Localization of TrkC to Schwann cells and effects of neurotrophin‐3 signaling at neuromuscular synapses

Abstract

Neurotrophins and their receptors, the Trks, are differentially expressed among the cell types that make up neuromuscular and other synapses, but the function and directionality of neurotrophin signaling at synapses are poorly understood. Here we demonstrate, via immunostaining, Western blotting, and RT-PCR analyses, that TrkC, the receptor for neurotrophin-3 (NT3), is expressed by mouse perisynaptic and myelinating Schwann cells from birth through adulthood and is unaltered after denervation. Analyses of transgenic mice in which the NT3 coding sequence is replaced by lacZ showed that NT3 is expressed in motor neurons and Schwann cells during perinatal development, but not in adult mice. In muscle, NT3 is expressed by intrafusal muscle fibers within spindles, as has been previously reported. Surprisingly, NT3 is also expressed in extrafusal muscle fibers during perinatal life and in adults. Genetic approaches were used to explore the roles of NT3 and TrkC signaling at neuromuscular synapses. Overexpression of NT3 in muscle fibers during development resulted in an increased number of perisynaptic Schwann cells at neuromuscular synapses, without altering synaptic size, suggesting that muscle-derived NT3 might act as a mitogen or trophic factor for Schwann cells. Conditional deletion of NT3 from motor neurons did not alter the number of Schwann cells or other aspects of neuromuscular synaptic structure, suggesting that motor-neuron-derived NT3 is not required for normal development of perisynaptic Schwann cells or synapses. Together, these results demonstrate that NT3 expression is developmentally regulated in skeletal muscle and may modulate the number of Schwann cells at neuromuscular synapses.

Heterogeneity in Synaptic Vesicle Release at Neuromuscular Synapses of Mice Expressing SynaptopHluorin

“…Detailed methods for NMJ immunostaining and imaging have been described previously (cf. Hess et al, 2007). For most confocal imaging, young adult mice were transcardially perfused with 4% paraformaldehyde (PFA) in PBS, pH 7.4, and the sternomastoid muscles were removed and washed with PBS.…”

Targeted Depletion of TDP-43 Expression in the Spinal Cord Motor Neurons Leads to the Development of Amyotrophic Lateral Sclerosis-like Phenotypes in Mice

“…For instance, expression of HB9:Cre has also been observed in cells other than the ventral motor neurons, which could be due to HB9 promoter activity in the visceral motor neurons or in motor neuron progenitors (34). Other studies have shown that Cre is expressed in motor neurons as well as in some ventral interneurons at the embryonic stage (35,36). In any case, as shown and described below, the use of HB9:Cre mice eliminated TDP-43 expression mainly in ChAT(ϩ) motor neurons in the spinal cords of our conditional knock-out mice.…”

“…Also, HB9:Cre mice express the cre gene specifically in the spinal cord motor neurons and have been used previously to manipulate gene expression in the motor neurons (33). The motor neuron specificity of the HB9-driven Cre expression have also been verified by crossing the HB9:Cre mouse line to a ROSA26:LacZ reporter mice in several studies (33)(34)(35)(36)(37)(38). Examination of the GFP signals in HB9:GFP transgenic mice have indicated that expression of HB9 is restricted in the motor neurons, but not in the sensory neurons (39).…”

To build a synapse: signaling pathways in neuromuscular junction assembly

“…A recent study demonstrates that muscle fibers express neurotrophin 3 to modulate the number of Schwann cells, the myelinating glial cells of the peripheral nervous system, in developing NMJs (Hess et al, 2007). Motoneurons release neuregulin 1 to promote Schwann cell survival and development (Hayworth et al, 2006;Trachtenberg and Thompson, 1996) (reviewed by Fischbach and Rosen, 1997;Lemke, 1993).…”