Acharya M, Martirosian V, Chmielewski N, Hanna N, Tran K, Liao A, Christie L, Parihar V, Limoli C. "Stem Cell Transplantation Reverses Chemotherapy-Induced Cognitive Dysfunction."Cancer Research 75;676-686, 2015.
Prepared by: Anushka Sindkar, Advance Stem Cell, Fall 2015
LAYMAN’S SUMMARY
Today’s oncological treatments have improved the outlook of survival for many advanced stage cancers however, cognitive decline is often noted as a side effect of chemotherapy, especially in late stage cancer patients. “Chemobrain” or “chemofog” is considered to be a result of brain cells being negatively affected by chemotherapy drugs. Patients tend to have slower processing speeds, problems with memory and difficulties paying attention. Prior research has shown that neurons in the hippocampus region of the brain (which controls memory formation and storage) are less complex in structure than their healthy counterparts. There is also indication of residual inflammation. Such damage is correlated with cognitive deterioration.
The central purpose of this paper was to reverse the negative effects of a chemotherapy drug, cyclophosphamide (CYP), by transplanting human neural stem cells (hNSC) into the brain after CYP treatment. It was hypothesized that stem cell transplantation would decrease the inflammation caused by drugs like CYP, as well as preserve the shape and integrity of neuronal cells. Findings indicated that this was indeed the case; cognitive decline caused by CYP treatment was significantly improved upon when human neural stem cells were transplanted.
The method of this study began with three sets of rats: one group which was the control and received no CYP or hNSCs, one group which underwent CYP treatment, and one group that underwent CYP treatment, followed by hNSC transplantation. All the groups underwent behavior testing after the mentioned procedures, which consisted of a series of four tasks to examine cognitive function. At the end of the experiment, the rats were euthanized and the brains were analyzed through immunohistochemistry and neuron morphology analyses.
The results of the behavior experiments showed a marked improvement in cognition in the rats that received hNSCs after receiving CYP injections. For each of the tasks, cognitive performance of the CYP group as compared to the control group was low. More importantly, the cognitive performance of the CYP + hNSC group was significantly better than the performance of the CYP group. Data showed that the transplanted cells had mostly retained their “stemness”, but many had differentiated into various types of cells present in the brain. In addition, the CYP + hNSC had a healthier morphology than that of the CYP group. Further data analysis showed that while the CYP group had far greater indicators of inflammation than the control, these indicators were significantly reduced in the CYP + hNSC group. However, it is of note that the indicators of inflammation used by the researchers may not have sufficiently illustrated how the host immune system had reacted to the foreign transplant. This may be the subject of further study. In the future, it may also be worthwhile to determine exactly which mechanism may have led to improved cognition after hNSC transplantation: the reduction of inflammation or the improvements in neuronal structure?
In the long run, this research could lead to the development of post cancer treatment regimens to reduce the effects of chemobrain, but more work is required to create effective clinical therapies. Based on the positive results of this paper however, it is clear that there is much hope for patients in restoring cognition after cancer therapy.
SCIENTIFIC SUMMARY
Cancer therapy has lasting negative effects on the cognition of survivors. Research has shown that chemotherapy affects memory, processing speed, attention and executive function. It is hypothesized that cancer treatment has an effect on the neural progenitors in the brain, especially in the hippocampus region. Dendrite complexity of newly made and developed neurons in the hippocampus is reduced, as is spine density and synaptic protein levels. The sum of these results leads to what is popularly called “chemo brain”—a cognitive effect that affects more than 75% of cancer survivors. There is no current treatment for this cognitive decline. In light of this, the researchers of this paper sought to find a method to reverse the neuronal effects of chemotherapy. Human neural stem cells (hNSC) were grafted into the brain of rats to determine whether cognitive impairments resulting from chemotherapy could be improved. The study found that this was indeed the case.
Athymic nude rats were divided into three groups of eight: the control group (CON) that received a sham surgery (saline injections), a group that received injections of the chemotherapeutic drug, cyclophosphamide (CYP) and a group that received the CYP injections and then a graft of hNSCs after the last CYP injection (CYP + hNSC). Behavior testing was used to analyze the effects of the graft on cognition. Four tasks were used: the novel place recognition task, the temporal order task, the object in place task and the fear-conditioning task. Outcomes were measured in the form of an exploration ratio for the first three tasks (tnovel/(tnovel + tfamiliar) and percent time spent freezing in the fear task. Finally, the animals were euthanized and brain tissue was analyzed with respect to graft survival, differentiation, neuroinflammation and structural analysis.
The results of the novel place recognition task showed that the baseline time of exploration did not statistically vary among the groups. In the test phase, the CYP mice spent a lower proportion of time at the novel place as compared to the other two groups. The control and CYP + hNSC groups did not statistically differ. In the temporal order task, CYP rats spent a lower proportion of time exploring the first object than the controls and the CON and CYP+hNSC groups did not vary statistically. For the object in place test, the CON and CYP+hNSC did show a preference to explore the new objects, however there was not a statistically significant difference. These trends in data showed that the outcomes for the CYP + hNSC group were comparable to the CON group, and were significantly better than the CYP group. In the fear-conditioning task, the groups did not vary in baseline measure, pre-cue or post-cue measures. In the context phase, the CYP animals spent less time freezing while the CON and CYP+hNSC groups had similar freezing times. The tone shock data in the post-training phase showed that every group had significant increases in freezing. Fear conditioning data therefore indicates that hippocampus function had deteriorated, but that sensory function was intact. The addition of hNSCs seemed to improve contextual memory in the fear task as well.
The confocal microscopy data showed that the grafted cells were present near the grafting site. Many had differentiated into neurons, astrocytes and oligodendrocytes (as measured by their markers, DCX/Neun, GFAP/S100B+, NG2/Olig2, respectively), however the vast majority expressed Sox2, the marker of the grafted human neural stem cells. Neuroinflammation was measured by the amount of activated microglia in three different regions. It was found that CYP had increased the amount of activated microglia in the dentate hilus, and the CA1/3 subfield. The number of activated microglia significantly decreased in the CYP+hNSC group in all three regions. It is under question why the researchers only measured activated microglia as a source of inflammation. It was not explained whether there was an immune response from the rats against the grafted cells, and therefore this idea could be explored more.
Neuronal morphology analysis in the dentate gyrus indicated that total dendritic length, volume and complexity was reduced in the CYP group, but there were no significant difference in the parameters of the CON and CYP+hNSC groups. The same outcome was found in the pyramidal neuronal architecture of the CA1 region as well as in the dendritic architecture in the dentate gyrus and CA1 regions. After two months, 8% of the grafted cells remained, and it was concluded that the CYP treatment did not negatively affect the microenvironment and prevent the benefits of the grafting from occurring. Further studies could be done to determine whether it was a decrease in inflammation or an increase in neuronal complexity that caused the amelioration of cognitive decline.
It can be interpreted from this wealth of data that there is indeed a positive effect of using stem cell therapy for chemo-brain. It was established that the decreases in neuronal structure and complexity led to a decrease in cognitive function and that the grafting of human neuronal stem cells reversed this consequence. In light of this, it can be surmised that more research will illuminate whether this therapy could be implemented for cancer survivors in order to improve cognitive function. |
