Biochemical characterisation of the C1QTNF5 gene associated with late-onset retinal degeneration. A genetic model of age-related macular degeneration

HtrA1 Mediated Intracellular Effects on Tubulin Using a Polarized RPE Disease Model

Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss. The protein HtrA1 is enriched in retinal pigment epithelial (RPE) cells isolated from AMD patients and in drusen deposits. However, it is poorly understood how increased levels of HtrA1 affect the physiological function of the RPE at the intracellular level. Here, we developed hfRPE (human fetal retinal pigment epithelial) cell culture model where cells fully differentiated into a polarized functional monolayer. In this model, we fine-tuned the cellular levels of HtrA1 by targeted overexpression. Our data show that HtrA1 enzymatic activity leads to intracellular degradation of tubulin with a corresponding reduction in the number of microtubules, and consequently to an altered mechanical cell phenotype. HtrA1 overexpression further leads to impaired apical processes and decreased phagocytosis, an essential function for photoreceptor survival. These cellular alterations correlate with the AMD phenotype and thus highlight HtrA1 as an intracellular target for therapeutic interventions towards AMD treatment.

Loss of CTRP5 improves insulin action and hepatic steatosis

The gene that encodes C1q/TNF-related protein 5 (CTRP5), a secreted protein of the C1q family, is mutated in individuals with late-onset retinal degeneration. CTRP5 is widely expressed outside the eye and also circulates in plasma. Its physiological role in peripheral tissues, however, has yet to be elucidated. Here, we show that Ctrp5 expression is modulated by fasting and refeeding, and by different diets, in mice. Adipose expression of CTRP5 was markedly upregulated in obese and diabetic humans and in genetic and dietary models of obesity in rodents. Furthermore, human CTRP5 expression in the subcutaneous fat depot positively correlated with BMI. A genetic loss-of-function mouse model was used to address the metabolic function of CTRP5 in vivo. On a standard chow diet, CTRP5-deficient mice had reduced fasting insulin but were otherwise comparable with wild-type littermate controls in body weight and adiposity. However, when fed a high-fat diet, CTRP5-deficient animals had attenuated hepatic steatosis and improved insulin action. Loss of CTRP5 also improved the capacity of chow-fed aged mice to respond to subsequent high-fat feeding, as evidenced by decreased insulin resistance. In cultured adipocytes and myotubes, recombinant CTRP5 treatment attenuated insulin-stimulated Akt phosphorylation. Our results provide the first genetic and physiological evidence for CTRP5 as a negative regulator of glucose metabolism and insulin sensitivity. Inhibition of CTRP5 action may result in the alleviation of insulin resistance associated with obesity and diabetes.

The macular degeneration-linked C1QTNF5 (S163) mutation causes higher-order structural rearrangements

The C1q-tumor necrosis factor 5 (C1QTNF5) protein plays a significant role in retinal pigmented epithelium (RPE) cellular adhesion. The C1QTNF5 gene is co-transcribed with the frizzled-related protein (MFRP) gene. A Ser-to-Arg mutation at site 163 (S163R) in C1QTNF5 is known to cause late-onset retinal macular degeneration (L-ORMD). Here we also found that C1QTNF5 monomers can multimerize into a bouquet-like octadecamer. We found that a novel intermolecular hydrogen-bond network of S163 that glues adjacent globular heads of C1QTNF5 together was weakened or abolished by the R163 pathogenic mutation. These findings could underlie the structural basis of this protein's adhesive function and relate to the pathogenesis of its S163R mutation. Additionally, the fact that C1QTNF5 immobilized to a resin selectively enriched detergent extracted membrane-bound MFRP, further confirmed their interaction, implying functions other than cellular adhesion for C1QTNF5.

Crystal structure of the globular domain of C1QTNF5: Implications for late-onset retinal macular degeneration

Autosomal dominant late-onset retinal macular degeneration (L-ORMD) is caused by a single S163R mutation in the C1q and tumor necrosis factor-related protein 5 (C1QTNF5) gene. The C1QTNF5 gene encodes a secreted and membrane-associated protein involved in adhesion of retinal pigmented epithelial cells (RPE) to Bruch's membrane. The crystal structure of the trimeric globular domain of human C1QTNF5 at 1.34Å resolution reveals unique features of this novel C1q family member. It lacks a Ca²⁺-binding site, displays a remarkable non-uniform distribution of surface electrostatic potentials and possesses a unique sequence (F₁₈₁F₁₈₂G₁₈₃G₁₈₄W₁₈₅P₁₈₆) that forms a hydrophobic plateau surrounded by Lys and Arg residues with a solvent cavity underneath. S₁₆₃ forms a hydrogen bond with F₁₈₂ in a hydrophobic area extending to the hydrophobic plateau. The pathogenic mutation S163R disrupts this hydrogen bonding and positively charges these hydrophobic areas. Thus, our analysis provides insights into the structural basis of the L-ORMD disease mechanism.

Genetic insights into age-related macular degeneration: controversies addressing risk, causality, and therapeutics

Age-related macular degeneration (AMD) is a common condition among the elderly population that leads to the progressive central vision loss and serious compromise of quality of life for its sufferers. It is also one of the few disorders for whom the investigation of its genetics has yielded rich insights into its diversity and causality and holds the promise of enabling clinicians to provide better risk assessments for individuals as well as to develop and selectively deploy new therapeutics to either prevent or slow the development of disease and lessen the threat of vision loss. The genetics of AMD began initially with the appreciation of familial aggregation and increase risk and expanded with the initial association of APOE variants with the disease. The first major breakthroughs came with family-based linkage studies of affected (and discordant) sibs, which identified a number of genetic loci and led to the targeted search of the 1q31 and 10q26 loci for associated variants. Three of the initial four reports for the CFH variant, Y402H, were based on regional candidate searches, as were the two initial reports of the ARMS2/HTRA1 locus variants. Case-control association studies initially also played a role in discovering the major genetic variants for AMD, and the success of those early studies have been used to fuel enthusiasm for the methodology for a number of diseases. Until 2010, all of the subsequent genetic variants associated with AMD came from candidate gene testing based on the complement factor pathway. In 2010, several large-scale genome-wide association studies (GWAS) identified genes that had not been previously identified. Much of this historical information is available in a number of recent reviews (Chen et al., 2010b; Deangelis et al., 2011; Fafowora and Gorin, 2012b; Francis and Klein, 2011; Kokotas et al., 2011). Large meta analysis of AMD GWAS has added new loci and variants to this collection (Chen et al., 2010a; Kopplin et al., 2010; Yu et al., 2011). This paper will focus on the ongoing controversies that are confronting AMD genetics at this time, rather than attempting to summarize this field, which has exploded in the past 5 years.

Characterisation of a C1qtnf5 Ser163Arg knock-in mouse model of late-onset retinal macular degeneration

A single founder mutation resulting in a Ser163Arg substitution in the C1QTNF5 gene product causes autosomal dominant late-onset retinal macular degeneration (L-ORMD) in humans, which has clinical and pathological features resembling age-related macular degeneration. We generated and characterised a mouse “knock-in” model carrying the Ser163Arg mutation in the orthologous murine C1qtnf5 gene by site-directed mutagenesis and homologous recombination into mouse embryonic stem cells. Biochemical, immunological, electron microscopic, fundus autofluorescence, electroretinography and laser photocoagulation analyses were used to characterise the mouse model. Heterozygous and homozygous knock-in mice showed no significant abnormality in any of the above measures at time points up to 2 years. This result contrasts with another C1qtnf5 Ser163Arg knock-in mouse which showed most of the features of L-ORMD but differed in genetic background and targeting construct.