Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

Critical role of GFRα1 in the development and function of the main olfactory system

Glial cell line-derived neurotrophic factor (GDNF) and its receptor GFRα1 are prominently expressed in the olfactory epithelium (OE) and olfactory bulb (OB), but their importance for olfactory system development is completely unknown. We have investigated the consequences of GFRα1 deficiency for mouse olfactory system development and function. In the OE, GFRα1 was expressed in basal precursors, immature olfactory sensory neurons (OSNs), and olfactory ensheathing cells (OECs), but was excluded from mature OSNs. The OE of newborn Gfra1 knock-out mice was thinner and contained fewer OSNs, but more dividing precursors, suggesting deficient neurogenesis. Immature OSN axon bundles were enlarged and associated OECs increased, indicating impaired migration of OECs and OSN axons. In the OB, GFRα1 was expressed in immature OSN axons and OECs of the nerve layer, as well as mitral and tufted cells, but was excluded from GABAergic interneurons. In newborn knock-outs, the nerve layer was dramatically reduced, exhibiting fewer axons and OECs. Bulbs were smaller and presented fewer and disorganized glomeruli and a significant reduction in mitral cells. Numbers of tyrosine hydroxylase-, calbindin-, and calretinin-expressing interneurons were also reduced in newborn mice lacking Gfra1. At birth, the OE and OB of Gdnf knock-out mice displayed comparable phenotypes. Similar deficits were also found in adult heterozygous Gfra1(+/-) mutants, which in addition displayed diminished responses in behavioral tests of olfactory function. We conclude that GFRα1 is critical for the development and function of the main olfactory system, contributing to the development and allocation of all major classes of neurons and glial cells.

L-DOPA-induced increase in TH-immunoreactive striatal neurons in parkinsonian mice: insights into regulation and function

Tyrosine hydroxylase (TH)-immunoreactive (ir) neurons have been found in the striatum after dopamine depletion; however, little is known about the mechanism underlying their appearance or their functional significance. We previously showed an increase in striatal TH-ir neurons after L-DOPA treatment in mice with unilateral 6-OHDA lesions in the striatum. In the present study, we further examined the time-course and persistence of the effects of chronic L-DOPA treatment on the appearance and regulation of TH-ir neurons as well as their possible function. We found that the L-DOPA-induced increase in striatal TH-ir neurons is dose-dependent and persists for days after L-DOPA withdrawal, decreasing significantly 10 days after L-DOPA treatment ends. Using hemiparkinsonian D1 receptor knock-out (D1R-/-) and D2 receptor knock-out (D2R-/-) mice, we found that the D1R, but not the D2R, is required for the L-DOPA-induced appearance of TH-ir neurons in the dopamine-depleted striatum. Interestingly, our experiments in aphakia mice, which lack Pitx3 expression in the brain, indicate that the L-DOPA-dependent increase in the number of TH-ir neurons is independent of Pitx3, a transcription factor necessary for the development of mesencephalic dopaminergic neurons. To explore the possible function of L-DOPA-induced TH-ir neurons in the striatum, we examined dopamine overflow and forelimb use in L-DOPA-treated parkinsonian mice. These studies revealed a tight spatio-temporal correlation between the presence of striatal TH-ir neurons, the recovery of electrically stimulated dopamine overflow in the lesioned striatum, and the recovery of contralateral forelimb use with chronic L-DOPA treatment. Our results suggest that the presence of TH-ir neurons in the striatum may underlie the long-duration response to L-DOPA following withdrawal. Promotion of these neurons in the early stages of Parkinson's disease, when dopamine denervation is incomplete, may be beneficial for maintaining motor function.

Differential expression of RET receptor isoforms in the olfactory system

The glial cell line-derived neurotrophic factor (GDNF) family supports neurons by activating the tyrosine kinase receptor RET. The two main isoforms of RET, RET9 and RET51, differ in their carboxyl termini and have been implicated with distinct functions in the enteric and central nervous systems. Previously we reported the cellular localization of GDNF, neurturin and RET9 in the olfactory system [Maroldt H, Kaplinovsky T, Cunningham AM (2005) J Neurocytol 34:241-255]. In the current study, we examined immunohistochemical expression of RET9 and RET51 in neonatal and adult rat olfactory neuroepithelium (ON) and bulb to explore their potential functional roles. In the ON, RET9 was expressed by olfactory receptor neurons (ORNs) throughout the olfactory neuroepithelial sheet, whereas RET51 was restricted to ORNs situated in ventromedial and ventrolateral regions. Within these regions, RET51 was expressed by a subset of RET9-expressing ORNs. In olfactory bulb, RET9 expression was primarily on cell bodies, including olfactory ensheathing and periglomerular cells, and again, RET51 was expressed by a subset of RET9-expressing cells. RET51 was identified on axons in the olfactory nerve layer and glomerular neuropil, but only in the ventromedial and ventrolateral regions of the bulb. This regionalization correlated with the predicted axonal projection from expressing regions of the ON. RET51 was also expressed on dendrites in the external plexiform layer and glomerular neuropil. These results suggest RET9 may be the predominant functional isoform in the ON while RET51 plays a more selective role in a restricted region of the olfactory neuroepithelial sheet. In the bulb, RET9 is likely the main functional isoform while RET51 may be important in axonal and dendritic function/targeting.

The fine-structural distribution of G-protein receptor kinase 3, beta-arrestin-2, Ca2+/calmodulin-dependent protein kinase II and phosphodiesterase PDE1C2, and a Cl(-)-cotransporter in rodent olfactory epithelia

The sequentially activated molecules of olfactory signal-onset are mostly concentrated in the long, thin distal parts of olfactory epithelial receptor cell cilia. Is this also true for molecules of olfactory signal-termination and -regulation? G-protein receptor kinase 3 (GRK3) supposedly aids in signal desensitization at the level of odor receptors, whereas beta-arrestin-2, Ca2+/calmodulin-dependent protein kinase II (CaMKII) and phosphodiesterase (PDE) PDE1C2 are thought to do so at the level of the adenylyl cyclase, ACIII. The Na+, K(+)-2Cl(-)-cotransporter NKCC1 regulates Cl(-)-channel activity. In an attempt to localize the subcellular sites olfactory signal-termination and -regulation we used four antibodies to GRK3, two to beta-arrestin-2, five to CaMKII (one to both the alpha and beta form, and two each specific to CaMKII alpha and beta), two to PDE1C2, and three to Cl(-)-cotransporters. Only antibodies to Cl(-)-cotransporters labeled cytoplasmic compartments of, especially, supporting cells but also those of receptor cells. For all other antibodies, immunoreactivity was mostly restricted to the olfactory epithelial luminal border, confirming light microscopic studies that had shown that antibodies to GRK3, beta- arrestin-2, CaMKII, and PDE1C2 labeled this region. Labeling did indeed include receptor cell cilia but occurred in microvilli of neighboring supporting cells as well. Apical parts of microvillous cells that are distinct from supporting cells, and also of ciliated respiratory cells, immunoreacted slightly with most antibodies. When peptides were available, antibody preabsorption with an excess of peptide reduced labeling intensities. Though some of the antibodies did label apices and microvilli of vomeronasal (VNO) supporting cells, none immunoreacted with VNO sensory structures.