Hydroxyurea Treatment and Development of the Rat Cerebellum: Effects on the Neurogenetic Profiles and Settled Patterns of Purkinje Cells and Deep Cerebellar Nuclei Neurons

5-Bromo-2'-deoxyuridine labeling: historical perspectives, factors influencing the detection, toxicity, and its implications in the neurogenesis

The halopyrimidine 5-bromo-2'-deoxyuridine (BrdU) is an exogenous marker of DNA synthesis. Since the introduction of monoclonal antibodies against BrdU, an increasing number of methodologies have been used for the immunodetection of this synthesized bromine-tagged base analogue into replicating DNA. BrdU labeling is widely used for identifying neuron precursors and following their fate during the embryonic, perinatal, and adult neurogenesis in a variety of vertebrate species including birds, reptiles, and mammals. Due to BrdU toxicity, its incorporation into replicating DNA presents adverse consequences on the generation, survival, and settled patterns of cells. This may lead to false results and misinterpretation in the identification of proliferative neuroblasts. In this review, I will indicate the detrimental effects of this nucleoside during the development of the central nervous system, as well as the reliability of BrdU labeling to detect proliferating neuroblasts. Moreover, it will show factors influencing BrdU immunodetection and the contribution of this nucleoside to the study of prenatal, perinatal, and adult neurogenesis. Human adult neurogenesis will also be discussed. It is my hope that this review serves as a reference for those researchers who focused on detecting cells that are in the synthetic phase of the cell cycle.

Purine nucleoside phosphorylase deficiency induces p53-mediated intrinsic apoptosis in human induced pluripotent stem cell-derived neurons

Purine nucleoside phosphorylase (PNP) is an important enzyme in the purine degradation and salvage pathway. PNP deficiency results in marked T lineage lymphopenia and severe immunodeficiency. Additionally, PNP-deficient patients and mice suffer from diverse non-infectious neurological abnormalities of unknown etiology. To further investigate the cause for these neurologic abnormalities, induced pluripotent stem cells (iPSC) from two PNP-deficient patients were differentiated into neurons. The iPSC-derived PNP-deficient neurons had significantly reduced soma and nuclei volumes. The PNP-deficient neurons demonstrated increased spontaneous and staurosporine-induced apoptosis, measured by cleaved caspase-3 expression, together with decreased mitochondrial membrane potential and increased cleaved caspase-9 expression, indicative of enhanced intrinsic apoptosis. Greater expression of tumor protein p53 was also observed in these neurons, and inhibition of p53 using pifithrin-α prevented the apoptosis. Importantly, treatment of the iPSC-derived PNP-deficient neurons with exogenous PNP enzyme alleviated the apoptosis. Inhibition of ribonucleotide reductase (RNR) in iPSC derived from PNP-proficient neurons with hydroxyurea or with nicotinamide and trichostatin A increased the intrinsic neuronal apoptosis, implicating RNR dysfunction as the potential mechanism for the damage caused by PNP deficiency. The findings presented here establish a potential mechanism for the neurological defects observed in PNP-deficient patients and reinforce the critical role that PNP has for neuronal viability.

Developmental timetables and gradients of neurogenesis in cerebellar Purkinje cells and deep glutamatergic neurons: A comparative study between the mouse and the rat

The aim of this report is to determine whether the times of neuron origin and neurogenetic gradients of PCs and Deep cerebellar nucli (DCN) glutamatergic neurons are different between mice and rats. Purkinje cells (PCs) were analyzed in each compartment of the cerebellar cortex (vermis, paravermis, medial, and lateral hemispheres), and deep glutamatergic neurons at the level of the medialis, interpositus, and lateralis nuclei. Tritiated thymidine ([³ H]TdR) autoradiography was applied on sections. The experimental rodents were the offspring of pregnant dams injected with [³ H]TdR on embryonic days (E) 11-12, E12-13, E13-14, E14-15, E15-16, and E16-17. Our results indicate that systematic differences exist in the pattern of neurogenesis and the spatial location of cerebellar PCs and deep glutamatergic neurons between mice and rats. In mice, PCs and deep glutamatergic neurons neurogenesis extend from E10 to E14, with a predominance of neurogenesis on E12 for PCs, and on E12, E11, and E10 for the medialis, interpositus, and lateralis neurons, respectively. When neurogenesis in rats was considered, the data reveal that PCs and deep glutamatergic neurons production extends from E12 to E16, with a peak of production on E14 for PCs, and on E14, E13, and E12 for the medialis, interpositus, and lateralis neurons, respectively. Current data also indicate that, both in mice and rats, both types of macroneurons are generated according to a lateral-to-medial gradient. Thus, the lateral hemisphere and the lateralis nucleus present more early-generated neurons than the vermis and the medialis nucleus, which in their turn have more late-produced neurons.

Administration of 5-bromo-2'-deoxyuridine interferes with neuroblast proliferation and promotes apoptotic cell death in the rat cerebellar neuroepithelium

The current study was conducted to assess whether a single administration of 5-bromo-2'-deoxyuridine (BrdU) interferes with cell proliferation and leads to the activation of apoptotic cellular events in the prenatal cerebellum. BrdU effects across a wide range of doses (25-300 μg/g b.w.) were analyzed using immunohistochemical and ultrastructural procedures. The pregnant rats were injected with BrdU at embryonic day 13, and their fetuses were sacrificed from 5 to 35 hr after exposure. The quantification of several parameters such as the density of mitotic figures, and BrdU and proliferating cell nuclear antigen (PCNA)-reactive cells showed that, in comparison with the saline injected rats, the administration of BrdU impairs the proliferative behavior of neuroepithelial cells. The above-mentioned parameters were significantly reduced in rats injected with 100 μg/g b.w. of BrdU. The reduction was more evident using 200 μg/g b.w. The most severe effects were found with 300 μg/g b.w. of BrdU. The present findings also revealed that high doses of BrdU lead to the activation of apoptotic cellular events as evidenced by both terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay and immunohistochemistry for active caspase-3. In comparison with saline rats, many apoptotic cells were found in rats injected with 100 μg/g b.w. of BrdU. The number of dying cells increased with 200 μg/g b.w. The most important number of apoptotic cells were observed in animals injected with 300 μg/g b.w. of BrdU. Ultrastructural studies confirmed the presence of neuroblasts at different stages of apoptosis.

An immunocytochemical approach to the analysis of the cell division cycle in the rat cerebellar neuroepithelium

Cerebellar neurons are generated from the rhombic lip and the neuroepithelium. In this study, we analyze the histogenesis of the cerebellar neuroepithelium in terms of cellular kinetics. The experimental animals are the offspring of pregnant dams injected with 5-bromo-2'-deoxyuridine (BrdU) on embryonic day 13. We infer the fraction of S-phase cells by examining a range of survival times after a single BrdU-exposure and a cumulative BrdU-labeling sequence, which allow for the derivation of cell-cycle parameters and phase durations. The current results indicate that the dose of BrdU employed (35 mg/kg) provides saturation S-phase labeling from at least 1 h after marker delivery. The duration of G2, mitotic phase, and G1 are 1.2, 0.5, and 6.9 h, respectively. The duration for the S-phase, growth fraction, and the whole cycle are obtained on the basis of two proliferative models, steady-state and exponential growth. Both models provided similar results. In conclusion, our results indicate that the steady-state and the cumulative S-phase labeling paradigms can be adopted to analyze cell cycle parameters in the cerebellar neuroepithelium. Current results can help in understanding the regulatory mechanisms of cerebellar histogenesis and the cell biological mechanisms of the proliferative cycle of the neuroepithelium.

An Animal Model for Assessing the Effects of Hydroxyurea Exposure Suggests That the Administration of This Agent to Pregnant Women and Young Infants May Not Be as Safe as We Thought

The cytostatic agent hydroxyurea (HU) has proven to be beneficial for a variety of conditions in the disciplines of oncology, hematology, infectious disease and dermatology. It disrupts the S phase of the cell cycle by inhibiting the ribonucleotide reductase enzyme, thus blocking the transformation of ribonucleotides into deoxyribonucleotides, a rate limiting step in DNA synthesis. HU is listed as an essential medicine by the World Health Organization. Several studies have indicated that HU is well tolerated and safe in pregnant women and very young pediatric patients. To our knowledge, only a few controlled studies on the adverse effects of HU therapy have been done in humans. Despite this, the prevalence of central nervous system abnormalities, including ischemic lesions and stenosis have been reported. This review will summarize and present the effects of HU exposure on the prenatal and perinatal development of the rat cerebellar cortex and deep cerebellar nuclei neurons. Our results call for the necessity to better understand HU effects and define the administration of this drug to gestating women and young pediatric patients.

Hydroxyurea Exposure and Development of the Cerebellar External Granular Layer: Effects on Granule Cell Precursors, Bergmann Glial and Microglial Cells

The current paper presents a histological analysis of the cell death in the cerebellar external granular layer (EGL) following the treatment with a single dose (2 mg/g) of hydroxyurea (HU). The rats were examined at postnatal days (P) 5, 10, and 15, and sacrificed at appropriate times ranging from 6 to 48 h after treatment administration. Studies were done in each cortical lobe (anterior, central, posterior, and inferior). The quantification of several parameters, such as density of 5-bromo-2'-deoxyuridine, TUNEL, vimentin, and tomato lectin-stained cells, revealed that HU compromises the viability of EGL cells. Our results indicate that P10 is a time of high vulnerability to injury. We also show here that the anterior and central lobes are the cortical regions most susceptible to the action of the HU. Additionally, our data also indicate that from 6 to 24 h after HU-exposure is a time-window of high sensibility to this agent. On the other hand, our ultrastructural analysis confirmed that HU administration produces the activation of apoptotic cellular events in the EGL, resulting in a substantial number of dying cells. Different stages of apoptosis can be observed in all cortical lobes at all investigated postnatal ages and survival times. Moreover, we observed that dying neuroblasts were covered by laminar processes of Bergmann glia, and that these unipolar astrocytes presented cytological features of phagocytes engulfing apoptotic bodies and cell debris. The electron microscopy study also revealed the participation of ameboid microglial cells in the phagocytosis of apoptotic cells in the regions of the EGL with extensive cell death.

Effects of Hydroxyurea Exposure on the Rat Cerebellar Neuroepithelium: an Immunohistochemical and Electron Microscopic Study Along the Anteroposterior and Mediolateral Axes

We present a histological study of the cell death of cerebellar neuroepithelial neuroblasts following treatment with the cytotoxic agent hydroxyurea (HU) during the embryonic life. Pregnant rats were treated with a single dose of HU (300 mg/kg) at embryonic days 13, 14, or 15 of gestation, and their fetuses were studied from 5 to 35 h after treatment to elucidate the mechanisms of HU-induced fetotoxicity. Quantification of several parameters such as the density of pyknotic, mitotic, and PCNA-immunoreactive cells indicated that HU compromises the survival of the cerebellar neuroepithelium neuroblasts. On the other hand, our light and electron microscopic investigations during the course of prenatal development indicated that HU leads to two types of cell death: apoptosis and cells presenting cytoplasmic vacuolization, altered organelles, and a recognizable cell nucleus. Both modalities of cell death resulted in a substantial loss of cerebellar neuroepithelium cells. Current results suggest that HU exposure during gestation is toxic to the cerebellar neuroepithelium. Moreover, they allow to examine the mechanisms of HU-induced toxicity during the early development of the central nervous system. Our data also suggest that it is essential to avoid underestimating the adverse effects of HU when administered during early prenatal life.

Cell cycle analysis in the rat external granular layer evaluated by several bromodeoxyuridine immunoperoxidase staining protocols

An important step in bromodeoxyuridine (BrdU) immunohistochemistry is the production of single-stranded DNA to make the incorporated BrdU accessible to the antibodies. This paper examines the effect of distinct DNA denaturation pretreatments (DNase I, sodium citrate buffer, endonuclease Eco RI and exonuclease III, and HCl hydrolysis) on detection of BrdU. We found that all the methods used in the partial denaturation of DNA combined good nuclear immunostaining with acceptable tissue integrity. We also observed that these immunohistochemical protocols revealed a spatial pattern in the distribution of DNA-synthesizing cells within the cerebellar external granular layer (EGL) of 10-day-old rats, allowing us to estimate the fraction of S-phase cells. Our results indicate that detection of BrdU-stained cells is affected by the distinct histological procedures used in such detection. Additionally, as the duration and phases of the cell cycle in EGL neuroblasts are estimated in accordance with BrdU detection, an effect on this detection can render the measurement of cell cycle inaccurate. The present work shows that DNase I and citrate buffer, at appropriate conditions, may be good alternatives for acid denaturation. However, they are less sensitive than autoradiographic techniques that use ³H-thymidine administration. Finally, current data reveal that short survival times after a single BrdU exposure do not seem to affect the cell cycle progression of the EGL neuroblasts.

Developmental Injury to the Cerebellar Cortex Following Hydroxyurea Treatment in Early Postnatal Life: An Immunohistochemical and Electron Microscopic Study

Postnatal development of the cerebellar cortex was studied in rats administered with a single dose (2 mg/g) of the cytotoxic agent hydroxyurea (HU) on postnatal day (P) 9 and collected at appropriate times ranging from 6 h to 45 days. Quantification of several parameters such as the density of pyknotic, mitotic, BrdU-positive, and vimentin-stained cells revealed that HU compromises the survival of the external granular layer (EGL) cells. Moreover, vimentin immunocytochemistry revealed overexpression and thicker immunoreactive glial processes in HU-treated rats. On the other hand, we also show that HU leads to the activation of apoptotic cellular events, resulting in a substantial number of dying EGL cells, as revealed by TUNEL staining and at the electron microscope level. Additionally, we quantified several features of the cerebellar cortex of rats exposed to HU in early postnatal life and collected in adulthood. Data analysis indicated that the analyzed parameters were less pronounced in rats administered with this agent. Moreover, we observed several alterations in the cerebellar cortex cytoarchitecture of rats injected with HU. Anomalies included ectopic placement of Purkinje cells and abnormities in the dendritic arbor of these macroneurons. Ectopic granule cells were also found in the molecular layer. These findings provide a clue for investigating the mechanisms of HU-induced toxicity during the development of the central nervous system. Our results also suggest that it is essential to avoid underestimating the adverse effects of this hydroxylated analog of urea when administered during early postnatal life.