Deciphering the Mechanism Underlying Adipose deficiency in a Drosophila model of Mucopolysaccharidosis Type VII

Basu, Indrani (2022) Deciphering the Mechanism Underlying Adipose deficiency in a Drosophila model of Mucopolysaccharidosis Type VII. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Indrani Basu (14RS017)) 14RS017.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

Mucopolysaccharidosis Type VII (MPSVII) or Sly syndrome is an autosomal recessive inherited disorder caused due to deficiency of the β-glucuronidase (β-GUS) enzyme that leads to abnormal accumulation of undegraded and partially degraded glycosaminoglycans (dermatan sulfate, keratan sulfate, chondroitin sulfate) within lysosomes. MPSVII patients mostly display cognitive impairment, skeletal dysplasia, hepatosplenomegaly, hydrops fetalis, coarse facial features, corneal clouding, mental retardation, behavioral abnormalities, musculoskeletal deformities and heart, pulmonary and renal dysfunction, which subsequently causes death at a premature age. Apart from all these aforementioned clinical manifestations, MPSVII is also characterized as an adipose storage deficiency disorder and this decreased adiposity is reported for four other Lysosomal storage diseases (LSDs): MPSI, MPSIIIB, Niemann-Pick type A/B, and infantile neuronal ceroid lipofuscinosis. MPS I, MPS IIIB, and MPSVII mice exhibited lean phenotype and reduced triglyceride levels in liver and muscle tissues. One of the important cardinal features observed for β-GUS deficient mice is striking reduction in body weight. Correspondingly, for human patients, males and females alike, lower body mass index (BMI) was also reported upon loss of β-GUS function. However, such studies are inconclusive till date. The underlying molecular basis behind the paucity of these adipose stores remains unexplained, and hence, proves to be an intriguing clinical challenge. Therefore, the primary goal of this thesis is to elucidate the mystery behind this lean phenotype and decreased adiposity in MPSVII. In order to address this underlying issue of impaired adiposity in MPSVII, we have used a Drosophila MPSVII model, where the β-GUS homolog, CG2135 is genetically ablated. This invertebrate model recapitulated most of the phenotypes observed for MPSVII patients and murine model. Hence, to ascertain the underlying cause behind reduced adipose stores in MPSVII, we first investigated the fate of adipogenesis in these CG2135-/- flies. Loss of function of β-GUS in Drosophila also displayed stark difference in the arrangement of the lobes of fat tissues, reduced body weight and buoyancy, and significant loss of adipocyte cells and downregulation of adipogenic genes, which corroborated with the phenotypes being observed for MPSVII murine model and patients. These findings thereby provided an explanation for the lean phenotype of the MPVII patients or the β-GUS deficient MPS VII mouse model and raised further questions about the underlying mechanism behind the dearth of these adipose tissues. Hence, the plausible mechanistic link underlying this adipose storage defect upon abolition of β-GUS function in Drosophila was delineated next by looking at the fate of autophagy in the fat tissues. It was earlier observed that functional autophagy plays a critical role in the adipogenic process and genetic disruption of autophagy related genes resulted in gross reduction in white adipose tissue mass in mice. Implication of impaired autophagic flux has been reported in patient-derived fibroblast cells and animal models for different LSDs such as in Niemann Pick disease, Danon disease, Gaucher disease, Batten disease, Pompe disease, Mucolipidosis II, MPS IIIA, IV and VI. Intrigued by these aforementioned pieces of literature, highlighting the significance of autophagy in regulating white adipogenesis along with the fact that autophagy impairment has emerged as a common feature in many LSDs, prompted us to check the status of autophagy in the CG2135-/- larvae. Dysregulated autophagy network with an increased abundance of autophagosomes was also observed in the CG2135-/- fat tissues due to defective autophagosome-lysosome fusion. Finally, we aimed to bridge the gap between adipose deficiency and impaired autophagy in this invertebrate model through the master regulator Ser/Thr kinase, mTORC1; since it is involved in the regulation of lipid metabolism, lysosomal biogenesis and autophagy pathway. Decreased lysosome-mediated turnover resulted in attenuation of mTOR activity in the CG2135-/- larvae. Interestingly, nutritional and pharmacological stimulation of mTOR with mTOR stimulators, glucose and 3-BDO could significantly ameliorate adipose deficiency as well as impaired autophagy in these larvae. Hence, this resulted in the restoration of normal adipose tissue deposition and functional autophagic flux in CG2135-/- flies. In recent years, the nexus between mTORC1 with lysosomal-autophagy network has gained momentum in the field of lysosome biology and studies are underway to decipher the mechanisms involved in the cross-talk between these signalling cascades. However, to date, whether autophagic impairment may have a role in the pathogenesis of adipocyte dysfunction in MPSs is still unknown and hence our primary goal was to decode the unknown mechanistic link between autophagy dysfunction, mTOR downregulation and reduced adiposity using this MPSVII fly model.

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