Final Report Summary - GABA NETWORKS (Maturation of functional cortical GABAergic microcircuits)
Network development provides an interesting and unique environment to dissect how microcircuits are organized to produce the various types of network dynamics associated with cortical function (Feldt-Muldoon et al. TINS 2011). Therefore, our proposal was centred on the integration of cortical GABAergic microcircuits into a functional network during brain development in health and disease. To describe cortical networks at the scale of microcircuits, we had developed a multi-angle experimental approach combining two-photon imaging of network dynamics in vitro with mathematics, electrophysiology and neuroanatomy. Collaboration with the Fishell lab (NYU, USA) had introduced us to the use of inducible genetic fate mapping approaches to label or manipulate cells depending on their spatio-temporal embryonic origins. With that compound approach we had established that superconnected GABA hub neurons gate and transmit network synchronization in the developing hippocampus (Bonifazi et al. Science 2009). The scientific we have obtained build upon this finding and all contribute to feed the sole concept that when a neuron is born during embryogenesis critically determines its developmental journey (Allene et al. J. Neurosc. 2012) and ultimately its adult fate and function in health (Picardo et al. Neuron 2011) and disease (Marissal et al. Nature Comm. 2012, Feldt-Muldoon et al. PNAS 2013, Feldt-Muldoon et al. in revision). More specifically, we have shown that an early birthdate confers neurons with a stronger network weight (Picardo et al. Neuron 2011, Marissal et al. Nature Comm. 2012) and a resistance to epileptogenesis (Feldt-Muldoon et al. in revision). Hence, early generated GABAergic cells form a subpopulation of hub neurons in the immature hippocampus (Picardo et al. Neuron 2011) that develop into a network of long-range projecting GABAergic neurons linking the adult hippocampus to the septum and entorhinal cortex (Guigue, Villette et al. submitted). In addition, early born GABAergic neurons are recruited in synapse-driven network dynamics and express firing diversity before later born ones (Allene et al. J. Neuroscience 2012). We have recently analysed the perinatal development of early born GABA hub cells and show that the maturation of their axonal connectivity nicely parallels the emergence of GDPs with a growth discontinuity around P3 (Guigue, Picardo et al. isubmitted). Such observations can be extended from GABAergic to glutamatergic neurons in the hippocampus. Indeed, we established that early generated glutamatergic neurons carry an important pacemaker function in the disinhibited adult hippocampus and develop into a specific subpopulation of CA3 pyramidal neurons (Marissal et al. Nature Comm. 2012).
These recent findings have oriented many of our projects towards analyzing the network function of early born GABA and glutamatergic neurons, not in development but rather in the adult healthy or epileptic hippocampus, in vivo conditions, where the extensive and long-range connectivity of these cells is preserved. To this aim, we have achieved several experimental developments. We have adapted a previously described chronic window on the brain (Dombeck et al. 2010) to allow for large scale calcium imaging from hundreds of CA1 neurons simultaneously in head restrained adult mice. Mice are free to run on a treadmill allowing for self-paced changing of tactile cues. Network oscillations are simultaneously recorded using an extracellular electrode. Viral induction of a genetically encoded calcium indicator (GCaMP5G) is used to monitor the activity of GABAergic neurons and pyramidal cells. This allows us to image as deep as the hippocampal fissure and we have established collaboration with the Mosaic lab at Fresnel and ONERA to use adaptive optics in order to image as deep as the Dentate Gyrus in vivo. We show that the activity of CA1 neuronal assemblies is modulated by time, location or distance depending on the external cues provided on the treadmill (Villette, Malvache et al. Nature Neurosc. in revision).
Altogether, the results obtained during this Starting Grant clearly demonstrate how studying cortical network development can fuel our understanding of structure-function relationships in adult circuits both in health and disease.They are the conceptual and experimental basis of our Consolidator Grant proposal that was successfully evaluated by the ERC 2014 panel.
These recent findings have oriented many of our projects towards analyzing the network function of early born GABA and glutamatergic neurons, not in development but rather in the adult healthy or epileptic hippocampus, in vivo conditions, where the extensive and long-range connectivity of these cells is preserved. To this aim, we have achieved several experimental developments. We have adapted a previously described chronic window on the brain (Dombeck et al. 2010) to allow for large scale calcium imaging from hundreds of CA1 neurons simultaneously in head restrained adult mice. Mice are free to run on a treadmill allowing for self-paced changing of tactile cues. Network oscillations are simultaneously recorded using an extracellular electrode. Viral induction of a genetically encoded calcium indicator (GCaMP5G) is used to monitor the activity of GABAergic neurons and pyramidal cells. This allows us to image as deep as the hippocampal fissure and we have established collaboration with the Mosaic lab at Fresnel and ONERA to use adaptive optics in order to image as deep as the Dentate Gyrus in vivo. We show that the activity of CA1 neuronal assemblies is modulated by time, location or distance depending on the external cues provided on the treadmill (Villette, Malvache et al. Nature Neurosc. in revision).
Altogether, the results obtained during this Starting Grant clearly demonstrate how studying cortical network development can fuel our understanding of structure-function relationships in adult circuits both in health and disease.They are the conceptual and experimental basis of our Consolidator Grant proposal that was successfully evaluated by the ERC 2014 panel.