Experimental studies on the olfactory marker protein. IV. Olfactory marker protein in the olfactory neurons transplanted within the brain

Grafting the olfactory epithelium to the olfactory bulb

BACKGROUND

Impaired olfactory function leads to a decrease in the quality of life for many patients. Surgical treatment options are limited, especially for those suffering from hyposmia or anosmia after posttraumatic injury to the olfactory nerves. Stem cells located in the olfactory epithelium (OE) have the capacity to grow new neurons, making the OE an ideal candidate for restorative tissue grafting. This study was performed to determine if strips of OE survive transplantation directly to the olfactory bulb (OB).

METHODS

Transgenic mice, expressing a green fluorescent protein (GFP), were used to obtain the donor graft tissue. Strips of OE from GFP donor mice were transplanted directly to sites in the OB and cerebral cortex (CC; control sites) of wild-type mice. Graft survival rates at 30 days were determined for transplant sites in the OB and CC.

RESULTS

Strips of OE from transgenic mice survived transplantation to the OB and continued to express the GFP marker protein. The 30-day survival rate in the OB (83%, 5 of 6 grafts) was the same as in the CC (10 of 12 grafts). The morphology of the graft revealed characteristics found in normal OE.

CONCLUSION

We showed that strips of OE can be successfully grafted to both the OB and CC. Grafts of the OE, if strategically positioned on the ventral surface of the bulb and given access to the nasal cavity, could provide the basis for new surgical treatments to restore olfactory function.

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.

LacZ and OMP are co-expressed during ontogeny and regeneration in olfactory receptor neurons of OMP promoter-lacZ transgenic mice

The ontogeny and cellular specificity of expression of beta-galactosidase activity and olfactory marker protein (OMP) are compared in olfactory tissue of the H-OMP-lacZ-3 line of transgenic mice. In this line the expression of lacZ is driven by a 0.3 kb fragment of the rat OMP promoter. During fetal development, lacZ expression is detectable in olfactory receptor neurons (ORNs) shortly after the initial appearance of endogenous OMP. The beta-galactosidase marker was observed only in mature olfactory receptor neurons where it co-localized with endogenous OMP. It was absent from immature neurons that express the growth associated phosphoprotein B50/GAP43. Lesion of the peripheral olfactory pathway by intranasal irrigation with Triton X-100 eliminated expression of both OMP and lacZ in the olfactory neuroepithelium. Subsequent regeneration of the full complement of olfactory receptor neurons was associated with co-expression of both OMP and beta-galactosidase activity. Neither OMP nor beta-galactosidase activity was induced in any other cell type of the regenerating olfactory mucosa. Thus, as little as 0.3 kb of the OMP promoter has the ability to target lacZ expression to olfactory receptor neurons in a temporally and spatially defined manner. We discuss the potential utility of this transgenic line for future studies of the olfactory system.

Histochemical and immunocytochemical study of the migration of neurons from the rat olfactory placode

Immunocytochemical and histochemical methods have been used to describe the neuronal population migrating from the rat olfactory placode and to analyze the spatio-temporal evolution of this neuronal migration during development. Several neuronal markers, such as binding to the lectin Ulex europaeus (UEA I) and the presence of neuron-specific enolase (NSE), olfactory marker protein (OMP), and luteinizing hormone-releasing hormone (LHRH), have been tested in order to determine whether migrating neurons originate from both the medial and the lateral parts of the placode and whether they all express LHRH. Our data show that a large population of differentiated migrating neurons can be identified with an antibody against NSE from the 14th day of gestation and with UEA I one day later. Migrating neurons are closely associated with both the vomeronasal axon fascicles emerging from the medial pit and the olfactory axons originating from the lateral pit. However, the neuron migration from the lateral pit appears to be more discrete than that from the medial pit. No LHRH immunoreactivity has been detected among neurons migrating from the lateral pit. Some neurons accompanying the olfactory axon fascicles exhibit a high level of maturation as shown by their OMP-positivity. Numerous neurons positive for both NSE and UEA I have also been observed within the presumptive olfactory nerve layer in early embryonic stages.

Formation of an olfactory glomerulus: morphological aspects of development and organization

We have studied the development of olfactory nerves in the rat from their first contact with the telencephalic vesicle until the formation of glomerular structures in the olfactory bulb at early postnatal period. The study is based on serial semithin and ultrathin sections of material prepared for electron microscopy and antibodies to label radial glial cells, glial fibrillary acidic protein and Rat-401. Beginning on embryonic day 12, developing olfactory axons from the olfactory placode are accompanied by migratory cells, also derived from the olfactory placode, that reach the prospective olfactory bulb by embryonic day 13. The mass of migratory cells accumulate superficial to the telencephalic vesicle. The cells increase in number by mitotic divisions. The majority of these cells represent precursor elements that will later develop into the ensheathing cells of the olfactory nerves and olfactory nerve layer of the adult. Some migratory cells penetrate into the prospective olfactory bulb early during development. The first synaptic contacts of olfactory axons with dendritic processes in the olfactory bulb were observed at embryonic day 18. Glomerular formation is initiated by penetration of cells from the migratory mass into the prospective glomerular layer by embryonic day 20 to postnatal day 0. These cells form walls surrounding zones of high synaptic density forming protoglomeruli. Postnatally, the peripheral processes of radial glial cells branch profusely delimiting glomerular formations and transform into periglomerular astrocytes. Rat-401 stains radial glial cells from embryonic day 14. Immunoreactivity becomes restricted to the olfactory glomeruli during the first postnatal weeks and it virtually disappears by the end of the first postnatal month. We conclude that the early penetration of cells from the migratory mass into the prospective olfactory bulb, observed immediately after the first synaptic contacts were established, initiates the formation of olfactory glomeruli which becomes completed by the transformation of radial glial cells into periglomerular astrocytes.

CNS glial cells support in vitro survival, division, and differentiation of dissociated olfactory neuronal progenitor cells

Olfactory receptor neurons (ORNs) are replaced and differentiate in adult animals, but differentiation in dissociated cell culture has not been demonstrated. To test whether contact with the CNS regulates maturation, neonatal rat olfactory cells were grown on a culture substrate or on CNS astrocytes. Mature ORNs, immunopositive for olfactory marker protein (OMP), disappeared rapidly from both systems. Neurons positive for neuron-specific tubulin (immature and mature) disappeared from substrate-only cultures, but remained abundant in the cocultures. OMP-positive neurons reappeared after 10 days in vitro. Pulse labeling with [3H]thymidine showed extensive neurogenesis of both immature and mature olfactory neurons. This demonstrates, in vitro, both division and differentiation of olfactory progenitor cells.

PNS-CNS transitional zone of the first cranial nerve

This study examined the ultrastructure of the region of transition where fascicles of olfactory axons leave the peripheral nervous system (PNS) to enter the central nervous system (CNS), the so-called PNS-CNS transitional zone. Adult rats were transcardially perfused with a solution of 1% glutaraldehyde and 1% paraformaldehyde, decapitated, and the heads decalcified over a period of several weeks in a solution of 1% glutaraldehyde in 0.1 M tetrasodium ethylenediamine tetraacetic acid; the latter solution was changed daily. It was found that astrocytes did not form the glia limitans at the nerve entry zone, unlike the situation that exists in other cranial and spinal nerves. Rather, the glia limitans in this region of the olfactory bulb was formed by a special type of glial cell, referred to as an ensheathing cell. Ensheathing cells are found only in the nerve fiber layer of the olfactory bulb. They possess a mixture of Schwann cell and astrocytic features and are more likely to be of placodal than of CNS origin. The meningeal coverings of the olfactory nerve rootlets and of the olfactory bulb are also described and the functional implications of the findings discussed.

Regulation of gene expression in the olfactory neuroepithelium: a neurogenetic matrix

The olfactory neuroepithelium exhibits neurogenesis throughout adult life, and in response to lesions, a phenomenon that distinguishes this neural tissue from the rest of the mammalian brain. The newly formed primary olfactory neurons elaborate axons into the olfactory bulb. Thus, denervation and subsequent reinnervation of olfactory bulb neurons may occur throughout life. This unique ability of the olfactory neuroepithelium to generate new neurons from a population of precursor cells present in the basal cell layer of this tissue makes it a valuable model in the study of neural development and regeneration. The molecular processes underlying the neurogenic properties of the olfactory neuroepithelium are poorly understood. Here we have reviewed our studies on the expression of B50/GAP43 during ontogeny of the olfactory system and following lesioning. This analysis includes the characterization of the expression of OMP, a protein expressed in mature olfactory neurons, as well as PKC and calmodulin. The latter two molecules are of particular interest to the function of B50/GAP43 since the degree of phosphorylation of B50/GAP43 appears to determine B50/GAP43's ability to bind calmodulin (see also Storm, chapter 4, this volume). In the mature olfactory epithelium B50/GAP43 expression is restricted to a subset of cells located in the basal region. Since the expression of B50/GAP43 is high in developing and regenerating nerve cells we are confident that the B50/GAP43 positive cells are new neurons derived from the stem cells in the basal region of the epithelium. B50/GAP43 is absent from the stem cells themselves and also from the mature OMP-expressing neurons. On the basis of the patterns of B50/GAP43 and OMP expression two stages could be discriminated in the regeneration of the olfactory epithelium. First, as an immediate response to lesioning a large population of B50/GAP43 positive, OMP negative neurons are formed. Subsequently, during the second stage, these newly formed differentiating neurons mature as evidenced by a decrease in B50/GAP43 and an increase in OMP expression. The second stage in the regeneration process is only manifested if the regenerating neurons can reach their target cells in the olfactory bulb. Hence, bulbectomy results in the arrest of the reconstituted olfactory epithelium in an immature state. The differential patterns of B50/GAP43 expression following peripheral lesioning and bulbectomy suggest the existence of a target derived signal molecule involved in the down-regulation of B50/GAP43 expression in olfactory neurons that have established synaptic contacts in the olfactory bulb (see also Willard, chapter 2, this volume, "the suppressor hypothesis").(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroplasticity in the olfactory system: differential effects of central and peripheral lesions of the primary olfactory pathway on the expression of B-50/GAP43 and the olfactory marker protein

The regeneration of the olfactory neuroepithelium following olfactory bulbectomy or peripheral deafferentation was studied with mRNA probes and antibodies for B-50/GAP43 and for olfactory marker protein (OMP). Two stages in the regeneration of the olfactory epithelium could be discerned with these reagents. The first stage occurs following either peripheral deafferentation of the olfactory epithelium with Triton X-100 (TX-100) or after bulbectomy and is characterized by the formation of a large population of immature olfactory receptor neurons. These newly formed neurons express B-50/GAP43, a phosphoprotein related to neuronal growth and plasticity. During the second stage of the regeneration process the newly formed olfactory neurons mature, as evidenced by a decrease in their expression of B-50/GAP43 and an increase in the expression of OMP. This stage is only manifested if the developing neurons have access to the target olfactory bulb. Formation of a full complement of OMP-expressing neurons occurs only after peripheral lesion with TX-100. In contrast, following bulbectomy the reconstituted olfactory epithelium lacks its normal target and is compromised in its ability to recover from nerve damage, as evidenced by the presence of a large number of B-50/GAP43-expressing neurons up to 3 months after the lesion and its failure to establish a full complement of OMP-expressing neurons. These results demonstrate that the olfactory epithelium is capable of replacing its sensory neurons independently of the presence of its target, the olfactory bulb. However, the differential patterns of expression of B-50/GAP43 and OMP at long times after peripheral lesion with TX-100 or bulbectomy illustrate the profound effect the olfactory bulb has on neuronal maturation in reconstituted olfactory neuroepithelium.

Glial influences on axonal growth in the primary olfactory system

Neurogenesis in the olfactory epithelium continues throughout the entire life of mammals, and it is the axons of these newly formed olfactory receptor neurons that grow into the target tissue after the first cranial nerve is injured, not the regenerating axons of mature cells. These axons are able to enter and grow within the CNS of adult animals, unlike regenerating axons in injured dorsal roots, the majority of which are prevented from penetrating very far into the spinal cord. One reason why the olfactory axons are so successful in entering the CNS may be due, at least partially, to the fact that they are ensheathed by a type of glial cell (the ensheathing cell) that expresses phenotypic features of both astrocyte and Schwann cells. The presence of both L1/Ng-CAM and N-CAM in the plasma membranes of both ensheathing cells and immature olfactory receptor neurons would enable the olfactory axons to use the glial cell surfaces as a substratum on which to grow. It is probably also true that ensheathing cells synthesize and secrete laminin, thus providing an additional adhesive substrate for the olfactory axons, as well as glia-derived nexin and nerve growth factor, both of which are neurite-promoting agents.

Electron-microscopic demonstration of olfactory-marker protein with protein G-gold in freeze-substituted, Lowicryl K11M-embedded rat olfactory-receptor cells

In this study electron-microscopic immunocytochemistry was used to localize olfactory marker protein in olfactory epithelia. Rat olfactory-epithelial samples were rapidly frozen, freeze-substituted with acetone, embedded at low temperatures with Lowicryl K11M and labelled on the sections with polyclonal antibodies raised against olfactory marker protein and with protein G conjugated to colloidal gold. Apart from the aforementioned use of acetone, substitution was carried out in the complete absence of chemical fixation, i.e., neither aldehydes nor OsO4 were used. This procedure resulted in localization concurrent with a good ultrastructural preservation. Olfactory-marker protein was present throughout the cytoplasmic compartments of dendrites and dendritic endings of olfactory-receptor cells, but it was not found in organelles such as mitochondria. Olfactory-marker protein was found only in dendritic endings of olfactory-receptor cells mature enough to have given rise to cilia, but these cilia displayed less labelling than dendrites and dendritic endings. Olfactory-marker protein was not found in apices and microvilli of neighboring olfactory-supporting cells.

Experimental studies on the olfactory marker protein. V. Olfactory marker protein in the olfactory neurons transplanted within the olfactory bulb

The olfactory mucosa of neonatal rats was transplanted within the olfactory bulb of littermates to investigate whether the olfactory bulb would have played a role in the differentiation of the olfactory neurons and whether the olfactory axons, growing out from the transplant, would have interacted with the olfactory glomeruli of the host. The observations were conducted on sections stained with Gill's hematoxylin, Loots' silver method, and the immunohistochemical technique for the demonstration of the olfactory marker protein (OMP). The olfactory neurons of the transplant (those localized in the neuroepithelium and those migrating from it into the bulbar parenchyma) could become fully differentiated but only few of them were OMP positive. Numerous sensory axons originated from the transplanted olfactory mucosa, however, they did not form ectopic glomeruli nor did they interact with the glomeruli of the host. These results indicate that the olfactory bulb, in vivo, does not affect the number of olfactory neurons expressing OMP and that the ectopically located neurons lack the cues to recognize the host glomeruli.