Emergence of a synaptic neuronal network within primary striatal cultures seeded in serum-free medium

Deficits in coordinated neuronal activity and network topology are striatal hallmarks in Huntington's disease

Background

Network alterations underlying neurodegenerative diseases often precede symptoms and functional deficits. Thus, their early identification is central for improved prognosis. In Huntington’s disease (HD), the cortico-striatal networks, involved in motor function processing, are the most compromised neural substrate. However, whether the network alterations are intrinsic of the striatum or the cortex is not fully understood.

Results

In order to identify early HD neural deficits, we characterized neuronal ensemble calcium activity and network topology of HD striatal and cortical cultures. We used large-scale calcium imaging combined with activity-based network inference analysis. We extracted collective activity events and inferred the topology of the neuronal network in cortical and striatal primary cultures from wild-type and R6/1 mouse model of HD. Striatal, but not cortical, HD networks displayed lower activity and a lessened ability to integrate information. GABA_A receptor blockade in healthy and HD striatal cultures generated similar coordinated ensemble activity and network topology, highlighting that the excitatory component of striatal system is spared in HD. Conversely, NMDA receptor activation increased individual neuronal activity while coordinated activity became highly variable and undefined. Interestingly, by boosting NMDA activity, we rectified striatal HD network alterations.

Conclusions

Overall, our integrative approach highlights striatal defective network integration capacity as a major contributor of basal ganglia dysfunction in HD and suggests that increased excitatory drive may serve as a potential intervention. In addition, our work provides a valuable tool to evaluate in vitro network recovery after treatment intervention in basal ganglia disorders.

Selective Inactivation of Striatal FosB/ΔFosB-Expressing Neurons Alleviates L-DOPA-Induced Dyskinesia

BACKGROUND

ΔFosB is a surrogate marker of L-DOPA-induced dyskinesia (LID), the unavoidable disabling consequence of Parkinson's disease L-DOPA long-term treatment. However, the relationship between the electrical activity of FosB/ΔFosB-expressing neurons and LID manifestation is unknown.

METHODS

We used the Daun02 prodrug-inactivation method associated with lentiviral expression of β-galactosidase under the control of the FosB promoter to investigate a causal link between the activity of FosB/ΔFosB-expressing neurons and dyskinesia severity in both rat and monkey models of Parkinson's disease and LID. Whole-cell recordings of medium spiny neurons (MSNs) were performed to assess the effects of Daun02 and daunorubicin on neuronal excitability.

RESULTS

We first show that daunorubicin, the active product of Daun02 metabolism by β-galactosidase, decreases the activity of MSNs in rat brain slices and that Daun02 strongly decreases the excitability of rat MSN primary cultures expressing β-galactosidase upon D1 dopamine receptor stimulation. We then demonstrate that the selective, and reversible, inhibition of FosB/ΔFosB-expressing striatal neurons with Daun02 decreases the severity of LID while improving the beneficial effect of L-DOPA.

CONCLUSIONS

These results establish that FosB/ΔFosB accumulation ultimately results in altered neuronal electrical properties sustaining maladaptive circuits leading not only to LID but also to a blunted response to L-DOPA. These findings further reveal that targeting dyskinesia can be achieved without reducing the antiparkinsonian properties of L-DOPA when specifically inhibiting FosB/ΔFosB-accumulating neurons.

GABAA receptor activity shapes the formation of inhibitory synapses between developing medium spiny neurons

Basal ganglia play an essential role in motor coordination and cognitive functions. The GABAergic medium spiny neurons (MSNs) account for ~95% of all the neurons in this brain region. Central to the normal functioning of MSNs is integration of synaptic activity arriving from the glutamatergic corticostriatal and thalamostriatal afferents, with synaptic inhibition mediated by local interneurons and MSN axon collaterals. In this study we have investigated how the specific types of GABAergic synapses between the MSNs develop over time, and how the activity of GABA_A receptors (GABA_ARs) influences this development. Isolated embryonic (E17) MSNs form a homogenous population in vitro and display spontaneous synaptic activity and functional properties similar to their in vivo counterparts. In dual whole-cell recordings of synaptically connected pairs of MSNs, action potential (AP)-activated synaptic events were detected between 7 and 14 days in vitro (DIV), which coincided with the shift in GABA_AR operation from depolarization to hyperpolarization, as detected indirectly by intracellular calcium imaging. In parallel, the predominant subtypes of inhibitory synapses, which innervate dendrites of MSNs and contain GABA_AR α1 or α2 subunits, underwent distinct changes in the size of postsynaptic clusters, with α1 becoming smaller and α2 larger over time, while both the percentage and the size of mixed α1/α2-postsynaptic clusters were increased. When activity of GABA_ARs was under chronic blockade between 4–7 DIV, the structural properties of these synapses remained unchanged. In contrast, chronic inhibition of GABA_ARs between 7–14 DIV led to reduction in size of α1- and α1/α2-postsynaptic clusters and a concomitant increase in number and size of α2-postsynaptic clusters. Thus, the main subtypes of GABAergic synapses formed by MSNs are regulated by GABA_AR activity, but in opposite directions, and thus appear to be driven by different molecular mechanisms.

Neurochemical and electrophysiological characteristics of rat striatal neurons in primary culture

Neurons maintained in dispersed primary culture offer a number of advantages as a model system and are particularly well-suited for studies of the intrinsic electrical properties of neurons by patch clamp. We have characterized the immunocytochemical and electrophysiological properties of cultured rat striatal neurons as they develop in vitro in order to compare this model system with the known properties found in vivo. We found a high abundance of cells in vitro corresponding to the principal striatal output neuron, the medium spiny neuron. Immunocytochemical studies indicate that these cells have both dopamine-1 and dopamine-2 receptors and that there is overlap in their expression within the population of neurons. Semiquantitative analysis revealed bimodal distributions of dopamine receptor expression among the population of neurons. The principal peptide neurotransmitters substance P and enkephalin were present but at reduced levels compared with adult preparations. Other striatal markers such as calbindin, calretinin, and the cannabinoid-1 receptor were abundant. An immunocytochemical survey of voltage-gated K(+) channel subunits characteristic of adult tissue demonstrated the presence in vitro of Kv1.1, Kv1.4, Kv4.2, Kv4.3, and Kvbeta1.1, which have been associated with the rapidly inactivating currents. Electrophysiological studies employing voltage clamp revealed that outward currents had a large inactivating (A-type) component characteristic of mature basal ganglia. Current clamp studies reveal complex spontaneous firing patterns in a subset of neurons, including bursting behaviors superimposed on a slow depolarization. The inward rectifying channels Kir2.1 and Kir2.3, which are specific to particular compartments in adult striatum, were present in culture.

Definition of the alpha-KTx15 subfamily

Three novel scorpion toxins, Aa1 from Androctonus australis, BmTX3 from Buthus martensi and AmmTX3 from Androctonus mauretanicus were shown able to selectively block A-type K+ currents in cerebellum granular cells or cultured striatum neurons from rat brain. In electrophysiology experiments, the transient A-current completely disappeared when 1 microM of the toxins was applied to the external solution whereas the sustained K+ current was unaffected. The three toxins shared high sequence homologies (more than 94%) and constituted a new 'short-chain' scorpion toxin subfamily: alpha-KTx15. Monoiododerivative of 125I-sBmTX3 specifically bound to rat brain synaptosomes. Under equilibrium binding conditions, maximum binding was 14 fmol/mg of protein and the dissociation constant (Kd) was 0.21 nM. This Kd value was confirmed by kinetic experiments (kon = 6.0 x 10(6) M(-1) s(-1) and koff = 6.0 x 10(-4) s(-1)). Competitions with AmmTX3 and Aa1 with 125I-sBmTX3 bound to its receptor on rat brain synaptosomes showed that they fully inhibited the 125I-sBmTX3 binding (Ki values of 20 and 44 pM, respectively), demonstrating unambiguously that the three molecules shared the same target in rat brain. A panel of toxins described as specific ligands for different K+, Na+ and Ca2+ channels were not able to displace 125I-sBmTX3 from its binding site. Thus, 125I-sBmTX3 is a new ligand for a still unidentified target in rat brain. In autoradiography, the distribution of 125I-sBmTX3 binding sites in the adult rat brain indicated a high density of 125I-sBmTX3 receptors in the striatum, hippocampus, superior colliculus, and cerebellum.

Formation of dendritic spines in cultured striatal neurons depends on excitatory afferent activity

The role of afferent innervation in the formation of dendritic spines was studied in cultured rat striatum. The striatum is a unique structure in that it contains highly spiny GABAergic projection neurons, with no known local excitation. Grown alone in culture, striatal neurons did not express spontaneous network activity and were virtually devoid of dendritic spines. Adding GFP-expressing mouse cortical neurons to the striatal culture caused the appearance of spontaneous and evoked excitatory synaptic currents in the striatal neurons and a 10-fold increase in the density of spines on their dendrites. This effect was blocked by a continuous presence of TTX in the growth medium, while removal of the drug caused a rapid appearance of spines. Exposure to glutamate, or the presence of cortex-conditioned medium did not mimic the effect of cortical neurons on formation of spines in the striatal neurons. Also, the cortical innervation did not cause a selective enhancement of survival of specific subtypes of spiny striatal neurons. These experiments demonstrate that excitatory afferents are necessary for the formation of dendritic spines in striatal neurons.

Functional consequences of deleting the two C-terminal residues of the scorpion toxin BmTX3

We deleted the two C-terminal residues of the scorpion toxin BmTx3, a peptidyl inhibitor of a transient A-type K(+) current in striatum neurons in culture, to assess their contribution to receptor recognition. The sBmTX3-delYP analog was shown to have a native-like structure in one-dimensional 1H-nuclear magnetic resonance (NMR) spectroscopy. We found that sBmTX3-delYP bound to its receptor less efficiently than the wild-type molecule (by a factor of about 10(5)) in binding assays with rat brain membranes, and that this molecule did not block the A-type K(+) current (at a concentration of 35 microM) in whole-cell patch clamp experiments with striatum neurons. Also, these results show that the A-type K(+) channel blocked by BmTX3 should have a canonical K(+) channel pore structure.

Expanding the scorpion toxin alpha-KTX 15 family with AmmTX3 from Androctonus mauretanicus

A novel toxin, AmmTX3 (3823.5 Da), was isolated from the venom of the scorpion Androctonus mauretanicus. It showed 94% sequence homology with Aa1 from Androctonus australis and 91% with BmTX3 from Buthus martensi which, respectively, block A-type K+ current in cerebellum granular cells and striatum cultured neurons. Binding and displacement experiments using rat brain synaptosomes showed that AmmTX3 and Aa1 competed effectively with 125I-labelled sBmTX3 binding. They fully inhibited the 125I-labelled sBmTX3 binding (Ki values of 19.5 pm and 44.2 pm, respectively), demonstrating unambiguously that the three molecules shared the same target in rat brain. The specific binding parameters of 125I-labelled AmmTX3 for its site were determined at equilibrium (Kd = 66 pm, Bmax = 22 fmol per mg of protein). Finally, patch-clamp experiments on striatal neurons in culture demonstrated that AmmTX3 was able to inhibit the A-type K+ current (Ki = 131 nm).

A new class of scorpion toxin binding sites related to an A-type K+ channel: pharmacological characterization and localization in rat brain

A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensi venom, sequenced and chemically synthesized (sBmTX3). The A-type current of striatum neurons in culture completely disappeared when 1 microM sBmTX3 was applied (Kd=54 nM), whereas the sustained K+ current was unaffected. 125I-sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol x mg(-1) of protein, Kd=0.21 nM). A panel of toxins yet described as specific ligands for K+ channels were unable to compete with 125I-sBmTX3. A high density of 125I-sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.

Potassium chloride depolarization mediates CREB phosphorylation in striatal neurons in an NMDA receptor-dependent manner

Potassium chloride (KCl)-depolarization has been used to study the properties of L-type Ca2+ channel-mediated signal transduction in hippocampal neurons. Calcium influx through L-type Ca2+ channels stimulates a second messenger pathway that transactivates genes under the regulatory control of the Ca2+-and cyclic AMP-responsive element (CRE). Here, we show that in striatal neurons, but not in hippocampal neurons, CRE binding protein (CREB) phosphorylation and CRE-mediated gene expression after KCl-depolarization depends on functional NMDA receptors. This difference in NMDA receptor dependence is not due to different properties of L-type Ca2+ channels in either neuronal type, but rather to different neuron-intrinsic properties. Despite this variation, the second messenger pathway activated by KCl requires Ca2+/calmodulin (CaM) kinase for CREB phosphorylation in both neuronal types. We conclude that depolarization by KCl works differently in striatal and hippocampal neurons.

Agonist stimulation provokes dendritic and axonal dopamine D(1) receptor redistribution in primary cultures of striatal neurons

To investigate the influence of neurotransmitter on G-protein-coupled receptor trafficking and compartimentalization in neurons, we have developed a model of primary neuronal cultures from fetal rat striatum on which we have studied the cellular and subcellular distribution and trafficking of the D(1) dopaminergic receptor. This receptor is known to be somatodendritic and axonal targeted in vivo, mostly to extrasynaptic locations. Immunohistochemical studies at the light and electron microscopic levels showed that, in cultures, the D(1) dopaminergic receptor is expressed in the absence of dopamine stimulation. The pattern of D(1) dopaminergic receptor immunostaining after stimulation by the D(1) dopaminergic receptor agonist SKF 82958 (1 microM) is dramatically modified with a decrease of the number of labeled D(1) dopaminergic receptor puncta (-40%) and an increase of their size in both dendrites (+120%) and axons (+240%). Seven hours after removal of the agonist, return to normal pattern was observed. The D(1) dopaminergic receptor antagonist SCH 23390 (2 microM) abolishes the effect of SKF 82958. Electron microscopy demonstrated, in dendrites, a translocation of the labeling from the plasma membrane to endosomes. Axonal D(1) dopaminergic receptor redistribution after acute stimulation indicates that the D(1) dopaminergic receptor is membrane targeted and responsive to stimulation. These results validate primary culture of striatal neurons to study subcellular localization and intraneuronal trafficking of G-protein-coupled receptors. This preparation will be useful to address various questions concerning the behavior and the trafficking of these receptors in neurons in relation to the neurotransmitter environment.

Methylmalonate toxicity in primary neuronal cultures

Several inhibitors of mitochondrial complex II cause neuronal death in vivo and in vitro. The goal of the present work was to characterize in vitro the effects of malonate (a competitive blocker of the complex) which induces neuronal death in a pattern similar to that seen in striatum in Huntington's disease. Exposure of striatal and cortical cultures from embryonic rat brain for 24 h to methylmalonate, a compound which produces malonate intracellularly, led to a dose-dependent cell death. Methylmalonate (10 mM) caused >90% mortality of neurons although cortical cells were unexpectedly more vulnerable. Cell death was attenuated in a medium containing antioxidants. Further characterization revealed that DNA laddering could be detected after 3 h of treatment. Morphological observations (videomicroscopy and Hoechst staining) showed that both necrotic and apoptotic cell death occurred in parallel; apoptosis was more prevalent. A decrease in the ATP/ADP ratio was observed after 3 h of treatment with 10 mM methylmalonate. In striatal cultures it occurred concomitantly with a decline in GABA and a rise in aspartate content and the aspartate/glutamate ratio. Changes in ion concentrations were measured in similar cortical cultures from mouse brain. Neuronal [Na+]i increased while [K+]i and membrane potential decreased after 20 min of continuous incubation in 10 mM methylmalonate. These changes progressed with time, and a rise in [Ca2+]i was also observed after 1 h. The results demonstrate that malonate collapses cellular ion gradients, restoration of which imposes an additional load on the already compromised ATP-generation machinery. An early elevation in [Ca2+]i may trigger an increase in activity of proteases, lipases and endonucleases and production of free radicals and DNA damage which, ultimately, leads to cells death. The data also suggest that maturational and/or extrinsic factors are likely to be critical for the increased vulnerability of striatal neurons to mitochondrial inhibition in vivo.

Appearance of neurofilament subunit epitopes correlates with electrophysiological maturation in cortical embryonic neurons cocultured with mature astrocytes

E14 rat cortical neurons which have almost no glial progenitors were cocultured with a homogeneous population of mature type 1 astrocytes at a 4/1 ratio in serum free medium. Maturation of neurons was evaluated using a set of well characterized antibodies and two new monoclonal antibodies (MN2E4 and MN3H6) raised against various neurofilament subunits and whole-cell patch clamp experiments. We observed that this coculture method leads to a well-timed and very homogeneous neuronal maturation and that sequential appearance of neurofilament subunits in developing neurons correlates with the electrophysiological maturation. This sequence, early expression of the 68 kDa neurofilament subunit and late appearance of the 200 kDa neurofilament subunit, occurs in normal brain development, which validates this culture model as a useful tool for studying neuronal maturation and differentiation. MN2E4 staining (non-phosphorylated 200 kDa cytoskeletal protein antibody) appeared just before the neurons became excitable. It could thus be used as a functional neuronal marker. MN3H6 staining (phosphorylated 160-200 kDa neurofilament subunit antibody) appeared just after the neurons made synaptic contacts and generated synaptically driven spike bursts. This finding indicated that some phosphorylated epitopes of 160-200 kDa neurofilament followed synaptogenesis. These processes may play a key role in stabilizing the synapses to achieve a functional neuronal network.