Ischemia/reperfusion (I/R) injury is a common cause of AKI in clinical settings such as kidney transplantation, major surgery, and shock (Chen et al., 2025). Specific regions of the kidney, notably the S3 segment of the proximal tubule and the thick ascending limb of the loop of Henle in the outer medulla, are particularly susceptible to I/R injury due to their high metabolic activity and oxygen demand (Kwiatkowska et al., 2023). The ischemic phase induces rapid depletion of adenosine triphosphate (ATP), causing cellular energy failure, mitochondrial dysfunction, and a massive generation of reactive oxygen species (ROS). While reperfusion is essential for cell survival, it paradoxically exacerbates tissue damage through a cascade of molecular events (Nakamura et al., 2024). Damaged cells release damage-associated molecular patterns (DAMPs), such as high mobility group box-1, which, in conjunction with pathogen-associated molecular patterns in septic AKI, activate pattern recognition receptors on immune and renal parenchymal cells (Zhang et al., 2024; Tang et al., 2012). This activation triggers a massive release of pro-inflammatory cytokines (e.g., TNF-α, IL-6) and chemokines (e.g., MCP-1/CCL2), establishing a self-perpetuating, pro-inflammatory feed-forward circuit (He et al., 2025; Rabb et al., 2016). Severe energy depletion can induce necroptosis in TECs, a process mediated by the necroptosome, a complex formed by receptor-interacting protein kinase 1 (RIPK1), RIPK3, and their substrate, mixed lineage kinase domain-like protein (MLKL) (Li et al., 2024; Li et al., 2022; Gardner et al., 2023). The DAMPs released from necroptotic cells activate inflammatory responses, creating a “necrosis-inflammation” vicious cycle that accelerates kidney damage (Roger et al., 2023; Frank and Vince, 2019).

During AKI, hypoxia and inflammation are synergistic and antagonistic. Hypoxia, a consequence of microvascular dysfunction, not only directly damages cells and exacerbates endothelial dysfunction but also sensitizes the tissue to inflammatory insults (Takeda and Fong, 2007). Conversely, inflammatory mediators can inhibit mitochondrial respiration, further impair ATP production and perpetuating the cycle of injury (Gouriou et al., 2020). At the molecular level, the hypoxia-inducible factor-1α (HIF-1α)-driven hypoxia-sensing pathway exhibits significant crosstalk with the nuclear factor-κB (NF-κB)-regulated inflammatory pathway (Li et al., 2021). Inflammatory stimuli can enhance HIF-1α transcription, sustaining a state of functional hypoxia that promotes disease progression and maladaptive repair (Taylor et al., 2016). In clinical scenarios like kidney transplantation where I/R injury is unavoidable, ischemic preconditioning (IPC), namely, applying brief, non-lethal episodes of ischemia/reperfusion before a sustained ischemic event, can induce tissue tolerance to subsequent injury (Ortega-Trejo and Bobadilla, 2023). The protective mechanisms include mitigating oxidative stress, upregulating antioxidant enzymes, and inhibiting inflammatory pathways. Although results from large clinical trials have been mixed, IPC nonetheless provides a valuable conceptual framework for the prevention and treatment of AKI.

The development of PTCs rarefaction is driven by complex cellular and molecular mechanisms, primarily encompassing ECs injury, pericyte detachment, and the synergistic effects of the local hypoxic and inflammatory microenvironment (Figure 3). The pathological process of PTCs rarefaction in AKI commences with multifaceted injury to ECs, triggered by insults such as ischemia, sepsis, or nephrotoxins (Molema et al., 2022). Morphologically, ECs injury manifests as cell swelling, loss of fenestrae, cytoskeletal disruption, and compromised vascular permeability (Finch et al., 2023). Damage to the endothelial glycocalyx, a protective proteoglycan-polysaccharide layer on the luminal surface of ECs, is a critical early event in ECs dysfunction. Inflammatory cytokines like TNF-α, in concert with enzymes such as heparanase and matrix metalloproteinases, can specifically degrade glycocalyx components, causing their shedding into the circulation (Wang et al., 2025). This process not only compromises the vascular permeability barrier but also exposes endothelial adhesion molecules, thereby promoting leukocyte adhesion and infiltration and exacerbating local ischemia (Saunders et al., 2005).

ECs injury also activates programmed cell death pathways. In septic AKI, for instance, elevated levels of the key necroptosis protein RIPK3 induce ECs membrane perforation via the RIPK1/RIPK3-MLKL pathway. Ferroptosis, characterized by decreased glutathione peroxidase 4 activity and accumulation of lipid peroxidation products, leads to ECs death through an iron-dependent lipid peroxidation mechanism (Liu et al., 2024; Jiang et al., 2025). Furthermore, ECs injury results in reduced nitric oxide bioavailability and increased ROS production, shifting the endothelial phenotype towards a pro-thrombotic and pro-inflammatory state (Yeh et al., 2024).

Pericytes are critical for maintaining the structural stability of PTCs, secreting matrix components such as type IV collagen and laminin, and regulating local blood flow through contraction. Consequently, pericyte detachment and dysfunction further aggravate microvascular injury (Venkatachalam and Weinberg, 2017). Endothelial damage disrupts the core signaling networks responsible for maintaining vascular stability. For example, dysregulation of the platelet-derived growth factor-B (PDGF-B)/PDGFRβ axis inhibits pericyte recruitment and attachment to the vessel wall (Cossutta et al., 2019). Concurrently, the excessive release of Angiopoietin-2 (Ang-2), a competitive antagonist of Ang-1, blocks Ang-1/Tie2-mediated vascular maturation signals, ultimately causing pericyte detachment from the capillary basement membrane (Park et al., 2017). Once detached, pericytes migrate into the renal interstitium and undergo a phenotypic switch upon stimulation by pro-fibrotic factors such as TGF-β (Tanaka et al., 2023). These cells then activate α-SMA expression and differentiate into myofibroblasts, which drive interstitial fibrosis through the secretion of extracellular matrix proteins like type I collagen and fibronectin (Yeh et al., 2024). Indeed, lineage tracing studies have confirmed that over 80% of α-SMA-positive myofibroblasts in fibrotic kidneys originate from the pericyte pool (Alarcon-Martinez et al., 2021). Their detachment therefore not only leads to a loss of vascular support and accelerates PTCs regression but also directly supplies the effector cells for fibrosis (Baker and Cantley, 2025).

The progression from AKI to CKD is determined by the outcome of a “race” between adaptive repair and maladaptive fibrosis. When microvascular injury remains unresolved, the resultant persistent PTCs rarefaction creates a state of chronic tubulointerstitial hypoxia, which in turn drives the renal fibrotic process (Kida, 2020). This state of sustained hypoxia and inflammation foster a pro-fibrotic microenvironment conducive to the activation and proliferation of myofibroblasts (Kellum et al., 2021). Maladaptive repair, the core mechanism driving fibrosis, is characterized by the dysfunctional regulation of signaling pathways that possess dual roles in tissue regeneration. For example, while transient activation of the Wnt/β-catenin pathway post-injury promotes epithelial repair, its sustained or excessive activation aberrantly drives fibrotic gene expression and pathological remodeling (Bülow and Boor, 2019). The regulatory pattern of HIF-1α also demonstrates this duality: although its early, transient activation is an adaptive response to acute hypoxia, but chronic, sustained activation transforms it into a potent pro-fibrotic signal (Zhang et al., 2021). Chronic HIF-1α stabilization promotes fibrosis through multiple mechanisms, including the metabolic reprogramming of renal cells towards a pro-fibrotic phenotype and synergistic activation with pathways such as TGF-β and Notch (Liu et al., 2024).

As a high-perfusion organ, the kidney receives approximately 20%–25% of the cardiac output. However, even under physiological conditions, a steep oxygen gradient exists between the cortex and medulla, leaving the medulla in a state of borderline hypoxia (10–20 mmHg) due to its countercurrent exchange system (Dyson et al., 2011; Bai et al., 2022). This fragile oxygenation balance is critically dependent on the integrity of the post-glomerular microcirculation (Zhao et al., 2022). Following an AKI event, hemodynamic disturbances and increased oxygen consumption by injured and regenerating cells exacerbate this hypoxia. Hypoxia, through activation of the HIF signaling pathway, upregulates pro-angiogenic factors such as VEGF to initiate a pro-angiogenic program (Roger et al., 2023). Simultaneously, AKI triggers a robust inflammatory cascade, that leads to the infiltration of numerous immune cells, such as neutrophils and macrophages, into the damaged tissue (Yeh et al., 2024). These cells, along with injured TECs, release DAMPs, cytokines, and growth factors, forming a complex signaling network that collectively regulates angiogenesis (DeWolf et al., 2022).

Notably, the post-AKI microenvironment exerts a dual impact on angiogenesis (Tanaka and Nangaku, 2013). Early hypoxic and inflammatory signals are necessary to initiate this reparative process. However, if the inflammatory response becomes excessive or prolonged, it can damage nascent blood vessels and activate fibrotic pathways, thereby counteracting these reparative effects. Therefore, the efficacy of angiogenesis depends on the timely transition from an injurious microenvironment to a pro-reparative one.

HIF-1α, the master regulator of the hypoxic response, is rapidly stabilized within injured TECs and ECs (Roger et al., 2023). Activated HIFs translocate to the nucleus and, by binding to hypoxia-response elements, systematically upregulate a suite of target genes, including VEGF-A, erythropoietin (EPO), and various angiopoietins, which collectively promote ECs proliferation, migration, and new vessel formation (Thévenod et al., 2022). Furthermore, HIF activation can synergize with other signaling pathways, such as Wnt/β-catenin to promote kidney repair (Xu et al., 2022). However, the effect of HIF activation is context-dependent. While HIF signaling generally exerts a protective role in AKI by promoting angiogenesis and cell survival, its overactivation or activation in specific cell types can provoke fibrosis (Tanaka and Nangaku, 2013).

VEGF-A stimulates PTCs neogenesis by acting on VEGFR1/2 receptors on adjacent ECs; this crosstalk is mediated with greater efficiency by VEGF-A-rich extracellular vesicles secreted from TECs (Chen B. et al., 2022; Zhong et al., 2022). Renal VEGF-A expression exhibits a biphasic pattern, peaking during early AKI and again during the transition to CKD. Exogenous supplementation with VEGF-A in early AKI can be renoprotective by maintaining microvascular integrity and alleviating secondary tubular hypoxic injury. However, its dysregulation or overactivation can lead to increased permeability of nascent vessels and exacerbated inflammation (Huang et al., 2023). Moreover, different VEGF isoforms and their receptors have distinct effects: following cardiac surgery, elevated soluble VEGFR-1 increases the odds of AKI by 56%, whereas high levels of VEGF are protective. (Mansour et al., 2019).

The Ang-1/Ang-2/Tie2 signaling axis is a primary regulator of vessel stability and maturation (Liu et al., 2014). Ang-1 activates its receptor, Tie2, to promote vessel stabilization. In contrast, Ang-2, which is upregulated by ECs during AKI, functions as a context-dependent Tie2 antagonist. Initially, this antagonism facilitates vessel sprouting, but its sustained high levels disrupt vascular integrity. Vascular endothelial protein tyrosine phosphatase (VE-PTP), a key negative regulator of Tie2, is also elevated following renal ischemia (Li et al., 2023). Inhibition of VE-PTP or administration of an Ang-1 mimetic effectively mitigates AKI. Clinical data corroborate these findings, confirming that a higher Ang-1/Ang-2 ratio at hospital discharge is associated with improved long-term renal and cardiovascular outcomes (Mansour et al., 2022).

Several other pathways are also implicated in this complex process. For example, Notch signaling ensures orderly vessel growth by regulating ECs tip/stalk cell specification, but its sustained activation in TECs can induce cellular senescence and impede repair (Bakleh and Al, 2025; Sörensen-Zender et al., 2014). The SDF-1 (CXCL12)/CXCR4 axis is crucial for mobilizing endothelial progenitor cells (EPCs) to the site of injury (Cao et al., 2022). Following ischemic AKI, renal SDF-1expression increases, creating a chemotactic gradient that recruits CXCR4-positivecells to the damaged area to participate in PTCs repair.

PTCs remodeling is the result of concerted actions among multiple cell types. As the primary building blocks of new vessels, ECs become activated and initiate sprouting in response to angiogenic signaling cues (Almazmomi et al., 2023). The detachment of pericytes from the vessel wall is a prerequisite for initial vessel sprouting, while their subsequent re-recruitment to and ensheathment of nascent vessels is critical for vascular maturation and stabilization (Tanaka and Nangaku, 2013). However, if this pericyte recruitment fails, the detached cells can differentiate into myofibroblasts, directly driving renal fibrosis (Xu et al., 2025). EPCs are mobilized to the site of injury following AKI, where they contribute to repair either by directly integrating into the vasculature or by supporting resident ECs function through paracrine signaling (Kraińska et al., 2021). The phenotypic switch of infiltrating macrophages is a critical determinant of the repair outcome. During the repair phase, pro-inflammatory M1 macrophages must polarize toward a pro-reparative M2 phenotype for adaptive repair to proceed (Chen H. et al., 2022). These M2 macrophages actively support adaptive angiogenesis and tissue remodeling by secreting various pro-angiogenic factors and regulating the extracellular matrix.

PTCs neogenesis during the course of AKI exhibits a tightly regulated spatio-temporal pattern that dictates its ultimate effect. Temporally, the early injury phase is characterized by PTCs structural damage; while hypoxia induces the initial release of HIF-1α and VEGF, the highly inflammatory microenvironment inhibits effective neogenesis (Baker and Cantley, 2025). The disparity between the sharp drop in ATP in proximal tubules and the more gradual decline in distal cells further confirms this injury heterogeneity (Yamamoto et al., 2020). The subsequent maintenance phase presents a divergent path: activation of repair signals can initiate mobilization of EPCs, while persistent inflammation exacerbates PTCs rarefaction (Gonsalez et al., 2019). A successful repair phase depends on the M1-to-M2 polarization of macrophages, which is required to drive ECs proliferation, lumen formation, and pericyte recruitment for vascular maturation (Chen H. et al., 2022). Furthermore, evidence suggests that the degree of ATP recovery in proximal tubules during the acute phase is negatively correlated with the extent of fibrosis in the chronic phase (Yamamoto et al., 2020). Spatially, the renal outer medulla exhibits a greater demand for neogenesis due to its high ischemic sensitivity, yet the precise spatial dynamics of this process in human AKI require further investigation (Kwiatkowska et al., 2023; Tanaka and Nangaku, 2013).

Adaptive PTCs neogenesis restores oxygen supply and facilitates the removal of metabolic waste by rebuilding the microcirculation; it also directly promotes TECs repair through paracrine factors secreted from the endothelium (Zhong et al., 2022). Clinically, elevated levels of pro-angiogenic markers such as VEGF are significantly associated with improved AKI outcomes (Mansour et al., 2019; Vakhshoori et al., 2025). Conversely, dysregulated angiogenesis can be a core driver of the AKI-to-CKD transition. Structurally immature, “leaky” new vessels can exacerbate inflammatory cell infiltration, while uncontrolled VEGF signaling can paradoxically promote M1 macrophage polarization by activating NF-κB, creating a “leakage-inflammation” vicious cycle (Tanaka and Nangaku, 2013; Ourradi et al., 2017). Persistent hypoxia and inflammation activate fibroblasts and induce excessive extracellular matrix deposition, while the aberrant, sustained activation of certain HIF and VEGF isoforms can directly promote the fibrotic process (Jiang et al., 2023). Additionally, if nascent vessels lack adequate pericyte coverage or fail to integrate into the functional circulation, their inherent defects will consume resources needed for repair, underscoring that the quality of PTCs neogenesis is as important as its quantity (Wang et al., 2023; Chou et al., 2024).

Therapeutic strategies for AKI are undergoing a paradigm shift, moving from single-target interventions toward multi-dimensional, precise regulation of vascular repair. Growth factor-based therapies, for instance, are evolving from a simple “pro-proliferative” approach to one focused on restoring “vascular homeostasis”. Although preclinical studies show that exogenous supplementation of VEGF-A in early AKI can be renoprotective by preserving the microvasculature, its “double-edged sword” effect and the off-target risks of non-specific delivery have shifted research focus (Huang et al., 2023). Attention is now on kidney-targeted delivery and controlled-release technologies using nanocarriers, such as VEGF fused to elastin-like polypeptides. Clinical translation of these nanocarrier strategies, however, faces significant hurdles, including poor homing efficiency (Engel et al., 2019). Consequently, researchers are now focusing more on the Ang-1/Tie2 pathway, which is central to vascular stabilization (Li et al., 2023). Ang-1 mimetics, such as Vasculotide and Hepta-ANG1, have demonstrated superior protective effects and safety profiles compared to VEGF-based therapies in various animal models of AKI by restoring vascular homeostasis (Kumpers et al., 2011).

Cell-based therapies using EPCs represent another strategy. As progenitor cells capable of differentiating into mature ECs, EPCs are mobilized and home to the injured kidney following AKI, where they participate in repair through the dual mechanisms of direct integration into PTCs and paracrine signaling that improves the microenvironment (Becherucci et al., 2009; Basile et al., 2001). Notch signaling exhibits classic context-dependency in angiogenesis and kidney repair. Inhibiting Notch signaling in late-stage AKI with γ-secretase inhibitors (GSIs) can reduce fibrosis, whereas early inhibition may interfere with cell proliferation and tissue regeneration. However, the clinical use of GSIs is limited by off-target effects from systemic γ-secretase inhibition (De Chiara et al., 2021; Wyss et al., 2018). (For more details, please see the Supplementary Material).

Despite extensive research revealing the mechanisms of post-AKI angiogenesis, the field faces a significant “translational gap.” Numerous therapeutic strategies that proved effective in preclinical animal models have repeatedly failed in human trials (Gerhardt et al., 2021). This disconnect stems from multiple factors. First, a therapeutic window gap exists, wherein promising preclinical strategies such as VEGF-targeted therapy and IPC have not reproduced their protective effects in clinical trials. This failure is attributed to the narrow therapeutic window created by the dual role of VEGF, the difficulty in precisely timing interventions, and the confounding effects of comorbidities and anesthetic methods on IPC within heterogeneous patient populations (Gouriou et al., 2020; Jiang et al., 2025; Bakleh and Al, 2025; H et al., 2010). Second, preclinical models suffer from a “fidelity gap.” The unilobar kidney anatomy, divergent metabolic rates and renal hemodynamics of rodents, and the absence of human-specific inflammatory pathways (e.g., involving IL-8) make it difficult for these models to fully recapitulate the immunological complexity and multi-factorial injuries seen in human AKI with comorbidities (Kida, 2020; Yamamoto et al., 2020; Gonsalez et al., 2019; Meng et al., 2023). Third, diagnostic tools, primarily serum creatinine, have a diagnostic lag. While novel biomarkers like neutrophil gelatinase-associated lipocalin can detect injury earlier, they have not yet been translated into clinically actionable guidance for therapies (Kellum et al., 2021; Wang et al., 2023; Chou et al., 2024). Similarly, while angiogenic markers like VEGF have shown prognostic value, they still lack diagnostic and therapeutic utility (Noel et al., 2024; Huang et al., 2024; Lundy et al., 2024). Collectively, these challenges constitute the translational gap for AKI angiogenesis therapies and necessitate breakthroughs in preclinical model design, the development of precise diagnostic and staging tools, and the formulation of personalized therapeutic strategies.