Review ArticleFunctions for the cardiomyokine, MANF, in cardioprotection, hypertrophy and heart failure
Highlights
► Unfolded protein response. ► Endoplasmic reticulum stress. ► Cardiomyokine secretion. ► Cardioprotection. ► Mesencephalic Astrocyte-derived Neurotrophic Factor.
Introduction
Secreted proteins, including hormones and cytokines, are critical for the development, growth and maintenance of all tissues and cells. Numerous proteins are secreted from various tissues, such as skeletal muscle; these proteins, such as the cardioprotective peptide, Fstl1 [1], [2], are called myokines [3], [4]. By analogy, we propose the term cardiomyokine (CMK) to describe heart-derived myokines that exert paracrine, autocrine and/or endocrine effects.
The rough endoplasmic reticulum (ER) is the location of the biosynthesis of proteins secreted via the classical or conventional secretory pathway [5], [6], as are many CMKs. Conventionally-secreted proteins are inserted into the ER lumen co-translationally, and then, upon completion of their synthesis and folding in the lumen, they are transported to the Golgi on their way to secretory vesicles, from which they are released into the extracellular space [7]. Accordingly, a robust protein synthesis and folding environment in the rough ER [8], and perhaps the sarcoplasmic reticulum (SR) of myocardial cells, is crucial for efficient biosynthesis and secretion of most CMKs.
Certain myocardial injuries, such as infarction or ischemia, impair protein folding in the ER [9]. Although not yet shown in the heart, studies in other cell and tissue types have demonstrated that impaired protein folding in the ER during hypoxic or ischemic stress reduces secretion of adiponectin [10], apolipoprotein B100 [11] and procollagen [12], [13]. Additionally, impaired ER-protein folding in hypoxic tumor cells inhibits Wnt secretion [14], and in a model of renal epithelial ischemia, mis-folding of secreted proteins decreases their secretion [15]. Thus, since myocardial ischemia impairs ER-protein folding, it is likely that ischemia also impacts the secretion of many CMKs. However, there is the potential that there is a group of CMKs whose secretion may not be reduced under such conditions, but may actually be preserved, or even enhanced by virtue of their induction during the ER stress response [16], [17].
There are several features of the ER stress response, which is sometimes called the unfolded protein response, that are cell-survival oriented. One feature involves the degradation of mis-folded ER-proteins by ER-associated degradation, or ERAD [18]. If ERAD is not sufficient to remove mis-folded ER-proteins, the continued accumulation of mis-folded proteins in the ER leads to activation of a conserved signal transduction program, which at the outset is oriented toward resolving the stress, usually by restoring ER-protein folding capacity. However, if the stress is severe enough, later events in ER stress signaling trigger apoptosis [19].
In the healthy heart, ER-protein folding is efficient (Fig. 1A). However, upon ischemia, mis-folded proteins accumulate in the ER (Fig. 1B). These mis-folded proteins are detected by several ER-transmembrane proteins that serve central roles in sensing and initiating UPR signaling; activation of transcription factor 6, ATF6 is one such ER stress sensor that may be the most important ER stress sensor in mammals [20], [21]. In all mammalian cells, ER stress stimulates ATF6 translocation from the ER to the Golgi, where it is cleaved by site 1 and site 2 proteases, which liberates a soluble N-terminal fragment called N-ATF6. N-ATF6, which is generated on the cytoplasmic face of the Golgi, translocates to the nucleus where it binds to several forms of ER stress response elements (ERSEs), and induces transcription of ATF6-dependent ER stress response genes (Fig. 1C).
In the cardiac context, ischemia induces ER stress, as well as ATF6 translocation from the ER to the nucleus, where it serves as a transcription factor, upregulating genes that protect cardiac myocytes from ischemic damage [22]. Using a novel line of transgenic mice that express activated ATF6 in the heart, it was shown that ATF6 reduces myocardial damage and enhances left ventricular function after I/R [23]. It is presumably through ATF6-dependent genes, some of which may encode known or novel CMKs, that this protective aspect of ER stress is mediated in the heart. However, until recently, there had been no identification of ATF6-dependent genes in the heart.
Microarray analyses of ATF6 transgenic mouse hearts identified numerous ATF6-regulated genes, including a group of genes that encode putative ATF6-dependent CMKs [24]. The proteins encoded by this group of genes are potentially unique, since they are induced, synthesized and folded in the ER during adverse conditions that actually inhibit the expression of other CMKs. Accordingly, since the synthesis and folding of ATF6-dependent CMKs are increased, even in the sub-optimal environment of the stressed ER, we postulate that these ER stress-inducible CMKs may serve previously unappreciated roles under conditions that induce ER/SR stress, such as myocardial ischemia. One ER stress-inducible CMK gene that was identified in the ATF6 transgenic mouse hearts is arginine-rich mutated in early tumors, or ARMET, also named mesencephalic astrocyte-derived neurotrophic factor, or MANF (Fig. 1D).
When it was identified as an ATF6-inducible gene in the heart, the ARMET gene sequence was known, but the protein encoded by this gene had not been characterized [25]. In unrelated studies that were designed to identify neuronal growth factors in astrocyte-conditioned medium, a new dopaminergic neurotrophic factor was discovered. Based on its origin and function, it was named mesencephalic astrocyte-derived neurotrophic factor, or MANF. Sequence analysis of MANF suggested that it was encoded in an open reading frame in the ARMET gene. Cloning and sequencing of the MANF cDNA verified that the MANF mRNA is encoded on a 4.3 kb region of human chromosome 3 previously identified as the ARMET gene [26].
The MANF primary transcript encompasses 1109 bp, which codes for a predicted 179 amino acid protein (Fig. 2A). The N-terminal 21 amino acids serve as the signal sequence that targets the nascent protein to the ER/SR in cardiac myocytes. Sequencing of the mature MANF protein isolated from astrocyte-conditioned culture medium verified the absence of the putative signal sequence from the mature protein [26], consistent with its removal by signal peptidase while the nascent MANF protein is co-translationally imported through the ER membrane and into the ER lumen (Fig. 2B). Although many other secreted proteins undergo post-translational proteolytic modification before they are secreted, it is believed that the secreted form of MANF is the full length protein without the signal sequence, i.e. 158 amino acids, and that it has no transmembrane domains and four intramolecular disulfide bonds [27] (Fig. 2B).
The MANF sequence is highly conserved across species. For example, human and fruit fly (D. melanogaster) MANF exhibit 54% identity. Although this conservation exists across a broad range of species, other than the recently-described, ciliary-derived neurotrophic factor (CDNF) [28], [29], [30], which exhibits a 59% amino acid identity with MANF, when comparing the full length protein to others, MANF shows little similarity with any other protein. This uniqueness makes it difficult to predict the mechanism of MANF action by analogy to previously reported examples. Accordingly, to date, most hypotheses concerning the mechanism of MANF function are based on domains that MANF shares with other proteins.
Crystallographic analyses have shown that the N-terminus of MANF adopts a configuration similar to saposin-like proteins [27], which are known to bind membrane and free lipids. In this regard, while the putative receptor for MANF has not yet been identified, it has been proposed that extracellular MANF may exert its function by binding to cell-surface membrane lipids [27]. The C-terminus of MANF has a C-X-X-C motif, which is common in thiol/disulfide oxidoreductases, also called protein disulfide isomerases (PDIs). ER-targeted PDIs catalyze the formation of intramolecular protein disulfide bonds in this organelle. ER-targeted PDIs are often induced during ER stress, where they restore protein folding and, in so doing, exert protective functions [31], [32]. Thus, it has been proposed that in the ER lumen, intracellular MANF may serve a PDI-like function to facilitate disulfide bond formation of proteins folding in this organelle [30]. However, in refute of that hypothesis is a study which showed that MANF does not exhibit oxidoreductase activity [33]. It has been proposed that extracellular MANF may, in part, exert its action by altering disulfide bond status of cell-surface proteins; via this post-translational modification, MANF could affect the function of intracellular signaling processes [30].
Section snippets
MANF expression in tissues and cells
Soon after its discovery in cultured astrocyte-conditioned medium, it was shown that MANF is expressed in several other cell and tissue types, and in some cases, expression was shown to be relatively high, even in the absence of ER stress. For example, in the absence of any ER stress-inducing maneuvers, MANF expression was low in the brain and heart, but relatively high in the stomach, and skeletal muscle [33]. Additionally, in several cultured cell lines, including mouse 3T3 fibroblasts, HeLa
Translational potential
Although there are many studies yet to be carried out, based on our current understanding of MANF structure and function, one would predict it to have therapeutic potential. Due to its potentially broad spectrum of action, a MANF-based therapeutic is predicted to be of high impact. All studies carried out to date that have examined the effects of MANF have demonstrated that it is protective in culture and in vivo. Moreover, this protection has been seen in multiple cell and tissue types,
Disclosure statement
None declared.
Acknowledgments
Research in the author's laboratory is supported by the National Institutes of Health (PO1 HL085577, RO1 HL75573, RO1 HL104535, and RO3 EB011698), the California Institute for Regenerative Medicine (TB1-01193), the Rees-Stealy Research Foundation, the San Diego Chapter of the Achievement Rewards for College Scientists (ARCS) Foundation, and the American Heart Association (10PRE3410005).
The author wishes to thank Shirin Doroudgar for her critical reading of the manuscript, insightful
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