BMP-7/OP-1, a member of the transforming growth factor-beta (TGF-beta) family of secreted growth factors, is expressed during mouse embryogenesis in a pattern suggesting potential roles in a variety of inductive tissue interactions. The present study demonstrates that mice lacking BMP-7 display severe defects confined to the developing kidney and eye. Surprisingly, the early inductive tissue interactions responsible for establishing both organs appear largely unaffected. However, the absence of BMP-7 disrupts the subsequent cellular interactions required for their continued growth and development. Consequently, homozygous mutant animals exhibit renal dysplasia and anophthalmia at birth. Overall, these findings identify BMP-7 as an essential signaling molecule during mammalian kidney and eye development (Dudley, 1995).

BMP-7-deficient mice die shortly after birth because of poor kidney development. Histological analysis of mutant embryos at several stages of development reveal that metanephric mesenchymal cells fail to differentiate, resulting in a virtual absence of glomerulus in newborn kidneys. The absence of BMP-7 affects the expression of molecular markers of nephrogenesis, such as Pax-2 and Wnt-4 between 12.5 and 14.5 days postcoitum. This identifies BMP-7 as an inducer of nephrogenesis. In addition, BMP-7-deficient mice have eye defects that appear to originate during lens induction. BMP-7-deficient mice also have skeletal patterning defects restricted to the rib cage, the skull, and the hindlimbs (Luo, 1995).

While generating bcl2 alpha transgenic mice, some F2 offspring from one of the transgenic lines were found which were very small and had closed eyes at the time of weaning. These pups die within 1 month after birth. In order to determine the molecular basis of this phenotype, a genomic library of the above transgenic line was screened with a transgene-specific probe. It was found that the Bmp7 gene, a member of the TGF beta superfamily, had been inactivated by insertional mutagenesis due to transgene integration. The Bmp7 homozygous null condition in mice is a postnatal lethal mutation and is associated with various developmental defects: holes in the basisphenoid bone and the xyphoid cartilage, retarded ossification of bones, fused ribs and vertebrae, underdeveloped neural arches of the lumbar and sacral vertebrae, polydactyly of the hind limbs, a kinked tail, a reduced number of nephrons, polycystic kidney, lack of retinal pigmentation, and retarded lens development. These findings indicate that BMP7 is an important signaling molecule for normal development of the mammalian skeleton, kidney, and eye (Jena, 1997).

Bmp6, a member of the 60A subgroup of bone morphogenetic proteins (BMPs), is expressed in diverse sites in the developing mouse embryo from preimplantation stages onward. To evaluate roles for Bmp6 signaling in vivo, gene targeting was used to generate a null mutation at the Bmp6 locus. The resulting Bmp6 mutant mice are viable and fertile, and show no overt defects in tissues known to express Bmp6 mRNA. The skeletal elements of newborn and adult mutants are indistinguishable from wild-type. However, careful examination of skeletogenesis in late gestation embryos reveals a consistent delay in ossification, strictly confined to the developing sternum. In situ hybridization studies in the developing long bones and sternum show that other BMP family members are expressed in overlapping domains. In particular Bmp2 and Bmp6 are coexpressed in hypertrophic cartilage, suggesting that Bmp2 may functionally compensate in Bmp6 null mice. The defects in sternum development in Bmp6 null mice are likely to be associated with a transient early expression of Bmp6 in the sternal bands, prior to ossification. These sternal defects are slightly exacerbated in Bmp5/6 double mutant animals (Solloway, 1998).

Despite the importance of BMP signaling in normal development, very little is known about the mechanisms that control the synthesis and distribution of BMP signals in vertebrates. A large array of cis-acting control sequences have been identified that lay out expression of the mouse Bmp5 gene in specific skeletal structures and soft tissues. Some of these elements show striking specificity for particular anatomical features within the skeleton, rather than for cartilage and bone in general. These data suggest that the vertebrate skeleton is built from the sum of many independent domains of BMP expression, each of which may be controlled by separate regulatory elements driving expression at specific anatomical locations. Surprisingly, some of the regulatory sequences in the Bmp5 gene map over 270 kb from the Bmp5 promoter; this distance between regulatory elements and the promoter is one of the longest yet identified in studies of eukaryotic gene expression (DiLeone, 1998).

Ventral midline cells at different rostrocaudal levels of the central nervous system exhibit distinct properties but share the ability to pattern the dorsoventral axis of the neural tube. Ventral midline cells acquire distinct identities in response to the different signaling activities of underlying mesoderm. Signals from prechordal mesoderm control the differentiation of rostral diencephalic ventral midline cells, whereas notochord induces floor plate cells caudally. Sonic hedgehog (SHH) is expressed throughout axial mesoderm and is required for the induction of both rostral diencephalic ventral midline cells and floor plate. However, prechordal mesoderm also expresses BMP7, whose function is required coordinately with Shh to induce rostral diencephalic ventral midline cells. BMP7 acts directly on neural cells, modifying their response to Shh so that they differentiate into rostral diencephalic ventral midline cells rather than floor plate cells. A model is suggested whereby axial mesoderm both induces the differentiation of overlying neural cells and controls the rostrocaudal character of the ventral midline of the neural tube (Dale, 1997).

To investigate the role of BMPs in neural development, the expression of five Bmp genes belonging to the Drosophila Decapentaplegic (Bmp2 and Bmp4) and 60A subgroups (Bmp5, Bmp6 and Bmp7) have been compared. A striking co-expression of these Bmps is observed within the dorsomedial telencephalon, coincident with a future site of choroid plexus development. Bmp co-expression overlaps that of Msx1 and Hfh4, and is complementary to that of Bf1. The domain of Bmp co-expression is also associated with limited growth of the neuroectoderm, as revealed by morphological observation, reduced cell proliferation, and increased local programmed cell death. In vitro experiments using explants from the embryonic lateral telencephalic neuroectoderm reveal that exogenous BMP proteins (BMP4 and BMP2) induce expression of Msx1 and inhibit Bf1 expression, a finding consistent with their specific expression patterns in vivo. Moreover, BMP proteins locally inhibit cell proliferation and increase apoptosis in the explants. These results provide evidence that BMPs function during regional morphogenesis of the dorsal telencephalon by regulating specific gene expression, cell proliferation and local cell death (Furuta, 1997).

Proper dorsal-ventral patterning in the developing central nervous system requires signals from both the dorsal and ventral portions of the neural tube. Data from multiple studies have demonstrated that bone morphogenetic proteins (BMPs) and Sonic hedgehog protein are secreted factors that regulate dorsal and ventral specification, respectively, within the caudal neural tube. In the developing rostral central nervous system Sonic hedgehog protein also participates in ventral regionalization; however, the roles of BMPs in the developing brain are less clear. It was hypothesized that BMPs also play a role in dorsal specification of the vertebrate forebrain. To test this hypothesis, beads soaked in recombinant BMP5 or BMP4 were implanted into the neural tube of the chicken forebrain. Experimental embryos show a loss of the basal telencephalon that results in holoprosencephaly (a single cerebral hemisphere), cyclopia (a single midline eye), and loss of ventral midline structures. In situ hybridization using a panel of probes to genes expressed in the dorsal and ventral forebrain reveals the loss of ventral markers, although dorsal markers are maintained. Furthermore, the loss of the basal telencephalon is the result of excessive cell death and not a change in cell fates. These data provide evidence that BMP signaling participates in the dorsal-ventral patterning of the developing brain in vivo, and that disturbances in dorsal-ventral signaling result in specific malformations of the forebrain (Golden, 1999).

Sympathetic neurons from perinatal rat pups extend only a single axon when maintained in culture in the absence of glia and serum. Exposure to recombinant osteogenic protein-1 (OP-1) selectively induces the formation of dendrites that correctly segregate and modify cytoskeletal and membrane proteins and form synaptic contacts of appropriate polarity. OP-1 requires nerve growth factor (NGF) as a cofactor, and, in the presence of optimal concentrations of NGF, OP-1-induced dendritic growth from cultured perinatal neurons is comparable to that observed in situ. Sympathetic neuroblasts that have not formed dendrites in situ also respond to OP-1 in culture, indicating that OP-1 can cause de novo formation as well as regeneration of dendrites. These data imply that specific signals can regulate the development of neuronal shape and polarity (Lein, 1995).

The growth patterns of axons and dendrites differ with respect to their number, length, branching, and spatial orientation; therefore, it is likely that these processes differ in their growth requirements. To examine this hypothesis, an analysis has been carried out of the responses of cultured rat sympathetic neurons to three types of stimuli: large structural proteins of the extracellular matrix, matrix-associated growth factors, and neurotrophins. Purified structural proteins such as laminin and collagen IV have been found to promote only axonal growth; whereas the matrix associated growth factor, osteogenic protein-1, selectively stimulates dendritic growth. In contrast, nerve growth factor modulates the growth of both types of processes. These data suggest that process-specific interactions with the extracellular environment may be critical determinants of cell shape in neurons. Perinatal rat sympathetic neurons grown in culture in the absence of serum or glial cells extend a single process, which is axonal in nature. Exposure to osteogenic protein-1 causes the formation of additional processes that express the morphological, cytoskeletal, and ultrastructural characteristics of dendrites. Consistent with observations on the regulation of dendritic growth in sympathetic neurons in situ, the dendrite-promoting activity of osteogenic protein-1 is independent of synaptic or electrical activity, but is modulated by nerve growth factor. In the presence of optimal concentrations of osteogenic protein-1 and nerve growth factor, the size of the dendritic arbor extended by cultured sympathetic neurons approximates that seen in situ at comparable developmental stages. Osteogenic protein-1 does not promote dendritic growth in cultured neurons obtained from embryonic ciliary, dorsal root, trigeminal or nodose ganglia, suggesting that its morphogenetic effects are cell selective. Since mRNA for osteogenic protein-1 is expressed in mature as well as embryonic target tissues of the sympathetic nervous system, the effects of osteogenic protein-1 on cultures of sympathetic neurons derived from adult rats were also examined. Consistent with results obtained with perinatal neurons, osteogenic protein-1 selectively promotes dendritic growth in adult neurons. These data suggest that this matrix-associated growth factor could play a role not only in the morphogenesis of the developing nervous system, but also in the maintenance and remodeling of dendritic structures in the mature animal (Lein, 1996).