Retrograde signaling onto Ret during motor nerve terminal maturation

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.

Differential survival response of neurons to exogenous GDNF depends on the presence of skeletal muscle

Glial cell line-derived neurotrophic factor (GDNF) is known as a potent survival factor for neurons in vitro and in vivo. The current study investigated the effects of a single in utero injection with GDNF in both wild-type and Myf5-/-:MyoD-/- embryos. The embryos in the latter group, denoted double mutants (DM), do not contain skeletal muscle and associated neurotrophic factors due to lack of myogenesis and, therefore, neurons of the central and peripheral nervous system undergo excessively occurring programmed cell death (EPCD). We found that treatment with GDNF had no effect on wild type neuronal numbers in any of the anatomic locations investigated. However, GDNF rescued the neurons of the facial motor nucleus, the mesencephalic nucleus and the median motor column in the absence of skeletal muscle. The findings of the current study agree with previous reports that compromised mouse neurons have increased survival response to GDNF.

Genetic model system studies of the development of the enteric nervous system, gut motility and Hirschsprung's disease

The enteric nervous system (ENS) is the largest and most complicated subdivision of the peripheral nervous system. Its action is necessary to regulate many of the functions of the gastrointestinal tract including its motility. Whilst the ENS has been studied extensively by developmental biologists, neuroscientists and physiologists for several decades it has only been since the early 1990s that the molecular and genetic basis of ENS development has begun to emerge. Central to this understanding has been the use of genetic model organisms. In this article, we will discuss recent advances that have been achieved using both mouse and zebrafish model genetic systems that have led to new insights into ENS development and the genetic basis of Hirschsprung's disease.

Direct muscle delivery of GDNF with human mesenchymal stem cells improves motor neuron survival and function in a rat model of familial ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease in which there is a progressive loss of motor neurons and their connections to muscle, leading to paralysis. In order to maintain muscle connections in a rat model of familial ALS (FALS), we performed intramuscular transplantation with human mesenchymal stem cells (hMSCs) used as "Trojan horses" to deliver growth factors to the terminals of motor neurons and to the skeletal muscles. hMSCs engineered to secrete glial cell line-derived neurotrophic factor (hMSC-GDNF) were transplanted bilaterally into three muscle groups. The cells survived within the muscle, released GDNF, and significantly increased the number of neuromuscular connections and motor neuron cell bodies in the spinal cord at mid-stages of the disease. Further, intramuscular transplantation with hMSC-GDNF was found to ameliorate motor neuron loss within the spinal cord where it connects with the limb muscles receiving transplants. While disease onset was similar in all the animals, hMSC-GDNF significantly delayed disease progression, increasing overall lifespan by up to 28 days, which is one of the largest effects on survival noted for this rat model of FALS. This preclinical data provides a novel and practical approach toward ex vivo gene therapy for ALS.

GDNF and GFRalpha: a versatile molecular complex for developing neurons

The GDNF family ligands (GFLs) signal through the canonical signaling receptor Ret and a glycosyl-phosphatidylinositol-anchored co-receptor, GFRalpha. In recent years, signaling by GFLs has been shown to be more complex than originally assumed. The discrepant expression between GFRalphas and Ret has suggested the existence of additional signal-transducing GDNF receptors, such as NCAM. Here we summarize novel functions and Ret-independent signaling mechanisms for GDNF and GFRalpha, focusing on developing neurons. Emerging evidence indicates a prominent role of GDNF and GFRalpha in the control of neuroblast migration and chemoattraction and in the formation of neuronal synapses by a new mechanism of ligand-induced cell adhesion. Therefore, these data highlight the importance of this versatile molecular complex for nervous system development, function and regeneration.