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Institut für Humangenetik
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Institut für Humangenetik / Arbeitsbereiche / Funktionelle Genetik / Funktionelle Genetik Neu

FUNCTION UNRAVELED BY DYSFUNCTION

We are an international team of physicians, biologists, biochemists and related disciplines, who share an interest for the pathophysiology of monogenic hereditary disorders. To unravel the chain of events that finally results in a specific pathology we use different model systems such as cell lines, iPSC derived organoids, and mouse models. This approach allowed us to unravel some basic cellular mechanisms by studying the consequences of the dysfunction of disease-associated genes. We have a large repertoire of different approaches such as CRISPR/Cas mediated genome engineering, high-throughput sequencing technologies, and advanced microscopy techniques.

 

Past Research and ongoing work

Axonopathies

One of the topics addressed by the group are monogenic axonopathies, neurodegenerative disorders primarily affecting the axon, the primary output process of neurons. The degeneration can primarily affect afferent or efferent axons and thus predominantly result in sensory or motor deficits as observed in polyneuropathies. If the axons projecting from cortical motoneurons to spinal cord motoneurons degenerate, this causes spasticity and muscle weakness of the legs as observed in hereditary spastic paraplegia (HSP). We have previously shown that FAM134B is mutated in Hereditary Sensory and Autonomic Neuropathy 2 (HSAN2) (Kurth et al. 2009). We could show that FAM134B is an ER-resident protein and characterized by the presence of a Reticulon Homology Domain (RHD), which is able to induce membrane curvature. Further functional studies together with the lab of Ivan Dikić allowed us to identify FAM134B as a receptor for the selective degradation of ER fragments (ER-phagy) (Khaminets et al. 2015). We could show that this process requires the recruitment of other membrane shaping proteins such as ARL6IP1, another protein associated with axon degeneration, to remodel ER-membranes efficiently (Foronda et al. 2023).

Figure 1: Use of tdTomato, a fluorescent protein, to label parvalbumin (PVALB)-expressing neurons in the brain of genetically modified mice (in purple) as revealed by Structured Illumination Microscopy (SIM), a super-resolution based-fluorescence microscopy technique. Neurofilament NF-H (the high molecular weight neurofilament subunit) as an intermediate filaments found in neurons in labeled in green. Overall, this technique allows studying PVALB neurons, which are critical inhibitory interneurons in brain regions such as the neocortex and hippocampus, and are implicated in neurological disorders or pathologies such as axonopathy.

Hereditary disorders of ion transport

Ion channels and ion transporters allow the passage of specific ions and electrical charge across membranes. While ion channels mediate the passive flux of ions along the electrochemical gradient, ion transporters can transport ion against existing gradients. The function of several ion transport proteins emerged from mutations in humans or mice. We have previously shown different roles of members of the cation chloride cotransporter family or the family of bicarbonate transporters of the SLC4 for epithelial transport (Boettger et al. 2002, Leviel et al. 2010), synaptic transmission (Hübner et al. 2001) and brain development (Pfeffer et al. 2009).

<em>Figure 2:</em> Localization of the ion transporter Slc4a10 in the mouse brain, visualized by immunohistochemical staining. <b>A:</b> Coexpression of the ion transporter Slc4a10 and parvalbumin (PVALB) in purkinje cells of the cerebellum <b>B:</b> In the choroid plexus, Slc4a10 (NCBE) is expressed in the basolateral membrane, while Slc12a2 (NKCC1) is expressed on the apical side. Nuclei are stained with DAPI. Scale bar: 20 µm
Figure 2: Localization of the ion transporter Slc4a10 in the mouse brain, visualized by immunohistochemical staining. A: Coexpression of the ion transporter Slc4a10 and parvalbumin (PVALB) in purkinje cells of the cerebellum B: In the choroid plexus, Slc4a10 (NCBE) is expressed in the basolateral membrane, while Slc12a2 (NKCC1) is expressed on the apical side. Nuclei are stained with DAPI. Scale bar: 20 µm

Recently, we were able to show that variants in SLC4A10, encoding a Na+ coupled chloride/bicarbonate exchanger are associated with a rare neurodevelopmental disorder with severe cognitive impairment (Fasham et al. 2023).

<em>Figure 3:</em> Workflow for intra- and/or extracelluar electrophysiological recordings from acute brain slicest to reveal changes in ion transport systems as result of genetic changes. <i>Created in BioRender. Hübner, C. (2025) rgmcicq https://BioRender.com</i>
Figure 3: Workflow for intra- and/or extracelluar electrophysiological recordings from acute brain slicest to reveal changes in ion transport systems as result of genetic changes. Created in BioRender. Hübner, C. (2025) rgmcicq https://BioRender.com

Congenital disorders of glycosylation

Previously, we identified mutations in GMPPA in a rare syndrome characterized by alacrima, achalasia and mental retardation (AAMR) (Koehler et al. 2013). Subsequent studies allowed us to show that GMPPA serves as an allosteric feedback inhibitor of GMPPB. This enzyme facilitates the conversion of Mannose-1-phosphate and GTP into GDP-Mannose, which serves as a sugar donor for glycosylation. Consequently, loss of GMPPA increased the production of GDP-Mannose, which leads to hypermannosylation of different proteins including e.g. Dystroglycan. Because Dystroglycan is important for the stability of the sarcolemma and its turnover increased when hypergylcosylated, a mouse model of AAMR syndrome develops progressive motor impairments. Importantly, dietary restriction for Mannose can partially prevent the progressive motor impairments in a mouse model of AAMR-syndrome (Franzka et al. 2021).

 

Congenital kidney disorders

In collaboration with kidney experts in France, our aim is to unravel the disease mechanisms of congenital kidney disorders involving ion-transporting proteins. We have previously generated a mouse model with Clcnkb inactivation that recapitulates Bartter syndrome, characterised by renal salt wasting, hypokalaemia, and metabolic alkalosis. Using patch-clamp techniques on isolated renal tubules, we demonstrated that Clcnkb acts as the primary basolateral chloride channel in both the thick ascending limb and the distal nephron (Hennings et al., 2017). Regarding another protein expressed in the kidney, the ATP6V0A4 subunit of the V-type ATPase, we demonstrated that it plays an unexpected role in the proximal tubule (Hennings et al., 2012). Ongoing studies with Atp6v0a4 mice, acid-loaded C57BL/6 mice and kidney organoids revealed that metabolic acidosis itself impairs proximal tubule function through impaired trafficking and metabolic rewiring (under review). These findings extend the scope of our studies from rare genetic disorders to chronic kidney disease, which affects around 10% of the global population.

 
<em>Figure 4:</em> Lipid droplets in proximal tubule cells under sustained metabolic acidosis, as highlighted by adipophilin immunofluorescence staining (red signal). The nuclei are shown in blue. Scale bar: 5 µm
Figure 4: Lipid droplets in proximal tubule cells under sustained metabolic acidosis, as highlighted by adipophilin immunofluorescence staining (red signal). The nuclei are shown in blue. Scale bar: 5 µm
 

Links:

DFG-funded research projects of the group: https://gepris.dfg.de/gepris/person/1587967

All publications of Prof. Christian A. Hübner: https://tinyurl.com/2bex4a99

 

References:

  • Boettger, T., C. A. Hübner, H. Maier, M. B. Rust, F. X. Beck and T. J. Jentsch (2002). "Deafness and renal tubular acidosis in mice lacking the K-Cl co-transporter Kcc4." Nature 416(6883): 874-878.

  • Fasham, J., A. K. Hübner, L. Liebmann, R. Khalaf-Nazzal, R. Maroofian, N. Kryeziu, S. B. Wortmann, J. S. Leslie, N. Ubeyratna and G. M. Mancini (2023). "SLC4A10 mutation causes a neurological disorder associated with impaired GABAergic transmission." Brain 146(11): 4547-4561.

  • Foronda, H., Y. Fu, A. Covarrubias-Pinto, H. T. Bocker, A. González, E. Seemann, P. Franzka, A. Bock, R. M. Bhaskara, L. Liebmann, M. E. Hoffmann, I. Katona, N. Koch, J. Weis, I. Kurth, J. G. Gleeson, F. Reggiori, G. Hummer, M. M. Kessels, B. Qualmann, M. Mari, I. Dikić and C. A. Hübner (2023). "Heteromeric clusters of ubiquitinated ER-shaping proteins drive ER-phagy." Nature 618(7964): 402-410.

  • Franzka, P., H. Henze, M. J. Jung, S. C. Schüler, S. Mittag, K. Biskup, L. Liebmann, T. Kentache, J. Morales, B. Martínez, I. Katona, T. Herrmann, A.-K. Hübner, J. C. Hennings, S. Groth, L. Gresing, R. Horstkorte, T. Marquardt, J. Weis, C. Kaether, O. M. Mutchinick, A. Ori, O. Huber, V. Blanchard, J. von Maltzahn and C. A. Hübner (2021). "GMPPA defects cause a neuromuscular disorder with α-dystroglycan hyperglycosylation." The Journal of Clinical Investigation 131(9).

  • Hennings, J. C., O. Andrini, N. Picard, M. Paulais, A. K. Hübner, I. K. L. Cayuqueo, Y. Bignon, M. Keck, N. Cornière and D. Böhm (2017). "The ClC-K2 chloride channel is critical for salt handling in the distal nephron." Journal of the American Society of Nephrology 28(1): 209-217.

  • Hennings, J. C., N. Picard, A. K. Hübner, T. Stauber, H. Maier, D. Brown, T. J. Jentsch, R. Vargas‐Poussou, D. Eladari and C. A. Hübner (2012). "A mouse model for distal renal tubular acidosis reveals a previously unrecognized role of the V‐ATPase a4 subunit in the proximal tubule." EMBO Molecular Medicine 4(10): 1057-1071.

  • Hübner, C. A., V. Stein, I. Hermans-Borgmeyer, T. Meyer, K. Ballanyi and T. J. Jentsch (2001). "Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition." Neuron 30(2): 515-524.

  • Khaminets, A., T. Heinrich, M. Mari, P. Grumati, A. K. Hübner, M. Akutsu, L. Liebmann, A. Stolz, S. Nietzsche, N. Koch, M. Mauthe, I. Katona, B. Qualmann, J. Weis, F. Reggiori, I. Kurth, C. A. Hübner and I. Dikic (2015). "Regulation of endoplasmic reticulum turnover by selective autophagy." Nature 522(7556): 354-358.

  • Koehler, K., M. Malik, S. Mahmood, S. Gießelmann, C. Beetz, J. C. Hennings, Antje K. Hübner, A. Grahn, J. Reunert, G. Nürnberg, H. Thiele, J. Altmüller, P. Nürnberg, R. Mumtaz, D. Babovic-Vuksanovic, L. Basel-Vanagaite, G. Borck, J. Brämswig, R. Mühlenberg, P. Sarda, A. Sikiric, K. Anyane-Yeboa, A. Zeharia, A. Ahmad, C. Coubes, Y. Wada, T. Marquardt, D. Vanderschaeghe, E. Van Schaftingen, I. Kurth, A. Hübner and Christian A. Hübner (2013). "Mutations in GMPPA Cause a Glycosylation Disorder Characterized by Intellectual Disability and Autonomic Dysfunction." The American Journal of Human Genetics 93(4): 727-734.

  • Kurth, I., T. Pamminger, J. C. Hennings, D. Soehendra, A. K. Hübner, A. Rotthier, J. Baets, J. Senderek, H. Topaloglu, S. A. Farrell, G. Nürnberg, P. Nürnberg, P. De Jonghe, A. Gal, C. Kaether, V. Timmerman and C. A. Hübner (2009). "Mutations in FAM134B, encoding a newly identified Golgi protein, cause severe sensory and autonomic neuropathy." Nature Genetics 41(11): 1179-1181.

  • Leviel, F., C. A. Hübner, P. Houillier, L. Morla, S. El Moghrabi, G. Brideau, H. Hatim, M. D. Parker, I. Kurth, A. Kougioumtzes, A. Sinning, V. Pech, K. A. Riemondy, R. L. Miller, E. Hummler, G. E. Shull, P. S. Aronson, A. Doucet, S. M. Wall, R. Chambrey and D. Eladari (2010). "The Na+-dependent chloride-bicarbonate exchanger SLC4A8 mediates an electroneutral Na+ reabsorption process in the renal cortical collecting ducts of mice." The Journal of Clinical Investigation 120(5): 1627-1635.

  • Pfeffer, C. K., V. Stein, D. J. Keating, H. Maier, I. Rinke, Y. Rudhard, M. Hentschke, G. M. Rune, T. J. Jentsch and C. A. Hübner (2009). "NKCC1-dependent GABAergic excitation drives synaptic network maturation during early hippocampal development." Journal of Neuroscience 29(11): 3419-3430.