Increased excitatory regulation of the hypothalamic paraventricular nucleus and circulating vasopressin results in the hypertension observed in polycystic kidney disease

The Kynurenine Pathway in Acute Kidney Injury and Chronic Kidney Disease

BACKGROUND

The kynurenine pathway (KP) is the major catabolic pathway for tryptophan degradation. The KP plays an important role as the sole de novo nicotinamide adenine dinucleotide (NAD+) biosynthetic pathway in normal human physiology and functions as a counter-regulatory mechanism to mitigate immune responses during inflammation. Although the KP has been implicated in a variety of disorders including Huntington's disease, seizures, cardiovascular disease, and osteoporosis, its role in renal diseases is seldom discussed.

SUMMARY

This review summarizes the roles of the KP and its metabolites in acute kidney injury (AKI) and chronic kidney disease (CKD) based on current literature evidence. Metabolomics studies demonstrated that the KP metabolites were significantly altered in patients and animal models with AKI or CKD. The diagnostic and prognostic values of the KP metabolites in AKI and CKD were highlighted in cross-sectional and longitudinal human observational studies. The biological impact of the KP on the pathophysiology of AKI and CKD has been studied in experimental models of different etiologies. In particular, the activation of the KP was found to confer protection in animal models of glomerulonephritis, and its immunomodulatory mechanism may involve the regulation of T cell subsets such as Th17 and regulatory T cells. Manipulation of the KP to increase NAD+ production or diversion toward specific KP metabolites was also found to be beneficial in animal models of AKI. Key Messages: KP metabolites are reported to be dysregulated in human observational and animal experimental studies of AKI and CKD. In AKI, the magnitude and direction of changes in the KP depend on the etiology of the damage. In CKD, KP metabolites are altered with the onset and progression of CKD all the way to advanced stages of the disease, including uremia and its related vascular complications. The activation of the KP and diversion to specific sub-branches are currently being explored as therapeutic strategies in these diseases, especially with regards to the immunomodulatory effects of certain KP metabolites. Further elucidation of the KP may hold promise for the development of biomarkers and targeted therapies for these kidney diseases.

The subfornical organ drives hypertension in polycystic kidney disease via the hypothalamic paraventricular nucleus

AIMS

Hypertension is a prevalent yet poorly understood feature of polycystic kidney disease. Previously we demonstrated that increased glutamatergic neurotransmission within the hypothalamic paraventricular nucleus produces hypertension in the Lewis Polycystic Kidney rat model of polycystic kidney disease. Here we tested the hypothesis that augmented glutamatergic drive to the paraventricular nucleus in Lewis Polycystic Kidney rats originates from the forebrain lamina terminalis, a sensory structure that relays blood-borne information throughout the brain.

METHODS AND RESULTS

Anatomical experiments revealed that 38% of paraventricular nucleus-projecting neurons in the subfornical organ of the lamina terminalis expressed Fos/Fra, an activation marker, in Lewis Polycystic Kidney rats while <1% of neurons were Fos/Fra+ in Lewis control rats (P = 0.01, n = 8). In anaesthetised rats, subfornical organ neuronal inhibition using isoguvacine produced a greater reduction in systolic blood pressure in the Lewis Polycystic Kidney versus Lewis rats (-21 ± 4 vs. -7 ± 2 mmHg, P < 0.01; n = 10), which could be prevented by prior blockade of paraventricular nucleus ionotropic glutamate receptors using kynurenic acid. Blockade of ionotropic glutamate receptors in the paraventricular nucleus produced an exaggerated depressor response in Lewis Polycystic Kidney relative to Lewis rats (-23 ± 4 vs. -2 ± 3 mmHg, P < 0.001; n = 13), which was corrected by prior inhibition of the subfornical organ with muscimol but unaffected by chronic systemic angiotensin II type I receptor antagonism or lowering of plasma hyperosmolality through high-water intake (P > 0.05); treatments that both nevertheless lowered blood pressure in Lewis Polycystic Kidney rats (P < 0.0001).

CONCLUSION

Our data reveal multiple independent mechanisms contribute to hypertension in polycystic kidney disease, and identify high plasma osmolality, angiotensin II type I receptor activation and, importantly, a hyperactive subfornical organ to paraventricular nucleus glutamatergic pathway as potential therapeutic targets.

TRANSLATIONAL PERSPECTIVE

Hypertension is a significant comorbidity for all forms of chronic kidney disease and for individuals with polycystic kidney disease, often an early presenting feature. Nevertheless, the cause(s) of hypertension in polycystic kidney disease are poorly defined. Here we define the contribution of a neural pathway that contributes to hypertension in polycystic kidney disease. Critically, targeting this pathway may provide an additional antihypertensive effect beyond that achieved with current conventional antihypertensive therapies. Future work identifying the drivers of this neural pathway will aid in the development of newer generation antihypertensive medication.

Role of Mineralocorticoid and Angiotensin Type 1 Receptors in the Paraventricular Nucleus in Angiotensin-Induced Hypertension

The hypothalamic paraventricular nucleus (PVN) is an important site where an interaction between circulating angiotensin (Ang) and mineralocorticoid receptor (MR) activity may modify sympathetic nerve activity (SNA) to influence long-term elevation of blood pressure. We examined in conscious Ang II-treated rabbits, the effects on blood pressure and tonic and reflex renal SNA (RSNA) of microinjecting into the PVN either RU28318 to block MR, losartan to block Ang (AT₁) receptors or muscimol to inhibit GABA_A receptor agonist actions. Male rabbits received a moderate dose of Ang II (24 ng/kg/min subcutaneously) for 3 months (n = 13) or sham treatment (n = 13). At 3 months, blood pressure increased by +19% in the Ang II group compared to 10% in the sham (P = 0.022) but RSNA was similar. RU28318 lowered blood pressure in both Ang II and shams but had a greater effect on RSNA and heart rate in the Ang II-treated group (P < 0.05). Losartan also lowered RSNA, while muscimol produced sympatho-excitation in both groups. In Ang II-treated rabbits, RU28318 attenuated the blood pressure increase following chemoreceptor stimulation but did not affect responses to air jet stress. In contrast losartan and muscimol reduced blood pressure and RSNA responses to both hypoxia and air jet. While neither RU28318 nor losartan changed the RSNA baroreflex, RU28318 augmented the range of the heart rate baroreflex by 10% in Ang II-treated rabbits. Muscimol, however, augmented the RSNA baroreflex by 11% in sham animals and none of the treatments altered baroreflex sensitivity. In conclusion, 3 months of moderate Ang II treatment promotes activation of reflex RSNA principally via MR activation in the PVN, rather than via activation of AT₁ receptors. However, the onset of hypertension is independent of both. Interestingly, the sympatho-excitatory effects of muscimol in both groups suggest that overall, the PVN regulates a tonic sympatho-inhibitory influence on blood pressure control.