A toxic mutant huntingtin species is resistant to selective autophagy

Protein misfolding is a common theme in neurodegenerative disorders including Huntington's disease (HD). The HD-causing mutant huntingtin protein (mHTT) has an expanded polyglutamine (polyQ) stretch that may adopt multiple conformations, and the most toxic of these is the one recognized by antibody 3B5H10. Here we show that the 3B5H10-recognized mHTT species has a slower degradation rate due to its resistance to selective autophagy in human cells and brains, revealing mechanisms of its higher toxicity.

A striatal-enriched intronic GPCR modulates huntingtin levels and toxicity

Huntington's disease (HD) represents an important model for neurodegenerative disorders and proteinopathies. It is mainly caused by cytotoxicity of the mutant huntingtin protein (Htt) with an expanded polyQ stretch. While Htt is ubiquitously expressed, HD is characterized by selective neurodegeneration of the striatum. Here we report a striatal-enriched orphan G protein-coupled receptor(GPCR) Gpr52 as a stabilizer of Htt in vitro and in vivo. Gpr52 modulates Htt via cAMP-dependent but PKA independent mechanisms. Gpr52 is located within an intron of Rabgap1l, which exhibits epistatic effects on Gpr52-mediated modulation of Htt levels by inhibiting its substrate Rab39B, which co-localizes with Htt and translocates Htt to the endoplasmic reticulum. Finally, reducing Gpr52 suppresses HD phenotypes in both patient iPS-derived neurons and in vivo Drosophila HD models. Thus, our discovery reveals modulation of Htt levels by a striatal-enriched GPCR via its GPCR function, providing insights into the selective neurodegeneration and potential treatment strategies.

Preconditioning stimuli induce autophagy via sphingosine kinase 2 in mouse cortical neurons

Sphingosine kinase 2 (SPK2) and autophagy are both involved in brain preconditioning, but whether preconditioning-induced SPK2 up-regulation and autophagy activation are linked mechanistically remains to be elucidated. In this study, we used in vitro and in vivo models to explore the role of SPK2-mediated autophagy in isoflurane and hypoxic preconditioning. In primary mouse cortical neurons, both isoflurane and hypoxic preconditioning induced autophagy. Isoflurane and hypoxic preconditioning protected against subsequent oxygen glucose deprivation or glutamate injury, whereas pretreatment with autophagy inhibitors (3-methyladenine or KU55933) abolished preconditioning-induced tolerance. Pretreatment with SPK2 inhibitors (ABC294640 and SKI-II) or SPK2 knockdown prevented preconditioning-induced autophagy. Isoflurane also induced autophagy in mouse in vivo as shown by Western blots for LC3 and p62, LC3 immunostaining, and electron microscopy. Isoflurane-induced autophagy in mice lacking the SPK1 isoform (SPK1(-/-)), but not in SPK2(-/-)mice. Sphingosine 1-phosphate and the sphingosine 1-phosphate receptor agonist FTY720 did not protect against oxygen glucose deprivation in cultured neurons and did not alter the expression of LC3 and p62, suggesting that SPK2-mediated autophagy and protections are not S1P-dependent. Beclin 1 knockdown abolished preconditioning-induced autophagy, and SPK2 inhibitors abolished isoflurane-induced disruption of the Beclin 1/Bcl-2 association. These results strongly indicate that autophagy is involved in isoflurane preconditioning both in vivo and in vitro and that SPK2 contributes to preconditioning-induced autophagy, possibly by disrupting the Beclin 1/Bcl-2 interaction.

Behavioral deficits, early gliosis, dysmyelination and synaptic dysfunction in a mouse model of mucolipidosis IV

Mucolipidosis IV (MLIV) is caused by mutations in the gene MCOLN1. Patients with MLIV have severe neurologic deficits and very little is known about the brain pathology in this lysosomal disease. Using an accurate mouse model of mucolipidosis IV, we observed early behavioral deficits which were accompanied by activation of microglia and astrocytes. The glial activation that persisted during the course of disease was not accompanied by neuronal loss even at the late stage. In vivo [Ca²⁺]-imaging revealed no changes in resting [Ca²⁺] levels in Mcoln1^−/− cortical neurons, implying their physiological health. Despite the absence of neuron loss, we observed alterations in synaptic plasticity, as indicated by elevated paired-pulse facilitation and enhanced long-term potentiation. Myelination deficits and severely dysmorphic corpus callosum were present early and resembled white matter pathology in mucolipidosis IV patients. These results indicate the early involvement of glia, and challenge the traditional view of mucolipidosis IV as an overtly neurodegenerative condition.

Drugging unconventional targets: insights from Huntington's disease

Classical targeted drug discovery is based on targeting druggable targets, typically kinases and receptors of which the function can be agonized or antagonized. This strategy meets difficulties in cases such as Huntington's disease (HD) and similar neurodegenerative disorders, where the pathological function of the protein causing the disease is not clear. HD is caused by mutant HTT protein (mHTT) containing an expanded polyglutamine (polyQ) stretch, but the function of mHTT and how mHTT causes HD are unknown, thus preventing efforts to screen for mHTT 'inhibitors'. However, HD is appealing for drug discovery because the genetic mutation is clear, as compared with other major neurodegenerative disorders. Although mHTT is not a conventional 'druggable' target, one approach that appears promising is lowering its level, which might be applicable to other neurodegenerative disorders and proteinopathies linked to aberrant accumulation of proteins. Here we review mHTT lowering strategies that might provide promising avenues for drugging such diseases.

SUMO-2 and PIAS1 modulate insoluble mutant huntingtin protein accumulation

A key feature in Huntington disease (HD) is the accumulation of mutant Huntingtin (HTT) protein, which may be regulated by posttranslational modifications. Here, we define the primary sites of SUMO modification in the amino-terminal domain of HTT, show modification downstream of this domain, and demonstrate that HTT is modified by the stress-inducible SUMO-2. A systematic study of E3 SUMO ligases demonstrates that PIAS1 is an E3 SUMO ligase for both HTT SUMO-1 and SUMO-2 modification and that reduction of dPIAS in a mutant HTT Drosophila model is protective. SUMO-2 modification regulates accumulation of insoluble HTT in HeLa cells in a manner that mimics proteasome inhibition and can be modulated by overexpression and acute knockdown of PIAS1. Finally, the accumulation of SUMO-2-modified proteins in the insoluble fraction of HD postmortem striata implicates SUMO-2 modification in the age-related pathogenic accumulation of mutant HTT and other cellular proteins that occurs during HD progression.

Identification of NUB1 as a suppressor of mutant Huntington toxicity via enhanced protein clearance

Huntington's disease is caused by expanded CAG repeats in HTT, conferring toxic gain of function on mutant HTT (mHTT) protein. Reducing mHTT amounts is postulated as a strategy for therapeutic intervention. We conducted genome-wide RNA interference screens for genes modifying mHTT abundance and identified 13 hits. We tested 10 in vivo in a Drosophila melanogaster Huntington's disease model, and 6 exhibited activity consistent with the in vitro screening results. Among these, negative regulator of ubiquitin-like protein 1 (NUB1) overexpression lowered mHTT in neuronal models and rescued mHTT-induced death. NUB1 reduces mHTT amounts by enhancing polyubiquitination and proteasomal degradation of mHTT protein. The process requires CUL3 and the ubiquitin-like protein NEDD8 necessary for CUL3 activation. As a potential approach to modulating NUB1 for treatment, interferon-β lowered mHTT and rescued neuronal toxicity through induction of NUB1. Thus, we have identified genes modifying endogenous mHTT using high-throughput screening and demonstrate NUB1 as an exemplar entry point for therapeutic intervention of Huntington's disease.

The regulation of N-terminal Huntingtin (Htt552) accumulation by Beclin1

AIM

Huntingtin protein (Htt) was a neuropathological hallmark in human Huntington's Disease. The study aimed to investigate whether the macroautophagy regulator, Beclin1, was involved in the degradation of Htt.

METHODS

PC12 cells and primary cultured brain neurons of rats were examined. pDC316 adenovirus shuttle plasmid was used to mediate the expression of wild-type Htt-18Q-552 or mutant Htt-100Q-552 in PC12 cells. The expression of the autophagy-related proteins LC3 II and Beclin1, as well as the lysosome-associated enzymes Cathepsin B and L was evaluated using Western blotting. The locations of Beclin1 and Htt were observed with immunofluorescence and confocal microscope.

RESULTS

Htt552 expression increased the expression of LC3 II, Beclin1, cathepsin B and L in autophagy/lysosomal degradation pathway. Treatment with the autophagy inhibitor 3-MA or the proteasome inhibitors lactacystin and MG-132 increased Htt552 levels in PC12 cells infected with Ad-Htt-18Q-552 or Ad-Htt-100Q-552. The proteasome inhibitor caused a higher accumulation of Htt552-18Q than Htt552-100Q, and the autophagy inhibitor resulted in a higher accumulation of Htt552-100Q than Htt552-18Q. Similar results were observed in primary cultured neurons infected with adenovirus. In Htt552-expressing cells, Beclin1 was redistributed from the nucleus to the cytoplasm. Htt siRNA prevented Beclin1 redistribution in starvation conditions. Blockade of Beclin1 nuclear export by leptomycin B or Beclin1 deficiency caused by RNA interference induced the formation of mHtt552 aggregates.

CONCLUSION

Beclin1 regulates the accumulation of Htt via macroautophagy.

Multiple phenotypes in Huntington disease mouse neural stem cells

Neural stem (NS) cells are a limitless resource, and thus superior to primary neurons for drug discovery provided they exhibit appropriate disease phenotypes. Here we established NS cells for cellular studies of Huntington's disease (HD). HD is a heritable neurodegenerative disease caused by a mutation resulting in an increased number of glutamines (Q) within a polyglutamine tract in Huntingtin (Htt). NS cells were isolated from embryonic wild-type (Htt(7Q/7Q)) and "knock-in" HD (Htt(140Q/140Q)) mice expressing full-length endogenous normal or mutant Htt. NS cells were also developed from mouse embryonic stem cells that were devoid of Htt (Htt(-/-)), or knock-in cells containing human exon1 with an N-terminal FLAG epitope tag and with 7Q or 140Q inserted into one of the mouse alleles (Htt(F7Q/7Q) and Htt(F140Q/7Q)). Compared to Htt(7Q/7Q) NS cells, HD Htt(140Q/140Q) NS cells showed significantly reduced levels of cholesterol, increased levels of reactive oxygen species (ROS), and impaired motility. The heterozygous Htt(F140Q/7Q) NS cells had increased ROS and decreased motility compared to Htt(F7Q/7Q). These phenotypes of HD NS cells replicate those seen in HD patients or in primary cell or in vivo models of HD. Huntingtin "knock-out" NS cells (Htt(-/-)) also had impaired motility, but in contrast to HD cells had increased cholesterol. In addition, Htt(140Q/140Q) NS cells had higher phospho-AKT/AKT ratios than Htt(7Q/7Q) NS cells in resting conditions and after BDNF stimulation, suggesting mutant htt affects AKT dependent growth factor signaling. Upon differentiation, the Htt(7Q/7Q) and Htt(140Q/140Q) generated numerous Beta(III)-Tubulin- and GABA-positive neurons; however, after 15 days the cellular architecture of the differentiated Htt(140Q/140Q) cultures changed compared to Htt(7Q/7Q) cultures and included a marked increase of GFAP-positive cells. Our findings suggest that NS cells expressing endogenous mutant Htt will be useful for study of mechanisms of HD and drug discovery.

Expression of mutant N-terminal huntingtin fragment (htt552-100Q) in astrocytes suppresses the secretion of BDNF

Huntington's disease (HD) is an inheritable neurological disorder caused by an abnormal expansion of the polyglutamine tract in the N-terminus of the protein huntingtin (htt). Mutant htt (mhtt) leads to selective neurodegeneration that preferentially affects striatal medium spiny neurons. Although mhtt is also expressed in astrocytes, whether and how astrocyte derived mhtt contributes to the neurodegeneration in HD remains largely unknown. In this study, a glia HD model, using an adenoviral vector to express wild-type and mutant N-terminal huntingtin fragment 1-552 aa (htt552) in rat primary cortical astrocytes, was generated. The influence of htt552 on the protein level of brain-derived neurotrophic factor (BDNF) in astrocytes was evaluated. Immunofluorescence showed that htt552-100Q formed aggregates in some astrocytes. These mhtt aggregates sequestered clathrin immunoreactivities and dispersed the Golgi complex. ELISA and immunofluorescence demonstrated an increase in BDNF levels in the astrocytes expressing htt552-100Q. Western blot analysis showed that there was an increase in pro-BDNF, but a decrease in mature BDNF in the astrocytes expressing htt552-100Q. Furthermore, medium collected from astrocytes expressing htt552-100Q showed a lower level of mature BDNF and less activity in supporting neurite development of primary cortical neurons. These results suggest that aggregates formed by mutant htt552 affect processing and secretion of the BDNF in astrocytes, which might contribute to the neuronal dysfunction and degeneration in HD.

Huntingtin cleavage product A forms in neurons and is reduced by gamma-secretase inhibitors

Background

The mutation in Huntington's disease is a polyglutamine expansion near the N-terminus of huntingtin. Huntingtin expressed in immortalized neurons is cleaved near the N-terminus to form N-terminal polypeptides known as cleavage products A and B (cpA and cpB). CpA and cpB with polyglutamine expansion form inclusions in the nucleus and cytoplasm, respectively. The formation of cpA and cpB in primary neurons has not been established and the proteases involved in the formation of these fragments are unknown.

Results

Delivery of htt cDNA into the mouse striatum using adeno-associated virus or into primary cortical neurons using lentivirus generated cpA and cpB, indicating that neurons in brain and in vitro can form these fragments. A screen of small molecule protease inhibitors introduced to clonal striatal X57 cells and HeLa cells identified compounds that reduced levels of cpA and are inhibitors of the aspartyl proteases cathepsin D and cathepsin E. The most effective compound, P1-N031, is a transition state mimetic for aspartyl proteases. By western blot analysis, cathepsin D was easily detected in clonal striatal X57 cells, mouse brain and primary neurons, whereas cathepsin E was only detectible in clonal striatal X57 cells. In primary neurons, levels of cleavage product A were not changed by the same compounds that were effective in clonal striatal cells or by mRNA silencing to partially reduce levels of cathepsin D. Instead, treating primary neurons with compounds that are known to inhibit gamma secretase activity either indirectly (Imatinib mesylate, Gleevec) or selectively (LY-411,575 or DAPT) reduced levels of cpA. LY-411,575 or DAPT also increased survival of primary neurons expressing endogenous full-length mutant huntingtin.

Conclusion

We show that cpA and cpB are produced from a larger huntingtin fragment in vivo in mouse brain and in primary neuron cultures. The aspartyl protease involved in forming cpA has cathepsin-D like properties in immortalized neurons and gamma secretase-like properties in primary neurons, suggesting that cell type may be a critical factor that specifies the aspartyl protease responsible for cpA. Since gamma secretase inhibitors were also protective in primary neurons, further study of the role of gamma-secretase activity in HD neurons is justified.

The mTOR kinase inhibitor Everolimus decreases S6 kinase phosphorylation but fails to reduce mutant huntingtin levels in brain and is not neuroprotective in the R6/2 mouse model of Huntington's disease

Background

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion within the huntingtin gene. Mutant huntingtin protein misfolds and accumulates within neurons where it mediates its toxic effects. Promoting mutant huntingtin clearance by activating macroautophagy is one approach for treating Huntington's disease (HD). In this study, we evaluated the mTOR kinase inhibitor and macroautophagy promoting drug everolimus in the R6/2 mouse model of HD.

Results

Everolimus decreased phosphorylation of the mTOR target protein S6 kinase indicating brain penetration. However, everolimus did not activate brain macroautophagy as measured by LC3B Western blot analysis. Everolimus protected against early declines in motor performance; however, we found no evidence for neuroprotection as determined by brain pathology. In muscle but not brain, everolimus significantly decreased soluble mutant huntingtin levels.

Conclusions

Our data suggests that beneficial behavioral effects of everolimus in R6/2 mice result primarily from effects on muscle. Even though everolimus significantly modulated its target brain S6 kinase, this did not decrease mutant huntingtin levels or provide neuroprotection.

Disruption of Rab11 activity in a knock-in mouse model of Huntington's disease

The Huntington's disease (HD) mutation causes polyglutamine expansion in huntingtin (Htt) and neurodegeneration. Htt interacts with a complex containing Rab11GDP and is involved in activation of Rab11, which functions in endosomal recycling and neurite growth and long-term potentiation. Like other Rab proteins, Rab11GDP undergoes nucleotide exchange to Rab11GTP for its activation. Here we show that striatal membranes of HD(140Q/140Q) knock-in mice are impaired in supporting conversion of Rab11GDP to Rab11GTP. Dominant negative Rab11 expressed in the striatum and cortex of normal mice caused neuropathology and motor dysfunction, suggesting that a deficiency in Rab11 activity is pathogenic in vivo. Primary cortical neurons from HD(140Q/140Q) mice were delayed in recycling transferrin receptors back to the plasma membrane. Partial rescue from glutamate-induced cell death occurred in HD neurons expressing dominant active Rab11. We propose a novel mechanism of HD pathogenesis arising from diminished Rab11 activity at recycling endosomes.

Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes

Research in autophagy continues to accelerate,(1) and as a result many new scientists are entering the field. Accordingly, it is important to establish a standard set of criteria for monitoring macroautophagy in different organisms. Recent reviews have described the range of assays that have been used for this purpose.(2,3) There are many useful and convenient methods that can be used to monitor macroautophagy in yeast, but relatively few in other model systems, and there is much confusion regarding acceptable methods to measure macroautophagy in higher eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers of autophagosomes versus those that measure flux through the autophagy pathway; thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from fully functional autophagy that includes delivery to, and degradation within, lysosomes (in most higher eukaryotes) or the vacuole (in plants and fungi). Here, we present a set of guidelines for the selection and interpretation of the methods that can be used by investigators who are attempting to examine macroautophagy and related processes, as well as by reviewers who need to provide realistic and reasonable critiques of papers that investigate these processes. This set of guidelines is not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to verify an autophagic response.

Striatal neurochemical changes in transgenic models of Huntington's disease

Transgenic mouse models of Huntington's disease (HD) were examined following the onset of overt behavioral symptoms. The HD transgenic mice demonstrated profound striatal losses in D1, D2, and D3 dopamine (DA) receptor proteins in comparison with their nonsymptomatic, age-matched littermate controls. In parallel, a robust increase in the striatal D5 DA receptor subtype occurred in the transgenic compared with the wild-type control mice. This receptor elevation was accompanied by heightened cyclic AMP levels, which may be induced by the adenylyl cyclase-linked D5 receptor. This is a unique result; normal striatal D5 protein levels are modest and not thought to contribute substantially to cyclic AMP-mediated DA signaling mechanisms. Simple compensatory up-regulation of D5 DA receptors in response to D1 receptor subtype loss does not explain our findings, because genetic inactivation of the D1 DA receptor does not alter levels of D5 DA receptor expression. Immunofluorescent detection of tyrosine hydroxylase showed that nigrostriatal DA containing terminals were reduced, further supporting that disturbances in DA signaling occurred in HD transgenic models. The substance P-containing striatal efferent pathway was more resistant to the HD mutation than met-enkephalin-producing striatal projection neurons in the transgenics, based on neuropeptide immunofluorescent staining. Analogous findings in multiple transgenic models suggest that these changes are due to the presence of the transgene and are not dependent on its composition, promotor elements, or mouse strain background. These findings suggest modifications in the striatal DA system and that its downstream signaling through cyclic AMP mechanisms is disrupted severely in HD following onset of motor symptoms.

Forskolin and dopamine D1 receptor activation increase huntingtin's association with endosomes in immortalized neuronal cells of striatal origin

Huntingtin is a cytoplasmic protein of unknown function that associates with vesicle membranes and microtubules. Its protein interactions suggest that huntingtin has a role in endocytosis and organelle transport. In this study we sought to identify factors that regulate the transport of huntingtin in striatal neurons, which are the cells most affected in Huntington's disease. In clonal striatal cells derived from fusions of neuroblastoma and embryonic striatal neurons, huntingtin localization is diffuse and slightly punctate in the cytoplasm. When these neurons were differentiated by treatment with forskolin, huntingtin redistributed to perinuclear regions, discrete puncta along plasma membranes, and branch points and terminal growth cones in neurites. Huntingtin staining overlapped with clathrin, a coat protein involved in endocytosis. Immunoblot analysis of subcellular membrane fractions separated by differential centrifugation confirmed that huntingtin immunoreactivity in differentiated neurons markedly increased in membrane fractions enriched with clathrin and with huntingtin-interacting protein 1. Dopamine treatment altered the subcellular localization of huntingtin and increased its expression in clathrin-enriched membrane fractions. The dopamine-induced changes were blocked by the D1 antagonist SCH 23390 and were absent in a clonal cell line lacking D1 receptors. Results suggest that the transport of huntingtin and its co-expression in clathrin and huntingtin-interacting protein 1-enriched membranes is influenced by activation of adenylyl cyclase and stimulation of dopamine D1 receptors.

Analysis of Huntingtin-associated protein 1 in mouse brain and immortalized striatal neurons

Huntingtin, the protein product of the Huntington's disease (HD) gene, is expressed with an expanded polyglutamine domain in the brain and in nonneuronal tissues in patients with HD. Huntingtin-associated protein 1 (HAP-1), a brain-enriched protein, interacts preferentially with mutant huntingtin and thus may be important in HD pathogenesis. The function of HAP-1 is unknown, but recent evidence supports a role in microtubule-dependent organelle transport. We examined the subcellular localization of HAP-1 with an antibody made against the NH2-terminus of the protein. In immunoblot assays of mouse brain and immortalized striatal neurons, HAP-1 subtypes A and B migrated together at about 68 kD and separately at 95 kD and 110 kD, respectively. In dividing clonal striatal cells, HAP-1 localized to the mitotic spindle apparatus, especially at spindle poles and on vesicles and microtubules of the spindle body. Postmitotic striatal neurons had punctate HAP-1 labeling throughout the cytoplasm. Western blot analysis of protein extracts obtained after subcellular fractionation and differential centrifugation of the clonal striatal cells showed that HAP-1B was preferentially enriched in membrane fractions. Electron microscopic study of adult mouse basal forebrain and striatum showed HAP-1 localized to membrane-bound organelles including large endosomes, tubulovesicular structures, and budding vesicles in neurons. HAP-1 was also strongly associated with an unusual large "dense" organelle. Microtubules were labeled in dendrites and axonal fibers. Results support a role for HAP-1 in vesicle trafficking and organelle movement in mitotic cells and differentiated neurons and implicate HAP-1B as the predominant molecular subtype associated with vesicle membranes in striatal neurons.

Regional expression and subcellular localization of the tyrosine-specific phosphatase SH-PTP2 in the adult human nervous system

The protein tyrosine phosphatase SH-PTP2 has been implicated in a variety of cell signaling cascades, including those mediating neuronal survival. We therefore investigated the expression of SH-PTP2 in the adult human nervous system using Western blotting, immunohistochemistry and immunoelectron microscopy. SH-PTP2 immunoreactivity was noted only in neurons, but was not restricted to a specific neuronal type or location. Immunohistochemistry showed perikaryal staining, whereas Western blotting and ultrastructural analysis suggested that SH-PTP2 is present in axons as well. While immunohistochemistry showed a Nissl-like pattern in large motor neurons, immunoelectron microscopy demonstrated a diffuse pattern of cytoplasmic staining, without apparent preferential localization. The presence of the SH2 domain-containing tyrosine-specific phosphatase SH-PTP2 in diverse neurons in the adult nervous system suggests that SH-PTP2 may play a role in a broad spectrum of neuronal responses.

Fluorescent and biotin probes for dopamine receptors: D1 and D2 receptor affinity and selectivity

Fluorophor and biotin derivatives of dopamine agonist and antagonist drugs were synthesized and evaluated for binding affinity and selectivity at D1 and D2 dopamine receptors in membranes prepared from monkey (Macaca fascicularis) caudate putamen. Binding was measured using [3H]SCH 23390 to label D1 receptors and [3H]spiperone to label D2 receptors. The selective D1 antagonist SKF 83566, whether coupled to 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD), to fluorescein, or to biotin retained high affinity for D1 dopamine receptors (Ki, 5.3 16 and 3.5 nM, respectively) and high D1/D2 receptor selectivity (130-, 300, and 600-fold, respectively). The selective D2 antagonist derivative N-(p-aminophenethyl)spiperone, (NAPS) coupled either to biotin or to NBD via the N-aminoethylphenyl group, likewise retained high D2 receptor affinity (Ki, 0.58 and 0.66 nM, respectively) and high D2/D1 selectivity (190- and 150-fold, respectively). The affinity of the NBD-coupled derivative of (S)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin hydrochloride [(S)-PPHT], a selective D2 agonist, was actually higher than that of the parent compound (Ki, 0.30 versus 2.1 nM), whereas the affinity of fluorescein-coupled (S)-PPHT was lower (Ki, 4.8 nM). Sensitivity to GTP, a characteristic of agonist binding at dopamine receptors, was demonstrated for NBD-coupled (S)-PPHT, because D2 receptor affinity was somewhat reduced in the presence of GTP. PPHT-fluorescein fluorescence labeling rimmed cells in monkey and rat anterior pituitary and outlined cells in the striatum. Fluorescent and biotin probes based on selective high affinity ligands for dopamine receptors may expedite studies of receptor localization and mobility at the cellular level.

Synaptic organization of the globus pallidus

The synaptic organization of the globus pallidus is reviewed with respect to present knowledge about neurons, fibers, axon terminals, and their intrinsic synaptic relationships. Information derived from studies employing Nissl stains, Golgi impregnations, lesion degeneration techniques, immunohistochemistry, and anterograde axonal labeling in various species are presented along with ultrastructural data. Studies indicate that the globus pallidus contains a principal efferent neuron with smooth or spiny dendrites and simple or complex terminal dendritic arborizations. This cell type receives convergent inputs from intrinsic and extrinsic sources and uses gamma-aminobutyric acid as a transmitter. A smaller and separate population of pallidal projection neurons contains acetylcholine. Two other less frequent neuronal types, of small and medium size, have also been recognized. Three to six types of axonal boutons forming synaptic contacts with pallidal neurons have been recognized in various studies. Among these, three types (types I, II, and III) are the most prevalent. Studies indicate that the most frequent category (type I) originates from neostriatal neurons via radial fiber projections and contains immunoreactive GABA and enkephalins. The synaptic architecture of the globus pallidus is dominated by a mosaic-like arrangement of long dendrites that are ensheathed by longitudinally oriented axons making synapses en passant. Triadic synapses involving dendrites that are pre- and postsynaptic are encountered infrequently. Because both striatopallidal and pallidothalamic connections are inhibitory, pallidal target neurons in the thalamus may be "disinhibited" when the neostriatum is activated.

Localization of immunoreactive GABA and enkephalin and NADPH-diaphorase-positive neurons in fetal striatal grafts in the quinolinic-acid-lesioned rat neostriatum

Fetal striatal tissue grafts have been shown to partially reverse the biochemical and behavioral deficits induced by excitotoxic lesions. To determine if grafted striatal neurons contain neurochemical markers similar to those in neurons in the caudate nucleus and to establish the morphological characteristics and relative frequency of labeled neurons in the grafts, the localization of immunoreactive GABA and leucine-enkephalin (ENK) and of NADPH-diaphorase (NADPH-d) activity was examined in fetal striatal grafts at the light and electron microscopic levels. Striatal tissue from 17-day fetuses was grafted into the caudate nucleus of adult rats 1 week after intracaudate injections of either a low or high dose of quinolinic acid. At the light microscopic level, immunoreactive GABA and ENK and NADPH-d-positive neurons, processes, and punctate structures were present within adjacent sections of the same grafts. The frequency and morphological features of these labeled cell populations were similar in grafts placed into either minimally or extensively lesioned striata. Immunoreactive GABA and ENK neurons in the grafts constituted 28% and 13.5%, respectively, of the neuronal population of the graft and their mean diameters were 22 and 14% larger, respectively, than neostriatal neurons that contained the same chemical markers. NADPH-d-positive neurons in the grafts formed 3.5% of total grafted neurons and exhibited characteristics of neostriatal NADPH-d-containing aspiny cells, including medium-sized somata, indented nuclei, and varicose dendrites. At the electron microscopic level most GABA-positive neurons in the grafts contained indented nuclei and most immunoreactive ENK somata had unindented nuclei. Dendrites and dendritic spines with GABA or ENK immunoreactivity were present in the grafts where they were postsynaptic to unlabeled axons. Immunoreactive GABA and ENK axon terminals formed synapses with unlabeled neuronal profiles in the grafts. These findings demonstrate that fetal striatal grafts contain chemically defined neuronal populations that form synaptic connections within the graft and share some features with corresponding cell groups in the neostriatum. These results provide an anatomical basis for the graft-induced recovery from behavioral and biochemical deficits caused by instrastriatal lesions reported in other studies.

A Golgi study of the monkey paraventricular nucleus: neuronal types, afferent and efferent fibers

The neuronal organization of the paraventricular nucleus (PVN) was examined in Golgi impregnations of adult monkey. Results showed that at least six types of neurons could be identified in the nucleus on the basis of morphological features of the somata, dendrites, and axons. Four types of neurons with sparse to densely spined cell bodies and dendrites exhibited long axons and included large neurons (types I and II), medium-sized to large neurons (type III), and small to medium-sized cells (type IV). Axons of type I, III, and IV neurons had different diameters and were followed out of the PVN. Axon collaterals that arborized within the PVN were seen on the axons of types III and IV cells. Two types of interneurons with small somata were also found. One (type V) exhibited varicose dendrites and a profusely arborizing local axon. The other cell (type VI) had recurved dendrites with long appendages and no impregnated axon. Afferent fibers were also identified. Type 1 was a fine-caliber axon that coursed long distances in the PVN and exhibited numerous short branches. Additional observations suggested that type 1 afferents originated from the stria terminalis. The other afferent axon (type 2) was thicker and gave rise to terminal arborizations containing clusters of small swellings. The efferent fibers of the PVN were also examined in impregnations of the paraventriculosupraopticohypophysial tract. Fibers formed an extensive plexus as they coursed ventrally and passed through the lateral hypothalamus. Axons coursing more laterally in the tract were much larger than those more medially located. Our findings show a diverse organization of neuronal types within the monkey PVN with evidence for intrinsic connections through axon collaterals of efferent neurons and the locally arborizing axons of interneurons. Correlations are proposed between morphological subtypes of neurons seen in this Golgi study and the known functional output pathways of the PVN.

Cytochrome oxidase activity in the rat caudate nucleus: light and electron microscopic observations

Cytochrome oxidase (CO) activity was examined in the neostriatum of normal adult rats at the light and electron microscopic level. At the light microscopic level a heterogeneous distribution of CO activity was observed and was characterized by patches of high activity ranging in size from 200 to 800 microns surrounded or adjacent to regions of lower activity. The most dorsomedial and ventromedial regions of the caudate nucleus appeared to be consistently high in activity in all animals. At the ultrastructural level CO reaction product was localized to the membranes and intracristal spaces of mitochondria. The most reactive mitochondria (those containing the denest precipitates of reaction product) were found within the dendrites of spiny neurons in all caudate regions. In areas of high CO activity the mitochondria within bundles of myelinated fibers and in many axon terminals were also highly reactive whereas those in neuronal somata, primary dendrites, and glial cells and processes exhibited relatively little activity. Quantitative study showed that mitochondria within dendrites accounted for most of the CO activity in caudate neuropil. The mitochondria within dendrites and axon terminals were more reactive in regions of high CO activity than in regions of low CO activity. No differences in the density of synapses or in the proportions of axospinous and axodendritic synapses were observed between CO-rich and CO-poor areas. Heterogeneity in the distribution of CO activity in the caudate nucleus may be related to the "patchy" pattern of localization previously observed for some neostriatal afferents, enzymes, transmitters, peptides, and receptor ligands.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of immunoreactive enkephalins in GABA synthesizing neurons of the rat neostriatum

The localization of immunoreactive glutamic acid decarboxylase (GAD) and enkephalin-like immunoreactivity was examined in serial, 4-micron frozen sections of the caudate nucleus from rats pretreated with colchicine. Colocalization was found in numerous caudate neurons of medium size. Cell counts of corresponding labeled neurons in paired adjacent sections showed that GAD and enkephalin-like immunoreactivity coexist in about one half of the caudate cell populations containing each of these substances.

Ultrastructural localization of immunoreactive neurotensin in the monkey superficial dorsal horn

Neurotensin, a tridecapeptide, has been proposed to have a role in sensory systems, especially those mediating pain. The light microscopic and ultrastructural localization of neurotensin immunoreactivity in neurons of the monkey spinal cord was studied with the aim of examining their synaptic interactions. At the light microscopic level, neurotensin-containing cells were located in laminae II and III and immunoreactive axons and terminals were found in laminae I, II, and III. Neurotensin-positive axons were mostly thin and unmyelinated and their boutons contained both clear and large granular vesicles. Boutons varied considerably in size (1-3 micron) and in their relative content of large granular vesicles, which appeared occasionally in presynaptic locations. In lamina I neurotensin-immunoreactive terminals formed synapses with cell bodies which varied both in size and subcellular features. Some large dendrites in lamina I were contacted by numerous neurotensin-positive axons and also unlabeled terminals. In lamina II boutons with neurotensin immunoreactivity formed synapses mostly with small unlabeled dendrites some of which contained vesicles. The present results together with recent anatomical and physiological findings suggest that spinal cord neurons which contain neurotensin synapse with cells in the superficial dorsal horn that receive either input from primary afferents conveying nociceptive information or form part of the spinothalamic tract, or both. The diversity observed both in the morphology of neurotensin-positive terminals and in their synaptic patterns may indicate that they arise from more than one type of dorsal horn cell.

A Golgi and ultrastructural study of the monkey globus pallidus

Golgi preparations reveal that the most frequent type of pallidal neuron (principal cell), which has been recognized in all previous reports, is large (20-50 microns), fusiform, with dendrites up to 700 microns long. Large neurons of globular shape are less frequently impregnated. The morphology of dendrites varies considerably within the same neuron. Some exhibit numerous spines and protrusions and are seen to terminate in elaborate arborizations. A small interneuron (12 microns), with relatively short dendrites, up to 150 microns, and a short sparsely branching axon is observed less frequently. At least two types of afferent axons are present. A small-diameter fiber from the neostriatum enters the pallidum in bundles and gives rise to numerous thin branching processes with varicosities about 1 micron in size. The axon collaterals are oriented orthogonal to the main axon and parallel to the dendrites of principal cells. A large-caliber fiber with clusters of 2-3 microns swellings can also be seen in close proximity to large pallidal dendrites. Ultrastructurally, principal cell dendrites (trunks, spines, and protrusions) are totally covered by synapsing axon terminals. In contrast, some small dentrites, presumed to belong to interneurons, form very few synapses. At least six categories of profiles containing vesicles are observed. One group has cytologic features of dendrites and participates in serial and triadic synapses with other profiles in the pallidal neuropil. Results suggest that the synaptic organization of the globus pallidus may be viewed as a repetitive, geometric arrangement of striatal and other afferent axons ensheathing and synapsing with the dendrites of principal cells. This pattern is interrupted by the presence of presynaptic dendrites, probably belonging to interneurons, which participate in complex synaptic arrangements.

Ultrastructure of Golgi-impregnated and gold-toned spiny and aspiny neurons in the monkey neostriatum

Golgi-impregnated, gold-toned spiny and aspiny neurons in the monkey neostriatum were deimpregnated and examined at the electron microscope level. Spiny type I neurons have relatively large nuclei with few indentations and aggregates of chromatin under the nuclear membrane which in some regions give the appearance of a dark rim. The small quantity of surrounding cytoplasm is poor in organelles. Aspiny type I neurons have eccentric, highly indented nuclei. The relatively large proportion of cytoplasm is rich in organelles especially Golgi apparatus and rough endoplasmic reticulum which often appears in stacks. Synapses with symmetric membrane densities are common on the somata of spiny type I neurons. Those on the proximal and distal dendritic shafts are few in number and asymmetric, and those on spines more frequent and primarily asymmetric. Aspiny type I neurons have few synapses on their cell bodies. Proximal and distal dendrites, however, are contacted by numerous profiles which contain small round vesicles and make both symmetric and asymmetric synapses. The same axon terminals also synapse with dendritic spines of spiny neurons, indicating that an input, most likely of afferent origin, is shared by both cell types. Other less frequently occurring profiles forming symmetric membrane densities also contact the dendrites of aspiny and spiny neurons. The axon hillocks and initial segments of both neuronal types receive a synaptic input, which is more common on spiny cells. Results offer unequivocal evidence for the differences in the ultrastructure of these two most common categories of medium-size neostriatal neurons, which may help in their proper identification in standard material, as well as information on the types and distributions of synaptic inputs onto these neurons. Moreover, the findings clarify some controversies in the literature probably originating from observations on a mixed population of cells of medium size.

Early postnatal development of the monkey neostriatum: a Golgi and ultrastructural study

Paired specimens of the neostriatum were taken from monkeys at zero (newborn), one, two, four, eight, and 16 weeks of age, and prepared for Golgi impregnations and electron microscopy. Light microscopy shows that in the first postnatal week, the structure contains the five neuronal types and four categories of afferent axons described in the adult, as well as some cells too undifferentiated to classify. Most neurons exhibit immature dendritic features, including local enlargements, terminal growth cones with filopodia, and filiform processes. In spiny type I cells, various levels of maturity may coexist in regions of a single dendrite, in different dendrites of the same neuron, and among individual cells. Spine density increases progressively with age, but the relative distribution of spine types remains about the same. Spiny type II neurons show some decline in spine density, and generally mature sooner than spiny type I cells. The long axons of spiny neurons have varicosities which disappear at about eight weeks. In younger animals (newborn and one week), the dendrites of aspiny neurons (types I, II, and III) may have a "spiny" appearance, exhibiting many spine-like and filiform processes. Concurrently, the short axons vary in degree of arborization from very immature to well developed. Electron microscopy corroborates the developmental features recognized in the Golgi material: dendritic and axonal growth cones, filopodia and varicosities, as well as various stages of maturation in somata and dendrites. Degenerating elements, mostly of an axonal nature, are seen up to eight weeks. The synapses which reach maturity at birth are of the asymmetric axospinous type, in which the axonal profile contains small round vesicles, and of the symmetric axodendritic class, with the presynaptic elements having pleomorphic vesicles. Some synapses are slower to mature and appear at one to eight postnatal weeks. These include those made by profiles with pleomorphic vesicles, forming either symmetric contacts with somata and axon initial segments, or asymmetric contacts with spines. The same applies to the asymmetric axodendritic synapses made by elements containing small round vesicles. Finally, profiles containing large round or flat vesicles are the latest to participate in mature synapses formation. Findings indicate that a considerable degree of qualitative and quantitative change takes place in the monkey neostriatal neuropil during early postnatal development, especially in the first eight-week period.