Ciliary neurotrophic factor is not required for terminal sprouting and compensatory reinnervation of neuromuscular synapses: Re-evaluation of CNTF null mice

Abstract

Loss of synaptic activity or innervation induces sprouting of intact motor nerve terminals that adds or restores nerve-muscle connectivity. Ciliary neurotrophic factor (CNTF) and terminal Schwann cells (tSCs) have been implicated as molecular and cellular mediators of the compensatory process. We wondered if the previously reported lack of terminal sprouting in CNTF null mice was due to abnormal reactivity of tSCs. To this end, we examined nerve terminal and tSC responses in CNTF null mice using experimental systems that elicited extensive sprouting in wildtype mice. Contrary to the previous report, we found that motor nerve terminals in the null mice sprout extensively in response to major sprouting-stimuli such as exogenously applied CNTF per se, botulinum toxinelicited paralysis, and partial denervation by L4 spinal root transection. In addition, the number, length and growth patterns of terminal sprouts, and the extent of reinnervation by terminal or nodal sprouts, were similar in wildtype and null mice. tSCs in the null mice were also reactive to the sprouting-stimuli, elaborating cellular processes that accompanied terminal sprouts or guided reinnervation of denervated muscle fibers. Lastly, CNTF was absent in quiescent tSCs in intact, wildtype muscles and little if any was detected in reactive tSCs in denervated muscles. Thus, CNTF is not required for induction of nerve terminal sprouting, for reactivation of tSCs, and for compensatory reinnervation after nerve injury. We interpret these results to support the notion that compensatory sprouting in adult muscles is induced primarily by contact-mediated mechanisms, rather than by diffusible factors.

Degeneration of Phrenic Motor Neurons Induces Long-Term Diaphragm Deficits following Mid-Cervical Spinal Contusion in Mice

“…Each junction was categorized as 1) normally innervated by the original axon, 2) completely denervated (no occupation of AChRs by nerve terminal), 3) partially denervated (partial occupation of AChRs by nerve terminal), 4) thinner diameter preterminal axons, and 5) multiply innervated NMJs. Both of these final categories are characteristic of recently reinnervated NMJs by regenerating axons (Wright and Son, 2007). For each muscle examined, a minimum of 100 NMJs were characterized.…”

Early Phrenic Motor Neuron Loss and Transient Respiratory Abnormalities after Unilateral Cervical Spinal Cord Contusion

“…Labeled muscles were analyzed for total numbers of neuromuscular junctions (NMJs), and were further categorized as: ''intact'' NMJs with full overlapping of pre-and postsynaptic domains, denervated NMJs (partial or complete), and NMJs with signs of reinnervation. 13 …”

“…2 Hemidiaphragm muscle was dissected from each animal for whole-mount immunohistochemistry, as previously described. 2,13 Motor axons and their presynaptic terminals were labeled with SMI-312R (Covance) and SV2-s (Developmental Studies Hybridoma Bank [DSHB]), respectively, and both labelings were detected with FITC anti-mouse Immunoglobulin G (IgG) secondary ( Jackson ImmunoResearch). Postsynaptic acetylcholine receptors were labeled with Alexa Fluor 647-conjugated a-bungarotoxin (Invitrogen).…”

Neuron–glia interactions: the roles of Schwann cells in neuromuscular synapse formation and function

“…It will be of interest to test the reaction of terminal Schwann cells after nerve injury in these animals [50]. Beside NRG1, application of CNTF (ciliary neur-otrophic factor) induces nerve terminal sprouting [51]; however, CNTF-null mice show both terminal Schwann cells and axonal sprouting after nerve injury or muscle inactivity [52]. Thus, the involvement of CNTF is unlikely.…”