SCNN1A

Key takeaways

SCNN1A belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SCNN1A to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SCNN1A from memory before moving on to harder problems.

Reference excerpt

The SCNN1A gene encodes for the α subunit of the epithelial sodium channel ENaC in vertebrates. ENaC is assembled as a heterotrimer composed of three homologous subunits α, β, and γ or δ, β, and γ. The other ENAC subunits are encoded by SCNN1B, SCNN1G, and SCNN1D. ENaC is expressed in epithelial cells and is different from the voltage-gated sodium channel that is involved in the generation of action potentials in neurons. The abbreviation for the genes encoding for voltage-gated sodium channel starts with three letters: SCN. In contrast to these sodium channels, ENaC is constitutively active and is not voltage-dependent. The second N in the abbreviation (SCNN1A) represents that these are NON-voltage-gated channels. In most vertebrates, sodium ions are the major determinant of the osmolarity of the extracellular fluid. ENaC allows transfer of sodium ions across the epithelial cell membrane in so-called "tight-epithelia" that have low permeability. The flow of sodium ions across epithelia affects osmolarity of the extracellular fluid. Thus, ENaC plays a central role in the regulation of body fluid and electrolyte homeostasis and consequently affects blood pressure. As ENaC is strongly inhibited by amiloride, it is also referred to as an "amiloride-sensitive sodium channel".

History The first mRNA encoding the alpha subunit of ENaC was isolated by two independent groups by screening a rat colon cDNA library.

Gene structure The human gene SCNN1A is located in the short arm of chromosome 12 (12p3). Human SCNN1A includes 13 exons spanning about 29,000 bp. The protein coding region is located in exons 2-13. The positions of introns are conserved in all four human ENaC genes. The positions of the introns are also highly conserved across vertebrates See: Ensembl GeneTree. Analysis of α subunit mRNA from human lung and kidney showed that during transcription of SCNN1A gene different mRNAs are produced as a result of alternative translation initiation and splicing sites. The isoforms translated from these differ in their activities.

Tissue-specific expression SCNN1A, SCNN1B, and SCNN1G are commonly expressed in tight epithelia that have low water permeability. The major organs where ENaC is expressed include parts of the kidney tubular epithelia, the respiratory airway, the female reproductive tract, male reproductive tract, including testis, spermatogonia in the seminiferous tubules, Sertoli cells, and spermatozoa, epididymis, colon and salivary glands. In the skin, SCNN1A is expressed in the keratinocytes in the epidermal layer, in the sebaceous sweat glands, and the smooth muscle cells mostly within the cytoplasm. In contrast, in the eccrine sweat glands ENaC is mostly located on the luminal surface of eccrine duct epithelia. ENaC is also expressed in the tongue, where it has been shown to be essential for the perception of salt taste. The expression of ENaC subunit genes is regulated mainly by the mineralocorticoid hormone aldosterone that is activated by the renin-angiotensin system.

Protein structure The primary structures of all four ENaC subunits show strong similarity. Thus, these four proteins represent a family of proteins that share a common ancestor. In global alignment (meaning alignments of sequences along their entire length and not just a partial segment), the human α subunit shares 34% identity with the δ subunit and 26-27% identity with the β and γ subunits. All four ENaC subunit sequences have two hydrophobic stretches that form two transmembrane segments named as TM1 and TM2. In the membrane-bound form, the TM segments are embedded in the membrane bilayer, the amino- and carboxy-terminal regions are located inside the cell, and the segment between the two TMs remains outside of the cell as the extracellular region of ENaC. This extracellular region includes about 70% of the residues of each subunit. Thus, in the membrane-bound form, the bulk of each subunit is located outside of the cell. The structure of ENaC has not been yet determined. Yet, the structure of a homologous protein ASIC1 has been resolved. The chicken ASIC1 structure revealed that ASIC1 is assembled as a homotrimer of three identical subunits. The authors of the original study suggested that the ASIC1 trimer resembles a hand holding a ball. Hence distinct domains of ASIC1 have been referred to as palm, knuckle, finger, thumb, and β-ball. Alignment of ENaC subunit sequences with ASIC1 sequence reveals that TM1 and TM2 segments and palm domain are conserved, and the knuckle, finger and thumb domains have insertions in ENaC. Site-directed mutagenesis studies on ENaC subunits provide evidence that many basic features of the ASIC1 structural model apply to ENaC as well.

Associated diseases The disease most commonly associated with mutations in SCNN1A is the multi-system form of type I pseudohypoaldosteronism (PHA1B) that was first characterized by A. Hanukoglu as an autosomal recessive disease. This is a syndrome of unresponsiveness to aldosterone in patients that have high serum levels of aldosterone but suffer from symptoms of aldosterone deficiency with a high risk of mortality due to severe salt loss. Initially, this disease was thought to be a result of a mutation in the mineralocorticoid receptor (NR3C2) that binds aldosterone. But homozygosity mapping in 11 affected families revealed that the disease is associated with two loci on chromosome 12p13.1-pter and chromosome 16p12.2-13 that include the genes for SCNN1A and SCNN1B and SCNN1G respectively. Sequencing of the ENaC genes identified mutation in affected patients, and functional expression of the mutated cDNAs further confirmed that identified mutations lead to the loss of activity of ENaC. In the majority of the patients with multi-system PHA1B a homozygous mutation or two compound heterozygous mutations have been detected. A stop mutation in the SCNN1A gene has been shown to be associated with female infertility.

Interactions SCNN1A has been shown to interact with:

NEDD4L, NEDD4, and Ubiquitin C

Illustrations

Worked examples

Example 1 — a first encounter with SCNN1A

Start with the simplest possible case. Write down what SCNN1A claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SCNN1A before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SCNN1A ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SCNN1A

In research: SCNN1A appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SCNN1A in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SCNN1A is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome 12, Sodium channels, Wikipedia articles with corresponding academic peer reviewed articles, so understanding it makes those chapters shorter.
In everyday life: Look for SCNN1A outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SCNN1A in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SCNN1A means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SCNN1A out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SCNN1A in simple terms?

The SCNN1A gene encodes for the α subunit of the epithelial sodium channel ENaC in vertebrates. ENaC is assembled as a heterotrimer composed of three homologous subunits α, β, and γ or δ, β, and γ.

Why does SCNN1A matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SCNN1A?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SCNN1A.