Opposite effects of astrocyte-derived soluble factor(s) on the functional expression of fetal peptidergic neurons in aggregate cultures: enhancement of neuropeptide Y and suppression of somatostatin

Continuous exposure to brain-derived neurotrophic factor is required for persistent activation of TrkB receptor, the ERK signaling pathway, and the induction of neuropeptide Y production in cortical cultures

We have previously demonstrated that brain-derived neurotrophic factor (BDNF) induces persistent neuropeptide Y (NPY) production in cortical cultures in an ERK1/2-dependent manner. In some studies, it was shown that BDNF leads to the downregulation of TrkB receptor and some of its downstream responses, whereas in others it does not. We examined whether the BDNF requirement for induction of persistent NPY production correlates with that for induction of phosphorylation of TrkB and ERK1/2. Continuous 24-h exposure to BDNF led to a 2- to 3-fold increase in NPY production (maximal level). While 1 h of BDNF exposure induced NPY production at a half maximal level, 8 h was required for induction of a maximal level. BDNF-induced NPY production was completely inhibited by co-exposure to TrkB-Fc fusion protein (TrkB extracellular domain fused to Fc) and partially inhibited by TrkB-Fc added 1 h after BDNF; TrkC-Fc did not do so. Activation of TrkB receptor was analyzed at two potential tyrosine phosphorylated sites, the activation loop and the Shc binding. BDNF led to coordinated phosphorylation of the two sites that persisted for 6-8 h, and this was not associated with changes in the content of TrkB protein. The presence of BDNF throughout the 6- to 8-h period was required for the persistent phosphorylation of TrkB and ERK1/2. Thus, continuous BDNF activation of TrkB is required for persistent activation of the ERK1/2 pathway and induction of NPY production. We propose that, within the time frame analyzed in this study, BDNF does not lead to the downregulation of TrkB receptor or of the biological responses leading to NPY production.

Development of pro-TRH gene expression in primary cultures of fetal hypothalamic cells

Little is known about the temporal relationship and the sequential steps for peptide biosynthesis during the terminal differentiation of the peptide phenotype in central nervous system. Analysis of the TRH phenotype in primary cultures of rat fetal day 17 hypothalamic cells has shown that TRH levels start increasing only after a week in culture, in contrast with in vivo data showing a steady increase during late fetal life. The purpose of this study was to compare the developmental patterns of TRH and pro-TRH mRNA levels in vitro to determine whether the initial low and steady levels of TRH are due to deficient transcription. Pro-TRH mRNA levels were detected by semi-quantitative RT-PCR through the development of primary cultures of serum-supplemented hypothalamic fetal cells from 17 day old embryos. Pro-TRH mRNA levels per dish increased steadily since the beginning of the culture. In contrast, TRH levels per dish were low and stable during the first week increasing afterwards, but remaining low compared to equivalent in vivo values. Pro-TRH mRNA levels per hypothalamus increased between fetal day 17 and postnatal 14, suggesting that the in vitro pattern of pro-TRH mRNA development mimics that occurring in vivo. These data show that pro-TRH gene expression does not limit TRH accumulation in vitro suggesting that the transcriptional and post-transcriptional programs leading to peptide accumulation are established independently.

Interleukin-1beta induces expression of neuropeptide Y in primary astrocyte cultures in a cytokine-specific manner: induction in human but not rat astrocytes

Previous studies have demonstrated that astrocyte cultures express neuropeptide Y (NPY) in a regulated manner, namely, phorbol ester leads to an increase in proNPY-mRNA and NPY production. In this respect, the behavior of astrocytes derived from the human fetal or rat neonatal brain is similar (Regul. Pept. 75 (1998) 293). Since astrocytes can be exposed to high levels of IL-1beta, we addressed the question: Does IL-1beta regulate NPY expression by the astrocytes? Primary astrocytes derived from the human fetal or rat neonatal cortex were cultured in serum-free medium. IL-1beta, but not IL-6 or TNF-alpha, led to an increase in NPY production dose-dependently. IL-1beta action manifested in the human but not in the rat astrocytes and it was completely abolished by IL-1 receptor antagonist. The responsiveness to IL-1beta did not diminish upon sub-culture of the astrocytes (five passages). In addition, IL-1beta led to an increase in the abundance of proNPY-mRNA, which was preceded by a rapid and transient increase in cFos-mRNA and a rapid and sustained increase in JunB-mRNA. In contrast to cFos/JunB, IL-1beta did not alter the abundance of cJun-mRNA. In summary, we demonstrate that IL-1beta induction of NPY expression in astrocytes is species- and cytokine-specific and that IL-1 receptor is involved. Moreover, induction of NPY expression is preceded by a rapid increase in the expression of two transcription factors (cFos, JunB) that have been previously (Oncogene 9 (1994) 2369; J. Neurochem. 70 (1998) 1887) implicated in transcriptional regulation of the human NPY gene.

An improved method for dissociation and aggregate culture of human fetal brain cells in serum-free medium

Moscona, in the early sixties [A.A. Moscona, Recombination of dissociated cells and the development of cell aggregates, in: B.M. Willmer (Ed.), Cells and Tissues in Culture, Academic Press, New York, 1965, pp. 489-529.] [16], discovered that aggregation of dissociated cells is a property of embryonal cells. Several features of the aggregate culture system are particularly attractive for the conduct of biochemical and molecular studies on the human fetal brain. (i) All the pertinent procedural parameters can be readily controlled and standardized, resulting in a consistently reproducible system suitable for quantitative analyses. (ii) Neuronal enriched aggregates can be readily obtained, with minimal neurotoxicity. (iii) Aggregates can be easily harvested for biochemical and molecular studies. Aggregate cultures, generated from rodent fetal brains, have been extensively utilized as a tool to study regulation of aminergic neurons [P. Honegger, E. Richelson, Biochemical differentiation of mechanically dissociated brain in aggregating cell culture, Brain Res. 109 (1976) 335-354; P. Honegger, E. Richelson, Biochemical differentiation of aggregating cell cultures of different fetal rat brain regions, Brain Res. 133 (1977) 329-339.] [11,12] and peptidergic neurons (neuropeptide Y (NPY) and somatostatin (SRIF) [A. Barnea, E. Anthony, G. Lu, G. Cho, Morphological differentiation of neuropeptide Y neurons in aggregate cultures of dissociated fetal cortical cells: a model system for glia-neuron paracrine interactions, Brain Res. 625 (1993) 313-322; A. Barnea, G. Cho, G. Lu, M. Mathis, Brain-derived neurotrophic factor induces functional expression and phenotypic differentiation of cultured fetal neuropeptide Y producing neurons, J. Neurosci. Res. 42 (1995) 638-647; A. Barnea, A. Hajibeigi, G. Cho, P. Magni, Regulated production and secretion of immunoreactive neuropeptide Y by aggregating fetal brain cells in culture, Neuroendocrinology 54 (1991) 7-13; P. Magni, A. Barnea, Forskolin and phorbol ester stimulation of neuropeptide Y (NPY) production and secretion by aggregating fetal brain cells in culture: evidence for regulation of NPY biosynthesis at transcriptional and posttranscriptional levels, Endocrinology 130 (1992) 976-984.]) [4-6,14]. However, very few studies have utilized this system to study regulatory processes of human fetal neurons/glia [M. McCarthy, L. Resnik, F. Taub, R.V. Stewart, R.D. Dix, Infection of human neural cell aggregate cultures with a clinical isolate of cytomegalovirus, J. Neuropathol. Exp. Neurol. 50 (1991) 441-450; L. Pulliam, M.E. Berens, M.L. Rosenblum, A normal human brain cell aggregate model for neurobiological studies, J. Neurosci. Res. 21 (1988) 521-530.] [15,17]. In a series of studies in our laboratory [N. Aguila-Mansilla, A. Barnea, Human fetal brain cells in aggregate culture: a model system to study regulatory processes of the developing human neuropeptide Y (NPY) producing neuron, Int. J. Dev. Neurosci. 14 (1996) 531-539; A. Barnea, N. Aguila-Mansilla, H.T. Chute, A.A. Welcher, Comparison of neurotrophin regulation of human and rat neuropeptide Y (NPY) neurons: induction of NPY production in aggregate cultures derived from rat but not from human fetal brains, Brain Res. 732 (1996) 52-60; A. Barnea, N. Aguila-Mansilla, G. Lu, R.H. Ho, Opposite effects of astrocyte-derived soluble factor(s) on the functional expression of fetal peptidergic neurons in aggregate cultures: enhancement of neuropeptide Y and suppression of somatostatin, J. Neurosci. Res. 50 (1997) 605-617; A. Barnea, J. Roberts, R.H. Ho, Evidence for a synergistic effect of the HIV-1 envelope protein gp120 and brain-derived neurotrophic factor (BDNF) leading to enhanced expression of somatostatin neurons in aggregate cultures derived from the human fetal cortex, Brain Res. 815 (1999) 349-357.] [1-3,7], we have established a human-derived aggregate culture system, maintained in serum-free medium for up to 28 days, in which expression

Evidence for a synergistic effect of the HIV-1 envelope protein gp120 and brain-derived neurotrophic factor (BDNF) leading to enhanced expression of somatostatin neurons in aggregate cultures derived from the human fetal cortex

Changes in the expression of somatostatin (SRIF) have been observed in the brains of HIV encephalitis. Since gp120 is thought to play a major role in AIDS-associated abnormalities in the brain, we addressed the question: Does gp120 alter the functional expression of human fetal SRIF neurons in culture and if so, is this effect fetal-age dependent? Aggregate cultures, obtained from cortices of nine fetuses (14.9-20.7 weeks), were exposed for 7 days to BDNF or BDNF+gp120; BDNF induced production of SRIF during the subsequent 24-48 h was assessed. Similar effects of BDNF and gp120 were observed in the 9 brain-cultures. A 7-day exposure to BDNF alone led to a significant increase in SRIF production (p=0.014), whereas exposure to gp120 alone did not. Co-exposure to BDNF and gp120 led to an increase in BDNF-induced SRIF production which was significantly greater than that after BDNF alone (p=0.006). These effects were BDNF- and gp120-dose dependent and they were not accompanied by changes in DNA content of the aggregates nor in lactate dehydrogenase activity in the medium; indicating that gp120 did not lead to a major loss of cell integrity. These results are consistent with a synergistic effect of BDNF and gp120 leading to enhanced functional expression of the signalling pathway(s) mediating BDNF induction of SRIF production; an effect expressed by fetal brains throughout the 2nd trimester of gestation. Thus, this culture system can serve as a model to study the mechanism(s) underlying the early interactions between gp120 BDNF in the developing human brain.

Changes in GAD- and GABA- immunoreactivity in the spinal dorsal horn after peripheral nerve injury and promotion of recovery by lumbar transplant of immortalized serotonergic precursors

We have utilized RN46A cells, an immortalized neuronal cell line derived from E13 brainstem raphe, as a model for transplant of bioengineered serotonergic cells. RN46A cells require brain-derived neurotrophic factor (BDNF) for increased survival and serotonin (5HT) synthesis in vitro and in vivo. RN46A cells were transfected with the rat BDNF gene, and the 46A-B14 cell line was subcloned. These cells survive longer than 7 weeks after transplantation into the subarachnoid space of the lumbar spinal cord and synthesize 5HT and BDNF. Chronic constriction injury (CCI) of the sciatic nerve was used to induce chronic neuropathic pain in the affected hindpaw in rats. Transplants of 46A-B14 cells placed 1 week after CCI alleviated chronic neuropathic pain, while transplants of 46A-V1 control cells, negative for 5HT and without the BDNF gene, had no effect on the induction of thermal and tactile nociception. When endogenous cells of the dorsal horn which contain the neurotransmitter gamma-aminobutyric acid (GABA) and its synthetic enzyme glutamate decarboxylase (GAD) were immunohistochemically quantified in the lumbar spinal cord 3 days and 1-8 weeks after CCI, the number of GABA- and GAD-immunoreactive (ir) cells decreased bilateral to the nerve injury as soon as 3 days after CCI. At 1 week after CCI, the number of GABA-ir cells continued to significantly decline bilaterally, returning to near normal numbers on the side contralateral to the nerve injury by 8 weeks after the nerve injury. The number of GAD-ir cells began to increase bilaterally to the nerve injury at 1 week after CCI and continued to significantly increase in numbers over normal values by 8 weeks after the nerve injury. When examined 2 and 8 weeks after CCI plus cell transplants, the transplants of 46A-B14 cells reversed the increase in GAD-ir cell numbers and the decrease in GABA-ir cells by 1 week after transplantation, while 46A-V1 control cell transplants after CCI had no effect on the changes in numbers of GAD-ir or GABA-ir cells. Collectively, these data suggest that altered 5HT levels, and perhaps BDNF secretion, related to the transplants ameliorate chronic pain and reverse the induction and maintenance of an endogenous pain mechanism in the dorsal horn. This induction mechanism is likely dependent on altered GAD regulation and GABA synthesis, initiated by CCI.

Evidence for regulated expression of neuropeptide Y gene by rat and human cultured astrocytes

A series of studies from our laboratory have established that fetal rat and human neuropeptide Y (NPY) cortical neurons in aggregate cultures are differentially regulated. In a preliminary study we found that primary astrocytes produce substantial amounts of immunoreactive (IR) NPY. We addressed the question: Is astrocyte production of NPY-IR a regulated process? The effects of brain-derived neurotrophic factor (BDNF, 50 ng/ml), basic fibroblast growth factor (bFGF), substance P (1 microM), forskolin (10 microM), or phorbol 12-myristate-13-acetate (PMA, 20 nM) on NPY-IR production was tested on rat and human primary astrocyte cultures. Of these agents, PMA and bFGF markedly induced NPY-IR production by rat as well as human astrocytes, forskolin induced NPY-IR production by human but not rat astrocytes, and neither BDNF nor substance P induced NPY-IR production by rat or human astrocytes. The molecular size of PMA-induced NPY-IR was found to be consistent with that of proNPY. Moreover, PMA induced the accumulation of mRNA corresponding in size to the neuronal NPY-mRNA. Immunocytochemical analysis of human post-mortem neocortex revealed co-existence of NPY-IR with astrocyte markers. These results indicate that cultured astrocytes express NPY gene in a regulated manner and they support our proposition that in situ reactive astrocytes may express NPY gene under some physiological/pathological conditions.