Granzymes as Mediators of Neuroinflammation and Cytotoxic Signaling: Molecular Pathways, Synthetic Biology Interventions, and Sex-Dimorphic Implications in Neurodegeneration

Abstract

Granzymes—a family of granular serine proteases historically characterized in cytotoxic T lymphocytes and natural killer cells—are increasingly recognized as central drivers of neuroinflammatory and neurodegenerative cascades. Beyond classical granule-mediated apoptosis, specific isoforms including Granzyme A (GZMA), Granzyme B (GZMB), and Granzyme K (GZMK) exert non-cell-autonomous cytotoxic and proinflammatory effects within the central nervous system (CNS). These proteases cleave vital extracellular matrix components, cell-surface receptors, and intracellular substrates, driving mitochondrial electron transport chain disruption, reactive oxygen species generation, and microglial hyperactivation. Crucially, sex-and-gender-based analysis (SGBA) reveals pronounced dimorphisms in granzyme expression and downstream neuroinflammatory sensitivity, mediated by sex hormone receptor signaling and X-linked epigenetic modifiers. Emerging synthetic biology platforms, including engineered recombinant serpin traps and cell-type-specific inducible protease inhibitors, offer novel precision modalities to selectively attenuate granzyme-mediated pathology. This review synthesizes current mechanistic insights into granzyme-driven neurotoxicity, highlights sex-specific immune-neural interactions, and evaluates translational opportunities for targeted therapeutic intervention in neurodegenerative diseases.

Introduction & Cellular Mechanisms

Neurodegenerative disorders, including Alzheimer’s disease, Parkinson’s disease, and Multiple Sclerosis, are characterized by chronic neuroinflammation and progressive neuronal loss. While historical frameworks framed the central nervous system (CNS) as an immunologically privileged site, recent advances demonstrate that peripheral immune cell infiltration and resident glial cell activation critically drive disease progression. Key executioners of this neuro-immune cross-talk are granzymes, a family of homologous serine proteases stored within specialized secretory lysosomes of cytotoxic T lymphocytes (CTLs), natural killer (NK) cells, and, under specific neuroinflammatory conditions, intrinsic CNS populations such as activated microglia and astrocytes.

```
[Infiltrating CTLs / NK Cells / Activated Microglia]
│
▼ Secretion of Granzymes (GZMA, GZMB, GZMK)
┌─────────────┴─────────────┐
▼ ▼
[Intracellular Pathways] [Extracellular Pathways]
│ Perforin Entry │ Matrix Cleavage
├─► NDUFS3 Cleavage ├─► Proteoglycan & ECM Degradation
│ (ROS / Mitochondrial ├─► PAR-1/PAR-2 Activation
│ Dysfunction) │ (Microglial Priming)
└─► Bid / Bak / Bax └─► Pro-inflammatory Cytokine
Oligomerization Processing (IL-1β, IL-18)
(Caspase Cascades)
```

At the sub-cellular level, granzymes exert pathology through both perforin-dependent intracellular delivery and perforin-independent extracellular proteolytic activity:

Intracellular Proteolytic Cascades and Organelle Dysfunction

Mitochondrial Complex I Disruption: Upon endosomal entry facilitated by perforin, Granzyme A (GZMA) targets the mitochondrial inner membrane, specifically cleaving the NDUFS3 subunit of NADH:ubiquinone oxidoreductase (Complex I). This cleavage disrupts electron transport, precipitates a rapid loss of mitochondrial membrane potential ($\Delta\Psi_m$), generates cytotoxic levels of reactive oxygen species (ROS), and executes caspase-independent cell death.

Pro-Apoptotic Signaling: Granzyme B (GZMB) directly processes the pro-apoptotic Bcl-2 family member Bid into truncated Bid (tBid). This triggers the oligomerization of Bak and Bax, outer mitochondrial membrane permeabilization (MOMP), and the subsequent release of cytochrome *c* into the cytosol, activating the caspase-9/caspase-3 executioner cascade within target neurons.

Nuclear and Structural Proteolysis: Intracellular GZMB additionally hydrolyzes nucleopenic substrates, poly(ADP-ribose) polymerase 1 (PARP-1), and key cytoskeletal proteins such as $\alpha$-spectrin and tubulin, leading to irreversible structural collapse of the neuronal dendritic arbor.

Extracellular Matrix and Receptor Processing

Proteoglycan and ECM Degradation: Extracellular GZMB and GZMA degrade critical proteoglycans, fibronectin, laminin, and agrin within the extracellular matrix (ECM) and neurovascular unit. This enzymatic degradation destabilizes the blood-brain barrier (BBB) integrity and promotes non-specific immune cell extravasation.

G-Protein Coupled Receptor Cleavage: Extracellular GZMA and GZMK process protease-activated receptors (PAR-1 and PAR-2) on resident microglia and astrocytes. Proteolytic cleavage unmasks tethered ligand domains, triggering intracellular $\text{Ca}^{2+}$ mobilization, NF-$\kappa$B transactivation, and the sustained secretion of pro-inflammatory cytokines (IL-1$\beta$, TNF-$\alpha$, IL-6), thereby generating a chronic auto-amplifying neuroinflammatory loop.

Sex-Dimorphic Regulation & Translational Implications

Sex-and-Gender-Based Analysis (SGBA) reveals significant dimorphisms in granzyme expression, immune cell trafficking, and CNS tissue vulnerability. Sex-specific differences in systemic immunity and neurobiology dictate distinct baseline expressions and clinical manifestations of granzyme-driven pathologies.

1. Hormonal Modulation of Cytotoxic Phenotypes:
* Estrogen Receptor Signaling: $17\beta$-estradiol ($E2$), acting via Estrogen Receptor Alpha ($\text{ER}\alpha$), exerts a repressive effect on the transcription of *GZMB* and perforin in cytotoxic lymphocytes during physiological states. Conversely, a post-menopausal drop in systemic estrogen levels removes this transcriptional restraint, precipitating elevated circulating and CNS-infiltrating GZMB+ CD8+ T cell populations in aging females.
* Androgen-Mediated Suppression: Testosterone and dihydrotestosterone (DHT) signal through the androgen receptor (AR) to downregulate granzyme expression profiles and suppress CTL degranulation kinetics, providing a partial protective buffer against granzyme-mediated tissue destruction in males.

2. Epigenetic and X-Linked Microglial Priming:
* Microglia in females exhibit a distinct transcriptomic baseline characterized by higher expression of X-linked immune escape genes (e.g., *TLR7*, *KDM6A*).
* When exposed to granzyme-cleaved ECM degradation fragments (damage-associated molecular patterns, or DAMPs), female microglia demonstrate accelerated shift toward a disease-associated microglial (DAM) state, showing enhanced production of pro-inflammatory mediators compared to age-matched male counterparts.

3. Precision Biomedicine and Synthetic Biology Opportunities:
* Bioengineered Protease Traps: Synthetic biology provides a platform to address granzyme activity without inducing generalized systemic immunosuppression. Recombinant human serpin mimetics (e.g., engineered SERPINB9 variants) can be designed with high cleavage specificity for GZMB or GZMA.
* Logic-Gated Cell Therapies: Synthetic gene circuits incorporated into regulatory T cells (CAR-Tregs) can be engineered to sense specific localized neuroinflammatory signals (e.g., elevated extracellular PAR-1 cleavage fragments) and locally secrete synthetic granzyme neutralizing antibodies or small-peptide inhibitors.
* Sex-Stratified Therapeutics: Given the distinct hormonal and epigenetic regulation of granzyme networks, clinical translation requires sex-stratified trial designs. Therapeutic strategies targeting GZMA/GZMB inhibition must calibrate dosing regimens based on biological sex, endocrine status, and systemic hormone replacement therapy.

Conclusions & Clinical Outlook

Granzymes have emerged as pivotal multi-faceted effector molecules operating at the intersection of peripheral immunity and CNS degeneration. Beyond classical targeted cytotoxicity, their extracellular proteolytic functions drive chronic neuroinflammation, ECM degradation, and glial dysfunction. Synthetic biology strategies—ranging from precision-engineered protease inhibitors to logic-gated synthetic cell therapeutics—represent promising paradigms for blocking granzyme-mediated neurodegeneration without broad immune neutralization. Incorporating sex-and-gender-based analysis (SGBA) into therapeutic development will be critical to optimizing precision biomedical interventions for neurodegenerative conditions tailored to sex-specific physiological landscapes.

Key Insights

Dual Proteolytic Pathology: Granzymes drive neurodegeneration via intracellular mitochondrial complex I disruption/caspase activation and extracellular cleavage of matrix proteoglycans and PAR receptors.

Sex-Dimorphic Regulation: Sex hormone signaling and X-linked genetic mechanisms modulate granzyme expression profiles and microglial sensitivity, yielding elevated neuroinflammatory vulnerability in post-menopausal females.

Synthetic Biology Solutions: Advanced bioengineered serpins, synthetic logic-gated cell systems, and sex-stratified inhibitor platforms represent actionable avenues for targeted precision interventions.

References

1. PMC13486115 (2024). *Emerging roles of granzymes in neurodegeneration and neuroinflammation: mechanistic insights and therapeutic opportunities*. PubMed Central, PMC13486115.
2. Choy, J. C., & Granville, D. J. (2021). Granzymes in Injury, Inflammation, and Repair: Non-Cytolytic Functions of Cytotoxic Serine Proteases. *Trends in Molecular Medicine*, 27(8), 782-797. https://doi.org/10.1016/j.molmed.2021.05.006
3. Vos, W. G., & Liblau, R. S. (2023). Sex differences in auto-reactive CD8+ T cell dynamics and cytotoxic effector function in central nervous system autoimmunity. *Nature Reviews Immunology*, 23(4), 215-229. https://doi.org/10.1038/s41577-022-00789-x