Biomedical subjects

J A Raper

Publications and source records attributed to J A Raper.

At least 19 recordsLinked to original sources

PubMed2002

A model to describe the settling behavior of fractal aggregates.

A model to predict fractal dimension from sedimentating fractal aggregates has been successfully developed. This model was developed using the settling rate and size data of fractal aggregates. In order to test the validity of the model, a purpose-built settling rig, equipped with lens with magnification of 1200x, which can capture images of particles/flocs down to 2 microm in diameter was used. The performance and technique of the settling rig were validated by comparing the measured settling rates of 30- and 50.7-microm standard particles with their theoretical settling rates calculated using Stokes' law. The measured settling rates were within 10% agreement with the calculated Stokes' velocities. The settling rates and sizes of the particles/flocs were analyzed using image analysis software called WiT 5.3. The maximum temperature gradient across the settling column was 0.1 degrees C, which effectively eliminated convective currents due to temperature differences in the settling column. A total of 1000 calcium phosphate flocs were analyzed. Calcium phosphate flocs with fractal dimensions varying from 2.3 to 2.8 were generated via orthokinetic aggregation. Measurements of fractal dimensions, using light scattering, were done simultaneously with the settling experiments and they were found to be constant. The fractal dimensions calculated using the model agreed with those obtained by light scattering to within 12%.

Journal Article↗

PubMed2001

Targeted disruption of semaphorin 3C leads to persistent truncus arteriosus and aortic arch interruption.

Semaphorin 3C is a secreted member of the semaphorin gene family. To investigate its function in vivo, we have disrupted the semaphorin 3C locus in mice by targeted mutagenesis. semaphorin 3C mutant mice die within hours after birth from congenital cardiovascular defects consisting of interruption of the aortic arch and improper septation of the cardiac outflow tract. This phenotype is similar to that reported following ablation of the cardiac neural crest in chick embryos and resembles congenital heart defects seen in humans. Semaphorin 3C is expressed in the cardiac outflow tract as neural crest cells migrate into it. Their entry is disrupted in semaphorin 3C mutant mice. These data suggest that semaphorin 3C promotes crest cell migration into the proximal cardiac outflow tract.

Amino Acid Sequence↗

PubMed2001

PlexinA2 and semaphorin signaling during cardiac neural crest development.

Classic studies using avian model systems have demonstrated that cardiac neural crest cells are required for proper development of the cardiovascular system. Environmental influences that perturb neural crest development cause congenital heart defects in laboratory animals and in man. However, little progress has been made in determining molecular programs specifically regulating cardiac neural crest migration and function. Only recently have complex transgenic tools become available that confirm the presence of cardiac neural crest cells in the mammalian heart. These studies have relied upon the use of transgenic mouse lines and fate-mapping studies using Cre recombinase and neural crest-specific promoters. In this study, we use these techniques to demonstrate that PlexinA2 is expressed by migrating and postmigratory cardiac neural crest cells in the mouse. Plexins function as co-receptors for semaphorin signaling molecules and mediate axon pathfinding in the central nervous system. We demonstrate that PlexinA2-expressing cardiac neural crest cells are patterned abnormally in several mutant mouse lines with congenital heart disease including those lacking the secreted signaling molecule Semaphorin 3C. These data suggest a parallel between the function of semaphorin signaling in the central nervous system and in the patterning of cardiac neural crest in the periphery.

Animals↗

PubMed2000

Slit2 is a repellent for retinal ganglion cell axons.

We set out to isolate inhibitory guidance cues that affect retinal ganglion cell (RGC) axons in vitro and that could potentially be involved in RGC pathfinding decisions. Here we describe the biochemical purification of an RGC growth cone collapsing factor from bovine brain membranes and its identification as Slit2. Recombinant human Slit2 collapses and repels RGC growth cones from all quadrants of the chick retina. In the developing mouse visual system, slit2 is expressed in the eye, in the optic stalk, and in the ventral diencephalon. Slit2 expression is strong in anterior ventral diencephalic structures but is absent from the ventral midline where the optic chiasm forms. The putative receptors for Slits, robo1 and robo2, are expressed in the inner retinal layer in which RGCs are located. A comparison of the expression patterns of Slit2 and retinal axon trajectories suggests that slit2 acts as a short range repellent for retinal ganglion cell axons.

Animals↗

PubMed2000

Olfactory sensory axons expressing a dominant-negative semaphorin receptor enter the CNS early and overshoot their target.

Sensory axons extend from the chick olfactory epithelium to the telencephalon well before the maturation of their target, the olfactory bulb. During a waiting period of several days, olfactory axons arrive and accumulate outside the CNS while the bulb differentiates beneath them. Semephorin-3A is expressed in the tel-encephalon during this period and has been proposed to prevent their entry into the CNS. We show that the misexpression of a dominant-negative neuropilin-1 that blocks SEMA-3A-mediated signaling in olfactory sensory axons induces many of them to enter the tel-encephalon prematurely and to overshoot the olfactory bulb. These results suggest that chemorepellents can prevent the premature innervation of immature targets.

Animals↗

PubMed2000

Semaphorins and their receptors in vertebrates and invertebrates.

The semaphorins are a family of intercellular signaling proteins that has grown to include 19 identified members in higher vertebrates. Several of its members act as axonal guidance molecules. One participates in signaling in the immune system. The majority, however, do not yet have known biological functions. Recent studies have shown that neuropilins and plexins act as receptors for semaphorins. The most important challenge for the future is to define the biological roles of semaphorins in vivo.

Animals↗

PubMed2000

SEMA3A regulates developing sensory projections in the chicken spinal cord.

The present study explores the role of SEMA3A (collapsin-1) in the temporal and spatial regulation of developing sensory projections in the chick spinal cord. During development, SEMA3A mRNA (SEMA3A) is first expressed throughout the spinal gray matter, but disappears from the dorsal region when small caliber (trkA(+)) sensory axon collaterals first grow into the dorsal horn. In explant cultures of spinal cord segments with attached sensory ganglia, the spatial extent of SEMA3A expression varied in different explants, but in each case the growth of trkA(+) sensory collaterals was largely excluded from areas of SEMA3A expression. To test if SEMA3A had a direct effect on sensory axon growth, we injected recombinant protein into the explants before placing them in culture. Increased levels of SEMA3A substantially reduced the ingrowth of trkA(+) axons, whereas trkC(+) axon collaterals were not affected. Consistent with the insensitivity of trkC(+) collaterals to SEMA3A, these collaterals did not express neuropilin-1, a receptor for SEMA3A. The inhibitory effects of SEMA3A on trkA(+) axons within the spinal cord suggests that the fall in SEMA3A expression in the dorsal horn may contribute to the initiation of growth of these axons into gray matter. In addition, the observation that trkA(+) axons frequently grew close to but rarely over areas of SEMA3A expression suggests that semaphorin may act principally as a short-range guidance cue within the spinal cord.

Afferent Pathways↗

PubMed1999

A dominant negative receptor for specific secreted semaphorins is generated by deleting an extracellular domain from neuropilin-1.

Neuropilins have recently been characterized as receptors for secreted semaphorins. Here, we report the generation of a dominant negative form of neuropilin-1 by the deletion of one of its extracellular domains. Expression of this variant in cultured primary sympathetic neurons blocks the paralysis of growth cone motility normally induced by SEMA-3A (collapsin-1, semaphorin III, semaphorin D) and SEMA-3C (collapsin-3, semaphorin E) but not that induced by SEMA-3F (semaphorin IV). A truncated form of neuropilin-1 that is missing its cytoplasmic domain fails to act as a dominant negative receptor component. These results suggest that neuropilin-1 is a necessary component of receptor complexes for some, but not all, secreted semaphorin family members. Overexpression of dominant negative neuropilins should provide a powerful new method of blocking the functions of secreted semaphorins.

Animals↗

PubMed1999

Collapsin-1/semaphorin D is a repellent for chick ganglion of Remak axons.

Chick collapsin-1/human semaphorin III/mouse semaphorin D is believed to guide the extension of specific axons by a repellent mechanism. Here we examine its role in the guidance of axons of the ganglion of Remak (Remak) in the developing chick intestine. Early in embryogenesis Remak axons extend parallel to, but do not enter, the intestine when collapsin-1 is expressed in the adjacent rectal wall. Remak axons later penetrate the peripheral portions of the rectal wall when collapsin-1 expression retreats from the outer muscle layer to the more internal submucosal and mucosal layers of the rectum. Extension of Remak neurites is repelled in vitro by rectum explants and also by 293T cells expressing collapsin-1. The rectal chemorepellent activity is blocked by anti-collapsin-1 antibodies. Our results suggest that collapsin-1 may help prevent Remak axons from projecting into the intestinal wall at early developmental times and later restricts Remak axon trajectories to the outer part of the intestinal muscle layer.

Alkaline Phosphatase↗

PubMed1999

Neuropilin-1 mediates collapsin-1/semaphorin III inhibition of endothelial cell motility: functional competition of collapsin-1 and vascular endothelial growth factor-165.

Neuropilin-1 (NRP1) is a receptor for two unrelated ligands with disparate activities, vascular endothelial growth factor-165 (VEGF165), an angiogenesis factor, and semaphorin/collapsins, mediators of neuronal guidance. To determine whether semaphorin/collapsins could interact with NRP1 in nonneuronal cells, the effects of recombinant collapsin-1 on endothelial cells (EC) were examined. Collapsin-1 inhibited the motility of porcine aortic EC (PAEC) expressing NRP1 alone; coexpressing KDR and NRP1 (PAEC/KDR/NRP1), but not parental PAEC; or PAEC expressing KDR alone. The motility of PAEC expressing NRP1 was inhibited by 65-75% and this inhibition was abrogated by anti-NRP1 antibody. In contrast, VEGF165 stimulated the motility of PAEC/KDR/NRP1. When VEGF165 and collapsin-1 were added simultaneously to PAEC/KDR/NRP1, dorsal root ganglia (DRG), and COS-7/NRP1 cells, they competed with each other in EC motility, DRG collapse, and NRP1-binding assays, respectively, suggesting that the two ligands have overlapping NRP1 binding sites. Collapsin-1 rapidly disrupted the formation of lamellipodia and induced depolymerization of F-actin in an NRP1-dependent manner. In an in vitro angiogenesis assay, collapsin-1 inhibited the capillary sprouting of EC from rat aortic ring segments. These results suggest that collapsin-1 can inhibit EC motility as well as axon motility, that these inhibitory effects on motility are mediated by NRP1, and that VEGF165 and collapsin-1 compete for NRP1-binding sites.

Actins↗

PubMed1999

Collapsin-1/semaphorin-III/D is regulated developmentally in Purkinje cells and collapses pontocerebellar mossy fiber neuronal growth cones.

Most axons in the CNS innervate specific subregions or layers of their target regions and form contacts with specific types of target neurons, but the molecular basis of this process is not well understood. To determine whether collapsin-1/semaphorin-III/D, a molecule known to repel specific axons, might guide afferent axons within their cerebellar targets, we characterized its expression by in situ hybridization and observed its effects on mossy and climbing fiber extension and growth cone size in vitro. In newborn mice sema-D is expressed by cerebellar Purkinje cells in parasagittal bands located medially and in some cells of the cerebellar nuclei. Later, sema-D expression in Purkinje cells broadens such that banded expression is no longer prominent, and expression is detected in progressively more lateral regions. By postnatal day 16, expression is observed throughout the cerebellar mediolateral axis. Collapsin-1 protein, the chick ortholog of sema-D, did not inhibit the extension of neurites from explants of inferior olivary nuclei, the source of climbing fibers that innervate Purkinje cells. In contrast, when it was applied to axons extending from basilar pontine explants, a source of mossy fiber afferents of granule cells, collapsin-1 caused most pontine growth cones to collapse, as evidenced by a reduction in growth cone size of up to 59%. Moreover, 63% of pontine growth cones arrested their extension or retracted. Its effects on mossy fiber extension and its distribution suggest that sema-D prevents mossy fibers from innervating inappropriate cerebellar target regions and cell types.

Animals↗

PubMed1999

Myelin and collapsin-1 induce motor neuron growth cone collapse through different pathways: inhibition of collapse by opposing mutants of rac1.

Precise growth cone guidance is the consequence of a continuous reorganization of actin filament structures within filopodia and lamellipodia in response to inhibitory and promoting cues. The small GTPases rac1, cdc42, and rhoA are critical for regulating distinct actin structures in non-neuronal cells and presumably in growth cones. Collapse, a retraction of filopodia and lamellipodia, is a typical growth cone behavior on contact with inhibitory cues and is associated with depolymerization and redistribution of actin filaments. We examined whether small GTPases mediate the inhibitory properties of CNS myelin or collapsin-1, a soluble semaphorin, in chick embryonic motor neuron cultures. As demonstrated for collapsin-1, CNS myelin-evoked growth cone collapse was accompanied by a reduction of rhodamine-phalloidin staining most prominent in the growth cone periphery, suggesting actin filament disassembly. Specific mutants of small GTPases were capable of desensitizing growth cones to CNS myelin or collapsin-1. Adenoviral-mediated expression of constitutively active rac1 or rhoA abolished CNS myelin-induced collapse and allowed remarkable neurite extension on a CNS myelin substrate. In contrast, expression of dominant negative rac1 or cdc42 negated collapsin-1-induced growth cone collapse and promoted neurite outgrowth on a collapsin-1 substrate. These findings suggest that small GTPases can modulate the signaling pathways of inhibitory stimuli and, consequently, allow the manipulation of growth cone behavior. However, the fact that opposite mutants of rac1 were effective against different inhibitory stimuli speaks against a universal signaling pathway underlying growth cone collapse.

Actins↗

PubMed1999

Analysis of a Zebrafish semaphorin reveals potential functions in vivo.

The semaphorin/collapsin gene family is a large and diverse family encoding both secreted and transmembrane proteins, some of which are thought to act as repulsive axon guidance molecules. However, the function of most semaphorins is still unknown. We have cloned and characterized several semaphorins in the zebrafish in order to assess their in vivo function. Zebrafish semaZ2 is expressed in a dynamic and restricted pattern during the period of axon outgrowth that indicates potential roles in the guidance of several axon pathways. Analysis of mutant zebrafish with reduced semaZ2 expression reveals axon pathfinding errors that implicate SemaZ2 in normal guidance.

Amino Acid Sequence↗

PubMed1998

Collapsin-1 covalently dimerizes, and dimerization is necessary for collapsing activity.

Chick collapsin-1, the first identified vertebrate member of the semaphorin family of axon guidance proteins, repels specific growth cones. Like all family members, collapsin-1 contains within its sequence a semaphorin domain that is necessary for specifying activity. Two additional structural domains of collapsin-1, the immunoglobulin (Ig) domain and the basic tail, each potentiate collapsin-1 activity. We identify in this study another structural feature of collapsin-1 that is necessary for its function. Collapsin-1 covalently dimerizes, and dimerization is necessary for collapse activity. This dimerization is mediated through a cysteine at residue 723, between the Ig domain and basic tail. The semaphorin domain alone is not active since it cannot dimerize. The collapsing activity of the semaphorin domain can be reconstituted when made as a chimeric construct with an immunoglobin Fc domain, which promotes dimerization.

Chromatography, High Pressure Liquid↗

PubMed1997

A role for collapsin-1 in olfactory and cranial sensory axon guidance.

Collapsin-1 is a member of the semaphorin family of signaling molecules that acts as a repellent for growing spinal sensory axons. We have constructed a chimeric collapsin-1/alkaline phosphatase probe to visualize putative collapsin-1 receptors in vitro and in situ. As predicted by the activity profile of collapsin-1, the probe binds spinal sensory tracts, ventral spinal roots, and the sympathetic chain but does not bind retinal axons. In addition, we find that the probe binds sensory axons arising from the olfactory epithelium and some, but not all, cranial sensory nerves. As predicted by these binding studies, in vitro assays demonstrate that primary olfactory sensory, trigeminal, and jugular ganglion growth cones collapse in the presence of soluble collapsin-1. Comparing the expression pattern of collapsin-1 with the trajectories of collapsin-1 responsive axons suggests that in both the spinal cord and the olfactory bulb, collapsin-1 prevents premature entry of sensory axons into their target and helps determine the final location of sensory terminations.

Alkaline Phosphatase↗

PubMed1997

A 70 amino acid region within the semaphorin domain activates specific cellular response of semaphorin family members.

The semaphorin family contains secreted and transmembrane signaling proteins that function in the nervous, immune, and cardiovascular systems. Chick collapsin-1 is a repellent for specific growth cones. Two other secreted members of the semaphorin family, collapsin-2 and -3, are structurally similar to collapsin-1 but have different biological activities. Semaphorins contain a 500 amino acid family signature semaphorin domain. We show in this study that (1) the semaphorin domain of collapsin-1 is both necessary and sufficient for biological activity, (2) the semaphorin domain contains a 70 amino acid region that specifies the biological activity of the three family members, and (3) the positively charged carboxy terminus potentiates activity without affecting specificity. We propose that semaphorins interact with their receptors through two independent binding sites: one that mediates the biological response and one that potentiates it.

Amino Acid Sequence↗

PubMed1997

Secreted chick semaphorins bind recombinant neuropilin with similar affinities but bind different subsets of neurons in situ.

Collapsin-1, a member of the semaphorin family, activates receptors on specific growth cones, thereby inhibiting their motility. Neuropilin, a previously cloned transmembrane protein, has recently been identified as a candidate receptor for collapsin-1. We have completed the cloning of chick collapsin-3 and -5 and show that collapsin-1, -2, -3, and -5 bind to overlapping but distinct axon tracts. We infer that in situ, there are distinct receptors with different affinities for collapsin-1, -2, -3, and -5. In contrast, these four collapsins all bind recombinant neuropilin with similar affinities. Strong binding to neuropilin is mediated by the carboxy third of the collapsins, while the semaphorin domain confers their unique binding patterns in situ. We propose that neuropilin is a common component of a semaphorin receptor complex, and that additional differentially expressed receptor components interact with the semaphorin domains to confer binding specificity.

Animals↗

PubMed1997

A sensory axon repellent secreted from ventral spinal cord explants is neutralized by antibodies raised against collapsin-1.

During embryogenesis, different subclasses of sensory neurons extend central projections to specific locations in the spinal cord. Muscle and cutaneous afferents initially project to the same location in the dorsal cord. Later, specific muscle afferents leave other afferents behind and project into the ventral cord. Previous studies have shown that ventral spinal cord explants secrete a repellent for sensory neurites. We now find that antibodies to collapsin-1 neutralize this repellent activity. Additional data suggest that all afferents respond to collapsin-1 when they are first confined to the dorsal cord, but that ventrally projecting muscle afferents become collapsin-1 insensitive as they project into the ventral cord. Our results suggest that the transient dorsal expression of collapsin-1 prevents all efferents from entering the cord early and sustained ventral expression prevents dorsally terminating afferents from entering the ventral cord later.

Age Factors↗