It was known that HIBD may lead to neuropsychotic disorders such as cerebral palsy, seizures and learning difficulties in children. More and more studies have shown the effectiveness of hUC-MSCs in the treatment of HIBD and related sequelae (Boruczkowski and Zdolinska-Malinowska, 2019; Dong et al., 2018; Xie et al., 2020). However, so far, most of the studies are still in the initial clinical trial stage (Shariati et al., 2022), which has to be attributed to the fact that there are still many problems related to hUC-MSCs that have not been solved. For example, the most suitable dose, the most appropriate time window and the best effective way of stem cell transplantation.
There are many theories about the optimal dose and time window of stem cells transplantation. Some researchers believe that the minimum effective dose of stem cells in nasal transplantation is 0.5×106, the higher dose will not be more effective, but the lower dose has no obvious therapeutic effect (Donega et al., 2013). Because the Hamilton syringe is needed in nasal or lateral ventricle transplantation and the volume of transplanting fluid is limited, the concentration of cell suspension should also be taken into account in the selection of the number of cell transplants. Higher concentration may aggregate into cell mass and affect cell activity, and lower concentration may not active therapeutic effect. 5×104 cells/ul is considered to be the most appropriate cell transplantation concentration(Zheng and Weiss, 2022). Therefore, the number of cells selected in this experiment was 0.5×106 cells. The problem of transplantation time also needs to be considered. Premature transplantation may not be conducive to the survival of stem cells in the brain microenvironment, late transplantation may result in glial scar formation and a large number of irreversible necrosis of neurons. So we decided to do the transplantation on the 7th day after the injury(Yasuhara et al., 2008). In order to make the results more reliable, this experiment unified the dose and time of transplantation of stem cells in three groups.
It was known that perinatal hypoxia-ischemic encephalopathy mainly causes motor and cognitive impairment in children(Herz et al., 2022). And the cognitive function of the individual mainly includes feeling, perception, memory, thinking, and so on. Through the construction of HIBD rat model, we found that the behavior of rats after hypoxia and ischemia was also manifested in the decline of motor balance and cognitive function (learning and memory). The motor and balance ability of rats in each group were compared by hanging wire and vertical pole test (Fig. 1). It was found in PN10 that the rats in HIBD group showed obvious motor backwardness and lack of balance ability. On the 7th day after transplantation of stem cells (PN21), the three ways showed obvious therapeutic effect, especially the IC transplantation showed better motor function improvement effect than IV transplantation in the hanging wire test, but there was no significant difference compared with IN transplantation. It is unclear whether this is related to the IC-transplanted cells can reach the local part of brain injury more. Similar results have been shown in other experiments (Noh et al., 2020b; Zhang et al., 2012), and stem cells tracking techniques have been used to determine that IC transplantation is faster and more numerous than IV transplantation (Doeppner et al., 2012; Li et al., 2010). In the vertical pole test, transplantation of stem cells through three ways improved the motor balance ability of rats, but the degree of improvement among the three pathways did not show significant statistical difference. Therefore, we speculate that the functional areas of the brain affected by intervention of stem cells in different ways may be different. This was also verified by the MWM test. HIBD rats exhibit significant impairment in learning memory function, and stem cells intervention reduces the extent of this functional impairment, especially through the IC and IN transplantation, and even the IC transplantation has a more pronounced improvement in short-term memory retention ability. It indicates that the intervention of stem cells through IC and IN has a more significant therapeutic effect on the brain tissue area in charge of learning and memory function in HIBD rats.
The brain areas in charge of learning and memory function are mainly located in the hippocampus, especially the CA1, CA3 and DG regions, while the cortex is the advanced center for further processing memory and behavioral movement. According to the overall brain tissue specimens of our experiment, the injury area of HIBD to rat brain is also mainly manifested in hippocampus and cortex. Therefore, from the microcosmic point of view, we choose to quantify the damage of hippocampus and cortex by pathological staining to compare the therapeutic effect. Glial cells in the central nervous system mainly include astrocytes, microglia and oligodendrocytes, which play an important role in maintaining human health activities. Among them, astrocytes seem to play a more extensive role (Giovannoni and Quintana, 2020). When HIBD occurred, the release of various harmful factors increased, astrocytes and microglia were activated, and the expression of inflammatory factors was up-regulated, resulting in neuronal necrosis and apoptosis(Mota-Rojas et al., 2022). In the process, astrocytes proliferation is thought to last longer, play a more significant role, and damage more(Xian et al., 2019). Therefore, the protection of the nervous system from the reactive proliferation of astrocytes seems to play a greater role. The expression of GFAP in astrocytes is up-regulated after central nervous system injury, so it is widely used as a marker of the responsiveness of astrocytes (Sofroniew, 2014). We detected the repair of neuronal necrosis in cerebral cortex and hippocampus by hUC-MSCs transplantation by HE staining, and detected the reactive proliferation of astrocytes by GFAP. We found that the transplantation of stem cells through IV, IN and IC could significantly repair the injured and necrotic neurons in the cerebral cortex and hippocampus of rats, and decrease the proliferation of reactive astrocytes. The improvement of the area of astrocytes in DG area in HIBD + IN group was more obvious than that in HIBD + IV group. Besides, as previously studied(Sato and Tsuji, 2021), the blow to rats by HIBD also includes inducing neuronal apoptosis. In this study, the number of apoptotic cells in the cerebral cortex and hippocampus of rats decreased significantly after stem cell transplantation, indicating that stem cell transplantation can reduce the apoptosis of nerve cells induced by HIBD. Among them, the reduction of apoptotic neurons in the hippocampus was more obvious in the HIBD + IC and HIBD + IN group than in the HIBD + IV group, whereas it was more obvious in the HIBD + IC group for apoptotic neurons in the cortex. It means that the intervention of hUC-MSCs through lateral ventricle and nasal cavity has a better effect on improving the apoptosis of nerve cells in the brain tissue of HIBD rats than through intravenous intervention, which is basically consistent with our behavioral results.
To sum up, our study shows that transplantation of stem cells by three ways has therapeutic effect on HIBD. In terms of neurological recovery, damaged tissue repair, astrocytes proliferation and neuronal apoptosis, the therapeutic effect of IC and IN interventions was stronger in HIBD rats compared to the IV intervention route, while there was no significant difference between the first two intervention routes. Previous studies on the optimal pathway for stem cells intervention have also been conducted, focusing on the peripheral (venous, arterial) pathway and the local (lateral ventricular) pathway. During the same 7-week time frame, IC transplantation maybe is preferable to IV transplantation for delivery of hUC-MSCs during subacute phases of stroke(Noh et al., 2020b). This is similar to our results. IC intervention is the direct delivery of all stem cells into the brain using a brain stereotaxic instrument and is undoubtedly the fastest and the way to maximize the number of cells reaching the local area of brain injury. And clinical experiments have shown that interventional therapy through IC route has certain curative effect on the short-term movement (Chiu et al., 2022). However, when performing lateral ventricular interventions, we also need to be aware of the side effects associated with the intervention route itself. In addition to direct puncture injuries and infectious complications, the sudden delivery of fluid and extracorporeal cells directly into the brain may also cause secondary damage by triggering an immune response in the brain. Besides, IC transplantation requires a high level of instrumentation and operator skill. IN transplantation is a non-invasive treatment method, which will hardly cause damage to the body and is easy to be repeated. It is a promising treatment method. And it has now been shown to have significant therapeutic effects in neurological disorders (Farfán et al., 2020; Yu-Taeger et al., 2019). IN-transplanted cells can ignore the blood-brain barrier and rapidly pass through the olfactory nerve into the brain (Vaes et al., 2021; Wu et al., 2021). Moreover, IN-transplanted stem cells could still be detected locally in brain tissue several months later (Alizadeh et al., 2021). This seems to indicate that the IN pathway is more advantageous in terms of long-term therapeutic effects. Therefore, the combination of safety, efficacy and compliance may make IN intervention of stem cells more valuable in clinical applications compared to IC intervention. However, research on IN-transplanted stem cells is still mainly focused on animals, including non-human primates, and has been preliminarily shown to be effective (Galeano et al., 2018), although the relevant clinical evidence is still lacking.
The main purpose of our study on the effect of different stem cell intervention pathways on the repair of HIBD is to provide further reference value for the application of stem cells in clinical practice. Therefore, in addition to taking into account the therapeutic effect, we also need to include enforceability, acceptability and low adverse reactions in our thinking. Our study shows that IC as well as IN are superior to IV routes in terms of therapeutic efficacy, while there is no statistically significant difference between the first two comparisons. When applied to clinic, IN transplantation may be a more performant, more acceptable, and less invasive approach. However, IN intervention with stem cells is still an emerging treatment, and further research is needed to determine whether it is feasible to apply it in humans, as well as the appropriate dose, duration, and preparation before the intervention. Additionally, even we have established a systematic evaluation method and system through this experiment, due to financial and time constraints, we have not done further research on the differentiation and homing of hUC-MSCs from each transplantation pathway in rats, nor the optimal transplantation dose and time window. And in the future, we can investigate more deeply in terms of therapeutic mechanisms, the optimal transplantation dose and time window, to facilitate the construction of clinical guidelines.
Overall, hUC-MSCs transplantation can reduce brain injury and motor and cognitive impairment caused by HIBD, and transplantation through the intracerebral and intranasal is more effective. In the future, if applied to clinical practice, intranasal transplantation may be an option that can be treated multiple times and is more compliant and convenient. However, it is worth noting that the stability and safety of intranasal transplantation still need to be further studied.