An Evolutionary Microcircuit Approach to the Neural Basis of High Dimensional Sensory Processing in Olfaction

Timing Matters: Lessons From Perinatal Neurogenesis in the Olfactory Bulb

In the olfactory bulb (OB), odorant receptor-specific input converges into glomeruli. Subsequently, the coding of odor information is fine-tuned by local synaptic circuits within the glomeruli and the deeper external plexiform layer (EPL) in the OB. Deciphering the organization of inhibitory granule cells (GCs) in the EPL relative to the secondary dendrites of projection neurons is pivotal for understanding odor processing. We conducted a detailed investigation of GCs, focusing on the timing of neurogenesis, laminar distribution, and synaptogenesis between GCs and projection neurons. In summary, GCs develop following a developmental continuum with an outside-in maturation pattern from embryogenesis to adulthood. GCs born 1 week after birth display a unique sublayer-specific distribution pattern, marking a transition between embryonic or neonatal and adult stages. Integration into reciprocal synaptic circuits occurred 10 days post-neurogenesis. We conclude that the timing of neurogenesis dictates the anatomical configuration of GCs within the OB, which, in turn, regulates preferential synaptic integration with either mitral cell or tufted cell secondary dendrites.

Olfactory bulb interneurons - The developmental timeline and targeting defined by embryonic neurogenesis

The generation of mouse olfactory bulb (OB) interneurons (INs) is initiated in the embryo but continues throughout life. It is generally agreed that OB INs generated postnatally affect the connectivity of the OB, depending on the timeline of neurogenesis. Here, we focused on OB INs generated embryonically, which have generally received less attention than those generated in the adult. Birthdates of embryonic INs were differentiated by maternal injections of thymidine analogs and their final destinations and phenotypes in the OB analyzed by immunohistochemistry. We found that the first embryonic INs were generated at embryonic day 10 (E10) and continued through the entire embryonic development. Analysis in adult tissues showed that embryonic INs were retained and were distributed across all layers of the OB. Interestingly, an initial lateral preference in cell density was seen in INs generated during E11-E13. Although INs are broadly distributed in the OB, we found that within the granule cell layer (GCL), OB INs distributed mostly in the superficial GCL. Immunostaining for calbindin, parvalbumin, tyrosine hydroxylase, 5T4 and calretinin were lacking co-expression with thymidine analogs labeled cells, suggesting that maturation of embryonic INs occurred slowly following birth. We studied the embryonic neuroblasts migration and differentiation by labeling IN progenitor cells in the lateral ganglionic eminence using in utero electroporation. We found that IN neuroblasts reached the primordial OB as early as E13 and began to differentiate apical dendrites by E15, which extended into the developing external plexiform layer. We established E16 as the embryonic stage at which the prototypical chain of migrating neuroblasts denoting the embryonic rostral migratory stream (RMS) was visible. Collectively, our data highlight the importance of studying OB INs in isolated time windows to better understand the formation of circuits that define the olfactory system function.

Olfactory Development and Dysfunction: Involvement of Microglia

Olfactory deficits are increasingly recognized in a variety of neurological, neurodevelopmental, psychiatric, and viral diseases. While the pathology underlying olfactory loss is likely to differ across diseases, one shared feature may be an immune response mediated by microglia. Microglia orchestrate the brain's response to environmental insults and maintain neurodevelopmental homeostasis. Here, we explore the potential involvement of microglia in olfactory development and loss in disease. The effects of microglia-mediated immune response during development may be of special relevance to the olfactory system, which is unique in both its vulnerability to environmental insults as well as its extended period of neurogenesis and neuronal migration.

Inflammatory Response and Defects on Myelin Integrity in the Olfactory System of K18hACE2 Mice Infected with SARS-CoV-2

Viruses, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), use respiratory epithelial cells as an entry point for infection. Within the nasal cavity, the olfactory epithelium (OE) is particularly sensitive to infections which may lead to olfactory dysfunction. In patients suffering from coronavirus disease 2019, deficits in olfaction have been characterized as a distinctive symptom. Here, we used the K18hACE2 mice to study the spread of SARS-CoV-2 infection and inflammation in the olfactory system (OS) after 7 d of infection. In the OE, we found that SARS-CoV-2 selectively targeted the supporting/sustentacular cells (SCs) and macrophages from the lamina propria. In the brain, SARS-CoV-2 infected some microglial cells in the olfactory bulb (OB), and there was a widespread infection of projection neurons in the OB, piriform cortex (PC), and tubular striatum (TuS). Inflammation, indicated by both elevated numbers and morphologically activated IBA1+ cells (monocyte/macrophage lineages), was preferentially increased in the OE septum, while it was homogeneously distributed throughout the layers of the OB, PC, and TuS. Myelinated OS axonal tracts, the lateral olfactory tract, and the anterior commissure, exhibited decreased levels of 2',3'-cyclic-nucleotide 3'-phosphodiesterase, indicative of myelin defects. Collectively, our work supports the hypothesis that SARS-CoV-2 infected SC and macrophages in the OE and, centrally, microglia and subpopulations of OS neurons. The observed inflammation throughout the OS areas and central myelin defects may account for the long-lasting olfactory deficit.

α-Synuclein Pathology and Reduced Neurogenesis in the Olfactory System Affect Olfaction in a Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by multiple symptoms including olfactory dysfunction, whose underlying mechanisms remain unclear. Here, we explored pathologic changes in the olfactory pathway of transgenic (Tg) mice of both sexes expressing the human A30P mutant α-synuclein (α-syn; α-syn-Tg mice) at 6-7 and 12-14 months of age, representing early and late-stages of motor progression, respectively. α-Syn-Tg mice at late stages exhibited olfactory behavioral deficits, which correlated with severe α-syn pathology in projection neurons (PNs) of the olfactory pathway. In parallel, olfactory bulb (OB) neurogenesis in α-syn-Tg mice was reduced in the OB granule cells at six to seven months and OB periglomerular cells at 12-14 months, respectively, both of which could contribute to olfactory dysfunction. Proteomic analyses showed a disruption in endocytic and exocytic pathways in the OB during the early stages which appeared exacerbated at the synaptic terminals when the mice developed olfactory deficits at 12-14 months. Our data suggest that (1) the α-syn-Tg mice recapitulate the olfactory functional deficits seen in PD; (2) olfactory structures exhibit spatiotemporal disparities for vulnerability to α-syn pathology; (3) α-syn pathology is restricted to projection neurons in the olfactory pathway; (4) neurogenesis in adult α-syn-Tg mice is reduced in the OB; and (5) synaptic endocytosis and exocytosis defects in the OB may further explain olfactory deficits.SIGNIFICANCE STATEMENT Olfactory dysfunction is a characteristic symptom of Parkinson's disease (PD). Using the human A30P mutant α-synuclein (α-syn)-expressing mouse model, we demonstrated the appearance of olfactory deficits at late stages of the disease, which was accompanied by the accumulation of α-syn pathology in projection neurons (PNs) of the olfactory system. This dysfunction included a reduction in olfactory bulb (OB) neurogenesis as well as changes in synaptic vesicular transport affecting synaptic function, both of which are likely contributing to olfactory behavioral deficits.

Lineage Relationships Between Subpallial Progenitors and Glial Cells in the Piriform Cortex

The piriform cortex is a paleocortical area, located in the ventrolateral surface of the rodent forebrain, receiving direct input from the olfactory bulb. The three layers of the PC are defined by the diversity of glial and neuronal cells, marker expression, connections, and functions. However, the glial layering, ontogeny, and sibling cell relationship along the PC is an unresolved question in the field. Here, using multi-color genetic lineage tracing approaches with different StarTrack strategies, we performed a rigorous analysis of the derived cell progenies from progenitors located at the subpallium ventricular surface. First, we specifically targeted E12-progenitors with UbC-StarTrack to analyze their adult derived-cell progeny and their location within the piriform cortex layers. The vast majority of the cell progeny derived from targeted progenitors were identified as neurons, but also astrocytes and NG2 cells. Further, to specifically target single Gsx-2 subpallial progenitors and their derived cell-progeny in the piriform cortex, we used the UbC-(Gsx-2-hyPB)-StarTrack to perform an accurate analysis of their clonal relationships. Our results quantitatively delineate the adult clonal cell pattern from single subpallial E12-progenitors, focusing on glial cells. In summary, there is a temporal pattern in the assembly of the glial cell diversity in the piriform cortex, which also reveals spatio-temporal progenitor heterogeneity.