Universität zu Köln

Montoro Gamez, Carolina (2023). Unravelling the mechanism of axonal degeneration in Hereditary Spastic Paraplegia type 7. PhD thesis, Universität zu Köln.

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Abstract

Hereditary spastic paraplegia (HSP) is an inherited neurodegenerative disease which affects mainly the upper motor neuron axons in the corticospinal tract. Due to mutations in any of the more than 80 genes that are known to cause the disease, these long axons get compromised and undergo a dying-back phenomenon, whereby the axonal distal part degenerates while the soma maintains its integrity. Mutations in SPG7 are among the most common causes of autosomal recessive HSP, yet the mechanisms underlying its pathogenesis are still not clear. SPG7 encodes for paraplegin, a protein that assembles in mouse with AFG3L2 and AFG3L1 to build up the m-AAA protease in the inner membrane of the mitochondria. These ATP-dependent proteases constitute a quality control system that ensures organellar homeostasis by degrading misfolded or damaged proteins and by processing specific substrates. In this thesis I aimed to decipher the function of paraplegin within the CNS and shed light on the mechanisms that lead to axonal degeneration in HSP. In order to investigate this, a new mouse model lacking both paraplegin and AFG3L1 was generated (DKO model). This model recapitulates closely the human SGP7-HSP condition, as it displayed early motor deficits, abnormal mitochondria in anterior spinal cord tracts, cerebellar axonal fibers and cerebellar granule cells, and a prominent axonal degeneration at 28 weeks of age. The ER compartment appeared also altered, in form of transverse, swollen structures in spinal cord tracts. Both astroglia and microglia displayed a reactive morphology in the affected areas indicating a contribution of neuroinflammation to the SPG7 pathology. Proteomic analysis at 16 weeks revealed the accumulation of certain inner-membrane mitochondrial proteins, such as UQCC2, suggesting a potential role of the paraplegin/AFG3L1 m-AAA proteases in their processing. Moreover, metabolite analysis revealed an increase of cADPR and a dysregulated NAD+/NADH ratio upon paraplegin/AFG3L1 loss. Indeed, deletion of the NADase SARM1 partially rescued the in vivo DKO phenotype, improving early weight and motor impairments up to 32 weeks of age. As opposed to most evidence, SARM1 absence did not improve the abnormal mitochondria nor the axonal degeneration observed in DKO spinal cord. However, it restored the mitochondrial phenotype of the granule cells and the loss of parallel fibers in the cerebellum. Proteomic analyses at 28 weeks revealed an alteration of a great amount of OXPHOS subunits and protein import components, as well as an alteration of the actin cytoskeleton, cell adhesion and synaptic transmission in DKO tissue. Interestingly, these protein changes were either not observed or showed an opposite direction in absence of SARM1. Moreover, this study reveals tissue-specific differences within the CNS of both paraplegin/AFG3L1 complexes and SARM1. The loss of SARM1 in absence of damage rewired the cellular proteome, particularly in the cerebellum, where proteomic analysis showed a substantial amount of OXPHOS subunits altered as well as proteins involved in cell adhesion, the immune system and neuronal morphogenesis.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Montoro Gamez, Carolina cmontoro@smail.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-654599
Date:	20 April 2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	CECAD - Cluster of Excellence Cellular Stress Responses in Aging-Associated Diseases
Subjects:	Life sciences Medical sciences Medicine
Uncontrolled Keywords:	Keywords Language Mitochondria English Motor neuron English Neurodegeneration English SARM1 English Ataxia English Protease English NAD English
Date of oral exam:	19 September 2022
Referee:	Name Academic Title Rugarli, Elena I. Prof. Kononenko, Natalia Prof.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/65459