Precommitment low-level Neurog3 expression defines a long-lived mitotic endocrine-biased progenitor pool that drives production of endocrine-committed cells

Bechard et al. show that a cell population defined as Neurog3 transcriptionally active and Sox9⁺ and often containing nonimmunodetectable Neurog3 protein has a relatively high mitotic index and prolonged epithelial residency. They propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny.

The current model for endocrine cell specification in the pancreas invokes high-level production of the transcription factor Neurogenin 3 (Neurog3) in Sox9⁺ bipotent epithelial cells as the trigger for endocrine commitment, cell cycle exit, and rapid delamination toward proto-islet clusters. This model posits a transient Neurog3 expression state and short epithelial residence period. We show, however, that a Neurog3^TA.LO cell population, defined as Neurog3 transcriptionally active and Sox9⁺ and often containing nonimmunodetectable Neurog3 protein, has a relatively high mitotic index and prolonged epithelial residency. We propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny. A novel BAC transgenic Neurog3 reporter detected two types of mitotic behavior in Sox9⁺Neurog3^TA.LO progenitors, associated with progenitor pool maintenance or derivation of endocrine-committed Neurog3^HI cells, respectively. Moreover, limiting Neurog3 expression dramatically increased the proportional representation of Sox9⁺Neurog3^TA.LO progenitors, with a doubling of its mitotic index relative to normal Neurog3 expression, suggesting that low Neurog3 expression is a defining feature of this cycling endocrine-biased state. We propose that Sox9⁺Neurog3^TA.LO endocrine-biased progenitors feed production of Neurog3^HI endocrine-committed cells during pancreas organogenesis.

Insm1 promotes endocrine cell differentiation by modulating the expression of a network of genes that includes Neurog3 and Ripply3

Insulinoma associated 1 (Insm1) plays an important role in regulating the development of cells in the central and peripheral nervous systems, olfactory epithelium and endocrine pancreas. To better define the role of Insm1 in pancreatic endocrine cell development we generated mice with an Insm1^GFPCre reporter allele and used them to study Insm1-expressing and null populations. Endocrine progenitor cells lacking Insm1 were less differentiated and exhibited broad defects in hormone production, cell proliferation and cell migration. Embryos lacking Insm1 contained greater amounts of a non-coding Neurog3 mRNA splice variant and had fewer Neurog3/Insm1 co-expressing progenitor cells, suggesting that Insm1 positively regulates Neurog3. Moreover, endocrine progenitor cells that express either high or low levels of Pdx1, and thus may be biased towards the formation of specific cell lineages, exhibited cell type-specific differences in the genes regulated by Insm1. Analysis of the function of Ripply3, an Insm1-regulated gene enriched in the Pdx1-high cell population, revealed that it negatively regulates the proliferation of early endocrine cells. Taken together, these findings indicate that in developing pancreatic endocrine cells Insm1 promotes the transition from a ductal progenitor to a committed endocrine cell by repressing a progenitor cell program and activating genes essential for RNA splicing, cell migration, controlled cellular proliferation, vasculogenesis, extracellular matrix and hormone secretion.

Generation of Nkx2.2:lacZ mice using recombination-mediated cassette exchange technology

Nkx2.2 encodes a homeodomain transcription factor required for the correct specification and/or differentiation of cells in the pancreas, intestine, and central nervous system (CNS). To follow the fate of cells deleted for Nkx2.2 within these tissues, we generated Nkx2.2:lacZ knockin mice using a recombination-mediated cassette exchange (RMCE) approach. Expression analysis of lacZ and/or β-galactosidase in Nkx2.2(lacZ/+) heterozygote embryos and adults demonstrates that lacZ faithfully recapitulates endogenous Nkx2.2 expression. Furthermore, the Nkx2.2(lacZ/lacZ) homozygous embryos display phenotypes indistinguishable from the previously characterized Nkx2.2(-/-) strain. LacZ expression analyses in the Nkx2.2(lacZ/lacZ) homozygous embryos indicate that Nkx2.2-expressing progenitor cells within the pancreas are generated in their normal numbers and are not mislocalized within the pancreatic ductal epithelium or developing islets. In the CNS of Nkx2.2(lacZ/lacZ) embryos, LacZ-expressing cells within the ventral P3 progenitor domain display different migration properties depending on the developmental stage and their respective differentiation potential.

A recombinase-mediated cassette exchange-derived cyan fluorescent protein reporter allele for Pdx1

Fluorescent protein (FP) reporter alleles are useful both for identifying and purifying specific cell populations in the mouse. Here, we report the generation of mouse embryonic stem cells that contain a pancreatic and duodenal homeobox 1 (Pdx1) loxed cassette acceptor (Pdx1(LCA)) allele and the use of recombinase-mediated cassette exchange to derive mice that contain a Pdx1(CFP) (Cerulean) reporter allele. Mice with this allele exhibited cyan fluorescence within the previously well-characterized Pdx1 expression domain in posterior foregut endoderm. Immunolabeling showed that endogenous Pdx1 was coexpressed with CFP at all time points examined. Furthermore, fluorescence-activated cell sorting was used to isolate CFP-positive cells from E11.5 and E18.5 embryonic tissues using both 405 and 445 nm lasers, although the latter resulted in a nearly 50-fold increase in emission intensity. The Pdx1(CFP) allele will enable the isolation of specific foregut endoderm and pancreatic cell populations, both alone and in combination with other FP reporter alleles.

Nkx2.2 repressor complex regulates islet β-cell specification and prevents β-to-α-cell reprogramming

Regulation of cell differentiation programs requires complex interactions between transcriptional and epigenetic networks. Elucidating the principal molecular events responsible for the establishment and maintenance of cell fate identities will provide important insights into how cell lineages are specified and maintained and will improve our ability to recapitulate cell differentiation events in vitro. In this study, we demonstrate that Nkx2.2 is part of a large repression complex in pancreatic β cells that includes DNMT3a, Grg3, and HDAC1. Mutation of the endogenous Nkx2.2 tinman (TN) domain in mice abolishes the interaction between Nkx2.2 and Grg3 and disrupts β-cell specification. Furthermore, we demonstrate that Nkx2.2 preferentially recruits Grg3 and HDAC1 to the methylated Aristaless homeobox gene (Arx) promoter in β cells. The Nkx2.2 TN mutation results in ectopic expression of Arx in β cells, causing β-to-α-cell transdifferentiation. A corresponding β-cell-specific deletion of DNMT3a is also sufficient to cause Arx-dependent β-to-α-cell reprogramming. Notably, subsequent removal of Arx in the β cells of Nkx2.2(TNmut/TNmut) mutant mice reverts the β-to-α-cell conversion, indicating that the repressor activities of Nkx2.2 on the methylated Arx promoter in β cells are the primary regulatory events required for maintaining β-cell identity.

Striking In vivo phenotype of a disease-associated human SCN5A mutation producing minimal changes in vitro

BACKGROUND

The D1275N SCN5A mutation has been associated with a range of unusual phenotypes, including conduction disease and dilated cardiomyopathy, as well as atrial and ventricular tachyarrhythmias. However, when D1275N is studied in heterologous expression systems, most studies show near-normal sodium channel function. Thus, the relationship of the variant to the clinical phenotypes remains uncertain.

METHODS AND RESULTS

We identified D1275N in a patient with atrial flutter, atrial standstill, conduction disease, and sinus node dysfunction. There was no major difference in biophysical properties between wild-type and D1275N channels expressed in Chinese hamster ovary cells or tsA201 cells in the absence or presence of β1 subunits. To determine D1275N function in vivo, the Scn5a locus was modified to knock out the mouse gene, and the full-length wild-type (H) or D1275N (DN) human SCN5A cDNAs were then inserted at the modified locus by recombinase mediated cassette exchange. Mice carrying the DN allele displayed slow conduction, heart block, atrial fibrillation, ventricular tachycardia, and a dilated cardiomyopathy phenotype, with no significant fibrosis or myocyte disarray on histological examination. The DN allele conferred gene-dose-dependent increases in SCN5A mRNA abundance but reduced sodium channel protein abundance and peak sodium current amplitudes (H/H, 41.0±2.9 pA/pF at -30 mV; DN/H, 19.2±3.1 pA/pF, P<0.001 vs. H/H; DN/DN, 9.3±1.1 pA/pF, P<0.001 versus H/H).

CONCLUSIONS

Although D1275N produces near-normal currents in multiple heterologous expression experiments, our data establish this variant as a pathological mutation that generates conduction slowing, arrhythmias, and a dilated cardiomyopathy phenotype by reducing cardiac sodium current.

Informatic and functional approaches to identifying a regulatory region for the cardiac sodium channel

RATIONALE

Although multiple lines of evidence suggest that variable expression of the cardiac sodium channel gene SCN5A plays a role in susceptibility to arrhythmia, little is known about its transcriptional regulation.

OBJECTIVE

We used in silico and in vitro experiments to identify possible noncoding sequences important for transcriptional regulation of SCN5A. The results were extended to mice in which a putative regulatory region was deleted.

METHODS AND RESULTS

We identified 92 noncoding regions highly conserved (>70%) between human and mouse SCN5A orthologs. Three conserved noncoding sequences (CNS) showed significant (>5-fold) activity in luciferase assays. Further in vitro studies indicated one, CNS28 in intron 1, as a potential regulatory region. Using recombinase-mediated cassette exchange (RMCE), we generated mice in which a 435-base pair region encompassing CNS28 was removed. Animals homozygous for the deletion showed significant increases in SCN5A transcripts, Na(V)1.5 protein abundance, and sodium current measured in isolated ventricular myocytes. ECGs revealed a significantly shorter QRS (10.7±0.2 ms in controls versus 9.7±0.2 ms in knockouts), indicating more rapid ventricular conduction. In vitro analysis of CNS28 identified a short 3' segment within this region required for regulatory activity and including an E-box motif. Deletion of this segment reduced reporter activity to 3.6%±0.3% of baseline in CHO cells and 16%±3% in myocytes (both P<0.05), and mutation of individual sites in the E-box restored activity to 62%±4% and 57%±2% of baseline in CHO cells and myocytes, respectively (both P<0.05).

CONCLUSIONS

These findings establish that regulation of cardiac sodium channel expression modulates channel function in vivo, and identify a noncoding region underlying this regulation.

Optimal effectiveness of BDNF for fetal nigral transplants coincides with the ontogenic appearance of BDNF in the striatum

Transplantation of fetal nigral dopamine neurons into the caudate and putamen of Parkinson's disease patients produces limited symptomatic relief. One approach to augment the outgrowth and function of nigral grafts includes exposure of the graphs to neurotrophic factors; however, the temporal requirements for optimizing these actions are unknown. The present study characterized the ontogeny of brain-derived neurotrophic factor (BDNF) in the rat striatum and used this information to define and evaluate three distinct periods of BDNF infusion into fetal nigral grafts transplanted into the striatum of rats with experimental Parkinson's disease. At postnatal day 1 (P1), BDNF and dopamine were measured at 17 and 27% of peak levels, respectively, that occurred at P27 for both. Both compounds showed their greatest surge between P7 and P20, increasing from 40% to approximately 95% of peak levels. Exogenous BDNF infused into transplants during weeks 1 and 2 after the transplantation, which coincide with the developmental period embryonic day 14 (E14)-P7 for transplanted tissue, did not improve rotational behavior or enhance fiber outgrowth of transplanted dopamine neurons. Delaying the BDNF infusion until transplanted tissue was approximately P8-P21 greatly enhanced the effect on rotational behavior and doubled the area of dopamine fiber outgrowth from the transplants. Delaying the infusion until transplanted tissue was approximately P36-P49 failed to augment fiber outgrowth and decreased the behavioral function of transplants. Thus, the optimal effect of exogenous BDNF on the development of dopamine neurons in fetal nigral transplants occurs at a postnatal age when endogenous dopamine and BDNF show the greatest increases during the normal development of the striatum.

Age-related decline in striatal dopamine release and motoric function in brown Norway/Fischer 344 hybrid rats

The Brown Norway/Fischer 344 F1 hybrid rats (F344BNF1) is a newer rat model and is emerging as an important rodent model of aging. In the present study we used motoric performance tests, intracerebral microdialysis, and neurochemical measures of postmortem brain tissue to investigate the effects of aging in young (4-5 months), middle-aged (18-19), and old (24-25 months) F344BNF1 hybrid rats. We observed that old F344BNF1 rats exhibited decreased motoric performance, and lower levels of spontaneous and d-amphetamine-induced locomotor activity than those observed in young F344BNF1 rats. Microdialysis measures of extracellular basal levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenylacetic acid (HVA) were significantly diminished in the striata of the middle-aged and old rats as compared to levels in young animals. In addition, d-amphetamine-evoked overflow of DA was significantly decreased in the middle-aged and aged rat striatum as compared to DA overflow in young F344BNF1 rats. Studies of postmortem brain tissue showed that the changes in overflow of DA correlated with significantly lower DA tissue content in ventral striatum and midbrain. Moreover, both dopamine turnover ratios (DOPAC/DA, HVA/DA) and the serotonin turnover ratio (5-HIAA/5-HT) were significantly elevated in the ventral striatum and nucleus accumbens. The results of this study demonstrate a correlation between reductions in striatal DA neurochemistry and diminished motor function in aged F344BNF1 rats.

Intranigral injections of SCH 23390 inhibit SKF 82958-induced rotational behavior

The selective D1 receptor antagonist, SCH 23390, was injected into the pars reticulata region of the lesioned substantia nigra at various concentrations (3.0, 1.5, 1.0, 0.6 or 0.3 mM) just before a s.c. injection of either the selective D1 agonist, SKF 82958; the selective D2 agonist, quinpirole; or the mixed D1-D2 receptor agonist, apomorphine. SCH 23390 pretreatment (1) had no significant effect on quinpirole rotational behavior, (2) attenuated apomorphine rotational behavior and (3) dose-dependently inhibited SKF 82958 rotational behavior with the highest SCH 23390 doses completely blocking SKF 82958 rotational behavior in some animals. These data provide further evidence that dopamine release in the midbrain may act as a neuromodulator of motor behavior, and that D1 receptors play a functional role in this process.

Intranigral injections of SCH 23390 inhibit amphetamine-induced rotational behavior

Rats were given unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway and permanent indwelling cannula were surgically implanted into the non-lesioned side of the brain; cannula were used for direct injections of dopamine antagonists into the pars reticulata region of the non-lesioned substantia nigra. The selective D1 receptor antagonist, SCH 23390, was injected intranigrally at various concentrations (3.0, 1.5, 1.0, 0.6, or 0.3 mM) just prior to an intraperitoneal injection of amphetamine. SCH 23390 dose-dependently inhibited amphetamine-induced rotational behavior with the highest doses completely blocking rotational behavior in some animals. An intranigral injection of the selective D2 receptor antagonist, (-)-sulpiride (1.0 mM), did not produce a significant reduction in amphetamine-induced rotational behavior whereas an equivalent molar concentration of SCH 23390 (1.0 mM) produced a significant 62% reduction in amphetamine-induced rotational behavior. A concentration of SCH 23390 that produced a 50% reduction in rotational behavior when injected directly into the substantia nigra was unable to produce a significant reduction in rotational behavior when injected directly into the striatum. The effects of intranigral injections of SCH 23390 on apomorphine-induced rotational behavior were directly opposite to that observed for amphetamine-induced rotational behavior; contralateral rotational behavior increased relative to baseline measures. These data support the hypothesis that dopamine release in the midbrain may act as a neuromodulator of motor behavior, and that D1 receptors play a functional role in this process.