Combined neonatal stress and young-adult glucocorticoid stimulation in rats reduce BDNF expression in hippocampus: effects on learning and memory.

Epidemiological studies suggest that multiple developmental disruptions are involved in the etiology of psychiatric illnesses including schizophrenia. In addition, altered expression of brain-derived neurotrophic factor (BDNF) has been implicated in these illnesses. In the present study, we examined the combined long-term effect of an early stress, in the form of maternal deprivation, and a later stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone, on BDNF expression in the hippocampus of rats. To assess whether there were behavioral effects, which may correlate with the BDNF changes, learning and memory was tested in the Y-maze test for short term spatial memory, the Morris water maze for long-term spatial memory, and the T-maze test for working memory. Four groups of rats received either no stress, maternal deprivation, corticosterone treatment, or both. Dorsal hippocampus sections obtained from parallel groups were used for BDNF mRNA in situ hybridization. Rats which had undergone both maternal deprivation and corticosterone treatment displayed a unique and significant 25-35% reduction of BDNF expression in the dentate gyrus (DG), and similar trends in the CA1 and CA3 regions of the hippocampus. These "two-hit" animals exhibited a learning delay in the Morris water maze test, a marked deficit in the Y-maze, but little change in the T-maze test. However, some aspects of cognition were also altered in rats with either maternal deprivation or corticosterone treatment. This study demonstrates a persistent effect of two developmental disruptions on BDNF expression in the hippocampus, with parallel, but not completely correlative changes in learning and memory.

c-Myb is required for neural progenitor cell proliferation and maintenance of the neural stem cell niche in adult brain.

Ongoing production of neurons in adult brain is restricted to specialized neurogenic niches. Deregulated expression of genes controlling homeostasis of neural progenitor cell division and/or their microenvironment underpins a spectrum of brain pathologies. Using conditional gene deletion, we show that the proto-oncogene c-myb regulates neural progenitor cell proliferation and maintains ependymal cell integrity in mice. These two cellular compartments constitute the neurogenic niche in the adult brain. Brains devoid of c-Myb showed enlarged ventricular spaces, ependymal cell abnormalities, and reduced neurogenesis. Neural progenitor cells lacking c-Myb showed a reduced intrinsic proliferative capacity and reduction of Sox-2 and Pax-6 expression. These data point to an important role for c-Myb in the neurogenic niche of the adult brain.

Differential effects of SOCS2 on neuronal differentiation and morphology.

Neuronal differentiation of neural progenitor cells is regulated by a variety of growth and transcription factors, that not only regulate cell fate of the progenitor cells but that can also regulate neuronal morphology. Suppressor of cytokine signaling-2 (SOCS2) is an intracellular regulator of Growth Hormone (GH) signaling that is expressed in neural stem cells and neurons during development and is required to overcome the inhibitory effects of GH on neuronal differentiation. SOCS2 also promotes neurite outgrowth, however, whether the mechanism by which SOCS2 regulates neuronal differentiation and neurite outgrowth is the same is not clear. Furthermore, whether the over-expression of SOCS2 has physiological in addition to morphological effects is unknown. To address these questions, we differentiated adult neural progenitor cells derived from wildtype C57BL/6 or SOCS2 over-expressing transgenic mice (SOCS2Tg) in the presence or absence of GH and determined effects on neuronal differentiation and morphology. Compared to wildtype cells, differentiation of SOCS2Tg neurospheres resulted in increased neurogenesis, which was not inhibited by GH. The neurons derived from these cells appeared more complex, with increased neurite outgrowth and number. GH did not, however, have any effect on neurite outgrowth of wildtype or SOCS2Tg neurons. Furthermore, basic electrophysiological analysis of wildtype and SOCS2Tg neurons derived from the neurospheres showed that they were both of an immature electrophysiological neuronal phenotype, indicating that although SOCS2 expression can regulate neuronal morphology, it appears to have little effect on neuronal ion channel expression.

Glucocorticoid regulation of glial responses during hippocampal neurodegeneration and regeneration.

Glucocorticoids can prevent or accelerate neurodegeneration in the adult rat hippocampus. To investigate these actions of glucocorticoids, we previously cloned genes from the hippocampus. Adrenalectomy specifically increased glial fibrillary acidic protein and transforming growth factor (TGF)-beta1 mRNAs in the dentate gyrus and these effects were dependent on induced apoptosis. Corticosterone treatment prevented apoptosis, and decreased glial activation and the influx of activated microglia. Since these effects are opposite to injury and neurodegeneration, we propose that they represent adaptive actions of glucocorticoids, preventing cellular defense mechanisms from overshooting. We used adrenalectomy as a model to investigate how adult granule neurons die in vivo and the effects of neurotrophic factors in protecting against apoptosis. Neurotrophin-4/5 and TGF-beta1 protected granule neurons against adrenalectomy-induced apoptosis. Since neurogenesis is also greatly increased in the dentate gyrus following adrenalectomy, we compared the time course of birth and death with glial responses. TGF-beta1 mRNA increased before the detection of dying cells in the dentate gyrus, which was coincident with increased proliferation in the neurogenic zone. Glucocorticoids also increased Ndrg2 mRNA in glia in the neurogenic zone; Ndrg2 is a member of a novel gene family involved in neural differentiation and synapse formation. Therefore, studying the effects of glucocorticoid manipulation on the dentate gyrus is increasing our understanding of how mature neurons die by apoptosis and the role of glia in induced apoptosis and neurogenesis. Discovering how endocrine and inflammatory responses regulate neuron birth and survival is important for developing successful neuron replacement strategies to treat neurodegenerative diseases.

Ndrg2, a novel gene regulated by adrenal steroids and antidepressants, is highly expressed in astrocytes.

Previously, we cloned a gene from rat hippocampus that now shows homology to Ndrg2, a member of the N-myc downregulated gene (NDRG) family with putative roles in neural differentiation, synapse formation, and axon survival. Following adrenalectomy, hippocampal Ndrg2 mRNA increased in response to glucocorticoids. Ndrg2 mRNA was also upregulated by corticosterone in cerebral cortex and heart. Since Ndrg2 mRNA increased in response to glucocorticoid treatment of cultured astrocytes, we examined its cellular localization in adult brain by in situ hybridization. Ndrg2 mRNA is a prevalent message that is widely expressed throughout the brain, but is more abundant in gray matter than in white matter. Predominant mRNA expression was found in neurogenic regions of the adult brain. Furthermore, Ndrg2 mRNA in these regions was localized to GFAP-positive astrocytes or radial glia. In one of these regions, the subgranular zone of the dentate gyrus, Ndrg2 expression was decreased after adrenalectomy, and was restored to sham-operated levels by corticosterone, indicating that it is under positive regulation by glucocorticoids in vivo. Recently, another group reported that Ndr2/Ndrg2 transcripts in rat frontal cortex were decreased by chronic antidepressant treatment. Because antidepressants may alleviate symptoms of depression by reversing the effects of glucocorticoids, these data suggest that further study of Ndrg2 regulation and function in glia could contribute to understanding the pathogenesis and treatment of depression.

Acute intrarenal infusion of ANG II does not stimulate immediate early gene expression in the kidney.

ANG II is capable of stimulating expression of immediate early genes such as egr-1 and c-fos in a variety of cultured cells, including cells of renal origin. To investigate whether ANG II can stimulate early growth response gene expression in vivo, we studied the effects of acute renal artery infusion of low-dose ANG II (2.5 ng small middle dot kg(-1) small middle dot min(-1)) or vehicle on the renal expression of c-fos and egr-1 genes in rats. ANG II infusion for 30 or 240 min decreased renal vascular conductance by approximately 13 and 8%, respectively, compared with the vehicle group. Expression of the early growth response genes c-fos and egr-1 was analyzed using Northern blot hybridization. No significant upregulation of c-fos or egr-1 mRNA levels was detected in rats that received ANG II for either 30 or 240 min, compared with the vehicle groups. We conclude that ANG II, at doses that cause significant physiological effects, does not increase the renal expression of c-fos or egr-1 genes over periods of up to 4 h in vivo.