Transient Cerebral Ischemia Induces Aberrant Neuronal Cell Cycle Re-entry and Alzheimer's Disease-like Tauopathy in Female Rats

Abstract

Aberrant mitosis occurs in many tauopathy-related neurodegenerative diseases and is believed to precede the formation of neurofibrillary tangles. In this study, we report for the first time that transient cerebral ischemia induces aberrant mitotic proteins and hyperphosphorylation of tau protein with neurofibrillary tanglelike conformational epitopes in adult female rat cortex. Following transient cerebral ischemia in rats, initiation of apoptosis precedes and is potentially integrated with subsequent aberrant mitosis and tau hyperphosphorylation. Furthermore, inhibition of mitosis-related cyclindependent kinases (Cdks) by roscovitine significantly reduced the hyperphosphorylation of tau. Administration of the female sex steroid and potent neuroprotective agent, 17␤-estradiol, reduced ischemia-reperfusioninduced cerebral damage and the subsequent aberrant mitosis and tauopathies. These results provide a neuropathological basis for the higher prevalence of dementia in stroke patients and support the hypothesis that apoptosis and aberrant mitosis are integrated pathological events in neurons that may play a critical role in the development of Alzheimer's disease and other tauopathy-related neuropathology.The prevalence of dementia in ischemic stroke patients is nine times higher than controls at 3 months (1) and 4 -12 times higher than in controls at 4 years after a lacunar infarct (2). Many of these dementias developed progressively, and cerebral damage is believed to be the direct cause of cognitive decline in only half of these cases (3). Alzheimer's disease (AD) 1 is the most prevalent dementia (4) and shares common neuropathology features with stroke. Amyloid angiopathy can lead to cerebral hemorrhage (5), Alz-50-immunoreactive granules are found around cerebral infarction after a stroke (6), amyloid precursor protein accumulates following transient focal ischemia (7), and ApoE4 is a genetic risk factor for both AD and stroke (8).Neurofibrillary tangles (NFTs), whose major component is hyperphosphorylated tau (9), are observed in many neurodegenerative diseases (10, 11). Recent reports indicate that aberrant mitotic activation, as well as DNA replication, in terminally differentiated neurons appears to be involved in the pathogenesis of AD (12, 13). Many cyclin-dependent kinases, including Cdc2 and cyclin B1, as well as many other mitotic markers, are elevated in nearly all tauopathy-related neurodegenerative diseases (14). These protein kinases regulate cell cycle progression into mitosis in developing and proliferating cells. If cell cycle re-entrance is forced by ectopically driving an oncogene with a neuronal-specific promotor, the targeted neurons will die rather than divide (15-18). In AD and other tauopathy diseases, induction of Cdc2 and cyclin B1, as well as aberrant DNA replication, is observed in neurons containing NFTs (13). However, completion of successful nuclear divisions in differentiated neurons has never been reported. These inappropriately induced mitotic protein kinases can phosphorylat...

Role of Protein Phosphatases in Estrogen-Mediated Neuroprotection

“…The present data may provide a mechanism by which estrogens ameliorate the effect of insult on tau hyperphosphorylation in vitro (Alvarez-de-la-Rosa et al, 2005) and in ischemia/ reperfusion-induced injury in vivo (Wen et al, 2004). By preventing insult-induced decreases in protein phosphatases, estrogens could allow for the dephosphorylation of tau even in the face of an insult stimulation of tau phosphorylation.…”

Hypoxia-Ischemia Induces DNA Synthesis without Cell Proliferation in Dying Neurons in Adult Rodent Brain

“…For example, the MCAO stroke model induces expression of CDK4, cyclin D1, cyclin B1, E2F1, phosphorylated RB, and a large number of TUNEL-positive cells (Osuga et al, 2000: Wen et al, 2004, but a similar study showed that only Ͻ1% of the TUNEL-positive nuclei after MCAO had incorporated BrdU (Katchanov et al, 2001). The small percentage of BrdU-and TUNEL-double-positive cells suggests that, using this particular model of focal ischemia, mature neurons very rarely pass the G 1 /S-phase checkpoint, even though they reenter the cell cycle.…”

“…For example, although transient middle cerebral artery occlusion (MCAO) induces aberrant cell cycle reentry, the incorporation of a thymidine analog, bromodeoxyuridine (BrdU), is only found in Ͻ1% of the terminal deoxynucleotidyl transferasemediated biotinylated UTP nick end labeling (TUNEL)-positive cells (Katchanov et al, 2001;Wen et al, 2004). One interpretation of the small percentage of TUNEL-and BrdU-doublelabeled cells is that most neurons are inherently incapable of passing the G 1 /S-phase checkpoint even after reentering the cellcycle.…”

Endovascular middle cerebral artery occlusion in rats as a model for studying vascular dementia

“…For example, cerebral ischemia up-regulates the expression of amyloid precursor protein (APP) in rats (Jin et al 2001a;Nihashi et al 2001), and enhances the cleavage of Aβ from APP (Saido et al 1994). Moreover, hyperphosphorylation of tau protein, another hallmark of AD, is induced by global ischemia (Sinigaglia-Coimbra et al 2002) and focal cerebral ischemia in rats (Wen et al 2004b). The significant overlap of cerebrovascular disease and AD pathology makes the distinction between VaD and AD less clear, and it is commonly known that the differential diagnosis of VaD and AD on the basis of clinical evidence is, at best, very difficult.…”

“…For example, cerebral ischemia induced by transient endovascular MCA occlusion in rats caused an increase in both amyloid precursor protein (APP) production and BACE 1 activity (Shi et al 2000;Wen et al 2004a). In addition, after transient MCA occlusion, the cortex showed high levels of both phosphorylated tau and AD-type tau conformational epitopes (Wen et al 2004b). Further, this tau hyperphosphorylation occurred predominantly in neurons that were undergoing apoptosis, as evidenced by TUNEL staining (Wen et al 2004b).…”