Effects of long-term and brain-wide colonization of peripheral bone marrow-derived myeloid cells in the CNS.

BACKGROUND: Microglia, the primary resident myeloid cells of the brain, play critical roles in immune defense by maintaining tissue homeostasis and responding to injury or disease. However, microglial activation and dysfunction has been implicated in a number of central nervous system (CNS) disorders, thus developing tools to manipulate and replace these myeloid cells in the CNS is of therapeutic interest. METHODS: Using whole body irradiation, bone marrow transplant, and colony-stimulating factor 1 receptor inhibition, we achieve long-term and brain-wide (~ 80%) engraftment and colonization of peripheral bone marrow-derived myeloid cells (i.e., monocytes) in the brain parenchyma and evaluated the long-term effects of their colonization in the CNS. RESULTS: Here, we identify a monocyte signature that includes an upregulation in Ccr1, Ms4a6b, Ms4a6c, Ms4a7, Apobec1, Lyz2, Mrc1, Tmem221, Tlr8, Lilrb4a, Msr1, Nnt, and Wdfy1 and a downregulation of Siglech, Slc2a5, and Ccl21a/b. We demonstrate that irradiation and long-term (~ 6 months) engraftment of the CNS by monocytes induces brain region-dependent alterations in transcription profiles, astrocytes, neuronal structures, including synaptic components, and cognition. Although our results show that microglial replacement with peripherally derived myeloid cells is feasible and that irradiation-induced changes can be reversed by the replacement of microglia with monocytes in the hippocampus, we also observe that brain-wide engraftment of peripheral myeloid cells (relying on irradiation) can result in cognitive and synaptic deficits. CONCLUSIONS: These findings provide insight into better understanding the role and complexity of myeloid cells in the brain, including their regulation of other CNS cells and functional outcomes.

Prolonged stimulus exposure reveals prolonged neurobehavioral response patterns.

Although it has been shown repeatedly that minimum response times in sensory systems can be quite short, organisms more often continue to respond to sensory stimuli over considerably longer periods of time. The continuing response to sensory stimulation may be a more realistic assessment of natural sensory responses, so we determined for how long a stimulus would evoke a response in naïve, freely moving animals. Specifically, we determined for how long such rats responded to odorants during continuous passive exposures by monitoring their sniffing with whole-body plethysmography. We found that naïve rats continue to sniff odorants vigorously for up to 3 minutes, much longer than what has been reported for highly trained, highly motivated rats. Patterns of 2-deoxyglucose (2-DG) uptake in the glomerular layer of the rat olfactory bulb also were seen after only 1-5 minutes of odorant exposure, overlapping with the period of increased respiration to odorants. Moreover, these 2-DG uptake patterns closely resembled the patterns that emerge from prolonged odorant exposures, suggesting that activity mapping over prolonged periods can identify areas of activity that are present when rats are still attending and responding to odorant stimuli. Given these findings, it seems important to consider the possibility that prolonged exposure to other sensory stimuli will reveal more realistic neural response patterns.

Broad activation of the glomerular layer enhances subsequent olfactory responses.

Early olfactory experience with a specific odorant enhances the subsequent response of the glomerular layer of the rat olfactory bulb to that same odorant. Because different odorants activate different glomerular layer regions, it seemed plausible that experience with a large number of odorants might result in enhanced glomerular activation during subsequent exposure to both the previously experienced odorants and the novel odorants evoking activity in regions that overlapped with those previously stimulated by different odorants. To this end, 7 odorants were selected using our glomerular response data archive that together stimulated much of the glomerular layer (alpha-phellandrene, benzaldehyde, L-carvone, decanal, pentanol, santalol, and valeric acid). Young rats were exposed to a different odorant each day for 7 days, and this cycle was repeated 3 times from postnatal days 1-21. The [(14)C]2-deoxyglucose technique was used to measure neural activity in response to both previously experienced and novel odorants. The 2 novel odorants (alpha-ionone and L-menthone) activate regions of the glomerular layer that overlap with those stimulated by the 7 enrichment odorants. Our results indicate that early experience with multiple odorants results in increased responsiveness both to previously experienced odorants and to novel odorants that stimulate previously activated regions of the bulb.

Exposure to a broad range of odorants decreases cell mortality in the olfactory bulb.

Experience with multiple odorants during early postnatal development increases the number of cells in the olfactory bulb of rats. In this study, we asked whether at least part of this increase was due to decreased cell death. We selected 30 natural odorants or synthetic odorant mixtures to stimulate a broad area of the bulb during postnatal days 1-15, and counted the number of cells with DNA damage associated with cell death in both the glomerular and the granule cell layers of the main olfactory bulb. Early olfactory enrichment significantly decreased cell death in both bulbar laminae. Thus, olfactory enrichment can spare bulbar cells during early development, possibly leading to increased efficacy in bulb function and enhanced bulbar responses.

Functional mapping of the rat olfactory bulb using diverse odorants reveals modular responses to functional groups and hydrocarbon structural features.

In an effort to understand the olfactory code of rats, we collected more than 1,500,000 measurements of glomerular activity in response to 54 odorants selected to provide differences in functional groups and hydrocarbon structure. Each odorant evoked a unique response pattern by differentially stimulating clusters of glomeruli, called modules. Odorants sharing specific aspects of their structure activated the same modules, allowing us to relate responses to structure across approximately 80% of the glomerular layer. The most obvious relationship was between the presence of particular oxygen-containing functional groups and the activity of glomeruli within dorsal modules. Functional group-specific responses were observed for odorants possessing a wide range of hydrocarbon structure, including aliphatic, cyclic, and aromatic features. Even formic acid and acetone, the simplest odorants possessing acid or ketone functional groups, respectively, stimulated modules specific for these functional groups. At the same time, quantitative analysis of pattern similarities revealed relationships in activation patterns between odorants of similar hydrocarbon structure. The odorant responses were reliable enough to allow us to predict accurately specific aspects of odorant molecular structure from the evoked glomerular activity pattern, as well as predicting the location of glomerular activity evoked by novel odorants.