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Mike Karl Group

Unraveling mechanisms of neuronal degeneration and regeneration in the retina

Portrait Mike Karl


Overall goals

Our overall goals are to decipher and manipulate mechanisms that control the fate of retinal cells throughout their life.  Specifically, we like to find new ways to prolong neuronal live and to replace neuronal cells.

Specific aims

Two of our major current aims are to understand and overcome the barriers of neuronal regeneration in the adult mammalian retina.  By doing so we might learn how to utilize the surviving cells capacity to provide therapeutic benefit.  Further, we want to find out how degenerative processes interfere with endogenous neuroprotective mechanisms and self-repair.

Our approach

Towards our goals we study retinal cells throughout their life, from embryonic stem cells to retinal progenitors to differentiating and mature neurons and glia — with one eye on neuronal regeneration (meaning de-novo neurogenesis) in the mammalian retina — and with another eye on retinal disease pathomechanisms.  We investigate cellular and molecular mechanisms of retinal degeneration and regeneration using rodents and human cells as model systems.

Neuronal regeneration of the retina

Retinal regeneration by de-novo neurogenesis, a damage induced process by endogenous cell sources, is well established in retina of some species, like fish and chick, but does not occur spontaneously in mice and humans.  So far, two major endogenous cell sources for neuronal regeneration have been described: the Müller glia (MG) and retinal pigment epithelial (RPE) cells.

Our goals are to understand and overcome the limitations of regeneration in the mammalian retina, so that endogenous retinal cells are reprogrammed into an adult stem cell and generate adequate progenitor progeny to provide sufficient cell numbers and types for neuronal regeneration. We hypothesize that upon retinal neuronal damage MG or RPE cells undergo defined and controlled changes in cellular and molecular phenotype towards a cell with progenitor properties – this process that we are studying we call regenerative reprogramming.  In detail, currently we investigate the underlying mechanisms that may lead to cell cycle re-entry, so that Müller glia divide to self-renew and generate progenitor and neuronal progeny (see Figure).

Retina degeneration

Along our research on neuronal regeneration we investigate whether or not in neurodegenerative diseases the surviving cells have any capacity that may be utilized to therapeutic benefit.  In essence, most pathological conditions in the mammalian retina lead to changes in Müller glia – often called reactive gliosis, which might be analogous to the response of astrocytes upon injury of other brain regions.  Gliosis is a non-stereotypical response of glia associated with a pathological state that serves as an umbrella term describing various phenotypic changes.  The role of gliosis is controversial since it has been shown to provide neuroprotection and repair as well as being harmful by forming a glial scar (particularly the proliferative form) – or possibly both.  Here one of our major hypotheses is whether gliosis is an aberrant regenerative response or an independent entity.  Further, we currently investigate the interactions between vascular and gliotic pathologies in retinal degeneration and the consequences for neuronal survival in mice.

Mike Karl Research: Figure
Figure: NEURONAL REGENERATION versus SCARRING OF THE RETINA – working model. We are currently using the retina as a model to understand the molecular and cellular mechanisms that limit neuronal regeneration and possibly exacerbate retinal degeneration in mammals. We hypothesize that upon neuronal damage and disease Müller glia respond specifically by de-differentiation, i.e. change in cell phenotype. We developed a robust mouse retina regeneration ex vivo assay to study the mechanisms underlying the proposed processes in our model depicted at the top. Whole retina explant after organ culture stained for BrdU (black dots) is shown in the lower left. Further we developed mouse and human pluripotent stem cell derived retina organoid models to facilitate our studies. Human retina organoid shown in the lower right. Images kindly provided by mokalab members.

Future Projects and Goals

  1. In ongoing research our major aims are to decipher the molecular programs and cellular mechanisms that control and limit neuronal regeneration in mammals.
  2. Müller glia are activated in almost all types of retinal disease, also called gliosis. Here our major question is whether gliosis is an aberrant regenerative response or an independent entity.
  3. We hypothesize that neuronal repair by de-novo neurogenesis (regeneration) is not the only endogenous mechanism to rescue neuronal loss. Here we will investigate neuronal degenerations and the complex interaction of various cell types in the retina, including neurons, glia and vasculature to reveal novel strategies to protect the retina.
  4. We thrive to develop novel model systems to study and manipulate retinal regeneration and degeneration.

Methodological and Technical Expertise

  • investigation of embryonic retina development
  • genetic /toxin induced neuronal degeneration
  • embryonic and adult retina organ culture
  • pluripotent stem cell derived retinogenesis
  • patch-clamp electrophysiology


since 2012
Research Group Leader, DZNE, Helmholtz, Dresden, Germany

since 2010
Research Group Leader, CRTD, TU Dresden, Germany

Postdoctoral Work, University of Washington, Seattle, USA

Research Assistant, Univ. Hamburg, Germany

Junior Research Fellow, Univ. of Pennsylvania, Philadelphia, USA

Doctoral Thesis, Univ. Hamburg, Germany

MD, Univ. Hamburg, Germany

More Information

Karl Group at TUD CRTD

Selected Publications

Santos-Ferreira T.*, Völkner M.*, Borsch O., Haas J., Cimalla P., Vasudevan P., Corbeil D., Michalakis S., Koch E., Karl M.O.° & Ader M.° (*shared first & °corresponding authors)
Differential integration of stem cell-derived photoreceptor transplants into mouse models of cone-rod dystrophy.
Investigative Ophthalmology & Visual Science. 57: 3509–3520. (2016)

Völkner M.V., Zschätzsch M., Rostovskaya M., Overall R.W., Busskamp V., Anastassiadis K. & Karl M.O.
Retinal organoids from pluripotent stem cells efficiently recapitulate retinogenesis.
Stem Cell Reports. 6(4):525–38. (2016) (featured in

Löffler K, Schäfer P, Völkner M, Holdt T & Karl MO.
Age-dependent Müller glia stem cell competences in the mouse retina.
Glia. 63(10):1809–24 (2015)

Karl M.O. & Reh T.A.
Regenerative Medicine for Retinal Diseases: Activating the Endogenous Repair Mechanisms.
Trends Mol Med Apr;16(4):193–202 (2010)

Karl M.O., Hayes S, Nelson B.R., Tan K., Buckingham B. and Reh T.A.
Stimulation of neural regeneration in the mouse.
Proc Natl Acad Sci U S A. 105(49):19508–13. (2008)


Center for Regenerative Therapies Dresden (CRTD)
TU Dresden
Fetscherstraße 105
Dresden, Germany