Research reportEffects of prenatal exposure to ethanol on the cyclin-dependent kinase system in the developing rat cerebellum
Introduction
Prenatal ethanol exposure is deleterious to the developing central nervous system (CNS) [19], [35], [49]. Gestational exposure to ethanol can alter the number of neurons and glia in the mature brain. Such damage varies among CNS structures. Cerebellum is one of the CNS regions that is especially vulnerable to ethanol [21], [40], [41], [42]. A feature of early ethanol exposure is the depletion of cerebellar neurons. Animal studies have consistently shown that ethanol exposure reduces the numbers of both Purkinje cells and granule cells in cerebellum [7], [8], [40], [42], [63]. The number of neurons within cerebellum depends upon the sum of two opposing processes: the additive effect of cell acquisition (neurogenesis) and the subtractive effect of neuronal death. Ethanol affects both processes [6], [8], [10], [37], [55], [75], [78]. The cellular and molecular mechanisms underlying perturbation of these processes are yet to be determined.
The cerebellum is emerging as being more crucial for cognition than previously thought. Damage to the cerebellum results in motor function impairments [30], [33], [38], [60] and cognitive disorders [45], [69] seen in fetal alcohol syndrome. Elucidating cellular and molecular mechanisms of ethanol action would greatly enhance our understanding of how ethanol produces its various detrimental effects, and may provide insight necessary to develop therapies to improve the outcome of maternal ethanol abuse.
Cyclin-dependent kinases (CDKs) are a family of protein kinases that play multiple roles in the regulation of cell proliferation, differentiation and survival [13], [27], [28], [34], [66]. The activity of CDKs is positively regulated by a group of proteins (CDK activators) called cyclins, and negatively regulated by CDK inhibitors (CKDIs) [2]. CDKs have been shown to play a critical role in neurogenesis as well as neuronal differentiation [11], [20], [22], [34], [66], [77]. Ethanol inhibits cell proliferation and induces apoptosis in cultured cerebellar granule progenitors and this damage is accompanied by a severe disruption of CDK system [32]. The present study is designed to determine whether prenatal ethanol exposure affects the developmental expression of CDKs and their activators/inhibitors in the postnatal cerebellum.
Section snippets
Animals and ethanol exposure
Pregnant Long–Evans rats were obtained from Harlan–Sprague–Dawley (Indianapolis, IN, USA) on gestational day (G) 4. The animals were maintained in a temperature/humidity controlled facility with a 12-h light/dark cycle. Beginning on G6, rats were arbitrarily assigned to one of two treatment groups, and fed one of two liquid diets, an ethanol diet (Et), or a control diet (Ct). Both liquid diets were protein-enriched diets that were nutritionally balanced and met the requirements of pregnant rats
Expression of cyclin-dependent kinases
Developmental expression of three cyclin-dependent kinases, CDK2, CDK4 and CDK6 was examined in the cerebellum. In control pups, the expression of CDK2 increased between P0 and P6, and it decreased thereafter (Fig. 1). By P21, the expression of CDK2 was barely detectable. In contrast, the amounts of CDK4 declined from P0 to P3, and then expression stabilized during the rest of developmental period (Fig. 2). CDK6 expression remained quite constant, that is, there was little change of expression
Cyclin-dependent kinase system and the developing cerebellum
The development of rodent cerebellum occurs postnatally [3]. The production of major cerebellar neurons in rats, granule cells, begins at birth and ends after approximately 3 weeks. The cerebellar granule cells, the most abundant cells in the CNS (estimated at 1011), are generated in the proliferative external germinal layer (EGL) and migrate to their final destination in the internal granule layers (IGL) where they differentiate into neurons. From postnatal day 0 (P0) to P6, the cells in the
Acknowledgements
This research was supported by grants from the National Institutes of Health (AA07658, AA12968 and CA 90385).
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