Practical Modeling Concepts for Connective Tissue Stem Cell and Progenitor Compartment Kinetics

Stem cell activation and development is central to skeletal development, maintenance, and repair, as it is for all tissues. However, an integrated model of stem cell proliferation, differentiation, and transit between functional compartments has yet to evolve. In this paper, the authors review current concepts in stem cell biology and progenitor cell growth and differentiation kinetics in the context of bone formation. A cell-based modeling strategy is developed and offered as a tool for conceptual and quantitative exploration of the key kinetic variables and possible organizational hierarchies in bone tissue development and remodeling, as well as in tissue engineering strategies for bone repair

Nel positively regulates the genesis of retinal ganglion cells by promoting their differentiation and survival during development

Peer reviewedPublisher PD

Transposon-Mediated Stable Suppression of Gene Expression in the Developing Chick Retina

The embryonic chick has long been a favorite model system for in vivo studies of vertebrate development. However, a major technical limitation of the chick embryo has been the lack of efficient loss-of-function approaches for analyses of gene functions. Here, we describe a methodology in which a transgene encoding artificial microRNA sequences is introduced into embryonic chick retinal cells by in ovo electroporation and integrated into the genome using the Tol2 transposon system. We show that this methodology can induce potent and stable suppression of gene expression. This technique therefore provides a rapid and robust loss-of-function approach for studies of gene function in the developing retina

Nell2 regulates the contralateral-versus-ipsilateral visual projection as a domain-specific positional cue

In mammals with binocular vision, retinal ganglion cell (RGC) axons from each eye project to eye-specific domains in the contralateral and ipsilateral dorsal lateral geniculate nucleus (dLGN), underpinning disparity-based stereopsis. Although domain-specific axon guidance cues that discriminate contralateral and ipsilateral RGC axons have long been postulated as a key mechanism for development of the eye-specific retinogeniculate projection, the molecular nature of such cues has remained elusive. Here, we show that the extracellular glycoprotein Nell2 (neural epidermal growth factor-like-like 2) is expressed in the dorsomedial region of the dLGN, which ipsilateral RGC axons terminate in and contralateral axons avoid. In Nell2 mutant mice, contralateral RGC axons abnormally invaded the ipsilateral domain of the dLGN, and ipsilateral axons terminated in partially fragmented patches, forming a mosaic pattern of contralateral and ipsilateral axon-termination zones. In vitro, Nell2 exerted inhibitory effects on contralateral, but not ipsilateral, RGC axons. These results provide evidence that Nell2 acts as a domain-specific positional label in the dLGN that discriminates contralateral and ipsilateral RGC axons, and that it plays essential roles in the establishment of the eye-specific retinogeniculate projection

Nell2 regulates the contralateral-versus-ipsilateral visual projection as a domain-specific positional cue

In mammals with binocular vision, retinal ganglion cell (RGC) axons from each eye project to eye-specific domains in the contralateral and ipsilateral dorsal lateral geniculate nucleus (dLGN), underpinning disparity-based stereopsis. Although domain-specific axon guidance cues that discriminate contralateral and ipsilateral RGC axons have long been postulated as a key mechanism for development of the eye-specific retinogeniculate projection, the molecular nature of such cues has remained elusive. Here, we show that the extracellular glycoprotein Nell2 (neural epidermal growth factor-like-like 2) is expressed in the dorsomedial region of the dLGN, which ipsilateral RGC axons terminate in and contralateral axons avoid. In Nell2 mutant mice, contralateral RGC axons abnormally invaded the ipsilateral domain of the dLGN, and ipsilateral axons terminated in partially fragmented patches, forming a mosaic pattern of contralateral and ipsilateral axon-termination zones. In vitro, Nell2 exerted inhibitory effects on contralateral, but not ipsilateral, RGC axons. These results provide evidence that Nell2 acts as a domain-specific positional label in the dLGN that discriminates contralateral and ipsilateral RGC axons, and that it plays essential roles in the establishment of the eye-specific retinogeniculate projection

Identification and characterization of a novel axon guidance receptor in the layer-specific visual projection

Correct functioning of the nervous system critically depends on the formation of the precise neuronal network. In the vertebrate visual system, axons of retinal ganglion cells project to the specific layers on the target regions in the brain, such as the lateral geniculate nucleus in the thalamus and the superior colliculus/tectum in the midbrain. In mammals, the layer-specific visual projection provides the anatomical basis for disparity-based stereopsis (perception of depth). Our previous studies have provided evidence that Nell2 (Neural epidermal growth factor (EGF)-like-like 2), an extracellular glycoprotein predominantly expressed in the nervous system, acts as an inhibitory guidance molecule for retinal axons in establishment of the layer-specific visual projection. A crucial question at the next step is how Nell2 is recognized by retinal axons and how downstream signals are mediated. However, the receptor(s) for Nell2 has not been identified in retinal axons. The current work aims to identify and functionally characterize the receptor(s) that transduces Nell2-mediated signaling in retinal axons. By using immunohistochemistry, we have recently found that the receptor tyrosine kinase Ros1, which is known to bind to Nell2 in the testis, is expressed in developing chick retinal ganglion cells, suggesting that Ros1 may act as a receptor for Nell2 in the visual system. To analyze the function of Ros1 in the visual projection, we have designed and generated DNA oligonucleotides that encode pre-micro-RNA (pre-miRNA) sequences against Ros1 cDNA. The pre-miRNA sequences will be introduced into and stably expressed in the developing retina, by using the Tol2 transposon system and in vivo electroporation, and effects of Nell2 knockdown on the layer-specific visual projection will be analyzed by axon tracing. We expect that this project will elucidate the function of Nell2-Ros1 interaction in the layer-specific visual projection and provide novel insights into the molecular mechanisms of neuronal network formation

In vitro guidance of retinal axons by a tectal lamina-specific glycoprotein Nel

Nel is a glycoprotein containing five chordin-like and six epidermal growth factor-like domains and is strongly expressed in the nervous system. In this study, we have examined expression patterns and in vitro functions of Nel in the chicken retinotectal system. We have found that in the developing tectum, expression of Nel is localized in specific laminae that retinal axons normally do not enter, including the border between the retinorecipient and non-retinorecipient laminae. Nel-binding activity is detected on retinal axons both in vivo and in vitro, suggesting that retinal axons express a receptor for Nel. In vitro, Nel inhibits retinal axon outgrowth and induces growth cone collapse and axon retraction. These results indicate that Nel acts as an inhibitory guidance cue for retinal axons, and suggest its roles in the establishment of the lamina-specificity in the retinotectal projection