NDR2 Kinase Regulate Microglial Metabolic Adaptation and Inflammatory Response: Critical Role in Glucose-Dependent Functional Plasticity

Diabetes and its retinal complication, diabetic retinopathy (DR), are a rapidly increasing health, societal and economic burden. Diabetic retinopathy is a complex disease with a chronic inflammatory component mediated by retinal microglial cells. Recent studies have demonstrated the importance of the Hippo pathway kinases, Ndr1/Stk38 and Ndr2/Stk38l , in the regulation of macrophages, immune cells that share similarities with microglial cells. However, the role of NDR2 kinases in microglial inflammatory response and in the pathophysiology of diabetic retinopathy has not yet been uncovered. This study investigates the role of NDR2 kinase in microglial cells, particularly in response to high glucose (HG) conditions. Using CRISPR-Cas9, we downregulated Ndr2 kinase in BV-2 microglial cells and analyzed the impact on cellular metabolism, phagocytosis and migratory capabilities.

We demonstrate that microglial cells expressed NDR2 kinase protein, especially in HG conditions, suggesting its importance in regulating microglial functions during hyperglycemia. Ndr2 downregulated cells present a decreased basal respiration, indicating an impaired mitochondrial function. They also showed decreased metabolic flexibility to stress conditions, such as adaptation to HG conditions. Functionally, Ndr2 downregulation led to decreased phagocytic capacity and migration of microglial cells, both cytoskeleton-based functions. Furthermore, Ndr2 downregulation resulted in altered cytokine and chemokine secretion profiles. Notably, increased levels of pro-inflammatory cytokines such as IL-6, TNF, IL-17 and IL-12p70 were observed in Ndr2 downregulated cells, even under normal glucose conditions. In conclusion, our findings indicate that NDR2 kinase is crucial for microglial metabolic adaptation to stress, such as high glucose exposure and for influencing microglial inflammatory responses. Therefore, NDR2 kinase plays a vital role in maintaining microglial functional plasticity in response to glucose variations, suggesting potential implications for neuroinflammatory processes in conditions like diabetic retinopathy.

Research in Context

What is already known about this subject?

• The pathogenesis of diabetic retinopathy (DR) involves chronic inflammation mediated by retinal microglial cells, which contribute to vascular damage and neurodegeneration.

• Microglial dysfunction under high glucose (HG) conditions exacerbates cytokine release and oxidative stress, driving DR progression.

• NDR kinases regulate inflammatory pathways in macrophages, but their role in microglia during DR was previously unexplored.

What is the key question?

• How do NDR2 kinase regulate microglial inflammatory responses and functional adaptability in diabetic retinopathy?

What are the new findings?

• NDR2 expression is upregulated in microglia exposed to HG.

• Ndr2 downregulation in microglia impairs metabolic flexibility, phagocytosis, and migration.

• Ndr2 downregulation disrupts cytoskeleton-dependent microglial functions, limiting their ability to adapt to metabolic stress.

• Ndr2 downregulation in microglia increases pro-inflammatory cytokines (IL-17, TNF) and reduces anti-inflammatory factors (sTNFRI, VEGF), exacerbating inflammation.

How might this impact clinical practice?

• Targeting Ndr2 signaling could emerge as a therapeutic strategy to modulate microglial-driven inflammation, potentially slowing DR progression and complementing existing glycemic control approaches.