The pH of jimpy glia is increased: intracellular measurements using fluorescent laser cytometry

The jimpy mutation lies in the gene which codes for myelin proteolipid protein, and the brains and spinal cords of jimpy mice contain little myelin and no measurable proteolipid protein. It has been thought that the mutation affected only the myelin forming oligodendroglial cells, but there is now considerable evidence that astroglia are also a target of the mutation since jimpy astrocytes exhibit a prominent gliosis along with defects in metabolism and proliferation. Because cell proliferation is associated with an increase in intracellular pH, we investigated whether the pH of jimpy glia was abnormal. Using a pH sensitive fluorescent dye and a laser cytometry system we measured the intracellular pH of individual cells in cultures derived from both jimpy and normal brains. The relative pH of flat astrocytes in jimpy cultures was higher than in normal cultures by an average of 0.24 pH units, and these increased values were evident 2-3 days after plating. At this in vitro age the cultures contain only a few oligodendrocytes, none of which express detectable proteolipid protein. The pH of the process-bearing cell population, which contains the oligodendrocytes as well as some astrocytes and presumptive glial precursors, was also increased but not until 7 days in culture. The finding that a mutation in the myelin proteolipid protein gene can alter the normal pH of astrocytes is quite unexpected since, as far as is known, astrocytes do not make proteolipid protein. These results and others discussed in this paper support the hypothesis that either proteolipid protein itself, or some other product of the gene, may have an important role in central nervous system development.

Jimpy mutation affects astrocytes: lengthening of the cell cycle in vitro

The jimpy mutation has been identified as a point mutation in the gene coding for the major myelin proteolipid protein. The most prominent effect of the mutation is an extreme reduction in central nervous system myelin in affected mice. However, both oligodendrocytes and astrocytes, the two types of central nervous system macroglia, have been shown to exhibit more subtle developmental and metabolic changes as a result of the mutation. These include early death and proliferation abnormalities in jimpy oligodendrocytes, and hypertrophy, increased pH and abnormal responses to high K+ in jimpy astrocytes. In the present study, we examine the effect of the mutation of the cell cycle of astrocytes. Using an immunocytochemical method to chart the percent of labeled mitoses, we find the total cell cycle to be lengthened in jimpy astrocytes by 5-6 h, with increases in several different phases. Since there is no evidence that astrocytes make myelin proteolipid protein, the results support previous studies which suggest that this gene may code for other proteins playing an important role in the development of many cell types.

The cell cycle of glial cells grown in vitro: an immunocytochemical method of analysis

Studies of cell cycles have traditionally employed [3H]- and [14C]-thymidine to label the DNA of proliferating cells and autoradiography to reveal the thymidine label. The development of antibodies to the thymidine analogue 5-bromodeoxyuridine (BrdU) has allowed the development of an immunocytochemical method analogous to the thymidine autoradiographic technique. In direct comparisons, we found that the immunocytochemical method consistently detected a larger number of proliferating cells. This suggests that it may be a more sensitive index of proliferation than thymidine autoradiography in some systems. We used the BrdU method to analyze the cycle of astroglia cultured from neonatal mouse cerebral cortex. Cells were exposed to BrdU for 1 hr to label a discrete subpopulation of proliferating cells. At 2-36 hr after the pulse, a combination of anti-BrdU immunocytochemistry and counterstaining with propidium iodide was used to identify proliferating cells. The length of the cell cycle was determined by charting the percent of BrdU-labeled mitotic cells vs time after the pulse. We found the average length of the cell cycle of astrocytes grown in vitro to be 20.5 hr. The combined G2 + M phases were 2-3 hr. These values are virtually identical with those found for glial cells in vivo, suggesting that the culture environment does not interfere with the normal control of cell cycle length.

Cellular dimensions affecting the nucleocytoplasmic volume ratio

Although it has long been appreciated that larger eukaryotic cells have larger nuclei, little is known about how this size relationship is maintained. Here we describe a method for measuring the aqueous volume ratio of nucleus to cytoplasm, two compartments which are interconnected via the pores in the nuclear envelope. We then use that method to identify proportional cellular dimensions in variously treated cells and in different cell types. Cells were scrape loaded with a mixture of fluorescent dextrans: Texas red dextran, average mol wt = 10,000 (TRDx10), and fluorescein isothiocyanate dextran, average mol wt = 70,000 (FDx70). After introduction into the cytoplasmic space, the TRDx10 distributed into both the nucleus and cytoplasm, whereas the FDx70 was restricted to cytoplasm, due to size exclusion by the nuclear pores. The aqueous nucleocytoplasmic volume ratio (RN/C) was determined by measuring, from fluorescence images of spread cells, total cellular fluorescence of each of the two probes and the fluorescence ratio of those probes in the cytoplasm. RN/C was unaffected by the measurement procedure or by varying temperatures between 23 degrees and 37 degrees C. Loading excess unlabeled dextrans had little effect on RN/C, with the single exception that high concentrations of large dextrans could lower RN/C in endothelial cells. Expanding intracellular membranous compartments of macrophages by phagocytosis of latex beads decreased RN/C. Expanding the same compartment by pinocytosis of sucrose, which nearly doubled total cell volume, had little effect on RN/C, indicating that nuclear volume was more closely linked to the cytoplasmic volume, exclusive of vesicular organelles, than to total cell volume. RN/C was the same in mononucleate and binucleate endothelial cells. Finally, measurements of RN/C in murine bone marrow-derived macrophages, bovine aortic endothelial cells, Swiss 3T3 fibroblasts, PtK2 cells, and CV-1 cells revealed that nuclear volume scaled allometrically with cell volume. The allometric relationship indicated that cell volume was proportional to nuclear surface area.

Studies of glial lineage and proliferation in vitro using an early marker for committed oligodendrocytes

The potential of immature glial cells to differentiate into astrocytes (ASs) or oligodendrocytes (OLs) has been examined using a monoclonal antibody (007) that is specific for OLs in vivo. Cells were dissociated from 2-day postnatal mouse cortex and labeled with the 007 antibody 2 hr after plating. The cells which were labeled during this single, brief exposure to the antibody retained the antibody on their surfaces over the course of the experiments. Cells were double stained at various timepoints for residual 007 antibody and either galactocerebroside (GC) or glial fibrillary acidic protein (GFAP). Shortly after plating, most 007+ cells were GC- and none expressed GFAP. These cells were round, although some had begun to extend very short processes. After 96 hr, greater than 95% of cells with residual 007 on their surfaces also expressed GC. By this time, all the 007+ cells had several processes of varying lengths extending from their cell bodies. Cells expressing both 007 and GFAP were never seen. The 007+/GC+ OLs were not induced to differentiate from 007+ bipotential progenitors since they were grown in fetal calf serum. These results show that under our culture conditions the 007 antibody is OL specific. Immunostaining for bromodeoxyuridine, a marker for dividing cells, revealed that some 007+ cells were proliferating. The majority of these proliferating cells had already extended three or more processes. We therefore conclude that immature, process-bearing cells can be committed to the OL lineage at times before they express detectable amounts of GC. Since these young 007+ OLs are actively proliferating, committed cells can serve as an important source of new OLs.

Division of astroblasts and oligodendroblasts in postnatal rodent brain: evidence for separate astrocyte and oligodendrocyte lineages

What precursor cells are the source of the macroglia generated during postnatal development? In order to answer this question, we studied the expression of glial specific antigens in proliferating neuroglia in postnatal rodent brain and optic nerve. Immunocytochemistry using antibodies to oligodendrocyte (OL) specific markers (sulfatide and galactocerebroside) and an astrocyte (AS) specific marker (glial fibrillary acidic protein) was combined with thymidine autoradiography. During the first week of postnatal development when most ASs are being generated, one third to one half of the proliferating cells in the optic system are positive for glial fibrillary acidic protein after a 1 h injection of thymidine (Skoff, Dev. Biol., 139:149-168, 1990). During the second postnatal week when OLs are being generated, 30 to 100% of the proliferating cells in presumptive white matter tracts are sulfatide positive and at least 10% are galactocerebroside positive. This finding demonstrates that ASs and OLs divide during postnatal development. These results confirm previous electron microscopic autoradiographic studies showing that the vast majority of proliferating cells in postnatal rat optic nerve have the morphologic characteristics of differentiating ASs or OLs (Skoff, J. Comp. Neurol., 169:291-312, 1976). Since proliferating ASs (astroblasts) and OLs (oligodendroblasts) constitute the majority of the dividing cells at the time that ASs and OLs are being generated, these glioblasts must be the major source for the macroglia generated postnatally. The findings strongly suggest that separate lineages exist for ASs and OLs during postnatal development. There is no compelling in vivo evidence for a bipotential progenitor cell that generates the majority of OLs and certain ASs in postnatal rodent brain. There may, of course, be distinct lineages for the subtypes of ASs and possibly even for subtypes of OLs. We review the concepts of commitment and plasticity and apply these terms to glial differentiation. In situ, the presence of oligodendroblasts and astroblasts demonstrates the COMMITMENT of proliferating cells to a specific glial lineage during normal development. Culture conditions may provide an environment that permits proliferating glial cells to vacillate in their selection of a specific lineage. This situation demonstrates developmental PLASTICITY and the ability of glia to adapt to an altered environment. Whether committed glial cells in situ can be induced to switch their lineage when normal CNS conditions are altered is an intriguing question that remains to be answered.

Plasticity of the tubular lysosomal compartment in macrophages

Because bone marrow-derived macrophages differentiate in culture, their lysosomal compartment is largely devoid of the undigested particles that are common in macrophages removed from tissues. The morphology of this nearly vacant lysosomal compartment was observed, after labeling with fluorescent endocytic tracers such as Lucifer Yellow, to be an extensive, tubuloreticular network, which underwent extensive rearrangements in accommodating endocytic loads. It was converted to spherical organelles when the lysosomal compartment was loaded with osmotically active solutes such as sucrose or Acridine Orange. Enzymatic degradation of intravacuolar sucrose by pinocytosed invertase resulted in the shrinkage of vacuoles and the re-formation of the tubular network. After phagocytosis of opsonized erythrocytes or latex beads, tubular lysosomes wrapped around the phagosomes, then merged to form phagolysosomes. The disappearance of tubules was proportional to the total surface area of particles ingested. Degradation of the phagocytosed contents permitted shrinkage of the phagolysosome and concomitant re-formation of the tubuloreticular network. Nondegradable contents such as latex beads prevented re-formation of the tubular network. These rearrangements of the lysosomal compartment indicate that the organelle exhibits considerable plasticity and interconnectedness, and that maturation of lysosomes after endocytosis does not necessarily entail irreversible morphological changes.

Death of individual oligodendrocytes in jimpy brain precedes expression of proteolipid protein

Immunocytochemistry and thymidine autoradiography were combined to determine the time elapsed between cell division and the expression of proteolipid protein (PLP) in individual oligodendrocytes in normal mouse brain. In jimpy (jp) brains, autoradiography was used to determine the time elapsed between cell division in an individual oligodendrocyte and evidence of cell death. Oligodendrocytes in normal mouse brain do not express PLP until 72 h after a single injection of [3H]-thymidine. In contrast, oligodendrocytes in jp brains begin to die within 9-11 h after an injection of thymidine. The jp mouse is one of several X-linked, hypomyelinated mutants in which a defect has been demonstrated in the gene coding for PLP. It has been presumed that the lack of this protein in the myelin sheath is responsible for the jp phenotype. However, the present study shows that individual jp oligodendrocytes begin to die long before they would normally have synthesized detectable levels of PLP. Therefore, it seems unlikely that the death of jp oligodendrocytes is due to the absence of PLP in myelin sheaths. Oligodendrocyte death and other early jp abnormalities may be due to the presence of abnormal PLP message which may interfere with glial differentiation. Alternatively, the PLP message may code for another protein which is important for normal development of neuroglia.

Differences in levels of neuroglial cell death in jimpy male mice and carrier females

Jimpy (jp) is an X-linked disorder which results in a variety of glial cell abnormalities and the virtual absence of myelin in the central nervous system (CNS) of affected males. Female heterozygote carriers of the jp gene are mosaics. The wild-type genome will be expressed in roughly half of their cells, while in the other half the jp genome will be expressed. We have exploited this characteristic in order to determine whether the premature oligodendroglial death previously described in jp males is a primary effect of the mutation. Mosaic spinal cords were examined at the light-microscopic level for the presence of pyknotic cells at ages when oligodendroglial death is quite pronounced in jp males. The percentage of pyknotic glial cells (less than 0.6%) is not statistically different in normal and mosaic females at 5 and 30 days. At 14-15 days there is a slight increase in mosaic cords (1.2 vs. 0.75%) but there are still 10 times more dying glia in jp male cords of the same age. The low level of oligodendroglial death in mosaic spinal cords suggests that it is secondary to some other jp abnormality. Small increases in glial death over a protracted period of time could result in lower numbers of oligodendrocytes in older mosaics. Even if this occurs, the data still support the idea that the phenotype of genetically jp cells can be favorably altered by the mosaic environment.

Differential expression of galactocerebroside, myelin basic protein, and 2',3'-cyclic nucleotide 3'-phosphohydrolase during development of oligodendrocytes in vitro

This paper describes the differential expression and localization of myelin components within membrane sheets produced by oligodendrocytes in vitro. In double-labeling experiments using antibodies to the myelin antigens 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and galactocerebroside (GC), the two antigens were coexpressed in at least 95% of oligodendrocytes at all ages examined. A small population of relatively undifferentiated cells expressed one antigen before the other. Within the membrane sheets produced by the cultured cells, CNP and GC are distributed differently. CNP is highly concentrated in cell bodies, in a network of processes extending from the cell body into the sheets, and around the perimeter of the sheets. CNP staining cannot be detected in some areas within the body of the sheet. When present, it is of low intensity. Under our labeling conditions, GC staining is found throughout the membrane sheets, except in the network of veins which are CNP+. GC and myelin basic protein (MBP) staining are seen in similar membrane domains even though GC is a surface component while MBP resides on the cytoplasmic face. Both the timing and localization of CNP immunostaining show that CNP is as early a marker for oligodendrocytes as GC, and support the idea that CNP may play a structural role in the myelin membrane. Double-labeling studies with GC and CNP antibodies also show that the true shape of a cell and the extent of its development are not always revealed by a single antigen. The differential distribution of antigens within membrane sheets illustrates that they contain areas of structural specialization that may reflect the situation in intact myelin.

Glial conditioned medium enables jimpy oligodendrocytes to express properties of normal oligodendrocytes: production of myelin antigens and membranes

Cultures of cerebral cells from 2-day-old jimpy (jp) and normal animals were immunocytochemically stained with antibodies to galactocerebroside, 2', 3'-cyclic nucleotide 3'-phosphohydrolase and proteolipid protein (PLP). As the normal cultures mature, oligodendrocytes express these markers in cell bodies, in processes and in expansive membrane sheets produced along the lengths of cell processes. In contrast, oligodendrocytes from jp cultures do not produce large membrane sheets and their total numbers are reduced. Only rarely do jp oligodendrocytes exhibit immunostaining for PLP. Thus, jp oligodendrocytes grown in vitro mimic deficiencies of jp CNS in situ. In order to examine the effect of environment and cell-cell interactions on expression of the jp mutation we grew jp cerebral cells in the presence of medium conditioned for short periods by normal cerebral cells. Under these conditions jp oligodendroglia appeared nearly normal by immunostaining criteria. Their numbers were increased; they were able to produce and maintain membrane sheets; and some cells expressed PLP. These results show that jp oligodendrocytes have the capacity to express certain normal phenotypic parameters, and when given an appropriate environment they do so. The effects of normal conditioned medium may be due to a secreted factor, possibly produced by the astrocytes, which constitute the vast majority of cells in the conditioning cultures. The dramatic effect of normal glial conditioned medium on jp oligodendrocytes suggests that the steps leading to hypomyelination in jp are exceedingly complex and may involve glial-glial interactions.

A defect in the cell cycle of neuroglia in the myelin deficient jimpy mouse

One h after receiving a single pulse of [3H]thymidine, spinal cords from jimpy and control animals were examined for labeled cells and for cells in mitosis. Although the number of labeled cells was significantly higher in jimpy spinal cords at 14 and 20 days postnatal, the number of mitotic cells was not. In normal animals, the ratio of the number of labeled cells to the number of mitotic cells (L:M) was close to 4:1 regardless of the age of the animal or the labeling index. In jimpy animals, the L:M ratio was always close to 8:1. Previous ultrastructural studies have shown that the great majority of [3H]thymidine-labeled cells in jimpy central nervous system (CNS) at these ages are oligodendrocytes. Therefore, the unusually high L:M ratio indicates that there is an abnormality in some aspect of the jimpy oligodendroglial cell cycle.

Cultured oligodendrocytes mimic in vivo phenotypic characteristics: cell shape, expression of myelin-specific antigens, and membrane production

Primary cultures of neonatal mouse cerebra were maintained for up to 4 weeks in the absence of neurons. Oligodendrocytes in these cultures pass through a sequence of cytoarchitectural change and antigen expression which mimics the differentiation of oligodendrocytes in vivo. The cell bodies and processes of oligodendrocytes first express the myelin-specific antigen galactocerebroside (GC) by 2 days in vitro. Myelin basic protein (MBP) appears several days later. The majority of oligodendrocytes then proceed to elaborate large sheets of membranous material from the tips and lengths of cell processes. These membranous sheets, which contain GC and MBP, are reminiscent of unwrapped myelin profiles in vivo. As with the cell bodies and processes, GC is inserted into the sheets several days before MBP. Our results establish that oligodendrocytes cultured without neurons are able to produce extensive membranes containing myelin-specific antigens. They also suggest that oligodendrocyte shape and membrane production are, in part, regulated from within the oligodendrocyte itself.

Oligodendroglial cell death in jimpy mice: an explanation for the myelin deficit

Neuroglial cell death was investigated in 3 white matter tracts of jimpy and normal mice. In normal animals, glial death during development ranged from 0.5 to 2.7% of the total glial population. The number of dying glial cells was significantly higher in jimpy animals at times corresponding with oligodendroglial proliferation and the onset of myelination in each tract. At certain ages, over 10% of the glial population were pyknotic at the light-microscopic level. Dying glial cells that were identified ultrastructurally presented the characteristics of oligodendrocytes. Premature death of oligodendrocytes presents a simple explanation for the gross deficits of myelin in jimpy animals. The shortened life span of the jimpy oligodendrocyte may preclude the elaboration of a normal myelin sheath. The jimpy model may prove to be a valuable tool in delineating a role for normal neuroglial cell death during the development of the nervous system.