Early postnatal development of glial cells in the canine cervical spinal cord

Transmission Electron Microscopy and Morphometry of the CNS White Matter

Transmission electron microscopy of central nervous system white matter has provided unparalleled access to the ultrastructural features of axons, their myelin sheaths, and the major cells of white matter; namely, oligodendrocytes, oligodendrocyte precursors, astrocytes, and microglia. In particular, it has been invaluable in elucidating pathological changes in axons and myelin following experimentally induced injury or genetic alteration, in animal models. While also of value in the examination of human white matter, the tissue is rarely fixed adequately for the types of detailed analyses that can be performed on well-preserved samples from animal models, perfusion fixed at the time of death. In this chapter we describe methods for obtaining, processing, and visualizing white matter samples using transmission electron microscopy of perfusion fixed tissue and for unbiased morphometry of white matter, with particular emphasis on axon and myelin pathology. Several advanced electron microscopy techniques are now available, but this method remains the most expedient and accessible for routine ultrastructural examination and morphometry.

MC4R Is Involved in Neuropathic Pain by Regulating JNK Signaling Pathway After Chronic Constriction Injury

Background

Neuropathic pain can develop after nerve injury, when deleterious changes occur in injured neurons and glia cells. Melanocortin 4 receptor (MC4R) is involved in the regulation of pain due to its high expressions in brain. Moreover, MC4R could mediate the c-Jun N-terminal kinase (JNK) signaling pathway, but whether the MC4R-regulated JNK signaling pathway participated in neuropathic pain after chronic constriction injury (CCI) is still unclear.

Methods

A total of 128 Sprague-Dawley rats were allocated into four experiment groups: the SHAM group, CCI + NaCl group, CCI + HS group, and CCI + SP + HS group. For the CCI + NaCl group, the sciatic nerves were ligated. For the SHAM group, an identical manner to the CCI without ligation was performed. For CCI + HS and CCI + SP + HS groups, rats were injected with MC4R inhibitor (HS014) and HS014 plus JNK inhibitor (SP600125), respectively, from days 3 to 14 after CCI. Paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) were used to assess the nociceptive behavior. ELISA was used to detect the levels of inflammatory cytokines. qRT-PCR and Western blots (WB) were utilized to examine the mRNA and protein expressions of JNK signaling pathway-related genes. Meanwhile, the expression levels of MC4R and p-JNK were further evaluated by immunohistochemistry (IHC) and immunofluorescence (IF) experiments. Finally, in order to confirm the in vivo results, astrocytes were isolated and transfected with MC4R-overexpression plasmid. Furthermore, the protein expressions of JNK signaling pathway-related genes were tested by WB.

Results

It was showed that the values of PWL and PWT were significantly increased in CCI + HS group and CCI + SP + HS group compared with CCI + NaCl group. The increased interleukin-6 (IL-6), IL-1β, and tumor necrosis factor-α (TNF-α) secretion in CCI + NaCl group was lowered by HS and SP + HS. MC4R, p-JNK, ATF3, and c-Jun levels were up-regulated with CCI surgery, but down-regulated with HS and SP + HS treatments. Moreover, the IHC and IF results further revealed that MC4R and p-JNK expressions in CCI + NaCl group were remarkably higher than those in HS group and HS + SP group. In vitro data also indicated that HS, SP, and SP + HS could down-regulate the expressions of MC4R, p-JNK, ATF3, and c-Jun in M1830 astrocytes.

Conclusion

Our findings indicated that MC4R is involved in neuropathic pain by regulating JNK signaling pathway after CCI.

Myelin and oligodendrocyte development in the canine spinal cord

We studied the developmental pattern of oligodendrocyte differentiation and myelin formation in the fetal canine spinal cord from E40 to P0. The pattern of development matches what has been described in the spinal cord of humans, rodents, and many other species. Oligodendrocytes were first found at E40, close to the central canal, with their spread in a tangential manner to the ventral and then lateral columns. Myelin development followed the same pattern but was not seen until E46. A clear subpial zone lacking glial cells and myelin was seen in the lateral column in early development, suggesting that there may also be a radial component of migration of oligodendrocyte progenitor cells (OPCs) from a ventral site. This spatial-temporal developmental pattern seen in wild type matches a delay in myelination of the superficial tracts of the spinal cord seen in a canine myelin mutant, suggesting that the mutation prevents the distribution and differentiation of OPCs at an early, but narrow, window of time during fetal development.

Phenotype overlap in glial cell populations: astroglia, oligodendroglia and NG-2(+) cells

The extent to which NG-2(+) cells form a distinct population separate from astrocytes is central to understanding whether this important cell class is wholly an oligodendrocyte precursor cell (OPC) or has additional functions akin to those classically ascribed to astrocytes. Early immuno-staining studies indicate that NG-2(+) cells do not express the astrocyte marker GFAP, but orthogonal reconstructions of double-labeled confocal image stacks here reveal a significant degree of co-expression in individual cells within post-natal day 10 (P10) and adult rat optic nerve (RON) and rat cortex. Extensive scanning of various antibody/fixation/embedding approaches identified a protocol for selective post-embedded immuno-gold labeling. This first ultrastructural characterization of identified NG-2(+) cells revealed populations of both OPCs and astrocytes in P10 RON. NG-2(+) astrocytes had classic features including the presence of glial filaments but low levels of glial filament expression were also found in OPCs and myelinating oligodendrocytes. P0 RONs contained few OPCs but positively identified astrocytes were observed to ensheath pre-myelinated axons in a fashion previously described as a definitive marker of the oligodendrocyte lineage. Astrocyte ensheathment was also apparent in P10 RONs, was absent from developing nodes of Ranvier and was never associated with compact myelin. Astrocyte processes were also shown to encapsulate some oligodendrocyte somata. The data indicate that common criteria for delineating astrocytes and oligodendroglia are insufficiently robust and that astrocyte features ascribed to OPCs may arise from misidentification.

Embryonic development of central nervous system myelination in a reptilian species, Eumeces fasciatus

The myelin proteolipid proteins are a vital component of the vertebrate central nervous system (CNS), contributing essential functions to the development of the myelinating cells of the CNS and to the structure of CNS myelin. Alternative splicing of the proteolipid protein (PLP) gene to produce two related isoforms occurs in Mammalia, Aves, and Reptilia, but not Amphibia. As part of a long-term investigation into the function of the different isoforms of PLP, embryonic development, myelination, and PLP gene expression in reptilian CNS were examined. PLP gene expression was already substantial by day 19 (stage 39) of the 27-day Eumeces fasciatus egg incubation period. By day 21 of incubation, also stage 39, PLP mRNA was at peak levels; there was a significant amount of CNS myelination as demonstrated by electron microscopy of the spinal cord; and the reflexive motor response was evident. Although most axons were myelinated by the time of hatching, myelin sheaths continued to increase in size and compactness after hatching. The correlation of physiological development, CNS myelination, and expression of the PLP gene in the lizard corresponded well with the developmental pattern seen in mammals.

Modification of astrocytes in the spinal cord following dorsal root or peripheral nerve lesions

Glial fibrillary acidic protein (GFAP) immunocytochemistry was used to monitor the response of astrocytes in the rat spinal cord to either dorsal root or sciatic nerve lesions. Image analysis methods were used to provide a quantitative correlate of the reactive gliosis. Multiple dorsal root section elicited a rapid increase in GFAP immunoreactivity of astrocytes unilaterally within the spinal cord along the pathway of the degenerating dorsal root axons in the dorsal and ventral horns and this gliosis persisted in the dorsal horn beyond the time at which active phagocytosis of degenerative debris occurred. Labeling of proliferating cells using [3H]thymidine revealed that none of the dividing cells contained detectable GFAP, suggesting that the increased GFAP labeling represents primarily a hypertrophy rather than a proliferation of astrocytes. Comparison of animals that had been deafferented in the early neonatal period with those deafferented as adults indicated that the GFAP immunoreactive response persisted following neonatal lesions but that it was markedly less intense than after adult lesions. Sciatic nerve section in adults does not result in extensive frank degeneration but it does evoke a rapid and marked increase in staining of astrocytes both in the dorsal horn and in the ventral horn. Transganglionic changes in GFAP staining in the dorsal horn occur by 3 days post-operatively, which is much earlier than the time of dorsal root ganglion neuron death caused by the sciatic nerve lesion. These experiments indicate that astrocytes can respond to signals from a variety of changes in neurons, including not only Wallerian degeneration, but also retrograde and transganglionic changes.

Gliogenesis in rat optic nerve: astrocytes are generated in a single wave before oligodendrocytes

The time of origin for astrocytes in the rat optic nerve was investigated to determine whether this cell type is generated in two waves, a first wave which occurs before the formation of oligodendrocytes and a second wave which occurs after the peak period of oligodendrocyte formation. To answer this question, multiple injections of radioactive thymidine were administered to rats after the peak period of oligodendrocyte production in the optic nerve and the animals were sacrificed several weeks after the first injection. Thymidine-labeled cells in the optic nerve were identified with the electron microscope. Of the labeled cells, greater than 80% are oligodendrocytes, 4% are microglia, 2% are astrocytes, and the remainder are unclassifiable. The thymidine-labeled cells in the nerve were not immunostained for glial fibrillary acidic protein (GFAP), a marker characteristic of astrocytes. The number of thymidine-labeled glia generated after the second postnatal week is a small fraction of the total number of glia generated neonatally. No evidence exists for a second wave of astrocyte formation in the rat optic nerve as has been suggested in a study by Miller et al. (1985, Dev. Biol. 111, 35-41); rather, the vast majority of astrocytes are generated during the first 2 postnatal weeks and these data are in keeping with classical studies of gliogenesis. The question of whether astrocytes in the rat optic nerve arise directly from division of an undifferentiated, common progenitor cell or from a cell committed to the astrocyte lineage was addressed by combining thymidine autoradiography with GFAP immunocytochemistry. Rats were sacrificed 1 hr after an injection of thymidine and their nerves were processed for GFAP immunocytochemistry and autoradiography. During the first postnatal week, many thymidine-labeled cells are immunostained for GFAP. These observations demonstrate that cells committed to the astrocyte lineage divide neonatally and give rise to additional astrocytes.

Region-specific appearance of myelin constituents in the developing rat spinal cord

The appearance of myelin-specific glycolipids and of myelin basic protein was studied with regard to the detailed anatomy of the rat cervical spinal cord. The expression of these constituents in particular fibre tracts and regions occurs at specific times of development between postnatal days 1 and 14. This mosaic-like appearance started in the ventral funiculus (day 1) followed by fasciculus cuneatus and ventro-lateral funiculus (day 2), and fasciculus gracilis and dorso-lateral funiculus (days 3 to 4). Cortico-spinal tract (day 11), Lissauer tract (day 14) and the commissures started to acquire myelin very late. In the grey matter, myelin constituents appeared around days 11 to 14 in a patchy pattern. These results support a concept of highly local interactions regulating oligodendrocyte differentiation. In addition, a general rostro-caudal gradient of myelin development exists in the spinal cord, which is independent of the ascending or descending nature of the fibre tracts. Appearance of myelin constitutents in the caudal spinal cord was not prevented by a neonatal transection at mid-thoracic levels.