Origin and function of olfactory bulb interneuron diversity

Abstract

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.

Figure 1

The subventricular zone (SVZ)–olfactory bulb pathway represents one of the few constitutive neurogenic areas in the adult CNS. Diagram in the center shows a sagittal section of the rodent forebrain. The arrow indicates the tangential migration of neuroblasts (purple dots) toward the olfactory bulb (OB). New neurons recruited into the OB continually replace local interneurons. Panel on the right illustrates the wiring of the OB. Each olfactory sensory neuron (OSN) expresses only one of the ~1000 odorant receptor genes, and the axons from all cells expressing that particular receptor converge onto one or a few glomeruli (GL) in the OB. The nearly 2000 glomeruli in the rodent OB are spheroid knots of neuropil ~50–100 μm in diameter which contain the incoming axons of sensory neurons (OSN), the apical dendrites of the main projection neuron of the OB plus axon and dendrites from PGCs. The activity of projection neurons in the OB occurs through two populations of interneurons: periglomerular cells (PGC) and granule cells (GC) (both in purple) and short axon cells (represented in red). Left panel shows the neurogenic niche. Here, proliferation in the SVZ takes place in the walls of the lateral ventricle (LV), where stem cells (in green, type B cells) divide to generate transit-amplifying cells (in brown, type C cells), which in turn give rise to neuroblasts (in purple, type A cells) that migrate in the rostral migratory stream (RMS) (bottom panel) to their final destination in the OB, where they differentiate into interneurons.

Figure 2

Main microcircuits in the olfactory bulb. The three top panels show distinct glomerular microcircuits: (a) OSN→MC→PGC, (b) OSN→PGC→MC, (c) OSN→ET→PGC→MC. Microcircuits in (a) and (c) refer to type II PGCs whereas the microcircuit in (b) makes use of type I PGCs. The two lower panels show lateral connections that support interglomerular circuits. (d) OSN→ET→SA→PGC→MC and (e) show all glomerular microcircuits together with the mitral–granule–mitral (MC→GC→MC) lateral circuit. Note that all intraglomerular microcircuits (a–c) and lateral long-range interglomerular circuits (d,e) of the OB are mediated through local interneurons and are affected by adult neurogenesis. OSN = olfactory sensory neuron; MC = mitral cell; PGC = periglomerular cell; GC = granule cell; ET = external tufted cell; SA = short axon cell. (See also Ref. [5].).

Figure 3

Schematic depicting the contribution of different SVZ regions to the heterogeneity of olfactory bulb interneurons. Interneurons shown on the left are camera lucida traces of cells derived from virally labeled stem cells within restricted regions of the SVZ (for experimental details, see Ref. [43]). Postnatal stem cell-labeling experiments have shown that stem cells in the dorsal SVZ (dashed oval and black arrows) produce mostly superficial GCs (green) and TH⁺ PGCs (red). TH⁺ and a subpopulation of CR⁺ OB interneurons are derived from Emx1-expressing progenitors [22,46] (red arrow and red area). Deep GCs (blue) and CalB⁺ PGCs (purple) are produced primarily by stem cells targeted by viral injections in the ventral SVZ [43] (dashed oval and black arrows). Some stem cells in this region express the transcription factors Gsh2 and Nkx2.1 (light blue and purple areas, respectively). Lineage-tracing experiments suggest that Gsh2⁺ progenitors produce the CalB⁺ OB neurons (light blue arrow) [46].