Differential expression of the GDNF family receptors RET and GFRalpha1, 2, and 4 in subsets of motoneurons: a relationship between motoneuron birthdate and receptor expression

Hox Proteins Coordinate Motor Neuron Differentiation and Connectivity Programs through Ret/Gfrα Genes

The accuracy of neural circuit assembly relies on the precise spatial and temporal control of synaptic specificity determinants during development. Hox transcription factors govern key aspects of motor neuron (MN) differentiation; however, the terminal effectors of their actions are largely unknown. We show that Hox/Hox cofactor interactions coordinate MN subtype diversification and connectivity through Ret/Gfrα receptor genes. Hox and Meis proteins determine the levels of Ret in MNs and define the intrasegmental profiles of Gfrα1 and Gfrα3 expression. Loss of Ret or Gfrα3 leads to MN specification and innervation defects similar to those observed in Hox mutants, while expression of Ret and Gfrα1 can bypass the requirement for Hox genes during MN pool differentiation. These studies indicate that Hox proteins contribute to neuronal fate and muscle connectivity through controlling the levels and pattern of cell surface receptor expression, consequently gating the ability of MNs to respond to limb-derived instructive cues.

Rapid transient isoform-specific neuregulin1 transcription in motor neurons is regulated by neurotrophic factors and axon-target interactions

The neuregulins (NRGs) are a family of alternatively spliced factors that play important roles in nervous system development and disease. In motor neurons, NRG1 expression is regulated by activity and neurotrophic factors, however, little is known about what controls isoform-specific transcription. Here we show that NRG1 expression in the chick embryo increases in motor neurons that have extended their axons and that limb bud ablation before motor axon outgrowth prevents this induction, suggesting a trophic role from the developing limb. Consistently, NRG1 induction after limb bud ablation can be rescued by adding back the neurotrophic factors BDNF and GDNF. Mechanistically, BDNF induces a rapid and transient increase in type I and type III NRG1 mRNAs that peak at 4h in rat embryonic ventral spinal cord cultures. Blocking MAPK or PI3K signaling or blocking transcription with Actinomycin D blocks BDNF induced NRG1 gene induction. BDNF had no effect on mRNA degradation, suggesting that transcriptional activation rather than message stability is important. Furthermore, BDNF activates a reporter construct that includes 700bp upstream of the type I NRG1 start site. Protein synthesis is also required for type I NRG1 mRNA transcription as cycloheximide produced a super-induction of type I, but not type III NRG1 mRNA, possibly through a mechanism involving sustained activation of MAPK and PI3K. These results reveal the existence of highly responsive, transient transcriptional regulatory mechanisms that differentially modulate NRG1 isoform expression as a function of extracellular and intracellular signaling cascades and mediated by neurotrophic factors and axon-target interactions.

Elucidation of target muscle and detailed development of dorsal motor neurons in chick embryo spinal cord

The avian cervical spinal cord includes motoneurons (MNs) that send their axons through the dorsal roots. They have been called dorsal motoneurons (dMNs) and assumed to correspond to MNs of the accessory nerve that innervate the cucullaris muscle (SAN-MNs). However, their target muscles have not been elucidated to date. The present study sought to determine the targets and the specific combination of transcription factors expressed by dMNs and SAN-MNs and to describe the detailed development of dMNs. Experiments with tracing techniques confirmed that axons of dMNs innervated the cucullaris muscle. Retrogradely labeled dMNs were distributed in the ventral horn of C3 and more caudal segments. In most cases, some dMNs were also observed in the C2 segment. It was also demonstrated that SAN-MNs existed in the ventral horn of the C1-2 segments and the adjacent caudal hindbrain. Both SAN-MNs and dMNs expressed Isl1 but did not express Isl2, MNR2, or Lhx3. Rather, these MNs expressed Phox2b, a marker for branchial motoneurons (brMNs), although the intensity of expression was weaker. Dorsal MNs and SAN-MNs were derived from the Nkx2.2-positive precursor domain and migrated dorsally. Dorsal MNs remain in the ventral domain of the neural tube, unlike brMNs in the brainstem. These results indicate that dMNs and SAN-MNs belong to a common MN population innervating the cucullaris muscle and also suggest that they are similar to brMNs of the brainstem, although there are differences in Phox2b expression and in the final location of each population. J. Comp. Neurol. 521: 2987-3002, 2013. © 2013 Wiley Periodicals, Inc.

Examining the combinatorial model of motor neuron survival by expression profiling of trophic factors and their receptors in the embryonic Gallus gallus

During embryogenesis, limb-innervating lateral motor column (LMC) spinal motor neurons (MN) are generated in excess and subsequently nearly half of them die. Many motor neuron survival factors (MnSFs) have been shown to suppress this default programmed cell death (PCD) program through their receptors (MnSFRs), raising the possibility that they are involved in matching specific MNs with their target muscles. Published observations suggest a combinatorial model of MnSF/Rs function, which assumes that during the PCD phase, MNs are expressing combinations of MnSFRs, whereas the limb muscles innervated by these MNs express cognate combinations of MnSFs. We tested this model by expression profiling of MnSFs and their receptors in the avian lumbosacral spinal cord and limb muscles during the peak PCD period. Our findings highlight the complexity of MnSF/Rs function in the control of LMC motor neuron survival.

Organotypic specificity of key RET adaptor-docking sites in the pathogenesis of neurocristopathies and renal malformations in mice

The receptor tyrosine kinase ret protooncogene (RET) is implicated in the pathogenesis of several diseases and in several developmental defects, particularly those in neural crest-derived structures and the genitourinary system. In order to further elucidate RET-mediated mechanisms that contribute to these diseases and decipher the basis for specificity in the pleiotropic effects of RET, we characterized development of the enteric and autonomic nervous systems in mice expressing RET9 or RET51 isoforms harboring mutations in tyrosine residues that act as docking sites for the adaptors Plcgamma, Src, Shc, and Grb2. Using this approach, we found that development of the genitourinary system and the enteric and autonomic nervous systems is dependent on distinct RET-stimulated signaling pathways. Thus, mutation of RET51 at Y1062, a docking site for multiple adaptor proteins including Shc, caused distal colon aganglionosis reminiscent of Hirschsprung disease (HSCR). On the other hand, this mutation in RET9, which encodes an isoform that lacks the Grb2 docking site present in RET51, produced severe abnormalities in multiple organs. Mutations that abrogate RET-Plcgamma binding, previously shown to produce features reminiscent of congenital anomalies of kidneys or urinary tract (CAKUT) syndrome, produced only minor abnormalities in the nervous system. Abrogating RET51-Src binding produced no major defects in these systems. These studies provide insight into the basis of organotypic specificity and redundancy in RET signaling within these unique systems and in diseases such as HSCR and CAKUT.

Gamma motor neurons express distinct genetic markers at birth and require muscle spindle-derived GDNF for postnatal survival

BACKGROUND

Gamma motor neurons (gamma-MNs) selectively innervate muscle spindle intrafusal fibers and regulate their sensitivity to stretch. They constitute a distinct subpopulation that differs in morphology, physiology and connectivity from alpha-MNs, which innervate extrafusal muscle fibers and exert force. The mechanisms that control the differentiation of functionally distinct fusimotor neurons are unknown. Progress on this question has been limited by the absence of molecular markers to specifically distinguish and manipulate gamma-MNs. Recently, it was reported that early embryonic gamma-MN precursors are dependent on GDNF. Using this knowledge we characterized genetic strategies to label developing gamma-MNs based on GDNF receptor expression, showed their strict dependence for survival on muscle spindle-derived GDNF and generated an animal model in which gamma-MNs are selectively lost.

RESULTS

In mice heterozygous for both the Hb9::GFP transgene and a tau-lacZ-labeled (TLZ) allele of the GDNF receptor Gfralpha1, we demonstrated that small motor neurons with high Gfralpha1-TLZ expression and lacking Hb9::GFP display structural and synaptic features of gamma-MNs and are selectively lost in mutants lacking target muscle spindles. Loss of muscle spindles also results in the downregulation of Gfralpha1 expression in some large diameter MNs, suggesting that spindle-derived factors may also influence populations of alpha-MNs with beta-skeletofusimotor collaterals. These molecular markers can be used to identify gamma-MNs from birth to the adult and to distinguish gamma- from beta-motor axons in the periphery. We also found that postnatal gamma-MNs are also distinguished by low expression of the neuronal nuclear protein (NeuN). With these markers of gamma-MN identity, we show after conditional elimination of GDNF from muscle spindles that the survival of gamma-MNs is selectively dependent on spindle-derived GDNF during the first 2 weeks of postnatal development.

CONCLUSION

Neonatal gamma-MNs display a unique molecular profile characterized by the differential expression of a series of markers - Gfralpha1, Hb9::GFP and NeuN - and the selective dependence on muscle spindle-derived GDNF. Deletion of GDNF expression from muscle spindles results in the selective elimination of gamma-MNs with preservation of the spindle and its sensory innervation. This provides a mouse model with which to explore the specific role of gamma-fusimotor activity in motor behaviors.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Glial cell line-derived neurotrophic factor family ligands enhance capsaicin-stimulated release of calcitonin gene-related peptide from sensory neurons

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are a group of peptides that have been implicated as important factors in inflammation, since they are released in increased amounts during inflammation and induce thermal hyperalgesia upon injection. Mouse isolated sensory neurons in culture and freshly dissociated spinal cord slices were used to examine the enhancement in stimulated-release of the neuropeptide, calcitonin gene-related peptide (CGRP), as a measure of sensitization. Exposure of isolated sensory neurons in culture to GDNF, neurturin, and artemin enhanced the capsaicin-stimulated release of immunoreactive calcitonin gene-related peptide (iCGRP) two- to threefold, but did not increase potassium-stimulated release of iCGRP. A similar profile of sensitization was observed in freshly dissociated spinal cord slices. Persephin, another member of the GFL family thought to be important in development, was unable to induce an enhancement in the release of iCGRP. These results demonstrate that specific GFLs are important mediators affecting sensory neuronal sensitivity, likely through modulation of the capsaicin receptor. The sensitization of sensory neurons during inflammation, and the pain and neurogenic inflammation resulting from this sensitization, may be due in part to the effects of these selected GFLs.

Expression pattern of LRR and Ig domain-containing protein (LRRIG protein) in the early mouse embryo

The combination of leucine-rich repeat (LRR) and immunoglobulin-like (Ig) domains is found in the domain architecture of the Trk neurotrophin receptor protein. Recently dozens of such proteins simultaneously carrying LRR and Ig domains as the Trk receptors have been identified. Given the significant biological roles of Trk and such newly identified proteins, we have searched the public database for human proteins with LRR and Ig domains (collectively termed the leucine-rich repeat and Ig domain-containing protein, LRRIG protein, in this study), and have analyzed the mRNA expression pattern of mouse orthologs of obtained human LRRIG proteins at embryonic day 10. The list of the LRRIG proteins includes 36 human proteins: four LINGO, three NGL, five SALM, three NLRR, three Pal, two ISLR, three LRIG, two GPR, two Adlican, two Peroxidasin-like proteins, three Trk neurotrophin receptors, a yet unnamed protein AAI11068, and three AMIGO. Some molecules (LINGO2, LINGO4, NGL1, SALM1, SALM5, and TrkB) were expressed exclusively in neuronal tissues, whereas others (ISLR1, GPR124, and Adlican2) exhibited non-neuronal expression profiles. However, the majority of LRRIG protein family exhibited broad mRNA tissue-expression profiles.

Tissue distribution of Ret, GFRalpha-1, GFRalpha-2 and GFRalpha-3 receptors in the human brainstem at fetal, neonatal and adult age

Occurrence and localization of receptor components of the glial cell line-derived neurotrophic factor (GDNF) family ligands, the Ret receptor tyrosine kinase and the GDNF family receptor (GFR) alpha-1 to -3, were examined by immunohistochemistry in the normal human brainstem at fetal, neonatal, and adult age. Immunoreactive elements were detectable at all examined ages with uneven distribution and consistent pattern for each receptor. As a rule, the GFRalpha-1 and GFRalpha-2 antisera produced the most abundant and diffuse tissue labelling. Immunoreactive perikarya were observed within sensory and motor nuclei of cranial nerves, dorsal column nuclei, olivary nuclear complex, reticular formation, pontine nuclei, locus caeruleus, raphe nuclei, substantia nigra, and quadrigeminal plate. Nerve fibers occurred within gracile and cuneate fasciculi, trigeminal spinal tract and nucleus, facial, trigeminal, vestibular and oculomotor nerves, solitary tract, medial longitudinal fasciculus, medial lemniscus, and inferior and superior cerebellar peduncles. Occasionally, glial cells were stained. Age changes were appreciable in the distribution pattern of each receptor. On the whole, in the grey matter, labelled perikarya were more frequently observed in pre- and perinatal than in adult specimens; on the other hand, in discrete regions, nerve fibers and terminals were abundant and showed a plexiform arrangement only in adult tissue; finally, distinct fiber systems in the white matter were immunolabelled only at pre- and perinatal ages. The results obtained suggest the involvement of Ret and GFRalpha receptors signalling in processes subserving both the organization of discrete brainstem neuronal systems during development and their functional activity and maintenance in adult life.

Onset of ETS expression is not accelerated by premature exposure to signals from limb mesenchyme

The ETS transcription factors ER81 and PEA3 are expressed in discrete populations of sensory and motor neurons and regulate late events in neuronal development and limb innervation. Although initiation of ETS expression requires limb-derived signals, we show here that precocious axon growth into transplanted older donor limbs, which prematurely exposes neurons to limb-derived signals, does not accelerate the onset of expression of Er81 or Pea3. Similarly, neither MN-cadherin, which is reportedly regulated by ER81, nor T-cadherin is expressed precociously in neurons innervating older donor limbs. Thus, neurons must attain a particular level of differentiation to respond to inducing signals from limb. We also show that signals emanating from limb mesenchyme are sufficient to initiate Er81 and Pea3 expression in sensory and motor neurons in the absence of myogenic cells in Sp(d) mutant mice and that induction of ETS expression is unlikely to directly involve retinoid signaling from limb mesenchyme.

Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury

RET (for "rearranged during transfection") is a transmembrane tyrosine kinase signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands. We used RET immunohistochemistry (IHC), double-labeling immunofluorescence (IF), and in situ hybridization (ISH) in adult naïve and nerve-injured rats to study the distribution of RET in the spinal cord. In the dorsal horn, strong RET-immunoreactive (-ir) fibers were abundant in lamina II-inner (II(i)), although this labeling was preferentially observed after an antigen-unmasking procedure. After dorsal rhizotomy, RET-ir fibers in lamina II(i) completely disappeared from the dorsal horn, indicating that they were all primary afferents. After peripheral axotomy, RET-ir in primary afferents decreased in lamina II(i) and appeared to increase slightly in laminae III and IV. RET-ir was also observed in neurons and dendrites throughout the dorsal horn. Some RET-ir neurons in lamina I had the morphological appearance of nociceptive projection neurons, which was confirmed by the finding that 53% of RET-ir neurons in lamina I colocalized with neurokinin-1. GDNF-ir terminals were in close proximity to RET-ir neurons in the superficial dorsal horn. In the ventral horn, RET-ir was strongly expressed by motoneurons, with the strongest staining in small, presumably gamma-motoneurons. Increased RET expression following peripheral axotomy was most pronounced in alpha-motoneurons. The expression and regulation pattern of RET in the spinal cord are in line with its involvement in regenerative processes following nerve injury. The presence of RET in dorsal horn neurons, including nociceptive projection neurons, suggests that RET also has a role in signal transduction at the spinal level. This role may include mediating the effects of GDNF released from nociceptive afferent fibers.

Differential responses to the application of exogenous NT-3 are observed for subpopulations of motor and sensory neurons depending on the presence of skeletal muscle

We examined the effects of a single injection of exogenous NT-3, administered at embryonic day (E) 13.5, on the survival of two populations of motor neurons and two populations of sensory neurons. Both wild-type and double knockout, Myf5-/-:MyoD-/-, mutant embryos were examined to determine the effects of the aforementioned neurotrophin on motor and sensory neuron survival in the presence and absence, respectively, of skeletal muscle. We found that, although NT-3 rescues select populations of motor neurons in the absence of muscles, there is a lack of increase in neuron survival when skeletal muscle is present. Additionally, NT-3 was found to rescue a select population of proprioceptive sensory neurons in the absence of target tissue, while, at times, exacerbating neuron cell death when target tissues are present. Lastly, we found that neurons in the spinal cord and brainstem show both a regional and functional specificity in their response to the administration of NT-3 in utero. Our results indicate the possibility that different pathways are involved in the survival of neurons during naturally occurring programmed cell death and during excessively occurring programmed cell death.

Ablation of persephin receptor glial cell line-derived neurotrophic factor family receptor alpha4 impairs thyroid calcitonin production in young mice

Glial cell line-derived neurotrophic factor family receptor (GFRalpha) 4, the binding receptor for persephin, is coexpressed with the signaling Ret receptor tyrosine kinase predominantly in thyroid calcitonin-producing C cells. We show by in situ hybridization and immunohistochemistry that the functional, glycolipid-anchored form of GFRalpha4 is produced in mouse only in the C cells but not in parathyroid gland or in the brain. C cells expressed functional GFRalpha4 throughout postnatal development, whereas Ret expression in these cells decreased postnatally and was undetectable in adults. To understand the physiological role of GFRalpha4, we produced GFRalpha4-deficient [knockout (KO)] mice. No differences were observed between wild-type and GFRalpha4-KO littermate animals in growth, gross behavior, or viability. The number and morphology of the thyroid C cells were indistinguishable between the genotypes in both newborn and adult age. However, thyroid tissue calcitonin content was reduced by 60% in newborn and by 45% in 3-wk-old GFRalpha4-KO mice compared with wild-type controls. In contrast, thyroid calcitonin levels were similar in adult animals. Consistent with the reduced calcitonin levels, bone formation rate in juvenile GFRalpha4-KO mice was increased. In conclusion, this study indicates a novel role for endogenous GFRalpha4 signaling in regulating calcitonin production in thyroid C cells of young mice.

Glial cell-line derived neurotrophic factor-dependent fusimotor neuron survival during development

Glial cell-line derived neurotrophic factor (GDNF) is a potent survival factor for motor neurons. Previous studies have shown that some motor neurons depend upon GDNF during development but this GDNF-dependent motor neuron subpopulation has not been characterized. We examined GDNF expression patterns in muscle and the impact of altered GDNF expression on the development of subtypes of motor neurons. In GDNF hemizygous mice, motor neuron innervation to muscle spindle stretch receptors (fusimotor neuron innervation) was decreased, whereas in transgenic mice that overexpress GDNF in muscle, fusimotor innervation to muscle spindles was increased. Facial motor neurons, which do not contain fusimotor neurons, were not changed in number when GDNF was over expressed by facial muscles during their development. Taken together, these data indicate that fusimotor neurons depend upon GDNF for survival during development. Since the fraction of cervical and lumbar motor neurons lost in GDNF-deficient mice at birth closely approximates the size of the fusimotor neuron pool, these data suggest that motor neuron loss in GDNF-deficient mice may be primarily of fusimotor neuron origin.

Ficolin A and ficolin B are expressed in distinct ontogenic patterns and cell types in the mouse

Ficolins are a group of proteins characterized by the presence of collagen-like and fibrinogen-like domains. Two of three human ficolins, L-ficolin and H-ficolin, are serum lectins that form complexes with mannose-binding lectin-associated serine proteases (MASPs) and play important roles in the lectin complement pathway. The other human ficolin, M-ficolin, is a non-serum-type ficolin that is expressed in monocytes. Little is known about the physiological roles of ficolins. In this study, we delineated the ontogeny and cell types that express ficolins in mice. RT-PCR analysis showed that the expression pattern of ficolin A expression was closely similar to that of Masps, suggesting that these molecules may function in coordination as components of the lectin complement pathway. The cell types that express ficolin A mRNA in both adult liver and spleen were identified as macrophages by in situ hybridization. Ficolin B exhibited a distinct ontogeny pattern that switched from embryonic liver to postnatal bone marrow and spleen. The cells that express ficolin B mRNA were identified as belonging to the myeloid cell lineage by magnetic sorting and by subsequent RT-PCR in bone marrow cells. Thus, the different spatial-temporal expression patterns of ficolins A and B suggest that these molecules play distinct roles in the prenatal and postnatal stages.

Overexpression of glial cell line-derived neurotrophic factor in the CNS rescues motoneurons from programmed cell death and promotes their long-term survival following axotomy

To study the role of one of the most potent motoneuron (MN) survival factors, glial cell line-derived neurotrophic factor (GDNF) derived from the CNS, we generated transgenic animals overexpressing GDNF under the control of an astrocyte-specific GFAP promoter. In situ hybridization revealed that GDNF was expressed at high levels in astrocytes throughout the brain and spinal cord. We analyzed the effects of CNS-derived GDNF on MN survival during the period of programmed cell death (PCD) and after nerve axotomy. In GFAP-GDNF mice at E15, E18, and P1, the survival of brachial MNs was increased on average by 30%, lumbar MNs by 20%, and thoracic MNs at P1 by 33%. GDNF also prevented MN PCD in several cranial motor nuclei. We demonstrated for the first time that the number of MNs in the mouse abducens nucleus was also increased by 40%, thus extending known MN populations that are responsive to GDNF. Next, we tested if GDNF could support complete and relatively long-term survival of MNs following neonatal facial nerve axotomy. We found that virtually all MNs (91%) in GFAP-GDNF mice survived for up to 18 weeks post-axotomy. This is the longest GDNF-mediated survival of neonatal MNs reported following axotomy. Most of surviving MNs were not atrophic, and MN-specific ChAT and neurofilament immunoreactivity (IR) were preserved. Furthermore, GDNF attenuated axotomy-induced astroglial activation. These data demonstrate that overexpression of GDNF in the CNS has very profound effects on MN survival both during the PCD period and after neuronal injury. GFAP-GDNF mice will be valuable to study the effects of CNS-derived GDNF in mouse models of MN degenerative diseases and axonal regeneration in vivo.

Motoneurons crave glial cell line-derived neurotrophic factor

This is a commentary on the developmental and therapeutic relevance of recent studies in the glial fibrillary acid protein (GFAP)-glial cell line-derived neurotrophic factor (GDNF) transgenic mouse reported by Zhao et al. (2004). This interesting study demonstrated that increased expression of GDNF in astrocytes increases the number of neighboring motoneurons of certain motoneuron subpopulations by diminishing programmed cell death during development. In addition, astrocyte-derived GDNF was shown to protect facial motoneurons from injury-induced cell death. Since this is the first direct demonstration that secretion of GDNF from astrocytes in the CNS can affect motoneuron development in utero and motoneuron survival after axotomy, novel approaches for motor neuron disease are suggested. The known target neurons that respond to GDNF are reviewed, as are studies using GDNF gene delivery in animal models of amyotrophic lateral sclerosis (ALS). It is postulated that GDNF is a factor to which many motoneurons respond along their whole extent from soma to axon to terminal.

Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

The Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wild-type Ret activity, including its activation of AKT. All RetDN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The RetDN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the RetDN/+ mice only had defects in the parasympathetic nervous system. A small proportion of RetDN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules,maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

The function of neurotrophic factor receptors expressed by the developing adductor motor pool in vivo

We examined the spatio-temporal relationship between neurotrophic factor receptor (NTF-R) expression and motoneuron (MN) survival in the developing avian spinal cord and observed heterogeneity in the expression of NTF-Rs between, but not within, pools of MNs projecting to individual muscles. We then focused on the role of NTFs in regulating the survival of one motor pool of MNs, all of which innervate a pair of adductor muscles in the thigh and hence compete for survival during the period of programmed cell death (PCD). The complete NTF-R complement of these MNs was analyzed and found to include many, but not all, NTF-Rs. Treatment with exogenous individual NTFs rescued some, but not all, adductor MNs expressing appropriate NTF-Rs. In contrast, administration of multiple NTFs completely rescued adductor MNs from PCD. Additionally, adductor MNs were partially rescued from PCD by NTFs for which they failed to express receptors. NTF-Rs expressed by the nerve but not in the muscle target were capable of mediating survival signals to MNs in trans. Finally, the expression of some NTF-Rs by adductor MNs was not required for MN survival. These studies demonstrate the complexity in NTF regulation of a defined subset of competing MNs and suggest that properties other than NTF-R expression itself can play a role in mediating trophic responses to NTFs.

Distinct muscle targets do not vary in the developmental regulation of brain-derived neurotrophic factor. J Comp Neurol 470:317-329,2004

Developing neurons depend on many target-derived signals. One of these signals is the neurotrophin brain-derived neurotrophic factor (BDNF). Exogenous application of BDNF in vitro and in vivo rescues a population of lumbar motoneurons from programmed cell death. Given that BDNF does not rescue all motoneurons and that motoneurons differ in trophic factor receptor expression, subpopulations of motoneurons may have different sensitivities to the factor. These differences may be reflected in distinct target muscles specialized to produce different protein concentrations, or muscles may contain equal amounts of the factor and receptor expression determines motoneuron responsiveness. By using a sensitive electrochemiluminescent immunoassay (ECLIA), we measured normal developmental changes in BDNF protein concentration in anatomically and functionally distinct chick embryonic thigh muscles from E6 to E18. We found that there were no significant differences in BDNF protein concentration between muscles classified according to function (fast vs. slow) or anatomical position (flexor vs. extensor) and that the quantity of BDNF in the target did not appear to be activity dependent. These results suggest that, during development, the differences in the response of motoneurons to BDNF are not due to the anatomical or functional diversity of muscle targets. J. Comp. Neurol. 470:330-337, 2004.