Pierre-Marie Lledo

Adult neurogenesis and functional plasticity in neuronal circuits

The adult brain is a plastic place. To ensure that the mature nervous systems control of behaviour is flexible in the face of a varying environment, morphological and physiological changes are possible at many levels, including that of the entire cell. In two areas of the adult brain — the olfactory bulb and the dentate gyrus — new neurons are generated throughout life and form an integral part of the normal functional circuitry. This process is not fixed, but highly modulated, revealing a plastic mechanism by which the brains performance can be optimized for a given environment. The functional benefits of this whole-cell plasticity, however, remain a matter for debate.

Becoming a new neuron in the adult olfactory bulb.

New neurons are continually recruited throughout adulthood in certain regions of the adult mammalian brain. How these cells mature and integrate into preexisting functional circuits remains unknown. Here we describe the physiological properties of newborn olfactory bulb interneurons at five different stages of their maturation in adult mice. Patch-clamp recordings were obtained from tangentially and radially migrating young neurons and from neurons in three subsequent maturation stages. Tangentially migrating neurons expressed extrasynaptic GABAA receptors and then AMPA receptors, before NMDA receptors appeared in radially migrating neurons. Spontaneous synaptic activity emerged soon after migration was complete, and spiking activity was the last characteristic to be acquired. This delayed excitability is unique to cells born in the adult and may protect circuits from uncontrolled neurotransmitter release and neural network disruption. Our results show that newly born cells recruited into the olfactory bulb become neurons, and a unique sequence of events leads to their functional integration.

Neuronal fate determinants of adult olfactory bulb neurogenesis.

Adult neurogenesis in mammals is restricted to two small regions, including the olfactory bulb, where GABAergic and dopaminergic interneurons are newly generated throughout the entire lifespan. However, the mechanisms directing them towards a specific neuronal phenotype are not yet understood. Here, we demonstrate the dual role of the transcription factor Pax6 in generating neuronal progenitors and also in directing them towards a dopaminergic periglomerular phenotype in adult mice. We present further evidence that dopaminergic periglomerular neurons originate in a distinct niche, the rostral migratory stream, and are fewer derived from precursors in the zone lining the ventricle. This regionalization of the adult precursor cells is further supported by the restricted expression of the transcription factor Olig2, which specifies transit-amplifying precursor fate and opposes the neurogenic role of Pax6. Together, these data explain both extrinsic and intrinsic mechanisms controlling neuronal identity in adult neurogenesis.

Origin and function of olfactory bulb interneuron diversity

In adult rodents, subventricular zone (SVZ) astrocytes (B cells) function as primary progenitors in the generation of new neurons that migrate to the olfactory bulb (OB), where they differentiate into multiple types of interneurons. It has been generally considered that individual adult SVZ stem cells are capable of generating different types of neurons and glial cells. However, recent studies indicate that these adult SVZ primary progenitors are heterogeneous and predetermined to generate specific types of neurons. Surprisingly, OB interneurons are generated by stem cells not only in the walls of the lateral ventricle facing the striatum but also in the rostral migratory stream and walls of the lateral ventricle facing the cortex and the septum. SVZ B cells in different locations within this extensive germinal region generate different kinds of interneurons. General physiological characteristics of major classes of OB interneurons have begun to emerge, but the functional contribution of each subtype remains unknown. The mosaic organization of the SVZ offers a unique opportunity to understand the origin of interneuron diversity and how this assortment of neurons contributes to plasticity of postnatal olfactory circuits.

Tenascin-R mediates activity-dependent recruitment of neuroblasts in the adult mouse forebrain

Neuroblasts arising in the adult forebrain that travel to the olfactory bulb use two modes of migration: tangentially, along the rostral migratory stream, and radially, in the core of the olfactory bulb where they start to ascend to the outer layers. Although the mechanisms of tangential migration have been extensively studied, the factors controlling radial migration remain unexplored. Here we report that the extracellular matrix glycoprotein tenascin-R, expressed in the adult mouse olfactory bulb, initiates both the detachment of neuroblasts from chains and their radial migration. Expression of tenascin-R is activity dependent, as it is markedly reduced by odor deprivation. Furthermore, grafting of tenascin-R-transfected cells into non-neurogenic regions reroutes migrating neuroblasts toward these regions. The identification of an extracellular microenvironment capable of directing migrating neuroblasts provides insights into the mechanisms regulating radial migration in the adult olfactory bulb and offers promising therapeutic venues for brain repair.

The GTPase Rab3a negatively controls calcium-dependent exocytosis in neuroendocrine cells.

There is accumulating evidence that small GTPases of the rab family regulate intracellular vesicle traffic along biosynthetic and endocytotic pathways in eukaryotic cells. It has been suggested that Rab3a, which is associated with synaptic vesicles in neurons and with secretory granules in adrenal chromaffin cells, might regulate exocytosis. We report here that overexpression in PC12 cells of Rab3a mutant proteins defective in either GTP hydrolysis or in guanine nucleotide binding inhibited exocytosis, as measured by a double indirect immunofluorescence assay. Moreover, injection of the purified mutant proteins into bovine adrenal chromaffin cells also inhibited exocytosis, as monitored by membrane capacitance measurements. Finally, the electrophysiological approach showed that bovine chromaffin cells which were intracellularly injected with antisense oligonucleotides targeted to the rab3a messenger exhibited an increasing potential to respond to repetitive stimulations. In contrast, control cells showed a phenomenon of desensitization. These results provide clear evidence that Rab3a is involved in regulated exocytosis and suggest that Rab3a is a regulatory factor that prevents exocytosis from occurring unless secretion is triggered. Furthermore, it is proposed that Rab3a is involved in adaptive processes such as response habituation.

Olfactory Discrimination Learning Increases the Survival of Adult-Born Neurons in the Olfactory Bulb

In the olfactory bulb (OB), new neurons are added throughout life, forming an integral part of the functioning circuit. Yet only some of them survive more than a month. To determine whether this turnover depends on olfactory learning, we examined the survival of adult newborn cells labeled with the cell division marker BrdU, administered before learning in an olfactory discrimination task. We report that discrimination learning increases the number of newborn neurons in the adult OB by prolonging their survival. Simple exposure to the pair of olfactory cues did not alter neurogenesis, indicating that the mere activation of sensory inputs during the learning task was insufficient to alter neurogenesis. The increase in cell survival after learning was not uniformly distributed throughout angular sectors of coronal sections of the OB. Monitoring odor activation maps using patterns of Zif268 immediate early gene expression revealed that survival was greater in regions more activated by the non-reinforced odorant. We conclude that sensory activation in a learning context not only controls the total number of newborn neurons in the adult OB, but also refines their precise location. Shaping the distribution of newborn neurons by influencing their survival could optimize the olfactory information processing required for odor discrimination.

Adult neurogenesis promotes synaptic plasticity in the olfactory bulb.

To explore the functional consequences of adult neurogenesis in the mouse olfactory bulb, we investigated plasticity at glutamatergic synapses onto GABAergic interneurons. We found that one subset of excitatory synapses onto adult-born granule cells showed long-term potentiation shortly after their arrival in the bulb. This property faded as the newborn neurons matured. These results indicate that recently generated adult-born olfactory interneurons undergo different experience-dependent synaptic modifications compared with their pre-existing mature neighbors and provide a possible substrate for adult neurogenesis–dependent olfactory learning.

Integrating new neurons into the adult olfactory bulb: joining the network, life–death decisions, and the effects of sensory experience

In contrast to the situation in the developing brain, neurons born during adulthood must integrate into established neuronal networks characterized by ongoing activity. For sensory systems, this neuronal activity is driven mainly by external stimuli that can lead to experience-dependent morpho-functional changes in adult circuits. Here, we describe new insights into the mechanisms by which sensory experience might govern the targeting of adult-generated neurons to appropriate regions, their differentiation into distinct neuronal subtypes, and finally their survival in the adult olfactory bulb. We propose not only that neurogenesis depends on the degree of sensory experience, but also that new neurons bring unique features to the operational network, allowing a continuous adjustment of information processing in response to an ever-changing external word.

Interplay between Local GABAergic Interneurons and Relay Neurons Generates γ Oscillations in the Rat Olfactory Bulb

Olfactory stimuli have been known for a long time to elicit oscillations in olfactory brain areas. In the olfactory bulb (OB), odors trigger synchronous oscillatory activity that is believed to arise from the coherent and rhythmic discharges of large numbers of neurons. These oscillations are known to take part in encoding of sensory information before their transfer to higher subcortical and cortical areas. To characterize the cellular mechanisms underlying γ (30–80 Hz) local field potential (LFP) oscillations, we simultaneously recorded multiunit discharges, intracellular responses, and LFP in rat OB slices. We showed that a single and brief electrical stimulation of olfactory nerve elicited LFP oscillations in the mitral cell body layer lasting >1 sec. Both action potentials and subthreshold oscillations of mitral/tufted cells, the bulbar output neurons, were precisely synchronized with LFP oscillations. This synchronization arises from the interaction between output neurons and granule cells, the main population of local circuit inhibitory interneurons, through dendrodendritic synapses. Interestingly enough, the synchronization exerted by reciprocal synaptic interactions did not require action potentials initiated in granule cell somata. Finally, local application of a GABAA receptor antagonist at the mitral cell and external plexiform layers confirmed the exclusive role of the granule cell reciprocal synapses in generating the evoked oscillations. We concluded that interneurons located in the granule cell layer generate synaptic activity capable of synchronizing activity of output neurons by interacting with both their subthreshold and spiking activity.