Everything you need to know about distal renal tubular acidosis in autoimmune disease

The secretion of protons over the apical membrane is for 90 % achieved by the so-called Na⁺-H⁺ exchanger isoform 3 (NHE3) that exchanges sodium for protons over the apical membrane. This transporter is present in the proximal tubule, thick ascending limb and distal convoluted tubule and is dependent on the basolateral Na⁺/K⁺ pump activity [16]. A second mechanism to secrete protons is carried out by the vacuolar H⁺-ATPase located in the distal tubule (10 %). The vacuolar H⁺-ATPase is limited to create a chemical gradient of 10³ of H⁺ over the apical membrane. This limitation is caused by a lack of ATP to keep the transporter functioning at a higher gradient. The maximally reached gradient over the apical membrane is reflected by a decrease in urinary pH from 7.5 to 4.5 [17].

The kidney filters about 4,320 mmol/day of bicarbonate, of which 99.9 % is reabsorbed [16]. The proximal convoluted tubule is responsible for the reabsorption of 80–85 % of filtered HCO₃ ⁻ [18]. Remaining HCO₃ ⁻ is reabsorbed further downstream in the nephron. All intraluminal bicarbonate can be protonated and subsequently reabsorbed. This means that the complete reabsorption of filtered HCO₃ ⁻ requires 4320 mmol/day of secreted protons, which is considerably more than the 70–100 mmol/day of proton secretion required for neutralizing of nonvolatile acids. However, the process of HCO₃ ⁻ reabsorption is not accompanied by net H⁺ excretion.

Secreted protons will also interact with buffers other than HCO₃ ⁻. These buffers originate from metabolic reactions. The most significant buffers are phosphate (pKa = 6.8), urate (pKa = 5.8) and creatinine (pKa = 5.0) [16]. With a lower urinary pH, a higher percentage of the buffer will be protonated, regardless of the pKa of each buffer.

In the proximal convoluted tubule are the so-called sodium-phosphate cotransporters (NaPi) located that are responsible for phosphate reabsorption. Early studies already showed that these transporters are down-regulated in periods of metabolic acidosis [19]. Recent studies indicate that these transporters are directly inhibited by protons, resulting in hyperphosphaturia [20]. Because of its relative high pKa and the pH-dependent reabsorption of phosphate, phosphate is an important buffer.

The amount of buffer that is ultimately excreted in the urine is largely dependent on the GFR and the plasma concentration of the buffer. For example, an average individual with a normal plasma phosphate concentration and normal GFR will excrete approximately 30 mmol/day of phosphate.

Ammonia (NH₃) is extremely important as urinary buffer, because of its high pKa of 9, which means that almost all the ammonia will be protonated to ammonium (NH₄ ⁺). NH₄ ⁺ is in equilibrium with NH₃ and H⁺ in both the intra- and extracellular space of the nephron. Ammonia is produced in every segment of the nephron, but predominantly in the proximal tubule by the metabolism of mitochondrial glutamine (Fig. 1) [21]. Produced ammonium is secreted by the proximal tubule by NHE3-mediated Na⁺/H⁺ exchange and Ba²⁺-sensitive K⁺ channels (ROMK) [22, 23]. Additionally, NH₃ is transported over the apical membrane by still undefined channels. Secreted ammonium will be reabsorbed in the thick ascending limb of Henle’s loop either via the K⁺/H⁺(NH₄ ⁺) exchanger, or by the Ba²⁺-sensitive K⁺ channels (ROMK) or by the Na⁺-K⁺-(2Cl⁻) cotransporter (NKCC2) [24]. Electroneutral K⁺/NH₄ ⁺ exchange and diffusive NH₃ transport across the apical plasma membrane by undefined channels take also place, but are less important. Cytosolic NH₄ ⁺ will mainly exit the tubulus cell via the basolateral NHE4 transporter [25]. A second mechanism of basolateral NH₄ ⁺ exit may involve dissociation of NH₄ ⁺ to NH₃ and H⁺. Transport of NH₃ over the basolateral membrane in the thick ascending limb is presumed to be via diffusion as evidence for a gas transporter for NH₃ in the thick ascending limb is lacking. However, the concept that gasses (NH₃ and CO₂) and water diffuse over the membranes has been questioned over the last years. Instead of diffusion, gasses and water are carried over the membrane by transporters, such as aquaporins and the recently discovered rhesus glycoproteins [26].

The thick ascending limb buffers intracellular produced protons via basolateral bicarbonate transport. This is mediated by the sodium-bicarbonate cotransporter (NBCn1) leading to the formation of H₂CO₃ [27]. H₂CO₃ will be dissociated into H₂O and CO₂, after which CO₂ will be transported over the basolateral membrane into the peritubular lumen.

Ammonium in the peritubular space will be transported in the collecting duct via Na⁺-K⁺-ATPase and Rhesus glycoproteins Rhbg and Rhcg [26, 28]. Intracellular ammonia will be secreted over the apical membrane via the Rhcg glycoprotein and becomes available to buffer secreted protons [26]. Formed ammonium in the collecting tubular lumen is trapped and will be excreted.

The complex system of ammonia transport through the nephron provides the collecting tubule a chemical and concentration gradient over the apical membrane. By altering these gradients, ammonia secretion over the apical membrane in the collecting tubule can be regulated to buffer the secreted protons.

As outlined before, an electronegative luminal potential in the collecting tubule contributes to proton secretion. The ENaC is responsible for this driving force by reabsorbing Na⁺. ENaC’s activity is predominantly regulated by aldosterone. Apart from regulation of ENaC activity aldosterone can enhance distal urinary acidification by increasing the activity of H⁺-ATPase in the cortical collecting tubule [30, 31].

Both genetic and acquired forms of decreased ENaC activity exist. Genetic causes are related to mutations in genes encoding for the alpha, beta or gamma subunit of the channel (respectively, SCNN1A, SCNN1B, SCNN1G genes), resulting in autosomal recessive pseudohypoaldosteronism type 1. An autosomal dominant form in which the genetic defect (NR3C2) affects the mineralocorticoid receptor is also known [32].

Acquired forms of decreased ENaC activity are more common. They are common due to hypoaldosteronism. The most common cause of hypoaldosteronism is hyporeninemia as can occur in diabetes mellitus, renal insufficiency or use of nonsteroidal anti-inflammatory drugs or calcineurin inhibitors [33]. Furthermore, aldosterone is diminished in Addison’s disease. Additionally, medication can directly or indirectly decrease ENaC activity (e.g., amiloride, cyclosporine, tacrolimus, lithium, ACE-inhibitors, angiotensin II receptor blocker, aldosterone receptor blockers and heparin) [32, 33].

Alpha-intercalated cells are responsible for both generation and secretion of protons. The intracellular enzyme CAII catalyzes the reaction leading to the formation of protons and bicarbonate ions. The main proton transporter is the vacuolar H⁺ ATPase, built in the apical membrane. The bicarbonate ion is transported over the basolateral membrane by the AE1. A defect in one of those subparts of the H⁺ secreting machinery can lead to dRTA.

Primary causes for a defect in one of the compartments are due to mutations in genes encoding subunits of the vacuolar H⁺ ATPase (ATP6V1B1 and ATPV6V0A4), resulting in impaired transporter function. These mutations lead to autosomal recessive forms of dRTA that can coexist with and without deafness. Also an autosomal dominant form of dRTA is known, caused by a mutation of a gene coding for the AE1 (SLC4A1), leading to a decreased number of this transporter in the basolateral membrane. Carbonic anhydrase enzyme type 2 deficiency by genetic mutations leads to both proximal and distal RTA [34]. Medullary sponge kidney is also a primary cause of dRTA, related to the malformation of the distal tubules. The presence of dRTA in these patients depends on the number of nephrons affected [35].

Acquired impaired transporter function of the H⁺ secreting machinery is often associated with autoimmune diseases like Sjögren syndrome and SLE. In patients with primary Sjögren syndrome, inhibitory autoantibodies against the CAII enzyme have been reported [36]. Also certain medications, such as topiramate and acetazolamide, can inhibit the function of the CAII enzyme [37].

Proton secretion is dependent on the H⁺ gradient over the apical membrane, which is achieved by vacuolar H⁺ ATPase. Notwithstanding an appropriately working vacuolar H⁺-ATPase, creating of such gradient is not always successful. This is the case in leaky membrane, sometimes seen in patients using amphotericin B [38, 39]. In experimental models, amphotericin B increases the permeability for protons of the apical membrane in the collecting duct, causing back diffusion of the secreted protons [40, 41].

Ammonium plays a major role in renal urinary acidification. In case of low availability of ammonium in urine, urinary acid excretion is impaired to a certain pH. The most important cause of decreased urinary ammonium is hyperkalemia [42]. Hyperkalemia reduces the expression of ammoniagenic enzymes and acid transport proteins [43]. Additionally, hyperkalemia decreases the secretion of ammonia in the loop of Henle and the collecting duct. This probably is due to competition between NH₄ ⁺ and potassium. NH₄ ⁺ and potassium use the same binding spot on the transporters in the thick ascending limb (respectively, NKCC2 and Na⁺-K⁺-ATPase) [44]. Hyperkalemia will also drive protons from intracellular to extracellular, leading to a decreased concentration of protons in the distal tubule cells.

Of the RTA syndromes, also an incomplete form of dRTA is known, including patients with nephrocalcinosis or urolithiasis but without metabolic acidosis. Patients with incomplete dRTA cannot acidify their urine, but a higher amount of NH₄ ⁺ excretion compensates for the acid secretion defect. Donnelly et al. hypothesized that this increased NH₄ ⁺ excretion originates from an increased production and secretion of ammonium in the proximal convoluted tubule. Additionally, hypocitraturia in these patients is often present. Diagnosis and treatment is the same as for complete dRTA [49].

It is suggested that dRTA is more prevalent in autoimmune diseases. Shearn et al. [50] reported in 1965 the first case of dRTA revealing Sjögren syndrome. Both primary and secondary Sjögren syndrome is associated with dRTA [4, 51‐53]. Other autoimmune diseases such as SLE [54], primary biliary cirrhosis (PBC) [55], autoimmune hepatitis (AIH) [56] and autoimmune thyroiditis (AIT) [53] are less common associated with dRTA. The prevalence of dRTA in Sjögren syndrome is currently estimated to be 25 % [4]. The clinical presentation of dRTA in patients with an autoimmune disease is similar to that of those patients without a systemic disease.

The pathophysiological mechanism of dRTA in relation to autoimmunity remains unclear. Several reports suggest that autoantibodies against the CAII enzyme [36, 57] or the acid–base transporters are involved in the pathogenesis of dRTA in autoimmune disease [58]. Recently, Espinosa et al. [59] reported that anti-Ro52 autoantibodies from patients with Sjögren syndrome inhibit Ro52 E3 ligase activity. In vitro inhibition of the ubiquitination process may increase the transcription of pro-inflammatory genes leading to local inflammation and tissue damage [59]. Interstitial inflammation is often found in renal biopsies.

It is unknown whether treatment with corticosteroids in autoimmune disease has a positive effect on dRTA. We advise to treat dRTA in autoimmune diseases with potassium citrate. Potassium citrate is an effective treatment for both the symptoms and complications of dRTA, by restoring acid–base balance (see below). Studies about prognosis of dRTA in autoimmune diseases are lacking.