Applied Microbiology and Biotechnology

, Volume 89, Issue 5, pp 1323–1332

Current findings, future trends, and unsolved problems in studies of medicinal mushrooms

Authors

    • Institute of Evolution & Department of Evolutionary and Environmental Biology, Faculty of Natural SciencesUniversity of Haifa
    • N.G. Kholodny Institute of BotanyNational Academy of Sciences of Ukraine
Mini-Review

DOI: 10.1007/s00253-010-3067-4

Cite this article as:
Wasser, S.P. Appl Microbiol Biotechnol (2011) 89: 1323. doi:10.1007/s00253-010-3067-4

Abstract

The target of the present review is to draw attention to many critically important unsolved problems in the future development of medicinal mushroom science in the twenty-first century. Special attention is paid to mushroom polysaccharides. Many, if not all, higher Basidiomycetes mushrooms contain biologically active polysaccharides in fruit bodies, cultured mycelium, and cultured broth. The data on mushroom polysaccharides are summarized for approximately 700 species of higher Hetero- and Homobasidiomycetes. The chemical structure of polysaccharides and its connection to antitumor activity, including possible ways of chemical modification, experimental testing and clinical use of antitumor or immunostimulating polysaccharides, and possible mechanisms of their biological action, are discussed. Numerous bioactive polysaccharides or polysaccharide–protein complexes from medicinal mushrooms are described that appear to enhance innate and cell-mediated immune responses and exhibit antitumor activities in animals and humans. Stimulation of host immune defense systems by bioactive polymers from medicinal mushrooms has significant effects on the maturation, differentiation, and proliferation of many kinds of immune cells in the host. Many of these mushroom polymers were reported previously to have immunotherapeutic properties by facilitating growth inhibition and destruction of tumor cells. While the mechanism of their antitumor actions is still not completely understood, stimulation and modulation of key host immune responses by these mushroom polymers appears central. Particularly and most importantly for modern medicine are polysaccharides with antitumor and immunostimulating properties. Several of the mushroom polysaccharide compounds have proceeded through phases I, II, and III clinical trials and are used extensively and successfully in Asia to treat various cancers and other diseases. A total of 126 medicinal functions are thought to be produced by medicinal mushrooms and fungi including antitumor, immunomodulating, antioxidant, radical scavenging, cardiovascular, antihypercholesterolemia, antiviral, antibacterial, antiparasitic, antifungal, detoxification, hepatoprotective, and antidiabetic effects.

Keywords

Medicinal mushroomsPolysaccharidesbeta-GlucansAntitumorImmunomodulatingAntioxidant activities

Introduction

For millennia, mushrooms have been valued as edible and medical provisions for humankind. Medicinal mushrooms (MM) have an established history of use in traditional ancient therapies. Contemporary research has validated and documented much of the ancient knowledge. The interdisciplinary field of science that studies MMs has been developed and increasingly demonstrates potent and unique properties of compounds extracted from a range of mushroom species in the last three decades. Modern clinical practice in Japan, China, Korea, Russia, and several other countries rely on mushroom-derived preparations (Mizuno 1999; Chang 1999; Wasser and Weis 1999; Reshetnikov et al. 2001; Van Griensven 2009; Wasser 2010a).

Ancient oriental traditions have stressed the importance of several mushroom species, namely Ling Zhi or Reishi (Ganoderma lucidum (W. Curt.: Fr.) P. Karst.) and Shiitake mushrooms (Lentinus edodes (Berk.) Singer). Mushrooms have also played an important role in the treatment of ailments affecting rural populations of eastern European countries. The most important species in these countries were Inonotus obliquus (Pers.: Fr.) Pilát (Chaga), Fomitopsis officinalis (Vill.: Fr.) Bond. et Singer (Wood Conk or Agaricon), Piptoporus betulinus (Bull.: Fr.) P. Karst. (Birch Polypore), and Fomes fomentarius Fr.: Fr (Tinder Bracket; Pöder 2005). These species were used in the treatment of gastrointestinal disorders, various forms of cancers, bronchial asthma, night sweats, etc. There is also a long history of traditional use of mushrooms as curatives in Mesoamerica (especially for species of the genus Psilocybe) in Africa (Yoruba populations in Nigeria and Benin), Algeria, and Egypt. A very special role was found in Fly Agaric (Amanita muscaria (L.: Fr.) Pers.) in Siberia and Tibetan shamanism, Buddhism, and Celtic myths (Wasson 1968; Wasser and Weis 1999; Van Griensven 2009; Wasser 2007a, 2010a).

Moreover, fungi and mushrooms are extremely abundant and diverse worldwide. Recent estimates of the number of fungi on Earth range from 500,000 to 10 million species, and the generally accepted working figure is 1.5 million. Meanwhile, the total number of described fungi of all kinds is currently 100,000 species. The figure is based on the total reached by summing the numbers of species in each genus given in the last edition of the Dictionary of the Fungi (Kirk et al. 2008) and includes all organisms traditionally studied by mycologists: slime molds, chromistan fungi, chytridiaceous fungi, lichen-forming fungi, filamentous fungi, molds, and yeasts. Out of these, mushrooms constitute 14,000 species, calculated from the Dictionary of the Fungi (Kirk et al. 2008). The number of mushroom species on Earth is currently estimated at 150,000, yet perhaps only 10% (approximately 15,000 named species) are known to science or go as high as 22,000 (Hawksworth 2001; Mueller and Schmit 2007; Kirk et al. 2008). An analysis of the localities from which fungi new to science have been described and catalogued in the Index of Fungi in the last 10 years revealed that about 60% of all newly described fungi are from the tropics. This is also the case for mushrooms, especially those species forming ectomycorrhizas with native trees, although new species continue to be discovered in Europe and North America. In various tropical areas, 22–55% (in some cases up to 73%) of mushroom species have proved to be undescribed (Hawksworth 2001; Mueller and Schmit, 2007).

Mushrooms are currently evaluated for their nutritional value and acceptability as well as for their pharmacological properties. They make up a vast and yet largely untapped source of powerfully new pharmaceutical products. In particular and most importantly for modern medicine, MMs present an unlimited source of polysaccharides and polysaccharide–protein complexes with anticancer and immunostimulating properties. Many, if not all, higher Basidiomycetes mushrooms contain biologically active polysaccharides in fruit bodies, cultured mycelia, and cultured broth (Wasser 2002, 2010a; Smith et al. 2003).

Current status

Recently, studied medicinal actions of mushrooms included antitumor, immunomodulating, antioxidant, radical scavenging, cardiovascular, antihypercholesterolemia, antiviral, antibacterial, antiparasitic, antifungal, detoxification, hepatoprotective, and antidiabetic effects (Gao et al. 2002, 2003, 2004; Didukh et al. 2003; Rowan et al. 2003; Sullivan et al 2006; Zhang et al. 2007; Dai et al. 2009; Ichinohe et al. 2010; Wasser 2010a).

Mushroom polysaccharides prevent oncogenesis, show direct antitumor activity against various synergetic tumors, and prevent tumor metastasis. Their activity is especially beneficial when used in conjunction with chemotherapy. The antitumor action of polysaccharides requires an intact T cell component; their activity is mediated through a thymus-dependent immune mechanism. They activate cytotoxic macrophages, monocytes, neutrophiles, natural killer cells, dendritic cells, and chemical messengers (cytokines, such as interleukins, interferons, and colony-stimulating factors) that trigger complementary and acute phase responses. Also, mushroom polysaccharides can be considered as multicytokine inducers able to induce gene expression of various immunomodulatory cytokines and cytokine receptors (Chihara et al. 1969; 1970; Wasser 2002, 2010a; Smith et al. 2003; Gao et al. 2003, 2004; Sullivan et al. 2006; Radic et al. 2010).

The benefits of mushroom compounds on different clinical conditions have attracted the interest of the scientific community in the last decade in order to understand the molecular mechanisms responsible for their actions. Several classes of mushroom compounds such as proteins, polysaccharides, lipopolysaccharides, and glucoproteins have been classified as molecules that have potent effects on the immune system. They may restore and augment immunological responses of host immune effector cells, but they have no direct cytotoxic effect on tumors (Chihara et al. 1969, 1970).

Polysaccharides from other less known but promising mushroom species also show positive results in treating cancers in vitro and in vivo. These species include Agaricus brasiliensis S. Wasser et al. (= Agaricus blazei sensu Heinem.), Phellinus linteus (Berk. et W. Curt.) Teng, Grifola frondosa (Dicks.: Fr.) S.F. Gray, c Retz.: Fr., Hypsizygus marmoreus (Peck) Bigel., Flammulina velutipes (W. Curt.: Fr.) P. Karst., and others. A new class of antitumor MM drugs has been called biological response modifiers (BRMs). The application of BRMs has become the new kind of cancer treatment together with surgery, chemotherapy, and radiotherapy (Mizuno 1999; Wasser 2002, Gao et al. 2002; Zhang et al. 2007).

Immunoceuticals isolated from more than 30 MM species have demonstrated antitumor activity in animal treatments. However, only a few have been tested for their anticancer potential in humans. The few that have been tested are β-d-glucans or β-d-glucans linked to proteins. Moreover, the latter have shown greater immunopotentiation activity than the free glucans. There are plenty clinical studies proving the cancer inhibitory effects of L. edodes (Chihara et al. 1969, 1970; Hobbs 2000; Wasser 2010b), G. frondosa (Zhuang and Wasser 2004; Boh and Berivic 2007), Schizophyllum commune Fr.: Fr. (Hobbs 2005), G. lucidum (Yuen and Gohel 2005; Zhou et al. 2005; Lin 2009; Mahajna et al. 2009; Wasser 2010c), Trametes versicolor (L.: Fr.) Lloyd (Yang 1999; Hobbs 2004), I. obliquus (Mizuno et al. 1999; Park et al. 2005), Ph. linteus (Berk. et M.A. Curt.) Teng (Kim et al. 2007), F. velutipes ( Maruyama and Ikekawa 2007), Cordyceps sinensis (Holliday and Cleaver 2008), etc. Mushroom immunoceuticals act mainly by elevating the host immune system. This process includes activation of dendritic cells, NK cells, T cells, macrophages, and production of cytokines. Several MM products, mainly polysaccharides and especially β-glucans, were developed with clinical and commercial purposes: Krestin (PSK) and polysaccharide peptide from T. versicolor; Lentinan, isolated from L. edodes; Schizopyllan (Sonifilan, Sizofiran, or SPG), from Sch. commune; Befungin from I. obliquus; d-fraction, from G. frondosa, G. lucidum polysaccharides fraction from G. lucidum; active hexose correlated compound; and many others.

Other mushroom compounds of therapeutic interests are the secondary metabolites as lectins, lactones, terpenoids, alkaloids, antibiotics, and metal chelating agents, which are also important for the immune function of the organism. MMs also contain a number of enzymes such as laccase, superoxide dismutase, glucose oxidase, and peroxidase. It has been shown that enzyme therapy plays an important role in cancer treatment preventing oxidative stress and inhibiting cell growth (Wasser and Weis 1999; Zaidman et al. 2005).

It has been documented that fungi produce a huge number of biologically active compounds that not only stimulate the immune system but also modulate specific cellular responses by interfering in particular transduction pathways. For instance, the caffeic acid phenethyl ester (CAPE), which specifically inhibits DNA binding of NF-κB and has shown some promising results in human breast cancer MCF-7 cells, was found to be produced by Agaricus bisporus, Marasmius oreades, L. edodes, and Ph. linteus. Also, a methanol extract of F. fomentarius was reported to inhibit iNOS and COX expression due to the downregulation of NF-κB binding activity to DNA. Panepoxydone, a compound isolated from Panus spp. but also found in Lentinus crinitus, interferes with the NF-κB-mediated signal by inhibiting the phosphorylation of IκBα. These reports demonstrate the fact that such substances can be used as molecular targets in malignant cells in order to combat cancer. Because low molecular sizes help them to penetrate the cell membrane, these substances have also been classified as low molecular weight compounds; among them are lectins, lactones, terpenoids, alkaloids, antibiotics, and metal chelating agents. Many fungal species have already been reported to produce various metabolites capable of modulating different intracellular pathways thus playing an essential role in cancer treatment (Zaidman et al. 2005, 2008; Yassin et al. 2008; Petrova et al. 2008, 2009; Rouhana-Toubi et al. 2009; Dotan et al. 2010; Mahajna et al. 2010; Ruimi et al. 2010a, b).

MMs produce beneficial effects not only as drugs but also as a novel class of products by a variety of names: dietary supplements (DSs), functional foods, nutriceuticals, mycopharmaceuticals, and designer foods that produce healthy benefits through everyday use as part of a healthy diet (Chang and Buswell 2003; Chang 2006; Wasser and Akavia 2008). The increased interest in traditional remedies for various physiological disorders and the recognition of numerous biological activities of mushroom products have led to the coining of the term “mushroom nutriceuticals”, which should not be confused with nutraceuticals, functional foods, and pharmaceuticals. A mushroom nutriceutical is a refined, or partially refined, extract or dried biomass from either mycelium or the fruiting body of a mushroom, which is consumed in the form of capsules or tablets as a DS (not a food) and has potentially therapeutic applications. Regular intake may enhance the immune response of the human body thereby increasing resistance to disease and in some cases causing regression of the disease state. Thus, acting as immunopotentiators, MM preparations modify host biological responses (also known as BRMs).

There is no doubt that MM-based products can serve as superior DS. The market of DS from mushrooms is quickly growing and is valued at more than US $15 billion today (representing 10% of the general market of dietary supplements). Every year data are collected as new evidence on the beneficial effects from DS made from MMs (Chang 1999; Wasser and Akavia 2008). A new product for dementia (especially for Alzheimer’s disease) based on a proprietary standardized extract that contains hericenones and amyloban (both from the Hericium erinaceus—Lion’s Mane mushroom) currently exists on the market.

Future trends and unsolved problems

On the one hand, MM science made great progress in the last 30 years, embracing traditional Chinese medicines (TCM) as well as Chinese herbal medicines along with their commercial derivatives. On the other hand, there are many unsolved but very important problems in future MM development, which in turn can also affect the continuation of MM science in the twenty-first century. Below, I have listed the most critically important problems in the continuing development of MM science.

Taxonomy and nomenclature of MMs

Many species of MMs are critically misunderstood. Without the correct scientific name of MM from the onset, future investigative studies will have no validity. Together with classical taxonomical methods, DNA bar coding may be useful and helpful for the correct identification of MM species, including the study of type material and standardized MM products. For example, there is now mounting evidence that shows most species previously reported as Ling Zhi or Reishi (Ganoderma lucidum) in most pharmacological studies were mistakenly identified. G. lucidum presents a taxon-linneon or species-complex of which future subdivision requires caution (Wasser et al. 2006). Publications, patents, and products are also at risk. Through the years, at least 166 laccate Ganoderma species have been described worldwide (Moncalvo and Rivarden 1997). Approximately 100 Ganoderma species are known from China (Zhao 1989). It is not known what the taxonomic positions of so-called medicinal Blue Ling Zhi, or Red Ling Zhi, or White Ling Zhi are for example.

Another example of mistaken identity was revealed for Agaricus blazei, well-known in literature as a MM. A. blazei is a North American endemic species described only from three localities and does not exist in culture; it therefore cannot be listed as a MM. Two concepts of A. blazei exist: A. blazei sensu Murrill, reported from three localities of the USA, and A. blazei sensu Heinem., reported from Brazil and cultivated in Japan (Wasser et al. 2002). We studied type material of A. blazei sensu Murrill, A. blazei sensu Heinem., A. subrufescens from the New York Botanical Garden (New York), and other species of this group as well as cultivated strains from different countries and material from nature in Brazil. A. blazei sensu Murrill and A. blazei sensu Heinem. represent two different species. A. blazei sensu Murrill differs from A. blazei sensu Heinem. in size, shape of fruit bodies, pileal surface, type of pileal covering, presence of cheilocystidia, and spore size. It was determined that the widely cultivated culinary-medicinal mushroom known as A. blazei had nothing in common with the A. blazei described by Murrill from the USA, and therefore, a new species for science was described as A. brasiliensis (Wasser et al. 2002). A. blazei is no longer known as a culinary-medicinal mushroom. Later, using morphological data with molecular and biological data, the differences between A. blazei and A. brasiliensis were proven (Didukh et al. 2004). Kerrigan (2005), and our researchers (Wasser et al. 2002, 2005; Didukh et al. 2004; Wasser 2007b) published several articles in the last few years to clarify distinctions among A. brasiliensis, A. subrufescens, A. fiardii, A. praemagniceps, and A. blazei. These species are now classified with distinct morphological, molecular, and biological characteristics and different geographical distribution. The misclassification of A. blazei caused many problems in MM science but has since been corrected.

The study of culinary-medicinal mushrooms in pure culture

More attention must be paid to the study of culinary-medicinal mushrooms in pure culture. The study of cultures is necessary to provide stability and continuity in scientific work. The teleomorph stage is the most essential criterion for the identification of cultures, but every so often MMs do not produce fruit bodies in pure culture. Vegetative mycelia of MMs in pure culture have received too little attention in mycological literature. Many species of MMs cannot be identified correctly without the study of vegetative mycelia. Vegetative mycelia of MM cultures is a complex of branched hyphae, which differ only within narrow limits of width, length, number of nuclei, thickness of cell wall, and branching. The accumulation of information on an increasing number of studies of vegetative mycelia of MM species provides new material for the study and comparison of morphological characteristics of mycelia and for the estimation of their potential use for taxonomic purposes and purity control in biotechnological processes (Buchalo and Didukh 2005; Buchalo et al. 2009).

Because there are no type strains of MMs, we need to choose correctly identified type strains of many species of MMs. We need to organize a world culture collection of culinary-medicinal mushrooms with depository activity following patent procedures according to the Budapest Treaty. This issue must be discussed with the World Federation of Culture Collections (Wasser 2010a).

Medicinal mushroom dietary supplement problems

Recently, there has been growing popularity in developing mushroom carbohydrate polymers as DS or functional foods. Significant questions arise with establishing DS and MM products including their safety, standardization, regulation, efficacy, and mechanism of action.

Unfortunately, standardization around the world of DS from MMs is still in its early stages, including an insufficient understanding of DS bioactive effects. We do not have internationally recognized standards and protocols for production and the testing of MMs products. Only proper standards and protocols can guarantee product quality. Without consistency in the quality of MM products, commercially available MM preparations of mushrooms will be dramatically different and vary enormously in composition and affectivity. It is not known whether bioactive effects are caused by a single component or are the result of a synergistic impact from several ingredients. There is insufficient data to determine which components better affect mushroom fruiting bodies or submerged mycelia powder vs. extracts. Are simple dried fruiting bodies and mycelium powders as effective as hot water, alcoholic, or hydro-alcoholic extracts? Between crude extracts and isolated fractions, which one is more effective and has a higher safety profile (some companies are selling fraction-like G. frondosa Maitake D-Fraction or GLPS fraction of G. lucidum)? The role of low molecular weight compounds for MM extracts is still unclear.

What is more effective—the combination of components containing biomass or extracts of two to ten different species in one pill or having one species in one pill? How can one assess the effectiveness of different mushroom products when blending in many species in one product (“shotgun” approach)? Since mushroom products can be cytokine stimulants, from what age are they safe for young children to take since their immune systems are not yet mature? What dosages are safe and effective during pregnancy and nursing? The absence of insufficiently elaborated standards for the recommended use of MM DSs, including precise doses and duration of administration, needs very serious investigation. Some research shows that too high a dose could lead to immune suppression and too low a dose might not trigger an immune response. Furthermore, major problems associated with mushroom-based DSs are due to their wide variability, the current lack of standards for production, and the lack of testing protocols necessary to guarantee product quality. The active ingredients of many present-day commercial mushroom products have not been indicated.

Adulteration of MM products with spurious species (for instance, different Ganoderma species for G. lucidum, Stereum species for Trametes species, and different Cordyceps species for C. sinensis) is very common. Difficulties are found in producing pure β-glucans for the market (90–95% of β-glucan on the market is considered counterfeit and adulterated). Adulteration has led to a number of adverse effects resulting in nephropathy, acute hepatitis, coma, and fever. There have also been neurological, cardiovascular, and gastrointestinal problems reported with TCM (McKenna et al. 2002; Bagchi and Preuss 2005).

We still have not solved the problems concerning the safety of several well-known MM products. For example, there have been several reports from hospitals in Japan where the intake of MM A. brasiliensis products caused liver dysfunctions (Nagaoka et al. 2006). On the basis of a study of a phase I/II trial polysaccharide extract of G. frondosa in breast cancer patients, it was concluded that the Maitake mushroom produces more complex effects than presumed and may depress as well as enhance immune function (Deng et al. 2009).

What is the role of fresh mushroom consumption? The consumption of fresh mushrooms was found to increase, for example, anti-β-glucan antibodies in the serum of humans. It was also suggested to provide better defense against pathogenic fungi by the Ohno group from Japan (Ishibashi et al. 2005). Information is still lacking on the use of MM DSs and their inter-crossing or interaction with some drugs.

Medicinal mushroom natural products are an unclaimed source for drug discovery

Cancer is the major public health problem in the world. Currently, one in four deaths in the USA is due to cancer. Cancer is the second most common cause of death among children between the age of 1 and 14 years in the USA. A total of approximately 1.5 million are projected to occur each year in the USA (Jemal et al. 2008).

The development of real immunomodulating and anticancer drugs from MM polysaccharides (e.g., Lentinan, Schizophyllan, and Krestin) was hampered by the fact that high molecular weight compounds are used. All MM drugs were developed from high molecular weight polysaccharides from 100,000 to 0.5 million Da. These compounds cannot be synthesized; therefore, their production is restricted to extraction from fruit bodies, cultured mycelium, or cultured broth. Such an approach imposes high market prices. Today, science should concentrate on the beneficial medicinal effects of low weight molecular compounds produced by MMs, i.e., low molecular weight secondary metabolites targeting processes such as apoptosis, angiogenesis, metastasis, cell cycle regulation, and signal transduction cascades (Zaidman et al. 2005). Western pharmaceutical companies are more interested in relatively easily synthesized compounds that can be produced for markets.

Historically, the majority of new drugs have been generated from natural products (secondary metabolites). MMs are an unclaimed source for drug discovery. By 1990, about 80% of drugs were either natural products or analogs inspired by them. “Blockbuster drugs” like antibiotics (penicillin, tetracycline, and erythromycin), antiparasitics (avermectin), antimalarials (quinine, artemisinin), lipid control agents (lovastatin and analogs), immunosuppressants for organ transplants (cyclosporin, rapamycins), and anticancer drugs (taxol, doxorubicin) revolutionized medicine (Li and Vederas 2009). Many of the abovementioned drugs were discovered from components found in fungi.

Modern pharmaceutical trends in preventing cancer include the development of new drugs with (a) growth-factor inhibitors of cancer cells (drugs such as Herceptin, Erbitux, and Terceva). They block a cancer cell’s link to critical proteins that help it divide and grow; (b) hormone blockers (drugs such as Tamoxifen) keep cells from dividing by binding to estrogen receptors, which are overexpressed in some tumor cells; (c) signal blockers working inside a cell. These drugs interrupt communication among enzymes that regulate growth and development; and (d) angiogenesis inhibitors, e.g., Avastin, was the first drug to inhibit the formation of new blood vessels around cancer cells, starving them of nourishment (Ammerpohl et al. 2010).

About 860 cancer drugs are being tested on humans. This number is more than twice the number of experimental drugs for heart disease and stroke combined, nearly twice as many for AIDS and all other infectious diseases combined and nearly twice as many for Alzheimer’s and all other neurological diseases combined (Pollack 2009). Cancer drugs have been the biggest category of drugs in terms of sales worldwide since 2006 and in the USA since 2008, according to market research by IMS Health. Today, drug companies see a future in treating cancer. The world’s largest pharmaceutical company (Pfizer), for example, was focused on cardiovascular drugs, the cholesterol lowering buster Lipitor (Endo 2004), and the blood pressure reduction pill Norvasc (Pollack 2009). Recently, Pfizer hired about 1,000 researchers for an all-out effort to develop drugs for cancer, a disease the company once largely ignored. Pfizer has now scaled back on cardiovascular research and has made cancer drugs one of its six focus areas. About 20% of Pfizer’s more than $7 billion budget for research and development spending is on cancer research, and 22 of the roughly 100 drugs being tested are anticancer drugs (Pollack 2009).

Progress in research of MMs must include genomics, proteomics, metabolomics, and systems pharmacology. Studying molecular mechanisms to determine the medicinal effects of MMs should be the focus of new investigations using modern methods in the above approaches.

Another important source for substances of therapeutic interest can be found in the pool of secondary metabolites produced by MMs. These substances can be classified according to five main metabolic sources (Zaidman et al. 2005): amino acid-derived pathways; the shikimic acid pathway for the biosynthesis of aromatic amino acids; acetate-malonate pathway from acetyl coenzyme A; the mevalonic acid pathway from acetyl coenzyme A, which functions in primary metabolism for the synthesis of sterols; and polysaccharides and peptidopolysaccharides. The polyketide and the mevalonic acid pathways are most often involved, and they produce a greater variety of compounds than the other pathways.

Every effort should be made to find new sources for anticancer drugs using low molecular weight secondary metabolites from MM that can inhibit or trigger specific responses, i.e., activating or inhibiting the NF-κB, inhibiting protein and especially tyrosine kinases, aromatase and sulfatase, matrix metalloproteinases, cyclooxygenases, DNA topoisomerases and DNA polymerase, anti-angiogenic substances, etc. (Zaidman et al. 2005, 2008; Yassin et al. 2008; Petrova et al. 2008, 2009; Rouhana-Toubi et al. 2009).

Fungal low molecular weight compounds directly influencing NF-κB inhibitory effects are CAPE, cordycepin, panepoxydone, and cycloepoxydon. Low molecular weight CAPE produced, for example, by Ph. linteus and M. oreades, show specific cytotoxicity against tumor cells, show NF-κB inhibitor activity, and can be a candidate for antitumor drugs, especially against breast cancer (Petrova et al. 2009).

Pharmaceutical companies involved in drug discovery need new sources of natural products. MMs are the best unclaimed gifts of nature that in a short amount of time can be used in the production of new pharmaceuticals.

Unsolved problems in the study of structural characteristics, isolation process, receptor-mediated mechanism, and antitumor activity of MM β-glucans

The success of β-glucans and other mushroom carbohydrate polymers’ application requires active research in addressing the structure–activity relationship of mushroom carbohydrate polymers, especially in terms of molecular conformation and receptor-mediated mechanisms (Chen and Seviour 2007). Clarification of water solubility, size of molecules and molecular weight, structure, and molecular mechanisms of β-glucan action takes into consideration that not all β-glucans contained in a MM exhibit pharmaceutical activity (Ohno 2005; Chen and Seviour 2007; Zhang et al. 2007).

We still do not know the role of molecular weights in pharmaceutical activity of β-glucans. Problems of affectivity of high molecular weight β-glucans vs. low molecular weight β-glucans still exist. Scleroglucan high molecular weight preparations are most effective (Ohno 2005). But, for example, only low molecular weight Lentinan has higher antitumor activity (Chihara et al. 1969; Zhang et al. 2007). We must take into account the different reactivity of β-glucans in each individual (anti-β-glucan titer and increments of the titer by β-glucan administration are different; reactivity of peripheral blood leucocytes to β-glucan is significantly different in each individuals; reactivity to β-glucans is, for example, significantly different in various strains of mice; Ohno 2005; Chen and Seviour 2007; Zhang et al. 2007).

Solubility in water is one of the more important characteristics of β-glucans. We still do not know what the major factors are affecting the solubility and pharmaceutical activity of β-glucans: Molecular weight, length of side chain, number of side chains on the main chain, rations of (1,4), (1,6), and (1,3) linkages, and ionization by acid must be considered (Wasser 2002; Ohno 2005; Zhang et al. 2007). Soluble β-glucans appear to be stronger immunostimulators than insoluble β-glucans. The reasons for this are not totally clear. We do not know the exact mechanism of intestinal absorption of orally administrated β-glucan (nonspecific intestinal absorption; passage of β-glucans through the gap junction in the intestinal epithelial membrane; absorption through intestinal M cells; absorption after binding with Toll-like receptor proteins on the intestinal lumen, and dendritic cell probing; Miller et al. 2007; Pamer 2007). It is possible that orally administrated insoluble β-glucans are subsequently degraded into smaller bioactive oligomers after ingestion (Lehmann and Kunze 2000).

We need to clarify differences between plant β-glucans (Tada et al. 2008; Tiwari and Cummins 2009), yeasts β-glucans (Liu et al. 2009; Verticka and Vetrickova 2009), and β-glucans from MMs (Ohno 2005; Chen and Seviour 2007; Zhang et al. 2007; Wasser 2010a). What is the difference in structure, solubility, and biological activity? For example, the structure of cereal β-glucan is essentially β-1,3 and β-1,4-linkages, not β-1,6-linkages. In addition, plant β-glucans are linear, not branched. Usually, the molecular weights of plant β-glucans are smaller than that of MM β-glucans. Biological activity has not been fully examined in the case of plant β-glucans. Usually, yeast β-glucans are only partly water soluble, and many MM β-glucans are water insoluble. Why do they have different biological activity? What are the key advantages of MM β-glucans compared to cereal β-glucans for example, or yeast β-glucans?

We know a lot about the function of receptor dendritic-cell-associated C-type lectin-1 (Dectin-1) of β-glucans (Taylor et al. 2007; Harada and Ohno 2008; Graham and Brown 2009). However, we still do not know how β-glucans bind to receptor Dectin-1. How does the side chain affect binding to the receptor? We know a lot about the function of receptor Dectin-1 of β-glucans, but the function of receptor Dectin-2 is still unclear (Geijtenbeek and Gringhuis 2009).

Why do β-glucans have the triple-helix conformation, and does the triple-helix structure have an advantage for MMs that have a single strand? (Ohno 2005; Chen and Seviour 2007). Unfortunately, we do not understand what structural features are best for inducing specific activities and, even more importantly, what the presence of hydrophilic groups located on the outside surface of the helix is. We can see contradictory data in literature on the biological activity of triple-helix and single-helix structures of the same β-glucan—for example, Schizophyllan (Ohno 2005; Chen and Seviour 2007). We still do not know which has the stronger biological activity—the closed triple-helix or a partially opened triple-helix (Mizuno 1999; Falch et al. 2000).

Important issues of MM studies adapting to the twenty-first century

  1. 1.

    The role of polysaccharide–protein or polysaccharide–peptide complexes in pharmacological activity of MM needs further investigation.

     
  2. 2.

    More studies are needed to demonstrate which mushroom extracts or compounds are most effective for specific ailments against viral infections, bacterial infections, metabolic syndromes, cancer, cholesterol, etc.

     
  3. 3.

    The development of new methods and processes in the study of MMs must be a priority.

    For example, a new method was developed in 2009 for nanoparticle extraction of water soluble β-glucans from MMs by the Park group (Park et al. 2009) from South Korea. A novel process for nanoparticle extraction of Sparan, the β-d-glucan from Sparassis crispa, and Phellian, the β-d-glucan from Ph. linteus, was investigated using insoluble tungsten carbide as a model for nanoknife technology. This was the first report showing that the nanoknife method results in high yields of Sparan (70.2%) and Phellin (65.2%) with an average particle size of 150 and 390 nm, respectively. The Park group (Park et al. 2009) proposed the nanoknife method to be used in producing β-glucans for food, cosmetics, and pharmaceutical industries.

     
  4. 4.

    High-quality, long-term double-blinded, placebo-controlled studies with large trial populations are definitely needed to procure safety and efficacy of MMs with statistical power.

     
  5. 5.

    More attention must be paid to research on farm animals and MMs. On the one hand, there are research areas that could potentially be advanced by using farm animals as biomedical models including obesity, diabetes, aging, cardiovascular diseases, infectious diseases, neurobiology, cancer, nutrition, immunology, ophthalmology, and reproduction. On the other hand, we can revolutionize farm animal research, which is now in crisis (Roberts et al. 2009) by proposing new types of food and DS, antibiotic replacement, and antiviral agents for farm animals.

     
  6. 6.

    Protecting intellectual properties (IP) of MMs’ genetic resources for invention and innovation is a problem that needs more attention.

    Mushroom genetic resources are currently being utilized and exploited by the pharmaceutical, cosmetic, agricultural, food, enzyme, chemical, and waste-treatment industries. Nevertheless, the role of IP advantages in today’s knowledge-driven enterprises is frequently overlooked, despite their potential as sources of monetary value and financial gain. IPs are often under-managed or under-leveraged. The challenge is how to create, protect, and extract value from IP assets for invention and innovation (Jong 2005).

     
  7. 7.

    We must continue to educate the society and consumers on MM science.

    It is our responsibility as scientists to do much more in educating the public at large on the health benefits of MMs. Interest in and advancements made in current research are not always visible or available to the public. It is unbelievable that in 2010, many people all over the world are completely unaware of the health benefits of medicinal mushrooms. Scientists should be recruiting deans, presidents, chancellors, alumni associations, and journalists to help them communicate with their local community and business leaders, as well as with many important governmental people. We need to create opportunities to invite entrepreneurs into our laboratories. We need to provide the answers to questions and concerns because the positive impact of our research is not always “newsworthy”. We need to create different venues to bring this knowledge across the board—from schools to medical professionals, to consumers and industries (Cicerone 2009). By publishing more information on the benefits of medicinal mushrooms through advertisements, commercials, pamphlets, etc., the public will learn about the safety and availability of MM products.

     
  8. 8.

    It is to our advantage to bridge the gap between Western and Eastern medicine.

    Western and Eastern medicines have adopted different regulatory systems for herbal and mushroom preparations. Most Western countries follow the rules of the WHO and DSHEA in which plant or MM extracts are defined as DSs, and clinical studies are not required before DSs are introduced to the market (Wasser and Akavia 2008). China and several other Asian countries define many of the same herbs and some MMs as drugs, and therefore, clinical studies are needed. Western medicine has made little use of medicinal mushroom products partly due to their complex structure and lack of acceptable pharmacological purity (Sullivan et al. 2006). Our target for the future should be to adopt those regulations, standards, and practices from Western and Eastern medicine that have proven to be the most valuable in the quest for health benefits in the twenty-first century.

     

Acknowledgments

I would like to thank all of my colleagues who have contributed their time and effort in sending information and data to help me with the preparation of this paper: Dr. John Holiday (USA), Christopher Hobbs (USA), Professor Ha Won Kim (South Korea), Professor Naohito Ohno (Japan), Professor Peter C.K. Cheung (Hong Kong), Professor Shufeng Zhou (Australia), Professor Reinhold Pöder (Austria), Dr. Daniel Job (Switzerland), Dr. Cun Zhuang (USA), Professor G. Guzmàn (Mexico), Dr. Roumiana Petrova (Bulgaria), and Dr. Jamal Mahajna (Israel).

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© Springer-Verlag 2010