Session: Stem cells
NEUROGENESIS IMPAIRMENT AND CELL CYCLE ALTERATIONS IN DOWN
SYNDROME
Renata Bartesaghi1*, Andrea Contestabile1, Tatiana Fila1, Laura Bonapace1, Claudio
Ceccarelli2, Paola Bonasone2, Donatella Santini2, Elisabetta Ciani1
1Department
of Human and General Physiology, University of Bologna, Italy and 2St.
Orsola-Malpighi Hospital, Bologna, Italy. * Tel. +39 051 2091727; Fax: +39 051
251731; e-mail: renata.bartesaghi@unibo.it
Down syndrome (DS; trisomy 21) is the most frequent genetic cause of mental
retardation compatible with postnatal survival. Individuals affected by DS display several
neuropathological abnormalities, including reduction in brain volume and neuron number.
Cerebral atrophy is present in fetuses and children with DS, suggesting that the reduced
neuron number in adult DS subjects may be due to early developmental defects rather than
to late-occurring, neurodegenerative processes. This hypothesis, however, is largely
speculative, as no study has addressed so far the issue of neurogenesis in the DS brain. The
goal of the present investigation was to ascertain whether DS subjects exhibit neurogenesis
impairment during early phases of brain building and to identify possible mechanisms
underlying this defect. To this purpose we analyzed neurogenesis in the hippcampal
dentate gyrus and ventricular germinal matrix of 18-21 week-old DS human fetuses and in
the dentate gyrus and cerebellum of neonate (P2) Ts65Dn mice, an animal model of DS.
Results showed that, both in human fetuses with DS and in Ts65Dn mice, proliferation of
progenitor cells occurred at a significantly lower rate compared to controls in all
investigated structures. Following the fate of the newborn cells in the DS mice model, we
found that at P30 the number of surviving cells was smaller in Ts65Dn mice compared to
controls and that, though most cells had acquired neuronal phenotype, there was a slight
but significant shift towards acquisition of an astrocytic phenotype. In parallel with the
reduced neurogenesis, we observed a notable reduction in the stereological features of the
cerebellum and dentate gyrus. Analysis of phosphorylated-histone-H3 (pHH3) expression
showed that DS fetuses and P2 Ts65Dn mice had a higher number of cells in the G 2 phase
of cell cycle and a smaller one in the M phase, suggesting that a longer time spent by
proliferating cells in the G2 phase may underlie their reduced proliferation rate. We
performed a microarray screening in P2 mice cerebellum, to analyze the expression of key
genes involved in cell cycle and neurogenesis/differentiation. In Ts65Dn mice, there was a
significant down-regulation of CyclinB1 and the cell cycle kinase Skp2, both of which are
involved in the regulation of G2/M progression. We additionally found a significant downregulation of Bmi1, a gene involved in maintenance of the proliferative capacity of
progenitors cells and an up-regulation of genes involved in the exit from the replicative
state and induction of differentiation towards either a neuronal (Mash1, NeuroD,
Neurogenin2) or a glial (Olig1, S100b) fate.
We demonstrate here for the first time impairment of neural precursor proliferation in the
brain of DS subjects during the early phases of brain development. Results point at a defect
in cell cycle progression as a likely determinant of impaired proliferation and highlight
some genes as possible contributors to cell cycle and replication deregulation. Collectively,
our observations suggest that neurogenesis impairment starting from the earliest stages of
development may underlie the widespread brain atrophy of DS.
Key-words: Down syndrome; neural precursors; hippocampus; cerebellum; cell cycle