, 2021 - Published by Cold Spring Harbor Laboratory Press Expression patterns of the homeo box- containing genes En-1 and En-2 and the proto-oncogene int-1 diverge during mouse development Claytus A. Davis and Alexandra L. Joyner Division of Molecular and Developmental Biology, Mount Sinai Hospital Research Institute and the Department of Medical Genetics, Toronto, Ontario, Canada, M5G-1X5 We have compared the expression of the murine genes En-l,En-2, and int-1 during development by in situ hybridization. Expression of all three genes was first detected at 8.0 days in overlapping bands of the anterior neural folds. By 12.0 days the expression patterns diverged. En-1 and En-2 were expressed in a similar ring of cells in the central nervous system (CNS) at the midbrain/hindbrain junction. En-1 was also expressed de novo in two lateral stripes extending the length of the hindbrain and spinal cord, in the developing vertebral column, in two lateral stripes of dermatome-derived cells, and in the tail and limb buds. By 12.0 days int-1 expression showed little overlap with the En genes and could not be detected at later stages. At 15.5 days En gene expression was primarily limited to the midbrain/hindbrain in overlapping but nonidentical sets of differentiated cells. In the adult, En-1 and En-2 marked the same sets of cells in the pons, but En-2 alone was detected in the granular layer of the cerebellum. The results are consistent with int-1 and the En genes playing a role early in development in defining spatial domains in the CNS. Later in development the En genes may have an additional function in neurogenesis. En-1 expression in the developing pericordal tube suggests that it may also be involved in vertebral assembly. [Key Words: Homeo box; engrafted; rot-l; RNA in situ; mouse development; brain; vertebrae] Received August 31, 1988; revised version accepted November 10, 1988. The ease with which mutants may be isolated and char- pressed in the same region of the developing and adult acterized in Drosophila melanogaster has led to the iso- brain (Davis et al. 1988). Northern blot analysis of ex- lation and study of a large number of genes controlling pression of En-1 in embryo tissues showed that its ex- pattern formation in development. One conserved se- pression pattern is not restricted to the brain but can quence motif that is both unique to these genes and also be detected in the spinal cord, limbs, and carcass prevalent among them is the homeobox. Compared with (Joyner and Martin 1987}. Drosophila, knowledge of the genetic control of develop- In Drosophila, mutational analysis has shown that en ment in vertebrates has been slow to accumulate due to is crucial for generation of the posterior compartments the difficulty of directly identifying and mutating the of each segment [Garcia-Bellido and Santamaria 1972; genes involved. However, many genes potentially im- Lawrence and Morata 1976; Komberg 1981}. en and a re- portant in vertebrate development, for instance, the lated gene, invected (inv}, are co-expressed early in em- homeo box-containing genes {e.g., the Hox genes; for re- bryogenesis in the cells of the posterior compartments of view, see Fienberg et al. 1987; Holland and Hogan the segments IDinardo et al. 1985; Fjose et al. 1985; 1988a), have been isolated by using Drosophila develop- Komberg et al. 1985; Coleman et al. 1987} and later in mental genes as probes. development in the developing central nervous system We have been studying the two murine genes, En-I (CNS} (DiNardo et al. 1985; Coleman et al. 1987). The and En-2, which were isolated on the basis of their ex- regulation of expression of en in Drosophila is complex. tensive sequence homology to the Drosophila segmen- One of the genes involved in maintenance of expression tation gene engrailed (en) (Joyner et al. 1985; Joyner and oi en and which also may be capable of initiating its de Martin 1987). Our in situ hybridization analysis of the novo expression is the segment polarity gene wingless expression of En-2 during development has shown that (wg} (DiNardo et al. 1988). wg is expressed in a stripe of it is expressed early in embryogenesis in a band of cells cells anterior to the en/inv-expressing posterior com- across the anterior neural folds and continues to be ex- partment cells in the cellular blastoderm {Baker 1987; 1736 GENES& DEVELOPMENT 2:1736-1744 © 1988 by Cold Spring Harbor Laboratory ISSN 0890-9369/88 $1.00 Downloaded from genesdev.cshlp.org on September 27, 2021 - Published by Cold Spring Harbor Laboratory Press En gene expression during mouse development Rijsewijk et al. 1987). Recently it has been shown that each of the three antisense probes. Sense-strand negative wg is the Drosophila homolog of the murine oncogene control probes were included in each series. int-1 (Rijsewijk et al. 1987). In the mouse, the expression Whole embryos for four different stages of gestation, of int-1 has been shown by in situ analysis to be limited from 7.7 to 15.5 days, as well as 17.5-day and adult brain to the CNS during embryogenesis (Wilkinson et al. were examined. When the sense strand-negative control 1987) and the testes in the adult (Jackobovits et al. 1986; probes were used, occasional artifactual binding oc- Shackleford and Varmus 1987). curred near the borders of the sections or lumens within Considering that the two related genes en and inv are the embryos. Nonspecific binding was also consistently co-expressed in the same cells during embryogenesis, it observed around the edges of the cerebellum as well as was of interest to determine the pattern of expression of to the dentate gyrus and pyramidal cell layer of the adult mouse En-1 during development and to compare it to hippocampus. that of the related gene En-2. Furthermore, because in Drosophila wg plays a role in the regulation of expres- sion of at least en and is expressed at the cellular blasto- Expression of En-1, En-2, and int-1 was first detected in derm stage adjacent to the en/inv-expressing cells, it was overlapping domains during formation of the early important to determine whether the expression patterns somites of En-1, En-2, and int-1 suggest a similar interaction be- In our previous in situ study we were first able to detect tween these genes. In this paper, we have used in situ En-2 transcripts in outbred CD-1 mice during a 6-hr pe- hybridization to examine the pattern of expression of riod covering the formation of the first six somites En-1 at stages throughout development and to compare around day 8.0 (Davis et al. 1988). Similarly, the earliest the expression patterns of En-1, En-2, and int-1. These detectable expression of int-1 by in situ analysis was be- studies demonstrate that expression of the three genes tween days 8 and 9 in CBA mice (Wilkinson et al. 1987). begins within the same 6-hr period of early somite for- This time corresponds to 7.5-8.5 days in other mouse mation and is restricted to similar overlapping domains strains, including CD-1. within the neural folds. By 12 days of embryonic devel- To determine the degree of coincidence of expression opment, however, the patterns of expression of these of En-1, En-2, and int-1, 7.7-day (CD-1 outbred mice) three genes have greatly diverged and any overlap in ex- embryos which possessed a neural plate and foregut pression is restricted to a small region of the metence- pocket, but lacked any somites, and 8.0-day embryos phalon. Also, although both en genes are expressed in which had between 4 and 6 somites were hybridized in similar regions of the developing brain, En-1 is also ex- situ to the probes specific for each of the genes. The 7.7- pressed transiently at midgestation in the spinal cord, in day embryos showed no hybridization specific to any of the limb and tail buds, and in two somite-derived the probes (C.A. Davis, data not shown). However, by tissues. 8.0 days all three probes hybridized to the embryos (Fig. 1). En-1 and En-2 transcripts were only detected in a band of the neural folds at the level of the foregut Results pocket. Although it is not possible from this analysis to be certain that the two expression patterns are identical, Experimental strategy they are very similar and the slight variations we ob- To analyze the expression of En-1, En-2, and int-1 in situ served could be due to slight differences between neigh- during mouse embryogenesis, we used SP6 transcripts of boring sections. The int-1 probe gave a higher back- small regions of the three genes. The En-2 probe was the ground than the En probes in all tissues from all stages same 250-bp fragment of 3' untranslated cDNA that was examined. Nevertheless, from examining six embryos in used previously (Davis et al. 1988). For En-1, a 180-bp 3' sagittal or cross sections, it was clear that the same re- untranslated cDNA fragment was used. This probe hy- gion of the neural folds that hybridized to the En probes bridized to a single copy sequence on Southern blots of was also the major site of int-1 expression in the 8.0-day mouse genomic DNA.