In this study we found that in an in vitro model system of adult hippocampal neurogenesis in AD, abnormal stimulation of the CDK5/p35/p25 signaling pathway resulted in impaired maturation of NPCs, as reflected by reduced neurite outgrowth in NPC-derived neural progeny. These effects were rescued by downmodulation of CDK5 activity by Roscovitine or siRNA specific for CDK5. Similarly, in the APP tg mouse model of AD, we found that deficits in neurogenesis in the adult hippocampus were reversed by genetic or pharmacological inhibition of CDK5.
Previous studies have implicated CDK5 in embryonic neuronal development20
and neuroblast migration in the cerebral cortex.21, 22
In healthy mature neurons, CDK5 regulates synaptic plasticity28
and neurite outgrowth;20
however, less is known about its role in adult neurogenesis. Several recent studies have implicated CDK5 in the maintenance of a non-cycling state in postmitotic neurons.29
This function appears to be dependent on the subcellular localization of CDK5, which is regulated by cell cycle stage29
and interactions with the cell cycle inhibitor p27,30
among other factors. Consistent with previous observations,29
in this study we have detected some CDK5 expression in both the nuclear and cytoplasmic compartments in both in vitro
and in vivo
models, which is likely a reflection of variability in cell cycle stage of cycling NPCs and immature neural progeny.
Recent studies have implicated CDK5 in neuronal maturation and dendritic development during the process of adult neurogenesis.6, 23
Although loss of CDK5 activity does not appear to alter neuronal fate, it severely disrupts dendritic development of NPCs in vivo
Targeted ablation of CDK5 expression in adult hippocampal NPCs reduces the numbers of DCX-positive immature neurons in the SGZ.23
Similarly, the results of this study show in an alternative model of CDK5 deficiency that the numbers of hippocampal DCX- and BrdU-positive cells were reduced.
It is important to note that although too little CDK5 activity is detrimental to embryonic and adult neurogenesis,6, 22, 23
pathological activation of CDK5 associated with AD has a significant negative impact as well. Notably, we observed similar defects in neurogenesis in the brains of APP tg mice that were rescued by CDK5 inhibition, suggesting that abnormal CDK5 activation could reflect a loss of normal function and/or a gain of atypical function; however, future studies will be necessary to elucidate the mechanisms involved. A previous review paper aptly labeled CDK5 as a ‘Jekyll and Hyde kinase',2
reflecting both the important physiological role of CDK5 in development22, 28
and the abnormal CDK5 activity associated with neurodegeneration in the pathogenesis of AD.9
Although physiological CDK5 activity is required for the critical functions noted above, abnormal stimulation of CDK5 in the pathogenesis of AD has been shown to contribute to the neurodegenerative process and mature neuronal cell death.9
In this neurotoxic pathway, Aβ
triggers calcium influx, which activates calpain activity and subsequent cleavage of p35 to p25, a stable activator of CDK5 that promotes the hyperphosphorylation of downstream substrates of CDK5.9
In this context, for this study we focused on the activation of this pathway to further probe the role of CDK5 in adult hippocampal neurogenesis and neuronal maturation in AD. Our studies in an in vitro
model of aberrant CDK5 activation in adult neurogenesis in AD show that p35 overexpression combined with Aβ
treatment promoted the generation of abnormal neural progeny and impaired neurite outgrowth.
It was an unexpected finding that under conditions of abnormal CDK5 activation in our in vitro
NPC model, the proportions of progeny coexpressing markers traditionally associated with proliferation or neuronal differentiation stages were more prominent than under control conditions. We hypothesize that these disease-like conditions might arrest cells in an early transitional state where expression of progenitor cell markers might be prolonged or atypical, reflecting an impairment in the neuronal maturation process. In support of this possibility, a previous study has shown that in the DG of AD brains, progenitor cells fail to differentiate into mature microtubule-associated protein-2 (MAP2)-positive neurons.14
Further studies will be necessary to delineate the precise mechanisms and downstream targets through which abnormal CDK5 activation interferes with the maturation of NPCs in AD. Several substrates of CDK5 are worth further investigation because a number of these are involved in cytoskeletal development, and could be functionally related to neurite outgrowth. Taken together, this study supports a role for abnormal activation of CDK5 in impaired maturation of NPCs in the adult hippocampus in AD. Therapeutic approaches targeting at normalizing, but not eradicating, CDK5 activity may prove useful in promoting the appropriate maturation of hippocampal NPCs in AD.