Abstract by Mia Langbøl Hjerrild

Glaucoma is the leading cause of irreversible blindness affecting nearly 80 million people worldwide. The definition of glaucoma is the progressive loss of retinal ganglion cells (RGC) that transmit signals to the brain to create our vision. Untreated, loss of RGCs can cause visual field defects and, ultimately, blindness. A major risk factor for the development and aggravation of glaucoma is increased intraocular pressure (IOP), and current evidence-based treatments solely aim to lower IOP. The actual cause of the disease remains unknown, and no cure exists. Glaucoma is divided into subtypes, of which primary open-angle glaucoma (POAG) is the most common, further subdividing depending on whether a high IOP has been measured or not. Patients who have never measured a high IOP belong to the subgroup normal-tension glaucoma (NTG), while patients with a high IOP belong to the subgroup high-tension glaucoma (HTG). Furthermore, a group of patients has high IOP but no glaucomatous damage. This group belongs to the subgroup of ocular hypertension (OHT). The various clinical phenotypes indicate the existence of other important risk factors, supported by research suggesting that glaucoma is a multifactorial and systemic disease manifesting in the eye due to the high demand for energy and nutrients in the retina and optic nerve. Several studies have observed vascular dysregulation and mitochondrial dysfunction in patients with glaucoma, possibly reflecting a systemic metabolic vulnerability, oxidative stress, and inflammation. The present thesis evaluates biomarkers for the systemic status of metabolism, oxidative stress, and inflammation in plasma from patients with NTG and OHT compared to healthy controls.

We have developed a human experimental model in which participants are exposed to hypoxia for two hours, followed by 30 minutes of recovery with normal breathing. The hypoxic intervention stresses the systemic vascular and metabolic systems, enabling a comparison of baseline and stress responses in the included groups. To assess the systemic status of the investigated parameters, plasma samples have been selected.

The first study investigates levels of metabolic substrates, including glucose, lactate, and amino acids, by high-performance liquid chromatography (HPLC) and blood gas analyzer measurements. Regulations of amino acids in controls during hypoxia suggest the utilization of alternative energy substrates to glucose. NTG patients may have switched to lactic fermentation due to low levels of oxygen. Energy substrates, including lactate and amino acids, seem limited in NTG patients. The previously suggested neuroprotective role of lactate may help explain the vulnerability of RGCs in patients with NTG. Additionally, upregulation of energy substrates during hypoxia and recovery indicates that patients with NTG may be unable to either uptake or metabolize amino acids for energy production.

The second study examines markers for oxidative stress and antioxidant defense through colorimetric assays and lipidomics. The study suggests that patients with OHT possess a high antioxidant capacity and elevated levels of lipid mediators with assumed antioxidant and anti-inflammatory properties. A lack of hypoxic regulation of the identified lipid mediators in OHT patients indicates a balanced baseline defense that does not require regulation under stress.

The third study evaluates the entire metabolome through non-targeted metabolomics. The results suggest a disrupted energy metabolism in patients with NTG and OHT due to dysregulation of several metabolites. Metabolites associated with fatty acid metabolism and the kynurenine pathway may uniquely contribute to substrate utilization for energy production and protection in OHT patients.

The fourth study utilizes proteomic and cytokine profiling to reveal alterations in immune- and hemostasis-related pathways in NTG and OHT patients. A unique expression of apolipoproteins in OHT patients suggests that cholesterol metabolism is involved. The differentially expressed apolipoproteins are components of high-density lipoprotein 3 (HDL3) with suggested antioxidant and anti-inflammatory properties. OHT patients may have a steadier immune response since hypoxia induces decreased levels of tumor necrosis factor-a (TNF-a) compared to controls. Patients with NTG, on the other hand, may have an inability to optimally activate the immune response, as hypoxia induces decreased levels of interleukin-1b (IL-1b) and C-reactive protein (CRP) compared to controls.

Overall, the present thesis implies that patients with NTG and OHT exhibit altered molecular regulation patterns. The included studies suggest metabolic vulnerability and altered immune response in NTG pathology, whereas OHT patients may possess resilience to glaucoma development despite high IOP. This resilience is attributed to superior antioxidant defense and anti-inflammatory mechanisms. Different classes of lipids and apolipoproteins appear crucial through their proposed antioxidant and anti-inflammatory properties in OHT patients, combating oxidative stress and inflammation while compensating for potential metabolic vulnerability.