A diametric mode of neuronal circuitry-neurogenesis coupling in the adult hippocampus via parvalbumin interneurons

Abstract

Adult neurogenesis occurs within a specialized microenvironment (or niche), and is dynamically regulated by experience. However, the niche mechanism that couples neuronal circuitry activity to the production of new neurons is not well-understood. In our recent two studies published in Nature¹ and Nature Neuroscience,² we identified a novel niche mechanism involving parvalbumin (PV⁺) interneurons that couples local circuit activity to diametric regulation of the activation of quiescent neural stem cells (NSC) and the survival of their proliferating neural progeny, two critical early phases of adult hippocampal neurogenesis. Using a combination of optogenetics, lineage-tracing of adult NSCs, and retroviral mediated approach for birthdating of neural progenitors, we demonstrated that activation of PV⁺ interneurons, but not somatostatin (SST⁺) or vasoactive intestinal peptide (VIP⁺) interneurons, suppresses the activation of quiescent NSCs, while simultaneously promotes the survival of proliferative newborn progeny in the adult mouse dentate gyrus. In this commentary, we review our findings and discuss a potential role for PV⁺ neuron-mediated GABA signaling as a key neuronal activity sensor that translates animal's experience into regulation of adult hippocampal neurogenesis.

Keywords: :

• parvalbumin interneurons
• optogenetics
• GABAergic inputs
• adult hippocampal neurogenesis
• proliferating neural progenitors

This article refers to:

Introduction

Neurogenesis occurs throughout life in discrete regions of the adult mammalian brain and substantial evidence supports critical roles of adult-born neurons in specific brain functions, such as learning, memory and olfactory processing (reviewed in refs. 3–6). Under normal physiological conditions, active adult neurogenesis is limited to two brain regions, the olfactory bulb where newborn neurons arise from the subventricular zone (SVZ) of the lateral ventricles, and the dentate granule cell layer of the hippocampus where newborn neurons are generated locally within the subgranular zone (SGZ). Our current understanding of what makes these two neurogenic regions receptive to the integration of de novo neuronal populations in the adult brain is very limited. Therefore, identifying critical features of the neurogenic niche that foster synaptic integration and survival of newborn progeny may lead to novel strategies to enhance functional repair from endogenous or transplanted neurons after injury and degenerative neurological disorders.

Significant progress has been made in identifying major milestones and processes underlying adult neurogenesis (reviewed in ref. 7). In the adult mouse dentate gyrus, using cell-stage dependent markers (nestin, Tbr2, DCX, and NeuN) and cell proliferation marker MCM2 via lineage-tracing, we showed that quiescent nestin⁺MCM2⁻ NSCs give rise to highly proliferative Tbr2⁺MCM2⁺ intermediate progenitors, which in turn generate mitotic DCX⁺MCM2⁺ neuroblasts to become DCX⁺MCM2⁻ immature post-mitotic neurons and finally DCX⁻NeuN⁺ mature dentate granule neurons (Fig. 1). Electrophysiological studies have further revealed a stereotypic process of synaptic integration of new neurons into the existing circuitry. In the adult mice, newborn neurons are tonically activated by ambient GABA soon after birth, followed by depolarizing GABAergic synaptic inputs when cells are around 1 wk old, and finally glutamatergic synaptic inputs that emerge 1 wk later.⁸

Figure 1. Adult neurogenesis in the dentate gyrus of the hippocampus. Shown is a schematic summary of the development of newborn cells as characterized by expression of specific molecular markers at each stage.

It is generally believed that there exists a specialized local environment, or niche, that both houses somatic stem cells and regulates their development in vivo. Niche constituents that support adult SVZ or SGZ neurogenesis include endothelial cells, ependymal cells, astrocytes, microglia, and mature neurons (reviewed in refs. 9 and 10). Niche mechanisms underlying activity-dependent regulation of adult neurogenesis are not well understood. We recently showed that PV⁺ interneurons in the adult hippocampus, via a non-synaptic mechanism through GABA, suppress quiescent radial neural stem cell activation.¹ In sharp contrast to this inhibitory role of PV⁺ neuron activity on quiescent NSC activation, optogenetic analyses further showed that PV⁺ neuron activation promotes the survival of proliferating neuronal progeny.² Together, these studies identified a novel activity-dependent niche signaling mechanism through which adult neurogenesis is dynamically controlled, and revealed a diametric mode of regulation of two critical early phases of adult hippocampal neurogenesis via PV⁺ neuron activity. Here, we comment on how GABA signaling mediated by PV⁺ neurons regulates adult neurogenesis at distinct developmental stages, and discuss a potential role for PV⁺ neuron-mediated GABA signaling as a sensor of local neuronal circuitry activity and as a key regulator of the speed and extent of adult mammalian neurogenesis.

PV⁺ Interneurons as a Major Source of GABAergic Influence for Adult NSCs and Progenitors

Hippocampal microcircuits recruit different types of GABAergic interneurons, which exert inhibitory control at the soma, proximal dendrites, distal dendrites, and the axonal initial segment of the principal cells.^11,12 The hippocampal interneurons are remarkably diverse in their location, morphology, targets, and expression of proteins, such as calcium binding proteins.^11,13 A number of interneurons with their axons in close proximity to the SGZ neurogenic niche can potentially exert functional impact on adult neurogenesis. These interneurons include molecular layer perforant path interneurons (MOPP), hilar commissural-associational pathway related interneurons (HICAP), hilar perforant path-associated interneurons (HIPP), stratum lacunosum–moleculare (L–M) interneurons, and basket cells.^11,12,14

In contrast to embryonic neurogenesis, one hallmark of adult neurogenesis is its dynamic regulation by experience at specific stages. For example, exposure to enriched environment promotes adult hippocampal neurogenesis by increasing new neuron survival.¹⁵ Using an optogenetic approach, we identified PV⁺ interneurons as a critical niche component that couples neuronal circuitry activity to direct regulation of early stages of adult neurogenesis in vivo under both normal physiological conditions and after experience, such as social isolation and enriched environment.¹⁶ While SST⁺ and VIP⁺ interneurons also extend elaborate processes in the subgranular zone and surrounding areas, their activities did not modulate quiescent NSC behavior under identical experimental conditions, suggesting a specific role of PV⁺ interneurons as a niche component. PV⁺ interneurons are abundant in the hippocampus, representing ~20% of all GABAergic neurons, and have been implicated in higher brain function and cognitive dysfunction.^11,17 In the adult dentate gyrus, PV⁺ interneurons receive excitatory inputs from dentate granule cells and, to a lesser extent, from the entorhinal cortical input.¹⁸ They project axons to the dentate granule cell layer with extensive coverage across both transverse and septotemporal axes.¹⁹ A common feature of PV⁺ interneurons is the formation of ensembles coupled by both electrical, through GAP junctions, and chemical connections through reciprocal innervations.²⁰ Among multiple subtypes of interneurons in the adult dentate gyrus, PV⁺ interneurons may be uniquely poised to convey relevant information about environmental stimuli and hippocampal activity to adult neurogenesis. PV⁺ neurons are involved in feedforward and feedback inhibition of local principal neurons in the hippocampus, they are also coupled with each other, and can fire synchronously. PV⁺ neurons can also indirectly regulate adult hippocampal neurogenesis via modulating its downstream targets. For example, decreased PV⁺ neuron activity may increase activation of mature dentate granule neurons, which in turn promotes adult hippocampal neurogenesis via release of niche factors.^21,22

PV⁺ Neuron-Mediated GABA Signaling in Early Stages of Adult Neurogenesis Regulation

Tonic GABA signaling regulates quiescent NSCs

Using a combination of optogenetics to selectively target local interneuron populations and a clonal lineage-tracing system to selectively target individual quiescent NSCs (also referred as “radial glia-like cell”), we identified PV⁺ interneurons as a critical and unique niche component among several different interneuron subtypes that couples neuronal circuitry activity to regulate NSC quiescence and activation through γ₂-containing GABA_ARs in the adult mouse dentate gyrus. In contrast to the direct synaptic inputs identified onto immature neurons in the adult mouse dentate gyrus,²³ no apparent functional GABAergic synaptic responses were detected when quiescent NSCs were recorded in acute slices in this and previous studies,²⁴ suggesting that GABA spillover from activated PV⁺ interneuron-mature granule cell synapses indirectly regulates nearby NSCs. The tonic signal has two very intriguing attributes that make its involvement in neurogenesis regulation especially attractive. First, because tonic signaling transcends individual pre-synaptic/post-synaptic cellular pairs, it provides a means for acting on cells that might be located some distance from the proximity of the signaling synapse. Second, recurrent connections are extensive between granule cells and interneurons in the SGZ microenvironment and since the ambient transmitter level is an integrated signal, it is an especially attractive candidate signal that reflects the overall local network activity for potential translation by the neural progenitors. The neurogenic SGZ region is in a mesh of interneuron terminals; therefore, adult NSCs are subject to dynamic regulation of various interneuron types possibly through neurotransmitter GABA. Interestingly, a recent study showed that tonic and phasic GABA activation of neural progenitor cells and immature neurons is modulated by chemokine stromal cell-derived factor 1 co-released with GABA from local interneurons.²⁵ While the mechanisms underlying such a regulation remain to be determined, these findings suggest that additional specificity of regulation could be achieved by co-factors in the local niche.

Synaptic GABA signaling regulates proliferating progenitor survival

Significant progress has been made in characterizing synaptic integration of newborn neurons into the existing neuronal circuitry (reviewed in ref. 26), yet little is known about the properties of the first functional synaptic inputs formed onto newborn progeny during adult hippocampal neurogenesis. Previous electron microscopy and electrophysiological analyses have suggested the presence of morphological synapses onto early precursors in the postnatal SGZ.^24,27,28 In agreement with these early observations, we further identified one source of presynaptic inputs from PV⁺ neurons, and these inputs are acting on the proliferating progenitors. This result is surprising because previous studies have suggested that PV⁺ neurons make functional synaptic contacts with neurons largely upon maturation of target neurons.²⁹ In contrast to highly efficient synapses from PV⁺ interneurons onto mature dentate granule neurons where GABA release can be induced with activation of three or fewer Ca²⁺ channels,³⁰ these initial PV⁺ synapses onto newborn progeny exhibit variable structural forms and appear to be functionally immature and are only activated upon a train of stimuli.

What is the functional role of these immature synapses in regulation of neural progenitors at the proliferating phase? Computational models have suggested advantages of circuit activity-neurogenesis coupling for temporal storage and memory clearing in the adult hippocampus^31,32 and for tuning olfactory bulb activity to improve odorant discrimination.³³ Recent experimental evidence also suggests that a proper rate of both addition and elimination of new neurons optimizes behavioral outcomes in animal models.^34-36 At the cellular level, previous studies of adult hippocampal neurogenesis have identified an activity-dependent phase of competitive survival of 2- to 3-wk-old post-mitotic new neurons via glutamate-NMDAR signaling.³⁷ This critical phase of neuronal elimination occurs during glutamatergic synapse formation and is input-specific. In addition to this activity-dependent fine-tuning of post-mitotic new neuron number, we show a large-scale regulation of neuronal production at the proliferative phase that is also circuitry activity-dependent, but via GABA signaling. Notably, this activity-dependent mechanism appears to be distinct from that during embryonic neurogenesis when cell death of proliferating neural progenitors occurs before any synapse formation. During adult SVZ neurogenesis, significant cell death has also been observed in the rostral migratory stream,^38,39 although the role of neuronal circuitry activity in this process is unknown. In addition, a second phase of new neuron number regulation occurs during experience-regulated synaptic integration into olfactory bulb circuitry.⁴⁰ Thus, multi-phasic activity-dependent control of new neuron numbers may be a general rule for adult neurogenesis, starting with robust regulation during the proliferative phase and followed by refinement of individual neuron survival during the maturation phase.

Local Circuitry Control of Adult Hippocampal Neurogenesis—Diametric Regulation of Adult Neurogenesis by Local PV⁺ Interneurons

PV⁺ neuron activity that promotes proliferating neural progeny survival is in sharp contrast to its inhibitory role on quiescent neural stem cell activation in adult dentate gyrus.¹ Together, these results reveal a diametric mode of adult hippocampal neurogenesis regulation via PV⁺ neuron activity (Fig. 2): during heightened activity within dentate gyrus, activation of PV⁺ neurons promotes the survival of proliferating neuronal progenitors and inhibits quiescent neural stem cell activation; conversely, when the activity in the dentate gyrus is low, decreased PV⁺ neuron activity suppresses the survival of proliferating neuronal progenitors and simultaneously promotes the activation of quiescent NSCs via symmetric cell division. Consequently, sustained low PV⁺ neuron activity is expected to expand the quiescent NSC pool at the expenses of decreased neurogenesis presumably by decreasing proliferating progenitor survival. In contrast, sustained high PV⁺ neuron activity is expected to increase neurogenesis by promoting proliferating progenitor survival and preventing quiescent NSCs from activation thereby preserving the quiescent stem cell pool for long-term sustainable neurogenesis.

Figure 2. Diametric regulation of two sequential proliferative processes of adult hippocampal neurogenesis by PV⁺ neuron activity. Shown is a model of diametric regulation of quiescent neural stem cell activation, and survival and maturation of their proliferative neuronal progeny: during low activity within dentate gyrus (A), decreased PV⁺ neuron activity suppresses the survival of proliferating neuronal progenitors and simultaneously promotes expansion of the neural stem cell pool via symmetric cell division; conversely, when the activity in the dentate gyrus is high (B), activation of PV⁺ neurons promotes the survival and maturation of proliferating neuronal progenitors and inhibits quiescent neural stem cell activation.

The potential advantage of such a diametric mode of neuronal circuitry–neurogenesis coupling may be associated with adaptive adult neurogenesis regulation in response to dynamic network activity. Low levels of neuronal activity in the dentate gyrus may make it unnecessary for additional newborn neurons to integrate into the existing circuitry. Meanwhile, expanding the NSC pool via symmetric cell division during periods of low PV⁺ neuron activity may prepare the dentate gyrus for future adaptive neurogenesis where the activity returns and NSCs may undergo asymmetric cell division to produce more new neurons. Conversely, heightened dentate gyrus activity leads to a decrease of quiescent NSC activation. Such decreased activation of NSCs can potentially prevent stem cell depletion to ensure lifelong neurogenesis.^41,42

In addition, it may facilitate time-stamping of a specific cohort of newborn neurons during heightened activity of dentate gyrus by increasing the cohort of recently born neuronal progeny while simultaneously suppressing the generation of new neurons of a different age from quiescent NSC activation. This could be particularly important, given recent behavioral studies suggesting a critical contribution of newborn neurons at a particular developmental stage to hippocampal function.^43-45

Experience-Dependent Regulation of Adult Neurogenesis through Local Interneurons

Different from other somatic stem cell compartments where morphogens and growth factors generally serve as niche signals, adult neurogenesis is well known to be dynamically regulated by neuronal activity and experience. Accumulating evidence suggests that different steps of adult neurogenesis are differentially regulated by physiological and pathological stimuli, including an enriched environment, stress, learning, and seizures (reviewed in ref. 9). Although these studies indicate that neuronal activity is essential for this adult form of structural plasticity, how electrical activity directly regulates neural stem cells and their progeny is largely unknown. As adult hippocampal neurogenesis occurs within a dynamic neuronal network, it is conceivable that the local circuit activity could serve as an effective readout of current tissue demands and provide a signal to tune new neuron production. Indeed, our results suggest a model that PV⁺ neurons serve as a unique niche component to couple environmental stimuli to the regulation of adult NSCs and neural progenitors. Specifically, decreased PV⁺ neuron activity by chronic social isolation promotes excessive activation and symmetric cell division of NSCs at the expenses of decreased neuronal production,¹ whereas increased PV⁺ neuron activity by exposure to an enriched environment promotes the survival of both proliferating neural progenitors and subsequent mature neurons.² Though the causal link between altered neurogenesis and animal behavior has not been established, these findings point to the possibility that neuronal activity modulation by local circuits may provide the fine control of synaptic integration of adult-born neurons required for specific neurogenic events in response to experiences, environmental influence, and pathological conditions (Fig. 3).

Figure 3. Experience-dependent regulation of adult neurogenesis mediated by local interneurons. Shown is an illustration of how local interneurons may couple experiential activity to the regulation of adult neurogenesis at distinct developmental stages. Entorhinal cortical inputs activate dentate granule neurons, which in turn activate local PV⁺ interneurons. Local PV⁺ interneurons serve as intermediary cells which relay signals to adult neural stem cells and their progeny.

Summary

Our studies identified a critical role of PV⁺ neurons as a key sensor of local neuronal circuitry activity to regulate early phases of adult hippocampal neurogenesis in response to experience. Given the extensive evidence of regulations in both the number and properties of PV⁺ interneurons in the adult hippocampus under many physiological and pathological conditions, such as aging, Alzheimer diseases, chronic stress, schizophrenia and other severe psychiatric illness, and epilepsy,⁴⁶ these findings have significant and broad implications. In addition, a lack of such niche mechanisms in “non-neurogenic” regions may impede functional integration and survival of new neurons that have migrated to injury sites. Mimicking basic features of this niche mechanism may constitute a novel strategy to overcome deficits in early integration and neuronal survival of either endogenous or transplanted cells to promote adult plasticity and regeneration in brain disorders and after injury.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

The research in the authors’ laboratories was supported by UNC start-up fund (J.S.), NARSAD (J.S. and G.M.), Whitehall Foundation (J.S.), American Heart Association (J.S.), UNC BBSP and Neurobiology Curriculum training grant (A.J.C.), UNC Department of Pharmacology training grant (R.H.J.O.), NIH (G.M. and H.S.), and MSCRF (G.M.).