Distinct pathways leading to TDP-43-induced cellular dysfunctions

TAR DNA-binding protein of 43 kDa (TDP-43) is the major component protein of inclusions found in brains of patients with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP). However, the molecular mechanisms by which TDP-43 causes neuronal dysfunction and death remain unknown. Here, we report distinct cytotoxic effects of full-length TDP-43 (FL-TDP) and its C-terminal fragment (CTF) in SH-SY5Y cells. When FL-TDP was overexpressed in the cells using a lentiviral system, exogenous TDP-43, like endogenous TDP-43, was expressed mainly in nuclei of cells without any intracellular inclusions. However, these cells showed striking cell death, caspase activation and growth arrest at G2/M phase, indicating that even simple overexpression of TDP-43 induces cellular dysfunctions leading to apoptosis. On the other hand, cells expressing TDP-43 CTF showed cytoplasmic aggregates but without significant cell death, compared with cells expressing FL-TDP. Confocal microscopic analyses revealed that RNA polymerase II (RNA pol II) and several transcription factors, such as specificity protein 1 and cAMP-response-element-binding protein, were co-localized with the aggregates of TDP-43 CTF, suggesting that sequestration of these factors into TDP-43 aggregates caused transcriptional dysregulation. Indeed, accumulation of RNA pol II at TDP-43 inclusions was detected in brains of patients with FTLD-TDP. Furthermore, apoptosis was not observed in affected neurons of FTLD-TDP brains containing phosphorylated and aggregated TDP-43 pathology. Our results suggest that different pathways of TDP-43-induced cellular dysfunction may contribute to the degeneration cascades involved in the onset of ALS and FTLD-TDP.

RP58 regulates the multipolar-bipolar transition of newborn neurons in the developing cerebral cortex

Accumulating evidence suggests that many brain diseases are associated with defects in neuronal migration, suggesting that this step of neurogenesis is critical for brain organization. However, the molecular mechanisms underlying neuronal migration remain largely unknown. Here, we identified the zinc-finger transcriptional repressor RP58 as a key regulator of neuronal migration via multipolar-to-bipolar transition. RP58(-/-) neurons exhibited severe defects in the formation of leading processes and never shifted to the locomotion mode. Cre-mediated deletion of RP58 using in utero electroporation in RP58(flox/flox) mice revealed that RP58 functions in cell-autonomous multipolar-to-bipolar transition, independent of cell-cycle exit. Finally, we found that RP58 represses Ngn2 transcription to regulate the Ngn2-Rnd2 pathway; Ngn2 knockdown rescued migration defects of the RP58(-/-) neurons. Our findings highlight the critical role of RP58 in multipolar-to-bipolar transition via suppression of the Ngn2-Rnd2 pathway in the developing cerebral cortex.

RP58 controls neuron and astrocyte differentiation by downregulating the expression of Id1-4 genes in the developing cortex

Appropriate number of neurons and glial cells is generated from neural stem cells (NSCs) by the regulation of cell cycle exit and subsequent differentiation. Although the regulatory mechanism remains obscure, Id (inhibitor of differentiation) proteins are known to contribute critically to NSC proliferation by controlling cell cycle. Here, we report that a transcriptional factor, RP58, negatively regulates all four Id genes (Id1-Id4) in developing cerebral cortex. Consistently, Rp58 knockout (KO) mice demonstrated enhanced astrogenesis accompanied with an excess of NSCs. These phenotypes were mimicked by the overexpression of all Id genes in wild-type cortical progenitors. Furthermore, Rp58 KO phenotypes were rescued by the knockdown of all Id genes in mutant cortical progenitors but not by the knockdown of each single Id gene. Finally, we determined p57 as an effector gene of RP58-Id-mediated cell fate control. These findings establish RP58 as a novel key regulator that controls the self-renewal and differentiation of NSCs and restriction of astrogenesis by repressing all Id genes during corticogenesis.

The 5'-flanking region of the RP58 coding sequence shows prominent promoter activity in multipolar cells in the subventricular zone during corticogenesis

Pyramidal neurons of the neocortex are produced from progenitor cells located in the neocortical ventricular zone (VZ) and subventricular zone (SVZ) during embryogenesis. RP58 is a transcriptional repressor that is strongly expressed in the developing brain and plays an essential role in corticogenesis. The expression of RP58 is strictly regulated in a time-dependent and spatially restricted manner. It is maximally expressed in E15-16 embryonic cerebral cortex, localized specifically to the cortical plate and SVZ of the neocortex, hippocampus, and parts of amygdala during brain development, and found in glutamatergic but not GABAergic neurons. Identification of the promoter activity underlying specific expression patterns provides important clues to their mechanisms of action. Here, we show that the RP58 gene promoter is activated prominently in multipolar migrating cells, the first in vivo analysis of RP58 promoter activity in the brain. The 5.3 kb 5'-flanking genomic DNA of the RP58 coding region demonstrates promoter activity in neurons both in vitro and in vivo. This promoter is highly responsive to the transcription factor neurogenin2 (Ngn2), which is a direct upstream activator of RP58 expression. Using in utero electroporation, we demonstrate that RP58 gene promoter activity is first detected in a subpopulation of pin-like VZ cells, then prominently activated in migrating multipolar cells in the multipolar cell accumulation zone (MAZ) located just above the VZ. In dissociated primary cultured cortical neurons, RP58 promoter activity mimics in vivo expression patterns from a molecular standpoint that RP58 is expressed in a fraction of Sox2-positive progenitor cells, Ngn2-positive neuronal committed cells, and Tuj1-positive young neurons, but not in Dlx2-positive GABAergic neurons. Finally, we show that Cre recombinase expression under the control of the RP58 gene promoter is a feasible tool for conditional gene switching in post-mitotic multipolar migrating young neurons in the developing cerebral cortex.

SNARE conformational changes that prepare vesicles for exocytosis

When cells release hormones and neurotransmitters through exocytosis, cytosolic Ca(2+) triggers the fusion of secretory vesicles with the plasma membrane. It is well known that this fusion requires assembly of a SNARE protein complex. However, the timing of SNARE assembly relative to vesicle fusion--essential for understanding exocytosis--has not been demonstrated. To investigate this timing, we constructed a probe that detects the assembly of two plasma membrane SNAREs, SNAP25 and syntaxin-1A, through fluorescence resonance energy transfer (FRET). With two-photon imaging, we simultaneously measured FRET signals and insulin exocytosis in beta cells from the pancreatic islet of Langerhans. In some regions of the cell, we found that the SNARE complex was preassembled, which enabled rapid exocytosis. In other regions, SNARE assembly followed Ca(2+) influx, and exocytosis was slower. Thus, SNARE proteins exist in multiple stable preparatory configurations, from which Ca(2+) may trigger exocytosis through distinct mechanisms and with distinct kinetics.

A systems approach reveals that the myogenesis genome network is regulated by the transcriptional repressor RP58

We created a whole-mount in situ hybridization (WISH) database, termed EMBRYS, containing expression data of 1520 transcription factors and cofactors expressed in E9.5, E10.5, and E11.5 mouse embryos--a highly dynamic stage of skeletal myogenesis. This approach implicated 43 genes in regulation of embryonic myogenesis, including a transcriptional repressor, the zinc-finger protein RP58 (also known as Zfp238). Knockout and knockdown approaches confirmed an essential role for RP58 in skeletal myogenesis. Cell-based high-throughput transfection screening revealed that RP58 is a direct MyoD target. Microarray analysis identified two inhibitors of skeletal myogenesis, Id2 and Id3, as targets for RP58-mediated repression. Consistently, MyoD-dependent activation of the myogenic program is impaired in RP58 null fibroblasts and downregulation of Id2 and Id3 rescues MyoD's ability to promote myogenesis in these cells. Our combined, multi-system approach reveals a MyoD-activated regulatory loop relying on RP58-mediated repression of muscle regulatory factor (MRF) inhibitors.

The transcriptional repressor RP58 is crucial for cell-division patterning and neuronal survival in the developing cortex

The neocortex and the hippocampus comprise several specific layers containing distinct neurons that originate from progenitors at specific development times, under the control of an adequate cell-division patterning mechanism. Although many molecules are known to regulate this cell-division patterning process, its details are not well understood. Here, we show that, in the developing cerebral cortex, the RP58 transcription repressor protein was expressed both in postmitotic glutamatergic projection neurons and in their progenitor cells, but not in GABAergic interneurons. Targeted deletion of the RP58 gene led to dysplasia of the neocortex and of the hippocampus, reduction of the number of mature cortical neurons, and defects of laminar organization, which reflect abnormal neuronal migration within the cortical plate. We demonstrate an impairment of the cell-division patterning during the late embryonic stage and an enhancement of apoptosis of the postmitotic neurons in the RP58-deficient cortex. These results suggest that RP58 controls cell division of progenitor cells and regulates the survival of postmitotic cortical neurons.

Newly‐formed axonal branches of rat sciatic neurons sprouting in the spinal cord after peripheral axotomy

A retrograde method of nerve tracing using a recombinant adenovirus was applied to experimental regeneration of peripheral nerves to study the sprouting position of the regenerating axon. This enabled us to see the entire length of the axons on a whole-mount neural specimen. The peroneal nerve was transsected and infected with this virus, and the tibial nerve was transsected and sutured in eight Wistar rats. Four to five weeks later, labelled axons appeared in the tibial nerve, some of which could be traced from the tibial nerve to the spinal cord without making a connection with other labelled fibres. Control experiments negated the possibilities of transneuronal immigration or contamination of the virus. When the peroneal and tibial nerves were double-labelled with fluorescent tracers four weeks after their transsection, double-labelled motor neurons appeared. Based on these findings, we conclude that regenerating branches do sprout in the spinal cord after axotomy.

Co-localization of a novel transcriptional repressor simiRP58 with RP58

We have cloned a novel transcriptional repressor protein, termed simiRP58, which has high homology to RP58. Both simiRP58 and RP58 belong to the POZ domain and Kruppel Zn finger (POK) family of proteins. Using the luciferase assay system, we found that simiRP58 also has transcriptional repressor activity like RP58. Northern blotting and quantitative RT-PCR showed that simiRP58 was expressed in testes at the highest level. In situ hybridization of testes showed that simiRP58 is expressed by spermatocytes in only a portion of the seminiferous tubules. In contrast, expression of RP58 by spermatocytes was ubiquitous in all seminiferous tubules. Using COS-7 cells, we observed that simiRP58 was localized in the cytoplasm, which is in contrast to RP58 that was localized in the nucleus. Interestingly, co-transfection with simiRP58 and RP58 induced changes in the localization patterns of both proteins.

Ca2+-permeable AMPA receptors regulate growth of human glioblastoma via Akt activation

Evidence has been accumulated that glioblastoma cells release and exploit glutamate for proliferation and migration by autocrine or paracrine loops through Ca2+-permeable AMPA-type glutamate receptors. Here, we show that Ca2+ signaling mediated by AMPA receptor regulates the growth and motility of glioblastoma cells via activation of Akt. Ca2+ supplied through Ca2+-permeable AMPA receptor phosphorylated Akt at Ser-473, thereby facilitating proliferation and mobility. A dominant-negative form of Akt inhibited cell proliferation and migration accelerated by overexpression of Ca2+-permeable AMPA receptor. In contrast, introduction of a constitutively active form of Akt rescued tumor cells from apoptosis induced by the conversion of Ca2+-permeable AMPA receptor to Ca2+-impermeable receptors by the delivery of GluR2 cDNA. Therefore, Akt functions as downstream effectors for Ca2+-signaling mediated by AMPA receptor in glioblastoma cells. The activation of the glutamate-AMPA receptor-Akt pathway may contribute to the high degree of anaplasia and invasive growth of human glioblastoma. This novel pathway might give an alternative therapeutic target.

Spatial and temporal expression of RP58, a novel zinc finger transcriptional repressor, in mouse brain

RP58, a novel zinc finger protein containing a POZ domain, is a sequence-specific transcriptional repressor. To understand the role of this protein, we examined RP58 gene expression in the developing mouse brain by quantitative polymerase chain reaction (PCR) and in situ hybridization. RP58 mRNA expression was detected at embryonic day (E) 10 in the neuroepithelium, and subsequently in the ventricular zones of the cerebral cortex in the E12 embryo. Strong expression was observed in the preplate in the cerebral cortex from this stage onward. High levels of expression continued to be detected in the cortical plate and subventricular zone of the neocortex, hippocampus, and parts of the amygdala, but not in the thalamus or striatum. These results suggest that RP58 plays a crucial role in neuronal proliferation, migration, and differentiation in the developing cerebral cortex. RP58 is also expressed in the adult mouse neocortex, hippocampus, parts of the amygdala, and granule cells in the cerebellum. Double in situ hybridization using GAD67 or VGLUT1 probes revealed that RP58 is expressed in glutamatergic excitatory neurons.

Expression of alpha-synuclein, a presynaptic protein implicated in Parkinson's disease, in erythropoietic lineage

The present study investigated expression of alpha-synuclein (alpha-syn), a presynaptic protein involved in the pathogenesis of Parkinson's disease, in erythroid cells. Using various immunological procedures, immunoreactivity of alpha-syn was unambiguously demonstrated in erythroid lineage in murine bone marrows and peripheral erythrocytes. Expression of alpha-syn mRNA was also confirmed by in situ hybridization. Furthermore, flow cytometry analysis revealed that approximately 80% of erythroid cells in murine bone marrows expressed alpha-syn, while more than 90% of peripheral erythrocytes expressed alpha-syn. Nonetheless, alpha-syn null mice exhibited apparently no phenotypic changes in erythroid cells as was the case in their brains, suggesting that there might be underlying some redundant mechanisms. Together with previous reports showing the expression of alpha-syn in lymphocytes and platelets, the present finding supports a contention that alpha-syn might play some important functions in hematopoietic system.

Glial Nax Channels Control Lactate Signaling to Neurons for Brain [Na+] Sensing

Sodium (Na) homeostasis is crucial for life, and Na levels in body fluids are constantly monitored in the brain. The subfornical organ (SFO) is the center of the sensing responsible for the control of salt-intake behavior, where Na(x) channels are expressed in specific glial cells as the Na-level sensor. Here, we show direct interaction between Na(x) channels and alpha subunits of Na(+)/K(+)-ATPase, which brings about Na-dependent activation of the metabolic state of the glial cells. The metabolic enhancement leading to extensive lactate production was observed in the SFO of wild-type mice, but not of the Na(x)-knockout mice. Furthermore, lactate, as well as Na, stimulated the activity of GABAergic neurons in the SFO. These results suggest that the information on a physiological increase of the Na level in body fluids sensed by Na(x) in glial cells is transmitted to neurons by lactate as a mediator to regulate neural activities of the SFO.

Retrograde infection of precerebellar nuclei neurons by injection of a recombinant adenovirus into the cerebellar cortex of normal and reeler mice

The reeler mouse is an autosomal recessive mutant mouse caused by mutation of the reelin gene and characterized by cerebellar ataxia. To determine whether the distribution pattern of precerebellar nuclei neurons in the brainstem of the reeler mouse changes, we injected a small volume of a replication-defective recombinant adenovirus carrying E. coli beta-galactosidase (lacZ) into the cerebellar cortex of normal and reeler mice. Five days later, the mice were transcardially perfused by a fixative solution. X-gal staining of coronal or sagittal sections of the brainstem revealed that many origins for reticulocerebellar, cuneocerebellar, trigeminocerebellar, and pontocerebellar projections were retrogradely labeled, but only a few olivocerebellar neurons were labeled. Retrogradely labeled neurons in the lateral reticular nucleus tended to locate more laterally and be more condensed into a small compartment in the reeler compared with their normal counterparts. Retrogradely labeled neurons in the external cuneate nucleus were more dorsally shifted in the reeler mice compared with their normal counterparts. We could not find any differences between the normal and reeler mice in the distribution patterns of their trigeminocerebellar projection neurons. Retrogradely labeled pontocerebellar neurons in the basilar pons of the reeler mouse were reduced in number compared with their normal counterparts in addition to being more ventrally and laterally shifted. These findings strongly suggest that the migration of some precerebellar nuclei neurons from the rhombic lip to their final loci may be obstructed in the reeler mice.

Sprouting of sensory neurons in dorsal root ganglia after transection of peripheral nerves

Morphological reaction of sensory neurons of dorsal root ganglia after peripheral nerve transection was investigated by a nerve tracing method using E. coli lacZ (beta-galactosidase) gene recombinant adenovirus. The sciatic nerve of the rat was transected and inoculated with the gene recombinant adenovirus from the cutting end of nerve fibers. The fixation was accomplished from one to six weeks after inoculation. A whole mount specimen was observed after the reaction in a X-galactocidase substrate. Newly formed sprouting processes of dorsal root ganglion (DRG) cells appeared, all of them sprouting from the primary segment of DRG cells. Developed branches were morphologically categorized in to two types: one was the "linear type" which showed diverged branches running straightly along the major axis of the DRG; the other was the "winding type" which exhibited a random running pattern to the original axons and wound and extended in all directions in dorsal root ganglia with many branches. Many of this type encircled other cell bodies and formed a ring-like structure. There was no difference in the size of cell bodies in either type or between the ring-like structure forming the cells and those cells encircled by them.

A newly modified SCG10 promoter and Cre/loxP-mediated gene amplification system achieve highly specific neuronal expression in animal brains

We designed a new promoter that drives transgene expression in an exclusively neuron-specific manner. The promoter of superior cervical ganglion10 (SCG10), expressed in neurons, was further modified to enhance its neuron specificity and activity by changing its length and fusing a multiple neuronal restrictive silencer element (NRSE) to its upstream or downstream regions. The promoter, which contained 2 kb original promoter length and two extra NRSEs in its downstream region, eventually exhibited remarkable neuron specificity as well as strong activity. To further amplify the promoter activity, the promoter was introduced into a Cre recombinase (Cre)-expressing adenovirus, and subsequent combination with Cre-inducible enhanced green fluorescence protein (EGFP)-expressing adenovirus vector, which has much stronger general promoter, resulted in a remarkably strong gene expression exclusively in neuronal cells of mixed cultures and in an animal model. This system is also applicable to astrocyte-specific expression; for instance, by changing the Cre promoter cassette to an astrocyte-specific promoter. The present relatively compact promoter combined with Cre/loxP system could be useful for a wide range of transgene experiments in vivo as well as for clinical applications.