Florence Rage

Physiology of BDNF: focus on hypothalamic function.

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family which interacts with high-affinity protein kinase receptors (Trk) and the unselective p75(NGFR) receptor. The BDNF gene has a complex structure with multiple regulatory elements and four promoters that are differentially expressed in central or peripheral tissue. BDNF expression is regulated by neuronal activity or peripheral hormones. Neurotrophins regulate the survival and differentiation of neurons during development but growing evidence indicates that they are also involved in several functions in adulthood, including plasticity processes. BDNF expression in the central nervous system (CNS) is modified by various kinds of brain insult (stress, ischemia, seizure activity, hypoglycemia, etc.) and alterations in its expression may contribute to some pathologies such as depression, epilepsy, Alzheimers, and Parkinsons disease. Apart from very traumatic situations, the brain functioning is resilient to stress and capable of adaptive plasticity. Neurotrophins might act as plasticity mediators enhancing this trait which seems to be crucial in adaptive processes. In addition to documenting all of the topics mentioned above in the CNS, we review the state of the art concerning neurotrophins and their receptors, including our personal contribution which is essentially focused on the stress response.

Age-related changes in brain-derived neurotrophic factor and tyrosine kinase receptor isoforms in the hippocampus and hypothalamus in male rats.

A large amount of aging individuals show diminished cognitive and endocrine capabilities. The main brain areas involved in these changes are the hippocampus and hypothalamus, two regions possessing high plasticity and implicated in cognitive and endocrine functions, respectively. Among neurotrophins (considered as genuine molecular mediators of synaptic plasticity), brain-derived neurotrophic factor (BDNF) exhibits in adult rats, the highest concentrations in the hippocampus and hypothalamus. Most of neuronal effects of BDNF are mediated through high-affinity cell surface BDNF tyrosine kinase receptors (TrkB). Different TrkB isoforms are issued by alternative splicing of mRNA encoding for TrkB (trkB mRNA) generating at least three different TrkB receptors with different signaling capabilities. The goal of this study was to examine simultaneously the expression (mRNAs and proteins) of BDNF and its three specific receptors, in the hippocampus and hypothalamus throughout lifespan in rats. We observed that BDNF essentially increased during the first 2 postnatal weeks in the hippocampus and hypothalamus, with no close correlation to its mRNA levels. In these regions, mRNA encoding for BDNF full-length catalytic receptor (trkB.FL mRNA) showed no important changes throughout life but of the mRNA truncated forms of TrkB receptors (trkB.T1 mRNA and trkB.T2 mRNA) trkB.T1 mRNA strongly increased after birth, then remaining stable during aging. trkB.T2 mRNA gradually decreased from 1 postnatal week becoming undetectable in the hippocampus in old-rats. Proteins issued from these mRNAs showed substantial quantitative modifications with aging. From 2 months old, the BDNF full-length catalytic receptor (TrkB.FL) gradually and significantly decreased in the hippocampus and the hypothalamus. Of the truncated forms of TrkB receptors (TrkB.T1 and TrkB.T2) TrkB.T1, which is essentially localized in glial cells, significantly increased from the first postnatal week in the hippocampus and in the hypothalamus, remaining stable during aging but reduced in old rats. TrkB.T2 which similarly to TrkB.FL has a neuronal localization also gradually decreased in the hippocampus and in the hypothalamus throughout lifespan. These reductions were significant at 21 and 30 days old, respectively. All the changes reported here could contribute to the reduced plasticity of these regions observed in old rats.

Neuroendocrine and Autonomous Mechanisms Underlying Thermoregulation in Cold Environment

This review focuses on the central regulation of thermoregulatory responses with special attention to the participation of thyrotropin-releasing hormone (TRH) in both autonomous and endocrine responses to a cold environment. Besides a direct projection of TRH neurons from paraventricular nuclei (PVN) to the median eminence, and the subsequent activation of the thyroid axis, there are direct projections from the PVN to the autonomic preganglionic neurons controlling autonomous responses. There projections convey information to peripheral targets involved in thermogenesis through the dorsal vagal complex and the spinal cord, for parasympathetic and sympathetic neurotransmissions respectively. Furthermore, cold exposure increases TRH mRNA levels in the PVN but also in dorsal motor and caudal raphe nuclei, thus providing strong evidence for a functional link between autonomous and neuroendocrine systems involved in thermoregulation. The review also focuses on neuroendocrine regulation of cold-induced TRH/TSH release associated with modifications in somatostatin release, with special reference to the participation of several central neurotransmitters (catecholamines, serotonin or GABA) or the influence of sex steroids.

Dendrites of Mammalian Neurons Contain Specialized P-Body-Like Structures That Respond to Neuronal Activation

Intracellular mRNA transport and local translation play a key role in neuronal physiology. Translationally repressed mRNAs are transported as a part of ribonucleoprotein (RNP) particles to distant dendritic sites, but the properties of different RNP particles and mechanisms of their repression and transport remain largely unknown. Here, we describe a new class of RNP-particles, the dendritic P-body-like structures (dlPbodies), which are present in the soma and dendrites of mammalian neurons and have both similarities and differences to P-bodies of non-neuronal cells. These structures stain positively for a number of P-body and microRNP components, a microRNA-repressed mRNA and some translational repressors. They appear more heterogeneous than P-bodies of HeLa cells, and they rarely contain the exonuclease Xrn1 but are positive for rRNA. These particles show motorized movements along dendrites and relocalize to distant sites in response to synaptic activation. Furthermore, Dcp1a is stably associated with dlP-bodies in unstimulated cells, but exchanges rapidly on neuronal activation, concomitantly with the loss of Ago2 from dlP-bodies. Thus, dlP-bodies may regulate local translation by storing repressed mRNPs in unstimulated cells, and releasing them on synaptic activation.

A single brain-derived neurotrophic factor injection modifies hypothalamo–pituitary–adrenocortical axis activity in adult male rats

Immobilization stress induces in adult male rats rapid activation of brain derived neurotrophic factor (BDNF) expression in the hypothalamic paraventricular nucleus (PVN) preceding the increases in corticotropin releasing hormone (CRH) and arginin-vasopressin (AVP) expression. The BDNF mRNA signal belatedly co-localizes with CRH and AVP mRNA signals in the PVN, as determined by in situ hybridization. Intracerebroventricular BDNF injections (5 microg/rat) in non-anesthetized adult male rats induce a gradual increase in the CRH mRNA signal whereas AVP mRNA signal progressively decreases in the parvocellular and magnocellular PVN portions. At the same time, the CRH hypothalamic content decreases while the AVP content increases. These variations are accompanied by increases in ACTH and corticosterone plasma concentrations. These results strongly suggest that BDNF could be a stress-responsive intercellular messenger since when it is exogenously administered acts as an important and early component in the activation and recruitment of hypothalamic CRH and AVP neurons.

Immobilization stress rapidly modulates BDNF mRNA expression in the hypothalamus of adult male rats

We demonstrated that short times (15 min) of immobilization stress application induced a very rapid increase in brain-derived neurotrophic factor (BDNF) mRNA expression in rat hypothalamus followed by a BDNF protein increase. The early change in total BDNF mRNA level seems to reflect increased expression of the BDNF transcript containing exon III, which was also rapidly (15 min) modified. The paraventricular and supraoptic nuclei, two hypothalamic nuclei closely related to the stress response and known to express BDNF mRNA, were analyzed by in situ hybridization following immobilization stress. In the parvocellular region of the paraventricular nucleus, BDNF mRNA levels increased very quickly as early as 15 min. In contrast, in the two other regions examined, the lateral and ventral magnocellular regions of the paraventricular nucleus, as well as in the supraoptic nucleus, signals above control were increased later, at 60 min. After stress application, plasma adrenocorticotropic hormone and corticosterone levels were strongly and significantly increased at 15 min. These studies demonstrated that immobilization stress challenge very rapidly enhanced BDNF mRNA levels as well as the protein, suggesting that BDNF may play a role in plasticity processes related to the stress response.

Effects of alcohol on brain-derived neurotrophic factor mRNA expression in discrete regions of the rat hippocampus and hypothalamus

Chronic alcohol consumption has adverse effects on the central nervous system, affecting some hippocampal and hypothalamic functions. In this study we tempted to demonstrate that some of these modifications could involve impairment of neurotrophic factors. Three experimental groups of male Sprague Dawley rats were studied: one control group, one chronically treated with alcohol vapor according to a well‐established model that induces behavioral dependence, and a third group treated similarly but killed 12 hr after alcohol withdrawal. In all groups, changes in brain‐derived neurotrophic factor mRNA expression occurring in the hippocampus and supraoptic nucleus were first analyzed by reverse transcription‐polymerase chain reaction and then by in situ hybridization. In parallel, we used ribonuclease protection assay to measure mRNA levels encoding trkB in the two central nervous system regions. We showed that chronic alcohol intoxication decreases brain‐derived neurotrophic factor mRNA expression in discrete regions of the rat hippocampus (CA1 region and dentate gyrus) and in the supraoptic nucleus of the hypothalamus. We also showed a global up‐regulation of trkB mRNA expression encoding the high‐affinity brain‐derived neurotrophic factor receptor (TrkB), after applying the same treatment. Following 12 hr of alcohol withdrawal, a significant increase in BDNF mRNA expression was observed in the dentate gyrus and CA3 region of hippocampus and in the hypothalamic supraoptic nucleus. These findings suggest that chronic alcohol intake may modify hippocampal and hypothalamic neuronal functions through modifications in growth factors and its receptors. J. Neurosci. Res. 63:200–208, 2001.

Impaired minor tri-snRNP assembly generates differential splicing defects of U12-type introns in lymphoblasts derived from a type I SMA patient

The survival of motor neuron (SMN) protein is essential for cytoplasmic assembly of spliceosomal snRNPs. Although the normal proportion of endogenous snRNAs is unevenly altered in spinal muscular atrophy (SMA) tissues, the biogenesis of individual snRNPs is not dramatically affected in SMN-deficient cells. The SMN protein is also required for normal Cajal body (CB) formation, but the functional consequences of CB disruption upon SMN deficiency have not yet been analyzed at the level of macromolecular snRNPs assembly. Here, we show that the SMN protein is required for tri-snRNPs formation and that the level of the minor U4atac/U6atac/U5 tri-snRNPs is dramatically decreased in lymphoblasts derived from a patient suffering from a severe form of SMA. We found also that splicing of some, but not all, minor introns is inhibited in these cells, demonstrating links between SMN deficiency and differential alterations of splicing events mediated by the minor spliceosome. Our results suggest that SMA might result from the inefficient splicing of one or only a few pre-mRNAs carrying minor introns and coding for proteins required for motor neurons function and/or organization.

Immobilization Stress Rapidly and Differentially Modulates BDNF and TrkB mRNA Expression in the Pituitary Gland of Adult Male Rats

Brain-derived neurotrophic factor (BDNF) is a neurotrophin involved in neuronal survival and plasticity that binds to high-affinity receptors named TrkB. In the central nervous system, brain insults, including stress, induce modifications in BDNF messenger RNA (mRNA) expression. The present study attempted to determine in the adult rat pituitary, a peripheral structure relevant for the stress response: (1) whether BDNF and TrkB mRNA expression is influenced by different durations (15, 30, 60, 180 and 300 min) of single immobilization stress; (2) the expression of BDNF transcripts containing the different exons and their possible variations after stress exposure. Plasma corticotropin (ACTH) and corticosterone concentrations were strongly and significantly increased as early as 5 min after the stress stimulus. Using RNAse protection assay and in situ hybridization, a rapid increase in BDNF mRNA occurred at 15 min. This was accompanied by an increase in BDNF protein at 60 min, and by a rapid and significant decrease in TrkB mRNA expression observed at 15 and 30 min after stress application. RT-PCR analysis of BNDF transcripts showed strong basal expression of exons III and IV, whereas transcripts containing exons I and II seemed weakly expressed. After stress application, transcripts containing exons III and IV were rapidly and significantly increased at 30 min, whereas transcripts containing exons I and II remained unchanged. These results show that pituitary BDNF transcripts expression is differentially affected by immobilization stress.

Expression of mRNAs encoding BDNF and its receptor in adult rat hypothalamus.

WE used a digoxigenin-UTP-labeled cRNA probe with in situ hybridization and Northern blot analysis to investigate the localization of brain derived neurotrophic factor (BDNF) mRNA and expression of its different transcripts in adult rat hypothalamus. As the BDNF gene is under the control of alternative multiple promoters, which provide tissue-specific gene expression, we studied whether these transcripts were expressed in adult hypothalamus. Our data revealed two novel sources of hypothalamic BDNF mRNA: the supraoptic and periventricular nuclei. In addition, we observed the expression of transcripts from exons I, II and III as well as the presence of 1.6 and 4.2 kb BDNF mRNAs. Finally, our findings confirmed expression of mRNA encoding neurotrophins receptors in the hypothalamus.