Unit: Odor Perception and Memory
Director: LLEDO Pierre-Marie
The ability of adult brain to produce new neurons has excited much interest not only because of its fundamental significance, but also in part because of the potential to treat neurodegenerative diseases. While proliferation of neuronal precursors in the mature brain has been extensively studied in the past, very little was known about how neuronal progenitors migrate and integrate to the appropriate target area to become truly functional neurons and what are the factors controlling their recruitment into pre-existing neuronal networks.
We are currently investigating the mechanisms that regulate production of new neurons in the adult brain, their targeting to the appropriate regions, and their differentiation and survival in these areas. Understanding of these mechanisms will have profound advance in the cell replacement therapies by offering new strategies aimed to control not only the dispersal of grafted cells, but also the production and recruitment of endogenous precursor neurons.
The discovery of neuronal recruitment into adult brain microcircuits has been one of the major recent breakthroughs in developmental Neuroscience. Clearly, the next forthcoming steps are expected to bring new insights into the mechanisms that govern the production of new neurons, their targeting to appropriate regions, their differentiation into distinct neuronal subtypes and finally, their survival. In 2005, we have been investigating these fundamental issues by taking the olfactory bulb as a model. We have achieved this through a multidisciplinary, fully integrated approach. Methodologically this had included morphological, electrophysiological, and molecular characterization of new neurons in vitro and in vivo as well as behavior approaches to investigate the functional contribution of new neurons to pre-existing brain circuitry. Special emphasis was given to investigate the relative contribution of intrinsic (cell-autonomous) and extrinsic (environmental) cues regulating this process.
1) Characterization of adult-generated neurons (cellular physiology, morphology, immuno-histochemical profiles)
We have demonstrated that newly formed neurons become functional neurons in the adult brain that could be either dopaminergic or GABAergic (de Chevigny, Saghatelyan). A description of the progressive maturation of newly formed cells has also been provided in the adult brain (the lentiviral vector necessary for assessing the role of different factors influencing neurogenesis is provided by Pierre Charneau, Pasteur Institute). The production of adult-formed neurons has been compared with the one of early postnatal (Lemasson, Saghatelyan).
Tracking the migratoring cells in the adult forebrain in vivo (Davenne, Saghatelyan).
Investigating the functional consequences of sensory neurons turnover for sensory information processing (Grubb) (Photo1).
The cellular targets and the way by which newly generated cells incorporate into the existing neuronal circuitry according to some molecular cues (extracellular matrix components and neurotransmitters) have been investigated (Alonso, de Chevigny, Gabellec, Saghatelyan, Violet) (Photo2).
2) Understanding the molecular and environmental factors affecting neuronal proliferation, migration and differentiation of adult-generated neurons
We have started to unravel the molecular mechanisms that determine how and when the fate of newborn cells is specified along the SVZ-OB pathway. Two transcription factors Pax6 and Olig2 are expressed in the SVZ in adulthood, with Olig2 being exclusively expressed in SVZ transit-amplifying precursor cells. Viral vector-mediated overexpression of mouse Olig2 in the SVZ facilitated oligodendrocyte maturation, thereby repressing neuronal development. By contrast, Pax6 was scarce in the SVZ but more abundant in migrating neuroblasts, and viral vector-mediated repression of Pax6 in the SVZ led to a substantial reduction in the proportion of neuronal precursors. These results point to an important role for Pax6 in promoting neurogenesis (de Chevigny, Saghatelyan).
The role of ionotropic neurotransmitter receptors in regulating neuroblast proliferation/migration are currently investigated both in vivo and in vitro experiments on explants and on neurospheres (Murray, Katagiri).
3) Identification of environmental factors affecting neuronal proliferation, migration and differentiation of adult-generated neurons
The role of sensory deprivation is particularly explored on the maturation of newly generated neurons in the adult olfactory bulb (Saghatelyan).
4) Understanding the functional consequences of adult neurogenesis at the behavioral level
We have performed electrophysiological analysis of neural synchronization in the olfactory bulb after modification of neuroblast proliferation, migration and/or survival (using mutant mice and sensory deprived rats) (Gheusi, Lagier, Lemasson).
We compared the odor sensitivity (threshold tests), odor discrimination and odor memory in different types of mutant mice with altered neurogenesis pattern (Gheusi).
5) Behavioral relevance of bulbar neurogenesis during brain development and adulthood
In order to fully appreciate the relevance of neurogenesis, it is important to take advantage of naturally occurring situations in which the production of neurons could play a role. We are exploring different forms of learning and memory that could take place in newborn mice during odor-guided social transmission of offspring and mate recognition. This should undoubtedly constitute a critical step in understanding the role of bulbar neurogenesis in olfaction (Viollet, Alonso, Gheusi).
6) Investigating the mechanisms underlying spatial patterning in the olfactory bulb
Our study focused on gamma network oscillations in the bulb, which is a form of temporal patterning in bulbar activity elicited by olfactory stimuli. We used computational modeling combined with electrophysiological recordings to investigate the basic synaptic organization necessary and sufficient to generate sustained gamma rhythms. We found that features of gamma oscillations obtained in vitro were identical to those of a model based on lateral inhibition as the coupling modality. We could precisely tune the oscillation frequency by changing the kinetics of inhibitory events supporting the lateral inhibition. Moreover, gradually decreasing GABAergic synaptic transmission decreased the degree of relay neuron synchronization in response to sensory inputs, both theoretically and experimentally. Thus, we have found that lateral inhibition provides a mechanism by which the dynamic processing of odor information might be finely tuned within the OB circuit (Lagier).
Photo 1: Glomeruli in the olfactory bulb are morphological entities (it is a location where sensory neurons penetrate the CNS) as well as functional units (where sensory neurons contact both relay neurons and bulbar interneurons). Blue : DAPI staining; Green : OMP staining.
Photo 2: Dendritic arbors of virus infected newly-generated cells in the adult olfactory bulb. Mature neurons are stained with a NeuN antibody.
Keywords: Neural stem cells, Olfaction, Perception, Memory, Neuroscience