Jukka Kallijärvi

The TRIM37 Gene Encodes a Peroxisomal RING-B-Box-Coiled-Coil Protein: Classification of Mulibrey Nanism as a New Peroxisomal Disorder

Mulibrey nanism is a rare growth disorder of prenatal onset caused by mutations in the TRIM37 gene, which encodes a RING-B-box-coiled-coil protein. The pathogenetic mechanisms of mulibrey nanism are unknown. We have used transiently transfected cells and antibodies raised against the predicted TRIM37 protein to characterize the TRIM37 gene product and to determine its intracellular localization. We show that the human TRIM37 cDNA encodes a peroxisomal protein with an apparent molecular weight of 130 kD. Peroxisomal localization is compromised in mutant protein representing the major Finnish TRIM37 mutation but is retained in the protein representing the minor Finnish mutation. Colocalization of endogenous TRIM37 with peroxisomal markers was observed by double immunofluorescence staining in HepG2 and human intestinal smooth muscle cell lines. In human tissue sections, TRIM37 shows a granular cytoplasmic pattern. Endogenous TRIM37 is not imported into peroxisomes in peroxin 1 (PEX1(-/-)) and peroxin 5 (PEX5(-/-)) mutant fibroblasts but is imported normally in peroxin 7 (PEX7(-/-)) deficient fibroblasts, giving further evidence for a peroxisomal localization of TRIM37. Fibroblasts derived from patients with mulibrey nanism lack C-terminal TRIM37 immunoreactivity but stain normally for both peroxisomal matrix and membrane markers, suggesting apparently normal peroxisome biogenesis in patient fibroblasts. Taken together, this molecular evidence unequivocally indicates that TRIM37 is located in the peroxisomes, and Mulibrey nanism thus can be classified as a new peroxisomal disorder.

RNAi screening for kinases and phosphatases identifies FoxO regulators

Forkhead box class O (FoxO) transcription factors are key regulators of growth, metabolism, life span, and stress resistance. FoxOs integrate signals from different pathways and guide the cellular response to varying energy and stress conditions. FoxOs are modulated by several signaling pathways, e.g., the insulin-TOR signaling pathway and the stress induced JNK signaling pathway. Here, we report a genome wide RNAi screen of kinases and phosphatases aiming to find regulators of dFoxO activity in Drosophila S2 cells. By using a combination of transcriptional activity and localization assays we identified several enzymes that modulate dFoxO transcriptional activity, intracellular localization and/or protein stability. Importantly, several currently known dFoxO regulators were found in the screening, confirming the validity of our approach. In addition, several interesting new regulators were identified, including protein kinase C and glycogen synthase kinase 3β, two proteins with important roles in insulin signaling. Furthermore, several mammalian orthologs of the proteins identified in Drosophila also regulate FOXO activity in mammalian cells. Our results contribute to a comprehensive understanding of FoxO regulatory processes.

Apolipoprotein E includes a binding site which is recognized by several amyloidogenic polypeptides.

Inheritance of the apolipoprotein E (apoE) epsilon 4 allele is a risk factor for late-onset Alzheimers disease (AD). Biochemically apoE is present in AD plaques and neurofibrillary tangles of the AD brain. There is a high avidity and specific binding of apoE and the amyloid beta-peptide (A beta). In addition to AD apoE is also present in many other cerebral and systemic amyloidoses, Downs syndrome and prion diseases but the pathophysiological basis for its presence is still unknown. In the present study we have compared the interaction of apoE with A beta, the gelsolin-derived amyloid fragment AGel(183-210) and the amyloidogenic prion fragments PrP(109-122) and PrP(109-141). We show that, similar to A beta, also AGel and PrP fragments can form a complex with apoE, and that the interaction between apoE and the amyloidogenic protein fragments is mediated through the same binding site on apoE. We also show that apoE increases the thioflavin-T fluorescence of PrP and AGel and that apoE influences the content of beta-sheet conformation of these amyloidogenic fragments. Our results indicate that amyloids and amyloidogenic prion fragments share a similar structural motif, which is recognized by apoE, possibly through a single binding site, and that this motif is also responsible for the amyloidogenicity of these fragments.

Characterization of the structural and functional determinants of MANF/CDNF in Drosophila in vivo model.

Mammalian MANF and CDNF proteins are evolutionarily conserved neurotrophic factors that can protect and repair mammalian dopaminergic neurons in vivo. In Drosophila, the sole MANF protein (DmManf) is needed for the maintenance of dopaminergic neurites and dopamine levels. Although both secreted and intracellular roles for MANF and CDNF have been demonstrated, very little is known about the molecular mechanism of their action. Here, by using a transgenic rescue approach in the DmManf mutant background we show that only full-length MANF containing both the amino-terminal saposin-like and carboxy-terminal SAP-domains can rescue the larval lethality of the DmManf mutant. Independent N- or C-terminal domains of MANF, even when co-expressed together, fail to rescue. Deleting the signal peptide or mutating the CXXC motif in the C-terminal domain destroys the activity of full-length DmManf. Positively charged surface amino acids and the C-terminal endoplasmic reticulum retention signal are necessary for rescue of DmManf mutant lethality when DmManf is expressed in a restricted pattern. Furthermore, rescue experiments with non-ubiquitous expression reveals functional differences between the C-terminal domain of human MANF and CDNF. Finally, DmManf and its C-terminal domain rescue mammalian sympathetic neurons from toxin-induced apoptosis in vitro demonstrating functional similarity of the mammalian and fly proteins. Our study offers further insights into the functional conservation between invertebrate and mammalian MANF/CDNF proteins and reveals the importance of the C-terminal domain for MANF activity in vivo.

Tissue expression of the mulibrey nanism-associated Trim37 protein in embryonic and adult mouse tissues

Mutations in the TRIM37 gene underlie mulibrey nanism (muscle–liver–brain–eye nanism), a rare monogenic developmental disorder characterized by severe growth failure, characteristic dysmorphic features, cardiopathy, failure of sexual maturation, and metabolic syndrome. The TRIM37 protein, a member of the tripartite motif subfamily of RING finger proteins, is highly conserved between human and mouse. High evolutionary conservation is seen also at the gene level. We here show that the mouse Trim37 gene presents several alternative splice variants, including a testis-specific transcript with an additional 3′ exon. By Northern blot analysis the highest level of Trim37 mRNA was detected in testis and brain. In embryonic tissues, the Trim37 protein was detected in epithelia, including ducts of the developing pancreas, epithelium of the midgut and nasal epithelium. In adult mouse tissues, Trim37 immunoreactivity was detected in the central and peripheral nervous systems, including enteric ganglia, retina, and the adrenal medulla. Moreover, specific cellular populations in the adenohypophysis, pancreatic islets, intestine and gonads showed intense Trim37 staining. Both nuclear and granular cytoplasmic staining patterns were observed. These findings are in agreement with the clinical manifestations of mulibrey nanism and provide a basis for the future analysis of Trim37 knock-out mice.

Characterization of Drosophila GDNF Receptor-Like and Evidence for Its Evolutionarily Conserved Interaction with Neural Cell Adhesion Molecule (NCAM)/FasII

Background Glial cell line-derived neurotrophic factor (GDNF) family ligands are secreted growth factors distantly related to the TGF-β superfamily. In mammals, they bind to the GDNF family receptor α (Gfrα) and signal through the Ret receptor tyrosine kinase. In order to gain insight into the evolution of the Ret-Gfr-Gdnf signaling system, we have cloned and characterized the first invertebrate Gfr-like cDNA (DmGfrl) from Drosophila melanogaster and generated a DmGfrl mutant allele. Results We found that DmGfrl encodes a large GPI-anchored membrane protein with four GFR-like domains. In line with the fact that insects lack GDNF ligands, DmGfrl mediated neither Drosophila Ret phosphorylation nor mammalian RET phosphorylation. In situ hybridization analysis revealed that DmGfrl is expressed in the central and peripheral nervous systems throughout Drosophila development, but, surprisingly, DmGfrl and DmRet expression patterns were largely non-overlapping. We generated a DmGfrl null allele by genomic FLP deletion and found that both DmGfrl null females and males are viable but display fertility defects. The female fertility defect manifested as dorsal appendage malformation, small size and reduced viability of eggs laid by mutant females. In male flies DmGfrl interacted genetically with the Drosophila Ncam (neural cell adhesion molecule) homolog FasII to regulate fertility. Conclusion Our results suggest that Ret and Gfrl did not function as an in cis receptor-coreceptor pair before the emergence of GDNF family ligands, and that the Ncam-Gfr interaction predated the in cis Ret-Gfr interaction in evolution. The fertility defects that we describe in DmGfrl null flies suggest that GDNF receptor-like has an evolutionarily ancient role in regulating male fertility and a previously unrecognized role in regulating oogenesis. Significance These results shed light on the evolutionary aspects of the structure, expression and function of Ret-Gfrα and Ncam-Gfrα signaling complexes.

Gynecological tumors in Mulibrey nanism and role for RING finger protein TRIM37 in the pathogenesis of ovarian fibrothecomas

Mulibrey nanism is an autosomal recessive growth disorder caused by mutations in the TRIM37 gene encoding a protein of unknown function. More than half of female patients with Mulibrey nanism develop benign mesenchymal tumors of ovarian sex cord–stromal origin. In this work, we characterize the gynecological tumors of female patients with Mulibrey nanism in detail. In addition to tumors of the fibrothecoma group, 18% (4/22) of the patients were observed with epithelial neoplasias, including 2 ovarian adenofibromas, 1 ovarian poorly differentiated adenocarcinoma and 1 endometrial adenocarcinoma. To investigate the possible involvement of TRIM37 alterations in the pathogenesis of sporadic fibrothecomas, we analyzed the TRIM37 cDNA for mutations and alternatively spliced transcripts and TRIM37 expression in fibrothecomas of women without Mulibrey nanism. No mutations in the open-reading frame of TRIM37 were detected. Two alternatively spliced variants were found, one lacking exon 23 and one exon 2. TRIM37del2 was also found in normal ovary but in a proportion of sporadic fibrothecomas, the TRIM37del2:TRIM37 ratio was increased. In normal ovary, TRIM37 was localized in the cytoplasm of stromal cells, especially theca cells surrounding developing follicles. TRIM37 transcript was found in all sporadic fibrothecomas examined, but 80% (20/25) of the tumors showed reduced or absent expression of TRIM37 protein. Allelic loss at the TRIM37 locus (17q22–23) was observed in 6% of sporadic fibrothecomas. Nearly half of the sporadic fibrothecomas showed evidence of CpG promoter methylation, suggesting promoter downregulation as one mechanism of reduced TRIM37 expression. In conclusion, inherited biallelic inactivation of TRIM37 (Mulibrey nanism) predisposes to both mesenchymal and epithelial ovarian tumors and dysregulation of TRIM37 may also be involved in the pathogenesis of sporadic fibrothecomas.

Exploring the Conserved Role of MANF in the Unfolded Protein Response in Drosophila melanogaster

Disturbances in the homeostasis of endoplasmic reticulum (ER) referred to as ER stress is involved in a variety of human diseases. ER stress activates unfolded protein response (UPR), a cellular mechanism the purpose of which is to restore ER homeostasis. Previous studies show that Mesencephalic Astrocyte-derived Neurotrophic Factor (MANF) is an important novel component in the regulation of UPR. In vertebrates, MANF is upregulated by ER stress and protects cells against ER stress-induced cell death. Biochemical studies have revealed an interaction between mammalian MANF and GRP78, the major ER chaperone promoting protein folding. In this study we discovered that the upregulation of MANF expression in response to drug-induced ER stress is conserved between Drosophila and mammals. Additionally, by using a genetic in vivo approach we found genetic interactions between Drosophila Manf and genes encoding for Drosophila homologues of GRP78, PERK and XBP1, the key components of UPR. Our data suggest a role for Manf in the regulation of Drosophila UPR.

Ret receptor tyrosine kinase sustains proliferation and tissue maturation in intestinal epithelia

Expression of the Ret receptor tyrosine kinase is a defining feature of enteric neurons. Its importance is underscored by the effects of its mutation in Hirschsprung disease, leading to absence of gut innervation and severe gastrointestinal symptoms. We report a new and physiologically significant site of Ret expression in the intestine: the intestinal epithelium. Experiments in Drosophila indicate that Ret is expressed both by enteric neurons and adult intestinal epithelial progenitors, which require Ret to sustain their proliferation. Mechanistically, Ret is engaged in a positive feedback loop with Wnt/Wingless signalling, modulated by Src and Fak kinases. We find that Ret is also expressed by the developing intestinal epithelium of mice, where its expression is maintained into the adult stage in a subset of enteroendocrine/enterochromaffin cells. Mouse organoid experiments point to an intrinsic role for Ret in promoting epithelial maturation and regulating Wnt signalling. Our findings reveal evolutionary conservation of the positive Ret/Wnt signalling feedback in both developmental and homeostatic contexts. They also suggest an epithelial contribution to Ret loss‐of‐function disorders such as Hirschsprung disease.

Mulibrey Nanism - a Novel Peroxisomal Disorder

Mulibrey nanism (MUscle-LIver-BRain-EYe nanism, MIM 253250) is an autosomal recessive disorder with severe prenatal-onset growth failure and multi-organ manifestations including cardiovascular, hepatic, endocrine, skeletal and opthalmologic abnormalities (Perheentupa et al, 1973; Lipsanen-Nyman, 1986; Lapunzina et al., 1995). Mulibrey nanism occurs worldwide, and some 100 patients have been identified of whom more than 80 are Finnish. Mutations in the TRIM37 gene (previously designated MUL) underlie mulibrey nanism (Avela et al., 2000). The TRIM37 gene product localizes to peroxisomes in cultured cells, which suggests that mulibrey nanism can be classified as a novel peroxisomal disorder (Kallijarvi et al., 2002). The fact that mulibrey nanism shares symptoms with known peroxisomal disorders supports the molecular evidence. Here, we discuss the clinical features of the disorder, as well as the current knowledge on the TRIM37 gene, its mutations and the protein it encodes.