Inositol phospho-oligosaccharide stimulates cell proliferation in the early developing inner ear

Identification and analysis of genes from the mouse otic vesicle and their association with developmental subprocesses through in situ hybridization

The otic vesicle (otocyst) occupies a pivotal position in inner ear development, bridging the gap between otic placode determination, and morphogenesis of vestibular and auditory compartments. The molecular mechanisms underlying the progressive subdivision of the developing inner ear into different compartments, and the molecular control and execution of the different developmental processes involved, are largely unknown. Since relatively few genes have been implicated in these processes, we have undertaken this study to identify genes involved in these early embryonic stages. We have used cDNA subtractions of mouse otic vesicle against adult liver cDNA, and describe a set of 280 candidate genes. We have also performed otic vesicle RNA hybridizations against DNA chips to not only confirm the efficacy of the library approach, but also to investigate the utility of DNA array alternatives. To begin to dissect potential developmental roles, we investigated the spatial pattern of gene expression for a selected set of 80 genes in developing mouse embryos at mid-gestation by whole-mount in situ hybridization. These data illustrate the compartmentalisation of gene expression in the otic vesicle for the majority of genes tested, and furthermore, implicate many of the genes tested with distinct developmental subprocesses.

Induction of cell growth by insulin and insulin-like growth factor-I is associated with Jun expression in the otic vesicle

The present report investigates the cellular mechanisms involved in the regulation of cell proliferation by insulin and insulin-like growth factor-I (IGF-I) in the developing inner ear. The results show that insulin and IGF-I stimulate cell proliferation in the otic vesicle. This effect is associated with the induction of the expression of the nuclear proto-oncogene c-jun. The temporal profile of Jun expression coincided with the proliferative period of growth of the otic vesicle. IGF-I promoted the hydrolysis of a membrane glycosyl-phosphatidylinositol, which was characterised as the endogenous precursor for inositol phosphoglycan (IPG). Both purified IPG and a synthetic analogue, 6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-D-myoinositol-1,2-cyclic phosphate (C3), were able to mimic the effects of IGF-I on Jun expression. Anti-IPG antibodies blocked the effects of IGF-I, which were rescued by the addition of IPG or its analogue. These results suggest that the sequence involving the hydrolysis of membrane glycolipids and the expression of c-jun and c-fos proto-oncogenes is part of the mechanism that activates cell division in response to insulin and IGF-I during early organogenesis of the avian inner ear. The implications of these observations for otic development and regeneration are briefly discussed.

The role of glycosyl-phosphatidylinositol in signal transduction

Glycosyl-phosphatidylinositol (GPI) lipids have a structural role as protein anchors to the cell surface. In addition, they are implicated in hormone, growth factor and cytokine signal transduction. Their phosphodiesteric hydrolysis mediated by an activated phospholipase results in the generation of water soluble oligosaccharide species termed the inositol phosphoglycan (IPG). This product has been demonstrated to possess biological properties when added exogenously to cells mimicking the biological effects of a variety of extracellular ligands. This may be accomplished since IPG is generic for a family of closely related species which are released in a tissue-specific manner and additionally have cell-specific targets. Micro-organic synthesis has recently been able to shed new light on this topic by the introduction of defined oligosaccharide analogues of IPG for the assessment of their biological activity. These have complemented the findings observed with purified IPG from biological sources thus strengthening the belief that the GPI/IPG signalling system represents a truly novel aspect of transmembrane signalling.

The development of the vertebrate inner ear

The inner ear is a complex sensory organ responsible for balance and sound detection in vertebrates. It originates from a transient embryonic structure, the otic vesicle, that contains all of the information to develop autonomously into the mature inner ear. We review here the development of the otic vesicle, bringing together classical embryological experiments and recent genetic and molecular data. The specification of the prospective ectoderm and its commitment to the otic fate are very early events and can be related to the expression of genes with restricted expression domains. A combinatorial gene expression model for placode specification and diversification, based on classical embryological evidence and gene expression patterns, is discussed. The formation of the otic vesicle is dependent on inducing signals from endoderm, mesoderm and neuroectoderm. Ear induction consists of a sequence of discrete instructions from those tissues that confer its final identity on the otic field, rather than a single all-or-none process. The important role of the neural tube in otic development is highlighted by the abnormalities observed in mouse mutants for the Hoxa1, kreisler and fgf3 genes and those reported in retinoic acid-deficient quails. Still, the nature of the relation between the neural tube and otic development remains unclear. Gene targeting experiments in the mouse have provided evidence for genes potentially involved in regional and cell-fate specification in the inner ear. The disruption of the mouse Brn3.1 gene identifies the first mutation affecting sensory hair-cell specification, and mutants for Pax2 and Nkx5.1 genes show their requirement for the development of specific regions of the otic vesicle. Several growth-factors contribute to the patterned cell proliferation of the otic vesicle. Among these, IGF-I and FGF-2 are expressed in the otic vesicle and may act in an autocrine manner. Finally, little is known about early mechanisms involved in guiding ear innervation. However, targeted disruption of genes coding for neurotrophins and Trk receptors have shown that once synaptic contacts are established, they depend on specific trophic interactions that involve these two gene families. The accessibility of new cellular and molecular approaches are opening new perspectives in vertebrate development and are also starting to be applied to ear development. This will allow this classical and attractive model system to see a rapid progress in the near future.

Isolation and partial characterisation of insulin-mimetic inositol phosphoglycans from human liver

Extracts of human liver were found to contain activities which copurified and coeluted with the two major subtypes of mediators (type A and type P) isolated from insulin-stimulated rat liver. The putative type A mediator from human liver inhibited cAMP-dependent protein kinase from bovine heart, decreased phosphoenolypyruvate carboxykinase mRNA levels in rat hepatoma cells, and stimulated lipogenesis in rat adipocytes. The putative type P mediator stimulated bovine heart pyruvate dehydrogenase phosphatase. Both fractions were able to stimulate proliferation of EGFR T17 fibroblasts and the type A was able to support growth in organotypic cultures of chicken embryo cochleovestibular ganglia. Both activities were resistant to Pronase treatment and the presence of carbohydrates, phosphate, and free-amino groups were confirmed in the two fractions. These properties are consistent with the structure/ function characteristics of the type A and P inositolphosphoglycans (IPG) previously characterized from rat liver. Further, the ability of the human-derived mediators to interact with rat adipocytes and bovine-derived metabolic enzymes suggests similarity in structure between the mediators purified from different species. Galactose oxidase-susceptible membrane-associated glycosylphosphatidylinositols (GPI) have been proposed to be the precursors of IPG. GPI was purified from human liver membranes followed by treatment with galactose oxidase and reduction with NaB3H4. Serial t.l.c. revealed three radiolabeled bands which comigrated with the putative GPI precursors found in rat liver. These galactose-oxidase-reactive lipidic compounds, however, were only partially susceptible to hydrolysis with phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis and were resistant to glycosylphosphatidylinositol-specific phospholipase C from Trypanosoma brucei. These data indicate that IPG molecules with insulin-like biological activities are present in human liver.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.

Synthesis and investigation of the possible insulin-like activity of 1D-4-O- and 1D-6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-myo-inositol 1-phosphate and 1D-6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-myo-inositol 1,2-(cyclic phosphate)

The synthesis of the glycosyl-myo-inositol 1-phosphates 1 and 2 and of the glycosyl-myo-inositol 1,2-(cyclic phosphate) 3, starting from previously synthesized intermediates, is reported. Compound 3 was found to display proliferative effects on the early developing inner ear of chick embryo.

Diffusible factors regulate hair cell regeneration in the avian inner ear

Damage to the avian inner ear results in up-regulation of mitotic activity resulting in regeneration of hair cells. The objective of this investigation was to determine whether the damaged inner ear epithelium releases a soluble mitogen that is responsible for the up-regulation of proliferation. The sensory epithelium from normal and drug-damaged avian inner ears was cultured alone or in the presence of other cultures. As previously shown in vivo and in vitro, damaged organs displayed increased supporting cell proliferation compared with undamaged organs, leading to eventual morphologic and functional recovery. When damaged organs were cocultured with an undamaged organ, proliferation was increased in the undamaged tissue. When undamaged organs were cultured together, proliferation was decreased. These results indicate that a soluble factor released from the damaged inner ear epithelium stimulates proliferation and suggest the release of a factor from normal tissue that suppressed mitotic activity. Thus, reparative hair cell regeneration in the inner ear appears to be regulated by a balance between proliferative and antiproliferative paracrine factors.

Multifunctional roles of glycosyl-phosphatidylinositol lipids

The results presented here indicate that GPI lipids are a structurally and functionally diverse molecular family. Despite new detailed information on the structures of GPI-anchored proteins, there is relatively scant information on the structure of free-GPI. Thus, little is known of the relationships between GPI structures and the mechanism of their biological effects. For example, there is no distinction at the structural level between hormone-sensitive free-GPI and those that serve as precursors for protein-GPI. Nor is there precise biochemical data on the mechanism and importance of free-GPI in hormone signaling, or the signaling roles that GPI anchors play in protein function. The T-cell activation cascade is an ideal system for studying both forms of GPI and their derivatives. The study of GPI molecules in T lymphocytes offers the exciting possibility of addressing questions on the structure, function, genesis, and regulation of both free- and protein-GPI molecules in a single cell type. The detection of multiple protein-GPI and free-GPI forms, and of hormone-sensitive GPI, provides the first approach to these issues. For the moment, the potential for biochemical signaling by intact GPI or its metabolites is enormous. If significant progress is to be made, the structures of hormone sensitive free-GPI must be elucidated. Only then can we precisely define the roles of these molecules in the regulation of cell metabolism and proliferation.

Role of the glycosylphosphatidylinositol/inositol phosphoglycan system in human fibroblast proliferation

The involvement of the glycosylphosphatidylinositol/inositol phosphoglycan (gly-PtdIns/IPG) system in the stimulation of macromolecular syntheses in human fibroblasts has been investigated. The study demonstrates that an insulin sensitive gly-PtdIns/IPG system is present in human fibroblasts, that IPG can significantly stimulate DNA, RNA, and protein synthesis, and that the action of insulin on DNA synthesis as well as that of IPG can be significantly reduced by a specific anti-IPG antibody. These results strongly support the hypothesis that the gly-PtdIns/IPG system is involved in the signal transduction pathway leading to the stimulation of cell proliferation.

Glycosyl-phosphatidylinositol/inositol phosphoglycan: a signaling system for the low-affinity nerve growth factor receptor

Nerve growth factor (NGF) exerts a variety of actions during embryonic development. At the early stages of inner ear development, NGF stimulates cell proliferation, an effect mediated through low-affinity receptors. We have studied the possibility that the glycosyl-phosphatidylinositol/inositol phosphoglycan (glycosyl-PtdIns/IPG) system is involved in transmitting this NGF signal. Endogenous glycosyl-PtdIns was characterized in extracts of cochleovestibular ganglia (CVGs) that incorporated [3H]glucosamine, [3H]galactose, [3H]myristic acid, and [3H]palmitic acid. Incubation of CVG with NGF produced a rapid and transient hydrolysis of glycosyl-PtdIns. Hydrolysis was complete at 100 ng/ml, and the half-maximal effect occurred at 25 ng/ml, overlapping with the concentration dependence of the mitogenic effect of NGF. An IPG was isolated from embryonic extracts. It had biological effects similar to those reported for the insulin-induced IPG in other tissues. It exerted a powerful mitogenic effect on CVG, comparable to that of NGF. Both the IPG- and NGF-induced cell proliferation were blocked by anti-IPG antibodies that recognized the endogenous IPG on a silica plate immunoassay. These results show that CVG possesses a fully active glycosyl-PtdIns/IPG signal transduction system and that the proliferative effects associated with NGF binding to low-affinity receptors require IPG generation.