Age-dependent changes in Fibroblast growth factor 2 (FGF-2) expression in mouse cerebellar neurons

Down-Regulation of Fibroblast Growth Factor 2 (FGF2) Contributes to the Premature Senescence of Mouse Embryonic Fibroblast

Background

Freshly isolated mouse embryonic fibroblasts (MEFs) have great proliferation capacity but quickly enter senescent state after several rounds of cell cycle, a process called premature senescence. Cellular senescence can be induced by various stresses such as telomere erosion, DNA damage, and oncogenic signaling. But the contribution of other molecules, such as growth factors, to cellular senescence is incompletely understood. This study aimed to compare the gene expression difference between non-senescent and senescent MEFs to identify the key molecule(s) involved in the spontaneous senescence of MEFs.

Material/Methods

Primary MEFs were isolated from E12.5 pregnant C57/BL6 mice. The cells were continuously cultured in Dulbecco’s Modified Eagle Medium for 9 passages. SA-β-Gal staining was used as an indicator of cell senescence. The supernatant from primary MEFs (P1 medium) or Passage 6 MEFs (P6 medium) were used to culture freshly isolated MEFs to observe the effects on cell senescence state. Gene expression profiles of primary and senescent MEFs were investigated by RNA-Seq to find the key genes involved in cell senescence. Adipocyte differentiation assay was used to evaluate the stemness of MEFs cultured in FGF2-stimulated medium.

Results

The senescence of MEFs cultured in the P1 medium was alleviated when compared to the P6 medium. Downregulation of FGF2 expression was revealed by RNA-Seq and further confirmed by real-time quantitative polymerase chain reaction and western blot. FGF2-stimulated medium also had anti-senescence function and could maintain the differentiation ability of MEFs.

Conclusions

The premature senescence of MEFs was at least partially caused by FGF2 deficiency. Exogenous FGF2 could alleviate the senescent phenotype.

Altered expression of glutamate signaling, growth factor, and glia genes in the locus coeruleus of patients with major depression

Several studies have proposed that brain glutamate signaling abnormalities and glial pathology have a role in the etiology of major depressive disorder (MDD). These conclusions were primarily drawn from post-mortem studies in which forebrain brain regions were examined. The locus coeruleus (LC) is the primary source of extensive noradrenergic innervation of the forebrain and as such exerts a powerful regulatory role over cognitive and affective functions, which are dysregulated in MDD. Furthermore, altered noradrenergic neurotransmission is associated with depressive symptoms and is thought to have a role in the pathophysiology of MDD. In the present study we used laser-capture microdissection (LCM) to selectively harvest LC tissue from post-mortem brains of MDD patients, patients with bipolar disorder (BPD) and from psychiatrically normal subjects. Using microarray technology we examined global patterns of gene expression. Differential mRNA expression of select candidate genes was then interrogated using quantitative real-time PCR (qPCR) and in situ hybridization (ISH). Our findings reveal multiple signaling pathway alterations in the LC of MDD but not BPD subjects. These include glutamate signaling genes, SLC1A2, SLC1A3 and GLUL, growth factor genes FGFR3 and TrkB, and several genes exclusively expressed in astroglia. Our data extend previous findings of altered glutamate, astroglial and growth factor functions in MDD for the first time to the brainstem. These findings indicate that such alterations: (1) are unique to MDD and distinguishable from BPD, and (2) affect multiple brain regions, suggesting a whole-brain dysregulation of such functions.

Expression and Functions of Fibroblast Growth Factor 2 (FGF-2) in Hippocampal Formation

Among the 23 members of the fibroblast growth factor (FGF) family, FGF-2 is the most abundant one in the central nervous system. Its impact on neural cells has been profoundly investigated by in vitro and in vivo studies as well as by gene knockout analyses during the past 2 decades. Key functions of FGF-2 in the nervous system include roles in neurogenesis, promotion of axonal growth, differentiation in development, and maintenance and plasticity in adulthood. From a clinical perspective, its prominent role for the maintenance of lesioned neurons (e.g., ischemia and following transection of fiber tracts) is of particular relevance. In the unlesioned brain, FGF-2 is involved in synaptic plasticity and processes attributed to learning and memory. The focus of this review is on the expression of FGF-2 and its receptors in the hippocampal formation and the physiological and pathophysiological roles of FGF-2 in this region during development and adulthood.

An evaluation of the neuroprotective effects of melatonin in an in vitro experimental model of age-induced neuronal apoptosis

The neuroprotective effects of melatonin in an experimental model of aging-induced apoptosis have been examined. Cerebellar granule neurons show characteristics of apoptosis after 17 days in culture (DV). The addition of melatonin to neuronal cell cultures (100-500 mum) resulted in neuroprotective and antiapoptotic effects, which were revealed by nuclear condensed cell counting. In a thorough analysis by Western-blot of the potential pathways responsible for melatonin's neuroprotective effects, we found an increase in the activation of prosurvival Akt. Subsequently GSK3beta inhibition and an increase in p-FOXO1 phosphorylation occurred. In this model of aging, apoptosis was associated with an elevated DNA damage, as demonstrated by an increase in the activation of ataxia telangiectasia muted (ATM). Subsequently, downstream targets such as p53 were activated. Furthermore, the process of DNA damage was coupled to an increase in the expression of certain proteins involved in cell cycle regulation; these were cyclin D and the proapoptotic transcription factor E2F-1. We conclude that the antiapoptotic effects of melatonin were mediated by two potential mechanisms: by increasing the activity of prosurvival pathways via Akt and by the prevention of DNA damage (via ATM inhibition) followed by the reduction of cell cycle re-entry.

Environmental enrichment alters dentate granule cell morphology in oldest-old rat

The hippocampus of aged rats shows marked age-related morphological changes that could cause memory deficits. Experimental evidence has established that environmental enrichment attenuates memory deficits in aged rats. We therefore studied whether environmental enrichment produces morphological changes on the dentate granule cells of aged rats. Fifteen male Sprague-Dawley rats, 24 months of age, were randomly distributed in two groups that were housed under standard (n = 7) or enriched (n = 8) environmental conditions for 26 days. Quantitative data of dendritic morphology from dentate gyrus granule cells were obtained on Golgi-Cox stained sections. Environmental enrichment significantly increased the complexity and size of dendritic tree (total number of segments increased by 61% and length by 116%), and spine density (88% increase). There were large interindividual differences within the enriched group, indicating differential individual responses to environmental stimulation. Previous studies in young animals have shown changes produced by environmental enrichment in the morphology of dentate gyrus granule cells. The results of the present study show that environmental enrichment can also produce changes in dentate granule cell morphology in the senescent brain. In conclusion, the hippocampus retains its neuroplastic capacity during aging, and enriched environmental housing conditions can attenuate age-related dendritic regression and synaptic loss, thus preserving memory functions.

Neurobin/TMPRSS11c, a novel type II transmembrane serine protease that cleaves fibroblast growth factor-2 in vitro

TTSPs [type II TMPRSSs (transmembrane serine proteases)] are a growing family of trypsin-like enzymes with, in some cases, restricted tissue distribution. To investigate the expression of TTSPs in the nervous system, we performed a PCR-based screening approach with P10 (postnatal day 10) mouse spinal cord mRNA. We detected the expression of five known TTSPs and identified a novel TTSP, which we designated neurobin. Neurobin consists of 431 amino acids. In the extracellular part, neurobin contains a single SEA (sea-urchin sperm protein, enterokinase and agrin) domain and a C-terminal serine protease domain. RT-PCR (reverse transcription-PCR) analysis indicated the expression of neurobin in spinal cord and cerebellum. Histochemical analysis of brain sections revealed distinct staining of Purkinje neurons of the cerebellum. Transiently overexpressed neurobin was autocatalytically processed and inserted into the plasma membrane. Autocatalytic activation could be suppressed by mutating Ser(381) in the catalytic pocket to an alanine residue. The protease domain of neurobin, produced in Escherichia coli and refolded from inclusion bodies, cleaved chromogenic peptides with an arginine residue in position P(1). Serine protease inhibitors effectively suppressed the proteolytic activity of recombinant neurobin. Ca2+ or Na+ ions did not significantly modulate the catalytic activity of the protease. Recombinant neurobin processed 17-kDa FGF-2 (fibroblast growth factor-2) at several P(1) lysine and arginine positions to distinct fragments, in a heparin-inhibitable manner, but did not cleave FGF-7, laminin or fibronectin. These results indicate that neurobin is an authentic TTSP with trypsin-like activity and is able to process FGF-2 in vitro.

Nuclear trafficking of secreted factors and cell-surface receptors: new pathways to regulate cell proliferation and differentiation, and involvement in cancers

Secreted factors and cell surface receptors can be internalized by endocytosis and translocated to the cytoplasm. Instead of being recycled or proteolysed, they sometimes translocate to the nucleus. Nuclear import generally involves a nuclear localization signal contained either in the secreted factor or its transmembrane receptor, that is recognized by the importins machinery. In the nucleus, these molecules regulate transcription of specific target genes by direct binding to transcription factors or general coregulators. In addition to the transcription regulation, nuclear secreted proteins and receptors seem to be involved in other important processes for cell life and cellular integrity such as DNA replication, DNA repair and RNA metabolism.

Nuclear secreted proteins and transmembrane receptors now appear to induce new signaling pathways to regulate cell proliferation and differentiation. Their nuclear localization is often transient, appearing only during certain phases of the cell cycle. Nuclear secreted and transmembrane molecules regulate the proliferation and differentiation of a large panel of cell types during embryogenesis and adulthood and are also potentially involved in wound healing. Secreted factors such as CCN proteins, EGF, FGFs and their receptors are often detected in the nucleus of cancer cells. Nuclear localization of these molecules has been correlated with tumor progression and poor prognosis for patient survival. Nuclear growth factors and receptors may be responsible for resistance to radiotherapy.

Age-related changes in cell density and the proliferation rate of olfactory ensheathing cells in the lamina propria of postnatal mouse olfactory mucosa

We investigated age-related changes in the distribution and proliferation of olfactory ensheathing cells (OECs) in postnatal mouse olfactory mucosa. In contrast to reported data on other glial cell types in the peripheral and central nervous systems, OEC cell density in the olfactory nerve bundles in the lamina propria remained almost constant from 10 days through 16 months of age. Electron microscopy of the nerve bundles revealed that axon packing density also was constant during that period. These findings suggest that the ratio of the number of OECs to the unit length of the olfactory neuron axons ensheathed by them does not change markedly throughout the lifetime of mice in an undisturbed condition. By contrast, OEC proliferative density decreased rapidly in the 10-day to 1-month-old period, showing a significant difference, and for the rest of life remained at low level, similar to previous values reported for other glial cell types.