Cdk5 (Cyclin-Dependent Kinase 5)

Cdk5 (Cyclin-Dependent Kinase 5)

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Cdk5 — Cyclin-Dependent Kinase 5</th>
</tr>
<tr> [@shah2022]
<td class="label">Protein Name</td> [@tarricone2001]
<td><strong>Cdk5 (Cyclin-Dependent Kinase 5)</strong></td> [@seo2017]
</tr> [@bhatt2023]
<tr> [@cheung2006]
<td class="label">Gene</td> [@lopes2010]
<td><a href="https://www.ncbi.nlm.nih.gov/gene/1020" target="_blank">CDK5</a> (7q36.1)</td> [@tau]
</tr> [@taua]
<tr> [@gskb]
<td class="label">UniProt</td> [@amyloidbeta]
<td><a href="https://www.uniprot.org/uniprot/Q00535" target="_blank">Q00535</a></td> [@alzheimers]
</tr> [@parkinsons]
<tr> [@nflprotein]
<td class="label">PDB Structures</td> [@excitotoxicity]
<td><a href="https://www.rcsb.org/structure/1H4L" target="_blank">1H4L</a>, <a href="https://www.rcsb.org/structure/1UNL" target="_blank">1UNL</a>, <a href="https://www.rcsb.org/structure/3O0G" target="_blank">3O0G</a></td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~33.3 kDa (291 amino acids)</td>
</tr>
<tr>
<td class="label">Localization</td>
<td>Cytoplasm, nucleus, synapses, growth cones, perinuclear region</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>CMGC kinase group, CDK family</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9

Cdk5 (Cyclin-Dependent Kinase 5)

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Cdk5 — Cyclin-Dependent Kinase 5</th>
</tr>
<tr> [@shah2022]
<td class="label">Protein Name</td> [@tarricone2001]
<td><strong>Cdk5 (Cyclin-Dependent Kinase 5)</strong></td> [@seo2017]
</tr> [@bhatt2023]
<tr> [@cheung2006]
<td class="label">Gene</td> [@lopes2010]
<td><a href="https://www.ncbi.nlm.nih.gov/gene/1020" target="_blank">CDK5</a> (7q36.1)</td> [@tau]
</tr> [@taua]
<tr> [@gskb]
<td class="label">UniProt</td> [@amyloidbeta]
<td><a href="https://www.uniprot.org/uniprot/Q00535" target="_blank">Q00535</a></td> [@alzheimers]
</tr> [@parkinsons]
<tr> [@nflprotein]
<td class="label">PDB Structures</td> [@excitotoxicity]
<td><a href="https://www.rcsb.org/structure/1H4L" target="_blank">1H4L</a>, <a href="https://www.rcsb.org/structure/1UNL" target="_blank">1UNL</a>, <a href="https://www.rcsb.org/structure/3O0G" target="_blank">3O0G</a></td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~33.3 kDa (291 amino acids)</td>
</tr>
<tr>
<td class="label">Localization</td>
<td>Cytoplasm, nucleus, synapses, growth cones, perinuclear region</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>CMGC kinase group, CDK family</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">300 edges</a></td>
</tr>
</table>

Cdk5 (Cyclin-Dependent Kinase 5)

Pathway Diagram

Introduction

[Cdk5](/proteins/cdk5-protein) — Cyclin Dependent Kinase 5 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

[Cdk5](/genes/cdk5) ([cdk5](/proteins/cdk5) is a proline-directed serine/threonine kinase that, despite its name, is primarily active in postmitotic [neurons](/entities/neurons) rather than in dividing cells. Unlike other CDK family members, [Cdk5](/proteins/cdk5) is not activated by cyclins but instead requires binding to its neuron-specific activators p35 (CDK5R1) or p39 (CDK5R2) for kinase activity ([Tsai et al., 1994](https://doi.org/10.1038/368461a0)). Under physiological conditions, Cdk5 is essential for brain development, neuronal migration, synaptic function, and neuronal survival. However, pathological conversion of p35 to p25 by calpain-mediated cleavage results in sustained Cdk5 hyperactivation, which drives [tau-protein](/proteins/tau) hyperphosphorylation, neuronal death, and neurodegeneration in [alzheimers](/diseases/alzheimers-disease), [parkinsons](/diseases/parkinsons-disease), [als](/diseases/amyotrophic-lateral-sclerosis), and other conditions ([Shah & Bhatt, 2022](https://doi.org/10.1186/s12929-021-00774-y)).

Cdk5 is now recognized as one of the most important kinases in neurodegeneration and a prime therapeutic target. Its dual nature — essential for neuronal health yet destructive when dysregulated — poses both challenges and opportunities for drug development.

Structure

Kinase Domain Architecture

Cdk5 is a 291-amino-acid protein (~33.3 kDa) with the canonical bilobal kinase fold:

• N-terminal lobe (small lobe): consists of a five-stranded antiparallel beta-sheet and a single alpha-helix (the PSTAIRE-equivalent helix, which in Cdk5 reads PSSALRE). Contains the ATP-binding pocket and the glycine-rich loop for nucleotide positioning.
• C-terminal lobe (large lobe): predominantly alpha-helical; contains the activation segment (T-loop), the catalytic loop with the conserved aspartate residue (Asp144), and substrate recognition elements.
• Hinge region: connects the two lobes and forms part of the ATP-binding cleft; important for inhibitor design.

Activation Mechanism

Unlike other CDKs, Cdk5 does not require T-loop phosphorylation for activation. Instead, binding of p35 or p39 to the PSSALRE helix induces a conformational change that opens the substrate-binding cleft and positions the catalytic residues for phosphotransfer. This unique activation mechanism explains why Cdk5 escapes the cell-cycle regulatory machinery that controls other CDKs ([Tarricone et al., 2001](https://doi.org/10.1016/S1097-2765(01)00350-0)).

The p35-to-p25 Conversion

p35 is a short-lived protein (half-life ~20 minutes) normally degraded by the [ubiquitin-proteasome system](/cell-types/ubiquitin-proteasome-system), keeping Cdk5 activity tightly controlled. Under neurotoxic stress — including [Amyloid-Beta](/proteins/amyloid-beta) exposure, oxidative stress, excitotoxicity, or ischemia — the calcium-activated protease calpain cleaves p35 into p10 (N-terminal, membrane-anchoring fragment) and p25 (C-terminal, Cdk5-binding fragment). p25 has a much longer half-life (~60 minutes), causing prolonged Cdk5 activation. Moreover, p25 lacks the N-terminal myristoylation signal that anchors p35 to membranes, so the Cdk5/p25 complex is mislocalized from the membrane to the cytoplasm and nucleus, where it accesses pathological substrates including [tau-protein](/proteins/tau), leading to neurodegeneration ([Patrick et al., 1999](https://doi.org/10.1038/46547)).

Normal Function

Neuronal Migration and Cortical Development

Cdk5 is essential for proper cortical lamination during embryonic development. Cdk5 knockout mice die at birth with severe cortical layering defects due to failed neuronal migration. Cdk5 phosphorylates multiple substrates required for the cytoskeletal reorganization underlying neuronal migration, including:

• Doublecortin (DCX): regulation of microtubule bundling during migration
• Ndel1/Lis1: coupling of centrosome-nucleus movement to the leading process
• FAK: focal adhesion dynamics for motility
• CRMP-2: growth cone guidance and axon specification

Synaptic Function and Plasticity

At mature synapses, Cdk5 modulates both presynaptic and postsynaptic functions:

• Presynaptic: phosphorylates synapsin I, Munc18, and dynamin I, regulating synaptic vesicle cycling and neurotransmitter release
• Postsynaptic: phosphorylates NR2A and NR2B subunits of [nmda-receptor](/entities/nmda-receptor) receptors, PSD-95, and DARPP-32, modulating [long-term-potentiation](/mechanisms/long-term-potentiation)
• [LTP](/mechanisms/long-term-potentiation)/LTD: Cdk5 activity is required for certain forms of [long-term potentiation](/mechanisms/long-term-potentiation) and long-term depression

Neuronal Survival

Physiological Cdk5/p35 activity promotes neuronal survival through:

• Phosphorylation and inactivation of pro-apoptotic proteins (e.g., p53 family members)
• Maintenance of cytoskeletal integrity via neurofilament phosphorylation
• DNA damage response signaling
• Cell cycle suppression in postmitotic [neurons](/entities/neurons) — loss of Cdk5 activity allows aberrant cell cycle re-entry, leading to neuronal death

Neurofilament Phosphorylation

Cdk5 directly phosphorylates the KSP repeats in [nfl-protein](/proteins/nfl-protein) medium (NEFM) and heavy (NEFH) chain tail domains. This phosphorylation increases neurofilament sidearm extension, regulating inter-filament spacing and thus axonal caliber. Cdk5-mediated neurofilament phosphorylation is a major determinant of axonal diameter and nerve conduction velocity.

Role in Disease

Alzheimer's Disease

Cdk5/p25 hyperactivation is a central pathogenic mechanism in [alzheimers](/diseases/alzheimers-disease):

• [tau-protein](/proteins/tau) hyperphosphorylation]: Cdk5/p25 phosphorylates [tau-protein](/proteins/tau) at multiple disease-associated sites including Ser202, Thr205, Ser235, and Ser404. These phosphorylations reduce [tau](/proteins/tau)'s affinity for microtubules, promote its aggregation into neurofibrillary tangles, and correlate with [braak-staging](/mechanisms/braak-staging) of AD ([Patrick et al., 1999](https://doi.org/10.1038/46547)).
• [Amyloid-Beta](/proteins/amyloid-beta) processing: Cdk5 can phosphorylate [bace1-protein](/proteins/bace1-protein) (https://www.ncbi.nlm.nih.gov/gene/930, increasing its activity and promoting amyloidogenic processing of [app-protein](/entities/app-protein).
• Neuronal cell cycle re-entry: Cdk5/p25 drives aberrant cell cycle activation in postmitotic [neurons](/entities/neurons), a lethal event.
• DNA damage: Cdk5 hyperactivation in the nucleus causes widespread DNA damage and genomic instability, contributing to neuronal death ([Kim et al., 2008](https://doi.org/10.1016/j.cell.2008.02.032)).

Parkinson's Disease

In [parkinsons](/diseases/parkinsons-disease), Cdk5 contributes to [dopaminergic-neurons-snpc](/cell-types/dopaminergic-neurons-snpc) degeneration through:

• Phosphorylation and modulation of parkin ([parkin](/proteins/parkin) activity
• Phosphorylation of [alpha-synuclein](/proteins/alpha-synuclein) at Ser87
• Regulation of [lrrk2-protein](/proteins/lrrk2-protein) activity
• Promotion of [oxidative-stress](/mechanisms/oxidative-stress) and [mitochondrial-dysfunction](/mechanisms/mitochondrial-dysfunction) via [drp1](/proteins/drp1) phosphorylation

ALS

In [als](/diseases/amyotrophic-lateral-sclerosis), Cdk5/p25 activity is elevated in spinal cord motor [neurons](/entities/neurons). Cdk5 hyperactivation contributes to:

• Hyperphosphorylation of neurofilaments, disrupting axonal transport
• Phosphorylation of [sod1-protein](/proteins/sod1-protein) mutants, modulating their toxicity
• Dysregulation of [rna-metabolism](/mechanisms/rna-metabolism) in motor neurons

Frontotemporal Dementia

In [ftd](/diseases/frontotemporal-dementia) [tauopathies](/mechanisms/tauopathies), Cdk5/p25 inhibition attenuates [tau](/proteins/tau) pathology, neuroinflammation, and neuronal loss. Inhibition of the p25/Cdk5 complex in mouse and iPSC models of FTD reduces tau phosphorylation, [neuroinflammation](/mechanisms/neuroinflammation), DNA damage, and cell cycle re-entry ([Seo et al., 2017](https://pubmed.ncbi.nlm.nih.gov/28965760/)).

Huntington's Disease

Cdk5 phosphorylates [huntingtin](/proteins/huntingtin) at Ser434, which may modulate [huntingtin](/proteins/huntingtin) toxicity and aggregate formation in [huntington-pathway](/mechanisms/huntington-pathway).

Therapeutic Targeting

Small Molecule Inhibitors

• Roscovitine: a broad CDK inhibitor that attenuates Cdk5 hyperactivity and ameliorates p25-associated pathologies including DNA damage, tau phosphorylation, and neuronal death. However, its lack of selectivity for Cdk5 over other CDKs limits clinical utility.
• Dinaciclib: another CDK inhibitor with activity against Cdk5, used primarily in oncology but studied in neurodegeneration contexts.

Peptide-Based Approaches

A 12-amino-acid Cdk5-derived peptide (CIP) that specifically disrupts the Cdk5/p25 complex without affecting Cdk5/p35 activity represents a more targeted approach. This peptide selectively inhibits the pathological Cdk5/p25 interaction while preserving the physiological Cdk5/p35 function, ameliorating neurodegenerative phenotypes in cell and mouse models ([Bhatt et al., 2023](https://doi.org/10.1073/pnas.2217864120)).

p35/p25 Modulation

Strategies to prevent p35-to-p25 conversion by inhibiting calpain activity or stabilizing p35 are under investigation. Calpain inhibitors reduce p25 generation and tau hyperphosphorylation in AD models.

Brain Atlas Resources

• Allen Human Brain Atlas: [Cdk5 — Cyclin-Dependent Kinase 5 expression search](https://human.brain-map.org/microarray/search/show?search_term=Cdk5+%E2%80%94+Cyclin-Dependent+Kinase+5)
• Allen Mouse Brain Atlas: [Cdk5 — Cyclin-Dependent Kinase 5 search](https://mouse.brain-map.org/search/index.html?query=Cdk5+%E2%80%94+Cyclin-Dependent+Kinase+5)
• Allen Cell Type Atlas: [Transcriptomic cell type reference](https://portal.brain-map.org/atlases-and-data/rnaseq)
• BrainSpan Developmental Transcriptome: [Cdk5 — Cyclin-Dependent Kinase 5 developmental expression](https://www.brainspan.org/rnaseq/search/index.html?search_term=Cdk5+%E2%80%94+Cyclin-Dependent+Kinase+5)

See Also

• [neurons](/cell-types/neurons)
• [Diseases Index
• [Entities Index
• [Mechanisms Index

External Links

• [UniProt: Q00535](https://www.uniprot.org/uniprot/Q00535)
• [PDB: 1UNL](https://www.rcsb.org/structure/1UNL)
• [KEGG: cdk5 pathway](https://www.kegg.jp/kegg-bin/show_pathway?map=map05010)
• [UniProt: Q00535](https://www.uniprot.org/uniprot/Q00535)
• [PDB: 1UNL](https://www.rcsb.org/structure/1UNL)
• [KEGG: cdk5 pathway](https://www.kegg.jp/kegg-bin/show_pathway?map=map05010)
• [UniProt: Q00535](https://www.uniprot.org/uniprot/Q00535)
• [PDB: 1UNL](https://www.rcsb.org/structure/1UNL)
• [KEGG: cdk5 pathway](https://www.kegg.jp/kegg-bin/show_pathway?map=map05010)
• [PubMed](https://pubmed.ncbi.nlm.nih.gov/) - Literature database
• [NCBI](https://www.ncbi.nlm.nih.gov/) - National Center for Biotechnology Information

Background

The study of Cdk5 — Cyclin Dependent Kinase 5 has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

Brain Atlas Resources

References

Pathway Diagram

The following diagram shows the key molecular relationships involving Cdk5 (Cyclin-Dependent Kinase 5) discovered through SciDEX knowledge graph analysis: