The family of Hsp70 includes more than 10 members (Table 1). Generally, they enable adequate folding for both newly synthesized or mature proteins as well as refolding for denaturated/aggregated proteins (Hartl and Hayer-Hartl, 2002). In addition, some constitutively expressed or induced members of the family possess relatively specific functions, such as regulation of apoptosis, mitochondrial function (mtHsp70), and metabolic pathways (Lackie et al., 2017). However, Hsp70 are specified to ensure folding via ATP-dependent machinery, preventing denatured proteins from aggregating (Jores et al., 2018).

Heat shock proteins are found in almost all cellular compartments, including the nucleus and cytoplasm (Hsc70), as well as in mitochondria (Wentink et al., 2020). Moreover, Hsc70 may be found in a lysosome-specific isoform (Ciechanover and Kwon, 2017). General protective properties of Hsp70 determine both pro- and eukaryotic biology, and even plants express their own Hsp70 analogs (Chaudhary et al., 2019). However, some features are shared by principal chaperones of the group for mammals, including humans, mice, and rats (Figure 1). Hsc70, which is the most common one among the Hsp70 in healthy and undamaged state, has the largest number of unique interacting proteins among all the Hsp70 family members (Rai et al., 2021).

Hsp70 expression is controlled by heat shock factor 1 (HSF-1), which receives downstream signals from a number of stimuli, such as pathway of extracellular signal-regulated kinases and mitogen-activated protein-kinases (ERK/MAPK), highly influenced by increased air pressure and temperature (Matsathit et al., 2016). HSF-1 is also a thermosensor whose leucin zipper domains change in temperature bounces. Moreover, Hsp70-induced feedback permits to monomerize trimers of HSF-1 and separate it from DNA molecules (Kmiecik and Mayer, 2022).

The activity of Hsp70 is driven by small Hsp with a molecular weight of 40 kDa (Hsp40/DnaJ family), especially DnaJB11 and DnaJC5 (Braun, 2023), which are relevant for some types of tauopathies (Kampinga and Craig, 2010; Zhang et al., 2023, M). Although the activity of Hsp70 is firstly dependent on its expression, some enzymes also may introduce modifications, regulating Hsp70 in kinase dependent manner. For instance, it has been reported that cyclin-dependent kinase 1 (Cdk1) is able to temporarily downregulate chaperonic functions of Hsp70 by phosphorylation of serine in a region between ATP- and substrate-binding domains (Kao et al., 2020). Interestingly, in addition to de novo synthesis of Hsp70, neurons tend to uptake it from neighboring astrocytes (Tytell et al., 1986; Hightower and Guidon, 1989; Guzhova et al., 1998, 2001; Kalmar and Greensmith, 2017).

Hsp70 play an important role in the nervous system in health and in disease. It is especially remarkable for aging brain. The aging itself is a crucial risk factor as any senescent cell exhibits a lower chaperone protein translation while its markers for labeling of misfolded proteins are not expressed well (Llewellyn et al., 2023). For instance, Hsp70 production falls by 50% in aged rat liver cells in stress (Heydari et al., 1994). Basal Hsp70 content in cells stays higher in long-living animals (de Toda et al., 2016). Furthermore, co-chaperones of Hsp70 in mice are expressed less in development of age-dependent neurodegeneration (Lackie et al., 2020). Higher levels of oxidant and lower ones of antioxidant actors may aggravate the state of chaperone action in aged individuals (Martínez de Toda and De la Fuente, 2015).

Despite these facts, not all the heat shock proteins decrease at the same rate as Hsp70 do in aging. Specifically, ATP-recruiting Hsp (foldases) experience a strong hypoexpression, while ATP-independent (holdases) continue to be actively synthesized (de Graff et al., 2020). As a result, proteostasis and preventing of aggregation suffers less than the protection of nascent proteins. Sirtuins, which are well-known for their controlling role in aging regulation, are probably responsible for proper Hsp70 expression after the stimulation by HSF-1. Common in young age, sirtuin 1 has been evidenced to potentiate binding of HSF-1 to DNA, thereby modifying levels of Hsp70 synthesis (Karvinen et al., 2016).

Alzheimer’s disease (AD), known as the most widespread ND, mainly affects memory and other cognitive functions related to the synaptic loss and deposition of neurofibrillary tangles (NFT) and peptide plaques (Lane et al., 2018). The plaques are principally formed by amyloid beta (Aβ; Abeta) deriving from Aβ precursor (APP) after enzyme-mediated cleavage, especially under presenilin-1 (PS1; PSEN1) action (Russo et al., 2000; Rajesh and Kanneganti, 2022). Despite being debatable, this Aβ hypothesis of AD is still considered to be generally accepted (Breijyeh and Karaman, 2020). Recent studies also claim soluble Aβ oligomers (AβOs) to have more harmful impact than their insoluble counterparts. For instance, excitotoxicity and abnormal long-term potentiation are already seen at very early stages of AD before detectable plaques (Huang and Liu, 2020). In fact, multifaceted pathways of AD imply numerous links described in detail in excellent reviews (Perrin et al., 2009).

Elevated levels of Hsp70 are found in AD, whereas the chaperones obviously attenuate the disease (Romi et al., 2011). For example, protein aggregation and neuronal death together with AD-like symptoms, caused by the use of paraquat, a popular herbicide in many countries, are accompanied by a reduced Hsp70 expression (Moyano et al., 2021). On the other hand, an upregulation of Hsp70 has correlated with a decline in Aβ content (Sun et al., 2017).

Excessive endogenous Hsp70 increases the efficiency of Aβ-degrading enzymes (Hoshino et al., 2011). Rivera and coll. Have observed Hsp70 affecting the Aβ assembling process in vitro preventing oligomer formation. Moreover, the presence of Hsp70 reduced the Aβ peptide-induced toxicity of cultured neurons (Rivera et al., 2018). In drosophila genetic models, hyper-expressing secreted Hsp70 (secHsp70) together with mutant Aβ42, reveal that the extracellular Hsp70 only provided a protective action (De Mena et al., 2017). Some studies have found a similar pattern of Hsp70 impacts for both localizations. Thus, the potential for cognitive protection remained the same for both extra- and intracellular Hsp70 for drosophilae with Aβ accumulation regardless of their age or exposure (Martín-Peña et al., 2018).

Cytoskeletal instability as a result of the production of impaired tau protein is another important component of AD pathology (Choi et al., 2020). It is generally accepted that an increased activity of tau kinases, especially of glycogen-synthase kinase 3 (GSK-3), extracellular signal-regulated kinase (ERK), and p38, leads to the hyperphosphorylation of tau (Hartz et al., 2023). As a result, tau molecules aggregate into double threads forming a net of NFT, that further intensifies the Aβ accumulation (Uematsu et al., 2018). It is shown that valosin-containing protein (p97; VCP) can eliminate the NFT with the help of Hsp70 (Saha et al., 2023). Overexpressed Hsp70 enables degradation or dephosphorylation of pathological tau improving stability of microtubules (Lackie et al., 2017).

Despite all the positive effects, at the late stages of cellular pathology Hsp70 loses its protective functions and forms epichaperomes representing inert long-living scaffolds (Bolaender et al., 2021). This phenomenon enables protein connectivity-based dysfunction and might aggravate the neurodegeneration (Inda et al., 2020). Additionally, Aβ and tau suppressive activity of Hsp70 has been shown to be abolished in the presence of ε4 isoform of apolipoprotein E (ApoE), one of the most recognizable factors contributing to AD (Osorio et al., 2007; Serrano-Pozo et al., 2021). Finally, although the subchronic intranasal administration of human Hsp70 has improved the course of disease in familial AD murine models, innate immunity and antigen presentation have been upregulated at the same time (Evgen’ev et al., 2019).

Parkinson’s disease (PD) is a common neurodegeneration, clinically displayed in bradykinesia, postural instability, rigidity, and tremor (Ascherio and Schwarzschild, 2016). The disorder is caused by a progressive death of dopaminergic neurons in the substantia nigra (Beal, 2010). The hallmark of the disease required to confirm the diagnosis is intracellular inclusions called Lewy bodies (LB) (Spillantini et al., 1997; Sipilä et al., 2023). LB are composed of ubiquitinated abnormal protein called α-synuclein (α-Syn) (George, 2002). In addition to α-Syn, some other causative players have been shown to contribute to the disease: Parkin, encoded by PARK2 gene; phosphatase and tensin homolog-induced kinase 1, encoded by PINK1 or PARK6; protein deglycase J-1 (DJ-1), encoded by PARK7; and leucine-rich repeat kinase 2 (LRRK2), encoded by PARK8 (Poewe et al., 2017).

In PD, Hsp70 overexpression is common in damaged cells, generally in surviving neurons (Dickson, 2018; Jellinger, 2019). Consistently, Drosophila models have disclosed Hsp70 to delay death of dopaminergic cells (Auluck and Bonini, 2002). On one hand, the benefit of Hsp70 might be explained by its ability to enhance autophagy (Moors et al., 2017; McKinnon et al., 2020). Nonetheless, mitophagy and elimination of Parkin also require HspA1 presence, and of HspA1A mainly, as HspA1L potential is weaker (Hasson et al., 2013). However, Hsp70 machinery in PD is not acting alone, but strongly depends on Hsp40, Hsp90, and Hsp70-Hsp90-organizing protein (HOP) (Ebrahimi-Fakhari et al., 2012; Wu et al., 2019; Hu et al., 2021). Moreover, small molecular chaperones, e. g. clusterin (ApoJ), are probably more prominent to manage α-Syn aggregation in PD (Lenzi et al., 2020).

Huntington’s disease (HD) is a complex motor (choreal signs are rather common), cognitive, and mental neurodegenerative disorder, which is inherited via an autosomal dominant pattern (Joshi et al., 2021). HD is the most common representative of a wide range of polyglutamine-associated diseases (polyQ), including also spinocerebellar ataxias and dentatorubral-pallidoluysian atrophy (Carroll et al., 2018). All of them imply a translation of polyQ-chains forming Htt protein after posttranslational modification (Bates et al., 2015). Htt aggregates in essentially all neuronal and astrocytic compartments (Joshi et al., 2021; Lange et al., 2023).

Hsp70 may promote the collapse of polyQ chains (Choudhury et al., 2016; Davis et al., 2020; Gupta et al., 2020). In insects, Hsc70 is responsible for the effect (Rai and Tapadia, 2022). However, there is a lack of data supporting active Hsp70 engagement in the treatment of these disorders in mammals (Pratt et al., 2015; Reis et al., 2017). Maheshwari and coll. Have tried steroid hormones to manage Hsp70 machinery in HD, but no specific action is shown (Maheshwari et al., 2014). Some studies propose an anti-apoptotic role of Hsp70 in HD (Sabirzhanov et al., 2012). Besides, an indirect explanation of Hsp70 relevance in HD is given by the fact that progenitor nerve cells express more Hsp40 (co-chaperone of Hsp70) than mature neurons, whereas polyQ-associated disorders usually manifest in adult persons and not in childhood (Thiruvalluvan et al., 2020). Perhaps, the Hsp40 is the key factor to trigger polyQ removal.

Amyotrophic lateral sclerosis (ALS) is a progressive motor neuronal disorder (Amico and Antel, 1981). Despite a long history of studies, the interactions between its key actors are still not completely clear (Al-Chalabi et al., 2017). Superoxide dismutase 1 (SOD-1) is often considered to be crucial in pathology (Cleveland, 1999). However, products of fused-in-sarcoma (FUS) gene are important contributors to protein aggregation, and such faulty proteins as transactive response DNA binding protein 43 kDa (TDP-43), C9orf72, and ubiquilin-2 are also involved (Thomas et al., 2013). Moreover, there is a second row of proteins, which are able to aggravate the course of the disorder. So, NF-kappa-B activator-binding kinase 1 of TNF receptor-associated factor’s family member (TBK1), optineurin, and p62 participate in impaired autophagy, while vesicle-associated membrane protein-associated protein B/C (VAP-B) is typical for proteasomal failure. Although, VCP takes part in both machineries (Kalmar and Greensmith, 2017).

ALS and frontotemporal dementia (FTD) may be regarded together because of similar genomics, proteomics, metabolomics, and transcriptomics (Menéndez-González, 2023). Mandrioli et al. (2019) also mention that both diseases are accompanied by neuroinflammation with inflammasomes. FTD is a group of similar threatening and rapidly progressing cognitive disorders with a rather high mortality (Wang et al., 2022). The pathology comprises a storage of pathological tau, Fus, and TDP-43 with predominant harm to the neurons in the frontal and temporal cortex (Josephs et al., 2011). Furthermore, repeated sequences of G4C2 in C9orf72 genes are notorious for the synthesis of dipeptide repeat proteins (DPR) or poly-Gly-Ala (poly-GA), tending to aggregate (Lee et al., 2023). Interestingly, DPR complexes are able to be transported to neighboring cells, thereby hindering their clearance (Khosravi et al., 2020).

Hsp70 is found to assist the elimination of the abnormal proteins, especially in the DPR-related pathology (Deng et al., 2011). Generally, in ALS and FTD Hsp70 act via either autophagy to prevent aggregation of RNA-binding proteins (Mandrioli et al., 2019) or proteasomal machinery to fix DPR-induced damage. In proteasomes, ubiquilin-2 provides a rapid Hsp70 binding to poly-GA (Renaud et al., 2019). The intermediate region of ubiquilin-2 molecule includes a flexible proline-X-X-proline (PXXP) motif, which is perhaps accessible for concurrent catalysis or inhibition (Zhang et al., 2021). Meanwhile, these RNA-binding proteins comprise TDP-43 and Fus, and may be cleaved by such kinds of autophagy as chaperone-mediated autophagy and aggrephagy (Thomas et al., 2013). Aggrephagy implies a consumption of labeled stress granules from aberrant RNA (Ripin and Parker, 2022). Interestingly, TDP-43 and ubiquilin-2 seem to be tightly interacting despite their different roles in proteasomal machinery and autophagy (Seelaar et al., 2007; Wang et al., 2008).

Heat shock factors are first possible tools to rule Hsp70 in NDs (Verma et al., 2014). So, HSF-1 takes part in numerous pathways, thereby being a key point of regulation (Kim et al., 2017). In particular, HSF-1 is trimerized to bind a sequence called the heat shock element in Hsp70 gene promotors (Kondo et al., 2013). However, an excessive production of HSF-1 in cerebellar neurons has decreased Htt-related toxicity in rats not via a direct Hsp70 upregulation, but via such coactor as BAG-3 (Verma et al., 2014). In Hsp70-knockout (ko) mice, increased HSF-1 levels have also improved the number of dopaminergic neurons in treatment by U-133, a compound derived from sea urchins (Ekimova et al., 2018). Independently of direct or indirect Hsp70 engagement, geranylgeranyl acetone (GGA; teprenone) mitigates neuronal damage via HSF-1 activation through the ERK/p38 MAPK pathway (Sun et al., 2017) (Table 3).

Multivector drugs, that affect different Hsp groups, attract a lot of interest. These are mainly geldanamycin and its derivatives, especially 17-AAG, 17-DMAG, IPI-504, and radicicol, which work as Hsp90-inhibitors and HSF-1-mediated Hsp70 inducers at the same time (Alam et al., 2017). Although, a predominance of Hsp70 or Hsp90 is still debatable (Rutledge et al., 2022). Several mechanisms of neuronal action can also be combined in a single medication. Thus, riluzole, which is known as anti-ALS basic treatment, is officially described by its producers as an antagonist of glutamate-associated excitotoxicity, but its effects are also provided by HSF-1 upregulation (Miller et al., 2012; Petri et al., 2023). Yang and coll. Report that riluzole may also enable Htt cleavage (Yang et al., 2008). Actually, there is no consensus on dynamics of riluzole action in literature.

Furthermore, some well-known medications may take part in the Hsp70-associated management of NDs. For instance, colchicine is actively tested together with riluzole due to its upregulation of HspB8 expression and blocking of TDP-43 accumulation (Mandrioli et al., 2019). Konturek et al. (2005) also studied pioglitazone and discovered its ability to improve Hsp70 content in the pancreas. However, a concomitant intake of pioglitazone and riluzole exhibits no clinical benefit in trials for ALS (Dupuis et al., 2012). In addition, fenofibrate causes a synchronous decrease in Hsp90 levels and elevation of Hsp70 levels, and cognitive dysfunction regresses in a rat model of dementia (Rizk et al., 2022). However, valproic acid enhances Hsp70 levels with no impact on Hsp90-machinery. Finally, carbenoxolone is efficient in proteasomal cleavage of aggregates via HSF-1 upregulation in PD rat models in the presence of rotenone (Thakur and Nehru, 2014).

Some Hsp70 enhancers with no clear mechanism found are suggested. For example, sleep deprivation-induced learning/memory impairment has been shown to recover after intracerebroventricular luteolin administration, while an increase in Hsp70 levels have accompanied the treatment (Rahimpour et al., 2023). Then, FLZ or phenlarmide alleviates motor dysfunction in animal PD models with an overexpression of Hsp70 (Kong et al., 2011; Bao et al., 2017). Additionally, some mechanisms are being developed with no pharmaceutical compounds proposed yet. Thus, cysteine string protein α (CSPα) phosphorylation by protein kinase C-γ promotes Hsp70 activity (Shirafuji et al., 2018).

Surprisingly, relative rare NDs tend to be more easily managed by Hsp70-recruiting drug options. So, arimoclomol (BRX-220), which is a low-molecular-weight hydroxylamine derivative Hsp70 enhancer, is tested in animal models of Fabry, Sandhoff, and Niemann-Pick type C (NPC) diseases (Kirkegaard et al., 2016). Mice of Gla−/− (Fabry), Hexb−/− (Sandhoff), and Npc1−/− (NPC) lines have shown a clinical improvement after arimoclomol intake. Hsp70 inducers/coinducers are generally convincing for ALS treatment (Kalmar et al., 2014; Kalmar and Greensmith, 2017), and it is especially true for arimoclomol (Phukan, 2010). Interestingly, that arimoclomol alone is effective but not efficient. It requires a preliminary heat shock induction to exhibit its potential (Kalmar et al., 2008).

Hsp70 inducers, mentioned above, are however very different by their biochemical features. This diversity has to be respected in practical implementation. For example, the compounds of geldanamycin group, in addition to their initial Hsp-related activity in cells, are able to prevent a transfer of receptors to steroid hormones into the nucleus, thereby providing a longer immune association (Czar et al., 1997). Geranyl geranylacetone acts via NF-κB-COX-2 axis to rule gene promoters, so cyclooxygenase-related adverse effects may be provoked (Nishida et al., 2007).

Exogenous recombinant Hsp70 (rHsp70ex) is a direct form of the chaperones that can be easily delivered into the body. It seems to be beneficial in NPC and AD models, particularly in familial AD (Kakimura et al., 2002; Mengel et al., 2021). For instance, rHsp70ex enhances memory and learning in AD models in vivo (Zatsepina et al., 2021). Then, cerebral and hippocampal cortex accumulates labeled rHsp70ex after intranasal administration (Yurinskaya et al., 2015). Further, mice with familial AD display declined Aβ levels and partial cognitive recovery after rHsp70ex administration (Bobkova et al., 2014; Evgen’ev et al., 2017). In addition, murine models of AD reflect a downregulation of neuroinflammatory markers after rHsp70ex in transcriptomic analysis of hippocampal neurons (Heppner et al., 2015; Yurinskaya et al., 2016). Moreover, neuroblasts proliferate and differentiate after intranasal rHSP70ex administration, perhaps due to the activation of cAMP 26 responsive element binding protein (CREB) cascade (Kwon et al., 2019). The treatment by rHsp70ex results in resistance to oxidative stress via mature endosomes and lysosomes with decreased apoptotic activity (Subrizi et al., 2015).

In a gene therapy study, rHsp70 has reached impaired dopaminergic neurons via an adenoviral vector with further decrease of neuronal loss (Dong et al., 2005). A similar effect is seen in Drosophila models for Hsp70 boosts both via gene therapy and induction by tanespimycin, a geldanamycin derivative (Zhang et al., 2016). Severity of PD signs has mitigated in sirtuin-1-transgenic mice due to the activation of HspA4 (Yang et al., 2022). However, HspA4 is not traditionally considered to belong to Hsp70 (Kaneko et al., 1997), although many controversial data appear recently (Shang et al., 2021; Abd El-Fadeal et al., 2023). A separate field of study includes epigenetic modifications of the chaperome, but it still remains weakly studied (Taldone et al., 2014).

Allosteric modification represents an alternative way to control Hsp70 function. Surprisingly, practically all modulators can behave as Hsp inhibitors, although their overall impact depends on many factors. Avoiding concurrence, allosteric binding stays selective, supporting a comprehensive understanding of Hsp70-involving cascades (Ekimova and Plaksina, 2016; Gleixner et al., 2016; Ferraro et al., 2019). First, rhodacyanine-derived compounds are found to modify Hsp70 cooperation with its coactors in an allosteric site, inhibiting a reversive transformation of the chaperone’s molecule from ADP- into ATP-binding state and thereby improving protein holding (Li et al., 2016). The group includes MKT-077 and YM-01, which have been investigated for anti-AD activity as they may provoke Hsp70-mediated decline in pathological tau content in vitro (Abisambra et al., 2013; Martin et al., 2016). Furthermore, YM-08 is the next generation with a milder but also a less toxic action (Miyata et al., 2013), whereas its halogen-recruiting modification (JG-23) is even more chemically stable (Chang et al., 2021; Shao et al., 2021).

Allosteric Hsp70 regulation has been also proposed for some other pharmacological groups. For instance, phenothiazines such as methylene blue and azure C decrease total tau and phospho-tau levels due to the anti-ATP-ase activity, while the benefit of this effect is debatable (Martin et al., 2016; Lo Cascio and Kayed, 2018). Also, the neurotrophic compound J147 prevents synaptic loss in a transgenic murine AD model (Chen et al., 2011). However, J147 directly affects ATP-synthase, thereby triggering a bounce in intracellular calcium levels with a launch of 5′-adenosine monophosphate-activated protein kinase and mammalian target of rapamycin (AMPK/mTOR) pathway by kinase β of calcium/calmodulin-dependent protein kinase (CAMKK2) affecting mitochondrial metabolism (Goldberg et al., 2018). There is no proven Hsp70-involving action for J147.

Despite the new horizons of Hsp70 usage, current clinical trials focus mainly on Hsp90-modifiers or nonselective Hsp-controllers because of a better understanding of Hsp90 machinery (Thirstrup et al., 2016). Even more, pridopidine has been studied for HD treatment with a profound search on the impact of S1R chaperones and few data about Hsp engagement (Shenkman et al., 2021). However, chemical and biological modifications of Hsp70-involving pathways is a mighty impact spot in future management of NDs (Fontaine et al., 2016).

Perhaps, combining chemically different medications would provide additional options. So, Hsp70 overexpression strengthens in concomitant administration of Hsp90 inhibitors together with histone deacetylase inhibitors (Kuta et al., 2020). Some drugs are slightly effective with no strict pathway clear yet. That is, for example, the phenomenon of myricetin, bortezomib, and MG-132, counterparts that affect proteasome-mediated Hsp70 action (Joshi et al., 2019). Generally, these proteasome modulators (or JG substances) are studied for tumor cell management, as Grp78 and mtHsp70 machinery suffers in their presence (Cagala et al., 2020; Ferguson et al., 2022).

Curiously, physical methods might also contribute to Hsp70 machinery. For example, near-infrared irradiation tends to improve Hsp70 activity in the splenic and hepatic regions of mice (Escudero-Duch et al., 2023). The idea has also been developed for sauna heating inducing Hsp70 activation (Hunt et al., 2020). Hsp70 levels are generally elevated in increased physical activity (Kim et al., 2022). Phytotherapy also may represent a feasible approach. So, an extract of African walnut, or Tetracarpidium conophorum, is recently shown to improve the PD-like signs in rats, probably via Hsp70 modulation (Tokunbo et al., 2023). Neferine, derived from seeds of lotus plants, seems to increase neuronal tolerance to ischemia via Hsp70-induction machinery (Sengking et al., 2022).