Stefan Herlitze

Evolving the lock to fit the key to create a family of G protein-coupled receptors potently activated by an inert ligand

We evolved muscarinic receptors in yeast to generate a family of G protein-coupled receptors (GPCRs) that are activated solely by a pharmacologically inert drug-like and bioavailable compound (clozapine-N-oxide). Subsequent screening in human cell lines facilitated the creation of a family of muscarinic acetylcholine GPCRs suitable for in vitro and in situ studies. We subsequently created lines of telomerase-immortalized human pulmonary artery smooth muscle cells stably expressing all five family members and found that each one faithfully recapitulated the signaling phenotype of the parent receptor. We also expressed a Gi-coupled designer receptor in hippocampal neurons (hM4D) and demonstrated its ability to induce membrane hyperpolarization and neuronal silencing. We have thus devised a facile approach for designing families of GPCRs with engineered ligand specificities. Such reverse-engineered GPCRs will prove to be powerful tools for selectively modulating signal-transduction pathways in vitro and in vivo.

Modulation of Ca2+ channels by G-protein beta gamma subunits.

CALCIUM ions entering cells through voltage-gated Ca2+ channels initiate rapid release of neurotransmitters and secretion of hormones. Ca2+ currents can be inhibited in many cell types by neurotransmitters acting through G proteins via a membrane-delimited pathway independently of soluble intracellular messengers1–4. Inhibition is typically caused by a positive shift in the voltage dependence and a slowing of channel activation and is relieved by strong depolarization resulting in facilitation of Ca2+currents1,4–6. This pathway regulates the activity of N-type and P/ Q-type Ca2+ channels1,2,7, which are localized in presynaptic terminals8,9 and participate in neurotransmitter release10–13. Synaptic transmission is inhibited by neurotransmitters through this mechanism1,4. G-protein a subunits confer specificity in receptor coupling1–4,14–17, but it is not known whether the Gα or Gβγ subunits are responsible for modulation of Ca2+channels. Here we report that Gβγ subunits can modulate Ca2+ channels. Transfection of Gβγ into cells expressing P/Q-type Ca2+ channels induces modulation like that caused by activation of G protein-coupled receptors, but Gα subunits do not. Similarly, injection or expression of Gβγ subunits in sympathetic ganglion neurons induces facilitation and occludes modulation of N-type channels by noradrenaline, but Gα subunits do not. In both cases, the Gγ subunit is ineffective by itself, but overexpression of exogenous Gβ subunits is sufficient to cause channel modulation.

Argiotoxin detects molecular differences in AMPA receptor channels

Argiotoxin, a component of the spider venom from Argiope lobata, blocks AMPA receptor channels expressed in homomeric and heteromeric configuration in Xenopus oocytes. Argiotoxin acts as an open channel blocker in a voltage-dependent manner and discriminates between the functionally diverse AMPA receptors. Importantly, a transmembrane region 2 determinant for divalent cation permeability also determines argiotoxin sensitivity. Subunit-specific differences in the time courses of block and recovery demonstrate that heteromeric AMPA receptors can assemble in variable ratios. Thus, argiotoxin can be used as a tool in analyzing the subunit composition of AMPA receptors in native membranes.

Location of a threonine residue in the α−subunit M2 transmembrane segment that determines the ion flow through the acetylcholine receptor channel

By the combination of cDNA manipulation and functional analysis of normal and mutant acetylcholine receptor (AChR) channels of Torpedo expressed in Xenopus laevis oocytes determinants of ion flow were localized in the bends bordering the putative M2 transmembrane segment (Imoto et al. 1988). We now report that in the rat muscle AChR, substitution of a threonine residue in the α-subunit localized in the M2 transmembrane segment increases or decreases the channel conductance, depending on the size of the amino acid side chain located at this position. This threonine residue (αT264) is located adjacent to the cluster of charged amino acids that form the intermediate anionic ring (Imoto et al. 1988). This effect is pronounced for the large alkali cations Cs+, Rb+, K+ whereas for Na+ the effect is much smaller. Taken together the results suggest that the threonine residues at position 264 in the two α-subunits together with the amino acids of the intermediate anionic ring form part of a narrow region close to the cytoplasmic mouth of the AChR channel.

Light-Induced Rescue of Breathing after Spinal Cord Injury

Paralysis is a major consequence of spinal cord injury (SCI). After cervical SCI, respiratory deficits can result through interruption of descending presynaptic inputs to respiratory motor neurons in the spinal cord. Expression of channelrhodopsin-2 (ChR2) and photostimulation in neurons affects neuronal excitability and produces action potentials without any kind of presynaptic inputs. We hypothesized that after transducing spinal neurons in and around the phrenic motor pool to express ChR2, photostimulation would restore respiratory motor function in cervical SCI adult animals. Here we show that light activation of ChR2-expressing animals was sufficient to bring about recovery of respiratory diaphragmatic motor activity. Furthermore, robust rhythmic activity persisted long after photostimulation had ceased. This recovery was accomplished through a form of respiratory plasticity and spinal adaptation which is NMDA receptor dependent. These data suggest a novel, minimally invasive therapeutic avenue to exercise denervated circuitry and/or restore motor function after SCI.

Pet-1 is required across different stages of life to regulate serotonergic function.

Transcriptional cascades are required for the specification of serotonin (5-HT) neurons and behaviors modulated by 5-HT. Several cascade factors are expressed throughout the lifespan, which suggests that their control of behavior might not be temporally restricted to programming normal numbers of 5-HT neurons. We used new mouse conditional targeting approaches to investigate the ongoing requirements for Pet-1 (also called Fev), a cascade factor that is required for the initiation of 5-HT synthesis, but whose expression persists into adulthood. We found that Pet-1 was required after the generation of 5-HT neurons for multiple steps in 5-HT neuron maturation, including axonal innervation of the somatosensory cortex, expression of appropriate firing properties, and the expression of the Htr1a and Htr1b autoreceptors. Pet-1 was still required in adult 5-HT neurons to preserve normal anxiety-related behaviors through direct autoregulated control of serotonergic gene expression. These findings indicate that Pet-1 is required across the lifespan of the mouse and that behavioral pathogenesis can result from both developmental and adult-onset alterations in serotonergic transcription.

A general and rapid mutagenesis method using polymerase chain reaction.

The construction of deletions, insertions and point mutations in DNA sequences is a powerful approach to analysing the function and structure of genes and their products. Here, we present a fast and efficient method using the polymerase chain reaction to introduce mutations into cDNAs coding for the alpha-, gamma- and epsilon-subunit of the rat muscle acetylcholine receptor. Two flanking primers and one mutant oligo, in conjunction with supercoiled plasmid DNA and a fragment of the target DNA are sufficient to introduce the mutation by two PCR amplifications. Our method permits directing the location of mutations anywhere in the target gene with a very low misincorporation rate, as no substitution could be detected within 9600 bp. The utility of this approach is demonstrated by the rapid introduction and analysis of eleven mutations into three different cDNAs. Any kind of mutation can be introduced with an efficiency of at least 50%.

Substitution of 5-HT1A receptor signaling by a light-activated G protein-coupled receptor.

Understanding serotonergic (5-HT) signaling is critical for understanding human physiology, behavior, and neuropsychiatric disease. 5-HT mediates its actions via ionotropic and metabotropic 5-HT receptors. The 5-HT1A receptor is a metabotropic G protein-coupled receptor linked to the Gi/o signaling pathway and has been specifically implicated in the pathogenesis of depression and anxiety. To understand and precisely control 5-HT1A signaling, we created a light-activated G protein-coupled receptor that targets into 5-HT1A receptor domains and substitutes for endogenous 5-HT1A receptors. To induce 5-HT1A-like targeting, vertebrate rhodopsin was tagged with the C-terminal domain (CT) of 5-HT1A (Rh-CT5-HT1A). Rh-CT5-HT1A activates G protein-coupled inward rectifying K+ channels in response to light and causes membrane hyperpolarization in hippocampal neurons, similar to the agonist-induced responses of the 5-HT1A receptor. The intracellular distribution of Rh-CT5-HT1A resembles that of the 5-HT1A receptor; Rh-CT5-HT1A localizes to somatodendritic sites and is efficiently trafficked to distal dendritic processes. Additionally, neuronal expression of Rh-CT5-HT1A, but not Rh, decreases 5-HT1A agonist sensitivity, suggesting that Rh-CT5-HT1A and 5-HT1A receptors compete to interact with the same trafficking machinery. Finally, Rh-CT5-HT1A is able to rescue 5-HT1A signaling of 5-HT1A KO mice in cultured neurons and in slices of the dorsal raphe showing that Rh-CT5-HT1A is able to functionally compensate for native 5-HT1A. Thus, as an optogenetic tool, Rh-CT5-HT1A has the potential to directly correlate in vivo 5-HT1A signaling with 5-HT neuron activity and behavior in both normal animals and animal models of neuropsychiatric disease.

New optical tools for controlling neuronal activity.

A major challenge in understanding the relationship between neural activity and development, and ultimately behavior, is to control simultaneously the activity of either many neurons belonging to specific subsets or specific regions within individual neurons. Optimally, such a technique should be capable of both switching nerve cells on and off within milliseconds in a non-invasive manner, and inducing depolarizations or hyperpolarizations for periods lasting from milliseconds to many seconds. Specific ion conductances in subcellular compartments must also be controlled to bypass signaling cascades in order to regulate precisely cellular events such as synaptic transmission. Light-activated G-protein-coupled receptors and ion channels, which can be genetically manipulated and targeted to neuronal circuits, have the greatest potential to fulfill these requirements.

RGS2 Determines Short-Term Synaptic Plasticity in Hippocampal Neurons by Regulating Gi/o- Mediated Inhibition of Presynaptic Ca2+ Channels

RGS2, one of the small members of the regulator of G protein signaling (RGS) family, is highly expressed in brain and regulates G(i/o) as well as G(q)-coupled receptor pathways. RGS2 modulates anxiety, aggression, and blood pressure in mice, suggesting that RGS2 regulates synaptic circuits underlying animal physiology and behavior. How RGS2 in brain influences synaptic activity is unknown. We therefore analyzed the synaptic function of RGS2 in hippocampal neurons by comparing electrophysiological recordings from RGS2 knockout and wild-type mice. Our study provides a general mechanism of the action of the RGS family containing RGS2 by demonstrating that RGS2 increases synaptic vesicle release by downregulating the G(i/o)-mediated presynaptic Ca(2+) channel inhibition and therefore provides an explanation of how regulation of RGS2 expression can modulate the function of neuronal circuits underlying behavior.