Rosa M. Rios

hpttg, a human homologue of rat pttg, is overexpressed in hematopoietic neoplasms: Evidence for a transcriptional activation function of hPTTG

We have isolated a human cDNA clone encoding a novel protein of 22 kDa that is a human counterpart of the rat oncoprotein PTTG. We show that the corresponding gene (hpttg) is overexpressed in Jurkat cells (a human T lymphoma cell line) and in samples from patients with different kinds of hematopoietic malignancies. Analysis of the sequence showed that hPTTG has an amino-terminal basic domain and a carboxyl-terminal acidic domain, and that it is a proline-rich protein with several putative SH3-binding sites. Subcellular fractionation studies show that, although hPTTG is mainly a cytosolic protein, it is partially localized in the nucleus. In addition we demonstrate that the acidic carboxyl-terminal region of hPTTG acts as a transactivation domain when fused to a heterologous DNA binding domain, both in yeast and in mammalian cells.

Microtubule nucleation at the cis-side of the Golgi apparatus requires AKAP450 and GM130

We report that microtubule (MT) nucleation at the Golgi apparatus requires AKAP450, a centrosomal γ‐TuRC‐interacting protein that also forms a distinct network associated with the Golgi. Depletion of AKAP450 abolished MT nucleation at the Golgi, whereas depletion of the cis‐Golgi protein GM130 led to the disorganisation of AKAP450 network and impairment of MT nucleation. Brefeldin‐A treatment induced relocalisation of AKAP450 to ER exit sites and concomitant redistribution of MT nucleation capacity to the ER. AKAP450 specifically binds the cis‐side of the Golgi in an MT‐independent, GM130‐dependent manner. Short AKAP450‐dependent growing MTs are covered by CLASP2. Like for centrosome, dynein/dynactin complexes are necessary to anchor MTs growing from the Golgi. We further show that Golgi‐associated AKAP450 has a role in cell migration rather than in cell polarisation of the centrosome–Golgi apparatus. We propose that the recruitment of AKAP450 on the Golgi membranes through GM130 allows centrosome‐associated nucleating activity to extend to the Golgi, to control the assembly of subsets of MTs ensuring specific functions within the Golgi or for transporting specific cargos to the cell periphery.

GMAP-210 Recruits γ-Tubulin Complexes to cis-Golgi Membranes and Is Required for Golgi Ribbon Formation

Mammalian cells concentrate Golgi membranes around the centrosome in a microtubule-dependent manner. The mechanisms involved in generating a single Golgi ribbon in the periphery of the centrosome remain unknown. Here we show that GMAP-210, a cis-Golgi microtubule binding protein, recruits gamma-tubulin-containing complexes to Golgi membranes even in conditions where microtubule polymerization is prevented and independently of Golgi apparatus localization within the cell. Under overexpression conditions, very short microtubules, or tubulin oligomers, are stabilized on Golgi membranes. GMAP-210 depletion by RNA interference results in extensive fragmentation of the Golgi apparatus, supporting a role for GMAP-210 in Golgi ribbon formation. Targeting of GMAP-210 or its C terminus to mitochondria induces the recruitment of gamma-tubulin to their surface and redistribution of mitochondria to a pericentrosomal location. All our experiments suggest that GMAP-210 displays microtubule anchoring and membrane fusion activities, thus contributing to the assembly and maintenance of the Golgi ribbon around the centrosome.

The Golgi apparatus at the cell centre

In non-polarised mammalian cells, the Golgi apparatus is localised around the centrosome and actively maintained there. Microtubules and molecular motor activity are required for determining both the localisation and organisation of the Golgi apparatus. Other factors, however, also appear necessary for regulating both the static steady-state distribution of this organelle and its relationship with microtubule minus-end-anchoring activities of the centrosome. Several non-motor microtubule-binding proteins have now been found to be associated with the Golgi apparatus. Recent advances suggest that, in addition to important roles in cell motility, polarisation and differentiation, the interplay between Golgi apparatus and centrosome could participate in other physiological processes such as intracellular signalling, mitosis and apoptosis.

Disconnecting the Golgi ribbon from the centrosome prevents directional cell migration and ciliogenesis

AKAP450 is a critical determinant of Golgi ribbon integrity, positioning, and function.

Identification of a high affinity binding protein for the regulatory subunit RII beta of cAMP-dependent protein kinase in Golgi enriched membranes of human lymphoblasts.

Immunocytochemical evidence of an association between the regulatory subunit RII of the cAMP‐dependent protein kinase (cAMP‐PK) and the Golgi apparatus in several cell types has been reported. In order to identify endogenous Golgi proteins binding RII, a fraction enriched in Golgi vesicles was isolated from human lymphoblasts. Only the RII beta isoform was detected in the Golgi‐rich fraction, although RII alpha has also been found to be present in these cells. A 85 kDa RII‐binding protein was identified in Golgi vesicles using a [32P]RII overlay of Western blots. The existence of an endogenous RII beta‐p85 complex in isolated Golgi vesicles was demonstrated by two independent means: (i) co‐immunoprecipitation of both proteins under non‐denaturing conditions with an antibody against RII beta and (ii) co‐purification of RII beta‐p85 complexes on a cAMP‐analogue affinity column. p85 was phosphorylated by both endogenous and purified catalytic subunits of cAMP‐pKII. Extraction experiments and protease protection experiments indicated that p85 is an integral membrane protein although it partitioned atypically during Triton X‐114 phase separation. We propose that p85 anchors RII beta to the Golgi apparatus of human lymphoblasts and thereby defines the Golgi substrate targets most accessible to phosphorylation by C subunit. This mechanism may be relevant to the regulation of processes involving the Golgi apparatus itself, such as membrane traffic and secretion, but also relevant to nearby nuclear events dependent on C subunit.

The Overexpression of GMAP-210 Blocks Anterograde and Retrograde Transport Between the ER and the Golgi Apparatus

Golgi Microtubule‐Associated Protein (GMAP)‐210 is a peripheral coiled‐coil protein associated with the cis‐Golgi network that interacts with microtubule minus ends. GMAP‐210 overexpression has previously been shown to perturb the microtubule network and to induce a dramatic enlargement and fragmentation of the Golgi apparatus (Infante C, Ramos‐Morales F, Fedriani C, Bornens M, Rios RM. J Cell Biol 1999; 145: 83–98). We now report that overexpressing GMAP‐210 blocks the anterograde transport of both a soluble form of alkaline phosphatase and the hemagglutinin protein of influenza virus, an integral membrane protein, between the endoplasmic reticulum and the cis/medial (mannosidase II‐positive) Golgi compartment. Retrograde transport of the Shiga toxin B‐subunit is also blocked between the Golgi apparatus and the endoplasmic reticulum. As a consequence, the B‐subunit accumulates in compartments positive for GMAP‐210. Ultrastructural analysis revealed that, under these conditions, the Golgi complex is totally disassembled and Golgi proteins as well as proteins of the intermediate compartment are found in vesicle clusters distributed throughout the cell. The role of GMAP‐210 on membrane processes at the interface between the endoplasmic reticulum and the Golgi apparatus is discussed in the light of the property of this protein to bind CGN membranes and microtubules.

The centrosome–Golgi apparatus nexus

A shared feature among all microtubule (MT)-dependent processes is the requirement for MTs to be organized in arrays of defined geometry. At a fundamental level, this is achieved by precisely controlling the timing and localization of the nucleation events that give rise to new MTs. To this end, MT nucleation is restricted to specific subcellular sites called MT-organizing centres. The primary MT-organizing centre in proliferating animal cells is the centrosome. However, the discovery of MT nucleation capacity of the Golgi apparatus (GA) has substantially changed our understanding of MT network organization in interphase cells. Interestingly, MT nucleation at the Golgi apparently relies on multiprotein complexes, similar to those present at the centrosome, that assemble at the cis-face of the organelle. In this process, AKAP450 plays a central role, acting as a scaffold to recruit other centrosomal proteins important for MT generation. MT arrays derived from either the centrosome or the GA differ in their geometry, probably reflecting their different, yet complementary, functions. Here, I review our current understanding of the molecular mechanisms involved in MT nucleation at the GA and how Golgi- and centrosome-based MT arrays work in concert to ensure the formation of a pericentrosomal polarized continuous Golgi ribbon structure, a critical feature for cell polarity in mammalian cells. In addition, I comment on the important role of the Golgi-nucleated MTs in organizing specialized MT arrays that serve specific functions in terminally differentiated cells.

GMAP210 and IFT88 are present in the spermatid golgi apparatus and participate in the development of the acrosome–acroplaxome complex, head–tail coupling apparatus and tail

We describe the localization of the golgin GMAP210 and the intraflagellar protein IFT88 in the Golgi of spermatids and the participation of these two proteins in the development of the acrosome–acroplaxome complex, the head–tail coupling apparatus (HTCA) and the spermatid tail. Immunocytochemical experiments show that GMAP210 predominates in the cis‐Golgi, whereas IFT88 prevails in the trans‐Golgi network. Both proteins colocalize in proacrosomal vesicles, along acrosome membranes, the HTCA and the developing tail. IFT88 persists in the acrosome–acroplaxome region of the sperm head, whereas GMAP210 is no longer seen there. Spermatids of the Ift88 mouse mutant display abnormal head shaping and are tail‐less. GMAP210 is visualized in the Ift88 mutant during acrosome–acroplaxome biogenesis. However, GMAP210–stained vesicles, mitochondria and outer dense fiber material build up in the manchette region and fail to reach the abortive tail stump in the mutant. In vitro disruption of the spermatid Golgi and microtubules with Brefeldin‐A and nocodazole blocks the progression of GMAP210‐ and IFT88‐stained proacrosomal vesicles to the acrosome–acroplaxome complex but F‐actin distribution in the acroplaxome is not affected. We provide the first evidence that IFT88 is present in the Golgi of spermatids, that the microtubule‐associated golgin GMAP210 and IFT88 participate in acrosome, HTCA, and tail biogenesis, and that defective intramanchette transport of cargos disrupts spermatid tail development. Developmental Dynamics 240:723–736, 2011.

Dysfunction of the unfolded protein response increases neurodegeneration in aged rat hippocampus following proteasome inhibition

Dysfunctions of the ubiquitin proteasome system (UPS) have been proposed to be involved in the aetiology and/or progression of several age‐related neurodegenerative disorders. However, the mechanisms linking proteasome dysfunction to cell degeneration are poorly understood. We examined in young and aged rat hippocampus the activation of the unfolded protein response (UPR) under cellular stress induced by proteasome inhibition. Lactacystin injection blocked proteasome activity in young and aged animals in a similar extent and increased the amount of ubiquitinated proteins. Young animals activated the three UPR arms, IRE1α, ATF6α and PERK, whereas aged rats failed to induce the IRE1α and ATF6α pathways. In consequence, aged animals did not induce the expression of pro‐survival factors (chaperones, Bcl‐XL and Bcl‐2), displayed a more sustained expression of pro‐apoptotic markers (CHOP, Bax, Bak and JKN), an increased caspase‐3 processing. At the cellular level, proteasome inhibition induced neuronal damage in young and aged animals as assayed using Fluorojade‐B staining. However, degenerating neurons were evident as soon as 24 h postinjection in aged rats, but it was delayed up to 3 days in young animals. Our findings show evidence supporting age‐related dysfunctions in the UPR activation as a potential mechanism linking protein accumulation to cell degeneration. An imbalance between pro‐survival and pro‐apoptotic proteins, because of noncanonical activation of the UPR in aged rats, would increase the susceptibility to cell degeneration. These findings add a new molecular vision that might be relevant in the aetiology of several age‐related neurodegenerative disorders.