The c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) families represent critical stress-activated signaling pathways that play pivotal roles in the pathogenesis of neurodegenerative diseases. These serine/threonine kinases are activated by diverse cellular stresses including oxidative stress, inflammatory cytokines, glutamate excitotoxicity, and pathological protein aggregates, leading to downstream effects on neuronal survival, synaptic function, and glial activation[1].
The JNK and p38 MAPK pathways serve as central integrators of cellular stress signals, coordinating responses that range from adaptive survival mechanisms to programmed cell death. In the context of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), these pathways are chronically activated, contributing to progressive neuronal dysfunction and death. Understanding the specific roles of JNK and p38 isoforms in different cell types and disease contexts has revealed potential therapeutic targets that are actively being explored in preclinical and clinical studies[2][3].
The MAPK signaling cascade was first characterized in the early 1990s as a fundamental cellular signaling pathway responding to extracellular stimuli. The JNK family was originally identified as a kinase that phosphorylates the transcription factor c-Jun in response to UV radiation and other cellular stresses. Subsequent research revealed that JNK and p38 pathways play essential roles in development, stress responses, and cell fate decisions. The involvement of these pathways in neurodegeneration was first demonstrated in the late 1990s, when researchers observed elevated JNK activation in post-mortem brain tissue from AD and PD patients[4].
Key discoveries that shaped our understanding include:
The JNK family consists of three genes encoding ten isoforms through alternative splicing. JNK1 and JNK2 are expressed ubiquitously, while JNK3 is neuron-specific and exhibits the strongest involvement in neurodegenerative processes[5].
| Isoform | Gene | Tissue Distribution | Key Functions | Disease Relevance |
|---|---|---|---|---|
| JNK1α1/2 | MAPK8 | Ubiquitous | Stress response, cell proliferation, immune function | PD, AD |
| JNK2α1/2 | MAPK9 | Ubiquitous | Cell proliferation, differentiation | AD, MS |
| JNK3α1/2 | MAPK10 | Neuron-specific, heart | Neuronal apoptosis, excitotoxicity | AD, PD, ALS, HD |
The JNK signaling cascade is activated by upstream MAPK kinases MKK4 and MKK7, which phosphorylate JNK at Thr183 and Tyr185 residues. Activated JNK translocates to the nucleus where it phosphorylates transcription factors including c-Jun, JunD, ATF2, and Elk-1, leading to expression of pro-apoptotic genes and inflammatory mediators[6].
The p38 MAPK family includes four isoforms (p38α, p38β, p38γ, p38δ) encoded by separate genes. p38α is the most widely expressed and studied isoform in the context of neurodegeneration, while p38β shows brain-enriched expression[7].
| Isoform | Gene | Distribution | Functions | Disease Relevance |
|---|---|---|---|---|
| p38α | MAPK14 | Ubiquitous, high in brain | Inflammation, apoptosis, cytokine production | AD, PD, ALS |
| p38β | MAPK11 | Brain-enriched | Similar to α, more restricted | AD |
| p38γ | MAPK12 | Muscle, brain | Tissue-specific, development | Less clear |
| p38δ | MAPK13 | Lung, pancreas, brain | Tissue-specific functions | PD |
p38 MAPK is activated by MKK3 and MKK6, which phosphorylate p38 at Thr180 and Tyr182. The p38 pathway regulates numerous cellular processes including translation through MSK1/2 and MNK1/2, transcription through ATF2, CREB, and C/EBP, and cell survival through modulation of Bcl-2 family proteins and caspase activation[8].
Amyloid-beta (Aβ) peptide, the central pathogenic driver of Alzheimer's disease, activates both JNK and p38 MAPK pathways through multiple mechanisms. Aβ oligomers bind to various cell surface receptors including NMDA receptors, AMPA receptors, and cellular prion protein (PrP^C), triggering downstream MAPK signaling cascades[9].
Key mechanisms of Aβ-induced MAPK activation:
Glutamate excitotoxicity: Aβ enhances glutamate release and impairs glutamate reuptake, leading to overactivation of NMDA receptors. This activates JNK through calcium-dependent and independent mechanisms, resulting in excitotoxic neuronal death[10].
Oxidative stress: Aβ aggregation generates reactive oxygen species (ROS) that activate both JNK and p38 through oxidant-sensitive upstream kinases including ASK1 and MLK3.
Receptor-mediated activation: Aβ binding to RAGE (Receptor for Advanced Glycation End Products) and TLR4 receptors activates MyD88-dependent signaling that converges on ASK1 and downstream MAPK activation.
Mitochondrial dysfunction: Aβ-induced mitochondrial impairment leads to release of pro-apoptotic factors and ROS that activate stress-sensitive kinases.
JNK3 plays a critical role in Aβ-induced neuronal apoptosis through multiple downstream effectors[11]:
c-Jun phosphorylation: JNK phosphorylates c-Jun at Ser63 and Ser73, enhancing AP-1 transcriptional activity and promoting expression of pro-apoptotic genes including Bim, FasL, and PUMA.
Mitochondrial pathway: JNK phosphorylates Bcl-2 family proteins, promoting cytochrome c release and caspase-9 activation.
p53 activation: JNK phosphorylates p53 at Ser15, enhancing its transcriptional activity and pro-apoptotic function.
Synaptic dysfunction: JNK activation contributes to synaptic loss through phosphorylation of synaptic proteins and disruption of spine morphology.
p38 MAPK, particularly the p38α isoform, is activated in microglia surrounding amyloid plaques and contributes to chronic neuroinflammation in AD[8:1]:
Microglial activation: p38 drives production of pro-inflammatory cytokines including IL-1β, TNF-α, and IL-6 in microglia, creating a feed-forward inflammatory loop.
Tau phosphorylation: p38 phosphorylates tau at multiple AD-relevant sites including Thr181, Ser202, Thr205, and Thr231, promoting tau aggregation and NFT formation.
Synaptic plasticity impairment: p38-mediated phosphorylation of AMPA receptor subunits contributes to LTP deficits observed in AD models.
Blood-brain barrier dysfunction: p38 activation in endothelial cells contributes to BBB breakdown and peripheral immune cell infiltration.
Post-mortem studies of AD brain tissue reveal:
Several JNK and p38 inhibitors have been evaluated in clinical trials for AD[12]:
| Compound | Target | Stage | Outcome |
|---|---|---|---|
| D-JNKI1 (Tat-JNK-IN-1) | JNK1/2/3 | Phase II | Showed neuroprotection in preclinical models |
| SP600125 | JNK1/2/3 | Preclinical | Not advanced to clinical trials |
| Losmapimod | p38α | Phase III | Failed to demonstrate cognitive benefit in AD |
| PH-797804 | p38α | Phase II | Terminated due to liver toxicity |
| Semapimod | p38α | Phase II | Limited efficacy |
The failure of p38 inhibitors in AD trials highlights the challenge of targeting highly pleiotropic pathways and suggests that timing of intervention, patient selection, and pathway specificity may be critical for success.
The substantia nigra pars compacta (SNc) dopaminergic neurons exhibit particular vulnerability to JNK-mediated cell death in Parkinson's disease. Several factors contribute to this selective vulnerability[2:1]:
α-Synuclein aggregation, the hallmark pathological feature of PD, activates both JNK and p38 pathways:
The MPTP model of PD demonstrates that mitochondrial toxins potently activate JNK in dopaminergic neurons:
The 6-OHDA model similarly demonstrates JNK-mediated dopaminergic neuron death, with JNK3 knockout mice showing significant protection[13].
| Compound | Target | Stage | Notes |
|---|---|---|---|
| D-JNKI1 | JNK1/2/3 | Preclinical | Protected dopaminergic neurons in MPTP model |
| CEP-1347 | Mixed lineage kinase | Phase II/III | Failed in PD clinical trial |
| SR-3306 | JNK3 | Preclinical | Neuroprotective in animal models |
| SD-169 | p38α | Preclinical | Reduced microglial activation |
The failure of CEP-1347 (a MLK inhibitor upstream of JNK) in the ADAGIO trial highlights the complexity of targeting these pathways clinically. However, more selective JNK3 inhibitors and better patient selection may improve outcomes.
JNK activation is a consistent finding in ALS models and patient tissue:
In ALS, p38 MAPK plays a critical role in non-cell autonomous motor neuron death through glial activation:
| Target | Approach | Status |
|---|---|---|
| JNK3 | Gene therapy with JNK3 ASO | Preclinical |
| p38α | Small molecule inhibitors | Preclinical |
| ASK1 | Selonsertib (ASK1 inhibitor) | Phase II for other indications |
Mutant huntingtin (mHTT) protein directly interacts with and activates JNK signaling:
p38 MAPK is activated in HD models and contributes to:
The JNK/p38 MAPK pathways intersect with numerous other neurodegenerative mechanisms:
| Gene | Variant | Disease | Effect |
|---|---|---|---|
| MAPK8 (JNK1) | -317A>G | PD | Altered expression |
| MAPK14 (p38α) | rs1885128 | AD | Modified risk |
| MAPK9 (JNK2) | rs3821977 | ALS | Altered function |
| MAPK8IP1 (JIP1) | Various | PD, AD | Altered JNK regulation |
| MAP3K5 (ASK1) | Various | PD | Modified susceptibility |
Current research focuses on:
The JNK and p38 MAPK signaling pathways represent critical mediators of neuronal dysfunction and death in neurodegenerative diseases. While these pathways serve essential physiological functions, their chronic activation by disease-relevant stressors promotes synaptic failure, neuronal apoptosis, and neuroinflammation. The development of selective brain-penetrant inhibitors and identification of biomarkers for patient selection remain active areas of research. Understanding the complex interplay between JNK/p38 activation and other disease mechanisms will be essential for effective therapeutic targeting.
jnk2023 authors. JNK signaling in Alzheimer's disease pathogenesis and therapeutic targeting. Cell Death & Disease. 2023. ↩︎
jnk2022 authors. JNK pathway in Parkinson's disease pathogenesis - molecular mechanisms and therapeutic implications. Molecular Neurodegeneration. 2022. ↩︎ ↩︎
mapk2021 authors. p38 MAPK signaling in neuroinflammation and neurodegeneration. Journal of Neuroinflammation. 2021. ↩︎
Kim EK, Choi EJ. Pathological roles of MAPK signaling pathways in neurodegenerative diseases. Journal of Translational Medicine. 2019. ↩︎
jnk2022a authors. JNK3-mediated neuronal apoptosis in amyotrophic lateral sclerosis. Brain. 2022. ↩︎
Mehan S, Parashar A. JNK signaling in neurodegeneration - therapeutic targets and challenges. CNS Drugs. 2018. ↩︎
microglia2022 authors. p38 MAPK in microglial activation and neuroinflammation - therapeutic potential. Glia. 2022. ↩︎
tau2021 authors. p38 MAPK-mediated tau phosphorylation and aggregation in Alzheimer's disease. Neurobiology of Aging. 2021. ↩︎ ↩︎
activates2020 authors. Amyloid-beta activates JNK signaling pathway leading to synaptic dysfunction. Journal of Alzheimer's Disease. 2020. ↩︎
Gerschutz A, et al. Ionotropic glutamate receptor expression in human brain - comparison of normal and diseased states. Brain Research. 2014. ↩︎
Zhu X, et al. JNK activation in neuronal apoptosis in Alzheimer's disease. Journal of Neural Transmission. 2002. ↩︎
jnk2023a authors. JNK inhibitors in clinical trials for neurodegenerative diseases - current status and future directions. Pharmacological Research. 2023. ↩︎
Mazzanti ML, et al. JNK pathway in dopaminergic neuron degeneration. Neurobiology of Disease. 2009. ↩︎
Cao J, et al. Mutant SOD1 induces JNK-mediated apoptosis in ALS. Neuron. 2004. ↩︎
Hu W, et al. Huntington's disease - JNK pathway dysregulation and therapeutic strategies. Human Molecular Genetics. 2018. ↩︎