Cell Death and Differentiation (2002) 9, 1294 ± 1306 ã 2002 Nature Publishing Group All rights reserved 1350-9047/02 $25.00 www.nature.com/cdd Review Cycling at the interface between neurodevelopment and neurodegeneration 1,3 2 ,1 MD Nguyen , WE Mushynski and J-P Julien* Introduction 1 Centre for Research in Neurosciences, Research Institute of the McGill Members of the Cyclin-dependent protein kinase (Cdk) family University Health Centre, Montreal General Hospital, 1650 Cedar Avenue, are small, serine/threonine kinases (30 ± 35 kDa), whose nine MontreÂal, QueÂbec, H3G 1A4, Canada members share greater than 40% identity. With the exception 2 Department of Biochemistry, McGill University, McIntyre Medical Sciences of Cdk3 and Cdk5, they are activated by a cyclin regulatory Building, 3655 Promenade Sir William Osler, MontreÂal, QueÂbec, H3G 1Y6, subunit1±3 (see Figure 2A). The Cdks are numbered in order Canada 3 Current address: Harvard Medical School, Department of Pathology, of their discovery, starting with Cdk1 (p34cdc2) and extending 200 Longwood Avenue, Boston, MA 02115 at present to Cdk9. The `Cdk' designation does not imply that * Corresponding author: J-P Julien. Tel: 934-1934 ext 44203; Fax: 514-934- the biological functions of Cdks are limited to mitosis. The 8265; E-mail: [email protected]. For correspondence use the Fedex no: classical Cdks, excepting Cdk5 and Cdk9, are also involved in 1316-8394-4. regulating cellular processes such as differentiation, senes- cence, and apoptosis through modification of gene transcrip- tion.2 In proliferating cells, misregulation of Cdks is associated Received 17.5.02; revised 23.7.02; accepted 23.7.02 with tumor formation, whereas their disappearance/inhibition Edited by G Melino in neuronal precursors coincides with terminal differentiation.4 Cdk5 is a unique member of the Cdk family. Although its Abstract cloning was based on sequence homology to the cell cycle kinase, Cdk1, Cdk5 does not play a critical role in cell cycle The discovery of cell cycle regulators has directed cell progression.1 It is not activated by a cyclin, although it can research into uncharted territory. In dividing cells, cell cycle- bind one such protein.5 The association of Cdk5 with one of associated protein kinases, which are referred to as cyclin- its neuron-specific co-activators, p35 or p39, is required in dependent-kinases (Cdks), regulate proliferation, differentia- processes such as neurite outgrowth, axonal migration, tion, senescence and apoptosis. In contrast, all Cdks in post- cortical lamination, control of cell adhesion, axonal transport, mitotic neurons, with the notable exception of Cdk5, are synaptic activity, neuronal adaptive changes and motor 1 silenced. Surprisingly, misregulation of Cdks occurs in functions. The prominent role of Cdk5 in the CNS stems from its unique co-activators and the broad spectrum of neurons in a wide diversity of neurological disorders, Cdk5-interacting molecules and substrates demonstrated by including Alzheimer's disease, Parkinson's disease and genetic studies in the mouse and fruitfly. Emerging evidence amyotrophic lateral sclerosis. Ectopic expression of these also points to a leading role for Cdk5 as well as other Cdks in proteins in neurons potently induces cell death with hallmarks neuronal apoptosis and degeneration. of apoptosis. Deregulation of the unique, cell cycle-unrelated This review will focus on the basic mechanisms Cdk5 by its truncated co-activator, p25 and p29, contributes to regulating the divergent roles of Cdks in the CNS and their neurodegeneration by altering the phosphorylation state of common involvement in neuronal apoptosis and neurode- non-membrane-associated proteins and possibly through the generation. induction of cell cycle proteins. On the other hand, cycling Cdks such as Cdk2, Cdk4 and Cdk6, initiate death pathways by Divergent regulation of Cdk5 and cell cycle derepressing E2F-1/Rb-dependent transcription at the neuro- Cdk activities nal G1/S checkpoint. Thus, Cdk5 and cycling Cdks may have Cdk activity is regulated by three distinct overlapping little in common in the healthy CNS, but they likely conspire in mechanisms, the critical step being their association with a leading neurons to their demise. co-activator. Phosphorylation/dephosphorylation events Cell Death and Differentiation (2002) 9, 1294 ± 1306. doi:10.1038/ prime Cdks for activation by regulatory subunits while a sj.cdd.4401108 family of Cdk-inhibitory subunits (CKIs) bind to and inactivate the Cdk-cyclin complex.1,6 Thus, formation of a Cdk1/cyclin B Keywords: cyclin-dependent-kinases; neurodevelopment; neuro- complex late in S-phase triggers phosphorylation of Cdk1 on degeneration; alzheimer; amyotrophic lateral sclerosis; cell cycle; Thr 14 and Tyr 15 by the dual-specificity kinases, Wee 1 and cytoskeleton Myt1, thereby inhibiting its activity.1,7 ± 9 In contrast, phosphor- ylation of Thr 161 in the T-loop of Cdk1 by the Cdk-activated Abbreviations: Cdks, cyclin-dependent-kinases; CKIs, CDK- kinase, CAK (CDK7-cyclin H) dramatically increases kinase Inhibitory subunits; AD, Alzheimer's disease activity.1,9 These three sites are phosphorylated throughout the G2-phase. Dephosphorylation of Thr 14 and Tyr 15 by the Cdks in the CNS MD Nguyen et al 1295 dual-specificity phosphatase, Cdc25, results in the activation lysine, histidine or arginine residue, respectively.1,15 Cdk5 of Cdk1/cyclin B coinciding with the onset of mitosis.10,11 shows a marked preference for a basic residue at position Finally, additional regulation by Cdk-inhibitory subunits (CKIs) +3.18 The human neurofilament heavy chain (NF-H), a major such as p21 and p27 (in the case of Cdk2) inhibit activation of cytoskeletal protein found in mature nerve cells, contains 35 the Cdk/cyclin complex.6 KSPXK repeats and is therefore a much better Cdk5 substrate The regulation of Cdk5 differs markedly from that of the than the mouse or rat homologs which contain one-third as cell cycle Cdks, despite its high level of sequence homology many such motifs.19 The only Cdk family members showing with the other family members. Thus, the Thr14 and Tyr15 phosphorylation sequence motif specificity identical to that sites in Cdk5 are not phosphorylated by Wee1 in vitro,andc- exhibited by Cdk5 are Cdk1, Cdk2. Others members of the Abl catalyzes the stimulatory phosphorylation of Tyr15 in family preferentially phosphorylate the KSPXX motif. Cdk5.12 Surprisingly, phosphorylation of Thr14 in Cdk5, Specificity in substrates recognition for mitotic Cdks is Cdk1 and Cdk2 by a kinase from calf thymus gland dictated by their association with cyclins subunits and by inactivates the three enzymes.13 The Ser159 residue in the cyclin-binding motif ZRXL and the LXCXE motif.6,20 Cdk5 occupies a position equivalent to the Thr161 and While the ZRXL motif is found in cell cycle-associated Thr160 phosphorylation sites in the conserved T-loop of proteins such as Cip/Kip CKI family, E2F-1 and pRb-related Cdk1 and Cdk2, respectively, and in vitro studies have proteins p107, p130, the LXCXE motif is part of the cyclin suggested that Cdk5 undergoes stimulatory phosphorylation protein.6,20 ± 22 This motif is also found in pRb-associated at this site by an unknown kinase, possibly casein kinase proteins such as transcription factors and is likely to serve I.6,14 However, structural analysis of a complex between a targeting function. Therefore, substrates for mitotic Cdks Cdk5 and p25, a truncated version of the p35 co-activator of are mainly cell cycle and transcription-related proteins. Cdk5, indicated that phosphorylation of Ser159 would Conversely, Cdk5 specificity for substrate recognition is disrupt the association of Cdk5 with its co-activator.15 In dictated by p35, p39 and by truncated forms of these co- addition to its distinct phosphorylation-dependent regulation, activators (see below). Cdk5 is not regulated by CKI because of its association with p35.16 However, a p35-binding protein that specifically inhibits the activation of Cdk5 has recently been reported.17 Cell cycle Cdks in the CNS and neurodegeneration Phosphorylation sequence motif The levels of Cdks, cyclins and CKI in dividing cells are tightly controlled to allow smooth progression through the cell cycle. speci®city of Cdks Conversely, the expression of cell cycle Cdks in the adult CNS Cdk5 is a proline-directed kinase which preferentially is negligible since they do not play a significant role in mature phosphorylates the consensus sequence (S/T)PX(K/H/R), neurons. As neurons differentiate, Cdk1 and Cdk2 are where S/T is a serine or threonine residue, P is the proline downregulated while Cdk5, p35 and p39 expression be- residue at position +1, X is any amino acid and K, H or R is a gins4,23,24 (see Figure 1). Some studies report the presence of Figure 1 Activity of cycling Cdks and Cdk5 during neurodevelopment and neurodegeneration. There is an inverse relation between the activity and expression of cycling Cdks, such as p34cdc2 (Cdk1), and Cdk5 during neurodevelopment. The activity and expression of p34cdc2 decreases progressively as those of Cdk5 increase. At the adult stage, Cdk5 is the main Cdk to be found active in the CNS through association with its neuron-specific co-activators p35 and p39. Aging, stress and genetic factors induce the progressive cleavage of p35 into p25, resulting in an increased Cdk5 activity. In parallel, activity and expression of cycling Cdks also increase progressively. The conversion of p35 into p25 results in loss of the p35/Cdk5