Mutation in AQP5, encoding aquaporin 5, causes palmoplantar keratoderma Bothnia type

Aquaporin-5 Dynamic Regulation

Aquaporin-5 (AQP5), belonging to the aquaporins (AQPs) family of transmembrane water channels, facilitates osmotically driven water flux across biological membranes and the movement of hydrogen peroxide and CO₂. Various mechanisms have been shown to dynamically regulate AQP5 expression, trafficking, and function. Besides fulfilling its primary water permeability function, AQP5 has been shown to regulate downstream effectors playing roles in various cellular processes. This review provides a comprehensive overview of the current knowledge of the upstream and downstream effectors of AQP5 to gain an in-depth understanding of the physiological and pathophysiological processes involving AQP5.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

Aquaporins in Skin

The skin is the largest organ of our body and plays a protective role against the external environment. The skin functions as a mechanical and water permeability barrier, assisting with thermoregulation and defending our body against a variety of stresses such as ultraviolet radiation, microbial infection, physical injuries, and chemical hazards. The structure of the skin consists of three main layers: the hypodermis, the dermis, and the epidermis. Aquaporins (AQPs) are a family of integral membrane proteins whose function is to regulate intracellular fluid hemostasis by facilitating the transportation of water, and in some cases small molecules, across the cell membranes. Up to six different AQPs (AQP1, 3, 5, 7, 9, and 10) are expressed in a variety of cell types in the skin. The AQP family plays an important role in these various locations, contributing to many key functions of the skin including hydration, wound healing, and immune responses. The involvement of different aquaporin family members in skin is discussed.

Bothnian Palmoplantar Keratoderma: Further Delineation of the Associated Phenotype

Bothnian palmoplantar keratoderma (PPKB, MIM600231) is an autosomal dominant form of diffuse non-epidermolytic PPK characterized by spontaneous yellowish-white PPK associated with a spongy appearance after water-immersion. It is due to AQP5 heterozygous mutations. We report four patients carrying a novel AQP5 heterozygous mutation (c.125T>A; p.(Ile42Asn)), and belonging to the same French family. Early palmoplantar swelling (before one year of age), pruritus and hyperhidrosis were constant. The PPK was finally characterized as transgrediens, non-progrediens, diffuse PPK with a clear delineation between normal and affected skin. The cutaneous modifications at water-immersion test, "hand-in-the-bucket sign", were significantly evident after 3 to 6 min of immersion in the children and father, respectively. AQP5 protein is expressed in eccrine sweat glands (ESG), salivary and airway submucosal glands. In PPKB, gain of function mutations seem to widen the channel diameter of ESG and increase water movement. Thus, swelling seems to be induced by hypotonicity with water entrance into cells, while hyperhidrosis is the result of an increased cytosolic calcium concentration.

Physiological Cooperation between Aquaporin 5 and TRPV4

Aquaporins—among them, AQP5—are responsible for transporting water across biological membranes, which is an important process in all living organisms. The transient receptor potential channel 4 (TRPV4) is a cation channel that is mostly calcium-permeable and can also be activated by osmotic stimuli. It plays a role in a number of different functions in the body, e.g., the development of bones and cartilage, and it is involved in the body’s osmoregulation, the generation of certain types of sensation (pain), and apoptosis. Our earlier studies on the uterus and the literature data aroused our interest in the physiological role of the cooperation of AQP5 and TRPV4. In this review, we focus on the co-expression and cooperation of AQP5 and TRPV4 in the lung, salivary glands, uterus, adipose tissues, and lens. Understanding the cooperation between AQP5 and TRPV4 may contribute to the development of new drug candidates and the therapy of several disorders (e.g., preterm birth, cataract, ischemia/reperfusion-induced edema, exercise- or cold-induced asthma).

Palmoplantar Keratoderma: A Molecular Genetic Analysis of Family Cases

Palmoplantar keratoderma is a clinically polymorphic disorder with a heterogeneous etiology characterized by marked hyperkeratotic lesions on the surface of palms and soles. Hereditary forms of palmoplantar keratoderma usually have autosomal dominant inheritance and are caused by mutations in dozens of genes, most of which belong to the keratin family. We carried out clinical and molecular genetic analysis of the affected and healthy members of four families with autosomal dominant palmoplantar keratoderma. In three out of four family cases of autosomal dominant palmoplantar keratoderma, the following molecular genetic causes were established: in two families—previously non-described missense mutations in the AQP5 gene (NM_001651.4): c.369C>G (p.(Asn123Lys)) and c.103T>G (p.(Trp35Gly)); in one family—a described splice site mutation in the KRT9 gene (NM_000226.4): c.31T>G. In one family, the possible cause of palmoplantar keratoderma was detected—a variant in the KRT1 gene (NM_006121.4): c.931G>A (p.(Glu311Lys)).

AQP5 pathogenic variants induce palmoplantar keratoderma Bothnia type in two Chinese families

Palmoplantar keratoderma Bothnia type (PPKB) is caused by AQP5 pathogenic variants. The mechanisms of this disease and the genotype-phenotype correlation are still not fully understood. We report two pedigrees with PPKB caused by a recurrent variant c.367A>T and a novel variant c.530T>A in the AQP5 gene, respectively. We also summarize the cases with AQP5 variants identified, and found that there seemed to be no significant genotype-phenotype correlation of this disease. Moreover, we noticed that the epidermis of the patient had strong proliferation and immature differentiation potential as well as recognizing the possible important role of TRPV4 in the pathogenesis of PPKB.

Molecular dynamics study of water transport through AQP5-R188C mutant causing palmoplantar keratoderma (PPK) using the gating mechanism concept

It is widely known that any disruption to the water regulation in aquaporins (AQPs) leads to numerous important diseases. However, studies of dynamics and energetics of disease-causing mutations in the aquaporins on the molecular level are still limited. In the present work, the effects of a skin disease-causing mutant, R188C, on the structure of AQP5 and water transport mechanism within this mutated aquaporin are investigated using the concept of gating mechanism. Our results have revealed that the R188C mutation causes a remarkable increase in the pore radius inside the selectivity filter (SF) region facilitating the passage of water molecules. This observation is supported by plotting the free energy profiles of water molecules transport and calculating permeability values through AQP5-R188C, such that the energy barrier in the SF region of the pores was substantially reduced by this mutation, and therefore, the translocation of water molecules was improved. The total averaged osmotic permeability for R188C has been computed as about 11-fold of the wild-type permeability. However, a comparison between the osmotic permeability values related to the open conformation of CE revealed that this coefficient for AQP5-R188C is about 6.5 times larger than that of wt-AQP5, which can be a more accurate value according to the gating mechanism associated with the constriction region of the aquaporin.

Human Aquaporins: Functional Diversity and Potential Roles in Infectious and Non-infectious Diseases

Aquaporins (AQPs) are integral membrane proteins and found in all living organisms from bacteria to human. AQPs mainly involved in the transmembrane diffusion of water as well as various small solutes in a bidirectional manner are widely distributed in various human tissues. Human contains 13 AQPs (AQP0-AQP12) which are divided into three sub-classes namely orthodox aquaporin (AQP0, 1, 2, 4, 5, 6, and 8), aquaglyceroporin (AQP3, 7, 9, and 10) and super or unorthodox aquaporin (AQP11 and 12) based on their pore selectivity. Human AQPs are functionally diverse, which are involved in wide variety of non-infectious diseases including cancer, renal dysfunction, neurological disorder, epilepsy, skin disease, metabolic syndrome, and even cardiac diseases. However, the association of AQPs with infectious diseases has not been fully evaluated. Several studies have unveiled that AQPs can be regulated by microbial and parasitic infections that suggest their involvement in microbial pathogenesis, inflammation-associated responses and AQP-mediated cell water homeostasis. This review mainly aims to shed light on the involvement of AQPs in infectious and non-infectious diseases and potential AQPs-target modulators. Furthermore, AQP structures, tissue-specific distributions and their physiological relevance, functional diversity and regulations have been discussed. Altogether, this review would be useful for further investigation of AQPs as a potential therapeutic target for treatment of infectious as well as non-infectious diseases.

Nagashima-Type Palmoplantar Keratosis: Clinical Characteristics, Genetic Characterization, and Clinical Management

Nagashima-type palmoplantar keratosis (NPPK) is the most prevalent palmoplantar keratoderma (PPK) in East Asia. Homozygous or compound heterozygous loss-of-function mutations in serpin peptidase inhibitor, clade B (ovalbumin), and member 70 (SERPINB7), which encodes members of the serine protease inhibitor superfamily, have been identified as the cause of NPPK. Clinical manifestations of NPPK include well-demarcated erythema, mild to moderate hyperkeratosis on the whole palm, and sole with transgrediens, extending to the dorsal surfaces of the hands and feet, inner wrists, ankles, and the Achilles tendon areas. In this study, we perform a review of relevant clinical cases aimed at elucidating the clinical characteristics, genetic characterization, differential diagnoses, and clinical management of NPPK. A better understanding of the clinical characteristics and pathogenic gene characterization of NPPK will enhance the diagnosis of NPPK, identify related diseases, and inform on the precise therapy and prognosis. Moreover, it will promote the awareness of NPPK in non-Asian regions.

Symmetrical acral keratoderma revisited: proposal for a new term, 'pigmented carpotarsal hyperkeratosis'

First reported from Taiwan mistakenly as acral acanthosis nigricans in 1991, pigmented carpotarsal hyperkeratosis or hyperkeratosis nigricans carpi et tarsi displays a peculiar distribution of velvety brown-grey hyperpigmented plaques symmetrically on the flexural side of the wrists and ankles and on the dorsal sides of the hands and feet. A marked epidermal hyperkeratosis with typically mild acanthosis and papillomatosis is observed in histology. Whitish maceration upon perspiration or water exposure, with exacerbation in summer but remission in winter, is common. The association with obesity, endocrine disorders, atopic dermatitis, ichthyosis or malignancy is unknown. Familial occurrence and hereditary patterns are ill-defined. There is preliminary evidence indicating a pathogenic role of missense mutation in the transcription factor 4 gene. Treatment is empirical, with good outcome with topical retinoids and keratolytic agents. Recurrence is common, and long-term prognosis is unclear. To be distinguished are acral acanthosis nigricans, palmoplantar keratoderma of the Nagashima type, palmoplantar keratoderma of the Bothnian type and aquagenic palmoplantar keratoderma. Most reported cases are from Southern China and are predominantly observed in men between the ages of 20 and 40 years. The currently used term 'symmetrical acral keratoderma' is non-specific and misleading and may lead to global unawareness, underreporting or misdiagnosis of this phenomenon. Further genetic and molecular studies are required to clarify its pathogenesis and relation to palmoplantar keratoderma.

Hereditary palmoplantar keratodermas. Part I. Non-syndromic palmoplantar keratodermas: classification, clinical and genetic features

The term palmoplantar keratoderma (PPK) indicates any form of persistent thickening of the epidermis of palms and soles and includes genetic as well as acquired conditions. We review the nosology of hereditary PPKs that comprise an increasing number of entities with different prognoses, and a multitude of associated cutaneous and extracutaneous features. On the basis of the phenotypic consequences of the underlying genetic defect, hereditary PPKs may be divided into the following: (i) non-syndromic, isolated PPKs, which are characterized by a unique or predominant palmoplantar involvement; (ii) non-syndromic PPKs with additional distinctive cutaneous and adnexal manifestations, here named complex PPKs; (iii) syndromic PPKs, in which PPK is associated with specific extracutaneous manifestations. To date, the diagnosis of the different hereditary PPKs is based mainly on clinical history and features combined with histopathological findings. In recent years, the exponentially increasing use of next-generation sequencing technologies has led to the identification of several novel disease genes, and thus substantially contributed to elucidate the molecular basis of such a heterogeneous group of disorders. Here, we focus on hereditary non-syndromic isolated and complex PPKs. Syndromic PPKs are reviewed in the second part of this 2-part article, where other well-defined genetic diseases, which may present PPK among their phenotypic manifestations, are also listed and diagnostic and therapeutic approaches for PPKs are summarized.

Peripheral neuropathic changes in pachyonychia congenita

We compared patterns of intraepidermal nerve fibers and mechanoreceptors from affected and unaffected plantar skin from patients with pachyonychia congenita (PC) and control subjects. Plantar biopsies from 10 genetically confirmed patients with PC (with a mutation in KRT6A) were performed at the ball of the foot (affected skin) and the arch (unaffected) and were compared to biopsies from corresponding locations in 10 control subjects. Tissue was processed to visualize intraepidermal nerve fibers (IENF) (PGP9.5), subsets of IENF (CGRP, substance P, tyrosine hydroxylase), myelinated nerve fiber (neurofilament H, NFH), blood vessels (CD31), Meissner corpuscles, and Merkel cells (MCs). Structures were quantified using stereology or validated quantification methods. We observed that PC-affected plantar skin had significantly lower sweat gland innervation (sweat gland nerve fiber density) and reduced numbers of Meissner corpuscles compared to PC-unaffected or anatomically matched control skin. In contrast, Merkel cell densities and blood vessel counts were higher in PC-affected skin compared to either control or PC-unaffected skin. There were no differences in myelinated nerve fiber densities, SP, or CGRP between the groups. Pressure pain thresholds in PC-affected skin were lower compared to PC-unaffected and anatomically matched control skin. Additionally, MC densities in callused plantar skin from healthy runners with callus and one subject with a nonpainful palmoplantar keratoderma (AQP5 mutation) were similar to PC-unaffected and control skin consistent with callus alone not being sufficient to increase MC number. These findings suggest that alterations in PC extend beyond keratinocytes and may provide strategies to study neuropathic pain in PC.

Aquaporins and Gland Secretion

Aquaporins (AQPs ) are expressed in most exocrine and endocrine secretory glands. Consequently, summarizing the expression and functions of AQPs in secretory glands represents a daunting task considering the important number of glands present in the body, as well as the number of mammalian AQPs - thirteen. The roles played by AQPs in secretory processes have been investigated in many secretory glands. However, despite considerable research, additional studies are clearly needed to pursue our understanding of the role played by AQPs in secretory processes. This book chapter will focus on summarizing the current knowledge on AQPs expression and function in the gastrointestinal tract , including salivary glands, gastric glands, Duodenal Brunner's gland, liver and gallbladder, intestinal goblets cells, exocrine and endocrine pancreas, as well as few other secretory glands including airway submucosal glands, lacrimal glands, mammary glands and eccrine sweat glands.

Palmoplantar keratodermas: clinical and genetic aspects

Palmoplantar keratodermas comprise a diverse group of acquired and hereditary disorders marked by excessive thickening of the epidermis of palms and soles. Early onset and positive family history suggest a genetic cause. While hereditary forms of palmoplantar keratoderma (PPK) may represent the sole or dominant clinical feature, they may also be associated with other ectodermal defects or extracutaneous manifestations. In recent years, much progress has been made in deciphering the genetic basis of PPK, which has led to the emergence of new disorders and syndromes. The elucidation of disease mechanisms has opened new avenues for specific therapies, increasingly sparking interest in this field. Given the high heterogeneity with respect to clinical features, genetic defects, and disease mechanisms, the classification of PPK is based on various criteria. These include extent of disease manifestations, morphology of palmoplantar skin involvement, inheritance patterns, and molecular pathogenesis. Though not always feasible, the clinical distinction of various PPK entities is based on fine-tuned criteria or clues. Remarkably, apparently distinct disorders have been shown to be allelic, as they are caused by mutations in the same gene. By contrast, similar clinical pictures may result from mutations in different genes. Because of this complexity, mutation analysis is required to determine the precise type of PPK. The best-defined entities are described in this review.

The first Danish family reported with an AQP5 mutation presenting diffuse non-epidermolytic palmoplantar keratoderma of Bothnian type, hyperhidrosis and frequent Corynebacterium infections: a case report

Background

An autosomal dominant form of diffuse non-epidermolytic palmoplantar keratoderma, palmoplantar keratoderma of Bothnian type, is caused by mutations in the AQP5 gene encoding the cell-membrane water channel protein aquaporin 5 leading to defective epidermal-water-barrier function in the epidermis of the palms and soles.

Case presentation

We report the first Danish family diagnosed with diffuse non-epidermolytic palmoplantar keratoderma of Bothnian type in which fourteen individuals are potentially affected. The proband, a 36-year-old male had since childhood been affected by pronounced hyperhidrosis of the palms and soles along with palmoplantar keratoderma. He reported a very distinctive feature of the disorder, aquagenic wrinkling, as he developed pronounced maceration of the skin with translucent white papules and a spongy appearance following exposure to water. The patient presented recurrent fungal infections, a wellknown feature of the condition, but also periodic worsening with pitted keratolysis and malodour due to bacterial infections.

Conclusions

Palmoplantar keratoderma of Bothnian type, which may be associated with hyperhidrosis, is frequently complicated by fungal infections and may be complicated by Corynebacterium infections.

Electronic supplementary material

The online version of this article (doi:10.1186/s12895-016-0044-3) contains supplementary material, which is available to authorized users.

Aquaporin-5: from structure to function and dysfunction in cancer

Aquaporins, a highly conserved group of membrane proteins, are involved in the bidirectional transfer of water and small solutes across cell membranes taking part in many biological functions all over the human body. In view of the wide range of cancer malignancies in which aquaporin-5 (AQP5) has been detected, an increasing interest in its implication in carcinogenesis has emerged. Recent publications suggest that this isoform may enhance cancer cell proliferation, migration and survival in a variety of malignancies, with strong evidences pointing to AQP5 as a promising drug target and as a novel biomarker for cancer aggressiveness with high translational potential for therapeutics and diagnostics. This review addresses the structural and functional features of AQP5, detailing its tissue distribution and functions in human body, its expression pattern in a variety of tumors, and highlighting the underlying mechanisms involved in carcinogenesis. Finally, the actual progress of AQP5 research, implications in cancer biology and potential for cancer detection and prognosis are discussed.

Regulation of the perilymphatic-endolymphatic water shunt in the cochlea by membrane translocation of aquaporin-5

Volume homeostasis of the cochlear endolymph depends on radial and longitudinal endolymph movements (LEMs). LEMs measured in vivo have been exclusively recognized under physiologically challenging conditions, such as experimentally induced alterations of perilymph osmolarity or endolymph volume. The regulatory mechanisms that adjust LEMs to the physiological requirements of endolymph volume homeostasis remain unknown. Here, we describe the formation of an aquaporin (AQP)-based "water shunt" during the postnatal development of the mouse cochlea and its regulation by different triggers. The final complementary expression pattern of AQP5 (apical membrane) and AQP4 (basolateral membrane) in outer sulcus cells (OSCs) of the cochlear apex is acquired at the onset of hearing function (postnatal day (p)8-p12). In vitro, hyperosmolar perfusion of the perilymphatic fluid spaces or the administration of the muscarinic agonist pilocarpine in cochlear explants (p14) induced the translocation of AQP5 channel proteins into the apical membranes of OSCs. AQP5 membrane translocation was blocked by the muscarinic antagonist atropine. The muscarinic M3 acetylcholine (ACh) receptor (M3R) was identified in murine OSCs via mRNA expression, immunolabeling, and in vitro binding studies using an M3R-specific fluorescent ligand. Finally, the water shunt elements AQP4, AQP5, and M3R were also demonstrated in OSCs of the human cochlea. The regulation of the AQP4/AQP5 water shunt in OSCs of the cochlear apex provides a molecular basis for regulated endolymphatic volume homeostasis. Moreover, its dysregulation or disruption may have pathophysiologic implications for clinical conditions related to endolymphatic hydrops, such as Ménière's disease.

Aquaporin-5 is expressed in adipocytes with implications in adipose differentiation

Aquaporins (AQPs) are membrane channels widely distributed in nature. Typically, multiple isoforms are expressed in a single tissue. The adipose tissue is no exception where several AQP members have been identified. The importance of overlapped AQPs expression is unclear, yet interisoforms interactions might be required for key cellular functions. Recently, AQP5 was described as a regulator of other AQP isoforms. Therefore, we hypothesized for a role of AQP5 in adipocyte biology. Gene expression analysis revealed the presence of AQP5 in both 3T3-L1 fibroblasts and adipocytes, being more abundant in the later. AQP5 depletion impaired adipocyte differentiation, which was confirmed by decreased expression of specific differentiation markers. By overexpressing the human AQP5 in mature adipocytes it was possible to ascertain its role as a water channel in a gain-of-function scenario. To our knowledge, this is the first time that AQP5 is reported on adipose tissue. Our data revealed AQP5 as a new player in adipose tissue biology.

Defective channels lead to an impaired skin barrier

Channels are integral membrane proteins that form a pore, allowing the passive movement of ions or molecules across a membrane (along a gradient), either between compartments within a cell, between intracellular and extracellular environments or between adjacent cells. The ability of cells to communicate with one another and with their environment is a crucial part of the normal physiology of a tissue that allows it to carry out its function. Cell communication is particularly important during keratinocyte differentiation and formation of the skin barrier. Keratinocytes in the skin epidermis undergo a programme of apoptosis-driven terminal differentiation, whereby proliferating keratinocytes in the basal (deepest) layer of the epidermis stop proliferating, exit the basal layer and move up through the spinous and granular layers of the epidermis to form the stratum corneum, the external barrier. Genes encoding different families of channel proteins have been found to harbour mutations linked to a variety of rare inherited monogenic skin diseases. In this Commentary, we discuss how human genetic findings in aquaporin (AQP) and transient receptor potential (TRP) channels reveal different mechanisms by which these channel proteins function to ensure the proper formation and maintenance of the skin barrier.

Aquaporins: important but elusive drug targets

The aquaporins (AQPs) are a family of small, integral membrane proteins that facilitate water transport across the plasma membranes of cells in response to osmotic gradients. Data from knockout mice support the involvement of AQPs in epithelial fluid secretion, cell migration, brain oedema and adipocyte metabolism, which suggests that modulation of AQP function or expression could have therapeutic potential in oedema, cancer, obesity, brain injury, glaucoma and several other conditions. Moreover, loss-of-function mutations in human AQPs cause congenital cataracts (AQP0) and nephrogenic diabetes insipidus (AQP2), and autoantibodies against AQP4 cause the autoimmune demyelinating disease neuromyelitis optica. Although some potential AQP modulators have been identified, challenges associated with the development of better modulators include the druggability of the target and the suitability of the assay methods used to identify modulators.

Mutations in SERPINB7, encoding a member of the serine protease inhibitor superfamily, cause Nagashima-type palmoplantar keratosis

"Nagashima-type" palmoplantar keratosis (NPPK) is an autosomal recessive nonsyndromic diffuse palmoplantar keratosis characterized by well-demarcated diffuse hyperkeratosis with redness, expanding on to the dorsal surfaces of the palms and feet and the Achilles tendon area. Hyperkeratosis in NPPK is mild and nonprogressive, differentiating NPPK clinically from Mal de Meleda. We performed whole-exome and/or Sanger sequencing analyses of 13 unrelated NPPK individuals and identified biallelic putative loss-of-function mutations in SERPINB7, which encodes a cytoplasmic member of the serine protease inhibitor superfamily. We identified a major causative mutation of c.796C>T (p.Arg266(∗)) as a founder mutation in Japanese and Chinese populations. SERPINB7 was specifically present in the cytoplasm of the stratum granulosum and the stratum corneum (SC) of the epidermis. All of the identified mutants are predicted to cause premature termination upstream of the reactive site, which inhibits the proteases, suggesting a complete loss of the protease inhibitory activity of SERPINB7 in NPPK skin. On exposure of NPPK lesional skin to water, we observed a whitish spongy change in the SC, suggesting enhanced water permeation into the SC due to overactivation of proteases and a resultant loss of integrity of the SC structure. These findings provide an important framework for developing pathogenesis-based therapies for NPPK.