The early post-natal development of the neuronal lysosome

Changes in some enzymic activities of separated neuronal and glial cell-enriched fractions from rat brains during development

Bulk-prepared neuronal perikarya and glial fractions have been used to study developmental changes of some enzymic activities concerning glycolysis and synthesis of neurotransmitters. Somewhat higher pyruvate kinase activity was found in neuronal perikarya than in glial cells, and its rapid rise was observed during early developmental stages. Increased K(+) concentration orD-glutamate addition to the incubation medium enhanced consumption of phosphoenolpyruvate. This activation of the enzyme was small just after birth, but it increased in parallel with development to adult level, where the activation in glial fractions was over twice that in neuronal fractions. Choline acetyltransferase activity was found in purified neuronal fractions and increased with age; glutamate decarboxylase was also found in high activity in purified neuronal fractions and increased with development. However, some enzymic activities were also found in glial fractions, and possibilities of contamination by synaptosomal, myelin, or other subfractions are discussed.

Mechanisms of cellular distribution of psychotropic drugs. Significance for drug action and interactions

Distribution of a drug in the body is dependent on its permeation properties, the blood flow rates in various tissues, and on plasma and tissue uptake. The distribution of drugs in vivo is largely determined by uptake competitions between blood and tissues, as well as competitions among individual tissues. Basic lipophilic drugs are characterized by extensive accumulation in tissues, which leads to a high volume of distribution. Nonspecific binding to cellular membranes and uptake by acidic compartments (mainly lysosomes) are responsible for such a distribution pattern. Lysosomal trapping is an important mechanism of distribution of basic psychotropic drugs; however, the tissue distribution of the aliphatic-type phenothiazine neuroleptic promazine, tricyclic antidepressants (TADs) and selective serotonin reuptake inhibitors (SSRIs) depends more on phospholipid binding than on lysosomal trapping, whereas in the case thioridazine and perazine, lysosomal trapping is as important for the tissue uptake as is phospholipid binding. Neuroleptics and antidepressants mutually inhibit their lysosomal uptake. A decrease in the intralysosomal drug concentrations in vivo leads to a shift of the drug from organs abundant in lysosomes (lungs, liver and kidneys) to those poor in these organella, e.g., the heart, which may be of clinical importance (cardiotoxicity). The brain is not a homogenous organ, i.e., the phospholipid pattern and density of lysosomes vary in its different regions. Therefore, the contribution of the two mechanisms (lysosomal trapping and tissue binding) to total drug uptake is different in areas of the brain. Both lysosomal trapping and binding to cellular elements for psychotropics are higher in the grey matter and neurons than in the white matter and astrocytes, respectively. Lysosomal trapping and distribution interactions of psychotropics take place mainly in neurons. A decrease (via a distributive interaction) in the concentration of psychotropics in lysosomes (depot) may lead to an increase in their level in membranes and tissue fluids (i.e., in concentrations and compartments relevant to their pharmacological action) and, in consequence, to enhancement of the drug binding to neurotransmitter receptors and/or transporters.

Intracellular distribution of psychotropic drugs in the grey and white matter of the brain: the role of lysosomal trapping

1. Since the brain is not a homogenous organ (i.e. the phospholipid pattern and density of lysosomes may vary in its different regions), in the present study we examined the uptake of psychotropic drugs by vertically cut slices of whole brain, grey (cerebral cortex) and white (corpus callosum, internal capsule) matter of the brain and by neuronal and astroglial cell cultures. 2. Moreover, we assessed the contribution of lysosomal trapping to total drug uptake (total uptake=lysosomal trapping+phospholipid binding) by tissue slices or cells conducting experiments in the presence and absence of 'lysosomal inhibitors', i.e., the lysosomotropic compound ammonium chloride (20 mM) or the Na(+)/H(+)-ionophore monensin (10 microM), which elevated the internal pH of lysosomes. The initial concentration of psychotropic drug in the incubation medium was 5 microM. 3. Both total uptake and lysosomal trapping of the antidepressants investigated (imipramine, amitriptyline, fluoxetine, sertraline) and neuroleptics (promazine, perazine, thioridazine) were higher in the grey matter and neurones than in the white matter and astrocytes, respectively. Lysosomal trapping of the psychotropics occurred mainly in neurones where thioridazine sertraline and perazine showed the highest degree of lysosomotropism. 4. Distribution interactions between antidepressants and neuroleptics took place in neurones via mutual inhibition of lysosomal trapping of drugs. 5. A differential number of neuronal and glial cells in the brain may mask the lysosomal trapping and the distribution interactions of less potent lysosomotropic drugs in vertically cut brain slices. 6. A reduction (via a distribution interaction) in the concentration of psychotropics in lysosomes (depot), which leads to an increase in their level in membranes and tissue fluids, may intensify the pharmacological action of the combined drugs.

beta-Glucosidase and beta-galactosidase in primary cultures of rat astrocytes: comparison to the brain enzymes

In primary astrocyte cultures beta-glucosidase (EC 3.2.1.21) and beta-galactosidase (EC 3.2.1.23) showed pH optima and Km values identical to rat brain enzymes, using methylumbelliferyl glycosides and labeled gluco- and galactocerebrosides as substrates. The activities of both glycosidases increased in culture up to 3-4 weeks. In rat brain only galactosidase increased; glucosidase activity declined between 12-20 days after birth. The specific activities were two- to sixfold higher in astrocyte cultures than in rat brain. These activities were not due to uptake of enzymes from the growth medium. Secretion of beta-galactosidase, but not beta-glucosidase nor acid phosphatase could be demonstrated. These results support the suggestion of a degradative function for astrocytes in the brain.

Distribution and protein synthetic activities of neuronal free and membrane-bound ribosomes during postnatal development of rat cerebral cortex

The distribution and amino acid incorporative activities of free and membrane-bound ribosomes in neuronal perikarya of rat cerebral cortex at successive stages of postnatal development were investigated. The relative proportion of neuronal membrane-bound ribosomes increased significantly between 6 and 18 days of age, reaching 50% of the total ribosomal population around day 18. In contrast to the neuronal fraction, the membrane-bound ribosomes in unfractionated cerebral cortex decreased from 50% at birth to 35% in 18-day-old pups. When tested in a cell-free amino acid incorporation system the activities of both free and membrane-bound ribosomes increased up to day 10 post partum. However, whereas the activity of free ribosomes reached a constant level at this age, that of the membrane-bound fraction continued to rise until 18th day after birth. Crossover experiments employing homologous or heterologous cell sap fractions showed that the preparation from the 18-day-old animals was more efficient in supporting protein synthesis in vitro, if compared with similar preparation from the neonatal animals. This was attributed to an enhanced aminoacylation of transfer RNA in neuronal cell sap of older compared with younger rats. The observed developmental alterations in the distribution and activities of free and membrane-bound ribosomes are discussed in relation to changes that occur in morphology and function of brain during the early postnatal period.

Development-dependent regulation of N-acetyl-beta-D-hexosaminidase of cerebellum and cerebrum of normal and staggerer mutant mice

Distinctive activities of various glycosidases were expressed in the cerebellum and cerebral cortex of mice during their development. In particular, N-acetyl-beta-D-hexosaminidase (EC 3.2.1.30) appeared to be developmentally regulated. A transient peak of enzyme activity at postnatal day 7 was characteristic for the cerebellum, whereas the activity in the cerebral cortex gradually increased through the 1st postnatal month and was maintained at a high level of activity throughout adulthood. The regulation of N-acetylhexosaminidase activity in the developing cerebellum of the staggerer mouse deviated clearly from enzyme activities in the wild-type, whereas the activity pattern in the staggerer cerebral cortex remained unaffected. In experiments mixing wild-type and staggerer cerebellum homogenates, the specific activity was additive. Thus, involvement of inhibitors or activating molecules can be excluded. This developmentally controlled regulation or disregulation in staggerer appears to be enzyme specific, sine beta-glucosidase, alpha-glucosidase, and beta-galactosidase did not exhibit such a pattern in either normal or staggerer mice. In the mutation weaver that, like staggerer, loses the majority of its cerebellar granule cells, N-acetyl-beta-hexosaminidase activity of the cerebellum was not elevated, indicating a specific defect in staggerer rather than a general effect on lysosomal enzymes due to cell death.

Postnatal changes in the activity ratio of specific and nonspecific cholinesterases from neuronal perikarya

A soluble fraction from rat brain neuronal perikarya was shown to contain both the specific and nonspecific forms of the enzyme acetylcholinesterase (EC's 3.1.1.7. and 3.1.1.8., respectively). The ratio of the enzyme activities varied along the course of brain development: the nonspecific form being predominant from 1 to 15 days of age and the specific one showing the pattern of rising activity from day 15 onward. We suggest a possible relationship between this changing in cholinesterase activities and the establishment of synapses within the rat cerebral cortex.

Specific binding of kainic acid to purified subcellular fractions from rat brain

The subcellular distribution of kainic acid (KA) binding sites in rat brain has been studied using a microcentrifugation assay. KA did not bind to myelin or brain cytosol and had few or no binding sites in the nuclear fraction. However, it bound to microsomal components (Kd = 128-136 nM; 2.5-4.8 pmol/mg protein), purified synaptic plasma membranes (SPM)(Kd = 45-71 nM; 5.8-6.5 pmol/mg), and purified cell-body and intraterminal mitochondria (Kd = 11-31 nM; 0.4-1.1 pmol/ng). Bound KA could be totally displaced by L-glutamate or L-asparate, but several putative antagonists of these amino acids (nuciferin, compound HA-966, 2-amino-4-phosphonobutyrate, and 2-amino-3-phosphonoproprionate) failed to displace KA or did so at very high concentrations (greater than or equal to 4 mM). Glutamic acid diethyl ester (GDEE) and D,L-alpha-aminoadipate (alpha-AA) were more effective (IC50, 0.2-0.8 mM) and showed differential effects in their capacity to displace KA bound to the various subcellular fractions. Thus, GDEE only displaced 40-60% of the KA bound by SPM or mitochondria and did not prevent the binding of KA to microsomes. alpha-AA, on the other hand, was more effective in preventing the binding of KA at high concentrations and displaced between 80 and 100% of the drug. Both compounds showed biphasic curves of KA displacement from synaptic plasma membranes and mitochondria. The overall results indicate the presence of multiple binding sites for KA in brain cells and suggest that KA does not act exclusively as synaptic glutamate receptors. The mechanism of KA action is most likely quite complex, and the drug probably acts at multiple binding sites affecting a number of processes.

Lysosomal properties during thyroxine-induced lateral motor column neurogenesis

This investigation illustrates that thyroid hormones and not prolactin regulate lysosomal activity during hormone-stimulated lateral motor column (LMC) development in Rana pipiens larvae. Administering a single dose of DL-thyroxine to normal or hypophysectomized larvae induces a 4-8 fold increase in lysosomal acid hydrolase activity within 5 or 6 days. During this interval, the physical properties of lysosomes are dramatically altered with the granules becoming extremely labile to treatments known to disrupt membranes. Nevertheless, thyroxine does not directly influence lysosomal stability; it apparently induces the synthesis and packaging of enzymes through a mechanism requiring de movo production of RNA and protein. The subcellular distribution and physical characteristics of the enzymes are then modified during neuronal death. Such information suggests that changes in lysosomal activity which accompany LMC neurogenesis are indicative of neuronal death, and that these events are controlled by thyroid hormones.

Neuronal N-acetyl-beta-D-glucosaminidase. Evidence for its biosynthesis in vitro

Neuronal cell bodies, isolated in bulk from 8-day-old rat cerebral cortices, were incubated in the presence of a 3H-labelled amino acid mixture, and subcellular fractions isolated by differential centrifugation. The particulate fractions were frozen/thawed in 0.20 M-sucrose/0.1 M-KCl [Selling et al. (1973) Biochim. Biophys. Acta 315, 128-146] and the profiles of acid-insoluble radioactivity and N-acetyl-beta-D-glucosaminidase (glucosaminidase) activity compared in the resulting non-sedimentable fractions by DEAE-cellulose chromatography and cellulose acetate electrophoresis. Radioactivity and glucosaminidase activity co-migrated to a significant extent. Electrophoresis revealed that after 1 min of incubation 42% of the radioactivity of the non-sedimentable microsomal fraction after freezing and thawing co-migrated with an intensely fluorescent band of glucosaminidase activity. Since the pellet fraction obtained on freezing/thawing the microsomal fraction contained up to 75% of the RNA, 95% of the radioactivity and 45% of the glucosaminidase, a detailed study of the association between its radioactivity and nascent glucosaminidase activity was undertaken. After 1 and 2 min of incubation, followed by centrifugation of the microsomal pellet on 35-60% (w/v) sucrose density gradients, radioactivity and glucosaminidase activity exhibited parallel profiles in the region of heavy polyribosomes and at the top of the gradient which contains spontaneously released nascent polypeptide chains. DEAE-cellulose chromatography of these chains revealed glucosaminidase A to be the principal nascent glucosaminidase component, with glucosaminidases B and C as minor peaks. After 2 min of incubation, all of the glucosaminidase components appeared labelled, and glucosaminidase A exhibited two distinct sub-components. The pattern of glucosaminidase labelling in the soluble and microsomal fractions suggested that newly formed glucosaminidase molecules traverse both the cellular sap and the lumen of the endoplasmic reticulum. Only glucosaminidase A reacted specifically with concanavalin A and radioactive glucosaminidase A could be successfully regenerated by treatment with alpha-methyl-D-glucoside. Glucosaminidase A and a substantial portion of the radioactivity associating with it could be readily converted into glucosaminidase B by re-chromatography on DEAE-cellulose and by reaction of the concanavalin A-glucosaminidase A complex with methyl glucosides.

Aspects of biochemical differentiation in the central nervous system

A demonstration of cell-specific patterns of development in the immature CNS is provided by examples of characteristic, cell-specific time-courses of enzyme development in different classes of brain cells isolated in highly purified form by bulk-separation from the cerebral and cerebellar cortex of the growing rat. The enzymatic analysis was carried out at the level of the nerve and glial cell lysosomes and mitochondria, two subcellular organelles crucial to the economy of all cells. The findings reveal rather similar developmental patterns for the lysosomal hydrolase N-acetyl-beta-D-glucosaminidase in neurons and glial cells of the cerebral cortex as well as in two different cerebellar nerve cell types, the Purkinje and the granule cell. However, significant differences in the post-natal chronology of development of the mitochondrial enzyme alpha-glycerophosphate dehydrogenase were noted between cortical nerve and glial cells, the glial enzyme exhibiting 6-fold higher levels of activity than the neuronal one throughout the first month of postnatal life. The findings emphasize the feasibility as well as the necessity of studies aimed at the elucidation of the cell-specific aspects of the biochemistry of developing nerve and glial cells.