Biomedical subjects

Marten P Smidt

Publications and source records attributed to Marten P Smidt.

At least 19 recordsLinked to original sources

PubMed2007

How to make a mesodiencephalic dopaminergic neuron.

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

Animals↗

PubMed2006

Molecular programming of stem cells into mesodiencephalic dopaminergic neurons.

In a screen for homeobox transcription factors expressed in the embryonic ventral midbrain, Andersson et al. recently identified Lmx1a and Msx1. Using in ovo electroporation in chick embryos, they showed that these factors are crucial for initiating the differentiation of neuroepithelial progenitor neurons into mesodiencephalic dopaminergic (mdDA) neurons. Lmx1a also initiated a developmental program that drove an mdDA phenotype in mouse embryonic stem cells. This indicates that these factors can be exploited in cell-replacement strategies for treatment of Parkinson's disease.

Animals↗

PubMed2006

Slow progressive degeneration of nigral dopaminergic neurons in postnatal Engrailed mutant mice.

The homeobox transcription factors Engrailed-1 and Engrailed-2 are required for the survival of mesencephalic dopaminergic neurons in a cell-autonomous and gene-dose-dependent manner. Because of this requirement, the cells die by apoptosis when all four alleles of the Engrailed genes are genetically ablated (En1-/-;En2-/-). In the present study, we show that viable and fertile mice, heterozygous null for Engrailed-1 and homozygous null for Engrailed-2 (En1+/-;En2-/-), have an adult phenotype that resembles key pathological features of Parkinson's disease. Specifically, postnatal mutant mice exhibit a progressive degeneration of dopaminergic neurons in the substantia nigra during the first 3 mo of their lives, leading to diminished storage and release of dopamine in the caudate putamen, motor deficits similar to akinesia and bradykinesia, and a lower body weight. This genetic model may provide access to the molecular etiology for Parkinson's disease and could assist in the development of novel treatments for this neurodegenerative disorder.

Animals↗

PubMed2006

Insulin signaling in the central nervous system: learning to survive.

Insulin is best known for its role in peripheral glucose homeostasis. Less studied, but not less important, is its role in the central nervous system. Insulin and its receptor are located in the central nervous system and are both implicated in neuronal survival and synaptic plasticity. Interestingly, over the past few years it has become evident that the effects of insulin, on neuronal survival and synaptic plasticity, are mediated by a common signal transduction cascade, which has been identified as "the PI3K route". This route has turned out to be a major integrator of insulin signaling in the brain. A pronounced feature of this insulin-activated route is that it promotes survival by directly inactivating the pro-apoptotic machinery. Interestingly, it is this same route that is required for the induction of long-term potentiation and depression, basic processes underlying learning and memory. This leads to the hypothesis that the PI3K route forms a direct link between learning and memory and neuronal survival. The implications of this hypothesis are far reaching, since it provides an explanation why insulin has beneficial effects on learning and memory and how synaptic activity can prevent cellular degeneration. Applying this knowledge may provide novel therapeutic approaches in the treatment of neurodegenerative diseases such as Alzheimer's disease.

Animals↗

PubMed2006

In control of biology: of mice, men and Foxes.

Forkhead proteins comprise a highly conserved family of transcription factors, named after the original forkhead gene in Drosophila. To date, over 100 forkhead genes have been identified in a large variety of species, all sharing the evolutionary conserved 'forkhead' DNA-binding domain, and the cloning and characterization of forkhead genes have continued in recent years. Forkhead transcription factors regulate the expression of countless genes downstream of important signalling pathways in most, if not all, tissues and cell types. Recent work has provided novel insights into the mechanisms that contribute to their functional diversity, including functional protein domains and interactions of forkheads with other transcription factors. Studies using loss- and gain-of-function models have elucidated the role of forkhead factors in developmental biology and cellular functions such as metabolism, cell division and cell survival. The importance of forkhead transcription factors is underlined by the developmental defects observed in mutant model organisms, and multiple human disorders and cancers which can be attributed to mutations within members of the forkhead gene family. This review provides a comprehensive overview of current knowledge on forkhead transcription factors, from structural organization and regulatory mechanisms to cellular and developmental functions in mice and humans. Finally, we will discuss how novel insights gained from involvement of 'Foxes' in the mechanisms underlying human pathology may create new opportunities for treatment strategies.

Amino Acid Sequence↗

PubMed2006

Pitx3 deficiency in mice affects cholinergic modulation of GABAergic synapses in the nucleus accumbens.

We investigated to what extent Pitx3 deficiency, causing hyperdopaminergic transmission in the nucleus accumbens microcircuitry, may lead to developmental changes. First, spontaneous firing activity of cholinergic interneurons in the nucleus accumbens was recorded in vitro. Firing patterns in the Pitx3-deficient mice were more variable and intrinsically different from those observed in wild-type mice. Next, to test whether the irregular firing patterns observed in mutant mice affected the endogenous nicotinic modulation of the GABAergic input of medium spiny neurons, we recorded spontaneous GABAergic inputs to these cells before and after the application of the nicotinic receptor blocker mecamylamine. Effects of mecamylamine were found in slices of either genotype, but in a rather inconsistent manner. Possibly this was attributable to heterogeneity in firing of nearby cholinergic interneurons. Thus paired recordings of cholinergic interneurons and medium spiny neurons were performed to more precisely control the experimental conditions of the cholinergic modulation of GABAergic synaptic transmission. We found that controlling action potential firing in cholinergic neurons leads to a conditional increase in GABAergic input frequency in wild-type mice but not in Pitx3-deficient mice. We conclude that Pitx3-deficient mice have neural adaptations at the level of the nucleus accumbens microcircuitry that in turn may have behavioral consequences. It is discussed to what extent dopamine release in the nucleus accumbens may be a long-term gating mechanism leading to alterations in cholinergic transmission in the nucleus accumbens, in line with previously reported neural adaptations found as consequences of repeated drug treatment in rodents.

Action Potentials↗

PubMed2006

Developmental origin and fate of meso-diencephalic dopamine neurons.

Specific vulnerability of substantia nigra compacta neurons as compared to ventral tegmental area neurons, as emphasized in Parkinson's disease, has been studied for many years and is still not well understood. The molecular codes and mechanisms that drive development of these structures have recently been studied through the use of elegant genetic ablation experiments. The data suggested that specific genes at specific anatomical positions in the ventricular zone are crucial to drive development of young neurons into the direction of the dopaminergic phenotype. In addition, it has become clear the these dopaminergic neurons are present in the diencephalon and in the mesencephalon and that they may contain a specific molecular signature that defines specific subsets in terms of position and function. The data indicate that these specific subsets may explain the specific response of these neurons to toxins and genetic ablation.

Amino Acid Sequence↗

PubMed2005

Identification of forkhead transcription factors in cortical and dopaminergic areas of the adult murine brain.

The murine forkhead family of transcription factors consists of over 30 members, the vast majority of which is important in embryonic development. Implicated in processes such as proliferation, differentiation and survival, forkhead factors show highly restricted expression patterns. In search for forkhead genes expressed in specific neural systems, we identified multiple family members. We performed a detailed expression analysis for Foxj2, Foxk1 and the murine orthologue of the human ILF1 gene, which show a remarkable preference for complex cortical structures. In addition, a comprehensive examination of forkhead gene expression in dopamine neurons of the ventral tegmental area and substantia nigra pars compacta, revealed Ilf1 as a novel transcriptional regulator in midbrain dopamine neurons. These forkhead transcription factors may play a role in maintenance and survival of developing and adult neurons.

Aging↗

PubMed2005

Spatial and temporal expression of FoxO transcription factors in the developing and adult murine brain.

In order to obtain leads to molecular mechanisms of signal transduction pathways and controlled gene expression in neuronal development we have screened the adult mouse brain for expressed forkhead transcription factors using a degenerate RT-PCR approach. Here, we focus on three FoxO genes found to be expressed in the brain: FoxO1, FoxO3 and FoxO6. The FoxO subfamily of forkhead transcription family is emerging as a central keypoint in an array of cellular functions, such as metabolism, differentiation and transformation. In situ hybridization experiments on adult and embryonic mouse brain showed differential expression patterns for three FoxO members. FoxO1 was strongly expressed in the striatum and neuronal subsets of the hippocampus (dentate gyrus and the ventral/posterior part of the CA regions), whereas FoxO3 was more diffusely expressed throughout the brain including all hippocampal areas, cortex and cerebellum. FoxO6 expression was eminent in various parts of the adult mouse brain, including the entire hippocampus, the amygdalohippocampal area and the shell of the nucleus accumbens. Remarkably, all three FoxO transcription factors were expressed relatively late in the developing murine brain, starting between E12.5 and E14. In summary, the presented data show FoxO factors to be expressed in the adult and developing mouse brain, in a spatially end temporally restricted manner.

Animals↗

PubMed2005

FoxO6 transcriptional activity is regulated by Thr26 and Ser184, independent of nucleo-cytoplasmic shuttling.

Forkhead members of the 'O' class (FoxO) are transcription factors crucial for the regulation of metabolism, cell cycle, cell death and cell survival. FoxO factors are regulated by insulin-mediated activation of PI3K (phosphoinositide 3-kinase)-PKB (protein kinase B) signalling. Activation of PI3K-PKB signalling results in the phosphorylation of FoxO factors on three conserved phosphorylation motifs, which are essential for the translocation of FoxO factors from the nucleus to the cytosol. FoxO6, however, remains mostly nuclear due to the fact that its shuttling ability is dramatically impaired. FoxO1, FoxO3 and FoxO4 all contain an N- and C-terminal PKB motif and a motif located in the forkhead domain. FoxO6 lacks the conserved C-terminal PKB motif, which is the cause of the shuttling impairment. Since FoxO6 can be considered constitutively nuclear, we investigated whether it is also a constitutively active transcription factor. Our results show that FoxO6 transcriptional activity is inhibited by growth factors, independent of shuttling, indicating that it is not constitutively active. The PKB site in the forkhead domain (Ser184) regulated the DNA binding characteristics and the N-terminal PKB site acted as a growth factor sensor. In summary, FoxO6 is not a constitutively active transcription factor and can be regulated by growth factors in a Thr26- and Ser184-dependent manner, independent of shuttling to the cytosol.

Amino Acid Motifs↗

PubMed2005

Cloning and analysis of the murine Foxi2 transcription factor.

Forkhead transcription factors comprise a large family of key regulators of embryonic development. Here, we describe the cloning and analysis of the murine Foxi2 gene, coding for a putative 311 amino acid protein resembling Foxi subfamily members in mice and other species. Expression analysis during the final stages of embryonic development revealed that Foxi2 expression is mainly confined to subsets of cells in epithelial structures and particular ducts, in addition to the developing forebrain and neural retina. Since FoxI factors are thought to be implicated in the regulation of cell fate, the highly restricted expression pattern of Foxi2 suggestive of a possible role in controlling cellular identity.

Amino Acid Sequence↗

PubMed2005

Molecular and cellular alterations in the Pitx3-deficient midbrain dopaminergic system.

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra compacta (SNc). In order to provide insights into adaptive mechanisms of the mDA system in pathology, specific molecular and cellular parameters of the mDA system were studied in Pitx3-deficient Aphakia (ak) mice, which suffer from severe developmental failure of SNc mDA neurons. Here, we demonstrate differential changes in striatal gene expression, reflecting the specific neuronal loss in these mice. In addition, the neuronal activity of remaining mDA neurons in the ventral tegmental area (VTA) was significantly increased in ak mice. In conclusion, ak mice display specific molecular and cellular alterations in the mDA system that provide new insights in compensatory mechanisms present in mDA-associated disorders such as PD.

Adaptation, Physiological↗

PubMed2005

Regulation of FoxO activity by CBP/p300-mediated acetylation.

Forkhead box, class O (FoxO) transcription factors are inhibited by insulin-induced FoxO phosphorylation. Recently, acetylation of FoxO factors by calcium response element-binding (CREB)-binding protein (CBP) and/or p300 has been identified as a novel regulatory pathway, although the exact consequences of acetylation remain unclear. We propose that binding of CBP/p300 to FoxO factors is essential for FoxO-mediated transcription. CBP and p300 act as FoxO cofactors by weakening histone-DNA interactions. Acetylation of FoxO factors, however, attenuates FoxO-mediated transcriptional activity by disrupting the interaction between FoxO factors and target DNA. Therefore, acetylation shifts the function of FoxO from cell-cycle arrest and protection against oxidative stress towards cell death.

Acetylation↗

PubMed2005

U-SPECT-I: a novel system for submillimeter-resolution tomography with radiolabeled molecules in mice.

UNLABELLED: A major advance in biomedical science and diagnosis was accomplished with the development of in vivo techniques to image radiolabeled molecules, but limited spatial resolution has slowed down applications to small experimental animals. Here, we present a SPECT system (U-SPECT-I) dedicated to radionuclide imaging of murine organs at a submillimeter resolution. METHODS: The high performance of U-SPECT-I is based on a static triangular detector setup, with a cylindric imaging cavity in the center and 75 gold micropinhole apertures in the cavity wall. The pinholes are focused on a small volume of interest such as the mouse heart or spine to maximize the detection yield of gamma-photons. Three-dimensional molecular distributions are iteratively estimated using the detector data and a statistical reconstruction algorithm that takes into account system blurring and data noise to increase resolution and reduce image noise. RESULTS: With 0.6-mm-diameter pinholes, the maximum fraction of detected photons emitted by a point source (peak sensitivity) is 0.22% for a 15%-wide energy window and remains higher than 0.12% in the central 12 mm of the central plane. In a resolution phantom, radioactively filled capillaries as small as 0.5 mm and separated by 0.5 mm can be distinguished clearly in reconstructions. Projection data needed for the reconstruction of cross sections of molecular distributions in mouse organs can readily be obtained without the need for any mechanical movements. Images of a mouse spine show 99mTc-hydroxymethylene diphosphonate uptake down to the level of tiny parts of vertebral processes. These are separated clearly from the vertebral and intervertebral foramina. Using another tracer, one can monitor myocardial perfusion in the left and right ventricular walls, even in structures as small as the papillary muscles. CONCLUSION: U-SPECT-I allows discrimination between molecular concentrations in adjacent volumes of as small as about 0.1 muL, which is significantly smaller than can be imaged by any existing SPECT or PET system. Our initial in vivo images of the mouse heart and spine show that U-SPECT-I can be used for novel applications in the study of dynamic biologic systems with a clear projection to clinical applications. The combination of high resolution and detection efficiency of U-SPECT-I opens up new possibilities for the suborgan-level study of radiotracers in mouse models.

Animals↗

PubMed2004

Homeobox gene Pitx3 and its role in the development of dopamine neurons of the substantia nigra.

The homeobox gene Pitx3 plays an important part in the development and function of vertebrate midbrain dopaminergic neurons. Re-localization of the genetic defect in the mouse mutant aphakia to the Pitx3 locus, together with the subsequent identification of two deletions causing the gene to be silent, has been the hallmark of several studies into the role of Pitx3. In this review, we summarize the data and reflect on the role of Pitx3 in the development of dopamine neurons in the midbrain. The data indicate that Pitx3 is essential for the survival of dopamine neurons located in the substantia nigra compacta during development. Molecular analysis of the underlying mechanisms might provide new insights for understanding the selective degeneration observed in Parkinson patients.

Animals↗

PubMed2004

Species differences in brain pre-pro-neurotensin/neuromedin N mRNA distribution: the expression pattern in mice resembles more closely that of primates than rats.

We have used in situ hybridization to determine the distribution of pre-pro-neurotensin/neuromedin N (NT/N) mRNA in the brain of mice and rats. In rats and mice, expression was observed in the lateral septal nucleus, nucleus accumbens, medial preoptic area, bed nucleus of the stria terminalis, lateral hypothalamus, central amygdaloid nucleus and the subicilum. However, several differences in the NT/N mRNA distribution were observed between rats and mice in other brain areas. In mice, NT/N expression was detected in the subthalamic nucleus and geniculate nucleus, whereas expression was not observed in these brain areas in rats. Surprisingly, expression was not observed in mouse mesencephalic dopaminergic (mesDA) neurons and the CA1 area of the hippocampus, areas known to contain NT/N mRNA in the rat brain. Taken together, these results show that although the brain NT/N mRNA distribution largely overlaps in mice and rats, species differences exist in specific brain areas in rodents. Moreover, these data indicate that the distribution in mice resembles most that of primates than rats.

Animals↗

PubMed2004

The ins and outs of FoxO shuttling: mechanisms of FoxO translocation and transcriptional regulation.

FoxO (forkhead box O; forkhead members of the O class) are transcription factors that function under the control of insulin/insulin-like signalling. FoxO factors have been associated with a multitude of biological processes, including cell-cycle, cell death, DNA repair, metabolism and protection from oxidative stress. Central to the regulation of FoxO factors is a shuttling system, which confines FoxO factors to either the nucleus or the cytosol. Shuttling of FoxO requires protein phosphorylation within several domains, and association with 14-3-3 proteins and the nuclear transport machinery. Description of the FoxO-shuttling mechanism contributes to the understanding of FoxO function in relation to signalling and gene regulation.

Amino Acid Sequence↗

PubMed2004

The neuregulin receptor, ErbB4, is not required for normal development and adult maintenance of the substantia nigra pars compacta.

Degeneration of dopaminergic neurons in the substantia nigra is associated with one of the most prominent human neurological disorders, Parkinson's disease. It is therefore of high interest to identify molecules with trophic effects on this neuronal population. We show here that the neuregulin receptor ErbB4 is differentially expressed in mesencephalic dopaminergic neurons, found in the substantia nigra and in a subregion of the ventral tegmentum but not in the retrorubral field. Early developmental onset and continued expression of ErbB4 into the adult and the presence of two high affinity ligands, neuregulin-1 and betacellulin, in the basal ganglia, suggested that these molecules might participate in the differentiation and/or maintenance of the nigrostriatal system. In order to address this hypothesis, we used a loxP flanked ErbB4 allele in combination with a nestin-Cre transgene and generated brain-specific ErbB4 null mice. These mutant animals survived into adulthood. The distribution of dopaminergic cell bodies in the midbrain, the expression of numerous genes specific to mesencephalic dopaminergic neurons, and the axonal projection to the basal ganglia all appeared normal. Finally, an assessment of their motor function revealed no behavioral deficits. The apparent lack of any mutant phenotype suggests the presence of a strong compensatory mechanism.

Aging↗