Retinoids and Hox genes

The vertebrate embryonic body plan is constructed through the interaction of many developmentally regulated genes that supply cells with the essential positional and functional information they require to migrate to their appropriate destination and generate the proper structures. Some molecular cues involved in patterning the central nervous system, particularly in the hindbrain, are interpreted by the Hox homeobox genes. Retinoids can affect the expression of Hox genes in cells lines and embryonic tissues; the hindbrain and branchial region of the head are particularly sensitive to the teratogenic effects of retinoic acid. The presence of endogenous retinoic acid, together with the distribution of retinoid binding proteins and nuclear receptors in the developing embryo, strongly suggest that retinoic acid is a natural morphogen in vertebrate development. The molecular basis for the interaction between retinoic acid and the Hox genes has been aided in part by approaches using deletion analysis in transgenic mice carrying lacZ reporter constructs. Such studies have identified functional retinoic acid response elements within flanking sequences of some of the most 3' Hox genes, suggesting a direct interaction between the genes and retinoic acid. Furthermore, as demonstrated using transgenic mice carrying Hoxb-1/lacZ constructs, multiple retinoic acid response elements may cooperate with positive and negative regulatory enhancers to specify pattern formation in the vertebrate embryo. These types of studies strongly support the normal roles of retinoids in patterning vertebrate embryogenesis through the Hox genes.     

Genetic interactions of Hox genes in limb development: learning from compound mutants

Some investigators have noted an increased incidence of suicidal ideation and attempts in individuals with panic attacks. The direct temporal relationship between the panic state and suicidal thoughts and behaviors has not been well elucidated however. Furthermore, although aggressive behavior is often manifested in individuals with suicidal behavior, the relationship between aggression and panic has received little attention. The aim of this study was to assess the frequency and type of reported suicidal and aggressive ideation and behaviors that occur during the panic state in patients with panic disorder. In order to evaluate the contribution of depression, individuals with pure (i.e. uncomplicated) panic disorder were compared with individuals who had comorbid panic and major depression. Nineteen patients with a diagnosis of pure panic disorder and 28 patients with comorbid panic plus major depression were included in the study. All patients were given the Panic, Suicide and Aggression Scale (PSAS), a questionnaire specifically designed to assess reported suicidal and aggressive thoughts and behaviors that occur during panic attacks. Other scales given to all patients included overall measures of impulsivity, suicide risk and violence risk. Patients with pure panic disorder reported high rates of suicidal and aggressive ideation and behavior during panic. The presence of comorbid depression resulted in a doubling of the rate of reported panic-associated suicidal ideation, property destruction and assaults, and a five-fold increase in the rate of homicidal ideation. The rate of reported suicide attempts was equal in the pure panic and comorbid group. There were also high correlations in all panic patients between measures of panic-associated suicide and aggression with the psychometric measures of impulsivity, suicide risk and violence risk.     

Hox genes: from master genes to micromanagers

We still have little idea how the differential expression of one 'master' gene can control the morphology of complex structures, but recent studies suggest that the Drosophila Hox gene Ultrabithorax micromanages segment development by manipulating a large number of different targets at many developmental stages.     

Lhx2, a vertebrate homologue of apterous, regulates vertebrate limb outgrowth

apterous specifies dorsal cell fate and directs outgrowth of the wing during Drosophila wing development. Here we show that, in vertebrates, these functions appear to be performed by two separate proteins. Lmx-1 is necessary and sufficient to specify dorsal identity and Lhx2 regulates limb outgrowth. Our results suggest that Lhx2 is closer to apterous than Lmx-1, yet, in vertebrates, Lhx2 does not specify dorsal cell fate. This implies that in vertebrates, unlike Drosophila, limb outgrowth can be dissociated from the establishment of the dorsoventral axis.     

Vertebrate evolution: something fishy about Hox genes

The complete Hox gene complement of the Japanese pufferfish has now been determined, together with the genomic organisation of all four Hox gene clusters. One of the many surprises is that this strange fish has lost an unusually large number of Hox genes.     

Analysis of Hoxd-13 and Hoxd-11 misexpression in chick limb buds reveals that Hox genes affect both bone condensation and growth

Hox genes are important regulators of limb pattern in vertebrate development. Misexpression of Hox genes in chicks using retroviral vectors provides an opportunity to analyze gain-of-function phenotypes and to assess their modes of action. Here we report the misexpression phenotype for Hoxd-13 and compare it to the misexpression phenotype of Hoxd-11. Hoxd-13 misexpression in the hindlimb results in a shortening of the long bones, including the femur, the tibia, the fibula and the tarsometatarsals. Mutations in an alanine repeat region in the N-terminus of Hoxd-13 have recently been implicated in human synpolydactyly (Muragaki, Y., Mundlos, S., Upton, J. and Olsen, B. R. (1996) Science 272, 548-551). N-terminal truncations of Hoxd-13 which lack this repeat were constructed and were found to produce a similar, although slightly milder, misexpression phenotype than the full-length Hoxd-13. The stage of bone development regulated by Hox genes has not previously been examined. The changes in bone lengths caused by Hoxd-13 misexpression are late phenotypes that first become apparent during the growth phase of the bones. Analysis of tritiated thymidine uptake by the tibia and fibula demonstrates that Hox genes can pattern the limb skeleton by regulating the rates of cell division in the proliferative zone of growing cartilage. Hoxd-11, in contrast to Hoxd-13, acts both at the initial cartilage condensation phase in the foot and during the later growth phase in the lower leg. Ectopic Hoxd-13 appears to act in a dominant negative manner in regions where it is not normally expressed. We propose a model in which all Hox genes are growth promoters, regulating the expression of the same target genes, with some Hox genes being more effective promoters of growth than other Hox genes. According to this model, the overall rate of growth in a given region is the result of the combined action of all of the Hox genes expressed in that region competing for the same target genes.     

Paralogous mouse Hox genes, Hoxa9, Hoxb9, and Hoxd9, function together to control development of the mammary gland in response to pregnancy

Although the role of Hox genes in patterning the mammalian body plan has been studied extensively during embryonic and fetal development, relatively little is known concerning Hox gene function in adult animals. Analysis of mice with mutant Hoxa9, Hoxb9, and Hoxd9 genes shows that these paralogous genes are required for mediating the expansion and/or differentiation of the mammary epithelium ductal system in response to pregnancy. Mothers with these three mutant genes cannot raise their own pups, but the pups can be rescued by fostering by wild-type mothers. Histologically, the mammary glands of the mutant mothers seem normal before pregnancy but do not develop properly in response to pregnancy and parturition. Hoxa9, Hoxb9, and Hoxd9 are expressed normally in adult mammary glands, suggesting a direct role for these genes in the development of mammary tissue after pregnancy. Because loss-of-function mutations in these Hox genes cause hypoplasia of the mammary gland after pregnancy, it may be productive to look for misexpression of these genes in mammary carcinomas.     

Rescue of Caenorhabditis elegans pharyngeal development by a vertebrate heart specification gene

Development of pharyngeal muscle in nematodes and cardiac muscle in vertebrates and insects involves the related homeobox genes ceh-22, nkx2.5, and tinman, respectively. To determine whether the nematode and vertebrate genes perform similar functions, we examined activity of the zebrafish nkx2.5 gene in transgenic Caenorhabditis elegans. Here, we report that ectopic expression of nkx2.5 in C. elegans body wall muscle can directly activate expression of both the endogenous myo-2 gene, a ceh-22 target normally expressed only in pharyngeal muscle, and a synthetic reporter construct controlled by a multimerized CEH-22 binding site. nkx2.5 also efficiently rescues a ceh-22 mutant when expressed in pharyngeal muscle. Together, these results indicate that nkx2.5 and ceh-22 provide a single conserved molecular function. Further, they suggest that an evolutionarily conserved mechanism underlies heart development in vertebrates and insects and pharyngeal development in nematodes.     

Pattern formation in epithelial development: the vertebrate limb and feather bud spacing

The ectoderm of the vertebrate limb and feather bud are epithelia that provide good models for epithelial patterning in vertebrate development. At the tip of chick and mouse limb buds is a thickening, the apical ectodermal ridge, which is essential for limb bud outgrowth. The signal from the ridge to the underlying mesoderm involves fibroblast growth factors. The non-ridge ectoderm specifies the dorsoventral pattern of the bud and Wnt7a is a dorsalizing signal. The development of the ridge involves an interaction between dorsal cells that express radical fringe and those that do not. There are striking similarities between the signals and genes involved in patterning the limb ectoderm and the epithelia of the Drosophila imaginal disc that gives rise to the wing. The spacing of feather buds involves signals from the epidermis to the underlying mesenchyme, which again include Wnt7a and fibroblast growth factors.     

The role of homeobox genes in limb development

Intensive study of the development of the vertebrate limb has led to a conceptual framework for understanding the specification of a limb primordium, the outgrowth of those cells and their organization and differentiation into a functional appendage. During the past few years, a number of homeobox-containing genes have been identified that are likely to play controlling roles in each of these events.     

Functional dominance among Hox genes: repression dominates activation in the regulation of Dpp

Here we investigate the mechanisms by which Hox genes compete for the control of positional identity. Functional dominance is often observed where different Hox genes are co-expressed, and frequently the more posteriorly expressed Hox gene is the one that prevails, a phenomenon known as posterior prevalence. We use dpp674, a visceral mesoderm-specific enhancer of decapentaplegic (dpp), to investigate functional dominance among Hox genes molecularly. We find that posterior prevalence does not adequately describe the regulation of dpp by Hox genes. Instead, we find that abdominal-A (abd-A) dominates over the more posterior Abdominal-B (Abd-B) and the more anterior Ultrabithorax (Ubx). In the context of the dpp674 enhancer, abd-A functions as a repressor whereas Ubx and Abd-B function as activators. Thus, these results suggest that other cases of Hox competition and functional dominance may also be understood in terms of competition for target gene regulation in which repression dominates over activation.     

The PBC domain contains a MEINOX domain: coevolution of Hox and TALE homeobox genes?

The diagnosis of inadequacy of the respiratory apparatus conditioning function (RACFI) necessitates carrying out an appreciable volume of calculations to determine observable and required values for its parameters, together with drawing up a concluding statement according to the existing RACFI classification. The above rapid method permits the 2.5-fold cut in man-power to be streamlined into research on respiratory heat-exchange in patients with cardiopulmonary disorders. The suggested method is recommended for use in therapeutic, pulmonological, cardiological departments and functional-diagnosis units in hospitals.     

Radical fringe positions the apical ectodermal ridge at the dorsoventral boundary of the vertebrate limb

Vertebrate limb outgrowth requires a structure called the apical ectodermal ridge, formation of which follows the previous establishment of the dorsoventral limb axis. Radical fringe is expressed in the dorsal ectoderm before the ridge appears, and is repressed by Engrailed-1, which is expressed in the ventral ectoderm. Misexpression of these genes indicates that a ridge is formed wherever there is a boundary between cells expressing and not expressing Radical fringe. Thus, as in Drosophila, Radical fringe positions the ridge at the dorsoventral limb boundary.     

The role of Alx-4 in the establishment of anteroposterior polarity during vertebrate limb development

We have determined that Strong's Luxoid (lstJ) [corrected] mice have a 16 bp deletion in the homeobox region of the Alx-4 gene. This deletion, which leads to a frame shift and a truncation of the Alx-4 protein, could cause the polydactyly phenotype observed in lstJ [corrected] mice. We have cloned the chick homologue of Alx-4 and investigated its expression during limb outgrowth. Chick Alx-4 displays an expression pattern complementary to that of shh, a mediator of polarizing activity in the limb bud. Local application of Sonic hedgehog (Shh) and Fibroblast Growth Factor (FGF), in addition to ectodermal apical ridge removal experiments suggest the existence of a negative feedback loop between Alx-4 and Shh during limb outgrowth. Analysis of polydactylous mutants indicate that the interaction between Alx-4 and Shh is independent of Gli3, a negative regulator of Shh in the limb. Our data suggest the existence of a negative feedback loop between Alx-4 and Shh during vertebrate limb outgrowth.