STAT3-Ser/Hes3 Signaling: A New Molecular Component of the Neuroendocrine System?

The endocrine system involves communication among different tissues in distinct organs, including the pancreas and components of the Hypothalamic-Pituitary-Adrenal Axis. The molecular mechanisms underlying these complex interactions are a subject of intense study as they may hold clues for the progression and treatment of a variety of metabolic and degenerative diseases. A plethora of signaling pathways, activated by hormones and other endocrine factors have been implicated in this communication. Recent advances in the stem cell field introduce a new level of complexity: adult progenitor cells appear to utilize distinct signaling pathways than the more mature cells in the tissue they co-reside. It is therefore important to elucidate the signal transduction requirements of adult progenitor cells in addition to those of mature cells. Recent evidence suggests that a common non-canonical signaling pathway regulates adult progenitors in several different tissues, rendering it as a potentially valuable starting point to explore their biology. The STAT3-Ser/Hes3 Signaling Axis was first identified as a major regulator of neural stem cells and, subsequently, cancer stem cells. In the endocrine/neuroendocrine system, this pathway operates on several levels, regulating other types of plastic cells: (a) it regulates pancreatic islet cell function and insulin release; (b) insulin in turn activates the pathway in broadly distributed neural progenitors and possibly also hypothalamic tanycytes, cells with important roles in the control of the adrenal gland; (c) adrenal progenitors themselves operate this pathway. The STAT3-Ser/Hes3 Signaling Axis therefore deserves additional research in the context of endocrinology.

Expression profiles of the nuclear receptors and their transcriptional coregulators during differentiation of neural stem cells

Neural stem cells (NSCs) are pluripotent precursors with the ability to proliferate and differentiate into 3 neural cell lineages, neurons, astrocytes and oligodendrocytes. Elucidation of the mechanisms underlying these biologic processes is essential for understanding both physiologic and pathologic neural development and regeneration after injury. Nuclear hormone receptors (NRs) and their transcriptional coregulators also play crucial roles in neural development, functions and fate. To identify key NRs and their transcriptional regulators in NSC differentiation, we examined mRNA expression of 49 NRs and many of their coregulators during differentiation (0-5 days) of mouse embryonic NSCs induced by withdrawal of fibroblast growth factor-2 (FGF2). 37 out of 49 NRs were expressed in NSCs before induction of differentiation, while receptors known to play major roles in neural development, such as THRα, RXRs, RORs, TRs, and COUP-TFs, were highly expressed. CAR, which plays important roles in xenobiotic metabolism, was also highly expressed. FGF2 withdrawal induced mRNA expression of RORγ, RXRγ, and MR by over 20-fold. Most of the transcriptional coregulators examined were expressed basally and throughout differentiation without major changes, while FGF2 withdrawal strongly induced mRNA expression of several histone deacetylases (HDACs), including HDAC11. Dexamethasone and aldosterone, respectively a synthetic glucocorticoid and natural mineralocorticoid, increased NSC numbers and induced differentiation into neurons and astrocytes. These results indicate that the NRs and their coregulators are present and/or change their expression during NSC differentiation, suggesting that they may influence development of the central nervous system in the absence or presence of their ligands.

Matrix metalloproteinase-7 modulates synaptic vesicle recycling and induces atrophy of neuronal synapses

Matrix metalloproteinase-7 (MMP-7) belongs to a family of zinc dependent endopeptidases that are expressed in a variety of tissues including the brain. MMPs are known to be potent mediators of pericellular proteolysis and likely mediators of dynamic remodelling of neuronal connections. While an association between proteases and the neuronal synapse is emerging, a full understanding of this relationship is lacking. Here, we show that MMP-7 alters the structure and function of presynaptic terminals without affecting neuronal survival. Bath application of recombinant MMP-7 to cultured rat neurons induced long-lasting inhibition of vesicular recycling as measured by synaptotagmin 1 antibody uptake assays and FM4-64 optical imaging. MMP-7 application resulted in reduced abundance of vesicular and active zone proteins locally within synaptic terminals although their general levels remained unaltered. Finally, chronic application of the protease resulted in synaptic atrophy, including smaller terminals and fewer synaptic vesicles, as determined by electron microscopy. Together these results suggest that MMP-7 is a potent modulator of synaptic vesicle recycling and synaptic ultrastructure and that elevated levels of the enzyme, as may occur with brain inflammation, may adversely influence neurotransmission.

Stem cell biology and neurodegenerative disease

The fundamental basis of our work is that organs are generated by multipotent stem cells, whose properties we must understand to control tissue assembly or repair. Central nervous system (CNS) stem cells are now recognized as a well-defined population of precursors that differentiate into cells that are indisputably neurons and glial cells. Work from our group played an important role in defining stem cells of the CNS. Embryonic stem (ES) cells also differentiate to specific neuron and glial types through defined intermediates that are similar to the cellular precursors that normally occur in brain development. There is convincing evidence that the differentiated progeny of ES cells and CNS stem cells show expected functions of neurons and glia. Recent progress has been made on three fundamental developmental processes: (i) cell cycle control; (ii) the control of cell fate; and (iii) early steps in neural differentiation. In addition, our work on CNS stem cells has developed to a stage where there are clinical implications for Parkinson's and other degenerative disorders. These advances establish that stem cell biology contributes to our understanding of brain development and has great clinical promise.

Sequential actions of BMP receptors control neural precursor cell production and fate

Bone morphogenetic proteins (BMPs) have diverse and sometimes paradoxical effects during embryonic development. To determine the mechanisms underlying BMP actions, we analyzed the expression and function of two BMP receptors, BMPR-IA and BMPR-IB, in neural precursor cells in vitro and in vivo. Neural precursor cells always express Bmpr-1a, but Bmpr-1b is not expressed until embryonic day 9 and is restricted to the dorsal neural tube surrounding the source of BMP ligands. BMPR-IA activation induces (and Sonic hedgehog prevents) expression of Bmpr-1b along with dorsal identity genes in precursor cells and promotes their proliferation. When BMPR-IB is activated, it limits precursor cell numbers by causing mitotic arrest. This results in apoptosis in early gestation embryos and terminal differentiation in mid-gestation embryos. Thus, BMP actions are first inducing (through BMPR-IA) and then terminating (through BMPR-IB), based on the accumulation of BMPR-IB relative to BMPR-IA. We describe a feed-forward mechanism to explain how the sequential actions of these receptors control the production and fate of dorsal precursor cells from neural stem cells.

In vitro generation and transplantation of precursor-derived human dopamine neurons

The use of in vitro expanded human CNS precursors has the potential to overcome some of the ethical, logistic and technical problems of fetal tissue transplantation in Parkinson disease. Cultured rat mesencephalic precursors proliferate in response to bFGF and upon mitogen withdrawal, differentiate into functional dopamine neurons that alleviate motor symptoms in Parkinsonian rats (Studer et al. [1998] Nat. Neurosci. 1:290-295). The successful clinical application of CNS precursor technology in Parkinson disease will depend on the efficient in vitro generation of human dopaminergic neurons. We demonstrate that human dopamine neurons can be generated from both midbrain and cortical precursors. Transplantation of midbrain precursor-derived dopamine neurons into Parkinsonian rats resulted in grafts rich in tyrosine hydroxylase positive neurons 6 weeks after transplantation. No surviving tyrosine hydroxylase positive neurons could be detected when dopamine neurons derived from cortical precursors were grafted. Our data demonstrate in vitro derivation of human dopamine neurons from expanded CNS precursors and encourage further studies that systematically address in vivo function and clinical potential.

Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus

Knowing the rate of addition of new granule cells to the adult dentate gyrus is critical to understanding the function of adult neurogenesis. Despite the large number of studies of neurogenesis in the adult dentate gyrus, basic questions about the magnitude of this phenomenon have never been addressed. The S-phase marker bromodeoxyuridine (BrdU) has been extensively used in recent studies of adult neurogenesis, but it has been carefully tested only in the embryonic brain. Here, we show that a high dose of BrdU (300 mg/kg) is a specific, quantitative, and nontoxic marker of dividing cells in the adult rat dentate gyrus, whereas lower doses label only a fraction of the S-phase cells. By using this high dose of BrdU along with a second S-phase marker, [(3)H]thymidine, we found that young adult rats have 9,400 dividing cells proliferating with a cell cycle time of 25 hours, which would generate 9,000 new cells each day, or more than 250,000 per month. Within 5-12 days of BrdU injection, a substantial pool of immature granule neurons, 50% of all BrdU-labeled cells in the dentate gyrus, could be identified with neuron-specific antibodies TuJ1 and TUC-4. This number of new granule neurons generated each month is 6% of the total size of the granule cell population and 30-60% of the size of the afferent and efferent populations (West et al. [1991] Anat Rec 231:482-497; Mulders et al. [1997] J Comp Neurol 385:83-94). The large number of the adult-generated granule cells supports the idea that these new neurons play an important role in hippocampal function.

Neurotrophins act at presynaptic terminals to activate synapses among cultured hippocampal neurons

We have recently demonstrated that embryonic E16 hippocampal neurons grown in cultures are unable to form fast synaptic connections unless treated with BDNF or NT-3. This experimental system offers an opportunity to define the roles of neurotrophins in processes leading to formation of functional synaptic connections. We have used ultrastructural and electrophysiological methods to explore the cellular locations underlying neurotrophin action on synaptic maturation. The rate of spontaneous miniature excitatory postsynaptic currents (mEPSCs) evoked by hyperosmotic stimulation was 7-16-fold higher in neurotrophin-treated cells than in controls. In addition, the potent neurotransmitter-releasing drug alpha-latrotoxin was virtually ineffective in the control cells while it stimulated synaptic events in neurotrophin-treated cells. Likewise, the membrane-bound dye FM1-43 was taken up by terminals in neurotrophin-treated cultures five-fold more than in controls. Both the total number and the number of docked synaptic vesicles were increased by neurotrophin treatment. Activation of synaptic responses by neurotrophins occurred even when postsynaptic glutamate receptors and action potential discharges were pharmacologically blocked. These results are consistent with a presynaptic locus of action of neurotrophins to increase synaptic vesicle density which is critical for rapid synaptic transmission. They also suggest that neurotrophins can activate synapses in the absence of pre- and postsynaptic neuronal activity.

Long-term survival, migration, and differentiation of neural cells without functional NMDA receptors in vivo

The NMDA receptor, one of the two major ionotropic glutamate receptors, has been proposed to play fundamental roles in the survival, migration, differentiation, and activity-dependent maturation of neural cells. The NR1 gene encodes the major subunit that is responsible for channel function, and NR1 -/- mice die at birth, inhibiting the study of glutamate signaling in postnatal neurons. The properties of cells lacking the NR1 subunit of NMDA receptors were studied by transplanting dissociated telencephalic, diencephalic, and mesencephalic cells of E14 mouse embryos with a targeted deletion of the NR1 gene into the ventricles of embryonic rats using intrauterine transplantation (Brüstle et al., 1995, Neuron 15, 1275-1285). The transplanted cells took part in the normal development of the host brain where they survived after migration into a large number of brain structures. Morphological and immunohistochemical analysis suggests that NR1 -/- cells can differentiate normally in these sites. The results provide evidence that NMDA-receptor-initiated signals are not required for the postnatal differentiation and survival of many types of neurons in the central nervous system, in a noncell autonomous fashion after transplantation into a wild-type environment.

Ascorbic acid increases the yield of dopaminergic neurons derived from basic fibroblast growth factor expanded mesencephalic precursors

CNS precursors derived from E12 rat mesencephalon proliferate in the presence of basic fibroblast growth factor and differentiate in vitro into functional dopaminergic neurons, which upon transplantation alleviate behavioral symptoms in a rat model of Parkinson's disease. Here we show that the efficiency of dopaminergic differentiation decreases in the mesencephalic precursors that were proliferated or passaged for extended periods in vitro. Ascorbic acid treatment restored dopaminergic differentiation in these precursors and led to a greater than 10-fold increase in dopamine neuron yield compared with untreated cultures. The effect of ascorbic acid was stereospecific and could not be mimicked by any other antioxidants. The expression of sodium-dependent vitamin C transporter, a recently identified stereospecific ascorbic acid transporter, was maintained in mesencephalic precursors for extended in vitro periods. Pre-treatment of in vitro expanded mesencephalic precursors with ascorbic acid might facilitate the large-scale generation of dopaminergic neurons for clinical transplantation.

A glia-derived signal regulating neuronal differentiation

Astrocytes are present in large numbers in the nervous system, are associated with synapses, and propagate ionic signals. Astrocytes influence neuronal physiology by responding to and releasing neurotransmitters, but the mechanisms that establish the close interaction between these cells are not defined. Here we use hippocampal neurons in culture to demonstrate that vasoactive intestinal polypeptide (VIP) promotes neuronal differentiation through activity-dependent neurotrophic factor (ADNF), a protein secreted by VIP-stimulated astroglia. ADNF is produced by glial cells and acts directly on neurons to promote glutamate responses and morphological development. ADNF causes secretion of neurotrophin 3 (NT-3), and both proteins regulate NMDA receptor subunit 2A (NR2A) and NR2B. These data suggest that the VIP-ADNF-NT-3 neuronal-glial pathway regulates glutamate responses from an early stage in the synaptic development of excitatory neurons and may also contribute to the known effects of VIP on learning and behavior in the adult nervous system.

Efficient generation of midbrain and hindbrain neurons from mouse embryonic stem cells

Embryonic stem (ES) cells are clonal cell lines derived from the inner cell mass of the developing blastocyst that can proliferate extensively in vitro and are capable of adopting all the cell fates in a developing embryo. Clinical interest in the use of ES cells has been stimulated by studies showing that isolated human cells with ES properties from the inner cell mass or developing germ cells can provide a source of somatic precursors. Previous studies have defined in vitro conditions for promoting the development of specific somatic fates, specifically, hematopoietic, mesodermal, and neurectodermal. In this study, we present a method for obtaining dopaminergic (DA) and serotonergic neurons in high yield from mouse ES cells in vitro. Furthermore, we demonstrate that the ES cells can be obtained in unlimited numbers and that these neuron types are generated efficiently. We generated CNS progenitor populations from ES cells, expanded these cells and promoted their differentiation into dopaminergic and serotonergic neurons in the presence of mitogen and specific signaling molecules. The differentiation and maturation of neuronal cells was completed after mitogen withdrawal from the growth medium. This experimental system provides a powerful tool for analyzing the molecular mechanisms controlling the functions of these neurons in vitro and in vivo, and potentially for understanding and treating neurodegenerative and psychiatric diseases.

Cell contact regulates fate choice by cortical stem cells

Cell fate is determined by intrinsic programs and external cues, such as soluble signals and cell-cell contact. Previous studies have demonstrated the roles of soluble factors in the proliferation and differentiation of cortical stem cells and cell-cell contact in maintaining stem cells in a proliferative state. In the present study, we focused on the effect of cell-cell interaction on cell-fate determination. We found that density could exert a strong influence on the cell-type composition when cortical stem cells differentiate. Multipotent stem cells, which normally gave rise to neurons, astrocytes, and oligodendrocytes under high-density culture condition, differentiated almost exclusively into smooth muscle at low density. Clonal analysis indicated that smooth muscle and astrocytes were derived from a common precursor and that the density effect on cell types used an instructive mechanism on the choice of fate rather than an effect of selective survival and/or proliferation. This instructive mechanism depended on the local and not the average density of the cells. This local signal could be mimicked by membrane extract. These findings demonstrate the importance of membrane-bound signals in specifying lineage and provide the first evidence for a short-range regulatory mechanism in cortical stem cell differentiation.

Transplanted CNS stem cells form functional synapses in vivo

An understanding of developmental mechanisms and new cell therapies can be achieved by transplantation into the nervous system. Multipotential stem cells have been isolated from the foetal and adult central nervous system (CNS). Immortalized and primary precursor cells integrate into the developing brain generating both neurons and glia as defined by immunological and morphological criteria. Here we show for the first time that in vitro-expanded CNS precursors, upon transplantation into the brains of rats, form electrically active and functionally connected neurons. These neurons exhibit spontaneous and evoked postsynaptic events and respond to focal glutamate application. Donor cells were grafted into the foetal hippocampus, and the amplitude and frequency of spontaneous synaptic events were monitored in the grafted cells in area CA1 for the first month of postnatal life. The formation of synapses onto grafted neurons indicates that grafted CNS stem cells can be used to study synaptic development in vivo and has important implications for clinical cell replacement therapies.

Hippocampal stem cells differentiate into excitatory and inhibitory neurons

Stem cell technology promises new and rapid advances in cell therapy and drug discovery. Clearly, the value of this approach will be limited by the differentiated functions displayed by the progeny of stem cells. The foetal and adult central nervous system (CNS) harbour stem cells that can be expanded in vitro and differentiate into immature neurons and glia. Surprisingly, we do not know if neurons derived from stem cells form synapses, a definitive feature of neuronal function. Neuronal differentiation is a complex process and in this paper we establish conditions that permit extensive maturation of neurons in the presence of neurotrophins. These conditions permit the differentiation of rat hippocampal stem cells into both excitatory (glutamatergic) and inhibitory (GABAergic) neurons. The proportion of excitatory and inhibitory synapses was strongly influenced by specific neurotrophins, and these responses reflect the region of origin of the stem cells in the brain. These data show that stem cells can be used to study mechanisms of excitation and inhibition in the nervous system.

Early cortical precursors do not undergo LIF-mediated astrocytic differentiation

Multipotential stem cells have been isolated from the developing and adult CNS. Similar identified factors control the differentiation of these cells. A striking example is the instructive action of CNTF/LIF activating the JAK/STAT pathway to induce astrocytic differentiation in both fetal and adult CNS stem cells. Here we show that E12 cortical precursors express functional LIF receptors but do not exhibit this differentiation response to CNTF/LIF either in explant or in dissociated cell culture. The lack of response to LIF-induced astrocytic differentiation is maintained in cocultures with LIF responsive cells derived from E15 cortex. This suggests cell intrinsic differences between early and late stage precursors in the interpretation of LIF-mediated signaling; however, the early nestin-positive precursor population differentiates into both neurons and neural crest derivatives. These data define differences between CNS stem cells from different stages of cortical development. J. Neurosci. Res. 59:301-311, 2000. Published 2000 Wiley-Liss, Inc.

Restoring production of hippocampal neurons in old age

The production of hippocampal granule neurons continues throughout adulthood but dramatically decreases in old age. Here we show that reducing corticosteroid levels in aged rats restored the rate of cell proliferation, resulting in increased numbers of new granule neurons. This result indicates that the neuronal precursor population in the dentate gyrus remains stable into old age, but that neurogenesis is normally slowed by high levels of corticosteroids. The findings further suggest that decreased neurogenesis may contribute to age-related memory deficits associated with high corticosteroids, and that these deficits may be reversible.

Embryonic stem cell-derived glial precursors: a source of myelinating transplants

Self-renewing, totipotent embryonic stem (ES) cells may provide a virtually unlimited donor source for transplantation. A protocol that permits the in vitro generation of precursors for oligodendrocytes and astrocytes from ES cells was devised. Transplantation in a rat model of a human myelin disease shows that these ES cell-derived precursors interact with host neurons and efficiently myelinate axons in brain and spinal cord. Thus, ES cells can serve as a valuable source of cell type-specific somatic precursors for neural transplantation.

Suppression of epileptogenesis by modification of N-methyl-D-aspartate receptor subunit composition

The effects of altered N-methyl-D-aspartate (NMDA) receptor subunit composition on seizure development in kindling epilepsy were assessed in transgenic mice expressing high neuronal levels of NR2D under control of the calcium/calmodulin kinase II alpha subunit (alphaCaMKII) promoter. The NR2D subunit is normally present at very low levels in the mature forebrain. Transgenic mice showed a marked reduction of amygdala kindling development. Spread of epileptic activity was retarded and generalized seizures appeared later in animals overexpressing NR2D compared with wild-type mice. The progressive lengthening of epileptiform activity, which normally occurs in kindling, was also dampened in transgenic animals. We conclude that NMDA receptor subunit composition determines the progression of experimental epilepsy.

An immortalized, type-1 astrocyte of mesencephalic origin source of a dopaminergic neurotrophic factor

Rat embryonic d 14 (E14) mesencephalic cells, 2.5% of which are glioblasts, were incubated in medium containing 10% of fetal bovine serum for 12 h and subsequently expanded in a serum-free medium using basic fibroblast growth factor (bFGF) as the mitogen. On a single occasion, after more than 15 d in culture, several islets of proliferating, glial-like cells were observed in one dish. The cells, when isolated and passaged, proliferated rapidly in either a serum-free or serum-containing growth medium. Subsequent immunocytochemical analysis showed that they stained positive for GFAP and vimentin, and negative for A2B5, O4, GalC, and MAP2. Serum-free conditioned medium (CM) prepared from these cells caused a fivefold increase in survival and promoted neuritic expansion of E14 mesencephalic dopaminergic neurons in culture. These actions are similar to those exerted by CM derived from primary, mesencephalic type-1 astrocytes. The pattern of expression of the region-selective genes; wnt-1, en-1, sis showed that 70% of the cells were heteroploid, and of these, 50% were tetraploid. No apparent decline in proliferative capacity has been observed after 25 passages. The properties of this cell line, named ventral mesencephalic cell line one (VMCL1), are consistent with those of an immortalized, type-1 astrocyte. The mesencephalic origin of the cell line, and the pattern and potency of the neurotrophic activity exerted by the CM, strongly suggest that the neurotrophic factor(s) identified are novel, and will likely be strong candidates with clinical utility for the treatment of Parkinson's disease.

Chimeric brains generated by intraventricular transplantation of fetal human brain cells into embryonic rats

Limited experimental access to the central nervous system (CNS) is a key problem in the study of human neural development, disease, and regeneration. We have addressed this problem by generating neural chimeras composed of human and rodent cells. Fetal human brain cells implanted into the cerebral ventricles of embryonic rats incorporate individually into all major compartments of the brain, generating widespread CNS chimerism. The human cells differentiate into neurons, astrocytes, and oligodendrocytes, which populate the host fore-, mid-, and hindbrain. These chimeras provide a unique model to study human neural cell migration and differentiation in a functional nervous system.

Neurotrophins induce formation of functional excitatory and inhibitory synapses between cultured hippocampal neurons

Cell cultures were used to analyze the role of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the development of synaptic transmission. Neurons obtained from embryonic day 18 (E18) rat hippocampus and cultured for 2 weeks exhibited extensive spontaneous synaptic activity. By comparison, neurons obtained from E16 hippocampus expressed very low levels of spontaneous or evoked synaptic activity. Neurotrophin treatment produced a sevenfold increase in the number of functional synaptic connections in the E16 cultures. BDNF induced formation of both excitatory and inhibitory synapses, whereas NT-3 induced formation of only excitatory synapses. These effects were independent of serum or the age of the glia bed used for the culture. They were not accompanied by significant changes in synaptic-vesicle-associated proteins or glutamate receptors. Treatment of the cultures with the neurotrophins for 3 d was sufficient to establish the maximal level of functional synapses. During this period, neurotrophins did not affect the viability or the morphology of the excitatory neurons, although they did produce an increase in the number and length of dendrites of the GABAergic neurons. Remarkably, only BDNF caused an increase in the number of axonal branches and in the total length of the axons of the GABAergic neurons. These results support a unique and differential role for neurotrophins in the formation of excitatory and inhibitory synapses in the developing hippocampus.

Platelet-derived growth factor induces chemotaxis of neuroepithelial stem cells

The ability of differentiating cells to migrate within the developing central nervous system (CNS) depends on extrinsic guidance signals, some of which are growth factors. In this study we have investigated the chemotactic response of cultured stem cells from the embryonic rat cortex to platelet-derived growth factor (PDGF). Nestin-positive stem cells from the developing CNS can be maintained and expanded in vitro under serum-free conditions in the presence of basic fibroblast growth factor (bFGF). Northern blot analysis of PDGF receptor expression revealed both alpha- and beta-receptors on bFGF-treated neural stem cells. Both PDGF-AA and PDGF-BB readily induced directed migration of cultured neuroepithelial cells as measured in a microchemotaxis assay. Blocking of the migratory response was achieved by incubation with PDGF isoform-specific antibodies. More than 90% of the migrating cells were nestin-positive and incorporation of BrdU was also seen suggesting the cells to be immature and not yet committed to a specific cell lineage. These findings suggest a role for PDGF in cell migration in the developing cortex.

POU transcription factors control expression of CNS stem cell-specific genes

Multipotential stem cells throughout the developing central nervous system have common properties. Among these is expression of the intermediate filament protein nestin and the brain fatty acid binding protein (B-FABP). To determine if common mechanisms control transcription in CNS stem cells, the regulatory elements of these two genes were mapped in transgenic mice. A 257 basepair enhancer of the rat nestin gene is sufficient for expression throughout the embryonic neuroepithelium. This enhancer contains two sites bound by the class III POU proteins Brn-1, Brn-2, Brn-4, and Tst-1. Only one of the two POU sites is required for CNS expression. An adjacent hormone response element is necessary for expression in the dorsal midbrain and forebrain. The regulatory sites of the B-FABP gene are strikingly similar to those of the nestin gene. A hybrid POU/Pbx binding site is recognized in vitro by Pbx-1, Brn-1 and Brn-2. This site is essential for expression in most of the CNS. In addition, a hormone response element is necessary for forebrain expression. Both the nestin and B-FABP genes therefore depend on POU binding sites for general CNS expression, with hormone response elements additionally required for activity in the anterior CNS. These data indicate that regulation by POU proteins and hormone receptors is a general mechanism for CNS stem cell-specific transcription.

Transplantation of expanded mesencephalic precursors leads to recovery in parkinsonian rats

In vitro expansion of central nervous system (CNS) precursors might overcome the limited availability of dopaminergic neurons in transplantation for Parkinson's disease, but generating dopaminergic neurons from in vitro dividing precursors has proven difficult. Here a three-dimensional cell differentiation system was used to convert precursor cells derived from E12 rat ventral mesencephalon into dopaminergic neurons. We demonstrate that CNS precursor cell populations expanded in vitro can efficiently differentiate into dopaminergic neurons, survive intrastriatal transplantation and induce functional recovery in hemiparkinsonian rats. The numerical expansion of primary CNS precursor cells is a new approach that could improve both the ethical and the technical outlook for the use of human fetal tissue in clinical transplantation.

Regulation of neurogenesis by growth factors and neurotransmitters

The generation of neurons and glia in the developing nervous system is likely to be regulated by extrinsic factors, including growth factors and neurotransmitters. Evidence from in vivo and/or in vitro systems indicates that basic fibroblast growth factor, transforming growth factor (TGF)-alpha, insulin-like growth factor-1, and the monoamine neurotransmitters act to increase proliferation of neural precursors. Conversely, glutamate, gamma-aminobutyric acid, and opioid peptides are likely to play a role in down-regulating proliferation in the developing nervous system. Several other factors, including the neuropeptides vasoactive intestinal peptide and pituitary adenylate cyclase-activating peptide, as well as the growth factors platelet-derived growth factor, ciliary neurotrophic factor, and members of the TGF-beta family, have different effects on proliferation and differentiation depending on the system examined. Expression of many of these factors and their receptors in germinal regions of the central nervous system suggests that they can act directly on precursor populations to control their proliferation. Together, the findings discussed here indicate that proliferation and cell fate determination in the developing brain are regulated extrinsically by complex interactions between a relatively large number of growth factors and neurotransmitters.

A rod end deletion in the intermediate filament protein nestin alters its subcellular localization in neuroepithelial cells of transgenic mice

Neuroepithelial and radial glial cells span between the ventricular and the pial surfaces of the neural tube and express two intermediate filaments (IFs), nestin and vimentin, which form a filamentous network throughout the length of the cells. In this report we study the polymerization characteristics of nestin and examine how mutations affect the assembly and localization of the nestin protein in cultured cells and in the developing CNS of transgenic mice. A wild-type rat nestin gene transfected into the IF-free SW13 cell line failed to assemble into a filamentous network but was incorporated into the existing IF network of a subclone expressing vimentin, demonstrating that nestin requires vimentin for proper assembly. In transgenic mice, rat nestin formed a network indistinguishable from that formed by endogenous nestin and vimentin, but a mutant form lacking five amino acids at the carboxy terminus of the rod domain was largely restricted to the pial endfeet. Since nestin mRNA is localized to the pial endfoot region we propose that both transgenes are translated there, but that the wild-type protein is preferentially incorporated into the IF network. These observations provide evidence for hierarchical assembly and a complex organization of the IF network along the ventricular-pial axis in the early CNS.

Survival and synaptogenesis of hippocampal neurons without NMDA receptor function in culture

Physiological and morphological properties of cultured hippocampal neurons were measured to investigate whether NMDA receptors play a role in survival and differentiation. Neurons dissociated from mouse embryos with different NMDAR1 genotypes were grown in culture. Electrophysiological analysis verified the absence of NMDA receptor-mediated currents in neurons taken from homozygous mutant (NR1-/-) embryos. The number of surviving hippocampal neurons was 2.5-fold higher in cultures from the NR1-/- embryos compared with wild type (NR1 +/+) and heterozygous (NR1+/-) controls. Despite the lack of NMDA receptor function, NR1-/- neurons formed synapsin I-positive presynaptic boutons associated with MAP2ab-positive dendrites in culture. Confocal microscopic analysis of Dil labelled neurons confirmed the presence of dendritic spines on NR1-/- neurons with 80% of the density found in NR1 +/+ neurons. These results suggest that the NMDA receptor has little effect on general features of neuronal differentiation. In contrast, there is clear effect on neuronal survival. This finding establishes neuron number in standard culture conditions as a measure of NMDA receptor activity.

Hippocampal synaptic plasticity in mice overexpressing an embryonic subunit of the NMDA receptor

The effects of changing NMDA receptor subunit composition on synaptic plasticity in the hippocampus were analyzed by creating transgenic mice overexpressing NR2D, a predominantly embryonic NMDA receptor subunit. NMDA-evoked currents in the transgenic mice had smaller amplitudes and slower kinetics. The transgenics also displayed age-dependent deficits in synaptic plasticity in area CA1 of the hippocampus. Long-term depression was selectively impaired in juvenile mice when NR2D overexpression was moderate. In mature mice, overexpression of NR2D was associated with a reduction of both NR2B and Ca2+-independent activity of Ca2+- and calmodulin-dependent protein kinase II. These biochemical changes were correlated with a marked impairment of NMDA-dependent long-term potentiation, but spatial behavior was normal in these mice. These results show that the developmental regulation of NMDA receptor subunit composition alters the frequency at which modification of synaptic responses occur after afferent stimulation.

Multiple routes to astrocytic differentiation in the CNS

Ciliary neurotrophic factor (CNTF) acts instructively to switch multipotent stem cells of the CNS to an astrocytic fate. Here we show that CNTF causes activation of janus kinase-signal transducers and activators of transcription and mitogen-activated protein kinase (MAPK) pathways with differential kinetics in these cells. Inhibition studies indicate that activation of the MAPK pathway is required early in the differentiation process, whereas activation of signal transducer and activator of transcription (STAT) proteins is required for commitment to an astrocytic fate. Bone morphogenetic proteins have also been shown to cause astrocytic differentiation but do not cause STAT activation or astrocytic differentiation in fibroblast growth factor 2-expanded fetal stem cells used here. These results show that there are two distinct routes to initiate astrocytic commitment in multipotent CNS precursors.

In vitro-generated neural precursors participate in mammalian brain development

During embryogenesis, pluripotent stem cells segregate into daughter lineages of progressively restricted developmental potential. In vitro, this process has been mimicked by the controlled differentiation of embryonic stem cells into neural precursors. To explore the developmental potential of these cell-culture-derived precursors in vivo, we have implanted them into the ventricles of embryonic rats. The transplanted cells formed intraventricular neuroepithelial structures and migrated in large numbers into the brain tissue. Embryonic-stem-cell-derived neurons, astrocytes, and oligodendrocytes incorporated into telencephalic, diencephalic, and mesencephalic regions and assumed phenotypes indistinguishable from neighboring host cells. These observations indicate that entirely in vitro-generated neural precursors are able to respond to environmental signals guiding cell migration and differentiation and have the potential to reconstitute neuronal and glial lineages in the central nervous system.

Neurotrophins stimulate chemotaxis of embryonic cortical neurons

During mammalian cortical development, neuronal precursors proliferate within ventricular regions then migrate to their target destinations in the cortical plate, where they organize into layers. In the rat, most cortical neuronal migration occurs during the final week of gestation (Bayer et al, 1991; Jacobson, 1991). At this time (E15-E21), reverse transcriptase-polymerase chain reaction demonstrated that cortical homogenates contain mRNA encoding brain derived neurotrophic factor (BDNF) and the catalytic form of its high-affinity receptor, TrkB. Immunocytochemistry and in situ hybridization of sections revealed that the catalytic TrkB receptors predominantly localize to regions containing migratory cells. Many TrkB+ cells exhibited the classic morphology of migrating neurons, suggesting that TrkB ligands play a role in cortical neuronal migration. We analysed whether TrkB ligands influence the motility of embryonic cortical cells (from E15-E21) using a quantitative in vitro chemotaxis assay. High-affinity TrkB ligands (BDNF and NT4/5) stimulated chemotaxis (directed migration) of embryonic neurons at concentrations ranging from 1 to 100 ng/ml. NT-3, a low-affinity TrkB ligand, only stimulated significant migration at high concentrations (> or =100 ng/ml). Peak migration to BDNF was observed at gestational day 18 (E18). BDNF-induced chemotaxis was blocked by either tyrosine kinase inhibitor, K252a, or the Ca2+-chelator, BAPTA-AM, suggesting that BDNF-induces motility via autophosphorylation of TrkB receptor proteins and involves Ca2+-dependent mechanisms. BDNF-stimulation of increased cytosolic Ca2+ was confirmed with optical recordings of E18 cortical cells loaded with Ca2+ indicator dye. Thus, signal transduction through the TrkB receptor complex directs neuronal migration, suggesting that, in vivo, BDNF exerts chemotropic effects that are critical to morphogenesis of the cortex.

Novel differentially expressed genes induced by kainic acid in hippocampus: putative molecular effectors of plasticity and injury

Systemic kainic acid administration in rats induces acute limbic status epilepticus and subsequent neuronal degeneration and development of chronic hyperexcitability with similarities to human temporal lobe epilepsy. The mechanisms mediating the responses to kainic acid likely involve transcriptional changes in genes of importance for cellular injury, protection, and plasticity. We have used an arbitrarily primed PCR technique to identify such changes in the rat dentate gyrus. Three previously uncharacterized transcripts were found to be upregulated in the dentate gyrus 4 h following systemic kainic acid. In situ hybridization using riboprobes transcribed from the cloned PCR fragments were used to confirm differential expression specifically in dentate granule neurons following seizure. Basal expression for all three transcripts is widespread throughout the rat brain, with the highest levels seen in the hippocampal pyramidal and granule cell layers. The novel sequences do not match any known full-length cDNAs and may belong to novel gene families. However, they all showed high homology to human partial cDNA sequences (ESTs) that are expressed in brain as well as several other tissues. Two additional transcripts identified in this study corroborate earlier findings on differential expression of heat-shock proteins after seizure. The novel transcripts found in this study may be involved in epileptogenesis and neuronal responses to damage following seizure.

Survival, integration, and differentiation of neural stem cell lines after transplantation to the adult rat striatum

The in vivo properties of four different neural stem cell lines, generated from embryonic striatum or hippocampus by immortalization with the temperature-sensitive (s) A58/U19 allele of the SV40 Large T-antigen, have been studied with respect to their ability to survive, differentiate, and integrate after transplantation to the adult rat striatum. The cells were labeled with [3H]thymidine prior to grafting, and combined autoradiography and immunohistochemistry was used to characterize their phenotypic differentiation within the adult brain environment. The results show that all four types of cells survived well, up to at least 1.5-6 months postgrafting, without any signs of tissue perturbation or tumor formation. The cells underwent, on average, 2-3 cell divisions during the first 5 days after implantation and exhibited extensive migration over a distance of 1-1.5 mm from the injection site to become morphologically integrated with the surrounding host striatum. The cell number and tissue distribution attained by 2 weeks remained stable for up to 6 months postgrafting with the exception of one cell line, which showed a 40% loss of cells between 2 and 6 weeks. Twice the number of [3H]thymidine-labeled cells were recovered when the cells were grafted into a 1-week-old excitotoxic striatal lesion, probably due to an increased proliferation of the cells in response to the neuron-depleting depleting lesion. The immortalized cells behaved as multipotent neural progenitors. The vast majority of the cells developed a glial-like morphology, 6-14% being clearly GFAP-positive; however, a small but consistent proportion of them (1-3%) expressed MAP-2 and exhibited neuron-like morphology. In mature transplants about 75-80% of the grafted cells were located in the striatal grey matter, and 10-15% in white matter, some of which are proposed to have differentiated into oligodendrocytes. Remaining 5-10% occurred around small blood vessels (resembling pericytes) and in the subventricular zone underneath the ependyma of the lateral ventricle. It is concluded that the ts cell lines are highly suitable for intracerebral transplantation and that they allow the creation of a regionally confined cellular chimeras where the graft-derived glial cells become stably integrated with the resident glial cell matrix.

Id genes encoding inhibitors of transcription are expressed during in vitro astrocyte differentiation and in cell lines derived from astrocytic tumors

Id proteins belong to a class of nuclear transcription factors known as helix-loop-helix proteins. It has been reported that Id genes function as negative regulators of differentiation, and Id gene expression is down-regulated during cell differentiation. We examined the regulation of Id genes during astrocyte differentiation in a murine nervous system precursor cell line, NSEHip2-28, which is able to differentiate along the astroglial lineage, as well as in human astroglial tumor cell lines. Upon induction of NSEHip2-28 differentiation, at a time when glial fibrillary acidic protein expression became detectable, the expression of all four Id family members initially increased dramatically, and subsequently decreased. Furthermore, varying levels of Id gene expression were found in astroglial tumor cell lines displaying variable degrees of lineage-specific differentiation. These results suggest that the expression of Id family members may play an important role in the control of astrocyte differentiation.

Single factors direct the differentiation of stem cells from the fetal and adult central nervous system

Identifying the signals that regulate stem cell differentiation is fundamental to understanding cellular diversity in the brain. In this paper we identify factors that act in an instructive fashion to direct the differentiation of multipotential stem cells derived from the embryonic central nervous system (CNS). CNS stem cell clones differentiate to multiple fates: neurons, astrocytes, and oligodendrocytes. The differentiation of cells in a clone is influenced by extracellular signals: Platelet-derived growth factor (PDGF-AA, -AB, and -BB) supports neuronal differentiation. In contrast, ciliary neurotrophic factor and thyroid hormone T3 act instructively on stem cells to generate clones of astrocytes and oligodendrocytes, respectively. Adult stem cells had remarkably similar responses to these growth factors. These results support a simple model in which transient exposure to extrinsic factors acting through known pathways initiates fate decisions by multipotential CNS stem cells.

Neuronal progenitors as tools for cell replacement in the nervous system

The clinical prospect of using neural precursor cells for reconstructive approaches in the nervous system has received strong impetus from a recent series of important experimental findings. Transplantation studies in the developing brain have demonstrated that migration and differentiation of neural precursor cells are regulated predominantly by environmental signals. Several observations suggest that the mature CNS retains at least some of these guidance cues. These findings, together with recent evidence for the persistence of neural stem cells in the adult mammalian brain, have made precursor cell recruitment a new focus in CNS reconstruction.

Development of neuronal precursor cells and functional postmitotic neurons from embryonic stem cells in vitro

To understand the mechanism of the sequential restriction of multipotency of stem cells during development, we have established culture conditions that allow the differentiation of neuroepithelial precursor cells from embryonic stem (ES) cells. A highly enriched population of neuroepithelial precursor cells derived from ES cells proliferates in the presence of basic fibroblast growth factor (bFGF). These cells differentiate into both neurons and glia following withdrawal of bFGF. By further differentiating the cells in serum-containing medium, the neurons express a wide variety of neuron-specific genes and generate both excitatory and inhibitory synaptic connections. The expression pattern of position-specific neural markers suggests the presence of a variety of central nervous system (CNS) neuronal cell types. These findings indicate that neuronal precursor cells can be isolated from ES cells and that these cells can efficiently differentiate into functional post-mitotic neurons of diverse CNS structures.

Host-guided migration allows targeted introduction of neurons into the embryonic brain

The stereotyped positions occupied by individual classes of neurons are a fundamental characteristic of CNS cytoarchitecture. To study the regulation of neuronal positioning, we injected genetically labeled neural precursors derived from dorsal and ventral mouse forebrain into the telencephalic vesicles of embryonic rats. Cells from both areas were found to participate in the generation of telencephalic, diencephalic, and mesencephalic brain regions. Donor-derived neurons populated the host brain in distinct patterns and acquired phenotypic features appropriate for their final location. These observations indicate that neuronal migration and differentiation are predominantly regulated by non-cell-autonomous signals. Exploiting this phenomenon, intrauterine transplantation allows generation of controlled chimerism in the mammalian brain.

Cerebellar precursors transplanted to the neonatal dentate gyrus express features characteristic of hippocampal neurons

During the development of the CNS, a salient issue is whether neuronal phenotype is defined by the lineage or by the environment of precursor cells. Transplants permit these two possibilities to be tested, as cell fate can be examined in a new location. Dissociated cerebellar cells from newborn rats treated with tritiated thymidine or from NSE-lacZ transgenic mice were grafted into the dentate gyrus of the developing hippocampus. Implanted cells integrated into the granule cell layer, which contains the cell bodies of host granule neurons. Immunohistochemistry showed that grafted cells in the granule cell layer, like the host hippocampal granule neurons, were calbindin positive and upregulated FOS in a seizure paradigm. Electron microscopic analysis also showed that cells grafted to the dentate gyrus share features with host dentate neurons. These assays indicate that transplanted cerebellar cells acquired morphological and antigenic features characteristic of hippocampal neurons. These results show that metencephalic precursors are capable of differentiating in response to signals in the telencephalon, suggesting that the environment controls the regional fate of neuronal precursor cells during neurogenesis.

CNS-derived neural progenitor cells for gene transfer of nerve growth factor to the adult rat brain: complete rescue of axotomized cholinergic neurons after transplantation into the septum

A CNS-derived conditionally immortalized temperature-sensitive neural progenitor (CINP) cell line was used to generate NGF-secreting cells suitable for intracerebral transplantation. The cells were transduced by repeated retroviral infection, using a vector containing the mouse NGF cDNA under the control of the LTR promoter. Subcloning at the permissive temperature (33 degrees C) identified a highly NGF-secreting clone (NGF-CINP), which contained multiple copies of the transgene and released NGF at a rate of 2 ng/hr/10(5) cells in vitro, both at 33 and 37 degrees C, which was approximately 1 order of magnitude higher than what was possible to achieve in the heterogeneously infected cell cultures. After transplantation to the brain, the NGF-CINPs differentiated into cells with a predominant glia-like morphology and migrated for a distance of 1-1.5 mm from the implantation site into the surrounding host tissue, without any signs of overgrowth and tumor formation. Grafts of NGF-CINP cells implanted into the septum of adult rats with complete fimbria-fornix lesion blocked over 90% of the cholinergic cell loss in the medial septum and grafts placed in the intact striatum induced accumulation of low-affinity NGF receptor positive fibers around the implantation site. Expression of the NGF transgene in vivo was demonstrated by RT-PCR at 2 weeks after grafting. It is concluded that the immortalized neural progenitors have a number of advantageous properties that make them highly useful experimental tools for gene transfer to the adult CNS.

Functions of basic fibroblast growth factor and neurotrophins in the differentiation of hippocampal neurons

Restrictions in neuronal fate occur during the transition from a multipotential to a postmitotic cell. This and later steps in neuronal differentiation are determined by extracellular signals. We report that basic fibroblast growth factor is mitogenic for stem cells and is a differentiation factor for calbindin-expressing hippocampal neurons. The neurotrophin NT-3 is a differentiation factor for the same neurons but does not affect proliferation. NT-3 and brain-derived neurotrophic factor promote the maturation of neurons derived from stem cells that have been grown in vitro. These results define functions for basic fibroblast growth factor and neurotrophins in the differentiation processes that direct a multipotential stem cell to a specific neuronal fate.

Dopaminergic microtransplants into the substantia nigra of neonatal rats with bilateral 6-OHDA lesions. I. Evidence for anatomical reconstruction of the nigrostriatal pathway

Reconstruction of the nigrostriatal pathway by long axon growth derived from dopamine-rich ventral mesencephalic (VM) transplants grafted into the substantia nigra may enhance their functional integration as compared to VM grafts implanted ectopically into the striatum. Here we report on a novel approach by which fetal VM grafts are implanted unilaterally into the substantia nigra (SN) of 6-hydroxydopamine (6-OHDA)-lesioned neonatal pups at postnatal day 3 (P3) using a microtransplantation technique. The results demonstrate that homotopically placed dopaminergic neurons survive and integrate well into the previously 6-OHDA-lesioned neonatal SN region. Moreover, the tyrosine hydroxylase (TH)-positive neurons extended axons rostrally along the white matter tract of the internal capsule closely following the course of the original nigrostriatal pathway. The graft reestablished a TH-positive axon terminal network in the ipsilateral caudate-putamen, with the highest density in the medial and central parts. Retrograde labeling with Fluoro-Gold from the host striatum demonstrated that most of the transplant neurons giving rise to the graft-derived fiber outgrowth were TH-positive, but revealed also a small proportion of projecting neurons which were TH-negative. Amphetamine-induced striatal Fos expression was normalized in the caudate-putamen ipsilateral to the intranigral VM grafts, showing hyperexpression in some areas of the striatum, and the apomorphine-induced Fos expression seen in the 6-OHDA-lesioned animals was completely reversed on the grafted side. These findings indicate that the graft-derived dopaminergic reinnervation of the striatum is functional. The microtransplantation strategy may provide new avenues for the exploration of morphological and functional integration of fetal dopamine neurons in the nigrostriatal system and give new insights into the mechanisms controlling long-distance axon growth in the brain.

Re-expression of the intermediate filament nestin in reactive astrocytes

The intermediate filament nestin is highly expressed in multipotential stem cells of the developing central nervous system (CNS). During neuro- and gliogenesis, nestin is replaced by cell type-specific intermediate filaments, e.g. neurofilaments and glial fibrillary acidic protein (GFAP). In this study, we demonstrate that nestin expression is re-induced in reactive astrocytes in the lesioned adult brain. Following ischaemic and mechanical lesioning, a strong and sustained expression of nestin was noted in GFAP-positive cells surrounding the lesion site. Lesion experiments in transgenic mice carrying the lacZ gene under control of regulatory sequences from the nestin gene suggested that the upregulation of nestin in reactive astrocytes is mediated via the same sequences that control nestin expression during CNS development. These observations and recent data on the co-expression of glial and neuronal marker antigens in reactive astrocytes point to a close relationship between proliferating astrocytes and neuroepithelial precursor cells.

Dopaminergic regulation of transcription factor expression in organotypic cultures of developing striatum

Dopamine is a major neurotransmitter in neural systems innervating the striatum, and dopamine receptors are expressed during early pattern formation in the developing striatum. To test for the functional responsiveness of developing striatal neurons to dopaminergic stimulation, we established an organotypic slice culture of newborn rat striatum. We analyzed the effects of dopamine receptor agonists and of adenylate cyclase and protein kinase activation on striatal neurons by measuring the induction of Fos-like and Fra-like proteins in the cultured striatum. Fos-like and Fra-like proteins were induced in striatal neurons by activation of D1-like dopamine receptors but not by activation of D2-like receptors. The induction of Fos-like protein was mainly in striosomes and a medial compartment next to the ventricular zone, whereas Fra-like protein was induced in the striatal matrix as well. cAMP analogs and forskolin induced widespread expression of both Fos-like and Fra-like proteins. Our findings thus suggest that neurons of developing striosome and matrix compartments not only have different functional coupling of D1-like receptors to adenylate cyclase, but also have distinct maturational programs for dopaminergic regulation of individual transcription factors. Finally, despite evidence that protein kinase was involved in the induction of Fos-like protein, experiments with kinase inhibitors suggested that the induction of Fos-like protein had unusual pharmacological characteristics and raised the possibility that a novel protein kinase A-like molecule may have been involved in the induction. The cultured striatal slice preparation should provide a valuable tool for analyzing the molecular determinants of striatal development and function.

Morphological specification of trigeminal neurites depends on target fields

Primary sensory neurons bridge the sensory periphery to the central nervous system (CNS) via their two axonal processes. The morphological patterning of the terminals of each process in its respective target is unique. Whether the differences between peripheral and central axons result from an intrinsic developmental program of the ganglion cell body, or from target-derived signals is not known. To explore this issue, we have used an explant coculture system in which embryonic (E15) trigeminal ganglion explants were placed between a vibrissa pad and a brainstem explant, but the explants were rotated 180 degrees relative to their normal orientation. In other experiments, individual ganglia were placed between two vibrissa pad explants or between two slices taken through the brainstem. The cultures were fixed after several days and ganglion cell processes were labeled with the lipophilic tracer DiI. Results of the ganglion rotation experiments suggest that trigeminal axons which would be directed centrally in vivo can regenerate into peripheral targets, and peripheral axons can grow into CNS tissue. Similarly, in cocultures with two peripheral or two central targets, both processes of trigeminal ganglion cells can simultaneously invade vibrissa pad explants or project into brainstem slices. Moreover, in all cocultures the differentiation of each set of processes is specific to the target innervated by it. These results show that the axons of embryonic sensory neurons are not selective in their choice of targets, and that their morphological patterning is dictated by target-derived signals.

5-Hydroxytryptamine type 2A receptors regulate cyclic AMP accumulation in a neuronal cell line by protein kinase C-dependent and calcium/calmodulin-dependent mechanisms

The effects of 5-hydroxytryptamine (5-HT)2A receptor activation on cAMP formation were studied in a cell line derived from embryonic rat cortex (A1A1). 5-HT (EC50 = 0.87 microM) amplified the amount of cAMP formed in response to 5'-N-ethylcarboxamidoadenosine (an adenosine A2 receptor agonist), cholera toxin, and forskolin after 15 min of coincubation in the presence of the phosphodiesterase inhibitor rolipram. This effect of 5-HT was blocked by 10 nM ketanserin as well as by 10 nM spiperone, indicating a response mediated by the 5-HT2A receptor subtype. Similarly, cAMP accumulation was enhanced by coincubation with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and the calcium ionophore A23187. After exposure to PMA for 24 hr (PKC-depleted cells), 5-HT and A23187 still enhanced cAMP formed in response to forskolin and 5'-N-ethylcarboxamidoadenosine, whereas the amplifying effects of PMA were abolished. Analysis by Western blots and PKC activity measurements revealed that, of three PKC isoforms detected in A1A1 cells (alpha, delta, and epsilon), only the calcium-independent isoform PKC-epsilon remained in membrane fractions after long term PMA treatment. In PKC-depleted cells, 5-HT-mediated amplification was greatly reduced after treatment with the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl)-ester or the calmodulin antagonists calmidazolium and N-(6-aminohexyl)-5-chloro-1-napthalenesulfonamide hydrochloride. In addition, 5-HT-mediated amplification of cAMP accumulation was reduced by the PKC inhibitor staurosporine in normal cells but was unaffected in PKC-depleted cells. In conclusion, these data suggest that 5-HT2A receptor activation can amplify cAMP formation in A1A1 cells by two distinct pathways coupled to the hydrolysis of inositol phosphates, i.e., PKC and calcium/calmodulin.