Chaperone-Mediated Autophagy in Neurodegenerative Diseases And Acute Neurological Insults in The Central Nervous System Part 3

6. Acute Neurological Insults and CMA

6.1. Traumatic Brain Injury

TBI initiates a cascade of multiple pathophysiological processes, including the degradation pathway of aberrant proteins, such as macroautophagy and UPS [96,97]. These degradation systems are considered to be activated in response to various stress conditions after TBI. Decreasing toxic aberrant proteins via the autophagic process may provide a neuroprotective effect following TBI [98,99]. 

Pathophysiological processes are processes in which abnormal changes occur in an organ or system of the human body, leading to dysfunction. Usually, such changes lead to various problems in the body, but they do not necessarily affect memory.

However, in some pathophysiological processes, memory may indeed be affected. For example, Alzheimer's disease is a common neurodegenerative disease in which patients gradually lose brain function and their memory is severely affected. This is because the disease causes the continuous formation of tangles and deposits of nerve fibers in the brain, which destroy the connection between neurons.

Another problem related to pathophysiological processes is stroke. A stroke is a disease caused by the rupture or blockage of a blood vessel in the brain. In this case, some brain cells die due to a lack of oxygen and blood supply. Similar to Alzheimer's disease, this disease may also affect memory.

However, we should emphasize that most pathophysiological processes do not affect memory. Maintaining a good state of health is one of the important factors in ensuring the normal function of the brain. Whether it is controlling high blood pressure and diabetes, quitting smoking, or eating a healthy diet, these habits can help us maintain functional vitality in our later years and protect our memory.

Finally, we should also talk about the nature of memory. Memory is a very complex function that involves the coordinated work of multiple brain regions. Even if we suffer from a certain pathological physiological process, we can still improve our brain power by actively exercising the brain and training memory. This positive attitude and practice is what we should always keep in mind in life. It can be seen that we need to improve memory, and Cistanche can significantly improve memory because Cistanche can also regulate the balance of neurotransmitters, such as increasing the levels of acetylcholine and growth factors, which are very important for memory and learning. In addition, Cistanche can also improve blood flow and promote oxygen delivery, which can ensure that the brain obtains sufficient nutrition and energy, thereby improving brain vitality and endurance.

Click now to improve brain function

Importantly, a previous study showed that LAMP2A expression increased in neurons and proliferated microglia in a rat model of TBI [16]. In that study, the upregulation of LAMP2A occurred from 3 to 15 days following TBI. 

Another study using a mouse model of TBI also demonstrated that LAMP2A expression was upregulated in the injured brain [100]. These findings suggested that the CMA pathway can be activated in damaged neural tissue after TBI. 

A recent study demonstrated that annexin A1 peptide Ac2-26 activated the CMA process to degrade IKKβ and consequently reduced TNF-α expression in microglial cultures [101]. 

These findings suggest that there is an anti-inflammatory mechanism associated with the CMA process in microglia [101]. Interestingly, silent information regulator 1 (Sirt1) activated CMA by upregulating DnaJ heat-shock protein family member B1 (Dnajb1) expression and consequently attenuated astrocyte activation and neuronal loss after TBI in mice [100]. 

Taken together, these findings suggest that the activation of the CMA pathway following TBI might exert a neuroprotective effect of attenuating inflammatory reactions and reducing neural tissue damage in the injured brain [100]. 

However, the actual function of CMA in TBI remains largely unknown. Further studies will thus be needed to elucidate the pathophysiological and neuroprotective mechanisms of CMA following TBI.

6.2. Cerebral Ischemia

Ischemic cerebral stroke is one of the leading causes of death and morbidity in humans. Previous studies have suggested that over-activation of autophagic pathways exerts a neuroprotective effect in ischemic brain injury [102,103]. 

Hsc70 and Hsp40 are reported to be synergistically expressed in the neurons of vulnerable areas in response to sub-lethal ischemia [104]. The combination of Hsc70 and Hsp40 suppresses aggregate formation and apoptosis in neurons [105].

Another study showed that the upregulation of LAMP-2A expression and the accumulation of LAMP-2A-positive lysosomes were induced under ischemic conditions in neuronal cells in vitro [17]. In an animal model of cerebral ischemia, LAMP-2A expression was slightly decreased until two days after ischemia and then the level increased significantly seven days after ischemia [17]. 

These findings suggest that CMA may be activated under ischemic conditions in the brain and may facilitate neuronal survival. Blocking LAMP-2A expression with siRNA increased neuronal cell death after brain ischemia. [17]. 

In addition, the administration of mycophenolic acid, a potent CMA activator, rescued hypoxia-mediated cell death in a brain ischemia model. Furthermore, a membrane-permeable peptide that specifically binds to cyclin-dependent kinase 5 (CDK5) with a CMA targeting motif (Tat-CDK5-CTM) can promote the degradation of CDK5, reducing neuronal cell death [106]. 

In addition, Tat-CDK5-CTM also reduced the infarction area and neuronal loss and improved the neurological functions in a cerebral infarction mouse model [106]. Taken together, these findings suggest that promoting CMA activity may lead to the acceleration of the removal of damaged protein, thereby contributing to the survival of neurons after cerebral ischemia.

6.3. Spinal Cord Injury

Degradation of dysfunctional intracellular components via the autophagic process is a crucial step in maintaining cellular homeostasis in response to various forms of stress, including nutrient deprivation, hypoxia, reactive oxygen species, DNA damage, and endoplasmic reticulum (ER) stress [15,99,107,108]. 

Many previous studies have provided experimental evidence that autophagy is an essential cytoprotective pathway for reducing secondary neural tissue damage and functional impairment after SCI [99,109–111]. 

We previously reported that LAMP2A protein expression was significantly upregulated in damaged neural tissue after SCI in mice [18]. The expression of LAMP2A was increased in various neural cells, such as neurons, astrocytes, oligodendrocytes, and microglia, in the injured spinal cord [18]. These results indicated that CMA was activated in damaged neural tissue following SCI. 

Interestingly, our results also showed that the number of LAMP2A-expressing cells increased from 24 h and peaked at 3 days, lasting for at least 7 days after injury. The time course of LAMP2A expression is similar to that of apoptosis after SCI [112–114]. 

Apoptosis is considered a major cause of secondary damage following SCI [112,114]. Therefore, CMA activity might be regulated in response to secondary neural tissue damage. 

A previous study showed that histone deacetylase-6 (HDAC6) has a molecular function of inducing Hsp90 deacetylation and increasing the interaction between LAMP2A and Hsp90, thereby upregulating CMA activity [115]. 

Another study showed that a deficiency in HDAC6 hindered CMA activity to resist oxidative stress in vitro [116]. In addition, inhibition of HDAC6 accelerated reactive oxygen species (ROS) generation and neuronal apoptosis in response to hypoxia-ischemia [116]. 

Importantly, both HDAC6 and LAMP2A expressions are upregulated in a mouse model of SCI [116]. Taken together, HDAC6 may have an important role in the regulation of CMA activity and may be a potential therapeutic target for the effective treatment of SCI. 

Further studies will be needed to clarify the pathophysiological and cytoprotective mechanisms of CMA after SCI. In summary, previous studies have shown evidence that CMA activity can be upregulated in damaged neural tissue following various types of acute neurological insults, such as cerebral infarction [17], TBI [16], and SCI [18]. 

Therefore, the CMA pathway may play an important biological role not only in neurogenerative diseases but also in acute neurological insults to the CNS.

7. Therapeutic Potential of CMA for Neurodegenerative Diseases

Major neurodegenerative diseases are generally caused by the accumulation of aberrant proteins, as described above. Aberrant proteins, such as α-synuclein and LRRK2 in PD, RCAN1 and Tau protein in AD, Htt in HD, and TDP-43 in ALS and FTLD, are the substrates of CMA [4,14]. Thus, the upregulation of CMA activity has therapeutic potential for treating neurodegenerative diseases caused by misfolded proteins [15]. 

As a therapeutic approach, CMA activity can be modulated by various molecular mechanisms, such as changing the LAMP2A level in lysosomes, changing the Hsc70 level, and changing the condition of the KFERQ-like motif. Many studies have suggested that enhancing LAMP2A expression to upregulate the activity of CMA can be an important therapeutic target. 

A previous study demonstrated that recombinant adeno-associated virus augmenting the LAMP2A level protected dopaminergic neurons in the substantia nigra from α-synuclein-induced degeneration [117]. 

In addition, it has also been reported that various compounds, such as geldanamycin [118], 6-aminonicotinamide [119], glucose-6-phosphate dehydrogenase inhibitor [119], silymarin [120], chronic caffeine [121], manganese [122], trehalose [123], b-asarone [124], and other compounds extracted from natural medicinal plants [125], or even combination treatments with bortezomib and suberoylanilide hydroxamic acid (SAHA) [126], can increase LAMP2A levels and activate the CMA pathway. 

However, these compounds are not able to specifically regulate the CMA pathway and have many other targets. Thus, it is important to develop selective CMA modulators that can be used to manage human diseases. Recent studies have revealed a novel molecular mechanism involving the effect of deacetylase and methyltransferase enzymes on the activity of chaperones in the CMA process. 

It was also reported that histone deacetylase 10 (HDAC10) deacetylates Hsc70 and upregulates the CMA pathway in vitro [127]. In addition, HDAC10 knock-out in cells results in the accumulation of LAMP2A-positive lysosomes around the nucleus, activating CMA to degrade a well-known CMA substrate, GAPDH [128]. These findings suggest therapeutic potential in the regulation of Hsc chaperones for CMA activation. 

Another therapeutic approach involves the modification of the condition of the KFERQ-like motif of pathological proteins to make them suitable for degradation via the CMA pathway. A recent study showed that tagging amyloid-β oligomers with multiple KFERQ motifs promoted their entering endosomes and lysosomes, thereby protecting human primary cultured cortical neurons from neurotoxicity [82]. 

In addition, the use of an adaptor containing two copies of polyQ binding sequences and two different KFERQ motifs specifically directed mutant Htt to CMA degradation, ameliorating symptoms in an HD disease model [62]. An artificial peptide containing two CMA recognition motifs fused to two copies of the polyglutamine-binding peptide 1 (QPB1) sequence enables Htt to be degraded by CMA, ameliorating Htt aggregation and toxicity [62]. 

Interestingly, a novel antibody containing a KFERQ-like motif was able to recognize TDP-43 and targeted it to lysosomes for CMA degradation [129]. These findings suggest that modification of the condition of the KFERQ-like motif of aberrant proteins may be a new therapeutic strategy for treating neurodegenerative diseases. The chemical enhancement of CMA can protect cells from oxidative stress and proteotoxicity. 

Signaling through retinoic acid receptor alpha (RARα) inhibits CMA activity. Synthetic derivatives of all-trans-retinoic acid can specifically neutralize this inhibitory effect [32]. Recently, it was also reported that human has molecular functions to antagonize endogenous CMA inhibitors and promote interaction between the CMA chaperone Hsp90 and the CMA receptor LAMP2A. 

Humanin and its analogs can enhance the CMA pathway by increasing substrate binding and translocation into lysosomes and exerting cytoprotective effects against hypoxia-induced cell death [130]. Another study found that metformin, a drug commonly prescribed for type 2 diabetes, can activate the CMA pathway and prevent the accumulation of amyloid-β plaque in an animal model of AD [85]. 

Different protein degradation systems are wired to maintain cellular proteostasis under various physiological and pathological conditions. Protein degradation via CMA is achieved through the lysosome-based autophagy system and therefore interacts with macroautophagy and UPS [4,40,131]. Thus, a therapeutic approach that activates the CMA, macroautophagy, and UPS pathways should provide complementary or synergistic effects in restoring protein homeostasis [40]. 

However, the molecular mechanism involved in the interplay between these different protein degradation pathways has not been fully elucidated. Exploring the mechanisms underlying the cross-talk between CMA, macroautophagy, and UPS may facilitate the development of an effective therapeutic strategy to restore proteostasis in various neurodegenerative diseases.

8. Therapeutic Potential of CMA for Acute Neurological Insults

Following acute neurological insults to the CNS, including cerebral infarction, TBI, and SCI, secondary injury can be induced by various molecular mechanisms, such as oxidative stress and neuroinflammation in the brain and spinal cord [132,133]. Such secondary injury is involved in multiple pathologies associated with neural cell death and neurodegeneration, aggravating the initial tissue damage of the CNS [112,113,132]. 

The secondary damage can be a potential therapeutic target for the effective treatment of acute neurological insults to the CNS. Many previous studies have shown that activation of the autophagic process can exert a neuroprotective effect against secondary damage after acute CNS injury [99,134]. Notably, several studies have suggested that the upregulation of CMA activity may help reduce secondary neural tissue damage following acute neurological insults to the CNS [6]. 

As mentioned above, mycophenolic acid administered to activate the CMA pathway rescued hypoxia-mediated cell death after brain ischemia in an in vitro model [17]. In addition, Tat-CDK5-CTM increases the CMA degradation of CDK5, reducing the infarction area and neuronal loss and improving the neurological functions in a mouse model of cerebral infarction [106]. 

Furthermore, HDAC6 can regulate Hsp90 acetylation to enhance CMA activity and exert a neuroprotective effect after SCI in mice [116]. The upregulation of Dnajb1 expression induced by Sirt1 activated CMA and consequently reduced neuronal loss in a mouse model of TBI [100]. 

Therefore, enhancing the CMA pathway to remove toxic proteins may be a novel therapeutic approach to reduce secondary neural tissue damage after acute neurological insults. Acute neurological insults in the CNS damage different types of neural cells, such as neurons, oligodendrocytes, astrocytes, and microglia. Such damage to these neural cells causes complex pathophysiological processes, including extensive neuronal cell loss, axonal injury, demyelination, and destruction of the blood-brain/spinal cord barrier [132,133]. 

Importantly, the activity of CMA is increased not only in neurons but also in microglia at the lesion site after TBI and SCI [16,18]. Microglia play various important roles in neuroprotection and neuroinflammation following acute CNS injury [135–137]. 

As described above, annexin A1 peptide enhances the CMA activity to degrade IKKβ and consequently reduces the TNF-α expression in microglia, suggesting an anti-inflammatory mechanism associated with CMA [101]. In addition, the activity of the CMA pathway is also upregulated in astrocytes and oligodendrocytes after SCI in mice [18]. 

CMA activation has been shown to reduce α-synuclein accumulation in astrocytes and oligodendrocytes in vitro [138,139]. The reduction in α-synuclein aggregation in the injured spinal cord has been reported to provide neuroprotective effects, attenuating axonal damage, neuronal loss, and neuroinflammation following SCI [140]. 

Previous studies have also suggested that autophagic activity contributes to the survival of oligodendrocytes and the prevention of myelin loss after SCI [141]. Taken together, modulation of CMA activity in various types of glial cells may affect multiple pathophysiological processes following acute neurological insults in the CNS. 

It is important to determine the molecular mechanisms underlying the interaction between CMA and various pathologies in the damaged CNS. Many studies have suggested that depositions of aberrant proteins, such as amyloid-β and Tau protein, are observed in the brains of patients after TBI [142]. The pathological accumulation of aberrant proteins after TBI can be a major risk factor for several progressive neurodegenerative diseases, such as AD and PD [142,143]. 

The aggregation of amyloid-β is accelerated in injured brains, and amyloid-β plaques can be a pathological cause of neurodegenerative diseases in chronic-stage TBI [144,145]. TBI can also reportedly induce the aggregation of Tau proteins, which is a common feature of several neurodegenerative disorders [146]. Importantly, enhancement of the CMA pathway can decrease the accumulation of amyloid-β and Tau proteins in the brain [4,6,85,117,147]. Thus, the upregulation of CMA may aid in removing toxic aberrant proteins causing late-onset neurodegeneration after TBI.

9. Concluding Remarks and Future Perspectives

In the past decade, the regulatory mechanisms involved in the CMA degradation pathway have become clearer, expanding our understanding of the importance of CMA in cellular functions [4,6,8]. 

There is increasing evidence that CMA dysfunction is associated with different pathologies in neurodegenerative diseases in the CNS [1,4,6,14,15]. Important pathogenic proteins have been identified as the substrates of CMA, such as α-synuclein in PD [60], Tau protein in AD [61], huntingtin (Htt) in HD [62,63], and TDP-43 in ALS and FTLD [64,65]. 

However, most previous studies related to CMA in the CNS have focused on neurodegenerative diseases rather than acute neurological insults, such as TBI and SCI [6,14]. 

The CMA function in acute neurological insults in the CNS is still an immature research field and limited evidence has been published thus far. As mentioned above, CMA activity is likely to be upregulated in damaged neural tissues after acute CNS injury [16,18,100,116]. The actual function of CMA activation following acute injury of the brain and spinal cord remains unknown. Therefore, further studies will be necessary to assess the possible association of CMA with acute neurological insults in the CNS.

Various compounds have been reported to increase LAMP2A levels and activate the CMA pathway [118–124], as described above. However, these compounds cannot selectively regulate the CMA pathway. 

Therefore, it is important to develop selective CMA modulators that can be used for clinical treatment of human diseases [4]. The development of pharmacological selective CMA modulators will be a crucial step towards the implementation of therapeutic strategies aimed at improving cellular homeostasis through the regulation of CMA in the CNS. 

Several currently available FDA-approved drugs and natural products have been found to promote CMA activity [85,121,148,149]. These drugs and products that enhance CMA might be able to be translated into novel clinical applications. Clinical trials involving autophagy as a therapeutic target for neurodegenerative diseases have focused on macroautophagy, not CMA [150,151]. 

No clinical trial has yet targeted CMA for the treatment of any neurodegenerative diseases. It will be important to develop new drugs that can selectively modulate CMA in target organs to maximize the therapeutic effect and minimize toxicity in clinical use. Modulation of the CMA pathway may be promising for developing novel treatments for neurodegenerative diseases as well as acute neurological insults in the CNS. However, this area of research remains largely unexplored. 

Further efforts are needed to clarify the actual biological function of CMA in various pathophysiological processes in the brain and spinal cord. It is also important to elucidate the interaction between CMA and other protein degradation systems. Future research on these issues will aid in the development of novel clinical applications of CMA for the treatment of neurodegenerative diseases and acute neurological insults in the CNS.

Author Contributions: Conceptualization, H.K. and K.H.; writing-original draft preparation, H.K. and K.H.; writing-review and editing, H.K. and K.H.; visualization, H.K., K.H., and T.M.; supervision, T.A. and H.O. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Li, W.; Nie, T.; Xu, H.; Yang, J.; Yang, Q.; Mao, Z. Chaperone-mediated autophagy: Advances from bench to bedside. Neurobiol. Dis. 2019, 122, 41–48. [CrossRef] [PubMed] 

2. Amanullah, A.; Upadhyay, A.; Joshi, V.; Mishra, R.; Jana, N.R.; Mishra, A. Progressing neurobiological strategies against proteostasis failure: Challenges in neurodegeneration. Prog. Neurobiol. 2017, 159, 1–38. [CrossRef] [PubMed] 

3. Hekmatimoghaddam, S.; Zare-Khormizi, M.R.; Pourrajab, F. Underlying mechanisms and chemical/biochemical therapeutic approaches to ameliorate protein misfolding neurodegenerative diseases. BioFactors 2016, 43, 737–759. [CrossRef] 

4. Auzmendi-Iriarte, J.; Matheu, A. Impact of Chaperone-Mediated Autophagy in Brain Aging: Neurodegenerative Diseases and Glioblastoma. Front. Aging Neurosci. 2021, 12, 630743. [CrossRef] [PubMed] 

5. Mizushima, N.; Levine, B.; Cuervo, A.M.; Klionsky, D.J. Autophagy fights disease through cellular self-digestion. Nature 2008, 451, 1069–1075. [CrossRef] [PubMed] 

6. Kaushik, S.; Cuervo, A.M. The coming of age of chaperone-mediated autophagy. Nat. Rev. Mol. Cell Biol. 2018, 19, 365–381. [CrossRef] [PubMed] 

7. Chiang, H.-L.; Terlecky, S.R.; Plant, C.P.; Dice, J.F. A role for a 70-kilodalton heat shock protein in lysosomal degradation of intracellular proteins. Science 1989, 246, 382–385. [CrossRef] [PubMed] 

8. Cuervo, A.M.; Wong, E. Chaperone-mediated autophagy: Roles in disease and aging. Cell Res. 2013, 24, 92–104. [CrossRef] 

9. Dice, J.F. Chaperone-mediated autophagy. Autophagy 2007, 3, 295–299. [CrossRef] [PubMed] 

10. Cuervo, A.N.A.M.; Knecht, E.; Terlecky, S.R.; Dice, J.F. Activation of a selective pathway of lysosomal proteolysis in rat liver by prolonged starvation. Am. J. Physiol. Physiol. 1995, 269, C1200–C1208. [CrossRef] [PubMed] 

11. Hubbi, M.E.; Hu, H.; Kshitiz, F.; Ahmed, I.; Levchenko, A.; Semenza, G.L. Chaperone-mediated Autophagy Targets Hypoxia-inducible Factor-1α (HIF-1α) for Lysosomal Degradation. J. Biol. Chem. 2013, 288, 10703–10714. [CrossRef] 

12. Kiffin, R.; Christian, C.; Knecht, E.; Cuervo, A.M. Activation of Chaperone-mediated Autophagy during Oxidative Stress. Mol. Biol. Cell 2004, 15, 4829–4840. [CrossRef] [PubMed]

For more information:1950477648nn@gmail.com