Chem. Senses 27: 569-572,
2002
© Oxford University Press 2002
SYMPOSIUM: Mechanisms of Differentiation and Migration of Olfactory Progenitors |
Molecular Mechanisms for Migration of Placodally Derived GnRH Neurons
Cellular and Developmental Neurobiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, USA
Correspondence to be sent to: Susan Wray, Chief, Cellular and Developmental Neurobiology Section, NINDS, NIH, Building 36, Room 5A-25, Bethesda, MD 20892-4156, USA. e-mail: swray{at}codon.nih.gov
Abstract
Gonadotropin-releasing hormone (GnRH) neurons, critical for reproduction, are derived from the nasal placode and migrate into the brain along nasal axons. GnRH neurons appear to diverge from olfactory sensory cells during early stages of nasal placode differentiation. However, GnRH neurons rely on olfactory/vomeronasal axons as their pathway to the central nervous system (CNS). A novel factor, termed nasal embryonic luteinizing hormone-releasing hormone factor (NELF), was discovered in a differential screen of migrating versus nonmigrating GnRH neurons. NELF is expressed in olfactory sensory cells and GnRH cells in nasal areas. Antisense experiments demonstrated that knock-down of NELF decreased olfactory axon outgrowth and GnRH neuronal migration. These results indicate that NELF plays a role as a guidance molecule for olfactory axon projections and migration of GnRH cells. We hypothesize that NELF acts via a homophilic interaction and that NELF expression is critical for reproduction by insuring that GnRH cells reach the CNS. Furthermore, down-regulation of NELF on GnRH cells as they enter the telencephalon may allow GnRH cells to distinguish a different pathway(s) in the CNS (from those leading to olfactory regions) and thereby facilitate establishment of the appropriate adult-like GnRH distribution.
Introduction
The neuroendocrine gonadotropin-releasing hormone (GnRH) system is
essential for vertebrate reproduction [reviewed in
(Fink, 1988
)]. These cells
express the `mammalian form' of GnRH transcript
(Kasten et al., 1996).
In mammals, these GnRH cells are distributed in a continuum from the olfactory
bulbs to the hypothalamus (Figure
1A). Independent of location, the majority of GnRH cells send
their axons to the median eminence, where they access the pituitary portal
capillary system. Release of GnRH at this site is pulsatile, and affects
secretion of gonadotropins from cells in the anterior pituitary and
consequently gonadal function [reviewed in
(Fink, 1988)].
|
25% of the GnRH
population in vivo. Inset: explant observed using phase microscopy.
Phase bright GnRH cells (arrowheads) are seen apposed to olfactory axons
(arrows). (D—F) NELF expression in nasal regions. Areas in these
panels are shown in (B). (D) Immunocytochemically stained section indicates
NELF expression in olfactory epithelia (oe) and olfactory/vomeronasal axons
(arrows). (E and F) Sections immunofluorescently labeled for GnRH (left) and
NELF (right). Cells exiting the olfactory pit (E) are positive for both GnRH
and NELF (arrows). Olfactory axons in this region were also positive for NELF
(arrowhead). (F) GnRH cells co-expressed NELF at the nasal/forebrain junction.
(G) Knock-down of NELF alters olfactory axon outgrowth and GnRH
migration (for location see C). Compare explants with antisense NELF probe to
control. ob = olfactory bulb, mot = medial olfactory tract, ms = medial
septum, db = diagonal band of Broca, mpoa = medial preoptic area, me = median
eminence.
Although located within the forebrain postnally, GnRH cells originate in
the nasal region and thereafter migrate into the forebrain
(Figure 1B). Developmental
mapping studies led to the hypotheses that (i) GnRH cells originate in the
nasal placode and (ii) GnRH cells migrate on olfactory/vomeronasal axons, i.e.
are neurophilic in nature [reviewed in
(Wray, 2001)]. Olfactory
placode ablation and transplantation studies, as well as chick/quail chimeras,
dye-labeling studies and in vitro models, support the nasal origin
for cells expressing the mammalian GnRH transcript [reviewed in
(Wray, 2001)]. The
relationship between the development of the GnRH system and olfactory systems
is illustrated by Kallmann syndrome, a mutation resulting in anosmia and
hypogonadism in humans. Examination of tissue from a Kallmann fetus revealed
olfactory axons and GnRH cells on the nasal side of the cribriform plate, not
within the forebrain (Schwanzel-Fukuda
et al., 1989). Taken together, these data indicate that
the cells that arise in nasal regions and express the mammalian GnRH
transcript are, in fact, the major neuroendocrine component of the
hypothalamo-pituitary-gonadal axis.
Lineage of neuroendocrine GnRH neurons
The nasal placodes are ectodermal thickenings that give rise to nonsensory
respiratory and sensory olfactory epithelia. The olfactory epithelium produces
the main olfactory as well as vomeronasal organ epithelia
(Halpern, 1987;
Farbman, 1992). In many
species, GnRH-expressing cells are first detected in the vomeronasal organ
epithelia. This observation, together with ablation/transplantation studies,
led to the hypothesis that GnRH cells originate in an area of the placode
associated with olfactory epithelial-derived structures. However, in chicks,
where presumptive olfactory and respiratory areas can be divided prior to
placode formation, ablation experiments suggest that GnRH progenitors are more
closely associated with respiratory rather than olfactory regions
(El Amrauoui and Dubois, 1993),
and in fact, GnRH cells have been detected in the respiratory epithelium of
normal chicks (Hilal et al.,
1996).
GnRH cells have also been detected in the respiratory epithelium of mice
possessing a mutation in the developmental transcription factor activator
protein 2
(Kramer et al.,
2000). In this same study, prior to GnRH expression in the nasal
placode, an area was identified between respiratory cells and sensory cells.
It was speculated that this intermediate region might be the location of GnRH
progenitor cells. In addition, GnRH neurons have been screened for olfactory
epithelial lineage markers (Kramer and
Wray, 2000a). Olfactory epithelial markers were not detected in
GnRH cells, while nestin, a marker of central nervous system (CNS) and neural
crest progenitor cells, was. Thus, several studies now argue that olfactory
epithelial progenitors are distinct from GnRH progenitors and that GnRH
neurons diverge from olfactory sensory cells during early stages of nasal
placode differentiation.
Migration of GnRH cells
The exact axonal pathway used by GnRH cells as they migrate across the
nasal region is unclear. A variety of molecules expressed on
olfactory/vomeronasal axons mark this pathway, but to date none exclusively
highlights the route along which GnRH cells move. Nasal explants have been
established (Figure 1C) in
which GnRH neurons migrate in a manner similar to GnRH cells in vivo;
for example, they show directed outgrowth of olfactory axons, differentiation
of GnRH neurons, migration of GnRH neurons in association with olfactory
derived axons, and directed movement of GnRH cells
(Fueshko and Wray, 1994). The
association of GnRH cells with axons in these explants, and directed movement
of GnRH cells, supported cell-adhesion molecules as key molecular mechanisms
of GnRH cell migration in nasal regions.
In nasal explants, single GnRH cells can be identified in situ
(Figure 1C). To determine
molecules directly involved in GnRH migration, we used this system to obtained
RNA from single GnRH cells (Kramer and
Wray, 2000b). A differential screen comparing a migrating GnRH
cell and non-migrating GnRH cells was performed. A novel protein, termed nasal
embryonic luteinizing hormone-releasing hormone factor (NELF), was identified.
NELF is expressed in peripheral nervous system and CNS tissues, including
olfactory sensory cells and GnRH cells during embryonic development
(Figure 1D-F). As in
vivo, NELF is expressed on olfactory axons and GnRH cells in nasal
explants (Kramer and Wray
2000b). Transfection experiments were performed in nasal explants
using NELF antisense probes. Knock-down of NELF decreased olfactory axon
outgrowth and GnRH cell migration into the periphery of the explant
(Figure 1G). These experiments
indicate that NELF plays a role as a common guidance mechanism for olfactory
axon projections and subsequently, either directly or indirectly, in the
neurophilic migration of GnRH cells across nasal regions.
NELF is not expressed on postnatal GnRH cells
(Kramer and Wray, 2000b).
Interestingly, NELF is down-regulated on GnRH neurons which enter the
forebrain. In particular, NELF is turned off on GnRH cells that migrate toward
the hypothalamus, but remains, albeit at low expression levels, on GnRH cells
that migrate toward the developing olfactory bulb
(Kramer and Wray, 2000b). This
raises the possibility that molecules such as NELF, by turning off expression,
play an important role in establishing the appropriate GnRH adult-like
distribution by insuring that GnRH cells do not end up in forebrain olfactory
regions.
NELF is not restricted in its expression pattern to the olfactory and GnRH
systems. During development it is robustly expressed in the cortex,
hippocampus and thalamus (Kramer and Wray,
2001), and expression is maintained in the brain postnatally in
these same regions, as well as in the olfactory bulb. Developmentally, many of
the regions that express NELF exhibit active neuronal migration. Thus,
although the role of NELF in these areas awaits further investigations, its
presence in these structures is consistent with it acting as a migratory
signal. The role of NELF postnatally is presently unknown.
Conclusion
It is clear that the development of the olfactory system and GnRH neuroendocrine system are intimately entwined, and probably utilize both cell surface recognition molecules and chemoattractant/repellent molecules. Chemoactive molecules with short diffusion properties are likely involved in directing olfactory axon outgrowth while cell adhesion molecules are more likely responsible for directing GnRH cell movement. NELF is a novel molecule, expressed on both nasal GnRH cells and olfactory axons. Further work is needed to understand the role of NELF in the development of the GnRH neuronal system as well as the olfactory system, but perturbation of this molecule alters olfactory axon outgrowth as well as GnRH cell migration.
Acknowledgments
The author would like to thank Dr P. Kramer for his contributions to the work summarized here.
References
El Amrauoui, A. and Dubois, P.M. (1993) Experimental evidence for an early commitment of gonadotropin-releasing hormone neurons, with special regard to their origin from the ectorderm of nasal cavity presumptive territory. Neuroendocrinology,57 , 991-1002.[Medline]
Farbman, A.I. (1992) Cell Biology of Olfaction. Cambridge University Press, Cambridge.
Fink, G. (1988) Gonadotropin secretion and its control. In Knobil, E. and Neil, J. (eds), The Physiology of Reproduction. Raven Press, New York, Ch.32 .
Fueshko, S. and Wray, S. (1994) LHRH cells migrate on peripherin fibers in embryonic olfactory explant cultures: an in vitro model for neurophilic neuronal migration.Devl Biol. , 166,331 -348.
Halpern, M. (1987) The organization and function of the vomeronasal system. Annu. Rev. Neurosci.,10 , 325-362.[Web of Science][Medline]
Hilal, E.M., Chen, J.H. and Silverman, A.-J. (1996) Joint migration of gonadotropin-releasing hormone (LHRH) and neuropeptide Y (NPY) neurons from olfactory placode to central nervous system. J. Neurobiol., 31,487 -502.[Medline]
Kasten T.L., White, S.A., Norton, T.T., Bond, C.T., Adelman, J.P. and Fernald, R.D. (1996) Characterization of two new preproGnRH mRNAs in the tree shrew: first direct evidence for mesencephalic GnRH gene expression in a placental mammal. Gen. Comp. Endocrinol., 104,7 -19.[Web of Science][Medline]
Kramer, P.K. and Wray, S. (2000a) Midline nasal tissue influences nestin expression in nasal-placode-derived luteinizing hormone-releasing hormone (LHRH) neurons during development.Devl Biol. , 227,343 -357.
Kramer, P.K. and Wray, S. (2000b)
Novel gene expressed in nasal regions influences outgrowth of olfactory
axons and migration of luteinizing hormone releasing hormone (LHRH)
neurons. Genes Dev., 14,1824
-1834.
Kramer, P.K. and Wray, S. (2001) Nasal embryonic LHRH factor (NELF) expression within the CNS and PNS of the rodent, Gene Expression Patterns,1 , 23-26.
Kramer, P.K., Guerrero, G., Krishnamurthy, R., Mitchell,
P.J. and Wray, S. (2000) Ectopic expression of
LHRH and peripherin in the respiratory epithelium of mice lacking
transcription factor AP-2. Mech. Dev.,94
, 79-94.[Medline]
Schwanzel-Fukuda, M., Bick, D. and Pfaff, D.W. (1989) Luteinizing hormone-releasing hormone (LHRH)-expressing cells do not migrate normally in an inherited hypogonadal (Kallmann) syndrome. Mol. Brain Res.,6 , 311-326.[Medline]
Wray, S. (2001) Development of luteinizing hormone releasing hormone neurones. J. Neuroendocrinol.,13 , 3-11, 2001.[Web of Science][Medline]
Accepted May 1, 2002
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