Among G protein-coupled receptors, G proteins are the core of the entire signal transduction pathway. According to the different functions they mediate, they are classified into four types: Gs, Gi, G_12/13, and Gq. The signal transduction of D1-like receptors mainly relies on coupling to the Gαs protein. The Gαs protein binds to the downstream adenylate cyclase, promoting the activity of adenylate cyclase. The activated adenylate cyclase catalyzes ATP in the cytoplasm into cAMP. As a second messenger, cAMP binds to downstream kinase proteins such as PKA, activating protein kinases like PKA, thus mediating the transduction of the signal pathway (Channer et al., 2023; Neve et al., 2004). In addition, D1-like receptors can also couple to the Gαq protein to drive the operation of the downstream signal pathway. When D1-like receptors couple to the Gαq protein instead of the Gαs protein, the Gαq protein binds to phospholipase C (PLC) and catalyzes the decomposition of phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate (IP3). IP3 binds to the inositol trisphosphate receptor (IP3R), which is a ubiquitous Ca²⁺-permeable channel that can mediate the release of Ca²⁺ from the endoplasmic reticulum. Then, DAG and Ca²⁺ jointly activate PKC, as shown in Figure 1 (Jackson et al., 2005; Paknejad and Hite, 2018).

Unlike D1-like receptors, the signal transduction of D2-like receptors mainly depends on coupling to the Gαi protein. The Gαi protein binds to the downstream adenylate cyclase to inhibit the activity of adenylate cyclase, thus preventing adenylate cyclase from catalyzing adenosine triphosphate (ATP) in the cytoplasm into cyclic AMP. Interestingly, the activation of PKC by D2-like receptors is mediated by the Gβγ subunit. In the D2-like receptor signal pathway, except for the different subtypes of G proteins, the pathways for activating PKC in the D1-like and D2-like receptor signal pathways are the same. The G_βγ subunit first binds to PLC and then catalyzes the decomposition of PIP2 into DAG and IP3. Since the IP3R is a ubiquitous Ca²⁺-permeable channel, after IP₃ binds to IP₃R, it can mediate the release of Ca²⁺ from the endoplasmic reticulum. Finally, DAG and Ca²⁺ jointly activate PKC, as shown in Figure 1 (Channer et al., 2023; Neve et al., 2004).

In the dopamine receptor signal pathway, when PKC is over-activated, it will affect physiological activities and even lead to diseases. Therefore, in a healthy system, the dopamine receptor pathway dynamically inhibits the activity of PKC to prevent diseases caused by over-activation of PKC. Research has shown that when PKC is activated by PMA, it binds to PDK 1 and undergoes membrane translocation. When dopamine stimulates D₂R, β -arrestin 2 inhibits the binding of PDK1 to PKC, thereby suppressing the translocation and activation of PKC. This process involves GRK 2 and 14-3-3 η. Among them, the phosphorylation of the dopamine receptor by GRK 2 is a key step in inhibiting PKC activation. And 14-3-3 η dissociates PKC from Pyruvate dehydrogenase kinase 1 (PDK1) by inhibiting the phosphorylation of PDK 1-S241. This process belongs to heterologous regulation, as shown on the right side of Figure 2 (Zhang et al., 2020). Another study shows that β-arrestin activates diacylglycerol kinase (DGK), and then DGK mediates the conversion of DAG to phosphatidic acid (PA). Through this pathway, the activity of PKC is inhibited, thus suppressing the regulatory pathways mediated by PKC, this process belongs to homologous regulation, as shown on the left side of Figure 2 (Nelson et al., 2007).

In dopaminergic neurons, dopamine is stored in vesicles at the axon terminals. PKC promotes the fusion of synaptic vesicles with the cell membrane by phosphorylating synaptic proteins such as synapsin and syntaxin, thus enhancing dopamine release. The release of dopamine depends on the phosphorylation modification by PKC (Harsing et al., 2022). Some studies have demonstrated that PKC affects dopamine release by regulating calcium channels and related proteins of synaptic vesicles in presynaptic neurons (Luderman et al., 2015; Shoji-Kasai et al., 2002; Xue et al., 2009). Meanwhile, the release of dopamine relies on the structural change of membrane proteins, which allows Ca²⁺ influx, and the fusion of vesicles with nerve terminals or dendrites, releasing dopamine into the synaptic cleft through exocytosis. In this process, PKC phosphorylates calcium channel proteins, increasing the opening frequency of calcium channels and enhancing the influx of calcium ions, thereby promoting the release of dopamine vesicles (Herlitze et al., 2001). PKC not only regulates the release of vesicular dopamine but also participates in the regulation of non-vesicular dopamine release (Harsing et al., 2022). In addition, when treated with various PKC inhibitors (such as calphostin C, chelerythrine, and Ro31-8220), the amphetamine-induced dopamine release is inhibited, which conversely proves that PKC plays a crucial role in dopamine release (Gnegy, 2003).

The reuptake of dopamine mainly depends on the dopamine transporter (DAT) (Mulvihill, 2019). After dopamine is released into the synaptic cleft through exocytosis, the dopamine transporter is activated to reduce the dopamine concentration in the synaptic cleft. PKC plays a key role in this process (Magee et al., 2021). PKC controls the expression level of DAT on the cell membrane by regulating its internalization and recycling, thus controlling the reuptake of dopamine (Foster and Vaughan, 2017; Julku et al., 2021). The reuptake function of DAT requires the co-regulation of phosphorylation and palmitoylation modifications (Julku et al., 2021). When the phosphorylation of DAT-Ser7 is low and the palmitoylation of DAT-C580 is high, the reuptake ability of DAT for dopamine is enhanced, and the PKC-mediated downregulation of DAT internalization is reduced. Conversely, when the phosphorylation of DAT-Ser7 is high and the palmitoylation of DAT-C580 is low, the reuptake ability of DAT for dopamine is weakened, and the PKC-mediated downregulation of DAT internalization is increased (Moritz et al., 2015). PKC can phosphorylate DAT, promoting its endocytosis and reducing the number of DAT expressed on the membrane surface, thereby decreasing the reuptake rate of dopamine and increasing the dopamine concentration in the synaptic cleft (Luis-Ravelo et al., 2021). In this process, when the amino acid residue at position 547 of DAT is mutated, the PKC-mediated phosphorylation of DAT is inhibited, indicating that the amino acid site at position 547 of DAT is the phosphorylation site of PKC (Quizon et al., 2016). Meanwhile, the PKC-mediated internalization of DAT depends on Ack1 and activated clathrin pits (Underhill and Amara, 2021). Interestingly, the activation of clathrin pits requires activated Cdc42 (cell division cycle 42)-associated tyrosine kinase 1 (Ack1), and PKC needs to inactivate Ack1 and dissociate it from clathrin before mediating the dissociation of DAT-Rit2 to accelerate DAT internalization (Fagan et al., 2020). In this process, the PKC-mediated internalization of DAT depends on its binding to Rit2, and the residues from position 587 to 596 of DAT are the sites that attract Rit2 binding (Fagan et al., 2020). At the same time, the residues from position 587 to 591 of DAT are the essential sites for PKC-induced accelerated internalization of DAT (Boudanova et al., 2008). By stimulating D3R with pramipexole, it was found that under short-term treatment, DAT translocates to the cell membrane under the mediation of PI3K and MAPK to reuptake dopamine in the synaptic cleft; while under long-term treatment, DAT on the cell membrane is phosphorylated and ubiquitinated, thus accelerating its internalization, reducing the expression of DAT on the cell membrane, and reducing the reuptake of dopamine by DAT. This process is PKCβ-dependent, as shown in Figure 3 (Luis-Ravelo et al., 2021). Conversely, when the activity of PKC is inhibited, the PKC-mediated internalization effect of DAT is also inhibited. PKC regulates the presynaptic dopamine release and phosphorylates the DAT to control the reuptake of dopamine, affecting the signal transmission and duration of dopamine, and ultimately influencing the downstream signal transduction of dopamine receptors.

In Parkinson’s disease patients and various animal models, the activity of PKC in the nigrostriatal pathway shows significant abnormalities (Lin et al., 2023; Liu et al., 2023). In the early stage, the PKC activity may increase compensatorily. It attempts to resist the damaging factors such as oxidative stress and mitochondrial dysfunction that neurons are facing, and maintain the normal function of dopaminergic neurons by activating a series of intracellular anti-apoptotic signaling pathways. These include upregulating the expression of the anti-apoptotic protein B-cell lymphoma-2 (Bcl-2), inhibiting the activation of the pro-apoptotic protein Bax, and enhancing mitochondrial stability to reduce the production of reactive oxygen species (ROS) (Roshdy et al., 2024). However, as the disease progresses continuously, the long-term stress stimulation deteriorates the intracellular environment, and the activation pattern of PKC becomes disordered, with over-activation or abnormal translocation. Instead of playing its protective role, it may exacerbate the damage to neurons. For example, abnormally activated PKC may over-phosphorylate certain key proteins, such as tau protein, promoting its aggregation into neurofibrillary tangles, which destroys the cytoskeleton structure of neurons and disrupts axonal transport, thus affecting the normal metabolism and function maintenance of dopaminergic neurons. At the same time, changes in PKC activity will also feedback-regulate the function and expression of dopamine receptors, weakening the efficiency of dopamine signal transduction, further reducing the synthesis and release of dopamine, forming a vicious cycle and accelerating the disease progression (Kim and Kornberg, 2022).

PKC also plays an important role in the pathological process of schizophrenia. Research shows that the activity and expression levels of PKC in regions such as the cerebral cortex and hippocampus of schizophrenia patients are changed, and over-activation of PKC is relatively common (Arnsten, 2011; Pandey et al., 2020). This abnormal activation may stem from the continuous stimulation caused by hyperactivity of the dopamine system. Because the enhancement of dopaminergic signals can increase the production of DAG through the activation of a series of upstream signal molecules, such as PLC, and then over-activate PKC (Dean et al., 1997; Yabuki et al., 2019). The over-activated PKC will further exacerbate the disorder of the dopamine system. For example, PKC phosphorylates the D3 receptor, changing its coupling efficiency with the G protein, weakening the inhibitory effect of the D3 receptor on AC, causing an abnormal increase in intracellular cAMP levels, disrupting intracellular signal homeostasis, forming a vicious cycle, and continuously promoting the progression of the disease, affecting multiple functional dimensions such as the patient’s cognition, emotion, and behavior (Min et al., 2023b; Zeng et al., 2024).

In the AD model of 5XFAD mice, PKCη is specifically enriched in reactive astrocytes in the cortex and hippocampus, and regulates neuroinflammation through a negative feedback mechanism. Mechanistic studies have shown that the PKCη-mTOR-PP2A signaling axis inhibits the NF-κB-mediated inflammatory response through the following pathways: (1) mTORC2 mediates the phosphorylation and activation of PKCη at the T655 site; (2) The activated PKCη enhances the activity of Protein phosphatase 2 (PP2A); (3) PP2A inhibits the nuclear translocation of NF-κB-related proteins by dephosphorylating them, thereby downregulating the transcription of pro-inflammatory factors such as Interleukin-6 (IL-6). This regulatory pathway significantly alleviates the neuroinflammatory response in the AD model (Muraleedharan et al., 2021). Another study has shown that the protein level of PKCα in the brains of AD patients is significantly increased (by approximately 20%), and the phosphorylation level of its substrate synapse-associated protein 97 (SAP97) is increased by fourfold. PKCα remodels the brain phosphoproteome through enhanced catalytic activity (such as the M489V variant) or upregulated expression, leading to synaptic degeneration, a decrease in dendritic spines, and a decline in cognitive function. Its effect is independent of amyloid β-protein (Aβ) and synergizes with Aβ to accelerate the pathological process of AD. This indicates that the excessive activation of the PKCα signaling pathway may be a common feature of AD (Lordén et al., 2022). Meanwhile, some scholars have pointed out that PKC is activated by Aβ oligomers through an integrin β1-dependent pathway, and then phosphorylates the NR2B subunit, increasing its density and function in synapses, resulting in calcium signaling disorders and synaptic damage. This mechanism may manifest as a compensatory response in the early stages of AD, but with the progression of the disease, it ultimately exacerbates neuronal damage (Ortiz-Sanz et al., 2022). Bryostatin-1, as an agonist of PKCδ and PKCε, has entered clinical trials for the treatment of AD due to its favorable effects in animal models. Bryostatin-1 inhibits the accumulation of Aβ by activating PKC, thereby improving the symptoms of AD (Tian et al., 2023).