Heart failure is a growing global health problem that burdens both healthcare and the economy [1, 2]. It is a clinical syndrome caused by a functional abnormality in the heart that results in elevated intracardiac pressure and/or inadequate cardiac output. This leads to cardiac symptoms during exercise or in advanced stages also at rest [3,4,5]. As the average age of the population increases, so does the incidence of heart failure [1], which is the most common reason for hospitalizations in the Western world [6]. The mortality rate of heart failure patients is high with the 5-year mortality rate after hospitalization being approximately 60% [7, 8].

The etiology of heart failure is multifactorial. Typically, heart failure is the result of myocardial dysfunction, which is caused by diastolic or systolic disorders, sometimes by both. In systolic heart failure the heart muscle does not contract efficiently. In diastolic heart failure the heart muscle fails to relax normally between the heart beats. Also, inappropriate function of heart valves, changes in the pericardium and endocardium, and cardiac malformations or arrhythmias, such as atrial fibrillation can cause heart failure [5, 9]. Patients with heart failure are frequently classified by the left ventricular ejection fraction, which can be reduced, preserved or in the mid-range. In the reduced heart failure (HFrEF) the left ventricular EF is under or equal to 40%, in the mid-range (HFmrEF) EF is from 41 to 49% and in the diastolic form (HFpEF) EF is more than 50% [5].

Currently, the treatment of heart failure is based on medication, lifestyle changes and for selected patients pacemakers or invasive procedures. Treatment aims to alleviate the symptoms and improve the prognosis of the patient, mainly by reducing the load on the left ventricle and suppressing neuroendocrine activation triggered by the heart failure. Medications that improve the prognosis of HFrEF are SGLT2-inhibitors and neuro-hormonal antagonists consisting of angiotensin-converting enzyme inhibitors (ACEI), mineralocorticoid receptor antagonists (MRAs) and beta-blockers. Diuretics and digilatis can also be used to reduce symptoms. Digitalis is used for HFrEF patients when they have a high-rate atrial fibrillation [10]. Nevertheless, conservative treatments do not restore damaged heart muscle. Also, in many patients, heart failure progresses beyond optimal conservative therapy. In more severe cases, surgery is required to open occluded coronary arteries or to replace heart valves. Patients with congestive heart failure can be treated with biventricular pacing therapy. Ventricular assistance devices can be used as a bridge to heart transplantation or as a treatment in lieu of the transplant. The only curative treatment is a heart transplant, which is rare in clinical practice [5]. Heart failure is a growing health problem, which still has a poor prognosis despite the improvements in pharmacological treatments. The need for new therapies is vitally important [4].

Therapeutic angiogenesis could become a future treatment for heart failure. In angiogenesis new blood vessels grow from pre-existing vessels. Vascular endothelial growth factors (VEGFs) are very promising transgenes for the therapeutic angiogenesis. VEGF family includes VEGF-A, VEGF-B, VEGF-C, VEGF-D, and placenta growth factor (PIGF). VEGFs signal by binding to tyrosine kinase receptors VEGF-R1, VEGF-R2 and VEGF-R3 and co-receptors neuropilin 1 and 2 (NRP-1 and −2) ([11]Fig. 1) [11]. VEGFs can improve perfusion resulting in improvement in the overall myocardial condition which is desirable for heart failure treatment [12]. They also influence energy metabolism [13] and possible stem and progenitor cell recruitment [14, 15] resulting in regenerative changes, especially VEGF-B and VEGF-D. These characteristics make VEGFs attractive in therapeutic settings of heart failure.

Ligands for vascular endothelial growth factor receptors and co-receptors. VEGFR-1 and VEGFR-2 are mainly expressed on vascular endothelial cells, while VEGFR-3 is widely expressed in lymphatic endothelial cells. Nrp-1 is expressed in many cell types such as neurons, vascular endothelium, and heart, whereas Nrp-2 is primarily found in the lymphatic endothelial cells. Created in BioRender [16]

VEGF-A has not been widely used for heart failure gene therapy whereas VEGF-B is relatively myocardium-specific, at least in larger mammals, which makes it an interesting target for gene therapy aimed at treating heart diseases [12]. VEGF-B is highly expressed in metabolically active tissues including the heart [13,14,15, 17]. Currently, two alternative spliced isoforms of VEGF-B have been identified: VEGF-B167 and VEGF-B186 [18], with VEGF-B167 being the predominant isoform accounting for 80% of the VEGF-B transcripts [19]. Both VEGF-B isoforms bind to VEGF-R1, NRP-1 and NRP-2, but VEGF-B186 binds to Nrp-1 and −2 receptors only after proteolytic modification [11].

VEGF-B is a cardioprotective growth factor. Besides angiogenesis, it regulates myocardial contractility, metabolism and protects cardiomyocytes from ischemic and apoptotic damages by downregulating the expression of proapoptotic genes [20,21,22]. VEGF-B167 induces α-MHC, SERCA2a, RYR, ANF, BNP, and PGC1α genes and represses the expression of β-MHC and α-actin 1 genes, which maintains the contractility of the heart muscle [23]. In heart failure the level of VEGF-B is reduced [12, 24]. VEGF-B promotes angiogenesis also in the subendocardial cardiac region [25].

The signaling pathways mediating VEGF-B-induced angiogenic effects have remained somewhat unclear despite extensive studies. It has been suggested that endogenous VEGF-A binds more efficiently to VEGF-R2 when VEGF-B is present, resulting in angiogenesis [20, 26]. Additionally, Lähteenvuo et al. found that VEGF-R1 and NRP-1 receptors are important for VEGF-B-induced angiogenic effects [12]. On the other hand, Korpela et al. showed in their study by using mutated VEGF-B186 isoforms that only the long form of VEGF-B186 (binds only to VEGF-R1) is able to induce angiogenesis, but the shorter isoforms VEGF-B127 (binds to both VEGF-R1 and NRP-1) and VEGF-B109 (binds only to VEGF-R1) are not angiogenic [27]. The results provided by Korpela et al. and Mallick et al. suggest that there are still unknown receptors and/or mechanism that mediate VEGF-B effects [27, 28].

Indeed, Mallick et al. showed that the long form of VEGF-B186 induces endothelial progenitor cell recruitment via the upregulation of proangiogenic and hematopoietic growth factors leading to angiogenesis [11]. Also, Sultan et al. described a distinct VEGF-B-induced endothelial cell population [29]. Recently, RGD-binding integrins were found as novel receptors of VEGF-B186 that can mediate VEGF-B-induced endoplasmic reticulum stress and support angiogenesis [28].

VEGF-B is co-expressed with a cluster of nuclear encoded mitochondrial genes which are involved in the oxidative phosphorylation [30]. Through binding to its receptors, VEGF-B regulates vascular fatty acid uptake by promoting the activity of transport proteins in endothelial cells. Lipid uptake of ECs is also closely related to mitochondrial lipid utilization [30]. Enhancing the VEGF-B signal has shown to interfere with low-density lipoprotein receptor recycling [31]. Excess activation of the fatty acid uptake and utilization in myocardium leads to inhibition of pyruvate dehydrogenase, which is caused by increased mitochondrial acetyl-CoA levels. This leads to glucose oxidation and subsequent glycolysis through several pathways, including insulin signaling [12, 32, 33]. The inhibition of VEGF-B signaling in mice decreased lipid accumulation and improved insulin sensitivity and glucose tolerance [34].

Metabolically active tissues, such as the heart muscle utilize fatty acids as the primary source of energy [35]. Imbalance of the fatty acid uptake and oxidation results in the accumulation of ceramides and diacylglycerols. This affects insulin mediated glucose uptake and related effects on gene expression and signaling in the heart [32, 36,37,38]. Through regulating fatty acid uptake and oxidation, VEGF-B is involved in mitochondrial dysfunction in the heart [24]. However, there is still uncertainty whether the fatty acid oxidation-related effects were observed due to high-fat diet-induced insulin resistance [39, 40]. Overexpression of VEGF-B in the hearts from transgenic mice showed shifting of the fatty acid oxidation towards glucose metabolism [20] and reduced lipid deposition together with an increased susceptibility to heart failure [41]. Meanwhile, VEGF-B186 overexpression in mouse adipose tissues showed increased insulin sensitivity, glucose tolerance and improved metabolic health [26].

VEGF-D has an essential role in lymphangiogenesis but can also cause strong angiogenesis. VEGF-D binds to VEGF-R2 and VEGF-R3, where the angiogenic effect is signaled through VEGF-R2 [42]. VEGF-D∆N∆C is a proteolytically processed form of VEGF-D, where the N and C terminal ends have been removed and this modification has shown to enhance angiogenicity of the growth factor through increased affinity to VEGF-R2 [42, 43]. VEGF-D∆N∆C has been taken to clinical testing in refractory angina pectoris patients in KAT301 phase 1 trial, which showed promising results after adenoviral VEGF-D∆N∆C gene transfer by improving perfusion reserve in the treated areas after one year follow-up [44,45,46]. Currently ongoing ReGenHeart phase 2 trial will test its efficacy in a randomized, double-blinded, placebo-controlled multicenter study. Primary endpoints are exercise tolerance and alleviation of symptoms at 6 months follow-up [47].

Stromal cell-derived factor 1 (SDF-1) is a chemokine facilitating tissue repair after injury through stimulation of the anti-inflammatory pathways, vascular density and stem cell guidance to the myocardium [48]. SDF-1 plasma levels can predict the risk of heart failure and mortality [49]. SDF-1 levels increase typically after myocardial infarction, leading to increased amount of cardiac stem cells in the infarct border zone, and improving cardiac function [48]. Expression of SDF-1 induces angiogenesis in both rats and large animals. However, large animal studies failed to improve left ventricular function and reduce infarct size, which was observed in rats [48, 50].

The calcium ion (Ca2 +) cycle plays a crucial role in the ability of cardiomyocytes to contract and relax [51]. Sarcoplasmic reticulum (SR) maintains the intracellular balance of Ca2 + together with SR transport proteins. Abnormality in the Ca2 + handling mechanisms is linked to the development of impaired cardiac contractility and repolarization [52].

SR ATPase (SERCA) plays a crucial role in Ca2 + signaling by transporting free Ca2 + ions back to the SR (Fig. 2). The main subtype present in the heart is SERCA2a, which functions during diastole [53]. Decreased expression and/or function of SERCA2a has been linked to diastolic dysfunction and HFpEF [54,55,56], making it an attractive target for therapeutic approaches [57].

SERCA2a plays a crucial role in Ca2 + homeostasis in cardiomyocytes. During systole Ca2 + ions are released from SR through RyR2 channels. During diastole Ca2 + ions are transported back to SR through SERCA2a. Abnormality in the activity or the amount of SERCA2a is connected to diastolic dysfunction and HFpEF. Created in BioRender [16]

Multiple studies in various animal models [55, 58] have shown improvements in cardiac function through enhancing or restoring SERCA2a activity. In a rat model Ad-SERCA2a gene transfer showed improvements in both systolic and diastolic performance in failing hearts as well as in survival and cardiac energetics [58]. The activity of SERCA2a is regulated by several factors, which include phospholamban (PLN), S100A1 and small ubiquitin related modifiers (SUMOs). PLN inhibits SERCA2a activity and phosphorylation of PLN activates SERCA2a through inhibiting the function of PLN [59].

PLN is known to be involved in the development of cardiac diseases, including heart failure [60, 61]. Mutation of arginine 14 in PLN causes severe arrhythmic cardiomyopathy and is relatively common in the Dutch population [62]. In heart failure patients, PLN levels seem to remain stable while PLN phosphorylation and SERCA2a levels decrease causing reduced Ca2 + cycling due to disrupted PLN to SERCA2a ratio [