Jean-Christophe Deloulme

S100B expression defines a state in which GFAP-expressing cells lose their neural stem cell potential and acquire a more mature developmental stage

During the postnatal development, astrocytic cells in the neocortex progressively lose their neural stem cell (NSC) potential, whereas this peculiar attribute is preserved in the adult subventricular zone (SVZ). To understand this fundamental difference, many reports suggest that adult subventricular GFAP‐expressing cells might be maintained in immature developmental stage. Here, we show that S100B, a marker of glial cells, is absent from GFAP‐expressing cells of the SVZ and that its onset of expression characterizes a terminal maturation stage of cortical astrocytic cells. Nevertheless, when cultured in vitro, SVZ astrocytic cells developed as S100B expressing cells, as do cortical astrocytic cells, suggesting that SVZ microenvironment represses S100B expression. Using transgenic s100b‐EGFP cells, we then demonstrated that S100B expression coincides with the loss of neurosphere forming abilities of GFAP expressing cells. By doing grafting experiments with cells derived from β‐actin‐GFP mice, we next found that S100B expression in astrocytic cells is repressed in the SVZ, but not in the striatal parenchyma. Furthermore, we showed that treatment with epidermal growth factor represses S100B expression in GFAP‐expressing cells in vitro as well as in vivo. Altogether, our results indicate that the S100B expression defines a late developmental stage after which GFAP‐expressing cells lose their NSC potential and suggest that S100B expression is repressed by adult SVZ microenvironment.

Disruption of CK2beta in embryonic neural stem cells compromises proliferation and oligodendrogenesis in the mouse telencephalon.

ABSTRACT Genetic programs that govern neural stem/progenitor cell (NSC) proliferation and differentiation are dependent on extracellular cues and a network of transcription factors, which can be regulated posttranslationally by phosphorylation. However, little is known about the kinase-dependent pathways regulating NSC maintenance and oligodendrocyte development. We used a conditional knockout approach to target the murine regulatory subunit (beta) of protein kinase casein kinase 2 (CK2β) in embryonic neural progenitors. Loss of CK2β leads to defects in proliferation and differentiation of embryonic NSCs. We establish CK2β as a key positive regulator for the development of oligodendrocyte precursor cells (OPCs), both in vivo and in vitro. We show that CK2β directly interacts with the basic helix-loop-helix (bHLH) transcription factor Olig2, a critical modulator of OPC development, and activates the CK2-dependent phosphorylation of its serine-threonine-rich (STR) domain. Finally, we reveal that the CK2-targeted STR domain is required for the oligodendroglial function of Olig2. These findings suggest that CK2 may control oligodendrogenesis, in part, by regulating the activity of the lineage-specific transcription factor Olig2. Thus, CK2β appears to play an essential and uncompensated role in central nervous system development.

Myelin basic protein functions as a microtubule stabilizing protein in differentiated oligodendrocytes.

Myelin basic protein (MBP) is an oligodendrocyte‐specific protein essential for oligodendrocyte morphogenesis at late stages of cell differentiation. There is evidence that the morphogenetic function of MBP is mediated by MBP interaction with the cytoskeleton. Thus, an MBP/cytoplasmic microtubule association has been reported, and MBP has Ca2+/calmodulin‐regulated microtubule cold‐stabilizing activity in vitro. However, the unambiguous demonstration of a microtubule‐stabilizing activity for MBP in cells has been difficult because oligodendrocytes contain variants of STOP (stable tubule only polypeptide) proteins, which are responsible for microtubule cold stability in different cell types. Herein, we have used genetic mouse models and RNA interference to assay independently the microtubule cold‐stabilizing activities of MBP and of STOP in developing oligodendrocytes. In wild‐type oligodendrocytes, microtubules were cold stable throughout maturation, which is consistent with the presence of STOP proteins from early stages of differentiation. In contrast, in oligodendrocytes from STOP‐deficient mice, microtubules were cold labile in the absence of MBP expression or when MBP expression was restricted to the cell body and became stable in fully differentiated oligodendrocytes, where MBP is expressed in cell extensions. The suppression of MBP by RNA interference in STOP‐deficient oligodendrocytes suppressed microtubule cold stability. Additionally, STOP suppression in oligodendrocytes derived from shiverer mice that lack MBP led to the complete suppression of microtubule cold stability at all stages of cell differentiation. These results demonstrate that both STOP and MBP function as microtubule‐stabilizing proteins in differentiating oligodendrocytes and could be important for the morphogenetic function of MBP.

Peripheral T Lymphocytes Recirculating Back into the Thymus Can Mediate Thymocyte Positive Selection

The thymus continuously produces T lymphocytes that contribute to the maintenance of the peripheral T cell pool. Since peripheral recirculating T cells represent a very minor population among total thymocytes in normal animals, the relationship between the thymus and secondary lymphoid organs is generally considered unidirectional. Recently, several reports have described the presence of recirculating T cells in the thymus, raising issues regarding their possible function. In this article, we show that the niche for recirculating T cells in the thymus, i.e., their absolute number, is the same in lymphopenic and normal mice. Using a novel combination of TCR-transgenic mice in which the ligand necessary for positive selection of host T cells is only expressed by transferred donor T cells, we show that mature T cells recirculating back to the thymus can mediate positive selection.

The antidepressant hyperforin increases the phosphorylation of CREB and the expression of TrkB in a tissue-specific manner.

Hyperforin is one of the main bioactive compounds that underlie the antidepressant actions of the medicinal plant Hypericum perforatum (St. Johns wort). However, the effects of a chronic hyperforin treatment on brain cells remains to be fully addressed. The following study was undertaken to further advance our understanding of the biological effects of this plant extract on neurons. Special attention was given to its impact on the brain-derived neurotrophic factor (BDNF) receptor TrkB and on adult hippocampal neurogenesis since they appear central to the mechanisms of action of antidepressants. The consequences of a chronic hyperforin treatment were investigated on cortical neurons in culture and on the brain of adult mice treated for 4 wk with a daily injection (i.p.) of hyperforin (4 mg/kg). Its effects on the expression of the cyclic adenosine monophosphate response element-binding protein (CREB), phospho-CREB (p-CREB), TrkB and phospho-TrkB (p-TrkB) were analysed by Western blot experiments and its impact on adult hippocampal neurogenesis was also investigated. Hyperforin stimulated the expression of TRPC6 channels and TrkB via SKF-96365-sensitive channels controlling a downstream signalling cascade involving Ca(2+), protein kinase A, CREB and p-CREB. In vivo, hyperforin augmented the expression of TrkB in the cortex but not in the hippocampus where hippocampal neurogenesis remained unchanged. In conclusion, this plant extract acts on the cortical BDNF/TrkB pathway leaving adult hippocampal neurogenesis unaffected. This study provides new insights on the neuronal responses controlled by hyperforin. We propose that the cortex is an important brain structure targeted by hyperforin.

The deletion of STOP/MAP6 protein in mice triggers highly altered mood and impaired cognitive performances.

J. Neurochem. (2012) 121, 99–114.

Microtubule-associated protein 6 mediates neuronal connectivity through Semaphorin 3E-dependent signalling for axonal growth

Structural microtubule associated proteins (MAPs) stabilize microtubules, a property that was thought to be essential for development, maintenance and function of neuronal circuits. However, deletion of the structural MAPs in mice does not lead to major neurodevelopment defects. Here we demonstrate a role for MAP6 in brain wiring that is independent of microtubule binding. We find that MAP6 deletion disrupts brain connectivity and is associated with a lack of post-commissural fornix fibres. MAP6 contributes to fornix development by regulating axonal elongation induced by Semaphorin 3E. We show that MAP6 acts downstream of receptor activation through a mechanism that requires a proline-rich domain distinct from its microtubule-stabilizing domains. We also show that MAP6 directly binds to SH3 domain proteins known to be involved in neurite extension and semaphorin function. We conclude that MAP6 is critical to interface guidance molecules with intracellular signalling effectors during the development of cerebral axon tracts.

Neuronal transport defects of the MAP6 KO mouse - a model of schizophrenia - and alleviation by Epothilone D treatment, as observed using MEMRI.

The MAP6 (microtubule-associated protein 6) KO mouse is a microtubule-deficient model of schizophrenia that exhibits severe behavioral disorders that are associated with synaptic plasticity anomalies. These defects are alleviated not only by neuroleptics, which are the gold standard molecules for the treatment of schizophrenia, but also by Epothilone D (Epo D), which is a microtubule-stabilizing molecule. To compare the neuronal transport between MAP6 KO and wild-type mice and to measure the effect of Epo D treatment on neuronal transport in KO mice, MnCl2 was injected in the primary somatosensory cortex. Then, using manganese-enhanced magnetic resonance imaging (MEMRI), we followed the propagation of Mn(2+) through axonal tracts and brain regions that are connected to the somatosensory cortex. In MAP6 KO mice, the measure of the MRI relative signal intensity over 24h revealed that the Mn(2+) transport rate was affected with a stronger effect on long-range and polysynaptic connections than in short-range and monosynaptic tracts. The chronic treatment of MAP6 KO mice with Epo D strongly increased Mn(2+) propagation within both mono- and polysynaptic connections. Our results clearly indicate an in vivo deficit in neuronal Mn(2+) transport in KO MAP6 mice, which might be due to both axonal transport defects and synaptic transmission impairments. Epo D treatment alleviated the axonal transport defects, and this improvement most likely contributes to the positive effect of Epo D on behavioral defects in KO MAP6 mice.

The deletion of the microtubule-associated STOP protein affects the serotonergic mouse brain network.

J. Neurochem. (2010) 115, 1579–1594.

Abnormal nociception and opiate sensitivity of STOP null mice exhibiting elevated levels of the endogenous alkaloid morphine

Background-Mice deficient for the stable tubule only peptide (STOP) display altered dopaminergic neurotransmission associated with severe behavioural defects including disorganized locomotor activity. Endogenous morphine, which is present in nervous tissues and synthesized from dopamine, may contribute to these behavioral alterations since it is thought to play a role in normal and pathological neurotransmission.Results-In this study, we showed that STOP null brain structures, including cortex, hippocampus, cerebellum and spinal cord, contain high endogenous morphine amounts. The presence of elevated levels of morphine was associated with the presence of a higher density of mu opioid receptor with a higher affinity for morphine in STOP null brains. Interestingly, STOP null mice exhibited significantly lower nociceptive thresholds to thermal and mechanical stimulations. They also had abnormal behavioural responses to the administration of exogenous morphine and naloxone. Low dose of morphine (1 mg/kg, i.p.) produced a significant mechanical antinociception in STOP null mice whereas it has no effect on wild-type mice. High concentration of naloxone (1 mg/kg) was pronociceptive for both mice strain, a lower concentration (0.1 mg/kg) was found to increase the mean mechanical nociceptive threshold only in the case of STOP null mice.Conclusions-Together, our data show that STOP null mice displayed elevated levels of endogenous morphine, as well as an increase of morphine receptor affinity and density in brain. This was correlated with hypernociception and impaired pharmacological sensitivity to mu opioid receptor ligands.