Mehran Salehpour

Dynamics of Hippocampal Neurogenesis in Adult Humans

Adult-born hippocampal neurons are important for cognitive plasticity in rodents. There is evidence for hippocampal neurogenesis in adult humans, although whether its extent is sufficient to have functional significance has been questioned. We have assessed the generation of hippocampal cells in humans by measuring the concentration of nuclear-bomb-test-derived ¹⁴C in genomic DNA, and we present an integrated model of the cell turnover dynamics. We found that a large subpopulation of hippocampal neurons constituting one-third of the neurons is subject to exchange. In adult humans, 700 new neurons are added in each hippocampus per day, corresponding to an annual turnover of 1.75% of the neurons within the renewing fraction, with a modest decline during aging. We conclude that neurons are generated throughout adulthood and that the rates are comparable in middle-aged humans and mice, suggesting that adult hippocampal neurogenesis may contribute to human brain function.

Neurogenesis in the Striatum of the Adult Human Brain

In most mammals, neurons are added throughout life in the hippocampus and olfactory bulb. One area where neuroblasts that give rise to adult-born neurons are generated is the lateral ventricle wall of the brain. We show, using histological and carbon-14 dating approaches, that in adult humans new neurons integrate in the striatum, which is adjacent to this neurogenic niche. The neuronal turnover in the striatum appears restricted to interneurons, and postnatally generated striatal neurons are preferentially depleted in patients with Huntingtons disease. Our findings demonstrate a unique pattern of neurogenesis in the adult human brain.

Dynamics of Cell Generation and Turnover in the Human Heart

The contribution of cell generation to physiological heart growth and maintenance in humans has been difficult to establish and has remained controversial. We report that the full complement of cardiomyocytes is established perinataly and remains stable over the human lifespan, whereas the numbers of both endothelial and mesenchymal cells increase substantially from birth to early adulthood. Analysis of the integration of nuclear bomb test-derived (14)C revealed a high turnover rate of endothelial cells throughout life (>15% per year) and more limited renewal of mesenchymal cells (<4% per year in adulthood). Cardiomyocyte exchange is highest in early childhood and decreases gradually throughout life to <1% per year in adulthood, with similar turnover rates in the major subdivisions of the myocardium. We provide an integrated model of cell generation and turnover in the human heart.

Dynamics of human adipose lipid turnover in health and metabolic disease

Adipose tissue mass is determined by the storage and removal of triglycerides in adipocytes. Little is known, however, about adipose lipid turnover in humans in health and pathology. To study this in vivo, here we determined lipid age by measuring 14C derived from above ground nuclear bomb tests in adipocyte lipids. We report that during the average ten-year lifespan of human adipocytes, triglycerides are renewed six times. Lipid age is independent of adipocyte size, is very stable across a wide range of adult ages and does not differ between genders. Adipocyte lipid turnover, however, is strongly related to conditions with disturbed lipid metabolism. In obesity, triglyceride removal rate (lipolysis followed by oxidation) is decreased and the amount of triglycerides stored each year is increased. In contrast, both lipid removal and storage rates are decreased in non-obese patients diagnosed with the most common hereditary form of dyslipidaemia, familial combined hyperlipidaemia. Lipid removal rate is positively correlated with the capacity of adipocytes to break down triglycerides, as assessed through lipolysis, and is inversely related to insulin resistance. Our data support a mechanism in which adipocyte lipid storage and removal have different roles in health and pathology. High storage but low triglyceride removal promotes fat tissue accumulation and obesity. Reduction of both triglyceride storage and removal decreases lipid shunting through adipose tissue and thus promotes dyslipidaemia. We identify adipocyte lipid turnover as a novel target for prevention and treatment of metabolic disease.

Dynamics of Oligodendrocyte Generation and Myelination in the Human Brain

The myelination of axons by oligodendrocytes has been suggested to be modulated by experience, which could mediate neural plasticity by optimizing the performance of the circuitry. We have assessed the dynamics of oligodendrocyte generation and myelination in the human brain. The number of oligodendrocytes in the corpus callosum is established in childhood and remains stable after that. Analysis of the integration of nuclear bomb test-derived (14)C revealed that myelin is exchanged at a high rate, whereas the oligodendrocyte population in white matter is remarkably stable in humans, with an annual exchange of 1/300 oligodendrocytes. We conclude that oligodendrocyte turnover contributes minimally to myelin modulation in human white matter and that this instead may be carried out by mature oligodendrocytes, which may facilitate rapid neural plasticity.

The age and genomic integrity of neurons after cortical stroke in humans

It has been unclear whether ischemic stroke induces neurogenesis or neuronal DNA rearrangements in the human neocortex. Using immunohistochemistry; transcriptome, genome and ploidy analyses; and determination of nuclear bomb test–derived 14C concentration in neuronal DNA, we found neither to be the case. A large proportion of cortical neurons displayed DNA fragmentation and DNA repair a short time after stroke, whereas neurons at chronic stages after stroke showed DNA integrity, demonstrating the relevance of an intact genome for survival.

Subattomole sensitivity in biological accelerator mass spectrometry

The Uppsala University 5 MV Pelletron tandem accelerator has been used to study (14)C-labeled biological samples utilizing accelerator mass spectrometry (AMS) technology. We have adapted a sample preparation method for small biological samples down to a few tens of micrograms of carbon, involving among others, miniaturizing of the graphitization reactor. Standard AMS requires about 1 mg of carbon with a limit of quantitation of about 10 amol. Results are presented for a range of small sample sizes with concentrations down to below 1 pM of a pharmaceutical substance in human blood. It is shown that (14)C-labeled molecular markers can be routinely measured from the femtomole range down to a few hundred zeptomole (10 (-21) mol), without the use of any additional separation methods.

Total molecular yields for fast heavy ion induced desorption of biomolecules

Abstract The total desorption yield for intact molecules of the amino acid leucine (131 amu) induced by 90MeV 127I14+ ions has been studied. A collector method was used where the collector samples were analysed using both an amino acid analyzer (via optical absorption) for absolute measurements and Plasma Desorption Mass Spectroscopy for relative measurements of the total number of unfragmented molecules. Results indicate high yields of intact molecules of the order of one thousand which corresponds to a removal of about 1 × 105 A3 of sample material per incident ion. It has been shown that fast heavy ion induced desorption of leucine is essentially a “neutral ejection” process with the generally studied ionized portion constituting a very small fraction (≈1:104).

Molecular size effects in fast heavy ion induced desorption of biomolecules

Abstract Fast ion induced desorption yields of positive secondary ions of different biomolecules have been studied with a time-of-flight technique. The yields were studied as a function of energy deposition of the primary ion. The samples used were an amino acid, two protected oligonucleotides and a small protein (bovine insulin). Different primary ions with constant velocity were used throughout the experiment. In this way the energy density or electronic stopping power was changed by using different primary ions. It was found that for low energy densities the yields increase with different powers of the energy density for the different systems. For high energy densities the yields are proportional to the energy density for all systems studied. The results indicate that for large molecules multiple bond breaking is needed for desorption to occur. For the amino acid sample (valine) an extensive clustering effect was also found. Cluster ions containing as many as 20 valine molecules were observed. This wor...

Damage cross sections for fast heavy ion induced desorption of biomolecules

Abstract The Uppsala EN-tandem accelerator combined with a time-of-flight mass spectrometer has been used to measure the damage cross sections for Fast Heavy Ion Induced Desorption (FHIID) of the amino acid valine (MW = 117) and the protein bovine insulin (MW = 5733). Time-of-flight spectra have been obtained after exposing the sample to a known radiation dose of 90 MeV 127 I 14+ ions and the yield of the quasi-molecular ions has been measured as a function of the radiation dose. The results are: 6.8(± 1.8) × 10 −13 cm 2 and 50(± 17) × 10 −13 cm 2 for positive ions of valine and insulin respectively. The cross section for valine is roughly one order of magnitude larger than previously published low energy (keV) damage cross sections for the amino acid leucine.