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The present study uses the embryonic chick to examine in vivo the mechanisms and regulation of Schwann cell programmed cell death (PCD) in spinal and cranial peripheral nerves. Schwann cells are highly dependent on the presence of axons for survival because the in ovo administration of NMDA, which excitotoxically eliminates motoneurons and their axons by necrosis, results in a significant increase in apoptotic Schwann cell death. Additionally, pharmacological and surgical manipulation of axon numbers also affects the relative amounts of Schwann cell PCD. Schwann cells undergoing both normal and induced PCD display an apoptotic-like cell death, using a caspase-dependent pathway. Furthermore, axon elimination results in upregulation of the p75 and platelet-derived growth factor receptors in mature Schwann cells within the degenerating ventral root. During early development, Schwann cells are also dependent on axon-derived mitogens; the loss of axons results in a decrease in Schwann cell proliferation. Axon removal during late embryonic stages, however, elicits an increase in proliferation, as is expected from these more differentiated Schwann cells. In rodents, Schwann cell survival is regulated by glial growth factor (GGF), a member of the neuregulin family of growth factors. GGF administration to chick embryos selectively rescued Schwann cells during both normal PCD and after the loss of axons, whereas other trophic factors tested had no effect on Schwann cell survival. In conclusion, avian Schwann cells exhibit many similarities to mammalian Schwann cells in terms of their dependence on axon-derived signals during early and later stages of development.

We have previously shown that chronic treatment of chick embryos [from embryonic day 5 (E5) to E9] with NMDA rescues spinal cord motoneurons (MNs) from programmed cell death. In this situation, MNs exhibit a reduced vulnerability to acute excitotoxic lesions and downregulate NMDA and AMPA-kainate receptors. Here, we report that this treatment results in long-lasting sublethal structural changes in MNs. In Nissl-stained sections from the spinal cord of NMDA-treated embryos, MNs display an area adjacent to an eccentrically positioned nucleus in which basophilia is excluded. Ultrastructurally, MNs accumulate tubulovesicular structures surrounded by Golgi stacks. Thiamine pyrophosphatase but not acid phosphatase was detected inside the tubulovesicular structures, which are resistant to disruption by brefeldin A or monensin. Immunocytochemistry reveals changes in the content and distribution of calcitonin gene-related peptide, the KDEL receptor, the early endosomal marker EEA1, and the recycling endosome marker Rab11, indicating that a dysfunction in membrane trafficking and protein sorting occurs in these MNs. FM1-43, a marker of the endocytic pathway, strongly accumulates in MNs from isolated spinal cords after chronic NMDA treatment. Changes in the distribution of cystatin C and presenilin-1 and an accumulation of amyloid precursor protein and beta-amyloid product were also observed in NMDA-treated MNs. None of these alterations involve an interruption of MN-target (muscle) connections, as detected by the retrograde tracing of MNs with cholera toxin B subunit. These results demonstrate that chronic NMDA treatment induces severe changes in the motoneuronal endomembrane system that may be related to some neuropathological alterations described in human MN disease.