Transcriptomics unravels molecular changes associated with cilia and COVID-19 in chronic rhinosinusitis with nasal polyps.

Chronic rhinosinusitis with nasal polyps (CRSwNP) is a common upper respiratory tract complication where the pathogenesis is largely unknown. Herein, we investigated the transcriptome profile in nasal mucosa biopsies of CRSwNP patients and healthy individuals. We further integrated the transcriptomics data with genes located in chromosomal regions containing genome-wide significant gene variants for COVID-19. Among the most significantly upregulated genes in polyp mucosa were CCL18, CLEC4G, CCL13 and SLC9A3. Pathways involving "Ciliated epithelial cells" were the most differentially expressed molecular pathways when polyp mucosa and non-polyp mucosa from the same patient was compared. Natural killer T-cell (NKT) and viral pathways were the most statistically significant pathways in the mucosa of CRSwNP patients compared with those of healthy control individuals. Upregulated genes in polyp mucosa, located within the genome-wide associated regions of COVID-19, included LZTFL1, CCR9, SLC6A20, IFNAR1, IFNAR2 and IL10RB. Interestingly, the second most over-expressed gene in our study, CLEC4G, has been shown to bind directly to SARS-CoV-2 spike's N-terminal domain and mediate its entry and infection. Our results on altered expression of genes related to cilia and viruses point to the de-regulation of viral defenses in CRSwNP patients, and may give clues to future intervention strategies.

Loss of the Mitochondrial Fission GTPase Drp1 Contributes to Neurodegeneration in a Drosophila Model of Hereditary Spastic Paraplegia.

Mitochondrial morphology, distribution and function are maintained by the opposing forces of mitochondrial fission and fusion, the perturbation of which gives rise to several neurodegenerative disorders. The large guanosine triphosphate (GTP)ase dynamin-related protein 1 (Drp1) is a critical regulator of mitochondrial fission by mediating membrane scission, often at points of mitochondrial constriction at endoplasmic reticulum (ER)-mitochondrial contacts. Hereditary spastic paraplegia (HSP) subtype SPG61 is a rare neurodegenerative disorder caused by mutations in the ER-shaping protein Arl6IP1. We have previously reported defects in both the ER and mitochondrial networks in a Drosophila model of SPG61. In this study, we report that knockdown of Arl6IP1 lowers Drp1 protein levels, resulting in reduced ER-mitochondrial contacts and impaired mitochondrial load at the distal ends of long motor neurons. Increasing mitochondrial fission, by overexpression of wild-type Drp1 but not a dominant negative Drp1, increases ER-mitochondrial contacts, restores mitochondrial load within axons and partially rescues locomotor deficits. Arl6IP1 knockdown Drosophila also demonstrate impaired autophagic flux and an accumulation of ubiquitinated proteins, which occur independent of Drp1-mediated mitochondrial fission defects. Together, these findings provide evidence that impaired mitochondrial fission contributes to neurodegeneration in this in vivo model of HSP.

NeurodegenERation: The Central Role for ER Contacts in Neuronal Function and Axonopathy, Lessons From Hereditary Spastic Paraplegias and Related Diseases.

The hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions whose characteristic feature is degeneration of the longest axons within the corticospinal tract which leads to progressive spasticity and weakness of the lower limbs. Though highly genetically heterogeneous, the majority of HSP cases are caused by mutations in genes encoding proteins that are responsible for generating and organizing the tubular endoplasmic reticulum (ER). Despite this, the role of the ER within neurons, particularly the long axons affected in HSP, is not well understood. Throughout axons, ER tubules make extensive contacts with other organelles, the cytoskeleton and the plasma membrane. At these ER contacts, protein complexes work in concert to perform specialized functions including organelle shaping, calcium homeostasis and lipid biogenesis, all of which are vital for neuronal survival and may be disrupted by HSP-causing mutations. In this article we summarize the proteins which mediate ER contacts, review the functions these contacts are known to carry out within neurons, and discuss the potential contribution of disruption of ER contacts to axonopathy in HSP.

ER-shaping proteins are required for ER and mitochondrial network organization in motor neurons.

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders characterized by degeneration of the longest motor neurons in the corticospinal tract, leading to muscle weakness and spasticity of the lower limbs. Pathogenic variants in genes encoding proteins that shape the endoplasmic-reticulum (ER) network are a leading cause of HSP, however, the mechanisms by which loss of ER-shaping proteins underpin degeneration of selective neurons in HSP remain poorly understood. To begin to address this, we have generated a novel in vivo model of HSP in Drosophila melanogaster by targeted knockdown of the ER-shaping protein Arl6IP1 Variants in the human homolog of this gene have recently been linked to HSP subtype SPG61. Arl6IP1 RNAi flies display progressive locomotor deficits without a marked reduction in lifespan, recapitulating key features of HSP in human patients. Loss of Arl6IP1 leads to fragmentation of the smooth ER and disrupted mitochondrial network organization within the distal ends of long motor neurons. Furthermore, genetically increasing mitochondrial fission, by overexpression of dynamin-related protein 1 (Drp1), restores mitochondrial network organization and rescues locomotor deficits in two independent Drosophila models of HSP. Taken together, these results propose a role for ER-shaping proteins in mitochondrial network organization in vivo and suggest that impaired mitochondrial organization may be a common mechanism underpinning some forms of HSP.

Rare disease models provide insight into inherited forms of neurodegeneration.

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions characterised by retrograde degeneration of the longest motor neurons in the corticospinal tract, resulting in muscle weakness and spasticity of the lower limbs. To date more than 70 genetic loci have been associated with HSP, however the majority of cases are caused by mutations that encode proteins responsible for generating and maintaining tubular endoplasmic reticulum (ER) structure. These ER-shaping proteins are vital for the long-term survival of axons, however the mechanisms by which mutations in these proteins give rise to HSP remain poorly understood. To begin to address this we have characterized in vivo loss of function models of two very rare forms of HSP caused by loss of the ER-shaping proteins ARL6IP1 (SPG61) and RTN2 (SPG12). These models display progressive locomotor defects, disrupted organisation of the tubular ER and length-dependant defects in the axonal mitochondrial network. Here we compare our findings with those associated with more common forms HSP including: Spastin, Atlastin-1 and REEP 1 which together account for over half of all cases of autosomal dominant HSP. Furthermore, we discuss recent observations in other HSP models which are directly implicated in mitochondrial function and localization. Overall, we highlight the common features of our rare models of HSP and other models of disease which could indicate shared mechanisms underpinning neurodegeneration in these disorders.