Henrik Gezelius

OLM interneurons differentially modulate CA3 and entorhinal inputs to hippocampal CA1 neurons

The vast diversity of GABAergic interneurons is believed to endow hippocampal microcircuits with the required flexibility for memory encoding and retrieval. However, dissection of the functional roles of defined interneuron types has been hampered by the lack of cell-specific tools. We identified a precise molecular marker for a population of hippocampal GABAergic interneurons known as oriens lacunosum-moleculare (OLM) cells. By combining transgenic mice and optogenetic tools, we found that OLM cells are important for gating the information flow in CA1, facilitating the transmission of intrahippocampal information (from CA3) while reducing the influence of extrahippocampal inputs (from the entorhinal cortex). Furthermore, we found that OLM cells were interconnected by gap junctions, received direct cholinergic inputs from subcortical afferents and accounted for the effect of nicotine on synaptic plasticity of the Schaffer collateral pathway. Our results suggest that acetylcholine acting through OLM cells can control the mnemonic processes executed by the hippocampus.

Vesicular Glutamate Transporter 2 Is Required for Central Respiratory Rhythm Generation But Not for Locomotor Central Pattern Generation

Glutamatergic excitatory neurotransmission is dependent on glutamate release from presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6–8, which function as vesicular glutamate transporters (VGLUTs). Here, we show that VGLUT2 (Slc17a6) is required for life ex utero. Vglut2 null mutant mice die immediately after birth because of the absence of respiratory behavior. Investigations at embryonic stages revealed that neural circuits in the location of the pre-Bötzinger (PBC) inspiratory rhythm generator failed to become active. However, neurons with bursting pacemaker properties and anatomical integrity of the PBC area were preserved. Vesicles at asymmetric synapses were fewer and malformed in the Vglut2 null mutant hindbrain, probably causing the complete disruption of AMPA/kainate receptor-mediated synaptic activity in mutant PBC cells. The functional deficit results from an inability of PBC neurons to achieve synchronous activation. In contrast to respiratory rhythm generation, the locomotor central pattern generator of Vglut2 null mutant mice displayed normal rhythmic and coordinated activity, suggesting differences in their operating principles. Hence, the present study identifies VGLUT2-mediated signaling as an obligatory component of the developing respiratory rhythm generator.

EphA4-Dependent Axon Guidance Is Mediated by the RacGAP α2-Chimaerin

Neuronal network formation in the developing nervous system is dependent on the accurate navigation of nerve cell axons and dendrites, which is controlled by attractive and repulsive guidance cues. ...

Identification of Novel Spinal Cholinergic Genetic Subtypes Disclose Chodl and Pitx2 as Markers for Fast Motor Neurons and Partition Cells

Spinal cholinergic neurons are critical for motor function in both the autonomic and somatic nervous systems and are affected in spinal cord injury and in diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy. Using two screening approaches and in situ hybridization, we identified 159 genes expressed in typical cholinergic patterns in the spinal cord. These include two general cholinergic neuron markers, one gene exclusively expressed in motor neurons, and nine genes expressed in unknown subtypes of somatic motor neurons. Further, we present evidence that chondrolectin (Chodl) is expressed by fast motor neurons and that estrogen‐related receptor β (ERRβ) is a candidate marker for slow motor neurons. In addition, we suggest paired‐like homeodomain transcription factor 2 (Pitx2) as a marker for cholinergic partition cells. J. Comp. Neurol. 518:2284–2304, 2010.

Netrin-1-Dependent Spinal Interneuron Subtypes Are Required for the Formation of Left-Right Alternating Locomotor Circuitry

Neuronal circuits in the spinal cord that produce the rhythmic and coordinated activities necessary for limb movements are referred to as locomotor central pattern generators (CPGs). The identities and preceding development of neurons essential for coordination between left and right limbs are not yet known. We show that the ventral floor plate chemoattractant Netrin-1 preferentially guides dorsally originating subtypes of commissural interneurons, the majority of which are inhibitory. In contrast, the excitatory and ventralmost V3 subtype of interneurons have a normal number of commissural fibers in Netrin-1 mutant mice, thus being entirely independent of Netrin-1-mediated attraction. This selective loss of commissural fibers in Netrin-1 mutant mice resulted in an abnormal circuitry manifested by a complete switch from alternating to synchronous fictive locomotor activity suggesting that the most ventral-originating excitatory commissural interneurons are an important component of a left-right synchrony circuit in the locomotor CPG. Thus, during development, Netrin-1 plays a critical role for the establishment of a functional balanced CPG.

DCC mediated axon guidance of spinal interneurons is essential for normal locomotor central pattern generator function

Coordinated limb rhythmic movements take place through organized signaling in local spinal cord neuronal networks. The establishment of these circuitries during development is dependent on the correct guidance of axons to their targets. It has previously been shown that the well-known axon guidance molecule netrin-1 is required for configuring the circuitry that provides left-right alternating coordination in fictive locomotion. The attraction of commissural axons to the midline in response to netrin-1 has been shown to involve the netrin-1 receptor DCC (deleted in Colorectal Cancer). However, the role of DCC for the establishment of CPG coordination has not yet been resolved. We show that mice carrying a null mutation of DCC displayed an uncoordinated left-right activity during fictive locomotion accompanied by a loss of interneuronal subpopulations originating from commissural progenitors. Thus, DCC plays a crucial role in the formation of spinal neuronal circuitry coordinating left-right activities. Together with the previously published results from netrin-1 deficient mice, the data presented in this study suggest a role for the most ventral originating V3 interneurons in synchronous activities over the midline. Further, it provides evidence that axon crossing in the spinal cord is more intricately controlled than in previously suggested models of DCC-netrin-1 interaction.

Role of glutamate in locomotor rhythm generating neuronal circuitry.

Central pattern generators (CPGs) are defined as neuronal circuits capable of producing a rhythmic and coordinated output without the influence of sensory input. The locomotor and respiratory neuronal circuits are two of the better-characterized CPGs, although much work remains to fully understand how these networks operate. Glutamatergic neurons are involved in most neuronal circuits of the nervous system and considerable efforts have been made to study glutamate receptors in nervous system signaling using a variety of approaches. Because of the complexity of glutamate-mediated signaling and the variety of receptors triggered by glutamate, it has been difficult to pinpoint the role of glutamatergic neurons in neuronal circuits. In addition, glutamate is an amino acid used by every cell, which has hampered identification of glutamatergic neurons. Glutamatergic excitatory neurotransmission is dependent on the release from glutamate-filled presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Recent data describe that Vglut2 (Slc17a6) null mutant mice die immediately after birth due to a complete loss of the stable autonomous respiratory rhythm generated by the pre-Bötzinger complex. Surprisingly, we found that basal rhythmic locomotor activity is not affected in Vglut2 null mutant embryos. With this perspective, we discuss data regarding presence of VGLUT1, VGLUT2 and VGLUT3 positive neuronal populations in the spinal cord.

Prenatal thalamic waves regulate cortical area size prior to sensory processing

The cerebral cortex is organized into specialized sensory areas, whose initial territory is determined by intracortical molecular determinants. Yet, sensory cortical area size appears to be fine tuned during development to respond to functional adaptations. Here we demonstrate the existence of a prenatal sub-cortical mechanism that regulates the cortical areas size in mice. This mechanism is mediated by spontaneous thalamic calcium waves that propagate among sensory-modality thalamic nuclei up to the cortex and that provide a means of communication among sensory systems. Wave pattern alterations in one nucleus lead to changes in the pattern of the remaining ones, triggering changes in thalamic gene expression and cortical area size. Thus, silencing calcium waves in the auditory thalamus induces Rorβ upregulation in a neighbouring somatosensory nucleus preluding the enlargement of the barrel-field. These findings reveal that embryonic thalamic calcium waves coordinate cortical sensory area patterning and plasticity prior to sensory information processing.

Firing properties of Renshaw cells defined by Chrna2 are modulated by hyperpolarizing and small conductance ion currents Ih and ISK

Renshaw cells in the spinal cord ventral horn regulate motoneuron output through recurrent inhibition. Renshaw cells can be identified in vitro using anatomical and cellular criteria; however, their functional role in locomotion remains poorly defined because of the difficulty of functionally isolating Renshaw cells from surrounding motor circuits. Here we aimed to investigate whether the cholinergic nicotinic receptor alpha2 (Chrna2) can be used to identify Renshaw cells (RCsα2) in the mouse spinal cord. Immunohistochemistry and electrophysiological characterization of passive and active RCsα2 properties confirmed that neurons genetically marked by the Chrna2‐Cre mouse line together with a fluorescent reporter mouse line are Renshaw cells. Whole‐cell patch‐clamp recordings revealed that RCsα2 constitute an electrophysiologically stereotyped population with a resting membrane potential of −50.5 ± 0.4 mV and an input resistance of 233.1 ± 11 MΩ. We identified a ZD7288‐sensitive hyperpolarization‐activated cation current (Ih) in all RCsα2, contributing to membrane repolarization but not to the resting membrane potential in neonatal mice. Additionally, we found RCsα2 to express small calcium‐activated potassium currents (ISK) that, when blocked by apamin, resulted in a complete attenuation of the afterhyperpolarisation potential, increasing cellular firing frequency. We conclude that RCsα2 can be genetically targeted through their selective Chrna2 expression and that they display currents known to modulate rebound excitation and firing frequency. The genetic identification of Renshaw cells and their electrophysiological profile is required for genetic and pharmacological manipulation as well as computational simulations with the aim to understand their functional role.

Genetic Labeling of Nuclei-Specific Thalamocortical Neurons Reveals Putative Sensory-Modality Specific Genes

Abstract The thalamus is a central brain structure with topographically ordered long‐range axonal projections that convey sensory information to the cortex via distinct nuclei. Although there is an increasing knowledge about genes important for thalamocortical (TC) development, the identification of genetic landmarks of the distinct thalamic nuclei during the embryonic development has not been addressed systematically. Indeed, a more comprehensive understanding of how the axons from the individual nuclei find their way and connect to their corresponding cortical area is called for. Here, we used a genetic dual labeling strategy in mice to purify distinct principal sensory thalamic neurons. Subsequent genome‐wide transcriptome profiling revealed genes specifically expressed in each nucleus during embryonic development. Analysis of regulatory regions of the identified genes revealed key transcription factors and networks that likely underlie the specification of individual sensory‐modality TC connections. Finally, the importance of correct axon targeting for the specific sensory‐modality population transcriptome was evidenced in a Sema6A mutant, in which visual TC axons are derailed at embryonic life. In sum, our data determined the developmental transcriptional profile of the TC neurons that will eventually support sensory processing.