Even though AKI is highly variable in its clinical presentation, damage to proximal tubule epithelium is a significant hallmark of the condition that can manifest through various mechanisms that elicit pre-renal, intrinsic, and post-renal injuries. For instance, the degree of injury, clinical severity, and progression of AKI are associated with the duration of IRI. The current understanding of AKI has been developed through extensive research using animal models, particularly mouse and rat species, and the ability to rapidly and reliably induce mild, moderate, and severe damage in the PCTs by administering nephrotoxins in various doses19 and modulating the duration needed to induce IRI20. Based on these facts, in this work, we demonstrate the ability of gray level co-occurrence matrix and discrete wavelet transform methods to detect subtle structural changes in PTC nuclei after mild pre-renal AKI.
The gold standard in the biochemical evaluation of AKI relies on serum creatinine levels. These peak serum creatinine elevations 24 hours post-ischemia corresponds to peak injury and is a hallmark of this model21. Using this approach, we verified the induction of mild AKI using a mild form of IRI, evidenced by the two-fold rise in this biomarker 24 hours after injury and its gradual return to baseline within a week. Moreover, biopsies also revealed that the bilateral pedicle clamp model supported mild tubular disruptions evidenced by histology The examination of images collected from sham and mildly injured animals allowed us to show that the most critical nuclear GLCM indicators such as angular second moment and inverse difference moment significantly change as the result of mild AKI, which indicates the rise of nuclear textural heterogeneity due to the injury-associated redistribution of euchromatin and heterochromatin. To our knowledge, this is the first study to combine GLCM indicators and DWT coefficient energies to reveal discrete AKI-related alterations in PTC nuclear architecture. We also propose hypothetical machine learning models based on support vector machines, random forest, and logistic regression, which might in the future be used as a part of accurate computational AI sensing systems for diagnostic purposes.
In our previous articles, we applied similar GLCM approaches to analyze alterations to the renal vascular architecture22, highlighting its potential application in the characterization of whole organ scaffolds23 that can be generated for bioartificial kidney development24. Using this technique, We also examined cell nuclei after the damage induced by exposure to a sublethal toxic dose of ethanol12. On an experimental model of saccharomyces cerevisiae, we calculated angular second moment, inverse difference moment, textural contrast, GLCM correlation, and variance and demonstrated that these features significantly change after alcohol treatment. This type of cell damage was also associated with the reduction of textural homogeneity and uniformity, leading us to believe that these changes in nuclear textural patterns are generally related to cell damage. Similarly, as in the present study, we proposed several machine learning models, such as the ones based on logistic regression, decision trees, and artificial neural networks12. Despite the apparent differences in the methodological approach and experimental protocol between the two works, this assumption is worthy of investigation in future research.
One of the earliest research articles on applying GLCM in the histological evaluation of kidney tissue was published in 2013. Indicators such as the angular second moment and inverse difference moment were quantified to assess chromatin architecture in macula densa cells during mice postnatal development and aging25. Textural features were evaluated in conjunction with fractal dimension and lacunarity as indicators of complexity. Although no statistically significant changes were detected in ASM and IDM values, the research nevertheless holds some value since it was the first to show that GLCM analysis is not only possible in kidney tissue but also applicable for scoring various aspects of textural heterogeneity in individual kidney cell nuclei.
Our current study is also not the first to use the GLCM computational method for the detection of AKI. Previously, textural features such as GLCM contrast and GLCM correlation were quantified in the kidney medulla after inducing IRI in rats by clamping both renal vascular pedicles and subsequent reperfusion with saline26. It was shown that both CON and COR features had excellent discriminatory power in separating injured from control medullar tissue, with the area under the receiver operating characteristic curve in both cases higher than 85%. The results identified both fractal and GLCM parameters as suitable candidates for the future development of computational biosensors in nephropathology.
There are several potential explanations for the AKI-related changes in PTC nuclear textural patterns, which were detected in our current study. First, it is possible that AKI led to the redistribution of euchromatin and heterochromatin in PTCs, and that the redistribution was the result of either direct damage to the cell or activation of a signaling pathway. AKI is indeed associated with sometimes profound epigenetic changes, as explained in detail by other authors6. Some of the genes that are upregulated during AKI may also influence chromatin integrity and remodeling on higher scales. Although these phenomena are generally not noticeable during the standard histopathological evaluation, the subsequent changes in textural patterns may have been detected with GLCM and DWT. Also, it should be considered that, sometimes, euchromatin and heterochromatin, at least in the ultrastructural sense, have different levels of fractal complexity27, and these differences in complexity may have reflected on GLCM and DWT features in this experimental setting as well.
Another possibility is that the mild AKI in the renal cortex is sometimes associated with programmed cell death. Indeed, PTCs are highly susceptible to apoptosis, as discussed earlier28, and this type of cell death contributes to the loss of kidney functionality during AKI. On the other hand, some previous works have suggested that GLCM indicators such as angular second moment and inverse difference moment significantly decrease after cell treatment with proapoptotic substances29. During the early stages of programmed cell death, phenomena such as (initial) condensation and marginalization of chromatin may perhaps lead to the increased textural heterogeneity, detectable using both GLCM and DWT. However, additional research is needed to confirm this assumption, particularly on PTCs and other cell populations in the renal cortex.
In the future, it will be possible to broaden this type of research by developing AI models based on artificial neural networks. This approach could include relatively simple perceptron networks, but also more complex recurrent (RNNs) and convolutional neural networks (CNNs). The input layer of neurons in these models could receive DWT and GLCM data, but also the data from a three-dimensional matrix of values based on red, green, and blue light intensities. Inclusion of various other image analysis quantifications, such as fractal dimension, lacunarity, and granularity might further benefit the network’s ability to distinguish damaged from intact cells. Convolutional neural networks are of particular interest since previously, they have been successfully applied for image classification on numerous occasions30, 31. Creating a complex CNN that combines DWT and GLCM with other input parameters could lead to developing a sensitive, accurate, and affordable computer-aided diagnostic system that might serve as an essential addition to current nephropathology practices.
Our study had several important limitations that need to be discussed and considered when conducting future research in this scientific area. First, GLCM and DWT methods are relatively new in terms of their applications in pathology and histology, so there is a lack of data on their quality assurance and validity. Different software platforms often produce different results, and based on our previous experience, indicators such as angular second moment and inverse difference moment can significantly vary depending on the software settings and parameters during micrograph acquisition. Second, one needs to stress that the AI models proposed in this research are only hypothetical since they were trained and tested on a very limited number of nuclear ROIs. To increase validity and test this approach's diagnostic value, one would need to develop the machine learning models on a very large sample of micrographs with one ROI corresponding to one individual micrograph or even one individual animal. Finally, also from our previous experience, values obtained through GLCM, and DWT analyses greatly depend on the histological staining applied to the tissue. In the future, one might consider repeating the experiments and using other techniques such as periodic acid–Schiff, Sirius Red, Feulgen, or Toluidine Blue. Only then would we be able to have full insight into the true scientific value of GLCM and DWT computational methods.
In conclusion, we present evidence that GLCM and DWT computational methods can detect subtle structural alterations in PTC nuclei associated with mild AKI. After quantifying textural features such as the angular second moment and inverse difference moment of nuclear architecture, we conclude that this form of injury leads to the rise of nuclear textural heterogeneity. This change is not clearly visible during a conventional histopathological evaluation. Since this syndrome rarely has a sole and distinct and is frequent among patients without critical illness, it is essential that health care professionals, especially those without specialization in renal disorders, detect it easily32. Thus, we propose creating AI models that use GLCM and DWT indicators as input data, capable of AKI identification and PTC classification with great accuracy and discriminatory power. The obtained results present a potentially valuable foundation for future research in AI applications in pathology, nephrology, and related disciplines and support current regimens used to address AKI.