Jochen Roeper

Unique Properties of Mesoprefrontal Neurons within a Dual Mesocorticolimbic Dopamine System

The mesocorticolimbic dopamine system is essential for cognitive and emotive brain functions and is thus an important target in major brain diseases like schizophrenia, drug addiction, and attention deficit hyperactivity disorder. However, the cellular basis for the diversity in behavioral functions and associated dopamine-release pattern within the mesocorticolimbic system has remained unclear. Here, we report the identification of a type of dopaminergic neuron within the mesocorticolimbic dopamine system with unconventional fast-firing properties and small DAT/TH mRNA expression ratios that selectively projects to prefrontal cortex and nucleus accumbens core and medial shell as well as to basolateral amygdala. In contrast, well-described conventional slow-firing dopamine midbrain neurons only project to the lateral shell of the nucleus accumbens and the dorsolateral striatum. Among this dual dopamine midbrain system defined in this study by converging anatomical, electrophysiological, and molecular properties, mesoprefrontal dopaminergic neurons are unique, as only they do not possess functional somatodendritic Girk2-coupled dopamine D2 autoreceptors.

Parkinson phenotype in aged PINK1-deficient mice is accompanied by progressive mitochondrial dysfunction in absence of neurodegeneration

Background Parkinsons disease (PD) is an adult-onset movement disorder of largely unknown etiology. We have previously shown that loss-of-function mutations of the mitochondrial protein kinase PINK1 (PTEN induced putative kinase 1) cause the recessive PARK6 variant of PD. Methodology/Principal Findings Now we generated a PINK1 deficient mouse and observed several novel phenotypes: A progressive reduction of weight and of locomotor activity selectively for spontaneous movements occurred at old age. As in PD, abnormal dopamine levels in the aged nigrostriatal projection accompanied the reduced movements. Possibly in line with the PARK6 syndrome but in contrast to sporadic PD, a reduced lifespan, dysfunction of brainstem and sympathetic nerves, visible aggregates of α-synuclein within Lewy bodies or nigrostriatal neurodegeneration were not present in aged PINK1-deficient mice. However, we demonstrate PINK1 mutant mice to exhibit a progressive reduction in mitochondrial preprotein import correlating with defects of core mitochondrial functions like ATP-generation and respiration. In contrast to the strong effect of PINK1 on mitochondrial dynamics in Drosophila melanogaster and in spite of reduced expression of fission factor Mtp18, we show reduced fission and increased aggregation of mitochondria only under stress in PINK1-deficient mouse neurons. Conclusion Thus, aging Pink1−/− mice show increasing mitochondrial dysfunction resulting in impaired neural activity similar to PD, in absence of overt neuronal death.

Alternative sulfonylurea receptor expression defines metabolic sensitivity of K-ATP channels in dopaminergic midbrain neurons.

ATP‐sensitive potassium (K‐ATP) channels couple the metabolic state to cellular excitability in various tissues. Several isoforms of the K‐ATP channel subunits, the sulfonylurea receptor (SUR) and inwardly rectifying K channel (Kir6.X), have been cloned, but the molecular composition and functional diversity of native neuronal K‐ATP channels remain unresolved. We combined functional analysis of K‐ATP channels with expression profiling of K‐ATP subunits at the level of single substantia nigra (SN) neurons in mouse brain slices using an RT–multiplex PCR protocol. In contrast to GABAergic neurons, single dopaminergic SN neurons displayed alternative co‐expression of either SUR1, SUR2B or both SUR isoforms with Kir6.2. Dopaminergic SN neurons expressed alternative K‐ATP channel species distinguished by significant differences in sulfonylurea affinity and metabolic sensitivity. In single dopaminergic SN neurons, co‐expression of SUR1 + Kir6.2, but not of SUR2B + Kir6.2, correlated with functional K‐ATP channels highly sensitive to metabolic inhibition. In contrast to wild‐type, surviving dopaminergic SN neurons of homozygous weaver mouse exclusively expressed SUR1 + Kir6.2 during the active period of dopaminergic neurodegeneration. Therefore, alternative expression of K‐ATP channel subunits defines the differential response to metabolic stress and constitutes a novel candidate mechanism for the differential vulnerability of dopaminergic neurons in response to respiratory chain dysfunction in Parkinsons disease.

Dissecting the diversity of midbrain dopamine neurons

Midbrain dopamine (DA) neurons are essential for controlling key functions of the brain, such as voluntary movement, reward processing, and working memory. The largest populations of midbrain DA neurons are localized in two neighboring nuclei, the substantia nigra (SN) and the ventral tegmental area (VTA). Regardless of their different axonal projections to subcortical and cortical targets, midbrain DA neurons have traditionally been regarded as a relatively homogeneous group of neurons, with a stereotypical set of intrinsic electrophysiological properties and in vivo pattern of activity. In this review, I highlight recent data supporting an unexpected degree of diversity among these midbrain DA neurons in the mammalian brain, ranging from their developmental lineages and different synaptic connectivity to their electrophysiological properties and behavioral functions.

Kvβ1 Subunit Binding Specific for Shaker-Related Potassium Channel α Subunits

Abstract Voltage-activated potassium (Kv) channels from mammalian brain are hetero-oligomers containing α and β subunits. Coexpression of Kv1α and Kvβ1 subunits confers rapid A-type inactivation on noninactivating potassium channels (delayed rectifiers) in expression systems in vitro. We have delineated a Kv1.5 amino-terminal region of up to 90 amino acids (residues 112–201) that is sufficient for interactions of Kv1.5α and Kvβ1 subunits. Within this region of the Kv1.5 amino terminus (residues 193–201), a Kvβ1 interaction site necessary for Kvβ1-mediated rapid inactivation of Kv1.5 currents was detected. This interaction site motif (FYE/QLGE/DEAM/L) is found exclusively in the Shaker -related subfamily (Kv1). The results show that hetero-oligomerization between α and Kvβ1 subunits is restricted to Shaker -related potassium channel α subunits.

Resting state fMRI reveals increased subthalamic nucleus–motor cortex connectivity in Parkinson's disease

Parkinsons disease (PD) is associated with abnormal hypersynchronicity in basal ganglia-thalamo-cortical loops. The clinical effectiveness of subthalamic nucleus (STN) high frequency stimulation indicates a crucial role of this nucleus within the affected motor networks in PD. Here we investigate alterations in the functional connectivity (FC) profile of the STN using resting state BOLD correlations on a voxel-by-voxel basis in functional magnetic resonance imaging (fMRI). We compared early stage PD patients (n=31) during the medication-off state with healthy controls (n=44). The analysis revealed increased FC between the STN and cortical motor areas (BA 4 and 6) in PD patients in accordance with electrophysiological studies. Moreover, FC analysis of the primary motor cortex (M1) hand area revealed that the FC increase was primarily found in the STN area within the basal ganglia. These findings are in good agreement with recent experimental data, suggesting that an increased STN-motor cortex synchronicity mediated via the so called hyperdirect motor cortex-subthalamic pathway might play a fundamental role in the pathophysiology of PD. An additional subgroup analysis was performed according to the presence (n=16) or absence (n=15) of tremor in patients. Compared to healthy controls tremor patients showed increased STN FC specifically in the hand area of M1 and the primary sensory cortex. In non-tremor patients, increased FC values were also found between the STN and midline cortical motor areas including the SMA. Taken together our results underline the importance of the STN as a key node for the modulation of BG-cortical motor network activity in PD patients.

K-ATP channels in dopamine substantia nigra neurons control bursting and novelty-induced exploration

Phasic activation of the dopamine (DA) midbrain system in response to unexpected reward or novelty is critical for adaptive behavioral strategies. This activation of DA midbrain neurons occurs via a synaptically triggered switch from low-frequency background spiking to transient high-frequency burst firing. We found that, in medial DA neurons of the substantia nigra (SN), activity of ATP-sensitive potassium (K-ATP) channels enabled NMDA-mediated bursting in vitro as well as spontaneous in vivo burst firing in anesthetized mice. Cell-selective silencing of K-ATP channel activity in medial SN DA neurons revealed that their K-ATP channel–gated burst firing was crucial for novelty-dependent exploratory behavior. We also detected a transcriptional upregulation of K-ATP channel and NMDA receptor subunits, as well as high in vivo burst firing, in surviving SN DA neurons from Parkinsons disease patients, suggesting that burst-gating K-ATP channel function in DA neurons affects phenotypes in both disease and health.

Regional diversity and developmental dynamics of the AMPA-receptor proteome in the mammalian brain.

UNLABELLED Native AMPA receptors (AMPARs) in the mammalian brain are macromolecular complexes whose functional characteristics vary across the different brain regions and change during postnatal development or in response to neuronal activity. The structural and functional properties of the AMPARs are determined by their proteome, the ensemble of their protein building blocks. Here we use high-resolution quantitative mass spectrometry to analyze the entire pool of AMPARs affinity-isolated from distinct brain regions, selected sets of neurons, and whole brains at distinct stages of postnatal development. These analyses show that the AMPAR proteome is dynamic in both space and time: AMPARs exhibit profound region specificity in their architecture and the constituents building their core and periphery. Likewise, AMPARs exchange many of their building blocks during postnatal development. These results provide a unique resource and detailed contextual data sets for the analysis of native AMPAR complexes and their role in excitatory neurotransmission. VIDEO ABSTRACT

NIP domain prevents N-type inactivation in voltage-gated potassium channels

Shaker-related voltage-gated K+ (Kv) channels, are assembled from ion-conducting Kvα subunits, which are integral membrane proteins, and auxiliary Kvβ subunits. This leads to the formation of highly diverse heteromultimeric Kv channels that mediate outward currents with a wide range of time courses for inactivation. Two principal inactivation mechanisms have been recognized: C-type inactivation correlated with carboxy-terminal Kvα-subunit structures, and N-type inactivation conferred by ‘ball’ domains in the amino termini of certain Kvα, and Kvβ subunits. Assembly of heteromultimers with one or more Kvα,- and/or Kvβ ball domains appears to be an essential principle of the generation of A-type Kv channel diversity. Here we show that, unexpectedly, the presence of Kvα- or Kvβ-ball domains does not dominate the gating phenotype in heteromultimers containing Kv1.6α subunits. These heteromultimers mediate non-inactivating currents because of the dominant-negative activity of a new type of N-type inactivation-prevention (NIP) domain present in the Kv1.6 amino terminus. Mutations in the NIP domain lead to loss of function, and its transfer to another Kvα subunit leads to gain of function. Our discovery of the NIP domain, which neutralizes the activity of Kvα- and Kvβ-inactivation gates, establishes a new determinant for the gating behaviour of heteromultimeric Kv channels.

Cloning and functional expression of rat ether-à-go-go-like K+ channel genes

1 Screening of rat cortex cDNA resulted in cloning of two complete and one partial orthologue of the Drosophilaether‐à‐go‐go‐like K+ channel (elk). 2 Northern blot and reverse transcriptase‐polymerase chain reaction (RT‐PCR) analysis revealed predominant expression of rat elk mRNAs in brain. Each rat elk mRNA showed a distinct, but overlapping expression pattern in different rat brain areas. 3 Transient transfection of Chinese hamster ovary (CHO) cells with rat elk1 or rat elk2 cDNA gave rise to voltage‐activated K+ channels with novel properties. 4 RELK1 channels mediated slowly activating sustained potassium currents. The threshold for activation was at −90 mV. Currents were insensitive to tetraethylammonium (TEA) and 4‐aminopyridine (4‐AP), but were blocked by micromolar concentrations of Ba2+. RELK1 activation kinetics were not dependent on prepulse potential like REAG‐mediated currents. 5 RELK2 channels produced currents with a fast inactivation component and HERG‐like tail currents. RELK2 currents were not sensitive to the HERG channel blocker E4031.