Horizontal cells are retinal interneurons that modulate the output from photoreceptors. Two horizontal cell (HC) subtypes are commonly identified in the vertebrate retina: axon-bearing and axon-less HCs. In this work, we have identified Isl1 as a novel HC marker and demonstrated that Lim1 and Isl1 distinguish axon-bearing and axon-less HCs, respectively. In the chick retina, axon-less HCs are furthermore split into two different subtypes based on the expression of GABA and TrkA. We have demonstrated that during early chick retinogenesis, HCs expressing either Lim1 or Isl1 are generated consecutively as two equally large sub-groups at different time points. Moreover, these newborn HCs undertake an unexpected bi-directional migration before settling in their final laminar position. Different HC subtypes complete this migration at different times. We investigated the role of activin signaling during HC subtype generation. Activin or its inhibitor follistatin was administrated during the main phase of HC generation and analyzed when HCs had completed migration. Activin caused a significant decrease in both HC subtypes and decreased the proliferation of retinal precursor cells. Follistatin increased the number of late born (Isl1+) HCs, which migrated to the HC-layer during a prolonged migration period. Both treatments affected retinal histology, but only activin influenced the generation of retinal populations other than HCs. These effects were most likely mediated by altered proliferation in certain retinal precursor cells. The data on HC subtype ratios, birth-dates, migration, apoptosis and extrinsic activin modulation favor a scenario where the mature proportions of HC subtypes are generated sequentially from a specific HC-precursor cell lineage early in development and remain stable thereafter. These proportions are not adjusted by apoptosis, but rather by the combined actions of transcription factors and extrinsic signaling. Our studies on HC subtypes and their development promises to facilitate future studies on HC development, evolution and function.

Gliomas are common primary brain tumours that occur as low-grade (LGG) and high-grade gliomas (HGG). Typically occurring in younger adults, LGG has an indolent course with a median survival of 5-10 years, but carries an inherent potential for transforming into HGG. The thesis focused on LGG in adults, with the aim of identifying prognostic biomarkers for LGG. Paper I. Epileptic seizures are common symptoms in LGG. In a retrospective study, the correlation between 11C-methionine (MET) uptake, measured by Positron Emission Tomography (PET), and seizure activity was assessed in 101 patients with LGG. Although there was no correlation between MET uptake and seizure activity, survival was longer in patients who were seizure-free before surgery. Paper II. This finding prompted the search for common genetic pathways for both tumour and seizure development and a review of genetic polymorphisms in focal epilepsy and glioma risk. Cell cycle and immune response genes affecting both glioma and seizure risk were identified, and genes involved in synaptic transmission presented potential candidates for future studies. Paper III. The transcription factor PROX1 plays a pivotal role in normal development and carcinogenesis of various organs. The prognostic value of PROX1, together with established clinical and molecular prognostic factors for survival, was retrospectively assessed in 116 patients with LGG. High PROX1 expression in the tumour was associated with shorter survival. Paper IV. DNA repair enzymes, such as ERCC6, are crucial for maintaining genomic stability in glioma response to radiotherapy. An association between the polymorphism rs4253079, mapped to ERCC6, and longer survival in patients with LGG and HGG was identified. Paper V. As LGG typically presented as non-contrast enhancing tumours on morphological MRI (magnetic resonance imaging), the value of combined MET PET with physiological MRI for preoperative diagnosis was assessed in a prospective study of 32 patients with suspected LGG. Representative tumour areas were identified through a combination of perfusion-MRI with MET PET, which can be used as a baseline investigation for follow-up over time. Conclusions: The parameters seizure-freedom before surgery, the polymorphism rs4253079 in ERCC6 and low PROX1 expression in the tumor were identified as favorable prognostic biomarkers.

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PROX1 étant un facteur de susceptibilité au diabète de type 2 (DT2), nousavons réalisé des études génétiques et moléculaires afin de comprendre son rôledans l'étiologie du DT2.Nous avons analysé l'impact de 80 SNPs de PROX1 sur des phénotypescliniques associés au DT2 dans l'étude HELENA (n=1155 adolescents) et montréque trois SNPs (rs340838, rs340837 et rs340836) sont associés à l'insulinémie àjeun. Nous avons évalué la fonctionnalité de 9 SNPs (les 3 SNPs associés et 6 SNPsen déséquilibre de liaison) en utilisant un gène rapporteur Luciférase dans descellules HepG2 et MIN6. Les allèles associés à la diminution de l'insulinémie desSNPs rs340874, rs340873 et rs340835 sont associés à une diminution del'expression du gène rapporteur Luciférase, suggérant que l'expression de PROX1est diminuée chez les individus porteurs des allèles à risque.Nous avons aussi montré que l'inhibition de l'expression de Prox1 par siRNAsdans les cellules INS-1E engendrait une diminution de 1,7 fois de la sécrétiond'insuline en réponse au glucose et qu'une concentration élevée en glucose modulaitpositivement l'expression de la protéine Prox1.Des analyses transcriptomiques réalisées dans les cellules INS-1E ont permisde montrer que certains des gènes cibles de PROX1 dans les cellules bêta sont desgènes impliqués dans des voies de sécrétion d'insuline.Enfin, nous avons également observé que l'agoniste de PPARgamma, latroglitazone, diminuait l'expression de Prox1 dans les cellules INS-1E.Ces résultats suggèrent qu'une altération de l'expression de Prox1 par desvariants cis-régulateurs pourrait conduire à une sécrétion d'insuline en réponse auglucose altérée des cellules bêta, conférant ainsi une susceptibilité au DT2.