Hochrainer Laboratory

Karin Hochrainer, Ph.D.
Assistant Professor of Neuroscience 

Lab Mission:

We aim to better understand how protein modifications impact neuronal viability in stroke and neurodegeneration, with the goal of harnessing this knowledge to improve the treatment of ischemic stroke and other neurodegenerative disorders.

Research Directions and Goals: 

Ischemic Stroke:

Ischemic stroke is a leading cause of death and disability worldwide, for which effective therapeutic interventions are lacking and thus direly needed. To achieve this, a better understanding of the molecular mechanisms of post-ischemic neuronal death is necessary. Ubiquitination, a post-translational protein modification with the small molecule ubiquitin, is essential for cell viability, particularly under stress conditions. Ischemic stroke poses a severe stress to affected brain cells, and we have found that ubiquitination is highly induced (Hochrainer et al, Stroke 2012, and JCBFM 2015). But how ubiquitination impacts post-ischemic protein function and in particular neuronal survival is unknown. By investigating this we hope to provide tools to modulate this response as a novel strategy for the treatment of ischemic stroke.

1. Post-ischemic ubiquitination targets:

To shed light on post-ischemic ubiquitination targets, we are performing proteomics on tissue derived from a mouse model of ischemic stroke. We are verifying targets and further characterizing them using cell culture, organotypic slice and animal stroke models, as well as gene transfer, gene knockdown and other molecular and biochemical wet lab techniques.

Our preliminary results suggest an enrichment of ubiquitinated proteins at the postsynaptic density (PSD), a highly specialized neuronal compartment whose function is essential for synaptic plasticity and neuronal survival. We are very excited about this finding as it implies a role for post-ischemic ubiquitination in neuronal communication, which may critically influence neuronal health after stroke. Much work lies ahead to dissect the impact ubiquitination has on ischemic targets.

2. Mechanisms leading to increased post-ischemic ubiquitination:

To be able to modify the ubiquitination response after ischemia we need to learn more about the involved mechanisms. Elevated ubiquitination in neurons often is caused by proteolysis failure, which is usually considered a sign of impending cell death. However, our recent work suggests that in case of ischemia the increase in ubiquitination is rather the result of a free radical-mediated reduction in intracellular deubiquitinase (DUB) activity (Kahles et al, CMLS 2021). This suggests a role of post-ischemic ubiquitination in protein regulation rather than proteostasis.

There are close to 100 DUBs in mammalian cells. Using siRNA screening approaches, we are now identifying those DUBs that impact post-ischemic ubiquitination and survival. The goal is to modify DUB activities and consequently neuronal death after stroke.

Tauopathy:

In collaboration with Dr Costantino Iadecola’s laboratory at WCM we investigate how the function of Tau, a protein involved in neurodegeneration, is altered early on in models of Tauopathy and Alzheimer’s disease. 

3. Postsynaptic Tau and protein palmitoylation in Tauopathy:

We unveiled a role of pathogenic Tau in disrupting PSD95-nNOS coupling at the postsynapse leading to decreased production of nitric oxide and neurovascular dysfunction (Park, Hochrainer et al, Nat Neuroscience 2020). These events occur before onset of neurodegeneration and provide a mechanism for neurovascular alterations that could be targeted for preclinical therapy. 

Current efforts concentrate on uncovering the molecular determinants of Tau binding to PSD95, which seem to involve protein palmitoylation, a lipid modification of proteins. To study this, we are using cell culture as well as animal models of Tauopathy, combined with biochemistry, cell, and molecular biology techniques. The goal of this research is to find ways to interrupt this interaction and restore neurovascular function to prevent eventual cognitive decline.

Achievements:

*Discovered and partly characterized the protein family of small HERC HECT domain E3 ligases (PhD and PostDoc work)

*Established that the inflammatory mediator NF-kappaB is regulated by mono- and poly-ubiquitination, as well as acetylation and phosphorylation of the RelA subunit in a gene-specific manner (PostDoc and Early Career work).

*Discovered that ischemic stroke induces protein ubiquitination in the ischemic periphery dependent on reperfusion and independent of tissue damage.

*Established that deubiquitinase (DUB) inhibition by free radicals, and not proteasomal inhibition, is responsible for increases in post-ischemic ubiquitination. 

*Revealed the insoluble accumulation of RNA-binding proteins commonly associated with neurodegeneration in acute ischemic stroke. 

Lab Members:

Current:

*Emma Hambright, Research Technician I
*Luvna Dhawka, Research Technician I
*Gianfranco Racchumi, Laboratory Supervisor (in collaboration with Anrather Laboratory)
*Alicia Savage, Laboratory Manager (in collaboration with Iadecola Laboratory)

Collaborators:

Costantino Iadecola (BMRI)
Josef Anrather (BMRI)
Ping Zhou (BMRI)
Ulrike Resch (Medical University of Vienna, Vienna, Austria)
Agnidipta Ghosh (Albert Einstein School of Medicine, NY, USA)