HECTD1 is both a positive regulator and substrate of caspase-3 activity during apoptotic cell death

Programmed cell death is a complex and tightly regulated sequence of events that determines cell fate during tissue homeostasis, development, and pathogenesis. The small protein modifier ubiquitin mediates important regulatory functions during cell death by regulating the stability and activity of checkpoint proteins and the assembly of cell death signalling complexes. The caspase family of cysteine aspartases are essential effectors of apoptotic cell death. Components of the ubiquitin system including RING ubiquitin ligases XIAP, MDM2, RBX1; RBR E3 ubiquitin ligases Parkin and LUBAC; and HECT E3 ubiquitin ligases NEDD4 and Itch are also substrates of caspase-mediated cleavage. In the case of NEDD4 and Itch, the single cleavage event occurs outside of the catalytic HECT domain and it remains unclear whether such cleavage events impact on ubiquitin ligase activity and/or function. Here, we identified the E3 ubiquitin ligase HECTD1 as the third HECT E3 cleaved by caspase-mediated cleavage during apoptotic cell death, in a manner which does not affect the integrity of the catalytic C-ter HECT domain. We mapped the single cleavage event to DFLD1664↓S and showed that the cleaved C-ter product, which contains the HECT ligase domain, is as stable as the endogenous full length protein. We also found that HECTD1 transient depletion led to reduced caspase-3 activity, but not caspase 8 nor 9. Furthermore, we also identified caspase-3 as the protease responsible for HECTD1 cleavage at Asp1664 suggesting that HECTD1 and caspase-3 might be part of a novel feedback loop mechanism during apoptotic cell death. This study highlight novel crosstalk between cell death mechanisms and the ubiquitin system and raises important questions on whether proteolytic cleavage of E3 ubiquitin ligases might represent an underappreciated mode of regulation during cell death mechanisms

PHYCOERYTHROCYANINS FROM Westiellopsis prolifica AND Nostoc rivulare: CHARACTERIZATION OF THE PHYCOVIOLOBILIN CHROMOPHORE IN BOTH STATES

Phycoerythrocyanin or fractions enriched in it have been isolated from the filamentous cyanobacteria, Westiellopsis prolifica ARM 365 and Nostoc rivulare ARM 212. Both show the photoreversible photochromism (difference maxima at 503 and 570 nm) characteristic of this pigment, which is related to the phycoviolobilin chromophore on the α-subunit. Native phycoerythrocyanin and its β-subunit show little if any reversible photochemistry in the 600–620 nm region, where the phycocyanobilin chromophores absorb maximally. Instead the phycocyanobilin chromophores are bleached irreversibly. At the same time, the data show that reversible photochemistry is a useful analytical tool to detect phycoerythrocyanin in cyanobacterial extracts. Fluorescence measurements indicate that: (i) the 510 nm absorbing isomer of the violobilin chromophore has only little fluorescence; and (ii) the energy transfer from the violobilin chromophores to the cyanin chromophores is efficient only in the 570 nm form

FLUORESCENCE AND CIRCULAR DICHROISM STUDIES ON THE PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM

Two phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobac-terium Westiellopsis prolifica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (α-84) to the cyanin (β-84, 155) chromophores is efficient in the trimer (αβ it is impeded after dissociation to the monomer (α,β). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol. 55,119–124, 1992), exhibited the spectral properties similar to that of the α-subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained

Targeting the Ubiquitin System in Glioblastoma

Glioblastoma is the most common primary brain tumor in adults with poor overall outcome and 5-year survival of less than 5%. Treatment has not changed much in the last decade or so, with surgical resection and radio/chemotherapy being the main options. Glioblastoma is highly heterogeneous and frequently becomes treatment-resistant due to the ability of glioblastoma cells to adopt stem cell states facilitating tumor recurrence. Therefore, there is an urgent need for novel therapeutic strategies. The ubiquitin system, in particular E3 ubiquitin ligases and deubiquitinating enzymes, have emerged as a promising source of novel drug targets. In addition to conventional small molecule drug discovery approaches aimed at modulating enzyme activity, several new and exciting strategies are also being explored. Among these, PROteolysis TArgeting Chimeras (PROTACs) aim to harness the endogenous protein turnover machinery to direct therapeutically relevant targets, including previously considered “undruggable” ones, for proteasomal degradation. PROTAC and other strategies targeting the ubiquitin proteasome system offer new therapeutic avenues which will expand the drug development toolboxes for glioblastoma. This review will provide a comprehensive overview of E3 ubiquitin ligases and deubiquitinating enzymes in the context of glioblastoma and their involvement in core signaling pathways including EGFR, TGF-β, p53 and stemness-related pathways. Finally, we offer new insights into how these ubiquitin-dependent mechanisms could be exploited therapeutically for glioblastoma

Spatial distribution and functional relevance of FGFR1 and FGFR2 expression for glioblastoma tumor invasion

Glioblastoma is the most lethal brain cancer in adults. These incurable tumors are characterized by profound heterogeneity, therapy resistance, and diffuse infiltration. These traits have been linked to cancer stem cells, which are important for glioblastoma tumor progression and recurrence. The fibroblast growth factor receptor 1 (FGFR1) signaling pathway is a known regulator of therapy resistance and cancer stemness in glioblastoma. FGFR1 expression shows intertumoral heterogeneity and higher FGFR1 expression is associated with a significantly poorer survival in glioblastoma patients. The role of FGFR1 in tumor invasion has been studied in many cancers, but whether and how FGFR1 mediates glioblastoma invasion remains to be determined. Here, we investigated the distribution and functional relevance of FGFR1 and FGFR2 in human glioblastoma xenograft models. We found FGFR1, but not FGFR2, expressed in invasive glioblastoma cells. Loss of FGFR1, but not FGFR2, significantly reduced cell migration in vitro and tumor invasion in human glioblastoma xenografts. Comparative analysis of RNA-sequencing data of FGFR1 and FGFR2 knockdown glioblastoma cells revealed a FGFR1-specific gene regulatory network associated with tumor invasion. Our study reveals new gene candidates linked to FGFR1-mediated glioblastoma invasion

Fibroblast growth factor receptor functions in glioblastoma

Glioblastoma is the most lethal brain cancer in adults, with no known cure. This cancer is characterized by a pronounced genetic heterogeneity, but aberrant activation of receptor tyrosine kinase signaling is among the most frequent molecular alterations in glioblastoma. Somatic mutations of fibroblast growth factor receptors (FGFRs) are rare in these cancers, but many studies have documented that signaling through FGFRs impacts glioblastoma progression and patient survival. Small-molecule inhibitors of FGFR tyrosine kinases are currently being trialed, underlining the therapeutic potential of blocking this signaling pathway. Nevertheless, a comprehensive overview of the state of the art of the literature on FGFRs in glioblastoma is lacking. Here, we review the evidence for the biological functions of FGFRs in glioblastoma, as well as pharmacological approaches to targeting these receptors

Mesenchymal Stem Cells Promote Oligodendroglial Differentiation in Hippocampal Slice Cultures

We have previously shown that soluble factors derived from mesenchymal stem cells (MSCs) induce oligodendrogenic fate and differentiation in adult rat neural progenitors (NPCs) in vitro. Here, we investigated if this pro-oligodendrogenic effect is maintained after cells have been transplanted onto rat hippocampal slice cultures, a CNS-organotypic environment. We first tested whether NPCs, that were pre-differentiated in vitro by MSC-derived conditioned medium, would generate oligodendrocytes after transplantation. This approach resulted in the loss of grafted NPCs, suggesting that oligodendroglial pre-differentiated cells could not integrate in the tissue and therefore did not survive grafting. However, when NPCs together with MSCs were transplanted in situ into hippocampal slice cultures, the grafted NPCs survived and the majority of them differentiated into oligodendrocytes. In contrast to the prevalent oligodendroglial differentiation in case of the NPC/MSC co-transplantation, naive NPCs transplanted in the absence of MSCs differentiated predominantly into astrocytes. In summary, the pro-oligodendrogenic activity of MSCs was maintained only after co-transplantation into hippocampal slice cultures. Therefore, in the otherwise astrogenic milieu, MSCs established an oligodendrogenic niche for transplanted NPCs, and thus, co-transplantation of MSCs with NPCs might provide an attractive approach to re-myelinate the various regions of the diseased CNS. Copyright (C) 2009 S. Karger AG, Base

TWO DIFFERENT TYPES OF PHOTOCHEMISTRY IN PHYCOERYTHROCYANIN α-SUBUNIT

The photochemical activities of phycoerythrocyanin α-subunits from Mastigocladus laminosus separated by isoelectric focusing were tested by irradiating at 500, 550, 577 and 600 nm. Two types of photoreversible photochromic responses have been characterized by absorption and absorption difference spectroscopy. Type I is the well-known absorption shift from 571 to 506 nm. Type II is a new response characterized by a line-broadening of the 570 nm absorption

Transcriptional control of embryonic and adult neural progenitor activity

Neural precursors generate neurons in the embryonic brain and in restricted niches of the adult brain in a process called neurogenesis. The precise control of cell proliferation and differentiation in time and space required for neurogenesis depends on sophisticated orchestration of gene transcription in neural precursor cells. Much progress has been made in understanding the transcriptional regulation of neurogenesis, which relies on dose- and context-dependent expression of specific transcription factors that regulate the maintenance and proliferation of neural progenitors, followed by their differentiation into lineage-specified cells. Here, we review some of the most widely studied neurogenic transcription factors in the embryonic cortex and neurogenic niches in the adult brain. We compare functions of these transcription factors in embryonic and adult neurogenesis, highlighting biochemical, developmental, and cell biological properties. Our goal is to present an overview of transcriptional regulation underlying neurogenesis in the developing cerebral cortex and in the adult brain