Activin A and follistatin expression in developing targets of ciliary ganglion neurons suggests a role in regulating neurotransmitter phenotype

TGF-β Superfamily Signaling in the Eye: Implications for Ocular Pathologies

The TGF-β signaling pathway plays a crucial role in several key aspects of development and tissue homeostasis. TGF-β ligands and their mediators have been shown to be important regulators of ocular physiology and their dysregulation has been described in several eye pathologies. TGF-β signaling participates in regulating several key developmental processes in the eye, including angiogenesis and neurogenesis. Inadequate TGF-β signaling has been associated with defective angiogenesis, vascular barrier function, unfavorable inflammatory responses, and tissue fibrosis. In addition, experimental models of corneal neovascularization, diabetic retinopathy, proliferative vitreoretinopathy, glaucoma, or corneal injury suggest that aberrant TGF-β signaling may contribute to the pathological features of these conditions, showing the potential of modulating TGF-β signaling to treat eye diseases. This review highlights the key roles of TGF-β family members in ocular physiology and in eye diseases, and reviews approaches targeting the TGF-β signaling as potential treatment options.

Expression and polymorphism of Follistatin (FST) gene and its association with growth traits in native and exotic chicken

Follistatin (FST), a member of the transforming growth factor beta super-family regulates body growth by inhibiting the binding of myostatin (an inhibitor of growth) with its receptor in chicken. An experiment was conducted to explore ontogenic expression of the follistatin gene, determine polymorphism at the coding region of the gene and estimate its effect on growth traits in native (Aseel) and exotic broiler (PD-1) and layer (White Leghorn) chicken. The significant differences of FST gene expression were observed among the breeds revealing significantly (p < 0.05) higher expression in PD-1 line followed by White Leghorn and Aseel breeds during both embryonic and post-hatch period. The polymorphism at the functional domain of the FST gene was identified with the presence of 4 haplotypes. The follistatin haplogroups had the significant effect on body weights (p < 0.05) at 42 days of age in the White Leghorn, PD-1 and Aseel breeds (h1h1 in PD-1, h1h4 in White Leghorn and h1h2 haplogroups in Aseel breeds had the highest body weights of 770.04 ± 12.96, 246.28 ± 7.60 and 270.00 ± 10.68 g, respectively). It is concluded that the follistatin gene expressed differently during the embryonic and post-embryonic period across the breeds and the coding region of the gene was polymorphic having significant effects on growth traits in chicken.

The diversity of neuronal phenotypes in rodent and human autonomic ganglia

Selective sympathetic and parasympathetic pathways that act on target organs represent the terminal actors in the neurobiology of homeostasis and often become compromised during a range of neurodegenerative and traumatic disorders. Here, we delineate several neurotransmitter and neuromodulator phenotypes found in diverse parasympathetic and sympathetic ganglia in humans and rodent species. The comparative approach reveals evolutionarily conserved and non-conserved phenotypic marker constellations. A developmental analysis examining the acquisition of selected neurotransmitter properties has provided a detailed, but still incomplete, understanding of the origins of a set of noradrenergic and cholinergic sympathetic neuron populations, found in the cervical and trunk region. A corresponding analysis examining cholinergic and nitrergic parasympathetic neurons in the head, and a range of pelvic neuron populations, with noradrenergic, cholinergic, nitrergic, and mixed transmitter phenotypes, remains open. Of particular interest are the molecular mechanisms and nuclear processes that are responsible for the correlated expression of the various genes required to achieve the noradrenergic phenotype, the segregation of cholinergic locus gene expression, and the regulation of genes that are necessary to generate a nitrergic phenotype. Unraveling the neuron population-specific expression of adhesion molecules, which are involved in axonal outgrowth, pathway selection, and synaptic organization, will advance the study of target-selective autonomic pathway generation.

Signaling via the transcriptionally regulated activin receptor 2B is a novel mediator of neuronal cell death during chicken ciliary ganglion development

The TGF-β ligand superfamily members activin A and BMP control important aspects of embryonic neuronal development and differentiation. Both are known to bind to activin receptor subtypes IIA (ActRIIA) and IIB, while in the avian ciliary ganglion (CG), so far only ActRIIA-expression has been described. We show that the expression of ACVR2B, coding for the ActRIIB, is tightly regulated during CG development and the knockdown of ACVR2B expression leads to a deregulation in the execution of neuronal apoptosis and therefore affects ontogenetic programmed cell death in vivo. While the differentiation of choroid neurons was impeded in the knockdown, pointing toward a reduction in activin A-mediated neural differentiation signaling, naturally occurring neuronal cell death in the CG was not prevented by follistatin treatment. Systemic injections of the BMP antagonist noggin, on the other hand, reduced the number of apoptotic neurons to a similar extent as ACVR2B knockdown. We therefore propose a novel pathway in the regulation of CG neuron ontogenetic programmed cell death, which could be mediated by BMP and signals via the ActRIIB.

Microarray characterization of human embryonic stem cell--derived retinal cultures

PURPOSE

A number of protocols have been published to induce retinal determination from human embryonic stem cells (hESC) and induced pluripotent stem cells (iPSC). Although all these studies have shown some degree of expression of markers of retinal cells, fewer than 30 markers are typically used to characterize the ESC-derived retinal cells. Hence, it is not known whether they express all the genes present in normal developing retinal cells. To assess the efficiency of their retinal determination protocol at the transcriptome level and to understand the changes in human retinal gene expression patterns during development, the authors conducted a microarray-based analysis comparing human retina to hESC-derived retinal cells.

METHODS

The authors extracted total RNA from 60-day, 80-day, and 96-day human fetal retina and hESC-derived retinal cells at 3 weeks and 9 weeks after induction. RNA was subjected to analysis using a commercial microarray. Data were normalized using Affymetrix Power Tools and analyzed using commercial microarray software.

RESULTS

On K-median clustering analysis, the authors found that overall there was a very high correlation between genes expressed in human fetal retina and those in ESC-derived retinal cultures. The cultures were at similar developmental ages to the corresponding fetal retinal ages. They found only 1% of the genes on the array to be expressed at a higher level in ESC-derived retinal cells than in fetal retina, and most of these were expressed in the retinal pigment epithelium and ciliary epithelium.

CONCLUSIONS

In sum, gene array profiling provides an effective method for characterization of the efficiency of directed differentiation of hESCs to retinal cells.

The cortistatin gene PSS2 rather than the somatostatin gene PSS1 is strongly expressed in developing avian autonomic neurons

Somatostatin and cortistatin are neuromodulators with divergent expression patterns and biological roles. Whereas expression and function of genes encoding somatostatin (PSS1) and the related peptide cortistatin (PSS2) have been studied in detail for the central nervous system (CNS) and immune system, relatively little is known about their expression patterns in the peripheral nervous system (PNS). We compare the expression patterns of PSS1 and PSS2 in chicken embryos. At E14, PSS1 is higher in the CNS versus PNS, whereas PSS2 is higher in the PNS. During early development, PSS1 is transiently expressed in lumbar sympathetic ganglia and is detectable at low levels throughout the development of dorsal root and ciliary ganglia. In contrast, PSS2 expression increases as development progresses in sympathetic and dorsal root ganglia, whereas levels in ciliary ganglia by E8 are more than 100-fold higher than in sympathetic ganglia. Activin, which induces somatostatin-like immunoreactivity in ciliary ganglion neurons in vivo and in vitro, controls PSS2 expression by stabilizing PSS2 but not PSS1 mRNA. We conclude that much of the somatostatin-like immunoreactivity in the developing avian peripheral nervous system is actually cortistatin, the PSS2 product, as opposed to true somatostatin, which is the PSS1 product. The identification of PSS2 as the predominantly expressed somatostatin gene family member in avian autonomic neurons provides a molecular basis for further functional and pharmacological studies.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Prostate stem cell antigen is an endogenous lynx1-like prototoxin that antagonizes alpha7-containing nicotinic receptors and prevents programmed cell death of parasympathetic neurons

Vertebrate alpha-bungarotoxin-like molecules of the Ly-6 superfamily have been implicated as balancers of activity and survival in the adult nervous system. To determine whether a member of this family could be involved in the development of the avian ciliary ganglion, we identified 6 Gallus genes by their homology in structure to mouse lynx1 and lynx2. One of these genes, an ortholog of prostate stem cell antigen (psca), is barely detectable at embryonic day (E) 8, before neuronal cell loss in the ciliary ganglion, but increases >100-fold as the number of neurons begins to decline between E9 and E14. PSCA is highly expressed in chicken and mouse telencephalon and peripheral ganglia and correlates with expression of alpha7-containing nicotinic acetylcholine receptors (alpha7-nAChRs). Misexpressing PSCA before cell death in the ciliary ganglion blocks alpha7-nAChR activation by nicotine and rescues the choroid subpopulation from dying. Thus, PSCA, a molecule previously identified as a marker of prostate cancer, is a member of the Ly-6 neurotoxin-like family in the nervous system, and is likely to play a role as a modulator of alpha7 signaling-induced cell death during development.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

The role of activin in neuropeptide induction and pain sensation

Signals from target tissues play critical roles in the functional differentiation of neuronal cells, and in their subsequent adaptations to peripheral changes in the adult. Sensory neurons in the dorsal root ganglia (DRG) provide an excellent model system for the study of signals that regulate the development of neuronal diversity. DRG have been well characterized and contain both neurons that convey information from muscles about limb position, as well as other neurons that provide sensations from skin about pain information. Sensory neurons involved in pain sensation can be distinguished physiologically and antigenically, and one hallmark characteristic is that these neurons contain neuropeptides important for their functions. The transforming growth factor (TGF) beta family member activin A has recently been implicated in neural development and response to injury. During sensory neuron development, peripheral target tissues containing activin or activin itself can regulate pain neuropeptide expression. Long after development has ceased, skin target tissues retain the capacity to signal neurons about changes or injury, to functionally refine synapses. This review focuses on the role of activin as a target-derived differentiative factor in neural development that has additional roles in response to cutaneous injuries in the adult.

Proteomic analysis of the Gallus gallus embryo at stage-29 of development

The chicken (Gallus gallus) is one of the primary models for embryological and developmental studies. In order to begin to understand the molecular mechanisms underlying the normal and abnormal development of the chicken, we used 2-DE to construct a whole-embryo proteome map. Proteins were separated by IEF on IPG strips, and by 11% SDS-PAGE) gels. Protein identification was performed by means of PMF with MALDI-TOF-MS. In all, 105 protein spots were identified, 35 of them implicated in embryo development, 10 related with some diseases, and 16, finally, being proteins that have never been identified, purified or characterized in the chicken before. This map will be updated continuously and will serve as a reference database for investigators, studying changes at the protein level under different physiological conditions.

Immunocytochemical properties of stellate ganglion neurons during early postnatal development

Neurotransmitter features in sympathetic neurons are subject to change during development. To better understand the neuroplasticity of sympathetic neurons during early postnatal ontogenesis, this study was set up to immunocytochemically investigate the development of the catecholaminergic, cholinergic, and peptidergic phenotypes in the stellate ganglion of mice and rats. The present study was performed on Wistar rats and Swiss mice of different ages (newborn, 10-day-old, 20-day-old, 30-day-old, and 60-day-old). To this end, double labeling for tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), vasoactive intestinal (poly)peptide (VIP), neuropeptide Y (NPY), galanin (GAL), and somatostatin (SOM) was applied. The results obtained indicate that the majority of the neurons in the stellate ganglion of both species were TH-positive from birth onward and that a large part of these neurons also contained NPY. The percentage of neurons containing TH and NPY invariably increased with age up to 60 days postnatally. A smaller portion of the stellate ganglion neurons contained other types of neuropeptides and showed a distinct chronological pattern. The proportion of VIP- and ChAT-positive neurons was maximal in 10-day-old animals and then decreased up to 60 days of age, whereas the number of SOM-positive cells in rats significantly decreased from birth onward. In newborn rats, VIP-, ChAT- and SOM-positive neurons were largely TH-positive, while their proportions decreased in 10-day-old and older rats. Accordingly, the largest part of VIP-positive neurons also expressed SOM immunoreactivity at birth, after which the number of neurons containing both peptides diminished. The VIP- and SOM-positive cells did not contain NPY in any of the age groups studied. In rats up to 10 days of life, GAL-immunoreactive (-IR) neurons were scarce, after which their number increased to reach a maximal value in 30-day-old animals and then declined again. The SOM-reactive cells had the smallest size in all rats, while the largest neurons were those containing ChAT. In the mouse stellate ganglion, VIP- and ChAT-IR neurons were larger in comparison to NPY- and TH-IR cells. Our study further revealed some species differences: compared to mice the proportion of neurons containing TH and NPY was higher in rats at all ages under study. Furthermore, no GAL-immunostained neurons were found in mice and the number of SOM-positive cells in mice was limited compared to that observed in rats. In conclusion, the development of neurotransmitter composition is complete in rats and mice by their second month of life. At this age, the percentages of immunopositive cells have become similar to those reported in adult animals.

Re-establishment of neurochemical coding of preganglionic neurons innervating transplanted targets

We investigated the effect on neurochemical phenotype of changing the targets innervated by sympathetic preganglionic neurons. In neonatal rats, the adrenal gland was transplanted into the neck, to replace the postganglionic neurons of the superior cervical ganglion. Transplanted adrenal glands survived, and contained noradrenergic and adrenergic chromaffin cells, and adrenal ganglion cells. Retrograde tracing from the transplants showed that they were innervated by preganglionic neurons that would normally have supplied postganglionic neurons of the superior cervical ganglion. The neurochemical phenotypes of preganglionic axons innervating transplanted chromaffin cells were compared with those innervating the normal adrenal medulla or superior cervical ganglion neurons. As in the normal adrenal gland, preganglionic nerve fibres apposing transplanted chromaffin cells were cholinergic. The peptide and calcium-binding protein content of preganglionic fibres was similar in normal and transplanted adrenal glands. In both cases, cholinergic fibres immunoreactive for enkephalin targeted adrenergic chromaffin cells, whilst cholinergic fibres with co-localised calretinin-immunoreactivity innervated noradrenergic chromaffin cells and adrenal ganglion cells. In contrast to the innervation of normal adrenal glands, these axons lacked immunoreactivity to nitric oxide synthase. In a set of control experiments, the superior cervical ganglion was subjected to preganglionic denervation in rat pups the same age as those that received adrenal transplants, and the ganglion was allowed to be re-innervated over the same time course as the adrenal transplants were studied. When the superior cervical ganglion was re-innervated by preganglionic nerve fibres, we observed that all aspects of chemical coding were restored, including cholinergic markers, nitric oxide synthase, enkephalin, calcitonin gene-related peptide and calcium binding proteins in predicted combinations, although the density of nerve fibres was always lower in re-innervated ganglia. These data show that the neurochemical phenotypes expressed by preganglionic neurons re-innervating adrenal chromaffin cells are selective and similar to those seen in the normal adrenal gland. Two explanations are advanced: either that contact of preganglionic axons with novel target cells has induced a switch in their neurochemical phenotypes, or that there has been target-selective reinnervation by pre-existing fibres of appropriate phenotype. Regardless of which of these alternatives is correct, the restoration of normal preganglionic codes to the superior cervical ganglion following denervation supports the idea that the target tissue influences the neurochemistry of innervating preganglionic neurons.

Target-mediated control of neural differentiation

The development of the nervous system entails the coordination of the spatial and chemical development of both pre- and postsynaptic elements. This coordination is accomplished by signals passing between neurons and the target cells that they innervate. This review focuses on well-characterized examples of target-mediated neuronal differentiation in the central and peripheral nervous systems. These include control of neurogenesis in the leech by male genitalia, presynaptic differentiation induced by postsynaptic molecules expressed by skeletal muscle, postsynaptic adhesion molecules that induce presynaptic differentiation in the central nervous system (CNS), target-mediated control of neurotransmitter phenotype in peripheral neurons, and target-regulated control of neuronal nicotinic acetylcholine receptors (nAChRs) and large conductance calcium-activated potassium channels (BK). The detailed understanding of these processes will uncover signals critical for the directed differentiation of stem cells as well as identify future targets for therapies in neural regeneration that promote the reestablishment of functional connections.

Transporters/receptors in the anterior chamber: pathways to explore ocular drug delivery strategies

Membrane transporters/receptors are involved in drug transport processes and play a key role in intestinal absorption, tissue distribution and elimination. Drug targeting to specific transporters and receptors using carrier-mediated absorption has immense clinical significance. Ocular drug delivery is a challenging task since it involves drug transport across various barriers in the eye. Specialised transport processes exist at these barriers, which control the entry of drugs and xenobiotics. Ocular drug therapy involving topical or systemic administration of drugs has various limitations. Transport processes in the eye have been targeted in an effort to increase ocular bioavailability of drugs following topical instillation. This review discusses various transport processes in the eye and drug delivery strategies utilising these transporters/receptors.

Developmental cell death in vivo: rescue of neurons independently of changes at target tissues

Programmed cell death is a prominent feature of neural development that is regulated by a variety of cell-cell interactions. We used the avian ciliary ganglion to dissect the relative contributions of target tissues vs. ganglionic inputs in regulating cell death. The two populations of the ciliary ganglion innervate different targets: choroid neurons innervate vasculature, whereas ciliary neurons innervate the iris and ciliary body. By counting after labeling all neurons with Islet-1 and choroid neurons with anti-somatostatin, we determined that alpha-bungarotoxin (alpha-btx) at 12.5 microg/day rescued only ciliary neurons, whereas 75 microg/day rescued both ciliary and choroid neurons. It is unlikely that alpha-btx acted by blocking nerve transmission at both targets because the choroid vasculature lacked transcripts for alpha-btx binding molecules. In addition, no inherent trophic activity could be ascribed to alpha-btx, and survival could not be attributed to differences in total trophic activity of eyes from saline vs. alpha-btx-treated embryos. In contrast, the alpha7 antagonist alpha-methyllycaconitine (MLA) rescued ciliary neurons at 2.6 microg/day, whereas 26 microg/day rescued choroid neurons. Nerve terminals of ciliary neurons rescued with alpha-btx were significantly larger; however, differences in nerve terminal size or branching of axons were not observed in ciliary neurons rescued with MLA or choroid neurons rescued by either MLA or alpha-btx. Our results suggest that neuronal survival can be promoted independently of changes at the target tissues when orthograde signals acting by means of neuronal alpha7 nicotinic receptors are blocked.

Chick ciliary neurotrophic factor is secreted via a nonclassical pathway

In contrast to mammalian ciliary neurotrophic factors (CNTFs), chick CNTF is secreted, although it lacks an N-terminal signal. We determined that a 52 aa region of chick CNTF containing an internal hydrophobic domain could direct secretion of rat CNTF. Using a stable cell line that overexpressed chick CNTF, we found that chick CNTF immunoreactivity was punctate throughout the cytosol. Cellular fractionation confirmed chick CNTF to be protected by vesicles. Chick CNTF did not colocalize with fibronectin, calreticulin, wheat germ agglutinin binding sites, or with transferrin receptor. The distribution of chick CNTF was altered neither by brefeldin A nor by chloroquine treatment. Although the punctate pattern of chick CNTF immunoreactivity was not due to reuptake, chick CNTF could be found in a cellular compartment labeled after a brief incubation with dextran microbeads. When synthesized in vitro, chick CNTF did not translocate into microsomes. We conclude that chick CNTF is secreted via a nonclassical pathway.

Activin type II receptors in embryonic dorsal root ganglion neurons of the chicken

Activin induces neuropeptide expression in chicken ciliary ganglion neurons. To determine if activin might also influence neuropeptide expression in developing sensory neurons, we examined whether type II activin receptors are expressed during embryonic development of the chicken dorsal root ganglia (DRG), and also examined the effects of activin on neuropeptide expression in cultured DRG neurons. Using reverse transcription polymerase chain reaction (rtPCR), we detected mRNAs for both the activin receptors type IIA (ActRIIA) and type IIB (ActRIIB) in DRG from embryonic day 7 through posthatch day 1. With in situ hybridization, we found that morphologically identifiable neurons express mRNAs for both ActRIIA and ActRIIB. With developmental age, a subset of neurons that hybridizes more intensely with riboprobes to these receptor mRNAs becomes evident. A similar pattern of expression is observed with immunocytochemical staining using antisera against activin type II receptors. To examine whether embryonic DRG cells respond to activin we treated dissociated cultures of DRG with activin A and assessed the expression of vasoactive intestinal peptide (VIP) and calcitonin gene related peptide (CGRP) mRNAs using semiquantitative rtPCR. Activin treatment results in an increase in VIP mRNA, but does not affect CGRP mRNA levels. These observations indicate that neurons in the embryonic chicken DRG can respond to activin and suggest that activin has the potential to play a role in the development and function of DRG sensory neurons.

Neurochemical differentiation of functionally distinct populations of autonomic neurons

The coeliac ganglion of guinea pigs displays a unique topographical arrangement of neurochemically and functionally distinct populations of sympathetic neurons. The authors used multiple-labeling immunohistochemistry to investigate the neurochemical differentiation of these neurons during embryonic and fetal development. Sympathoadrenal precursors, located on either side of the abdominal aorta, were intensely immunoreactive for tyrosine hydroxylase (TH-IR), neurofilament, and the human natural killer 1 antibody at midembryonic stages (Carnegie stages 16-19). During late embryonic stages (stages 20-23), a single bilobed ganglion had formed. At this time, neuropeptide Y immunoreactivity (NPY-IR) was widely expressed in sympathetic neurons (with moderate TH-IR) and chromaffin cells (with intense TH-IR). The onset of somatostatin (Som-IR) expression followed that of NPY-IR and was restricted to sympathetic neurons. However, at late embryonic stages, most TH-IR neurons with Som-IR also expressed NPY-IR (a combination of peptides not found in the mature coeliac ganglion). Between late embryonic stages and the end of the early fetal period, there was a significant increase in the proportion of neurons in lateral regions that had both NPY-IR and TH-IR. At the same time, there was an increase in the proportion of neurons in medial regions that had both Som-IR and TH-IR. Neurons expressing both Som-IR and TH-IR were rarely observed in lateral regions of the coeliac ganglion. Thus, a clear topography within the coeliac ganglion is established during late embryonic and early fetal stages of development and reflects that found in the mature animal by the end of the early fetal period.

BK-Type K(Ca) channels in two parasympathetic cell types: differences in kinetic properties and developmental expression

The intrinsic electrical properties of identified choroid and ciliary neurons of the chick ciliary ganglion were examined by patch-clamp recording methods. These neurons are derived from a common pool of mesencephalic neural crest precursor cells but innervate different target tissues and have markedly different action potential waveforms and intrinsic patterns of repetitive spike discharge. Therefore it is important to determine whether these cell types express different types of plasma membrane ionic channels, and to ascertain the developmental stages at which these cell types begin to diverge. This study has focused on large-conductance Ca(2+)-activated K(+) channels (K(Ca)), which are known to regulate spike waveform and repetitive firing in many cell types. Both ciliary ganglion cell types, identified on the basis of size and somatostatin immunoreactivity, express a robust macroscopic K(Ca) carried by a kinetically homogeneous population of large-conductance (BK-type) K(Ca) channels. However, the kinetic properties of these channels are different in the two cell types. Steady-state fluctuation analyses of macroscopic K(Ca) produced power spectra that could be fitted with a single Lorentzian curve in both cell types. However, the resulting corner frequency was significantly lower in choroid neurons than in ciliary neurons, suggesting that the underlying K(Ca) channels have a longer mean open-time in choroid neurons. Consistent with fluctuation analyses, significantly slower gating of K(Ca) channels in choroid neurons was also observed during macroscopic activation and deactivation at membrane potentials positive to -30 mV. Differences in the kinetic properties of K(Ca) channels could also be observed directly in single-channel recordings from identified embryonic day 13 choroid and ciliary neurons. The mean open-time of large-conductance K(Ca) channels was significantly greater in choroid neurons than in ciliary neurons in excised inside-out patches. The developmental expression of functional K(Ca) channels appears to be regulated differently in the two cell types. Although both cell types acquire functional K(Ca) at the same developmental stages (embryonic days 9-13), functional expression of these channels in ciliary neurons requires target-derived trophic factors. In contrast, expression of functional K(Ca) channels proceeds normally in choroid neurons developing in vitro in the absence of target-derived trophic factors. Consistent with this, extracts of ciliary neuron target tissues (striated muscle of the iris/ciliary body) contain K(Ca) stimulatory activity. However, K(Ca) stimulatory activity cannot be detected in extracts of the smooth muscle targets of choroid neurons.

Developmental regulation of neuronal KCa channels by TGFbeta 1: transcriptional and posttranscriptional effects mediated by Erk MAP kinase

An avian ortholog of transforming growth factor beta1 (TGFbeta1) is the target-derived factor responsible for the developmental expression of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in chick ciliary ganglion (CG) neurons developing in vivo and in vitro. Application of TGFbeta1 evokes an acute stimulation of K(Ca) that can be observed immediately after cessation of a 12 hr exposure to this factor, that persists in the presence of protein synthesis inhibitors, and that is therefore mediated by posttranslational events. Here we show that a single 3 hr exposure to TGFbeta1 can also induce long-lasting stimulation of macroscopic K(Ca) that persists for at least 3.5 d after the end of the treatment. In contrast to the acute stimulation, this sustained effect is dependent on the transcription and synthesis of new proteins at approximately the time of TGFbeta1 treatment. However TGFbeta1 does not cause increases in the levels of slowpoke alpha subunit transcripts in CG neurons, suggesting that induction of some other protein or proteins is required for sustained enhancement of macroscopic K(Ca). In addition, application of TGFbeta1 evoked an almost immediate but transient phosphorylation of the mitogen-activated protein kinase Erk in CG neurons. TGFbeta1-evoked Erk activation was blocked by the specific MEK1 inhibitor 2- (2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059). Moreover, application of PD98059 blocked both acute and sustained K(Ca) stimulation evoked by TGFbeta1. These results indicate that TGFbeta1 elicits a biphasic stimulation of K(Ca) via activation of an MEK1-Erk pathway and raise the possibility that other neuronal effects of TGFbeta superfamily members entail Erk activation.

Molecular cloning of testican-2: defining a novel calcium-binding proteoglycan family expressed in brain

We have screened a human cDNA library using an expressed sequence tag related to the BM-40/secreted protein, acidic and rich in cysteine (SPARC)/osteonectin family of proteins and isolated a novel cDNA. It encodes a protein precursor of 424 amino acids that consists of a signal peptide, a follistatin-like domain, a Ca2+-binding domain, a thyroglobulin-like domain, and a C-terminal region with two putative glycosaminoglycan attachment sites. The protein is homologous to testican-1 and was termed testican-2. Testican-1 is a proteoglycan originally isolated from human seminal plasma that is also expressed in brain. Northern blot hybridization of testican-2 showed a 6.1-kb mRNA expressed mainly in CNS but also found in lung and testis. A widespread expression in multiple neuronal cell types in olfactory bulb, cerebral cortex, thalamus, hippocampus, cerebellum, and medulla was detected by in situ hybridization. A recombinant fragment consisting of the Ca2+-binding EF-hand domain and the thyroglobulin-like domain of testican-2 showed a reversible Ca2+-dependent conformational change in circular dichroism studies. Testican-1 and -2 form a novel Ca2+-binding proteoglycan family built of modular domains with the potential to participate in diverse steps of neurogenesis.

Development of the avian iris and ciliary body: mechanisms of cellular differentiation during the smooth-to-striated muscle transition

The avian iris and ciliary body undergoes a transition from smooth-to-striated muscle during embryonic development. Using antibodies specific for smooth muscle-specific alpha-actin and myosin heavy chain, we confirm that a smooth-to-striated muscle transition occurs between E8 and E17 in both iris and ciliary body of the chick. To study the mechanisms regulating the transition in muscle type, we analyzed the fate of quail clones derived from E7 iris cells. When cells were cloned alone, 45/71 colonies differentiated into smooth muscle and 10/71 became striated muscle. None of the colonies were mixed with respect to muscle phenotype, indicating a lack of pluripotent stem cells. Furthermore, clones giving rise to nonstriated muscle could not be forced to incorporate into myotubes when cocultured with chick myocytes. Clones grown in coculture with chick embryo fibroblasts or E11 iris cells had very high cloning efficiencies (>98%). Significantly more clones differentiated into striated muscle when cocultured with E11 cells (60/156) than when cocultured with fibroblasts (29/108). This was due to an increased recruitment of undifferentiated cells into striated muscle, rather than a change in the percentage of cells differentiating into smooth muscle. In vivo and in vitro, various smooth and striated muscle-specific markers including contractile proteins, acetylcholine receptor subtypes, and transcription factors were colocalized in cells. Although our data argue against a multipotent stem cell for smooth and striated muscle cells, they cannot exclude a role for transdifferentiation. Cumulatively these results suggest that both smooth muscle and migratory myoblasts contribute to the development of myotubes in the avian iris and that this process is regulated in a non-cell-autonomous fashion by locally generated signals.

Activin A and follistatin influence expression of somatostatin in the ciliary ganglion in vivo

An important developmental question concerns whether neurotransmitter phenotype is an inherent property of neurons or is influenced by target tissues. This issue can be addressed in the avian ciliary ganglion (CG) which contains two cholinergic populations, ciliary and choroid neurons, that differentially express the peptide cotransmitter, somatostatin. The present study tests the hypothesis that differences in the level of expression of activin A and its endogenous inhibitor follistatin in CG neuron target tissues are responsible for selective expression of somatostatin in choroid neurons. Intraocular injection of activin A or follistatin (300 ng injected at E10/E11) in cultured embryos resulted in a 39% increase or a 23% decrease, respectively, in somatostatin-positive neurons relative to controls. Chorioallantoic membrane application of follistatin (1 microgram daily from E7 to E13) reduced somatostatin positive neurons by 54%. Neuron number, size, and target tissue morphology were unaffected by these treatments. Together with our previous studies, these data suggest that activin A and follistatin are target-derived molecules that regulate neuropeptide phenotype in the ciliary ganglion.

Activin co-operates with fibroblast growth factor 2 to regulate tyrosine hydroxylase expression in the basal forebrain ventricular zone progenitors

Activin and its cognate receptors are expressed during embryogenesis in the rapidly dividing cells of the basal forebrain ventricular zone. This finding prompted us to study the role of activin in regulating neurotransmitter phenotype expression and other aspects of the ventricular zone-derived progenitor cell differentiation. Although virtually ineffective alone, activin co-operated with fibroblast growth factor 2 to induce a rapid tyrosine hydroxylase-immunoreactivity in cultured ventricular zone progenitors. Northern analysis indicated that the increase in tyrosine hydroxylase-immunoreactivity was associated with increased tyrosine hydroxylase gene expression. Activin and fibroblast growth factor 2 action was specific to tyrosine hydroxylase, as it did not induce the expression of choline acetyltransferase, nor enhance the expression of glutamate decarboxylase. Cultures treated with the DNA replication marker bromodeoxyuridine revealed that both proliferating ventricular zone progenitors and their post-mitotic progeny were induced to express tyrosine hydroxylase. In these cultures, activin acted to reduce fibroblast growth factor 2 stimulated mitotic activity. Furthermore, activin permitted neuronal differentiation and survival of the ventricular zone progenitors after three days in vitro. Together these data demonstrate a novel role of activin and fibroblast growth factor 2 in regulating the fate of the embryonic basal forebrain ventricular zone progenitors.

Target-dependent development of the vesicular acetylcholine transporter in rodent sweat gland innervation

Descriptive studies have delineated a developmental change in neurotransmitter phenotype from noradrenergic to cholinergic in the sympathetic innervation of sweat glands in rodent footpads. Transplantation and culture experiments provide evidence that interactions with the target tissue induce this change. Recent studies with an antiserum that recognizes the vesicular acetylcholine transporter (VAChT) suggest, however, that the development of cholinergic function in sympathetic neurons, including those that innervate sweat glands, occurs prior to and does not require target contact. To clarify these apparently contradictory findings, we directly compared the appearance of VAChT immunoreactivity in the sympathetic neurons that innervate sweat glands with the time that axons contact this target. We find that VAChT immunoreactivity is not detectable in either the axons or cell bodies of sweat gland neurons until several days after target innervation. Before and during VAChT acquisition, the developing sweat gland innervation contains vesicular stores of catecholamines. An analysis of mutant mice that lack sweat glands was undertaken to determine whether VAChT expression requires target interactions and revealed that VAChT does not appear in the absence of glands. These findings, together with previous studies, confirm the target dependence of cholinergic function in the sympathetic neurons that innervate sweat glands.

Development of the avian iris and ciliary body: the role of activin and follistatin in coordination of the smooth-to-striated muscle transition

Although general principles have been established in the regulation of vetebrate organogenesis, the specific molecules responsible for such signaling are just being identified. We have studied differentiation in the avian iris and ciliary body which undergoes a transition from smooth to striated muscle. Using heterochronic cocultures, we have found that striated muscle differentiation in pretransition (E8) cells is induced by midtransition (E11) cells through a secreted and soluble activity. In addition, contact-mediated mechanisms among pretransition cells prevented precocious striated muscle differentiation. We have tested the role of activin and its antagonist follistatin, as candidate regulators of this muscle transition. Activin induced smooth muscle differentiation while repressing striated muscle development. Conversely, follistatin promoted the emergence of striated muscle, while inhibiting smooth muscle differentiation. Significantly, secreted follistatin activity was found to increase during the smooth-to-striated muscle transition. Moreover, the striated muscle inducing activity from midtransition iris and ciliary body cell conditioned medium was depleted with an activin-affinity column which binds follistatin. These results suggest that activin and follistatin coordinate differentiation in the avian iris and ciliary body.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

GDNF induces the calretinin phenotype in cultures of embryonic striatal neurons

Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-beta (TGF-beta) superfamily, is a potent neurotrophic factor for several neuron populations in the central and peripheral nervous system. Members of the neurotrophin, neurokine, and TGF-beta families of growth factors can affect neurons beyond their capacity to promote survival. They can play instructive roles including the determination of a particular transmitter phenotype. Here, we show that GDNF enhances the number of calretinin (CaR)-positive neurons in serum-free cultures of striatal cells isolated from embryonic rats. The effect is dose-dependent, can be elicited with concentrations as low as 0.1 ng/ml, and is not accompanied by increased incorporation of 5-bromo-2'-desoxyuridine and appearance of glial fibrillary acidic protein-positive cells. Similar, but weaker effects can be elicited by brain-derived neurotrophic factor, neurotrophin-3 and -4, fibroblast growth factor-2. Ciliary neurotrophic factor, nerve growth factor, and TGF-beta 1 do not affect striatal CaR expression. GDNF can augment CaR-positive cells at any time point and with a minimal exposure of 18 hr, suggesting induction of the phenotype rather than increased survival. By reverse transcription polymerase chain reaction (RT-PCR), we show that GDNF is expressed in the E16 striatum and in cultures derived from this tissue. GDNF also protected striatal CaR-positive neurons against glutamate toxicity. We conclude that striatal GDNF, in addition to its retrograde trophic role for nigrostriatal dopaminergic neurons, may also act locally within the striatum (e.g., by inducing the CaR phenotype and protecting these cells against toxic insult).

Activin exerts a neurotrophic effect on cultured hippocampal neurons

Activin is a member of the transforming growth factor (TGF)-beta superfamily, which comprises a growing list of multifunctional proteins that serve as regulators of cell proliferation and differentiation. Recently, activin was shown to regulate the neurotransmitter phenotype in peripheral neurons. It is also a potent survival factor for neurogenic clonal cell lines, retinal neurons and midbrain dopaminergic neurons. We have studied the effect of activin on hippocampal cells which show abundant expression of activin receptors or binding sites. Exposure of primary cultures of rat hippocampal neurons to activin supported neuronal survival. This neurotrophic action of activin was blocked by treatment with the tyrosine kinase inhibitor genistein or the protein kinase C inhibitor calphostin C. However, the Ca2+/calmodulin kinase inhibitor KN-62 had no effect. Nicardipine, a blocker of the L-type Ca2+ channel, also inhibited the neurotrophic effect of activin. Furthermore, activin potentiated the depolarization-induced elevation in intracellular Ca2+ concentration ([Ca2+]i). The neurotrophic effect and the potentiation of depolarization-induced increase of [Ca2+]i caused by activin were completely abolished by the protein synthesis inhibitor cycloheximide. These results suggest that activin supports neuronal survival by increasing the expression of voltage-dependent Ca2+ channel through the action of a tyrosine kinase and of protein kinase C, but not of Ca2+/calmodulin kinase.

Opposing effects of activin A and follistatin on developing skeletal muscle cells

Activin and the activin-binding protein follistatin modulate a variety of biological processes and are abundant at sites of muscle development. Activin and follistatin were expressed in developing chick pectoral muscle in vivo and in primary cell culture. Addition of recombinant activin inhibited muscle development in a dose-dependent manner as measured by the number of nuclei in myosin heavy chain positive cells and creatine phosphokinase activity. Conversely, follistatin potentiated muscle development. The effects of activin were found to be distinct from those of the related protein transforming growth factor (TGF) beta1. Muscle development was repressed by activin at all time points investigated and did not recover with the removal of activin following a limited exposure. In contrast, while myogenic differentiation in TGFbeta1 was initially repressed, muscle marker expression recovered to control levels--even in the continued presence of TGFbeta1. Fibroblast growth factor (FGF) had little effect on inhibiton of muscle development caused by activin A. However, inhibition of development produced by TGFbeta increased with increasing concentrations of FGF. Finally, early expression of myoD and myf5 mRNA by muscle cultures in the presence of activin and follistatin was analyzed. Activin-treated cultures expressed reduced myoD and myf5 levels at 1.5 days after plating. Myf5 levels in follistatin-treated cultures were elevated, but, surprisingly, these cultures showed a reduction in myoD levels. These data suggest that endogenously expressed activin and follistatin are important modulators of muscle development.

Target tissue influence on somatostatin expression in the avian ciliary ganglion

Activin as a neurodifferentiation factor. Our studies of neurotransmitter expression have focused on the expression of neuropeptide transmitters in the avian ciliary ganglion (CG) and have examined the influence of choroidal vascular smooth muscle cells in regulating the differential expression of somatostatin in the CG. In these activities we have identified activin A as a potential target-derived neurodifferentiation factor that can stimulate somatostatin expression in cultured CG neurons. In cultured CG neurons, activin can stimulate the expression of somatostatin in choroid neurons, the pattern of neurotransmitter expression found in vivo, and in the ciliary neurons that would normally not express somatostatin. In vivo, mRNA transcripts of the cActR-IIA appear to be expressed by both choroid and ciliary CG neurons. This suggests that activin might serve as an instructive factor in controlling neuropeptide phenotype. For activin to serve as an instructive factor requires that activin be produced by choroid smooth-muscle target cells. Indeed, activin mRNA and activin-like immunoreactivity are found in choroid cells, in vitro. However, the lack of somatostatin expression by ciliary neurons suggests that activin is not produced by their targets, the iris and ciliary body. This simple view is countered by the observation that activin A mRNA is also present in the iris and activin-like immunoreactivity is detectable in the iris and ciliary body. Instead, the production of the specific activin inhibitor follistatin in the iris and ciliary body is likely to limit the availability of activin to only those neurites innervating the choroid layer, thus accounting for the differential expression of somatostatin in only the choroid CG neurons. This somewhat more complicated arrangement is similar to the mechanism thought to be employed for primary induction during frog embryogenesis. The observations reviewed here are all consistent with the hypothesized role for activin as a molecule whose availability to neurites in the target regulates neurotransmitter expression. Additional in vivo perturbation experiments are needed to further examine this hypothesis; nevertheless, activin appears as a strong candidate for a target-derived neurotransmitter differentiation factor. Activin's potential roles in differentiation: A wide variety of biological effects have been ascribed to activin. Initially identified and purified as a gonadal hormone stimulating the production and release of FSH from the pituitary, activin is also implicated in the stimulation of erythroid differentiation, as a modulator of follicular granulosa cell differentiation, as a mesodermalizing factor in both amphibian and avian early development, and as a component in establishing left-right axial patterning in the chicken embryo. Activin has also been found to be a survival factor for several neuronal cell lines and for rat embryonic neural retina cells in culture. However, activin is not a survival factor for chicken CG neurons in culture. Our observation that activin may play a function in target-derived control of neuropeptide expression adds yet another aspect to the list of its potential biological functions. In addition, activin shares regions of amino acid sequence identity with members of the TGF-beta superfamily, which includes the TGF-betas, Mullerian inhibitory substance, Drosophila decapentaplegic gene product, dorsalin, bone morphogenetic proteins, inhibin, and glial-derived neurotrophic factor. Interestingly, these are all factors that have effects upon cellular differentiation. Effects of activin on other neurons. Activin A--as well as two other TGF-beta superfamily members, BMP-2 and BMP-6--has been shown to induce expression of mRNAs for several neuropeptides in cultured rat sympathetic neurons. In addition, activin A induces ChAT mRNA in cultured sympathetic neurons. (ABSTRACT TRUNCATED)

CNTF and LIF are not required for the target-directed acquisition of cholinergic and peptidergic properties by sympathetic neurons in vivo

During development, the sympathetic innervation of two targets, sweat glands and periosteum, changes the neurotransmitters it expresses from noradrenaline to acetylcholine and vasoactive intestinal peptide (VIP). The target-derived molecules that induce, these changes have not been identified. Neuropoietic cytokines, including ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF), induce the same phenotypic changes in sympathetic neurons in vitro as sweat glands and periosteum do in vivo, raising the possibility that one of these factors mediates induction of cholinergic traits and VIP by these target tissues. Because CNTF and LIF have overlapping functions and signalling pathways, they could act interchangeably or in concert to influence neurotransmitter expression. To determine whether CNTF or CNTF and LIF together are responsible for the induction of cholinergic and peptidergic function in vivo, we analyzed the neurotransmitter properties of sweat gland innervation in mice lacking CNTF or CNTF and LIF. We find that, as in wild-type mice, gland innervation in mice lacking one or both molecules appropriately expresses cholinergic properties and VIP immunoreactivity. Furthermore, footpads of mice lacking one or both genes contain choline acetyltransferase activity comparable to that of wild-type mice, and CNTF- or CNTF/LIF-deficient mice possess the normal complement of active sweat glands. We analyzed the innervation of a second, recently identified cholinergic sympathetic target, the periosteum, which is the connective tissue surrounding bone. Periosteal innervation of mice lacking CNTF, LIF, or both, like that of wild-type mice, is immunoreactive for the vesicular acetylcholine transporter, a recently identified cholinergic marker, and VIP. These results provide evidence that neither CNTF, LIF, nor a combination of the two are required for the developmental change from noradrenergic to cholinergic function that occurs in sympathetic innervation of sweat glands and periosteum.

Activin receptor mRNA expression by neurons of the avian ciliary ganglion

Previous studies have suggested that activin may serve as a neurodifferentiation factor regulating somatostatin expression in neurons of the avian ciliary ganglion (CG). As one aspect of examining the role of activin in CG development, we inquired whether any of the known activin receptors are expressed by developing CG neurons in vivo. In addition, we examined whether activin A mRNA is expressed in the choroid layer and iris of the chicken eye. Oligonucleotide primers were designed for the chicken activin receptor type IIA (cActR-IIA), type IIB (cActR-IIB), and activin A. In reverse-transcription-polymerase chain reaction (rtPCR), an appropriately sized product was amplified from CG cDNA using primers to the cActR-IIA but not the cActR-IIB. Sequencing confirmed the identity of the PCR product as a fragment of the cActR-IIA. It thus appears that mRNA for the type IIA but not the type IIB activin receptor is expressed in the chicken CG. An antisense strand digoxigenin-labeled riboprobe complimentary to a 358-bp portion of the cActR-IIA kinase region hybridized to cells within cryostat sections of embryonic CG. From E6.5-E18, hybridization of this probe appears to be specific for cells with a neuronal morphology. Using rtPCR with activin A-specific primers we detected activin mRNA in the choroid layer of E14 and E19 eyes, and from the iris at E14. Our results are consistent with a role for activin as a neurodifferentiation factor in vivo, and imply that within the CG, the cActR-IIA is specifically expressed by neurons, and that activin A is expressed in the targets of these neurons.

Changes in the regulatory effects of cell-cell interactions on neuronal AChR subunit transcript levels after synapse formation

Nicotinic acetylcholine receptors (AChRs) mediate excitatory synaptic transmission in the chick ciliary ganglion. AChR protein and mRNA levels are increased by both innervation and retrograde signals from target tissues during synapse formation. We now show that AChR alpha3, beta4, and alpha5 subunit transcript levels stop increasing after synaptogenesis. Moreover, maintenance of these mRNA levels requires the continued presence of regulatory signals from both pre- and postganglionic tissues. Unilateral preganglionic denervation or postganglionic axotomy causes declines in alpha3, beta4, and alpha5 transcript levels, ranging from twofold to 3. 5-fold, relative to contralateral control neuron values in newly hatched chicks. The reductions are not merely an injury response; cbeta4-tubulin mRNA levels are not affected by either axotomy or denervation. Further, similar decreases in AChR mRNA levels are observed after local application of colchicine to the postganglionic nerves, which blocks fast transport without disturbing axonal integrity. These results also demonstrate a developmental change in the regulatory effects of target tissues. Reductions in alpha5 mRNA levels caused by axotomy or colchicine treatment after peripheral synapse formation contrast with the lack of an effect on alpha5 when synapse formation with the target tissue is prevented. The ability of the target tissue to regulate alpha5 mRNA levels after synaptogenesis is interesting, because this subunit may be necessary for the formation of high-conductance AChRs. The specific regulatory effects of target tissues and inputs at different developmental stages demonstrate that neurons continually depend on signals from their pre- and postsynaptic tissues to accomplish mature levels of AChR subunit expression and optimal functioning of that neuronal circuit.

Information processing in mammalian olfactory system

In recent years, considerable progress has been made in understanding how the olfactory system uses neural space to encode sensory information. In this review, we focus on recent studies aimed at understanding the organizational strategies used by the mammalian olfactory system to encode information. The odorant receptor gene family is discussed in the context of its genomic organization as well as the specificity of olfactory sensory neurons. These data have important consequences for the mechanisms of odorant receptor gene choice by a given sensory neuron. Division of the olfactory epithelium into zones that express different sets of odorant receptors is the first level of input organization. The topographical relationship between periphery and olfactory bulb represents a further level of processing of information and results in the formation of a highly organized spatial map of information in the olfactory bulb. There, local circuitry refines the sensory input through various lateral interactions. Finally, the factors that may drive the development of such a spatial map are discussed. The onset of expression and the establishment of the zonal organization of odorant receptor genes in the epithelium are not dependent upon the presence of the olfactory bulb, suggesting that the functional identity of olfactory sensory neurons is determined independently of target selection.

Expression of a chicken ciliary neurotrophic factor in targets of ciliary ganglion neurons during and after the cell-death phase

Ciliary ganglion (CG) neurons, like other neuronal populations, become dependent on their targets for survival during development. We have previously purified and cloned a secreted ciliary neurotrophic factor that was called growth-promoting activity (GPA). We report here the expression and purification of a highly active form of recombinant GPA, the preparation of GPA-specific polyclonal and monoclonal antibodies, and the use of these antibodies to investigate the cellular location and timing of GPA expression in tissues innervated by CG neurons. Virtually all of the trophic activity in extracts of embryonic eyes could be depleted by GPA-specific antibodies. GPA-like immunoreactivity was found in both targets of the CG: the arterial vasculature of the choroid layer and the ciliary body of the eye. In the choroid layer, GPA was localized to smooth muscle cells surrounding the choroid arteries. Staining in the choroid layer was first detectable at embryonic day (E) 10, or about 2 days after cell death has begun in the ganglion, then increased in intensity through E19. Quantification of trophic activity from whole eye extracts at various ages showed a small increase in activity observed between E9 and E12 and at least a 10-fold increase between E12 and E18. The presence of GPA protein in target cells of CG neurons during the specific developmental period when these neurons undergo cell death is consistent with its proposed function as a target-derived ciliary neurotrophic factor.