Granulocyte-Colony Stimulating Factor (G-CSF) is a glycoprotein (protein with sugars attached to it) that signals to the bone marrow to produce granulated white blood cells (specifically neutrophils), and to release stem cells and progenitor cells into the peripheral circulation.
This function of G-CSF makes it a candidate treatment for patients who have recently experienced a heart attack, since the release of stem cells from the bone marrow could, in theory, bring more stem cells to the damaged heart to heal it. Additionally, G-CSF is known to induce the proliferation and enhance the survival of heart muscle cells.
In several experiments with laboratory animals showed that G-CSF treatments after a heart attack significantly reduced mortality (Moazzami K, Roohi A, and Moazzimi B. Cochrane Database Systematic Reviews 2013; 5: CD008844. However, in a clinical trial known as the REVIVAL-2 trial, a double-blind, placebo-controlled study, G-CSG treatment failed to influence the performance of the heart six months after administration.
Now Birgit Steppich and others have published a seven-year follow-up of the subjects in the original REVIVAL-2 study to determine if G-CSF had long-term benefits that were not revealed in the short-term study. These results were published in the journal Thrombosis and Haemostasis (115.4/2016).
Of the initially enrolled 114 patients, 106 patients completed the seven-year follow-up. The results of this trial showed that G-CSF treatment for five days in successfully revascularized heart attack patients did not alter the incidence of death, recurrent heart attacks, stroke, or secondary adverse heart events during the seven-year follow-up.
These results are similar to those of the STEMMI trial, which treated patients with G-CSF for six days 10-65 hours after the reperfusion. In a five-year follow-up of 74 patients, there were no differences in the occurrence of major cardiovascular events between the G-CSF-treated group and the placebo group (Achili F, et al., Heart 2014; 100: 574-581).
Therefore, it appears that even though G-CSF worked in laboratory rodents that had suffered heart attacks, this treatment does not consistently benefit human heart attack patients. Although why it does not work will almost certainly require more insights than we presently possess.
The migration of several different types of stem cells is regulated by a receptor called “CXCR4” and the molecule that binds to this receptor, SDF-1. SDF-1 is a powerful summoner of white blood cells. During early development, SDF-1 mediates the migration of hematopoietic cells from fetal liver to bone marrow and plays a role in the formation of large blood vessels. During adult hood, SDF-1 plays an important role in making new blood vessels by recruiting endothelial progenitor cells (EPCs) from the bone marrow. Consequently, SDF-1 has a role in tumor metastasis where cancer cells that express the receptor CXCR4 are attracted to metastasis target tissues that release SDF-1. SDF-1 also attracts mesenchymal stem cells and helps them suppress the breakdown of bone.
Hopefully, I have convinced you that SDF-1 and its receptor CXC4 are important molecules. Can overexpression the CXCR4 receptor improve the retention of stem cells within an injured tissue?
Xiao-Tao Wu and Feng Wang from Zhongda Hospital in Nanjing, China and their colleagues have used this CXCR4 receptor/SDF-1 system to test this question in the damaged spinal cord. This work was published in the journal DNA and Cell Biology (doi:10.1089/dna.2015.3118).
Isolated MSCs were treated with genetically engineered viruses to so that would overexpress the CXCR4 receptor. In order to track these cells under medical imaging scans, the MSCs were also labeled with superparamagnetic iron oxide (SPIO). Next, rabbits that had suffered injuries to their intervertebral discs that lie between the vertebrae were given infusions of these labeled, genetically engineered MSCs. Images of the spine were taken at 0, 8, and 16 weeks after the surgery. The degeneration of the damaged intervertebral discs were also evaluated by disc height (damaged, degenerating intervertebral discs tend to shrink and lose height).
The SPIO-labeled CXCR4-MSC could be detected within the intervertebral discs by MRI 16 weeks post-transplantation. The MSCs that had been engineered to overexpress CXCR4 showed better retention within the discs, relative to implanted MSCs that had not been engineered to overexpress CXCR4.
Did the implanted MSCs affect the integrity of the intervertebral discs? Indeed they did. Compared to the control group, loss of disc height was slowed in the animals that received the CXCR4-overexpressing MSCs. Also, the genetically engineered MSCs seemed to make more cartilage-specific materials, like the giant molecule aggrecan and type II collagen. There is a caveat here, since there is no indication that measured protein directly; only mRNAs. Until the quantities of these molecules can be directly shown to increase in the disc, the increases in these cartilage-building molecules can be said to be presumptive, but not proven.
From these experiments, it seems reasonable to conclude that CXCR4 overexpression promoted MSC retention within the damaged intervertebral discs and the increased stem cell retention enhanced stem cell-based disc regeneration. Therefore this SDF-1/CXCR4 signaling pathway might be a way to drive stem cell migration and infiltration within degenerated intervertebral discs.
A growth factor called Glial cell line-derived neurotrophic factor (GDNF) has the remarkable ability to supports the growth and survival of dopamine-using neurons. Dopamine-using neurons are the cells that die off in Parkinson’s disease (PD). Providing GDNF to dopamine-using neurons can help them survive , but getting GDNF genes into the central nervous system relies on invasive intracerebral injections in order to pass through the blood-brain barrier.
Typically, genes are placed into the central nervous system by means of genetically engineered viruses. Viruses, however, are often recognized by the immune system and are destroyed before they can deliver their genetic payload. Therefore, non-viral gene delivery that can pass through the blood-brain barrier is an attractive alternative, since it is non-invasive. Unfortunately, such a high-yield technique is not yet available.
A new study by workers in the laboratories of Hao-Li Liu from Chang Gung University and Chih-Kuang Yeh from National Tsing Hua University, Taiwan has utilized a novel, non-viral gene delivery system to deliver genes into the central nervous system.
In this study, Lui and Yeh and their research teams used tiny bubbles made from positively-charged molecules to carry genes across the blood brain barrier. These bubbles formed stable complexes with GDNF genes, and when the skulls of laboratory animals were exposed to focused ultrasound, the bubble-gene complexes permeated the blood brain barrier and induced local GDNF expression.
In fact, this technique outperformed intracerebral injection in terms of targeted GDNF delivery. The amount of GDNF expressed in these laboratory animals that received the GDNF gene/microbubbles + ultrasound protocol was significantly higher than those animals that had genes directly injected into their brains. Furthermore, these higher levels of GDNF genes increased the levels of neuroprotection from PD. Animals that had a form of PD and had received nonviral GDNF gene therapy showed reduced disease progression and restored behavioral function.
This interesting study explores the potential of using ultrasound-induced passage through the blood brain barrier to bring genes into the central nervous system. This noninvasive technique successfully delivered genes into the brain to delay the effects of, and possibly treat, a neurodegenerative disease.
This study was published in Scientific Reports6, Article number: 19579 (2016), doi:10.1038/srep19579.
Retinal degenerations are the leading cause of blindness and fixing a defective retina is not an easy task.
Fortunately, a model system in nonhuman primates that has been used to test retinal replacement with stem cell-derived retinal cells has seen some success. In several experiment in small animals, retinal transplantations helped blind animals regain their sight. However, small laboratory rodents are not terribly good model systems for human eye problems.
To address the clinical relevancy of this transplantation system, Shirai and colleagues confirmed in rats and in macaques that transplantion of human embryonic stem cell (hESC)–derived retinas integrate into the already-existing retina and develop as fully mature retinal grafts.
In this paper, Shirai and others established the developmental stage at which embryonic stem cell-derived retinal cells could integrate into the retina and replace damaged cells. By transplanting cells into nude rats that do not have the ability to reject transplanted tissue, they refined their cell-based technique to heal damaged retinas. Then they took their refined technique into macques to treat two newly established monkey models of retinal degeneration.
In the first model system, Shirai et al. exposed one group monkeys to retina-damaging chemicals, and the other group had their retinas damaged by lasers. In both cases, the result was photoreceptor degeneration. Anywhere from 46 to 109 days after injury, the human embryonic stem cell-derived retinal sheets were implanted into the damaged retinas.
The retinal grafts integrated into the primate eyes and continued to differentiate into cone and rod cells, which are the two types of photoreceptor cells in the retina. Functional studies are still being conducted, but if vision can be improved, but these new macaque models confirm the clinical potential of stem cell–derived grafts for retinal blindness that results from photoreceptor degeneration.
See H. Shirai et al., Transplantation of human embryonic stem cell-derived retinal tissue in two primate models of retinal degeneration. Proc. Natl. Acad. Sci. U.S.A.113, E81–E90 (2015).
Jan Nolta and her colleagues at the Stem Cell Program and Institute for Regenerative Cures at UC Davis have published a remarkable paper in the journal Molecular Therapy regarding Huntington’s disease and a potential stem cell-based strategy to delay the ravages of this disease.
Huntington’s disease (HD) is an inherited neurodegenerative disease. It is inherited as an autosomal dominant disease, which means that someone need only inherit one copy of the disease-causing allele of the HTT gene to have this disease. HD is characterized by progressive cell death in the brain, particularly in a portion of the brain known as the striatum and by widespread brain atrophy.
The portion of the brain known as the striatum lies underneath the surface of the forebrain (subcortical) and it receives neural inputs from the cerebral cortex and is the primary source of neural inputs to the basal ganglia system. The basal ganglia system (BGS) is located underneath the surface of the brain but even deeper within the cerebral hemispheres. The BGS is part of the corpus striatum, it consists of the subthalamic nucleus and the substantia nigra. The BGS help with voluntary motor control, procedural learning relating to routine behaviors. otherwise known as “habits,” eye movements, and cognitive, and emotional functions. The ventral striatum is very important in addiction because it is the reward center on consists of the nucleus accumbens, olfactory tubercle, and islands of Calleja.
HD takes its largest toll on the striatum, which affects voluntary movement, routine behaviors, and personality. Disturbances of both involuntary and voluntary movements occur in individuals with HD. Chorea, an involuntary movement disorder consisting of nonrepetitive, non-periodic jerking of limbs, face, or trunk, is the major sign of the disease. Chorea is present in more than 90% of individuals, increasing during the first ten years. The choreic movements are continuously present during waking hours, cannot be suppressed voluntarily, and are worsened by stress. HD patients show impaired voluntary motor function early on and show a clumsiness in common daily activities.
With advancing disease duration, other involuntary movements such as slowness of movement (bradykinesia), rigidity, and involuntary muscle contractions that cause repetitive or twisting movements (dystonia) occur. Eye movement becomes progressively worse. So-called “gaze fixation” is observed in ~75% of symptomatic individuals. Unclear speech occurs early and Swallowing difficulties occur later.
Animal models of HD used in the past have injected molecules into the brain that kill off striatal cells and mimic at least some of the characteristics of HD in laboratory animals. Unfortunately, such a model system is fat too clean, since implanted cells tend to survive perfectly well. However the brains of HD patients are like unto a toxic waste dumps and implanted cells are quickly killed off. Therefore, a better animal model system was required, and it came in the form of R6/2 and YAC128 mice. R6/2 mice have a part of the human HTT gene that has 150 CAG triplets, and show the characteristic cell death in the striatum and behavioral deficits. The only problem with this mouse strain is that the neurodegenerative decline is very rapid rather than slow and progressive. YAC128 mice have a full-length copy of the HTT gene and show a slower, more progressive neurological decline that more closely approximates the human clinical condition.
In this paper from the Nolta laboratory, they used a growth factor that is known to decrease precipitously in HD brains; a growth factor called Brain-Derived Neurotrophic Factor (BDNF). BDNF is known to mediate the survival and function of striatal neurons and the reduction of BDNF in the brains of HD patients correlates with the onset of symptoms and the greater the reduction in BDNF, the greater the severity of the disease (see Her LS & Goldstein LS, J Neurosci 2008; 28, 13662-13676).
However injecting BDNF into the brain is problematic, since the protein has a very short half-life. Delivering the growth factor by means of genetically engineered viruses shows promise, but most of the viral vectors used in such experiments are recognized by the immune system as foreign invaders. Therefore, Nolta and her colleagues decided to genetically engineer mesenchymal stem cells (MSCs) to overexpress BDNF and implant these cells into the brains of R6/2 and YAC128 mice.
MSCs have an added advantage over viral vectors: these cells migrate to damaged areas where they can exert their healing properties (see Olson SD et al., Mol Neurobiol 45; 2012: 87-98).
Nolta and her coworkers actually tested human MSCs in HD model mice. After completing all the necessary control experiments to ensure that their isolated and engineered MSCs were secreting BDNF, Nolta and others implanted them into the brains of R6/2 and YAC128 mice.
HD mice show greater anxiety, which is manifested in a so-called “open field assay” by not remaining the center of the field. The control HD mice did not stay long in the center of the open field, but the normal mice did. The MSC-BDNF-implanted mice spend far more time in the center of the field. Mind you, not as much as wild-type mice, but significantly more than their HD counterparts.
Next the volume of the striata of these mice were determined and compared to the normal mice. While all the HD mice showed shrinking of the striatum, the MSC-NDNF-implanted YAC128 mice show significantly less shrinking of the striatum. Then the degree of neurogenesis (formation of new neurons) was measured in normal, HD, HD + implanted MSCs, and HS + MSC-BDNF mice. This experiment measures the degree of healing that is occurring in the brain. The brain from HD + MSC and HD + MSC-BDNF mice showed significantly more new brain cell growth. This is probably the reason for the delayed onset of symptoms and the delayed shrinking of the striatum.
Finally, Nolta and others measured the lifespans of the R6/2 mice and compared them with R6/2 mice that had been implanted with MSCs-BDNF. Animals transplanted with the MSCs that made the most BDNF lived 15% longer than the nontreated R6/2 mice.
MSCs have been shown in several experiments to promote neuronal growth, decrease cell death and decrease inflammation through the secretion of trophic factors. MSCs can modify the toxic environment that is part of the brain of an HD patient and help damaged tissue out by inducing neural regeneration and protection (see Crigler L, et al., Experimental neurology, 198; 2–6, 54-64; Kassis I, et al., Archives of Neurology 65; 2008: 753-761).
The downside of using MSCs that they will only survive in the brain for a few months. However, several studies have shown that the benefits of modified MSC implantation persist after the MSCs are gone, since the neural reconstruction wrought by the secreted BDNF stay after the MSCs have died off (see Arregui L, et al., Cell Mol Neurobiol 31; 2011: 1229-1243 and many others).
At best this treatment would delay the ravages of HD, but delaying this disease might very well be the first step towards a cure. Hopefully, clinical trials will not be fat behind.
New papers in Science magazine and the journal Cell have addressed a long-standing question of how the descendants of hematopoietic stem cells in bone marrow make the various types of blood cells that course through our blood vessels and occupy our lymph nodes and lymphatic vessels.
Hematopoietic stem cells (HSCs) are partly dormant cells that self-renew and produce so-called “multipotent progenitors” or MPPs that have reduced ability to self-renew, but can differentiate into different blood cell lineages.
The classical model of how they do this goes like this: the MPPs lose their multipotency in a step-wise fashion, producing first, common myeloid progenitors (CMPs) that can form all the red and white blood cells except lymphocytes, or common lymphoid progenitors (CLPs) that can form lymphocytes (see the figure below as a reference). Once these MPPs form CMPs, for example, the CMP then forms either an MEP that can form either platelets or red blood cells, or a GMP. which can form either granulocytes or macrophages. The possibilities of the types of cells the CMP can form in whittled down in a step-by-step manner, until there is only one choice left. With each differentiation step, the cell loses its capacity to divide, until it becomes terminally differentiated and becomes platelet-forming megakarocyte, red blood cell, neutrophil, macrophage, dendritic cells, and so on.
These papers challenge this model by arguing that the CMP does not exist. Let me say that again – the CMP, a cell that has been identified several times in mouse and human bone marrow isolates, does not exist. When CMPs were identified from mouse and human none marrow extracts, they were isolated by means of flow cytometry, which is a very powerful technique, but relies on the assumption that the cell type you want to isolate is represented by the cell surface protein you have chosen to use for its isolation. Once the presumptive CMP was isolated, it could recapitulate the myeloid lineage when implanted into the bone marrow of laboratory animals and it could also produce all the myeloid cells in cell culture. Sounds convincing doesn’t it?
In a paper in Science magazine, Faiyaz Notta and colleagues from the University of Toronto beg to differ. By using a battery of antibodies to particular cell surface molecules, Notta and others identified 11 different cell types from umbilical cord blood, bone marrow, and human fetal liver that isolates that would have traditionally been called the CMP. It turns out that the original CMP isolate was a highly heterogeneous mixture of different cell types that were all descended from the HSC, but had different developmental potencies.
Notta and others used single-cell culture assays to determine what kinds of cells these different cell types would make. Almost 3000 single-cell cultures later, it was clear that the majority of the cultured cells were unipotent (could differentiate into only one cell type) rather than multipotent. In fact, the cell that makes platelets, the megakarocyte, seems to derive directly from the MPP, which jives with the identification of megakarocyte progenitors within the HSC compartment of bone marrow that make platelets “speedy quick” in response to stress (see R. Yamamoto et al., Cell 154, 1112 (2013); S. Haas, Cell Stem Cell 17, 422 (2015)).
Another paper in the journal Cellby Paul and others from the Weizmann Institute of Science, Rehovot, Israel examined over 2700 mouse CMPs and subjected these cells to gene expression analyses (so-called single-cell transriptome analysis). If the CMP is truly multipotent, then you would expect it to express genes associated with lots of different lineages, but that is not what Paul and others found. Instead, their examination of 3461 genes revealed 19 different progenitor subpopulations, and each of these was primed toward one of the seven myeloid cell fates. Once again, the presumptive CMPs looked very unipotent at the level of gene expression.
One particular subpopulation of cells had all the trappings of becoming a red blood cell and there was no indication that these cells expressed any of the megakarocyte-specific genes you would expect to find if MEPS truly existed. Once again, it looks as though unipotency is the main rule once the MPP commits to a particular cell lineage.
Thus, it looks as though either the CMP is a very short-lived state or that it does not exist in mouse and human bone marrow. Paul and others did show that cells that could differentiate into more than one cell type can appear when regulation is perturbed, which suggests that under pathological conditions, this system has a degree of plasticity that allows the body to compensate for losses of particular cell lineages.
Fetal HSCs, however, are a bird of a different feather, since they divide quickly and reside in fetal liver. Also, these HSCs seem to produce CMPs, which is more in line with the classical model. Does the environmental difference or fetal liver and bone marrow make the difference? In adult bone marrow, some HSCs nestle next to blood vessels where they encounter cells that hang around blood vessels known as “pericytes.” These pericytes sport a host of cell surface molecules that affect the proliferative status of HSCs (e.g., nestin, NG2). What about fetal liver? That’s not so clear – until now.
In the same issue of Science magazine, Khan and others from the Albert Einstein College of Medicine in the Bronx, New York, report that fetal liver also has pericytes that express the same cell surface molecules as the ones in bone marrow, and the removal of these cells reduces the numbers of and proliferative status of fetal liver HSCs.
Now we have a conundrum, because the same cells in bone marrow do not drive HSC proliferation, but instead drive HSC quiescence. What gives? Khan and others showed that the fetal liver pericytes are part of an expanding and constantly remodeling blood system in the liver and this growing, dynamic environment fosters a proliferative behavior in the fetal HSCs.
When umbilical inlet is closed at birth, the liver pericytes stop expressing Nestin and NG2, which drives the HSCs from the fetal liver to the other place were such molecules are found in abundance – the bone marrow.
These models give us a better view of the inner workings of HSC differentiation. Since HSC transplantation is one of the mainstays of leukemia and lymphoma treatment, understanding HSC biology more perfectly will certainly yield clinical pay dirt in the future.
Mesoblast Limited announced that the number of subjects treated in their ongoing Phase 3 clinical trial in chronic heart failure (CHF) that is testing their proprietary cell-based medicine MPC-150-IM will be substantially reduced.
CHF is characterized by an enlarged heart, coupled with insufficient blood supply to the organs and extremities of the body. Unfortunately, this is a progressing condition that tends to get worse with time. CHF is caused by many different factors such as chronic high blood pressure, faulty heart valves, infections, or congenital heart problems.
Mesoblast centers their company around the isolation and expansion of so-called mesenchymal precursor cells (MPCs) from bone marrow. Mesenchymal stem cells are found in many different tissues and organs throughout our bodies. They play vital roles in maintaining tissue health. However, relatively speaking, mesenchymal stem cells are rare cells. They are found around blood vessels and respond to signals associated with tissue damage. They secrete mediators and growth factors that promote tissue repair and control the immune response to prevent it from going out of control.
Mesoblast uses an array of monoclonal antibodies to isolate primitive mesenchymal stem cells that are actually precursors to mesenchymal stem cells or mesenchymal precursor cells (MPCs). These cells are then expanded in culture without being differentiated into any other cell type.
Initially, Mesoblast planned to test their product on 1,165 subjects, but have scaled that number back to approximately 600 patients.
Mesoblast’s development and commercial partner, Teva Pharmacueticals has communicated this reduction in the number of subjects to the US Food and Drug Administration (USFDA). “The reduction in the size of the Phase 3 trial may significantly shorten the time to trial completion,” said Mesoblast CEO Silviu Itescu.
The reduction in the number of patients was due to a proposed change in the primary endpoint of the trial. The revised primary endpoint is now a comparison of recurrent heart failure-related major adverse cardiovascular events (HF-MACE) between patients treated with Mesoblast’s MPC-150-IM cells and the control patients who were not treated with these cells.
Why the change in the primary endpoint? The reason lies in the success that MPC-150-IM cells had their Phase 2 clinical trial. In this trial, a single injection of MPC-150-IM cells successfully prevented HF-MACE over three years. This second, confirmatory study will be conducted in parallel with a patient population that has an identical clinical profile; approximately 600 of them using the same primary endpoint.
In the completed Phase 2 trial, patients treated with MPC-150-IM had no HF-MACE over 36 months of follow-up, compared with 11 HF-MACE in the control group. From this same clinical trial, of those patients who suffered from advanced heart failure (defined by baseline Left Ventricular Systolic Volume being greater than 100 milliliters), 71 percent of the controls (who received no cells) had at least on HF-MACE versus none of those who received a single injection of MPC-150-IM cells. As it turns out, this Phase 2 patient population closely resemble the patients being recruited in the Phase 3 trial.
“Patients with advanced heart failure continue to represent among the largest unmet medical needs, where existing therapies are inadequate and the economic burden is the greatest. The current Phase 3 trial targets this patient population, continues to recruit well across North America, and is now expanding to Europe,” said Itescu.