Abdeljabbar El Manira

Intrinsic function of a neuronal network — a vertebrate central pattern generator

The cellular bases of vertebrate locomotor behaviour is reviewed using the lamprey as a model system. Forebrain and brainstem cell populations initiate locomotor activity via reticulospinal fibers activating a spinal network comprised of glutamatergic and glycinergic interneurons. The role of different subtypes of Ca2+ channels, Ca2+ dependent K+ channels and voltage dependent NMDA channels at the neuronal and network level is in focus as well as the effects of different metabotropic, aminergic and peptidergic modulators that target these ion channels. This is one of the few vertebrate networks that is understood at a cellular level.

Histone H2AX-dependent GABA A receptor regulation of stem cell proliferation

Stem cell self-renewal implies proliferation under continued maintenance of multipotency. Small changes in numbers of stem cells may lead to large differences in differentiated cell numbers, resulting in significant physiological consequences. Proliferation is typically regulated in the G1 phase, which is associated with differentiation and cell cycle arrest. However, embryonic stem (ES) cells may lack a G1 checkpoint. Regulation of proliferation in the ‘DNA damage’ S/G2 cell cycle checkpoint pathway is known for its role in the maintenance of chromatin structural integrity. Here we show that autocrine/paracrine γ-aminobutyric acid (GABA) signalling by means of GABAA receptors negatively controls ES cell and peripheral neural crest stem (NCS) cell proliferation, preimplantation embryonic growth and proliferation in the boundary-cap stem cell niche, resulting in an attenuation of neuronal progenies from this stem cell niche. Activation of GABAA receptors leads to hyperpolarization, increased cell volume and accumulation of stem cells in S phase, thereby causing a rapid decrease in cell proliferation. GABAA receptors signal through S-phase checkpoint kinases of the phosphatidylinositol-3-OH kinase-related kinase family and the histone variant H2AX. This signalling pathway critically regulates proliferation independently of differentiation, apoptosis and overt damage to DNA. These results indicate the presence of a fundamentally different mechanism of proliferation control in these stem cells, in comparison with most somatic cells, involving proteins in the DNA damage checkpoint pathway.

Efficient production of mesencephalic dopamine neurons by Lmx1a expression in embryonic stem cells

Signaling factors involved in CNS development have been used to control the differentiation of embryonic stem cells (ESCs) into mesencephalic dopamine (mesDA) neurons, but tend to generate a limited yield of desired cell type. Here we show that forced expression of Lmx1a, a transcription factor functioning as a determinant of mesDA neurons during embryogenesis, effectively can promote the generation of mesDA neurons from mouse and human ESCs. Under permissive culture conditions, 75%–95% of mouse ESC-derived neurons express molecular and physiological properties characteristic of bona fide mesDA neurons. Similar to primary mesDA neurons, these cells integrate and innervate the striatum of 6-hydroxy dopamine lesioned neonatal rats. Thus, the enriched generation of functional mesDA neurons by forced expression of Lmx1a may be of future importance in cell replacement therapy of Parkinson disease.

Synaptic vesicle release regulates myelin sheath number of individual oligodendrocytes in vivo

The myelination of axons by oligodendrocytes markedly affects CNS function, but how this is regulated by neuronal activity in vivo is not known. We found that blocking synaptic vesicle release impaired CNS myelination by reducing the number of myelin sheaths made by individual oligodendrocytes during their short period of formation. We also found that stimulating neuronal activity increased myelin sheath formation by individual oligodendrocytes. These data indicate that neuronal activity regulates the myelinating capacity of single oligodendrocytes.

Sensory pathways and their modulation in the control of locomotion

Recent experiments have extended our understanding of how sensory information in premotor networks controlling motor output is processed during locomotion, and at what level the efficacy of specific sensory-motor pathways is determined. Phasic presynaptic inhibition of sensory transmission combined with postsynaptic alterations of excitatory and inhibitory synaptic transmission from interneurons of the premotor networks contribute to the modulation of reflex pathways and to the generation of reflex reversal. These mechanisms play an important role in adapting the operation of central networks to external demands and thus help optimize sensory-motor integration.

Vertebrate Locomotion-A Lamprey Perspectivea

Abstract: The forebrain, brain stem, and spinal cord contribution to the control of locomotion is reviewed in this chapter. The lamprey is used as an experimental model because it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre‐ and postsynaptic modulation. Neuropeptides target different cellular and synaptic mechanisms and cause long‐lasting changes (>24 h) in network function.

Afferents of the lamprey striatum with special reference to the dopaminergic system: a combined tracing and immunohistochemical study.

The origin of afferents to the striatum in lamprey (Lampetra fluviatilis) was studied by using fluorescein‐coupled dextran‐amines (FDA). Injection of FDA into the striatum retrogradely labeled several cell populations in the forebrain and the rostral rhombencephalon. No retrograde labeled cells were seen in the mesencephalon.

Ion channels of importance for the locomotor pattern generation in the lamprey brainstem-spinal cord.

The intrinsic function of the spinal network that generates locomotion can be studied in the isolated brainstem‐spinal cord of the lamprey, a lower vertebrate. The motor pattern underlying locomotion can be elicited in the isolated spinal cord. The network consists of excitatory glutamatergic and inhibitory glycinergic interneurones with known connectivity. The current review addresses the different subtypes of ion channels that are present in the cell types that constitute the network. In particular the roles of the different subtypes of Ca2+ channels and potassium channels that regulate integrated neuronal functions, like frequency regulation, spike frequency adaptation and properties that are important for generating features of the motor pattern (e.g. burst termination), are reviewed. By knowing the role of an ion channel at the cellular level, we also, based on previous knowledge of network connectivity, can understand which effect a given ion channel may exert at the different levels from molecule and cell to network and behaviour.

Organization of the lamprey striatum – transmitters and projections

The purpose of the present study is to characterize the striatum of the lamprey by immunohistochemical and tracing techniques. Cells immunoreactive for GABA and substance P (SP), and positive for acetylcholinesterase, are present in the lamprey striatum. Immunoreactive (ir) fibers were detected by antisera raised against SP, dopamine, enkephalin and serotonin. These immunoreactive fibers were mainly located in the periventricular neuropil that borders the striatum and in which GABAergic striatal neurons distributed their dendritic arbors. Putative connections between the striatum, the ventral part of the lateral pallium, and the diencephalic motor centers involved in the control of locomotion were studied by using fluorescein-coupled dextran amines (FDA) as a tracer. The striatum projects to the ventral part of the lateral pallium (lpv), where GABA-ir cells and SP-ir fibers were also present. The lpv in turn projects to the ventral thalamus, which has descending connections to the reticulospinal cells involved in the control of locomotion. These results, together with previous findings of histaminergic and neurotensin projections, suggest that the lamprey striatum and its inputs with regard to neurotransmitters/modulators are very similar to those of modem amniotes, including primates, and are thus conserved to a high degree.

Characterization of a high-voltage-activated IA current with a role in spike timing and locomotor pattern generation

Transient A-type K+ channels (IA) in neurons have been implicated in the delay of the spike onset and the decrease in the firing frequency. Here we have characterized biophysically and pharmacologically an IA current in lamprey locomotor network neurons that is activated by suprathreshold depolarization and is specifically blocked by catechol at 100 μM. The biophysical properties of this current are similar to the mammalian Kv3.4 channel. The role of the IA current both in single neuron firing and in locomotor pattern generation was analyzed. The IA current facilitates Na+ channel recovery from inactivation and thus sustains repetitive firing. The role of the IA current in motor pattern generation was examined by applying catechol during fictive locomotion induced by N-methyl-d-aspartate. Blockade of this current increased the locomotor burst frequency and decreased the firing of motoneurons. Although an alternating motor pattern could still be generated, the cycle duration was less regular, with ventral roots bursts failing on some cycles. Our results thus provide insights into the contribution of a high-voltage-activated IA current to the regulation of firing properties and motor coordination in the lamprey spinal cord.