The olfactory bulb (OB) has an extremely higher proportion of interneurons innervating excitatory neurons than other brain regions, which is evolutionally conserved across species. Despite the abundance of OB interneurons, little is known about the diversification and physiological functions of OB interneurons compared to cortical interneurons. In this review, an overview of the general developmental process of interneurons from the angles of the spatial and temporal specifications was presented. Then, the distinct features shown exclusively in OB interneurons development and molecular machinery recently identified were discussed. Finally, we proposed an evolutionary meaning for the diversity of OB interneurons.
Identification of the neuronal components in the brain provides important insight for understanding high-order and complicated behaviors, including logical thinking, emotional sensation, and interaction with external signals (Ramón y Cajal et al., 1988). Specifically, interneurons control neurotransmission by the intricate modulation of information processing (Bartolini et al., 2013; Paredes et al., 2016). To adapt to these diverse neuronal functions, interneurons are developed into morphologically, molecularly, and electrophysiologically diverse subtypes and are continuously generated from embryonic to even adult stages (Bartolini et al., 2013; Batista-Brito and Fishell, 2009; Kepecs and Fishell, 2014). Malformation of the interneurons during early development can lead to neurodevelopmental disorders, such as autism spectrum disorder (ASD) and Tourette’s syndrome (Ashwin et al., 2014; Marco et al., 2011). Thus, defining neuronal properties and classifying the myriad of diverse interneurons is essential for understanding complex brain physiologies (Maccaferri and Lacaille, 2003), as well as neurodevelopmental disorders (Fang et al., 2014).
Mammalian OB express the most abundant and varied interneurons in the brain, but they have received little attention compared to cortical interneurons. Approximately 90% of ASD patients having mental retardation have a high sensitivity to external auditory stimuli and some of patients are suffered from hallucinations of olfaction (Galle et al., 2013; Gomes et al., 2008; Tonacci et al., 2017). Furthermore, the abnormal structural development of OB interneurons in the early stage induces olfactory impairments (Kim et al., 2020; Yoshihara et al., 2014). These facts indicated that research on the development of interneurons in the OB is critical and fundamental. In this review, we introduced the distinct characteristics of OB interneuron development by comparing them to the common developmental features of other interneurons. We also discussed the recently identified mechanisms underlying OB interneurons development and their physiological functions.
AMAZING CELL TYPE: INTERNEURON
The mammalian brain contains dozens of distinct types of interneurons with very diverse morphologies, molecular markers, electrophysiological properties and connectivity that modulate and refine neuronal circuits (Bandler et al., 2017; Hu et al., 2017). Broadly, GABAergic cells in the forebrain are classified based on their progenitor origins, and which has been studied well in mice (Fertuzinhos et al., 2009; Hansen et al., 2013). In the progenitor zones of the three subcortical regions of the brain, the medial ganglionic eminence (MGE), the caudal ganglionic eminence (CGE), and the lateral ganglionic eminence (LGE), many inhibitory cell subtypes are produced during embryonic stages and migrate along stereotyped streams, then finally disperse throughout the forebrain. MGE and CGE-derived interneurons which are mainly generated during embryonic days 11-15 predominantly migrate into the cortex, hippocampus, amygdala, and striatum, whereas LGE-derived interneurons, which are generated from mid embryonic days 13.5-15.5 become the olfactory bulb (OB)- or striatum-interneurons (Bandler et al., 2017; Torigoe et al., 2016). To more detail, cortical interneurons are divided into up to 50 different types, which are characterized by a combination of molecular markers or other intrinsic factors (Lim et al., 2018; Wamsley and Fishell, 2017). The subdivided regions of ganglionic eminence can generate more specialized and differentiated interneurons (Rubenstein et al., 1994). That is, these regional domains are specified by transcriptional factors with a spatial bias for the generation of specific interneuron types (Puelles and Rubenstein, 1993). For instance, Nkx2.1 highly expressed in MGE, determines MGE-derived cell fate, and the MGE-derived cells become somatostatin (SST)- or parvalbumin (PV)-expressing interneurons. In the case of CGE, Pax6, Prox1, and Sp8 are predominantly expressed and the CGE-derived cells become vasoactive intestinal peptide (VIP)- or cholecystokinin (CCK)-expressing interneurons. These observations strongly indicate that spatial specification critically contributes to the diversification of interneurons.
TIMELY DEVELOPMENT AS A DETERMINANT OF INTERNEURON DIVERSITY
The temporally defined development of interneurons is also a key factor in the diverse specifications of interneurons (Kao and Lee, 2010; Osterhout et al., 2014). The temporally defined expression of CoupTF2 determines the cell fate of progenitor cells derived from the MGE in SST- and PV-expressing cortical interneurons (Hu et al., 2017). Even interneurons with the same molecular cell fates can form different functional circuits dependent on their temporally defined birth. In the hippocampus, early-born and late-born PV-expressing basket cells form synapses with different subpopulations of pyramidal neurons in CA1 and play differential roles in memory and learning (Donato et al., 2015). Especially, the timely development of interneurons is more closely correlated with their final positioning in the brain (Fairen et al., 1986; Rymar and Sadikot, 2007). Interneurons with similar fates determined by their same birthdate assemble with each other to form laminar structures and cooperate in modulating the signal responses of excitatory neurons (Bartolini et al., 2013). For example, early- and late-born MGE-derived interneurons predominantly settled in infragranular layers and supragranular layers of the neocortex, respectively (Ma et al., 2006; Rymar and Sadikot, 2007), establishing distinct neuronal innervated circuits. Furthermore, it has been reported that the final positioning of interneuron was not determined by their clonality or lineage, rather, it might be affected by birthdates or migration machinery (Mayer et al., 2015). However, integrative studies on the temporal specifications of interneurons are still lacking.
DISTINCT CHARACTERISTICS OF THE OLFACTORY BULB INTERNEURONS
The OB, like the cortex, striatum or hippocampus, is a recipient of the massive generation of GABAergic interneurons from the telencephalon. Although each region shares common features for interneuron development, OB has a few unique properties (Fig. 1): a) the OB has an extremely higher proportion of interneurons (I) to excitatory neurons (E), at a 100:1 ratio, compared to other brain regions at a 1:5 ratio (Bayer, 1983). The reason for the high conserved ratio of OB interneurons remains a mystery; b) neurogenesis for OB interneurons occurs not only in the embryonic stages but also in the adult stages. Cortical interneurons are primarily produced from the MGE or CGE from embryonic days 9.5-17.5. However, OB interneurons are continuously generated from the LGE or subventricular zone (SVZ) throughout life (Alvarez-Buylla et al., 2001). Specifically, approximately 73% of the interneurons are generated from the SVZ during the postnatal first or second week, 25% are born during the embryonic stage from the LGE (Bayer, 1983; Hinds, 1968), and only 2% are generated from adult neurogenesis; and c) in the migration of LGE or SVZ-derived cells into the OB, the interneuron precursors (neuroblast) tangentially migrate through the RMS (Lledo et al., 2008; Lois and Alvarez-Buylla, 1994; Mirich et al., 2002; Rall et al., 1966), whose distance is relatively very long. This implies that LGE- or SVZ-derived precursors might have distinct migratory machinery, unlike the MGE- or SGZ-derived precursors traveling short distances (Lepousez et al., 2015). Lastly, GABAergic interneurons in OB rarely express SST or PV, which are representative markers in cortical or hippocampal interneurons, implying that the molecular markers identified before are not sufficient to fully define or understand the diversity of the OB interneurons. Given these distinct developmental features of OB interneurons, different approaches or criteria should be considered to analyze OB interneurons.
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CONCLUSION AND PERSPECTIVES
The OB interneurons are extremely abundant and diverse. Here, we pointed out the unique characteristics of OB interneurons different from other interneurons. Most notably, OB interneurons are generated over a long period from the mid-embryonic to the adult stage, and migrate a long distance through the RMS into the OB. We also briefly summarized that special molecular machinery, such sensory input-mediated c-fos synthesis and Abl1-Dcx signaling, is reflected in the unique properties of postnatal early-born interneurons, including a high survival rate and integration into the sGCL forming the innate olfactory behaviors. Through our review, we suggest that OB interneurons might be diversified and clustered by a combination of their diverse and distinct properties, including precursor origins, developmental timing, sensory inputs, and migratory machinery.
Why OB interneurons are highly populated and diverse remains unsolved. This may be interpreted by some facts: 1) OB, as the first gating site of robust inputs from the external environment, tightly controls the E-I ratio (Anderson et al., 2000; D’Amour and Froemke, 2015). Furthermore, it must be associated with more tight or delicate modulation machinery, like interneurons, since the OB is a direct pathway for olfactory information processing to the cortex without thalamic relay (Kay and Sherman, 2007); 2) OB interneurons are continuously generated even during the adult stage (Alvarez-Buylla et al., 2001). During the development of OB interneurons, they are consistently exposed to various and unexpected sensory stimuli, implying that the diversity of OB interneurons might be evolutionary evidence of their adaptation to diverse environmental stimuli; and 3) despite the fact that there is a smaller odorant receptor repertoire than in other species, humans still can distinguish 1 trillion smells (Bushdid et al., 2014; Zozulya et al., 2001). This suggests that there must be other machinery for odor discrimination in the central nervous system beyond odor sensing by the odorant receptors. Based on the above facts, it is expected that the distinct developmental features of mouse OB interneurons might be conserved in human OB interneurons (Paredes et al., 2016; Zapiec et al., 2017).
In summary, considering these unanswered and intriguing questions about the diversity of OB interneurons, a deep focus on these issues would be of crucial importance. Furthermore, it may provide new insights into cures for neurodevelopmental disorder patients having sensory hallucination.