Demonstration of functional coupling between dopamine synthesis and its packaging into synaptic vesicles

Divergent properties and independent regulation of striatal dopamine and GABA co-transmission

Substantia nigra pars compacta (SNc) dopamine neurons play a key role in regulating the activity of striatal circuits within the basal ganglia. In addition to dopamine, these neurons release several other transmitters, including the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). Both dopamine and GABA are loaded into SNc synaptic vesicles by the vesicular monoamine transporter 2 (VMAT2), and co-release of GABA provides strong inhibition to the striatum by directly inhibiting striatal medium spiny projection neurons (MSNs) through activation of GABAA receptors. Here, we found that despite both dopamine and GABA being co-packaged by VMAT2, the properties of transmission, including Ca2+ sensitivity, release probability, and requirement of active zone scaffolding proteins, differ between the two transmitters. Moreover, the extent by which presynaptic neuromodulators inhibit co-transmission also varied. Differences in modulation and the mechanisms controlling release allow for independent regulation of dopamine and GABA signals despite both being loaded via similar mechanisms.

Neuroscience and Actometry: an example of the benefits of the precise measurement of behavior

PURPOSE

Assess the impact the force-plate actometer, invented by Stephen C. Fowler, has had on behavioral neuroscience so far and what may be possible for future progress.

METHODS

The web service Scopus was queried on April 28, 2021 for articles that cited the Journal of Neuroscience Methods paper titled "A force-plate actometer for quantitating rodent behaviors: illustrative data on locomotion, rotation, spatial patterning, stereotypies, and tremor" resulting in 134 articles. Articles were coded by the author for type (e.g., research, review, book chapter), phenomenon (e.g., stress, addiction), intervention (e.g., pharmacological), and measure (e.g., distance traveled, tremor).

CONCLUSIONS

Of the 134 citations, 116 were research articles, 10 were review articles, 7 were book chapters and one was an advertisement. The force-plate actometer has been used to study a variety of phenomena and its measurement capabilities were expanded. While primarily used for rats and mice, other species have been used.

DOPA Homeostasis by Dopamine: A Control-Theoretic View

Dopamine (DA) is an important signal mediator in the brain as well as in the periphery. The term "dopamine homeostasis" occasionally found in the literature refers to the fact that abnormal DA levels can be associated with a variety of neuropsychiatric disorders. An analysis of the negative feedback inhibition of tyrosine hydroxylase (TH) by DA indicates, with support from the experimental data, that the TH-DA negative feedback loop has developed to exhibit 3,4-dihydroxyphenylalanine (DOPA) homeostasis by using DA as a derepression regulator. DA levels generally decline when DOPA is removed, for example, by increased oxidative stress. Robust DOPA regulation by DA further implies that maximum vesicular DA levels are established, which appear necessary for a reliable translation of neural activity into a corresponding chemical transmitter signal. An uncontrolled continuous rise (windup) in DA occurs when Levodopa treatment exceeds a critical dose. Increased oxidative stress leads to the successive breakdown of DOPA homeostasis and to a corresponding reduction in DA levels. To keep DOPA regulation robust, the vesicular DA loading requires close to zero-order kinetics combined with a sufficiently high compensatory flux provided by TH. The protection of DOPA and DA due to a channeling complex is discussed.

GAD67-mediated GABA Synthesis and Signaling Impinges on Directing Basket Cell Axonal Projections Toward Purkinje Cells in the Cerebellum

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system, synthesized by two isoforms of glutamate decarboxylase (GAD): GAD65 and GAD67. GABA may act as a trophic factor during brain development, but its contribution to the development and maturation of cerebellar neural circuits is not known. To understand the roles of GABA in cerebellar organization and associated functions in motor coordination and balance, we examined GAD65 conventional knock out (KO) mice and mice in which GAD67 was eliminated in parvalbumin-expressing neurons (PV-Cre; GAD67^flox/flox mice). We found aberrant subcellular localization of the Shaker-type K channel Kv1.1 in basket cell collaterals of PV-Cre; GAD67 ^flox/flox mice and abnormal projections from basket cells to Purkinje cells in both mouse strains. We also found that altered synaptic properties of basket cell terminals to Purkinje cells in PV-Cre; GAD67^flox/flox mice. Furthermore, PV-Cre; GAD67 ^flox/flox mice exhibited abnormal motor coordination in the rotarod test. These results indicate that GABA signaling in the cerebellum is critical for establishing appropriate connections between basket cells and Purkinje cells and is associated with motor coordination in mice.

Analysis of C. elegans acetylcholine synthesis mutants reveals a temperature-sensitive requirement for cholinergic neuromuscular function

In Caenorhabditis elegans, the cha-1 gene encodes choline acetyltransferase (ChAT), the enzyme that synthesizes the neurotransmitter acetylcholine. We have analyzed a large number of cha-1 hypomorphic mutants, most of which are missense alleles. Some homozygous cha-1 mutants have approximately normal ChAT immunoreactivity; many other alleles lead to consistent reductions in synaptic immunostaining, although the residual protein appears to be stable. Regardless of protein levels, neuromuscular function of almost all mutants is temperature sensitive, i.e., neuromuscular function is worse at 25° than at 14°. We show that the temperature effects are not related to acetylcholine release, but specifically to alterations in acetylcholine synthesis. This is not a temperature-dependent developmental phenotype, because animals raised at 20° to young adulthood and then shifted for 2 hours to either 14° or 25° had swimming and pharyngeal pumping rates similar to animals grown and assayed at either 14° or 25°, respectively. We also show that the temperature-sensitive phenotypes are not limited to missense alleles; rather, they are a property of most or all severe cha-1 hypomorphs. We suggest that our data are consistent with a model of ChAT protein physically, but not covalently, associated with synaptic vesicles; and there is a temperature-dependent equilibrium between vesicle-associated and cytoplasmic (i.e., soluble) ChAT. Presumably, in severe cha-1 hypomorphs, increasing the temperature would promote dissociation of some of the mutant ChAT protein from synaptic vesicles, thus removing the site of acetylcholine synthesis (ChAT) from the site of vesicular acetylcholine transport. This, in turn, would decrease the rate and extent of vesicle-filling, thus increasing the severity of the behavioral deficits.

Influence of energy deficiency on the subcellular processes of Substantia Nigra Pars Compacta cell for understanding Parkinsonian neurodegeneration

Parkinson's disease (PD) is the second most prominent neurodegenerative disease around the world. Although it is known that PD is caused by the loss of dopaminergic cells in substantia nigra pars compacta (SNc), the decisive cause of this inexorable cell loss is not clearly elucidated. We hypothesize that "Energy deficiency at a sub-cellular/cellular/systems level can be a common underlying cause for SNc cell loss in PD." Here, we propose a comprehensive computational model of SNc cell, which helps us to understand the pathophysiology of neurodegeneration at the subcellular level in PD. The aim of the study is to see how deficits in the supply of energy substrates (glucose and oxygen) lead to a deficit in adenosine triphosphate (ATP). The study also aims to show that deficits in ATP are the common factor underlying the molecular-level pathological changes, including alpha-synuclein aggregation, reactive oxygen species formation, calcium elevation, and dopamine dysfunction. The model suggests that hypoglycemia plays a more crucial role in leading to ATP deficits than hypoxia. We believe that the proposed model provides an integrated modeling framework to understand the neurodegenerative processes underlying PD.

Subcellular distribution of human tyrosine hydroxylase isoforms 1 and 4 in SH-SY5Y cells

Tyrosine hydroxylase (TH) is the key enzyme that controls the rate of synthesis of the catecholamines. SH-SY5Y cells with stable transfections of either human tyrosine hydroxylase isoform 1 (hTH1) or human tyrosine hydroxylase isoform 4 (hTH4) were used to determined the subcellular distribution of TH protein and phosphorylated TH, under basal conditions and after muscarine stimulation. Muscarine was previously shown to increase the phosphorylation of only serine 19 and serine 40 in hTH1 cells. Under basal conditions, the hTH1 and hTH4 proteins, their serine 19 phosphorylated forms and hTH1 phosphorylated at serine 40 were all similarly distributed; with ~80% in the cytosolic fraction, ~20% in the membrane fraction, and less than 1%, or not detectable, in the nuclear fraction. However, hTH4 phosphorylated at serine 71 had a significantly different distribution with ~65% cytosolic and ~35% membrane associated. Muscarine stimulation led to hTH1 being redistributed from the cytosol and nuclear fractions to the membrane fraction and hTH4 being redistributed from the cytosol to the nuclear fraction. These muscarine stimulated redistributions were not due to TH phosphorylation at serine 19, serine 40, or serine 71 and were most likely due to TH binding to proteins whose phosphorylation was increased by muscarine. This is the first study to show a difference in subcellular distribution between two human TH isoforms under basal and stimulated conditions.

Presynaptic regulation of dopamine release: Role of the DAT and VMAT2 transporters

The signaling dynamics of the neurotransmitter dopamine has been established to have an important role in a variety of behavioural processes including motor control, cognition, and emotional processing. Key regulators of transmitter release and the signaling dynamics of dopamine are the plasma membrane reuptake transporter (DAT) and the vesicular monoamine transporter (VMAT2). These proteins serve to remove dopamine molecules from the extracellular and cytosolic space, respectively and both determine the amount of transmitter released from synaptic vesicles. This review provides an overview of how these transporter proteins are involved in molecular regulation and function together to govern the dynamics of vesicular release with opposing effects on the quantal size and extracellular concentration of dopamine. These transporter proteins are both focal points of convergence for a variety of regulatory molecular cascades as well as targets for many pharmacological agents. The ratio between these transporters is argued to be useful as a molecular marker for delineating dopamine functional subsystems that may differ in transmitter release patterns.

The Molecular Chaperone Hsc70 Interacts with Tyrosine Hydroxylase to Regulate Enzyme Activity and Synaptic Vesicle Localization

We previously reported that the vesicular monoamine transporter 2 (VMAT2) is physically and functionally coupled with Hsc70 as well as with the dopamine synthesis enzymes tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase, providing a novel mechanism for dopamine homeostasis regulation. Here we expand those findings to demonstrate that Hsc70 physically and functionally interacts with TH to regulate the enzyme activity and synaptic vesicle targeting. Co-immunoprecipitation assays performed in brain tissue and heterologous cells demonstrated that Hsc70 interacts with TH and aromatic amino acid decarboxylase. Furthermore, in vitro binding assays showed that TH directly binds the substrate binding and carboxyl-terminal domains of Hsc70. Immunocytochemical studies indicated that Hsc70 and TH co-localize in midbrain dopaminergic neurons. The functional significance of the Hsc70-TH interaction was then investigated using TH activity assays. In both dopaminergic MN9D cells and mouse brain synaptic vesicles, purified Hsc70 facilitated an increase in TH activity. Neither the closely related protein Hsp70 nor the unrelated Hsp60 altered TH activity, confirming the specificity of the Hsc70 effect. Overexpression of Hsc70 in dopaminergic MN9D cells consistently resulted in increased TH activity whereas knockdown of Hsc70 by short hairpin RNA resulted in decreased TH activity and dopamine levels. Finally, in cells with reduced levels of Hsc70, the amount of TH associated with synaptic vesicles was decreased. This effect was rescued by addition of purified Hsc70. Together, these data demonstrate a novel interaction between Hsc70 and TH that regulates the activity and localization of the enzyme to synaptic vesicles, suggesting an important role for Hsc70 in dopamine homeostasis.

The N-terminal sequence of tyrosine hydroxylase is a conformationally versatile motif that binds 14-3-3 proteins and membranes

Tyrosine hydroxylase (TH) catalyzes the rate-limiting step in the synthesis of catecholamine neurotransmitters, and a reduction in TH activity is associated with several neurological diseases. Human TH is regulated, among other mechanisms, by Ser19-phosphorylation-dependent interaction with 14-3-3 proteins. The N-terminal sequence (residues 1-43), which corresponds to an extension to the TH regulatory domain, also interacts with negatively charged membranes. By using X-ray crystallography together with molecular dynamics simulations and structural bioinformatics analysis, we have probed the conformations of the Ser19-phosphorylated N-terminal peptide [THp-(1-43)] bound to 14-3-3γ, free in solution and bound to a phospholipid bilayer, and of the unphosphorylated peptide TH-(1-43) both free and bilayer bound. As seen in the crystal structure of THp-(1-43) complexed with 14-3-3γ, the region surrounding pSer19 adopts an extended conformation in the bound state, whereas THp-(1-43) adopts a bent conformation when free in solution, with higher content of secondary structure and higher number of internal hydrogen bonds. TH-(1-43) in solution presents the highest mobility and least defined structure of all forms studied, and it shows an energetically more favorable interaction with membranes relative to THp-(1-43). Cationic residues, notably Arg15 and Arg16, which are the recognition sites of the kinases phosphorylating at Ser19, are also contributing to the interaction with the membrane. Our results reveal the structural flexibility of this region of TH, in accordance with the functional versatility and conformational adaptation to different partners. Furthermore, this structural information has potential relevance for the development of therapeutics for neurodegenerative disorders, through modulation of TH-partner interactions.

The peripheral binding of 14-3-3γ to membranes involves isoform-specific histidine residues

Mammalian 14-3-3 protein scaffolds include seven conserved isoforms that bind numerous phosphorylated protein partners and regulate many cellular processes. Some 14-3-3-isoforms, notably γ, have elevated affinity for membranes, which might contribute to modulate the subcellular localization of the partners and substantiate the importance of investigating molecular mechanisms of membrane interaction. By applying surface plasmon resonance we here show that the binding to phospholipid bilayers is stimulated when 14-3-3γ is complexed with its partner, a peptide corresponding to the Ser19-phosphorylated N-terminal region of tyrosine hydroxylase. Moreover, membrane interaction is dependent on salts of kosmotropic ions, which also stabilize 14-3-3γ. Electrostatic analysis of available crystal structures of γ and of the non-membrane-binding ζ-isoform, complemented with molecular dynamics simulations, indicate that the electrostatic potential distribution of phosphopeptide-bound 14-3-3γ is optimal for interaction with the membrane through amphipathic helices at the N-terminal dimerization region. In addition, His158, and especially His195, both specific to 14-3-3γ and located at the convex lateral side, appeared to be pivotal for the ligand induced membrane interaction, as corroborated by site-directed mutagenesis. The participation of these histidine residues might be associated to their increased protonation upon membrane binding. Overall, these results reveal membrane-targeting motifs and give insights on mechanisms that furnish the 14-3-3γ scaffold with the capacity for tuned shuffling from soluble to membrane-bound states.

Catecholamines up integrates dopamine synthesis and synaptic trafficking

The highly reactive nature of dopamine renders dopaminergic neurons vulnerable to oxidative damage. We recently demonstrated that loss-of-function mutations in the Drosophila gene Catecholamines up (Catsup) elevate dopamine pools but, paradoxically, also confer resistance to paraquat, an herbicide that induces oxidative stress-mediated toxicity in dopaminergic neurons. We now report a novel association of the membrane protein, Catsup, with GTP cyclohydrolase rate-limiting enzyme for tetrahydrobiopterin (BH(4)) biosynthesis and tyrosine hydroxylase, rate-limiting enzyme for dopamine biosynthesis, which requires BH(4) as a cofactor. Loss-of-function Catsup mutations cause dominant hyperactivation of both enzymes. Elevated dopamine levels in Catsup mutants coincide with several distinct characteristics, including hypermobility, minimal basal levels of 3,4-dihydroxy-phenylacetic acid, an oxidative metabolite of dopamine, and resistance to the vesicular monoamine transporter inhibitor, reserpine, suggesting that excess dopamine is synaptically active and that Catsup functions in the regulation of synaptic vesicle loading and release of dopamine. We conclude that Catsup regulates and links the dopamine synthesis and transport networks.

A biochemical and functional protein complex involving dopamine synthesis and transport into synaptic vesicles

Synaptic transmission depends on neurotransmitter pools stored within vesicles that undergo regulated exocytosis. In the brain, the vesicular monoamine transporter-2 (VMAT(2)) is responsible for the loading of dopamine (DA) and other monoamines into synaptic vesicles. Prior to storage within vesicles, DA synthesis occurs at the synaptic terminal in a two-step enzymatic process. First, the rate-limiting enzyme tyrosine hydroxylase (TH) converts tyrosine to di-OH-phenylalanine. Aromatic amino acid decarboxylase (AADC) then converts di-OH-phenylalanine into DA. Here, we provide evidence that VMAT(2) physically and functionally interacts with the enzymes responsible for DA synthesis. In rat striata, TH and AADC co-immunoprecipitate with VMAT(2), whereas in PC 12 cells, TH co-immunoprecipitates with the closely related VMAT(1) and with overexpressed VMAT(2). GST pull-down assays further identified three cytosolic domains of VMAT(2) involved in the interaction with TH and AADC. Furthermore, in vitro binding assays demonstrated that TH directly interacts with VMAT(2). Additionally, using fractionation and immunoisolation approaches, we demonstrate that TH and AADC associate with VMAT(2)-containing synaptic vesicles from rat brain. These vesicles exhibited specific TH activity. Finally, the coupling between synthesis and transport of DA into vesicles was impaired in the presence of fragments involved in the VMAT(2)/TH/AADC interaction. Taken together, our results indicate that DA synthesis can occur at the synaptic vesicle membrane, where it is physically and functionally coupled to VMAT(2)-mediated transport into vesicles.

Three-way interaction between 14-3-3 proteins, the N-terminal region of tyrosine hydroxylase, and negatively charged membranes

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, is activated by phosphorylation-dependent binding to 14-3-3 proteins. The N-terminal domain of TH is also involved in interaction with lipid membranes. We investigated the binding of the N-terminal domain to its different partners, both in the unphosphorylated (TH-(1-43)) and Ser(19)-phosphorylated (THp-(1-43)) states by surface plasmon resonance. THp-(1-43) showed high affinity for 14-3-3 proteins (K(d) approximately 0.5 microM for 14-3-3gamma and -zeta and 7 microM for 14-3-3eta). The domains also bind to negatively charged membranes with intermediate affinity (concentration at half-maximal binding S(0.5) = 25-58 microM (TH-(1-43)) and S(0.5) = 135-475 microM (THp-(1-43)), depending on phospholipid composition) and concomitant formation of helical structure. 14-3-3gamma showed a preferential binding to membranes, compared with 14-3-3zeta, both in chromaffin granules and with liposomes at neutral pH. The affinity of 14-3-3gamma for negatively charged membranes (S(0.5) = 1-9 microM) is much higher than the affinity of TH for the same membranes, compatible with the formation of a ternary complex between Ser(19)-phosphorylated TH, 14-3-3gamma, and membranes. Our results shed light on interaction mechanisms that might be relevant for the modulation of the distribution of TH in the cytoplasm and membrane fractions and regulation of L-DOPA and dopamine synthesis.

Direct binding of GTP cyclohydrolase and tyrosine hydroxylase: regulatory interactions between key enzymes in dopamine biosynthesis

The signaling functions of dopamine require a finely tuned regulatory network for rapid induction and suppression of output. A key target of regulation is the enzyme tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, which is activated by phosphorylation and modulated by the availability of its cofactor, tetrahydrobiopterin. The first enzyme in the cofactor synthesis pathway, GTP cyclohydrolase I, is activated by phosphorylation and inhibited by tetrahydrobiopterin. We previously reported that deficits in GTP cyclohydrolase activity in Drosophila heterozygous for mutant alleles of the gene encoding this enzyme led to tightly corresponding diminution of in vivo tyrosine hydroxylase activity that could not be rescued by exogenous cofactor. We also found that the two enzymes could be coimmunoprecipitated from tissue extracts and proposed functional interactions between the enzymes that extended beyond provision of cofactor by one pathway for another. Here, we confirm the physical association of these enzymes, identifying interacting regions in both, and we demonstrate that their association can be regulated by phosphorylation. The functional consequences of the interaction include an increase in GTP cyclohydrolase activity, with concomitant protection from end-product feedback inhibition. In vivo, this effect would in turn provide sufficient cofactor when demand for catecholamine synthesis is greatest. The activity of tyrosine hydroxylase is also increased by this interaction, in excess of the stimulation resulting from phosphorylation alone. Vmax is elevated, with no change in Km. These results demonstrate that these enzymes engage in mutual positive regulation.

Identification of major classes of cholinergic neurons in the nematode Caenorhabditis elegans

The neurotransmitter acetylcholine (ACh) is specifically synthesized by the enzyme choline acetyltransferase (ChAT). Subsequently, it is loaded into synaptic vesicles by a specific vesicular acetylcholine transporter (VAChT). We have generated antibodies that recognize ChAT or VAChT in a model organism, the nematode Caenorhabditis elegans, in order to examine the subcellular and cellular distributions of these cholinergic proteins. ChAT and VAChT are found in the same neurons, including more than one-third of the 302 total neurons present in the adult hermaphrodite. VAChT is found in synaptic regions, whereas ChAT appears to exist in two forms in neurons, a synapse-enriched form and a more evenly distributed possibly cytosolic form. We have used antibodies to identify the cholinergic neurons in the body of larval and adult hermaphrodites. All of the classes of putative excitatory motor neurons in the ventral nerve cord appear to be cholinergic: the DA and DB neurons in the first larval stage and the AS, DA, DB, VA, VB, and VC neurons in the adult. In addition, several interneurons with somas in the tail and processes in the tail or body are cholinergic; sensory neurons are generally not cholinergic. Description of the normal pattern of cholinergic proteins and neurons will improve our understanding of the role of cholinergic neurons in the behavior and development of this model organism.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA-) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 microM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA- lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA- had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 microM) was administered in perfusate, LNAA- still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.