The Need to Identify Novel Markers for Early Renal Injury in Cardiorenal Syndrome.

The term "Cardiorenal Syndrome" (CRS) refers to the complex interplay between heart and kidney dysfunction. First described by Robert Bright in 1836, CRS was brought to its modern view by Ronco et al. in 2008, who defined it as one organ's primary dysfunction leading to secondary dysfunction in the other, a view that led to the distinction of five different types depending on the organ of primary dysfunction and the temporal pattern (acute vs. chronic). Their pathophysiology is intricate, involving various hemodynamic, neurohormonal, and inflammatory processes that result in damage to both organs. While traditional biomarkers have been utilized for diagnosing and prognosticating CRS, they are inadequate for the early detection of acute renal damage. Hence, there is a pressing need to discover new biomarkers to enhance clinical outcomes and treatment approaches.

Cardiorenal Syndromes and Their Role in Water and Sodium Homeostasis.

Sodium is the main osmotically active ion in the extracellular fluid and its concentration goes hand in hand with fluid volume. Under physiological conditions, homeostasis of sodium and thus amount of fluid is regulated by neural and humoral interconnection of body tissues and organs. Both heart and kidneys are crucial in maintaining volume status. Proper kidney function is necessary to excrete regulated amount of water and solutes and adequate heart function is inevitable to sustain renal perfusion pressure, oxygen supply etc. As these organs are bidirectionally interconnected, injury of one leads to dysfunction of another. This condition is known as cardiorenal syndrome. It is divided into five subtypes regarding timeframe and pathophysiology of the onset. Hemodynamic effects include congestion, decreased cardiac output, but also production of natriuretic peptides. Renal congestion and hypoperfusion leads to kidney injury and maladaptive activation of renin-angiotensin-aldosterone system and sympathetic nervous system. In cardiorenal syndromes sodium and water excretion is impaired leading to volume overload and far-reaching negative consequences, including higher morbidity and mortality of these patients. Keywords: Cardiorenal syndrome, Renocardiac syndrome, Volume overload, Sodium retention.

Renal Sodium Avidity in Heart Failure.

BACKGROUND: Increased renal sodium avidity is a hallmark feature of the heart failure syndrome. SUMMARY: Increased renal sodium avidity refers to the inability of the kidneys to elicit potent natriuresis in response to sodium loading. This eventually causes congestion, which is a major contributor to hospital admissions and mortality in heart failure. KEY MESSAGES: Important novel concepts such as the renal tamponade hypothesis, accelerated nephron loss, and the role of hypochloremia, the sympathetic nervous system, inflammation, the lymphatic system, and interstitial sodium buffers are involved in the pathophysiology of renal sodium avidity. A good understanding of these concepts is crucially important with respect to treatment recommendations regarding dietary sodium restriction, fluid restriction, rapid up-titration of guideline-directed medical therapies, combination diuretic therapy, natriuresis-guided diuretic therapy, use of hypertonic saline, and ultrafiltration.

Dual-specificity phosphatase 1 interacts with prohibitin 2 to improve mitochondrial quality control against type-3 cardiorenal syndrome.

Type-3 cardiorenal syndrome (CRS-3) is acute kidney injury followed by cardiac injury/dysfunction. Mitochondrial injury may impair myocardial function during CRS-3. Since dual-specificity phosphatase 1 (DUSP1) and prohibitin 2 (PHB2) both promote cardiac mitochondrial quality control, we assessed whether these proteins were dysregulated during CRS-3-related cardiac depression. We found that DUSP1 was downregulated in heart tissues from a mouse model of CRS-3. DUSP1 transgenic (DUSP1Tg) mice were protected from CRS-3-induced myocardial damage, as evidenced by their improved heart function and myocardial structure. CRS-3 induced the inflammatory response, oxidative stress and mitochondrial dysfunction in wild-type hearts, but not in DUSP1Tg hearts. DUSP1 overexpression normalized cardiac mitochondrial quality control during CRS-3 by suppressing mitochondrial fission, restoring mitochondrial fusion, re-activating mitophagy and augmenting mitochondrial biogenesis. We found that DUSP1 sustained cardiac mitochondrial quality control by binding directly to PHB2 and maintaining PHB2 phosphorylation, while CRS-3 disrupted this physiological interaction. Transgenic knock-in mice carrying the Phb2S91D variant were less susceptible to cardiac depression upon CRS-3, due to a reduced inflammatory response, suppressed oxidative stress and improved mitochondrial quality control in their heart tissues. Thus, CRS-3-induced myocardial dysfunction can be attributed to reduced DUSP1 expression and disrupted DUSP1/PHB2 binding, leading to defective cardiac mitochondrial quality control.

Cell-Specific Targeting of the Endothelium in the Cardiorenal Syndrome.

BACKGROUND: The vascular endothelium serves as a semi-selective permeable barrier as a conduit for transport of fluid, solutes, and various cell populations between the vessel lumen and tissues. The endothelium thus has a dynamic role in the regulation of coagulation, immune system, lipid and electrolyte transport, as well as neurohumoral influences on vascular tone and end-organ injury to tissues such as the heart and kidney. SUMMARY: Within this framework, pharmacologic strategies for heart and kidney diseases including blood pressure, glycemic control, and lipid reduction provide significant risk reduction, yet certain populations are at risk for substantial residual risk for disease progression and treatment resistance and often have unwanted off-target effects leaving the need for adjunct, alternative targeted therapies. Recent advances in techniques in sequencing and spatial transcriptomics have paved the way for the development of new therapies for targeting heart and kidney disease that include various gene, cell, and nano-based therapies. Cell-specific endothelium-specific targeting of viral vectors will enable their use for the treatment of heart and kidney diseases with gene therapy that can avoid unwanted off-target effects, improve treatment resistance, and reduce residual risk for disease progression. KEY MESSAGES: The vascular endothelium is an important therapeutic target for chronic kidney and cardiovascular diseases. Developing endothelial-specific gene therapies can benefit patients who develop resistance to current treatments.

Cardiorenal Interactions in Acute Heart Failure: Renal Proximal Tubules in the Spotlight.

BACKGROUND: The maladaptive neurohormonal activation, an integral mechanism in the pathophysiology of heart failure (HF) and cardiorenal syndrome, has a profound impact on renal sodium handling. Congestion is the primary reason for hospitalization of patients with HF and the main target of therapy. As sodium is the main determinant of extracellular volume, the goal is to enhance urinary sodium excretion in order to address excess fluid. The interventions to increase natriuresis have conventionally focused on distal nephron as the primary segment that counterbalances the effects of loop diuretics. SUMMARY: Recent developments in the field of cardiorenal medicine have resulted in a shift of attention to renal proximal tubules (e.g., emerging evidence on proximal tubular dysfunction beyond handling of sodium). Herein, we discuss the three main mechanisms of sodium transport in the proximal tubules with emphasis on their intrinsic links to one another as well as to more distal transporters of sodium. Then, we provide an overview of the findings of the most recent clinical studies that have tried to enhance the conventional decongestive strategies through simultaneous blockade of these mechanisms. KEY MESSAGE: Interventions aiming at renal proximal tubules have the potential to significantly improve our ability to decongest patients with acute HF.