Effects of undernutrition on glial maturation

Mitochondrial dysfunction in alveolar and white matter developmental failure in premature infants

At birth, some organs in premature infants are not developed enough to meet challenges of the extra-uterine life. Although growth and maturation continues after premature birth, postnatal organ development may become sluggish or even arrested, leading to organ dysfunction. There is no clear mechanistic concept of this postnatal organ developmental failure in premature neonates. This review introduces a concept-forming hypothesis: Mitochondrial bioenergetic dysfunction is a fundamental mechanism of organs maturation failure in premature infants. Data collected in support of this hypothesis are relevant to two major diseases of prematurity: white matter injury and broncho-pulmonary dysplasia. In these diseases, totally different clinical manifestations are defined by the same biological process, developmental failure of the main functional units-alveoli in the lungs and axonal myelination in the brain. Although molecular pathways regulating alveolar and white matter maturation differ, proper bioenergetic support of growth and maturation remains critical biological requirement for any actively developing organ. Literature analysis suggests that successful postnatal pulmonary and white matter development highly depends on mitochondrial function which can be inhibited by sublethal postnatal stress. In premature infants, sublethal stress results mostly in organ maturation failure without excessive cellular demise.

Abbreviated exposure to hypoxia is sufficient to induce CNS dysmyelination, modulate spinal motor neuron composition, and impair motor development in neonatal mice

Neonatal white matter injury (nWMI) is an increasingly common cause of cerebral palsy that results predominantly from hypoxic injury to progenitor cells including those of the oligodendrocyte lineage. Existing mouse models of nWMI utilize prolonged periods of hypoxia during the neonatal period, require complex cross-fostering and exhibit poor growth and high mortality rates. Abnormal CNS myelin composition serves as the major explanation for persistent neuro-motor deficits. Here we developed a simplified model of nWMI with low mortality rates and improved growth without cross-fostering. Neonatal mice are exposed to low oxygen from postnatal day (P) 3 to P7, which roughly corresponds to the period of human brain development between gestational weeks 32 and 36. CNS hypomyelination is detectable for 2–3 weeks post injury and strongly correlates with levels of body and brain weight loss. Immediately following hypoxia treatment, cell death was evident in multiple brain regions, most notably in superficial and deep cortical layers as well as the subventricular zone progenitor compartment. PDGFαR, Nkx2.2, and Olig2 positive oligodendrocyte progenitor cell were significantly reduced until postnatal day 27. In addition to CNS dysmyelination we identified a novel pathological marker for adult hypoxic animals that strongly correlates with life-long neuro-motor deficits. Mice reared under hypoxia reveal an abnormal spinal neuron composition with increased small and medium diameter axons and decreased large diameter axons in thoracic lateral and anterior funiculi. Differences were particularly pronounced in white matter motor tracts left and right of the anterior median fissure. Our findings suggest that 4 days of exposure to hypoxia are sufficient to induce experimental nWMI in CD1 mice, thus providing a model to test new therapeutics. Pathological hallmarks of this model include early cell death, decreased OPCs and hypomyelination in early postnatal life, followed by dysmyelination, abnormal spinal neuron composition, and neuro-motor deficits in adulthood.

The challenge of understanding cerebral white matter injury in the premature infant

White matter injury in the premature infant leads to motor and more commonly behavioral and cognitive problems that are a tremendous burden to society. While there has been much progress in understanding unique vulnerabilities of developing oligodendrocytes over the past 30years, there remain no proven therapies for the premature infant beyond supportive care. The lack of translational progress may be partially explained by the challenge of developing relevant animal models when the etiology remains unclear, as is the case in this disorder. There has been an emphasis on hypoxia-ischemia and infection/inflammation as upstream etiologies, but less consideration of other contributory factors. This review highlights the evolution of white matter pathology in the premature infant, discusses the prevailing proposed etiologies, critically analyzes a sampling of common animal models and provides detailed support for our hypothesis that nutritional and hormonal deprivation may be additional factors playing critical and overlooked roles in white matter pathology in the premature infant.

GFAP Expression in Astrocytes of Suprachiasmatic Nucleus and Medial Preoptic Area are Differentially Affected by Malnutrition during Rat Brain Development

The aim of the present study was investigate, in young rats, the effects of malnutrition on astrocyte distribution of two hypothalamic regions, the circadian pacemaker suprachiasmatic nucleus (SCN) and the medial preoptic area (MPA). Control rats were born from mothers fed on commercial diet since gestation and malnourished rats from mothers fed on multideficient diet, from the beginning of gestation (GLA group) or from the onset of lactation (LA group). After weaning, pups received ad libitum the same diet as their mothers, and were maintained under a 12/12 h light/dark cycle. The animals were analyzed either at 30-33, or 60-63 days of life. Brain coronal sections (50 microm) were processed to visualize glial fibrillary acidic protein (GFAP) immunoreactivity. Compared to control rats, both malnourished groups of 30 and 60 days exhibited a reduced number of GFAP-immunoreactive astrocytes in the SCN. The total GFAP-immunoreactive area in the SCN of the GLA group differed from the control group at both age ranges analyzed. The GFAP expression as measured by the relative optical density (ROD) exhibited a 50-60% reduction in the MPA in both malnourished groups, compared to controls. The results suggest that malnutrition early in life leads to alterations in gliogenesis or glial cell proliferation in both nuclei, being these alterations greater in the MPA. Compensatory plasticity mechanisms in the GFAP-expression seem to be developed in the astrocyte differentiation process in the SCN, especially when the malnutrition is installed from the lactation.

Effect of protein malnutrition on the neurobehavioural toxicity of styrene in young rats

Neurotoxic effects of styrene on certain biochemical and behavioural indices were studied in young rats kept deficient in protein during the weaning period. Young rats fed a low-protein diet showed a significant decrease in the level of dopamine and an increase in frontocortical [3H]serotonin binding in comparison to the group of rats fed a normal protein diet. These rats also showed a significant increase in foot shock-induced aggressive behaviour, while no changes in amphetamine-induced locomotor activity, levels of norepinephrine and serotonin and binding of [3H]spiperone to striatal membrane were observed. On exposure to styrene, rats fed a normal protein diet showed a decrease in dopamine level and an increase in foot shock-induced aggressive behaviour only, with no significant change in other parameters, in comparison to the respective controls. It was, however, interesting to note that when rats fed a low-protein diet were exposed to styrene they showed a significant decrease in the levels of norepinephrine, dopamine and serotonin and an increase in the binding of [3H]spiperone and [3H]5-HT to striatal and frontocortical membranes, respectively. A significant increase in foot shock-induced aggressive behaviour and amphetamine-induced locomotor activity was also observed in this group of animals in comparison to those fed a low-protein diet. The biochemical and behavioural data indicate that protein deficiency makes young animals more vulnerable and it is an important predisposing factor in the neurobehavioural toxicity of styrene.

Clonidine treatment during gestation prevents functional deficits induced by prenatal malnutrition in the rat visual cortex

It has been shown that prenatal malnutrition results at birth in increased concentration of noradrenaline (NA) in the brain. Besides, it is known that NA is an important regulator of normal regressive processes occurring during synaptogenesis such as cell death, axonal pruning and synaptic elimination. The present study was designed to investigate (i) whether prenatal malnutrition enhances the NA release in the visual cortex and (ii) whether or not chronic administration of clonidine during gestation may prevent long-term deleterious effects of fetal malnutrition on functional properties of interhemispheric connections of the visual cortex and on the interhemispheric asymmetry of visual evoked responses. Prenatal malnutrition was induced by restricting food consumption to pregnant rats from Day 8 postconception to parturition. Results show that at birth, prenatally malnourished rats had higher NA release than normals. At 45-50 days of age, the malnourished group exhibited (a) reduced peak-to-peak amplitude and diminished extent of the projecting field of transcallosal evoked responses, and (b) abolished interhemispheric asymmetry of visual evoked responses. Clonidine administration to malnourished mothers from Day 14 postconception to parturition (10 g/kg/day s.c.), prevented in the offspring disorders induced by prenatal malnutrition on cortical NA release, on interhemispheric connectivity of visual areas and on interhemispheric bioelectrical asymmetry, probably by restoring the normal trophic role of NA during synaptogenesis. Results are discussed in relationship to normal regressive events occurring during early brain development.

Structural, neurochemical and behavioural consequences of neonatal blockade of NMDA receptor through chronic treatment with CGP 39551 or MK-801

Recent evidence suggests that NMDA receptors may be involved in survival of neurons and establishment of correct connectivity during development. We have treated rat pups from postnatal day 1 to 22 with daily s.c. injections of a competitive (CGP 39551) and a non-competitive (MK-801) antagonist of the NMDA receptor. Body weight of treated rats was decreased by 50-65% at postnatal day 24 and by 25-32% at 70 days of age. Brain weight was decreased by 16-24% at both ages. Among the different brain regions, the cerebellum and striatum appeared more decreased in size than the cortex and hippocampus. Only few minor, and in some cases transient, differences were measured in the cerebellum, the hippocampus and the cortex for a battery of neurochemical markers related to cholinergic, GABAergic and glutamatergic transmission as well as to astrocyte and oligodendrocyte activity. When tested in actometric cages from postnatal days 28 to 60, treated rats exhibited a dramatic increase of spontaneous locomotor activity which was maximal in 28-day-old animals (380% and 250% of control values in CGP 39551 and MK-801 groups, respectively) and was still significant at 60 days of age. Therefore, long-lasting alteration of motor behaviour is obtained by the schedule of chronic treatment adopted for the present experiments. Our results suggest that blockade of NMDA receptors during the critical period of brain maturation may result in permanent alteration of neural circuits.

Gangliosides and sialoglycoproteins in hypothalamus of normal, postnatal, and pre- and postnatal protein undernourished rats

Total ganglioside and sialoglycoprotein concentrations were determined in the hypothalamus of normal (diet: 25% casein), postnatal undernourished (diet: 8% casein since birth), and pre- and postnatal undernourished rats (diet: 8% casein since pregnancy). Hypothalamic weights for the two low protein diet groups were lower than for the normal diet groups at all ages studied. Total hypothalamic ganglioside and sialoglycoproteins (mumol NANA) of postnatal undernourished rats were lower than control at day 10, while in pre- and postnatal undernourished rats this difference occurred at day 7. The reduction in gangliosides and sialoglycoprotein contents was not solely a consequence of the decrease in hypothalamic weight since, when the data were expressed as nmol NANA/mg tissue, similar reductions were observed principally in the pre- and postnatal protein undernutrition group. These results suggest that the effects of pre- and postnatal undernutrition on hypothalamic gangliosides and sialoglycoproteins are more pronounced than those that occur as a result of postnatal undernutrition.

Undernutrition during the preweaning period changes calcium ATPase and ADPase activities of synaptosomal fractions of weanling rats

The presence of activities that hydrolyse externally added ATP to adenosine in synaptosomal preparations from various sources is well demonstrated. The hydrolysis of ATP to AMP can be mediated either by the concerted action of enzymes or by an ATP-diphosphohydrolase (EC 3.6.1.5; apyrase). Undernutrition during the preweaning period can delay the development of several enzymes involved in the metabolism of neurotransmitters or neuronal function. In young rats, the presence of an apyrase in synaptosomal preparations from cerebral cortex was investigated. The results suggested that the hydrolysis of externally added ATP and ADP can be mediated by a single enzyme. The effects of preweaning undernutrition on the hydrolysis of ATP and ADP were also investigated. In weanling rats, previous undernutrition caused a decrease of about 20% in the hydrolysis of both substrates in synaptosomal fractions.

Role of VSV G antigen in the development of experimental spongiform encephalopathy in mice

The ts G31 VSV mutant induced spongiform encephalopathy without any inflammatory response when injected i.c. into the mouse. In electron microscopy, no virions could be detected in spongiform lesions. In contrast, with the wild VSV strain inflammatory lesions were seen, which contained viral particles in great abundance. As previously shown in vitro, when using the ts G 31 mutant at the nonpermissive temperature, the G antigen can spread from membrane to membrane to distant sites, fusing a great number of cells even in the absence of virus multiplication. Therefore, we postulate that a comparable mechanism is responsible for extensive brain lesions originating probably from a relatively small number of G antigen-producing cells. Indeed, the spongious regions seen mainly in the grey matter contained vacuoles, whose walls were clearly stained by peroxidase-labelled immune serum to G antigen, without detectable virions or inflammatory lesions. The vacuoles probably represent altered and swollen dendritic cell membranes. The relationship between spongiosis development and antigen diffusion in the absence of significant virus replication is discussed.

Incorporation of [3H]thymidine into DNA and of [35S]sulfate into sulfatides of oligodendroglial cells during development: effect of malnutrition

Incorporation of [3H]thymidine into DNA and of [35S]sulfate into sulfatides of oligodendroglial cells isolated from brain slices incubated with the radioactive precursor was studied in normal and malnourished rats at different ages. The pattern and the values of incorporation of [3H]thymidine into DNA were similar in both groups of animals. The maximum value of incorporation was observed at 7 days of age decreasing rapidly thereafter and leveling off between 18-21 days. In both groups of animals labeling of sulfatides attained a maximum at 18 days of age, showing similar values of incorporation up to that age. However, at 21 days of age; the values corresponding to malnourished rats were found to be 40% lower in comparison to controls. The results suggest that (a) proliferation of oligodendroglial cells stops at similar ages in normal and malnourished rats, (b) expression of sulfatide synthesis by oligodendroglial cells is similar in both groups of animals up to 18 days, and (c) the starved rats seem to be unable to maintain a normal synthesis of these galactolipids throughout the entire period of active myelinogenesis.

Myelin development and nutritional insufficiency

Postnatal undernourishment does not greatly retard the generation of rat brain cells, although there is a slight reduction in total cell numbers and brain size. Possibly the maturation of cells is more severely affected. The ratio of myelinated to non-myelinated fibers is greatly reduced in the corpus callosum and pyramidal tract, and presumably in other areas as well. There is only a slight reduction in the numbers of myelin lamellae for axons of a given size. The recovery of brain myelin and the incorporation of radioactive precursors into purified myelin proteins and lipids are all greatly reduced, leading to a comparatively severe reduction in the brain myelin concentration. The myelin composition is only slightly altered, possibly as a result of delay in its normal chemical maturation. The actual vulnerable period that produces a lasting myelin deficit is the early period that includes oligodendroglia cell proliferation, whereas undernutrition restricted to a later period that includes the actual peak of myelin does not cause a lasting reduction in the brain myelin concentration. The belief that stunting the postnatal proliferation of oligodendroglia largely accounts for the myelin effect has not been substantiated by direct analysis of cell numbers. Consequently, the observed hypomyelination likely results from a failure of oligodendroglia to mature and to initiate myelin formation. The myelin deficit appears largely uniform throughout the brain. Indirect evidence in human studies indicate that comparable effects appear in undernourished infants.

Overt and hidden forms of chronic malnutrition in the rat and their relevance to man

We have examined the physiological weight changes seen in rat dams and their offspring as sequelae of either an overt or a hidden form of chronic protein malnutrition. In the overt model, which was produced by feeding dams a very low protein diet (6% casein) starting 5 weeks prior to conception and continued through lactation, the females showed significant weight losses at all ages compared to dams maintained on a normal diet (25% casein). This caused the malnourished 6% dams to have offspring that were categorized as small-for-date at birth in terms of their weight indices and peripheral metabolic profiles. Also, the inadequate milk production of these dams resulted in their pups displaying the almost total failure of growth (greater than 60% decreases in body weights) and peripheral imbalances characteristic of infantile marasmus by day 8 of lactation. Consequently, at all times examined the 6% dams and pups showed most of the typical responses seen in the more severe forms of in utero and lactational malnutrition of mankind. In contrast, the hidden form of malnutrition produced by feeding dams a somewhat higher protein diet (8% casein) throughout the same time periods caused no marked weight losses by these females during their pregnancy compared to the normal dams. Although the 8% pups had the same birth weight indices as the normal offspring, previous data from our group have indicated that the 8% progeny show many metabolic imbalances at birth which are indicators of severe gestational malnutrition in humans. Moreover, while the 8% dams displayed lactational insufficiencies as noted by their pups retarded postnatal growth, nursing of these offspring by 25% dams allowed them to maintain a normal lactational growth curve. However, not only was this cross-fostering unable to rehabilitate most of the prenatally determined biochemical alterations affecting the 8% pups but, additionally, this form of malnutrition will remain undetected if weight indices alone are used as assessors of normalcy. Thus, it appears that the 8% rats may serve as a useful model for the hidden forms of malnutrition in man.

The relative numbers of oligodendroglia in different brain regions of normal and postnatally undernourished rats

The relative numbers of oligodendroglia were compared in representative brain regions of 21 day old undernourished and control rats. As a result of postnatal undernutrition which produced half normal body weights and a 10-15 percent reduction in brain weight, the relative numbers of oligodendroglia were slightly increased in photomicrographs of corticospinal tract (a motor tract), medial lemniscus (a sensory tract), red nucleus (a motor nucleus) and somatosensory cortex. Relative numbers were reduced in the corpus callosum, and the thickness of the corpus callosum was significantly reduced. Cell sizes of oligodendroglia were essentially normal throughout the brain, although some reductions of 5 to 6 percent were observed. Areas of brain structures in cross section were essentially unchanged. We have previously hypothesized that nutritionally induced brain hypomyelination results from a reduction in the specific numbers of oligodendroglia and consequently a lasting reduction in the brain myelin concentration. The present results are inconsistent with this hypothesis, as both the density of oligodendroglia and sizes of brain regions are essentially normal. We know from prior work using the same model of nutritional deprivation that myelin synthesis is greatly reduced. Consequently an important depressant effect of undernourishment on oligodendroglia in the developing brain involves either the communication between axons and oligodendroglia leading to myelin induction or the synthetic capacity to make myelin.

Developmental changes of synapses in the cerebellar cortex of the rat. A quantitative analysis

Synaptogenesis in the molecular layer of the vermis cortex in Wistar rats between 1 and 25 days after birth was investigated. After staining with OSO4, the following parameters were measured: the density of synaptic profiles; the percentage of the neuropil area occupied by synapses; the mean diameter of the boutons; and the numerical density of synapses in a defined volume. The detailed topographical analysis enabled us to show the following: the first synapses appear diffusely in the molecular layer; after the 10th day a synaptic gradient is present between the depth and the surface area (this gradient can be found by the count of the synapses and by studying the spatial distribution of the synaptic area); this gradient is no longer seen at the 25th day, when the density of synapses is relatively uniform throughout the whole molecular layer.

Effects of undernutrition on gliogenesis and glial maturation in rat corpus callosum

Rats were undernourished by halving the mother's food intake from the sixth day of pregnancy onwards and through lactation. At weaning, the young rats were restricted to half the normal weight of food. A combination of light and electron microscopic techniques was used to study the effects of this regime on gliogenesis and glial maturation in the corpus callosum of animals aged between 15 and 48 days. A disturbance of neuroglial proliferation was suggested by the finding of an increased proportion of astroglia relative to oligodendroglia in the 15-day-old undernourished rats, indicating a delay in the acquisition of cells produced relatively late in development. Impaired differentiation of oligodendroglia was suggested by the finding in treated animals of increased light oligodendrocytes at 15 days and a deficit of dark oligodendrocytes at 48 days, relative to controls. The previously observed retardation in myelin acquisition seems thus to be related to a delay in the differentiation of oligodendroglia, although it seems likely that the proliferation of these cells is also disturbed in undernutrition.

Long-term effects of postnatal undernutrition and maternal malnutrition on mouse cerebral cortex. I. Cellular densities, cortical volume and total numbers of cells

Quantitative analysis of cellular densities as well as an estimate of the cortical volume and of its total cell population were performed on the cortex of postnatally undernourished mice (2--21 days) and on that of pups from malnourished mothers (gestation and lactation). Animals were followed until 180 days of age after more than 5 months of nutritional rehabilitation, and data were obtained at 10, 30, 60, and 180 days of age. The neuronal density was much higher in all cortical layers of the two experimental series, suggesting a delay in cortical maturation. Moreover, layers II, III, and IV were more delayed than layers I, V, and VI. Postnatal undernutrition had more severe effects than maternal malnutrition and the degree of recovery after a long rehabilitation was much less. The increase of the cortical volume was greatly reduced in postnatal undernutrition and to a lesser degree in maternal malnutrition. The total number of glial cells was also reduced more in postnatal undernutrition than in maternal malnutrition, but the total number of neurons was never smaller than in the controls.