Implications of Krüppel-like factor signaling in neuroinflammation for neurodegenerative diseases

Abstract

Neurodegenerative diseases (NDs) pose a formidable challenge in modern healthcare and are characterized by progressive neuronal dysfunction and loss. Emerging research underscores the intricate interplay between neuroinflammation and mechanisms underlying ND pathogenesis. This review delves into the complex role of Krüppel-like factors (KLFs) in the context of neuroinflammation and major NDs. KLFs exert diverse effects in the brain on cellular processes such as blood-brain barrier integrity, neuronal cell cycle progression, and glial cell activation. Modulation of KLF expression and signaling emerges as a promising strategy to mitigate ND progression. By elucidating KLFs’ multifaceted implications across diverse pathways and cellular processes implicated in ND progression, this review offers valuable insights into their therapeutic potential as targets for NDs.

Introduction

Krüppel-like Factors (KLFs) are a family of transcriptional factors that act as activators or/and repressors of gene transcription. They are zinc finger proteins that bind to CACCC, or a GT box, in target gene promoters. The C-terminal domain of KLFs contains the DNA-binding region and nuclear localization signals, and the N-terminal domain is the protein-interacting region. There are 18 members in the KLF family with various gene expression patterns [1]. KLFs are involved in the regulation of many cellular processes such as cell cycle progression, proliferation, migration, transformation and invasion [1]. Altered expression of KLFs is associated with a wide range of diseases, including metabolic abnormalities, heart failure, and cancer. Here we present KLFs’ involvement in the development of neurodegenerative diseases and the implications in the pathology behind the diseases.

Neurodegenerative Diseases (NDs) share common characteristics such as the propagation of aberrant protein aggregates, neuroinflammation, increased oxidative stress, impaired proteolysis, mitochondrial dysfunction, and ultimately neuronal cell death [2,3]. Currently, the focus on developing treatments for NDs has shifted from targeting cytoplasmic and extracellular proteins towards targeting the associated genes in the nucleus that encode for or regulate the proteins relevant to NDs [4,5].

Alzheimer’s Disease (AD), the most prevalent ND, is a progressive condition that causes the affected patients to present symptoms such as declining memory, aphasia, deteriorating cognitive impairment, and ultimately the development of dementia [6]. AD displays pathological changes in patients’ brains, such as neuronal loss in the hippocampal region, leading to defects in learning and memory [7]. Key characteristics of AD include the accumulation of extracellular amyloid-β (Aβ) protein plaques, intracellular neurofibrillary tangles of hyperphosphorylated tau protein, chronic neuroinflammation, and complex neuroimmune interactions that involve reactive microglia and astrocyte [8,9].

Parkinson’s Disease (PD), the second most common ND, presents protein aggregates the form of Lewy bodies. Lewy bodies are formed by accumulation of misfolded α-synuclein (α-syn) protein [10]. Tau pathology is also observed in PD and several other NDs. There is an overlap between the formation of Aβ plaques, tau tangles, and α-syn aggregates, suggesting that Aβ plaques contribute to α-syn spreading [11]. Furthermore, PD presents as the death of dopamine-producing neurons in the substantia nigra, resulting in the development of symptoms affecting the motor system such as bradykinesia, loss of balance, tremors, and stiffness [12,13].

Huntington’s disease (HD) is an autosomal dominant inherited condition. A CAG trinucleotide repeat in the huntingtin gene causes an aberrant protein phenotype. This aberrant protein disrupts a wide array of molecular and cellular processes including cellular homeostasis, neuronal transportation, gene expression, and function of mitochondria and synapsis. Consequently, the loss of corpus striatum GABAergic medium spiny neurons and cholinergic neurons occurs. Patients present a variety of symptoms disturbing motor and cognitive skills [3,14].

Roles of KLFs in the progression of NDs

ND progression begins with dysregulation of molecular signaling within the cellular network inside the brain. To understand the roles of KLFs in ND progression, we first looked at the peer-reviewed literature for the expression of KLFs in the brain. We found that all the 17 KLF family members are expressed in one or more CNS cell types and are relevant to NDs (Table 1). Notably, most of the data outlined in this table were obtained from experimental mice and more studies were done on some KLFs such as KLF4, KLF7, KLF9 and KLF11 than others considering the number of publications contributing to the studies. Nevertheless, analysis of data reported from the European Bioinformatics Institute’s database confirms that the KLF family members are indeed expressed in the human brain, in general and in NDs-associated regions such as the cerebral cortex and hippocampus, in particular (Table 2).

Table 1.

KLF family expression in the central nervous system

	Expression Time/Disease model	Model organism	Expression Location	Brain Cell Type	Reference
KLF1	During development	Mice	Cerebral cortex	Retinal ganglion cells	[125]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF2	During development	Mice	Cerebral cortex	Retinal ganglion cells	[125]
	Neural differentiation	-	In-vitro	Dental pulp-derived stem cells	[119]
	Alzheimer disease	C57/BL6 mice	Cerebral cortex	Endothelial	[25]
		Human	Temporal cortex	-
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Cerebral ischemia	Mice	Cerebral cortex	Endothelial	[19]
	Alzheimer disease	Tg2576 Mice	Brain tissue	-	[26]
		Human brain endothelial cells	In-vitro	Endothelial
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
	Alzheimer disease	Kunming mice	Hippocampus	Neuronal	[128]
	Sciatic nerve injury	Sprague-Dawley rats	Dorsal root ganglia	Neuronal	[129]
	Hypoxic-ischemic brain damage	Sprague-Dawley rats	Hippocampus	Neuronal	[130]
			Cortex
	Spinal cord injury	Sprague-Dawley rats	Spinal cord	Neuronal	[131]
KLF3	During development	Mice	Cerebral cortex	Retinal ganglion cells	[125]
	-	Mice	Forebrain	Neural stem cells	[132]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
KLF4	During development	Mice	Cerebral cortex	Retinal ganglion cells	[125]
	Glutamatergic stimulation	CD1 mice	Cerebral cortex	Neurons	[133]
	Developmental	Rats	Cerebral cortex	Neurons	[134]
	Neuroinflammation	BALB/c mice	Whole brain tissue	-	[87]
		-	In-vitro	BV-2 cells
	Developmental (Ontogeny?)	C57BL/6 mice	Hypothalamus	Neurons	[135]
	Developmental	Mice	E13.5 forebrain	Neuronal stem cells	[132]
			E15.5 cortices	Neurons
			P0 cortices	Astrocytes
	Neuroinflammation	Mice	Brain tissue	Microglial	[136]
		-	In-vitro	BV-2
		Mice	Brain tissue	Astrocytes
	Neuroinflammation	Mice	Brain tissue	Microglial	[137]
		-	In-vitro	BV-2
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Developmental	Mice	White matter	Neuronal stem cells	[138]
			Cerebral cortex	Glial
				Astrocytes
	Parkinson’s disease	-	In-vitro	M17 neuroblastoma cell line	[139]
	Neuroinflammation	BALB/c mice	Whole brain	Microglia	[140]
		-	In vitro	BV-2 cells
	Neuronal regeneration	-	In-vitro	COS7 cells	[58]
		Mice	Retina	-
			Optic nerve	Retinal ganglion cells
	Neuronal degeneration	C57BL/6NHSd mice	Hippocampus	Neurons	[141]
		-	In-vitro	PC12 cells
	Traumatic brain injury	Sprague Dawley rats	Optic nerve	Retinal ganglion cells	[142]
		-	In-vitro	RGC-5 cells
	Subarachnoid hemorrhage	Human subjects	Cerebrospinal fluid	-	[143]
	Psychological stress	Sprague Dawley rats	Cortex	In vivo: Tissue homogenate	[51]
			Hippocampus	In vitro: HT-22 cells
	Alzheimer disease	AD transgenic J20 mice	Brain tissue	Microglial cells	[86]
		-	In-vitro	BV-2 cells
	Parkinson’s disease	-	In-vitro	SH-SY5Y cells	[144]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
	Cerebral edema	Sprague-Dawley rats	Cerebral cortex	Microglial cells	[145]
	Cerebral ischemia	Mice	Cerebral cortex	Brain microvascular endothelial cells	[146]
		-	In-vitro	bEnd.3 cells
	Cerebral ischemia	C57BL/6 mice	Ischemic penumbra	Astrocytes	[40]
KLF5	Chronic Schizophrenia	Human	Prefrontal cortex	-	[147]
			Hippocampus
	During development	Mice	Cortex	Retinal ganglion cells	[125]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
	Alzheimer disease	-	In-vitro	SH-SY5Y, and HT22 cells	[148]
		APP/PS1 mice	Hippocampus	Neurons
			Cerebral cortex
		Human	Cerebrospinal fluid	-
KLF6	Developmental	Mice	Cortical plate	-	[149]
			Hypothalamus
			Forebrain
			midbrain
	Developmental	Zebrafish	Optic nerve	Retinal ganglion cells	[150]
	-	Mice	Olfactory bulb	-	[151]
			Cerebral cortex
			Septum
			Hippocampus
			Basal ganglia
			Amygdala
			Thalamus
			Hypothalamus
	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Developmental	Rats	Cerebral cortex	Neurons	[134]
	Status epilepticus	C57BL/6 mice	Hippocampus	Reactive astrocytes	[152]
				Active microglia
				Neurons
				Endothelial cells
	Neuronal regeneration	-	In-vitro	COS7 cells	[58]
		Mice	Retina	-
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF7	Developmental differences	-	Ventral horn of the spinal cord	-	[77]
			Dorsal root ganglia
			Sympathetic ganglia
			Cerebral cortex
			Cerebellum
			Dorsal root ganglia
	Developmental	Mice	Olfactory bulb	Neurons	[78]
			Optic nerve	Retinal ganglion cells
			Cerebral cortex	Neurons
	Developmental	Mice	Olfactory bulb	Neurons	[153]
	Nerve physical injury	Zebrafish	Optic nerve	Retinal ganglion cells	[150]
	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Neural differentiation	-	In-vitro	PC12 cells	[154]
				Embryonic stem cells
				Neural stem cells
	Developmental	Mice	Olfactory bulb	-	[155]
			Pons
			Ventral midbrain
	Developmental	Mice	Cerebral cortex	Neurons	[156]
			Corticospinal tract
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF8	During development	Mice	Cortex	Retinal ganglion cells	[125]
	Brain tumors	Human	Astrocytoma	-	[157]
			Glioblastoma
			Bain tissue
	Alzheimer’s disease	Wistar rats	Cerebral cortex	-	[46]
			Hippocampus
		Mice	Cerebral cortex	Glial cells
			Hippocampus	Neuronal cells
	-	C57BL/6 mice	Cerebral cortex	Neurons	[45]
			Olfactory bulb
			Hypothalamus
			Pallidum
			Striatum
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF9	-	-	In vitro	N2a cells	[158]
		Rats	Brain tissue	-
	Stressor	Tadpoles	Brain tissue	-	[159]
	Developmental	C57BL/6	Hippocampus	-	[160]
			cerebellum
		-	In-vitro	N2a cells
	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Neuronal maturation	Mice	Forebrain	-	[161]
			Hippocampus
			Cerebral cortex
	Developmental	Mice	cerebellum	Purkinje cells	[162]
	Developmental	-	In vitro	HT-22 cells	[163]
		C57/BL6J mice	Hippocampus	-
	Differentiation and myelination	-	In-vitro	Oligodendrocyte precursor cells	[164]
		C57/Bl6 mice	cerebellum	-
			Optic nerve
			Corpus callosum
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Developmental	Tadpoles	Middle brain region	-	[165]
			preoptic area
			diencephalon
	Induced oxidative stress	-	In vitro	Mes23.5, SH-SY5Y and N27 cell lines	[166]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
	Developmental	-	In-vitro	HT22 cells	[167]
	Parkinson’s disease	BL/6J mice	Substantia nigra	-	[168]
		-	In-vitro	SH-SY5Y cells
KLF10	During development	Mice	Cortex	Retinal ganglion cells	[125]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Alzheimer’s disease	C57BL/6J E16 mice	Cerebral cortex	Neurons	[169]
		Sprague Dawley E18 rat
		Human	Hippocampus	-
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF11	During development	Mice	Cortex	Retinal ganglion cells	[125]
	Focal cerebral ischemia	Mice	Cerebral microvessels	Cerebral vascular endothelial cells	[37]
	Nerve growth factor	Mice	Dorsal root ganglia	Neurons	[127]
		-	In-vitro	PC12 cells
	Chronic Stress and Depressive Disorders	Human	Prefrontal cortex	-	[170]
	Chronic stress	Mice	Frontal cortex	-
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF12	During development	Mice	Cortex	Retinal ganglion cells	[125]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF13	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
	Developmental	-	In-vitro	HT22 cells	[167]
KLF14	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF15	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Developmental	Mice	neocortical regions	Neural stem cells	[171]
	Nerve growth factor	-	In-vitro	PC12 cells	[127]
	Developmental	Mice	Cerebral cortex and Spinal cord white matter	Astrocyte	[172]
				Oligodendrocytes
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF16	Developmental	Mice	Cortex	Retinal ganglion cells	[125]
	Cerebral ischemia	Mice	Cerebral cortex	-	[126]
KLF17	Developmental	Mice	Cortex	Retinal ganglion cells	[125]

Table 2.

KLF family expression in the human brain as reported by the EMBL’s European bioinformatics institute

	KLF1	KLF2	KLF3	KLF4	KLF5	KLF6	KLF7	KLF8	KLF9	KLF10	KLF11	KLF12	KLF13	KLF14	KLF15	KLF16	KLF17
Amygdala	VL	√	√	√	√	√	√	√	√√	√	√	√	√√	VL	√√	√√√	VL
Basal ganglia	√	√√	√√	√	√	√√	√√	√√	√	√√	√√	√√	√√	VL	√	√√	VL
Brain tissue	√	√√	√√√	√√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√	√√	√√√	VL
Caudate nucleus	VL	√	√	√	√	√	√	√	√√	√	√	√	√√	VL	√√	√√√	VL
Cerebellum	√	√√	√√	√	√	√√	√√	√√	√√	√√	√√	√√	√√	VL	√√	√√√	VL
cerebral cortex	√	√√	√√	√	√	√√	√√	√√	√√	√√	√√	√√	√√	VL	√√	√√	VL
choroid plexus	√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√	√√	√√	√
Diencephalon	√	√√	√√	√√	√	√√	√√	√√	√	√√	√√	√√	√√	√	√	√√	√
Forebrain	√	√√	√√	√√	√√	√√	√√	√√	√	√√	√√	√√	√√	√	√	√√	VL
Globus pallidus	VL	√	√	VL	VL	VL	√	VL	√	VL	VL	VL	√	VL	VL	√	VL
Hindbrain	√	√	√√	√√	√	√√	√	√√	√	√√	√√	√√	√√	VL	√	√√	VL
Hippocampus	√	√√	√√	√	√	√√	√√	√√	√	√√	√√	√	√√	VL	√√	√√	VL
Hypothalamus	VL	√√	√	√	√	√√	√	√	√√	√	√	√	√√	VL	√√	√√	VL
Medulla oblongata	√	√√	√√	√	√	√√	√√	√√	√	√√	√√	√√	√√	VL	√√	√√	VL
Midbrain	√√	√√	√√	√√	√√	√√	√√	√√	√	√√	√√	√√	√√	VL	√	√√	√
Pituitary gland	VL	√√	√√√	√√√	√√√	√√	√	√	√√	√√	√	√	√√	VL	√√	√√√	VL
Pons	√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	√√	VL	√√	√√	VL
Putamen	VL	√√	√	√	VL	√	√	√	√√	√	√	√	√√	VL	√√	√√	VL
Substantia nigra	VL	√√	√√	√	√	√√	√	√	√√	√	√	√	√√	VL	√√	√	VL
Telencephalon	√	√√	√√	√	√	√√	√√	√√	√	√√	√√	√√	√√	VL	√	√√	VL
Thalamus	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	nda	√√√	nda

VL, very low expression; ü, √ü and √√ü, low, medium and high expression, respectively; nda, no data available.

KLFs and blood-brain barrier integrity

Dysregulated neurovascular units (NVUs) and the blood-brain barrier (BBB) are linked to NDs [15]. An NVU is a collection of cells composed of neurons, astrocytes, and endothelial cells of the BBB. These NVU cells work together to regulate neuroimmune response, brain blood flow, and waste clearance [16]. In NDs, aberrant Aβ and p-Tau protein aggregates present around cerebral blood vessels in the brain parenchyma can cause NVU dysfunction and loss of blood vessel integrity or BBB breakdown. BBB dysfunction is associated with increased vascular permeability, facilitated immune cell invasion, enhanced neuroinflammation, and ultimately degeneration of the NVU [17].

Several KLFs play a role in regulating BBB integrity (Figure 1). KLF2 may ameliorate BBB dysfunction by upregulating autophagic flux in endothelial cells. A study showed that KLF2 expression improves the blood-spinal cord barrier integrity and functional recovery from spinal cord injury by inducing tight junction (TJ) protein expression [18]. KLF2’s implication in cerebrovascular integrity is further emphasized by another study reporting that KLF2 expression reduces infarction size by improving BBB function in the focal cerebral ischemia mouse model [19]. In this study, KLF2 was found to induce the expression of several tight junction proteins, including occludin, claudin-12, and junction adhesion molecule-1 (JAM-1). These proteins play an important role in preserving endothelial barrier and vascular integrity [20,21]. KLF2 rescues TJ protein expression and stabilizes vasculature through mediating anti-inflammatory p53/KLF2 signaling and activating the angiopoietin-1/PI3K/Akt-myocyte enhancer factor-2 (MEF2)-KLF2 signaling in glia [22]. This signaling pathway counteracts the vascular endothelial growth factor (VEGF) inflammatory response [23,24]. Moreover, a study on mouse brain microvascular endothelial cells conveys the potential of KLF2 activation as a therapeutic strategy for cerebral vascular dysfunction in AD. In this study, KLF2 promotion leads to attenuated Aβ-induced oxidative stress, improved mitochondrial function, and reduced apoptosis, ultimately ameliorating AD progression [25]. Similarly, overexpressing KLF2 can rescue occludin expression that was priorly disrupted by Aβ [26]. Thus, KLF2 shows neuroprotective effects in the cerebrovascular system through various signaling pathways that seem to favor BBB integrity by ameliorating inflammation present in several NDs.

Figure 1.

KLFs play a role in blood-brain barrier integrity by modulating the expression of several tight junction proteins, enhancing mitochondrial function, and reducing apoptosis and oxidative stress (The arrow indicates activation, while the straight line with a “T-shape” end indicates inhibition. KLFs boxed in green indicate a neuroprotective effect. These labels apply to all the Figures that follow).

KLF4 is also suggested to have neuroprotective effects in the cerebral vascular system. KLF4 expression was found to increase with time in astrocytes after cerebral ischemia-reperfusion, and its activation modulates the nuclear-erythroid factor 2-related factor 2 (Nrf2)/thioredoxin 1 (Trx1) signaling and ameliorates BBB disruption [27]. Both KLF2 and KLF4 induce anti-inflammatory and vasoprotective phenotypes in endothelial cells by inhibiting the nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) activation and induces endothelial nitric oxide synthase (eNOS) expression [28,29]. Upregulated NF-κB expression is also involved in BBB breakdown. NF-κB causes pericyte activation and matrix metalloproteinases (MMP) secretion, leading to basement membrane degradation and opening of the BBB. NF-κB activation may also induce TJ disruption, increasing the permeability of the endothelial cell layer [30]. Like KLF2 and KLF4, other KLFs such as KLF8 and KLF11 have been shown to regulate NF-κB activity associated with BBB integrity [31,32].

KLF11 presents neuroprotective properties by protecting the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ) to repress the transcription of the pro-apoptotic miR-15a, resulting in cerebrovascular endothelial cell protection after ischemic insults [33,34]. PPAR-γ inhibits the inflammatory activation of the MAP kinases p38 and extracellular signal-regulated kinases (ERK)1/2 as well as NF-κB downstream of Toll-like receptors 2 and 4 (TLR2, TLR4) [33,34]. Genetic deletion of KLF11 resulted in increased infiltration of peripheral neutrophils and macrophages in mice with traumatic brain injury (TBI) and post-traumatic BBB disruption. Additionally, KLF11 seems to have a role in the prevention of astrocyte activation at the BBB [34]. Moreover, KLF11 increases the expression of TJ proteins including occludin and Zonula occludens-1 (ZO-1) [35]. These results suggest that KLF11 expression is important for the protection of the BBB integrity [36,37].

The effects of KLF10 on cerebral ischemia reperfusion were investigated in vitro [38]. Results revealed that the downregulation of KLF10 resulted in the suppression of apoptosis, and oxidative stress and ameliorated BBB dysfunction through activation of NRF-2/heme oxygenase (HO-1) signaling. This study suggests that KLF10 downregulation may reduce BBB permeability by modulating TJ proteins expression, including ZO-1, occludin and claudin-5 expression in endothelial cells under oxygen-glucose deprivation/reperfusion (OGD/R) conditions [38].

Clearly, several KLFs play a role in BBB stability and with more research, their related signaling pathways may be targeted to promote BBB reconstruction in NDs to help ameliorate neuroinflammation and reinstate homeostasis in the brain.

KLF signaling in neuroinflammation

Several KLF signaling pathways including NF-κB, iron and JAK-STAT pathways have been implicated to play a role for neuroinflammation by regulating various cellular events such as glial polarization and ferroptosis in the brain (Figure 2).

Figure 2.

KLFs modulate neuroinflammation through glial polarization and ferroptosis by regulating various signaling pathways such as NF-κB, iron and JAK-STAT pathway.

Activation of NF-κB in microglia promotes neuronal degeneration while its expression in neurons is neuroprotective. NF-κB is found to be upregulated in the spinal cords of amyotrophic lateral sclerosis (ALS) patients, and inhibition of NF-κB signaling in microglia rescues motor neurons and extends survival in a mouse model [39]. Multiple KLFs have been found to play a role in neuroinflammation via NF-κB signaling. It was shown that after OGD/R, astrocytic KLF4 inhibited the activation of the A1 pro-inflammatory subtype of astrocytes and promoted the polarization of A2 anti-inflammatory subtype of astrocytes via modulation of NF-κB [40]. Like M1 microglia, A1 astrocytes are a source of neuroinflammation that is present with most NDs [41]. Additionally, M1 microglia play a role in astrocyte activation and NF-κB plays a critical role in the switch of microglia from M2 to M1 subtype. Likewise, KLF6 regulates NF-κB expression in coactivation of the NF-κB mediated inflammatory response, which is responsible for making ischemic-reperfusion injury more severe in the kidney [42]. KLF6 was also reported to promote inflammatory bowel disease by co-activating NF-κB and suppressing the STAT3 pathway in macrophages, which confers anti-inflammatory signaling [43,44]. There is not much research on KLF6 and its role in microglial polarization in the central nervous system (CNS), although there is much research linking KLF6 to macrophage polarization towards M1 phenotype [44]. More research is needed to evaluate whether KLF6 behaves similarly with microglial cells via NF-κB modulation. KLF11 was shown to play a BBB protective role through PPAR-γ-mediated inhibition of the inflammatory NF-κB pathway in microglial polarization [36,37]. KLF8 is highly expressed and active in the cerebral neurons in various regions such as the cerebral cortex, hippocampus, and hypothalamus [45]. Decreased expression of KLF8 was found in the brain of AD patients with disrupted Wnt/β-catenin signaling [46]. Indeed, KLF8 is a known regulator of the Wnt/β-catenin signaling [47], and β-catenin interaction with NF-κB is highly expressed in brains of patients with NDs such as PD and AD [48]. These results suggest that KLF8 in the brain may play a critical role in neuronal protection.

Iron plays a part in multiple cellular processes, such as oxygen transportation, mitochondrial respiration, DNA synthesis, neurotransmitter synthesis, and more. Dysregulated iron homeostasis can lead to oxidative damage and cause neurotoxicity [49]. Increased iron load in the brain is found to accelerate the formation of Aβ plaques and p-tau tangles and enhance oxidative stress production, which of course is associated with pathology in NDs [50]. A study showed that the activation of KLF4-heme carrier protein 1 (HCP1) signaling induced an increase in heme uptake under psychological stress. This leads to iron accumulation and promotes the release of reactive oxygen species (ROS) and subsequent neuronal damage due to ferroptosis [51]. KLF4 represses the transcription of ELK-3. ELK-3 is a transcription repressor of heme oxygenase 1 (HO-1) that degrades heme into bilirubin and frees iron. Thus, the KLF4-HO-1 signaling promotes iron deposition, resulting in exacerbated oxidative stress and cell damage [52,53]. Given that this KLF4 enhanced iron accumulation takes place in hippocampal neurons [51], it is plausible that aberrant KLF4 signaling like this can be harmful to these neurons critical for cognition and memory leading to NDs like AD.

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling mediates many processes such as tissue repair, hemopoiesis, inflammation, and apoptosis. Disruption of the JAK/STAT signaling has been linked to neuroinflammation in AD [54]. Modulation of the JAK/STAT signaling has been demonstrated in mice to improve post-ischemic recovery from AD-like pathology such as aberrant protein accumulation, neuroinflammation, BBB damage and neuronal apoptosis [54-56]. Both KLF13 and KLF4 have been reported to inhibit JAK/STAT signaling essential for axon regeneration. In a study using the mouse hippocampus-derived cell line HT22 [57], KLF13 was shown to inhibit neurotrophic growth hormone induced JAK/STAT signaling by directly repressing the transcription of several genes in the pathway. Another report demonstrated that KLF4 physically binds phosphorylated STAT3 and prevents the STAT3 from DNA binding, resulting in the blockage of the JAK/STAT signaling downstream of axon regenerative cytokine [53,58]. Axon regeneration can be significantly enhanced by the cytokine treatment in KLF4 knockout mice [53,58]. These studies suggest that targeting KLF family members like KLF4 and KLF13 could help block neurodegenerative progression through JAK/STAT mediated axon regeneration.

KLFs and cell cycle regulation in the brain

Neuronal loss is linked to aberrant neuronal cell cycle progression with increased expression of cell-cycle related proteins found in pathologic areas in AD, HD and PD brains [59,60]. Half of the KLF family members are positive or negative regulators of cell cycle in brain cells including both neurons and non-neuronal cells [61,62] (Figure 3). KLF8 was originally identified as an activator of cyclin D1 transcription for cell cycle progression downstream of focal adhesion kinase (FAK) [63]. Research on KLF8 has been focused primarily on cancer [64-74]. However, recent studies have revealed that in the brain, KLF8 is predominantly expressed in the neurons [45] and its expression is significantly decreased in AD patient brains [46], suggesting a potential role for KLF8 in neuronal cell cycle progression. Other KLF members such as KLF2, KLF4, KLF6, KLF7, KLF12, KLF13, and KLF14 have also been shown to regulate cell cycle progression by regulating the expression of cyclins, cyclin-dependent kinase inhibitors [74-78], the PI3K/Akt/mTOR signaling activity [22,79]. Genetic deletion of KLF4 does not disrupt microglial cell proliferation during post-natal brain development in mouse models [80,81]. However, diminished expression of KLF4 seems to be responsible for the loss of expression of the rhythmic genes that are critical for aged microglial differentiation and reprogramming during protective immune responses [82].

Figure 3.

KLFs regulate cell cycle progression in neuronal and glial cells.

KLFs and cellular mechanisms of neuroinflammation

Neuroinflammation plays a critical role in NDs. Normally, it is carried out in a tight knit way with innate immune cells in the brain to protect neurons. However, an excessive neuroinflammatory response is a major contribution to NDs pathology [83]. KLFs play a role in neuroinflammatory progression through activation of inflammatory shifts of glial state, inflammasome formation and mitochondrial metabolism in microglia and autophagy or mitophagy in neurons (Figure 4).

Figure 4.

KLFs modulate inflammasome and mitochondrial oxidation in glial cells, and mitophagy/autophagy in neurons.

Microglia are innate immune cells in the CNS that present different phenotypes. Microglia in a classical activation state, known as M1, secrete proinflammatory cytokines, whereas microglia in alternative activation or acquired deactivation state, known as M2, secrete anti-inflammatory factors. M1 microglia are closely associated with the aggregation of misfolded proteins seen in PD, AD, HD and ALS [84]. Astrocytes play a big role in supporting neuronal function as they help regulate homeostasis and synaptic plasticity and may provide neuroprotection upon brain injury. However, the dysfunction of M2 astrocytes and their switch to A1 astrocytes are linked to NDs pathology [85]. KLF4 plays an essential role in the microglial M1/M2 switch. The switch of microglia from M1 to M2 can be achieved by inhibiting KLF4 interaction with histone deacetylase 1 and suppressing deacetylation. Moreover, oligomeric Aβ42 increases KLF4 expression in microglial BV2 cells. Conversely, overexpression of KLF4 exacerbates Aβ42-induced neuroinflammation [86]. KLF4 expression can be highly induced in activated microglia by lipopolysaccharides (LPS) stimulation, while KLF4 knockdown leads to significantly reduced production of the pro-inflammatory cytokines such as TNF-α and IL-6 as well as iNOS and Cox-2 [87]. A study using the BV2 microglial cell line investigating how the anti-neuroinflammatory agent, Agmatine, exerts its neuroprotective effect revealed that agmatine strongly binds to interferon regulatory factor 2 binding protein (IRF2BP2) in the cytoplasm. This interaction frees the IRF2 that enters the nucleus where it activates the transcription of KLF4 [88], suggesting an important role in activation of M2 microglia. Consistently, KLF4/STAT6 signaling was found to induce M2 macrophages as well [89]. A recent study showed that inhibition of KLF4 translation using miR-25802 resulted in activation of M1 microglia via NF-κB inflammatory signaling and AD pathology, which was reversed by overexpression of KLF4 [90]. KLF11 promotes TGF-β signaling [91] that is known to ameliorate AD pathology by targeting Aβ and Tau through decreasing the expression of pro-inflammatory cytokines and increasing neuronal survival factors [92]. Genetic deletion of KLF11 in mice enhances post-traumatic astrocyte activation, microglial polarization [37] and expression of various pro-inflammatory factors in a traumatic brain injury model of mice [34].

The nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3) inflammasome is activated by the aggregation of misfolded proteins of Aβ, p-tau or α-syn, which causes initiation and promotion of the neuroinflammatory response in NDs such as AD [93]. Research on KLF4 and KLF2 suggests their potential role for the activation of NLRP3 inflammasome [94,95]. Overexpression of KLF4 increases the liver X receptor α (LXRα) and cholesterol 25-hydroxylase (CH25H) expression, resulting in the inhibition of NLRP3 inflammasome components and the promotion of microglia polarization from the M1 to M2 phenotype [96]. KLF2 was also shown to upregulate CH25H mRNA expression [96,97]. Treatment with simvastatin, a cholesterol lowering drug, causes an increase in the expression of KLF2 and inactivation of the NLRP3 inflammasome [98]. These results indicate the KLF family members such as KLF2 and KLF4 may play a part in neuroinflammation by regulating the NLRP3 inflammasome.

Microglial metabolism is dysregulated in AD with disrupted oxidative phosphorylation and lipid metabolism and a shift into glycolysis that is thought to decrease their ability to phagocytose Aβ and mediate AD pathology [99]. In vitro studies showed that microglia are affected by fluctuation in glucose concentration. A low-to-high glucose shift in BV-2 cells leads to an increase in the expression of the pro-inflammatory factors including the tumor necrosis factor alpha (TNF-α), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX2), while a high-to-low glucose shift promotes autophagy and apoptosis [99,100]. A KLF10 knockdown study suggested that KLF10 is a key regulator of energy metabolism in mitochondria in the cerebellum [101]. Consistently, KLF10 knockout, albeit in the liver, results in disrupted glucose metabolism [102]. Emerging research has shed light on the relationship between mitochondrial metabolic dysfunction and NDs. For example, aggregates of aberrant proteins like Aβ42 and p-tau may be the consequence of insulin resistance in the brain, and drug treatment against diabetes and insulin resistance has been shown to help preserve cognitive functions in AD patients [103,104]. These results suggest a neuroprotective role for KLF10 through regulation of glucose metabolism [105]. KLF9 induction in the neurons by the anti-oxidative stress factor NRF2 has been shown to promote cell survival against high levels of oxidative stress in animal models of brain damage [106]. This could serve as a neuroprotective mechanism of regulation of glutathione and cytochrome P450 against NDs such as PD [106-113].

Autophagy, including the mitophahy form, is important for the maintenance of homeostasis in the brain as it eliminates damaging protein aggregates, but aberrant autophagy can drive ND progression [106,114,115]. Although it is unclear how autophagic dysfunction accelerates ND pathology, some research suggests that the accumulation of Aβ protein is found in autophagic vacuoles, which eventually contributes to the formation of Aβ plaques [116]. Other research suggests that downregulation of autophagic activity leads to impaired clearance of aberrant protein aggregation by autophagy [117]. Consistent with positive regulation of autophagy by KLF2 during osteoblast differentiation and osteoclastogenesis [118], KLF2 deficiency is shown to negatively impact autophagy and mitophagy during neural differentiation of dental pulp-derived stem cell (DPSC), with downregulated expression markers for both autophagy (i.e., LC3B, ATG5, and LAMP1) and mitophagy (i.e., PINK1, Parkin, DRP1, FIS1) [119]. Interestingly, loss-of-function mutations in PINK1 and Parkin are associated with PD [120]. Aberrant activation of Wnt signaling was also observed in development of DPSC [119], suggesting the Wnt signaling regulator KLF8 in the brain [46] perhaps also plays a role in autophagy/mitophagy associated with the disease. A study on neurokinin-1 receptor (NK1R) signaling indicates that activation of this pathway can cause autophagy through the ERK5/KLF4/p62/Nrf2 signaling axis, resulting in the restoration of balanced redox signaling and the subsequent reduction of α-Syn aggregates [121]. Further investigation into how autophagy is regulated by KLFs in the brain is important for understanding mechanisms of progression of NDs.

Considering the critical role of KLF signaling for neuroinflammatory progression and aberrant protein aggregation in NDs [84,99,122-124], manipulating KLF expression could help balance the pro- vs. anti-inflammatory states of glia to ameliorate neuronal inflammation.

Conclusion

The exploration of cerebral expression and roles of KLFs within the intricate landscape of neuroinflammation and neurodegenerative diseases reveals a promising avenue for therapeutic intervention. The multifaceted roles of KLFs in modulating cellular processes such as BBB integrity and glial activation via critical signaling through JAK/STAT, NF-κB, and Wnt/β-catenin axis underscore their potential as key regulators of neuroprotection and neurodegeneration. The elucidation of these KLFs’ implications offer valuable insights into the pathophysiology of NDs, making KLFs potential therapeutic targets. By harnessing the regulatory power of KLFs in the brain, particularly using brain-specific gain/loss-of-function cellular and mouse models, we may unlock novel therapeutic strategies aimed at blocking or even reversing the progression of NDs, offering new hope for improved patient outcomes and quality of life.

Disclosure of conflict of interest

None.

Abbreviations

Aβ

amyloid-beta

AD

Alzheimer’s disease

ALS

amyotrophic lateral sclerosis

ATG5

autophagy-related protein 5

BBB

blood-brain barrier

BDNF

brain-derived neurotrophic factor

BIG1

brefeldin A-inhibited guanine nucleotide-exchange protein 1

CAG

cytosine-adenine-guanine

CCM

cerebral cavernous malformations

CH25H

cholesterol 25-hydroxylase

CNS

central nervous system

COX2

cyclooxygenase-2

DPSC

dental pulp-derived stem cell

DRP1

dynamin-related protein 1

eNOS

endothelial nitric oxide synthase

ERK

extracellular signal-regulated kinase

FAK

focal adhesion kinase

FIS1

mitochondrial fission 1 protein

GSH

glutathione

HCP1

heme carrier protein

HD

Huntington’s disease

HO-1

heme oxygenase 1

iNOS

nitric oxide synthase

IRF2BP2

interferon regulatory factor 2 binding protein

JAK/STAT

Janus kinase/signal transducer and activator of transcription

JAM-1

junction adhesion molecule-1

KLF

Krüppel-like factor

LAMP1

lysosomal-associated membrane protein

LC3B

microtubule-associated proteins 1A/1B light chain 3B

LPS

lipopolysaccharide

LXR

liver like receptor

MEF2

myocyte enhancer factor 2

MMP

matrix metalloproteinases

MTP18

mitochondrial protein 18

ND

neurodegenerative disease

NF-κB

nuclear factor kappa-light-chain-enhancer of activated B cells

NK1R

neurokinin 1 receptor

NLRP3

nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3

Nrf2/Trx1

nuclear factor erythroid 2-related factor 2/thioredoxin 1

NVU

neurovascular unit

OGD/R

oxygen-glucose deprivation/reperfusion

PD

Parkinson’s disease

PI3K/Akt

Phosphoinositide 3-kinase/Protein Kinase B

PINK1

PTEN-induced putative kinase 1

PPAR-γ

peroxisome proliferator-activated receptor gamma

ROS

reactive oxygen species

TGF-β

transforming growth factor-beta

TJ

tight junction

TLR

toll like receptor

TNF-α

tumor necrosis factor alpha

TRIM59

tripartite motif containing 59

VEGF

vascular endothelial growth factor

Wnt

wingless/integrated

ZO-1

zonula occludens-1

α-syn

α-synuclein