Crucial role of the local micro-environment in fate decision of neonatal rat NG2 progenitors

OBJECTIVES

The fate choice of neural progenitor cells could be dictated by local cellular environment of the adult CNS. The aim of our study was to investigate the effect of hippocampal tissue on differentiation and maturation of oligodendrocyte NG2 precursor cells.

MATERIALS AND METHODS

Hippocampal slice culture was established from the brains of 7-day-old rats. NG2 precursor cells, obtained from a 12-day-old mixed primary culture of neonatal rat cerebral hemispheres, were labelled with chloromethyl-fluorescein-diacetete and seeded on the hippocampal slices. After 7-14 days in co-culture, cells were stained with neural markers.

RESULTS

NG2 cells differentiated predominantly into oligodendrocytes, presenting various stages of maturation: progenitors (NG2), pre-oligodendrocytes (O4) and finally mature GalC-positive cells. However, except for a few cells with astrocyte-specific S100b staining, a considerable number of these cells differentiated into neurons: TUJ(+) and even MAP-2(+) cells were frequently observed. Moreover, a certain population of these cells preserved proliferative properties of primary precursor cells, as revealed by Ki67 expression.

CONCLUSIONS

The neuronal micro-environment provided by the culture of hippocampal slices is potent for induction of neurogenesis from oligodendrocyte NG2(+)/PDGFRalpha(+)/CNP(+) progenitor cells and promotes their differentiation not only into macroglia but also into neurons. It also sustains their proliferative capacity. The results indicate the crucial role of the local cellular environment in fate decision of primary NG2(+) multipotent neural progenitor cells, which may affect their behaviour after transplantation into the central nervous system.

Intrathecal application of neuroectodermally converted stem cells into a mouse model of ALS: limited intraparenchymal migration and survival narrows therapeutic effects

Stem and progenitor cells provide a promising therapeutic strategy for amyotrophic lateral sclerosis (ALS). To comparatively evaluate the therapeutic potentials of human bone marrow-derived mesodermal stromal cells (hMSCs) and umbilical cord blood cells (hUBCs) in ALS, we transplanted hMSCs and hUBCs and their neuroectodermal derivatives (hMSC-NSCs and hUBC-NSCs) into the ALS mouse model over-expressing the G93A mutant of the human SOD1 gene. We used a standardized protocol similar to clinical studies by performing a power calculation to estimate sample size prior to transplantation, matching the treatment groups for gender and hSOD-G93A gene content, and applying a novel method for directly injecting 100,000 cells into the CSF (the cisterna magna). Ten days after transplantation we found many cells within the subarachnoidal space ranging from frontal basal cisterns back to the cisterna magna, but only a few cells around the spinal cord. hMSCs and hMSC-NSCs were also located within the Purkinje cell layer. Intrathecal cell application did not affect survival times of mice compared to controls. Consistently, time of disease onset and first pareses, death weight, and motor neuron count in lumbar spinal cord did not vary between treatment groups. Interestingly, transplantation of hMSCs led to an increase of pre-symptomatic motor performance compared to controls in female animals. The negative outcome of the present study is most likely due to insufficient cell numbers within the affected brain regions (mainly the spinal cord). Further experiments defining the optimal cell dose, time point and route of application and particularly strategies to improve the homing of transplanted cells towards the CNS region of interest are warranted to define the therapeutic potential of mesodermal stem cells for the treatment of ALS.

Neurogenic potential of human umbilical cord blood: neural-like stem cells depend on previous long-term culture conditions

In vitro studies conducted by our research group documented that neural progenitor cells can be selected from human umbilical cord blood (HUCB-NPs). Due to further expansion of these cells we have established the first human umbilical cord blood-derived neural-like stem cell line (HUCB-NSC) growing in serum-free (SF) or low-serum (LS) medium for over 3 years. The purpose of the study was to evaluate the neurogenic potential of HUCB-NSCs cultured in SF and LS condition in different in vitro settings before transplantation. We have shown that the number of cells attaining neuronal features was significantly higher for cultures expanded in LS than in SF condition. Moreover, the presence of neuromorphogens, cultured rat astrocytes or hippocampal slices promoted further differentiation of HUCB-NSCs into neural lineage much more effectively when the cells had derived from LS cultures. The highest response was observed in the case of co-cultures with rat primary astrocytes as well as hippocampal organotypic slices. However, the LS cells co-cultured with hippocampal slices expressed exclusively a set of early and late neuronal markers whereas no detection of cells with glial-specific markers was possible. In conclusion, certain level of stem/progenitor cell commitment is important for optimal response of HUCB-NSC on the neurogenic signals provided by surrounding environment in vitro.

Human medulloblastoma cell line DEV is a potent tool to screen for factors influencing differentiation of neural stem cells

The aim of our study was to investigate whether a human neural cell line could be used as a reliable screening tool to examine the functional conservation, in humans, of transcription factors involved in neuronal or glial specification in other species. Gain-of-function experiments were performed on DEV cells, a cell line derived from a human medulloblastoma. Genes encoding nine different transcription factors were tested for their influence on the process of specification of human DEV cells towards a neuronal or glial fate. In a first series of experiments, DEV cells were transfected with murine genes encoding transcription factors known to be involved in the neuronal differentiation cascade. Neurogenins-1, -2, and -3; Mash-1; and NeuroD increased the differentiation of DEV cells towards a neuronal phenotype by a factor of 2-3.5. In a second series of experiments, we tested transcription factors involved in invertebrate glial specification. In the embryonic Drosophila CNS, the development of most glial cells depends on the master regulatory gene glial cell missing (gcm). Expression of gcm in DEV cells induced a twofold increase of astrocytic and a sixfold increase of oligodendroglial cell types. Interestingly, expression of tramtrack69, which is required in all Drosophila glial cells, resulted in a fourfold increase of only the oligodendrocyte phenotype. Expression of the related tramtrack88 protein, which is not expressed in the fly glia, or the C. elegans lin26 protein showed no effect. These results show that the Drosophila transcription factor genes tested can conserve their function upon transfection into the human DEV cells, qualifying this cell line as a screening tool to analyze the mechanisms of neuronal and glial specification.

Relative contribution of the de novo and remodelling pathways to the synthesis of platelet-activating factor in brain areas and during ischemia

Two distinct pathways for the synthesis of platelet-activating factor (PAF) have been demonstrated in the nervous tissue. This potent lipid mediator is involved in physiological and pathological processes. The relative contribution of the two pathways to its synthesis during various conditions needs to be defined, thus the activities of the enzymes directly responsible for PAF synthesis, PAF-synthesizing phosphocholinetransferase (PAF-PCT) and lyso-PAF acetlytransferase (lyso-PAF AcT), have been assayed in rat brain areas. The former catalyses the last reaction of the de novo pathway and the latter that of the remodelling one. PAF-PCT activity was always more elevated than that of lyso PAF AcT. No differences were observed among different brain areas when enzyme activities were assayed in their homogenates. In microsomes, the highest PAF-PCT activity was found in cerebellum whereas lyso-PAF AcT activity was greater in cerebellum and in hippocampus than in the other brain areas. The activity of PAF-synthesizing enzymes was also studied in the gerbil during ischemia and reperfusion. After 6 min from bilateral occlusion of the carotid arteries, a significant increase of lyso-PAF AcT activity was observed in the hippocampus. This enzyme activity remained relatively high up to 3 days after reperfusion whereas, in other brain areas it reached basal levels much earlier. Since it has been shown that the PAF levels increase in the brain of animals during ischemia, these results suggest that the remodelling pathway may provide an important contribution to its synthesis particularly in the hippocampus, where a selective neuronal death is observed. In this area during reperfusion, a further contribution to PAF synthesis might be also provided by the de novo pathway.

Proteolipid/DM-20 proteins bearing the paralytic tremor mutation in peripheral nerves and transfected Cos-7 cells

Paralytic tremor (Plp-pt) is a missense mutation of the myelin proteolipid gene (Plp) in rabbits. The myelin yield in the Plp-pt brain is reduced and the protein and lipid composition of central nervous system (CNS) myelin is abnormal. We studied the intracellular transport of the normal and Plp-pt mutant PLP and DM-20 in transiently transfected Cos-7 cells. While the mutant PLP accumulates in the rough endoplasmic reticulum and does not reach the plasma membrane, the spliced isoform of PLP, mutant DM-20, is normally transported to the cell surface and integrated into the membrane. Analysis of rabbit sciatic nerves revealed that concentration of peripheral nervous system (PNS) myelin proteins is normal in Plp-pt myelin. In the PNS like in the CNS, the level of Plp gene products is subnormal. But this does not affect myelination in the PNS where PLP, present in low concentration, is not a structural component of compact myelin. The normal level of Plp gene expression in Schwann cells is low and these results suggest that, in the Plp-pt PNS, Schwann cell function is not affected by the deficiency in PLP and/or the impairment of intracellular PLP transport.

Brain ischaemia transiently activates Ca2+/calmodulin-independent protein kinase II

The activity of Ca2+/calmodulin-dependent protein kinase II (CaM-KII) during short-term global ischaemia was investigated in the gerbil brain hippocampus and cortex. Ischaemia of 0.5 min duration significantly stimulated Ca(2+)-independent 'autonomous' activity, indicating activation of the first step of intramolecular enzyme phosphorylation just after ischaemia has developed. Prolongation of the ischaemic period up to 5 min inhibited both Ca(2+)-dependent and, to a lesser extent, Ca(2+)-independent activities of CaM-KII. These last events coincide with an extensive translocation of CaM-KII protein from the soluble to the membranous fraction. In effect, in spite of inhibition of total CaM-KII activity, its Ca(2+)-independent, persistently active component can still remain more abundant at specific membrane regions.

Paralytic tremor (pt) rabbit: a sex-linked mutation affecting proteolipid protein-gene expression

Paralytic tremor (pt) is a neurological sex-linked recessive mutation in rabbits which is characterized by a coarse body tremor and limb paresis. Morphological studies showed that this mutation affects CNS myelination. Although the number of oligodendrocytes is not reduced, myelination is slower, irregular and defective. We have made a biochemical and molecular analysis of 4-wk-old mutant and normal rabbits. The amount of myelin in the mutant represents only approximately 25% of the normal level. Radioimmunoassay for myelin basic protein showed a reduction to approximately 40% in pt whole-brain homogenate but the difference was not significant in purified myelin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of brain homogenates followed by immunoblotting showed that all major myelin proteins are affected by the pt mutation, although to different degrees. While most of the myelin proteins are reduced to approximately 60-80% of the normal level, an important reduction to approximately 30%, was measured for the proteolipid protein (PLP). In purified myelin, the difference in PLP concentration was significant while the other specific proteins were less affected. A similar reduction in myelin-protein gene expression was detected at the mRNA level. Sex-linked transmission, low concentrations of PLP and its specific mRNA in the CNS indicate that the pt mutation primarily affects the expression of the Plp gene.

Coupling of adenosine receptors to adenylate cyclase in postischemic rat brain

The potential usefulness of adenosine receptor stimulation in the therapy for ischemic brain disease is dependent upon retention of adenosine receptors and their transduction mechanisms after ischemia. The receptors most clearly associated with adenosine-dependent cerebral inhibition are the A1-type (A1-AR), which work via a Gi protein to inhibit adenylate cyclase. In brain membranes from rats recovering at various times after 15 min of complete cardiac arrest-induced ischemia, the levels of A1-AR decreased temporarily to 60% of the control values. However, agonist affinities for A1-AR, as well as guanine nucleotide-sensitive high-affinity binding, remain unchanged. The significant decrease of agonist affinities to A1-AR produced by calcium depletion in control membranes was markedly attenuated after ischemia. Moreover, the A1-AR agonist-induced inhibition of cAMP production parallels the decrease in these receptor numbers. It was blocked in the postischemic membranes but reverts to control levels upon extending the recovery period to one week after the insult. It is concluded that in addition to the lowering of the number of A1-AR binding sites, the coupling of A1 receptor activation to adenylate cyclase response is inhibited after ischemia, but not at the level of receptor-Gi protein interaction.

Effect of lipid peroxidation on Na+,K(+)-ATPase, 5'-nucleotidase and CNPase in mouse brain myelin

In the presence of H2O2, solutions of Fe2+ were applied to brain homogenate and isolated myelin from adult SWV control mice and the shiverer dysmyelinating mutant mouse as a source of a reactive oxygen species (Fenton reaction). Under these conditions, lipid peroxidation was initiated and measured as thiobarbituric acid-reactive oxidation products (TBAR). This was accompanied by 85% inhibition of myelin-associated Na+,K(+)-ATPase and 25% inhibition of 5'-nucleotidase. In contrast, CNPase activity was not altered. Studies on the shiverer mutant brain revealed that in spite of hypomyelination and prevalence of premature, myelin-like membranes in the homogenate, the myelin-related enzymes reacted as normal enzymes to peroxidation. Differences in the resistance of Na+,K(+)-ATPase to peroxidation in the brain homogenate and myelin suggest that the myelin enzyme is extremely sensitive to reactive oxygen toxicity.

Energy utilization and changes in some intermediates of glucose metabolism in normal and hypoxic rat brain after decapitation

Energy metabolism was studied in the cerebral cortex of rats during and after hypoxia induced by breathing a gas mixture of 7% O2 in N2 for 2 h. Cortical energy stores (2ATP + ADP + phosphocreatine) remained unchanged after hypoxic treatment. Lactate rose over four-fold. Pyruvate, glucose and glucose 6-phosphate concentrations also increased significantly. Metabolic activity in the cortex expressed as the utilization of high-energy phosphates 5, 10 and 30 s after decapitation was decreased by 30% after hypoxia and remained lowered for 3 h during recovery. This was accompanied by elevated glucose consumption and lactate production, suggesting that the maintenance of the energy balance after hypoxia was partly due to activation of the glycolytic pathway. During the recovery period, these metabolic abnormalities returned towards control values, but, after 6 h of recovery the high-energy phosphate utilization increased transitorily above the control values.