Context-dependent regulation of NeuroD activity and protein accumulation

The prolyl isomerase Pin1 stabilizes NeuroD during differentiation of mechanoreceptors

The peptidyl prolyl cis-trans isomerase Pin1 plays vital roles in diverse cellular processes and pathological conditions. NeuroD is a differentiation and survival factor for a subset of neurons and pancreatic endocrine cells. Although multiple phosphorylation events are known to be crucial for NeuroD function, their mechanisms remain elusive. In this study, we demonstrate that zebrafish embryos deficient in Pin1 displayed phenotypes resembling those associated with NeuroD depletion, characterized by defects in formation of mechanosensory hair cells. Furthermore, zebrafish Pin1 interacts with NeuroD in a phosphorylation-dependent manner. In Pin1-deficient cell lines, NeuroD is rapidly degraded. However, the protein stability of NeuroD is restored upon overexpression of Pin1. These findings suggest that Pin1 functionally regulates NeuroD protein levels by post-phosphorylation cis-trans isomerization during neuronal specification.

ERK Regulates NeuroD1-mediated Neurite Outgrowth via Proteasomal Degradation

Neurogenic differentiation 1 (NeuroD1) is a class B basic helix-loop-helix (bHLH) transcription factor and regulates differentiation and survival of neuronal and endocrine cells by means of several protein kinases, including extracellular signal-regulated kinase (ERK). However, the effect of phosphorylation on the functions of NeuroD1 by ERK has sparked controversy based on context-dependent differences across diverse species and cell types. Here, we evidenced that ERK-dependent phosphorylation controlled the stability of NeuroD1 and consequently, regulated proneural activity in neuronal cells. A null mutation at the ERK-dependent phosphorylation site, S274A, increased the half-life of NeuroD1 by blocking its ubiquitin-dependent proteasomal degradation. The S274A mutation did not interfere with either the nuclear translocation of NeuroD1 or its heterodimerization with E47, its ubiquitous partner and class A bHLH transcription factor. However, the S274A mutant increased transactivation of the E-box-mediated gene and neurite outgrowth in F11 neuroblastoma cells, compared to the wild-type NeuroD1. Transcriptome and Gene Ontology enrichment analyses indicated that genes involved in axonogenesis and dendrite development were downregulated in NeuroD1 knockout (KO) mice. Overexpression of the S274A mutant salvaged neurite outgrowth in NeuroD1-deficient mice, whereas neurite outgrowth was minimal with S274D, a phosphomimicking mutant. Our data indicated that a longer protein half-life enhanced the overall activity of NeuroD1 in stimulating downstream genes and neuronal differentiation. We propose that blocking ubiquitin-dependent proteasomal degradation may serve as a strategy to promote neuronal activity by stimulating the expression of neuron-specific genes in differentiating neurons.

Expression and cellular localization of the transcription factor NeuroD1 in the developing and adult rat pineal gland

Circadian rhythms govern many aspects of mammalian physiology. The daily pattern of melatonin synthesis and secretion is one of the classic examples of circadian oscillations. It is mediated by a class of neuroendocrine cells known as pinealocytes which are not yet fully defined. An established method to evaluate functional and cytological characters is through the expression of lineage-specific transcriptional regulators. NeuroD1 is a basic helix-loop-helix transcription factor involved in the specification and maintenance of both endocrine and neuronal phenotypes. We have previously described developmental and adult regulation of NeuroD1 mRNA in the rodent pineal gland. However, the transcript levels were not influenced by the elimination of sympathetic input, suggesting that any rhythmicity of NeuroD1 might be found downstream of transcription. Here, we describe NeuroD1 protein expression and cellular localization in the rat pineal gland during development and the daily cycle. In embryonic and perinatal stages, protein expression follows the mRNA pattern and is predominantly nuclear. Thereafter, NeuroD1 is mostly found in pinealocyte nuclei in the early part of the night and in cytoplasm during the day, a rhythm maintained into adulthood. Additionally, nocturnal nuclear NeuroD1 levels are reduced after sympathetic disruption, an effect mimicked by the in vivo administration of α- and β-adrenoceptor blockers. NeuroD1 phosphorylation at two sites, Ser(274) and Ser(336) , associates with nuclear localization in pinealocytes. These data suggest that NeuroD1 influences pineal phenotype both during development and adulthood, in an autonomic and phosphorylation-dependent manner.

NeuroD1 mediates nicotine-induced migration and invasion via regulation of the nicotinic acetylcholine receptor subunits in a subset of neural and neuroendocrine carcinomas

Nicotine up-regulates NeuroD1 in bronchial epithelial cells and certain undifferentiated carcinomas. NeuroD1 enhances expression of nicotinic acetylcholine receptor subunits. Increased invasion in Matrigel depends on these receptor subunits. Nicotine may induce positive feedback through NeuroD1 and increased expression of its own receptor.

Cigarette smoking is a major risk factor for acquisition of small cell lung cancer (SCLC). A role has been demonstrated for the basic helix-loop-helix transcription factor NeuroD1 in the pathogenesis of neural and neuroendocrine lung cancer, including SCLC. In the present study we investigate the possible function of NeuroD1 in established tumors, as well as actions early on in pathogenesis, in response to nicotine. We demonstrate that nicotine up-regulates NeuroD1 in immortalized normal bronchial epithelial cells and a subset of undifferentiated carcinomas. Increased expression of NeuroD1 subsequently leads to regulation of expression and function of the nicotinic acetylcholine receptor subunit cluster of α3, α5, and β4. In addition, we find that coordinated expression of these subunits by NeuroD1 leads to enhanced nicotine-induced migration and invasion, likely through changes in intracellular calcium. These findings suggest that aspects of the pathogenesis of neural and neuroendocrine lung cancers may be affected by a nicotine- and NeuroD1-induced positive feedback loop.

Aluminum exposure impacts brain plasticity and behavior in Atlantic salmon (Salmo salar)

Aluminum (Al) toxicity occurs frequently in natural aquatic ecosystems as a result of acid deposition and natural weathering processes. Detrimental effects of Al toxicity on aquatic organisms are well known and can have consequences for survival. Fish exposed to Al in low pH waters will experience physiological and neuroendocrine changes that disrupt homeostasis and alter behavior. To investigate the effects of Al exposure to both brain and behavior, Atlantic salmon (Salmo salar) kept in water treated with Al (pH 5.7, 0.37±0.04 µmol 1-1 of Al) for 2 weeks were compared to fish kept in a control condition (pH 6.7, <0.04 µmol 1-1 of Al). Fish exposed to Al and acidic conditions had increased Al accumulation in the gills and decreased gill Na+, K+-ATPase activity, which impaired osmoreguatory capacity and caused physiological stress, indicated by elevated plasma cortisol and glucose levels. Here we show for the first time that exposure to Al in acidic conditions also impaired learning performance in a maze task. Al toxicity reduced the expression of NeuroD1 transcript levels in the forebrain of exposed fish. As in mammals, these data show that exposure to chronic stress, such as acidified Al, can reduce neural plasticity during behavioral challenges in salmon, and may impair coping ability to new environments.

Synergistic nuclear import of NeuroD1 and its partner transcription factor, E47, via heterodimerization

The transition from undifferentiated pluripotent cells to terminally differentiated neurons is coordinated by a repertoire of transcription factors. NeuroD1 is a type II basic helix loop helix (bHLH) transcription factor that plays critical roles in neuronal differentiation and maintenance in the central nervous system. Its dimerization with E47, a type I bHLH transcription factor, leads to the transcriptional regulation of target genes. Mounting evidence suggests that regulating the localization of transcription factors contributes to the regulation of their activity during development as defects in their localization underlie a variety of developmental disorders. In this study, we attempted to understand the nuclear import mannerisms of NeuroD1 and E47. We found that the nuclear import of NeuroD1 and E47 is energy-dependent and involves the Ran-mediated pathway. Herein, we demonstrate that NeuroD1 and E47 can dimerize inside the cytoplasm before their nuclear import. Moreover, this dimerization promotes nuclear import as the nuclear accumulation of NeuroD1 was enhanced in the presence of E47 in an in vitro nuclear import assay, and NLS-deficient NeuroD1 was successfully imported into the nucleus upon E47 overexpression. NeuroD1 also had a similar effect on the nuclear accumulation of NLS-deficient E47. These findings suggest a novel role for dimerization that may promote, at least partially, the nuclear import of transcription factors allowing them to function efficiently in the nucleus.

Rewiring the retinal ganglion cell gene regulatory network: Neurod1 promotes retinal ganglion cell fate in the absence of Math5

Retinal progenitor cells (RPCs) express basic helix-loop-helix (bHLH)factors in a strikingly mosaic spatiotemporal pattern, which is thought to contribute to the establishment of individual retinal cell identity. Here, we ask whether this tightly regulated pattern is essential for the orderly differentiation of the early retinal cell types and whether different bHLH genes have distinct functions that are adapted for each RPC. To address these issues, we replaced one bHLH gene with another. Math5 is a bHLH gene that is essential for establishing retinal ganglion cell (RGC) fate. We analyzed the retinas of mice in which Math5 was replaced with Neurod1 or Math3, bHLH genes that are expressed in another RPC and are required to establish amacrine cell fate. In the absence of Math5, Math5Neurod1-KI was able to specify RGCs, activate RGC genes and restore the optic nerve, although not as effectively as Math5. By contrast, Math5Math3-KI was much less effective than Math5Neurod1-KI in replacing Math5. In addition, expression of Neurod1 and Math3 from the Math5Neurod1-KI/Math3-KIallele did not result in enhanced amacrine cell production. These results were unexpected because they indicated that bHLH genes, which are currently thought to have evolved highly specialized functions, are nonetheless able to adjust their functions by interpreting the local positional information that is programmed into the RPC lineages. We conclude that, although Neurod1 and Math3 have evolved specialized functions for establishing amacrine cell fate, they are nevertheless capable of alternative functions when expressed in foreign environments.

Expression of Disabled 1 suppresses astroglial differentiation in neural stem cells

Disabled 1 (Dab1), a cytoplasmic adaptor protein expressed predominantly in the CNS, transduces a Reelin-initiated signaling that controls neuronal migration and positioning during brain development. To determine the role of Dab1 in neural stem cell (NSC) differentiation, we established a culture of neurospheres derived from the embryonic forebrain of the Dab1(-/-) mice, yotari. Differentiating Dab1(-/-) neurospheres exhibited a higher expression of GFAP, an astrocytic marker, at the expense of neuronal markers. Under Dab1-deficient condition, the expression of NeuroD, a transcription factor for neuronal differentiation, was decreased and the JAK-STAT pathway was evidently increased during differentiation of NSC, suggesting the possible involvement of Dab1 in astrocyte differentiation via JAK-STAT pathway. Notably, expression of neural and glial markers and the level of JAK-STAT signaling molecules were not changed in differentiating NSC by Reelin treatment, indicating that differentiation of NSC is Reelin-independent. Immunohistochemical analyses showed a decrease in the number of neurons and an increase in the number of GFAP-positive cells in developing yotari brains. Our results suggest that Dab1 participates in the differentiation of NSCs into a specific cell lineage, thereby maintaining a balance between neurogenesis and gliogenesis.

The protein kinases ERK1/2 and their roles in pancreatic beta cells

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) activities are modulated in a manner that reflects the secretory demand on beta cells to integrate long- and short-term nutrient sensing information. Our studies have focused on the mechanisms of ERK1/2 activation in beta cells and on the actions of ERK1/2 that regulate beta cell function. Insulin and growth factors regulate ERK1/2 in beta cells in a largely calcium-independent manner. Nutrients and anticipatory hormones, in contrast, activate ERK1/2 in a calcium-dependent manner in these cells. We are exploring the key intermediates in these distinct activation pathways and find that calcineurin is essential for the nutrient pathway but is not essential for the growth factor pathway. Using reporter assays, heterologous reconstitution, electrophoretic mobility shift assays, Northern analysis, Q-PCR and chromatin immunoprecipitation, we have examined several genes that are regulated by ERK1/2, primarily the insulin gene and the apoptotic factor C/EBP-homologous protein (CHOP)-10 (GADD153/DDIT-3), a bZIP protein. ERK1/2-sensitive transcriptional regulators common to these two genes are C/EBP-beta and MafA. The insulin promoter is both positively and negatively regulated by glucose and other nutrients. Exposure to glucose for minutes to hours causes an increase in the rate of insulin gene transcription. In contrast, exposure to elevated glucose for 48 h or more results in inhibition of the insulin gene promoter. Both of these processes depend on ERK1/2 activity. Expression of CHOP is induced by stresses including nutrient deprivation and endoplasmic reticulum stress. CHOP gene expression, especially that regulated by nutrients, is also ERK1/2-dependent in beta cells, These studies support the hypothesis that the genes regulated by ERK1/2 and the mechanisms employed are key to maintaining normal beta cell function.

NeuroD1: developmental expression and regulated genes in the rodent pineal gland

NeuroD1/BETA2, a member of the bHLH transcription factor family, is known to influence the fate of specific neuronal, endocrine and retinal cells. We report here that NeuroD1 mRNA is highly abundant in the developing and adult rat pineal gland. Pineal expression begins in the 17-day embryo at which time it is also detectable in other brain regions. Expression in the pineal gland increases during the embryonic period and is maintained thereafter at levels equivalent to those found in the cerebellum and retina. In contrast, NeuroD1 mRNA decreases markedly in non-cerebellar brain regions during development. Pineal NeuroD1 levels are similar during the day and night, and do not appear to be influenced by sympathetic neural input. Gene expression analysis of the pineal glands from neonatal NeuroD1 knockout mice identifies 127 transcripts that are down-regulated (>twofold, p < 0.05) and 16 that are up-regulated (>twofold, p < 0.05). According to quantitative RT-PCR, the most dramatically down-regulated gene is kinesin family member 5C ( approximately 100-fold) and the most dramatically up-regulated gene is glutamic acid decarboxylase 1 ( approximately fourfold). Other impacted transcripts encode proteins involved in differentiation, development, signal transduction and trafficking. These findings represent the first step toward elucidating the role of NeuroD1 in the rodent pinealocyte.