Obesity Drug Update: The Lost Decade?

The growing worldwide obesity epidemic and obesity-related disorders present a huge unmet medical need for safe and effective anti-obesity medications. The discovery of leptin in 1994 was rapidly succeeded by a wave of related discoveries leading to the elaboration of a hypothalamic melanocortinergic neuronal circuit regulated by leptin and other central and peripheral signaling molecules to control energy homeostasis. The identification of specific neuronal subtypes along with their unique connections and expression products generated a rich target menu for anti-obesity drug discovery programs. Over the course of the last decade, several new chemical entities aimed at these targets have reached various stages or successfully completed the drug discovery/regulatory process only to be dropped or taken off the market. There are now in fact fewer options for anti-obesity drug therapies in late 2010 than were available in 2000. The challenge to discover safe and effective anti-obesity drugs is alive and well

A Tet-on system for DRD1-expressing cells.

Cells expressing the dopamine D1 receptor (DRD1) have significant functional roles in diverse physiological processes including locomotion and drug addiction. The present work presents a novel in vivo DRD1-Bacterial Artificial Chromosome (BAC) Tet-on system allowing for the inducible activation of tet-operated transgenes specifically within DRD1-expressing cells of transgenic mice. It is shown that the DRD1-rtTA BAC-driven expression of a tet-operated reporter is under tight regulation by doxycycline and is restricted to DRD1-expressing brain regions. The model will be a useful research tool in studies of movement and reward and associated pathologies such as Parkinson's disease and addiction

Immunostaining of β-galactosidase in bi-transgenic mice labels DRD1-expressing cells and neuronal processes in DRD1-expressing brain regions.

<p>(a) Coronal section of labeled cells in striatum at 10X. Inset is at 4X. (b) Intense labeling of cells in the core and shell regions of nucleus accumbens at 10X. (c) The arborization of a single medium spiny neuron in striatum at 40X. (d) Cells (4X) and cells showing processes (40X, inset) are labeled in amygdala. (e) Retinal bipolar cells with projections to inner plexiform layer and sporadic ganglion cells are labeled. (f) Expression in deep cortical layers V and VI (10X) and (g) in the granule cell layer of the olfactory bulb (10X). (h) Sporadic labeling of hippocampal cells in pyramidal, CA3 and molecular layers (4X). Abbr: Amyg, amygdala; CA3, CA3 field of the hippocampus; Gl, glomerular layer; GP, globus pallidus; GR, granule cell layer; Ml, molecular layer, NAc, nucleus accumbens; Pir, piriform cortex, Pl, pyramidal layer; Str, striatum.</p

<i>Drd1a</i>-rtTA BAC/tetO-<i>lacZ</i> bi-transgenic mice express β-galactosidase after 1, 2 or 4 weeks of DOX treatment.

<p>Coronal sections through the forebrain show strong X-gal staining within 1 week of DOX treatment. X-gal staining in the striatum is most pronounced in the dorso-medial region of this structure. Images were taken using a 2.5X objective.</p

Nigrostriatal and striatonigral pathways.

<p>β-galactosidase immunostaining of sagittal sections from <i>Drd1a</i>-rtTA BAC/tetO-<i>lacZ</i> bi-transgenic mice on 4 weeks of DOX labeled the entire direct striatonigral pathway (green). The dopaminergic nigrostriatal pathway (red) was labeled by immunostaining for TH. Abbr: Str, striatum, NAc, nucleus accumbens, OT, olfactory tubercle, ac, anterior commissure, MFB, medial forebrain bundle, SNc, substantia nigra pars compacta, SNr, substantia nigra pars reticulata.</p

Expression of β-galactosidase is greatly reduced following a 2-week washout of DOX.

<p>Coronal sections of the dorso-medial region of striatum imaged at 40X following (a) 2-week DOX treatment or (b) 2-week DOX treatment followed by a 2-week period without DOX.</p

<i>Drd1a</i>-rtTA BAC TET-On system.

<p>(a) The <i>Drd1a</i> BAC (RP23-47M2) was modified by bacterial homologous recombination by insertion of the coding sequence for rtTA at the ATG start site for <i>Drd1a</i>. (b) The modified BAC makes rtTA in DRD1-expressing cells in transgenic mice. Arrows labeled F and R represent forward and reverse PCR primers for genotyping. (c) rtTA, in the presence of DOX, activates the tet operator to transcribe a tet-operated transgene.</p