New Neuron Formation Required for Maintenance of Olfactory Nerves in Mice

For many years, scientists and neurologists were convinced that neurons in the brain only formed during early development, and after that it was simply impossible for new neurons to be formed.  More recent work, however, has shown this to be largely untrue, since several regions of the brain possess resident stem cell populations that can divide to replenished damaged neurons and even augment learning and memory.  The capacity of neural stem cell populations to regenerate the central nervous system is a continuing field of intense research, and scientists at the National Institutes of Health (NIH) have reported one region of the central nervous system that can form new brain cells; the mouse olfactory system, which processes smells.  This work appeared in the October 8 issue of the Journal of Neuroscience.

“This is a surprising new role for brain stem cells and changes the way we view them,” said Leonardo Belluscio, Ph.D., a scientist at NIH’s National Institute of Neurological Disorders and Stroke (NINDS) and lead author of the study.

The olfactory bulb is at the front of the brain (shown as “A” in he picture below), and is rather small in humans, but somewhat larger in other animals.  This structure receives information directly from the nose about volatile odors.  Neurons in the olfactory bulb sort through this smelly information and relay neural signals to the rest of the brain.  This is the point at which we become aware of the smells in our surroundings.  The loss of the sense of smell is sometimes an early symptom in a variety of neurological disorders, including Alzheimer’s and Parkinson’s diseases.

Olfactory lobes in brain


Neurogenesis is the process by which neuroprogenitor cells are produced in the subventricular zone deep in the brain.  After birth, these cells migrate to the olfactory bulb, which becomes the final location of these cells.  Once they arrive at the olfactory bulb. the neuroprogenitor cells divide, differentiate, and form connections with existing cells to become integrated into the neural circuitry in the olfactory bulb and elsewhere.

Dr. Belluscio studies the olfactory system, and for this study, he collaborated with Heather Cameron, Ph.D., a neurogenesis researcher at the NIH’s National Institute of Mental Health.  The goal of this study was to better understand how the continuous addition of new neurons affects the neural organization of the olfactory bulb.  They used two different types of genetically engineered laboratory mice that had specifically genes knocked out.  Consequently, these mice lacked the specific stem cell populations that generate the new neurons during adulthood, without affecting the other olfactory bulb cells.  Previously, this remarkable level of specificity had not been achieved.

Belluscio and his coworkers had previously shown that plugging the nostrils of the animals so that they are not subject to olfactory stimulation causes the axonal extensions of the olfactory neurons to dramatically spread out and lose the precise network of connections with other cells that are normally observed under normal conditions.  They also showed that this widespread disrupted circuitry could re-organize itself and restore its original precision once the sensory deprivation was reversed.  Therefore, Belluscio and his team temporarily plugged a nostril in their lab animals to block olfactory sensory information from entering the brain.  However, if laboratory animals that do not produce new neuroprogenitors are subjected to this type of manipulation, once the nose is unblocked, new neurons are prevented from forming and entering the olfactory bulb, and, therefore, the neural circuits remain in disarray. “We found that without the introduction of the new neurons, the system could not recover from its disrupted state,” said Dr. Belluscio.

Further examination showed that elimination of the formation of adult-born neurons in mice that did not experience sensory deprivation also caused the organization of the olfactory bulb organization began to degenerate, eventually resembling the pattern observed in animals prevented from receiving sensory information from the nose.  Belluscio and his team also noticed that the extent of stem cell loss was directly proportional to the degree of disorganization in the olfactory bulb.

According to Belluscio, circuits of the adult brain are thought to be rather stable and that introducing new neurons alters the existing circuitry, causing it to re-organize. “However, in this case, the circuitry appears to be inherently unstable requiring a constant supply of new neurons not only to recover its organization following disruption but also to maintain or stabilize its mature structure. It’s actually quite amazing that despite the continuous replacement of cells within this olfactory bulb circuit, under normal circumstances its organization does not change,” he said.

Dr. Belluscio and his colleagues think that these new neurons in the olfactory bulb are important for the maintenance of activity-dependent changes in the brain, which help animals adapt to a constantly varying environment.

“It’s very exciting to find that new neurons affect the precise connections between neurons in the olfactory bulb. Because new neurons throughout the brain share many features, it seems likely that neurogenesis in other regions, such as the hippocampus, which is involved in memory, also produce similar changes in connectivity,” said Dr. Cameron.

The underlying basis of the connection between neurological disease and changes in the olfactory system is also unknown but may come from a better understanding of how the sense of smell works. “This is an exciting area of science,” said Dr. Belluscio, “I believe the olfactory system is very sensitive to changes in neural activity and given its connection to other brain regions, it could lend insight into the relationship between olfactory loss and many brain disorders.”

New Standard of Care for Umbilical Cord Blood Transplants

New research led by John Wagner, Jr., M.D., director of the Pediatric Blood and Marrow Transplantation program at the University of Minnesota and a researcher in the Masonic Cancer Center, University of Minnesota, has established a new standard of care for children who suffer from acute myeloid leukemia (AML).

In a recent paper in the New England Journal of Medicine, Wagner and his coworkers compared clinical outcomes in children who suffered from acute leukemia and myelodysplastic syndrome who received transplants of either one unit or two units of partially matched cord blood. This large study was conducted at multiple sites across the United States, between December 2006 and February 2012. Coordinating the study was the Blood and Marrow Transplant Clinical Trials Network (BMT CTN) in collaboration with the Pediatric Blood and Marrow Transplant Consortium and the Children’s Oncology Group.

Umbilical cord blood provides a wonderfully rich source of blood-forming stem cells, and has been demonstrated to benefit many diseases of the blood or bone marrow, as in the case of leukemia and myelodysplasia, or bone marrow failure syndromes, hemoglobinopathies, inherited immune deficiencies and certain metabolic diseases. For leukemia patients cord blood offers several advantages since there is no need for strict tissue-type matching (human leukocyte antigen or HLA matching) or for a prolonged search for a suitable donor.

Wagner and others discovered that the survival rates of children who received either one or two units of umbilical cord blood were about the same, but, overall, their recorded survival rates were better than those reported in prior published reports. These higher survival rates, therefore, represent a new standard of care for pediatric patients, for whom there is often an adequate single cord blood unit, and for adults for usually require double units, since a single unit with an adequate number of blood-forming stem cells simply may not exist at time.

“Based on promising early studies using two cord blood units in adults for whom one unit is often not sufficient, we designed this study in order to determine if the higher number of blood forming stem cells in two cord blood units might improve survival,” explained Wagner. “What we found, however, was that both treatment arms performed very well with similar rates of white blood cell recovery and survival.”

Children with blood cancers who receive transfusions of umbilical cord blood show quantifiable clinical benefits even though the blood may not match their own tissue types. The reason stems from the immaturity of cord blood stem cells and their ability to suppress rejection from the immune system. This is an important aspect of umbilical cord blood transplants, since patients who cannot find a matched unrelated donor also benefit from cord blood transplants. However, cord collection from the placenta after birth often results in a limited number of blood-forming stem cells, which decreases the potential benefits of cord blood. The “double UCB approach” was pioneered at the University of Minnesota as a strategy to overcome this inherent limitation in the use of umbilical cord blood.

Despite the similarities in survival rates between children who received one unit or two units of cord blood, some differences were noted. Children transplanted with a single cord unit had faster recovery rates for platelets and lower risks of Graft Versus Host Disease; a condition in which the transplanted donor blood immune cells attack the patient’s body, which causes several complications.

“This is helpful news for physicians considering the best treatment options for their patients,” said Joanne Kurtzberg, M.D., chief scientific officer of the Robertson Clinical and Translational Cell Therapy Program, director of the Pediatric Blood and Marrow Transplant Program, co-director of the Stem Cell Laboratory and director of the Carolinas Cord Blood Bank at Duke University Medical Center. “We found children who have a cord blood unit with an adequate number of cells do not benefit from receiving two units. This reduces the cost of a cord blood transplant for the majority of pediatric patients needing the procedure. However, for larger children without an adequately dosed single cord blood unit, using two units will provide access to a potentially life-saving transplant.”

“The involvement of multiple research partners was instrumental to the success of the study completion,” added Dennis Confer, M.D., chief medical officer for the National Marrow Donor Program® (NMDP)/Be The Match® and associate scientific director for CIBMTR. “This trial is a testament to the importance of the BMT CTN and the collaboration of partners like the Children’s Oncology Group.”

Mary Horowitz, M.D., M.S., chief scientific director of CIBMTR and professor of medicine at the Medical College of Wisconsin, concurred. “Because of this tremendous collaboration, we were able to expand the scale of this research to multiple transplant centers across the United States and Canada. And the results will undoubtedly improve clinical practice, and most importantly, patient outcomes.”

Interestingly, in this study, patients who received cord blood with significant HLA mismatches showed no detrimental effects on their outcomes. Future studies will examine a closer look at how the HLA match within the cord blood unit impacts outcomes for patients, particularly those within minority populations.