James E. Schwob

Neural regeneration and the peripheral olfactory system

The peripheral olfactory system is able to recover after injury, i.e., the olfactory epithelium reconstitutes, the olfactory nerve regenerates, and the olfactory bulb is reinnervated, with a facility that is unique within the mammalian nervous system. Cell renewal in the epithelium is directed to replace neurons when they die in normal animals and does so at an accelerated pace after damage to the olfactory nerve. Neurogenesis persists because neuron‐competent progenitor cells, including transit amplifying and immediate neuronal precursors, are maintained within the population of globose basal cells. Notwithstanding events in the neuron‐depleted epithelium, the death of both non‐neuronal cells and neurons directs multipotent globose basal cell progenitors, to give rise individually to sustentacular cells and horizontal basal cells as well as neurons. Multiple growth factors, including TGF‐α, FGF2, BMPs, and TGF‐βs, are likely to be central in regulating choice points in epitheliopoiesis. Reinnervation of the bulb is rapid and robust. When the nerve is left undisturbed, i.e., by lesioning the epithelium directly, the projection of the reconstituted epithelium onto the bulb is restored to near‐normal with respect to rhinotopy and in the targeting of odorant receptor‐defined neuronal classes to small clusters of glomeruli in the bulb. However, at its ultimate level, i.e., the convergence of axons expressing the same odorant receptor onto one or a few glomeruli, specificity is not restored unless a substantial number of fibers of the same type are spared. Rather, odorant receptor‐defined subclasses of neurons innervate an excessive number of glomeruli in the rough vicinity of their original glomerular targets. Anat Rec (New Anat) 269:33–49, 2002.

Adult olfactory epithelium contains multipotent progenitors that give rise to neurons and non-neural cells

We have infused replication‐incompetent retroviral vectors into the nasal cavity of adult rats 1 day after exposure to the olfactotoxic gas methyl bromide (MeBr) to assess the lineage relationships of cells in the regenerating olfactory epithelium. The vast majority of the retrovirus‐labeled clones fall into three broad categories: clones that invariably contain globose basal cells (GBCs) and/or neurons, clones that always include cells in the ducts of Bowmans glands, and clones that are composed of sustentacular cells only. Many of the GBC‐related clones contain sustentacular cells and horizontal basal cells as well. Most of the duct‐related clones contain gland cells, and some also include sustentacular cells. Thus, the destruction of both neurons and non‐neuronal cells that is caused by MeBr activates two distinct types of multipotent cells. The multipotent progenitor that gives rise to neurons and non‐neuronal cells is a basal cell, whereas the progenitor that gives rise to duct, gland, and sustentacular cells resides within the ducts, based on the pattern of sparing after lesion and the analysis of early regeneration by using cell type‐specific markers. We conclude that the balance between multipotency and selective neuropotency, which is characteristic of globose basal cells in the normal olfactory epithelium, is determined by which cell types have been depleted and need to be replenished rapidly. J. Comp. Neurol. 400:469–486, 1998.

Anterior distribution of human olfactory epithelium.

Objectives/Hypothesis To functionally investigate the distribution of the olfactory epithelium in humans by means of the electro‐olfactogram (EOG) and anatomically located biopsy specimens.

Multipotency of purified, transplanted globose basal cells in olfactory epithelium.

By comparison with the rest of the nervous system, the olfactory epithelium has an unparalleled ability to renew and repair itself throughout life. However, the identity and capacity of the various types of progenitor cells that underlie that ability are not well established. We used selective isolation, transplantation, and engraftment of various types of marker‐labeled cells into the epithelium of methyl bromide‐lesioned, unmarked host mice to dissect progenitor cell capacity. Globose basal cells were purified from other potential progenitors using the monoclonal antibody GBC‐2 (GBC, globose basal cell) and fluorescence activated cell sorting. Transplanted globose basal cells engraft and, in aggregate, give rise to globose basal cells, neurons, sustentacular cells, and several other kinds of non‐neuronal cells. Individual clones, derived from single engrafted globose basal cells, can consist of a mixture of neurons and non‐neuronal cells, only neurons, or only non‐neuronal cells. Neurons that arise after transplantation mature to the point of expressing odorant receptors and olfactory marker protein and of projecting axons to the olfactory bulb. In contrast, other kinds of epithelial cells are neither neurogenic nor multipotent. For example, sustentacular and duct cells give rise only to themselves after transplantation. Furthermore, horizontal basal cells do not engraft in mice, in which the endogenous population is spared after lesion. Thus, some subtype(s) of GBC is a multipotent progenitor cell, whose multipotency is activated after destruction of both neurons and non‐neuronal cells. The results suggest that progenitor cell transplantation may prove useful as a therapeutic modality as well as an analytical tool. J. Comp. Neurol. 469:457–474, 2004.

The aging olfactory epithelium: Neurogenesis, response to damage, and odorant-induced activity

Olfactory epithelium retains the capacity to recover anatomically after damage well into adult life and perhaps throughout its duration. None the less, olfactory dysfunctions have been reported widely for elderly humans. The present study investigates the effects of aging on the neurophysiological and anatomical status of the olfactory epithelium in barrier‐raised Fischer 344X Brown Norway F 1 hybrid rats at 7, 10, 25 and 32/35 months old. The posterior part of the olfactory epithelium in 32/35‐month‐old rats is well preserved. Globose basal cells are dividing, and new neurons are being born even at this advanced age. None the less, the numbers of proliferating basal cells and immature, GAP‐43 (+) neurons are significantly decreased. Neurophysiological status was evaluated using voltage‐sensitive dye techniques to assess inherent patterns of odorant‐induced activity in the epithelium lining the septum and the medial surface of the turbinates. In middle and posterior zones of the epithelium, there were neither age‐related changes in overall responsivity of this part of the olfactory epithelium to any of five odorants, nor shifts in the location of the odorant‐induced hotspots. The inherent activity patterns elicited by the different odorants do become more distinct as a function of age, which probably reflects the decline in immature neurons and a slight, but not statistically significant, increase in mature neurons as a function of age. In contrast with the excellent preservation of posterior epithelium, the epithelium lining the anterodorsal septum and the corresponding face of the turbinates is damaged in the 32/35‐month‐old animals: in this part, horizontal basal cells are reactive, more basal cells and sustentacular cells are proliferating than in younger animals or in posterior epithelium of the same animals, and the neuronal population is less mature on average. Our findings indicate that degeneration of the olfactory epithelium is not an inevitable or pre‐programmed consequence of the aging process, since the posterior zone of the epithelium is very well preserved in these barrier‐protected animals. However, the deterioration in the anterior epithelium suggests that environmental insults can accumulate or become more severe with age and overwhelm the regenerative capacity of the epithelium. Alternatively, the regenerative capacity of the epithelium may wane somewhat with age. Either of these mechanisms or some combination of them can account for the functional and anatomical deterioration of the sense of smell associated with senescence in humans.

The FXG: A presynaptic Fragile X granule expressed in a subset of developing brain circuits

The loss of Fragile X mental retardation protein (FMRP) causes Fragile X syndrome, the most common inherited mental retardation and single gene cause of autism. Although postsynaptic functions for FMRP are well established, potential roles at the presynaptic apparatus remain largely unexplored. Here, we characterize the expression of FMRP and its homologs, FXR1P and FXR2P, in the developing, mature and regenerating rodent nervous system, with a focus on presynaptic expression. As expected, FMRP is expressed in the somatodendritic domain in virtually all neurons. However, FMRP is also localized in discrete granules (Fragile X granules; FXGs) in a subset of brain regions including frontal cortex, hippocampal area CA3 and olfactory bulb glomeruli. Immunoelectron microscopy shows that FMRP is localized at presynaptic terminals and in axons within these FXG-rich regions. With the exception of the olfactory bulb, FXGs are prominent only in the developing brain. Experiments in regenerating olfactory circuits indicate that peak FXG expression occurs 2–4 weeks after neurogenesis, a period that correlates with synapse formation and refinement. Virtually all FXGs contain FXR2P, while region-selective subsets harbor FMRP and/or FXR1P. Genetic studies show that FXR2P is essential for FXG expression, while FMRP regulates FXG number and developmental profile. These findings suggest that Fragile X proteins play a distinct, presynaptic role during discrete developmental epochs in defined circuits of the mammalian CNS. We propose that the neurological defects in Fragile X syndrome, including the autistic features, could be due in part to the loss of FMRP function in presynaptic compartments.

Odorant Receptor Expression Patterns Are Restored in Lesion-Recovered Rat Olfactory Epithelium

Lesions of the olfactory periphery provide a means for examining the reconstitution of a diverse and highly regulated population of sensory neurons and the growth, en masse, of nascent axons to the bulb. The olfactory epithelium and its projection onto the bulb are reconstituted after ablation by methyl bromide gas, and some measure of olfactory function is restored. The extent to which the system regenerates the full repertoire of odorant receptor-expressing neurons, particularly their spatially restricted distribution across the epithelial sheet, is unknown, however, and altered odorant receptor expression might contribute to the persistent distortion of odorant quality that is observed in the lesioned-recovered animals. To address the question of receptor expression in the recovered epithelium, we performed in situ hybridization with digoxigenin-labeled riboprobes for eight odorant receptors on the olfactory epithelium from unilaterally methyl bromide-lesioned and control rats. The data demonstrate that the distribution of sensory neuron types, as identified and defined by odorant receptor expression, is restored to normal or nearly so by 3 months after lesion. Likewise, the numbers of probe-labeled neurons in the lesioned-recovered epithelium are nearly equivalent to the unlesioned side at this time. Finally, our evidence suggests that odorant receptors are distributed in multiple overlapping bands in the normal, unlesioned, and lesioned-recovered epithelium rather than in the conventionally accepted three or four zones. Thus, the primary sensory elements required for functional recovery of the olfactory system after damage are restored, and altered function implies the persistence of a more central failure in regeneration.

Globose basal cells are required for reconstitution of olfactory epithelium after methyl bromide lesion.

Despite a remarkable regenerative capacity, recovery of the mammalian olfactory epithelium can fail in severely injured areas, which subsequently reconstitute as aneuronal respiratory epithelium (metaplasia). We contrasted the cellular response of areas of the rat epithelium that recover as olfactory after methyl bromide lesion with those undergoing respiratory metaplasia in order to identify stem cells that restore lesioned epithelium as olfactory. Ventral olfactory epithelium is at particular risk for metaplasia after lesion and patches of it are rendered acellular by methyl bromide exposure. In contrast, globose basal cells (GBCs, marked by staining with GBC‐2) are preserved in surrounding ventral areas and uniformly throughout dorsal epithelium, which consistently and completely recovers as olfactory after lesion. Over the next few days, neurons reappear, but only in those areas in which GBCs are preserved and multiply. In contrast, parts of the epithelium in which GBCs are destroyed are repopulated in part by Bowmans gland cells, which pile up above the basal lamina. Electron microscopy confirms the reciprocity between gland cells and globose basal cells. By 14 days after lesion, the areas that are undergoing metaplasia are repopulated by typical respiratory epithelial cells. As horizontal basal cells are eliminated from all parts of the ventral epithelium, the data suggest that GBC‐2(+) cells are ultimately responsible for regenerating olfactory neuroepithelium. In contrast, GLA‐13(+) cells may give rise to respiratory metaplastic epithelium where GBCs are eliminated. Thus, we support the idea that a subpopulation of GBCs is the neural stem cell of the olfactory epithelium. J. Comp. Neurol. 460:123–140, 2003.

Olfactory uptake of manganese requires DMT1 and is enhanced by anemia

Manganese, an essential nutrient, can also elicit toxicity in the central nervous system (CNS). The route of exposure strongly influences the potential neurotoxicity of manganese‐containing compounds. Recent studies suggest that inhaled manganese can enter the rat brain through the olfactory system, but little is known about the molecular factors involved. Divalent metal transporter‐1 (DMT1) is the major transporter responsible for intestinal iron absorption and its expression is regulated by body iron status. To examine the potential role of this transporter in uptake of inhaled manganese, we studied the Belgrade rat, since these animals display significant defects in both iron and manganese metabolism due to a glycine‐to‐arginine substitution (G185R) in their DMT1 gene product. Absorption of intranasally instilled 54Mn was significantly reduced in Belgrade rats and was enhanced in iron‐deficient rats compared to iron‐sufficient controls. Immunohistochemical experiments revealed that DMT1 was localized to both the lumen microvilli and end feet of the sustentacular cells of the olfactory epithelium. Importantly, we found that DMT1 protein levels were increased in anemic rats. The apparentfunction of DMT1 in olfactory manganese absorption suggests that the neurotoxicity of the metal can be modified by iron status due to the iron‐responsive regulation of the transporter. Thompson, K., Molina, R. M., Donaghey, T., Schwob, J. E., Brain, J. D., Wessling‐Resnick, M. Olfactory uptake of manganese requires DMT1 and is enhanced by anemia. FASEB J. 21, 223–230 (2007)

Reinnervation of the rat olfactory bulb after methyl bromide-induced lesion: timing and extent of reinnervation.

We used the inhalation of methyl bromide gas to produce a near‐complete destruction of the rat olfactory epithelium and analyzed the reinnervation of the bulb during reconstitution of the epithelium. The degeneration of olfactory axons elicits a transient up‐regulation of glial cell proliferation and glial fibrillary acidic protein expression in the olfactory nerve and olfactory nerve layer of the bulb. Anterograde transport after intranasal infusion of wheat germ agglutinin conjugated horseradish peroxidase demonstrates that the first nascent axons reach the bulb within the first week after lesion. Subsequently, a massive wave of fibers arrives at the bulb between 1 and 2 weeks postlesion, and enters the glomeruli between 2 and 3 weeks postlesion. However, the olfactory projection does not stabilize until 8 weeks after lesion judging from the return in growth associated protein‐43 expression to control levels. The extent of reinnervation after lesion is correlated with the completeness with which the epithelium reconstitutes itself. In rats that are lesioned while fed ad libitum, there is near‐complete reconstitution of the neuronal population, and the projection onto the bulb fills the glomerular layer in its entirety. However, in rats that are lesioned while food restricted, a significant fraction of olfactory epithelium becomes respiratory during its reconstitution, and the population of reinnervating fibers is less. As a consequence, the posterior half of the bulb remains hypoinnervated overall and denervated at its caudal margin. The preferential reinnervation of the anterior bulb in the food‐restricted, methyl bromide gas–lesioned animals indicates that the mechanisms that guide the growth of the olfactory axons and restore receptotopy do not operate with the same precision in this setting as they do during development or during the lower level of turnover associated with the “normal” laboratory existence. Accordingly, we hypothesize that the persistence of a significant population of pre‐existing neurons is needed to preserve receptotopy during reinnervation. In addition, the results suggest that in the face of massive turnover and a reduced afferent population, there is a tendency for reinnervating axons to fill available synaptic space. J. Comp. Neurol. 412:439–457, 1999.